(719g) Design Stable Room-Temperature Metal-Sulfur Batteries

High-energy and inexpensive rechargeable battery systems based on earth-abundant materials are important for both mobile and stationary energy storage technologies. Rechargeable sodium-sulfur (Na-S) batteries that are able to operate stably at room temperature are among the most sought-after of these platforms because these cells take advantage of a two-electron-redox process to yield high storage capacity from inexpensive electrode materials. Realization of practical Na-S batteries has been fraught with multiple stubborn problems ranging from unstable electrodeposition of sodium during battery recharge to rapid loss of the active cathode material by dissolution into the electrolyte. In this study, we develop a room temperature Na-S battery that uses a sodium metal anode, a microporous carbon-sulfur composite cathode, and a liquid electrolyte containing a functional imidazolium-based ionic liquid as a deposition stabilizer. We show that the Na-S cells with this configuration can cycle stably for over 100 cycles at 0.5C (1C = 1675 mAh/g) with 600 mAh/g reversible capacity and nearly 100 percent Coulombic efficiency. By means of spectroscopic and electrochemical analysis, we find that the high stability and reversibility of the cells stem from at least two sources related both to the cathode and anode. First, the functional ionic liquid spontaneously forms a Na-ion conductive film on the anode, which appears to stabilize deposition of sodium by reducing the electric field near the electrode and prevent electrolyte decomposition and depletion. Second, on the cathode side, microporous carbon materials play a key role that can constrain the electrochemical reaction between sodium ion and sulfur to the solid state without the formation of the intermediate soluble sodium polysulfide species. This combination of electrolyte and carbon substrate are shown to provide sufficiently strong association of sulfur in the cathode and at the same time stabilize the surface of the highly reactive sodium metal anode.