The present invention relates to an improvement in lithium secondary batteries comprised of a negative electrode, a positive electrode, a separator, and a lithium-based electrolyte carried in an aprotic solvent and to the electrolyte compositions. The improvement resides in the use of a lithium electrolyte of the formula: Li.sub.2B.sub.12F.sub.xZ.sub.12-x wherein x=>5 and Z represents H, Cl, and Br. Preferably, when x is less than 12, Z is Cl.