This research will develop electrochemical storage materials based on three-dimensionally patterned carbon nanotube composites. These composites consist of patterned vertically aligned carbon nanotube (VACNT) templates coated with electrochemically active materials. A unique aspect of these composite materials is the ability to control simultaneously the structure on both a nano- and micro-level. This permits systematic variation of the different length scales of the vertically aligned hierarchical structures (i.e., the nano- and micro-porosity) in order to impact and control the relative influence of, for example, solid-phase electron transport and the transport of ions in the electrolyte. In addition, dimensional control at multiple length scales can be used to create structures that accommodate the morphological changes that occur during electrode cycling. In short, this work will allow the PIs to understand and optimize the influence of electrode structure on the performance of energy storage electrodes in a way that has not previously been possible. It also holds the promise of creating structurally engineered electrodes to enable a new generation of high-performance electrical storage devices.
This work is potentially transformative, but also high risk as the ability to create functional battery electrodes based on these novel materials has not yet been demonstrated. The purpose of this EAGER grant is to demonstrate the feasibility of such electrodes as the foundation for a more extensive study that will exploit the potential of these unique electrodes. This will be accomplished by: 1) Synthesis of the vertically structured composite including patterning and growth of VACNT templates and chemical vapor deposition of high energy density electrochemical materials, and 2) Electrochemical cycling and characterization of structured composites. Basic microstuctural characterization of these composites will also be performed. While VACNT-templated structures can be applied to a variety of different electrode chemistries, this work will focus on silicon anodes for lithium-ion batteries. Silicon anodes represent a very challenging system that has great potential.
An interdisciplinary team of experts from BYU with an established record of successful collaboration has been assembled to take advantage of this opportunity. The team includes two physicists with expertise in microfabrication, nanofabrication, and micro and nanoscale analysis; and a chemical engineer with expertise in electrochemistry and energy storage materials and systems.
The intellectual merit is that it introduces potentially transformative electrochemical materials based on templated carbon nanotube composites. The new materials to be developed have the potential to enable the development of energy storage systems with both high energy density and high power density, including 3D electrodes. Energy storage with these characteristics is needed to address a wide variety of energy related issues in energy generation and delivery systems. These materials also provide a well-controlled test bed for fundamental understanding of the structural factors that limit electrode performance.
The broader impacts include the development of new energy storage materials with the potential to have significant societal and environmental impact. In addition, the project will involve education of undergraduate and graduate students in a multidisciplinary environment where the specific training is in energy storage materials. The PIs have a long track record of involving undergraduates in a positive research mentoring environment and will continue this effort.
The research was focused on the development of vertically aligned carbon nanotube (VACNT) composite electrochemical storage materials formed by coating VACNT structures with electrochemical materials of interest via chemical vapor infiltration. The electrode that we chose for investigation in this study was a silicon anode for use in lithium-ion batteries. Efforts were focused in three areas: 1) fabrication of VACNT-templated silicon anodes, 2) microscale and nanoscale physical characterization of the anode structures, and 3) electrochemical characterization of structured composite electrodes. Educational activities for this EAGER focused on training graduate and undergraduate students. These structures showed high capacity storage measured several ways. Capacity per area was 9 mAh/cm2, capacity per gram was over 2700 mAh/g, and capacity per volume over 4600 mAh/cm3. Although other nanostructured silicon systems have demonstrated high capacities per gram this is the first material with these high capacities per area and per volume which are 3 times and 20 times larger than the corresponding capacities for commercial silicon batteries. The fabrication process used to generate these materials is as follows. Vertically aligned multi-walled carbon nanotubes (MWCNT) were grown on stainless steel foils with the aid of a thin film catalyst (a 5 nm iron layer film on a 30 nm alumina layer). Silicon infiltration of the carbon nanotube framework results in a robust Si/CNT nanocomposite. The geometry of the Si/CNT composites has several potential advantages as lithium battery anodes: 1) porosity among Si/CNTs provides the space for volume expansion, 2) MWCNTs provide high electrical conductivity pathways to the current collector, and 3) the Si/CNTs high aspect ratio results in high capacity per area films. This is a flexible approach that allows nanoscale porosity to be readily varied by changing the silicon infiltration time and Si/CNT height to be varied by changing the VACNT growth time.
