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

ABSTRACT

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 Fe ⁴⁺ migration at high voltages, causing rapid capacity decay and pronounced voltage hysteresis. In this work, a Li ⁺ –Ti ⁴⁺ co‐doped P2 cathode, Na _0.75 Li _0.2 Fe _0.15 Mn _0.55 Ti _0.1 O ₂ , is synthesized, which fundamentally stabilizes the Fe–Mn framework. A dynamic Li ⁺ suppresses Fe migration, while d ⁰ ‐Ti ⁴⁺ accommodates lattice strains preventing structural transitions. Additionally, Ti ⁴⁺ 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 cm ²  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.