Semiconductors for Li-ion batteries: Raman spectroscopy & electrode microfabrication

Li-ion batteries are the energy storage of choice for portable electronics and are of extreme interest for high-energy applications, such as long-range electric vehicles because they are less dense, have greater charge densities, as well as faster charging times than any comparable energy storage technology. While the best energy storage candidate, Li-ion batteries still need to obtain even higher charge densities, which necessitates alternate electrode materials need to be utilized. Semiconductors, such as Si and Ge, are theoretically capable of accommodating more Li ions per mass and volume than any other elements. The increased charge density of Li would lead to higher capacity anodes than traditional graphitic carbon. The main factor preventing Si and Ge anodes from being utilized is the drastic 300% volumetric expansion encountered upon lithiation. This dissertation explores the fundamental interaction of Li with crystalline Si and Ge electrodes utilizing in situ Raman spectroelectrochemistry and model microfabricated electrodes. The third chapter examines the effects of lithiation on the three low-index faces (111, 100, and 110) of crystalline Si electrodes. The (110) face of Si is lithiated at lower overpotentials than the other two low-index faces; with the (111) exhibiting the highest lithiation overpotential. The anisotropic lithiation is then utilized in creating self-limiting microbars of crystalline Si through microfabrication. Chapter 4 examines the effects of dopant type and concentration on the lithiation of crystalline Si electrodes. Phosphorus and boron doped wafers are found to insert lithium at different electrochemical potentials, with P doped Si requiring a 0.5 V more negative potential for lithiation than B doped Si. Computational work is performed to understand the overpotential and an electronic surface state explanation is found, which compares the differences in Fermi levels of the doped surfaces. Chapter 5 explores the crystallographic and dopant effects on lithiation of Ge electrodes and draws comparison to Si. It is found, in contrast to Si, that Ge has minimal changes in lithiation with varying crystallographic face and dopant type. Again a surface electronic state explanation is found which relates the differences between the band gaps of the two semiconductors. Finally, chapter 6 examines the performance of Ge electrodes with model microstructures. Microstructures of Ge exhibit improved cycle life performance relative to bulk Ge. While microfabrication of Si is capable of forming a self-limiting microbar through anisotropic lithiation, Ge does not. In order to limit the faces that Li can intercalate into the Ge microbars, a Li inactive coating is utilized to enforce selective lithiation for improved performance