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

Battery Primary cell Secondary cell History of Batteries Daniel cell (19 세기 말, Zn-Cu) Zn-Mn 건전지 Pb/PbO 2 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)

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%)

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 Fig. 2 Li-ion cell 구조 (Aprotic solvent + Li salt) Disadvantage of Li-ion Battery (liquid-type) possibility of the leakage of flammable electrolytes (microporous PE, PP)

Solid polymer electrolyte Li-ion Battery (Li-SPE) Fig. 3 Schematic representation of solid polymer electrolyte network Disadvantage : poor ionic conductivity ~ S/cm at 20 ℃ (liquid electrolyte ~ S/cm) Improve ionic conductivity : T g 가 낮고 Li salt 을 잘 dissociation 하는 polymer 를 선택 (-O-, -NH-, -CN 등 ) crystallinity and T g  (branching 을 도입, plasticizer 을 첨가 등 ) bulky anion and anion receptor (such as aza-compound) 을 사용 Li-ion

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

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

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

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

Salt Solvent polymer LiClO 4, LiPF 6, LiBF 4, LiAsF 6, LiCF 3 SO 3, LiN(CF 3 SO 2 ) 2, LiC(CF 3 SO 2 ) 2, Li + [CF 3 SO 2 NSO 2 CF 3 ] - (LiTFSI), etc. 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 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

Ionic conductivity of LPB Polymer systemPolymer electrolyte Conductivity (S/cm) at 20 ℃ Linear PEO(PEO) 8 -LiClO 4 (EC:PC, 20mol%)10 -3 Crosslinked PEO(PEO) 8 -LiClO 4 (PC, 50wt%) 8  PVdFPVdF-LiN(CF 3 SO 2 ) 2 (EC:PC, 75wt%) 1.5  PEGAPEGA-(LiClO 4 :PC, 1M)10 -3 Poly(ethylene imine)PEI-LiClO PPTAPPTA-(PC:EC:LiBF 4, 25:25:0.8mol%) 2.2  PEGDMAPEGDMA-(LiClO 4 :PC, 1M) 2  PANPAN-(EC:PC:LiClO 4 ), 33-38:21:8mol%10 -3 PMMA-g-PEOPMMA-g-PEO/PC-LiBF 4 (1M)-15Cr510 -3

Ionic conductivity and mechanical property Highly mechanically stable gel : polymer/solvent=70-80/10-12(wt) Conductivity ~10 -3 S/cm Fig. 4 ionic conductivity of pristine PMMA gel electrolytes. (LiClO 4 /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 -4 S/cm UV, electron beam 등

Ionic conductivity ~10 -3 Improvement methods of mechanical strength improve mechanical strength ? 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, Al 2 O 3,  -LiAlO 2 or glass fiber) Ionic conductivity <10 -3 Affect normal applications for batteries

Materials Letters, 4078 (2002)  Comb cross-linking polymer 1. Controlled cross-linking Improve mechanical property Increase the local chains mobility TDI + PPG ℃ HEMA (1mol) PEG (1mol) ℃, Cat Cat 1mol 2mol (Cat : dibutyltindilaurate) Urethane acrylate macromonomer AIBN 55 ℃ dioxane LiClO 4 /PC Gel electrolyte film (thickness mm) A-type : M th = 2078; B-type : M th = 3556

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

2. Modify with cross-linking polymer J. Power Sources, 109, 98 (2002) PMMA gel electrolyte modify with cross-linking PEGDMA PMMA PEGDMA LiClO 4 /EC/PC Dissolving Casting UV, I 2 Curing Gel electrolyte ( I 2 : Benzoin ethyl ether) Fig. 7 ionic conductivity of pristine PMMA gel electrolytes. (LiClO 4 /EC/PC=1/8/3.5) Free-standing film : PMMA wt % > 50 Fig. 8 Visual appearance of PMMA-based gel electrolytes modified with PEGDMA (n=4.0). (LiClO 4 /EC/PC=1/8/3.5) / wt % PEGDMA / wt % Free-standing & flexible Not free-standing Brittle Conductivity > Scm -1

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. Amount : higher donor number and higher chain flexibility MW : cross-linking density Modifying with lower MW PEGDMA is effective for increasing the mechanical strength of PMMA-based gel electrolytes.

3. Control polymer-solvent affinity Low affinity of polymer-solvent Microscopic phase separation Reasonable mecha- nical strength polymer-rich phase solvent-rich phase PMMA P(VdF-HFP) PVdF THF LiClO 4 /EC/PC Casting THF Evaporate PAN LiClO 4 /EC/PC 120 ℃ SolutionCasting Gel electrlyte film ( mm) Fig. 11 Surface AFM image of the polymer gel films in EC/PC (8:3.5 mol rate) solvent. Affinity : PVdF 30 wt%  P(VdF-HFP) 30 wt%  PAN 30 wt%  PMMA 50 wt% Electrochimica Acta. 46, 1323 (2001)

Fig. 12 Stress-strain curves of the polymer gel films (30 wt% of polymer and 70 wt% of EC/PC solvent ) 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) 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  at 20 ℃ The polymer affinity for solvent could be modulated by blending two polymers of different affinity

4. use a mechanical support Polymer gel electrolyte (P(VdF-HFP), P(AN-MMA-St)) be coated onto microporous PE Solid State Ionics 148, 443 (2002);138, 41 (2000) Microporous PE(25  m ) P(VdF-HFP) LiPF 6 -EC/DEC Dissolve 60 ℃ Immerse Cooled to r.t. 60 ℃ Gel electrolyte (thickness ~ 65  m) Conductivity : 1.5~2  S/cm at r.t. (5-20 wt % P(VdF-HFP)) Microporous PE (25  m ) P(AN-MMA-St)) LiPF 6 -EC/DEC/EMC Dissolve 60 ℃ Immerse Cooled to r.t. 60 ℃ Gel electrolyte film (thickness 30 ~ 35  m) Conductivity : 1.1  S/cm at r.t. (5 wt % Polymer)

5. Reinforce with glass fiber cloth J. Power Sources, 92, 272 (2001) PAN P(VdF-HFP) LiClO 4 -EC/PC/DEC 110 ℃ Dissolve Glass sheet covered with a GFC (38  m) CastingPGE-GFC film (thickness 40~ 90  m) Glass-fiber cloth (GFC) design : sampleGFC polymerplasticizer LiClO 4 PANP(VdF-HFP)ECPCDEC PGE PGE-GFC Table 1. Composition (wt %) of PGE-GFC film

Sample Thickness (  m) Burst strength (KPa) PGE-GFC90>1000 Celgard PP25105 PGE110< 2 SampleConductivity ( S/cm ) PGE-GFC 2.0  PGE 2.1  Table 2. Comparison of mechanical strength of polymer electrolytes and microporous Celgard membrane Table 3. Ionic conductivity of polymer electrolytes at room temperrature

6. Add inorganic fillers First demonstrate : Solid State Ionics, 7 (1), 75 (1982) Adding inorganic filler (  -Al 2 O 3 ) to PEO-LiClO 4 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 ZrO 2, TiO 2, Al 2 O 3, zeolite,  -LiAlO 2, hydrophobic fumed silica, glass fiber etc. Fig. 14 Arrhenius plotes of ionic conductivity for gel electrolytes of PAN/LiAsF 6 /EC-PC with ( ○ ) no zeolite, and 5 wt% additions of zeolite, ( ▱ ) 4 Å, 40  m, ( △ ) 10 Å, 40  m, and ( ▽ ) 10 Å, 2  m. LiAsF 6 /EC-PC PAN, or/and zeolite ℃ mixture Casting Gel electrolyte EC:PC:PAN:LiAsF 6 = 40/34.75/21/4.25 mol % Thickness ~ 0.25mm Conductivity ~10 -2 S/cm at r.t. J. Power Sources, 55 (1), 7 (1995) Affect slightly the ionic conductivity of the electrolytes (decrease polymer crystallinity)

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. Polymer structural modifications and synthesis novel polymer 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 Composite ceramic polymer gel electrolyte

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