Rechargeable Solid-State Copper Sulphide Cathodes for Alkaline Batteries
Authors: Lambert Timothy N. , Duay Jonathon , Kelly Maria , Pineda-Dominguez Ivan 
Published: February 2020

For energy storage to become ubiquitous in the electric grid, safe, reliable low-cost electrochemical storage technologies that can be manufactured at high volumes with low capital expenditures are needed. Rechargeable alkaline batteries based on abundant and low cost materials with high capacities, such as a Zinc (Zn) anode (820 mAh g-1) paired with a Sulphur (S) cathode (1675 mAh g-1, based on the idealized redox couple of S2-/S) could serve as high energy dense systems if sufficient cycle life and energy densities can in fact be realized. S has a suitable terrestrial abundance and overall economics for wide-spread battery use; however, to date Zn/S batteries have been primary in nature and hindered by crossover of sulphide, which reacts with the Zn anode during discharge, forming an insulating ZnS layer and rendering the Zn anode inert to re-charge.

In this talk we will report on the first use of a solid-state sulphide/polysulphide electrochemical couple(s) in alkaline electrolyte.1 Specifically, a new battery chemistry that consists of a solid-state Cu2S cathode paired with a Zn anode in alkaline electrolyte, which gives a nominal >1.2 V cell voltage based on the sulfide/polysulfide redox cathode chemistry, will be presented. 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 ~ 1500 mAh g-1 (based on mass of S) or ~ 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 life (~ 250 cycles). The failure mechanism of this electrode is proposed to be the conversion of the charged S species into irreversible sulphite and/or sulphide, 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 as shown in the accompanying Figure. These Zn/Cu2S batteries provided a grid storage relevant energy density of > 42 W h L−1 (at 65 wt. % Cu2S loading), despite only using a 3% depth of discharge (DOD) for the Zn anode. These results represent the first example of a rechargeable solid-state sulphur cathode in an alkaline battery and augur for the further development of a new class of energy dense grid storage batteries based on high capacity solid-state S-based cathodes. Various aspects of the battery chemistry and its cycling properties will be presented.
This work was supported by the U.S. Department of Energy, Office of Electricity, Energy Storage Program, Dr. Imre Gyuk, Program Manager, and the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA-0003525. The views expressed herein do not necessarily represent the views of the U.S. Department of Energy or the United States Government.
[1]. J. Duay, T. N. Lambert, M. A. Kelly, I. Pineda-Dominguez “Rechargeable Solid-State Copper Sulphide Cathodes for Alkaline Batteries: Importance of the Copper Valence State” J. Electrochem. Soc.
2019
166 (4), A687-A694. DOI: 10.1149/2.0261904jes.

Figure 1