Lithium-sulfur battery chemistry has garnered global attention as a promising next-generation energy storage technology due to its significantly higher theoretical capacity (450 Wh/kg) compared to lithium-ion (265 Wh/kg), and the fact that its elemental components are green, safe and abundant[1]. As opposed to lithium-ion, the cathode solution chemistry is rich, as elemental sulfur forms polysulfide chains during discharge which can transport and deposit on the metallic lithium anode during a dissolution-migration-deposition “shuttle” mechanism which in effect a) cause a constant internal shorting current proportional to the transport of polysulfides and b) cause a build-up of lithium- and sulfur-rich solid-electrolyte interphase (SEI) on the anode which irreversibly passivates the lithium metal anode. This effect must be supressed at all costs in conventional lithium-sulfur batteries, and is achieved by encouraging rapid precipitation of Li₂S salts by the use of low-donor number solvents for the electrolyte such as diglyme (DME) and dioxolane (DOL).

However, polysulfide chains (Li₂S_x, 3 ≤ x ≤ 8) have great potential as redox couples due to their stable, successive multistep redox behaviour and have been successfully demonstrated in hybrid redox-flow battery configurations[2], in particular enabled by lithium nitrate as an additive to the catholyte that forms a stable SEI on the lithium metal surface that greatly reduces the polysulfide deposition. The lithium-polysulfide redox flow battery in theory far outstrips current state of the art vanadium redox flow batteries due to the higher capacity density in the catholyte (50-150 Wh/L vs 30 Wh/L), and the energy dense lithium metal[2]. However, the solubility of polysulfides decrease with chain length and depth of discharge, and high polarity, high donor number solvents that can enable high polysulfide concentrations[3] are typically far more reactive towards lithium metal[4]. Moreover lithium nitrate have little effect as anode protectant in this class of solvents compared to low donor number, low polarity solvents such as DME and DOL, and the polysulfide reduction pathway is dependent on the stabilising property of the solvent[5].

In collaboration with our commercial partner StorTera under the Faraday Institute, we have developed novel techniques for catholyte analysis. We show the role of nitrate consumption rate on protection of the anode, and the relative corrosive rate of lithium in a high polarity, high donor number class solvent (DMSO) versus conventional low polarity, low donor number class solvent (DOL/DME). Further we explore avenues to protect metallic lithium in highly concentrated polysulfide catholyte that enables large-scale energy storage that surpasses lithium-ion and vanadium redox flow batteries for cost, safety, serviceability and environmental impact. Such factors will be key for commercial deployment, in particular suitable for developing countries where microgrids for remote communities rely on intermittent renewable power supply.

1. Zhang, G., Zhang, Z. W., Peng, H. J., Huang, J. Q. & Zhang, Q. A Toolbox for Lithium–Sulfur Battery Research: Methods and Protocols. Small Methods 1, 1–32 (2017).
2. Yang, Y., Zheng, G. & Cui, Y. A membrane-free lithium/polysulfide semi-liquid battery for large-scale energy storage. Energy Environ. Sci. 6, 1552–1558 (2013).
3. Pan, H. et al. On the Way Toward Understanding Solution Chemistry of Lithium Polysulfides for High Energy Li-S Redox Flow Batteries. Adv. Energy Mater. 5, (2015).
4. Gupta, A., Bhargav, A. & Manthiram, A. Highly Solvating Electrolytes for Lithium–Sulfur Batteries. Adv. Energy Mater. 9, 1–9 (2019).
5. Lu, Y. C., He, Q. & Gasteiger, H. A. Probing the lithium-sulfur redox reactions: A rotating-ring disk electrode study. J. Phys. Chem. C 118, 5733–5741 (2014).