A low-cost co-solvent, Ethyl Methylsulfone (EMS), is selected to design the high voltage electrolyte by forming an inorganic-rich CEI layer. The stable interface effectively protects the cathode by preventing the leaching of transition metal ions, enabling stable cycling of the sodium-ion battery at high voltages.

Abstract

Raising the upper cut-off voltage of cathode is an effective method to improve the energy density of sodium-ion batteries (SIBs). However, the high upper cut-off voltage could cause severe side reactions and injure the cycle life of SIBs as the absence of stable cathode-electrolyte interface. Some fluorinated co-solvents have been ever employed and proven effective in stabilizing the cathode-electrolyte interface to support the normal operation of SIBs under a high upper cut-off voltage. However, the high-cost of fluorinated co-solvents would notably improve battery expenses. In this study, a low-cost co-solvent called ethyl methylsulfone (EMS) is introduced into the electrolyte for the Na_0.67Mn_0.8Cu_0.2O₂ cathode with a high upper cut-off voltage of 4.5 V. It is found that a stable and uniform cathode-electrolyte interface (CEI) forms on the cathode, which mitigates the cathode degradation and enhances the cycling stability of cathode. Consequently, this cathode with the designed electrolyte achieves a high capacity retention of 83.2 % after 750 cycles at a current density of 1 C (1 C=110 mAh g⁻¹). This work provides valuable insights into the development of electrolytes for sodium-ion batteries working at high-voltage.