Is proton a charge carrier for δ-MnO_(2) cathode in aqueous rechargeable magnesium-ion batteries?

摘要

Manganese dioxide(MnO_(2)) is considered as a potential cathode material for aqueous magnesium-ion batteries. However, the charge/discharge mechanism of MnO_(2)in aqueous electrolyte is still unclear. In present study, highly porous δ-MnO_(2) is investigated, which delivers a high capacity of 252.1 m Ah g^(-1) at 0.05 A g^(-1) and excellent rate capability, i.e., 109.7 m Ah 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 Mg^(2+) insertion reaction, is verified, by comparing the electrochemical performance of δ-MnO_(2) in 1 M MgCl_(2) and 1 M MnCl_(2) aqueous electrolyte and analyzing detailedly the Mg content and the bonding state of Mn at different charge/discharge state. Furthermore, partial irreversibility of Mg^(-1) ion insertion/extraction is observed, which may be one of the major reasons leading to capacity decay.