Insights into Ti doping for stabilizing the Na_2/3Fe_1/3Mn_2/3O₂ cathode in sodium ion battery

Iron- and manganese-based layered metal oxides, as cathodes for sodium ion batteries, have received widespread attention because of the low cost and high specific capacity. However, the Jahn-teller effect of Mn³⁺ ions and the resulted unstable structure usually lead to continuously capacity decay. Herein, Titanium (Ti) has been successfully doped into Na_2/3Fe_1/3Mn_2/3O₂ to suppress the Jahn-Teller distortion and improve both cycling and rate performance of sodium ion batteries. In situ high-energy synchrotron X-ray diffraction study shows that Ti-doped compound (Na_2/3Fe_1/3Mn_0.57Ti_0.1O₂) can maintain the single P2 phase without any phase transition during the whole charging/discharging process. Various electrochemical characterizations are also applied to explore the better kinetics of sodium ions transfer in the Na_2/3Fe_1/3Mn_0.57Ti_0.1O₂. This work provides a comprehensive insight into the Ti-doping effects on the performance from both structural and electrokinetic perspectives.