MTL Annual Research Report 2011

Material properties are defined not only by the composition, but by structure as well. Such materials are suited for use in electrochemical energy storage such as lithium-ion batteries. Specifically, nanostructured silicon nanowires for use in lithium-battery technologies offer a high surface-to-volume ratio, which translates into enhanced charge/discharge rates in battery systems. Moreover, superior mechanical properties over bulk materials can be also be achieved ^{[1] [2]} .Synthesis of low-cost, room temperature processing of silicon nanowires has been achieved using metal-catalyzed etching (MCE).  This process consists of using thin, patterned metal films of gold, which serve as the site of the catalytic etching of silicon.  This etch process is carried out in an HF solution with an oxidant such as H₂O₂; etching occurs only at the metal-silicon interface.  This enables the creation of large (>1 cm²) arrays of perfectly ordered Si-NWs with periods down to 40 nm, diameters down to 20 nm, and aspect ratios up to 200 to 1 (Figure 1).  These novel high-volume filling arrays are being used for studies of lithiation, where silicon serves as anode materials. Silicon has the highest known Li storage capacity, at four Li atoms per Si atoms. Improved silicon nanowire arrays on metal are being pursued to reduce resistance losses and enhance cyclability (Figure 2). Analysis of silicon crystallinity and nanowire geometric configuration is being pursued. ^[3]