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Polymer Gels for Lithium-ion Battery

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  1. Polymer Gels for Lithium-ion Battery Fiber & Polymer Engineering Department Li Guang Hua

  2. History of Batteries Battery Secondary cell Primary cell Daniel cell(19세기 말, Zn-Cu) Zn-Mn건전지 Pb/PbO2 축전기 Portable (small volume, lightweight, high capacity) Alkaline cell Ni/Cd (1950’s) Ni/Metal hydride (1970’s) Li cell (1970’s) Li-ion cell (1991) (liquid-type) Polymer gel Li-ion (2000)

  3. Fig. 1 Comparison of the different battery technologies in terms of volumetric and gravimetric energy density. Share of worldwide for portable battery: Ni-Cd (23%); Ni-MH (14%); Li-ion (63%)

  4. Advantage of Li-ion Battery (LIB) • high energy density (~150Wh/kg; ~380Wh/l) • high operating voltage (>3.6V) • low self-discharge rate • high drain capability • wide temp. range of operation • quick-charge acceptance • longer cycle life (Aprotic solvent + Li salt) Fig. 2 Li-ion cell 구조 (microporous PE, PP) Disadvantage of Li-ion Battery (liquid-type) • possibility of the leakage of flammable electrolytes

  5. Solid polymer electrolyte Li-ion Battery (Li-SPE) Disadvantage : poor ionic conductivity ~ 10-5 S/cm at 20℃ (liquid electrolyte ~ 10-2 S/cm) Li-ion Fig. 3 Schematic representation of solid polymer electrolyte network Improve ionic conductivity : •Tg 가 낮고 Li salt을 잘 dissociation하는 polymer를 선택 (-O-, -NH-, -CN 등) •crystallinity and Tg (branching을 도입, plasticizer을 첨가 등) •bulky anion and anion receptor (such as aza-compound)을 사용

  6. Polymer gel electrolyte Li-ion Battery (LPB) Three component electrolyte system : Polymer-solvent-Li salt (gel electrolyte, hybrid electrolyte, plasticized electrolyte) Advantage : • higher gravimetric energy density (180Wh/kg) than LIB • no electrolyte leakage • thin • lower cost than LIB • excellent safety characteristics and flexibility of shape • high room temp. ionic conductivity ~10-3 S/cm

  7. Polymer gel Gel is a cross-linked polymer network swollen in liquid medium (physical cross-linking, chemical cross-linking) Coulomb’s force Hydrogen bond Coordination bond Formation of helix Covalent bond Hydrophobic bond

  8. Polymer gel electrolyte Complex formation between polar group in a polymer chain and Li+ Dissociation of Li salt and migration of Li+ 에 유리 Chemical cross-linking Gel electrolyte Semi-crystalline (such as PEO) Hybrid (gel) electrolyte Fig. A hybrid (gel) network consisting of a semi-crystalline polymer Amorphous (such as PMMA) Plasticized electrolyte Chain entanglement Dipole force

  9. Selection of polymer gel electrolyte • ionic conductivity and Li-ion transference number • electrochemical stability • thermal stability during charge/discharge cycles • mechanical stability Selection of polymer • complex formation between polymer and Li-ion (-O-, -NH-, -CN, =O, -F, 등 polar group) • good mechanical and thermal stability • low crystallinity and Tg Selection of solvent • polar aprotic solvent (easy dissociation of Li salt) • high dielectric constant • low vapor pressure and low viscosity Selection of Li salt • bulky and electrochemically stable anion을 사용 • appropriate salt concentration

  10. LiClO4, LiPF6, LiBF4, LiAsF6, LiCF3SO3, LiN(CF3SO2)2, LiC(CF3SO2)2, Li+[CF3SO2NSO2CF3]- (LiTFSI), etc. Salt Ethylene carbonate (EC), propylene carbonate (PC), Dimethyl formamide (DMF), diethyl phthalate (DEP), Dimethyl carbonate (DMC), Diethyl carbonate (DEC), methylethyl carbonate (MEC), -butyrolactone (- BL), Glycol sulfide (GS), alkyl phthalates, etc Solvent Poly(vinylidene fluoride) (PVdF), Poly(ethylene oxide) (PEO), Poly(acrylonitrile) (PAN), Poly(methyl methacrylate) (PMMA), Poly(vinylidene carbonate) (PVdC), Poly(vinyl chloride) (PVC), Poly(vinyl sulfone) (PVS), poly(ethylene glycol acrylate)(PEGA) Poly(p-phenylene terephthalamide) (PPTA), Poly(vinyl pyrrolidone) (PVP), etc. Mainly research polymer

  11. Ionic conductivity of LPB

  12. Ionic conductivity and mechanical property Highly mechanically stable gel : polymer/solvent=70-80/10-12(wt) Conductivity 10-4~10-3 S/cm Fig. 4 ionic conductivity of pristine PMMA gel electrolytes. (LiClO4/EC/PC=1/8/3.5) Ionic conductivity ~10-3 : solvent (wt %) >50 mechanical stability is not satisfied for high-speed processing Cross-linking Conductivity 10-5~10-4 S/cm UV, electron beam 등

  13. Improvement methods of mechanical strength improve mechanical strength ? Ionic conductivity ~10-3 Ionic conductivity <10-3 Affect normal applications for batteries Improvement methods : • controlled cross-linking • modify with cross-linking polymer • control polymer-solvent affinity • use a mechanical support such as micro-porous polyolefin membranes • reinforce with glass fiber cloth • add inorganic fillers (fumed silica, zeolite, Al2O3, -LiAlO2 or glass fiber)

  14. 1. Controlled cross-linking Comb cross-linking polymer Improve mechanical property Increase the local chains mobility PEG (1mol) HEMA (1mol) 50-75℃ TDI+ PPG Cat 50-65℃, Cat 1mol 2mol AIBN 55℃ dioxane LiClO4/PC Gel electrolyte film (thickness 0.7-1.0mm) A-type : Mth = 2078; B-type : Mth = 3556 (Cat : dibutyltindilaurate) Urethane acrylate macromonomer Materials Letters, 4078 (2002) 

  15. Fig. 6 Arrehenius plots of ionic conductivity of gel polymer electrolytes containing different content liquid electrolyte (1M LiClO4/PC) : (A1) 33, (A2) 50, (A3) 66 wt % 1. Ionic conductivity increase with increasing of electrolyte solution. 2. Conductivity 4 10-3 at 25 ℃ 3. Ions mainly transport in the solvent domain beyond 50wt% Fig. 5 surface AFM (atomic force microscopy) of the comb cross-linking polymer (a) and gel electrolyte film (50wt % 1M LiClO4/PC) (b). Higher network density microgels are uniform distribution

  16. 2. Modify with cross-linking polymer PMMA gel electrolyte modify with cross-linking PEGDMA (I2 : Benzoin ethyl ether) LiClO4 /EC/PC PMMA PEGDMA UV, I2 Dissolving Curing Casting Gel electrolyte (LiClO4/EC/PC=1/8/3.5) / wt % Conductivity > 10-3 Scm-1 Not free-standing Free-standing & flexible Brittle PEGDMA / wt % Fig. 7 ionic conductivity of pristine PMMA gel electrolytes. (LiClO4/EC/PC=1/8/3.5) Fig. 8 Visual appearance of PMMA-based gel electrolytes modified with PEGDMA (n=4.0). Free-standing film : PMMA wt % > 50 J. Power Sources, 109, 98 (2002)

  17. Fig. 9 Stress-strain curve of PMMA-based gel electrolytes modified with PEGDMA of different chain length. PMMA-based gel : PMMA / PEGDMA /Li salt solution = 20/20/60 PMMA gel : PMMA /Li salt solution = 55/45 Fig. 10 Ionic conductivity of PMMA-based gel electrolytes modified with PEGDMA of different chain length. Total polymer content : 40 wt% Conductivity increase with increasing an amount and MW of PEGDMA. Modifying with lower MW PEGDMA is effective for increasing the mechanical strength of PMMA-based gel electrolytes. Amount : higher donor number and higher chain flexibility MW : cross-linking density

  18. 3. Control polymer-solvent affinity Microscopic phase separation Low affinity of polymer-solvent polymer-rich phase solvent-rich phase Reasonable mecha- nical strength LiClO4 /EC/PC PAN Solution Casting 120℃ Gel electrlyte film (0.5-0.7mm) PMMA P(VdF-HFP) PVdF LiClO4 /EC/PC THF THF Casting Evaporate Affinity : PVdF 30 wt%  P(VdF-HFP) 30 wt%  PAN 30 wt%  PMMA 50 wt% Fig. 11 Surface AFM image of the polymer gel films in EC/PC (8:3.5 mol rate) solvent. Electrochimica Acta. 46, 1323 (2001)

  19. Fig. 13 Arrhenius polts of ionic conductivity for liquid electrolyte and polymer gel electrolytes (30 wt% of polymer and 70 wt% of LiClO4/EC/PC =1.0:8.0:3.5 solution) Fig. 12 Stress-strain curves of the polymer gel films (30 wt% of polymer and 70 wt% of EC/PC solvent ) Low affinity P(VdF-HFP) exhibit higher mechanical strength 1. The order of increasing conductivity : Liquid > P(VdF-HFP) > PVdF  PMMA > PAN 2. P(VdF-HFP) conductivity 2 10-3 at 20 ℃ The polymer affinity for solvent could be modulated by blending two polymers of different affinity

  20. 4. use a mechanical support Polymer gel electrolyte (P(VdF-HFP), P(AN-MMA-St)) be coated onto microporous PE Microporous PE(25 m) LiPF6-EC/DEC Cooled to r.t. Gel electrolyte Immerse P(VdF-HFP) Dissolve 60℃ 60℃ (thickness ~ 65 m) (5-20 wt % P(VdF-HFP)) Conductivity : 1.5~2 10-3 S/cm at r.t. Microporous PE (25 m) LiPF6-EC/DEC/EMC P(AN-MMA-St)) Immerse Cooled to r.t. Dissolve 60℃ 60℃ (5 wt % Polymer) Gel electrolyte film (thickness 30 ~ 35 m) Conductivity : 1.1 10-3 S/cm at r.t. Solid State Ionics 148, 443 (2002);138, 41 (2000)

  21. 5. Reinforce with glass fiber cloth Glass-fiber cloth (GFC) design : Glass sheet covered with a GFC (38 m) LiClO4-EC/PC/DEC PAN P(VdF-HFP) Dissolve Casting PGE-GFC film 110 ℃ (thickness 40~ 90 m) Table 1. Composition (wt %) of PGE-GFC film J. Power Sources, 92, 272 (2001)

  22. Table 2. Comparison of mechanical strength of polymer electrolytes and microporous Celgard membrane Table 3. Ionic conductivity of polymer electrolytes at room temperrature

  23. 6. Add inorganic fillers First demonstrate : Solid State Ionics, 7 (1), 75 (1982) Adding inorganic filler (-Al2O3) to PEO-LiClO4 polymer electrolytes can improve significantly in the mechanical stability (has a negligible effect on the ionic conductivity) Since then : Introduce high surface area particulate fillers into polymer electrolytes such as ZrO2, TiO2, Al2O3, zeolite, -LiAlO2, hydrophobic fumed silica, glass fiber etc. PAN, or/and zeolite mixture LiAsF6 /EC-PC 100-110 ℃ Gel electrolyte Casting EC:PC:PAN:LiAsF6 = 40/34.75/21/4.25 mol % Thickness ~ 0.25mm Conductivity 10-3 ~10-2 S/cm at r.t. Affect slightly the ionic conductivity of the electrolytes (decrease polymer crystallinity) Fig. 14 Arrhenius plotes of ionic conductivity for gel electrolytes of PAN/LiAsF6/EC-PC with (○) no zeolite, and 5 wt% additions of zeolite, (▱) 4Å, 40 m, (△) 10Å, 40m, and (▽) 10Å, 2 m. J. Power Sources, 55 (1), 7 (1995)

  24. Conclusions and future aspects • Polymer gel electrolytes have higher ionic conductivity at r.t. •The mechanically stable gel electrolytes were may obtained by the above several methods. Disadvantage of polymer gel electrolytes in Li-ion battery Most of the studied solvents have shown electrochemical instabilites at Li metal surfaces, such as highly polar PC or EC Combination with less polar solvents such as DMC, etc. The anions of the Li salt decompose at Li metal electrode Polyelectrolytes having the anion attached to their polymer backbones minimize self-discharge, salt-leakage, and disposal problems Polymer structural modifications and synthesis novel polymer Composite ceramic polymer gel electrolyte

  25. Polymer structural modifications and synthesis novel polymer 1. Low Tg and amorphous polymer + chemical cross-linking such as polysiloxane, branched PEO, P(VdF-co-propylvinyl ether) 2. Low Tg and amorphous polymer –block- crystalline polymer Composite ceramic polymer gel electrolyte Nano clay, control of composite structure