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

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

Synthesis-Controlled Cation Solubility in Solid Sodium Ion Conductors Na_2+<i>x</i>Zr_1–<i>x</i>In_<i>x</i>Cl₆

Mechanochemically synthesized sodium halide solid solutions with the general formula Na2+xZr1–xMxCl6, as a class of potential catholytes, show promising ionic transport in comparison to their parental materials such as Na3YCl6. However, the influence of subsequent heat treatment protocols on the structure and transport properties of these materials is still not fully understood. In this work, a series of Na2+xZr1–xInxCl6 solid solutions are prepared by ball milling with subsequent annealing at different temperatures. X-ray diffraction analyses show a full indium solubility in Na2+xZr1–xInxCl6 when synthesized at low temperatures and crystallizing in the P21/n phase. In contrast, at higher heat treatment temperatures, exsolution is observed as the indium-rich Na2+xZr1–xInxCl6 compound tends to partially transform to the trigonal P3̅1c phase. By assessing the ionic conductivity of the differently synthesized Na2+xZr1–xInxCl6 series, we can show the synergistic effect of the Na+/vacancy ratio and crystallinity on sodium ion transport in this class of materials

Exploring Layered Disorder in Lithium-Ion-Conducting Li₃Y_1–<i>x</i>In_<i>x</i>Cl₆

Li3Y1–xInxCl6 undergoes a phase transition from trigonal to monoclinic via an intermediate orthorhombic phase. Although the trigonal yttrium containing the end member phase, Li3YCl6, synthesized by a mechanochemical route, is known to exhibit stacking fault disorder, not much is known about the monoclinic phases of the serial composition Li3Y1–xInxCl6. This work aims to shed light on the influence of the indium substitution on the phase evolution, along with the evolution of stacking fault disorder using X-ray and neutron powder diffraction together with solid-state nuclear magnetic resonance spectroscopy, studying the lithium-ion diffusion. Although Li3Y1–xInxCl6 with x ≤ 0.1 exhibits an ordered trigonal structure like Li3YCl6, a large degree of stacking fault disorder is observed in the monoclinic phases for the x ≥ 0.3 compositions. The stacking fault disorder materializes as a crystallographic intergrowth of faultless domains with staggered layers stacked in a uniform layer stacking, along with faulted domains with randomized staggered layer stacking. This work shows how structurally complex even the “simple” series of solid solutions can be in this class of halide-based lithium-ion conductors, as apparent from difficulties in finding a consistent structural descriptor for the ionic transport