Thermal Stability of Na_<i>x</i>CrO₂ for Rechargeable Sodium Batteries; Studies by High-Temperature Synchrotron X‑ray Diffraction

Thermal stability and phase transition processes of NaCrO₂ and Na_0.5CrO₂ are carefully examined by high-temperature synchrotron X-ray diffraction method. O3-type NaCrO₂ shows anisotropic thermal expansion on heating, which is a common character as layered materials, without phase transition in the temperature range of 27–527 °C. In contrast, for the desodiated phase, in-plane distorted P3-type layered oxide (P′3 Na_0.5CrO₂), phase transition occurs in the following order. Monoclinic distortion associated with Na/vacancy ordering is gradually lost on heating, and its symmetry increases and changes to a rhombohedral lattice at 207 °C. On further heating, phase segregation to two P3 layered metastable phases, which have different interlayer distances (17.0 and 13.5 Å, presumably sodium-rich and sodium-free P3 phases, respectively) are observed on heating to 287–477 °C, but oxygen loss is not observed. Oxygen loss is observed at temperatures only above 500 °C, resulting in the formation of corundum-type Cr₂O₃ and O3 NaCrO₂ as thermodynamically stable phases. From these results, possibility of Na_<i>x</i>CrO₂ as a positive electrode material for safe rechargeable sodium batteries is also discussed

Stereoselective Alkylation of the Vinylketene Silyl <i>N</i>,<i>O</i>‑Acetal and Its Application to the Synthesis of Mycocerosic Acid

Stereoselective alkylation of the vinylketene silyl <i>N</i>,<i>O</i>-acetal possessing a chiral auxiliary has been achieved by using activated alkyl halides including allyl iodides, benzyl iodides, and propargyl iodide with Ag­(I) ion in the presence of BF₃·OEt₂. The reaction proceeded to give reduced polyketides in high stereoselectivity. The synthesis of mycocerosic acid, a component of the cell envelope of <i>Mycobacterium tuberculosis</i>, has been accomplished by this methodology. During the synthetic studies, 2-methylbenzimidazole was found to be a bulky proton source which worked in the presence of liquid ammonia

Understanding Particle-Size-Dependent Electrochemical Properties of Li₂MnO₃‑Based Positive Electrode Materials for Rechargeable Lithium Batteries

Electrochemical properties of Li-excess electrode materials, Li_1.2Co_0.13Ni_0.13Mn_0.54O₂, with different primary particle sizes are studied in Li cells, and phase transition behavior on continuous electrochemical cycles is systematically examined. Although the nanosize (<100 nm) sample delivers a large reversible capacity of 300 mAh g^–1 at the initial cycle, capacity retention is not sufficient as a positive electrode material. Moreover, unfavorable phase transition, gradual enrichment of trivalent manganese ions, and lowering structural symmetry is not avoidable on electrochemical cycles for a nanosize sample, which is confirmed by combined techniques of synchrotron X-ray diffraction, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy. A submicrosize sample also delivers a large reversible capacity of 250 mAh g^–1 even though a slow activation process is observed accompanied with partial oxygen loss and migration oxide ions in the crystal lattice coupled with transition metal migration on the initial charge process. Such an unfavorable phase transition at room temperature is effectively suppressed by the use of a submicrosize sample with low surface area. However, suppression of the phase transition is found to be a kinetically controlled phenomena and is, therefore, unavoidable at elevated temperatures

Electrical Characteristics of Pentacene Films on Cross-Linked Polymeric Insulators of Varying Thicknesses

Pentacene films vacuum-sublimed on a cross-linked polymeric insulator (CPVP–C₆) prepared using poly­(4-vinylphenol) (PVP) and 1,6-bis­(trichlorosilyl)­hexane (C₆) were studied with a special concern on possible influences of the CPVP–C₆ thickness on the electrical characteristics of the pentacene films. It was found that the conductivities of the pentacene films on a thin CPVP–C₆ film (10 nm) were approximately 2 orders of magnitude higher than those on a glass substrate and increased slightly with the increase in the thickness of the underlying CPVP–C₆ film. In addition, the X-ray diffraction measurements showed that the stacking structure of pentacene molecules was remarkably enhanced by increasing the thickness of the CPVP–C₆ film, suggesting that the increase in conductivity is due, at least in part, to the improvement in carrier mobilities caused by the growth of large pentacene grains. An attempt to directly evaluate carrier mobilities using pentacene/CPVP–C₆ field-effect transistors was made, and a seeming increase in the carrier mobilities observed with the increase in the CPVP–C₆ thickness was ascribed to a hygroscopic nature of the CPVP–C₆ film, which was evidenced by the capacitance and quartz crystal microbalance measurements. Possible reasons are discussed to explain the enhanced conductivities of the pentacene films on the increased thicknesses of CPVP–C₆

P′2-Na_2/3Mn_0.9Me_0.1O₂ (Me = Mg, Ti, Co, Ni, Cu, and Zn): Correlation between Orthorhombic Distortion and Electrochemical Property

P′2-Na_2/3Mn_0.9Me_0.1O₂ (Me = Mg, Ti, Co, Ni, Cu, and Zn): Correlation between Orthorhombic Distortion and Electrochemical Propert

Poly-γ-glutamate Binder To Enhance Electrode Performances of P2-Na_2/3Ni_1/3Mn_2/3O₂ for Na-Ion Batteries

P2-Na_2/3Ni_1/3Mn_2/3O₂ (P2-NiMn) is one of the promising positive electrode materials for high-energy Na-ion batteries because of large reversible capacity and high working voltage by charging up to 4.5 V versus Na⁺/Na. However, the capacity rapidly decays during charge/discharge cycles, which is caused by the large volume shrinkage of ca. 23% by sodium deintercalation and following electric isolation of P2-NiMn particles in the composite electrode. Serious electrolyte decomposition at the higher voltage region than 4.1 V also brings deterioration of the particle surface and capacity decay during cycles. To solve these drawbacks, we apply water-soluble sodium poly-γ-glutamate (PGluNa) as an efficient binder to P2-NiMn instead of conventional poly­(vinylidene difluoride) (PVdF) and examined the electrode performances of P2-NiMn composite electrode with PGluNa binder for the first time. The PGluNa electrode shows Coulombic efficiency of 95% at the first cycle and capacity retention of 89% after 50 cycles, whereas the PVdF electrode exhibits only 80 and 71%, respectively. The alternating current impedance measurements reveal that the PGluNa electrode shows a much lower resistance during the cycles compared with the PVdF one. From the surface analysis and peeling test of the electrodes, the PGluNa binder was found to cover the surface of the P2-NiMn particles and suppresses the electrolyte decomposition and surface degradation. The PGluNa binder further enhance the mechanical strength of the electrodes and suppresses the electrical isolation of the P2-NiMn particles during sodium extraction/insertion. The efficient binder with noticeable adhesion strength and surface coverage of active materials and carbon has paved a new way to enhance the electrochemical performances of high-voltage positive electrode materials for Na-ion batteries

Remote Asymmetric Bromination Reaction with Vinylketene Silyl <i>N</i>,<i>O</i>‑Acetal and Its Application to Total Synthesis of Pellasoren A

Stereoselective bromination of the <i>E</i>,<i>E</i>-vinylketene silyl <i>N</i>,<i>O</i>-acetal possessing a chiral auxiliary has been achieved and applied to introduction of heteroatom at γ-position of α,β-unsaturated imide. The reactions proceeded in high stereoselectivity. Total synthesis of pellasoren A, an antitumor propionate from the myxobacteriun <i>Sorangium cellulosum</i>, has been accomplished in short steps by this methodology and our method of reduced polypropionate synthesis

Unraveling the Role of Doping in Selective Stabilization of NaMnO₂ Polymorphs: Combined Theoretical and Experimental Study

Dopants are known to modify structural, electronic, chemical, and other properties of materials; therefore, analysis of doping effects is of great interest in the fields of fundamental and applied science. However, in many functional materials, particularly transition metal (TM) compounds, such analysis could be quite complex owing to subtle interplay between possible oxidation states of various types of TM, which is hard to elucidate experimentally and difficult to model theoretically. In this work, we performed a study of the role of 3d TM and some non-TM dopants in stabilization of structural polymorphs of NaMnO₂, a highly promising material for electrocatalysis and Na-ion battery applications. Our X-ray diffraction experiments and DFT+<i>U</i> modeling revealed the exclusive formation of α- or β-NaMnO₂ polymorphs via substitutional doping of NaMnO₂ by Ti or Cu cations, respectively, whereas doping with other elements results in formation of several structural polymorphs. In the most important case of stabilization of β-NaMnO₂ by Cu cations, we find that geometry of this structure allows 2+ oxidation state of Cu, unlike α-NaMnO₂, where Cu adopts a more artificial 3+ oxidation state, which explains lower stability of α-type polymorph

New Insight into Structural Evolution in Layered NaCrO₂ during Electrochemical Sodium Extraction

Electrochemical properties and structural changes during charge for NaCrO₂, whose structure is classified as α-NaFeO₂ type layered polymorph (also O3-type following the Delmas’ notation), are examined as a positive electrode material for nonaqueous Na-ion batteries. NaCrO₂ delivers initial discharge capacity of 110 mAh g^–1 at 1/20C rate in the voltage range of 2.5–3.6 V based on reversible Cr³⁺/Cr⁴⁺ redox without oxidation to hexavalent chromium ions, while the initial discharge capacity is only 9 mAh g^–1 when cutoff voltage is set to 4.5 V. Results from <i>ex-situ</i> X-ray diffraction, X-ray absorption spectroscopy, and DFT calculations reveal that the irreversible phase transition occurs after sodium extraction by charging over a voltage plateau at 3.8 V associated with the lattice shrinkage along the <i>c</i>-axis in the case of <i>x</i> > 0.5 in Na_1–<i>x</i>CrO₂, which originates from the migration of chromium ions from octahedral sites in CrO₂ slabs to both tetrahedral and octahedral sites in interslab layer. The irreversible structural change would disturb sodium insertion into the damaged layer structure during discharge, resulting in the loss of reversibility as electrode materials. Reversible cycle range with stable capacity retention is, therefore, limited to the compositional range of 0.0 ≤ <i>x</i> ≤ 0.5 in Na_1–<i>x</i>CrO₂

Black Phosphorus as a High-Capacity, High-Capability Negative Electrode for Sodium-Ion Batteries: Investigation of the Electrode/Electrolyte Interface

For a nonaqueous sodium-ion battery (NIB), phosphorus materials have been studied as the highest-capacity negative electrodes. However, the large volume change of phosphorus upon cycling at low voltage causes the formation of new active surfaces and potentially results in electrolyte decomposition at the active surface, which remains one of the major limiting factors for the long cycling life of batteries. In this present study, powerful surface characterization techniques are combined for investigation on the electrode/electrolyte interface of the black phosphorus electrodes with polyacrylate binder to understand the formation of a solid electrolyte interphase (SEI) in alkyl carbonate ester and its evolution during cycling. The hard X-ray photoelectron spectroscopy (HAXPES) analysis suggests that SEI (passive film) consists of mainly inorganic species, which originate from decomposition of electrolyte solvents and additives. The thicker surface layer is formed during cycling in the additive-free electrolyte, compared to that in the electrolyte with fluoroethylene carbonate (FEC) or vinylene carbonate (VC) additive. The HAXPES and time-of-flight secondary ion mass spectroscopy (TOF-SIMS) studies further reveal accumulation of organic carbonate species near the surface and inorganic salt decomposition species. These findings open paths for further improvement for the cyclability of phosphorus electrodes for high-energy NIBs