Structural, electronic, and chemical properties of surface and interfacial zones

Dissertation Abstract

Basic research is needed to further elucidate physical phenomena at surfaces as materials science applications progress to ever-shrinking length scales.  Ion bombardment, or sputtering, is a ubiquitous technique used in surface preparation for both basic research and applied projects, but our understanding is incomplete concerning the nature of the process as it relates to surface modification.  Investigations are presented in which electron spectroscopy and diffraction experiments and computer simulation inform a predictive model for the disorder sputtering imparts to atomically clean crystalline surfaces in pursuit of correcting this deficiency at a fundamental level. The results of Low Energy Electron Diffraction (LEED) and Inverse Photoemission Spectroscopy (IPES) studies indicate that atoms of a crystalline nickel sample have sufficient surface mobility at room temperature to self-anneal, partially healing the surface of damage inflicted by sputtering. When the sample is bombarded and held at low temperatures for analysis, the results of LEED and IPES experiments indicate the self-annealing effect is eliminated or drastically reduced.

A complex application of surface characterization with electron spectroscopy exists in the Surface Electrolyte Interphase (SEI) of lithium-ion batteries (LIBs), a protective film that grows on the electrodes of the LIB during charge/discharge cycling.  Silicon is a promising anodic material for next generation LIBs because it can store more energy per unit mass than current standard materials, though steps must be taken to mitigate the effects of its large volumetric fluctuations during the cycling process, which limits performance and can destroy the battery.  Investigations using Hard X-ray Photoelectron Spectroscopy (HAXPES) into two promising tactics for accommodating the silicon volume fluctuation are presented. HAXPES experiments permit characterization of the SEI at depths unavailable to conventional techniques and reveal unique information about the chemical compositions there.

The results of HAXPES experiments performed on binder-free silicon nanoparticle anodes of batteries cycled with various electrolyte additives are presented and trends are identified to explain electrochemical cycling data which favor the electrolyte additive fluoroethylene carbonate (FEC).  Anode material binders were excluded from these systems to motivate understanding of the effect each electrolyte formulation has on SEI growth and composition. The results suggest the SEI developed for FEC is thinner and of a more homogeneous nature than that of ethylene carbonate (EC).  Furthermore, the FEC-derived layer is more resistant to cracking, which will be necessary to successful deployment of Si-based anodes going forward.

Anode material binders improve battery performance and lifetime by helping to maintain conductivity in the electrode as the battery is cycled. Results of HAXPES measurements on four systems which were prepared and cycled, differing only by the type of binder material used in the anode, are presented and trends identified to explain electrochemical cycling data which favor the binder polyacrylic acid (PAA).