Wood-Based
Nanocellulose In
Lithium Ion
Batteries and
Electrochemical
Coatings
Hyeyun Kim
Doctoral Thesis,2020
KTH Royal Institute of Technology
School of Engineering Sciences in Chemistry, Biotechnology and Health
Department of Chemical Engineering
Applied Electrochemistry
SE- 100 44 Stockholm, Sweden
© Hyeyun Kim 2020
TRITA-CBH-FOU-2020:12
ISBN: 978-91-7873-453-5
Akademisk avhandling som med tillstånd av Kungliga Tekniska Högskolan
i Stockholm framlägges till offentlig granskning för avläggande av
teknologie doktorsexamen torsdag den 12 Mars 2020 kl. 10:00 i sal F3,
KTH, Lindstedtsvägen 26, Stockholm. Fakultetsopponent: Davide
Beneventi från CNRS
i
Abstract
Lithium ion batteries contain diverse functional polymeric materials, e.g.
binders and separators. Naturally self-assembled wood cellulose can be
disintegrated to nanosized particles with a diversity of morphology by top-
down processes, adjusting the manufacturing parameters. The
nanomaterials can then be reconstructed by bottom-up assembly to
structures similar to that of the polymeric materials in lithium ion batteries,
capable of replacing their functions and ensuring similar or improved
performance.
The aim of the thesis is to evaluate the feasibility of wood-based cellulose
nanofibers in lithium ion batteries and explore other possible applications.
The relationship between the characteristics of nanocellulose, treated by
different processes, and their performance as battery components were
investigated using electrochemical and in-operando measurements.
Development of electrode-integrated cellulose separators was enabled by a
non-aqueous drying method. This significantly improved the drying
efficiency and can be considered an eco-friendly process without using
hazardous chemicals. This study sheds the light on cellulose as a promising
separator material, satisfying the industrial needs without trade-off of
durability of the material and ion transport properties.
Other than lithium ion battery applications, cellulose nanofibrils are
introduced as a pH-responsive polymer and a precursor of hydrogel,
electrochemically coated on any conductive substrate. Not only hydrogel,
this electro-precipitation method also enables to fabricate single or multi-
layered composites. The hydrogel and the composites fabricated by this
technique can work as functional materials in the diverse electrochemical
applications.
In summary, the results indicate that using wood-based cellulose as a raw
material is beneficial to fabricate the functional materials by eco-friendly
manufacturing processes, available for a variety of electrochemical
applications, showing excellent performance.
Keywords
cellulose nanofibers, lithium ion battery, separator, electro-precipitation,
hydrogel
ii
Sammanfattning
Litiumjon-batterier innehåller komponenter, exempelvis bindemedel och
separatorer, som består av polymera material. Naturligt bildat cellulosa
från trä kan finfördelas till nanometerstora partiklar vars morfologi beror
på olika tillverkningsparameterar i sönderdelningsprocessen. Dessa
partiklar kan sedan användas för att återskapa nanomaterial med
strukturer liknande de i de polymera materialen i litiumjon-batterierna,
och som också kan ersätta de senare med likvärdiga eller bättre prestanda.
Det huvudsakliga målet med detta arbete är att utvärdera möjligheten att
använda nanofibrer av cellulosa från trä i litiumjon-batterier. Cellulosa
behandlat på olika sätt har undersökts som material i batterikomponenter,
genom elektrokemiska mätningar och in operando tekniker. Separatorer
av cellulosa integrerade med elektroder visade sig kunna tillverkas i ett
vattenfritt system, vilket avsevärt underlättar dess torkning, i en
miljövänlig process utan skadliga kemikalier. Denna studie lyfter fram
cellulosa som lovande material för separatorer, som motsvarar industriella
behov utan att försämra materialens livslängd och jonledande egenskaper.
Utöver applikationen litiumjon-batteri har nanofibrer av cellulosa
undersökts som pH-känslig polymer och utgångsmaterial för hydrogeler,
vilka har belagts på ledande ytor. Med denna metod kan man också
tillverka kompositmaterial innehållande nanopartiklar i enskilda eller i
flera skikt. Hydrogeler och kompositer syntetiserade genom denna teknik
kan fungera som funktionella material i olika elektrokemiska
tillämpningar.
Sammanfattningsvis, resultaten indikerar att användning av träbaserad
cellulosa som råmaterial är fördelaktigt vid tillverkning av funktionella
material genom miljövänliga processer, i olika elektrokemiska
användningar där de visar utmärkta prestanda.
Nyckelord
nanofibrer av cellulosa, litiumjonbatteri, separator, elektrokemisk
utfällning, hydrogel
iii
List of appended papers Paper I
Lithium ion battery separators based on carboxylated cellulose nanofibers
from wood.
Hyeyun Kim, Valentina Guccini, Huiran Lu, Germán Salazar-Álvarez,
Göran Lindbergh, Ann Cornell
ACS Applied Energy Materials 2018, 2, 1241–1250.
Paper II
Feasibility of chemically modified cellulose nanofiber membrane as
lithium ion battery separator.
Hyeyun Kim, Ulriika Mattinen, Valentina Guccini, Germán Salazar-
Álvarez, Rakel Wreland Lindström, Göran Lindbergh, Ann Cornell
Manuscript
Paper III
Spray-coated nanocellulose based separator/electrode assembly.
Hyeyun Kim, Carl Moser, Ulriika Mattinen, Gunnar Henriksson, Rakel
Wreland Lindström, Göran Lindbergh, Ann Cornell
Manuscript
Paper IV
Different manufacturing processes on tempo-oxidized carboxylated
cellulose nanofiber performance as binder for flexible lithium-ion batteries.
Huiran Lu, Valentina Guccini, Hyeyun Kim, German Salazar-Alvarez,
Goran Lindbergh, and Ann Cornell
ACS Applied Materials & Interfaces 2017, 9, 37712–37720.
Paper V
One-step electro-precipitation of nanocellulose hydrogels on conducting
substrates and its possible applications: coatings, composites, and energy
devices.
Hyeyun Kim, Balázs Endrődi, Germán Salazar-Alvarez, Ann Cornell.
ACS Sustainable Chemistry & Engineering 2019, 7, 24, 19415-19425.
iv
The contributions of the author to these papers are:
Paper I Performed the major experimental works. Oxidation of
the pulp was performed with Valentina Guccini.
Paper II Performed the major experimental works. Valentina
Guccini helped the oxidation of the pulp and the dialysis of the fibers. Gas
evolution measurements were conducted by Ulriika Mattinen.
Paper III Performed the major experimental works. Nanocellulose
sample was prepared by Carl Moser, and Ulriika Mattinen performed the
gas evolution analysis.
Paper IV Participated in the pulp oxidation and performed the
rheology measurements of TOCN gels.
Paper V Performed the major electrochemical works and physical
characterization. Hydrogel coating for HER selective layer was performed
with Balázs Endrődi.
v
List of abbreviations
CMC Carboxyl methyl cellulose
CNF Cellulose nanofibers
CV Cyclic voltammetry
DF Degree of fibrillation
ECSA Electrochemically active surface area
EDX Energy dispersive X-ray spectroscopy
FT-IR Fourier transform infrared
HER Hydrogen evolution reaction
IPA Isopropanol
LCO Lithium cobalt oxide (LiCoO2)
LFP Lithium iron phosphate (LiFePO4)
LIB Lithium ion batteries
LiPF6 Lithium hexafluorophosphate
LMO Lithium manganese oxide (LiMn2O4)
LTO Lithium titanium oxide (Li4Ti5O12)
NMC Lithium nickel manganese cobalt oxide (LiNi1/3Mn1/3Co1/3O2)
NMC622 LiNi0.6Mn0.2Co0.2O2
NMP N-Methyl-2-pyrrolidone
PVDF Polyvinylidene fluoride
PVDF-HFP Poly(vinylidene fluoride-co-hexafluoropropylene)
RDE Rotating disc electrode
SEA Separator-electrode assembly
SEI Solid electrolyte interface
SEM Scanning electron microscopy
TOCN TEMPO-mediated oxidized cellulose nanofibers
VC Vinylene carbonate
vi
Contents
1. Introduction ................................................................... 1
1.1. Scope of the thesis .................................................................. 2
2. Background ................................................................... 3
2.1. Lithium ion batteries ............................................................... 3
2.2. Lithium ion battery separators ............................................... 4
2.3. Wood based nanocellulose .................................................... 5
2.4. Nanocellulose in LIB ............................................................... 6
2.5. Separator-electrode assembly (SEA) .................................... 7
2.6. Gas evolution in LIB ................................................................ 9
2.7. Electrochemical coating of biopolymers and its
applications ........................................................................................ 10
3. Experimental ............................................................... 12
3.1. Preparation of cellulose nanofibers (CNF) .......................... 12
3.1.1. TEMPO-mediated oxidation of cellulose ..................... 12
3.1.2. Enzymatically treated CNF ............................................ 12
3.2. CNF-based LIB separators .................................................... 13
3.2.1. Fabrication of freestanding separators ....................... 13
3.2.2. Spray-coated cellulose-based SEA .............................. 13
3.2.3. Electrochemical characterization of LIB separators .. 14
3.2.4. Gas evolution analysis by mass spectrometry .......... 15
3.3. Rheology measurements of cellulose nanofibers.............. 16
3.4. Electro-precipitation of nanocellulose hydrogels and its
applications ........................................................................................ 16
3.4.1. Electro-precipitation of TOCN hydrogels .................... 16
3.4.2. Electro-precipitation of composites ............................ 16
3.4.3. Application I: Eco-friendly fabrication of electrode for
LIB 17
3.4.4. Application II: Selective layer for hydrogen production
17
3.5. Physical characterization of samples .................................. 18
4. Results and Discussions............................................ 19
4.1. CNF based separators with different charge density ........ 19
vii
4.1.1. Characterization of CNF and separators thereof ....... 19
4.1.2. Electrochemical performance of TOCN separators ... 22
4.1.3. CNF separators with different charge density ........... 25
4.2. Spray coated cellulose-based separator-electrode
assembly (SEA) .................................................................................. 27
4.2.1. Fabrication of SEA ........................................................ 27
4.2.2. Electrochemical performance of CNF-SEA ................ 31
4.2.3. Electrochemical performance of SiO2-CNF-SEA ........ 34
4.3. Gas evolution of LIBs with CNF separators ........................ 36
4.4. Discussion about CNF-based LIB separators ..................... 42
4.5. Rheological properties of TOCN ........................................... 44
4.6. Electro-precipitation of CNF hydrogel, composites thereof,
and its applications ............................................................................ 46
4.6.1. Electrochemical precipitation of TOCN Hydrogels .... 46
4.6.2. Electro-precipitation of composites ............................ 49
4.6.3. Application I: Fabrication of electrode for LIB ........... 51
4.6.4. Application II: Selective membrane for HER .............. 52
5. Conclusions and outlook ........................................... 55
5.1. CNF based separators with different charge density ......... 55
5.2. Spray coated cellulose-based separator-electrode
assembly ............................................................................................. 55
5.3. Electro-precipitation of TOCN hydrogel, composites
thereof, and their applications .......................................................... 56
5.4. Outlook .................................................................................... 56
6. Acknowledgements .................................................... 58
7. References .................................................................. 59
Introduction - 1
1. Introduction
Increase in energy consumption inevitably brings about global
environmental pollutions and climate change. Today, dramatic conversion
of energy sources from traditional fossil fuels to renewable energy has been
gradually achieved overcoming economical and technical barriers. Lithium
ion batteries (LIB) appeared in the 1990’s in market and have been rising
as the major power sources for small portable electronics to large scale in
transportation, electric vehicles and electric grid. They have been
highlighted as appealing energy storage systems, which can fulfill the
demands not only for low carbon emission but also for high energy density
and stable performance capable of long-term operation. The global LIB
market is forecasted to grow from 120 GWh of sales volume in 2017 to 215
GWh by 20251,2. However, the rapid growth of the automotive industry and
energy markets will entail the production of a giant amount of waste.
Namely, despite of endeavors to reduce the reliance on fossil fuels moving
toward energy production and storage systems with lower environmental
impacts, ironically, the alternative technologies are still dependent on high
consumption of petroleum-based materials and not environmentally
friendly processes. For instance, upstream material acquisition processes
including mining and extraction of inorganic compounds can result in
significant environmental pollution. Furthermore, the-state-of-the-art
manufacturing process of LIB uses plastics and organic components in
preparation of electrode slurry, binders, separators, electrolytes, and
packaging3. Therefore, urgent actions should be taken to ensure
sustainable consumption and production patterns, which is one of 17 goals
of United Nations to tackle climate change4.
A push to develop such sustainable manufacturing processes implies the
demands for applications of biodegradable materials emitting less carbon
dioxide and pollutants during the waste treatment processes5–8. However,
an important challenge of application of bio-based materials for large
scaled commercial batteries is their high risk of introducing trace water
into LIB, which influences on durability and safety. Nevertheless, it is
believed that a better understanding of the reactions of the materials in LIB
can make them more wide-spread in the market, applied for commercial
cells.
Apart from LIB, such biopolymers have obtained significant attention as
alternative to plastics, due to their similarities in terms of optical,
Introduction - 2
dimensional, chemical and mechanical characteristics. Their hygroscopic
properties make them more attractive in versatile applications, e.g.
biomedical engineering, drug delivery, tissue engineering and
adsorbents9,10. Not only replacing materials, but also developing facile and
efficient fabrication techniques to obtain materials with a desired property
by lower energy input is important. We believe that such research efforts
will be able to widen the spectrum of applications of biopolymers in a
variety of fields as functional materials.
1.1. Scope of the thesis
The main goal of this thesis was to examine the feasibility of wood-based
nanocellulose in LIB. Nanocellulose has been investigated as a promising
candidate replacing inactive components in LIB such as binders or
separators and reported good performance, comparable to conventional
materials. By tuning the manufacturing processes, by top-down treatments,
pulp can be disintegrated to nanosized materials with desirable
dimensions depending on its target applications. This study focuses on the
comparison of the characteristics of nanocellulose, produced by different
treatments, and their performance as functional materials in
electrochemical applications. Depending on the dimensional properties of
nanocellulose, the applications varied from separators to binders and their
electrochemical performance was evaluated in LIB. Microfibrillated
cellulose with nanofibril networks was highlighted as a promising
candidate for a lithium ion battery separator, particularly, for the fast
charging applications. The thesis includes failure and success cases of
nanocellulose separators. The cause of cell failure was elucidated by
electrochemical and in-operando gas measurements. Development of
facile and eco-friendly fabrication method for cellulose based separator-
electrode assembly (SEA) is introduced as well.
Other than LIB applications, individually disintegrated cellulose
nanofibrils were fabricated to hydrogel or composites by electrochemical
precipitation and further applications in electrochemical energy devices,
e.g. hydrogen production were investigated.
Background - 3
2. Background
2.1. Lithium ion batteries
The primary components of LIB are positive and negative electrodes,
separator, current collectors and electrolyte. A principle of LIB is that
transport of lithium ions between two electrodes generates free electrons
which can flow through an external circuit storing and providing electric
power during the charging and the discharging processes.
Three different types of positive electrodes mainly used are (1) layered
lithium transition metal layer oxides, e.g. lithium cobalt oxide (LiCoO2,
LCO), lithium nickel manganese cobalt oxide (LiNixMnyCo1-x-yO2, NMC), (2)
polyanions, e.g. lithium iron phosphate (LiFePO4, LFP), (3) spinel
materials, e.g. lithium manganese oxide (LiMn2O4, LMO)11. NMC as a
positive active material has taken the highest market share (29% in 2015)
followed by LCO (26%) and LFP (23%)2. Due to high nominal potential (≈
3.7 V vs Li+/Li), high capacity and energy density with good cycling stability,
it is expected that the global supply and demands of NMC will grow more
in the future. Graphite and carbonaceous materials have been used as
major negative electrodes in LIB, taking more than 90% of the market
share in 20152. Silicon based compounds and lithium titanium oxide
(Li4Ti5O12, LTO) are also used as negative active materials. Aluminum and
copper foils are used as the current collectors for the positive and the
negative electrodes, respectively.
An electrolyte is an ion conductor, facilitating transport of lithium ions
between the electrodes. The commonly used liquid electrolytes are
composed of lithium containing salts e.g. lithium hexafluorophosphate
(LiPF6), lithium bis(trifluoromethylsulfonyl)imide, or lithium perchlorate,
dissolved in carbonate solvents, such as ethyl carbonate or diethyl
carbonate. As binder, polymeric materials such as polyvinylidene fluoride
(PVDF), carboxyl methyl cellulose (CMC) and styrene butadiene have been
widely used12,13.
A separator is one of the major components in LIB and its primary role is
to prevent the direct contact between the positive and the negative
electrodes. The detailed process of manufacturing and characterization of
lithium ion battery separators and recent progress of developments will be
more explained in the following section.
Background - 4
2.2. Lithium ion battery separators
Lithium ion battery separator is a key safety system in LIB, preventing
short circuit and further incidents. It needs to be a thin porous membrane,
which can transport lithium ions between the positive and the negative
electrodes, while blocking their direct contact. To achieve low resistivity
and high volumetric/gravimetric capacity of LIB, commercial separators
have 7-25 µm of thickness preferably and ≈ 40% of porosity. The
development of thinner separators improves the performance of LIB but
also takes high risk of compromising the durability of and the safety of a
battery cell. For example, an exceptionally thin separator was blamed as
the contributory factor of fire or explosion incidents of electronic devices14.
The commercially available separators are predominantly made of
polyolefin materials, e.g. polyethylene (PE, melting point, tm, ≈ 135 °C),
polypropylene (PP, tm ≈ 160 °C), or PVDF (tm ≈ 177 °C) and polyethylene
terephthalate (PET, tm ≈ 260 °C) 3,15–18. The production of the porous
membranes is achieved by stretching the polymer sheets in wet and dry
status12,16. Since the separator is wound with the electrodes under high
tension during the cell assembly process, materials with high tensile
strength and Young’s modulus are required12. The polyolefin materials
with low melting temperature are advantageous for thermal shutdown
function, but susceptible to thermal runaway. Recent research efforts have
highlighted the importance of surface coatings of ceramic particles on the
commercial PE separators, which enhance their thermal stability and
dimensional integrity even at an elevated temperature, above 200 °C2,17,19.
As the demand for LIB rapidly increases, the total market for separators is
estimated to grow from 900 Mm2 in 2015 up to 2700 Mm2 in 202512. The
cost of the separators is expected to decrease considerably from 23% of a
cell cost in 200220 to ≈12% in 2030 (recalculated including the costs only
for materials) 1,21. It would be beneficial if separators could be produced
with lower costs, preferably < 2 $/m2 16,22,23. In addition, considering the
quantity of separators (that is, the high consumption of plastics) and the
manufacturing processes producing a large amount of waste solvents,
more efforts should be made to produce separators with environmentally
friendly materials and sustainable manufacturing processes2,12,24–26
Background - 5
2.3. Wood based nanocellulose
Cellulose is the most abundant biopolymer, primarily obtainable from
plant resources, e.g wood, which takes 30-40% of the cell wall components.
In the plant cell wall, polysaccharides are enzymatically synthesized to
cellulose microfibrils with ≈ 4 nm in width and assembled into micro-sized
fibers with highly oriented and hierarchical structure27,28. The cellulose
fibers are the main constituent of wood, making it strong and supporting
its structure.
The naturally assembled hierarchical structure of cellulose fibers is
advantageous to produce nanofibers, by delaminating the microfibrils.
However, strong interfibrillar bonding and the presence of crystalline
regions require high mechanical energy input for defibrillation. In order to
overcome such recalcitrance of biomass and obtain cellulose nanofibers
(CNF) with high efficiency, a variety of processes have been investigated
e.g. mechanical treatment 29, enzymatic treatment 30–32, chemical
modification such as (2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-
mediated oxidation 33 and carboxymethylation 34.
Enzymatic hydrolysis is a common biological treatment to soften pulp,
which degrades cellulose chains using cellulases. Endoglucanase is one of
the widely used enzymes for pulp treatment, which can access less
crystalline regions in the cellulose chains and specifically hydrolyze
glycosidic bonds 30. Consequently, such enzymatic hydrolysis contributes
to loosening strong interfibrillar fiber networks and facilitates the
production of cellulose nanofibers by successive mechanical disintegration 35,36. It has been reported that enzymatically treated cellulose is mostly
micro-fibrillated cellulose, composed of partially delaminated nanofibril
networks.
TEMPO-mediated oxidation is one of the most commonly used chemical
modification methods to produce finely dispersed cellulose nanofibers.
During the oxidation process, hydroxyl groups at C6 positions of the
glucose units of cellulose chains are substituted to carboxylated groups
with sodium counterions. Introduction of the negatively charged groups
increases the electrostatic repulsion between fibrils and facilitates
consecutive mechanical disintegration of nanofibers. Combined with such
chemical modification and successive mechanical disintegration, this
method even enables production of individual TEMPO-oxidized cellulose
nanofibers (TOCN) with 3- 4 nm in width and a few microns in length. Such
Background - 6
nanofibrils can be dispersed in water with high colloidal stability. Due to
its high dispersibility and novel physicochemical properties, the TOCN has
been investigated in a variety of applications, e.g. energy devices, photonics,
biomedical engineering, drug delivery, absorbents6,9,37,38. It is very unique
and advantageous that such highly dispersed nanofibrils with low
dimension in width can be obtained from plant cellulose, which is not
available from other resources, such as bacterial (10-20 nm) or algae
cellulose (5-60 nm) 27,28,39.
Dimensions of cellulose nanofibers and their characteristics can be varied,
depending on the disintegration strategies and the parameters of the
production processes, e.g. the charge density, the degree of fibrillation, the
dose of enzymes. That is, nanocellulose with a desired dimension for the
target application can be acquired by adjusting the manufacturing process 40–42. The following sections will introduce the applications of wood-based
nanocellulose with different dimensions to electrochemical energy devices,
e.g. LIB and H2 production, and an electrochemical coating technique.
2.4. Nanocellulose in LIB
Cellulose has been highlighted as a promising candidate for future eco-
friendly material, which can replace some portion of petroleum-based
plastic materials. Even in LIB research, cellulose and its derivatives have
been investigated as binder, separator or precursor of carbon
materials36,38,40,43–49.
The composites containing cellulose nanofibers and active materials can
be fabricated to free-standing paper based electrodes by a paper-making
filtration process 38,44,48. Thanks to its good dispersibility in water, LIB
electrodes can be fabricated by aqueous based processes.
Not only as binders, cellulose has also been used as a promising separator
material due to its highly porous structure allowing transport of ions,
hydrophilicity absorbing liquid electrolyte efficiently, high thermal
stability, abundance and low material cost. In this respect, cellulose-based
separators have been already widely used in primary batteries for many
years. In LIB, Kuribayashi45 first demonstrated a cellulose separator with
submicron sized pores, capable of working in a LiCoO2 /carbon full cell in
the potential range between 2.7 to 4.2 V vs Li+/Li. The cellulose based
separators can be easily fabricated by a paper-making filtration
process36,40,47. Zolin et al.40 demonstrated the effect of the degree of beating
Background - 7
on the morphology and the electrochemical performance of the cellulose
separators in a bendable paper based flexible battery assembly. Chemically
modified cellulose or its derivatives have been investigated as separators
or gel-polymer electrolytes48,49. Leijonmarck et al. reported a single-paper
flexible LIB cell using TOCN both as binder and separator materials38.
Not only plant based cellulose, but also ones from other natural resources
have been examined as LIB separators, e.g. bacterial or algae cellulose 47,50,51. They have great advantages of naturally assembled mesoporous
structures and high crystallinity, which can result in high mechanical
strength and low risk of trace water in the cellulose chains47,50. However,
supply chain and extra energy input required to eliminate organism prior
to mechanical dispersion 51 would be considered as the key factor of the
commercialization of such materials.
2.5. Separator-electrode assembly (SEA)
A separator-electrode assembly is an integrated form of those two
compartments, which can be found in electrochemical devices, e.g.
microbial fuel cells52 and lithium ion batteries53–63, as shown in Figure 2.1.
Figure 2. 1. Schematic illustration of (A) a typical battery cell assembly
and (B) a SEA with a separator integrated with an electrode
In the state-of-the-art battery manufacturing process, the separately
produced separator and electrodes are assembled and wound to cylindrical
or prismatic cells. To achieve high production yield, a separator must have
sufficiently high tensile strength and Young’s modulus not to be broken
Background - 8
and deformed during the winding process. Such requirement limits the
commercialization of highly porous functional materials with relatively low
mechanical strength.
On the contrary, SEA is beneficial as it improves the mechanical strength
of the separators, reinforced by the metallic current collector, in the in-
plane direction. This means that, as long as the separator is attached to the
surface of the electrode, the tensile strength of the separator relies on that
of the current collector, rather than its own lesser strength. For practical
purpose, Toshiba, a Japanese lithium ion battery manufacturer, recently
announced the production plan of their new skin-coated electrode (ScdE)63.
The company highlights the advantages of the thin highly porous separator
made by electrospinning; low resistivity and highly improved capacity (≈
20%) of LIB.
SEA is an appealing approach, capable of widening the spectrum of
commercially available materials. Most of SEA studies have been
predominantly using inorganic nanoparticles e.g. Al2O3, SiO2, ZrO2,
Li7La3Zr2O12, with polymer binders such as PVDF, poly(vinylidene
fluoride-co-hexafluoropropylene) (PVDF-HFP), PVA (polyvinyl alcohol)
and poly(phenyleneoxide), as the primary materials 53–63. The inorganic
nanoparticles are advantageous to construct a separator with high surface
porosity, whereas there is a high risk of non-uniform deposition, which can
in the end affect the durability of the materials and the electrochemical
performance as well. In terms of polymer binders, PVDF requires to be
dissolved in hazardous solvents such as N-Methyl-2-pyrrolidone (NMP) or
N, N-dimethylformamide (DMF).
Another challenge with SEA is the cell configuration and the selection of
an electrode, on which to coat the separator. In a typical battery cell
configuration, the size of the negative electrode is similar or slightly larger
than that of the positive electrode64,65. In previous studies, coating the
separator part on the negative electrode (graphite or LTO) has been
commonly implemented54–56,60–62. However, volume expansion of graphite,
≈10%, can influence on the durability of the separator in SEA. Replacing
graphite with LTO, which has much lower volume change during battery
cycling, can improve its stability, 56,57,61,62 but it compromises the potential
and energy densities of the battery. Rather, coating the separator on the
positive electrode would be preferred, but only a few such cases have been
presented58. Considering the high market share of graphite and NMC,
SEAs suitable for a full cell consisting of a pair of those electrodes would
Background - 9
be the most favorable form, more likely where the separator is coated on
the positive electrode side.
2.6. Gas evolution in LIB
In-situ monitoring of LIB has provided better understanding about
fundamental mechanisms related to aging and promising next generation
energy storage materials26,66–69. In-operando mass spectrometry has been
highlighted as a powerful technique in LIB studies, in particular to measure
gases evolved and consumed during the LIB operation67,70–74. Gas
evolution in LIB during storage and in use is the major cause of cell failure
and further accidents75,76. Gas accumulated in the cell can result in poor
contact between LIB materials, and their delamination. It can further
increase the internal pressure and swelling of the cell and, in the end, likely
lead to explosion, risking serious damage of life and property.
The mechanism of gas formation in LIB has been explained related to the
effects of trace water67,70,71, electrolyte additives72,73, and the decomposition
of battery materials74,77, operating parameters, and prolonged cycle life of
the batteries71,78 For example, Metzger et al.70 elucidated the origin of H2
gas in LIB and effect of vinylene carbonate (VC) additive on suppressing
gas evolution using on-line electrochemical mass spectrometry.
Gases mainly detected in LIB are H2, C2H4, CO2, and CO. In the presence
of trace water, a significantly high amount of H2 gas can be measured by
mass spectrometry in the beginning of the cycling, along with the reduction
of trace water and the solid electrolyte interface (SEI) layer formation71.
Therefore, the H2 gas evolution can be an indicator to evaluate the
electrochemical stability of a hygroscopic material or LIB materials
produced by aqueous processes and explain the cause of cell failure or
other undesirable side reactions in LIB. In addition to reduction of trace
water, protic carbonate decomposed products at the cathode can diffuse to
the graphite and be reduced generating H2 gas during the cycling70. The
evolution of C2H4 is accompanied by the decomposition of electrolyte
solvent, thus the high amount is generated in the first charging due to SEI
layer formation and its evolution slows down during the cycling70,71. The
evolution reactions of CO2 and CO are dependent on the temperature and
the upper cut-off voltage79.
To meet an increasing interest in sustainability issues in LIB and reducing
the environmental impact in manufacturing processes, a variety of bio-
Background - 10
based materials have been applied and shown good electrochemical
performance7,26,47,50. Such bio-based materials are likely to contain surface
functional groups which can give materials better dispersible properties in
a solvent but also hygroscopic properties, unfavorable in LIB. Therefore,
prior to using bio-based materials, more comprehensive and careful
investigations about their chemistry and reaction mechanisms in LIB are
required. In this respect, an in-operando gas analysis system may provide
crucial information about the chemical or electrochemical stability a
material, especially, when it has a risk of introducing trace water into LIB.
2.7. Electrochemical coating of biopolymers and its
applications
Electrodeposition or electro-precipitation techniques have received
attention as a method to construct biopolymer-based hydrogels on desired
substrates in response to electrical signals. Chitosan was the first
biopolymer used for electrodeposition and its applications have been
extended to biosensors, enzyme immobilization, and drug delivery80, 81.
Other than chitosan, alginate82,83, gelatin84, CMC85, and silk fibroin86 have
been reported as stimuli responsive biomaterials capable of electro-
precipitation.
The most widely used mechanisms of electro-precipitation of such
biopolymer hydrogels are the reactions induced by i) pH dependent sol-gel
transition82,86–88 and ii) metal-ions coordination80,81,83,89.
For the pH dependent sol-gel transition, electrochemical water splitting
can be used to generate pH gradients at an electrode surface. In a water
splitting electrochemical cell, the O2 evolution reaction (OER) takes place
at the anode (2H2O ⟶ O2(g) + 4H+ + 4e-), where protons generated
decrease the local pH near the electrode. On the other hand, H2 gas and
hydroxide ions are produced (H2 evolution reaction, HER), leading to a
local pH increase at the cathode (2H2O + 2e- ⟶ H2 (g) + 2OH-). Alginate,
containing sodium carboxylate groups, can aggregate in acidic conditions
as sodium ions are substituted to protons and the surface charge is
neutralized (COO–Na+ + H+ → COOH + Na+). Taking another example,
chitosan chains are neutralized at alkaline conditions, thus chitosan can be
coated on the cathode surface in response to a local pH changes induced
by water splitting (chitosan – NH3+ + OH- → chitosan – NH2 + H2O) 87. Silk
Background - 11
fibroin is known to be electro-precipitated at a pH below 4.486. Wang et al. 88 reported on the fabrication of alginate/chitosan layer-by-layer
composites by alternately changing the direction of the electric field and
the coating medium. Since the electrochemical reaction-induced
precipitations of biopolymers occur in the diffusion layer of an electrode,
the growth of the hydrogel can be simply controlled by varying the
experimental parameters, such as the applied current or voltage.
Metal-ions coordinated gelation takes place at the anode surface, since the
it requires metal ions generated by the anodic dissolution. For instance,
chitosan hydrogel can be coated on the cathode surface in pH-responsive
gelation88, while it can be formed on the anode surface by a metal-ion
coordinated sol-gel transition reaction81.
It is known that most polysaccharides can be electro-precipitated to
hydrogel films on the anode surface, due to charge neutralization by a
localized pH gradient83. Considering the presence of functional groups of
TOCN, similar to alginate, TOCN can be also regarded as a pH-responsive
polymer, showing immediate aggregation at acidic conditions at pH < ≈
pKa 4.8 due to the protonation and charge neutralization of cellulose chains,
in the same way as alginate90. Such stimuli responsive characteristic makes
TOCN a more attractive material to be tuned and fabricated to hydrogel
materials for versatile applications, such as biomedical engineering, drug
delivery, tissue engineering and adsorbents9,10,85,91.
As a further benefit of electro-precipitation, nano-structured composite
materials can be easily fabricated80,82,89,92. Nanoscale particles dispersed in
the coating dispersion with stimuli responsive biopolymers can get
immobilized on the conductive substrates, simultaneously with electro-
precipitation of the biopolymers.
Experimental - 12
3. Experimental
3.1. Preparation of cellulose nanofibers (CNF)
3.1.1. TEMPO-mediated oxidation of cellulose
Never dried cellulose pulp (Domsjo Fabriker AB, Ornskoldsvik, Sweden)
was oxidized, following the earlier reported protocols by Saito et al.93.
Briefly, 40 g of pulp washed by a HCl solution (pH 2) was mixed with a 4
mmol of TEMPO and 40 mmol of NaBr in DI water. Pulps with different
charge densities (350, 650 and 1550 µmol COO-/g cellulose) were produced by
adjusting the amount of NaClO added. A 0.5 M NaOH solution was added
dropwise to keep the pH of the suspension at 10 during the reaction. 1
wt % TEMPO-mediated oxidized cellulose pulps suspended in water were
disintegrated by a micro-fluidizer (M-110EH, Micro fluidics Corp, United
States). The number of passages through the micro-fluidizer, degree of
fibrillation (DF), varied depending on the desired dimension of
nanocellulose. The TEMPO-mediated oxidized cellulose nanofibers were
named depending on their charge density. For instance, TOCN with 350
µmol/g was denoted TOCN350, while the ones with 650 and 1550 µmol/g
were TOCN650 and TOCN1550, respectively.
The TOCN suspensions treated once in the microfluidizer (DF 1) are used
to fabricate LIB separator materials (Paper I and II). Depending on the
types of functional groups, they were specifically denoted, e.g. TOCN350-
COO-Na+ and TOCN650- COO-Na+.
TOCN with 350, 650, and 1550 µmol/g treated by 6, 9, 12 times are
presented in Paper IV and V. Detailed protocols are addressed in Papers
I, II, IV and V.
3.1.2. Enzymatically treated CNF
Enzymatically treated fibers were manufactured following the protocols
reported by M. Henriksson et al.30. The pulp was dispersed in DI water,
passed through the 400 μm- and 200 μm-chambers once (DF 1) and a 1
wt % of aqueous suspension was prepared (Paper II).
The enzymatically treated cellulose dispersed in isopropanol (IPA) was
prepare by pretreating at 10% consistency in a phosphate buffer (11 mM
Na2HPO4 and 9mM NaH2PO4) using 25 ECU endoglucanase per gram of
Experimental - 13
dry fiber for 1 hour at 55°C. The pretreated fibers were washed and diluted
with IPA to 1 wt % and then processed in a microfluidizer (DF 3). An IPA
suspension of 0.64 wt % of CNF was obtained and used in Paper III.
3.2. CNF-based LIB separators
3.2.1. Fabrication of freestanding separators
In Paper I, an aqueous suspension containing TOCN350-COO-Na+ was
diluted to 0.1 wt % using a 10 mM HCl aqueous solution, hence Na+
counterions of COO-Na+ functional groups were substituted to proton. The
protonated form is denoted TOCN350-COOH. On the other hand, in Paper
II, the TOCN650-COO-Na+ was protonated by dialysis. A semi-permeable
dialysis bag containing 0.3 wt % of diluted TOCN650-COO-Na+ suspension
was immersed in 10 mM of HCl aqueous solutions stirred for 2 days. The
dialysis bag was transferred to DI water and washed until its conductivity
decreased below 1 mS/cm. The protonated fibers with 650 µmol/g were
named TOCN650-COOH.
Different types of cellulose nanofibers-based separators were prepared by
vacuum-filtration through a 0.22 µm pore sized membrane (Durapore,
Merck) and sequential solvent exchange was carried out by using 30 ml of
ethanol, acetone and pentane, successively. The filtered membranes were
dried overnight in a vacuum drier at 110 °C. The mass loading of the TOCN
membrane was ≈ 1 mg/cm2. The side of a membrane in contact with the
Durapore membrane during the vacuum-filtration process is denoted as
the bottom side, while the other side is named as the top side (located on
the top during the filtration). The TOCN based separators were denoted as
TOC followed by the quantity of surface charge density and the functional
groups, e.g. TOC350-COO-Na+ or TOC650-COOH (Paper I and II).
3.2.2. Spray-coated cellulose-based SEA
Spray-coating of a CNF-IPA suspension was implemented onto a heated
electrode (up to 145 oC), as shown in Figure 3.1A, until the desired
thickness of the CNF separator layer was achieved. A PVDF-HFP solution
dissolved in acetone was then spray-coated on the CNF coated on the
electrode and acetone was evaporated on the heating plate. The CNF
separator-electrode assembly was denoted as CNF-SEA. Different SEAs
Experimental - 14
were prepared varying the types of electrodes integrated with the separator
layer: (1) graphite SEA (separator thickness 30 µm), (2) graphite SEA (15
µm) – NMC (LiNi1/3Mn1/3Co1/3O2) SEA (15 µm) and (3) NMC SEA (22 µm)
with extra CNF-PVDF composite around the circular form of the electrode,
as a short-circuit preventive film for the full cell assembly (denoted as wing,
shown in Figure 3.1 B). The SEAs dried at different conditions (170 °C
overnight or 60 °C for 30 minutes) in a vacuum dryer were assembled to
batteries in an argon filled glove box.
A CNF-SiO2 NMC SEA was prepared by spray-coating 1 wt % of fumed
silica suspension dispersed in IPA on the top of a ready prepared CNF-
NMC SEA (30 µm thick in total). Afterward, PVDF-HFP/acetone mixture
was put on the SiO2 layer as shown in Figure 3.1C. A similar separator
layer was deposited on the surface of a high energy LiNi0.6Mn0.2Co0.2O2
(NMC622) electrode with a short-circuit preventive film was as well
around the edge of the electrode (named SiO2-CNF-NMC622 SEA, Figure
3.1 D).
Figure3. 1. Schematic illustration of fabrication of CNF-SEA (A) spray
coating of CNF and PVDF-HFP, successively, (B) fabrication of wing
around NMC SEA for full cell, (C) spray coating of SiO2 on the top of CNF-
SEA, (D) fabrication of wing around SiO2-CNF-NMC622 SEA for full cell
3.2.3. Electrochemical characterization of LIB separators
Electrochemical performance of CNF-based LIB separators were evaluated
by galvanostatic cycling tests and rate capability tests in the pouch cell
assembled in an argon-filled glove box (<1 ppm of O2 and H2O). Celgard
Experimental - 15
2325 was used as a reference separator and its electrochemical
performance was compared with that of CNF separators. Lithium nickel
manganese cobalt oxide positive electrodes (NMC, LiNi1/3Mn1/3Co1/3O2)
with 1.7 mAh/cm2 of capacity and graphite negative electrodes (2.2
mAh/cm2) were used as positive and negative electrodes, respectively.
High energy NMC622 has 3.5 mAh/cm2 of capacity and its matched
graphite has 4 mAh/cm2. As a liquid electrolyte, Selectilyte LP 40 from
BASF, composed of 1 M LiPF6 in ethylene carbonate (EC): diethyl
carbonate (DEC) 1:1 by weight, was used. The cells were cycled in the
potential window between 2.6 and 4.1 V versus Li+/Li. Ionic conductivity
of LP40 liquid electrolyte in a LIB separator was measured by
electrochemical impedance spectroscopy (EIS). In Paper I and II the
discharging rate capability tests and the long-term cycling tests were
carried out only using the protonated separators. In Paper III, liquid
electrolyte with VC was used for all electrochemical measurements. Details
are given in Paper I, II and III.
3.2.4. Gas evolution analysis by mass spectrometry
In-operando mass spectrometry measurements were conducted to analyze
the amount of gas evolved during the formation cycling (0.1C for 3 cycles).
The full cells with NMC and graphite but different separators were
assembled in a custom-designed stainless steel assembly-cell type. Briefly,
the gas analysis system consists of a battery cell setup with gas inlet
connected to a mass spectrometer. Argon gas is purged through a gas inlet
tubing and the gases evolved during the battery operation are pumped with
argon flow gas through a microflow capillary inlet (sample flow rate 12
μ/min) into an ion source. The ionized gas molecules are separated
depending on their mass/charge ratio (m/z) in the quadrupole mass filter
and analyzed in a detector. The types of gases analyzed were H2 (m/z 2),
C2H4 (m/z 26), H2O (m/z 18), CO2 (m/z 44), CO (m/z 28) and liquid
electrolyte. The electrochemical tests and the gas evolution measurements
by mass spectrometry were started simultaneously. The volume of the one-
compartment cell head space is approximately 10 ml, and no gas was added
to replace the sampled gas, which resulted in a decrease of the pressure and
absolute signal in the cell over time, whereas the ppm signal relates to gas
ratios in an accumulative way. Details are given in Paper II.
Experimental - 16
3.3. Rheology measurements of cellulose nanofibers
Rheological properties of 0.95 ± 0.05 wt % of concentration of TOCN with
different charge density and DF were measured. A rheometer with a
cylindrical cone-and-plate geometry (cone angle 2 °, diameter 25 mm, gap
height 50 μm) was used at a constant temperature of 23 °C. Oscillatory
frequency sweep tests were performed at angular frequency from 1 to 100
rad/s with constant 2 % strain. Details are given in Paper IV.
3.4. Electro-precipitation of nanocellulose hydrogels
and its applications
3.4.1. Electro-precipitation of TOCN hydrogels
TOCN with 650 µmol/g and DF 6 was diluted to 0.1 wt % aqueous
suspension using Ultra Turrax D125 Basic disperser at 10000 rpm for 5
min and named TOCN sol. Electro-precipitation of TOCN hydrogel was
conducted in a 2-electrode electrochemical cell setup on various substrates.
The substrates were immersed in the TOCN sol and connected as anodes
and a Pt mesh as a counter electrode. Anodic current was applied for a
certain deposition time to induce water electrolysis and gelation. TOCN sol
mixed with a pH indicator solution was prepared to visualize the pH profile
of TOCN sol during electro-precipitation.
3.4.2. Electro-precipitation of composites
The TOCN-graphite mixed sol was prepared by mixing 0.1 wt % of TOCN
and 2 wt % of graphite, followed by 20 min of ultrasonic treatment (Vibra-
cellTM, Sonics). Electro-precipitation of TOCN-graphite composite
hydrogel was conducted in the same cell setup as above. The thickness of
hydrogel composites on a copper foil were measured varying the deposition
conditions and the composition of the coating medium.
Multi-layered composites were fabricated by coating a hydrogel layer on
the top of the previous layer(s). A two-layered hydrogel was prepared by
switching the coating medium. A 1st coating layer was made by gelation of
TOCN-graphite mixed sol and a 2nd was made using TOCN sol (j = 2.5
mA/cm2 was applied for 30 s for each layer).
Experimental - 17
3.4.3. Application I: Eco-friendly fabrication of electrode for LIB
An aqueous suspension, composed of 0.1 wt % of TOCN with charge
density 1500 µmol and DF 6, 0.05 wt % of Super-P, and 2.34 wt % of
graphite, was prepared by ultrasonic treatment for 20 min. A circular
copper foil with 15 mm diameter was used as a coating substrate and later
as a current collector in LIB. The electrochemical coating was conducted in
the same 2-electrode cell setup, employing the copper foil as a working
electrode and a Pt cage as a counter electrode. 11.3 mA/cm2 of current
density was applied for 10 sec and subsequently the electrode was dried at
110 °C in a vacuum dryer overnight. The graphite coated copper electrode
was assembled with a Celgard 2325 separator and lithium foil as both
negative electrode and reference electrode. The cycling test of the graphite
electrode was performed by charging/discharging at the voltage range
from 2 mV to 1.5 V.
3.4.4. Application II: Selective layer for hydrogen production
This study was to investigate TOCN hydrogel as a possible oxygen
reduction reaction (ORR) protective layer during hydrogen evolution
production (HER). Pt is the most commonly used co-catalyst in
photocatalyst, where HER takes place. As a proof-of-concept, a HER model
study was carried out in electrochemical cell setups, see Figure 3.2. First,
a Pt rotating disc electrode (RDE) anode and Pt cage (cathode) were
immersed in the same container in Figure 3.2A, filled with TOCN sol, and
1 mA/cm2 of direct current was applied for 1 second. Photograph of the gel
coated Pt RDE is presented in Figure 3.2.A. Hydrogen production
capability of the Pt RDE in the presence/absence of the hydrogel protective
layer and the ORR preventive reaction were evaluated in a 3-eletrode cell
setup in Figure 3.2B, composed of a Pt RDE as a working electrode, a Pt
cage as counter electrode, and a Ag/AgCl reference electrode saturated by
KCl. The current density applied for HER is set in the same order of the
current density generated by photocatalyst.
To understand how hydrogel affects the HER, 3 different supplementary
electrochemical experiments were performed. (1) Electrochemically active
surface area (ECSA) of platinum in the presence/absence of the hydrogel
coating was measured following the protocol94,95. (2) To examine the effect
of hydrogel on permeation of H2, H2 oxidation was carried out96. (3) The
Experimental - 18
permeability of ferricyanide was measured by a CV measurement97. Details
can be found in Paper V.
Figure3. 2. Schematic illustrations of (A) the electrochemical coating and
(B) the measurement setups
3.5. Physical characterization of samples
The morphology of the samples was analyzed by Field emission scanning
electron microscopy (FE-SEM, Hitachi S-4800), at 1 kV. Energy dispersive
X-ray spectroscopy (EDX, X-Max 80 SDD, Oxford Instruments) was
operated with FE-SEM as an accessory, at 15 kV accelerating voltage (Paper
V). The microscopic analysis of TOCN single nanofibers used in Paper I
and II was carried out using a scanning electron microscope (JEOL JSM-
7401F, 20 kV) in transmission mode (STEM). In Papers I-III, the size
distribution of nanocellulose and the pore size distribution at the surface
of the separators were analyzed using ImageJ software v.151j898 and the
binarized microscopic images. In Paper I and II, overall porosity of the TOC
membranes was calculated from the weight change of a membrane before
and after immersing in butanol for 2 hours99. A Fourier transform infrared
(FT-IR) spectrometer was used to elucidate the mechanism of electro-
precipitation of TOCN (Paper V). Tensile strength of a specimens was
measured by an Instron 5944 single column tensile tester, with a rate of 10%
per minute.
Results and discussion - 19
4. Results and Discussions
4.1. CNF based separators with different charge density
4.1.1. Characterization of CNF and separators thereof
So far, the most common process of producing lab-made cellulose based
separators is vacuum filtration36,40,47,100. Using fibrous cellulose as raw
material for LIB separators is advantageous since the pores can be
naturally constructed between fibers. Nonetheless, the key factor of
producing highly porous separators aiming for fast charging application is
to maintain the porous structure upon drying, which can easily collapse
without sophisticated drying processes, due to high affinity between
hydrogen bonds and water.
The morphology of TOCN varied depending on the surface charge density.
Visuals of TOCN with different charge densities (350 and 650 µmol/g) and
size distribution of nanofibers are displayed in Figure 4.1 A. Both TOCN
are partially disintegrated, consisting of microfibers and numerous
nanofibrils exfoliated from the main body of microfibers. A TOCN650
single fiber looks more transparent in the STEM image with a lower
contrast (lighter color) than that of the TOCN350, which means that
TOCN650 has a less dense and more fibrillated structure due to the
increased interfibrillar repulsive forces.
The average width of TOCN350 fibers was reported to be 130 nm, which is
eight times higher than that of TOCN650 (16 nm). TOCN350 have
submicron sized fibers ranged 100 – 500 nm, whereas TOCN650 larger
than 100 nm were barely found. The results are in agreement with the
literature that TOCN with higher charge density has more dispersed
nanofibrillated networks compared to the lower charged one, when treated
under the same mechanical disintegration condition33.
The surface morphology of TOC350 membranes is addressed in Paper I.
The difference in physical properties and morphology between TOC350-
COO-Na+ and TOC350-COOH membranes is insignificant. However, their
electrochemical performance was distinctively different. The cell with
TOC350-COO-Na+ separator showed fast capacity fade, while the TOC350-
COOH counterpart showed good cycling stability. The electrochemical
performance of the TOC separators will be discussed in the following
section 4.1.2 and Figure 4.2.
Results and discussion - 20
Figure 4. 1. Comparison of characteristics of TOCN with different charge
densities (350 and 650 µmol/g) and membranes thereof, (A) microscopic
images of TOCN and size distribution, (B) the pore size distributions of the
top and the bottom sides of the TOC350-COOH and TOC650-COOH
separators
In this respect, the morphology of the membranes made of the protonated
TOCN with different carboxyl contents are compared in Figure 4.1.B. It
is noticeable that the nanofibrillated structure of the TOCN (STEM images
in Figure 4.1.A) and the surface morphology of the dried membranes
observed by SEM (Figure 4.1.B) are very similar. It indicates that the
mesoporous structure of the TOCN membranes originates from the
nanofibrillated network and there was no significant shrinkage of
nanofibrils during the membrane fabrication (protonation, filtration and
Results and discussion - 21
drying). The results emphasize that the sequential solvent exchange
effectively removes water from the cellulose network, consequently,
preventing significant pore collapse upon drying.
The separators fabricated by vacuum filtration had an asymmetrical
structure, where the top sides had a multi-scaled porous structure with
both macro- and nano-scaled pores, whereas the very flat bottom sides
only had mesopores. Porosity and the average pore width of the top sides
of the two separators were similar. On the contrary, for the bottom sides,
the TOC650 separator had lower porosity and smaller average pore width
than that of TOC350. The influence of such asymmetric structure on the
rate capability is seen in Figures 4.3 and 4.4.
Table 4. 1. Physical properties of Celgard 2325 and TOC separators
Porosity
(%)
Thickness
(μm)
Ionic
conductivity
(mS/cm)
Electrolyte
uptake (%)
Tensile
strength
at break
(MPa)
Celgard
2325 46 ± 2 23 ± 2 0.81 ± 0.02 85 ± 4 170
TOC350
- COOH 62 ± 3 34 ± 4 1.06 ± 0.07 ≥200 3.5
TOC650
- COOH 58 ± 2 33 ± 2 0.66 ± 0.03 ≥200 9
The low ionic conductivity of the TOC650 separator, seen in Table 4.1, is
likely due to the smaller average pore width and lower porosity in the
bottom side of TOC650. The highly dispersed fine nanofibrils were
vulnerable to tolerate the strong capillary forces between the fibers and
water. Thus, it is likely to more aggregate, compared to the microfibrillated
fibers, resulting in the pore closure during the drying process. On the other
hand, due to the compact structure with less porosity resulted from the
presence of a large amount of nanofibrils, TOC650 membrane has 2.6
times higher tensile strength than that of TOC350. The results demonstrate
that increase in charge density of CNF leads to decrease in the porosity and
the average pore width of the TOC650 membrane. The effect of charge
densities and the following morphological changes on the electrochemical
performance of LIB separators will be more discussed in the following
section 4.1.2.
Results and discussion - 22
4.1.2. Electrochemical performance of TOCN separators
Figure 4. 2. Electrochemical performance of the LiNMC|Graphite cells
with different separators at 0.1 C charging/discharging rate: (A) TOCN-
COO−Na+, (B) TOC350-COOH, (C) galvanostatic fast charging and
discharging cycling test at 1 C and (D) chronopotentiogram of the cells at
1st, 20th, 50th, and 100th cycles (discharging).
Figure 4.2 (A-B) shows the dramatic improvement in electrochemical
stability of a TOC350 separator, after the removal of sodium counter ions
in the functional group of TOCN. The cycling performance of the cell with
the TOC350-COOH separator at 1C charging/discharging rate was
comparable to that of Celgard 2325, with high stability (Figure 4.2C).
Relatively higher ohmic loss and lower specific capacity of the cell
containing the TOC350 separator shown in Figure 4.2 D were seemingly
attributed to its higher thickness and the low porosity at the bottom side,
compared to that of Celgard 2325.
Figure 4.3 A depicts the effect of the asymmetric structure of the TOC350
separator on their rate capability. In the full cell named TOC350-Top, the
top side of the separator is in contact with the negative electrode, whereas
Results and discussion - 23
the bottom side contacts the negative electrode in the cell named TOC350-
Bottom. Results in Figure 4.3 B demonstrate how the transport of lithium
ions varies depending on the surface porosity. When the porous top side
was faced towards the negative electrode, the transport of lithium ions was
facilitated during the discharging process, resulting in high discharging
capacity, even at 5 C (7.3 mA/cm2).
Figure 4. 3. (A) Schematic description of the cell configurations
membranes and lithium ion flux through the surface of separators
considering the asymmetric structure of TOC350-COOH, (B) the
discharging rate capability of the cells varying the orientation of the
separators, charged at 0.2 C and discharged at different rates
The cell with a TOC650-COO-Na+ separator in Figure 4.4A showed
failure during cycling test, similar to that of TOC350-COO-Na+ (Figure
4.2A). It can be speculated that such failure was attributed to (1) the
presence of strongly bound water or, predominantly, (2) the deposition of
sodium on the graphite surface. Berthold et al. 101 reported that the amount
of bound water of carboxymethyl cellulose (CMC) with sodium counterions
in functional groups becomes 2−4 times higher than that of the protonated
Results and discussion - 24
counterpart. Furthermore, once sodium is deposited on the surface of
graphite, it is likely to interrupt lithium intercalation into the graphite.
Figure 4. 4. Electrochemical performance of NMC|graphite full cell in
pouch cells with TOC650 separators with different counterions in
functional groups and electrolyte (A) TOC650-COO-Na+ without additive,
(B) TOC650-COOH without additive, (C) TOC650-COOH with 2wt% of VC
additive, (D) discharging rate capability test (charging rate 0.2 C), (E)
cycling stability test at 0.5 C of charging and discharging rate
Differently from the case of TOC350-COOH (Figure 4.2B), the cell
containing the protonated TOC650-COOH showed poor cycling stability,
despite of the removal of sodium ion (Figure 4.4B). Interestingly, the
electrochemical stability of the TOC650-COOH separator was considerably
improved by adding VC, as shown in Figure 4.4C. Thus, the cycling
stability test of TOC650 were performed with the electrolyte containing 2
wt % of VC (Figure 4.4 D-E). For further discussion, see section 4.3
about the cause of cell failure in the presence of sodium counterions in
sodium carboxylate surface groups and the effect of VC related to the gas
evolution in LIB.
The difference in the discharging specific rate capability of the TOC350 and
650 separators was greater at fast discharging rate, higher than 2C (Figure
Results and discussion - 25
4.3B and 4.4D). It is probably attributed to lower porosity of the bottom
side of the TOC650 separator. At 1C cycling test, the specific capacity of the
TOC650 cell was ≈80% of that of Celgard 2325 (Paper II), however at 0.5
C they showed similar capacities (Figure 4.4E). Accordingly, the results
of the rate capability tests imply that, by locating the porous side of the
separator in contact with the desired electrode and adjusting the testing
process, better discharging rate capability can be obtained.
Figure 4. 5. (A) Cycling stability test of the cell containing TOC650-
COOH separator, tested at 0.6 C of charging rate of constant current and
2.3 C of discharging rate as a function of cycle numbers, (B)
chronopotentiogram of the cell at the 20th cycles, as a function of time (x-
axis, bottom) and specific capacity (x-axis, top).
Figure 4.5 shows the cycling performance of the cell with TOC650-
COOH-Top containing VC additive, tested at 0.6 C (0.74 mA/cm2) of slow
charging and 2.3 C (2.95 mA/cm2) of fast discharging. The specific capacity
of the initial discharging was 105 mAh/cm2, which is 90% of the initial
capacity at 0.6 C. The results demonstrate that the TOC650 separator can
be applied for the battery with high power, applying a current > 2 mA/cm2.
4.1.3. CNF separators with different charge density
The relationship between the characteristics of nanocellulose, treated by
different processes, and their performance as separators is discussed in
this section. Figure 4.6 displays the electrochemical performance of the
different separators at 1C, applying a constant current of 1.5 mA/cm2. The
enzymatically treated cellulose has the same chemical structure as pristine
cellulose without carboxylic functional groups and its charge density is 44
Results and discussion - 26
µmol/g102. The enzymatically treated CNF membrane has poor mechanical
integrity due to its inhomogeneous dispersion. TOC separators show not
only higher mechanical strength but also better electrochemical
performance. Introducing 350 µmol/g of charge density in cellulose chain
is shown as the best condition to produce the separator with high
performance.
Figure 4. 6. Effect of the charge density of nanocellulose (degree of
fibrillation, 1 time passed) on electrochemical performance as LIB
separators (bar, specific capacity at 1C charging rate), and the mechanical
strength of the CNF membranes (dotted red line with diamond marker, the
same mass loading, produced by the same vacuum filtration)
Increasing the charge density contributed to improvement of the
mechanical strength of the cellulose membranes. The lower specific
capacity and higher mechanical strength of TOC650, compared to TOC350,
can be related to its morphological properties, e.g. lower surface porosity
and average pore width and ionic conductivity. Besides, TOC650 separator
requires VC additive to achieve the stable cycling performance.
Nevertheless, TOC650 can be regarded as a separator with good
mechanical integrity and electrochemical performance, capable of high
current density, ≈ 2.95 mA/cm2. To acquire a cellulose membrane with a
larger amount of finely dispersed cellulose nanofibers and higher tensile
strength, cellulose with higher charge density can be prepared and treated
to a higher degree of fibrillation. However, it is likely to increase the
amount of bound water in cellulose structure with the risk of failure during
cell cycling. Accordingly, it is noted that cellulose separators should be
Results and discussion - 27
produced with consideration of a trade-off between the desired properties,
e.g. mechanical strength, porosity and electrochemical performance.
To meet the industrial needs for high power or energy density battery
applications, highly porous separator materials which can apply >2.2
mA/cm2 are required103,104. In this respect, both TOC separators can be
considered as the promising separator materials fulfilling such
requirements. Particularly TOC350, is capable of sufficiently good
capacities; 1.5 mA/cm2 for 1C, 2.9 mA/cm2 for 2.3 C, and 7.3 mA/cm2 for
5C (Paper I). Therefore, it can be concluded that introduction of 350 - 650
µmol/g of carboxylate groups was beneficial to efficiently disintegrate
cellulose to nanofibers and to produce highly porous membrane for fast
discharging LIB applications, showing good electrochemical performance.
Further studies to improve its mechanical strength will be required, by
additional mechanical and chemical treatment38,60,105,106. In addition, such
aqueous-based processes require high energy input for the drying process,
e.g. overnight drying time at elevated temperature (>110 °C). A way to
overcome such drawback of the cellulose based porous separator is the
development of new types of integrated battery structures. In the following
section a CNF based separator-electrode assembly fabricated using a fast
and simple fabrication method, is addressed (Paper III).
4.2. Spray coated cellulose-based separator-electrode
assembly (SEA)
4.2.1. Fabrication of SEA
In the state-of-art battery manufacturing process, electrodes and
separators are separately produced and assembled later during a winding
process. A separator must have sufficiently high tensile strength and high
Young’s modulus not to be broken and deformed while wound with the
electrodes as a high pulling force is applied. Tensile strength of cellulose
separators has been reported in the range of 12-55 MPa, which is 10-30%
of that of commercial polyolefin separators (≈ 170 MPa) 36,49,60,107. Thus, it
is likely very challenging to obtain freestanding cellulose-based separators
with tensile strength comparable to that of the polyolefin ones without
compromising the rate capability and increase in resistivity.
Results and discussion - 28
Figure 4. 7. (A) Single CNF building block dispersed in IPA and (B) its
magnified view with nanofibril network, (C) porous CNF separator layer in
SEA, (D) histogram of the fiber width of nanofibrillated cellulose and (E)
histogram of pore size distribution of the CNF membrane.
Here we introduce a CNF separator/electrode assembly (SEA) produced by
spray-coating method, which is advantageous to supplement the low
mechanical strength of the separator. The spraying medium is
enzymatically treated cellulose mechanically dispersed in IPA. Figure
4.7A demonstrates a CNF building block, with 20 to 30 µm long and 10
nm to 2 µm wide. The difference in polarity between water and IPA has a
significant impact on the mechanical disintegration by the microfluidizer,
since the hydrogen bonds will be more stable in IPA than in water,
therefore the fibers are cut rather than delaminated.
Figure 4.7C exhibits the porous CNF separator in SEA, in which the
nanofibrillated structure of CNF was well-maintained after solvent
evaporation during the spray-coating made using a normal cosmetic spray.
The lower surface tension between the fibers and IPA enables fabrication
of a porous membrane without pore closure upon fast drying108,109. On the
contrary, if the CNFs were dispersed in water, it required a sophisticated
drying process, as above, e.g. solvent exchange, vacuum or freeze drying
for a long time (overnight or more)110. Size distributions of the width of the
Results and discussion - 29
fibers and the pore diameter are displayed in Figure 4.7D-E. The CNF
has similar dimensional characteristics as those of the TOC separators, but
slightly larger average pore width and higher porosity.
Figure 4.8. (A) Schematic illustrations of spray-coated CNF separator
/NMC electrode assembly, (B) CNF coated on the electrode surface and its
plain-view and cross-sectional images, thickness < 3 µm, (C) thickness ≈
22 µm, (D) PVDF-HFP spray-coated on the top of a 22 µm thick CNF layer
The CNF-SEA was fabricated by repeatedly spray-coating a CNF-IPA
suspension on the top of an electrode, until a desired thickness was
achieved, as shown in Figure 4.8. A layer of less than 3 µm was too thin
to be used as a separator, not fully covering the surface of the electrode,
which can result in short-circuit in LIB (Figure 4.8B). The electrode
surface is not observed anymore in Figure 4.8C-D, when the thickness
achieved was ≈ 22 µm. PVDF-HFP plays a role improving the dimensional
Results and discussion - 30
integrity of the CNF-SEA, binding the CNFs and increasing their flexibility.
Here the development of a separator layer of 22 µm is achieved, which is
slightly thinner than that of previously reported cellulose separators of ≈
30 µm 40,111 or other SEAs55,58,61. Woo et al.60 reported on a SEA with a 12
µm separator, which implies that SEA is a promising process to obtain a
thin separator with low resistivity. There was no significant difference in
the in-plane view of the CNF membrane observed before/after PVDF-HFP
coating, macro and mesopores remained without pore closure. Detailed
physical properties of the CNF-PVDF-HFP separator are addressed in
Table 4.2.
Table 4. 2 Physical properties of CNF-PVDF-HFP separator
CNF PVDF-HFP Ionic
conductivity Wettability
Thickness
(µm)
weight
(mg)
Thickness
(µm)
weight
(mg) mS/cm %
CNF/PVDF-
HFP 21 2 2±1
0.3
±0.1 1.08 ±0.2
256.7 ±
0.4
A membrane made of the CNF fibers with low aspect ratio and by the same
vacuum filtration had very weak mechanical strength, even in comparison
with the previously mentioned TOC separators. Nonetheless, it was
advantageous to use the fast evaporating organic solvent suspension, since
the spray-coated cellulose building block can adhere to the surface of the
electrode very well during solvent evaporation. It enables fast fabrication
of highly porous membranes, preventing adsorption of water into the
cellulose structure, which can cause detrimental side reactions in LIB. It
was also convenient to acquire a desired thickness, staking the CNF
building block by a bottom-up approach. The separator integrated with an
electrode can tolerate high tensile stress, when pulled in the in-plane
direction, since the mechanical strength of the SEA is more determined by
that of the current collector, to which the separator is attached. Tensile
strength of the NMC electrode used in this study was ≈30 MPa.
Accordingly, we can assume that the tensile strength of the SEA is
improved to that of NMC electrode composite.
Note that it is not easy to find suitable solvents and new materials for SEA,
other than NMP or dimethylformamide and inorganic nanoparticles54–
56,59,62. Presumably, there is less freedom when designing a separator for a
SEA if it requires extra pore forming processes such as phase separation,
Results and discussion - 31
using different polarity of solvents and solubility of materials. Besides, the
stability of the electrode materials to the solvents must be carefully
considered, not to deteriorate the cell performance. For instance, aqueous
suspensions are not preferred to be in contact with the normal electrodes,
since the active materials coated are likely detached from the current
collector as water pervades between the active materials and the binders.
Such limitation can be a major barrier for application of diverse materials
for SEAs. Casting a polymer solution without pore-forming agents, e.g.
inorganic nanoparticles, is likely to form a non-porous film, which is not
favorable as separators for high energy or power LIB applications. In this
respect, using CNF as a ready-porous building block and IPA breaks
through such obstacle, enabling the fabrication of SEAs capable of fast
charging (discussed in the following section 4.2.2), without degrading
the performance of the electrode. Here we can put more emphasis on the
advantages of this technique using cellulose as biodegradable material and
using non-hazardous solvents, which can be considered as an eco-friendly
process.
4.2.2. Electrochemical performance of CNF-SEA
The cycling performance of the cells depends on the configuration of the
SEAs. When the CNF separator was coated on the graphite electrode, the
cell showed poor cycling stability with a sudden drop in specific capacity
and low coulombic efficiency (or cell failure), due to high volume
expansion of graphite (Paper III). Volumetric change of graphite is known
as 10%112, while LTO is 0.2%113, LiMn1.5Ni0.5O4 (NMO)114 3-6.3%, LFP 7%114,
LCO 2%115, and NMC 2%11,115. To achieve successful cycling performance of
the SEAs with high volumetric change electrodes, such as carbonaceous
materials, tough and durable materials which can tolerate stress and strain
are necessitated, e.g. PVDF, PVDF-HFP, poly(phenylene oxide), or
inorganic nanoparticles 54,55,60,62. It seems that the CNF based composite
structure with low loading of PVDF-HFP is vulnerable to high stress and
strain from the volume expansion of the electrode.
Seeing the configuration of normal LIB, the sizes of the negative electrode
and the separator are in general slightly larger than that of the positive
electrode 64,65. Therefore, in SEA, it has been preferred to deposit a
separator on the negative electrode side54,55,60–62. LTO seems an
advantageous substrate, but because of its high operating potential ≈ 1.5 V
Results and discussion - 32
vs Li+/Li, it compromises the cell voltage and consequently power or
energy density of LIB56,57,61. In this respect, NMC seems a favorable
electrode material for CNF-SEA. To deposit a separator on the NMC side,
a cell design preventing direct contact between the edges of the electrodes
is required. Thus, extra CNF-PVDF-HFP was decorated along the edge of a
disc-type NMC electrode, as shown in Figure 4.9 A.
Figure 4. 9. (A) Photograph of CNF-NMC SEA and microscopic image of
the edge of the wing and the porous separator, (B-D) electrochemical
performance of lithium ion battery containing CNF-NMC SEA and the one
with Celgard 2325, (B) EIS of the CNF-NMC SEA after formation, (C)
cycling stability at 1C of charging rate, (D) rate capability test
After the SEI formation, the SEA had a lower resistivity than the cell
containing Celgard 2325 (Figure 4.9B), showing good electrochemical
performance as well (Figure 4.9C-D). Particularly, the CNF-SEA dried at
60 °C for 30 min in vacuum right after fabrication, showed excellent
electrochemical performance, similar to the one dried at 170 °C. It implies
that this manufacturing process can considerably reduce the energy and
time input for LIB production. After cycling tests, the separator layer
Results and discussion - 33
remained on the surface of the NMC electrode without significant
morphological changes and pore closure (Paper III).
Another advantage of the spray-coating method is that the CNF separator
can be coated on cells of any size and shape. A proof-of-concept of a small
sized SEA cell was made, inspired by a commercial prismatic cell, and
cycled under bent condition as shown in Figure 4.10. The CNF-NMC SEA
exhibited good cycling stability as 91.5% of the specific capacity remained
after 100 cycles, whereas the Celgard separator containing cell lost 18% of
the specific capacity during the same time. It is presumably attributed to
the good adhesion between the separator and the electrode in the SEA.
Figure 4. 10. Proof-of-concept of a small sized-prismatic CNF-NMC
SEA|graphite full cell cycled under bent condition (A) the image of the cell
configuration (the electrode located along the red line in the scheme) and
(B) cycling stability of the CNF-NMC SEA|graphite full cell at 0.25 C of
charging/discharging rate (marker open for charging, the filled for
discharging, x and + for coulombic efficiency)
Results and discussion - 34
4.2.3. Electrochemical performance of SiO2-CNF-SEA
Ceramic coated separators have been applied for commercial cells,
believed as a key component for high performance LIB 2,14,17,19. SiO2 can
tolerate external mechanical pressure and so enhance the rigidity of the
SiO2 coated separator.
Figure 4. 11. Schematic illustration of the effect of volume change of
electrodes on the separator in SEA during the cycling (A) CNF-SEA and (B)
SiO2-CNF-SEA and cycling performance of the full cell
Targeting the application for a high energy density battery, CNF-NMC622
SEAs were made and cycling tests were carried out. Since the NMC622
electrode was developed aiming for high energy density, the electrode and
the matched graphite also have ≈ 2 times higher mass loading and capacity
per unit area than that of NMC. It is assumed that the high volumetric
changes of NMC622 can increase the non-uniform pressure applied to the
CNF separator, which can lead to fracture and delamination of the
separator, as described in Figure 4.11A. Consequently, it caused poor
cycling performance with fluctuating capacity and internal short circuit
(Paper III). When a thin SiO2 layer was coated on the top of the CNF
Results and discussion - 35
separator, the layer could effectively tolerate the stress and the pressure
against the separator. Accordingly, it is believed that the SiO2 layer
contributes to the highly improved cycling stability of the SiO2-CNF
NMC622 SEA (Figure 4.11B). Furthermore, it seems that the SiO2 layer
not only enhances the mechanical stability of SEA but also facilitates
transport of lithium ions, displaying increased specific capacity.
Figure 4. 12. Schematic illustration of structure of SiO2-CNF NMC SEA
and morphology, electrochemical performance of SiO2-CNF-NMC SEA|Li-
metal battery (B) cycling stability test at 0.2 C of charging/discharging rate
(0.2 mA/cm2, marker open for charging, the filled for discharging, x and +
for Coulombic efficiency) and (C) the rate capability test
Lithium metal is ultimately the most favorable negative electrode, due to
its high specific capacity >3000 mAh/g and low redox potential, even
though its instability makes its commercialization challenging.
Nonetheless, it is still worth to put more efforts to develop stable and safe
lithium metal batteries. Unfortunately, the wood-based CNF separator
used in this study was not suitable for a lithium metal battery. This was
probably because of undesirable side reactions between lithium metal and
Results and discussion - 36
water residues, and highly porous and soft surface properties leading to
dendrite growth. The separator in contact with lithium metal got yellow
coloration on the surface, as shown in Figure 4.12A. On the contrary, a
thin SiO2 coating overlaid on the top of the CNF separator prevents direct
contact between CNF and lithium metal and consequently enables
successful cycling of a proof-of-concept lithium metal battery (Figure
4.12B-C). The cycling performance and the rate capability were
comparable to that of Celgard 2325.
4.3. Gas evolution of LIBs with CNF separators
Figure 4. 13. Gas evolution of NMC|Graphite full cells with different
separators and electrolyte components as a function of time and cycle
numbers (for 3 cycles at 0.1 C, C: charging, D: discharging), (A) H2 and (B)
C2H4, and the cells named after separator thereof
Results showing that protonation of TOC350 separator dramatically
improves the battery performance (Paper I) may indicate that the presence
of sodium ion causes failure of the cell. It was presumed to be due to bound
water (Paper I), according to the literature that the amount of bound water
varies depending on the type of counter ion in the carboxylate groups of
Results and discussion - 37
cellulose derivatives 101,116. Another possible explanation is that sodium
ions can be dissociated into the LIB electrolyte, and deposited on the
graphite electrode, which can interrupt lithium intercalation.
In-operando gas evolution measurements can help to better understand
the effect of sodium counterions in the surface functional groups of the
TOCN on the electrochemical performance of the LIB containing the TOC
separators. Note that the cycling behavior of the cells differed depending
on the cell design. Even though the cell with the TOC650-COO-Na+
separator showed a fast capacity fade in the pouch cell (Figure 4.4A), the
same cell assembly in the mass spectrometry measurement setup had
stable performance without fast capacity fading during the first 3 cycles at
0.1C of the cycling test (≈ 60 hours). It is most probably related to
continuous evacuation of gas during the mass spectrometry analysis.
Besides, the mass spectrometry cell setup required four times higher
amount of electrolyte, than that of the pouch cell, which can dilute the
sodium ion contents and their effects on the cell performance is attenuated.
On the contrary, the pouch cell is a closed system, where gas can
accumulate as the cycling test proceeds and it is likely to have more
aggressive side reactions, followed by cell failure.
Figure 4.13 demonstrates the H2 and C2H4 gas evolution measurements
with different separators and the addition of VC to the electrolyte. Since
there was no significant trend in the CO2 and CO gassing reactions at
potentials below 4.2 V, those results are not included. The highest amount
of H2 and C2H4 gases were released from the cell containing TOC650-
COOH separator, without VC, but the amount of gases evolved in the
TOC650-COO-Na+ cell was similar to that of Celgard 2325. The staircase-
like graph of H2 signal intensity indicates that the increase in the rate of
the gas evolution is related to the potential changes, which disagrees with
the literature that H2 gassing is a potential independent reaction79. See
Figure 4.13A, the rate of signal intensity rapidly increased in the time
ranges of 0-10, 20-30, and 40-50 hours during the charging (C), compared
to the rest of the time ranges, corresponding to the discharging (D).
The amount of gas evolved from the Celgard 2325 cell can be considered as
a reference where the “normal” gas evolution reactions take place without
significant side reactions between the electrode/electrolyte and the
separator. In the Celgard 2325 containing cell, a sudden increase of C2H4
are observed during the first charging (up to ≈ 10 hours in Figure 4.13B)
and a continuous gas evolution (H2 and C2H4) during the following cycling
Results and discussion - 38
process as well. We assume that, in the Celgard 2325 cell, gas evolution
reactions related to the decomposition of electrolyte only take place, as
proposed in the literature70 and it is schematically illustrated in Figure
4.14. At the positive electrode, the electrolyte solvent can decompose by
ring opening and the protic electrolyte decomposition compounds are
produced (Figure 4.14 (A)). These electrolyte decomposition products
can diffuse to the graphite electrode surface (Figure 4.14 (B)) and be
reduced and generate H2 gas (Figure 4.14 (C)), which causes the
continuous H2 gas evolution over the cycling. A sudden increase in the C2H4
signal in the beginning of the cycling test (0 ~ 10 hours) is because of the
decomposition of electrolyte solvents during the SEI layer formation
(Figure 4.14D)70,79. Note that the same gas evolution reactions take place
even in the cells containing the TOC650 separator.
Figure 4. 14, Schematic illustrations of electrolyte decomposition
reaction and gas evolution occurring in the cell containing Celgard 22325
separator as a reference cell (without VC)
That is, when a higher amount of gas was generated in the cells containing
TOC650 separators, it can be assumed that there were some additional side
reactions between the separator and the battery components. The possible
reactions taking place with cellulose components in LIB are the following
and described in Figures 4.15 and 16;
Reaction 1: reduction of H2O to H2 (H2O + e- → OH- + ½ H2 (g)) 70,71,117
Reaction 2: deposition of Na (Na++ e- → Na, 0.3 V vs Li+/Li)
Reaction 3: substitution of COO- and OH to COO-Li+118–120
Reaction 4: the reduction of H+ from COOH and OH to H2121.
Results and discussion - 39
More detailed reaction schemes are addressed in Paper II.
Figure 4. 15. Schematic illustrations of possible reactions in the cell
containing TOC650-COO-Na+ separator (without VC)
Figure 4.15 schematically describes the possible reactions taking place in
the cell containing a TOC650-COO-Na+ separator. The reduction of H2O
and protons at the graphite surface are likely to cause the high amount of
H2 evolution in the cell. However, seeing Figure 4.13 A, less H2 gas was
released from the TOC650-COO-Na+cell, compared to that of its
protonated counterpart, or similar to that of the Celgard 2325 cell. During
the first charging (~10 hours), the TOC650-COO-Na+ cell produced slightly
less C2H4 gas (red line, Figure 4.13B), compared to that of the Celgard
2325 reference cell (blue line, Figure 4.13B). Accordingly, it can be
speculated that a reduced active surface area on the graphite restricts the
formation of a stable SEI layer and other surface reactions as well. It
implies that probably not trace water but another reason, possibly sodium
deposition on the graphite surface, is a dominant cause for failure of the
pouch cell containing TOC650 separator with COO-Na+ functional groups.
Sodium ions in carboxylate groups can be replaced by lithium ions forming
(-COO-Li+) and dissociated to the electrolyte by ion exchange119,120.
Considering the electrochemical potential of sodium (-2.7 v vs SHE), the
sodium ions released would be deposited on the graphite surface at a
higher potential than lithium intercalation into graphite (0.3 V vs Li+/Li),
possibly causing sodium plating at the lower potential with little
intercalation 122,123. Thus, sodium deposition is likely to interrupt the
intercalation of lithium into graphite. A low concentration of sodium ions
(< 0.22 M) was found to enhance a stable SEI layer formation, and
Results and discussion - 40
consequently electrochemical stability of LIB thereof, whereas higher
concentrations of sodium added (> 0.44 M) deteriorated the performance
of the cell124. Thus, it is speculated that restricted lithium intercalation
possibly hinders the cell cycling, showing poor specific capacity and cycling
behavior as shown in the pouch cell, Figure 4.4A.
Figure 4. 16. Schematic illustrations of possible reactions in the cell
containing TOC650-COOH separator (without VC)
The highest amount of H2 gas was generated from the cell with a TOC650-
COOH separator and no VC addition. Figure 4.16 illustrates the possible
reactions taking place in the cell. A sudden increase of H2 in the initial
cycling (0-10 hours, black line in Figure 4.13A) can be possibly attributed
to Reactions 1, 3 and 4 at the graphite surface, and further continuous
evolution is due to the decomposition of electrolyte, the same as in the
Celgard 2325 reference cell (Figure 4.14). Due to the complicated
reactions of gas evolution, it was difficult to quantitatively elucidate the
origin of the H2 gas.
Interestingly, having 2 wt % VC in the electrolyte, overall gassing in the cell
with TOC-COOH separator was considerably suppressed, including H2 and
C2H4. A suggested mechanism of VC suppressing the gas evolution and
further side reactions in LIB is that a VC containing SEI layer prevents the
reactions described in Figure 4.16, and consequently much less H2 and
C2H4 gases are evolved 70,71.
Potential dependent H2 gas evolution shown in Figure 4.13A indicates
that CNF based separators likely have higher risk of cell failure without VC
when used in LIB with an electrode operated at high potential, e.g. NMC,
compared to low potential counterpart such as LFP or LTO. Nonetheless,
Results and discussion - 41
adding VC, the TOC separators are sufficiently stable to be applied for NMC
despite of such risk, and possibly capable of other high potential LIB, such
as lithium nickel cobalt aluminum oxide (LiNiCoAlO2) or high nickel NMC.
In this respect, the addition of VC is necessarily required when applying
nanocellulose to LIB. These reaction dynamics imply the need of careful
investigations about detailed chemistry of cellulose based material in LIB,
especially with high voltage positive electrode materials.
Figure 4. 17. H2 gas evolution of the CNF-NMC SEA|graphite LIBs dried
at 60 °C for 30 min, in the presence/absence of 2 wt % VC in the electrolyte.
As shown in Figure 4.17, the gas evolution from a CNF-SEA LIB was
analyzed in the same way as above. Here we considered H2 as the most
significant indicator to evaluate the electrochemical stability of a material
in a LIB. The enzymatically treated cellulose separator is expected to have
the similar reactions to that of TOC (Reactions 1,3 and 4). Even though
CNF was fabricated in a water-free environment, a high amount of gas was
released from the CNF-SEA cell without VC (Figure 4. 17). Nonetheless,
the amount of H2, even slightly higher than that of Celgard 2325 in Figure
4.13, is likely attributed to the reduction of the trace water and hydroxyl
surface groups, as previously mentioned (Reactions 1, 3 and 4). Water
could probably be introduced to the nanocellulose structure during the
Results and discussion - 42
enzymatic treatment conducted in aqueous environment and remain after
the short-term drying process. Nevertheless, the amount of H2 gas
generated from CNF-SEA (black line in Figure 4.17) was even less than
that of the aqueous based TOC separator, dried overnight (black line in
Figure 4.13A). The result greatly highlights the advantage of using CNF
treated in organic solvent which considerably improves drying efficiency.
By adding VC, the total amount of H2 gas evolved from the cell containing
CNF-SEA was significantly suppressed, ≈ 29% less than that of the Celgard
2325 reference cell (without VC). Thus, it can be concluded that the
combination of water-free conditions using CNF dispersed in IPA with the
addition of VC greatly increase the drying efficiency of the CNF based
separator manufacturing process.
4.4. Discussion about CNF-based LIB separators
To sum up, we have demonstrated wood based TOCN as LIB separator with
excellent electrochemical performance and high rate capability. Varying
the surface charge density of the cellulose, the effects of the carboxylate
surface group and its counterions on the morphology, physical properties
and electrochemical performance of the separators were studied.
Microfibrillated cellulose with nanofibril networks were obtained by
mechanically disintegrating the carboxylated pulps at mild conditions.
Increasing the charge densities of TOCN from 350 to 650 µmol/g, more
loose and fibrillated networks were acquired. Such a larger amount of
finely dispersed nanofibrils in TOCN650 contribute to the increase in the
tensile strength of the TOC650 separator, whereas it compromises its rate
capability and stability of LIB. Nevertheless, considering the asymmetric
structure of the membranes fabricated by vacuum filtration and adjusting
the cycling test conditions, better rate capability could be accomplished at
the charging rate > 2 mA/cm2. Electrochemical stability of the TOC350-
COO-Na+ separator was improved by substituting sodium ions to protons.
However, the TOC650-COOH separator necessarily required VC additive
for stable electrochemical cycling performance in LIB.
Even though the rate capability of TOC separators was sufficiently high, the
low mechanical strength remains a major drawback of the cellulose
separator. To overcome such disadvantage, a CNF based SEA was
developed. The CNF separator is integrated with the electrode and current
collectors and has highly enhanced dimensional stability. The CNF-SEA
Results and discussion - 43
was fabricated by spray-coating a CNF-IPA dispersion on a target electrode.
The method can be considered as an eco-friendly process, since it uses
biodegradable materials without using hazardous solvents. Additional
advantages of the CNF-SEA are (1) facile manufacturing of the separator
due to fast evaporation of IPA, (2) porous structure obtained without
additional pore-forming process, (3) less water adsorption, and (4) high
thermal stability. NMC SEA shows excellent electrochemical stability,
comparable to that of the cell with a Celgard 2325 separator, not only in a
normal disc-type small cell, but also in a proof-of-concept prismatic cell,
operated in bending. A SiO2 coating layer makes the CNF-SEA work not
only for high energy density batteries but also for lithium metal batteries.
We believe that the development of such SEA cell designs enables
applications of a variety of functional porous materials for separators,
which were considered as not attractive materials due to low mechanical
strength.
In-operando gas evolution measurements were carried out to investigate
the cause of failure of the cell containing cellulose based separators. It is
speculated that not trace water but sodium deposition on the graphite
surface is a dominant cause for failure of the cell containing TOC
separators with COO-Na+ functional groups. Noticeably high amounts of
H2 released from the TOC650-COOH cell seems to originate from the
reduction of (1) the trace water and (2) the degradation of electrolyte
decomposition products and (3) the reduction of protons, released from
COOH and OH, into H2 gas at the graphite surface. Such significant gas
generation measured by the mass spectrometry study can be related to
failure of the cell containing a TOC650-COOH separator in the closed
pouch cell system. The gas evolution was considerably suppressed by the
addition of VC, and the cells showed excellent electrochemical
performance with high cycling stability. Therefore, it is concluded that the
addition of VC is compulsory in the LIB likely to contain trace water (e.g.
produced by an aqueous process or containing hygroscopic materials,
biopolymers). CNF-SEA also demonstrated the same trend in the presence
/ absence of VC. Dispersing CNF in IPA, and adding of VC in CNF-SEA,
saves time and energy for drying process, without compromising the
electrochemical stability.
Results and discussion - 44
4.5. Rheological properties of TOCN
In the previous section, to obtain a desirable mesoporous structure as
separators, cellulose was treated to a low degree of fibrillation (DF), by
passing through the microfluidizer for 1-3 times. When increasing the DF
and the surface charge density, fibers become more dispersed to nanosized
individual fibrils. Such nanofibrils are not suitable for separators, since
during the drying process they are more likely to aggregate forming thin
and compact films with nanopores. However, due to highly enhanced
dispersibility, cellulose nanofibrils have unique optical, mechanical and
colloidal properties, different from that of microfibrillated TOCN. Then the
material is more likely suited as building block for hydrogels, available for
a variety of versatile soft matter applications, e.g. biomedical or tissue
engineering, or drug delivery80–83,88,89,125. In LIB applications it can be used
as a binder material, instead of PVDF.
Figure 4. 18. Effect of different manufacturing parameter of 0.95 wt % of
CNF suspension, the charge density (350, 650 and 1550 µmol/g) and the
degree of fibrillation (DF 6, 9, 12)
The strength of TOCN-nanofibril networks varies depending on the
pretreatment parameters (e.g. degree of fibrillation, the surface charge,
solid content) and the physical properties of the nanofibrils. It is well
known that high charge density, high aspect ratio and high concentration
can lead to higher storage modulus, with stronger interfibrillar interactions.
Oscillatory shear measurements were performed to study the viscoelastic
properties of the structures. The viscoelastic behavior of TOCN nanofibril
suspensions with different charge densities and different DF are shown in
Figure 4.18, expressed as storage modulus (G’). The high storage
modulus of the TOCN with 350 and 650 µmol/g of charge density and
treated at the mildest mechanical condition (DF 6) indicates that the TOCN
Results and discussion - 45
were less disintegrated compared to those with higher charge density and
higher DF.
The difference in G’ of TOCN nanofibrils with 350 µmol/g, when varying
DF, was negligible due to weak repulsive forces from the low charge density.
The storage modulus G’ of TOCN with high charge density, 650 and 1550
µmol/g, decreased as DF increased from 6 to 9. It is in agreement with the
literature that G’ decreases when increasing the degree of homogenization
due to reduced interfibrillar interactions between individually dispersed
nanofibrils126. The dispersion and the morphology of the individual
nanofibrils were analyzed by atomic force microscope and are presented in
Paper IV. The average widths of TOCN were 2.5 nm, which means that the
fibrillation of fibers to individual fibrils was already accomplished by
treating the pulp for 6 times in the microfluidizer. There was no difference
of G’ when DF increased from 9 to 12, regardless of the charge density. The
dispersibility of the nanofibrils was similarly good in DF 6 and 9, but fiber
lengths got shorter when increasing DF. It indicates that TOCN treated at
a too high degree of fibrillation will be mechanically cut, consequently have
low aspect ratio.
Such viscoelastic behavior of TOCN and their dispersibility in aqueous
suspension will influence the properties of a hydrogel or a composite
thereof. For example, TOCN1550 treated at DF6, which has high surface
charge density and low DF, showed the best performance as a flexible LIB
electrode binder, which seems attributed to its good dispersibility (Paper
IV).
Results and discussion - 46
4.6. Electro-precipitation of CNF hydrogel, composites
thereof, and its applications
4.6.1. Electrochemical precipitation of TOCN Hydrogels
Figure 4. 19. Schematic illustrations of the primary mechanism of pH
responsive sol-gel transition of TOCN induced by water oxidation at an
anode
The most common technique to make a cellulose based hydrogel is a pH
induced sol-gel transition process, typically performed by adding an acidic
solution to a TOCN suspension in a container with a desired shape. In this
study (Paper V), we introduce a pH induced sol-gel transition of TOCN but
triggered by an electrochemical reaction inducing a local pH change at the
surface of an electrode. Figure 4.19 schematically describes the
electrochemical precipitation process and changes in the colloidal stability
of TOCN at the surface of an anode.
The gelation of TOCN proceeds as the concentration of protons increases
in the vicinity of the anode. Basically, the electrochemical precipitation of
TOCN can occur at the surface of any conductive material which can
generate protons during an electrochemical reaction, as shown in Figure
4.20A. For example, carbon paper, copper and platinum can be used as
substrates, whereas aluminium and nickel with passive layers cannot. FT-
IR spectra confirm that the sol-gel transition takes place due to protonation
of TOCN. The peak at 1724 cm−1 corresponds to stretching of the
protonated carboxylate groups (Pt and carbon paper in Figure 4.20B). It
indicates that the gelation of TOCN was accomplished only by charge
neutralization due to protonation, without any other components inducing
crosslinking of TOCN. On the contrary, the pristine TOCN freeze-dried
Results and discussion - 47
hydrogel (aerogel) has a peak at 1607 cm−1, which corresponds to carbonyl
absorption of COO-Na+ functional groups. In terms of the gel on the copper
substrate, the peak at 1724 cm-1 is much less intense than that at 1607 cm-
1, implying that a smaller portion of the carboxyl groups are protonated in
this case. It is therefore assumed that the gel on copper was formed by two
processes occurring in parallel: 1) pH induced gelation, 2) copper ion
mediated cross linking.
Figure 4. 20. Anodically electro-precipitated TOCN hydrogels on
conductive substrates (A) copper wire, brass wire, platinum plate and
carbon paper (clockwise from the top left), (B) FT-IR spectra of freeze-
dried hydrogels of pristine TOCN (no template) as well as
electrochemically precipitated on different conductive substrates. In (B),
the spectra were shifted along the y-axis for better visibility.
The growth of a TOCN hydrogel at the anode was observed by using a
TOCN sol mixed with a universal pH indicator (red at pH < 3, yellow at pH
4, light green at pH 5-7, dark green at pH 8-9, pink at pH > 10), as shown
in Figure 4.21A. During the sol-gel transition of TOCN along with water
splitting, local pH changes at the electrode surfaces were indicated by color
changes. Red at the anode means the generation of protons forming acidic
conditions around the anode. Pink at the cathode means the production of
hydroxide ions forming alkaline conditions, while the bulk remained
neutral. The colored regions were propagated away from the surface to the
bulk, as the water splitting proceeded (at 15 sec in Figure 4.21A). It is
because of diffusion of protons and hydroxide ions generated from anode
and cathode, respectively. It implies that the gelation of TOCN can be
mainly controlled by diffusion of protons, adjusting the operating variables,
e.g. current, voltage or time. Figure 4.21B exhibits an almost linear growth
Results and discussion - 48
of the hydrogel, but the growth rate decreased when the applied charge
density was higher than 30 mC/cm2. Besides, due to flocculation of TOCN
at high charge density, the standard deviation also increased.
The advantage of this electrochemical precipitation is the freedom of the
experimental setup, which is not limited to the shape of the container,
rather directly immobilizing hydrogel on the surface of conductive
substrates with complex-shapes81,85,125.
Figure 4. 21. (A) Photographs of water splitting caused local pH change
induced TOCN sol-gel transition with universal pH indicator (red when pH
<3, pink when pH > 10), as a function of time (1, 5 and 15 sec) on a copper
foil anode. (B) Time dependent thickness growth as a function of current
density (1, 3, and 5 mA/cm2) and time (up to 20 sec) and photographs of
the gels made at the given deposition conditions
Results and discussion - 49
4.6.2. Electro-precipitation of composites
Electrochemical deposition of biopolymer-based composite coatings has
been studied using a variety of materials including carbon nanotubes,
metal/metal oxide nanoparticles and enzymes80–82,88,92,125. Such polymers
work as binders, adhering nanomaterials dispersed in the coating medium
on the target substrates. In this respect, TOCN can also be regarded as a
promising coating agent to fabricate hydrogel/aerogel composites,
including functional materials, by a one-pot electro-precipitation
technique.
Figure 4. 22. (A) Mass loading increase and thickness growth (measured
in dried status) of graphite/TOCN composite hydrogel with different
composition, as a function of current densities and time, (B) photographs
of the gels (Graphite 0.95%, TOCN 0.1% sol) made at the given deposition
conditions and (C) TGA results of graphite/TOCN composites varying the
compositions and the deposition conditions
The thickness and the mass loading of the composites (Figure 4.22A)
increase as a function of time and current applied, similarly to the TOCN
based hydrogel in Figure 4.22B. A poor coverage of graphite particles on
the substrate surface was observed when a too low current density (< 3
Results and discussion - 50
mA/cm2) was applied for very short time, compared to that of higher values
(>5 mA/cm2) for a longer deposition time. TGA results showed that the
composition of the composites can be controlled by the composition of the
coating medium.
Not only single layered hydrogels and composites, but also multiple
layered hydrogel structures can be built on a target substrate by the same
electro-precipitation methodology. As described in Figure 4.23, the
layers can be deposited on the top of each other, by switching coating
medium. The two-layered hydrogel on the bottom right side in Figure
4.23 was fabricated by applying a 1st layer of graphite/TOCN composite
(20:1) hydrogel on a copper substrate, and consecutively a 2nd layer in a
TOCN coating medium. The graphite hydrogel is clearly observed under
the TOCN 2nd layer. Likewise, a tri-layered composite, made in a similar
way, replacing coating medium and coating a new layer on the top of an
existing hydrogel layer, was demonstrated as well in Paper V.
Figure 4. 23. Schematic description of fabrication of multi-layer hydrogel
composite materials on conductive substrates by electrochemical
precipitation.
A limitation of the technique is a loss in colloidal stability of the coating
medium after each coating trial, due to aggregation of TOCN. Moreover,
coating efficiency of the micro/nanoparticles which can be
electrochemically oxidized, e.g. transition metals, is very restricted.
Nevertheless, it was suitable for coating of carbon materials and oxide
Results and discussion - 51
nanoparticles, e.g. silica or aluminum nanoparticles. Particularly, graphite
is an inert material at the given experimental conditions, not easily
oxidized. Another limitation is the necessity of an electronically conducting
substrate. In addition, the coating medium is preferably an aqueous
suspension, containing low solids content, diluted from a nanocellulose gel,
which has the limitation that it cannot be directly tuned from gel status and
requires large volume for storage as well as energy for preparation.
4.6.3. Application I: Fabrication of electrode for LIB
A fast and simple preparation of electrochemically deposited TOCN-
graphite composites gives an inspiration to its application as graphite
electrodes in LIB (Figure 4.24A). As shown in Figure 4.24B, the
composite has a microstructure suitable as LIB electrode - a porous
structure and active materials coated by binders. Chronopotentiogram of
the graphite negative electrode in LIB are displayed in Figure 4.24C,
which has very similar voltage profiles to that of typical graphite electrodes,
with the plateaus below 0.2 V. The specific capacity was slightly lower than
the theoretical value for graphite (372 mAh/g) and constantly faded
(Figure 4.24D). Further optimization could improve the electrochemical
performance of the electrode.
State-of-the-art electrode fabrication processes for lithium ion batteries
need hazardous solvents, e.g. NMP and fluorine containing plastic, e.g.
PVDF. Compared to that, this technique might be an appealing
environmentally friendly way of producing electrodes for different kinds of
batteries.
Figure 4. 24. Images of graphite electrode coated on the PEI-coated
copper electrode, (B) microscopic image of the graphite, (C)
chronopotentiogram of graphite electrode and (D) galvanostatic cycling
test of the cell at 0.1 C
Results and discussion - 52
4.6.4. Application II: Selective membrane for HER
With increasing demands for clean energy, hydrogen has been considered
as a promising energy carrier for renewable energy technologies, as a
possible replacement of fossil fuels127. Photocatalytic water splitting is one
promising future hydrogen production technique. To improve its efficiency,
the backward reaction of recombination of oxygen and hydrogen to water
on the cocatalyst surface, must be circumvented128. Here as a proof-of-
concept study, we investigated the hydrogen evolution reaction (HER)
selectivity in the absence/presence of oxygen on platinum (a typical co-
catalyst in photocatalysis) electrode coated with a selective hydrogel.
Figure 4. 25. Schematic illustrations of HER with/without ORR (oxygen
reduction reaction) protective layer
Figure 4.25 is a schematic description of HER with (left) and without
(right) a barrier layer against ORR, which is an unwanted side reaction in
this case. To prevent such backward reaction, HER selective protective
layers have been studied, mostly composed of an amorphous oxide layer
including chromium oxide129, titanium dioxide130, silicon dioxide96 and
vanadium oxide97. Other than those, biopolymer-based hydrogels may
possibly be used as ORR protective layers, if permeable to water and
smaller ions but blocking the transport of oxygen.
Electrochemically coated TOCN and TOCN-Triton X-100 hydrogels on the
surface of Pt rotating discs were prepared to evaluate their performance as
selective layers during HER. 0.01 wt % of Triton X-100 surfactant was
added to the 0.1% TOCN deposition solution, to enhance the stability of the
gels.
Results and discussion - 53
Figure 4. 26. Electrochemical measurements of gel coated Pt electrodes
in O2 saturated NaClO4 solution (pH=4) at rotating speed of 1000 rpm, ,
the magnified view of the green region (j=-2.5 – 0.5 mA/cm2) as shown in
the right side (scan rate 10 mV/s) (A) TOCN-Triton X-100 hydrogel, (B)
TOCN- hydrogel, at 2000 rpm (C) TOCN-Triton X-100 hydrogel and (D)
HER measurement of gel coated Pt
Figures 4.26A and B show the electrochemical measurements carried
out at a rotation rate of 1000 rpm. The current plateaus of the gel coated
electrodes observed between -0.2 V and -1 V are related to the diffusion
limited ORR. In comparison with bare platinum, both TOCN and TOCN-
Triton X-100 hydrogel coated Pt electrodes showed ≈60% reduced ORR
current. While ORR was being prevented, HER could take place on the gel
coated electrodes at potentials below -0.8 V. The Triton X-100 containing
hydrogel had slightly better ORR blocking (67%), which indicates that the
surfactant contributes to better coating of the Pt electrode and/or further
blocking the O2 transport. An oxidation peak appeared on the backward
anodic scan at -0.8 V and seems attributed to the back-oxidation of H2
trapped in the hydrogel. This indicates that HER takes place at the
interface between the Pt-RDE and the TOCN gel, hence water can
penetrate the gel, whereas O2 cannot and some H2 is also being trapped.
Results and discussion - 54
The short term stability of the layers was evaluated by cyclic
voltammograms recorded before and after 10 min of polarization at -5
mA/cm2 (Paper V and Figure 4.26A-B).
At the increased rotation rate of 2000 rpm or at higher current density (-
10 mA/cm2), Triton X-100 was a necessary additive to the TOCN gel to
obtain a stable and protective layer (Figure 4.26C and D). On the
contrary, the TOCN hydrogel without Triton X-100 was destroyed. TOCN
Triton X-100 hydrogels blocked 80% of the ORR reaction at 2000 rpm (at
potential -0.74 V), and still 75% after galvanostatic HER (Figure 4.26C
and D). We believe that the Triton X-100 contributes to improved
plasticity of the hydrogel, with higher tolerance against shear forces and
the through transport of hydrogen.
Figure 4. 27. Electrochemical measurements of performance of Pt
electrode in the presence/absence of the TOCN hydrogel surface coating in
the range of (A) hydrogen adsorption evaluating ECSA, (B) hydrogen
oxidation reaction and (C) K3Fe(CN)6 reduction reaction
To better understand the effect of hydrogels on HER, which selectively
block the transport of gas, supplementary experiments were carried out, as
shown in Figures 4.27A-C. The difference in ECSA between a Pt electrode
in the presence/absence of the TOCN hydrogel surface coating (Figure
4.27 A) was negligible. It indicates that the hydrogel is physically bound
to the electrode surface, without blocking active sites. The hydrogel can
suppress the hydrogen oxidation reaction (HOR) as well as ferricyanide
reduction. During the hydrogen oxidation (Figure 4.27B), 82% of HOR
was suppressed. The limiting current density of ferricyanide reduction on
bare Pt was j ≈ -4.4 mA/cm2, which is 3 times higher than for gel coated Pt,
j ≈ -1.5 mA/cm2 (67% suppressed, Figure 4.27C). It is assumed that the
hydrogel restricts the accessibility of molecules from the bulk electrolyte,
working as a barrier.
Conclusions and outlook- 55
5. Conclusions and outlook
5.1. CNF based separators with different charge density
The primary goal of this work is to evaluate the feasibility of wood-based
cellulose in LIB and look for further applications in electrochemical energy
devices. The chemically modified wood-based TOCN-materials have a
good potential as an eco-friendly alternative to conventional fossil fuel-
derived separators. Raw materials for separators were produced by
chemical treatment and consecutive mechanical treatment of cellulose
pulp at relatively mild condition compared to previous studies. Results
from electrochemical measurements and gas analyses using in-operando
mass spectrometry ensure the electrochemical stability of nanocellulose
based separators. The separators show good performance with high
stability and rate capability, even at fast charging conditions. Vinylene
carbonate significantly reduces the gas evolved in LIB, hence it is
necessitated when cellulose or other hygroscopic materials produced by
aqueous processes are applied for LIB.
5.2. Spray coated cellulose-based separator-electrode
assembly
The major advantage of a separator-electrode assembly is that the current
collector reinforces the mechanical strength of the separator, so it
alleviates the concern about low tensile strength of porous materials. In
this work, we demonstrate SEAs which consist of wood-based cellulose
nanofibers, fabricated by a facile and eco-friendly process. Spray-coating
of CNF dispersed in IPA enables fast fabrication of SEA without additional
pore-forming processes in a water-free condition. Such fabrication of SEA
can be considered as a highly promising technique to acquire thinner
separator, achieving low resistivity and better electrochemical
performance of LIB. NMC SEA shows excellent electrochemical stability,
especially at fast charging/discharging rate, not only in a normal pouch cell,
but also in a proof-of-concept prismatic cell, high density batteries and
lithium metal battery as well.
Conclusions and outlook- 56
5.3. Electro-precipitation of TOCN hydrogel,
composites thereof, and their applications
TOCN as a pH-responsive biopolymer undergoes sol−gel transition at
acidic conditions (pH < 4), when an electrochemical reaction generates a
local pH decrease at the electrode surface. The hydrogel can be coated on
any electronically conducting material, even with a complex 3D shape,
during aqueous electrolysis. Furthermore, micro- or nanosized particles
containing TOCN hydrogel can be coated on the electrode surface to a
single or multiple layered composite. This simple and facile electrocoating
technique can be subject to various applications, such as coatings making
electrodes selective for the hydrogen evolution reaction, as well as a new
eco-friendly aqueous-based synthesis of Li-ion battery electrodes.
5.4. Outlook
In this thesis, we aim to fabricate wood-based lithium ion battery
separators capable of fast charging and with high electrochemical stability.
Despite successful development of the separators, it is still challenging to
fulfill other industrial needs, e.g. mechanical strength. Therefore, in order
to challenge such barrier, further mechanical treatment or chemical
modification can be implemented. The key is that the mechanical strength
of the porous membrane should be enhanced with little compromising the
electrochemical performance and the rate capability as separator in LIB.
Moreover, chemical modification of cellulose can open the possibility of
development of functionalized separators. In addition, fabrication of CNF-
composite separators or gel polymer electrolytes, blended with
reinforcement materials, can be tested.
We have speculated on the cause of cell failure related to the presence of
sodium counterions. In this respect, a more direct analysis can be carried
out for a better understanding of effects of sodium and lithium ions co-
dissolved in the full cell. Possibly, further surface characterization using
XPS or other analytical techniques can be conducted. Since it is assumed
that sodium deposition on the graphite surface is the major cause for
failure, replacing the negative electrode to LTO or a lithium metal electrode
is likely to attenuate the effect of sodium ions in LIB. The wood-based
separators presented in this thesis cannot be used with lithium metal
batteries, due to some deleterious side reactions, possibly between water
Conclusions and outlook- 57
and lithium. Nonetheless, this study also found out that hierarchical
structure of cellulose is beneficial to acquire cellulose with different
dimensions, capable of diverse applications. For instance, combined with
characteristics of microfibrillated fibers as a fast charging separator and
TOCN nanofibrils as binder, multi layered separators with functional
layers e.g. ceramic or conductive carbon coatings, can be possibly
fabricated.
As a next step, compatibility of the cellulosed based separator for the next
generation of batteries, e.g. lithium metal battery and sodium ion battery
(SIB) can be evaluated. Besides, applications of sodium carboxylate groups
containing materials for a sodium ion battery is expected to be beneficial
since it can increase the number of mobile sodium in SIB, facilitating
transport of sodium ions.
Other than LIB and H2 production, TOCN hydrogels can be applied in
diverse electrochemical energy devices applications. As the follow-up of
our H2 production study, actual implementation with photocatalyst coated
by TOCN hydrogels can be carefully suggested. In addition to this, we can
find possible applications in other types of electrochemical energy devices,
e.g. fuel cells or electrode fabrications, or carrying out the similar studies
to other polysaccharides, e.g. hemicellulose. Cellulose will have a wider
spectrum of applications in the fields of bioengineering, including tissue
engineering, drug deliveries, scaffolds, wound healings. Here, we have
studied CNF-only hydrogels and CNF-nanoparticle composites,
immobilized on the substrates. In order to move toward hydrogels
available in a diversity of applications, it needs to investigate more about
fabrication of free-standing hydrogels. Moreover, further strategies to
make hydrogels firmer can be conducted, accompanied with rheological
studies, e.g. in-situ or post polymerization electrolytes.
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