摘要
随着便携式电子设备、电动汽车、可植入医疗设备对电池性能要求的不断提高,开发高容量、长寿命的电池越来越迫切。锂离子电池的整体性能主要取决于电池材料,所以开发新型的电池材料是提高电池性能的关键。目前,商品化的锂离子电池负极主要采用石墨材料,而石墨负极材料的理论比容量并不高(只有372mAh/g)。锡作为锂离子电池负极材料具有低的充放电电位和高的理论比容量而受到广泛的关注。它的理论比容量为994mAh/g,几乎是石墨的三倍,但是锡的应用却受到很大的限制,因为锡与锂合金化过程中,体积膨胀率很大(超过360%),引起电极材料的粉化和电接触变坏,进而导致容量急剧衰减。
为了改善锡用作锂离子电池负极材料的循环性能,必须对电极结构进行改进。人们对锡合金用作锂离子电池负极材料进行了广泛的研究。相对于纯锡,这些合金材料具有更好的循环性能和更长的循环寿命,但是经过较长的充放电循环后,容量还是会急剧衰减。
与薄膜和体状材料相比,使用三维纳米棒阵列结构电极能有效改善电池的循环性能和倍率性能,这一点已从人们制备出的Si, SnO2, Co3O4, TiO2等纳米棒结构电极得到证实。
本课题通过氧化铝模板辅助生长方法制备了用于锂离子电池的锡基纳米棒阵列电极。通过扫描电子显微镜,能谱色散X射线分析,X射线衍射分析等手段对锡基纳米棒电极进行了形貌和成份表征。扫描电子显微镜观察显示铜集流体表面均匀分布着锡基纳米棒阵列,纳米棒平均直径约250 nm。通过循环伏安和恒流充放电方法对锡基纳米棒电极的电化学性能进行了表征,结果表明使用锡镍合金纳米棒阵列结构电极得到了良好的倍率性能和循环性能。
课题首先对纯锡纳米棒阵列电极进行了研究。相对锡平面薄膜电极,锡纳米棒阵列电极表现出更好的循环性能和倍率性能。电化学性能的提高可能是由我们设计的三维纳米棒阵列电极结构所致。锡纳米棒阵列电极具有短的锂离子扩散路径,与电解质有大的接触界面,物质传输、电荷传输迅速,可以减小极化,提高充放电倍率,也提高了活性材料利用率。另外,锡纳米棒阵列电极能够在一定程度上缓冲金属锡负极材料与锂合金化过程中引起的体积膨胀。但是,锡纳米棒电极循环性能的提高是相当有限的,10次充放电循环以后,锡纳米棒电极的容量出现大幅度的剧烈衰减。
然后我们对锡镍合金纳米棒阵列电极进行了研究。相对锡镍合金平面薄膜电极,纯锡平面薄膜电极和纯锡纳米棒阵列电极,锡镍合金纳米棒电极表现出更好的循环性能和倍率性能。三维纳米棒阵列结构可以很好的缓冲锡基材料在充放电过程中大的体积变化,改善循环性能。另外,锡镍合金中的非活性相镍可以对活性相锡起到分散缓冲的作用,一定程度上能提高锡基材料的循环性能。
最后,我们研究了锡含量对锡镍合金电极电化学性能的影响。结果显示,30次循环内,锡含量为30%的锡镍合金纳米棒阵列电极表现出比较好的综合电化学性能。
There is great interest in developing rechargeable lithium ion batteries with higher energy capacity and longer cycle life for application in portable electronic devices, electric vehicles and implantable medical devices. Sn is an attractive anode material for lithium ion batteries, because it has a low discharge potential and a high theoretical charge capacity (994mAh/g). Although this is nearly three times existing graphite anodes, Sn anodes have limited application because of large volume change up to 360% in the process of insertion and extraction of lithium ion which results in pulverization and capacity fading.
In order to improve the performance of cyclability, modification of the electrode structure is a vital factor. Extensive attention has been conducted on tin based intermetallic compounds. These materials exhibited longer cyclability than that of pure Sn. However, long-term cycle will still result in rapid capacity loss.
The concept of using three-dimensional nanorod array materials has been demonstrated with Si, SnO2, Co3O4, TiO2, and has shown improvement in the rate capability and cycling performance compared to film and bulk materials.
In this paper, we report our investigation of a new Sn based nanorod electrode, which is expected to accommodate large strain and shorten the Li-ion diffusion length. Sn based nanorods were deposited onto copper current collectors by an anodic aluminum oxide (AAO) template-assisted growth method. Using such electrodes, we achieved good cycling performance and improvement in rate capability compared to planar electrodes.
First, the pure tin nanorod array electrode was studied. Compared with the tin planar thin film electrode, the tin nanorod array electrode exhibited better cycle performance and rate capability. The improvement of the electrochemical performance is due to our design of three-dimensional nanorod array electrode structure. Tin nanorod array electrode has a short Lithium ion diffusion distance, and allows for fast surface reaction resulting from large electrode/electrolyte interface area. In addition, the small nanorod diameter allows for better accommodation of the large volume change of metallic anode materials occurring in the process of charge and discharge. However, the improvement of tin nanorod array electrode is quite limited. The capacity of tin nanorod electrode decays dramatically, after 10 charge-discharge cycles.
Then, we studied the Sn-Ni alloy nanorod electrode. Compared with the Sn-Ni alloy planar thin film electrode, pure Sn planar thin film electrode, and pure Sn nanorod electrode, Sn-Ni alloy nanorod electrode showed better cycle performance and rate capability. The nanorod array structure is able to allow for better accommodation of the large volume change of metallic anode materials occurring in the process of charge and discharge, and the inactive element Ni can buffer the large volume change and as a barrier against the aggregation of Sn into large grains during Li-ion insertion and extraction process. The two factors both contribute to the improvement of cycle performance.
Finally, the effect of concentration of Sn on the electrochemical performance of the Sn-Ni alloy nanorod alloy electrode was investigated. The results showed that the Sn-Ni alloy nanorod electrode with 70% Sn content exhibited the best overall electrochemical performance.
引文
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