Vanadium solid-salt battery: Solid state with two redox couples

We present the “vanadium solid-salt battery” (VSSB), which has high energy density, is low cost, is easily recycled, operates at ambient temperature, and has no requirement for special solvents. The VSSB contains two types of vanadium solid salts that are supported on carbon felts with a minimal amount of hydrosulfuric acid added to moisten the ion-exchange membrane. The optimized VSSB shows a cell potential of 1.34 V, excellent reproducibility for charging and discharging for nearly 100 cycles, a high energy efficiency (87%) and a high energy density (77 W h kg⁻¹ at 5 mA cm⁻² using the carbon felt XF208). The energy density is enhanced by 250-350% compared with conventional vanadium redox-flow batteries.