High-voltage anode-free sodium-sulfur batteries

Room-temperature sodium-sulfur (Na-S) batteries offer a sustainable energy storage solution to conventional lithium (Li)-based systems^1-3, owing to the high element abundances and theoretical electrochemical performance^4,5. However, their practical applications have been severely hindered by the low discharge voltages and the need for largely excessive Na metal anode^6-8. Here we report a 3.6 V class Na-S battery featuring a high-valence sulfur/sulfur tetrachloride (S/SCl₄) cathode chemistry and anode-free configuration. We show that sodium dicyanamide (NaDCA) can simultaneously unlock reversible S/SCl₄ conversion and Na plating/stripping in a non-flammable chloroaluminate electrolyte. This design enables the maximum energy and power densities of 1,198 Wh kg^-1 and 23,773 W kg^-1, respectively, calculated on the basis of the total electrode mass including both the cathode and the anode. Also, we demonstrate facilitated S/SCl₄ conversion by incorporating a bismuth-coordinated covalent organic framework (Bi-COF) catalyst (8 wt% loading) into the S cathode, which realizes an impressive discharge capacity of 1,206 mAh g_{(sulfur+catalyst)}^-1, contributing to a maximum energy density of 2,021 Wh kg^-1 calculated on the basis of the total electrode mass. With an estimated cost of US$5.03 per kWh and excellent scalability, our anode-free Na-S battery shows promise in grid energy storage and wearable electronics.