The Preparation And Properties Research Of Ion-doped LiFePO₄Battery Anode Materials

The Li-ion battery has conquered the portable electronic market, such as mobilephones, laptops, cameras and other portable electronic devices at present, the nextgeneration of electromobiles will provide more space for the Li-ion battery develop-ment. Olivine-structured LiFePO₄is considered as a promising cathode material forlithiμm-ion batteries due to its advantages such as abundant raw materials, environ-mentally friendliness, high capacity, good cycle performance and preferable safety.Using carbothermic reduction method to prepare LiFePO₄, was proposed in this paper.Besides, we also studied the doping modification through different ions. The crystallinestructure, morphology, carbon contents, specific surface area and electrochemicalperformances were investigated by XRD, SEM, specific surface area analysis meter andelectrochemical tests.The cathode material LiFePO₄was synthesized by carbothermic reductionreaction, using different iron sources, LiH₂PO₄and some carbon sources as rawmaterials. The influence of crystalline structure, morphology, specific surface area andelectrochemical performances by different iron and carbon sources were studied. Theresults demonstrated that pyrogenation as carbon source and ferric nitrate as iron sourcecould optimize the performance of LiFePO₄effectively. The material LiFePO₄whichwas synthesized in this condition had higher charge-discharge capacity, higher specificsurface area, more uniform particle size distribution. Then, the influence of electro-chemical performances by different heating time and different carbon contents wasresearched. The optimized synthesis conditions with sintering temperature of350℃,sintering time of2h, then700℃, for6h; and the amount of carbon in precursorcompound of10wt%were obtained. LiFePO₄, which was synthesized by theseconditions, showed that the initial discharge capacity was127.351mAh/g at0.1C rateand124.512mAh/g after20cycles, reduced2.2%, showing better cyclical performance.The preparation of LiFePO₄through vanadiμm doping. In this synthesis, using rawmaterials which were LiH₂PO₄, Fe（NO3）3, C6H8O7and NH₄VO₃prepared the materialsLiFe_1-xV_xPO₄/C （x=0.002、0.04、0.06）.The XRD analysis showed that the samples by doping V（V） were pure well-ordered olivine phase with no impurities. When thevanadiμm doping amount x=0.04, the materials LiFe_0.96V_0.04PO₄/C showed the specificsurface area was40.08m²/g, the initial discharge capacity was141.065mAh/g at0.1Crate, improved10.77%than no vanadiμm doping material（127.351mAh/g）, and140.183mAh/g after20cycles. The modification effect of aliovalent doping on Fe site wasdiscussed from the viewpoint of defect chemistry. The dopant substituting on the Fesites can create lithiμm vacancies in olivine LiFePO₄lattice by charge-compensationmechanism. The existences of lithiμm vacancy may expand Li（I） diffusion channels inthe structure, weak the bound of oxygen to lithiμm,and increase lithiμm mobility.The preparation of LiFePO₄through fluorine doping. Through studying theinfluence of different fluorine compound doping to the electrochemical performance ofLiFePO₄, we determined that using LiF doping prepared LiFePO₄composite material.We compared the carbothermic reduction reaction with mechanical ball milling methodcombining carbothermic reduction for the material performance. The resultsdemonstrated that the two methodes both could synthesis olivine-structured LiFePO₄,and the composite material had favouraable discharge performance, cycle performanceand uniform size distribution. When the F doping amount x=0.03, LiFe（PO₄）0.97F0.03/Cwhich were made by two methodes showed the specific surface area respectively were84.27m²/g and40.50m²/g, the initial discharge capacity wre153.278mAh/g and137.113mAh/g at0.1C rate, improved20.34%and7.66%than no vanadiμm dopingmaterial（127.351mAh/g）,151.512mAh/g and136.571mAh/g after16cycles. Researchresults showed that the electrochemical performance of LiFePO₄was improved throughfluorine doping, and the composite material that was made by the mechanical ballmilling method combining carbothermic reduction had better electrochemicalperformance than the material made by the carbothermic reduction reaction, thepotential difference of the （de）oxidation was samller, so were solution resistance（Re）and charge transfer resistance（Rct）.The preparation of LiFePO₄through metal cladding on surface. Adding a littleamount of alkali carbonate compound which containing Cu（II）or Ni（II） ion to LiFePO₄which had been prepared was studied. The electrochemical performance of materialwhich was added Ni （II） carbonate was more apparent, had stable voltage platform,nosignificant polarization phenomenon, the initial discharge capacity was153.926mAh/gat0.1C rate, improved20.896%than no vanadiμm doping material（127.351mAh/g）. But the electrochemical performance of material which was added Cu （II） carbonatewas unobvious, and the initial charge-discharge capacity was only a little increased.