Barium ions act as defenders to prevent water from entering prussian blue lattice for sodium-ion battery

Prussian blue (PB) and its analogs (PBAs) have attracted significant attention as cathode materials for sodium-ion batteries due to their facile synthetic procedure, low cost as well as high capacity. Although a large amount of effort has been made on material innovations, the alkali-site alternative design strategy for PBAs is still elusive. A new strategy of adding multivalent cations in the electrolyte is proposed to improve the structural stability of PBA and promote sodium ions diffusion rate. In particular, Ba2+ can insert in the PBA lattice via in-situ elec-trochemical reaction when the battery is performed in charging-discharging process. Therefore, Ba2+ acts as a "defender" to maintain the frame stability and prevent residual water from entering the lattice. By reasonably controlling the additive amount of Ba2+ in the electrolyte, the as-prepared cathode shows remarkably enhanced structural stability with a capacity retention of 96.6% after 150 cycles and delivers a capacity of 83.41 mAh g-1 even at 6 C. This work provides important insights into the development of alkaline-site substitution strategies for high-performance cathode materials in sodium-ion batteries.