Effects of Li ion-solvent interaction on ionic transport and electrochemical properties in highly concentrated cyclic carbonate electrolytes

In recent research, the importance of electrolytes with high Li⁺ transference number (t_Li) and ionic conductivity (σ_ion) has been emphasized to realize rapid charge for Li secondary batteries. Simultaneously fulfilling high t_Li and σ_ion is still unsolved in liquid electrolytes; however, highly concentrated electrolytes (HCEs) of weakly coordinating solvents and Li salts will be promising for addressing this challenge. This idea is inspired by a recent study by Angell et al. on superprotonic ionic liquids comprising a weak Brønsted base and a superacid; highly labile and exchangeable H⁺ can be formed between significantly weak proton accepting sites. Here, we studied weakly coordinating fluoroethylene carbonate (FEC)-based electrolytes with lithium bis(fluorosulfonyl)amide (Li[FSA]) and compared with ethylene carbonate (EC)-based electrolytes. Experimental and computational studies indicated that solvent and ion exchange is more pronounced in the FEC-based HCE, resulting in higher t_Li^PP (0.73) and ionic conductivity (1.02 mS cm⁻¹) compared to those of the EC-based HCE (t_Li^PP= 0.53 and σ_ion= 0.84 mS cm⁻¹). However, the FEC-based HCE exhibited lower electrochemical stability due to the intrinsically lower reductive stability of FEC and the oxidative decomposition of the liberated solvent in the HCE. Despite the superior transport properties, the Li/LiCoO₂ cell with the FEC-based electrolyte showed lower discharge capacities and lower Coulombic efficiencies at higher current densities due to side reactions of the electrolyte. This study demonstrates that weak Li-solvent interactions can simultaneously enhance t_Li and σ_ion of HCEs, but they have the potential to sacrifice the electrochemical stability.