Organic carbonyl compounds show potential as cathode materials for lithium-ion batteries (LIBs) but the limited capacities (<600 mA h g-1 ) and high solubility in electrolyte restrict their further applications. Herein we report the synthesis and application of cyclohexanehexone (C6 O6 ), which exhibits an ultrahigh capacity of 902 mA h g-1 with an average voltage of 1.7 V at 20 mA g-1 in LIBs (corresponding to a high energy density of 1533 Wh kg-1 ). A preliminary cycling test shows that C6 O6 displays a capacity retention of 82 % after 100 cycles at 50 mA g-1 because of the limited solubility in high-polarity ionic liquid electrolyte. Furthermore, the combination of DFT calculations and experimental techniques, such as Raman and IR spectroscopy, demonstrates the electrochemical active C=O groups during discharge and charge processes.