A three-in-one strategy of residual alkali removal, TiO2-coating, and Ti4+-doping to enhance sodium storage properties of P2/O3 biphasic layered oxide cathodes.

Unstable surface/interface structure and serious irreversible phase transitions greatly hinder the commercial application of sodium layered oxides. Herein, a TiO2-coated and Ti4+-doped P2/O3 biphasic Na0.8Mn0.55Ni0.35Cu0.1O2 cathode material is designed by immersing Na0.8Mn0.55Ni0.35Cu0.1O2 in a purpose-made solution mixed with tetrabutyl titanate, acetic acid and ethanol, followed by solid-phase sintering. During the immersing process, residual alkali on the surface of the material is removed using low-concentration acetic acid, and a layer of tetrabutyl titanate is adsorbed onto the surface. During the subsequent high-temperature sintering process, a TiO2 protective layer is formed, and Ti elements are simultaneously doped into the material, which protects the material from H2O, CO2 attack, and electrolyte corrosion, and enlarges the layer spacing. Remarkably, the effect of Ti4+-doping reduces the proportion of Mn3+ and eliminates the irreversible P3-O″3 phase transition, which is firmly validated by in-situ X-ray diffraction. This as-modified material demonstrates a capacity retention rate of 90.02 % over 200 cycles at 1C (1C = 150 mA g-1) within the voltage range of 1.5-4.2 V, achieving a 24.2 % improvement over the pristine Na0.8Mn0.55Ni0.35Cu0.1O2 (72.5 %). Furthermore, the optimized material delivers superior rate capability, maintaining a discharge capacity of 74.9 mAh g-1 at 6C, whereas the unmodified counterpart achieves only 63.8 mAh g-1 under the same rate.