Improved cycle and air stability of P3-Na0.65Mn0.75Ni0.25O2 electrode for sodium-ion batteries coated with metal phosphates

Cathode materials are considered to be the most critical component of the sodium-ion batteries, which determine the overall electrochemical performance of the battery to some degree. Currently, the layered oxides receive significant attention due to the high operating voltage and high capacity. However, the instability in the air and unsatisfactory cycling performance limit their commercial application. Based on excellent electrochemical performance of the P3-type Na0.65Mn0.75Ni0.25O2 material, here AlPO4 and Mg3(PO4)2 protective coatings are employed to modify P3-type Na0.65Mn0.75Ni0.25O2. The metal phosphate layers cover the surfaces of the pristine particles via a co-precipitation route. It has been found that the two modified samples exhibit an obvious improvement of cycling performance with the capacity retentions about 93% at a current density of 0.2 C within 100 cycles. In addition, the coated metal phosphate layer can effectively suppress the irreversible oxidation of oxygen in the high voltage redox region. Furthermore, after exposure to the air, the coating samples still retain a high electrochemical activity. Therefore, the metal phosphates coating provides a meaningful exploration for the research and development of long-term cycling life and high air stability of cathode materials in sodium-ion batteries.