Insights on the degradation mechanism of 7 Ah sodium ion batteries at different aging modes

Abstract

Sodium ion batteries (SIBs) are considered to have significant advantages in the field of energy storage due to their abundant resources. However, SIBs are exposed to complex and adverse environments, making it particularly important to study the capacity degradation mechanism under extreme conditions. In this study, the 7.3 Ah SIBs with Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) and hard carbon (HC) as cathode and anode are taken as the research objects. The capacity degradation experiment of SIBs is conducted under different aging modes. Subsequently, comprehensive non-destructive and post-mortem analyses are combined to explore the failure mechanism of SIBs. This study indicates that the loss of active sodium and the increase of interface impedance are the main reasons for the capacity decay of SIBs. In particular, during the room temperature cycling, sodium plating is the main source of capacity loss. Additionally, during the high-temperature aging process, polarization caused by interface side reactions is the main reason. Surprisingly, throughout the entire aging test, the structures of NFPP and HC do not show significant deterioration, indicating that SIBs also have great potential for application in high-temperature scenarios. This study reveals the failure mechanism of SIBs, which provides reference for battery design and material research.