A highly reversible neutral zinc/manganese battery for stationary energy storage†
Manganese (Mn) based batteries have attracted remarkable attention due to their attractive features of low cost, earth abundance and environmental friendliness. However, the poor stability of the positive electrode due to the phase transformation and structural collapse issues has hindered their validity for rechargeable batteries. Here we presented a highly reversible and stable two electron transfer solid–liquid reaction based on MnO2 and soluble Mn(CH3COO)2(Mn(Ac)2) under neutral medium. Benefiting from the coordination effect of Ac−, the Mn2+ can directly deposit on the electrode in the form of MnO2, which is completely different from other manganese salts (MnSO4 or MnCl2). Compared with the common intercalation mechanism cathode, the dissolution/deposition reaction completely avoided the structure collapse issue, which results in a dramatic improvement in stability. Furthermore, in contrast to the redox pair of Mn3+/Mn2+, the intrinsic problems caused by the disproportionation of Mn3+ can be totally avoided. The proof of concept can be confirmed by a neutral Zn–Mn flow battery with an optimized electrolyte. The MnO2 could be perfectly deposited on the graphite fiber with an areal capacity of 20 mA h cm−2, which is the highest value ever reported. Unlike the alkaline electrolytes, a neutral flow system can effectively avoid the zinc dendrite issues. As a result, a Zn–Mn flow battery demonstrated a CE of 99% and an EE of 78% at 40 mA cm−2 with more than 400 cycles. Combined with excellent electrochemical reversibility, low cost and two-electron transfer properties, the Zn–Mn battery can be a very promising candidate for large scale energy storage.