Annalise E. Maughan

Lithium metal chloride halospinels of the general formula Li₂ MCl₄ are a promising class of earth-abundant ion conductors for all-solid-state batteries. However, poor room-temperature ionic conductivity has historically limited their use in practical applications. Here, we substitute Zr⁴⁺ into Li₂ZnCl₄ along the series Li_2–2x/3Zn_1–x Zr_2x/3Cl₄ (x = 0, 0.1, 0.3, 0.4, 0.6, 0.9, and 1.0) to understand how cation disorder and vacancy tuning impact ion transport in “normal” halospinels. Aliovalent Zr⁴⁺ substitution increases ionic conductivity by nearly 5 orders of magnitude, from 1.320(3) × 10^–9 S cm^–1 in Li₂ZnCl₄ to 6.74(1)× 10^–5 S cm^–1 for x = 0.6. Average and local structure characterization through synchrotron X-ray diffraction (SXRD) and neutron pair distribution function (nPDF) analysis reveal that Zr⁴⁺ redistributes the Zn²⁺ and Li⁺ sublattices into previously unoccupied interstitial sites, which form new low-energy hopping pathways that facilitate ion transport. We rationalize the dramatic rearrangement of the cation local structure by considering the coordination preferences of the cations and the potential electrostatic penalties incurred by the higher-valent Zr⁴⁺ cations. This work delivers an atomistic understanding of substitution-induced cation disorder and ion transport properties in a new family of earth-abundant halospinels.

Ball milling is a powerful synthetic tool for discovering new inorganic materials. Inspired by the high ionic conductivity in Li₂ZrCl₆ synthesized via mechanochemistry, we synthesized MgZrCl₆ with a similar method. High resolution synchrotron X-ray diffraction shows that MgZrCl₆ is poorly crystalline after ball milling, but crystallizes in a layered hexagonal structure (P31c) after heat treatment. In situ synchrotron X-ray diffraction reveals a narrow temperature window around 400 °C in which crystallization occurs. At higher temperatures the phase decomposes. Pair distribution function analysis shows 2D sheets of MgZrCl₆ form after milling, with heating driving 3D crystallization. Raman spectroscopy also shows evidence of MgZrCl₆ after milling. Electrochemical impedance spectroscopy does not reveal ionic conductivity in MgZrCl₆ (limit of detection ca. 1.4 × 10^-8 S/cm). In addition to supporting existing design rules for Mg-based solid electrolytes, this work shows the power of ball milling to synthesize temperature-sensitive inorganic compounds with high yield.

Argyrodites are a compositionally diverse family of materials that exhibit remarkable ion transport properties. While the average crystal structures of argyrodites have been extensively studied, ion transport in these materials is governed by a confluence of dynamic processes spanning the cation, anion, and polyanionic sublattices. This Perspective synthesizes recent advances in understanding the role of dynamics in structural behavior and ion transport properties. We examine the compositional and structural motifs that govern order-disorder transitions within the argyrodite family and further explore how ion hopping is facilitated by lattice dynamics, from long-range phonons to local rotational dynamics of polyanionic species. Through the lens of dynamics spanning multiple time and length scales, we establish guiding principles that govern transport phenomena and highlight avenues of future study for the argyrodite family of ion conductors.

Mixed ionic-electronic conductors (MIEC) are exciting options for battery materials, encompassing next generation cathodes and catholytes. Unfortunately, control over mixed transport parameters in sodium metal chloride MIECs has not been well explored. Herein we investigate the isostructural substitution series of NaAl _1−x Fe _x Cl ₄ to explore the transition from a single ion conductor to an MIEC. We observe an increase in electronic conductivity due to the inclusion of the Fe ³⁺ 3 d states, which is accompanied by a slight decrease in ionic conductivity in the metal site-mixed materials. To explore the electronic evolution, band calculations suggest that the addition of 3 d states lead to color and electronic changes in this material system, which is in agreement with optical absorption measurements. This work presents one pathway to tune the ionic and electronic transport of MIECs.

Lithium metal chloride halospinels of the general formula Li2MCl4 are a promising class of earth-abundant ion conductors for all-solid-state batteries. However, poor room-temperature ionic conductivity has historically limited their use in practical applications. Here, we substitute Zr4+ into Li2ZnCl4 along the series Li2−2x/3Zn1−xZr2x/3Cl4 (x = 0, 0.1, 0.3, 0.6, 0.9, and 1.0) to understand how cation disorder and vacancy tuning impacts ion transport in “normal” halospinels. Aliovalent Zr4+ substitution increases ionic conductivity by nearly five orders of magnitude, from 1.320(3) × 10−9 S cm−1 in Li2ZnCl4 to 6.74(1) × 10−5 S cm−1 for x = 0.6. Average and local structure characterization through synchrotron X-ray diffraction (SXRD) and neutron pair distribution function (nPDF) analysis reveal that Zr4+ redistributes the Zn2+ and Li+ sublattices into previously unoccupied interstitial sites that form new low-energy hopping pathways that facilitate ion transport. We rationalize the dramatic rearrangement of the cation local structure by considering the coordination preferences of the cations and the potential electrostatic penalties incurred by the higher-valent Zr4+ cations. This work delivers an atomistic understanding of substitution-induced cation disorder and ion transport properties in a new family of earth-abundant halospinels.

The advantageous properties of (pseudo)­halide argyrodite ion conductors of the formula Li₆PS₅ X (X = Cl^–, Br^–, I^–, CN^–) have motivated extensive studies of their structure-transport relationships, particularly as they pertain to the role of atomic site disorder. The argyrodite structure can accommodate additional configurational complexity to promote ion transport via extended three-anion site mixing and the potential for orientational disorder of molecular anions. In this work, we explore a ternary anion system including the cyanide anion, expanding site disorder and introducing dipolar orientations as an additional degree of freedom. We prepared the series Li₆PS₅(CN)_1–x Br _x , in which the dipolar cyanide anions are systematically diluted with bromide. We find that anion disorder, as determined by synchrotron and neutron diffraction and quantified by configurational entropy (S _config), is correlated with lowered activation barriers and increased lithium ion conductivity. We propose that S _config describes the electrostatic heterogeneity of the Li environments, flattening the energetic landscape for ion transport. While anion substitution strongly impacts the activation barrier for transport, the temperature-independent Arrhenius prefactor does not follow the same trend. Through heat-capacity measurements of attempt frequency and deconvolution of terms within the prefactor, we rationalize the apparent decoupling of activation energy and prefactor to strong cyanide-lithium interactions that increase the entropy of migration. Together, these results expand the structure–property relationships in the argyrodite family to encompass multiple facets of disorder and the subsequent impact on lithium ion transport.