Synthesis-Controlled Polymorphism and Anion Solubility in the Sodium-Ion Conductor Na₃InCl_6–<i>x</i>Br_<i>x</i> (0 ≤ <i>x</i> ≤ 2)

Motivated by the significant transport property improvement of the anion-substituted lithium metal halides, a series of anion mixed solid solutions of Na3InCl6–xBrx (0 ≤ x ≤ 2) are successfully synthesized by ball milling and subsequent annealing. By milling, the Na3InCl6–xBrx solid solution series crystallizes in a monoclinic P21/n phase, while the subsequently annealed Na3InCl6–xBrx series transforms into a trigonal P3̅1c phase. Through annealing and changes of the structure type, greater anion solubility can be achieved. The halide substitution slightly improves the ionic conductivity in the Na3InCl6–xBrx series, indicating that mixed halide compositions and their structural changes affect the ionic transport albeit less strongly than in the lithium analogues such as Li3YCl6–xBrx and Li3InCl6–xBrx

Bottleneck of Diffusion and Inductive Effects in Li₁₀Ge_1–<i>x</i>Sn_<i>x</i>P₂S₁₂

The lithium-ion conductor Li₁₀GeP₂S₁₂ (LGPS) is known to exhibit ionic conductivity values as high as 12 mS·cm^–1. Unfortunately, counter to chemical intuition, many attempts to enhance the ionic transport in LGPS, e.g., by increasing the Sn fraction in Li₁₀Ge_1–<i>x</i>Sn_<i>x</i>P₂S₁₂, have even led to a reduction in the conductivity. Employing a combination of Rietveld refinements against X-ray diffraction data, speed of sound measurements, and electrochemical impedance spectroscopy, we investigate the structure–property relationships governing this behavior. Herein, it is shown that with increasing Sn⁴⁺ fraction in Li₁₀Ge_1–<i>x</i>Sn_<i>x</i>P₂S₁₂ a structural bottleneck along the diffusion channels in the <i>z</i>-direction begins to tighten, and with the concomitant increase in the lattice softness, the local ionic bonding interactions between Li⁺ and S^2– become stronger, further increasing the activation barrier. This work provides a likely explanation for the lower conductivity exhibited by Li₁₀SnP₂S₁₂ and demonstrates that there is more to the underlying lithium diffusion mechanism in the Li₁₀MP₂S₁₂ structure

Bottleneck of Diffusion and Inductive Effects in Li₁₀Ge_1–<i>x</i>Sn_<i>x</i>P₂S₁₂

The lithium-ion conductor Li₁₀GeP₂S₁₂ (LGPS) is known to exhibit ionic conductivity values as high as 12 mS·cm^–1. Unfortunately, counter to chemical intuition, many attempts to enhance the ionic transport in LGPS, e.g., by increasing the Sn fraction in Li₁₀Ge_1–<i>x</i>Sn_<i>x</i>P₂S₁₂, have even led to a reduction in the conductivity. Employing a combination of Rietveld refinements against X-ray diffraction data, speed of sound measurements, and electrochemical impedance spectroscopy, we investigate the structure–property relationships governing this behavior. Herein, it is shown that with increasing Sn⁴⁺ fraction in Li₁₀Ge_1–<i>x</i>Sn_<i>x</i>P₂S₁₂ a structural bottleneck along the diffusion channels in the <i>z</i>-direction begins to tighten, and with the concomitant increase in the lattice softness, the local ionic bonding interactions between Li⁺ and S^2– become stronger, further increasing the activation barrier. This work provides a likely explanation for the lower conductivity exhibited by Li₁₀SnP₂S₁₂ and demonstrates that there is more to the underlying lithium diffusion mechanism in the Li₁₀MP₂S₁₂ structure

Local Tetragonal Structure of the Cubic Superionic Conductor Na₃PS₄

The sodium superionic conductor Na₃PS₄ is known to crystallize in one of two different structural polymorphs at room temperature (i.e., cubic or tetragonal, depending on the synthetic conditions). Experimentally, the cubic structure is known to exhibit a higher ionic conductivity than the tetragonal structure, despite theoretical investigations suggesting that there should be no difference at all. Employing a combination of Rietveld and pair distribution function (PDF) analyses, as well as electrochemical impedance spectroscopy, we investigate the open question of how the crystal structure influences the ionic transport in Na₃PS₄. Despite the average structures of Na₃PS₄ prepared via ball-milling and high-temperature routes being cubic and tetragonal, respectively, the structural analysis by PDF indicates that both compounds are best described by the structural motifs of the tetragonal polymorph on the local scale. Ultimately, the high ionic conductivity of Na₃PS₄ prepared by the ball-milling approach is confirmed to be independent of the crystal structure. This work demonstrates that even in ionic conductors differences can be observed between the average and local crystal structures, and it reasserts that the high ionic conductivity in Na₃PS₄ is not related to the crystal structure but rather differences in the defect concentration

Thermal Conductivities of Lithium-Ion-Conducting Solid Electrolytes

Solid electrolytes and solid-state batteries have gathered attention in recent years as a potential alternative to state-of-the-art lithium-ion batteries, given the promised increased energy density and safety following the replacement of flammable organic electrolytes with solids. While ongoing research focuses mainly on improving the ionic conductivities of solid electrolytes, little is known about the thermal transport properties of this material class. This includes fundamental studies of heat capacities and thermal conductivities, application-oriented investigations of porosity effects, and the modeling of the temperature distribution in solid-state batteries during operation. To expand the understanding of transport in solid electrolytes, in this work, thermal properties of electrolytes in the argyrodite family (Li6PS5X with X = Cl, Br, I, and Li5.5PS4.5Cl1.5) and Li10GeP2S12 as a function of temperature and porosity are reported. It is shown that the thermal conductivities of solid electrolytes are in the range of liquid electrolytes. Utilizing effective medium theory to describe the porosity-dependent results, an empirical predictive model is obtained, and the intrinsic (bulk) thermal conductivities for all electrolytes are extracted. Moreover, the temperature-independent, glass-like thermal conductivities found in all materials suggest that thermal transport in these ionic conductors occurs in a nontextbook fashion

Effect of Isovalent Substitution on the Thermoelectric Properties of the Cu₂ZnGeSe_4–<i>x</i>S_<i>x</i> Series of Solid Solutions

Knowledge of structure–property relationships is a key feature of materials design. The control of thermal transport has proven to be crucial for the optimization of thermoelectric materials. We report the synthesis, chemical characterization, thermoelectric transport properties, and thermal transport calculations of the complete solid solution series Cu₂ZnGeSe_4–<i>x</i>S_<i>x</i> (<i>x</i> = 0–4). Throughout the substitution series a continuous Vegard-like behavior of the lattice parameters, bond distances, optical band gap energies, and sound velocities are found, which enables the tuning of these properties adjusting the initial composition. Refinements of the special chalcogen positions revealed a change in bonding angles, resulting in crystallographic strain possibly affecting transport properties. Thermal transport measurements showed a reduction in the room-temperature thermal conductivity of 42% triggered by the introduced disorder. Thermal transport calculations of mass and strain contrast revealed that 34% of the reduction in thermal conductivity is due to the mass contrast only and 8% is due to crystallographic strain

Influence of Lattice Dynamics on Na⁺ Transport in the Solid Electrolyte Na₃PS_4–<i>x</i>Se_<i>x</i>

Li⁺- and Na⁺-conducting thiophosphates have attracted much interest because of their intrinsically high ionic conductivities and the possibility to be employed in solid-state batteries. Inspired by the recent finding of the influence of changing lattice vibrations and induced lattice softening on the ionic transport of Li⁺-conducting electrolytes, here we explore this effect in the Na⁺ conductor Na₃PS_4–<i>x</i>Se_<i>x</i>. Ultrasonic speed of sound measurements are used to monitor a changing lattice stiffness and Debye frequencies. The changes in the lattice dynamics are complemented by X-ray diffraction and electrochemical impedance spectroscopy. With systematic alteration of the polarizability of the anion framework, a softening of the lattice can be observed that leads to a reduction of the activation barrier for migration as well as a decreased Arrhenius prefactor. This work shows that, similar to Li⁺ transport, the softening of the average vibrational frequencies of the lattice has a tremendous effect on Na⁺-ionic transport and that ion–phonon interactions need to be considered in solid electrolytes

Correlating Transport and Structural Properties in Li_1+<i>x</i>Al_<i>x</i>Ge_2–<i>x</i>(PO₄)₃ (LAGP) Prepared from Aqueous Solution

Li_1+<i>x</i>Al_<i>x</i>Ge_2–<i>x</i>(PO₄)₃ (LAGP) is a solid lithium-ion conductor belonging to the NASICON family, representing the solid solution of LiGe₂(PO₄)₃ and AlPO₄. The typical syntheses of LAGP either involve high-temperature melt-quenching, which is complicated and expensive, or a sol–gel process requiring costly organic germanium precursors. In this work, we report a simple method based on aqueous solutions without the need of ethoxide precursors. Using synchrotron and neutron diffraction, the crystal structure, the occupancies for Al and Ge, and the distribution of lithium were determined. Substitution of germanium by aluminum allows for an increased Li⁺ incorporation in the material and the actual Li⁺ content in the sample increases with the nominal Li⁺ content and a solubility limit is observed for higher aluminum content. By means of impedance spectroscopy, an increase in the ionic conductivity with increasing lithium content is observed. Whereas the lithium ionic conductivity improves, due to the increasing carrier density, the bulk activation energy increases. This correlation suggests that changes in the transport mechanism and correlated motion may be at play in the Li_1+<i>x</i>Al_<i>x</i>Ge_2–<i>x</i>(PO₄)₃ solid solution

Correlating Transport and Structural Properties in Li_1+<i>x</i>Al_<i>x</i>Ge_2–<i>x</i>(PO₄)₃ (LAGP) Prepared from Aqueous Solution

Li_1+<i>x</i>Al_<i>x</i>Ge_2–<i>x</i>(PO₄)₃ (LAGP) is a solid lithium-ion conductor belonging to the NASICON family, representing the solid solution of LiGe₂(PO₄)₃ and AlPO₄. The typical syntheses of LAGP either involve high-temperature melt-quenching, which is complicated and expensive, or a sol–gel process requiring costly organic germanium precursors. In this work, we report a simple method based on aqueous solutions without the need of ethoxide precursors. Using synchrotron and neutron diffraction, the crystal structure, the occupancies for Al and Ge, and the distribution of lithium were determined. Substitution of germanium by aluminum allows for an increased Li⁺ incorporation in the material and the actual Li⁺ content in the sample increases with the nominal Li⁺ content and a solubility limit is observed for higher aluminum content. By means of impedance spectroscopy, an increase in the ionic conductivity with increasing lithium content is observed. Whereas the lithium ionic conductivity improves, due to the increasing carrier density, the bulk activation energy increases. This correlation suggests that changes in the transport mechanism and correlated motion may be at play in the Li_1+<i>x</i>Al_<i>x</i>Ge_2–<i>x</i>(PO₄)₃ solid solution

Dependence of the Li-Ion Conductivity and Activation Energies on the Crystal Structure and Ionic Radii in Li₆MLa₂Ta₂O₁₂

Inspired by the promising ionic conductivities of the lithium conducting garnets, we present a comparative study on the influence of the ionic radius of M²⁺ on the 8-coordinate site and the crystal structure on the ionic transport in the solid solution Li₆MLa₂Ta₂O₁₂. Neutron diffraction and synchrotron diffraction in combination with AC impedance measurements are employed to understand the systematic substitution with different-sized alkaline earth cations M²⁺. As may be expected, the unit-cell parameters increase linearly with increasing ionic radius from Ca²⁺ over Sr²⁺ to Ba²⁺, accompanied by an increase in the polyhedral volumes of the dodecahedral, and tetrahedral positions and the ionic conductivities. While the TaO₆ octahedral volume remain constant, the anisotropic thermal parameters of the coordinating oxygen anions suggest a high degree of rotational freedom with increasing unit-cell size. These structural parameters lead to lower activation energies because of broader Li conduction pathways and a higher flexibility in the crystal lattice, ultimately controlling the ionic conductivities in this class of materials