Electrochemical Stability and Reversibility upon Cycling of Aqueous Polysulfide Electrodes Beyond the Solubility Limit

Abstract

The low cost, high solubility, and high specific capacity of aqueous polysulfide electrodes are attractive in battery applications. Our group recently developed a redox flow battery system that used an air (oxygen) cathode and an aqueous polysulfide anode, aiming at low-cost large scale grid storage[1]. Surprisingly, for low cost electrochemically active materials such as aqueous polysulfide, the cost of auxiliary components including the supporting electrolytes and the tanks is ~2 times that of the active materials themselves. Accordingly, to further decrease the energy cost, it is imperative to increase the volumetric capacity (Ah/L) of aqueous polysulfide electrolytes. One natural way to increase the capacity of the redox active solution is to increase the “effective” concentration of the active species, which is bounded by the solubility of the active species. However, the solubility of aqueous polysulfides in the reduced form (Na2S2) at room temperature is ~2.5M (5M total sulfur), equivalent to 67 Ah/L when utilizing capacity range between Na2S2 and Na2S4. One strategy to overcome this limitation is to leverage the redox activity of the precipitated polysulfide hydrate (Na2S2 xH2O) during the reduction reaction, which subsequently can be oxidized to the fully soluble higher-order polysulfide (Na2S4). Using this approach, we doubled the capacity to 134 Ah/L by using 10M total sulfur in the reversible cycling between Na2S2 and Na2S4. Long-term stability of this “precipitation-solution” conversion is studied using galvanostatic cycling for more than 1600 hours. In addition, XPS and SEM are utilized to measure the overall oxidation state of the precipitates and to understand the fast dissolution of the polysulfide hydrate. The overall oxidation state of the precipitates is particularly important in inferring the stability of the polysulfide species remained in the solution as the overall nominal solution composition needs to remain between Na2S2 to Na2S4 to avoid capacity fading associated with chemical decays. Furthermore, aqueous polysulfide using carbon suspension electrodes is demonstrated to alleviate large overpotential associated with insulating precipitate formation. This work was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. [1] Li et al., Joule 1, 306-327 (2017).