Advancements in the Realm of Structural Engineering for Sodium-Ion Batteries via Elemental Doping: A Focus on P2-Phase Nickel–Manganese Layered Oxides

In recent decades, lithium-ion batteries (LIBs) have been widely adopted for large-scale energy storage due to their long cycle life and high energy density. However, the high cost and limited natural abundance of lithium highlight the urgent need to develop alternative devices, such as sodium-ion batteries (SIBs), which utilize abundant and readily available resources. Among SIB cathode materials, P2-phase Ni–Mn materials have emerged as commercially viable candidates because of their high operating voltage, good specific capacity, excellent sodium-ion conductivity, and robust stability under environmental conditions. Nevertheless, the Jahn–Teller effect triggered by high-voltage phase transitions, Na⁺/vacancy ordering, and the presence of Mn³⁺ at low voltages collectively lead to structural degradation and performance decline during cycling. By varying the macroscopic structural design and surface coating, elemental doping introduces one or more ions at the atomic scale, adjusting the valence states and reducing the band gap. This effectively alters the electronic structure and the intrinsic lattice of the cathode material, thereby accelerating reaction kinetics and yielding high-performance material characteristics. This review delves into the research advancements pertaining to tailored structural engineering strategies to address these challenges for P2-phase Ni–Mn layered oxides.