Aqueous metal-sulfide battery with enhanced stability achieved by a four-electron Ag2S electrode with a Cu2+/Cu+ redox charge carrier

Sulfur with high specific capacity is a promising candidate for electrode material of aqueous/nonaqueous metal-sulfide batteries, however, it suffers from poor cycle stability due to its poor conductivity, large volume change and shuttle effect of polysulfide. For the first time, this work employs a semiconductor Ag2S with good electronic conductivity as cathode of aqueous metal-sulfide batteries, which gives a four-electron electrode reaction through a three-step all-solid-state conversion reaction of Ag2S→Ag3CuS2→AgCuS→Cu2S with a specific capacity of 432 mAh g−1. Besides, low volume expansion and the elimination of shuttle effect due to the small Ksp of Ag2S contribute to satisfactory cycle performance. The as-proposed Cu-Ag2S battery shows an extremely low overpotential of 50 mV and excellent cycle performance of 9000 cycles, which are the lowest and highest values respectively among all the reported metal-sulfide batteries. By pairing with Zn of lower potential, a cascade Zn-Ag2S battery is designed and achieves high working voltage up to1.22 V successfully. After Ag2S fully reducing into Cu2S in discharge process, Cu2+ in the electrolyte can provide additional capacity for the battery, thus, the cascade battery design can achieve a higher energy density, representing a promising route to energy-dense aqueous batteries.