In-Situ Raman Spectroscopic Analysis of Factors Improving Discharge Rate Capability of Na-Ion Batteries with FSA-Based Ionic Liquids

Na-ion batteries (NIBs) with ionic liquid (IL) electrolytes are promising candidates for large-scale energy storage devices owing to the abundance of Na resources and the safety of ILs. In our previous study, we have demonstrated the improved rate capability of NIBs consisting of a hard carbon negative electrode, an NaCrO2 positive electrode, and an FSA-based IL electrolyte (FSA = bis(fluorosulfonyl)amide) by increasing the Na+ ion concentration in the IL. However, this phenomenon is not consistent with the trend observed for the ionic conductivities of bulk ILs. In this study, to clarify the unexplained behavior particularly for electrolytes with high Na+ concentrations, we performed in-situ Raman spectroscopic analysis in the vicinity of the electrode/electrolyte interface. The results of discharge rate capability tests indicated that a rate-determining step existed in the reaction at the positive electrode, where Na+ insertion occurred during discharge. In-situ Raman spectroscopy for Na/NaCrO2 half-cells using an IL electrolyte of low Na+ concentration (∼1 mol dm−3) revealed that the Na+ ion concentration at the interface (inside the NaCrO2 composite electrode) locally decreased as the discharging proceeded. In contrast, a high Na+ concentration electrolyte (∼2.2 mol dm−3) considerably suppressed the decrease in the Na+ ion concentration at the interface. Therefore, the improved performance of the electrolyte with a high Na+ concentration can be explained by the local Na+ ion concentration near the electrode/electrolyte interface, rather than by the bulk properties of the IL electrolytes.