Insight into effects of divalent cation substitution stabilizing P2-Type layered cathode materials for sodium-ion batteries

Manganese-based P2-type cathodes appear to be promising candidates in practical applications of sodium-ion batteries owing to their low cost, exciting electrochemical performance, and feasibility for commercial production. However, the P2-type cathode usually suffers from undesired P2-O2 phase transition at high voltage during Na⁺ extraction which leads to a poor cycle life. Although their stability is greatly improved with elements substitution, the rationale behind still needs to be understood. Divalent cation substituted P2-type Na_0.67Ni_0.23Mn_0.67M_0.1O₂ materials were explored considering the similar ionic radii and same valence state. The substitution of divalent cations in P2-type cathodes enlarges the lattice parameters. Both Mg²⁺ and Cu²⁺ substitutions effectively improve cycling performances and suppress the P2-O2 phase transition. A combination of electrochemical profiles and in-situ X-ray diffraction analysis reveals the solid-solution reactions during Na⁺ extraction and insertion. Besides, fast Na-ion diffusion in the Mg-substituted P2-NaNM material leads to its high rate capability. Therefore, this study may give a new insight into the rational design of cathode materials with stable structure and high capacity for sodium-ion batteries.