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

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.

钠离子电池（Na-ion batteries，简称NIBs）配以离子液体（ionic liquid，简称IL）电解质，因其钠资源的丰富性和离子液体的安全性，成为大规模储能设备的理想候选者。在前一阶段的研究中，我们通过提升离子液体中的Na+离子浓度，已证实了由硬碳负极、NaCrO2正极及基于FSA（二氟磺酰亚胺，bis(fluorosulfonyl)amide）的离子液体电解质组成的钠离子电池（FSA = bis(fluorosulfonyl)amide）在充放电速率上的提升。然而，此现象与大量离子液体电导率的趋势观察并不一致。在本研究中，为了阐明特别是在高Na+浓度电解质中未解之谜的行为，我们在电极/电解质界面附近进行了原位拉曼光谱分析。放电速率性能测试结果表明，在正极的反应中存在一个决定速率的步骤，其中Na+离子在放电过程中发生嵌入。采用低Na+浓度（约1 mol dm−3）的离子液体电解质对Na/NaCrO2半电池进行原位拉曼光谱分析揭示，随着放电的进行，界面（NaCrO2复合电极内部）的Na+离子浓度局部降低。相反，高Na+浓度（约2.2 mol dm−3）的电解质显著抑制了界面Na+离子浓度的降低。因此，高Na+浓度电解质性能的提升可以归因于电极/电解质界面附近的局部Na+离子浓度，而非离子液体电解质的大体性质。