Influence of Co/Ca Codoping Induced Interlayer Structural Regulation on Sodium Storage of P2–Mn–Fe–Cu-Based Oxide Cathodes

P2-type Mn-Fe-Cu-based cathode materials have garnered enormous attention as potential candidates for sodium-ion batteries (SIBs). Nevertheless, the detrimental phase transition and irreversible oxygen release cause significant capacity degradation and poor cycling stability, thereby decelerating their application progress. Herein, we develop a novel P2-Na_0.65Ca_0.05Mn_0.55Co_0.05Fe_0.2Cu_0.2O₂ cathode material via codoping with Co and Ca ions. The incorporation of Co³⁺ ions into the Mn sites not only mitigates the Jahn-Teller distortion of Mn³⁺ ions to hinder the phase transition but also activates the anionic redox activity by establishing a stable CoO₆ octahedron. The substitution of Ca²⁺ ions into the Na sites enhances the stability of the Na⁺ transport pathway and suppresses the sliding of the TMO₂ layers by constructing a stronger Ca-O bond. Under the synergistic effect of the Co/Ca codoping, P2-Na_0.65Ca_0.05Mn_0.55Co_0.05Fe_0.2Cu_0.2O₂ shows enhanced Na⁺ diffusion kinetics, improved intrinsic conductivity, alleviated electrolyte corrosion, and decreased cell volume variation during the Na⁺ extraction/insertion. Consequently, the codoping electrode exhibits a high initial discharge capacity (125.9 mA h g^-1 at 0.2 C), an excellent rate performance (79.6 mA h g^-1 at 10 C), and an outstanding long-cycle stability (73.2% capacity retention after 1000 cycles at 10 C). This codoping strategy highlights a promising opportunity to advance the practical application of P2-type Mn-Fe-Cu-based cathode materials in high-performance SIBs.