Energy storage systems are crucial in the deployment of renewable energies. As one of the most promising solutions, redox flow batteries (RFBs) are still hindered for practical applications by low energy density, high cost, and environmental concerns. To breakthrough the fundamental solubility limit that restricts boosting energy density of the cell, we here demonstrate a new RFB system employing polysulfide and high concentrated ferricyanide (up to 1.6 M) species as reactants. The RFB cell exhibits high cell performances with capacity retention of 96.9% after 1,500 cycles and low reactant cost of $32.47/kWh. Moreover, neutral aqueous electrolytes are environmentally benign and cost-effective. A cell stack is assembled and exhibits low capacity fade rate of 0.021% per cycle over 642 charging-discharging steps (spans 60 days). This neutral polysulfide/ferricyanide RFB technology with high safety, long-duration, low cost, and feasibility of scale-up is an innovative design for storing massive energy.