The central goal of this research is to advance the understanding of Li insertion in battery electrode materials, with respect to mechanical limitations (i.e., stress and strain). The volume expansion of the candidate electrode materials are very large, and their basic deformation properties are not well characterized. For these reasons, we will begin with detailed studies of thin film configurations, with measurements of in-plane stresses (based on wafer curvature), out-of-plane expansions and lateral inhomogeneities (based on interferometry), and deformation properties (based on nano-indentation). By performing these experiments in situ during Li insertion and removal, we will be able to obtain detailed understanding of the materials behavior which are not currently available. These experiments will be closely coupled with model development at several different length scales. Continuum modeling is necessary for analysis of the experiments, and ultimately for developing improved design methodologies for battery electrodes. Atomistic modeling will provide detailed insight into the fundamental properties of the basic materials and the relevant interfaces. The integrated approach used in these efforts will be applicable to a wide range of battery electrodes, but in this program we will specifically focus on carbon and silicon materials in negative electrodes.
Improved Li ion battery performance is a critical need for the near-term development of hybrid and electric vehicles. The planned research, both the methodology and the actual results, are designed to make significant contributions to new battery technology by providing important fundamental information about materials performance. The direct involvement of General Motors scientists provides an important avenue for disseminating this knowledge. The graduate student and post-doctoral associate who are supported with the requested funds will develop interdisciplinary expertise with a variety of methods. Working directly with the industrial participants will provide an important added dimension to their education, and both of these individuals will be well positioned for future work in battery-related fields. We also plan to involve at least six undergraduate researchers and one or more high school students with different aspects of this work.
Silicon-carbon composite electrodes have emerged as leading candidate materials for applications in rechargeable Li-ion batteries. Silicon has more than ten times the capacity of the graphite electrodes that are employed in most current batteries, however, silicon undergoes a volume increase of more than 300% during full lithiation. This makes it extremely challenging to create Si containing electrodes, where repeated Li insertion and removal during charging and discharging leads to drastic size changes in the Si-Li alloy. Considerable progress has been made in recent years by employing composite architectures. However, even here the large volume changes in Si still create substantial challenges due to cracking, loss of contact between the active materials, and other related degradation mechanisms. A key finding from the NSF funded work in this project is that the large volume changes in Si electrodes can be accommodated by low resistance sliding between Si and other materials. Here, detailed experiments showed that thin Si layers can expand and contract reversibly without losing contact with a neighboring solid. The mechanisms that enable this remarkable behavior are not yet fully clear, although some of the experimental evidence obtained in this program shows that excess Li can pile up at the interface between the two solids. This Li segregation may be linked to the low sliding resistance that was observed. Several other significant discoveries about the chemical and mechanical interfaces in Li ion battery materials were also made with this NSF funding. In the course of conducting this research, a new type of interferometer was invented to conduct detailed measurements of the sliding phenomena and other related deformation processes. This device and the discovery of reversible, very low resistance sliding have potential applications to a much broader range of technologies where low friction is important (i.e., well beyond the specific Li ion battery materials that were studied directly in this program). The NSF-funded investigations outlined above were conducted jointly with researchers from General Motors. Many of the results are directly relevant to their ongoing efforts to develop new battery materials for applications in hybrid vehicles. This collaborative work also benefitted the students and post-doctoral associates who were trained with direct exposure to industrial practice, including time spent working at General Motors facilities.
