Salt cavern redox flow batteries with a large salt-cavern storing electrolyte are a promising large-scale energy storage technology. Here a low-cost iron/organic redox material system was proposed to explore salt cavern redox flow batteries. To avoid the formation of ferric hydroxide, ligand threonine was used to enhance the stability of the ferric ion in aqueous solution. Paired with a two electron viologen catholyte and saturated brine solution as supporting electrolyte, the battery delivered a high battery efficiency and cycling stability, 99% CE, 80% EE, and an average 99.5% capacity retention per cycle. The impressive battery performance provides a promising strategy for developing large-scale salt-cavern redox flow batteries.