Comparative Investigation of Single‐Crystalline and Polycrystalline NaNi _1/3 Fe _1/3 Mn _1/3 O ₂ Positive Materials for Sodium‐Ion Batteries

Abstract Layered oxide positive materials (Na _x TMO ₂ ) for sodium‐ion batteries have received widespread attention due to their advantages of high tap density, high theoretical specific capacity, and easy scale‐up preparation. However, the current research about single‐crystalline and polycrystalline materials is very limited, and the effect of the single‐crystalline and polycrystalline structures on the electrochemical performance is still unclear. Based on this, NaNi _1/3 Fe _1/3 Mn _1/3 O ₂ (NFM111) is used as the basic system to systematically investigate in depth the single‐crystalline and polycrystalline positive materials. Contrary to the traditional view, the single‐crystalline material demonstrates poorer cycling performance due to the multiple phase transitions at high voltage (>4.1 V vs Na ⁺ /Na), the presence of harmful monoclinic phases dominated by the P3′ phase during the first‐cycle discharge process, which produces a huge volume change and as resulted large cracks, making the material failure. On the other hand, the gas production is more serious due to the presence of fine primary particles in polycrystalline materials, which makes the cycling performance worse under the large‐scale application of pouch batteries. This work provides a new perspective for a comprehensive understanding of the difference between single‐crystalline and polycrystalline materials, and provides an important basis for improving the electrochemical performance of both materials.