Enhanced all-climate sodium-ion batteries performance in a low-defect and Na-enriched Prussian blue analogue cathode by nickel substitution

Cobalt hexacyanoferrate (CoHCF) is one of the most promising cathode materials for all-climate sodium-ion batteries (SIBs) due to its open three-dimensional (3D) framework structures, high theoretical specific capacity, good voltage platform and almost no Jahn-Teller effects. However, CoHCF still suffers from poor cycling stability and bad rate capability, which is closely related to the huge distortion of frame structure and poor conductivity. In this study, by choosing nickel (Ni) to partially replace cobalt (Co) in the CoHCF lattice, we successfully prepared low-defect and Na-enriched Na2Co0.7Ni0.3[Fe(CN)6] (Co0.7Ni0.3HCF) in chelate and sodium salt-assisted coprecipitation method. Both experiments and first-principles calculations demonstrate that Ni substitution can effectively suppress the lattice distortion during the charging and discharging process of CoHCF. Furthermore, the introduction of Ni increases ion mobility by reducing the ion migration barrier (0.31 eV versus 0.17 eV) and improves the electronic conductivity by reducing the bandgap. It is found that Co0.7Ni0.3HCF exhibits superior electrochemical performance compared with that of CoHCF in a wide temperature range (-30 to 60 & DEG;C). At 25 & DEG;C, Co0.7Ni0.3HCF delivers a high specific capacity of 142.2 mAh g-1 at 0.2 C, an ultrahigh rate capability with 126.2 mAh g-1 at 5 C and excellent cycling stability with 80.9% capacity retention after 500 cycles at 5 C. Even at-30 & DEG;C, Co0.7Ni0.3HCF can provide a high capacity of 109 mAh g-1 without an activation process. This work reveals the great application prospect of PBAs for all-climate SIBs, especially at low temperatures.