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1.
ACS Appl Mater Interfaces ; 11(45): 42280-42287, 2019 Nov 13.
Artigo em Inglês | MEDLINE | ID: mdl-31682096

RESUMO

Glass and glass-ceramic samples of metastable lithium thiophosphates with compositions of 70Li2S-30P2S5 and Li7P3S11 were controllably prepared by using a rapid assisted-microwave procedure in under 30 min. The rapid preparation times and weak coupling of the evacuated silica ampules with microwave radiation ensure minimal reactivity of the reactants and the container. The microwave-prepared samples display comparable conductivity values with more conventionally prepared (melt quenched) glass and glass-ceramic samples, on the order of 0.1 and 1 mS cm-1 at room temperature, respectively. Rietveld analysis of synchrotron X-ray diffraction data acquired with an internal standard quantitatively yields phase amounts of the glassy and amorphous components, establishing the tunable nature of the microwave preparation. X-ray photoelectron spectroscopy and Raman spectroscopy confirm the composition and the appropriate ratios of isolated and corner-sharing tetrahedra in these semicrystalline systems. Solid-state 7Li nuclear magnetic resonance (NMR) spectroscopy resolves the seven crystallographic Li sites in the crystalline compound into three main environments. The diffusion behavior of these Li environments as obtained from pulsed-field gradient NMR methods can be separated into one slow and one fast component. The rapid and tunable approach to the preparation of high quality "Li7P3S11" samples presented here coupled with detailed structural and compositional analysis opens the door to new and promising metastable solid electrolytes.

2.
J Am Chem Soc ; 141(42): 16706-16725, 2019 Oct 23.
Artigo em Inglês | MEDLINE | ID: mdl-31487157

RESUMO

TiNb2O7 is a Wadsley-Roth phase with a crystallographic shear structure and is a promising candidate for high-rate lithium ion energy storage. The fundamental aspects of the lithium insertion mechanism and conduction in TiNb2O7, however, are not well-characterized. Herein, experimental and computational insights are combined to understand the inherent properties of bulk TiNb2O7. The results show an increase in electronic conductivity of seven orders of magnitude upon lithiation and indicate that electrons exhibit both localized and delocalized character, with a maximum Curie constant and Li NMR paramagnetic shift near a composition of Li0.60TiNb2O7. Square-planar or distorted-five-coordinate lithium sites are calculated to invert between thermodynamic minima or transition states. Lithium diffusion in the single-redox region (i.e., x ≤ 3 in LixTiNb2O7) is rapid with low activation barriers from NMR and DLi = 10-11 m2 s-1 at the temperature of the observed T1 minima of 525-650 K for x ≥ 0.75. DFT calculations predict that ionic diffusion, like electronic conduction, is anisotropic with activation barriers for lithium hopping of 100-200 meV down the tunnels but ca. 700-1000 meV across the blocks. Lithium mobility is hindered in the multiredox region (i.e., x > 3 in LixTiNb2O7), related to a transition from interstitial-mediated to vacancy-mediated diffusion. Overall, lithium insertion leads to effective n-type self-doping of TiNb2O7 and high-rate conduction, while ionic motion is eventually hindered at high lithiation. Transition-state searching with beyond Li chemistries (Na+, K+, Mg2+) in TiNb2O7 reveals high diffusion barriers of 1-3 eV, indicating that this structure is specifically suited to Li+ mobility.

3.
Phys Chem Chem Phys ; 21(19): 10070-10074, 2019 May 15.
Artigo em Inglês | MEDLINE | ID: mdl-31049516

RESUMO

The complete 31P NMR chemical shift tensors for 22 inorganic phosphates obtained from ab initio computation are found to correspond closely to experimentally obtained parameters. Further improvement was found when structures determined by diffraction were geometry optimized. Besides aiding in spectral assignment, the cases where correspondence is significantly improved upon geometry optimization point to the crystal structures requiring correction.

4.
Chem Commun (Camb) ; 55(5): 588-591, 2019 Jan 10.
Artigo em Inglês | MEDLINE | ID: mdl-30480673

RESUMO

Perovskite-derived hybrid platinum iodides with the general formula A2PtIVI6 (A = formamidinium FA and guanidinium GUA) accommodate excess I2 to yield hydrogen-bond-stabilized compounds where the I2 forms catenates with I- anions on the PtI6 octahedra.

5.
Inorg Chem ; 57(16): 10375-10382, 2018 Aug 20.
Artigo em Inglês | MEDLINE | ID: mdl-30074384

RESUMO

We describe the solid-state structural evolution in four hybrid hexaiodoplatinate(IV) compounds, demonstrating the increasingly important role that extended hydrogen bonding plays in directing the structure across the series. The compounds are A2PtI6, where A is one of the following amines: ammonium, NH4+; methylammonium, CH3NH3+; formamidinium, CH(NH2)2+; guanidinium, C(NH2)3+. These are closely related in structure and properties to the hybrid halide perovskites of lead(II) that have recently established their prowess in optoelectronics. The first three of these compounds crystallize in the vacancy-ordered double perovskite A2Pt□I6 (□ indicates a vacant site) structure in the K2PtCl6 archetype, despite the relatively large perovskite tolerance factors involved. The last compound, (GUA)2PtI6, crystallizes in a vacancy-ordered variant of the hexagonal CsNiCl3 structure: the K2MnF6 structure. A combination of solid-state 195Pt and 1H NMR spectroscopy and detailed density functional theory calculations helps to reveal structural trends and establish the hydrogen-bonding tendencies. The calculations and measured optical properties support the surprising observation in these iodosalt compounds that, for smaller A cations, the conduction bands are considerably disperse, despite lacking extended I-Pt-I connectivity.

6.
Angew Chem Int Ed Engl ; 56(47): 15118-15122, 2017 11 20.
Artigo em Inglês | MEDLINE | ID: mdl-28984016

RESUMO

We demonstrate a novel crosslinked disulfide system as a cathode material for Li-S cells that is designed with the two criteria of having only a single point of S-S scission and maximizing the ratio of S-S to the electrochemically inactive framework. The material therefore maximizes theoretical capacity while inhibiting the formation of polysulfide intermediates that lead to parasitic shuttle. The material we report contains a 1:1 ratio of S:C with a theoretical capacity of 609 mAh g-1 . The cell gains capacity through 100 cycles and has 98 % capacity retention thereafter through 200 cycles, demonstrating stable, long-term cycling. Raman spectroscopy confirms the proposed mechanism of disulfide bonds breaking to form a S-Li thiolate species upon discharge and reforming upon charge. Coulombic efficiencies near 100 % for every cycle, suggesting the suppression of polysulfide shuttle through the molecular design.

7.
J Am Chem Soc ; 137(38): 12282-8, 2015 Sep 30.
Artigo em Inglês | MEDLINE | ID: mdl-26373838

RESUMO

We describe the synthesis of two conjugated macrocycles that are formed from the end-to-end linking of stilbenes. We have named these macrocycles cyclostilbenes. The two cyclostilbene isomers created in this study differ in the configuration of the double bond in their subunits. These macrocycles are formed selectively through a stepwise reductive elimination from a tetraplatinum precursor and subsequent photoisomerization. Single-crystal X-ray diffraction reveals the formation of channel architectures in the solid state that can be filled with guest molecules. The cyclostilbene macrocycles emit blue light with fluorescence quantum yields that are high (>50%) and have photoluminescence lifetimes of ∼0.8-1.5 ns. The breadth and large Stokes shift in fluorescence emission, along with broad excited-state absorption, result from strong electronic-vibronic coupling in the strained structures of the cyclostilbenes.

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