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Composites of LiFePO4 and activated carbon (LFP-AC) were prepared by a ball milling method. Their morphologies were investigated by scanning electronic microscopy and energy dispersive spectroscopy. Hybrid supercapacitors were assembled using the LFP-AC composites as the positive electrode and Li4Ti5O12 as the negative electrode (LFP-AC/Li4Ti5O12). Effects of the positive electrode composition (ratio of LiFePO4 to AC) on the performance of LFP-AC/Li4Ti5O12 were investigated via constant current charge-discharge tests. The results demonstrated that the specific capacity and the specific power of the LFP-AC/Li4Ti5O12 hybrid supercapacitor significantly depend upon the content of LiFePO4 in the LFP-AC positive electrode. At 0.5 A g-1 charge-discharge current, the specific capacity of the LFP-AC/Li4Ti5O12 with 30 wt. % LiFePO4 and 70 wt. % AC in the positive electrode is 42% higher than that of AC/Li4Ti5O12, and the two supercapacitors have the same specific power. After 500 charge-discharge cycles at 1.0 A g-1, the specific capacity of the LFP-AC/Li4Ti5O12 with 80 wt. % LiFePO4 and 20 wt. % AC in the positive electrode is 53% higher than that of AC/Li4Ti5O12. The hybrid device stores electric energy via both the double-layer electrostatic adsorption and the intercalation/deintercalation of lithium ions. [ABSTRACT FROM AUTHOR]