The Role of the Electrolyte in Lithium Ion Batteries

Transcription

1 The Role of the Electrolyte in Lithium Ion Batteries Drive-E Akademie Andrea Balducci Institute of Physical Chemistry, Westfälische Wilhelms-University Münster, Corrensstraße 28/30, Münster Andrea Balducci Page 1

2 Energy vs. Power Energy (kwh/g): the capacity to do work Power (kw/kg): how fast the energy is delivered Specific Power / (kw kg -1 ) Capacitors Supercapacitors Batteries Fuel Cells Specific Energy / (Wh kg -1 ) Andrea Balducci Page 1

3 Batteries LITHIUM-IN Andrea Balducci Page 1

4 Lithium-ion battery TDAY TMRRW HIGH MEDIUM LW Energy Power Life Safety Cost TMRRW: Green & High Performance Batteries Andrea Balducci Page 51 4

5 Lithium-ion batteries NEW APPLICATINS PSSIBLE Lithium-ion batteries in automotive industry Andrea Balducci Page 1

6 Battery of tomorrow TDAY TMRRW HIGH MEDIUM LW Energy Power Life Safety Cost How to improve the performance? Electrolyte Components Materials (Active, Inactive) Andrea Balducci Page 51 4

7 utlines Electrolyte in lithium-ion batteries: general aspect Electrolyte and ionic liquids Solid polymer electrolytes & ILs Conclusions Andrea Balducci Page 7 71

8 Lithium-ion batteries Electrodes Electrolyte Electrodes Andrea Balducci Page 1

9 Lithium-ion Battery Neg. Current Co ollector (Cu) Separ rator Pos. Curren nt Collector (Al) Anode Electrolyte Cathode Graphite, Li 4 Ti 5 12 Si, Si/C LiCo 2, LiMn 2 4, LiFeP Andrea Balducci Page 21

10 Lithium Ion Battery: Charge e - e - Pos. Current Collector (Al) Separator Neg. Current Collector (Cu) Anode Electrolyte Cathode Andrea Balducci Page 3 1

11 Lithium Ion Battery: Discharge e - e - e - e - Pos. Current Collector (Al) Separator Neg. Current Collector (Cu) Anode Electrolyte Cathode Andrea Balducci Page 4 4 1

12 Electrolyte materials Liquid Liquid organic solvent based electrolytes Liquid inorganic solvent based electrolytes Molten salts (low temperature = ionic liquids) "Solid" Solid polymer electrolytes Ceramic electrolytes Glassy electrolytes Composites Gel electrolytes Andrea Balducci Page 1

13 utlines Electrolyte: general aspect Electrolyte and ionic liquids Solid polymer electrolytes & ILs Conclusions Andrea Balducci Page 7 71

14 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: For lithium and lithium ion batteries: high ionic conductivity low melting and high boiling points chemical and electrochemical stabilities safety SEI Film forming ability!!! Andrea Balducci Page 8 81

15 Lithium Ion Battery: Exfoliation Pos. Current Collector (Al) Separatorr Neg. Current Collector (Cu) Anode Electrolyte Cathode Andrea Balducci Page 9 1

16 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 e - SEI Li + e- Li + Properties: - just permeable for Ions, high ionic conductivity - ideally electronically insulating no further decomposition - uniform morphology and chemical composition homogenious current distribution - good mechanical strength and flexibility allows expansion and contraction of the graphene lattice Anode Electrolyte Cathode 1 Emanuel Peled Journal of the Electrochemical Society 1979, 126, low solubility in electrolytes no dissolution Andrea Balducci Page 10 1

17 Lithium Ion Battery: SEI-Film Pos. Current Collector (Al) Separatorr SEI Neg. Current Collector (Cu) Anode Electrolyte Cathode Andrea Balducci Page 11 1

18 Electrolyte System Li Salt Solvents Additives Multi Component System Energy Power Cost Life Safety Design of the electrolyte components Andrea Balducci Page 13 1

19 Electrolyte System Environment - friendly Temperature range of use?? Conductivity Electrolyte System Vapor pressure? Film forming ability Electrochemical stability window? State of the art: rganic electrolytes Andrea Balducci Page 14 1

20 Lithium salts There is No Universally Superior Electrolyte Salt Properties from best to worst Ion mobility LiBF 4 LiCl 4 LiPF 6 LiAsF 6 LiTf 1) LiTFSI 2) Ion pair dissociation LiTFSI LiAsF 6 LiPF 6 LiCl 4 LiBF 4 LiTf Solubility LiTFSI LiPF 6 LiAsF 6 LiBF 4 LiTf Thermal stability LiTFSI LiTf LiAsF 6 LiBF 4 LiPF 6 Chemical inertness LiTf LiTFSI LiAsF 6 LiBF 4 LiPF 6 SEI formation LiPF 6 LiAsF 6 LiTFSI LiBF 4 Al corrosion LiAsF 6 LiPF 6 LiBF 4 LiCl 4 LiTf LiTFSI 1) LiTf lithium triflate 2) LiTFSI lithium bis(trifluoromethansulfonyl)imide Nakajima, T.; Groult H. (eds.), Fluorinated Materials for Energy Conversion, Elsevier, Amsterdam, Andrea Balducci Page 1

21 Electrolyte Solvents EC PC GBL DEC DMC EMC High dielectric solvents: HDS Properties from best to worst Low viscosity solvents: LVS Low melting point DEC EMC PC GBL DMC EC High boiling point EC PC GBL DEC EMC DMC High dielectric constant ε EC PC GBL DMC EMC DEC Low viscosity η DMC EMC DEC GLB PC EC High flash point EC PC GBL DEC DMC SEI formation EC Andrea Balducci Page 1

22 Electrolytes components Salts High dielectric solvents (HDS) Low viscosity solvents (LVS) Electrolyte additives LiPF 6 DME VC LiBF 4 LiAsF 6 LiCl 4 LiCF 3 S 3 LiN(S 2 CF 3 ) 2 Not Well- LiBB Conductive or Toxic or Explosive or Corrosive EC PC GBL Uneffective SEI Insufficient xidation DMC Stability DEC EMC Me F P Me Me VA FEC TMP BP Andrea Balducci Page 1

23 State of the Art - Salt: Li + F F F - P F F F - Solvents: LiPF 6 For conductivity reasons use of solvent mixtures of: - high dielectric solvents HDS: EC PC + SEI forming compound - low viscosity solvents LVS: DEC DMC EMC Andrea Balducci Page 1

24 State of the Art: lithium salt State of the art: LiPF 6 + Instability SEI forming agent, Al current collector protection + Conductivity - Instability Thermal and chemical LiPF 6 + H 2 LiF + PF 3 + 2HF T LiPF 6 LiF + PF 5 + LiPF 6 LiCo 2 T F F Toxic and corrosive Lewis-acids, Catalysts for polymerization Highly(!) toxic Not ideal, but "best" among commercially available candidates Alternative or at least partial replacement urgently needed Andrea Balducci Page 1

25 State of the Art: liquid solvents State of the art: EC (PC) + low viscosity solvent LVS (DEC, DMC, EMC, ) + Sufficient SEI film form ability + T R 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 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 Andrea Balducci Page 1

26 Conductivity: Salt Issuses 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., LiPF 6, LiBB In general: Conductivities are 2-3 orders of magnitude lower than aqueous battery electrolytes! Thin electrolyte films to keep resistance small! Andrea Balducci Page 1

27 Conductivity: Solvent Issues 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 Low viscosity solvent (LVS): Is a dilutant, thus lowers viscosity. But: Poor ion solvation ion pair formation (= lack of free charge carriers) Andrea Balducci Page 1

28 Importance of the SEI Film Anode: graphite Electrolyte: 1M LiCl 4 in PC Scan rate: 50 µv.s -1 Anode: graphite Electrolyte: 1M LiCl 4 in PC Scan rate: 50 µv.s wt.% ethyl isocyanate (Et-NC) C. Korepp et al. Journal of Power Sources 2007, 174, Andrea Balducci Page 1

29 Electrolyte Decomposition SEI Interface Li + Li + (solv) y Li + Li + Anode Reductive electrolyte decomposition mechanism Electrolyte Cathode xidative electrolyte decomposition mechanism Reaction products: Reactions depend on: - directly react with electrodes - diffuse and then react with the electrode - redox-shuttle between the electrodes, etc. - electrolyte composition - electrodes (bulk, surface) - potentials - temperature, etc Andrea Balducci Page 1

30 Knowledge about SEI Film What we do not know? 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 nly 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! Andrea Balducci Page 1

31 SEI: Formation Assembly Electrolyte, electrode, active and "inactive" Formation charge, potential, change of chemistry side reactions, etc. SEI Application Battery properties electrochemistry T behaviour safety, etc Andrea Balducci Page 1

32 SEI: Formation Characterization Understanding Assembly Electrolyte, electrode, active and "inactive" Formation charge, potential, change of chemistry side reactions, etc. SEI Application Battery properties electrochemistry T behaviour safety, etc. composition, impurities, surface monitoring, characterization in situ, on-line characterization ex situ A N A LY T I C S SEI Composition Thinking Formation Mechanism Andrea Balducci Page 1

33 SEI: Formation Characterization Understanding Improvement Assembly Electrolyte, electrode, active and "inactive" Formation charge, potential, change of chemistry side reactions, etc. SEI Application Battery properties electrochemistry T behaviour safety, etc. composition, impurities, surface monitoring, characterization in situ, on-line characterization ex situ characterization in situ & ex situ A N A LY T I C S SEI Composition Thinking Formation Mechanism Thinking Understanding Application Improved Performance Andrea Balducci Page 1

34 Characterize & Understand the SEI 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 do 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) "Heterogeneous", "mosaic", "complex" structure/composition Andrea Balducci Page 1

35 Characterize & Understand the SEI 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 SEI Globally applied analytical method will give only global (average) information of the SEI Andrea Balducci Page 1

36 Characterize & Understand the SEI Locally Different Electrolyte Decomposition Products / SEI on Graphite Relevant Methodology: Basal plane surface In situ Ex situ Li + AFM STM SEM TEM XPS Auger etc. Current Collector (C Cu) Li + Li + Li + Electrolyte decomposition products Prismatic surface Electrolyte decomposition products Andrea Balducci Page 1

37 Characterize & Understand the SEI Locally Different Electrolyte Decomposition Products on Basal Plane and Prismatic Surfaces of Graphite Elements Prismatic (%) Basal plane (%) XPS measurements of HPG after one cycle in 1.2 M LiAsF 6 in EC : DEC electrolyte Andrea Balducci Page 1

38 Characterize & Understand the SEI Use of diverse methods which allow to detect certain aspects of the SEI Andrea Balducci Page 1

39 Characterize & Understand the SEI Ex situ Approach Characterization of a "practical" electrode after 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 - Transport and transfer to the analytical chamber (under protective atmosphere) - ften conversion or destruction of the SEI by the specific analytical experiment (Vacuum, Beam) Andrea Balducci Page 1

40 Characterize & Understand the SEI In situ Approach Characterization of a non practical electrode during electrochemical experiment (SEI formation) Model electrodes Inert metals, glassy carbon, carbon fibers, "binder-free, instead of composite (binder/carbon) electrodes Model experimental conditions Slow/fast electrochemical experiments Cell housing, Inactive materials (grids, etc.) different Model electrolytes Excess of electrolyte Model electrolyte components Andrea Balducci Page 1

41 Characterize & Understand the SEI In situ Model Electrode In Situ vs. Ex Situ Ex situ Battery Electrode Model Electrochemical Conditions but Handling & Analysis Inside Non-Battery Environment Practical Battery Electrochemical Conditions but Handling & Analysis utside Battery Environment Results of Model Electrode! Results of Practical Electrode? Andrea Balducci Page 1

42 utlines Electrolyte: general aspect Electrolyte and ionic liquids Solid polymer electrolytes & ILs Conclusions Andrea Balducci Page 7 71

43 Battery of tomorrow TDAY TMRRW HIGH MEDIUM LW Energy Power Life Safety Cost Andrea Balducci Page 51 4

44 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 easily dissolve lithium salt (doping) green electrolyte ILs electrolytes in lithium batteries Andrea Balducci Page 15 1

45 ILs Chemical Physical Properties N-methyl-N-buthylpyrrolidinium bis(trifluoromethansulfonyl)imide N + F F C F S N S F C F F - PYR 14 TFSI N-methyl-N-propyl pyrrolidinium bis(fluorosulfonyl)imide N + F S N S F - PYR 13 FSI Very high purity (>95%), H 2 content < 1 ppm σ / ms cm -1 Arrhenius Conductivity plot T / C PC - LiTFSI 1M PYR 14 TFSI PYR 13 FSI 2,8 2,9 3,0 3,1 3,2 3,3 3,4 3, T -1 / K -1 Electrochemical stability window at RT Current density (ma cm -2 ) 2,0 1,5 1,0 0,5 0,0-0,5-1,0-1,5-2,0 Potential vs. Ag / AgCF 3 S 3 in PYR 14 TFSI (V) PYR 14 TFSI PYR 13 FSI Potential vs. Li / Li + (V) PYR 14 TFSI: better electrochemical stability PYR 13 FSI: higher conductivity (even higher than PC-LiTFSI 1M) Andrea Balducci Page 16 1

46 SEI in graphite electrodes SEI Solvent Li-salt Electrolyte Additive separator Cu Graphite + Li 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? Andrea Balducci Page 17 1

47 PYR 14 TFSI - LiTFSI as Electrolyte Selected graphite: KS6 (TIMCAL) very sensitive to the electrolyte properties 0.3 M LiTFSI in PYR 14 TFSI 0.3 M LiTFSI in PYR 14 TFSI + 5% wt. VC i / ma mg -1 0,15 0,10 0,05 0,00-0,05-0,10 1 st cycle 2 nd cycle 3 rd cycle i / ma mg -1 0,2 0,1 0,0-0,1 1 st cycle 2 nd cycle 3 rd cycle -0, E / mv -0, E / mv Poor electrochemical performance Efficiency (3 rd cycle): 74,5% Specific capacity: 132 mahg -1 Better cyclability because of VC, but low specific capacity Andrea Balducci Page 18 1

48 PYR 13 FSI - LiTFSI as Electrolyte 0,3 0,2 0.3 M LiTFSI in PYR 13 FSI 0.3 M LiTFSI in PYR 13 FSI + 5% wt. VC 0,3 0,2 i / ma mg -1 0,1 0,0-0,1-0,2-0,3 1 st st cycle 2 nd nd cycle 3 rd rd cycle E / mv i / ma mg -1 0,1 0,0-0,1-0,2-0,3 1 st cycle 2 nd cycle 3 rd cycle E / mv Efficiency (3 rd cycle): 83,5% Efficiency (3 rd cycle): 89,8% Specific capacity: 130 mahg -1 Specific capacity: ca. 130 mahg -1 Stability comparable with PYR 14 TFSI + 5% VC Low specific capacity VC improves the efficiency, but not the specific capacity Andrea Balducci Page 19 1

49 In Situ FTIRS Measurements Home-made IR cell, provided with an optical ZnSe window. Counter Electrode Reference Electrode Working Electrode = Glassy Carbon (ø=12 mm) Counter electrode = Li Reference electrode = Li Working electrode IR Glassy carbon (GC) has a good capability for IR beam reflection Reference spectra at CV (R 0 ) Stepwise to 0.4 V vs Li/Li + (R y ) SNIFTIR method (Subtractively Normalized Interfacial FTIRectroscopy) Andrea Balducci Page 20 1

50 In Situ FTIR Measurements 0.3 M LiTFSI in PYR 14 TFSI Trasmission CV 1000 mv 750 mv 500 mv PYR 14 TFSI DES NT DECMPSE ν / cm M LiTFSI in PYR 14 TFSI + 5% wt VC Trasmission CV 1000 mv 500 mv 400 mv * R E / R 0 1,035 1,030 1,025 1,020 1,015 1,010 1,005 1, ν / cm ν / cm M LiTFSI in PYR 13 FSI * PYR 14 TFSI DES NT DECMPSE VC DECMPSES Trasmission CV 1000 mv 750 mv 500 mv R E / R 0 1,025 1,020 1,015 1,010 1,005 1, ν / cm -1 F S N S F F S N S The FSI - anion DECMPSES Andrea Balducci Page 21 1 ν / cm -1 F

51 ILs and Additive Electrolytes PYR 14 TFSI HIGHER STABILITY Poor electrochemical performance Additives necessary: VC source for the SEI layer PYR 13 FSI LWER STABILITY Better electrochemical performance Additives maybe not necessary: FSI - source for the SEI layer Different SEI chemistry ptimization of the SEI chemistry Li salt Andrea Balducci Page 22 1

52 PYR 14 TFSI as Electrolyte with LiPF 6 Role of Li salt LiPF 6 i / ma mg -1 0,3 0,2 0,1 0,0-0,1-0,2 0.3 M LiPF 6 in PYR 14 TFSI 0.3 M LiPF 6 in PYR 14 TFSI + 5% wt. VC 1 st cycle 2 nd cycle 3 rd cycle i / ma mg -1 0,3 0,2 0,1 0,0-0,1-0,2 1 st cycle 2 nd cycle 3 rd cycle -0, E / mv -0, E / mv Poor electrochemical performance Efficiency (3 rd cycle): 92,1% Specific capacity: 180 mahg -1 Higher specific capacity compared to LiTFSI but lower cycling stability Andrea Balducci Page 23 1

53 PYR 13 FSI as Electrolyte with LiPF M LiPF 6 in PYR 13 FSI 0.3 M LiPF 6 in PYR 13 FSI + 5% wt. VC i / ma mg -1 0,3 0,2 0,1 0,0-0,1-0,2-0, E / mv 1 st cycle 2 nd cycle 3 rd cycle i / ma mg -1 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 2 nd cycle 3 rd cycle E / mv Efficiency (3 rd cycle): 88,7% Efficiency (3 rd cycle): 96,5% Specific capacity: 300 mahg -1 Specific capacity: 300 mahg -1 In PYR 13 FSI the use of LiPF 6 increases the specific capacity more than 2 times BUT lower cycling stability The VC improve the efficiency, but not the specific capacity Andrea Balducci Page 24 1

54 PYR 14 TFSI and PYR 13 FSI The selection of Li salt is critical also in ILs based electrolytes Specific capa acity / mahg How to obtain high specific capacity and high cycling stability? PYR 13 FSI + LiPF 6 Mixtures of PYR 14 TFSI / PYR 13 FSI Cycling stability PYR 14 TFSI + LiTFSI Low Medium High Mixtures of PYR 14 TFSI / PYR 13 FSI Wide electrochemical stability (from PYR 14 TFSI) High conductivity (from PYR 13 FSI) Intrinsic film form ability (from PYR 13 FSI) Andrea Balducci Page 25 1

55 Mix of ILs as electrolyte Several mixtures have been prepared: (x)pyr 14 TFSI/(1-x)PYR 13 FSI/LiTFSI (x)pyr 14 TFSI/(1-x)PYR 13 FSI/LiPF 6 Evaluation of electrochemical stability window and ionic conductivity (and cost..!) Selected Mixtures: (80%) PYR 14 TFSI / (20%) PYR 13 FSI / LiTFSI (80/20/LiTFSI) (80%) PYR 14 TFSI / (20%) PYR 13 FSI / LiPF 6 (80/20/LiPF 6 ) (50%) PYR 14 TFSI / (50%) PYR 13 FSI / LiTFSI (50/50/LiTFSI) (50%) PYR 14 TFSI / (50%) PYR 13 FSI / LiPF 6 (50/50/LiPF 6 ) VC was added to improve the efficiency * G.B. Appetecchi et al., Journal of Power Sources,192,2, Andrea Balducci Page 26 1

56 Mix of ILs as electrolyte The mixtures of (x)pyr 14 TFSI/(1-x)PYR 13 FSI good strategy Specific ca apacity / mahg PYR 13 FSI + LiPF 6 80/20/LiTFSI PYR 14 TFSI + LiTFSI Low Medium High Cycling stability capacity / mah g mahg discharge capacity 50 charge capacity 20 efficiency cycle number High specific capacity High cycling stability efficiency / % * S.F. Lux et al., Journal of Power Sources,192,2, Wide electrochemical stability (from PYR 14 TFSI) High conductivity (from PYR 13 FSI) Intrinsic film form ability (from PYR 13 FSI) Andrea Balducci Page 27 1

57 utlines Electrolyte: general aspect Electrolyte and ionic liquids Solid polymer electrolytes & ILs Conclusions Andrea Balducci Page 7 71

58 Solid polymer electrolytes & ILs Solid polymer electrolyte based on PE Poly(ethylene oxide) vercoming the conductivity drawback of PE electrolytes aprotic liq. & ionic liquids solid polymer ( ) S/cm PE-LiX-IL electrolytes Polymer matrix (PE) + 2 salts (LiX and IL) having the same anion (PE-PYR 14 TFSI-LiTFSI) Andrea Balducci Page 29 1

59 PE-LiX-IL electrolytes PE host (Cation) + (Anion) - Promoted lithium ion mobility Very low interactions between Cation and Anion of IL Li + (Anion) - Li + (Anion) - (Anion) - (Anion) - No interaction between Cation and PE host (IL does not interact with PE) No interaction between Cation and Li + Strong interaction between Li + and TFSI - lowers the strength of the Li + - PE coordination Li + (Cation) + PE host Conductivity enhancement The IL-LiTFSI interaction prevents the formation of the crystalline P(E) 6 LiTFSI phase * M. Castriota et al., J. Phys. Chem. A 109 (2005) 92, * I. Nicotera et al., J. Phys. Chem. B 109 (2005) *J.H. Shin et al., J. Electrochem. Soc., 152 (2005) A Andrea Balducci Page 30 1

60 PE-LiX-IL electrolytes Problem for the incorporation of ILs ver a certain content of IL the mechanical stability becomes poor Crosslinking of PE Increase the conductivity of the film while maintaining the mechanical stability Andrea Balducci Page 31 1

61 Crosslinking of PE PE is sensitive to β + γ radiation (fragmentation!) UV crosslinking with a photoinitiator is possible Benzophenone acts via H abstraction! hv * H -CH 2 -CH CH 2 -CH-- CH CH 2 HC H 2 C CH CH 2 CH H 2 C Thin films of PE (few µm) containing 5 %wt. of BPh showed an insoluble (gel) fraction W=80% after photo-crosslinking at 70 C under inert gas condition Film preparation: PE/PYR 14 TFSI/Benzophenone 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). PE/PYR 14 TFSI/Benzophenone Andrea Balducci Page 32 1

62 Crosslinking of PE/IL/Li salt 1,0 320nm 365nm Absorption 0,5 PYR 14 TFSI Benzophenone PE There is no need to remove the Mylar foil for illumination: transparent >320 nm At 365 nm NLY the benzophenone adsorb 0, Wavelength [nm] The insoluble fraction of PE is about 80% after 5 min of illumination Gelfraction [%] Crosslinking of the Polymer Irradiation time [s] Fragmentation of the Polymer PYR 14 TFSI DES NT participate in the photo-crosslinking reaction The addition of LiTFSI did not modify the crosslinking reaction Andrea Balducci Page 33 1

63 Mechanical stability of PE/IL/Li salt 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 PE / PYR 14 TFSI / LiTFSI = 10:2:1 (mol) UV-curing 365 nm 5 min per side With crosslinked PE is possible to obtain significantly improve the mechanical stability The transparency of the sample after curing indicates low crystallinity Andrea Balducci Page 34 1 * B. Rupp et al., European Polymer Journal 9,44 (2008) 436

64 DSC & conductivity measurements Heat flow [W/g] -0,2-0,3-0,4-0,5-0, C 36 C -0,7 (1) uncured 1:1:10 2 (2) cured 1:1:10-0,8 (3) uncured 2:1:10 (4) cured 2:1:10-0, Temperatur [ C] PYR 14 TFSI / LiTFSI / PE The crossilinkg process reduce significantly the crystalline fraction No crystalline domain are discernible in the sample (from SEM) Half order of magnitude increase in the ionic conductivity: C 1 40 C Andrea Balducci Page 35 1

65 Electrolyte Scale-up PE / LiTFSI / PYR 14 TFSI (10:1:2) + 5% (PE wt.) Benzophenone Mixtures are prepared through a solvent-free procedure: 1. Benzophenone and LiTFSI are dissolved in PYR C 2. PE is added to the solution and mixed to form a paste 3. The paste is stored in 120 C to homogenize 4. Thin films are made by hot-pressing the electrolyte 90 C 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) Andrea Balducci Page 36 1

66 Mechanical properties 8 by 8 cm polymer electrolytes thin films Fully amorphous & Highly adhesive Very good mechanical properties Elastomeric behavior Andrea Balducci Page 37 1

67 utlines Electrolyte and ionic liquids Solid polymer electrolytes & ILs Binders and lithium battery Conclusions Andrea Balducci Page

68 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 rganic electrolyte are the state of the art Ionic liquids display promising properties Solid polymer electrolytes & ILs possible alternative Andrea Balducci Page 44 1

69 Acknowledgment AK Winter Andrea Balducci Page 1