Selective Chloride-Mediated Neat Ethanol Oxidation to 1,1-Diethoxyethane via an Electrochemically Generated Ethyl Hypochlorite Intermediate.

Selective primary alcohol oxidation to form aldehydes products without overoxidation to carboxylic acids remains a key chemistry challenge. Using simple alkylammonium chloride as the electrolyte with a glassy carbon working electrode in neat ethanol solvent, 1,1-diethoxyethane (DEE) was prepared with >95% faradaic efficiency (FE). DEE serves as a storage platform protecting acetaldehyde from overoxidation and volatilization. UV-vis spectroscopy shows that the reaction proceeds through an ethyl hypochlorite intermediate as the sole chloride oxidation product, and that this intermediate decomposes unimolecularly (rate constant k = (6.896 ± 0.516) × 10-4 s-1) to form HCl catalyst and acetaldehyde, which undergoes rapid nucleophilic attack by ethanol solvent to form the DEE product. This indirect oxidation mechanism enables ethanol oxidation at much less positive potentials due to the fast kinetics for chloride anion oxidation.

Microwave Synthesis of Spinel MgFe2O4 Nanoparticles and the Effect of Annealing on Photocatalysis.

Through microwave heating in ethanol and with subsequent annealing, crystalline MgFe2O4 nanoparticles are produced rapidly and in high yields >99%. Under varied annealing temperatures, the degree of Mg and Fe site inversion changes the optical, electronic, and composition of the nanoparticles. A small particle size of ∼10 nm is achievable with the aid of an ammonium salt mineralizer that caps the particles during nucleation and growth. Particles with the lowest inversion parameter and limited sintering upon annealing (at 600 °C) exhibit the greatest production of hydroxyl radicals under visible light illumination. As such, these particles also facilitate the degradation of methylene blue dye faster than those particles annealed at higher temperature and show a rate constant of 0.061 h-1 for degrading 10 ppm methylene blue with 20 mg of catalyst.

pH-Dependence of Binding Constants and Desorption Rates of Phosphonate- and Hydroxamate-Anchored [Ru(bpy)3]2+ on TiO2 and WO3.

The binding constants and rate constants for desorption of the modified molecular dye [Ru(bpy)3]2+ anchored by either phosphonate or hydroxamate on the bipyridine ligand to anatase TiO2 and WO3 have been measured. In aqueous media at pH 1-10, repulsive electrostatic interactions between the negatively charged anchor and the negatively charged surface govern phosphonate desorption under neutral and basic conditions for TiO2 anatase due to the high acidity of phosphonic acid (p Ka,4 = 5.1). In contrast, the lower acidity of hydroxamate (p Ka,1 = 6.5, p Ka,2 = 9.1) leads to little change in adsorption/desorption properties as a function of pH from 1 to 7. The binding constant for hydroxamate is 103 in water, independent of pH in this range. These results are true for WO3 as well, but are not reported at pH > 4 due to its Arrhenius acidity. Kinetics for desorption as a function of pH are reported, with a proposed mechanism for phosphonate desorption at high pH being the electrostatic repulsion of negative charges between the surface and the anionic anchor. Further, the hydroxamic acid anchor itself is likely the site of quasi-reversible redox activity in [Ru(bpy)2(2,2'-bpy-4,4'-(C(O)N(OH))2)]2+, which does not lead to any measurable deterioration of the complex within 2 h of dark cyclic voltammogram scans in aqueous media. These results posit phosphonate as the preferred anchoring group under acidic conditions and hydroxamate for neutral/basic conditions.

Advancing the Chemistry of CuWO4 for Photoelectrochemical Water Oxidation.

Photoelectrochemical (PEC) cells are an ongoing area of exploration that provide a means of converting solar energy into a storable chemical form (molecular bonds). In particular, using PEC cells to drive the water splitting reaction to obtain H2 could provide a clean and sustainable route to convert solar energy into chemical fuels. Since the discovery of catalytic water splitting on TiO2 photoelectrodes by Fujishima and Honda, significant efforts have been directed toward developing high efficiency metal oxides to use as photocatalysts for this reaction. Improving the efficiency of PEC cells requires developing chemically stable, and highly catalytic anodes for the oxygen-evolution reaction (OER). This water oxidation half reaction requires four protons and four electrons coupling in two bond making steps to form O2, which limits the rate. Our group has accelerated efforts in CuWO4 as a candidate for PEC OER chemistry. Its small band gap of 2.3 eV allows for using visible light to drive OER, and the reaction proceeds with a high degree of chemoselectivity, even in the presence of more kinetically accessible anions such as chloride, which is common to seawater. Furthermore, CuWO4 is a chemically robust material when subjected to the highly oxidizing conditions of PEC OER. The next steps for accelerating research using this (and other), ternary phase oxides, is to move beyond reporting the basic PEC measurements to understanding fundamental chemical reaction mechanisms operative during OER on semiconductor surfaces. In this Account, we outline the process for PEC OER on CuWO4 thin films with emphasis on the chemistry of this reaction, the reaction rate and selectivity (determined by controlled-potential coulometry and oxygen-detection experiments). We discuss key challenges with CuWO4 such as slow kinetics and the presence of an OER-mediating mid-gap state, probed by electrochemical impedance spectroscopy. We propose that this mid-gap state imparts the observed chemoselectivity of OER on CuWO4. We introduce insights into the chemical mechanism of PEC OER on CuWO4 using Tafel analysis of electrochemical polarization. We measure Tafel slopes of ∼161 mV/dec, showing that PEC OER proceeds at a slower rate on CuWO4 than on common electrocatalysts for this reaction. Moreover, the observed photocurrent is independent of the borate buffer concentration, signaling that the buffer plays no role in the rate-determining elementary step of the reaction. Finally, we explore some recent developments in doping this material with Co (a known electrocatalytically active metal) and in coupling it with a transparent manganese phosphate (MnPO) electrocatalyst. We find that introducing Co into the wolframite structure leads to detrimental recombination of photogenerated charge carriers. However, coupling CuWO4 with MnPO increases the photocurrent density. Despite some of these challenges, CuWO4 proves to be a robust, visible light absorbing photoanode that can oxidize water with a high degree of selectivity and is therefore worthy of further exploration. Even if new compositions emerge that show better reactivity, this material serves as an excellent proving ground for the common challenges in developing ternary-phase oxides and other compositionally complex materials.

Determining the Fate of a Non-Heme Iron Oxidation Catalyst Under Illumination, Oxygen, and Acid.

We analyze the stability of the non-heme water oxidation catalyst (WOC), Fe(bpmcn)Cl2 toward oxygen and illumination under nonaqueous and acidic conditions. Fe(bpmcn)Cl2 has been previously used as a C-H activation catalyst, a homogeneous WOC, and as a cocatalyst anchored to WO3 for photoelectrochemical water oxidation. This paper reports that the ligand dissociates at pH 1 with a rate constant k = 19.8(2) × 10-3 min-1, resulting in loss of catalytic activity. The combination of UV-vis experiments, 1H NMR spectroscopy, and cyclic voltammetry confirm free bpmcn and Fe2+ present in solution under acidic conditions. Even under nonaqueous conditions, both oxygen and illumination together show slow oxidation of iron over the course of a few hours, consistent with forming an Fe3+-O2- intermediate as corroborated by resonance-enhanced Raman spectroscopy, with a rate constant of k = 3.03(8) × 10-3 min-1. This finding has implications in both the merits of non-heme iron complexes as WOCs as well as cocatalysts in photoelectrochemical schemes: the decomposition mechanisms may include both anchoring group hydrolysis and instability under illumination.

Anchoring a molecular iron catalyst to solar-responsive WO3 improves the rate and selectivity of photoelectrochemical water oxidation.

Molecular catalysts help overcome the kinetic limitations of water oxidation and generally result in faster rates for water oxidation than do heterogeneous catalysts. However, molecular catalysts typically function in the dark and therefore require sacrificial oxidants such as Ce(4+) or S2O8(2-) to provide the driving force for the reaction. In this Communication, covalently anchoring a phosphonate-derivatized complex, Fe(tebppmcn)Cl2 (1), to WO3 removes the need for a sacrificial oxidant and increases the rate of photoelectrochemical water oxidation on WO3 by 60%. The dual-action catalyst, 1-WO3, also gives rise to increased selectivity for water oxidation in pH 3 Na2SO4 (56% on bare WO3, 79% on 1-WO3). This approach provides promising alternative routes for solar water oxidation.

Triiodide Anion as a Magnesium-ion Transporter for Low Overpotential Battery Cycling in Iodine-Containing Mg(TFSI)2 Electrolyte.

Magnesium iodide (MgI2) solid-electrolyte interface (SEI) layers have previously been shown to protect Mg metal anodes from passivation through products formed during Mg(TFSI)2 electrolyte decomposition (TSFI = trifluorosulfonimide). MgI2 formed in situ from small quantities of I2 added to the electrolyte shows a drastic decrease in the overpotential for magnesium deposition and stripping. In this work, a MgI2 SEI layer was created in an ex situ fashion and then the electrochemical characteristics of this MgI2 SEI layer were probed both alone and with small quantities of I2 or Bu4NI3 additives to identify the electroactive species. Chronopotentiometry (CP) and cyclic voltammetry (CV) show that the MgI2 SEI alone is insufficient for low overpotential magnesium cycling. I(3d) XPS data show that I3- is formed within the SEI layer, which can serve as the electroactive species when ligated with Mg2+ for low overpotential (<50 mV at 0.1 mA cm-2 current density) cycling. Moreover, Raman shifts at 110 and 140 cm-1 are consistent with I3- formation, and these signatures are observed before and after CP experiments. The Mg0 deposition curves in the CV with additives are consistent with diffusive species. Finally, electrochemical impedance spectroscopy (EIS) shows that there is a large decrease in the charge-transfer resistance within the SEI when either I2 or Bu4NI3 additives are used, which supports a solvating effect that facilitates magnesium deposition and stripping.

Visible light water oxidation using a co-catalyst loaded anatase-structured Ti(1-(5x/4))Nb(x)O(2-y-δ)N(y) compound.

The photocatalytic activity of anatase-structured Ti(1-(5x/4))Nb(x)O(2-y-δ)N(y) (x = 0.25, y = 0.02; NbN-25) was examined for water oxidation under UV and visible light irradiation. The semiconductor was prepared by sol-gel processing followed by nitridation in flowing ammonia and exhibits an indirect optical gap of 2.2 eV. Ti(1-(5x/4))Nb(x)O(2-y-δ)N(y) was loaded with RuO2 by an impregnation technique, and optimized conditions reveal that 1 wt % RuO2 generates 16 µmol O2 from water with concomitant IO3(-) reduction after 3 h of illumination under simulated solar radiation at a flux of 600 mW/cm(2) illumination, which corresponds to 6-sun AM1.5G illumination (compared to no detectible O2 without the RuO2 cocatalyst). A series of cut-on filters shows that the catalyst-loaded semiconductor evolves O2 for λ ≤ 515 nm, and a gas-phase mass spectrometry isotope labeling experiment shows that irradiating an iodate solution in H2(18)O in the presence of 1 wt % RuO2 loaded on NbN-25 gives rise to catalytic water oxidation: both (36)O2 and (34)O2 are observed. It is unclear whether (16)O arises from IO3(-) or surface reconstruction on the photocatalyst, but ICP-AES analysis of the postirradiated solution shows no dissolved metal ions.

Template-free preparation of crystalline Ge nanowire film electrodes via an electrochemical liquid-liquid-solid process in water at ambient pressure and temperature for energy storage.

The direct electrodeposition of crystalline germanium (Ge) nanowire film electrodes from an aqueous solution of dissolved GeO(2) using discrete 'flux' nanoparticles capable of dissolving Ge(s) has been demonstrated. Electrodeposition of Ge at inert electrode substrates decorated with small (<100 nm), discrete indium (In) nanoparticles resulted in crystalline Ge nanowire films with definable nanowire diameters and densities without the need for a physical or chemical template. The Ge nanowires exhibited strong polycrystalline character as-deposited, with approximate crystallite dimensions of 20 nm and a mixed orientation of the crystallites along the length of the nanowire. Energy dispersive spectroscopic elemental mapping of individual Ge nanowires showed that the In nanoparticles remained at the base of each nanowire, indicating good electrical communication between the Ge nanowire and the underlying conductive support. As-deposited Ge nanowire films prepared on Cu supports were used without further processing as Li(+) battery anodes. Cycling studies performed at 1 C (1624 mA g(-1)) indicated the native Ge nanowire films supported stable discharge capacities at the level of 973 mA h g(-1), higher than analogous Ge nanowire film electrodes prepared through an energy-intensive vapor-liquid-solid nanowire growth process. The cumulative data show that ec-LLS is a viable method for directly preparing a functional, high-activity nanomaterials-based device component. The work presented here is a step toward the realization of simple processes that make fully functional energy conversion/storage technologies based on crystalline inorganic semiconductors entirely through benchtop, aqueous chemistry and electrochemistry without time- or energy-intensive process steps.

Structure, optical properties, and magnetism of the full Zn(1-x)Cu(x)WO4 (0 ≤ x ≤ 1) composition range.

Microcrystalline and submicrometer powders of Zn(1-x)Cu(x)WO(4) (0 ≤ x ≤ 1) have been prepared by a solid-state synthesis from stoichiometric quantities of the constituent d-block metal oxide and tungsten oxide as well as from a Pechini sol-gel synthesis starting from the d-block metal nitrate and ammonium metatungstate. The stoichiometry of the product is confirmed by inductively coupled plasma-atomic emission spectrometry (ICP-AES) analysis. X-ray diffraction shows that for the entire range of compositions, a single-phase product crystallizes in the wolframite structure, with a symmetry-lowering transition from P2/c to P1[overline] at x = 0.20, concomitant with the first-order Jahn-Teller distortion of Cu(2+). Far-IR spectroscopy corroborates that symmetry lowering is directly related to the tetragonal distortion within the CuO(6) octahedra, with the Zn-O A(u) symmetry mode at 320 cm(-1) (x = 0) splitting into two stretches at 295 and 338 cm(-1) (x = 0.3). UV-vis-NIR spectroscopy shows an optical absorption edge characteristic of an indirect band gap that linearly decreases in energy from 3.0 eV (x = 0) to 2.25 eV (x = 1). SQUID magnetometry shows that Zn(1-x)Cu(x)WO(4) (0.1 ≤ x ≤ 1) has an effective moment of 2.30 ± 0.19 µ(B) per mol copper, typical of Cu(2+) in extended solids. For high concentrations of copper (x ≥ 0.8), two transitions are observed: one at high-temperature, 82 K (x = 1.0) that decreases to 59 K (x = 0.8), and the Néel temperature, 23.5 K (x = 1.0) that decreases to 5.5 K (x = 0.8). For x < 0.8, no long-range order is observed. A physical 1:1 mixture of both CuWO(4):ZnWO(4) shows magnetic ordering identical to that of CuWO(4).

Syntheses and structures of three complexes of formulas [L3Co(µ2-O2P(Bn)2)3CoL'][L"], featuring octahedral and tetrahedral cobalt(II) geometries; variable-temperature magnetic susceptibility measurement and analysis on [(py)3Co(µ2-O2PBn2)3Co(py)][ClO4].

The syntheses and structural properties of three dinuclear complexes [L(3)Co(µ(2)-O(2)P(Bn)(2))(3)CoL'][L"] [one ionic L(3) = py(3), L' = py, L" = ClO(4)(-) (1) and two molecular L(3) = py(3), L' = Cl (2) and L(3) = py, µ(2)-NO(3)(-), L' = py (3)] are reported. Complexes feature octahedral Co(II) sites bridged by three dibenzylphosphinate ligands to a tetrahedrally ligated Co(II) site, with the remaining coordination sites occupied by py, nitrato, and Cl ligands. The Co-Co distances are 4.248 Å at 291 K and 4.265 Å at 100 K for 1 and 4.278 and 4.0313(7) Å for 2 and 3, respectively at 100 K. A fit of the low-temperature magnetic susceptibility data was derived for complex 1 with g = 2.25, TIP = 700 × 10(-6) cm(3) mol (-1), λ = -173 cm(-1), κ = 0.93, ν = -3.9, Δ = 630 cm(-1), J = 0.15 cm(-1), and θ = -1.8 resulting in R(χ(M)) = 2.5 × 10(-5) and R(χ(M)T) = 5.8 × 10(-5).

Magnetic exchange coupling in actinide-containing molecules.

Recent progress in the assembly of actinide-containing coordination clusters has generated systems in which the first glimpses of magnetic exchange coupling can be recognized. Such systems are of interest owing to the prospects for involving 5f electrons in stronger magnetic exchange than has been observed for electrons in the more contracted 4f orbitals of the lanthanide elements. Here, we survey the actinide-containing molecules thought to exhibit magnetic exchange interactions, including multiuranium, uranium-lanthanide, uranium-transition metal, and uranium-radical species. Interpretation of the magnetic susceptibility data for compounds of this type is complicated by the combination of spin-orbit coupling and ligand-field effects arising for actinide ions. Nevertheless, for systems where analogues featuring diamagnetic replacement components for the non-actinide spin centers can be synthesized, a data subtraction approach can be utilized to probe the presence of exchange coupling. In addition, methods have been developed for employing the resulting data to estimate lower and upper bounds for the exchange constant. Emphasis is placed on evaluation of the linear clusters (cyclam)M[(mu-Cl)U(Me(2)Pz)(4)](2) (M = Co, Ni, Cu, Zn; cyclam = 1,4,8,11-tetraazacyclotetradecane; Me(2)Pz(-) = 3,5-dimethylpyrazolate), for which strong ferromagnetic exchange with 15 cm(-1) < or = J < or = 48 cm(-1) is observed for the Co(II)-containing species. Owing to the modular synthetic approach employed, this system in particular offers numerous opportunities for adjusting the strength of the magnetic exchange coupling and the total number of unpaired electrons. To this end, the prospects of such modularity are discussed through the lens of several new related clusters. Ultimately, it is hoped that this research will be of utility in the development of electronic structure models that successfully describe the magnetic behavior of actinide compounds and will perhaps even lead to new actinide-based single-molecule magnets.

Photocatalytic primary alcohol oxidation on WO3 nanoplatelets.

With the aid of direct heating through microwave irradiation in non-aqueous media, nanocrystalline tungsten(vi) oxide is achievable in 30 minutes at 200 °C, faster and at a lower temperature than conventional synthesis methods. Forming in a platelet morphology, these particles are as small as 20 nm with a BET surface area of 37 m2 g-1 WO3. These nanoplatelets are active for the photocatalytic oxidation of the 1° alcohols benzyl alcohol (rate constant, k of 2.6 × 10-3 h-1) and 5-(hydroxymethyl)-2-furfural (k of 0.01 h-1) using 10 mg of WO3 with 2 mL of 0.250 M substrate in acetonitrile and a 150 mW cm-2 460 nm blue LED source. As expected, these rate constants are larger than those observed for commercially prepared, micron-sized WO3. XPS analysis shows that during catalysis, the concentration of W5+ on the surface increases, but the nanoplatelets are stable under these reaction conditions. The overall morphology and size of the particles are retained through the reactions. Moreover, the nanoplatelets are recyclable-showing no loss in activity for four reaction cycles.

CuWO4 as a photocatalyst for room temperature aerobic benzylamine oxidation.

The aerobic photochemical oxidation of benzylamine was carried out on the ternary oxides CuWO4 and BiVO4 as a test proton-coupled-electron-transfer reaction in acetonitrile. Both oxides give the coupled imine product, N-benzylidenebenzylamine, in near quantitative (98-99%) yield, with rate constants of 0.34 h-1 g-1 and 0.70 h-1 g-1 for CuWO4 and BiVO4, respectively.

Urea-glass preparation of titanium niobium nitrides and subsequent oxidation to photoactive titanium niobium oxynitrides.

Titanium niobium oxynitrides (TiNbON) are an attractive category of potential photocatalysts, but strategies for preparing them remain limited. We adapt the wet chemical "urea glass" method for pure transition metal nitrides to single-phase mixed-metal titanium niobium nitrides for a range of niobium mole fractions. We then oxidize the nitrides by heating in air to prepare titanium niobium oxynitride that absorbs visible light of λ≤ 550 nm. The materials are characterized by powder X-ray diffraction, scanning electron microscopy, and diffuse reflectance UV-vis spectroscopy. Their photochemical activity as a function of Nb fraction is benchmarked with methylene blue photomineralization promoted by full-spectrum AM 1.5G solar irradiation with and without a λ≥ 400 nm cut-on filter. First-order Langmuir-Hinshelwood rate constants for photomineralization reveal a composition with ∼8% Nb to have superior reactivity. Full compositional analysis by Kjeldahl chemical nitrogen determination and energy-dispersive X-ray spectroscopy yields a chemical formula of Ti0.92Nb0.08O1.97N0.03. Finally, electron paramagnetic resonance spectroscopy correlates a localized Nb4+ defect with increased photochemical reaction rate.

Experimental and modeling study of visible light responsive photocatalytic oxidation (PCO) materials for toluene degradation.

Only limited research has examined the development and application of visible light responsive photocatalytic oxidation (PCO), although such materials have great potential for mitigating concentrations of volatile organic compounds (VOCs) when applied to building surfaces. This study evaluates the performance and characteristics of a visible light responsive photocatalyst, specially, a co-alloyed TiNbON compound with a band energy of 2.3 eV. The PCO material was developed using urea-glass synthesis, characterized by scanning electron microscopy (SEM), diffuse reflectance spectra (DRS), powder X-ray diffraction (PXRD), and Brunauer-Emmett-Teller (BET) methods, and VOC removal efficiency was measured under visible light for toluene (1-5 ppm) at room temperature (21.5°C) and a range of relative humidity (RH: 25 to 65%), flow rate (0.78 to 7.84 cm/s), and irradiance (42 to 95 W/m2). A systematic parametric evaluation of kinetic parameters was conducted. In addition, we compared TiNbON with a commercial TiO2-based material under black light, estimated TiNbON's long-term durability and stability, and tested its ability to thermally regenerate. Using mass transfer and kinetic analysis, three different Langmuir-Hinshelwood (LH) type reaction rate expressions were proposed and evaluated. A LH model considering one active site and competitive sorption of toluene and water was superior to others. The visible-light driven catalyst was able to remove up to 58 % of the toluene, generated less formaldehyde than the commercial TiO2, could be fully regenerated at 150°C, and had reasonable durability and stability. This evaluation of TiNbON shows the potential to remove VOCs and improve air quality for indoor applications. Further research is needed to evaluate the potential for harmful by-products, to identify optimal conditions, and to use field tests to show real-world performance.

Fluorinated Alkoxide-Based Magnesium-Ion Battery Electrolytes that Demonstrate Li-Ion-Battery-Like High Anodic Stability and Solution Conductivity.

Based on DFT predictions, a series of highly soluble fluorinated alkoxide-based electrolytes were prepared, examined electrochemically, and reversibly cycled. The alcohols react with ethylmagnesium chloride to generate a fluoroalkoxy-magnesium chloride intermediate, which subsequently reacts with aluminum chloride to generate the electrolyte. Solutions starting from a 1,1,1,3,3,3-hexafluoro-2-methylpropan-2-ol precursor exhibit high anodic stability, 3.2 V vs Mg(2+/0), and a record 3.5 mS/cm solution conductivity. Excellent galvanostatic cycling and capacity retention (94%) is observed with more than 300 h of cycle time while employing the standard Chevrel phase-Mo6S8 cathode material.

Chemically bonded TiO2-bronze nanosheet/reduced graphene oxide hybrid for high-power lithium ion batteries.

Although Li-ion batteries have attracted significant interest due to their higher energy density, lack of high rate performance electrode materials and intrinsic safety issues challenge their commercial applications. Herein, we demonstrate a simple photocatalytic reduction method that simultaneously reduces graphene oxide (GO) and anchors (010)-faceted mesoporous bronze-phase titania (TiO2-B) nanosheets to reduced graphene oxide (RGO) through Ti(3+)-C bonds. Formation of Ti(3+)-C bonds during the photocatalytic reduction process was identified using electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) techniques. When cycled between 1-3 V (vs Li(+/0)), these chemically bonded TiO2-B/RGO hybrid nanostructures show significantly higher Li-ion storage capacities and rate capability compared to bare TiO2-B nanosheets and a physically mixed TiO2-B/RGO composite. In addition, 80% of the initial specific (gravimetric) capacity was retained even after 1000 charge-discharge cycles at a high rate of 40C. The improved electrochemical performance of TiO2-B/RGO nanoarchitectures is attributed to the presence of exposed (010) facets, mesoporosity, and efficient interfacial charge transfer between RGO monolayers and TiO2-B nanosheets.

Oxygen vacancies lead to loss of domain order, particle fracture, and rapid capacity fade in lithium manganospinel (LiMn2O4) batteries.

Spinel-structured lithium manganese oxide (LiMn2O4) has attracted much attention because of its high energy density, low cost, and environmental impact. In this article, structural analysis methods such as powder neutron diffraction (PND), X-ray diffraction (XRD), and high-resolution transmission and scanning electron microscopies (TEM & SEM) reveal the capacity fading mechanism of LiMn2O4 as it relates to the mechanical degradation of the material. Micro-fractures form after the first charge (to 4.45 V vs. Li(+/0)) of a commercial lithium manganese oxide phase, best represented by the formula LiMn2O3.88. Diffraction methods show that the grain size decreases and multiple phases form after 850 electrochemical cycles at 0.2 C current. The microfractures are directly observed through microscopy studies as particle cracks propagate along the (1 1 1) planes, with clear lattice twisting observed along this direction. Long-term galvanostatic cycling results in increased charge-transfer resistance and capacity loss. Upon preparing samples with controlled oxygen contents, LiMn2O4.03 and LiMn2O3.87, the mechanical failure of the lithium manganese oxide can be correlated to the oxygen vacancies in the materials, providing guidance for better synthesis methods.

Enhanced oxidative stability of non-Grignard magnesium electrolytes through ligand modification.

A series of non-Grignard Mg-electrolytes with various para-substituents was synthesized starting from commercially-available phenols. More electron-withdrawing substituents shift the anodic stability of the electrolyte by 400 mV. The p-CF3 substituted phenol exhibits the highest stability of 2.9 V vs. Mg(2+/0), and cycles reversibly with the Chevrel-phase Mo6S8 Mg-ion cathode.