Modulation of trace strontium as pillars in sodium layers for stable O3-type sodium-ion battery cathodes

O3-type NaNi 1/3 Fe 1/3 Mn 1/3 O 2 represents a highly promising cathode candidate for sodium-ion batteries, offering a combination of competitive cost and impressive capacity characteristics. However, its development faces challenges such as irreversible structural phase transitions and sluggish Na + transport kinetics. Herein, a trace substitution of Sr 2+ at Na sites was employed to enhance the structural stability of NaNi 1/3 Fe 1/3 Mn 1/3 O 2 via a facile solid-phase sintering method. Comprehensive characterization confirmed that Sr 2+ preferentially occupied the position of sodium sites due to favorable ionic radius matching. The larger ionic radius of Sr 2+ effectively facilitated expansion of the interlayer spacing, which weakened the Na + -O 2− electrostatic attraction and thereby boosted Na + transport. Simultaneously, the formed SrO 6 octahedra with strong Sr O bonds acted as pillar units within the framework that could enhance structural integrity and suppress phase transitions and microcrack initiation. Consequently, the optimized cathode material exhibited an exceptional cycling durability with 79.3% capacity retention over 200 cycles at 1C, and demonstrated excellent rate capability of 107.9 mAh g −1 at 5C. The full cells with hard carbon achieved an energy density of 360.2 Wh kg −1 (calculated based on the mass of the cathode active material) with a capacity of 86.5% maintained after 100 cycles at 1C. This study elucidates the role of Sr as a pillar-type dopant in stabilizing O3-type cathode structures and proposes a viable strategy for enhancing the performance of layered oxide cathodes.