Factors Affecting Li+ Transport Properties of Molten Li Salt Solvate Electrolytes

Abstract

Among Li+ transport properties of electrolytes, ionic conductivity and Li+ transference number (t Li) are key factors contributing to enhancement of the battery performance. High t Li enables rapid charging even if the ionic conductivity is lower than that of conventional organic electrolytes and would also suppress the lithium dendrite formation despite the relatively low mechanical properties of the electrolytes [1], [2]. Our group focuses on molten Li salt solvate electrolytes including glyme (G4)-based solvate ionic liquids (SILs) owing to their attractive thermal and electrochemical properties. Recently, we have reported that high t Li (= 0.77) resulting from unique Li ion hoppling/exchange conduction mechanism in sulfolane (SL)-based electrolytes allows for the improved charge-discharge rate performance of Li-ion and Li-S batteries [3], [4]. Few studies, however, have clarified the relationship between t Li, which is affected by the correlations of ionic motions and molecular structure for highly concentrated electrolyte systems like molten Li salt solvates. In this study, we attempted to understand the cross-correlations of ionic motions and clarify their effects on the transport properties in molten Li salt solvate electrolytes with different solvent and anion structures. Table 1 Two transference numbers of the electrolytes. Sample t Li NMR t Li PP [Li(G4)][TFSA] 0.52 0.03 [Li(SL)3][TFSA] 0.57 0.69 Table 1 illustrates two different t Li values of the electrolytes; one is estimated from self-diffusion coefficients measured by pulse field gradient (PFG)-NMR (t Li NMR ) and the other is estimated from the electrochemical method (t Li PP ) [5], [6]. In the case of [Li(G4)][TFSA], t Li PP is extremely low compared with t Li NMR , which implies the concentration polarisation in this electrolyte is more pronounced under the electric field. By contrast, for [Li(SL)3][TFSA], t Li PP is higher than t Li NMR . This suggests that the interionic correlations can enhance t Li PP in [Li(SL)3][TFSA]. We further discuss the interionic cross-correlations and their relationship with Li ion transport properties of other electrolytes in detail.