Recent progress of ion-doping methods in modifying layered oxide cathode materials for sodium-ion batteries

Typically, the cathode materials contribute the most in achieving the commercialization of sodium-ion batteries (SIBs) as the cathodes take up the most proportion of the full cells cost. Hence, developing cathode materials with a high specific capacity, perfect capacity retention, long cycling lifetime, and strong chemical/environmental stability is of undoubted importance. Researchers are now using many methodologies to improve the performance of different types of cathodes. Among all these, ion-doping used in modifying layered oxide (NaxTMO2, in which TM=transition metal; 0<x≤1) cathode materials seem to be one of the most popular methods. This is because that the NaxTMO2 cathodes offer perfect energy density and Na+ conductivity, while ion-doping methods could help mitigate the phase transition, sluggish kinetics, and air instability challenges. As a result, this review summarizes the recent progress of ion-doped NaxTMO2 cathodes modification strategies, divided into three parts in dealing with the above three challenges respectively. This work targets for facilitating the development of ion-doping methods used in commercial NaxTMO2 modification and pave the way for the next generation of energy storage systems.