Band Engineering of Mn-P Alloy Enables HER-suppressed Aqueous Manganese Ion Batteries.

Aqueous manganese ion batteries hold potential for stationary storage applications owing to their merits in cost, energy density, and environmental sustainability. However, the formidable challenge is the instability of metallic manganese (Mn) anodes in aqueous electrolytes due to severe hydrogen evolution reaction (HER), which is more serious than the commonly studied Zn metal anodes. Moreover, the mechanism of HER side reactions has remained unclear. Herein, we design a series of Mn-P alloying anodes by precisely regulating their energy band structures to mitigate the HER issue. It is found that the serious HER primarily originates from the spontaneous Mn-HO reaction driven by the excessively high HOMO energy level of Mn, rather than electrocatalytic water splitting. Owing to a reduced HOMO energy level and enhanced electron escape work function, the MnP anode achieves an evidently enhanced cycle durability (over 1000 hours at a high current density of 5 mA cm). The MnP||AgVO full cell with an N/P ratio of 4 exhibits better rate capability and extended cycle life (7000 cycles) with minimal capacity degradation than the cell using metallic Mn anode (less than 100 cycles). This study provides a practical approach for developing highly durable aqueous Mn ion batteries.