Study Of Preparation Process And Electrochemical Performance Of Graphene Modified LiFePO₄ Cathode Materials

Lithium iron phosphate is a kind of lithium ion battery cathode material with good performance, because it’s green, high energy density, good cycle life, good safety performance, no environmental pollution. From the beginning the discovery has become a hot spot and attracts widespred attention.The article mainly talk about the produce of iron phosphate by precipitation and change the reaction condtions to find about the best optimal synthesis conditions. We prepared lithium iron phosphate from the iron phosphate by carbon thermal method, the product was carbon-coated hollow spherical shell state, by means of ultrasonic crushing material prepared, to get small particles of LiFePO4. Through salt balance method using the graphene oxide load LiFePO4 small particles and reducing graphene oxide to obtain LiFePO4/G composites. First use the graphene oxide load Ag nanoparticles and then load the small particles of LiFePO4 and reduction of graphene oxide to obtain LiFePO4/Ag/G composites. By XRD, particle size distribution apparatus, UV-VIS, SEM, TEM, electrochemical workstation, blue-electric cell test systems for battery and materials testing. Specific summarized as follows:1. Use FeCl3·6H2O and H3PO4 as raw materials, CTAB surfactant, synthesized by co-precipitation method target product precursor iron phosphate containing water of crystallization. By changing the reaction of phosphorus, iron feed ratio, reaction temperature, pH value, the amount of surfactant, ultrasonic time and other conditions. The optimal reaction conditions for phosphorus iron feed ratio of 1:1, the reaction temperature is 85℃, reaction pH of 2.5, a surfactant added in an amount of 4% by mass of raw iron salts, ultrasonic time 15min. By XRD,SEM,TEM, particle size distribution, phosphorus iron content ratio analysis, FePO4 crystal prepared at this time was perfect, the microstructure of thin block of about 400nm.2. Use Carbon thermal reduction method, FePO4 as precursors to prepare, add the Li2CO3 and glucose grind mixed. Under a nitrogen atmosphere,650℃ calcination to obtain a carbon-coated LiFePO4 material. The materials were characterized by XRD, crystalline material perfectly. By SEM, TEM observation of material microstructure, LiFePO4 material prepared by carbon thermal reduction process rendered hollow spherical shell structure composed of many small particles, beads coated thick layer of this toner layer, small ball size 3μm-5μm. By ultrasonic means, in the aqueous solvent state pellets sonication, and a thicker peel toner surface layer, get the size 400nm of LiFePO4 small particles.3. Use the Hummers method by using graphene oxide to obtain graphene oxide nanosheets. And by UV-VIS, TEM preparation characterize graphene oxide. Use salt balance method using the graphene oxide load LiFePO4 small particles and reducing graphene oxide to obtain LiFePO4/G composites. By SEM, TEM observation of graphene load LiFePO4 get small particles morphology. The LiFePO4/C material prepared by carbothermal reduction LiFePO4 beads, sonication LiFePO4 small particles treated graphene with simple physical mixing of materials are prepared LiFePO4 CR2032 button batteries. By electrochemical workstation, blue electrical test system to test their electrochemical properties. LiFePO4 material after sonication than not broken before the electrochemical performance is improved, LiFePO4/G material graphene loaded than uncoated after small particles of LiFePO4 material has improved. Charge-discharge curve material, charge-discharge cycle performance, cyclic voltammetry, AC impedance, high rate discharge performance have been improved.4. First through the graphene oxide load Ag nanoparticles then load LiFePO4 small particles and reducing graphene oxide to obtain LiFePO4/Ag/G composites. Microscopic morphology of composite materials by TEM, SEM characterization methods, Ag nanoparticles uniformly negative load in the graphene on the surface and filling in small gaps LiFePO4 particles. Through the material first discharge capacity, charge-discharge cycle performance test, the electrochemical properties of the material has been improved. Safety performance LiFePO4/Ag/G composites were tested by hot box test, overcharge experiment, external short circuit test material, material puncture experimental test found that does not appear battery leakage, fire or explosion, safety clearance.