Mg/Li Co-Doping Activates Anionic Redox in Sodium-Ion Battery Layered Oxides

Thanks to their low cost and environmental sustainability, sodium-ion batteries have emerged as a highly attractive alternative to lithium-ion systems in the field of large-scale energy storage; however, issues such as insufficient energy density and poor cycle stability have hindered their widespread adoption. We have rationally designed a Mg/Li co-doped P2-type NLMMO (Na_0.8Mg_0.22Li_0.08Mn_0.7O₂) cathode material that enables reversible anion redox reactions through synergistic interactions, enhancing the stability of the layered framework. The material exhibits an exceptional initial discharge capacity of 158 mAh g^-1 at 2.0-4.4 V and retains 68% of its capacity after 400 cycles at 0.1 A g^-1, surpassing the performance of both lithium-doped (Na_0.8Li_0.3Mn_0.76O₂, NLMO) and magnesium-doped (Na_0.8Mg_0.3Mn_0.7O₂, NMMO) materials. In situ XRD shows that NLMMO has structural stability, and the Mg doping modification strongly inhibits the phase transition and stabilizes the interlayer structure. Ex situ XPS analysis indicates that the lattice oxygen in the cathode material underwent changes. This study provides a new path for designing cathodes with synergistic anionic and cationic redox reactions.