A Mediated Li–S Flow Battery for Grid-Scale Energy Storage

Lithium–sulfur is a “beyond-Li-ion” battery chemistry attractive for its high energy density coupled with low-cost sulfur. Expanding to the MWh required for grid scale energy storage, however, requires a different approach for reasons of safety, scalability, and cost. Here we demonstrate the marriage of the redox-targeting scheme to the engineered Li solid electrolyte interphase (SEI), enabling a scalable, high efficiency, membrane-less Li–S redox flow battery. In this hybrid flow battery architecture, the Li anode is housed in the electrochemical cell, while the solid sulfur is safely kept in a separate catholyte reservoir and electrolyte is pumped over the sulfur and into the electrochemical cell. Electrochemically facile decamethylferrocene and cobaltocene are chosen as redox mediators to kick-start the initial reduction of solid S into soluble polysulfides and final reduction of polysulfides into solid Li2S, precluding the need for conductive carbons. On the anode side, a LiI and LiNO3 pretreatment strategy encourages a stable SEI and lessens capacity fade, avoiding use of ion-selective separators. Complementary materials characterization confirms the uniform distribution of LiI in the SEI, while SEM confirms the presence of lower surface area globular Li deposition and UV–vis corroborates evolution of the polysulfide species. Equivalent areal loadings of up to 50 mgS cm–2 (84 mAh cm–2) are demonstrated, with high capacity and voltage efficiency at 1–2 mgS cm–2 (973 mAh gS –1 and 81.3% VE in static cells and 1142 mAh gS –1 and 86.9% VE in flow cells). These results imply that the fundamental Li–S chemistry and SEI engineering strategies can be adapted to the hybrid redox flow battery architecture, obviating the need for ion-selective membranes or flowing carbon additives, and offering a potential pathway for inexpensive, scalable, and safe MWh scale Li–S energy storage.