Is proton a charge carrier for d-MnO2 cathode in aqueous rechargeable magnesium-ion batteries?

Manganese dioxide (MnO2) is considered as a potential cathode material for aqueous magnesium-ion batteries. However, the charge/discharge mechanism of MnO2 in aqueous electrolyte is still unclear. In present study, highly porous delta-MnO2 is investigated, which delivers a high capacity of 252.1 mAh g(-1) at 0.05 A g(-1) and excellent rate capability, i.e., 109.7 mAh g(-1) at 1 A g(-1), but a low-capacity retention of 54.4% after 800 cycles at 1 A g(-1). The two-step discharging process, namely a consequent H+ and Mg2+ insertion reaction, is verified, by comparing the electrochemical performance of delta-MnO2 in 1 M MgCl2 and 1 M MnCl2 aqueous electrolyte and analyzing detailedly the Mg content and the bonding state of Mn at different charge/discharge state. Furthermore, partial irreversibility of Mg2+ ion insertion/extraction is observed, which may be one of the major reasons leading to capacity decay. (C)& nbsp;2021 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by ELSEVIER B.V. and Science Press. All rights reserved.

Automated summary

This study investigates the highly porous delta-MnO2 as a potential cathode material for aqueous magnesium-ion batteries. The results show that delta-MnO2 exhibits high capacity and excellent rate capability, but low-capacity retention. The two-step discharging process and the partial irreversibility of Mg2+ ion insertion/extraction are confirmed through detailed analysis of the electrochemical performance and element state.