Study On Synthesis And Modification Of LiFePO₄/C Used As Cathode Materials For Lithium-Ion Batteries

The increasing concerns on energy, environmental and the rapid development of modern science and technology have more demands on batteries. Olivine-structured LiFePO₄ is considered as a potential cathode material for lithium-ion batteries due to its many attractive characteristics, such as high safety, low cost, high capacity, good cycling capability and thermal stability. However, its rate performance is significantly restricted by sluggish kinetics of electrons and lithium ions. In this paper, LiFePO₄/C was synthesized by a combination method of sol-gel and gas-phase diffused permeation, and determined the conditions of material preparation. LiFePO₄/C was synthesized by carbon-coated method, the effects of carbon sources and different carbon-coated methods and carbon-coated content on electrochemical behavior of LiFePO₄ were studied. The researchs about structures and morphologies and electrochemical behavior of materials which were synthesized by doping Ag were also studied. We used X- Ray Diffraction (XRD) to analyze phase composition, Scanning Electron Microscope (SEM) to observe superficial morphology, charge-dischagre test to study specific capacity and cyclic properties, CyclicVoltammetry (CV) and Electrochemical Impedance Spectroscope (EIS) to discuss electrochemical reactive mechanism.LiFePO₄ was synthesized by a combination method of sol-gel and gas-phase diffused permeation. LiFePO₄ with aspace group symmetry of Pnmb was indexed in the orthorhombic system. LiFePO₄/C materials were synthesized by different carbon sources (glucose, sucrose, citric acid). Used XRD, SEM, and charge-discharge test on material characterization, the results showed that sucrose as carbon source, the electrochemical properties of the material were better than the others. The first special discharge capacity of this material is 91.44mAh·g-1, the capacity retention could attain 94% of the initial discharge capacity after 30 cycles. Studying the influence of different sucrose addition on the electrochemical properties of materials, the results show that LiFePO₄/C synthesized with about 70mass% sucrose has the higher specific discharge capacity than the other materials. The specific discharge capacity of this material is 139.0 mAh/g at 0.1C. In order to furtherly improve the performances of LiFePO₄/C, the effects of Ag doping on the structure, morphology and electrochemical properties were studied. Charge-discharge tests showed the rate and cyclic performances of LiFePO₄/C were improved obviously and the doped material of x=0.03 had the best electrochemical properties. The specific discharge capacity of the doped material of x=0.03 are all higher than the undoped material at different rates.