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The Role of the Electrolyte in
Lithium Ion Batteries
15.05.2009 | Andrea Balducci Page | 1
Lithium Ion Batteries
Drive-E Akademie
09.03.2010
Andrea Balducci
Institute of Physical Chemistry, Westfälische Wilhelms-University Münster,
Corrensstraße 28/30, 48149 Münster
1
102
103
104
kW
kg
-1)
Capacitors
Energy (kWh/g): the capacity to do workPower (kW/kg): how fast the energy is delivered
Energy vs. Power
15.05.2009 | Andrea Balducci Page | 1
10-2
10-1
100
101
102
103
10-3
10-2
10-1
100
101
Sp
ec
ific
Po
we
r /
(kW
kg
Specific Energy / (Wh kg-1)
Supercapacitors
Batteries FuelCells
Page 1
TODAY
MEDIUM
HIGH
LOW
Lithium-ion battery
TOMORROW
15.05.2009 | Andrea Balducci Page | 1
Energy SafetyLife CostPower
LOW
Page 4
TOMORROW: Green & High Performance Batteries
10.07.2009 5Page 5
Lithium-ion batteries
Lithium-ion batteries in automotive industry
NEW APPLICATIONS POSSIBLE
15.05.2009 | Andrea Balducci Page | 1
TODAY
Energy SafetyLife CostPower
MEDIUM
HIGH
LOW
Battery of tomorrow
TOMORROW
15.05.2009 | Andrea Balducci Page | 1Page 4
How to improve the performance?
10.07.2009 5Page 5
Electrolyte Components
Materials (Active, Inactive)
Outlines
• Electrolyte in lithium-ion batteries: general aspect
• Electrolyte and ionic liquids
• Solid polymer electrolytes & ILs
15.05.2009 | Andrea Balducci Page | 110.07.2009 7Page 7
• Conclusions
Colle
cto
r(C
u) P
os. C
urre
nt
Sep
ara
tor
Lithium-ion Battery
15.05.2009 | Andrea Balducci Page | 1
Neg
. C
urr
ent
Colle
cto
r Cu
rren
tC
olle
cto
r(A
l)
Sep
ara
tor
Anode CathodeElectrolyte
Graphite, Li4Ti5O12
Si, Si/CLiCoO2, LiMn2O4, LiFePO4
10.07.2009 2Page 2
e-
e-N
eg. C
urr
ent
Colle
cto
r (C
u) P
os. C
urre
nt C
olle
cto
r (Al)
Lithium Ion Battery: Charge
15.05.2009 | Andrea Balducci Page | 1
Se
pa
rato
r
Neg. C
urr
ent
Colle
cto
r (C
u) P
os. C
urre
nt C
olle
cto
r (Al)
Anode CathodeElectrolyte
10.07.2009 Page 3
e-
e- e-
e-
Neg. C
urr
ent
Colle
cto
r (C
u) P
os. C
urre
nt C
olle
cto
r (Al)
Lithium Ion Battery: Discharge
15.05.2009 | Andrea Balducci Page | 1
Se
pa
rato
r
Neg. C
urr
ent
Colle
cto
r (C
u) P
os. C
urre
nt C
olle
cto
r (Al)
Anode CathodeElectrolyte
10.07.2009 4Page 4
• Liquid
• Liquid organic solvent based electrolytes
• Liquid inorganic solvent based electrolytes
• Molten salts (low temperature = ionic liquids)
• "Solid"
Electrolyte materials
15.05.2009 | Andrea Balducci Page | 1
• "Solid"
• Solid polymer electrolytes
• Ceramic electrolytes
• Glassy electrolytes
• Composites
• Gel electrolytes
Outlines
• Electrolyte: general aspect
• Electrolyte and ionic liquids
• Solid polymer electrolytes & ILs
15.05.2009 | Andrea Balducci Page | 110.07.2009 7Page 7
• Conclusions
Electrolyte in General
Electrolyte: electrolytic solution-type consisting of salts („electrolyte solutes“)
dissolved in solvents
Dissociation due to thermodynamic interactions between solvent and solute
molecules = solvation
Function: medium for the transfer of charge in form of ions
between the electrodes
Requirements for electrochemical devices:
15.05.2009 | Andrea Balducci Page | 1
Requirements for electrochemical devices:
high ionic conductivity
low melting and high boiling points
chemical and electrochemical stabilities
safety
SEI Film forming ability!!!
For lithium and lithium ion batteries:
10.07.2009 8Page 8
Se
pa
rato
r
Lithium Ion Battery: Exfoliation
Neg. C
urr
ent
Colle
cto
r (C
u) P
os. C
urre
nt C
olle
cto
r (Al)
15.05.2009 | Andrea Balducci Page | 1
Se
pa
rato
r
Neg. C
urr
ent
Colle
cto
r (C
u) P
os. C
urre
nt C
olle
cto
r (Al)
Anode CathodeElectrolyte
10.07.2009 9Page 9
Solid Electrolyte Interphase
- passivation layer
- prevent exfoliation
- formed within the first few cycles via
electrolyte decomposition
- minimum of irreversible material and
charge loss, minimum of side reaction
Properties:
- just permeable for Ions, high
ionic conductivity
- ideally electronically insulating
⇒ no further decomposition
- uniform morphology and
chemical composition
SEIe-
Li+
e-
Li+
15.05.2009 | Andrea Balducci Page | 1
1 Emanuel Peled Journal of the Electrochemical Society 1979, 126, 2047.
chemical composition
⇒ homogenious current
distribution
- good mechanical strength and
flexibility
⇒ allows expansion and
contraction of the graphene lattice
- low solubility in electrolytes
⇒ no dissolutionAnode CathodeElectrolyte
10.07.2009 10Page 10
Se
pa
rato
r
Lithium Ion Battery: SEI-Film
Neg. C
urr
ent
Colle
cto
r (C
u) P
os. C
urre
nt C
olle
cto
r (Al)
15.05.2009 | Andrea Balducci Page | 1
Se
pa
rato
r
SEI
Neg. C
urr
ent
Colle
cto
r (C
u) P
os. C
urre
nt C
olle
cto
r (Al)
Anode CathodeElectrolyte
10.07.2009 11Page 11
Electrolyte System
Multi Component System
Li Salt Solvents Additives
15.05.2009 | Andrea Balducci Page | 1
Design of the electrolyte components
Energy SafetyLifeCostPower
10.07.2009 13Page 13
Electrolyte System
ConductivityEnvironment - friendly
Temperature range of use
Electrolyte System
����
?
?
15.05.2009 | Andrea Balducci Page | 1
Vapor pressure Film forming ability
Electrochemical stability window
State of the art:
Organic electrolytes
����
?
?
10.07.2009 Page 14
Properties from best ⇒ to worst
Ion mobility LiBF4 LiClO4 LiPF6 LiAsF6 LiTf1) LiTFSI2)
Ion pair dissociation LiTFSI LiAsF6 LiPF6 LiClO4 LiBF4 LiTf
Solubility LiTFSI LiPF6 LiAsF6 LiBF4 LiTf
There is No Universally Superior Electrolyte Salt
Lithium salts
15.05.2009 | Andrea Balducci Page | 1
Thermal stability LiTFSI LiTf LiAsF6 LiBF4 LiPF6
Chemical inertness LiTf LiTFSI LiAsF6 LiBF4 LiPF6
SEI formation LiPF6 LiAsF6 LiTFSI LiBF4
Al corrosion LiAsF6 LiPF6 LiBF4 LiClO4 LiTf LiTFSI
1) LiTf lithium triflate 2) LiTFSI lithium bis(trifluoromethansulfonyl)imide
Nakajima, T.; Groult H. (eds.), Fluorinated Materials for Energy Conversion, Elsevier, Amsterdam, 2005
Properties from best ⇒ to worst
OO
O
EC
OO
O
PC
O O
GBL
O O
O
DEC
O O
O
DMC
O O
O
EMC
High dielectric solvents: HDS Low viscosity solvents: LVS
Electrolyte Solvents
15.05.2009 | Andrea Balducci Page | 1
Low melting point DEC EMC PC GBL DMC EC
High boiling point EC PC GBL DEC EMC DMC
High dielectric constant εεεε EC PC GBL DMC EMC DEC
Low viscosity ηηηη DMC EMC DEC GLB PC EC
High flash point EC PC GBL DEC DMC
SEI formation EC
O O
O
OO
DME
DMC
Salts
�LiPF6
�LiBF4
�LiAsF6
�LiClO4
High dielectricsolvents (HDS)
OO
O
OO
O
EC
Low viscositysolvents (LVS)
Electrolyteadditives
OO
O
O
O
O
VC
VAInsufficientOxidation Stability
Electrolytes components
15.05.2009 | Andrea Balducci Page | 1
O O
O
O O
O
DMC
DEC
EMC
4
�LiCF3SO3
�LiN(SO2CF3)2
�LiBOB
OO
PC
O O
GBL
OO
F
P
O
MeO OMe
OMe
FEC
TMP
BPUneffective SEI
Stability
Not Well-Conductiveor Toxic or
Explosive orCorrosive
- Salt:
- Solvents:
For conductivity reasons use of solvent mixtures of: OO
O
OO
O
LiPF6
P
F
F FF
FF
-Li+
SEI forming compound
+
State of the Art
15.05.2009 | Andrea Balducci Page | 1
- high dielectric solvents HDS:
- low viscosity solvents LVS:
EC
O O
O
DEC
O O
O
DMC
O O
O
EMC
PC
compound
State of the art: LiPF6
+ Instability ⇒ SEI forming agent, Al current collector protection
+ Conductivity
- Instability ⇒ Thermal and chemical
LiPF6 + H2O LiF + POF3 + 2HF Toxic and corrosiveLewis-acids,
State of the Art: lithium salt
15.05.2009 | Andrea Balducci Page | 1
LiPF6 LiF + PF5
+ LiPF6
∆T
OFF
LiCoO2
∆T
Lewis-acids, Catalysts for polymerization
Highly(!) toxicOO
O
Not ideal, but "best" among commercially available candidates⇒⇒⇒⇒ Alternative or at least partial replacement urgently needed
State of the art: EC (PC) + low viscosity solvent LVS (DEC, DMC, EMC,…)
+ Sufficient SEI film form ability
+ TR conductivity
- Low and high T behavior (conductivity, wetting, viscosity,…)
- Safety: flammability of LVS!
reactivity with electrodes, in particular at higher T
- Liquid: immobilization, leakage,…? ⇒ safety and performance concern
State of the Art: liquid solvents
15.05.2009 | Andrea Balducci Page | 1
- Liquid: immobilization, leakage,…? ⇒ safety and performance concern
2 Possibilities:
� Keep liquid organic electrolyte, but- substitute or add novel solvent components and electrolyte additives- immobilize liquid electrolyte (polymer matrix ⇒ hybrid or "gel" electrolyte)
� Substitute liquid organic electrolyte, e.g., by ceramic, solid polymer, IL,…electrolytes
Electrochemical stability requirements: Cation: stable vs. reduction, anion: stable vs. oxidation
Solubility/dissociation requirements:Small Li+ cation + large anion ⇒ small lattice energy
⇒ good dissociation, e.g., LiPF6, LiBOB
Conductivity: Salt Issuses
15.05.2009 | Andrea Balducci Page | 1
In general:
Conductivities are 2-3orders of magnitude lower than aqueousbattery electrolytes!
⇒Thin electrolyte filmsto keep resistance small!
Liquid organic electrolytes for lithium ion batteries are based on solvent
mixtures for conductivity reasons.
High dielectric solvent (HDS):Solvates ions, thus favors electrolyte salt dissociation.
But: (Too) high viscosity
Conductivity: Solvent Issues
15.05.2009 | Andrea Balducci Page | 1
Low viscosity solvent (LVS):Is a dilutant, thus lowers viscosity.
But: Poor ion solvation⇒ ion pair formation(= lack of free charge carriers)
Anode: graphite
Electrolyte: 1M LiClO4 in PC
Scan rate: 50 µV.s-1
Anode: graphite
Electrolyte: 1M LiClO4 in PC
Scan rate: 50 µV.s-1
+2 wt.% ethyl isocyanate (Et-NCO)
Importance of the SEI Film
15.05.2009 | Andrea Balducci Page | 1
C. Korepp et al. Journal of Power Sources 2007, 174, 628.
Anode CathodeElectrolyte
Li+
Li+Li+(solv)y
SEI Interface
Li+
Electrolyte Decomposition
15.05.2009 | Andrea Balducci Page | 1
Reductive electrolyte
decomposition mechanismOxidative electrolyte
decomposition mechanism
Reaction products: - directly react with electrodes- diffuse and then react with the electrode- redox-shuttle between the electrodes, etc.
Reactions depend on: - electrolyte composition- electrodes (bulk, surface)- potentials- temperature, etc.
Anode CathodeElectrolyte
What we know?
- exist and has its function
(SEI determines cell safety, life, etc.)
- consists of electrolyte decomposition
products and Li+
- not perfect (no true solid electrolyte)
- influenced by many parameters
- there is no universal SEI!
- SEI grows and ages during storage/cycling
Knowledge about SEI Film
15.05.2009 | Andrea Balducci Page | 1
- SEI grows and ages during storage/cycling
What we do not know ?
⇒ Only aspects are known about SEI formation, growth, aging, etc. No clear picture! ⇒ Composition of SEI is unclear: many contradictory reports!⇒ No rule for SEI formation procedure and for finding a good SEI forming agent.
Chemically very similar compounds show a totally different SEI behavior!
What is a good SEI? What is a bad SEI? Empirical Approach!
SEI
AssemblyElectrolyte, electrode,active and "inactive"
Formation charge, potential,change of chemistry side reactions, etc.
SEI: Formation
15.05.2009 | Andrea Balducci Page | 1
SEI
Battery propertieselectrochemistry
∆T behavioursafety, etc.
Application
SEI
AssemblyElectrolyte, electrode,active and "inactive"
Formation charge, potential,change of chemistry side reactions, etc. Formation
Mechanism
composition,
impurities, surface
monitoring,
characterization
in situ, on-line
characterization
ex situ
ANALYT
SEI Composition
Thinking
SEI: Formation ⇒ Characterization
⇒⇒⇒⇒ Understanding
15.05.2009 | Andrea Balducci Page | 1
SEI
Battery propertieselectrochemistry
∆T behavioursafety, etc.
Application
Mechanismex situTICS
AssemblyElectrolyte, electrode,active and "inactive"
Formation charge, potential,change of chemistry side reactions, etc. Formation
Mechanism
composition,
impurities, surface
monitoring,
characterization
in situ, on-line
characterization
ANALY
SEI Composition
Thinking
SEI: Formation ⇒⇒⇒⇒ Characterization
⇒⇒⇒⇒ Understanding ⇒⇒⇒⇒ Improvement
15.05.2009 | Andrea Balducci Page | 1
SEI
Application
Mechanismex situLYTICS
characterization in situ & ex situ
Application
Improved Performance
Understanding
Thinking
Battery propertieselectrochemistry
∆T behavioursafety, etc.
Multi Component Electrolyte ⇒⇒⇒⇒ Multi Component SEI
⇒ Electrolyte: solvents, salt(s), additive(s), impurities
⇒ Anode (material, surface)⇒ Many different SEI products
⇒ SEI may vary in lateral dimensions
⇒ Anode-near SEI parts must be stable against reduction potentials close to/equal to 0 V vs. Li/Li+, anode-far (electrolyte-near) parts
Characterize & Understand the SEI
15.05.2009 | Andrea Balducci Page | 1
"Heterogeneous", "mosaic", "complex" structure/composition
close to/equal to 0 V vs. Li/Li , anode-far (electrolyte-near) partsdo not have to
⇒ SEI composition varies in depth
⇒ Electrode formulation ("inactive components", etc.)
⇒ Formation conditions: charge procedures, current densities, etc.
⇒ SEI growth and aging during storage and cycling (temperature)
Multi Component SEI ⇒⇒⇒⇒ Locally Different SEI
The SEI is heterogeneously composed in depth and in lateral dimensions.⇒ Locally different SEI: Locally applied analytical method will give only local information of the SEIGlobally applied analytical method will give only global (average) information of the SEI
Characterize & Understand the SEI
15.05.2009 | Andrea Balducci Page | 1
Locally Different Electrolyte Decomposition Products / SEI on Graphite
RelevantMethodology:
In situEx situ Li+(C
u)
Li+Electrolytedecomposition
products
Basal plane surface
Characterize & Understand the SEI
15.05.2009 | Andrea Balducci Page | 1
Ex situ
AFMSTMSEMTEMXPSAugeretc.
Li+
Curr
ent
Co
llecto
r(C
u)
Li+
Electrolyte decomposition
products
Prismatic surface
Locally Different Electrolyte Decomposition Products on Basal Plane and Prismatic Surfaces of Graphite
XPS measurements of HOPG
Prismatic
(%)
Basal plane
(%)Elements
Characterize & Understand the SEI
15.05.2009 | Andrea Balducci Page | 1
of HOPGafter one cycle in 1.2 M LiAsF6
in EC : DEC electrolyte
Use of diverse methods which allow to detect certain aspects of the SEI
Characterize & Understand the SEI
15.05.2009 | Andrea Balducci Page | 1
Characterization of a "practical" electrodeafter electrochemical experiment (SEI formation)
- Removal from cell (under protective atmosphere)
- Rinsing and cleaning (under protective atmosphere) to remove the electrolyte: also parts of the SEI can be removed
Ex situ Approach
Characterize & Understand the SEI
15.05.2009 | Andrea Balducci Page | 1
- Transport and transfer to the analytical chamber (under protective atmosphere)
- Often conversion or destruction of the SEI by the specific analytical experiment(Vacuum, Beam)
Characterization of a “non practical” electrodeduring electrochemical experiment (SEI formation)
Model electrodesInert metals, glassy carbon, carbon fibers, "binder-free“, instead of composite (binder/carbon) electrodes
In situ Approach
Characterize & Understand the SEI
15.05.2009 | Andrea Balducci Page | 1
Model experimental conditionsSlow/fast electrochemical experimentsCell housing, Inactive materials (grids, etc.) different
Model electrolytesExcess of electrolyteModel electrolyte components
Ex situ
Battery Electrode
Practical “Battery”Electrochemical Conditions
but
In situ
Model Electrode
“Model“ ElectrochemicalConditions
but
In Situ vs. Ex Situ
Characterize & Understand the SEI
15.05.2009 | Andrea Balducci Page | 1
Results of
Practical Electrode ?
butHandling & Analysis
Outside BatteryEnvironment
Results of
Model Electrode !
butHandling & AnalysisInside “Non-Battery”
Environment
Outlines
• Electrolyte: general aspect
• Electrolyte and ionic liquids
• Solid polymer electrolytes & ILs
15.05.2009 | Andrea Balducci Page | 110.07.2009 7Page 7
• Conclusions
TODAY
Energy SafetyLife CostPower
MEDIUM
HIGH
LOW
Battery of tomorrow
TOMORROW
15.05.2009 | Andrea Balducci Page | 1Page 410.07.2009 5Page 5
Ionic Liquids (ILs)
Room Temperature Ionic Liquids (ILs)Typically consist of organic cations and inorganic/organic anions. The low melting temperatures result from unfavourable crystal packing and ion flexibility.
ILs properties
• negligible vapor pressures • high ionic conductivities• wide electrochemical stability window• thermally stable
15.05.2009 | Andrea Balducci Page | 1
• thermally stable• easily dissolve lithium salt (doping)• “green electrolyte”
ILs electrolytes in lithium batteries
15Page 15
ILs Chemical Physical Properties
2,0-4 -3 -2 -1 0 1 2
Potential vs. Ag° / AgCF3SO
3 in PYR
14TFSI (V)
80 70 60 50 40 30 20 10
T / °C
Arrhenius Conductivity plot Electrochemical stability window at RT
Very high purity (>95%), H2O content < 1 ppm
N-methyl-N-buthylpyrrolidinium
bis(trifluoromethansulfonyl)imidePYR14TFSIN SS
O
O
O
O
CC
F
F
F
F
F
F
-
N
+
•
N-methyl-N-propyl pyrrolidinium bis(fluorosulfonyl)imide PYR13FSI
-
N
+
N SS
O
O
O
O
FF•
15.05.2009 | Andrea Balducci Page | 1
PYR14TFSI: better electrochemical stability
PYR13FSI: higher conductivity (even higher than PC-LiTFSI 1M)
0 1 2 3 4 5 6-2,0
-1,5
-1,0
-0,5
0,0
0,5
1,0
1,5
2,0 PYR
14TFSI
PYR13
FSI
Cu
rren
t d
en
sit
y (
mA
cm
-2)
Potential vs. Li / Li+ (V)
2,8 2,9 3,0 3,1 3,2 3,3 3,4 3,51
10
PC - LiTFSI 1M
PYR14
TFSI
PYR13
FSI
σσ σσ
/ m
S c
m-1
103 T
-1 / K
-1
Page 16
SEI in graphite electrodes
The selection of
Cu Graphite + Li
SEI
sep
ara
tor
Solvent Li-salt
Electrolyte
Additive
15.05.2009 | Andrea Balducci Page | 1
The selection of film forming electrolytes additives and Li salt
is crucial in the case of organic liquid electrolytes
What is the importance of additives and Li-salt in ILs?
Page 17
0.3 M LiTFSI in PYR14TFSI
PYR14TFSI - LiTFSI as Electrolyte
0.3 M LiTFSI in PYR14TFSI + 5% wt. VC
-0,1
0,0
0,1
0,2 1
st cycle
2nd
cycle
3rd cycle
i / m
A m
g-1
O O
O
Selected graphite: KS6 (TIMCAL) very sensitive to the electrolyte properties
-0,05
0,00
0,05
0,10
0,15
i / m
A m
g-1
1st cycle
2nd
cycle
3rd cycle
15.05.2009 | Andrea Balducci Page | 1
Poor electrochemical performance
0 50 100 150 200 250 300 350 400-0,2
-0,1
E / mV
Better cyclability because of VC,
but low specific capacity
Efficiency (3rd cycle): 74,5%Specific capacity: 132 mAhg-1
0 250 500 750 1000-0,15
-0,10
E / mV
Page 18
PYR13FSI - LiTFSI as Electrolyte
0.3 M LiTFSI in PYR13FSI 0.3 M LiTFSI in PYR13FSI + 5% wt. VC
-0,3
-0,2
-0,1
0,0
0,1
0,2
0,3
1st cycle
2nd
cycle
3rd cycle
i / m
A m
g-1
-0,3
-0,2
-0,1
0,0
0,1
0,2
0,3
1st cycle
2nd
cycle
3rd cycle
i / m
A m
g-1
-0,3
-0,2
-0,1
0,0
0,1
0,2
0,3
1st cycle
2nd
cycle
3rd cycle
i / m
A m
g-1
15.05.2009 | Andrea Balducci Page | 1
0 50 100 150 200 250 300 350 400-0,3
E / mV
VC improves the efficiency,
but not the specific capacity
Stability comparable
with PYR14TFSI + 5% VCLow specific capacity
Efficiency (3rd cycle): 83,5%Specific capacity: 130 mAhg-1
Efficiency (3rd cycle): 89,8%Specific capacity: ca. 130 mAhg-1
0 200 400 600 800 1000 1200 1400-0,3
E / mV
0 50 100 150 200 250 300 350 400-0,3
E / mV
Page 19
In Situ FTIRS Measurements
CounterElectrode
ReferenceElectrode
Workingelectrode
Home-made IR cell, provided with an optical ZnSe window.
Working Electrode = Glassy Carbon (ø=12 mm) Counter electrode = LiReference electrode = Li
Glassy carbon (GC) has a goodcapability for IR beam reflection
15.05.2009 | Andrea Balducci Page | 1
IRcapability for IR beam reflection
SNIFTIR method (Subtractively Normalized Interfacial FTIRectroscopy)
Reference spectra at OCV (R0) Stepwise to 0.4 V vs Li/Li+ (Ry)
Page 20
PYR14TFSI DOES NOT DECOMPOSE
PYR14TFSI
4000 3500 3000 2500 2000 1500 1000 500
OCV 1000 mV 750 mV
500 mV
Tra
sm
issio
n
ν / cm-1
0.3 M LiTFSI in PYR14TFSI
0.3 M LiTFSI in PYR14TFSI + 5% wt VC
Tra
sm
issio
n
*
In Situ FTIR Measurements
15.05.2009 | Andrea Balducci Page | 1
PYR14TFSI DOES NOT DECOMPOSE
VC DECOMPOSES
The FSI- anionDECOMPOSES
4000 3500 3000 2500 2000 1500 1000 500
OCV 1000 mV
500 mV 400 mV
Tra
sm
issio
n
ν / cm-1
**
O O
O
O O
O
1820 1810 1800 1790 1780
1,000
1,005
1,010
1,015
1,020
1,025
1,030
1,035
RE / R
0
νννν / cm-1
0.3 M LiTFSI in PYR13FSI
1240 1220 1200 1180 1160 1140 1120 1100
1,000
1,005
1,010
1,015
1,020
1,025
νννν / cm-1
RE / R
0
N SS
O
O
O
O
FF N SS
O
O
O
O
FF
4000 3500 3000 2500 2000 1500 1000 500
OCV
1000 mV
750 mV
500 mV
ν / cm-1
Tra
sm
issio
n
Page 21
PYR13FSI
LOWER STABILITYBetter electrochemical
performance
Additives maybe not
necessary:FSI- source for the SEI layer
PYR14TFSI
HIGHER STABILITYPoor electrochemical
performance
Additives necessary:VC source for the SEI layer
ILs and Additive Electrolytes
15.05.2009 | Andrea Balducci Page | 1
Different SEI chemistry
Optimization of the SEI chemistry
Li salt
Page 22
0.3 M LiPF6 in PYR14TFSI 0.3 M LiPF6 in PYR14TFSI + 5% wt. VC
-0,1
0,0
0,1
0,2
0,3 1
st cycle
2nd
cycle
3rd cycle
i / m
A m
g-1
PYR14TFSI as Electrolyte with LiPF6
Role of Li salt
-0,1
0,0
0,1
0,2
0,3
i / m
A m
g-1
1st cycle
2nd
cycle
3rd cycle
LiPF6
15.05.2009 | Andrea Balducci Page | 1
Poor electrochemical performance
0 50 100 150 200 250 300 350 400-0,3
-0,2
E / mV
Higher specific capacity compared to
LiTFSI but lower cycling stability
Efficiency (3rd cycle): 92,1%Specific capacity: 180 mAhg-1
0 50 100 150 200 250 300 350 400-0,3
-0,2
E / mV
Page 23
0.3 M LiPF6 in PYR13FSI 0.3 M LiPF6 in PYR13FSI + 5% wt. VC
0 50 100 150 200 250 300 350 400-0,5
-0,4
-0,3
-0,2
-0,1
0,0
0,1
0,2
0,3
0,4
0,5 1
st cycle
2nd
cycle
3rd cycle
i /
mA
mg
-1
PYR13FSI as Electrolyte with LiPF6
0 50 100 150 200 250 300 350 400-0,3
-0,2
-0,1
0,0
0,1
0,2
0,3
i / m
A m
g-1
1st cycle
2nd
cycle
3rd cycle
15.05.2009 | Andrea Balducci Page | 1
0 50 100 150 200 250 300 350 400
E / mV
The VC improve the efficiency,
but not the specific capacity
In PYR13FSI the use of LiPF6 increases
the specific capacity more than 2 times
BUT lower cycling stability
0 50 100 150 200 250 300 350 400
E / mV
Efficiency (3rd cycle): 88,7%Specific capacity: 300 mAhg-1
Efficiency (3rd cycle): 96,5%Specific capacity: 300 mAhg-1
Page 24
PYR14TFSI and PYR13FSI
PYR13FSI + LiPF6
Mixtures of
PYR14TFSI / PYR13FSI
How to obtain high specific capacity
and high cycling stability?
Specific
capacity /
mA
hg
-1
300
400
The selection of Li salt is critical also in ILs based electrolytes
15.05.2009 | Andrea Balducci Page | 1
� Wide electrochemical stability (from PYR14TFSI)� High conductivity (from PYR13FSI) � Intrinsic film form ability (from PYR13FSI)
PYR14TFSI+ LiTFSI
Cycling stability
Specific
capacity /
mA
hg
Low Medium High
100
200
0
Mixtures of PYR14TFSI / PYR13FSI
Page 25
Mix of ILs as electrolyte
� Several mixtures have been prepared:
(x)PYR14TFSI/(1-x)PYR13FSI/LiTFSI
(x)PYR14TFSI/(1-x)PYR13FSI/LiPF6
� Evaluation of electrochemical stability window and ionic conductivity(and cost..!)
15.05.2009 | Andrea Balducci Page | 1
� Selected Mixtures:
(80%) PYR14TFSI / (20%) PYR13FSI / LiTFSI (80/20/LiTFSI)(80%) PYR14TFSI / (20%) PYR13FSI / LiPF6 (80/20/LiPF6)(50%) PYR14TFSI / (50%) PYR13FSI / LiTFSI (50/50/LiTFSI)(50%) PYR14TFSI / (50%) PYR13FSI / LiPF6 (50/50/LiPF6)
VC was added to improve the efficiency
Page 26* G.B. Appetecchi et al., Journal of Power Sources,192,2,599-605
PYR13FSI + LiPF6
Specific
capacity /
mA
hg
-1
200
300
400
The mixtures of (x)PYR14TFSI/(1-x)PYR13FSI good strategy
80/20/LiTFSI
Mix of ILs as electrolyte
0
50
100
150
200
250
300
350
400
0
20
40
60
80
100
discharge capacity
charge capacity
cap
ac
ity
/ m
Ah
g-1
efficiency
effic
ien
cy / %
290 mAhg-1
15.05.2009 | Andrea Balducci Page | 1
PYR14TFSI+ LiTFSI
Cycling stability
Specific
capacity /
mA
hg
Low Medium High
100
0
� Wide electrochemical stability (from PYR14TFSI)� High conductivity (from PYR13FSI) � Intrinsic film form ability (from PYR13FSI)
0 10 20 30 40 500 0
cycle number
� High specific capacity � High cycling stability
Page 27* S.F. Lux et al., Journal of Power Sources,192,2,606-611
Outlines
• Electrolyte: general aspect
• Electrolyte and ionic liquids
• Solid polymer electrolytes & ILs
15.05.2009 | Andrea Balducci Page | 110.07.2009 7Page 7
• Conclusions
Solid polymer electrolytes & ILs
S/cmS/cm
1010--33
1010--55
aprotic liq.aprotic liq.
solid polymersolid polymer
(1979 (1979 ––2000)2000)
&&
ionic liquidsionic liquids
Solid polymer electrolyte based on PEO Poly(ethylene oxide)
Overcoming the conductivity drawback of PEO electrolytes
15.05.2009 | Andrea Balducci Page | 1
1010--88
PEO-LiX-IL electrolytes
� Polymer matrix (PEO) + 2 salts (LiX and IL) having the same anion(PEO-PYR14TFSI-LiTFSI)
Page 29
• Very low interactions between Cation and Anion of IL
• No interaction between Cation and PEO host (IL does
not interact with PEO)
• No interaction between Cation and Li+
• Strong interaction between Li+ and TFSI- lowers the
(Anion)-
Li+
Li+
(Anion)-
(Cation)+
(Anion)-
(Anion)-
PEO host
(Anion)-
Promoted lithium ion mobility
PEO-LiX-IL electrolytes
15.05.2009 | Andrea Balducci Page | 1
• Strong interaction between Li+ and TFSI- lowers the
strength of the Li+ - PEO coordination
PEO host
(Anion)-
(Cation)+Li+
(Anion)-
* M. Castriota et al., J. Phys. Chem. A 109 (2005) 92,* I. Nicotera et al., J. Phys. Chem. B 109 (2005) 22814.*J.H. Shin et al., J. Electrochem. Soc., 152 (2005) A978
� Conductivity enhancement
The IL-LiTFSI interaction prevents the formation of
the crystalline P(EO)6LiTFSI phase
Page 30
Problem for the incorporation of ILs
Over a certain content of IL the mechanical stability becomes poor
Crosslinking of PEO
PEO-LiX-IL electrolytes
15.05.2009 | Andrea Balducci Page | 1
Increase the conductivity of the filmwhile maintaining the mechanical stability
Crosslinking of PEO
Page 31
O
hv
O *
-CH2-CH2-O-
OH
-CH2-CH-O-+
CH2
CH
O
CH
O
H2CCH2
CH
O
HC
O
H2C
PEO is sensitive to β + γ radiation (fragmentation !)
Benzophenone acts
via H abstraction !
Thin films of PEO (few µm) containing 5 %wt. of BPh showed an insoluble (gel) fraction W=80%
UV crosslinking with a photoinitiator is possible
Crosslinking of PEO
15.05.2009 | Andrea Balducci Page | 1
Thin films of PEO (few µm) containing 5 %wt. of BPh showed an insoluble (gel) fraction W=80%after photo-crosslinking at 70°C under inert gas condition
PEO/PYR14TFSI/Benzophenone
Film preparation:
• Mixing in solid state 57/43/5 (in weight)• Hot pressing between 2 mylar foils at 70°C for 5 min
• 150 µm thickness
• Punching (disc)
• UV curing (365 nm).
PEO/PYR14TFSI/Benzophenone
Page 32
200 250 300 350 400 450 5000,0
0,5
1,0
365nm
Ab
sorp
tio
n
Wavelength [nm]
PYR14
TFSI
Benzophenone
PEO
320nm
Crosslinking of PEO/IL/Li salt
There is no need to remove the Mylar foil for illumination: transparent >320 nm
At 365 nm ONLY the benzophenone adsorb
15.05.2009 | Andrea Balducci Page | 1
Wavelength [nm]
� The insoluble fraction of PEO is about
80% after 5 min of illumination
� PYR14TFSI DOES NOT participate in the photo-crosslinking reaction
0 100 200 300 400 500 600 700 800 900 10000
20
40
60
80
100
Ge
lfra
ctio
n [%
]
Irradiation time [s]
Fragmentation
of the PolymerCrosslinking of
the Polymer
� The addition of LiTFSI did not modify the crosslinking reaction
Page 33
365 nm
UV-curing
With non crosslinked composites, the limiting composition for mechanical stable film
is approx. 10/1/1 (with higher IL content sticky gels are obtained)
crosslinked PEO / PYR14TFSI / LiTFSI = 10:2:1 (mol)
Mechanical stability of PEO/IL/Li salt
15.05.2009 | Andrea Balducci Page | 1
365 nm
5 min per side
The transparency of the sample after curing indicates low crystallinity
With crosslinked PEO is possible to obtain significantly improve the mechanical stability
* B. Rupp et al., European Polymer Journal 9,44 (2008) 436Page 34
DSC & conductivity measurements
-100 -80 -60 -40 -20 0 20 40 60 80 100-0,9
-0,8
-0,7
-0,6
-0,5
-0,4
-0,3
-0,2
He
at flo
w [W
/g]
(1) uncured 1:1:10
(2) cured 1:1:10
(3) uncured 2:1:10
(4) cured 2:1:10
4
1
3
2
41°C
36°C
� The crossilinkg process reduce significantly the crystalline fraction
PYR14TFSI / LiTFSI / PEO
� No crystalline domain are discernible in the sample (from SEM)
15.05.2009 | Andrea Balducci Page | 1
-100 -80 -60 -40 -20 0 20 40 60 80 100-0,9
Temperatur [°C]
Half order of magnitude increase in the ionic conductivity:0.4 mS/cm @ 20°C 1 mS/cm @ 40°C
Page 35
Electrolyte Scale-up
Thin films are vacuum sealed Thin films are UV cured at 70°C for
PEO / LiTFSI / PYR14TFSI (10:1:2) + 5% (PEO wt.) Benzophenone
Mixtures are prepared through a solvent-free procedure:1. Benzophenone and LiTFSI are dissolved in PYR14TFSI @ 70°C2. PEO is added to the solution and mixed to form a paste3. The paste is stored in oven @ 120°C to homogenize 4. Thin films are made by hot-pressing the electrolyte mixtures @ 90°C
15.05.2009 | Andrea Balducci Page | 1
Thin films are vacuum sealed in polyethylene envelopes
Thin films are UV cured at 70°C for
different time (3, 5, 7, and 9 minutes)
Page 36
Mechanical properties
8 by 8 cm polymer electrolytes thin films
Fully amorphous &Highly adhesive
15.05.2009 | Andrea Balducci Page | 1
Highly adhesive
Very good mechanical properties
Elastomeric behavior
Page 37
Outlines
• Electrolyte and ionic liquids
• Solid polymer electrolytes & ILs
• Binders and lithium battery
15.05.2009 | Andrea Balducci Page | 1Page 38
• Binders and lithium battery
• Conclusions
Page 43
Summary
� The electrolyte plays a crucial role in lithium ion batteries
� The electrolyte is a multi-component system
� The electrolyte can (and need) be design
� The formation of the SEI is necessary
15.05.2009 | Andrea Balducci Page | 1Page 44
� Organic electrolyte are the state of the art
� Ionic liquids display promising properties
� Solid polymer electrolytes & ILs possible alternative