Insights into the capacity fading and failure mechanism of an O3-NaNi_1/3Fe_1/3Mn_1/3O₂ layered oxide cathode material for sodium-ion batteries

Low-cost sodium-ion batteries hold great potential for large-scale energy storage owing to the abundance of sodium reserves. Despite the higher energy density and initial coulombic efficiency of O3-type transition metal layered oxide cathode materials, denoted as Na_xMO₂ (M = transition metal, 0.7 < x ≤ 1), their poor long-term cycling performance remains a significant obstacle to their wide-scale industrialization. Herein, we investigate the capacity fading and failure mechanisms of O3-type NaNi_1/3Fe_1/3Mn_1/3O₂ cathode materials after long-term cycling. The results from XRD, SEM, TEM, and ICP-OES etc. reveal the structural damage, surface phase transition, and migration and dissolution of transition metal ions of the material. It is revealed that surface phase evolution from R3̄m layered structure to the rock salt (Fm3̄m) phase after long-term cycling leads to a detrimental feedback loop between the bulk phase and interface. The repeated volumetric strain of the bulk phase disrupts the integrity of the cathode particles, thus hindering ion migration, continuously consuming the electrolyte through side reactions on the fresh surface, affecting the chemical composition of the cathode-electrolyte interface (CEI), and ultimately deteriorating electrochemical performance. This work offers fresh insights into the failure mechanism of O3-type layered oxide cathode materials to guide the performance enhancement of sodium-ion batteries.