Secondary alkali-metal salts in supporting electrolytes for lithium polysulfide flow batteries

Abstract

Lithium polysulfide (LiPS) flow batteries are a promising next-generation technology for long-duration energy storage that offer several advantages over the more conventional lithium-sulfur configuration. Using liquid-phase polysulfides in a single-liquid flow battery configuration mitigates the polysulfide shuttle by separating most of the active material from the cell whilst enabling a higher concentration to be used, thus increasing volumetric energy density; however, performance remains limited by instability of the lithium metal anode. Alkali-metal additives have previously been shown to help regulate anode plating in lithium-metal batteries, and in the present work we investigate the effects of secondary alkali metal bis(trifluoromethylsulfonyl)imide (MTFSI; M = Na, K, Rb, Cs) additives on the electrochemical behaviour of LiPS electrolytes under static coin cell conditions. We have synthesised a full series of MTFSI salts and report the first solvent free crystal structure of RbTFSI, completing the family of unsolvated alkali metal TFSI complexes. Electrochemical characterisation (CV, GCD, EIS) reveals that NaTFSI and RbTFSI improve key performance metrics, including electrolyte conductivity, capacity retention, and coulombic efficiency, despite evidence of side reactions with polysulfides. SEM EDX analysis confirms the formation of alkali metal sulfide deposits at the electrodes, indicating that parasitic reactions dominate over the anticipated self healing electrostatic shield (SHES) mechanism. These results demonstrate that carefully selected secondary cations, particularly Na+ and Rb+, can beneficially modify LiPS battery behaviour despite concurrent side reactions, providing a route toward the rational design and optimisation of electrolyte formulations for next generation LiPS flow battery technologies.