Lithium Transport in High-Capacity Encapsulated Silicon-Vacnt Electrodes

Abstract

Si-based anodes are being explored for use in advanced lithium ion batteries due to their capacity and energy density, which exceeds that of current graphite anodes. However, there are several important factors that influence and often limit the cycling performance of these electrodes including large silicon volume changes and extensive SEI formation. Nanostructured electrodes have been shown to effectively reduce the negative impact of volume expansion; however, the use of nanostructures can greatly increase the electrode/electrolyte surface area and lead to greater SEI formation. Consequently, several strategies have been developed to reduce the area available for SEI formation while maintaining the benefits of a nanostructured material. In this study, we use silicon-coated vertically aligned carbon nanotube (Si-VACNT) electrodes to examine the role of Li transport in a system where encapsulation has been used to reduce the surface area of the nanostructured electrode that is exposed to the electrolyte. This system is ideal for studying the impact of an electrolyte-blocking layer due to its well-defined geometry and high aspect ratio. Since transport plays an essential role in the cycling performance of these electrodes, it is desirable to understand the factors that govern the transport of Li into and out of the silicon anode in the absence of electrolyte. Two different transport directions and length scales are relevant—1) radial transport of Li in/out of each silicon-coated nanotube (~50nm diameter) and 2) lithium transport along the length of the nanotubes (~100 micron height). Experimental results indicate that the height of the Si-VACNT electrodes does not appear to limit Li transport, even though that height was orders of magnitude greater than the diameter of the tubes. This result has important implications for a variety of encapsulation strategies. Additional tests were performed to characterize transport in the radial direction, which appeared to be a limiting factor. These tests included GITT (galvanostatic intermittent titration technique) measurements and EIS measurements on differently sized electrodes at different conditions. Hysteresis was also examined. Finally, a model was developed to help explain the experimental observations.