Size‐, Water‐, and Defect‐Regulated Potassium Manganese Hexacyanoferrate with Superior Cycling Stability and Rate Capability for Low‐Cost Sodium‐Ion Batteries

Abstract Potassium manganese hexacyanoferrate (KMHCF) is a low‐cost Prussian blue analogue (PBA) having a rigid and open framework that can accommodate large alkali ions. Herein, the synthesis of KMHCF and its application as a high‐performance cathode in sodium‐ion batteries (NIBs) is reported. High‐quality KMHCF with low amounts of crystal water and defects and with homogeneous microstructure is obtained by controlling the nucleation and grain growth by using a high‐concentration citrate solution as a precipitation medium. The obtained KMHCF exhibits superior cycling and rate performance as a NIB cathode, showing 80% capacity retention after 1000 cycles at 1 C and a high capacity of 95 mA h g −1 at 20 C. Unlike conventional single‐cation batteries, the hybrid NIB with KMHCF as cathode and Na as anode in Na‐ion electrolyte displays three reversible plateaus that involve stepwise insertion/extraction of both K + and Na + in the PBA framework. In later cycling, the K + –Na + cointercalated phase is partially converted into a cubic sodium manganese hexacyanoferrate (NaMHCF) phase due to the increasing replacement of Na + for K + .