Synthesis of Li2FeSiO4 Composites Coated with Carbon as Cathode Materials for Lithium-ion Batteries

• Main Authors: Yu-Jyun Lin, 林郁竣
• Other Authors: Chu-Chin Hsieh
• Format: Others
• Language: zh-TW
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/14834870218729013057

碩士 === 國立雲林科技大學 === 環境與安全衛生工程系 === 103 === Abstract Lithium-ion batteries are widely used in electronic products in the current market. They also are used for electric vehicles and other related applications. Therefore, how to effectively improve the electrochemical performance and select on low polluting with materials process of these batteries are important concerns. In this study, we focused on the positive aspects of the lithium-ion batteries. Furthermore, we selected lithium iron silicate (Li2FeSiO4) as the cathode material. The advantages of Li2FeSiO4 are that it is an easily available chemical of component elements with low pollution. However, the lithium-ion transfer rate and conductivity are weak. This factor limits the performance of batteries. To improve the structure and conductivity, we utilized the sol-gel method with PEG-PPG-PEG Pluronic P-123, HO(CH2CH2O)20- -(CH2CH(CH3)O)70(CH2CH2O)20H as the carbon source for Li2FeSiO4 modification. Then, to explore the different parameters we tested the electrochemical and structural properties of the materials at the amount of carbon source added, various calcination temperatures and calcination times. The initial preparation conditions were obtained from the literature (Wu et al., 2012 ; Wu et al., 2013 ; Singh and Mitra, 2014 ; Zhu et al., 2014). We used the orthogonal experiment of the L4 Taguchi method to plan the preparation of four different ratios for the Li2FeSiO4 cathode materials in order to test the first charge-discharge electrochemical results at 0.1 C rate. The preferred operating conditions were when the theoretical carbon content (C) was 22.8 wt%, and the calcination temperature (CT) was 650 ℃ and the calcination time (T) was 10 hr for the preparation conditions of C22.8CT650T10 sample. Then, the experiment result was 154.6 mAh/g. This was followed by an analysis of variance to obtain the maximum contribution from the experiment which was the amount of carbon source added. The results show that the contribution was 96.3 %. The experiment was repeated with C22.8CT650T10. The result was 146.7 mAh/g, and this verified that remained at the preferred result in the other experiments. With a high rate at 1 C stability testing for 10 cycles, the retention capacity was greater than 60%. Using XRD analysis of the materials in the crystalline structure, we found that the Li2FeSiO4 cathode materials were modified by the theoretical carbon content was 10.2 wt%can prepare with the pure phase, and this inhibits its impurity phase produced. SEM observation was used for the surface morphology of the material, and we found that when theoretical carbon content was 22.8 wt%, it could produce less than 100 nm of surface uniformly nanoscale particles, and then the lithium ion transport distance was significantly shortened. The EDS analysis results were similar in proportion with Li2FeSiO4. The carbon-coated casing of the materials was penetrated by TEM analysis. The carbon-coating can be observed from samples by the amount of carbon source added to the modification. The theoretical carbon content was 22.8 wt% had better uniformity of the carbon-coating. Moreover, the conductive properties of the material were enhanced. The analysis of the actual carbon content was done by EA. The average of actual carbon content were 3.5、15.5 wt%. After deducting the actual carbon content was more realistic in terms of the capacity. The C22.8CT650T10's 0.1 C rate of the initial discharge capacity was 182.5 mAh/g, and when the sample was done at a high rate 1 C, then the result was 133.8 mAh/g. In this study, when using the preferred operating parameters to prepare the Li2FeSiO4 cathode materials, the results showed improvement of the structure and morphology, and we found that cathode materials can achieve good discharge capacity with lithium-ion batteries. Thus, we believe that development of Li2FeSiO4 cathode materials has been established as a new cathode material. Keywords：nanoscale particles, Pluronic P-123, electrochemical testing, sol-gel method