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 decamethyl ferrocene 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, safe, MWh scale Li-S energy storage.