Fast cycling of “anode-less”, redox-mediated Li-S flow batteries

Redox flow batteries (RFBs) that incorporate solid energy-storing materials are attractive for high-capacity grid-scale energy storage due to their markedly higher theoretical energy densities compared to their fully liquid counterparts. However, this promise of higher energy density comes at the expense of rate capability. In this work we exploit a ZnO nanorod-decorated Ni foam scaffold to create a high surface area Li metal anode capable of rates up to 10 mA cm⁻², a 10× improvement over traditional planar designs. The ZnO nanorods enhance Li metal wettability and promote uniform Li nucleation, allowing the RFB to be initially operated with a prelithiated (charged) anode, or with a safety-conscious, Li-less, fully discharged anode. 5 mg_S cm⁻¹ were cycled using a mediated S cathode, whereby redox mediators help oxidize and reduce solid S particles. At 2.4 mg_S cm⁻² and 10 mA cm⁻², the RFB becomes limited by the mediation of solid S. Nevertheless, a respectable energy density of 20.3 Wh L⁻¹ is demonstrated, allowing considerable increase if the S mediation rate can be further improved. Lessons learned here may be broadly applied to RFBs with alkali metal anodes, offering an avenue for safe, dense, grid-scale energy storage.