Preparation Of Li-ion Batteries Cathode Material LiFePO₄Using H₃PO₄as Phosphorus Source And Its Properties Investigation

With the rapid development of human society, the global energy demand is growing.Over the past few decades, the mining of mineral resources, not only becomes seriouspollution to the survival of our environment, but also makes the human entered the era ofenergy crisis. So, developing new green energy has become an important scientific researchwork. As a kind of environmental and efficient secondary energy, lithium ion battery hasattracted people’s attention. LiFePO4batteries, because of its stable voltage platform, greenenvironmental protection, thermal stability, the organisms the advantages of low toxicity, wasthe most outstanding battery in the lithium ion battery. This thesis has developed a kind ofpreparation method of lithium ion batteries cathode material LiFePO4using H3PO4asphosphorus source and studied its properties. X-ray diffraction(XRD), scanning electronmicroscopy (SEM), transmission electron microscopy(TEM) and energy dispersivespectroscopy (EDS) were used to characterize the samples. Electrochemical tests were carriedout by cyclic voltammeters (CV), impedance (EIS) and charge-discharge techniques.The main experiments as follows:1. Different kinds of LiFePO4samples have been prepared by changing three major facts,including the proportion of Li element, the proportion of Fe element and the time ofcalcination. IR, XRD, SEM and charge-discharge tests were used to measure the obtainedsamples. The results showed that the best preparation conditions were the stoichiometric ratioof LiOH H2O, H3PO4, α-Fe2O3equal to1.1:1:0.5,700℃, calcination5hours.2. For the first time, the influence of starting materials including Fe2O3andFe(NO3)39H2O on the properties of LiFePO4samples were probed. Phosphorus source isH3PO4. XRD showed that the sample prepared from Fe2O3has better crystal shape than thesample prepared from Fe(NO3)39H2O. SEM and TEM showed that the LiFePO4particlesprepared from Fe2O3are found to be smaller and more uniformity than those prepared fromFe(NO3)39H2O. CV test showed that the sample prepared from Fe2O3has betterelectrochemical reversibility than the sample prepared from Fe(NO3)39H2O.Charge/discharge test showed that the sample prepared from Fe2O3has higher specificcapacity and better cycle performance. The first discharge specific capacity is157mAh/g at 0.1C.3. The sulfate group doped LiFePO4samples were fabricated to improve the propertiesof the sample using (NH3)2SO4as sulfate group source. The obtained samples werecharacterized and were compared with the pure LiFePO4. The results showed that doping ofsulfate radical has some influnce on the properties of the samples. LiFe(SO4)0.04(PO4)2.92/3sample has smaller microscopic particles and better electrochemical performance than thepure LiFePO4sample particles. The first discharge specific capacity ofLiFe(SO4)0.04(PO4)2.92/3is126.7mAh/g at0.5C, while the pure LiFePO4is106.4mAh/g. Theresults indicated that doping proper amount of sulfate groups can improve the properties ofLiFePO4.4. The LiFePO4samples have been prepared by the way of liquid boiling reflux-hightemperature calcination, in which CNTs and activated carbon powder were added. Thesamples micro appearance and crystal structure were studied by IR, XRD and SEM. Theirelectrochemical performance was studied by CV, EIS and charging-discharging test. The testresults showed that the sample having CNTs showed bar shape, while the sample added bycarbon powder displayed a spherical shape. The first discharge specific capacities at1C ofabove two samples are116.71mAh/g and95.69mAh/g, respectively. And they all haveshowed good cycle performance.