Lithium‐Substituted Tunnel/Spinel Heterostructured Cathode Material for High‐Performance Sodium‐Ion Batteries

Abstract Sodium manganese oxides as promising cathode materials for sodium‐ion batteries (SIBs) have attracted interest owing to their abundant resources and potential low cost. However, their practical application is hindered due to the manganese disproportionation associated with Mn³⁺, resulting in rapid capacity decline and poor rate capability. Herein, a Li‐substituted, tunnel/spinel heterostructured cathode is successfully synthesized for addressing these limitations. The Li dopant acts as a pillar inhibiting unfavorable multiphase transformation, improving the structural reversibility, and sodium storage performance of the cathode. Meanwhile, the tunnel/spinel heterostructure provides 3D Na⁺ diffusion channels to effectively enhance the redox reaction kinetics. The optimized [Na_0.396Li_0.044][Mn_0.97Li_0.03]O₂ composite delivers an excellent rate performance with a reversible capacity of 97.0 mA h g^–1 at 15 C, corresponding to 82.5% of the capacity at 0.1 C, and a promising cycling stability over 1200 cycles with remarkable capacity retention of 81.0% at 10 C. Moreover, by combining with hard carbon anodes, the full cell demonstrates a high specific capacity and favorable cyclability. After 200 cycles, the cell provides 105.0 mA h g^–1 at 1 C, demonstrating the potential of the cathode for practical applications. This strategy might apply to other sodium‐deficient cathode materials and inform their strategic design.