This invention is related to novel moderately solvating electrolytes (MSEs) that have moderate solubility of polysulfide species and are stable with metal anode and polysulfides formed from the sulfur cathode in metal-sulfur batteries such as lithium-sulfur (Li-S), sodium-sulfur (Na-S), magnesium-sulfur (Mg-S), aluminum-sulfur (Al-S) batteries, etc. These designed electrolytes generate high-quality solid electrolyte interphase (SEI) layers on both metal anode and sulfur cathodes, thereby improving the long-term cycling stability of the metal-sulfur batteries. Furthermore, the designed electrolytes effectively decrease the polysulfide solubility in the electrolyte and suppress the polysulfide shuttle in the metal-sulfur batteries. As a result, these electrolytes significantly improve the Coulombic efficiency (CE) and decrease the self-discharge of metal-sulfur batteries during storage. This invention could be widely applied to a variety of electrochemical systems, including Li-S, Na-S, Mg-S and Al-S batteries. It is also applicable to regular Li metal batteries, Li-ion batteries, Li-O2 batteries, sodium metal and sodium ion batteries, magnesium ion batteries, supercapacitors, and sensors.