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 (Na₂S_<i>n</i> 4 ≤ <i>n</i> ≤ 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)