A series of polar and aprotic organic molecules, which, when used as solvents or additives in nonaqueous electrolytes, afford improved performance for electrochemical cells that operate at high voltages. These polar and aprotic solvents or additives may contain at least one unsaturated functionality per molecule. The unsaturated functionality is conjugated with the polar functionality of the molecule. The unsaturated functionality that is either a double or triple bond could be between carbon-carbon, or between carbon-heteroatom, or between hetroatom-heteroatom. Nonaqueous electrolyte solutions are provided comprising one or more lithium salts dissolved in the mixture solvents, which comprises, in all possible ratios, at least one of the polar, aprotic and unsaturated solvent or additives, one or more cyclic carbonic diesters such as ethylene carbonate, and one or more acyclic carbonic diesters such as dimethyl carbonate, diethyl carbonate, and ethylmethyl carbonate.