In this work, we synthesized a composite cathode material containing LiFePO4 and activated carbon (AC), which is abbreviated as LAC, by a solid-state reaction, and assembled a hybrid battery-capacitor LAC/Li4Ti5O12. The electrochemical performances of the hybrid battery-capacitor LAC/Li4Ti5O12 were characterized by cyclic voltammograms, constant current charge-discharge, rate charge-discharge, and cycle performance testing. The results show the hybrid battery-capacitor LAC/Li4Ti5O12 has advantages of the high rate capability of hybrid capacitor AC/Li4Ti5O12 and the high capacity of battery LiFePO4/Li4Ti5O12. It is also proven that the hybrid battery-capacitor LAC/Li4Ti5O12 is an energy storage device where the capacitor and the secondary battery coexist in one cell system.