Over-lithiated layered Li2+x Ru1−xCo x O3 (x = 0, 0.1, 0.2, and 0.5) cathodes were synthesized through a solid-state reaction, which exhibits a superlattice structure. We investigated the effects of aliovalent Co3+ doping with excess lithium on the structural and electrochemical properties of layered Li2RuO3 cathodes. X-ray diffraction suggests the formation of a solid-solution where Co3+ occupies the Ru4+ sites in the transition metal layer while maintaining the layered Li2RuO3 structure. The electrochemical results indicate an enhanced electrochemical lithium reversibility of the cathodes at high C-rates (1C) than the pristine Li2RuO3. In particular, the high-performance Li2.1Ru0.9Co0.1O3 cathode delivered an initial capacity of 250 mAh g−1 with an enhanced Li+ extraction of 0.66 mol (52% capacity retention) after 100 cycles at 1C compared to the 0.28 mol of Li+ extraction (21% capacity retention) associated with the pristine Li2RuO3. Low content of Co3+ could realize an enhanced reversible Li+ extraction that originated from the improved reversible anionic redox chemistry, cation ordering with superlattice structure, and facile charge transfer process. The role of Co3+ content on the lithium reversibility of over-lithiated layered cathodes, containing excess lithium were investigated.