Zn²⁺ Induced Phase Transformation of K₂MnFe(CN)₆ Boosts Highly Stable Zinc‐Ion Storage

Abstract

Abstract Prussian blue analogues (PBAs), featuring an open framework for accommodating large ions and tunable valence states, have garnered wide interest in the context of aqueous zinc‐ion batteries (ZIBs). However, PBAs in ZIBs currently still suffer from low capacity and poor cycling stability due to structural instability. Here a K 2 MnFe(CN) 6 cathode achieving a very stable capacity of 100 mAh g −1 is reported in a ZIB charged/discharged to 400 cycles. Interestingly, such a stable capacity is attributed to the fact that the K 2 MnFe(CN) 6 cathode is gradually transformed to rhombohedral K 2 Zn 3 [Fe(CN) 6 ] 2 , a process that is induced by Zn 2+ insertion. The mechanism of the phase transformation is further investigated through ab initio calculations and detailed characterizations. The inserted Zn 2+ is found to induce an intense Jahn‐Teller effect of the trivalent manganese, resulting in a strong lattice distortion. Coupled with the disproportionation reaction of manganese, the MnN 6 octahedra are replaced by ZnN 4 tetrahedra and produce the new K 2 Zn 3 [Fe(CN) 6 ] 2 phase eventually. The robust structure of the resulting K 2 Zn 3 [Fe(CN) 6 ] 2 phase contains wider channels for accommodating divalent ions and thus enables highly stable and reversible storage of Zn 2+ ions. The findings of this work lead to a new understanding on the evolution of PBAs in ZIBs, and provide a promising cathode material.