Research Of Tin Based Nano-structured Electrodes For Lithium Ion Batteries

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.