| Lithium iron phosphate battery has been widely attention because of its high safety, stability and good cycle performance. In this paper, the LiFePO4/C composites were prepared by means of the solid state reaction, mechanical activation and carbothermal reduction methods. The influences of the ratio of the acetylene black and graphene conductive agents on electrical properties of the LiFePO4/C composites were studied. The effects of the addition of copper nano-particles into the cathode materials on the grain size and electrical properties of the LiFePO4/C composites were investigated. In order to find the best condition for preparing the cathode materials, the FePO4 precursors were produced by sol-gel method under different PH values, which were further used to prepare LiFePO4/C composites. The microstructures and the electrochemical properties of the LiFePO4/C materials were analysised by XRD, EPR, laser particle size analyzer, SEM, Mossbauer spectroscopy, electrochemical test system, cyclice voltammetry and AC impedance techniques. The effects of the microstructures on electrochemical properties of the LiFePO4/C materials were discussed. The main results are as follows:(1) The LiFePO4/C composites were prepared by means of solid state reaction, mechanical activation and carbothermal reduction methods with Fe2O3, C12H22O10, Li2CO3 as raw materials and keeping the ratio of Fe and C atoms as 1:1.2. The XRD result indicated that the structure of the LiFePO4/C composite was a typical olivine structure.(2) In analysing the effect of the content of graphene conductive agent on the initial specific capacity of the samples at 0.5 C, it could be found that the C3 sample, which was doped with 50% of graphene, had a longer discharge platform, and the charge and discharge voltage were 3.5 V and 3.4 V respectively. The initial discharge capacity of the cell, which was prepared by using C3 sample as its conductive agent, reached a value of 140 mAh.g-1, and it was higher than that of the one prepared by using acetylene black as ones conductive agent (126.6 mAh.g-1). Thus the initial discharge capacity of the cell can be improved by using C3 sample as its conductive agent. The remaining capacities of the C3 sample at the rate of 0.5 C and 5 C after 50 times charge-discharge cycles kept relatively high values of 118 mAh.g-1 and 86 mAh.g-1, respectively. That is, the electrochemical properties of the LiFePO4/C composites can be improved by using suitable content of graphene as its conductive agent.(3) The XRD results showed that there were Cu diffraction peaks appearing in the XRD patern of the LiFePO4/C cathode materials doped with copper nano particle as its content higher than 1.5%. And the intensities of Cu diffraction peaks increased with the content of copper nano particle in the LiFePO4/C cathode materials. There was no significant C diffraction peaks in the XRD patern of the LiFePO4/C cathode materials when the the content of copper nano particle lower than 3.0%.(4) The EPR results indicated that the intensity of the the EPR signal increased with the content of copper nano particles in the LiFePO4/C cathode materials. It might be due to some Cu atoms were oxidized to form CuO, and the Cu2+ions would enter into the lattice of LiFePO4 then substitute for Fe3+ ions during the process of sintering. There is an unpair electron in Cu2+ ion, the outmost electronic configuration of which is 3d, and gave rise to the EPR signal. The fact that the intensity of the the EPR signal increased with the content of copper nano particle in the LiFePO4/C cathode materials showed that the addition of copper nano particles into the cathode materials would lead to some Cu atoms were oxidized to form CuO, and reducing the Fe3+, for the oxidation of Fe3+ was stronger than that of Cu.(5) The results of Mossbauer spectra of LiCuxFe1-xPO4/C (x=0.0-3.0at%) cathode materials showed that the Mossbauer signal of Fe3+ decreased with the increasing of the content of copper nano particles in the composites:the content of Fe3+ in LiFePO4/C was quite high, that in LiCu0.015Fe0.985PO4/C was the next, while that in LiCu0.03Fe0.97PO4/C was relatively low. This showed that the Fe3+ in LiCuxFe1-xPO4/C cathode materials could be reduced by doping with copper nano particles, that is, Cu atoms played a role in reducing Fe3+. The results of Mossbauer spectra of LiCuxFe1-xPO4/C cathode materials were in agrrement with that of the EPR spectra.(6) The charge-discharge cyclic property of the-LiCu0.015Fe0.985PO4/C cathode material was quite good, after 50 charge and discharge cycles at the rate of 0.5 C, its discharge capacity was 131mAh.g-1 and its remaining specific capacity retention was 89%. At the rate of 5 C, the first discharge capacity of the LiCu0.015Fe0.985PO4/C cathode material was 81.3 mAh.g-1, after 50 charge and discharge of cycles the remaining capacity of it was 91.8 mAhg-1, and its remaining percent of specific capacity retention was 110.3%.(7) The LiFePO4/C composites prepared under the PH values of 3.5,4.0, 4.5 and 5.0 were refered as P1, P2, P3 and P4, respectively. The XRD results indicated that the structure of the P1, P2, P3 and P4 was a typical olivine structure. The results obtained by the laser particle size analyzer showed that the content of the nano-scale particles in the P2 sample was relatively high, while the contents of the nano-scale particles in the P1, P3 and P4 samples were quite low.(8) The platform of the charge and discharge of the P2 sample were quite long, and the charge and discharge voltage of the P2 sample are about 3.45 V and 3.4 V respectively. The initial charge and discharge capacity of the P2 sample were 147.2 mAh.g-1 and 139.2 mAh.g-1, respectively. The ranking from high to low of the discharge capacities of the samples at the rates of 0.5 C and 5 C, after 50 times charge and discharge cicles, was P2, P3, P1, P4. The charge and discharge capacities of the P2 sample after 50 times charge-discharge cicles were relatively high, and their value were 124.7 mAh.g-1 and 91.6 mAh.g-1, respectively; and their remaining specific capacity retentions were 89.5% and 94.3% respectively. Thus, in comparision with other samples, the electrochemical properties of the LiFePO4/C cathode material prepared under the PH value of 4.0 was relavetively good. |
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