The synergy effect on Li storage of LiFePO₄ with activated carbon modifications

In this work, composite electrodes containing lithium iron phosphate (LiFePO₄) and activated carbon (AC) were prepared by physically mixing LiFePO₄ and AC with polyvinylidene fluoride (PVDF) as a binder and acetylene black (AB) as an electrically conductive agent. X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), nitrogen sorption, four-probe conductivity and vibrating densitometer techniques were employed to characterize samples. The characterization results showed that the presence of AC increased the electrical conductivity, reduced the tap density, and modified the porosity of the resultant composite electrode materials. Electrochemical data demonstrated that the composite electrode displayed a significantly improved electrochemical performance in comparison with the pure LiFePO₄ electrode. An electrode with 5 wt% AC exhibited specific discharge capacities of 70 mA h g^-1 at 20 C and 100 mA h g^-1 at 10 C without significant capacity decay after 400 cycles. Galvanostatic charge-discharge and cyclic voltammetry results revealed that energy was stored via both charge adsorption and lithium intercalation/deintercalation owing to the presence of both AC and LiFePO₄ in the composite electrode. Electrochemical impedance spectroscopy (EIS) was used to investigate the charge-discharge kinetics and mechanism of the composite electrode. The EIS results demonstrated that the two different active materials (LiFePO₄ and AC) displayed synergy in terms of both material structure and energy storage, contributing to the observed excellent electrochemical performance.