Anionic Effects on Li-Ion Transport and Electrochemical Properties of High-Concentration Li Salt/Sulfone Electrolytes

High-concentration electrolytes (HCEs) comprising Li salts and sulfones are regarded as promising materials for boosting the thermal stability, energy density, and rate capability of Li batteries. In this study, to obtain a deeper understanding of the correlation between electrolyte composition and physicochemical properties, the anionic effects in binary mixtures of various Li salts and 3-methylsulfolane (MSL) were systematically studied. The ionic conductivity of the Li salt/MSL mixtures increased as the Lewis basicity of the anions becomes weaker. In HCEs with a molar ratio of Li salt/MSL = 1/4, the fraction of free MSL decreased as the Lewis basicity of the anions weakened, leading to a higher activity of Li+. The Li ion transference number in the HCEs decreased with weakening of the Lewis basicity of the anion. Electrochemical impedance measurements revealed that the charge-transfer kinetics of the LiCoO2 and Li metal electrodes in the HCEs were enhanced using weaker Lewis basic anions. Our findings reveal that the utilization of Li salts with weak Lewis basic anions is essential for enhancing the power density of lithium batteries with HCEs. This work provides insights into the functions of anions in sulfone-based HCEs and the correlation between the HCE composition and battery performance, which may be helpful for the design of electrolytes of advanced lithium batteries.