Highly Reversible Room-Temperature Sulfur/Long-Chain Sodium Polysulfide Batteries.

In a room-temperature sodium-sulfur (RT Na-S) battery, the complicated reduction reaction of the sulfur cathode generally involves two main steps: (i) transformation of elemental sulfur into long-chain soluble sodium polysulfides (Na2Sn 4 ≤ n ≤ 8) and (ii) conversion of the long-chain sodium polysulfides into solid-state short-chain polysulfide Na2S2 or disulfide Na2S. It is found that the slow kinetics of the second step limits the efficiency of discharge and induces irreversible capacity loss during cycling. Accordingly, we present here a RT Na-S cell operated with the sulfur/long-chain sodium polysulfide redox couple to avoid the capacity fade. An advanced cathode structure has been developed by inserting a carbon nanofoam interlayer between the sulfur cathode and the separator to localize the soluble polysulfide species and prevent its migration to the anode. The highly reversible sulfur/long-chain sodium polysulfide cell presented here can provide a stable output energy density of 450 Wh kg(-1) at an extremely low energy cost of ∼$10 kWh(-1) (based on the active material of anode and cathode).