Lattice Dynamics in the NASICON NaZr2(PO4)3 Solid Electrolyte from Temperature-Dependent Neutron Diffraction, NMR, and Ab Initio Computational Studies.

Natrium super ionic conductor (NASICON) compounds form a rich and highly chemically tunable family of crystalline materials that are of widespread interest because they include exemplars with high ionic conductivity, low thermal expansion, and redox tunability. This makes them suitable candidates for applications ranging from solid-state batteries to nuclear waste storage materials. The key to an understanding of these properties, including the origins of effective cation transport and low, anisotropic (and sometimes negative) thermal expansion, lies in the lattice dynamics associated with specific details of the crystal structure. Here we closely examine the prototypical NASICON compound, NaZr2(PO4)3, and obtain detailed insights into such behavior via variable-temperature neutron diffraction and 23Na and 31P solid-state NMR studies, coupled with comprehensive density functional theory-based calculations of NMR parameters. Temperature-dependent NMR studies yield some surprising trends in the chemical shifts and the quadrupolar coupling constants that are not captured by computation unless the underlying vibrational modes of the crystal are explicitly taken into account. Furthermore, the trajectories of the sodium, zirconium, and oxygen atoms in our dynamical simulations show good qualitative agreement with the anisotropic thermal parameters obtained at higher temperatures by neutron diffraction. The work presented here widens the utility of NMR crystallography to include thermal effects as a unique probe of interesting lattice dynamics in functional materials.

Rapid and Tunable Assisted-Microwave Preparation of Glass and Glass-Ceramic Thiophosphate "Li7P3S11" Li-Ion Conductors.

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.

Ab initio computation for solid-state 31P NMR of inorganic phosphates: revisiting X-ray structures.

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.

Polymorphism in M(H2PO2)3 (M = V, Al, Ga) compounds with the perovskite-related ReO3 structure.

Trivalent metal hypophosphites with the general formula M(H2PO2)3 (M = V, Al, Ga) adopt the ReO3 structure, with each compound displaying two structural polymorphs. High-pressure synchrotron X-ray studies reveal a pressure-driven phase transition in Ga(H2PO2)3 that can be understood on the basis of ab initio thermodynamics.

Communication: Investigation of ion aggregation in ionic liquids and their solutions with lithium salt under high pressure.

X-ray scattering measurements were utilized to probe the effects of pressure on a series of ionic liquids, N-alkyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr1A-TFSI) (A = 3, 6, and 9), along with mixtures of ionic liquid and 30 mol. % lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt. No evidence was found for crystallization of the pure ionic liquids or salt mixtures even at pressures up to 9.2 GPa. No phase separation or demixing was observed for the ionic liquid and salt mixtures. Shifts in the peak positions are indicative of compression of the ionic liquids and mixtures up to 2 GPa, after which samples reach a region of relative incompressibility, possibly indicative of a transition to a glassy state. With the application of pressure, the intensity of the prepeak was found to decrease significantly, indicating a reduction in cation alkyl chain aggregation. Additionally, incompressibility of the scattering peak associated with the distance between like-charges in the pure ionic liquids compared to that in mixtures with lithium salt suggests that the application of pressure could inhibit Li+ coordination with TFSI- to form Li[TFSI2]- complexes. This inhibition occurs through the suppression of TFSI- in the trans conformer, in favor of the smaller cis conformer, at high pressures.

Connection between Lithium Coordination and Lithium Diffusion in [Pyr12O1 ][FTFSI] Ionic Liquid Electrolytes.

The use of highly concentrated ionic liquid-based electrolytes results in improved rate capability and capacity retention at 20 °C compared to Li+ -dilute systems in Li-metal and Li-ion cells. This work explores the connection between the bulk electrolyte properties and the molecular organization to provide insight into the concentration dependence of the Li+ transport mechanisms. Below 30 mol %, the Li+ -containing species are primarily smaller complexes (one Li+ cation) and the Li+ ion transport is mostly derived from the vehicular transport. Above 30 mol %, where the viscosity is substantially higher and the conductivity lower, the Li+ -containing species are a mix of small and large complexes (one and more than one Li+ cation, respectively). The overall conduction mechanism likely changes to favor structural diffusion through the exchange of anions in the first Li+ solvation shell. The good rate performance is likely directly influenced by the presence of larger Li+ complexes, which promote Li+ -ion transport (as opposed to Li+ -complex transport) and increase the Li+ availability at the electrode.

Investigation of dynamics in BMIM TFSA ionic liquid through variable temperature and pressure NMR relaxometry and diffusometry.

A comprehensive variable temperature, pressure and frequency multinuclear (1H, 2H, and 19F) magnetic resonance study was undertaken on selectively deuterated 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (BMIM TFSA) ionic liquid isotopologues. This study builds on our earlier investigation of the effects of increasing alkyl chain length on diffusion and dynamics in imidazolium-based TFSA ionic liquids. Fast field cycling 1H T1 data revealed multiple modes of motion. Through calculation of diffusion coefficient (D) values and activation energies, the low- and high-field regimes were assigned to the translational and reorientation dynamics respectively. Variable-pressure 2H T1 measurements reveal site-dependent interactions in the cation with strengths in the order MD3 > CD3 > CD2, indicating dissimilarities in the electric field gradients along the alkyl chain, with the CD2 sites having the largest gradient. Additionally, the α saturation effect in T1 vs. P was observed for all three sites, suggesting significant reduction of the short-range rapid reorientational dynamics. This reduction was also deduced from the variable pressure 1H T1 data, which showed an approach to saturation for both the methyl and butyl group terminal methyl sites. Pressure-dependent D measurements show independent motions for both cations and anions, with the cations having greater D values over the entire pressure range.

Do TFSA Anions Slither? Pressure Exposes the Role of TFSA Conformational Exchange in Self-Diffusion.

Multinuclear ((1)H, (2)H, and (19)F) magnetic resonance spectroscopy techniques as functions of temperature and pressure were applied to the study of selectively deuterated 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (EMIM TFSA) ionic liquid isotopologues and related ionic liquids. For EMIM TFSA, temperature-dependent (2)H T1 data indicate stronger electric field gradients in the alkyl chain region compared to the imidazolium ring. Most significantly, the pressure dependences of the EMIM and TFSA self-diffusion coefficients revealed that the displacements of the cations and anions are independent, with diffusion of the TFSA anions being slowed much more by increasing pressure than for the EMIM cations, as shown by their respective activation volumes (28.8 ± 2.5 cm(3)/mol for TFSA vs 14.6 ± 1.3 cm(3)/mol for EMIM). Increasing pressure may lower the mobility of the TFSA anion by hindering its interconversion between trans and cis conformers, a process that is coupled to diffusion according to published molecular dynamics simulations. Measured activation volumes (ΔV(‡)) for ion self-diffusion in EMIM bis(fluoromethylsulfonyl)amide and EMIM tetrafluoroborate support this hypothesis. In addition, (2)H T1 data suggest increased ordering with increasing pressure, with two T1 regimes observed for the MD3 and D2 isotopologues between 0.1-100 and 100-250 MPa, respectively. The activation volumes for T1 were 21 and 25 cm(3)/mol (0-100 MPa) and 11 and 12 cm(3)/mol (100-250 MPa) for the MD3 and D2 isotopologues, respectively.

An iodide-based Li7P2S8I superionic conductor.

In an example of stability from instability, a Li(7)P(2)S(8)I solid-state Li-ion conductor derived from ß-Li(3)PS(4) and LiI demonstrates electrochemical stability up to 10 V vs Li/Li(+). The oxidation instability of I is subverted via its incorporation into the coordinated structure. The inclusion of I also creates stability with the metallic Li anode while simultaneously enhancing the interfacial kinetics and ionic conductivity. Low-temperature membrane processability enables facile fabrication of dense membranes, making this conductor suitable for industrial adoption.