Excess of Zn to relieve the structural distortion of manganese hexacyanoferrate in aqueous Zn-ion battery†

Abstract

The electrochemical performance and reaction mechanism of manganese hexacyanoferrate (MnHCF) in aqueous rechargeable Zn-ion batteries (AZIBs) have been widely studied. A consistent compositional and structural change in MnHCF due to the irreversible intercalation of Zn²⁺ has been reported. In this article, a series of (3%, 10% and 35%) Zn-substituted MnHCF samples was synthesized. Their electrochemical responses were evaluated, and the effect of Zn substitution on the electrochemical performance and structure stability of MnHCF was comprehensively investigated using operando and ex situ synchrotron X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) techniques. After Zn substitution, the long-range crystal structure of MnHCF and the local structural environment of Mn were found to be modified. Although Zn-substituted samples exhibited lower specific capacity in AZIBs than the MnHCF sample, higher cycling stability was observed, notably in the 10% ZnMnHCF sample. The study of the working mechanism of the 10% ZnMnHCF electrode demonstrated that a new MnO₆ local structural unit was formed and remained stable after the first charging cycle. This rapid and steady modification of the Mn site could partially explain the higher cycling stability of the 10% ZnMnHCF AZIB upon cycling. The local structural environment of Zn changes upon the insertion/release of Zn²⁺ in the initial cycles, but after 20 cycles, a tetrahedrally coordinated Zn unit was detected, corresponding to the cubic ZnHCF phase, which was observed for all Zn-substituted electrodes after 100 cycles.