(LiFePO₄-AC)/Li₄Ti₅O₁₂ hybrid supercapacitor: The effect of LiFePO₄ content on its performance

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.