| Recently, lithium iron phosphate cathode material has been paid more attention due to the advantages of rich resources, cheap price, thermal stability, excellent cycling performance, environmental compatibility as well as good security performance. But lithium iron phosphate suffers from poor electric conductivity, lower chemical diffusion coefficient and bad tap density, which are large obstacles to restrain its application, specific capacity and capability during charge and discharge process. Through adding carbon in the raw material during the synthesis process, doping ion, optimizing the synthetic process and preparation of nanometer particles are effective methods of to improve their electrochemical performance of LiFePO4. On the basis of previous studies, this paper carried out the following job:1.The LiFePO4/C composite materials were synthesized by solid-state reaction using Iron(Ⅱ) oxalate dehydrate as iron source and adding acetylene black, porous carbon, graphite, glucose, sucrose, starch, polyvinyl alcohol, phenol resin and epoxy resin as the carbon sources to discuss the influence of combination carbon way on conductive mechanism of lithium iron phosphate. During the nine carbon sources, the LiFePO4/C composite material prepared by paralysis of polyvinyl alcohol obtained high electric conductivity value with1.88×10-1S·cm-1and has the best performances of discharge capacity, capability as well as cycling properties. It may relate with transferring channels provide for electronic transport during in charging and discharging process because of the carbon network is face package structure in lithium iron phosphate.2. Cathode material lithium iron phosphate were synthesized by carbon reduction method using spherical ferric phosphate dehydrate as iron source and the lithium source synthesis temperature and carbon content on the influence of lithium iron phosphate properties are studied.The morphology, electronic conductivity, tapping density and charge discharge capacity has more influence by lithium sources. The samples prepared by lithium hydroxide has the good rate performance with the discharge capacity of142.56,122.39,65.56,84.50mAh·g-1at the current of C/20, C/10, C/5and1.0C rate. The synthesis temperature greatly influenced on the microstructure of samples and has little effect on the electric conductivity, thus the sample’s charge and discharge characteristics significantly affects by microstructure and electronic conductivity. The650degree centigrade was the reasonable temperature.The value of electric conductivity about lithium iron phosphate is not proportional relations with the content of carbon. An excess of adding of carbon will not always benefit for the electrochemical of samples and the suitable amount of addition was the five percentages in weight.3. A series of doping lithium iron phosphate samples were synthesized by magnesium, titanium and chromium as the doping elements. Lithium iron phosphate and its doping compounds were studied by the first principle calculation. The results indicate that the electric conductivity of the material was improved significantly with the order of magnitude in10-4S·cm-1. Doping improves the tapping density and discharging rate capability of the materials. The sample doped0.02atom titanium per mole Fe sites has the best electrochemical properties especially the rate capability of which the discharging capacity was98.74mAh·g-1at the current of1.0C rate and83.08mAh·g-1at the current of3.0C rate. Doping reduces energy band gap of lithium iron phosphate and enhances the electric conductivity of the materials by analysis of the electronic structure of the sample. The high valence ion doping has certain degree boost the ion conductivity.4. The nanometer scale lithium iron phosphate cathode material was synthesized modifying by carbon coating and titanium doping with FePO4/PANI precursor as raw materials. Synthesizing nanometer materials shortens the ion diffusion pathway during solid phase reaction, has the advantage of doping ion enters the lattice of lithium iron phosphate and increases the even distribution probability of doping element. The nanometer LiFePO4/C composite doped by titanium has little effect on the electric conductivity, but in certain degree increased the ion diffusion velocity. Sample LiFeo.96Tio.o2P04/C abtained the excellent cabon coating struture and the nanometer particle size uniform with high crystalline degree and tiny grain size. The electrochemical properties has been increased significantly and has the outstanding platform performance and rate capability, with the discharging capacity159.02,154.43,148.49,136.88and121.95mAh·g-1at the current of C/10,C/2,1.0C3.0C and5.0C rate, respectively. |
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