Molybdenum (Mo)-based transition metal oxides normally delivery superior energy density in lithium ion batteries through electrochemical conversion reactions. The main challenge for this kind of electrode material is to promote reversible transformation of Li₂O and alleviate the volume expansion during lithiation/delithiation processes. Here, electrochemical reaction mechanism of CoMoO₄ investigated by in-situ X-ray diffraction and ex-situ Transmission electron microscopy reveals the irreversible transformation from CoMoO₄ to CoO and MoO₃ in the first cycle. During subsequent charge/discharge process, high valence state cobalt ions originated from Co²⁺ provide additional capacity and promote the electrochemical reversible reaction between MoO₃ and Li₂O, thus facilitating the release of irreversible capacity. Benefited by the contributor of high valence state cobalt ion, when CoMoO₄ was utilized as Li-storage anode, high reversible capacity (1585.5 mAh g⁻¹ at 200 mA h⁻¹ after 150 cycles) and excellent cyclic stability (~100% retention after 400 cycles at 1000 mA g⁻¹) are delivered. Such an optimized anode material expands the potential to develop lithium-ion batteries with significantly higher energy density.