Is proton a charge carrier for δ-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 δ-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 δ-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.