The Investigation Of LiFePO₄/Graphene Composites

As a promising cathode material for lithium ion battery, LiFePO₄has recentlysparked wide interests in the experimental scientific community given its relatively lowcost, environmentally friendly, high safety and good cycle ability, etc. However, theelectrochemical performance of LiFePO₄deteriorates due to the low intrinsic electronicconductivity and the poor lithium ions diffusivity, especially in high current densitysituations, which brings big obstacles to the application in high-rate battery. In thispaper, reduced graphene oxide （RGO）, as a new type of carbon source for thecarbon-coating, was synthesized using a green and effective method. LiFePO₄/RGOcomposite was prepared with mechanically mixing using graphene as carbon source.One step, in-site graphene-coating method with graphene oxide as carbon source wasdeveloped to prepare the LiFePO₄/RGO composite.1. The reductive ability and mechanism of using Fe（OH）₂to synthesize RGO wasinvestigated. The graphene was synthesized by reducing the graphene oxide （GO） usingFe（OH）₂at ambient temperature under the flowing nitrogen. The RGO was confirmedby X-ray diffraction, scanning electron microscopy, Raman spectroscopy, atomic forcemicroscopy and thermo-gravimetric analysis. The results of characterization showed thenumber of epoxide, hydroxyl, carbonyl and carboxyl groups which were located on theedge or surface of graphene oxide sheets were significantly decreased after beingreduced by Fe（OH）₂. Moreover, the thermal stability of the composite was improved.Compared to the traditional reducing agents such as hydrazine, NaBH4or hydroquinone,exfoliated GO was successfully reduced by Fe（OH）₂with the advantages ofenvironmentally friendly, inexpensive, short processing time, etc.2. RGO was used as conductive additives to prepare LiFePO₄/RGO composite.The RGO suspension was prepared by sonication, the RGO nanosheet was wellexfoliated with the SEM and AFM characterization. LiFePO₄/RGO composite wasobtained by mechanically mixing the LiFePO₄and RGO powers, the results ofmeasurement showed the introduced RGO does not destroy the structure of LiFePO₄,and the RGO conductive additives disperse well among LiFePO₄particles which improve the reversible capacity and good rate performance of the composite.3. GO was used as carbon source to synthesis LiFePO₄/RGO composite with anin-site carbon-coating method. The GO aqueous, ethanol and DMF suspension wasobtained by sonication of graphite oxide; the GO nanosheets were well exfoliated in theultrapure water, ethanol and DMF with the SEM and AFM characterization.LiFePO₄/RGO composite was synthesized by one step solid-state reaction in thereductive atmosphere. In this process, the GO was reduced and realize the in site carboncoated LiFePO₄particles and the obtained RGO formed3D conductive network. Theelectrochemical measurement showed the special discharge capacity of LiFePO₄/RGOcomposite was162and94.8mAh/g at0.2C and10C, respectively. The in-sitecarbon-coating method with GO have a promising application future.