In Situ Plastic-Crystal-Coated Cathode toward High-Performance Na-Ion Batteries

Cathode materials are critical for Na-ion batteries while facing challenges due to the instability of the structure and interfaces. In this work, we propose a strategy to achieve an in situ plastic-crystal Na3–3xAlxPO4 coating and bulk Al doping for an O3-NaNi0.4Fe0.2Mn0.4O2 cathode through a simple one-step method. Na3–3xAlxPO4 exhibits high ion transport performance due to its unique "paddle-wheel" mechanism. The in situ formed Na3–3xAlxPO4 could consume the residual alkali compounds and induce the formation of a Na-deficient phase, thus leading to enhanced Na+ transport kinetics. Furthermore, strong Al–O bonds formed in the bulk further enhance the crystal structure stability. In a full cell, the capacity retention rate reached 70% after 500 cycles, making its commercial operation possible. Altogether, these results suggest that the in situ plastic-crystal-coating strategy can significantly improve the surface and bulk structure stability of NaNi0.4Fe0.2Mn0.4O2, thus leading to improved electrochemical performance.