Stabilizing O3-type layered oxide cathodes via dual-site co-doping for long-life sodium-ion batteries

O3-type layered transition metal oxide cathode materials exhibit tremendous potential for commercial applications owing to their high theoretical capacity. However, their practical application is fundamentally limited by oxygen loss caused by irreversible oxygen redox reactions, as well as significant volume variation during Na⁺ intercalation/deintercalation under high voltage conditions. To address these challenges, Na_0.98Ca_0.01Ni_0.33Fe_0.28Ti_0.05Mn_0.315Sn_0.015O₂ (CST-NFM) was designed and synthesized by co-doping with Sn⁴⁺/Ti⁴⁺ in the transition metal layer and Ca²⁺ in the alkali metal layer. Dual-site doping modulates the band structure between TM and O through the synergistic effect of inert ions, effectively suppressing oxygen release, alleviating the severe volume fluctuations caused by irreversible phase transitions, and thereby enhancing the structural stability and cycling stability of the materials. As a result, the CST-NFM electrode exhibited a capacity retention of 72.69% after 500 cycles. Meanwhile, the corresponding full cell paired with hard carbon delivered an initial discharge specific capacity of 142.16 mAh g^-1 and maintained a capacity retention of 86.37% after 200 cycles. This work demonstrates an effective and scalable approach for designing highly stable, high-performance layered oxide cathode materials for sodium-ion batteries through the synergistic effects of inert ion co-doping.