No abstract

Because of its unprecedented theoretical capacity near 4000 mAh/g, which is approximately 10-fold larger compared to those of the current commercial graphite anodes, silicon has been the most promising anode for lithium ion batteries, particularly targeting large-scale energy storage applications including electrical vehicles and utility grids. Nevertheless, Si suffers from its short cycle life as well as the limitation for scalable electrode fabrication. Herein, we develop an electrospinning process to produce core-shell fiber electrodes using a dual nozzle in a scalable manner. In the core-shell fibers, commercially available nanoparticles in the core are wrapped by the carbon shell. The unique core-shell structure resolves various issues of Si anode operations, such as pulverization, vulnerable contacts between Si and carbon conductors, and an unstable sold-electrolyte interphase, thereby exhibiting outstanding cell performance: a gravimetric capacity as high as 1384 mAh/g, a 5 min discharging rate capability while retaining 721 mAh/g, and cycle life of 300 cycles with almost no capacity loss. The electrospun core-shell one-dimensional fibers suggest a new design principle for robust and scalable lithium battery electrodes suffering from volume expansion.

Mononuclear nonheme iron enzymes generate high-valent iron(IV)-oxo intermediates that effect metabolically important oxidative transformations in the catalytic cycle of dioxygen activation. In 2003, researchers first spectroscopically characterized a mononuclear nonheme iron(IV)-oxo intermediate in the reaction of taurine: α-ketogultarate dioxygenase (TauD). This nonheme iron enzyme with an iron active center was coordinated to a 2-His-1- carboxylate facial triad motif. In the same year, researchers obtained the first crystal structure of a mononuclear nonheme iron(IV)-oxo complex bearing a macrocyclic supporting ligand, [(TMC)Fe(IV)(O)](2+) (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecene), in studies that mimicked the biological enzymes. With these breakthrough results, many other studies have examined mononuclear nonheme iron(IV)-oxo intermediates trapped in enzymatic reactions or synthesized in biomimetic reactions. Over the past decade, researchers in the fields of biological, bioinorganic, and oxidation chemistry have extensively investigated the structure, spectroscopy, and reactivity of nonheme iron(IV)-oxo species, leading to a wealth of information from these enzymatic and biomimetic studies. This Account summarizes the reactivity and mechanisms of synthetic mononuclear nonheme iron(IV)-oxo complexes in oxidation reactions and examines factors that modulate their reactivities and change their reaction mechanisms. We focus on several reactions including the oxidation of organic and inorganic compounds, electron transfer, and oxygen atom exchange with water by synthetic mononuclear nonheme iron(IV)-oxo complexes. In addition, we recently observed that the C-H bond activation by nonheme iron(IV)-oxo and other nonheme metal(IV)-oxo complexes does not follow the H-atom abstraction/oxygen-rebound mechanism, which has been well-established in heme systems. The structural and electronic effects of supporting ligands on the oxidizing power of iron(IV)-oxo complexes are significant in these reactions. However, the difference in spin states between nonheme iron(IV)-oxo complexes with an octahedral geometry (with an S = 1 intermediate-spin state) or a trigonal bipyramidal (TBP) geometry (with an S = 2 high-spin state) does not lead to a significant change in reactivity in biomimetic systems. Thus, the importance of the high-spin state of iron(IV)-oxo species in nonheme iron enzymes remains unexplained. We also discuss how the axial and equatorial ligands and binding of redox-inactive metal ions and protons to the iron-oxo moiety influence the reactivities of the nonheme iron(IV)-oxo complexes. We emphasize how these changes can enhance the oxidizing power of nonheme metal(IV)-oxo complexes in oxygen atom transfer and electron-transfer reactions remarkably. This Account demonstrates great advancements in the understanding of the chemistry of mononuclear nonheme iron(IV)-oxo intermediates within the last 10 years.

Figure 5 . Digital-camera images of separators after thermal treatment at 140 ° C for 1 h. a) A polydopamine-treated-PE separator. b) A bare-PE separator.

The effect of mechanical surface modification on the performance of lithium (Li) metal foil electrodes is systematically investigated. The applied micro‐needle surface treatment technique for Li metal has various advantages. 1) This economical and efficient technique is able to cover a wide range of surface area with a simple rolling process, which can be easily conducted. 2) This technique achieves improved rate capability and cycling stability, as well as a reduced interfacial resistance. The micro‐needle treatment improves the rate capability by 20% (0.750 mAh at a rate of 7C) and increases the cycling stability by 200% (85% of the initial discharge capacity after 150 cycles) compared to untreated bare Li metal (0.626 mAh at a rate of 7C, 85% of the initial discharge capacity after only 70 cycles). 3) This technique efficiently suppresses Li formation of high surface area Li during the Li deposition process, as preferred sites for controlled Li plating are generated.

A number of pseudocontact shifts (PCS) in monolanthanide-substituted Calbindin D 9k (Ca 2 Cb hereafter), a protein of 75 amino acids, were measured for Ce(III), Yb(III), and Dy(III). The assignment of the shifts was obtained through the conventional assignment procedures for the Ce(III) derivative (CaCeCb), since the line broadening is not severe, whereas in the case of Dy(III) and Yb(III) the assignment was obtained by analyzing the temperature dependence of the 1 H-15 N HSQC shifts of the lanthanide derivatives and comparing the results with the 1 H-15 N HSQC spectrum of Ca 2 Cb or CaCeCb. The NOE-based solution structures of Ca 2 Cb or CaCeCb were then refined with PCS. Since the three lanthanides span a wide range of magnetic anisotropies, the refinement was effective in shells from the metal of ∼5-15 Å for Ce(III), ∼9-25 Å for Yb(III), and ∼13-40 Å for Dy(III), as useful PCS were observed in these shells. The root-mean-square deviation of 30 conformers from the average for CaCeCb was 0.74 and 1.10 Å for the backbone and all heavy atoms, respectively, obtained from 1539 NOEs, 39 3 J values, and 6 T 1 values. With 589 pseudocontact shifts for Ce(III) (out of which 280 were larger than 0.1 ppm), 92 PCS for Yb(III), and 74 for Dy(III) the RMSD decreased to 0.54 and 0.95 Å for Ce(III), 0.60 and 0.98 Å for Yb(III), and 0.66 and 1.04 Å for Dy(III) for the backbone and all heavy atoms, respectively. While for Ce(III) resolution improvements are mainly found for the metal binding site itself, Yb(III) and Dy(III) can further constrain regions far away from the metal. These results show that constructing a lanthanide binding site may be a general and convenient tool to "enlighten" shells at variable distances from the metal itself, and may be used for various purposes including the investigation of biomolecular complexes.

The photocatalytic water oxidation to evolve O 2 was performed by photoirradiation (l > 420 nm) of an aqueous solution containing [Ru(bpy) 3 ] 2+ (bpy ¼ 2,2 0 -bipyridine), Na 2 S 2 O 8 and water-soluble cobalt complexes with various organic ligands as precatalysts in the pH range of 6.0-10. The turnover numbers (TONs) based on the amount of Co for the photocatalytic O 2 evolution with [Co II (Me 6 tren)(OH 2 )] 2+ (1) and [Co III (Cp * )(bpy)(OH 2 )] 2+ (2) [Me 6 tren ¼ tris(N,N 0 -dimethylaminoethyl) amine, Cp * ¼ h 5 -pentamethylcyclopentadienyl] at pH 9.0 reached 420 and 320, respectively. The evolved O 2 yield increased in proportion to concentrations of precatalysts 1 and 2 up to 0.10 mM. However, the O 2 yield dramatically decreased when the concentration of precatalysts 1 and 2 exceeded 0.10 mM. When the concentration of Na 2 S 2 O 8 was increased from 10 mM to 50 mM, CO 2 evolution was observed during the photocatalytic water oxidation. These results indicate that a part of the organic ligands of 1 and 2 were oxidized to evolve CO 2 during the photocatalytic reaction. The degradation of complex 2 under photocatalytic conditions and the oxidation of Me 6 tren ligand of 1 by [Ru(bpy) 3 ] 3+ were confirmed by 1 H NMR measurements. Dynamic light scattering (DLS) experiments indicate the formation of particles with diameters of around 20 AE 10 nm and 200 AE 100 nm during the photocatalytic water oxidation with 1 and 2, respectively. The particle sizes determined by DLS agreed with those of the secondary particles observed by TEM. The XPS measurements of the formed particles suggest that the surface of the particles is covered with cobalt hydroxides, which could be converted to active species containing high-valent cobalt ions during the photocatalytic water oxidation. The recovered nanoparticles produced from 1 act as a robust catalyst for the photocatalytic water oxidation.

Redox-inactive metal ions play pivotal roles in regulating the reactivities of high-valent metal-oxo species in a variety of enzymatic and chemical reactions. A mononuclear non-heme Mn(IV)-oxo complex bearing a pentadentate N5 ligand has been synthesized and used in the synthesis of a Mn(IV)-oxo complex binding scandium ions. The Mn(IV)-oxo complexes were characterized with various spectroscopic methods. The reactivities of the Mn(IV)-oxo complex are markedly influenced by binding of Sc(3+) ions in oxidation reactions, such as a ~2200-fold increase in the rate of oxidation of thioanisole (i.e., oxygen atom transfer) but a ~180-fold decrease in the rate of C-H bond activation of 1,4-cyclohexadiene (i.e., hydrogen atom transfer). The present results provide the first example of a non-heme Mn(IV)-oxo complex binding redox-inactive metal ions that shows a contrasting effect of the redox-inactive metal ions on the reactivities of metal-oxo species in the oxygen atom transfer and hydrogen atom transfer reactions.