Direct Growth of Si, Ge, and Si-Ge Heterostructure Nanowires Using Electroplated Zn: An Inexpensive Seeding Technique for Li-Ion Alloying Anodes.

A scalable and cost-effective process is used to electroplate metallic Zn seeds on stainless steel substrates. Si and Ge nanowires (NWs) are subsequently grown by placing the electroplated substrates in the solution phase of a refluxing organic solvent at temperatures >430 °C and injecting the respective liquid precursors. The native oxide layer formed on reactive metals such as Zn can obstruct NW growth and is removed in situ by injecting the reducing agent LiBH4 . The findings show that the use of Zn as a catalyst produces defect-rich Si NWs that can be extended to the synthesis of Si-Ge axial heterostructure NWs with an atomically abrupt Si-Ge interface. As an anode material, the as grown Zn seeded Si NWs yield an initial discharge capacity of 1772 mAh g-1 and a high capacity retention of 85% after 100 cycles with the active participation of both Si and Zn during cycling. Notably, the Zn seeds actively participate in the Li-cycling activities by incorporating into the Si NWs body via a Li-assisted welding process, resulting in restructuring the NWs into a highly porous network structure that maintains a stable cycling performance.

Influence of Carbonate-Based Additives on the Electrochemical Performance of Si NW Anodes Cycled in an Ionic Liquid Electrolyte.

Addition of electrolyte additives (ethylene or vinylene carbonate) is shown to dramatically improve the cycling stability and capacity retention (1600 mAh g-1) of Si nanowires (NWs) in a safe ionic liquid (IL) electrolyte (0.1LiTFSI-0.6PYR13FSI-0.3PYR13TFSI). We show, using postmortem SEM and TEM, a distinct difference in morphologies of the active material after cycling in the presence or absence of the additives. The difference in performance is shown by postmortem XPS analysis to arise from a notable increase in irreversible silicate formation in the absence of the carbonate additives. The composition of the solid electrolyte interphase (SEI) formed at the active material surface was further analyzed using XPS as a function of the IL components revealing that the SEI was primarily made up of N-, F-, and S-containing compounds from the degradation of the TFSI and FSI anions.

Copper Silicide Nanowires as Hosts for Amorphous Si Deposition as a Route to Produce High Capacity Lithium-Ion Battery Anodes.

Herein, copper silicide (Cu15Si4) nanowires (NWs) grown in high densities from a metallic Cu substrate are utilized as nanostructured hosts for amorphous silicon (aSi) deposition. The conductive Cu15Si4 NW scaffolds offer an increased surface area, versus planar substrates, and enable the preparation of high capacity Li-ion anodes consisting of a nanostructured active material. The formation method involves a two-step process, where Cu15Si4 nanowires are synthesized from a Cu substrate via a solvent vapor growth (SVG) approach followed by the plasma-enhanced chemical vapor deposition (PECVD) of aSi. These binder-free anodes are investigated in half-cell (versus Li-foil) and full-cell (versus LCO) configurations with discharge capacities greater than 2000 mAh/g retained after 200 cycles (half-cell) and reversible capacities of 1870 mAh/g exhibited after 100 cycles (full-cell). A noteworthy rate capability is also attained where capacities of up to 1367 mAh/g and 1520 mAh/g are exhibited at 5C in half-cell and full-cell configurations, respectively, highlighting the active material's promise for fast charging and high power applications. The anode material is characterized prior to cycling and after 1, 25, and 100 charge/discharge cycles, by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), to track the effects of cycling on the material.

Enhancing the performance of germanium nanowire anodes for Li-ion batteries by direct growth on textured copper.

Herein, textured Cu foil is presented as an attractive current collector substrate for directly grown Ge nanowire (NW) anodes. Compared to planar stainless steel (SS) current collectors, textured Cu led to an increase in achievable mass loading, removal of the requirement for a catalyst deposition step, improved adhesion of the active material and dramatically enhanced capacity retention. When SS and textured Cu foil based anodes with similar areal loadings (∼1.4 mA h cm-2) were compared, the capacity after 250 cycles for textured Cu was 2.7 times higher than the SS anode, illustrating the key role of the current collector.

Tunable Core-Shell Nanowire Active Material for High Capacity Li-Ion Battery Anodes Comprised of PECVD Deposited aSi on Directly Grown Ge Nanowires.

Herein, we report the formation of core@shell nanowires (NWs) comprised of crystalline germanium NW cores with amorphous silicon shells (Ge@aSi) and their performance as a high capacity Li-ion battery anode material. The Ge NWs were synthesized directly from the current collector in a solvent vapor growth (SVG) system and used as hosts for the deposition of the Si shells via a plasma-enhanced chemical vapor deposition (PECVD) process utilizing an expanding thermal plasma (ETP) source. The secondary deposition allows for the preparation of Ge@aSi core@shell structures with tunable Ge/Si ratios (2:1 and 1:1) and superior gravimetric and areal capacities, relative to pure Ge. The binder-free anodes exhibited discharge capacities of up to 2066 mAh/g and retained capacities of 1455 mAh/g after 150 cycles (for the 1:1 ratio). The 2:1 ratio showed a minimal ∼5% fade in capacity between the 20th and 150th cycles. Ex situ microscopy revealed a complete restructuring of the active material to an interconnected Si1- xGe x morphology due to repeated lithiation and delithiation. In full-cell testing, a prelithiation step counteracted first cycle Li consumption and resulted in a 2-fold improvement to the capacity of the prelithiated cell versus the unconditioned full-cells. Remarkable rate capability was also delivered where capacities of 750 mAh/g were observed at a rate of 10 C.

Investigating the effects of amphipathic gastrointestinal compounds on the solution behaviour of salt and free base forms of clofazimine: An in vitro evaluation.

Interactions between hydrophobic drugs and endogenous gastrointestinal substances have the potential to manipulate drug concentration in the human gastrointestinal system, and thus likely play an important role in determining the rate of absorption for hydrophobic drugs. The effects of phospholipids, bile salts and digestive proteins on the solution behaviour of clofazimine in biorelevant media was demonstrated here using dissolution experiments and solid state analytical techniques. Clofazimine is a hydrophobic, anti-mycobacterial agent with virtually no detectable water solubility in its free base form. Salt forms of the drug offer improved aqueous solubility but are unstable in solutions at low pH (pH 1.6) or high pH (pH 6.5). At low pH and high chloride ion concentrations, CFZ in solution experiences a high driving force to crystallize from solution as a hydrochloride salt, which is insoluble, while at high pH CFZ does not dissolve to any extent. In this study, it is demonstrated that amphipathic compounds present in the gastric and intestinal systems can overcome the instability experienced by CFZ at these pH values. This is done by encapsulation of the hydrophobic drug in mixed bile salt phospholipid micelles in both the gastric and intestinal fluid, and by the drug actively binding with the digestive enzyme pepsin in the gastric system. Pepsin binds and solubilises the drug at even relatively low concentration (0.1 mg/mL). When pepsin concentration is increased in the gastric media, a corresponding increase in the solution stability of CFZ is observed.

Axial Si-Ge Heterostructure Nanowires as Lithium-Ion Battery Anodes.

Here, we report the application of axially heterostructured nanowires consisting of alternating segments of silicon and germanium with a tin seed as lithium-ion battery anodes. During repeated lithiation and delithiation, the heterostructures completely rearrange into a porous network of homogeneously alloyed Si1- xGe x ligaments. The transformation was characterized through ex situ TEM, STEM, and Raman spectroscopy. Electrochemical analysis was conducted on the heterostructure nanowires with discharge capacities in excess of 1180 mAh/g for 400 cycles (C/5) and capacities of up to 613 mAh/g exhibited at a rate of 10 C.

Direct Synthesis of Alloyed Si1-xGex Nanowires for Performance-Tunable Lithium Ion Battery Anodes.

Here we report the formation of high capacity Li-ion battery anodes from Si1-xGex alloy nanowire arrays that are grown directly on stainless steel current collectors, in a single-step synthesis. The direct formation of these Si1-xGex nanowires (ranging from Si0.20Ge0.80 to Si0.67Ge0.33) represents a simple and efficient processing route for the production of Li-ion battery anodes possessing the benefits of both Si (high capacity) and Ge (improved rate performance and capacity retention). The nanowires were characterized through SEM, TEM, XRD and ex situ HRSEM/HRTEM. Electrochemical analysis was conducted on these nanowires, in half-cell configurations, with capacities of up to 1360 mAh/g (Si0.67Ge0.33) sustained after 250 cycles and in full cells, against a commercial cathode, where capacities up to 1364 mAh/g (Si0.67Ge0.33) were retained after 100 cycles.

Nanowire Heterostructures Comprising Germanium Stems and Silicon Branches as High-Capacity Li-Ion Anodes with Tunable Rate Capability.

Here we report the rational design of a high-capacity Li-ion anode material comprising Ge nanowires with Si branches. The unique structure provides an electrode material with tunable properties, allowing the performance to be tailored for either high capacity or high rate capability by controlling the mass ratio of Si to Ge. The binder free Si-Ge branched nanowire heterostructures are grown directly from the current collector and exhibit high capacities of up to ∼1800 mAh/g. Rate capability testing revealed that increasing the Ge content within the material boosted the performance of the anode at fast cycling rates, whereas a higher Si content was optimal at slower rates of charge and discharge. Using ex-situ electron microscopy, Raman spectroscopy and energy dispersive X-ray spectroscopy mapping, the composition of the material is shown to be transient in nature, transforming from a heterostructure to a Si-Ge alloy as a consequence of repeated lithiation and delithiation.