A novel iron-polysulfide based redox flow battery system (Fe-S) is developed for energy storage applications. This system employs alkali metal ferricyanide/ferrocyanide and alkali metal polysulfide as the redox electrolytes. When proper electrodes, such as pretreated graphite felts, are used, over 75% energy efficiency and at least 93% charge efficiency can be retained with good stability over 50 charge-discharge cycles. The remarkable advantages of this system over the current state-of-the-art redox flow batteries include: 1) very low capital cost of materials (about $70/kWh for redox electrolytes); 2) less corrosive redox solutions used and relatively environmentally benign; 3) excellent energy and utilization efficiencies.