An electrolyte for a lithium-containing battery cell is described. The electrolyte includes a solvent having at least one carbonate ester, and at least one lithium salt having a concentration ranging from 3 mol/liter to 15 mol/liter in the solvent. The electrolyte also includes a diluent that includes an aromatic fluorocarbon. In some embodiments, the solution of the at least one lithium salt and the solvent is a supersaturated solution for at least some operating temperatures of the battery cell. Also described are lithium-containing battery cells that include a positive electrode, a negative electrode, and the electrolyte.