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Synthesis and Characterization of Nanocrystalline Olivine-Structured LiFePO4 and LiMnPO4 Cathodes for Lithium Ion Batteries
- Author(s)
- DINH HUNG CUONG
- Advisor
- 모선일
- Department
- 일반대학원 에너지시스템학과
- Publisher
- The Graduate School, Ajou University
- Publication Year
- 2014-02
- Language
- kor
- Alternative Abstract
- LiMPO4 family (M=Fe, Mn, Ni & Co) with an ordered olivine structure is considered as the most promising cathode materials for Li+ ion batteries, due to moderate theoretical capacity (~170mAhg-1), low cost of its raw materials, reversibility, thermal stability, environmental friendliness, and an increased level of safety. The limitation in its conductivity is to be overcome for the practical application of LiMPO4 to high rate Li+ batteries. In order to increase the electronic and ionic conductivity, much effort have been made such as optimizing particle size (nano-scale) as well as coating conductive polymer, carbon, metal, or metal oxide on the surface of LiMPO4 particles or doping supper-valance ions in crystalline lattice. In this study, LiMPO4 (M= Mn, Fe, Ni, Co) crystalline particles were synthesized using a hydrothermal method. The particle morphology can be controlled by a careful choice of surfactant, solvents, and reaction parameters in hydrothermal synthesis. The obtained nanosized powder was coated with conductive materials; polymer or carbon. Highly uniform hierarchical-microstructured LiFePO4 particles with dumbbell- and donut-shape and individual LiFePO4 nanocrystals were prepared by a hydrothermal method utilizing citric acid or a triblock copolymer (Pluronic P123) as a surfactant. The cathode composed of the individual nanocrystalline LiFePO4 particles exhibited higher specific capacity than the cathodes composed of the hierarchically assembled microparticles. Coating a conductive polymer, poly-3,4-ethylenedioxythiophene (PEDOT), on the surface of LiFePO4 particles improved the battery performances such as large specific capacities, high rate capability and an improved cycle stability. The nanocrystalline LiFePO4 particles coated with PEDOT (20wt%) exhibited the highest discharge capacities of 175 and 136mAhg−1 for the first battery cycle and 163 and 128 mAhg−1 after 1000 battery cycles, with a degradation rate of 6–7%, at the rates of 1 and 10 C, respectively. Nanocrystalline LiMnPO4 particles approximately 100 nm in size are synthesized for use as a cathode material using a hydrothermal method. Small charge transfer resistances (Rct) measured using electrochemical impedance spectroscopy (EIS) indicate that the particles are effectively coated with the conductive C layer by pyrolyzing sucrose in an inert atmosphere. The cathode composed of nanocrystalline C-coated LiMnPO4 exhibits the largest specific capacities of 171 mAhg-1 and 153 mAhg-1 for the first cycle and 166 mAhg-1 and 146 mAhg-1 after 110 battery cycles at a rate of 0.05 C at 55oC and 25oC, respectively, with a degradation rate of 4 ±1% after 110 battery cycles. The specific capacities are 165 mAhg-1 and 142 mAhg-1 for the first cycle at 55oC for the nanoparticles with high crystallinity, even at high discharging current of 0.5 C and 5C, respectively. Diffusion coefficients of Li+ in LiMnPO4 (the fully discharged state) and in MnPO4 (the fully charged state) are estimated to be 3.58x10-16 and 4.99x10-17 Cm2s-1, respectively, from EIS in the low frequency region.
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- Appears in Collections:
- Graduate School of Ajou University > Department of Energy Systems > 3. Theses(Master)
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