Capacity degradation study of NaNi1/3Fe1/3Mn1/3O2 cathode sodium-ion batteries induced by overcharge

Sodium-ion batteries are gradually being applied in low-speed electric vehicles and energy storage fields. However, inconsistencies caused by cell manufacturing or long-term use can lead to overcharging in some batteries. Overcharging not only poses safety risks but also threatens battery performance. This study investigates the degradation behavior of sodium-ion batteries with NaNi1/3Fe1/3Mn1/3O2 as the cathode material under different overcharging conditions. Firstly, overcharge induced thermal runaway experiments are performed to determine safe overcharge limit state of charge (SOC) of the battery. Results demonstrate that battery capacity hardly degrades before overcharging to 120 % SOC, while significant capacity degradation is overserved with overcharge exceeding 140 % SOC. In addition, electrochemical characterization methods are used to investigate electrochemical behavior changes of the battery under different overcharging conditions, it is found that with overcharging exceeding 130 % SOC, the electrode will show significant irreversible loss of sodium inventory, conductivity loss and active material loss. Finally, the electrode materials are recovered from the overcharged batteries, and advanced material characterization techniques are used to find that after overcharging more than 120 % SOC, the cathode began to appear particle breakage and structural collapse, the anode began to grow sodium plating.