CHEMICAL VAPOR DEPOSITION FOR REPRODUCIBLE FABRICATION OF HIGH-QUALITY, VERTICALLY-ALIGNED CARBON NANOTUBE ELECTRODES FOR LI-AIR BATTERIES
Scott Tan1, Mazdak Hashempour2, Thomas Batcho2, Carl Thompson2.
1Pomona College, Claremont, CA, 2Massachusetts Institute of Technology, Cambridge, MA.
Vertically-aligned carbon nanotubes (VACNTs) are ideal candidates for Li-air battery cathodes due to desirable characteristics such as high conductivity, porosity, and surface area, but the previous chemical vapor deposition (CVD) fabrication technique produced electrodes with inconsistent height and density. Reproducible height and density are necessary for electrodes to be coated via atomic layer deposition, a technique used to modify carbon nanotubes for potential improvement in battery stability. This project focused on optimizing the CVD technique to consistently produce VACNT electrodes with uniform height and areal density. Optical microscopy and weight measurements of VACNT electrodes grown via the previous CVD method displayed a gradient for height and areal density between successive samples in a batch. This gradient was dependent on proximity to the source of gas flow, which suggested a shortage of ethylene in the system. To counteract this, ethylene gas flow was increased, which successfully reduced variation between samples. However, consistent height measurements with inconsistent areal densities implied deposition of unwanted species onto the substrates at ethylene flow rates above 125 standard cubic centimeters per minute. Other parameters were also optimized for the updated CVD method. The system was automated via microprocessor control to reduce procedure variation between experiments. The relative standard deviation between samples grown under the same conditions was reduced 4-fold for height and 3-fold for areal density using the new fabrication method. VACNT electrodes have been assembled and tested in Li-air batteries and imaged via SEM and TEM to confirm high-quality carbon nanotube formation.