Role of zinc hydroxysulfates in the thermodynamics and kinetics of mild-acid Zn-MnO2 batteries

Mild-acid Zn-MnO2 batteries have emerged as a promising alternative to replace Li-ion batteries in large-scale energy storage systems, primarily due to their high safety and low cost. While there have been significant improvements in the electrochemical performance of Zn-MnO2 batteries, the reaction mechanism of the MnO2 cathode is not fully comprehended. Despite the ongoing debate regarding the reaction mechanism of MnO2, it is noteworthy that the reversible formation and dissolution of zinc hydroxysulfates (ZHS) have been unequivocally observed during cycling. Nevertheless, the specific role of ZHS in the electrochemical reaction of MnO2 remains incompletely understood. Therefore, a comprehensive elucidation of the contribution of ZHS to the MnO2 reaction is still required. Herein, the role of ZHS in the electrochemical reaction of Mn2+ into MnO2 is investigated in terms of thermodynamics and kinetics. The voltage profiles associated with Mn2+ oxidation into MnO2 are compared under conditions of ZHS presence and absence. Operando pH analysis is also established to clarify the role of ZHS in the redox potential of the Mn2+/Mn4+ couple. In addition, a purposefully designed electrochemical experiment is performed to elucidate that ZHS significantly contributes to the oxidation kinetics of Mn2+ into MnO2 during charge. Various hydroxysulfate compounds are also examined to demonstrate a correlation between OH− derived from ZHS and the oxidation kinetics of Mn2+ during charge. These findings provide evidence that the electrochemical performance of Zn-MnO2 batteries is remarkably influenced by the dissolution kinetics of ZHS during charge and the precipitation sites of ZHS during discharge.