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

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 K2MnFe(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 K2MnFe(CN)(6) cathode is gradually transformed to rhombohedral K2Zn3[Fe(CN)(6)](2), a process that is induced by Zn2+ insertion. The mechanism of the phase transformation is further investigated through ab initio calculations and detailed characterizations. The inserted Zn2+ 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 MnN6 octahedra are replaced by ZnN4 tetrahedra and produce the new K2Zn3[Fe(CN)(6)](2) phase eventually. The robust structure of the resulting K2Zn3[Fe(CN)(6)](2) phase contains wider channels for accommodating divalent ions and thus enables highly stable and reversible storage of Zn2+ ions. The findings of this work lead to a new understanding on the evolution of PBAs in ZIBs, and provide a promising cathode material.

Automated summary

This study reports a K2MnFe(CN)6 cathode achieving a very stable capacity in a ZIB, attributed to its gradual transformation to a more stable K2Zn3[Fe(CN)6](2) phase induced by Zn2+ insertion. The mechanism involves lattice distortion and formation of wider channels for highly stable storage and release of zinc ions, providing new insights into the evolution of PBAs in ZIBs.