Polymers vs. liquids, gels and ionic liquid electrolytes. A ny winners?

Polymers vs. liquids, gels and ionic liquid electrolytes. Any winners? M. Armand, P.G.Bruce, B. Scrosati, W.Wieczorek Alistore ERI | www.alistore.eu

Outline • General status quo of present Li (ion) batteryarchitecture • Introduction to the field of modern electrolytes (concepts, transport mechanisms, improvementstrategy) • Role of salt anions • New types of salts • IonicLiquids • CrystallineElectrolytes (P. Bruce) • Composites (Ceramic, Anion receptors) • Conclusions Alistore ERI | www.alistore.eu

Strategies for Li Batteries Medium capacity “high” voltage Inorganic: LiFePO4, Li2Fe(Mn)SiO4 Positives: Very large capacity “low” voltage Organic: Li2+xC6O6 Electrolytes: New solutes, polymer (gels) ± ILs Very high capacity Cu mandatory / binders? Inorganic: Si Negatives: High capacity, 0.8 V Al option, s elec ?? Organic dicarboxylates

Liquid electrolytes: transport of solvated species Polymer electrolytes: transport by solvation / desolvation No net displacement of the host matrix Ceramic electrolytes: transport by ion hopping Transport mechanisms

Gels The Donor Number of the polymer repeat units vs. that of the solvent DNsolvent > DNpolymer DNpolymer > DNsolvent Direct solvent-cation interaction  solvent drag Direct polymer-cation interaction  no solvent drag DNPVF≈ 0 < DNcarbonates≈ 15 < DNPEO≈ 22

Li+ PEO PolymerElectrolytes • Electrodonorpolymers • O,N,S (sufficient donor ability for complexation) • Sufficientdistancebetweensites • Amorphous • Polyethersgoodcandidates • Low Tg (flexibility) General classification PolymerComlexes PoymerGels Polyelectrolytes (Single Ion Conductors) Copyrights Marek Marcinek

Solid PolymerElectrolytesAdvantages • non volatility, • no decomposition at the electrodes, • no possibility of leaks, • use of metallic lithium in secondary cells (lithium dendrites growing on the electrode surface would be stopped by the non-porous and solid electrolyte), • lowering the cell price (PEO is cheaper than organic carbonates; it could be used as a binder for electrodes to improve the compatibility of consecutive layers; moreover fabrication of such a cell would be easier –cost), • strengthening of cells thanks to the all-solid-state construction, • shape flexibility, • lowering the cell weight – non-volatile, all-solid-state cells don’t need heavy steel casing, • improved shock resistance, • better overheat and overcharge allowance, • improved safety!!!

Solid PolymerElectrolytesAdvantages • The most important and universal properties of polymeric electrolytes for application in lithium cells: • chemical and mechanical stabilities over a wide temperature range, • electrochemical stability of at least 3-4 V versus a Li electrode; especially important for battery applications • low activation energies for conduction • high cationic transport numbers • good electrode ‑ electrolyte characteristics • ease of sample preparation.

Polymeric electrolytes modifications • Methods of modification of polymericelectrolytes: • - random copolymers • - blockcopolymers • - comb-likecopolymers • - polymerblends • - addition of liquidplasticizers • - crosslinking (UV, gamma, chemical) • - application of plasticizingsalts • - addition of anion receptors: • soluble Lewis acids • supramolecularreceptors - polymer-in-salt materials • well-designedcrystallinepolymerelectrolytes(NEW) • compositeswithinorganicfillers • ceramic-in-polymers • reversedphase systems (polymer-in-ceramic) (NEW)

Electrolyte solvents • Generally an ideal electrolyte solvent should meet the following criteria: • be able to dissolve lithium salts to sufficient concentration • its viscosity should be low so fast ion transport can occur within electrolyte • be inert to all cell components especially anode and cathode materials • it should remain liquid in a wide temperature range (low melting and high boiling temperature are desirable)

Electrolytesolvents • The role of electrolyte is two, or sometimes threefold: • It should provide ionic contact between electrodes allowing to close the circuit when the cell is operational • It should assure electronic and spatial separation of the positive and negative electrode in order to avoid short-circuit and as a result – self discharge of the cell, which in some cases can be very spectacular (as those of failed high power Li-ion cells) • In case of electrochemical systems where electrode components are not the only reactants appearing in the overall cell reaction, the electrolyte is the source (storage) of the remaining ones.

(a) Optimization of ion conductivity in mixed solvents: 1.0 M LiClO4 in PC/DME. (b) Dependence ofdielectric constant (ε) and fluidity (η-1) on solvent composition. Chemical Reviews, 2004, Vol. 104, No. 10 Alistore ERI | www.alistore.eu

Electrolytes additives According to the functions targeted, the numerous chemicals tested as electrolyte additives can be tentatively divided into the following three distinct categories: (1) those used for improving the ionconduction properties in the bulk electrolytes; (2)those used for SEI chemistry modifications; and (3)those used for preventing overcharging of the cells… ….and thusimprove SAFETY!!!!

Donor Numbers DN for solvents: propensity to give electrons pairs (H of interaction to a reference Lewis acid) - + + - The notion of DN for anions :  Role of   DN(X-) < DN(solvent) DN(X-) > DN(solvent)

Anions – „A Letter to Santa” • Properties of the salt used for battery applications are as follows: • it should be able to completely dissolve in the applied solvent at desired concentration and ions should be able to transfer through the solution • anion should be stable towards oxidative decomposition at the cathode • anion should be inert to electrolyte solvent • both anion and cation should be inert towards other cell components • anion should be nontoxic and remains thermally stable at the battery working conditions

Anions-Role •Control dissociation and conductivity •Control transport numbers t+ /t- •are an important part of SEI build-up at +/- electrodes •Control aluminium corrosion

ClO4- BF4- Explosive ! Toxic ! SbF6- PF6- AsF6- ClassicAnions Tendency to decompose according to equilibrium: LiBF4BF3+ <LiF> LiPF6  PF5+ <LiF> Fast reaction above 80°C Destruction of electrolyte and interfaces

Conceptual Approach to Anion Design “O” is not a favorable building block: Strong Li—O interactions  ion pairing, ≠ ClO4-, BOB- If O present, F or CnF2n+1 is required “N, C” are favorable: Weak interactions Li—N but easy oxidation

Diagonally OpposedInterests? Ionic processes - - + + - Li+ - I- = 2,2 Å Organic chemistry Electrochemistry Enhance the activity of anions (SN) Maximize the conductivity  design of polyatomic anions

Hückel anions… Aromaticity 4n + 2 «  » electrons pKA = 10-60 pKA = 10-20 X = N, C-CN, CRF, S(O)RF Gain of > 1 eV by resonance See P. Johansson et alPhysical Chemistry Chemical Physics, volume 6, issue 5, (2004).

Cyanocarbons pKA < -10!! Stronger than 100% sulfuric acid pKA < -3 corrodes glass

Hückel anions… DCTA Stable to 3.8 V (La Sapienza, KZ)inexpensive Gives quite fluid ILs

Gas Phase Ion Pair Dissociation Energies Ion pair (g) Li+ (g) + Anion- (g) LiTDI < LiPDI < LiDCTA < LiTFSI < LiPF6 LiTDI LiPDI LiDCTA LiTFSI LiPF6 ExcellentTheoreticalPrediction MP2/6-31G(d) Scheers et al. 2009

Most Stable Lithium Imidazole Configurations 1.93 Å 1.87 Å 1.88 Å 1.92 Å LiTDI LiPDI B3LYP/6-311+G(d) Scheers et al. 2009

LiTDI(2-trifluoromethyl-4,5-dicyanoimidazole lithium salt) ImportantBenefits • Easy, low‑demanding, inexpensive, one‑step, high yield syntheses; • Salts are pure, stable in air atmosphere, non‑hygroscopic, stable up to 250°C, easy to handle;

New salts- SynthetizedExamples

ExemplaryElectrolyteConductivities (20°C)

New Salts - Anodic limit (Pt, EC-DMC) Real Chance to be >4V ClassBattery

Anodic limit (Al, EC-DMC) Real Chance to be >4V ClassBattery

Cycling LiMn2O4 4.3 V (EC-DMC) Swagelok cell , Al plunger PromisingCycling Performance…

Ragone Signature ..as well as RateCapability and Power/Energy relation

Conductivity in PEO SS / PEO20LiX / SS cooling scan LiDCTA LiPDI LiTDI

PEO20LiTDI PEO20LiPDI Hot-Pressing PEO20LiTDI PEO20LiPDI PEO20LiBOB/ LiBF4 Hot-Pressing PEO20LiDCTA Hot-Pressing PEO20LiCF3SO3+ ZrO2SA Casting

Charge profile 4.3 V cut-off, Al collector

Anodic stability Li / PEO20LiX / Super P LiDCTA LiPDI LiTDI

Interphase resistance - PEO Li / PEO20LiX / Li LiTDI LiDCTA LiPDI

Cycling behaviour

Rate capability (PEO) % of capacity at C/20

New imidazole-derived salts • Easy, low‑demanding, inexpensive, one‑step, high yield syntheses; • Salts are pure, stable in air atmosphere, non‑hygroscopic, stable up to 250°C, easy to handle; • 20°C ionic conductivity exceeds: • 10‑3 S∙cm-1 inPC, 10‑4 S∙cm‑1 inPEGDME500 • 10‑6 S∙cm‑1 inPEO (10‑4 S∙cm‑1 at 50°C) • 6 mS∙cm‑1in EC:DMC • T+ at ionic conductivity maximum reaches: • 0.45 in PC, 0.40 in EC-DMC, 0.25 in PEGDME500 (but overallmax 0.62); • Stable over time against Li; • Stable up to4.4 V vs. Li against metalliclithium anode; • Stable up to5.0 V vs. Li againstaluminum; • Much smaller association rate than commercially available salts;

IonicLiquids (IL) 1/3 Typical cations used in ionic liquids: 1) Imidazole cation 2) Alkylpyridine cation 3) Dialkylpyrrole cation Ionic liquid with lithium cation example

The basic IL

First Alkali metal IL? K(CF3SO2NSO2F) 99°C Eutectics, re-investigation of “polymer in salt” (Angell)

Fig.1 The structure of PEO6:LiAsF6. Left, view of the structure showing rows of Li+ ions perpendicular to the page. Right, view of the structure showing the relative position of the chains and their conformation (hydrogens not shown). Thin lines indicate coordination around the Li+ cation. Fig.2 Conductivity of crystalline polymer electrolytes. Red - PEO6:LiAsF6; green - PEO6:Li(AsF6)0.9(SbF6)0.1; magenta - PEO6:(LiSbF6)0.99(Li2SiF6)0.01; blue - PEO6:(LiAsF6)0.95(LiTFSI) 0.05; black - (PEO0.75G40.25)6:LiPF6. G4 – tetraglyme, CH3O(CH2CH2O)4CH3. Crystalline Solid PolymerElectrolytes 1/2

Fig.3 The structure of PEO8:NaAsF6. Left, view of the structure showing rows of Na+ ions perpendicular to the page. Right, view of the structure showing the relative position of the chains and their conformation (hydrogens not shown). Thin lines indicate coordination around the Na+ cations. Crystalline Solid PolymerElectrolytes 2/2

Polymers vs. liquids, gels and ionic liquid electrolytes. A ny winners?