Successive electrochemical conversion reaction to understand the performance of aqueous Zn/MnO2 batteries with Mn2+ additive

Abstract

Rechargeable aqueous zinc-manganese oxide (Zn/MnO2) batteries using Mn2+ as the electrolyte additive have recently attracted remarkable attention owing to their largely improved cycling stability. Herein, we find that the Zn/MnO2 batteries with Mn2+ additive still exhibit rapid capacity fading when cycling between 0.8 and 1.6 V vs. Zn/Zn2+, its improved performance is only observed when charged to a higher slope region (>1.6 V), which suggests that the improved performance of Mn2+ added Zn/MnO2 is not caused by suppressing the dissolution of MnO2 cathode. Inspired by this discovery, successive electrochemcial conversion reactions are scrutinized and proved for evaluating the performance of the Zn/MnO2 batteries after using Mn2+ as the electrolyte additive. By adding a certain amount of Mn2+ into the electrolyte, the battery can improve the capacity and cycling abilitily through converted electrodepostion of Mn2+. Specifically, the zinc sulfate hydroxide hydrate (Zn4SO4·(OH)6·4H2O) large-flake can initiate the generation of zinc vernadite nanosheets (ZnxMnO(OH)y) during the charge process (around 1.5 V vs. Zn/Zn2+), and then the zinc vernadite nanosheets can reversibly re-back to Zn4SO4·(OH)6·4H2O during the discharge process. Importantly, part of zinc vernadite nanosheets irreversibly transform into tunnel-like MnO2 nanocrystalline material when charging higher than 1.6 V vs. Zn/Zn2+, which can improve the specific capacity of Zn/MnO2 batteries in subsequent cycles and then make the Zn/MnO2 batteries exhibit excellent cycles stability. Finally, through a special zinc/carbon nanotube (Zn/CNT) battery, this successive electrochemcial conversion reactions are further verified.