First-principles study of the titanium-doping effects on the properties of O3-type NaNi0.25Fe0.25Mn0.5O2 cathode material for sodium-ion batteries

We carried out first-principles calculations on titanium-doped O3-type NaNi0.25Fe0.25Mn0.5O2 oxide material for titanium substitution at 0.02, 0.04, 0.06, 0.08, and 0.1. The results obtained show that materials doped with titanium have higher structural stability than undoped material. Also after doping, the conductivity of the materials increases greatly but Na(Ni0.25Fe0.25Mn0.5)0.98Ti0.02O2 is slightly more conductive (ΔEg = 0.017 eV). Na(Ni0.25Fe0.25Mn0.5)1-xTixO2 (NNFM-Tix) (x = 0.02, 0.04, 0.06, 0.08, and 0.1) indicates a larger sodium layer, which allows easy migration of sodium ions during charge/discharge processes than NNFM. Sodium ions diffuse faster in NNFM-Ti0.06 and NNFM-Ti0.08 compared to other doped-NNFMs. In addition, the Pugh index value of NNFM (1.8222) is lower than that of NNFM-Ti0.02, NNFM-Ti0.04, NNFM-Ti0.06, NNFM-Ti0.08, and NNFM-Ti0.1 (2.1376), demonstrating enhancement of elastic ductility after doping. Titanium-doped materials exhibited improved stability under temperature effects between −20 and 60 °C, which represent the operating temperature range of sodium-ion batteries, although this effect did not vary much with the doping rate. The improved thermodynamic stability indicates that the doped materials can better resist degradation, have high efficiency, and a long cycle life. These Na(Ni0.25Fe0.25Mn0.5)1-xTixO2 materials, particularly those with the titanium rate at 0.06 and 0.08 are therefore powerful materials for the accomplishment of efficient sodium-ion batteries cathodes.