Researches On Synthesis And Electrochemical Performance Of LiFePO₄/rGO Hybrid As Cathode For Lithium Ion Battery

LiFePO4has been extensively recognized as the cathode material for lithium ion batteries because of low cost, non-toxicity, and relatively high theoretical specific capacity (170mAh g-1). However, the practical application of LiFePO4(such as hybrid electric vehicles) was limited by its intrinsically poor electric conductivity and low Li+transport velocity. Graphene is a new type of carbon materials with high conductivity and large specific surface area, and it can be used to improve the electrochemical performance of LiFePO4. In this paper, based on summarizing the development of the lithium ion batteries and LiFePO4preparation method, LiFePO4/reduced graphene oxide (rGO) was synthesized by homogeneous co-precipitation method and high temperature solid-state method. Meanwhile, a new type cathode material of FePO4/rGO was also synthesized by hydrothermal method. The work was shown as follows:(1) Graphite oxide (GO) was prepared following a modified Hummers’method. By XRD, SEM, the result shows that the prepared GO has a typical morphology of agglomerated graphene layers. rGO was prepared by high temperature thermal reduction method. Raman spectroscopy indicates that rGO has a high ID/IG value (ID/IG=1.3), which means that rGO has a high degree of disorder in carbon arrangement. As cathode material for lithium ion battery, rGO delivered48mAh g-1capacity at the current density of0.06C in the voltage of2-3.8V.(2) LiFePO4/rGO hybrids with different rGO contents were synthesized by homogeneous co-precipitation method and high temperature solid-state method. Firstly, FePO4/GO was prepared by homogeneous co-precipitation method. By XRD, SEM, TEM, it was found that FePO4was amorphous with a particle size of20nm, and FePO4nanosphere particles were homogenously adhered to the surface of rGO sheets. Then, LiFePO4/rGO hybrids with different rGO contents of5%,7%,15%,25%were synthesized using FePO4/GO and LiOH·H2O by high temperature calcinations. They were characterized by XRD, SEM, TEM, XPS, Raman spectrum, CV, electrochemical method. The results show that LiFePO4particle with the size of100nm has an ordered olivine structure, and LiFePO4particles were closely adhered to the surface of rGO sheets through some oxygen-containing functional groups in rGO. The hybrid with15 %rGO delivered a first discharge capacity of172mAh g-1, and has an initial columbic efficiency of107.5%at0.06C in the voltage range of2-3.8V. Especially, at high density of11.8C, it can also deliver139mAh g-1specific capacity. After50cycles, LiFePO4/rGO hybird still remains a specific capacity of168mAh g-1, which has97.5%retention of the first discharge specific capacity. It was obvious that LiFePO4/rGO hybrid has good electrochemical performance and excellent cycling stability.(3) FePO4/rGO hybrid material was synthesized by hydrothermal method using GO,(NH4)2Fe(SO4)2-6H2O, H3PO4, sodium dodecyl sulfate(SDS) as raw materials. It was characterized by XRD, SEM, electrochemical testing. The result shows that the as-prepared FePO4/rGO nanosphere particles with size of150nm were uniformly dispersed in the rGO sheet. FePO4/rGO hybrid has175.7mAh g-1and184.3mAh g-1of the initial discharge and charge capacities at0.11C in the voltage range of2-4.2V.