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.