Topic: Fracture of Silicon Nanopillars during Electrochemical Lithium Insertion

Speaker: Prof. William D. Nix

　　　　 Department of Materials Science and Engineering

　　　　 Stanford University, Stanford, USA

Abstract:

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 less constraint on the volume changes that occur and more resistance to fracture during lithium insertion.But still, facture can occur even innanostructured silicon. Here, we consider the fracture of Si nanopillars during lithiation and find surprising results. We find that fracture is initiated at the surfaces of the crystalline nanopillars and not in the interior, as had been predicted by analyses based on diffusion-induced stresses. 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, at the expense of a crystalline Si core, subjected to compression. We also show that the expansion of the nanopillars is highly anisotropic and that thefracture locations are also anisotropic. In addition, we find a critical fracture diameterfor initially crystalline nanopillars of about 300nm that appears to depend on the electrochemical reaction rate. Modeling the stress evolution in Si nanopillars during lithiation provides a way to understand and control these failure processes. In particular, we find that the stresses that develop during the lithiation of crystalline Si inhibit the lithiation reaction and, for sufficiently small particles, can stop the lithiation process altogether. Also, we show that initially amorphous Si nanopillars are much more resistant to failure, because the stresses at the surface are initially compressive in this situation compared to tension in the case of initially crystalline nanopillars. For sufficiently big amorphous Si nanopillars, cracking would be expected to occur in the interior, but we have not yet found this type of failure. The critical size for fracture of amorphous Si is apparently more than an order of magnitude greater than that for crystalline Si. It is hoped that these studies will be useful in the design of silicon electrodes for advanced battery systems.