High-voltage performance of P2-Na_xMn_0.5Co_0.5O₂ layered cathode material

Sodium-ion batteries (SIBs) are rapidly developing interest among post-Lithium-ion batteries (LIBs) research. High capacity and structural stable cathode materials are needed for viable SIBs. In this study, P2-Na_xMn_0.5Co_0.5O₂ (NMC55) was synthesized and characterized the sodium storage properties. The material shows a remarkable increase in capacity from 84.6 to 136 mAh g⁻¹ but decreases capacity retention from 74% to 67% when extends the voltage from 4.0 to 4.5 V. Moreover, the material exhibits impressing rate capability and high cyclability in the wide voltage range of 1.5 to 4.5 V. Ex situ X-ray diffraction (XRD) was used to monitor structure change demonstrated that P2-type structure is reserved during a cycle of charge/discharge and event after 100 cycles charge/discharge between 1.5 and 4.5 V. Besides, the fluoroethylene carbonate (FEC) additive also helps to improve the performance at such high voltage. The difference of the interface chemicals between FEC-free and FEC-content electrolytes revealed by Fourier transform infrared (FTIR) spectra and electrochemical impedance spectroscopy (EIS) were also discussed.