Influence of various co-solvents on ion transport in concentrated poly-(ethylene oxide)-based polymer electrolytes
Simon Buyting, Monika Schönhoff
Abstract
In order to improve the ion conducting properties of poly(ethylene oxide) (PEO/PEG)-based electrolytes for application in lithium ion batteries , plasticization of the polymer network with low molecular weight co-solvents can be a viable and easy solution. This study provides guidelines for promising structures by screening a range of co-solvents including 15-crown-5 (15C5), tetra- (G4) and diglyme (G2), sulfolane (SL), dimethyl sulfoxide (DMSO), dimethyl- formamide (DMF) and propylene (PC) and vinylene carbonate (VC), each of them investigated in a mixture of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and PEO ([Li]/[EO] = 0.25). Regarding the generally observed conductivity enhancement, a major contribution arises from reduced Li-anion correlations in case of strongly Li-coordinating solvents. For the four most promising co-solvents in-depth transport studies are performed by electrophoretic NMR , determining not only cation and anion migration, but even chain and co-solvent migrational volume fluxes, which prove to play a crucial role. In contrast to the bulky and strongly coordinating co-solvents 15C5 and G4, the smaller and more weakly coordinating co-solvents DMSO and DMF can better maintain the beneficial effects of a high salt concentration. Overall, co-solvent addition has beneficial effects on salt dissociation and diffusion, but it reduces the Li + transference number based on a reduction of a chain-supported cation drift. This contrast is rationalized in the picture of an anion-driven volume flux of the chains, which aids the lithium transference, but is reduced upon co-solvent addition. Thus, the nature and amount of co-solvents in PEO-based electrolytes requires careful balancing of these opposing trends.