Studies On The Synthesis And Properties Of LiFePO₄ Cathode For Lithium Ion Battery

The olivine LiFePO₄ has been a potential and active one of the second generation Li-ion battery cathode materials for its nontoxic, inexpensive and environmentally benign, which also has a high theoretical specific capacity, fine cycle performance and stable thermal nature. In this paper, we reviewed the now research programs on Li-ion battery and the cathode materials;especially, the second-generation cathode materials of LiFePO₄ was introduced in detail by discussing the positive and negative factors, which influence on their electrochemical performance. The significant drawbacks of LiFePO₄ are generally thinked as its low electronic/ionic conductivity and great capacity loss with increasing current density. Our reaserch also studied on how to improve and modify that drawback of LiFePO₄, main contents are listed as follows:1. Series LiFePOVC materials have been prepared by a so-called reformative solid-coordination method, which uses citric acid as the carbon source. A monodenate coordination band of -COO-M was proved to form gradually by the system IR detection. Changing the synthesis conditions could control the residue coating carbon content and generation of impure phase Fe₂P. Deep researches showed that: the amounts of carbon is help to modify the particle morphology and surface area, which can accelerate the diffuse speed of Li-ion and then increase their reversible capability;the electro-conductive Fe₂P phase among LiFePO₄/C composites acts as important role in increasing electronic conductivity and evidently improves the electrochemical performance of LiFePO₄/C including the less polarization phenomenon, comparatively high reversible capability, stable cycling performance and slight trend of less loss of rate capability.2. In order to modify the close-packed olivine structure of LiFePO₄4, we have prepared three series of Li_1-aCr_xFe_1-xPO₄ (x=0.002, 0.01, 0.02, 0.03, 0.05), Li_1-aCr_xFe_1-xPO₄/C (x=0.002, 0.02, 0.03, 0.05) and Li_1-aM_0.02Fe_0.98PO₄/C (M= Al³⁺ Cr³⁺, Sc³⁺, Y³⁺, La³⁺) samples with the solid-coordination method. More reaserchesshowed: suitable ion doping will obviously improve the first discharge specific capacity, and that the suitable carbon coating will ameliorate the cycle performance then the high first discharge specific capacity of ion-doped samples can be rested with cycles;different ions doping experiments indicate that the effect ion radius direct influence the cell performance of Lii-aMo.ozFeo^sPCVC samples, in fact Lii-aYo.o2Feo.98P04/C sample shows the optimal results;combined the results of phase homogeneity and crystal parameter, the reasonable mechanism was put forword to explain how the doped ions modify the inner structure and then improve thire electrochemical performance.3. Based on the reformative solid-coordination method for preparing LiFePCVC, we brought forward a new idea to prepare the precursor in one-step with molecule-level mixing of Li, P, Fe and C element. Along with that idea, a functional and new ligand L2 was prepared. The later experiments showed: that the protons in L2 are difficult to be ionized has result in the impossibility of preparing LiFeL2 complex;however, L2 can be used as the sources of P and C to prepare LiFePCVC successfully whereas optimal conditions should be explored for better properties;L2 also can be used to prepare series rare earth nitrate complexes, and furthermore, the Eu3+ and Tb3+ complexes among them show better fluorescence properties.