Robert Ionescu

Monodisperse Porous Silicon Spheres as Anode Materials for Lithium Ion Batteries

Highly monodisperse porous silicon nanospheres (MPSSs) are synthesized via a simple and scalable hydrolysis process with subsequent surface-protected magnesiothermic reduction. The spherical nature of the MPSSs allows for a homogenous stress-strain distribution within the structure during lithiation and delithiation, which dramatically improves the electrochemical stability. To fully extract the real performance of the MPSSs, carbon nanotubes (CNTs) were added to enhance the electronic conductivity within the composite electrode structure, which has been verified to be an effective way to improve the rate and cycling performance of anodes based on nano-Si. The Li-ion battery (LIB) anodes based on MPSSs demonstrate a high reversible capacity of 3105 mAh g−1. In particular, reversible Li storage capacities above 1500 mAh g−1 were maintained after 500 cycles at a high rate of C/2. We believe this innovative approach for synthesizing porous Si-based LIB anode materials by using surface-protected magnesiothermic reduction can be readily applied to other types of SiOx nano/microstructures.

Towards Scalable Binderless Electrodes: Carbon Coated Silicon Nanofiber Paper via Mg Reduction of Electrospun SiO2 Nanofibers

The need for more energy dense and scalable Li-ion battery electrodes has become increasingly pressing with the ushering in of more powerful portable electronics and electric vehicles (EVs) requiring substantially longer range capabilities. Herein, we report on the first synthesis of nano-silicon paper electrodes synthesized via magnesiothermic reduction of electrospun SiO2 nanofiber paper produced by an in situ acid catalyzed polymerization of tetraethyl orthosilicate (TEOS) in-flight. Free-standing carbon-coated Si nanofiber binderless electrodes produce a capacity of 802 mAh g−1 after 659 cycles with a Coulombic efficiency of 99.9%, which outperforms conventionally used slurry-prepared graphite anodes by over two times on an active material basis. Silicon nanofiber paper anodes offer a completely binder-free and Cu current collector-free approach to electrode fabrication with a silicon weight percent in excess of 80%. The absence of conductive powder additives, metallic current collectors, and polymer binders in addition to the high weight percent silicon all contribute to significantly increasing capacity at the cell level.

Bio-Derived, Binderless, Hierarchically Porous Carbon Anodes for Li-ion Batteries.

Here we explore the electrochemical performance of pyrolyzed skins from the species A. bisporus, also known as the Portobello mushroom, as free-standing, binder-free, and current collector-free Li-ion battery anodes. At temperatures above 900 °C, the biomass-derived carbon nanoribbon-like architectures undergo unique processes to become hierarchically porous. During heat-treatment, the oxygen and heteroatom-rich organics and potassium compounds naturally present in the mushroom skins play a mutual role in creating inner void spaces throughout the resulting carbon nanoribbons, which is a process analogous to KOH-activation of carbon materials seen in literature. The pores formed in the pyrolytic carbon nanoribbons range in size from sub-nanometer to tens of nanometers, making the nanoribbons micro, meso, and macroporous. Detailed studies were conducted on the carbon nanoribbons using SEM and TEM to study morphology, as well as XRD and EDS to study composition. The self-supporting nanoribbon anodes demonstrate significant capacity increase as they undergo additional charge/discharge cycles. After a pyrolysis temperature of 1100 °C, the pristine anodes achieve over 260 mAh/g after 700 cycles and a Coulombic efficiency of 101.1%, without the use of harmful solvents or chemical activation agents.

Synthesis of Atomically Thin

Atomically thin molybdenum disulfide (MoS2) triangles and hexagrams were prepared by a two-step growth ambient pressure chemical vapor deposition (APCVD) process. Molybdenum Trioxide (MoO3) nanobelts, a few microns in length and width, were prepared using a hydrothermal technique and utilized as the starting material. High temperature treatment of the MoO3 nanobelts followed by a rigorous sulfurization via APCVD processing provided different morphologies of MoS2 monolayers and bilayer (BL) sheets. Triangle and hexagram morphologies were characterized using Raman spectroscopy, photoluminescence (PL) measurements, scanning electron microscopy and atomic force microscopy (AFM). The regrowth step in the CVD process was proven to be ideal in enlarging the grain size. PL and Raman spectroscopy and AFM results confirmed the presence of monolayer and BL regions in the regrowth growth process. Triangle and hexagram domains are observed to be cooperatively nucleating and coalescing together to form large-area layers. Furthermore, the electrical transport properties of the synthesized MoS2 layers were studied. Electron mobility based on back gated field effect transistors was measured to be approximately 0.02 cm2/V. S.

{\bf MoS}_{\bf 2}

Silicon is produced in a variety of ways as an ultra-high capacity lithium-ion battery (LIB) anode material. The traditional carbothermic reduction process required is expensive and energy-intensive; in this work, we use an efficient magnesiothermic reduction to convert the silica-based frustules within diatomaceous earth (diatomite, DE) to nanosilicon (nanoSi) for use as LIB anodes. Polyacrylic acid (PAA) was used as a binder for the DE-based nanoSi anodes for the first time, being attributed for the high silicon utilization under high current densities (up to 4C). The resulting nanoSi exhibited a high BET specific surface area of 162.6 cm2 g−1, compared to a value of 7.3 cm2 g−1 for the original DE. DE contains SiO2 architectures that make ideal bio-derived templates for nanoscaled silicon. The DE-based nanoSi anodes exhibit good cyclability, with a specific discharge capacity of 1102.1 mAh g−1 after 50 cycles at a C-rate of C/5 (0.7 A gSi−1) and high areal loading (2 mg cm−2). This work also demonstrates the fist rate capability testing for a DE-based Si anode; C-rates of C/30 - 4C were tested. At 4C (14.3 A gSi−1), the anode maintained a specific capacity of 654.3 mAh g−1 – nearly 2x higher than graphite’s theoretical value (372 mAh g−1).

Triangles and Hexagrams and Their Electrical Transport Properties

It is of paramount importance to improve the control over large area growth of high quality molybdenum disulfide (MoS2) and other types of 2D dichalcogenides. Such atomically thin materials have great potential for use in electronics, and are thought to make possible the first real applications of spintronics. Here in, a facile and reproducible method of producing wafer scale atomically thin MoS2 layers has been developed using the incorporation of a chelating agent in a common organic solvent, dimethyl sulfoxide (DMSO). Previously, solution processing of a MoS2 precursor, ammonium tetrathiomolybdate ((NH4)2MoS4), and subsequent thermolysis was used to produce large area MoS2 layers. Our work here shows that the use of ethylenediaminetetraacetic acid (EDTA) in DMSO exerts superior control over wafer coverage and film thickness, and the results demonstrate that the chelating action and dispersing effect of EDTA is critical in growing uniform films. Raman spectroscopy, photoluminescence (PL), x-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM) and high-resolution scanning transmission electron microscopy (HR-STEM) indicate the formation of homogenous few layer MoS2 films at the wafer scale, resulting from the novel chelant-in-solution method.

Carbon-Coated, Diatomite-Derived Nanosilicon as a High Rate Capable Li-ion Battery Anode

Herein, we report on chemical vapor deposition (CVD) of partially oxidized graphene (POG) films on electropolished polycrystalline copper foils at relatively low temperature under near-atmospheric pressure. The structural, chemical, and electronic properties of the films are studied in detail using several spectroscopic and microscopic techniques. The content of carbon and oxygen in the films is identified by chemical mapping at near-atomic scale. Electron diffraction patterns of the films possess clear diffraction spots with a six-fold pattern that is consistent with the hexagonal lattice. The fine structure of the carbon K-edge signal in STEM-EELS spectra of the films is distinguishable from that of graphene and graphite. The presence of oxygen in the films is further supported by a clear oxygen K-edge. Raman spectroscopy and XPS results provide direct evidence for a lower degree of oxidation. The work function of the films is found to be much higher than that of graphene, using UPS measurements.

Chelant Enhanced Solution Processing for Wafer Scale Synthesis of Transition Metal Dichalcogenide Thin Films

Three distinctive pyrolytic carbon structures, derived from three specific tissues of Agaricus bisporus mushroom, were studied and characterized. The three structures discovered within the stalk, cap, and cap skin tissues were found to contain unique microarchitectures, which were preserved upon anoxic carbonization. Experiments also revealed the formation of salt pockets and deposits within each microarchitecture, leading to a potential natural hard-template method for porous carbon structures.