Achieving High Energy Density and Superior Electrochemical Performance in P2-Type Fe-Mn Cathodes via Li-Ti Co-Doping

Co- and Ni-free P2-type Fe-Mn based sodium layered oxides are promising cathodes for sustainable, low-cost sodium-ion batteries. Despite years of research, these cathodes still suffer from irreversible phase transitions and severe transition-metal migration, particularly Fe⁴⁺ migration at high voltages, causing rapid capacity decay and pronounced voltage hysteresis. In this work, a Li⁺-Ti⁴⁺ co-doped P2 cathode, Na_0.75Li_0.2Fe_0.15Mn_0.55Ti_0.1O₂, is synthesized, which fundamentally stabilizes the Fe-Mn framework. A dynamic Li⁺ suppresses Fe migration, while d⁰-Ti⁴⁺ accommodates lattice strains preventing structural transitions. Additionally, Ti⁴⁺ substitution expands Na-layer spacing improving Na-diffusivity. The co-doped cathode also accommodates higher Na content, delivering enhanced reversible capacity and a high energy density of 526 W h kg^-1. It exhibits accelerated Na⁺ diffusion (D_Na ⁺ ≈ 10^-10 cm² s^-1), low polarization, and superior cycling stability compared to undoped and singly doped analogues. Even under high-voltage operation, the unit-cell volume changes by <0.3%, confirming zero-strain behaviour and exceptional structural stability. The material also demonstrates excellent rate performance, retaining >85% capacity at 1C, >76% at 5C over 100 cycles and >90% capacity in full-cell operation at C/2. The study therefore demonstrates Li-Ti co-doping as an effective strategy to enable high-voltage, structurally stable P2-type Fe-Mn cathodes for low-cost sustainable sodium-ion batteries.