The function of Mn2+ additive in aqueous electrolyte for Zn/δ-MnO2 battery

Adding Mn 2+ in aqueous electrolyte is a well adopted approach to improve cycle performance for Zn/MnO 2 batteries. The function of Mn 2+ additive in aqueous electrolyte, however, is not well understood, which could affect revealing the real performance of the MnO 2 cathode itself. In this work, a battery system with Zn anode and birnessite MnO 2 cathode is constructed and systematically investigated by using MnSO 4 additive in the electrolyte with different concentrations. The function of Mn 2+ in the electrolyte, the structural evolution of the electrode, the reaction mechanism, and the cause for capacity fading are comprehensively investigated in this system. It is found that Mn 2+ can be continuously electro-oxidized at the cathode and form ε-MnO 2 when the charge voltage is above 1.7 V during cycling, leading to successive capacity increase up to hundreds of cycles. The capacity increase, however, stops after certain cycle numbers, and is followed by fast capacity fading, which is correlated to the competition between ε-MnO 2 formation and phase transition from birnessite to spinel ZnMn 2 O 4 . It is suggested that addition of Mn 2+ in the electrolyte cannot stabilize the MnO 2 cathode and the improved cycle performance is achieved at the expense of consuming the Mn 2+ in the electrolyte.