Structural stabilization of high-voltage P2-type layered oxide cathodes through Li/Ti co-doping for sodium-ion...

High-voltage P2-type Na_2/3Ni_1/3Mn_2/3O₂ is a promising cathode for sodium-ion batteries but its practical capacity retention is limited by Na⁺/vacancy ordering, layer gliding, P2-O2/OP4-type phase evolution, and oxygen-related interfacial degradation at deep desodiation. Herein, a Li/Ti co-doping strategy is developed to stabilize the transition-metal slab and regulate high-voltage charge compensation through conventional solid-state reaction. The optimized Na_0.72Li_0.1Ni_0.23Mn_0.5Ti_0.17O₂ (NL_0.10NMTO) retains the P2 framework, shows homogeneous elemental distribution, and delivers 131.1 mA h g^-1 at 0.2 C between 2.5 and 4.5 V. Compared with undoped NNMO, the co-doped cathode exhibits smoother charge/discharge profiles, improved rate capability, and markedly enhanced cycling stability, retaining 99.7% of its initial capacity after 50 cycles at 0.2 C. Galvanostatic intermittent titration, differential capacity analysis, potential-dependent impedance spectroscopy, and operando X-ray diffraction collectively reveal accelerated Na⁺ transport, suppressed impedance growth, and a predominantly solid-solution-like structural evolution. These results demonstrate that cooperative cation substitution effectively mitigates high-voltage structural degradation and provides a feasible strategy for developing durable layered oxide cathodes for sodium-ion batteries.