High-energy and inexpensive rechargeable battery systems based on earth-abundant materials are important for both mobile and stationary energy storage technologies. Rechargeable metal-sulfur batteries (Li-S or Na-S) that can 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 metal-sulfur batteries has been fraught with multiple stubborn problems ranging from unstable electrodeposition of metal anodes during battery recharge to rapid loss of the active cathode material by dissolution into the electrolyte. In the studies undertaken in this thesis, I aim to develop design principles for room temperature metal-S batteries that use a metal anode, a carbon-sulfur composite cathode, and a liquid electrolyte containing a functional ionic liquid/polymer as a deposition stabilizer. My work shows that the metal sulfur cells, particularly for sodium-sulfur (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, electrochemical analyses and in-situ visualization, my work demonstrates 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 additives spontaneously form a Na-ion conductive film on the anode or increase the viscosity of the electrolyte without sacrificing its conductivity. This combination of features appears to stabilize deposition of sodium by reducing the electric field near the electrode and eliminate unstable electroconvection. Second, on the cathode side, carbon materials play a key role that can constrain the electrochemical reaction between sodium ion and sulfur to the solid state, without formation of the intermediate soluble sodium polysulfide species. This combination of the 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 metal anodes to enable stable long-term cycling of the Na-S electrochemical cells.