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Hydrothermal Synthesis and Modification Study on Spherical LiFePO4 Used as Cathode Materials for Lithium-ion Batteries

Author: LiHuanYu
Tutor: WangGuiLing
School: Harbin Engineering University
Course: Applied Chemistry
Keywords: lithium-ion battery cathode material LiFePO4 Carbon coated ion doped
CLC: TM912
Type: Master's thesis
Year: 2011
Downloads: 36
Quote: 0
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Abstract


Olivine-structured LiFePO4 has gaining particular interest as a potential candidate cathode material for rechargeable lithium-ion batteries because of its advantages of abundant resources, low cost, environmental friendliness, high thermal stability, good electrochemical capability, etc. However LiFePO4 suffer from a poor electronic conductivity and a low tap-density. Therefore, researchers mainly tend to focus on how to improve the conductivity and enhance the tap-density of LiFePO4 Powders.The spherical LiFePO4 particles were synthesized by the hydrothermal synthesis process from starting reagents of CH3COOLi-2H2O, Fe(NO3)3·6H2O, NH4H2PO4, citric acid. The structure, morphology of particles and electrochemical performance were investigated by XRD, SEM and TG-DSC. The electrochemical performance was tested by galvanostatic charge-discharge, electrochemical impedance spectroscopy and cyclic voltammetry.The preparative process of LiFePO4 was as follows:first synthesized the spherical precursor, then the LiFePO4 by sintering at high temperature in a protected atmosphere. The result shows that the samples free of impurities and were synthesized with Pmnb structure. These microspheres show a quite uniform size distribution of 1-5μm and the cathode tap-density is 1.36g·cm-1. The initial discharge capacity of 112.8mAh·g-1 was obtained at 0.1C charge-discharge rate. However after 20 cycles, the capacity of the sample exhibited a rapid degration, retaining capacity was 93mAh·g-1 and electrode polarization appeared.In order to improve the electrochemical performance of products, LiFePO4/C by addition of sucrose as the carbon source was prepared and the effect of carbon content on LiFePO4/C was examined. The experimental results show that the addition of carbon reduced the LiFePO4 grain size and enhanced performance in terms of improved discharge capacity and cycle performance. The material coated 25% carbon showed the best electrochemical properties, the initial and 20th discharge capacity separately of 153.7mAh·g-1 and 151.6mAh·g-1 was obtained at 0.1C charge-discharge rate and the capacity loss was just 3.6%.LiFePO4 containing low concentration ion dopant was prepared by mixing precursors with transition metal Mn and Ni. The results indicate that the ion dopants do not affect the structure of the material but considerably improve its electrochemical performance, especially large current discharge behaviors. LiFe0.85Mn0.15PO4 exhibit the best electrochemical performances, with a discharge capacity of 130.4mAh·g-1 at 0.1C charge-discharge rate and the capacity loss was just 4.8% after 20 times charge-discharge cycles;Similarly LiFe0.85Ni0.15PO4 showed the best electrochemical properties, the initial and 20th discharge capacity separately of 124.7mAh·g-1 and 117.1mAh·g-1 was obtained at 0.1C charge-discharge rate and the capacity loss was only 6.1%.

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CLC: > Industrial Technology > Electrotechnical > Independent power supply technology (direct power) > Battery
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