Rechargeable Solid-State Copper Sulfide Cathodes for Alkaline Batteries: Importance of the Copper Valence State

Batteries for grid storage applications must be inexpensive, safe, reliable, as well as have a high energy density. Here, we utilize the high capacity of sulfur (S) (1675 mAh g(-1), based on the idealized redox couple of S2-/S) in order to demonstrate for the first time, a reversible high capacity solid-state S-based cathode for alkaline batteries. To maintain S in the solid-state, it is bound to copper (Cu), initially in its fully reduced state as the sulfide. Upon charging, the sulfide is oxidized to a polysulfide species which is captured and maintained in the solid-state by the Cu ions. This solid-state sulfide/polysulfide cathode was analyzed versus a zinc (Zn) anode which gives a nominal > 1.2 V cell voltage based on the sulfide/polysulfide redox cathode chemistry. It was found that in order for the S cathode to have the best cycle life in the solid-state it must not only be bound to Cu ions but bound to Cu ions in the + 1 valence state, forming Cu2S as a discharge product. Zn/Cu2S batteries cycled between 1.45 V and 0.4 V vs. Zn displayed capacities of similar to 1500 mAh g(-1) (based on mass of S) or similar to 300 mAh g(-1) (based on mass of Cu2S) and high areal (> 23 mAh cm(-2)) and energy densities (> 135 Wh L-1), but suffered from moderate cycle lifes (< 250 cycles). The failure mechanism of this electrode was found to be disproportionation of the charged S species into irreversible sulfite releasing the bound Cu ions. The Cu ions become free to perform Cu specific redox reactions which slowly changes the battery redox chemistry from that of S to that of Cu with a S additive. Batteries utilizing the Cu2S cathode and a 50% depth of charge (DOC) cathode cycling protocol, with 5 wt% Na2S added to the electrolyte, retained a cathode capacity of 838 mAh g(-1) (based on the mass of S) or 169 mA h g(-1) (based on mass of Cu2S) after 450 cycles with > 99.7% coulombic efficiency. These Zn/Cu2S batteries provided a grid storage relevant energy density of > 42Wh L-1 (at 65 wt% Cu2S loading), despite only using a 3% depth of discharge (DOD) for the Zn anode. This work opens the way to a new class of energy dense grid storage batteries based on high capacity solid-state S-based cathodes. (c) The Author(s) 2019. Published by ECS.