Jan Chapter Meeting – Mechanical Properties of Lithiated Silicon: A Candidate Electrode for Lithium Ion Batteries
January 11, 2017 | Michael's at Shoreline
Understanding the insertion of lithium into silicon electrodes for high capacity lithium-ion batteries is likely to have benefits for mobile energy storage, for both electronics and transportation. Silicon nanostructures have proven to be attractive candidates for electrodes because they provide more resistance to fracture during lithium insertion. But still, facture can occur even in nanostructured silicon. Here, we consider the fracture of Si nanopillars during lithiation and find surprising results. In situ transmission electron microscopy observations of initially crystalline Si nanoparticles shows that lithiation occurs by the growth of an amorphous lithiated shell, subjected to tension and leading to fracture. We show that the expansion of the nanopillars is highly anisotropic and that the fracture locations are also anisotropic. Also, we show that initially amorphous Si nanopillars are much more resistant to failure, having much larger critical fracture diameters. For sufficiently big amorphous Si nanopillars, cracking is expected to be initiated in the interior based on diffusion-induced stresses, but we have not yet observed that kind of fracture. The modeling we, and others, have done has been based largely on estimates or guesses about the mechanical properties of lithiated Si. Recent nanoindentation experiments show that the elastic modulus and hardness of lithiated amorphous Si depend strongly on the lithium content. When these more subtle effects are included in the modeling they may be helpful in the design of silicon electrodes for advanced battery systems
February Chapter Meeting: Nanoporosity and the Welcome Guest: Developing Metal-Organic Frameworks for Catalysis, Hydrogen Storage, and Electronic Devices
February 01, 2017 | Michael's at Shoreline 2960 Shoreline Blvd
Metal-Organic Frameworks (MOF)s are crystalline materials in which metal ions or metal-ion clusters are linked by rigid organic molecules, creating a supramolecular network that has permanent nanoporosity. Unwanted "guest" species, which can be solvent molecules or residual reactant, can be removed without pore collapse. Once a MOF is activated, it provides a highly ordered, chemically tailorable structure that can function as a nanoscale catalytic reactor, store gases such as hydrogen, or serve as an active component of electronic devices.