Probing Thermal and Chemical Stability of Na_xNi_1/3Fe_1/3Mn_1/3O₂ Cathode Material toward Safe Sodium-Ion Batteries

Because of the low cost and high abundance of sodium, room-temperature sodium-ion batteries have recently been considered as an alternative power source to lithium-ion batteries. In contrast to the electrochemical performance of the batteries, safety has been paid much less attention, but safety is a critical consideration because sodium-ion batteries are intended for large-scale electrochemical energy storage applications. Herein, we have reported a NaNi_1/3Fe_1/3Mn_1/3O₂/hard carbon full cell with a good cycling performance and high Coulombic efficiency. The energy density of this pouch cell is close to 95 Wh/kg, and the capacity retention of the NFM full cell attained at 92.6% after 100 cycle numbers. Moreover, we have further used accelerating rate calorimetry, scanning electron microscopy, and operando synchrotron high-energy X-ray diffraction to investigate the thermal/chemical stability of charged Na_xNi_1/3Fe_1/3Mn_1/3O₂ cathode material at both cell and component level. It is found that the thermal decomposition of desodiated Na_xNi_1/3Fe_1/3Mn_1/3O₂ is a redox reaction that can be facilitated with the presence of either a reductive environment, such as electrolytes, or a strong oxidative environment that can result from a higher degree of desodiation. The findings presented in this work can guide future development of advanced sodium-ion batteries for practical application.