Ternary Ni‐based Prussian blue analogue with superior sodium storage performance induced by synergistic effect of Co and Fe

Abstract Prussian blue analogue Na 2 Ni[Fe(CN) 6 ] (Ni–PB) has been widely studied as a cathode material for sodium‐ion battery due to its excellent cycling performance. However, Ni–PB has a low theoretical capacity of 85 mAh g −1 because of the electrochemical inertness of Ni. Herein, ternary Ni–PB is successfully synthesized by double doping with Co and Fe at Ni‐site, and the effect of doping with Co and Fe on the electrochemical performance of Ni–PB is systematically investigated through theoretical calculations and electrochemical tests. The first principles calculations confirm that double doping with Co and Fe can significantly reduce the energy barrier and bandgap of Ni–PB. X‐ray diffraction and composition analysis results indicate that ternary NiCoFe–PB composite not only has good crystallinity and high Na content but also has low defects and crystal water. Electrochemical tests reveal that, besides the capacity contribution of high‐spin Co/Fe and low‐spin Fe, Co‐doping enhances the electrochemical activity of low‐spin Fe and Fe‐doping improves the activity of high‐spin Co; moreover, double doping can decrease the diffusion resistance of Na + ions through solid electrolyte interface film, accelerate the kinetics for both ion diffusion process and Faradic reaction, and increase active sites. Under the synergistic effect of Co and Fe, this ternary NiCoFe–PB exhibits outstanding electrochemical performance with a high initial discharge capacity of 120.4 mAh g −1 at 20 mA g −1 and an extremely low capacity fading rate of 0.0044% per cycle at a high current density of 2 A g −1 even after 10,000 cycles, showing great application potential of ternary NiCoFe–PB in the field of large‐scale energy storage.