Calvin J. Curtis

Preparation and Characterization of LiMn2 O 4 Spinel Nanoparticles as Cathode Materials in Secondary Li Batteries

Nanoparticles of LiMn 2 O 4 spinels have been synthesized through a sol-gel method followed by annealing at various temperatures between 350 and 550°C. The particle sizes depend on annealing temperatures. At a low annealing temperature (350°C), the nanoparticles may have a size of 10 nm, estimated from transmission electron microscopy photographs. The annealing temperatures higher than 550°C give rise to particles larger than nanoscale. Some large porous Li 1.4 Mn 2 O 4 particles with 1 μm size were prepared for comparison through an emission process. The electrochemical tests indicate that those nanoparticles reduce capacities in the 4 V discharge region but improve rechargeability of the electrode materials. On the other hand, the nanoparticles significantly improve both capacities and cycling performance in the 3 V discharge region. The electrochemical properties of these materials are further investigated using cyclic voltammetry and ac impedance spectroscopy. The results reveal that the homogeneous-phase transition occurring in the 3 V region and at the first discharge plateau of the 4 V discharge region increases the difficulty of the Li + insertion/extraction process and affects rechargeability of the electrode materials. However, the nanoparticles and porous materials reduce charge-transfer resistances between the composite electrodes and electrolyte interface and increase the effectiveness of lithium intercalation into the active materials in comparison with those large but nonporous materials.

Hydrogen-transfer reactions which generate new imine, imido, and trimethylenemethane complexes of tantalum

The reactions of Cp*TaMe3C1 (Cp* = qs-CsMeS) with a variety of alkali-metal alkoxide, alkylamide, and alkyl reagents have been examined. Reaction with LiNMe2 produces Cp*Ta(NMez)Me3, which decomposes at 25 OC to an imine (or metallaazirane) complex, Cp*Ta(CH2NMe)Me2. The decomposition is a first-order, unimolecular process with a large kinetic isotope effect (kH/kD = 9.7). Monoalkylamides (LiNHR) react with Cp*TaMe3C1 to form imido complexes Cp*Ta(NR)Mez. Reaction of Cp*TaMe3C1 with lithium diisopropylamide forms a bridging methylene complex, Cp*Me2Ta(pCH2)(p-H)zTaMezCp*. The alkoxide compounds Cp*Ta(OR)Me3 (R = Me, CHMe2, CMe3) are very stable and decompose only over 100 OC. Alkyl complexes are stable only if the alkyl group does not have j3-hydrogens. Treatment of Cp*TaMe3C1 with (2-methylally1)magneium bromide affords an unstable tantalum 2-methylallyl compound, which decomposes cleanly to the trimethylenemethanec omplex Cp*TaMeZ(q4-C(CH2),)T. he rates of hydrogen abstraction or elimination processes in this system correlate with the nature of the atom bound to tantalum: for reactions involving a /3-hydrogen transfer the order is C > N > 0, while the facility of a-hydrogen abstraction reactions appear to decrease in the reverse order N > C. These reactivity patterns appear to reflect the variance in T a x , Ta-N, and Ta-O bond energies in this series. Hydrogenation of the imido compounds (Cp*Ta(NR)Me2) in the presence of phosphine ligands yields new examples of imido hydride complexes Cp*Ta(NR)H,(L) (L = PMe3, PMe2(C6Hs); R = CMe3, CHzCMe3). A moderately stable alkyl hydride complex, Cp*Ta(CH2NMe)Me(PMe3)H, has also been prepared.

NANOPARTICLE PRECURSOR ROUTE TO LOW-TEMPERATURE SPRAY DEPOSITION OF CDTE THIN FILMS

In this letter we report a nanoparticle‐derived route to CdTe thin films. CdTe nanoparticles 39±8 A in diameter, prepared by an organometallic route, were characterized by x‐ray diffraction, UV‐Vis spectroscopy, transmission electron microscopy, and energy dispersive x‐ray spectroscopy. CdTe thin‐film deposition was realized by spraying a nanoparticle/butanol colloid onto SnO2‐coated glass substrates at variable susceptor temperatures. The resultant CdTe films were characterized by atomic force microscopy, x‐ray diffraction, and UV‐Vis spectroscopy. Smooth and dense CdTe thin films were obtained using growth temperatures ∼200 °C less than conventional spray pyrolysis. A growth temperature dependence upon CdTe grain size formation and crystallinity was observed by atomic force microscopy and x‐ray diffraction. UV‐Vis characterization revealed a transformation in the optical properties of the CdTe thin films as a function of growth temperature.

Fast-Switching Electrochromic Li+-Doped NiO Films by Ultrasonic Spray Deposition

A low cost, high throughput deposition method for films of nickel oxide NiO and lithium-doped nickel oxide with improved electrochromic performance is demonstrated. This method is based on ultrasonic spray deposition of aqueous-based precursor solutions in air at atmospheric pressure, which represents a significant cost savings compared to vacuum deposition methods. The resultant materials are characterized by X-ray diffraction, Raman spectroscopy, electron microscopy, and electrochemical measurements. Electrochromic performance is demonstrated with in situ optical transmission measurements during electrochemical characterization. Nickel oxide materials color anodically and are thereby ideally suited to be used as counter electrode for the well-known tungsten oxide WO3 system in “smart” window applications. The coloration of nickel oxide materials is known to be slow when compared to WO3 and thereby limits the overall response time of a NiO/WO3 tandem device. The analysis of potential step response data shows that our lithium-doped nickel oxide material achieves 90% of its total coloration change in 29 s, which is comparable to reported measurements for WO3. These results significantly mitigate a potential bottleneck to the adoption of metal oxide electrochromic windows not only by demonstrating similar performance between NiO and WO3, but by achieving this result via low cost, highly scalable processing methods.

Influences of Treatment Temperature and Water Content on Capacity and Rechargeability of V 2 O 5 Xerogel Films

V 2 O 5 xerogel films prepared with spray deposition have been investigated for their lithium battery applications. The influences of spray deposition and postannealing temperatures on the water content, structure, and electrochemical performance of the films have been studied. Water contents of the V 2 O 5 .nH 2 O films depend on the deposition and postannealing temperatures. One film sprayed at 150°C and annealed at 190°C exhibited a large discharge capacity and the longest cycling life. By contrast, the films sprayed or annealed at high temperatures (>250°C) showed poor electrochemical performance. The film deposited at a substrate temperature of 420°C demonstrated structural features and electrochemical behavior of orthorhombic V 2 O 5 . X-ray diffraction patterns of the films treated at different temperatures reveal that removal of too much water at high temperatures results in crystallization or a small interlayer spacing, which may restrict the diffusion of lithium ions inside V 2 O 5 .nH 2 O films. 51 V solid-state nuclear magnetic resonance spectra show that the films annealed at low temperatures (i.e., 120°C or room temperature) have broad peaks and significant downfield isotropic chemical shifts, suggesting that the interlayered water produces a distorted square-pyramidal ligand field that may favor lithium intercalation and electron transfer within the matrix of the films.

Next generation V2O5 cathode materials for Li rechargeable batteries

We report on investigations of vanadium oxide thin film cathodes prepared by three different synthesis techniques. Our experimental results on PLD-grown, textured V 2 O 5 crystalline films concur with reports in the literature that there is a voltage threshold above which, cycling appears to be completely reversible and below which, cycling appears to be irreversible. Crystalline films discharged beyond the threshold to 2.0 V exhibited an immediate and continuous fade in capacity as well as a nearly 90% decrease in XRD peak intensity and a similar decrease in Raman signal intensity in as few as ten cycles. PLD-grown amorphous films show capacity loss of < 2% over 200 cycles. Amorphous plasma-enhanced chemical vapor deposition (PECVD) films have capacities as high as 1.5 Li/V with excellent stability over 3000 cycles. Solution-grown nanoparticles (< 100 nm) of VO 2 were spray-deposited and sintered at relatively low temperatures to produce nanoporous films. Cycling properties along with structural investigations by XRD and Raman scattering will be presented.

Direct write processing for photovoltaic cells

Direct writing of solar cell components is an attractive processing approach. We have fabricated a 6.8% Si solar cell using silver ink based electrodes. Ohmic contact through the antireflection (AR) coating was obtained with pure Ag electrodes at 850/spl deg/C. We also report on highly conductive silver metallizations and initial results on direct-write TCO demonstrating a 100-micron spatial resolution produced by inkjet printing.

Multi-Layer Inkjet Printed Contacts for Silicon Solar Cells

Ag, Cu, and Ni metallizations were inkjet printed with near vacuum deposition quality. The approach developed can be easily extended to other conductors such as Pt, Pd, Au, etc. Thick highly conducting lines of Ag and Cu demonstrating good adhesion to glass, Si, and printed circuit board (PCB) have been printed at 100-200degC in air and N2 respectively. Ag grids were inkjet-printed on Si solar cells and fired through the silicon nitride AR layer at 850degC, resulting in 8% cells. Next generation inks, including an ink that etches silicon nitride, have now been developed. Multi-layer inkjet printing of the etching ink followed by Ag ink produced contacts under milder conditions and gave solar cells with efficiencies as high as 12%

Spray and Inkjet Printing of Hybrid Nanoparticle-Metal-Organic Inks for Ag and Cu Metallizations

Metal-organic and hybrid metal-organic/metal nanoparticle inkswere evaluated for use in the inkjet printing of copper and silver conducting lines. Pure, smooth, dense, highly conductive coatings were produced by spray printing with (hexafluoroacetylacetonato)copper(I)-vinyltrimethylsilane Cu(hfa)·VTMS) and (hexafluoroacetylacetonato)silver(I)(1,5-cyclooctadiene) (Ag(hfa)COD) metal-organic precursors on heated substrates. Good adhesion to the substrates tested, glass, Kapton tape and Si, has been achieved without use of adhesion promoters. The silver metal-organic ink has also beenused to print metal lines and patterns with a commercial inkjet printer. Hybrid inks comprised of metal nanoparticles mixed with the metal-organic complexes above have also been used to deposit Cu and Ag films by spray printing.This approach gives dense, adherent films that are much thicker than those obtained using the metal-organic inks alone. The conductivities of the silvercoatings obtained by both approaches are near that of bulk silver (2 μΩ·cm). The copper coatings had conductivities at least an order ofmagnitude less than bulk.

Nickel oxide interlayer films from nickel formate–ethylenediamine precursor: influence of annealing on thin film properties and photovoltaic device performance

An organometallic ink based on the nickel formate–ethylenediamine (Ni(O2CH)2(en)2) complex forms high performance NiOx thin film hole transport layers (HTL) in organic photovoltaic (OPV) devices. Improved understanding of these HTLs functionality can be gained from temperature-dependent decomposition/oxidation chemistries during film formation and corresponding chemical structure-function relationships for energetics, charge selectivity, and transport in photovoltaic platforms. Investigations of as-cast films annealed in air (at 150 °C–350 °C), with and without subsequent O2-plasma treatment, were performed using thermogravimetric analysis, Fourier transform infrared spectroscopy, ultraviolet and X-ray photoelectron spectroscopy, and spectroscopic ellipsometry to elucidate the decomposition and oxidation of the complex to NiOx. Regardless of the anneal temperature, after exposure to O2-plasma, these HTLs exhibit work functions greater than the ionization potential of a prototype donor polymer poly(N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT), thereby meeting a primary requirement of energy level alignment. Thus, bulk-heterojunction (BHJ), OPV solar cells made on this series of NiOx HTLs all exhibit similar open circuit voltages (Voc). In contrast, the short circuit currents increase significantly from 1.7 to 11.2 mA cm−2 upon increasing the anneal temperature from 150 °C to 250 °C. Concomitantly, increased conductivity and electrical homogeneity of NiOx thin films are observed at the nanoscale using conductive tip-AFM. Similar Voc observed for all the O2-plasma treated NiOx interlayers and variations to nanoscale conductivity suggest that the HTLs all form charge selective contacts and that their carrier extraction efficiency is determined by the amount of precursor conversion to NiOx. The separation of these two properties: selectivity and conductivity, sheds further light on charge selective interlayer functionality.