Both silicon and germanium are leading candidates to replace the carbon anode of lithium ions batteries. Silicon is attractive because of its high lithium storage capacity while germanium, a superior electronic and ionic conductor, can support much higher charge/discharge rates. Here we investigate the electronic, electrochemical and optical properties of Si(1-x)Gex thin films with x = 0, 0.25, 0.5, 0.75, and 1. Glancing angle deposition provided amorphous films of reproducible nanostructure and porosity. The film's composition and physical properties were investigated by X-ray photoelectron spectroscopy, four-point probe conductivity, Raman, and UV-vis absorption spectroscopy. The films were assembled into coin cells to test their electrochemical properties as a lithium-ion battery anode material. The cells were cycled at various C-rates to determine the upper limits for high rate performance. Adjusting the composition in the Si(1-x)Gex system demonstrates a trade-off between rate capability and specific capacity. We show that high-capacity silicon anodes and high-rate germanium anodes are merely the two extremes; the composition of Si(1-x)Gex alloys provides a new parameter to use in electrode optimization.
Both nanocolumnar and dense germanium thin films, synthesized by evaporative deposition, were tested as a potential anode material for sodium-ion batteries. The reversible capacity of the nanocolumnar films was found to be 430 mAh/g, which is higher than the theoretical capacity of 369 mAh/g. The nanocolumnar films retained 88% of their initial capacity after 100 cycles at C/5, whereas the dense films began to deteriorate after ∼15 cycles. Additionally, the nanocolumnar films were stable at charge/discharge rates up to 27C (10 A/g). The diffusion coefficient for sodium in germanium was estimated, from impedance analysis of the dense films, to be ∼10 −13 cm 2 s −1 . Modeling of diffusion in the sodium-germanium system predicts that sodium diffusion in the near-surface layers of the material is significantly faster than in the bulk. These results show that small feature sizes are critical for rapid, reversible electrochemical sodiation of germanium.
Amorphous TiO 2 film electrodes of controllable and reproducible nanostructure and porosity were grown via evaporation of titanium in an oxygen ambient (i.e., reactive ballistic deposition (RBD)). The cyclability, rate capability, and Coulombic capacity of the electrodes depended on their morphology and porosity, which varied with the angle of incidence of the evaporated titanium. When films are deposited via evaporation at a glancing angle of 80°with respect to surface normal, nanocolumnar arrays with high internal porosity, high surface area, and optimal pore size and connectivity can be prepared. The optimized films deposited at 80°exhibit a reversible lithium capacity of ∼285 mA h g -1 at a low cycling rate (0.2 C) and maintain a reversible capacity near 200 mA h g -1 at rates as high as 5 C. About 70% of the theoretical capacity (235 mA h g -1 ) was retained with indiscernible capacity decay after 100 cycles at 1 C. The total charge stored in the TiO 2 RBD films involves both surface capacitive and diffusional processes.
Porous, high surface area materials have critical roles in applications including catalysis, photochemistry, and energy storage. In these fields, researchers have demonstrated that the nanometer-scale structure modifies mechanical, optical, and electrical properties of the material, greatly influencing its behavior and performance. Such complex chemical systems can involve several distinct processes occurring in series or parallel. Understanding the influence of size and structure on the properties of these materials requires techniques for producing clean, simple model systems. In the fields of photoelectrochemistry and lithium storage, for example, researchers need to evaluate the effects of changing the electrode structure of a single material or producing electrodes of many different candidate materials while maintaining a distinctly favorable morphology. In this Account, we introduce our studies of the formation and characterization of high surface area, porous thin films synthesized by a process called reactive ballistic deposition (RBD). RBD is a simple method that provides control of the morphology, porosity, and surface area of thin films by manipulating the angle at which a metal-vapor flux impinges on the substrate during deposition. This approach is largely independent of the identity of the deposited material and relies upon limited surface diffusion during synthesis, which enables the formation of kinetically trapped structures. Here, we review our results for the deposition of films from a number of semiconductive materials that are important for applications such as photoelectrochemical water oxidation and lithium ion storage. The use of RBD has enabled us to systematically control individual aspects of both the structure and composition of thin film electrodes in order to probe the effects of each on the performance of the material. We have evaluated the performance of several materials for potential use in these applications and have identified processes that limit their performance. Use of model systems, such as these, for fundamental studies or materials screening processes likely will prove useful in developing new high-performance electrodes.
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