Copper regulated facet dependent surface behavior promoting single crystal growth of sodium layered cathodes

Layered oxide cathodes for sodium-ion batteries are susceptible to structural degradation during cycling, which severely limits their long-term stability. Although single-crystal engineering has been demonstrated to significantly improve structural stability, the underlying growth mechanism remains unclear. In this work, NaNi 0.4 Fe 0.2 Mn 0.4 O 2 was selected as a model system to systematically investigate the role of Cu doping during the growth process. In-situ high-temperature XRD and SEM analyses demonstrate that Cu doping influences crystal-growth kinetics and facet-dependent growth modes, thereby promoting a morphological transition from polycrystalline aggregates to single-crystal-like particle structures. Electrochemical and thermal analyses under Na-deficient conditions demonstrated that the Cu-Ni solid solution formed during sintering, together with the relatively strong Cu-O bonds, serves as structural pinning centers during cycling, mitigating interlayer slab gliding and irreversible structural changes, thereby enhancing both cycling stability and thermal stability. From the perspectives of facet evolution and local structural stabilization, this work provides mechanistic insight into how Cu doping regulates single-crystal-like growth and enhances structural stability, offering new strategies for facet engineering and the rational design of sodium-ion battery cathodes.