A Low-Crossover and Fast-Kinetics Thiolate Negolyte for Aqueous Redox Flow Batteries

Aqueous redox flow batteries (ARFBs) are a promising technology for large-scale energy storage. Developing high-capacity and long-cycle negolyte materials is one of major challenges for practical ARFBs. Inorganic polysulfide is promising for ARFBs owing to its low cost and high solubility. However, it suffers from severe crossover resulting in low coulombic efficiency and limited lifespan. Organosulfides are more resistant to crossover than polysulfides owing to their bulky structures, but they suffer from slow reaction kinetics. Herein, we report a thiolate negolyte prepared by an exchange reaction between a polysulfide and an organosulfide, preserving low crossover rate of the organosulfide and high reaction kinetics of the polysulfide. The thiolate denoted as 2-hydroxyethyl disulfide+potassium polysulfide (HEDS+K 2 S 2 ) shows reduced crossover rate than K 2 S 2 , faster reaction kinetics than HEDS, and longer lifespan than both HEDS and K 2 S 2 . The 1.5 M HEDS+1.5 M K 2 S 2 static cell demonstrated 96 Ah L -1 negolyte over 100 and 200 cycles with a high coulombic efficiency of 99.2% and 99.6% at 15 and 25 mA cm -2 , respectively. The 0.5 M HEDS+0.5 M K 2 S 2 flow cell delivered a stable and high capacity of 30.7 Ah L -1 negolyte over 400 cycles (691 h) at 20 mA cm -2 . This study presents an effective strategy to enable low-crossover and fast-kinetics sulfur-based negolytes for advanced ARFBs.