Excess of Zn to Relieve the Structural Distortion of Manganese Hexacyanoferrate in Aqueous Zn-ion Battery

Abstract

The electrochemical performances and reaction mechanism of manganese hexacyanoferrate (MnHCF) in aqueous rechargeable Zn-ion batteries (AZIBs) have been widely studied. Due to the irreversible intercalation of Zn2+, a consistent compositional and structural change of MnHCF has been reported. In this article, a series of (3%, 10% and 35%) Zn-substituted MnHCF samples were synthesized. Their electrochemical response was evaluated, and the function of Zn-substitution on the electrochemical performance and structure stability of MnHCF were comprehensively investigated using operando and ex-situ synchrotron X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) techniques. After Zn substitution, both the long-range crystal structure of MnHCF and local structural environment of Mn resulted to be modified. Although the Zn-substituted samples exhibit lower specific capacity in AZIB, compared to MnHCF sample, higher cycling stability was observed, notably for the 10% ZnMnHCF sample. The working mechanism study of 10% ZnMnHCF electrode demonstrated that a new MnO6 local structural unit was formed and remained stable after the first charge process, and this rapid and steady modification of Mn site could partially explain the higher cycling stability of the 10% ZnMnHCF AZIB upon cycling. The local structural environment of Zn changes with the insertion/release of Zn2+ at the beginning 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.