Understanding the Multiple Effects of TiO₂ Coating on NaMn_0.33Fe_0.33Ni_0.33O₂ 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. First, the TiO2-coating layer prevents the side reactions on the surface, which can retain the structural integrity and stability. Second, TiO2-coating induces Ti4+ doping which enlarges Na–O and increases the interslab spacing d. This raises the Na+ diffusion coefficient and improves the rate performance. Third, because of the large Ti–O bond energy, the 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 structural stability and the cyclic property. Fourth, Ti doping accompanying TiO2 coating decreases Mn3+/Mn4+ and mitigates the Jahn–Teller effect. This increases the stability of the layered structure. The understanding of the multiroles of TiO2 coating is equally instructive for the exploration of other cathode materials for Na-ion or Li-ion batteries.