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We previously demonstrated that electrode architectures comprising nanoscale birnessite-like MnO x affixed to three-dimensional carbon nanofoam (CNF) scaffolds offer performance advantages when used as cathodes in rechargeable zinc-ion cells. To discern chemical and physical changes at the MnO x @CNF electrode upon deep charge/discharge in aqueous Zn2+-containing electrolytes, we deploy electroanalytical methods and ex situ characterization by microscopy, elemental analysis, x-ray photoelectron spectroscopy, x-ray diffraction, and x-ray pair distribution function analyses. Our findings verify that redox processes at the MnO x are accompanied by reversible precipitation/dissolution of crystalline zinc hydroxide sulfate (Zn4(OH)6(SO4)· x H2O), mediated by the more uniformly reactive electrode structure inherent to the CNF scaffold. [ABSTRACT FROM AUTHOR]

By applying differential pair distribution function (DPDF) analyses to the energy-storage relevant MnOx/carbon system―but in a 3D architectural rather than powder-composite configuration―we can remove contributions of the carbon nanofoam paper scaffold and quantify the multiphasic oxide speciation as the nanoscale, disordered MnOx grafted to the carbon walls (MnOx@CNF) structurally rearranges in situ from disordered birnessite AMnOx (A = Na+; Li+) to tetragonal Mn3O4 to spinel LiMn2O4. The first reaction step involves topotactic exchange of interlayer Na+ by Li+ in solution followed by thermal treatments to crystal engineer the ∼10 nm-thick 2D layered oxide throughout the macroscale nanofoam paper into a cubic phase. The oxide remains affixed to the walls of the nanofoam throughout the phase transformations. The DPDF fits are improved by retention of one plane of birnessite-like oxide after conversion to spinel. We support the DPDF-derived assignments by X-ray photoelectron spectroscopy and Raman spectroscopy, the latter of which tracks how crystal engineering the oxide affects the disorder of the carbon substrate. We further benchmark MnOx@CNF with nonaqueous electrochemical measurements versus lithium as the oxide converts from X-ray-amorphous birnessite to interlayer-registered LiMnOx to spinel. The lamellar AMnOx displays pseudocapacitive electrochemical behavior, with a doubling of specific capacitance for the interlayer-registered LiMnOx, while the spinel LiMn2O4@CNF displays a faradaic electrochemical response characteristic of Li-ion insertion. Our results highlight the need for holistic understanding when crystal engineering an (atomistic) charge-storing phase within the (architectural) structure of practical electrodes. [ABSTRACT FROM AUTHOR]

Vanadium-substituted iron oxide aerogels (2 : 1 Fe : V ratio; VFe2Ox) are synthesized using an epoxide-initiated sol–gel method to form high surface-area, mesoporous materials in which the degree of crystallinity and concentration of defects are tuned via thermal treatments under controlled atmospheres. Thermal processing of the X-ray amorphous, as-synthesized VFe2Ox aerogels at 300 °C under O2-rich conditions removes residual organic byproducts while maintaining a highly defective γ-Fe2O3-like local structure with minimal long-range order and vanadium in the +5 state. When as-synthesized VFe2Ox aerogels are heated under low partial pressure of O2 (e.g., flowing argon), a fraction of vanadium sites are reduced to the +4 state, driving crystallization to a Fe3O4-like cubic phase. Subsequent thermal oxidation of this nanocrystalline VFe2Ox aerogel re-oxidizes vanadium +4 to +5, creating additional cation vacancies and re-introducing disordered oxide domains. We correlate the electrochemical charge-storage properties of this series of VFe2Ox aerogels with their degree of order and chemical state, as verified by X-ray diffraction, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy. We find that the disordered O2-heated VFe2Ox aerogel yields the highest Li+- and Na+-insertion capacities among this series, approaching 130 mA h g−1 and 70 mA h g−1, respectively. Direct heat-treatment of the VFe2Ox aerogel in flowing argon to yield the partially reduced, nanocrystalline form results in significantly lower Li+-insertion capacity (77 mA h g−1), which improves to 105 mA h g−1 by thermal oxidation to create additional vacancies and structural disorder. [ABSTRACT FROM AUTHOR]

The syntheses of two noncentrosymmetric (NCS) vanadium oxide-fluoride compounds that originate from the same synthetic reagent concentrations are presented. Hydrothermal and low-temperature syntheses allow the isolation of metastable products that may form new phases (or decompose) upon heating and allow creation of chemically similar but structurally different materials. NCS materials synthesis has been a long-standing goal in inorganic chemistry: in this article, we compare two chemically similar NCS inorganic materials, NaVOF4(H2O) (I) and NaVO2-xF2+x (II; x = 1/3). These materials originate from the same, identical reagent mixtures but are synthesized at different temperatures: 100 °C and 150 °C, respectively. Compound I crystallizes in Pna21: a = 9.9595(4) Å, b = 9.4423(3) Å, and c = 4.8186(2) Å. Compound II crystallizes in P21: a = 6.3742(3) Å, b = 3.5963(2) Å, c = 14.3641(7) Å, and β = 110.787(3)°. Both materials display second-harmonic-generation activity; however, compound I is type 1 non-phase-matchable, whereas compound II is type 1 phase-matchable. [ABSTRACT FROM AUTHOR]

The potassium vanadium oxide-fluoride phase β-KVOF3 was prepared by in-situ reduction with hydrothermal methods and characterized by single-crystal X-ray diffraction and magnetic susceptibility measurements. The compound is orthorhombic [space group Pnma, a = 7.1596(2) Å, b = 7.5392(2) Å, c = 13.0506(4) Å, Z = 8]. The material is structurally similar and isoformulaic to d1 materials of formulae AVOF3 ( A+ = alkali metal, ammonium, or organic cation). Magnetic susceptibility data were successfully fit with a Heisenberg chain model. In-situ reduction of V5+ to V4+ during the hydrothermal reaction conditions was observed in the presence of Br- anions. The role of bromide ions in this reduction is explained and employed to reduce a d0 vanadium species to obtain a material with magnetic behavior. [ABSTRACT FROM AUTHOR]

The potassium vanadium oxide-fluoride phase β-KVOF3 was prepared by in-situ reduction with hydrothermal methods and characterized by single-crystal X-ray diffraction and magnetic susceptibility measurements. The compound is orthorhombic [space group Pnma, a = 7.1596(2) Å, b = 7.5392(2) Å, c = 13.0506(4) Å, Z = 8]. The material is structurally similar and isoformulaic to d1 materials of formulae AVOF3 ( A+ = alkali metal, ammonium, or organic cation). Magnetic susceptibility data were successfully fit with a Heisenberg chain model. In-situ reduction of V5+ to V4+ during the hydrothermal reaction conditions was observed in the presence of Br- anions. The role of bromide ions in this reduction is explained and employed to reduce a d0 vanadium species to obtain a material with magnetic behavior. [ABSTRACT FROM AUTHOR]

A series of pseudosymmetrical structures of formula K10(M2OnF11-n)3X (M = V and Nb, n = 2, X = (F2Cl)1/3, Br, Br4/2,I4/2>; M = Mo, n = 4, X = CI, Br4/2, I4/2) illustrates generation of polar structures with the use of A- shaped basic building units (BBUs). For a compound to belong to a polar space group, dipole moments of individual species must be partially aligned. Incorporation of d° early transition metal polyhedral BBUs into structures is a common method to create polar structures, owing to the second-order Jahn--Teller distortion these polyhedra contain. Less attention has been spent examining how to align the polar moments of BBUs. To address alignment, we present a study on previously reported bimetallic BBUs and synthesized compounds K10(M2OnF11_n)3X. These materials differ in their (non)centrosymmetry despite chemical and structural similarities. The vanadium compounds are centrosymmetric (space groups P3ml or C2/m) while the niobium and molybdenum heterotypes are noncentrosymmetric (Pmn21). The difference in symmetry occurs owing to the presence of linear, bimetallic BBUs or A-shaped bimetallic BBUs and related packing effects. These A-shaped BBUs form as a consequence of the coordination environment around the bridging anion of the metal oxide fluoride BBUs. [ABSTRACT FROM AUTHOR]

We present structural and electrochemical analyses of a new double-wolframite compound: AgNa(VO2F2)2 or SSVOF. SSVOF is fully ordered and displays electrochemical characteristics that give insight into electrode design for energy storage beyond lithium-ion chemistries. The compound contains trioxovanadium fluoride octahedra that combine to form one-dimensional chain-like basic building units, characteristic of wolframite (NaWO4). The 1D chains are stacked to create 2D layers; the cations Ag+ and Na+ lie between these layers. The vanadium oxide-fluoride octahedra are ordered by the use of cations (Ag+, Na+) that differ in polarizability. In the case of sodium-ion batteries, thermodynamically, the use of a sodium anode introduces a 300 mV loss in overall cell voltage as compared to a lithium anode; however, this can be counter-balanced by introduction of fluoride into the framework to raise the reduction potentials via an inductive effect. This allows sodium-ion batteries to have comparable voltages to lithium systems. With SSVOF as a baseline compound, we have identified new materials design rules for emerging sodium-ion systems that do not apply to lithium-ion systems. These strategies can be applied broadly to provide materials of interest for fundamental structural chemistry and appreciable voltages for sodium-ion electrochemistry. [ABSTRACT FROM AUTHOR]

Three new vanadium oxide-fluoride phases, Na2(M(H2O)2)(V2O4F6) (M = Co2+, Ni2+, and Cu2+), were prepared by hydrothermal techniques in hydrofluoric acid. The compounds contain the dioxo vanadium fluoride dimeric anion: [V2O4F6]4-. The harder (Na+) and softer (M = Co2+, Ni2+, and Cu2+) cations coordinate to the harder fluoride and softer oxide anions, respectively. In contrast to monoxo vanadium fluorides, the dioxo vanadium fluoride exhibits an increased nucleophilicity of the oxide anions. This increased nucleophilicity allows the anion to bond covalently with late transition metals to form 2D layers that contain both an early transition metal (V5+) and a late transition metal (Co2+, Ni2+, or Cu2+). [ABSTRACT FROM AUTHOR]

Three new vanadium oxide-fluoride phases, Na2(M(H2O)2)(V2O4F6) (M = Co2+, Ni2+, and Cu2+), were prepared by hydrothermal techniques in hydrofluoric acid. The compounds contain the dioxo vanadium fluoride dimeric anion: [V2O4F6]4-. The harder (Na+) and softer (M = Co2+, Ni2+, and Cu2+) cations coordinate to the harder fluoride and softer oxide anions, respectively. In contrast to monoxo vanadium fluorides, the dioxo vanadium fluoride exhibits an increased nucleophilicity of the oxide anions. This increased nucleophilicity allows the anion to bond covalently with late transition metals to form 2D layers that contain both an early transition metal (V5+) and a late transition metal (Co2+, Ni2+, or Cu2+). [ABSTRACT FROM AUTHOR]

A methodology for the design of polar, inorganic structures is demonstrated here with the packing of lambda (Λ)-shaped basic building units (BBUs). Noncentrosymmetric (NCS) solids with interesting physical properties can be created with BBUs that lack an inversion center and are likely to pack into a polar configuration; previous methods to construct these solids have used NCS octahedra as BBUs. Using this methodology to synthesize NCS solids, one must increase the coordination of the NCS octahedra with maintenance of the noncentrosymmetry of the bulk. The first step in this progression from an NCS octahedron to an inorganic NCS solid is the formation of a bimetallic BBU. This step is exemplified with the compound CuVOF4(H2O)7: this compound, presented here, crystallizes in an NCS structure with ordered, isolated [Cu(H2O)5]2+ cations and [VOF4(H2O)]2- anions into Λ-shaped, bimetallic BBUs to form CuVOF4(H2O)6·H2O, owing to the Jahn-Teller distortion of Cu2+. Conversely, the centrosymmetric heterotypes with the same formula MVOF4(H2O)7 (MII = Co, Ni, and Zn) exhibit ordered, isolated [VOF4(H2O)]2- and [M(H2O)6]2+ ionic species in a hydrogen bond network. CuVOF4(H2O)7 exhibits a net polar moment while the heterotypes do not; this demonstrates that Λ-shaped BBUs give a greater probability for and, in this case, lead to NCS structures. [ABSTRACT FROM AUTHOR]

Non-centrosymmetric NaVOF4(H2O) (I) and NaVO2-xF2+x (II; x = 1/3) are prepared from an 1:1:4 molar mixture of V2O5, NaCl, and HF (teflon container, 100 °C, 30 min for (I) and 150 °C, 24 h for (II)). [ABSTRACT FROM AUTHOR]

β-KVOF3 is prepared by in-situ reduction under hydrothermal conditions using a mixture of V2O5 and KBr (3:2) in aq. [ABSTRACT FROM AUTHOR]

The new compounds (IV) and (VI) crystallize isostructurally in the monoclinic space group P21/c with Z = 2 (single crystal XRD). [ABSTRACT FROM AUTHOR]

The cause of polar distortion orientation of ordered one-dimensional chains of basic building units is illustrated with subtle alterations in the heterotypic structures of the previously known compound CuNbOF5(H2O)2(pyz)3 and the new hybrid materials (III), (V), and (VI), whose crystal structures are determined by single crystal XRD. [ABSTRACT FROM AUTHOR]

The purely inorganic, polar compound (IIIb) (orthorhombic space group Pna21, Z = 4) and its centrosymmetric (triclinic space group P $\bar 1$, Z = 4) heterotypes (IIIa,c) and (V) are characterized by single crystal XRD, FTIR spectroscopy, magnetic susceptibility, impedance, and second harmonic generation measurements. [ABSTRACT FROM AUTHOR]