there are disclosed electrolytes comprising solutions of lithium salts with large anions in polar aprotic solvents with a particular concentration of background salts . the concentration of the background salts is selected to be equal or close to the concentration of a saturated solution of these salts in the aprotic solvents used . the electrolytes disclosed can be used in chemical sources of electric energy such as secondary cells and batteries comprising sulphur - based positive active materials . the use of such electrolytes increases cycling efficiency and cycle life of the cells and batteries .