context: Sodium-ion batteries (SIBs) have recently attracted increasing attention as an alternative to lithium-ion batteries, especially for large-scale energy storage in light of the low cost and high abundance of Na resources. Layer structured sodium transition metal oxides are an important group of cathode materials for SIBs due to their high theoretical capacities. However, the poor cycling stability at high voltage hinders their practical applications, which is due to the multiple phase transition induced structural instability. Our recent work reveals that phase transition induced cracking is a major cause of the performance decay for layered cathodes. Bulk elemental doping has been demonstrated to be an effective approach to improve the cycling performance, because doping can effectively suppress phase transition induced volume changes and even realize a zero-strain cathode or phase transition free cathode. Furthermore, dopants can act as pillars to prevent the layered structure from collapsing and they can tailor the plane spacing to achieve superior diffusion kinetics of alkaline ions.
answer: Layer structured sodium transition metal oxides