Synergistic Effect of Anchoring Transitional/Interstitial Sites on Boosting Structural and Electrochemical Stability of O3-Type Layered Sodium Oxides

O3-type layered oxides are considered promising cathode materials for next-generation high-energy-density sodium-ion batteries (SIBs). However, they face challenges, such as low rate capacity and poor cycling stability, which arise from structural deformation, sluggish Na⁺ diffusion kinetics, and interfacial side reactions. Herein, a synergistic substitution strategy for transitional and interstitial sites was adopted to improve the structure stability and Na⁺ diffusion kinetics of the O3-type NaNi_0.2Fe_0.4Mn_0.4O₂. Simulation results indicate that Co³⁺/B³⁺ codoping effectively lowers the Na⁺ migration energy barrier. In addition, the synergistic effect of Co³⁺/B³⁺ codoping provides ultralow lattice strain during repeated Na⁺ deintercalation/intercalation. In situ characterization verified that the complex phase transformation during charge and discharge was suppressed, thereby significantly improving the structural stability. At 1 and 3 C, the capacity retention of the modified O3–Na(Ni_0.2Fe_0.4Mn_0.4)_0.96Co_0.04B_0.02O₂ (NFMCB) improved from 29.6% and 1.7% to 86.7% and 88.6% after 200 cycles, respectively. Even at 10 C, it could still produce 107.2 mAh·g^–1. Furthermore, full cells assembled with this material and commercial hard carbon exhibit a high energy density of 316.2 Wh·kg^–1 and a capacity retention of 80.8% after 200 cycles at 1 C. It is expected that this strategy will facilitate the commercialization of O3-type layered oxides.