Electrochemically activated MnO cathodes for high performance aqueous zinc-ion battery

Abstract

Aqueous Zn-ion batteries (ZIBs) attract ever-growing attention due to low cost, high safety, and environmental friendliness. Among the limited cathode candidates, manganese-based oxides stand out owing to large ion channels and multiple valence of Mn. Notably, MnO, thought to be inactive before, actually exhibits high energy densities in AZIBs, thus arousing wide interests. Herein, the electrochemical behavior of MnO electrode is extensively investigated in ZnSO4 electrolytes with different concentration of MnSO4. It is confirmed that the inactive MnO is electrochemically activated during the initial charge process. Greatly, the initial capacity and the following cycling stability of MnO electrode strongly depends on the concentration of MnSO4, indicating that the activation of MnO could be attributed to Mn dissolution, leading to the higher valence of Mn as evidenced by ex-situ XPS analysis. Greatly, a H+/Zn2+ co-insertion mechanism is suggested for the as-activated Mn1-xO electrode according to the ex-situ XRD results. In addition, by constructing a core-sheath MnO@N-doped graphene scrolls (MnO@NGS) configuration through a modified hydrothermal process, the reversible capacity of MnO has been successfully improved to 288 mAh g−1 (0.1 A g−1), leading to high energy density of 367 mWh g−1. More importantly, benefiting from the graphene wrapping and sufficient internal voids, 98% of the initial capacity can be retained after 300 cycles (0.5 A g−1), much better than the unmodified MnO and MnO2 electrodes. This work is expected to deepen the understanding of the charge-storage mechanism of MnO and help to design durable manganese-based cathodes for ZIBs.