An alkaline S/Fe redox flow battery endowed with high volumetric-capacity and long cycle-life

The S/Fe redox flow battery (RFB) with abundant sulfide and iron as redox-active species shows promising applications for energy storage. It exhibits advantages including low cost, high safety, and flexible operation. However, limited by the low solubility limit of [Fe(CN)6]4-, the volumetric capacity of reported S/Fe RFBs is too low to meet commercial requirements. Herein, an alkaline S/Fe RFB with high volumetric energy density and improved cycling stability enabled by the diverse-ion effect in the catholyte is successfully demonstrated. In a 0.50 M KOH solution, the concentration of [Fe(CN)6]4- achieves 1.52 M under ambient temperature which is increased by two times than the solubility limit of K4[Fe(CN)6] (0.76 M) in deionized water. The corresponding theoretical volumetric capacity can reach as high as 40.74 Ah L−1. With 1.30 M [Fe(CN)6]4- in the catholyte, the alkaline S/Fe RFB displays high performance, including a long cycle-life of 3153 h (over 4 months), high Coulombic efficiency (CE) above 99 %, and slow capacity decay of only 0.0166 % per cycle (0.1134 % per day). In addition, at higher temperatures and larger scales, the concentrated catholyte maintains excellent cycling performance, verifying the present strategy is highly reliable in improving the performance of a cost-effective S/Fe RFB system for practical applications.