Interplay between Oxo and Fluoro in Vanadium Oxyfluorides for Centrosymmetric and Non-Centrosymmetric Structure Formation.

Herein, we report the syntheses of two lithium-vanadium oxide-fluoride compounds crystallized from the same reaction mixture through a time variation experiment. A low temperature hydrothermal route employing a viscous paste of V2O5, oxalic acid, LiF, and HF allowed the crystallization of one metastable phase initially, Li2VO0.55(H2O)0.45F5⋅2H2O (I), which on prolonged heating transforms to a chemically similar yet structurally different phase, Li3VOF5 (II). Compound I crystallizes in centrosymmetric space group, I2/a with a = 6.052(3), b = 7.928(4), c = 12.461(6) Å, and ß = 103.99(2)°, while compound II crystallizes in a non-centrosymmetric (NCS) space group, Pna21 with a = 5.1173(2), b = 8.612(3), c = 9.346(3) Å. Synthesis of NCS crystals are highly sought after in solid-state chemistry for their second-harmonic-generation (SHG) response and compound II exhibits SHG activity albeit non-phase-matchable. In this article, we also describe their magnetic properties which helped in unambiguous assignment of mixed valency of V (+4/+5) for Li2VO0.55(H2O)0.45F5⋅2H2O (I) and +4 valency of V for Li3VOF5 (II).

Proton-Induced Disproportionation of Jahn-Teller-Active Transition-Metal Ions in Oxides Due to Electronically Driven Lattice Instability.

The interfacial chemical reactivity of Jahn-Teller-active transition-metal oxides remains an enigmatic area, often leading to uncontrollable phase transformations in the oxide-based technological applications. In particular, the higher tendency of unwanted transition-metal-ion dissolution and side-reactions in Jahn-Teller-active oxides is accompanied by performance degradation in many electrochemical systems, for example, lithium-ion batteries. We show here that the fundamental electronic structure instability that leads to Jahn-Teller (lattice) distortion in an octahedral ligand field is the active chemical driving force for the spontaneous disproportionation, phase transformation, and metal-ion dissolution in transition-metal oxides upon exposure to protons. On the basis of electronic structure analyses and 18O isotope labeling, we present a mechanism comprising a coupled acid-base/redox reaction that leads to a proton-induced disproportionation of Jahn-Teller-active transition-metal ions, as exemplified by the broad classes of respective oxides.

A 3D Lithiophilic Mo2 N-Modified Carbon Nanofiber Architecture for Dendrite-Free Lithium-Metal Anodes in a Full Cell.

The pursuit for high-energy-density batteries has inspired the resurgence of metallic lithium (Li) as a promising anode, yet its practical viability is restricted by the uncontrollable Li dendrite growth and huge volume changes during repeated cycling. Herein, a new 3D framework configured with Mo2 N-mofidied carbon nanofiber (CNF) architecture is established as a Li host via a facile fabrication method. The lithiophilic Mo2 N acts as a homogeneously pre-planted seed with ultralow Li nucleation overpotential, thus spatially guiding a uniform Li nucleation and deposition in the matrix. The conductive CNF skeleton effectively homogenizes the current distribution and Li-ion flux, further suppressing Li-dendrite formation. As a result, the 3D hybrid Mo2 N@CNF structure facilitates a dendrite-free morphology with greatly alleviated volume expansion, delivering a significantly improved Coulombic efficiency of ≈99.2% over 150 cycles at 4 mA cm-2 . Symmetric cells with Mo2 N@CNF substrates stably operate over 1500 h at 6 mA cm-2 for 6 mA h cm-2 . Furthermore, full cells paired with LiNi0.8 Co0.1 Mn0.1 O2 (NMC811) cathodes in conventional carbonate electrolytes achieve a remarkable capacity retention of 90% over 150 cycles. This work sheds new light on the facile design of 3D lithiophilic hosts for dendrite-free lithium-metal anodes.

Soft chemical routes to electrochemically active iron phosphates.

New iron phosphates with related structures have been synthesized using hydrothermal and ion-exchange routes, and their electrochemical properties were investigated. First, NaFe(HPO4)2 was synthesized employing a hydrothermal route and its structure was determined from single-crystal X-ray diffraction data. Subsequent Na+ and partial proton ion exchange with Li+ ion produced a known phase, Li2Fe(H0.5PO4)2, and complete deprotonation of Li2Fe(H0.5PO4)2 with Li+ by employing a solid-state ion-exchange route produced the new phase Li3Fe(PO4)2. The structure of the latter was solved from synchrotron powder X-ray data by employing ab initio methods. All of these phases are highly crystalline, built up of similar connectivities between FeO6 octahedra and PO4 tetrahedral units. Magnetic susceptibility measurements and room-temperature 57Fe Mössbauer spectroscopic studies confirm the 3+ oxidation state of the compounds and their antiferromagnetic ordering with Li2Fe(H0.5PO4)2 showing some interesting metamagnetic behavior. The compounds are stable up to 400 °C and undergo facile electrochemical lithium/sodium insertion through the reduction of Fe3+ to Fe2+. Galvanostatic charge-discharge studies indicate that up to 0.6 lithium ion and 0.5 sodium ion per formula unit can be inserted at average voltages of 3.0 and 2.75 V for lithium and sodium ion batteries, respectively, for NaFe(HPO4)2. The partially Li ion exchanged compound Li2Fe(H0.5PO4)2 showed better cycle life and experimentally achievable capacities up to 0.9 Li insertion with strong dependence on particle size. The electrochemical Li insertion in Li3Fe(PO4)2 was also investigated. The electrochemistry of these three related phases were compared with each other, and their mechanism of Li insertion was investigated by ex situ PXRD.

Long-Life Lithium-Sulfur Batteries with a Bifunctional Cathode Substrate Configured with Boron Carbide Nanowires.

Developing high-energy-density lithium-sulfur (Li-S) batteries relies on the design of electrode substrates that can host a high sulfur loading and still attain high electrochemical utilization. Herein, a new bifunctional cathode substrate configured with boron-carbide nanowires in situ grown on carbon nanofibers (B4 C@CNF) is established through a facile catalyst-assisted process. The B4 C nanowires acting as chemical-anchoring centers provide strong polysulfide adsorptivity, as validated by experimental data and first-principle calculations. Meanwhile, the catalytic effect of B4 C also accelerates the redox kinetics of polysulfide conversion, contributing to enhanced rate capability. As a result, a remarkable capacity retention of 80% after 500 cycles as well as stable cyclability at 4C rate is accomplished with the cells employing B4 C@CNF as a cathode substrate for sulfur. Moreover, the B4 C@CNF substrate enables the cathode to achieve both high sulfur content (70 wt%) and sulfur loading (10.3 mg cm-2 ), delivering a superb areal capacity of 9 mAh cm-2 . Additionally, Li-S pouch cells fabricated with the B4 C@CNF substrate are able to host a high sulfur mass of 200 mg per cathode and deliver a high discharge capacity of 125 mAh after 50 cycles.

A highly fluorinated lithium iron phosphate with interpenetrating lattices: electrochemistry and ionic conductivity.

Li5Fe2PO4F8, a new member of the family of alkali transition metal fluorophosphates, has been synthesized and characterized using single-crystal X-ray diffraction, 57Fe Mössbauer spectroscopy and magnetic susceptibility measurements. The existence of an infinite {-[PO4(FeF4)2]-}∞ tetrahedral network in an inter-penetrated diamond lattice, along with the presence of seven unique Li sites, presents interesting structural features of this structure-type for energy storage applications. The initial results of (de)lithiation reveal that a relatively low fraction of theoretical capacity may be utilized reversibly (0.2 Li+ ion per formula unit), possibly due to the lack of available free volume for Li+ insertion. The high Li content and the existence of large channels in all 3-dimensions of space also offer opportunities to study this material as a candidate for solid-state electrolytes. The results from electro-impedance measurements reveal the reasonable activation energy of Li diffusion (0.70 eV), which is also supported by theoretical calculations.

Tetragonal versus Hexagonal: Structure-Dependent Catalytic Activity of Co/Zn Bimetallic Metal-Organic Frameworks.

Tetragonal and hexagonal phases of monometallic Zn and bimetallic Co/Zn metal-organic frameworks (MOFs), with secondary building units (SBUs) containing a M3O (M = metal) cluster, were synthesized from identical constituents using a benzenetricarboxylate (BTC(3-)) linker that forms decorated 3,6- and 3,5-connected networks, respectively. There exist subtle differences between the SBUs; one of the metal atoms in the M3O cluster in the tetragonal phase has one dissociable DMF solvent molecule while that in the hexagonal phase has three. Connectivities between the SBUs form one-dimensional channels in both MOFs. These MOFs catalyze the chemoselective addition of amines to epoxides, giving exclusively ß-hydroxyamine under heterogeneous conditions. A ring-opening reaction of a symmetrical epoxide showed that the hexagonal phase diastereoselectively yields trans-alcohol, exhibiting an exquisite model for structure-dependent activity.

Phosphite as Polyanion-Based Cathode for Li-Ion Battery: Synthesis, Structure, and Electrochemistry of LiFe(HPO3)2.

A new lithium containing iron(III) phosphite, LiFe(HPO3)2, has been synthesized via a solvent-free, low temperature, solid-state synthesis route. The crystal structure of this material has been determined employing single-crystal X-ray diffraction, which indicates that the compound has a three-dimensional structure formed by isolated FeO6 octahedral units joined together via bridging HPO3 pseudopyramidal moieties. This arrangement leads to the formation of channels along all the three crystallographic directions, where channels along the a- and b-axes host Li(+) ions. The compound was further characterized by TGA, IR, and Mössbauer spectroscopic techniques. Additionally, it has been demonstrated that this phase is electrochemically active toward reversible intercalation of Li(+) ions and thus can be used as a cathode material in Li-ion cells. An average discharge potential of 2.8 V and a practical capacity of 70 mAh·g(-1) has been achieved as indicated by the results of cyclic voltammetry and galvanostatic charge-discharge tests.

catena-Poly[[chlorido[1-(3-nitro-phen-yl)-2-(triphenyl-phospho-ranyl-idene)ethanone-κC(2)]mercury(II)]-µ-chlorido].

In the title organometallic polymer, [HgCl(2)(C(26)H(20)NO(3)P)](n), the monodentate 1-(3-nitro-phen-yl)-2-(triphenyl-phospho-ran-yl--idene)ethanone ligand is coordinated to the Hg(II) atom through the methine C atom. The Hg(II) atom is four-coordinated in a distorted tetra-hedral geometry by one terminal Cl atom, two bridging Cl atoms, and one C atom from the ylidic ligand, resulting in a polymeric chain parallel to [010]. The terminal Cl atom is more strongly bound to the Hg(II) ion [2.3916 (9) Å] than the bridging Cl atoms. The bridge is asymmetric, as indicated by the two different Hg-Cl(bridging) bond lengths [2.5840 (8) and 2.7876 (8) Å]. Intra-molecular C-H⋯O and weak C-H⋯Cl contacts stabilize the polymeric chain. In the crystal, adjacent chains inter-act via C-H⋯O hydrogen bonds.