Studies On The Synthesis And Electrochemical Properties Of LiFePO₄ And FePO₄ Positive Materials For Lithium Ion Battery

In recent year, lithium transition metal phosphates with polyanion-type structures, IJMPO₄ (M = Co, Ni, Mn, Fe, Cu), have attracted much attention as promising new positive materials for rechargeable lithium batteries. Among them, the olivine LiFePO₄ has been a potential and active one of the second generation Li-ion battery positive materials for its nontoxic, inexpensive and environmentally benign, which also has a high theoretical specific capacity, fine cycle performance and stable thermal nature. The material does, however, have the disadvantage. The significant drawbacks of LiFePO₄ are its low electronic/ionic conductivity and great capacity loss with increasing current density. At present, research about LiFePO₄ is focus on mdification ways on conduction and diffusion of Li ion. Our reaserch also studied on how to improve and modify that drawback of LiFePO₄. And base on studying LiFePO₄, we studied the synthesis and electrochemical properties of FePO₄ as cathode materials.Main contents are listed as follows:1. In this paper, we briefly reviewed the recent progress on Li-ion battery and the cathode materials; especially, the second-generation cathode materials LiFePO₄ was introduced in detail by discussing.2. Series LiFePO₄/Fe₂P materials have been prepared by solid-coordination method. We put forward due to the carbothermal reduction, the iron phosphide phase (Fe₂P) during the annealed process has been formed. Deep researches showed that: 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. But we directly added 15 % Fe₂P into the standard LiFePO₄ with solid-state method, experiment demonstrates that simple mixing Fe₂P is not effective in enhancing the electrochemical performance of LiFePO₄.3. The precursor composites of iron phosphate (III) was prepared by both homogeneous precipitation and coordinate precipitation methods. Then by thermal decomposition of organophosphonates Fe (III)OPO(OC₆H₄COO)₂·0.5H₂O and FePO₄·1.5H₂O in air atmosphere, iron phosphate (III) materials were prepared. The results verified that the coordinate precipitation method favors the synthesis of suitable cathode materials with low crystal structure, relatively small grain size and minor remains carbons that is beneficial for improving the electrochemical performance. And the sintering time and temperature had significant influence on the crystal size and the particle morphology. The materials obtained from Fe (III) carboxy-phenyl phosphate annealed at 380°C for 9h showed the highest discharge capacity.4. Based on a idea to prepare the precursor with molecule-level mixing of Li, P, Fe and C element, a functional and new ligand L³ was prepared. And we developed to available study the mothed of synthesis LiFeL³...