Insight into the influence of ether and ester electrolytes on the sodium-ion transportation kinetics for hard carbon

The electrochemical performance of hard carbon (HC) materials is closely related to the electrolyte used in the sodium ion batteries (SIBs). Conventional electrolytes carbonate (EC) demonstrates low initial Columbic efficiency (ICE) and poor rate performance, which is one of the main bottlenecks that limits the practical application of HCs. Ether electrolyte (diglyme) was reported to improve the rate performance of HCs. Nevertheless, the underlying mechanism for the excellent rate capability is still lack of in-depth study. In this work, the differences of sodium-ion diffusion between ether and carbonate-base electrolytes in HCs are analyzed layer by layer. Firstly, when sodium-ions are diffused in electrolyte, the diffusion coefficient of sodium-ion in ether electrolyte is about 2.5 times higher than that in ester electrolytes by molecular dynamics (MD) simulation and experimental characterization. Furthermore, when the solvated sodium-ions are diffused into the solid electrolyte interphase (SEI) interface and the HCs material, the enhanced charge transfer kinetics (thin SEI layer (4.6 vs. 12 nm) and low R_SEI (1.5 vs. 24 Ω)) at the SEI combined with low desolvation energy (0.248 eV) are responsible for high-rate performance and good cycling stability of HC in ether electrolyte. Therefore, high diffusion coefficient, low desolvation energy, and good interface are the intrinsic reasons for enhanced rate performance in ether electrolyte, which also has guiding significance for the design of other high-rate electrolytes.