A Hierarchically Engineered Microfabrication Approach for Advanced Anode Materials in Lithium Ion Batteries
Silicon is an advanced material with promise for use in next generation batteries. Among anode materials, silicon stands out because of its potential for extremely high capacities – more than ten times the specific capacity of the commercial graphite anodes. Two issues that negatively impact the usable capacity and performance of Si anodes are i) large volume changes during charging/discharging and ii) the formation of an unstable solid-electrolyte interphase (SEI). The use of nanostructured electrodes has been effective in accommodating the large volume changes that occur during cycling [1, 2]. However, nanostructuring leads to a very high surface area, which exacerbates problems associated with SEI formation and stability.
This study seeks to improve the performance of Si anodes through the use of hierarchically structured electrodes to provide the nanoscale framework needed to accommodate large volume changes while controlling the interfacial area – which affects SEI formation. To accomplish this, electrodes will be fabricated from vertically aligned carbon nanotubes (VACNT) infiltrated with silicon. On the nanoscale, these electrodes allow us to adjust the surface area, porosity, and silicon layer thickness. On the microscale, we have the ability to control the electrode thickness and the incorporation of micro-sized features. For instance, microscopic patterning of the CNT matrix to form vertical holes, as shown in Figure 1, has the potential to impact transport through the electrode – as well as its interfacial area and mechanical stability. Treatment of the surface between the nano- and micro-structured areas can be used to control the size and chemistry of the interface between the electrolyte and the electrode matrix.
VACNT-Si 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 deposited on the aligned carbon nanotubes via LPCVD at 200mTorr and 570°C. Figure 2 shows a cross-sectional SEM image of a synthesized VACNT-Si array. A thin copper film was used as the current collector. Electrochemical testing was performed on the electrodes assembled in a CR2025 coin cell with a metallic Li foil as the counter electrode. The impact of the electrode structure on the capacity at various C-rates was investigated. Experiment results demonstrate the influence of structurally engineered electrodes on the performance of silicon electrodes for next generation batteries.
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