New Strategy to Build a High-Performance P′2-Type Cathode Material through Oxygen Vacancies and Mg Substitution for Sodium-Ion Batteries

The series P2-type Mn–Fe-based layered oxide materials (Na_2/3Mn_xFe_1–xO₂) for sodium-ion batteries are regarded as potential commercial cathode materials due to their high specific capacity and low cost. However, the P2-type layered oxide cannot avoid some phase transformation during the charge–discharge process, which results in poor structural reversibility and low-capacity retention. Moreover, a high capacity usually means more insertion/extraction of Na⁺, which results in large changes in the lattice volume and poor cycling stability. Herein, a new strategy containing oxygen vacancies and Mg substitution is designed to obtain high-performance sodium storage of Mn–Fe-based layered oxide. The P′2-type Na_0.67Mn_0.85Fe_0.1Mg_0.05O_2−δ (Ovs & Mg-sub) cathode material is synthesized by the above-mentioned dual-modification strategy. The effect of oxygen vacancies and Mg substitution on the structural evolution during the charge–discharge process is further investigated by in situ X-ray diffraction techniques. It demonstrates that Ovs & Mg-sub has reversible structural transformation and smaller lattice volume changes, thus further exhibiting prominent electrochemical properties. The initial discharge specific capacity reaches 190.2 mAh g^–1 at a current density of 20 mA g^–1 and remains at 152.9 mAh g^–1 at a current density of 100 mA g^–1 after 100 cycles. In addition, it has a superior rate capability of 88.9 mAh g^–1 at a current density of 2000 mA g^–1.