Electrochemical Properties Of Lithium Iron Phosphate / Graphene Cathode Materials

In this dissertation, LiFePO₄/graphene（GR） used as cathode materials for lithium-ion battery were synthesized by hydrothermal method, and the effects of graphite oxide and reductant on microstructure and electrochemical properties of LiFePO₄/GR electrode were also investigated. The microstructure and morphology of the cathode materials were characterized by Fourier-transform infrared（IR） spectroscopy, X-ray diffraction（XRD）, scanning electron microscope（SEM）, energy dispersive spectroscopy（EDS） and transmission electron microscopy（TEM）. Electrochemical properties were tested by galvanostatic charge/discharge, electrochemical impedance spectroscopy（EIS） and cyclic voltammetry（CV）.The XRD analysis results showed that LiFePO₄/GR cathode materials, prepared through a hydrothermal LiFePO₄ system using graphene oxide（GO） as starting material and L-ascorbic acid（LAA） as an reductant, had an olivine-type crystal structure（space group pnmb, No.62）, indicating that the formation of reduced graphene did not change the crystal structure of LiFePO₄ electrode material. However, the addition of LAA as an reductant could affect the crystal cell parameters and cell size, leading to a decrease of cell parameter b value. A minimum value of b was achieved（0.5889 nm） with a LAA:GO ratio of 1:1, which is obviously smaller than that of the sample without LAA（0.6009 nm）. The SEM and TEM measurements suggested that synchronized preparation of graphene in the synthesis process resulting in a smaller particle size of LiFePO₄ electrode material from 1 μm to 100 nm.Electrochemical properties tests showed that the formation of reduced graphene could obviously improve the discharge capacity and cycle stability of LiFePO₄/GR cathode materials. The maximum discharge capacity of 145.5 mAh-g^-1, 161.7 mAh-g^-1 and 151.0 mAh-g^-1 were achieved with LAA:GO ratios of 2:1, 5:1 and 7:1, respectively. All these values were larger than that of the without LAA(116.4 mAh-g^-1). It is noteworthy that the maximum discharge capacity of LiFePO₄/GR（5:1） is about 1.4 times of the undoped sample and the discharge capacity as well as capacity retention were 160.6 mAh-g^-1 and 99.3% after 30 cycles. EIS results indicated that the formation of reduced graphene could decrease the value of charge transfer resistance（Rct） of electrochemical reaction on the electrode surface from 587.9 ? to 102.6 ?, leading to a significant improvement of the performance of electrochemical reaction kinetics.