Research On Modification And Properties Of LiFePO₄-based Composites As Cathode Materials For Lithium-ion Batteries

Olivine-structured LiFeP04 has been subject to extensive study as a positive electrode material for a new generation of Li-ion batteries because of its low toxicity,high theoretical capacity,long cycle ability,etc.A novel LiFePO4/graphene/carbon composite as a performance-improved cathode material for lithium-ion batteries had been prepared by using an in-situ solvothermal method to synthesize LiFeP04/graphene powders as precursors and then followed by a carbon coating process.In addition,a facile approach to the fabrication of LiFeP04 composite by synthesizing NH4FeP04 as precursors and then followed by an ion exchange technique was explored.The graphene nanosheets(GR)we used were synthesized by a chemical method.Firstly,graphite oxide(GO)was synthesized from natural graphite using Hummers method.Afterwards,the prepared GO was exfoliated by thermal expansion and ultrasonication to form graphene oxide.Hydrazine hydrate was added as a reducing agent to get graphene.The structure and morphology were characterized by X-ray diffraction(XRD),infrared(IR)and electron microscopy(TEM).The results demonstrated that the graphene nanosheets with high quality could be obtained.The prepared samples of LiFeP04/graphene/carbon(2 wt%graphene and 6 wt%carbon)was investigated comparatively with LiFeP04/carbon(8 wt%)and LiFeP04/graphene(2 wt%)composites by X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM)and various electrochemical testing techniques.The results indicated that the co-modification of LiFeP04 with graphene and carbon coating could construct an effective conducting network,which significantly enhanced the electrochemical activity of LiFeP04/carbon based composite.Particularly,the LiFeP04/graphene/carbon composite with a low content of graphene exhibited a high initial discharge capacity of 163.7 mAh·g-1 at 0.1 C and 114 mAh·g-1 at 5 C,as well as an excellent cycling stability.In addition,AC impedance analysis and cyclic voltammetry test indicated that the LiFeP04/graphene/carbon composite had a lower charge transfer resistance at the interface and a better reversible than other samples when Li+ insertion/extraction in it.The prepared sample of LiFeP04/C composite synthesized by NH4FeP04 as precursors was investigated by X-ray diffraction(XRD),scanning electron microscopy(SEM)and various electrochemical testing techniques.The results demonstrated that an olivine-type LiFePO4 could be prepared by this facile synthetic route.The LiFePO4/C composite exhibited an initial discharge capacity of 80.7 mAh·g-1 at 0.1 C.Therefore,an attractive technology prospect for lithium secondary batteries was proposed by the further modification on above synthesis route.