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Bicomponent Blend-electrospun Separator
for Lithium ion Battery
http://www.tjpu.edu.cn
TJPU
Yanbo Liu*, Dongyue Qi, Wenxiu Yang, Weiya Chen
School of Textiles, Tianjin Polytechnic University, Tianjin 300387, China
1. Introduction
2. Experimental
3. Results and discussion
4. Conclusion
5. Acknowledgement
6. Reference
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TJPU1. INTRODUCTION
3
Battery:A container in which chemical energy is
converted into electricity and used as a
source of power, composed of anode,
separator, electrolyte solution, cathode,
shell, etc.
Lithium-ion battery (LIB):An electrochemical system in which
lithium ions move from the negative
electrode to the positive electrode during
discharge and back when charging.Y. Nishi, Chem. Rec. 1(2001) 406-413.
• High energy capacity, high storage energy density
• Long life cycle (charging/discharging: 500-10000 times)
• High nominal voltage, favoring for battery power pack
• High power, low self-discharging rate
• Small volume, light weight
• Green and environmentally friendly tech
• Fast charging/discharging rate
• No memory effect
• Potential to form high capacity battery pack for applications
in electric vehicles
• High cost/price, low safety, easy to explode/get fire
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TJPUFeatures of Li Ion Battery
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TJPUThe Role of Separator in LIB
Cost: 30%; Profit: 70%
• Provide a barrier between the anode and the cathode while
enabling the exchange of lithium ions from one side to the other in
the battery, to prevent short-circuit.
• Has high porosity (>45%), high Li ion permeability, low internal
resistance
• Possesses high safety and thermal stability: mechanical; dimensional
• Cut off the electric current when temp of the battery is unreasonably
elevated as a smart safety apparatus: thermal shutdown
• Loose its integrity at high temp: thermal runaway/meltdown
• It is very important to design a battery separator having appropriate
shutdown temp and high meltdown temp.
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TJPUThe Role of Separator in LIB
Conventional manufacture methods:
• stretching (dry method); or thermally induced phase separation (wet
method)
• polymers: PP, PE, PP/PE/PP
Most recently developed methods:
• ceramic coating for enhanced performance of high-temp resistance and
dimensional stability, but easy to block the pores
• electrospinning with high melting point polymer such as PVDF, PI, etc.
Shut down temperature and melting point of polyolefin membrane
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TJPUExisting Separator Technology
• Monocomponent polymer based separator cannot impart separator
with thermal shutdown property and thermal runaway property at
the same time
• Monocomponent polymer based separator having high melting
point is not safe at elevated temp, because other materials in the
battery may change their properties at high temp due to
unexpected chemical reactions, causing fire or explosion
• Current PP/PE/PP trilayer separator does not have sufficiently high
meltdown temp and sufficiently low shutdown temp
• Proper shutdown temp ranging from 110~125℃; the higher the
better for the meltdown temp
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TJPUProblems with Current Separators
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TJPUProblems with the LIBs
• The thermal property of separator is a key role in LIB security performance
• Novel bico ES tech could impart the separator with thermal shutdown and thermal
meltdown functions at the same time
Exothermic and decomposed temperature of
cathode materials with electrolyte
Temperature range and enthalpy of various
exothermic reactions in LIBs
The Advantages of ES Separator
Technical features: simple process; easy to control product specification; wide
materials types; high P/C ratio
Product advantages:high electrolyte absorbent rate; high Li ion permeability;
excellent electrochemical performance; long life cycle;
favoring the applications in high power/capacity LIBs, after
the design for enhanced thermal stabilities
Product disadvantages: low mechanical strength
Application fields:separators for medium/high end LIBs, especially for power LIBs
Bico composite electrospun separators
Feature 1: The bico composite electrospun separators contains a lower melting point
thermal plastic polymer compared to the other component
Feature 2: The low melting point component plays as a part of fiber binder, which impart
the bico composite electrospun separator with enhanced mechanical property upon
thermal calender bonding
Feature 3: The low melting point component provides the low temperature shutdown
function, enhancing the security performance of the separator
A schematic for Electrostatic spinning or Electrospinning process
Components of ES setups
(1)DC/AC high voltage power source(2)Spinneret(needle/needleless)(3)Solution feeding system(4)Fiber receiving system
Process parameters
(1)Polymer and solution parameters(2)Equipment:ES type; spinneret specification; receiver type, etc.(3)Process variables:voltage; receiving distance; feeding rate(4)Atmospheric variables:temp; RH; surrounding gas; gas speed
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TJPUIntroduction to ES Process
Research objectives and methods
Bico polymer blend ES→ES separator→thermal calender
→ characterization of the ES separator
Bico polymer pair selection:PVdF-HFP)/(PI-TiO2)
For comparison, three samples to be prepared and evaluated:
(1) PI/PVdF-HFP (PI/PH, with no nanoparticles contained in PI),
(2) TiO2@PI/PVdF-HFP (T@PI/PH, with TiO2 mixed in PI)
(3) f-TiO2@PI/PVdF-HFP (f-T@PI/PH, with f-TiO2 blended in PI)
TiO2 is easy to get aggregated when blending with PI in solution,
Modification of TiO2 is required for successful co-electrospinning
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TJPU2. EXPERIMENTAL
• PVdF-HFP=Poly(vinylidene fluoride-co-hexafluoropropylene,PH), MW =600,000• PI=Polyimide,MW =180,000• f-TiO2=Functionalized TiO2, synthesized by the atom transfer radical polymerization
process• TiO2=Titanium dioxide powder, average particle sizes of 20-30 nm.• DMF=N,N-Dimethylformamide• NMP=N-methyl-2-Pyrrolidone• APTES=3-aminopropyl triethoxysilane ( 98%),• 2-brom-opropionyl bromide (97%),• PMDETA=N,N,N’,N’’,N’’’-pentamethyldiethylenetriamin (99%),• HEMA=2-hydroxyethyl methacrylate(96%) ,used to modify TiO2
• Other raw materials and reagents: triethylamine, acetone, toluene, methanol andCuBr(cupper bromide)
2.1. Materials and reagents
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TJPU2. EXPERIMENTAL
2.2 Synthesis of TiO2-HEMA(f-TiO2)
(a) Agglomerated nanoparticles in the polymer for electrospinning without grafted polymer and
(b) Separation of particles due to the existence of the grafted polymer.
Functionalized TiO2 (f-TiO2) was synthesized by the
atom transfer radical polymerization process
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TJPU2. EXPERIMENTAL
2.3. Preparation of separators
The schematic illustration of the preparation of electrospun separator and battery assembling.
Preparation of solutions of polymer solutions: • PI/PVdF-HFP solution (14 wt%) :using mixture solvent of DMF/acetone(7/3 w/w)
• Nanoparticles@PI/PVdF-HFP solution (23 wt%) : dissolving the polymer and nanoparticles in NMP
with magnetic stirring at the temperature of 70 ℃ for 10 h after 30 min ultrasonic treatment to
achieve dispersion liquid, the ratio of nanoparticles/PI =0, 1, 2, 3% w/w
Co-electrospinning setup:• Four-needle electrospinning apparatus was set up
• Two of which were fed with PVdF-HFP(PH) solution and others with nanoparticles@PI solution
• The two types of polymer solutions were arrayed alternately in a straight line to form a composite
membrane.
Conditions for co-electrospinning:
• Same voltage 20 kV applied for both solutions
• Injected flow rates of 1 ml/h and 1.5 ml/h, respectively.
• Fiber receiving distance: 20 cm(rotational speed of the drum is 2 RPM)
• Needle diameter: OD=0.91 mm; ID=0.61 mm
• Needle spacing: 2cm , moving reciprocally with the amplitude of 30 cm.
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TJPU2. EXPERIMENTAL
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TJPU2. EXPERIMENTAL
Bico co-electrospinning process Resultant co-electrospun membrane
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TJPU3. RESULTS AND DISCUSSION
3.1. Characterization of nanoparticles
The FTIR spectra of TiO2 and
f-TiO2 nanoparticles.
UV-vis absorption spectrum of TiO2 and
f-TiO2 nanoparticles in NMP
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TJPU3. RESULTS AND DISCUSSION
3.2. Preparation and characterization of
electrospun composite membrane
SEM images:
(A) PI/PH,
(B) T@PI/PH
(C) f-T@PI/PH
with different nanoparticle concentration: (a)
1%, (b) 2%, (c) 3%
EDX / Energy-dispersive X-ray spectra
(D) 2% T@PI/PH
(E) 2% f-T@PI/PH
Compared with T@PI/PH membrane, the content of
C and O element slightly increased for f-T@PI/PH
membrane demonstrated by higher intensity in EDX
spectrum, which indicated the presence of HEMA.
(a) Porosity, liquid electrolyte uptake and (b) liquid electrolyte contact angle of electrospun
composite membranes with different nanoparticle concentration (0%, 1%, 2% and 3%) after
thermal calendering process.
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TJPU3. RESULTS AND DISCUSSION
Thermal calendering:135 ℃, 0.8 for 5 min
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TJPU3. RESULTS AND DISCUSSION
3.3. Mechanical and thermal properties of electrospun composite membrane
3.3.1. Mechanical property
The morphologies:
(a) PI/PH
(b) 2% T@PI/PH
(c) 2% f-T@PI/PH membranes
after thermal calendering process
(d) the stress-strain curves before and after
thermal calendering process of membranes.
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TJPU3. RESULTS AND DISCUSSION
3.3. Mechanical and thermal properties of electrospun composite membrane
3.3.2 Thermal property of electrospun composite membrane
(a) DSC curves of PI/PH, 2% T@PI/PH and 2% f-T@PI/PH after thermal calendering
(b) Photos of composite electrospun membranes before and after thermal treatment.
SEM photos of co-ES membrane before and after thermal calendering process: tortuous pores
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TJPU3. RESULTS AND DISCUSSION
Mono directional stretched PP and PE
Celgard (dry method)Asahi Hipore-2 (wet method)
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TJPU3. RESULTS AND DISCUSSION
SEM of commercial separators: through holes
Co-electrospun separator after
drying for 30min at 150℃
SEM photos before and after drying for 30min at 150℃
Co-electrospun separator before
drying for 30min at 150℃
Thermal stabilityshutdown & meltdown
Asahi stretched film Asahi stretched film
Before and after heating at 150℃,for 30min
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TJPU3. RESULTS AND DISCUSSION
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TJPU3. RESULTS AND DISCUSSION
3.4 Electrochemical performances of composite separator
Nyquist plots of SS/electrolyte-soaked
separator/SS cells at room temperature.
Linear sweep voltammograms of different
electrospun separators.
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TJPU3. RESULTS AND DISCUSSION
3.5. Battery performances
The AC impedance spectra of the half-cell
with different electrospun separators.
First-cycle charge-discharge curves of Li/LiCoO2
cells containing PI/PH, 2% T@PI/PH and 2%
f-@PI/PH fibrous separator at 0.5C.
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TJPU3. RESULTS AND DISCUSSION
3.5. Battery performances
Cycling performance of Li/LiCoO2 cells containing PI/PH,
2%T@PI/PH and 2%f-T@PI/PH fibrous separator at 0.5C.
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TJPU4. CONCLUSION
• The TiO2 nanoparticles@PI/PH bico composite separators could be successfully
prepared by co-electrospinning process.
• 2% f-TiO2 nanoparticle with better dispersibility produced by ATRP method:
• reduced the PI fiber diameter
• improved the physical property of co-ES membrane: porosity, liquid electrolyte
uptake and wettability.
• excellent mechanical property: due to the existence of fiber binder with low
melting point
• superior ionic conductivity, interfacial and electrochemical stability
• excellent cycling performance
• thermal dimensional stability: due to the existence of PI blended with f-TiO2
• higher safety performance: low temp shutdown & high temp meltdown
• suitable for lithium ion battery separator, particularly power battery separator.
5. Acknowledgement
This study is supported by NSFC(Approval No. 51373121)
Thanks for the efforts and contributions from my graduate students
6. REFERENCE
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Engineering, 2010, 39(5): 936-940
2. Yi Tingfeng,Hu Xingguo,Gao Kun. Battery, 2005, 35(6): 468
3. Huang Haijiang. Shanghai: Shanghai Institute of Microsystem and
Information Technology, 2005:103
4. Kominato A. Journal of Power Sources, 1997, 68: 471
5. Gerardine Botte G. Journal of Power Sources, 2001, 97-98: 570
6. WeiyaChen, Yanbo Liu, Ying Ma, et al. Materials Letters, 2014, 133:
67–70
7. Weiya Chen, Yanbo Liu, Ying Ma, Wenxiu Yang. Journal of Power
Sources 2015, 273: 1127-1135
8. QI Dong-yue;LIU Yan-bo;MA Ying;XIAO You-hua;SONG Guo-wen.
9. Wu Kai, Zhang Yao, Zeng Yuquan, Yang Jun. Progress in Chemistry.
2011,23 (2 /3) : 401-409
TJPU