Tylan Watkins

Stable silicon-ionic liquid interface for next-generation lithium-ion batteries

We are currently in the midst of a race to discover and develop new battery materials capable of providing high energy-density at low cost. By combining a high-performance Si electrode architecture with a room temperature ionic liquid electrolyte, here we demonstrate a highly energy-dense lithium-ion cell with an impressively long cycling life, maintaining over 75% capacity after 500 cycles. Such high performance is enabled by a stable half-cell coulombic efficiency of 99.97%, averaged over the first 200 cycles. Equally as significant, our detailed characterization elucidates the previously convoluted mechanisms of the solid-electrolyte interphase on Si electrodes. We provide a theoretical simulation to model the interface and microstructural-compositional analyses that confirm our theoretical predictions and allow us to visualize the precise location and constitution of various interfacial components. This work provides new science related to the interfacial stability of Si-based materials while granting positive exposure to ionic liquid electrochemistry.

Designer Ionic Liquids for Reversible Electrochemical Deposition/Dissolution of Magnesium

Chelating ionic liquids (ILs), in which polyether chains are pendent from the organic pyrrolidinium cation of the ILs (PEGylated ILs), were prepared that facilitate reversible electrochemical deposition/dissolution of Mg from a Mg(BH4)2 source. Mg electrodeposition processes in two specific PEGylated-ILs were compared against that in the widely studied N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquid (BMPyrTFSI). The two chelating IL systems (one with a pendent polyether chain with three ether oxygens, MPEG3PyrTFSI, and the other with a seven-ether chain, MPEG7PyrTFSI) showed substantial improvement over BMPyrTFSI for Mg electrodeposition/dissolution. The best overall electrochemical performance was in MPEG7PyrTFSI. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS) were used to characterize galvanostatically deposited Mg, revealing production of pure, dendrite-free Mg deposits. Reversible Mg electrodeposition was achieved with high Coulombic efficiency (CE) of 90% and high current density (ca. 2 mA/cm(2) for the stripping peak). Raman spectroscopy was used to characterize Mg(2+) speciation in the PEGylated ILs and BMPyrTFSI containing Mg(BH4)2 by study of Raman modes of the coordinated and free states of borohydride, TFSI(-), and polyether COC groups. Quantitative analysis revealed that the polyether chains can displace both TFSI(-) and BH4(-) from the coordination sphere of Mg(2+). Comparison of the different IL electrolytes suggested that these displacement reactions may play a role in enabling Mg deposition/dissolution with high CE and current density in these PEGylated IL media. These results represent the first demonstration of reversible electrochemical deposition/dissolution of Mg in an ionic liquid specifically designed with this task in mind.

Nanosynthesis routes to new tetrahedral crystalline solids: Silicon-like Si3AlP

We introduce a synthetic strategy to access functional semiconductors with general formula A(3)XY (A = IV, X-Y = III-V) representing a new class within the long-sought family of group IV/III-V hybrid compounds. The method is based on molecular precursors that combine purposely designed polar/nonpolar bonding at the nanoscale, potentially allowing precise engineering of structural and optical properties, including lattice dimensions and band structure. In this Article, we demonstrate the feasibility of the proposed strategy by growing a new monocrystalline AlPSi(3) phase on Si substrates via tailored interactions of P(SiH(3))(3) and Al atoms using gas source (GS) MBE. In this case, the high affinity of Al for the P ligands leads to Si(3)AlP bonding arrangements, which then confer their structure and composition to form the corresponding Si(3)AlP target solid via complete elimination of H(2) at ∼500 °C. First principle simulations at the molecular and solid-state level confirm that the Si(3)AlP building blocks can readily interlink with minimal distortion to produce diamond-like structures in which the P atoms are arranged on a common sublattice as third-nearest neighbors in a manner that excludes the formation of unfavorable Al-Al bonds. High-resolution XRD, XTEM, and RBS indicate that all films grown on Si(100) are tetragonally strained and fully coherent with the substrate and possess near-cubic symmetry. The Raman spectra are consistent with a growth mechanism that proceeds via full incorporation of preformed Si(3)AlP tetrahedra with residual orientational disorder. Collectively, the characterization data show that the structuro-chemical compatibility between the epilayer and substrate leads to flawless integration, as expected for pseudohomoepitaxy of an Si-like material grown on a bulk Si platform.

Designer hydride routes to ‘Si–Ge’/(Gd,Er)2O3/Si(1 1 1) semiconductor-on-insulator heterostructures

We demonstrate Si?Ge integration on engineered M2O3/Si(1?1?1) (M = Gd,Er) dielectric buffer layers using non-traditional chemical precursors that provide new levels of functionality within the deposition process. Stoichiometric Si0.50Ge0.50 alloys and pure Si heterostructures are grown epitaxially via ultra-low-temperature chemical vapor deposition using SiH3GeH3 and Si3H8/Si4H10, respectively. In the case of Si on Gd2O3, an optimal growth processing window in the range of 500?600 ?C was found to yield planar layers with monocrystalline structures via a proposed coincidence lattice matching mechanism (2aSi?aGd2O3), while for the SiGe system (2% lattice mismatch) comparable quality films with fully relaxed strain states are deposited at a lower temperature range of 420?450 ?C. Extension of this growth process to Si on Er2O3 yields remarkably high-quality layers in spite of the even larger ~3% lattice mismatch. In all cases, the Si?Ge overlayers are found to primarily adopt an A?B?A epitaxial alignment with respect to the M2O3 buffered Si(1?1?1). A comparative study of the Si growth using Si3H8 and Si4H10 indicates that both compounds provide an efficient and straightforward process for semiconductor growth on Gd2O3/Si(1?1?1), which appears to be more viable than conventional approaches from the point of view of scalability and volume.