The aim of this research is to improve the performance of advanced battery material through the use of hierarchically engineered electrodes that simultaneously exploits the benefits of nano- and micro-length scales of vertically aligned carbon nanotubes (VACNTs) and high specific capacity of silicon. Si is a promising material for use in next generation Li-ion batteries due to its highest specific capacity. However, two issues that adversely impact the usable capacity and performance of Si anodes are i) large volume changes during electrochemical cycling and ii) the formation of an unstable/thick solid-electrolyte interphase (SEI). The former triggers mechanical strains in the electrodes, causing electrical disconnectivity and electrode disintegration; the latter hinders the Li transport, resulting in poor cyclability and loss of capacity. Thus, the principal purpose is to investigate the use of Si-VACNT composite electrodes that provide the nanoscale framework needed to accommodate large volume changes while sustaining the electronic conductivity. Moreover, the geometry enables us to study the reduction and stabilization of SEI by encapsulating the Si-VACNT electrodes with a two-step process—CNT spray and PECVD carbon. Another key aspect of the work is to increase our understanding of the SEI properties and how they can be influenced by encapsulation of the electrode to control the interfacial area of the Si-VACNT composite electrodes exposed to the electrolyte.
Regardless of the control of the interfacial area, this encapsulation layer on the Si-VACNT electrodes also allows us to study the Li transport limitation in the Si-VACNT system. The encapsulation layer is a great tool to study the Li transport because this layer alleviates SEI formation in the Si-VACNT electrodes. This observation was proven by the microscopic characterization that very little SEI products were detected in the encapsulated electrodes whereas a great amount of the SEI products was found in the unencapsulated ones. In our encapsulated Si-VACNT electrodes, Li ions diffuse into Si-VACNT electrode via surface and bulk diffusions. Namely, the surface diffusion involves diffusion on the external surface of each Si-VACNT from top to bottom; the bulk diffusion takes place from the outer layer into the Si-VACNT core. The control over Si-VACNT heights and silicon loadings enables us to study which diffusion mechanisms limits the Li transport since the surface diffusion is associated with the electrode height while the bulk diffusion is related to the thickness of the silicon layer on the VACNTs.
The Si-VACNT composite electrodes were prepared by first synthesizing VACNTs on Si wafers using photolithography for catalyst patterning, followed by aligned CNT growth. Nano-layers of silicon were then infiltrated on the aligned carbon nanotubes via LPCVD at 200 mTorr and 535°C. Electrochemical testing was performed on the electrodes with and without the encapsulation layer at various current densities. Experiment results demonstrated the importance of the control over the superficial area between the electrolyte and the electrode to improve the performance of silicon-based electrodes for next generation lithium ion batteries. In addition, the results shows that Si-VACNT heights is not limiting the Li transport but the thickness of the Si layer is controlling.
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