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

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 center dot(OH)(6)center dot 4H(2)O) 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 center dot(OH)(6)center dot 4H(2)O 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. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

Rechargeable aqueous zinc-manganese oxide batteries with Mn2+ additive show improved cycling stability by initiating the generation of zinc vernadite nanosheets during charging and reversible conversion back to zinc sulfate hydroxide hydrate during discharging, with some transformation into tunnel-like MnO2 nanocrystalline material at higher voltages to further enhance specific capacity.