The impact of charge/discharge rate on capacity fade on composite structured cathodes in lithium ion batteries

We develop a cohesive understanding the effect of rate of cycle on the composite structure of a lithium ion battery. Electrochemical techniques are used to evaluate capacity, resistance, and rate capability. Together, capacity and resistance measurements are used to segregate chemical-induced degradation (associated with resistance rise and capacity loss) from mechanical-induced degradation (primarily associated with capacity loss) in the cathode for different c-rate cycling. Meanwhile, rate capability measurements reveal the impact of chemical and physical degradation on utilization of the electrodes at various c-rates. Raman spectroscopy is used to directly measure Li+ inventory losses caused by film growth on the composite anode surface and evaluate how it contributes to capacity loss measured electrochemically. Lastly, microscopy techniques assess mechanical damage in LiCoO2 particles in the form of micro-cracks and dislocation defects which are thought to impede solid-state Li ion diffusion. This was demonstrated through diffusivity measurements using galvanostatic intermittent titration techniques. Through the use of various testing techniques, it is determined that chemical degradation accounts for the entirety of capacity loss at slow c-rates. Meanwhile, at high c-rates it is both chemical and mechanical degradation that contributes to fade; where mechanical degradation plays an increasing role as c-rate is raised. Pointedly, higher c-rates effectively increase the strain rate for lithiation of LiCoO2 particles resulting in diffusion induced stresses that can lead to micro-crack formation, defect generation, and eventual particle fracture. Ultimately, micro-crack and defect formation limits electrode utilization at moderate to high c-rates resulting in reduced discharge capacity. To this end, diffusivity measurements using galvanostatic intermittent titration techniques are presented. By showing that Li inventory can be accounted for fully using the Raman techniques and quasi-equilibrium rates, we can assign mechanical effect and damage accumulation on cycling to the cathode loss of capacity, and ascribe a mechanical impact of the damage to the composite on the microscale, including the loss of solid state diffusivity observed as the composite undergoes successive charging and discharging

Reactive Ni-Al nanostructured composites through electrochemical dispersion deposition

Reactive metal composites (RMC) films are used in various applications from precision joining and brazing to local high heat sources for power generation and local melting. They are gasless, high heat generating, high propagation speed reactions that involve the release of energy when elemental constituents are combined to make intermetallics. These films require solid state diffusion at the nanoscale to function well, and the current method of manufacture is to sputter 20-50nm thick alternating films of the reactant elements to build up a ~50um thick composite film. This process is slow and costly, and does not lend itself well to generating unusual or three dimensional shapes. Specialized shapes must be cut using femtosecond lasers, adding to cost and complexity of the manufacturing process. We are investigating an alternative method of using dispersion electroplating to contain a dispersed phase (Ni) within a continuous phase (Al) that is also nanostructured. The propagation speed and calorific output of the films is dependent upon the overall stoichiometry of the films, and so a 1:1 atomic ratio is the most energetic, resulting in a 40% volumetric ratio of the dispersed phase, requiring the electrodeposition of the Al. Use of ionic liquid (IL) based electrochemical baths support a clean, oxide free interface between the dispersed phase and the matrix phase with roughly the same degree of mixing at the interface as is observed in the sputtered films. However, challenges arise from the use of these ionic liquids, in that the interaction of particles dispersed in the plating solution versus particle distribution within the plated solid phase is not linear, and depends upon several other parameters, such as shear rate at the interface, viscosity of the plating fluid and suspension, and the distribution of electronic charge across the particles when interacting with ionic liquids. We investigate these plating parameters and their effect on film deposition, smoothness, and dispersed phase incorporation, revealing that the viscosity of the plating fluid is non-Newtonian, and shear thinning during deposition. We hypothesize that this effect is due to the breakup of electrostatic structure within the ionic liquid and between the ionic liquid and the dispersed particulate phase. We also investigate the electrochemistry of the Al deposition from a number of salts within the ionic liquid, since these salts affect the rate of deposition of the Al matrix phase, and by extension, the relative ratio of reactants in the film. Finally, we discuss briefly the effect of these plating properties on both the calorific output of the plated film, and the reaction rate of the resultant films

Understanding the evolution of the silicon electrode SEI through model lithium silicate thin film layers

Development of higher capacity anodes in lithium ion batteries for use in electric vehicles is necessary to further enhance their energy density. Silicon anodes are being considered for these lithium ion batteries due to their high specific capacity. One drawback to silicon anodes is the formation of an unstable solid electrolyte interface (SEI). A major cause of this instability is due to silicon anode volume expansion of up to 300% during cycling. To this end, there remains much to learn about the chemical reactions occurring at the silicon surface. Because of this expansion, composite Si-graphite electrodes exhibit poor cycling performance, as well as significant capacity loss even at open circuit, “shelf” conditions in the absence of electrochemical cycling. Implicated in these processes is the role of the solid/electrolyte interphase (SEI) region between the Si solid material and the electrolyte systems that forms upon initial exposure to the electrolyte, and evolves over time. Thermodynamic arguments suggest that the formation of lithium silicate (LiSixOy) phases from the decomposition of the electrolyte at the silicon electrochemical potential play a role in SEI formation and evolution. To better understand the evolution of the SEI layer and the nature of silicates formed prior to any cycling of the silicon anode and how it impacts the performance of the silicon anode, model SEI layers were deposited on silicon thin films using RF magnetron co-sputtering. Thin film chemistries from SiO2 to Li3SiOx were synthesized to model the proposed lithiation of the oxide layer during the first cycle. The composition and structure of these thin films prior to exposure to electrolyte were analyzed. In order to observe the chemical reactivity of these model silicate thin films, they were soaked in 1.2M LiPF6 in EC:EMC 3:7 wt% electrolyte for up to 3 days, removed, rinsed and studied using Attenuated Total Reflectance Infrared Spectroscopy (ATR IR), X-ray Photoelectron spectroscopy (XPS) and Focused Ion Beam Cross-sections (FIB CS). Half cells with these same silicate model films were cycled to observe any differences in SEI formation or cell performance during electrochemical cycling. Please click Additional Files below to see the full abstract

Crystal structure of melaminium cyanoacetate monohydrate

The asymmetric unit of the title compound, 2,4,6-tri­amino-1,3,5-triazin-1-ium cyano­acetate monohydrate, C3H7N6+·NCCH2COO−·H2O, consists of a melaminium cation, a cyano­acetate anion and a water mol­ecule, which are connected to each other via N—H⋯O and O—H⋯O hydrogen bonds, generating an eight-membered ring. In the crystal, the melaminium cations are connected by two pairs of N—H⋯N hydrogen bonds, forming tapes along [110]. These tapes develop a three-dimensional network through N—H⋯O, O—H⋯O, N—H⋯N and C—H⋯O hydrogen bonds between the cations, anions and water mol­ecules

Oxide film formation from Electron Cyclotron Resonance (ECR) plasmas

The formation of SiO{sub x} films and fluorine-doped SiO{sub x} films using electron cyclotron resonance (ECR) plasma deposition is described. Parametric studies of the film composition and hydrogen content as a function of feed gas composition and RF biasing are presented. By replacing SiH{sub 4} with SiF{sub 4} in the gas feed, samples with F content from 2 at.% F to 12 at.% F are deposited, and the dielectric constant of the deposited layers decrease linearly with increasing fluorine concentration. The stability of these low dielectric constant SiO{sub x}F{sub y} layers is examined under hydrating conditions, and conditions typically found for interlayer dielectric processing in microelectronics. The hydrogen content of the SiO{sub 2} and F-doped SiO{sub 2} is characterized as a function of deposition conditions, and a model is given to describe the thermal release of H from SiO{sub 2}

P3 microengine development at Washington State University.

There is a pressing need for miniaturized power systems for a variety of applications requiring a long life in the field of operations. Such power systems are required to be capable of providing power for months to years of operation, which all but eliminates battery technologies and technologies that bring their own fuel systems (except for nuclear fuel systems, which have their own drawbacks) due to constraints of having the all of the chemical fuel necessary for the entire life of the operational run available at the starting point of the operation. Alternatively, harvesting energy directly from the local environment obviates this need for bringing along all of the fuel necessary for operation. Instead, locally available energy, either in the form of chemical, thermal, light, or motion can be harvested and converted into electrical energy for use in sensor applications. The work from this LDRD is focused on developing a thermal engine that can take scavenged thermal gradients and convert them into direct electrical energy. The converter system is a MEMS based external combustion engine that uses a modified Stirling cycle to generate mechanical work on a piezoelectric generator. This piezoelectric generator then produced an AC voltage and current that can be delivered into an external load. The MEMS engine works on the conversion of a two phase working fluid trapped between two deformable membranes. As heat is added to the system, the liquid working fluid is converted to a gas, which exerts pneumatic pressure on the membranes, expanding them outward. This outward expansion continues after the heat input is removed when the engine is operated at resonance, since the membrane is expanded further due to inertial forces. Finally, the engine cools and heat rejection is accomplished through the membranes, closing the thermodynamic cycle. A piezoelectric generator stack is deposited on one of the membranes, and this generator extracts the strain energy work from the membrane expansion and generates electrical work. The overall system is pulsed by an electrical heater to generate the input heat pulse. Currently, the system has a resonant frequency that is in the low kilohertz regime, but operations under a dynamic damping have demonstrated operation at resonance and the existence of an open mechanical cycle of heat addition, expansion, and heat rejection. Power generation of direct thermal-to-electrical conversion show a 1.45W, 6mJ heat pulse can generate a 0.8 {micro}W power output pulse, and continuous operation generates a sustained power output of 0.8 {micro}W at 240Hz. Future improvements in the device will allow active heat rejection, allowing resonance with external damping to improve the thermal to electrical power efficiency

Iron-catalyzed depolymerizations of end-of-life silicones with fatty alcohols

AbstractDuring the last decades, polymers became one of the major materials in our society and a future without polymers is hardly imaginable. However, as negative issue of this success enormous amount of end-of-life materials are accumulated, which are mainly treated by landfill storage, thermal recycling or down-cycling. On the other hand, feedstock recycling can be an interesting option to convert end-of-life polymers to high quality polymers, via depolymerization reactions to low-molecular weight building blocks and subsequent transformation via polymerization reactions. In this regard, we present herein the depolymerization of polysiloxanes (silicones) applying fatty alcohols as depolymerization reagents. In more detail, in the presence of catalytic amounts of simple iron salts, low-molecular weight products with the motif R(OSiMe2)mOR (R = alkyl, m = 1–2) were attained. Remarkably, the reaction of R(OSiMe2)mOR with water showed the formation of new cyclic siloxanes, which are useful starting materials for long-chain silicones, and the corresponding fatty alcohol as side product, which can be directly reused in subsequent depolymerization reactions. Importantly, a recycling of the silicones and a straightforward recycling of the depolymerization reagent are feasible

tert-Butoxy­triphenyl­silane

The title compound, C22H24OSi or Ph3SiOtBu, shows a distorted tetra­hedral coordination sphere around the Si atom. The C—O—Si angle is 135.97 (12)° and the O—Si distance is 1.6244 (13) Å. The mol­ecules are held together by weak inter­actions only. An H⋯H distance of 2.2924 (7) Å is found between aryl H atoms and is the shortest inter­molecular distance in the structure. With regard to the broad applicability of R 3SiO structural motifs in all fields of chemistry, the mol­ecule demonstrates a common model system for silicon centers surrounded by sterically demanding substituents

A miniaturized mW thermoelectric generator for nw objectives: continuous, autonomous, reliable power for decades.

We have built and tested a miniaturized, thermoelectric power source that can provide in excess of 450 {micro}W of power in a system size of 4.3cc, for a power density of 107 {micro}W/cc, which is denser than any system of this size previously reported. The system operates on 150mW of thermal input, which for this system was simulated with a resistive heater, but in application would be provided by a 0.4g source of {sup 238}Pu located at the center of the device. Output power from this device, while optimized for efficiency, was not optimized for form of the power output, and so the maximum power was delivered at only 41mV. An upconverter to 2.7V was developed concurrently with the power source to bring the voltage up to a usable level for microelectronics

Metastable Se6 as a ligand for Ag+: from isolated molecular to polymeric 1D and 2D structures

Attempts to prepare the hitherto unknown Se6 2+ cation by the reaction of elemental selenium and Ag[A] ([A]- = [Sb(OTeF5)6]-, [Al(OC(CF3)3)4]-) in SO2 led to the formation of [(OSO)Ag(Se6)Ag(OSO)][Sb(OTeF5)6]2 1 and [(OSO)2Ag(Se6)Ag(OSO)2][Al(OC(CF3)3)4]2 2a. 1 could only be prepared by using bromine as co-oxidant, however, bulk 2b (2a with loss of SO2) was accessible from Ag[Al(OC(CF3)3)4] and grey Se in SO2 (chem. analysis). The reactions of Ag[MF6] (M= As, Sb) and elemental selenium led to crystals of 1/∞{[Ag(Se6)]∞[Ag2(SbF6)3]∞} 3 and {1/∞[Ag(Se6)Ag]∞}[AsF6]2 4. Pure bulk 4 was best prepared by the reaction of Se4[AsF6]2, silver metal and elemental selenium. Attempts to prepare bulk 1 and 3 were unsuccessful. 1–4 were characterized by single-crystal X-ray structure determinations, 2b and 4 additionally by chemical analysis and 4 also by X-ray powder diffraction, FT-Raman and FT-IR pectroscopy. Application of the PRESTO III sequence allowed for the first time 109Ag MAS NMR investigations of 4 as well as AgF, AgF2, AgMF6 and {1/∞[Ag(I2)]∞}[MF6] (M= As, Sb). Compounds 1 and 2a/b, with the very large counter ions, contain isolated [Ag(Se6)Ag]2+ heterocubane units consisting of a Se6 molecule bicapped by two silver cations (local D3d sym). 3 and 4, with the smaller anions, contain close packed stacked arrays of Se6 rings with Ag+ residing in octahedral holes. Each Ag+ ion coordinates to three selenium atoms of each adjacent Se6 ring. 4 contains [Ag(Se6)+]∞ stacks additionally linked by Ag(2)+ into a two dimensional network. 3 features a remarkable 3-dimensional [Ag2(SbF6)3]- anion held together by strong Sb–F … Ag contacts between the component Ag+ and [SbF6]- ions. The hexagonal channels formed by the [Ag2(SbF6)3]- anions are filled by stacks of [Ag(Se6)+]∞ cations. Overall 1–4 are new members of the rare class of metal complexes of neutral main group elemental clusters, in which the main group element is positively polarized due to coordination to a metal ion. Notably, 1 to 4 include the commonly metastable Se6 molecule as a ligand. The structure, bonding and thermodynamics of 1 to 4 were investigated with the help of quantum chemical calculations (PBE0/TZVPP and (RI-)MP2/TZVPP, in part including COSMO solvation) and Born–Fajans–Haber-cycle calculations. From an analysis of all the available data it appears that the formation of the usually metastable Se6 molecule from grey selenium is thermodynamically driven by the coordination to the Ag+ ions