Electrochemical and thermal properties of hard carbon anodes in ether-based and ester-based electrolytes for sodium ion batteries

To achieve widespread commercial viability, sodium ion batteries (SIBs) must ensure both safety and long cycle life. Although hard carbon (HC) is considered the most promising negative electrode material for SIBs, research on the thermal safety and electrochemical stability of HC electrode and electrolyte remains limited. The electrochemical performance of HC in ester-based and ether-based electrolytes is investigated in this work. Etherbased electrolytes exhibit superior cycling and rate performance compared to ester-based electrolytes, attributed to a thinner solid electrolyte interphase (SEI) film as evidenced by scanning electron microscopy (SEM), transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). Additionally, the flammability of electrolytes and the thermal stability of sodiated HC with various electrolytes are examined. Etherbased electrolytes surpass that of ester-based electrolytes, with sodium tetrafluoroborate (NaBF4)/tetraethylene glycol dimethyl ether (TEGDME) and sodium hexafluorophosphate (NaPF6)/TEGDME electrolytes showing optimal performance. These findings offer valuable insights for the selection of suitable electrolytes for HC SIBs.