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 NaNi0.2Fe0.4Mn0.4O2. 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(Ni0.2Fe0.4Mn0.4)0.96Co0.04B0.02O2 (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.