Insight Into Graphene Modified Cathode Materials For Lithium Ion Batteries

As two kinds of outstanding anode materials for lithium ion batteries, the developments of Li FePO4 and LiMn2O4 were limited by the low electronic conductivity, poor ionic conductivity and capacity deterioration in process of electrochemical reactions. The cathode materials of LiFePO4 and LiMn2O4 can be modified by graphene which possesses good electrical conductivity, mechanical properties and high chemical stability, to improve its electrical conductivity. The prepared graphene coated on LiMn2O4 or LiFePO4 as a protective layer can buffer the materials volume expansion, improve its cycling stability and reduce the capacity deterioration.In this paper, we studied the synthesized methods of canthode materials modified with graphene. One is doping the active material directly with graphene, another is in situ grown graphene by catalyst-assisted chemical vapor deposition(CVD). Then carbon source, the growth substrate, the choice of carrier gas and temperature control, etc.) were chosen as the variable subjects to investigate the influences on preparing canthode materials. The crystal structure and surface morphology of modified materials were characterized by X-ray diffraction(XRD), scanning electron microscope(SEM), elemental analyzer(EA), transmission electron microscope(TEM) and Raman spectra. Based on the aforementioned research, we also optimized the preparation conditions. The reaction temperature is 800 ℃, the C6H6 dosage is 0.2 ml and the mass percent of catalyst A is 0.1%, which is the optimizing condition of LiMn2O4/G prepared in situ grown graphene by catalyst-assisted chemical vapor deposition(CVD). The testing LiMn2O4/G composites shows the maximum the electrical conductivity of 4.65×10-4 s/cm which is twenty times higher than the pure LiMn2O4 of 2.48×10-5 s/cm.The electrochemical properties of LiFePO4/G composites was test. When the reaction temperature is 800 ℃, the C6H6 dosage is 0.2 ml and the mass percent of catalyst A is 0.1%, the resulting LiFePO4/G composites performs the maximum the electrical conductivity of 0.93 s/cm which is four orders of magnitude higher than the pure LiFePO4 of 4.70×10-4 s/cm. And at the current density of 0.1C, the modified canthode showed a discharge capacity of 153.3 mA·h/g which is higher than pure LiFePO4 of 133.9 mA·h/g. We found that the coated graphene can improve the cycle stability of the LiFePO4 cathode material. After 120 consecutive cycles, the specific discharge capacitance retention of Li FePO4/G composites still maintained 91.1% of its initial capacitance, while pristine LiFePO4 only kept 77.2% of its initial specific discharge capacitance. At different current density(0.1 C, 0.2 C, 0.5 C, 1 C, 5 C), the specific discharge capacity retention of LiFePO4/G composites performs better than the pristine LiFePO4. The impedance pectra of LiFePO4/G composites were obtained by applying a sine wave with an amplitude of 0.5 mV over the frequency range from 100 kHz to 0.01 Hz. Results show that the impedance of modified materials can reduce to 129 Ω, which is 47.60% of its initial value. Cyclic voltammetry measurements were performed on an electrochemical workstation at a scan rate of 0.1 mVs-1 over the potential range of 2.5 V~4.2 V(vs. Li+/Li). The PD( potential difference) can fall from 0.51 V to 0.38 V and polarization can be reduced. In conclusion, the in situ prepared graphene-coating by chemical vapor deposition(CVD) can reduce the resistance and polarization of the cathode material LiFePO4, as well as buffer the volume expansion to improve its cycling stability.