Sodium-Rich, Co-Ni-Free P2-Layered Manganese Oxide Cathodes for Sodium-Ion Batteries

The rapid capacity loss attributed to irreversible phase reactions and structural instability has consistently affected the development of P2-layered cathode materials. Moreover, the introduction of costly elements such as single or multiple dopants has failed to resolve the sustainability challenges in designing an optimal Mn-based layered oxide cathode. This study proposes a Co-Ni-free, poly-elemental doping strategy (Li, Mg, and Cu) combined with high sodium content for an Mn-based P2-layered cathode designed for Na⁺ ion storage. In situ X-ray diffraction analysis confirms the absence of P2 - O2 phase transitions during cycling for both NLMMC87 (Na_0.87Li_0.1Mg_0.1Mn_0.7Cu_0.1O₂) and NLMMC77 (Na_0.77Li_0.1Mg_0.1Mn_0.7Cu_0.1O₂). This can be attributed to the co-substitution of the electronegative Cu element and the stabilizing dopants Li and Mg, which suppress oxygen evolution. Simultaneously, the high sodium content within the host structure promotes high reversible capacity, enhances structural stability, and minimizes internal stress due to volume changes. Moreover, NLMMC87 demonstrates a reversible capacity of 167.9 mA h g^-1 at 0.1 C (97% Coulombic efficiency) and maintains excellent stability across various current rates. Further investigations into the practical application of NLMMC87 in a sodium full cell will be a critical step toward realizing an ideal cathode for sodium-ion batteries.