Anionic Effects on Li-Ion Activity and Li-Ion Intercalation Reaction Kinetics in Highly Concentrated Li Salt/Propylene Carbonate Solutions

Certain highly concentrated electrolytes (HCEs) enhance the charge-transfer reaction rate at the electrode/electrolyte interface in Li-ion batteries. The solvation structure of Li⁺ in HCEs significantly affects the electrochemical interfacial reaction kinetics. However, the effects of different anions on these kinetics have not yet been fully understood. Therefore, in this study, we investigated the effects of anionic species on the liquid structure and electrochemical reactions of HCEs composed of various Li salts and propylene carbonate (PC). Raman spectra revealed that both PC and anions were coordinated to Li⁺ ions in HCEs and that the concentration of uncoordinated (free) PC changed depending on the anionic species. Consequently, the activity of Li⁺ in the electrolyte changed depending on the anionic species. The use of Li salts with weakly Lewis basic anions increased the activity of Li⁺ and decreased the concentration of free PC in HCEs. The activity of Li⁺ in the electrolyte significantly affected the Li⁺ intercalation/deintercalation reaction rate of the LiCoO₂ thin-film electrode. Electrochemical impedance spectroscopy revealed that the interfacial reaction rate of LiCoO₂ was enhanced in the HCEs with anions having weaker Lewis basicity owing to the higher Li⁺ ion activity.