The Impact of Encapsulation on Lithium Transport and Cycling Performance for Silicon Electrodes on Aligned Carbon Nanotube Substrates

In this study, silicon-coated vertically aligned carbon nanotube (Si-VACNT) electrodes were used to examine the impact of encapsulation, which effectively reduced the surface area exposed to the electrolyte. This system is ideal for examining the influence of an electrolyte-blocking layer due to its well-defined geometry and high aspect ratio. The morphology, composition and electrochemical performance of electrodes cycled at different rates were characterized for a range of silicon loadings. Significant differences were observed in the morphology and composition of the electrodes. However, the electrochemical performance was similar, and capacity fading was still observed for the encapsulated electrodes. The impact of the encapsulation layer on lithium transport was examined. Two different transport directions and length scales are relevant-1) radial transport of Li in/out of each silicon-coated nanotube (similar to 50 nm diameter) and 2) lithium transport along the length of the nanotubes (similar to 100 mu m height). Experimental results indicate that the height of the Si-VACNT electrodes does not limit Li transport, even though that height was orders of magnitude greater than the diameter of the tubes. These results have important implications for a variety of encapsulation strategies. (C) The Author(s) 2017. Published by ECS. All rights reserved.