Separating the effects of the electrolyte and solid electrolyte interphase on alkali metal ion transfer resistance in non-graphitizable carbon

Abstract

Non-graphitizable carbon has received considerable attention owing to its larger alkali metal ion capacity than that of graphite. However, the behavior of alkali metal ions at the non-graphitizable carbon/electrolyte interface remains unclear. The effects of electrolyte and solid electrolyte interphase (SEI) on alkali metal ion transfer resistance in non-graphitizable carbon were here investigated separately. In lithium-ion transfer reactions, the resistance was quite small and independent of the electrolyte or SEI. In contrast, the resistance in sodium-ion transfer reactions showed a much greater dependence on the SEI than on the electrolyte, with the SEI derived from fluoroethylene carbonate (FEC) having a larger resistance than the SEI derived from ethylene carbonate. Analysis of the SEI by X-ray photoelectron spectroscopy revealed that this FEC-based SEI contained a large amount of NaF. There were no other significant differences between EC-derived SEI and FEC-derived SEI. This finding underscores the importance of designing an optimal SEI to reduce the sodium-ion transfer resistance in non-graphitizable carbon.