The effect of nonflammable electrolyte on Cu-substituted P2-type layered cathode for high safety sodium-ion batteries

Sodium-ion batteries (SIBs) have attracted a lot of attention due to abundant resources and environmental-friendly advantages. However, further improvements are needed to commercialize SIBs for poor air-stable property of cathodes and flammable risk of electrolyte. Here, we newly synthesize Na2/3Cu2/9Fe1/9Mn2/3O2, an air-stable P2-type layered cathode, using solution combustion method and investigate its electrochemical performance using triethyl phosphate (TEP), a well-known fire-retardant solvent. We confirm an air and moisture stability of cathode and nonflammable property of electrolyte. In addition, compared to a cell comprising a carbonate-based electrolyte, the TEP cell exhibits a higher capacity retention of 76.28% after 200 cycles at 2 C rate in a wide voltage range (1.5-4.6 V vs. Na/Na+). To find out the reason of superior electrochemical properties of the TEP cell, morphology and components of the cathode-electrolyte interphase (CEI) layer are characterized by morphological and spectroscopic studies.