Structural Evolution and Li Dynamics in Nanophase Li₃PS₄ by Solid-State and Pulsed-Field Gradient NMR

The ceramic lithium ion conductor β-Li₃PS₄ has a disordered and nanoporous structure that leads to an enhancement in ionic conductivity by some 3 orders of magnitude compared to the crystalline γ phase. The β phase is prepared by thermal treatment of an inorganic–organic complex based on Li₃PS₄ and THF. Multinuclear (¹H, ^6,7Li, ³¹P) solid-state NMR spectroscopy is used to characterize the structural phase evolution of the starting material at various steps in the thermal treatment. The β phase formed after high temperature treatment is recognized as spectroscopically distinct from the bulk γ-Li₃PS₄ compound. Also formed is an amorphous lithium thiophosphate phase that is metastable as verified by annealing over an extended period. Lithium ion self-diffusion coefficients are measurable by standard pulsed-field gradient NMR methods at 100 °C and with values consistent with the high ionic conductivity previously reported for this material

Influence of Solvent on Ion Aggregation and Transport in PY₁₅TFSI Ionic Liquid–Aprotic Solvent Mixtures

Molecular dynamics (MD) simulations using a many-body polarizable APPLE&P force field have been performed on mixtures of the <i>N</i>-methyl-<i>N</i>-pentylpyrrolidinium bis­(trifluoromethanesulfonyl)­imide (PY₁₅TFSI) ionic liquid (IL) with three molecular solvents: propylene carbonate (PC), dimethyl carbonate (DMC), and acetonitrile (AN). The MD simulations predict density, viscosity, and ionic conductivity values that agree well with the experimental results. In the solvent-rich regime, the ionic conductivity of the PY₁₅TFSI–AN mixtures was found to be significantly higher than the conductivity of the corresponding −PC and −DMC mixtures, despite the similar viscosity values obtained from both the MD simulations and experiments for the −DMC and −AN mixtures. The significantly lower conductivity of the PY₁₅TFSI–DMC mixtures, as compared to those for PY₁₅TFSI–AN, in the solvent-rich regime was attributed to the more extensive ion aggregation observed for the −DMC mixtures. The PY₁₅TFSI–DMC mixtures present an interesting case where the addition of the organic solvent to the IL results in an increase in the cation–anion correlations, in contrast to what is found for the mixtures with PC and AN, where ion motion became increasingly uncorrelated with addition of solvent. A combination of pfg-NMR and conductivity measurements confirmed the MD simulation predictions. Further insight into the molecular interactions and properties was also obtained using the MD simulations by examining the solvent distribution in the IL–solvent mixtures and the mixture excess properties

Diffusion Coefficients from ¹³C PGSE NMR MeasurementsFluorine-Free Ionic Liquids with the DCTA^– Anion

Pulsed-field gradient spin–echo (PGSE) NMR is a widely used method for the determination of molecular and ionic self-diffusion coefficients. The analysis has thus far been limited largely to ¹H, ⁷Li, ¹⁹F, and ³¹P nuclei. This limitation handicaps the analysis of materials without these nuclei or for which these nuclei are insufficient for complete characterization. This is demonstrated with a class of ionic liquids (or ILs) based on the nonfluorinated anion 4,5-dicarbonitrile-1,2,3-triazole (DCTA^–). It is demonstrated here that ¹³C-PGSE NMR can be used to both verify the diffusion coefficients obtained from other nuclei, as well as characterize materials that lack commonly scrutinized nuclei  all without the need for specialized NMR methods

An Iodide-Based Li₇P₂S₈I Superionic Conductor

In an example of stability from instability, a Li₇P₂S₈I solid-state Li-ion conductor derived from β-Li₃PS₄ 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

Liquid Structure with Nano-Heterogeneity Promotes Cationic Transport in Concentrated Electrolytes

Using molecular dynamics simulations, small-angle neutron scattering, and a variety of spectroscopic techniques, we evaluated the ion solvation and transport behaviors in aqueous electrolytes containing bis­(trifluoromethanesulfonyl)­imide. We discovered that, at high salt concentrations (from 10 to 21 mol/kg), a disproportion of cation solvation occurs, leading to a liquid structure of heterogeneous domains with a characteristic length scale of 1 to 2 nm. This unusual nano-heterogeneity effectively decouples cations from the Coulombic traps of anions and provides a 3D percolating lithium–water network, <i>via</i> which 40% of the lithium cations are liberated for fast ion transport even in concentration ranges traditionally considered too viscous. Due to such percolation networks, superconcentrated aqueous electrolytes are characterized by a high lithium-transference number (0.73), which is key to supporting an assortment of battery chemistries at high rate. The in-depth understanding of this transport mechanism establishes guiding principles to the tailored design of future superconcentrated electrolyte systems