The initial lithium loss in lithium-ion batteries (LIBs) reduces their energy density (e.g., 15% or higher for LIBs using a Si-based anode). Herein, we report in situ chemical formation of a conformal Li2O/Co nanoshell (∼20 nm) on LiCoO2 particles as a high-capacity built-in prelithiation reagent to compensate this initial lithium loss. We show a 15 mAh g–1 increase in overall charge capacity for the LiCoO2 with 1.5 wt % Li2O/Co in comparison to the pristine LiCoO2 in virtue of the irreversible lithium extraction from the nanoshell (4Li2O + 3Co → 8Li+ + 8e– + Co3O4, 2Li2O → 4Li+ + 4e– + O2↑). Paired with a graphite–SiO anode, a full cell using such a LiCoO2 cathode demonstrates 11% higher discharge capacity (2.60 mAh cm–2) than that using pristine LiCoO2 (2.34 mAh cm–2) at 0.1 C, as well as stable battery cycling. Moreover, the prelithiated LiCoO2 is compatible with the current battery fabrication process.