The effect of salt anion in ether-based electrolyte for electrochemical performance of sodium-ion batteries: A case study of hard carbon

Compared with the extensively used ester-based electrolyte, the hard carbon (HC) electrode is more compatible with the ether-based counterpart in sodium-ion batteries, which can lead to improved cycling stability and robust rate capability. However, the impact of salt anion on the electrochemical performance of HC electrodes has yet to be fully understood. In this study, the anionic chemistry in regulating the stability of electrolytes and the performance of sodium-ion batteries have been systematically investigated. This work shows discrepancies in the reductive stability of the anionic group, redox kinetics, and component/structure of solid electrolyte interface (SEI) with different salts (NaBF_4, NaPF₆, and NaSO₃CF₃) in the typical ether solvent (diglyme). Particularly, the density functional theory calculation manifests the preferred decomposition of PF₆⁻ due to the reduced reductive stability of anions in the solvation structure, thus leading to the formation of NaF-rich SEI. Further investigation on redox kinetics reveals that the NaPF₆/diglyme can induce the fast ionic diffusion dynamic and low charge transfer barrier for HC electrode, thus resulting in superior sodium storage performance in terms of rate capability and cycling life, which outperforms those of NaBF₄/diglyme and NaSO₃CF₃/diglyme. Importantly, this work offers valuable insights for optimizing the electrochemical behaviors of electrode materials by regulating the anionic group in the electrolyte.