Regulating local defect energy states of O3 type layered cathode materials for better sodium ion batteries

The O3-type layered cathode material NaNi_1/3Fe_1/3Mn_1/3O₂ has gained attention due to its cobalt-free composition, low cost, and structural stability. In this study, we employ a "local defect energy state construction" approach to precisely regulate the formation and migration of oxygen vacancies at the atomic scale. Zr⁴⁺ and Sm³⁺ are selected to achieve synergistic optimization of both the bulk and surface properties, precisely guiding the formation of local defects and establishing a mechanism that combines bulk defect regulation with surface protection. Zr⁴⁺ bonds strongly with oxygen through 4d-2p orbital hybridization, which helps keep lattice oxygen stable, makes oxygen vacancy distribution more uniform, and improves structural stability. Sm³⁺, with its relatively large ionic radius, facilitates the formation of a stable Sm₂O₃ surface layer, while the remaining Zr forms a ZrO₂ coating, together providing enhanced protection to the cathode material. High-temperature heat treatment enables the spatial confinement and diffusion of Zr⁴⁺, constructing a multi-scale defect distribution network that restricts the randomness of vacancies and improves the stability of the sodium ion migration pathways. Compared to the bare cathode material, the modified material exhibits a significant improvement, with the discharge specific capacity increasing from 16.5 mAh g^-1 to 107.5 mAh g^-1 after 300 cycles at 1C, and the capacity retention rising from 15.73 % to 85.66 %. In full-cell tests with hard carbon as the anode, the material maintained strong electrochemical performance, delivering an initial discharge capacity of 101.1 mAh g^-1 at 1C, with 63.47 % capacity retention after 300 cycles. Both theoretical calculations and experimental results demonstrate that this structural design balances surface protection with internal defect regulation, optimizing both structural stability and ionic transport performance.