Promoting Synthesis of Single Crystalline O3‐NaNi _1/3 Fe _1/3 Mn _1/3 O ₂ with Cu/Ti Doping for Sodium‐Ion Batteries

Abstract Single crystalline layered oxide material is key to suppress some drawbacks of polycrystalline counterparts such as grain cracking, electrolyte oxidation, and moisture‐stability in the large‐scale application of Na‐ion batteries. In this work, A well‐dispersed single crystalline O3‐NaNi 1/3 Fe 1/3 Mn 1/3 O 2 (NFM111) is synthesized through co‐doping of copper and titanium. It is disclosed that copper undergoes a valence transition from II to I at high‐temperature and migrates first due to weaker Cu─O bond energy, creating transition metal vacancies and accelerating ionic migration throughout the particle grain, thus leading to the formation of single‐crystal Na[Ni 1/3 Fe 1/3 Mn 1/3 ] 0.9 Cu 0.05 Ti 0.05 O 2 (NFMCT) cathode material. Due to the smaller specific surface area and absence of grain boundary in single crystalline materials, the side reactions between the cathode materials and electrolyte/moisture are limited to the crystalline surface. In addition, the intergranular cracks of active materials caused by anisotropic volume expansion are significantly suppressed. Therefore, the cycling stability and moisture stability of O3‐NFMCT are significantly improved. After 300 cycles at 0.5C, the capacity retention is 81.9% (1C═150 mA g −1 ), and after storage at 96% relative humidity for 72 h, O3‐NFMCT still maintains 86.3% of its initial capacity.