High-Operating Voltage, Long-Life Layered Oxides for Sodium Ion Batteries Enabled by Cosubstitution of Titanium and Magnesium

P2-type layered oxides are considered as promising cathode materials for rechargeable sodium ion batteries, but preparing P2-type cathodes with high-operating voltage and long-life is still a big challenge. Herein, spherical P2-type cathode Na0.67Ni0.17Co0.17Mn0.66Ti0.05Mg0.05O2 has been designed, and the critical roles of Ti and Mg on electrochemical performance of the cathodes are systematically investigated. The high-temperature XRD clearly exhibits the forming process of the pure phase material and suggests that the most suitable calcination temperature is 850 degrees C. The Ti/Mg cosubstitution does not break the long-range P2 structure and the spherical morphology of the material. In the electrochemical processes, the Na0.67Ni0.17Co0.17Mn0.56Ti0.05Mg0.05O2 electrode exhibits better electrochemical performance than that of the undoped Na0.67Ni0.17Co0.17Mn0.66O2. It delivers an initial reversible capacity of 151 mAh g(-1) (2-4.5 V) with an average voltage of 3.8 V and exhibits a high capacity retention of 87.7% after 300 cycles at 100 mA g(-1). The improved electrochemical performance benefits from the Ti/Mg cosubstitution; Ti improves the average voltage while Mg and Ti significantly mitigate the undesired P2 -> O2 phase transition of the cathode, and these two elements jointly promote the development of the electrochemical performance. This strategy is also applicable to the optimization design of layered transition oxides and provides a new approach to prepare high-voltage, long-life cathodes for sodium ion batteries.