Unlocking long-lifespan O3-type oxide cathode for sodium-ion batteries via inactive element substitution

O3-type NaNi 1/3 Fe 1/3 Mn 1/3 O 2 is considered as one of the most promising cathode candidates for sodium-ion batteries due to its high specific density and low cost. However, rapid capacity degradation and sluggish diffusion kinetics , caused by structural distortions and irreversible phase transitions, present significant challenges to its commercial application. In this study, we design a series of O3-NaNi 1/3-x Fe 1/3 Mn 1/3 Al x O 2 cathodes with varying Al 3+ doping concentrations, synthesized through a one-step liquid-phase modified precursor method followed by post-calcination. Notably, an optimal level of Al 3+ substitution at the transition metal sites significantly enhances the structural stability during long cycling. However, excessive Al doping at the Na-site hampers sodium-ions diffusion, thereby limiting the rate performance. The optimal performance is achieved with the O3-NaNi 1/3-0.005 Fe 1/3 Mn 1/3 Al 0.005 O 2 cathode, which exhibits remarkable overall electrochemical performance including a high specific capacity of 151 mAh g -1 at 1 C, superior cycle stability with 85 % capacity retention after 150 cycles in a half cell, and improved rate performance (74 mAh g -1 at 10 C). This work provides valuable insights into the behavior of inactive element substitution, offering a pathway for the development of long-cycle life O3-type oxide cathodes and contributing to novel materials design strategies.