Li2S/metal nanocomposites are synthesized via chemical conversion reactions, and their properties as cathode prelithiation additives have been investigated to offset the initial lithium loss in lithium-ion batteries. The Li2S/Co nanocomposite shows good stability, and delivers a high “donor” lithium-ion specific capacity of 670 mAh g−1 in the cutoff potential range of existing cathode materials in a carbonate-based electrolyte.