Synergistic bulk-interface engineering enables high-performance of O3-type NaNi _1/3 Fe _1/3 Mn _1/3 O ₂ cathodes under high voltage

The O3-type NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM) has emerged as a highly promising cathode material for sodium-ion batteries due to its facile synthesis and high theoretical capacity. However, it suffers from severe capacity and rate capability degradation caused by multiple coupled failure mechanisms, including irreversible phase transitions, structural deterioration at high voltages, and electrolyte-induced surface corrosion. This work addresses the challenge of high-voltage stability in NFM cathodes via a synergistic bulk-phase and interface engineering strategy. Firstly, Li, Ti, and Co are co-doped into the bulk lattice structure to suppress the Mn³⁺-induced Jahn-Teller distortion and improve Na⁺ diffusion kinetics. And then, an AlPO₄ protective coating layer is fabricated to mitigate electrolyte corrosion and interfacial side reactions. Consequently, the as-designed composite cathode (AP@NFMLTC) can effectively suppress the P3 to O3’ phase transition within the voltage range of 2.0 to 4.2 V, resulting in a highly reversible sodium storage mechanism. After 100 cycles at a rate of 1 C, the capacity retention rate significantly improves from 45.6% to 83.6%, with a minimal voltage decay of just 0.08 V. The dual bulk-interface synergistic strategy in this work provides valuable insights into achieving high stable operation for SIBs cathodes under enhanced voltage.