Deciphering cycling voltage-dependent failures of O3-layered cathode for sodium ion battery

Abstract

The high energy density and low-cost O3-layered NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM) is a representative layered cathode for sodium-ion batteries (SIBs). However, its long-term cycling stability needs further improvement and its high voltage usage is highly desired, which is quite challenging. The lack of full understanding of the cycling-induced failures hinders material optimization. Herein, we utilize advanced microanalysis techniques to comprehensively investigate the failure mechanisms of the O3-NFM layered cathode upon low-voltage (2.0–4.0 V) and high-voltage (2.0–4.3 V) cycling. We found that surface degradations play a dominant role during low-voltage cycling, and bulk failures become prominent upon high-voltage cycling. Surface cracking, corrosion, and structure transition together lead to slow charge transfer kinetics, resulting in chronic capacity decay. Bulk degradations such as intragranular cracking, void formation, and interlayer cation mixing severely deteriorate Na storage performance and Na diffusion kinetics, causing rapid capacity decay and voltage fading issues, which are the main challenges of the NFM layered cathode for high voltage usage. High charging cutoff voltage activates the cation migration and condensation, causing a highly disordered layered structure but no phase transition occurs in the bulk. Synergistically stabilizing the surface and bulk structure of the high-voltage O3-layered cathode is essential for achieving superior electrochemical performance.