Understanding the Multiple Effects of TiO2 Coating on NaMn0.33Fe0.33Ni0.33O2 Cathode Material for Na-Ion Batteries

O3-type NaTMO2 (TM = transition metal) as cathodes for Na-ion batteries have aroused much interest. But the structural instability during charge-discharge cycles and the inferior rate capability restricts their application. In this study, we report a synergetic modification method to simultaneously increase the rate capacity and cycling stability of O3-type NaMn0.33Fe0.33Ni0.33O2 (MFN) cathode material by integrating TiO2-coating and Ti4+ doping. Moreover, the synergetic mechanism has been put forward. Firstly, TiO2-coating layer prevents the side reactions on the surface, which can retain the structural integrity and stability. Secondly, TiO2-coating induces Ti4+ doping which enlarges Na-O and increases the interslab spacing d. This raises Na+ diffusion coefficient and improves the rate performance. Thirdly, because of the large Ti-O bond energy TM-O bond shrinks when Ti4+ is doped into the transition metal site. O-O is also shortened due to the Ti doping. TMO2 slabs are compressed which benefits the str...