Unlocking long-lifespan O3-Type Oxide Cathode for Sodium-ion batteries via inactive element substitution

O3-type NaNi_1/3Fe_1/3Mn_1/3O₂ 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-xFe_1/3Mn_1/3Al_xO₂ cathodes with varying Al³⁺ doping concentrations, synthesized through a one-step liquid-phase modified precursor method followed by post-calcination. Notably, an optimal level of Al³⁺ 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_1/3-0.005Fe_1/3Mn_1/3Al_0.005O₂ 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.