鋰離子二次電池橄欖石結構正極材料之相變化與電化學分析研究

Phase Transition and Electrochemical Studies of Olivine-type Cathode Materials for Lithium Ion Batteries

陳宜群(Yi-Chun Chen)

指導教授 ： 施漢章 ； 葉均蔚

國立清華大學/工學院/材料科學工程學系/博士(2010年)

https://doi.org/10.6843/NTHU.2010.00280

摘要

LiFePO4 was the most attractive cathode material for the lithium ion batteries due to its low cost, high capacity and high safety. However LiFePO4 had two major drawbacks: first was the lower conductivity (10-9 Scm-1) and second was the lower working voltage compared with other cathode materials. In this study, other transition metals (Co and Mn) were substituted at 4c site of the olivine structure to raise the average working voltage because of the Co2+/Co3+ and Mn2+/Mn3+ redox couples with higher chemical potentials than that of the Fe2+/Fe3+ redox couples. Olivine-type cathode materials with carbon coatings (LiMPO4/C) were formed to enhance the electrical conductivity and electrochemical performances. Electrochemical impedance spectrum and cyclic voltammetry were used to study the fundamental electrochemistry of cathode materials. Furthermore in-situ synchrotron X-ray diffraction and absorption were used to analyze the structure transformation and valence change during the cycling.

We had substituted Mn2+ at the 4c site of LiFePO4 to prepare the lithium bi-metal phosphate LiMnxFe1-xPO4. At 0.05C charge/discharge, X-ray patterns revealed that LiMn0.25Fe0.75PO4 undergoes two two-phase transformations during the delithiation, resulting from Fe2+/Fe3+ and then Mn2+/Mn3+ redox reactions. However, the phase transformation for lithiation is different, becoming a two-phase (Mn2+/Mn3+) reaction and single-phase (Fe2+/Fe3+) reaction. Even at a higher charge/discharge rate (0.5C), the results were the same. LiMn0.25Fe0.75PO4 also had a good cyclability, since there is no significant capacity fading during the cycling test. The X-ray patterns showed that LiMn0.25Fe0.75PO4 still maintains a good crystal structure after 40 cycles because of its stable olivine structure. By in-situ metal K-edge absorption analysis, it revealed that a raised voltage was contributed by the Mn2+/Mn3+ redox couples, however, the substituted metal Mn2+ did not work completely at a higher discharge rate, due to poor electrical conductivity and a serious Jahn-Teller effect.
For LiMn0.5Fe0.5PO4, at room temperature, its capacity was about 138 mAhg-1 and the average voltage had increased from the value for pure lithium iron phosphate to 3.7V due to the Mn substitution. As the temperature was raised to 55oC, the LiMn0.5Fe0.5PO4/C showed an excellent electrochemical performance. Its capacity increased to 160 mAhg-1 and the average voltage also increased to 3.75 V. Even the discharge rate increased to 1 C, and the capacity could still be maintained at about the same value of 160 mAhg-1. The electrochemical impedance and cyclic voltammetry showed that elevated temperature could enhance the activity of the Fe2+/Fe3+ and Mn2+/Mn3+ redox couples and reduce their electrochemical resistance. Those were why LiMn0.5Fe0.5PO4/C showed such good performance at 55 oC. However, Mn dissolution was also observed in the olivine-type LiMn0.5Fe0.5PO4 at elevated temperatures.

Furthermore, lithium multi-transition metal phosphate LiCo1/3Mn1/3Fe1/3PO4 was synthesized and studied, which had a high voltage of 3.72 V and a capacity of 140 mAhg-1 at a 0.05C rate due to the Mn2+ and Co2+ substitution. From the in-situ XRD analysis, LiCo1/3Mn1/3Fe1/3PO4 show a high stability during cell charge/discharge, even operating at 5 V, which was due to the stable olivine structure. Although all the transition metals Co2+, Mn2+ and Fe2+ were at the same 4c site of the LiCo1/3Mn1/3Fe1/3PO4 structure, they seemed to have different chemical activities and reflected on the electrochemical performance. The capacity contributed by the Co2+/Co3+ redox couple was only 20 mAhg-1, which was less than that of the Fe2+/Fe3+ and Mn2+/Mn3+ redox couples. This was because the diffusivity of lithium ion for the Co2+/Co3+ redox couple was 10-16 cm2s-1 which was one order less than that of the Fe2+/Fe3+ and Mn2+/Mn3+ redox couples in LiCo1/3Mn1/3Fe1/3PO4.

關鍵字

磷酸鋰鐵 ； 同步輻射光源 ； 鋰離子二次電池 ； 相轉變

並列摘要

無資料

並列關鍵字

LiFePO4 ； Synchrotron X-ray ； Lithium ion batteries ； phase transformation

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