Mechanistic Origin of Superionic Lithium Diffusion in Anion-Disordered Li6PS5 X Argyrodites

Disorder-Dependent Li Diffusion in Li6PS5Cl Investigated by Machine-Learning Potential

Solid-state electrolytes with argyrodite structures, such as Li6PS5Cl, have attracted considerable attention due to their superior safety compared to liquid electrolytes and higher ionic conductivity than other solid electrolytes. Although experimental efforts have been made to enhance conductivity by controlling the degree of disorder, the underlying diffusion mechanism is not yet fully understood. Moreover, existing theoretical analyses based on ab initio molecular dynamics (MD) simulations have limitations in addressing various types of disorder at room temperature. In this study, we directly investigate Li-ion diffusion in Li6PS5Cl at 300 K using large-scale, long-term MD simulations empowered by machine-learning potentials (MLPs). To ensure the convergence of conductivity values within an error range of 10%, we employ a 25 ns simulation using a 5 × 5 × 5 supercell containing 6500 atoms. The computed Li-ion conductivity, activation energies, and equilibrium site occupancies align well with experimental observations. Notably, Li-ion conductivity peaks when Cl ions occupy 25% of the 4c sites rather than at 50% where the disorder is maximized. In addition, Li-ion diffusion shows non-Arrhenius behavior, leading to different activation energies at high temperatures (>400 K). These phenomena are explained by the interplay between inter- and intracage jumps. By elucidation of the key factors affecting Li-ion diffusion in Li6PS5Cl, this work paves the way for optimizing ionic conductivity in the argyrodite family.

Ion Transport at Polymer-Argyrodite Interfaces

Solid-state electrolytes, particularly polymer/ceramic composite electrolytes, are emerging as promising candidates for lithium-ion batteries due to their high ionic conductivity and mechanical flexibility. The interfaces that arise between the inorganic and organic materials in these composites play a crucial role in ion transport mechanisms. While lithium ions are proposed to diffuse across or parallel to the interface, few studies have directly examined the quantitative impact of these pathways on ion transport and little is known about how they affect the overall conductivity. Here, we present an atomistic study of lithium-ion (Li+) transport across well-defined polymer-argyrodite interfaces. We present a force field for polymer-argyrodite interfacial systems, and we carry out molecular dynamics and enhanced sampling simulations of several composite systems, including poly(ethylene oxide) (PEO)/Li6PS5Cl, hydrogenated nitrile butadiene rubber (HNBR)/Li6PS5Cl, and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP)/Li6PS5Cl. For the materials considered here, Li-ion exhibits a preference for the ceramic material, as revealed by free energy differences for Li-ion between the inorganic and the organic polymer phase in excess of 13 kBT. The relative free energy profiles of Li-ion for different polymeric materials exhibit similar shapes, but their magnitude depends on the strength of interaction between the polymers and Li-ion: the greater the interaction between the polymer and Li-ions, the smaller the free energy difference between the inorganic and organic materials. The influence of the interface is felt over a range of approximately 1.5 nm, after which the behavior of Li-ion in the polymer is comparable to that in the bulk. Near the interface, Li-ion transport primarily occurs parallel to the interfacial plane, and ion mobility is considerably slower near the interface itself, consistent with the reduced segmental mobility of the polymer in the vicinity of the ceramic material. These findings provide insights into ionic complexation and transport mechanisms in composite systems, and will help improve design of improved solid electrolyte systems.

Correlated Anion Disorder in Heteroanionic Cubic TiOF2

Resolving anion configurations in heteroanionic materials is crucial for understanding and controlling their properties. For anion-disordered oxyfluorides, conventional Bragg diffraction cannot fully resolve the anionic structure, necessitating alternative structure determination methods. We have investigated the anionic structure of anion-disordered cubic (ReO3-type) TiOF2 using X-ray pair distribution function (PDF), 19F MAS NMR analysis, density functional theory (DFT), cluster expansion modeling, and genetic-algorithm structure prediction. Our computational data predict short-range anion ordering in TiOF2, characterized by predominant cis-[O2F4] titanium coordination, resulting in correlated anion disorder at longer ranges. To validate our predictions, we generated partially disordered supercells using genetic-algorithm structure prediction and computed simulated X-ray PDF data and 19F MAS NMR spectra, which we compared directly to experimental data. To construct our simulated 19F NMR spectra, we derived new transformation functions for mapping calculated magnetic shieldings to predicted magnetic chemical shifts in titanium (oxy)fluorides, obtained by fitting DFT-calculated magnetic shieldings to previously published experimental chemical shift data for TiF4. We find good agreement between our simulated and experimental data, which supports our computationally predicted structural model and demonstrates the effectiveness of complementary experimental and computational techniques in resolving anionic structure in anion-disordered oxyfluorides. From additional DFT calculations, we predict that increasing anion disorder makes lithium intercalation more favorable by, on average, up to 2 eV, highlighting the significant effect of variations in short-range order on the intercalation properties of anion-disordered materials.

An Effective Catholyte for Sulfide-Based All-Solid-State Batteries Utilizing Gas Absorbents

All-solid-state batteries (ASSBs) possess the advantage of ensuring safety while simultaneously maximizing energy density, making them suitable for next-generation battery models. In particular, sulfide solid electrolytes (SSEs) are viewed as promising candidates for ASSB electrolytes due to their excellent ionic conductivity. However, a limitation exists in the form of interfacial side reactions occurring between the SSEs and cathode active materials (CAMs), as well as the generation of sulfide-based gases within the SSE. These issues lead to a reduction in the capacity of CAMs and an increase in internal resistance within the cell. To address these challenges, cathode composite materials incorporating zinc oxide (ZnO) are fabricated, effectively reducing various side reactions occurring in CAMs. Acting as a semiconductor, ZnO helps mitigate the rapid oxidation of the solid electrolyte facilitated by an electronic pathway, thereby minimizing side reactions, while maintaining electron pathways to the active material. Additionally, it absorbs sulfide-based gases, thus protecting the lithium ions within CAMs. In this study, the mass spectrometer is employed to observe gas generation phenomena within the ASSB cell. Furthermore, a clear elucidation of the side reactions occurring at the cathode and the causes of capacity reduction in ASSB are provided through density functional theory calculations.

Superionic lithium transport via multiple coordination environments defined by two-anion packing

Fast cation transport in solids underpins energy storage. Materials design has focused on structures that can define transport pathways with minimal cation coordination change, restricting attention to a small part of chemical space. Motivated by the greater structural diversity of binary intermetallics than that of the metallic elements, we used two anions to build a pathway for three-dimensional superionic lithium ion conductivity that exploits multiple cation coordination environments. Li7Si2S7I is a pure lithium ion conductor created by an ordering of sulphide and iodide that combines elements of hexagonal and cubic close-packing analogously to the structure of NiZr. The resulting diverse network of lithium positions with distinct geometries and anion coordination chemistries affords low barriers to transport, opening a large structural space for high cation conductivity.

Design principles for sodium superionic conductors

Motivated by the high-performance solid-state lithium batteries enabled by lithium superionic conductors, sodium superionic conductor materials have great potential to empower sodium batteries with high energy, low cost, and sustainability. A critical challenge lies in designing and discovering sodium superionic conductors with high ionic conductivities to enable the development of solid-state sodium batteries. Here, by studying the structures and diffusion mechanisms of Li-ion versus Na-ion conducting solids, we reveal the structural feature of face-sharing high-coordination sites for fast sodium-ion conductors. By applying this feature as a design principle, we discover a number of Na-ion conductors in oxides, sulfides, and halides. Notably, we discover a chloride-based family of Na-ion conductors NaxMyCl6 (M = La-Sm) with UCl3-type structure and experimentally validate with the highest reported ionic conductivity. Our findings not only pave the way for the future development of sodium-ion conductors for sodium batteries, but also consolidate design principles of fast ion-conducting materials for a variety of energy applications.

Dynamic Lone Pairs and Fluoride-Ion Disorder in Cubic-BaSnF4

Introducing compositional or structural disorder within crystalline solid electrolytes is a common strategy for increasing their ionic conductivity. (M,Sn)F2 fluorites have previously been proposed to exhibit two forms of disorder within their cationic host frameworks: occupational disorder from randomly distributed M and Sn cations and orientational disorder from Sn(II) stereoactive lone pairs. Here, we characterize the structure and fluoride-ion dynamics of cubic BaSnF4, using a combination of experimental and computational techniques. Rietveld refinement of the X-ray diffraction (XRD) data confirms an average fluorite structure with {Ba,Sn} cation disorder, and the 119Sn Mössbauer spectrum demonstrates the presence of stereoactive Sn(II) lone pairs. X-ray total-scattering PDF analysis and ab initio molecular dynamics simulations reveal a complex local structure with a high degree of intrinsic fluoride-ion disorder, where 1/3 of fluoride ions occupy octahedral "interstitial" sites: this fluoride-ion disorder is a consequence of repulsion between Sn lone pairs and fluoride ions that destabilizes Sn-coordinated tetrahedral fluoride-ion sites. Variable-temperature 19F NMR experiments and analysis of our molecular dynamics simulations reveal highly inhomogeneous fluoride-ion dynamics, with fluoride ions in Sn-rich local environments significantly more mobile than those in Ba-rich environments. Our simulations also reveal dynamical reorientation of the Sn lone pairs that is biased by the local cation configuration and coupled to the local fluoride-ion dynamics. We end by discussing the effect of host-framework disorder on long-range diffusion pathways in cubic BaSnF4.

Exploring the Relationship Between Halide Substitution, Structural Disorder, and Lithium Distribution in Lithium Argyrodites (Li6-xPS5-xBr1+x)

Lithium argyrodite superionic conductors have recently gained significant attention as potential solid electrolytes for all-solid-state batteries because of their high ionic conductivity and ease of processing. Promising aspects of these materials are the ability to introduce halides (Li6-xPS5-xHal1+x, Hal = Cl and Br) into the crystal structure, which can greatly impact the lithium distribution over the wide range of accessible sites and the structural disorder between the S2- and Hal- anion on the Wyckoff 4d site, both of which strongly influence the ionic conductivity. However, the complex relationship among halide substitution, structural disorder, and lithium distribution is not fully understood, impeding optimal material design. In this study, we investigate the effect of bromide substitution on lithium argyrodite (Li6-xPS5-xBr1+x, in the range 0.0 ≤ x ≤ 0.5) and engineer structural disorder by changing the synthesis protocol. We reveal the correlation between the lithium substructure and ionic transport using neutron diffraction, solid-state nuclear magnetic resonance (NMR) spectroscopy, and electrochemical impedance spectroscopy. We find that a higher ionic conductivity is correlated with a lower average negative charge on the 4d site, located in the center of the Li+ "cage", as a result of the partial replacement of S2- by Br-. This leads to weaker interactions within the Li+ "cage", promoting Li-ion diffusivity across the unit cell. We also identify an additional T4 Li+ site, which enables an alternative jump route (T5-T4-T5) with a lower migration energy barrier. The resulting expansion of the Li+ cages and increased connections between cages lead to a maximum ionic conductivity of 8.55 mS/cm for quenched Li5.5PS4.5Br1.5 having the highest degree of structural disorder, an 11-fold improvement compared to slow-cooled Li6PS5Br having the lowest degree of structural disorder. Thereby, this work advances the understanding of the structure-transport correlations in lithium argyrodites, specifically how structural disorder and halide substitution impact the lithium substructure and transport properties and how this can be realized effectively through the synthesis method and tuning of the composition.

Direct microwave energy input on a single cation for outstanding selective catalysis

Microwave (MW)-driven catalytic systems are attracting attention not only as an aggressive electrification strategy of the chemical industry but also as creating a unique catalytic reaction field that conventional equilibrium heating cannot achieve. This study unlocked direct and selective heating of single alkali metal cations in the pores of aluminosilicate zeolites under MW. Selectively heated Cs+ cations in FAU zeolite exhibited selective CH4 combustion performance, that is, COx generation at the heated Cs+ cations selectively occurred while side reactions in the low-temperature gas phase were suppressed. The Cs-O pair distribution function revealed by synchrotron-based in situ x-ray total scattering gave us direct evidence of peculiar displacement induced by MW, which was consistent with the results of molecular dynamics simulation mimicking MW heating. The concept of selective monoatomic heating by MW is expected to open a next stage in "microwave catalysis" science by providing physicochemical insights into "microwave effects."

Anion-polarisation-directed short-range-order in antiperovskite Li2FeSO

Short-range ordering in cation-disordered cathodes can have a significant effect on their electrochemical properties. Here, we characterise the cation short-range order in the antiperovskite cathode material Li₂FeSO, using density functional theory, Monte Carlo simulations, and synchrotron X-ray pair-distribution-function data. We predict partial short-range cation-ordering, characterised by favourable OLi₄Fe₂ oxygen coordination with a preference for polar cis-OLi₄Fe₂ over non-polar trans-OLi₄Fe₂ configurations. This preference for polar cation configurations produces long-range disorder, in agreement with experimental data. The predicted short-range-order preference contrasts with that for a simple point-charge model, which instead predicts preferential trans-OLi₄Fe₂ oxygen coordination and corresponding long-range crystallographic order. The absence of long-range order in Li₂FeSO can therefore be attributed to the relative stability of cis-OLi₄Fe₂ and other non-OLi₄Fe₂ oxygen-coordination motifs. We show that this effect is associated with the polarisation of oxide and sulfide anions in polar coordination environments, which stabilises these polar short-range cation orderings. We propose that similar anion-polarisation-directed short-range-ordering may be present in other heterocationic materials that contain cations with different formal charges. Our analysis illustrates the limitations of using simple point-charge models to predict the structure of cation-disordered materials, where other factors, such as anion polarisation, may play a critical role in directing both short- and long-range structural correlations.

Priority and Prospect of Sulfide-Based Solid-Electrolyte Membrane

All-solid-state lithium batteries (ASSLBs) employing sulfide solid electrolytes (SEs) promise sustainable energy storage systems with energy-dense integration and critical intrinsic safety, yet they still require cost-effective manufacturing and the integration of thin membrane-based SE separators into large-format cells to achieve scalable deployment. This review, based on an overview of sulfide SE materials, is expounded on why implementing a thin membrane-based separator is the priority for mass production of ASSLBs and critical criteria for capturing a high-quality thin sulfide SE membrane are identified. Moreover, from the aspects of material availability, membrane processing, and cell integration, the major challenges and associated strategies are described to meet these criteria throughout the whole manufacturing chain to provide a realistic assessment of the current status of sulfide SE membranes. Finally, future directions and prospects for scalable and manufacturable sulfide SE membranes for ASSLBs are presented.

Fundamental investigations on the sodium-ion transport properties of mixed polyanion solid-state battery electrolytes

Lithium and sodium (Na) mixed polyanion solid electrolytes for all-solid-state batteries display some of the highest ionic conductivities reported to date. However, the effect of polyanion mixing on the ion-transport properties is still not fully understood. Here, we focus on Na1+xZr2SixP3-xO12 (0 ≤ x ≤ 3) NASICON electrolyte to elucidate the role of polyanion mixing on the Na-ion transport properties. Although NASICON is a widely investigated system, transport properties derived from experiments or theory vary by orders of magnitude. We use more than 2000 distinct ab initio-based kinetic Monte Carlo simulations to map the compositional space of NASICON over various time ranges, spatial resolutions and temperatures. Via electrochemical impedance spectroscopy measurements on samples with different sodium content, we find that the highest ionic conductivity (i.e., about 0.165 S cm-1 at 473 K) is experimentally achieved in Na3.4Zr2Si2.4P0.6O12, in line with simulations (i.e., about 0.170 S cm-1 at 473 K). The theoretical studies indicate that doped NASICON compounds (especially those with a silicon content x ≥ 2.4) can improve the Na-ion mobility compared to undoped NASICON compositions.

Insights into interfacial chemistry of Ni-rich cathodes and sulphide-based electrolytes in all-solid-state lithium batteries

All-solid-state lithium batteries (ASSLBs) have attracted increasing attention recently because they are more safe and have higher energy densities than conventional lithium-ion batteries. In particular, ASSLBs composed of Ni-rich cathodes, sulphide-based solid-state electrolytes (SSEs) and lithium metal anodes have been regarded as the most competitive candidates. Ni-rich cathodes possess high operating potential, high specific energy and low cost, and sulphide-based SSEs have excellent ionic conductivity comparable to that of liquid electrolytes. However, severe parasitic reactions and chemo-mechanical issues hinder their practical application. Herein, the structure, ionic conductivity, chemical or electrochemical stability and mechanical property of sulphide-based SSEs are introduced. Critical interfacial problems between Ni-rich cathodes and sulphide-based SSEs, including chemical or electrochemical parasitic reactions, space charge layer effect, mechanical stress and contact loss, are summarised. The corresponding solutions including coating layer construction and structure design are expounded. Finally, the remaining challenges are discussed, and perspectives are outlined to provide guidelines for the future development of ASSLBs.

Cation Disorder and Large Tetragonal Supercell Ordering in the Li-Rich Argyrodite Li7Zn0.5SiS6

A tetragonal argyrodite with >7 mobile cations, Li₇Zn_0.5SiS₆, is experimentally realized for the first time through solid state synthesis and exploration of the Li-Zn-Si-S phase diagram. The crystal structure of Li₇Zn_0.5SiS₆ was solved ab initio from high-resolution X-ray and neutron powder diffraction data and supported by solid-state NMR. Li₇Zn_0.5SiS₆ adopts a tetragonal I4 structure at room temperature with ordered Li and Zn positions and undergoes a transition above 411.1 K to a higher symmetry disordered F43m structure more typical of Li-containing argyrodites. Simultaneous occupation of four types of Li site (T5, T5a, T2, T4) at high temperature and five types of Li site (T5, T2, T4, T1, and a new trigonal planar T2a position) at room temperature is observed. This combination of sites forms interconnected Li pathways driven by the incorporation of Zn²⁺ into the Li sublattice and enables a range of possible jump processes. Zn²⁺ occupies the 48h T5 site in the high-temperature F43m structure, and a unique ordering pattern emerges in which only a subset of these T5 sites are occupied at room temperature in I4 Li₇Zn_0.5SiS₆. The ionic conductivity, examined via AC impedance spectroscopy and VT-NMR, is 1.0(2) × 10^-7 S cm^-1 at room temperature and 4.3(4) × 10^-4 S cm^-1 at 503 K. The transition between the ordered I4 and disordered F43m structures is associated with a dramatic decrease in activation energy to 0.34(1) eV above 411 K. The incorporation of a small amount of Zn²⁺ exercises dramatic control of Li order in Li₇Zn_0.5SiS₆ yielding a previously unseen distribution of Li sites, expanding our understanding of structure-property relationships in argyrodite materials.

Opening Diffusion Pathways through Site Disorder: The Interplay of Local Structure and Ion Dynamics in the Solid Electrolyte Li6+xP1–xGexS5I as Probed by Neutron Diffraction and NMR

Solid electrolytes are at the heart of future energy storage systems. Li-bearing argyrodites are frontrunners in terms of Li⁺ ion conductivity. Although many studies have investigated the effect of elemental substitution on ionic conductivity, we still do not fully understand the various origins leading to improved ion dynamics. Here, Li_6+xP_1–xGe_xS₅I served as an application-oriented model system to study the effect of cation substitution (P⁵⁺ vs Ge⁴⁺) on Li⁺ ion dynamics. While Li₆PS₅I is a rather poor ionic conductor (10^–6 S cm^–1, 298 K), the Ge-containing samples show specific conductivities on the order of 10^–2 S cm^–1 (330 K). Replacing P⁵⁺ with Ge⁴⁺ not only causes S^2–/I^– anion site disorder but also reveals via neutron diffraction that the Li⁺ ions do occupy several originally empty sites between the Li rich cages in the argyrodite framework. Here, we used ⁷Li and ³¹P NMR to show that this Li⁺ site disorder has a tremendous effect on both local ion dynamics and long-range Li⁺ transport. For the Ge-rich samples, NMR revealed several new Li⁺ exchange processes, which are to be characterized by rather low activation barriers (0.1–0.3 eV). Consequently, in samples with high Ge-contents, the Li⁺ ions have access to an interconnected network of pathways allowing for rapid exchange processes between the Li cages. By (i) relating the changes of the crystal structure and (ii) measuring the dynamic features as a function of length scale, we were able to rationalize the microscopic origins of fast, long-range ion transport in this class of electrolytes.

Influence of Br-/S2- site-exchange on Li diffusion mechanism in Li6PS5Br: a computational study

We investigate how low degrees of [Formula: see text] site-exchange influence the [Formula: see text] diffusion in the argyrodite-type solid electrolyte [Formula: see text] by ab initio molecular dynamics simulations. Based on the atomic trajectories of the defect-free material, a new mechanism for the internal [Formula: see text] reorganization within the [Formula: see text] cages around the [Formula: see text] sites is identified. This reorganization mechanism is highly concerted and cannot be described by just one rotation axis. Simulations with [Formula: see text] defects reveal that [Formula: see text] interstitials ([Formula: see text]) are the dominant mobile charge carriers and originate from Frenkel pairs. These are formed because [Formula: see text] defects on the [Formula: see text] sites donate one or even two [Formula: see text] to the neighbouring cages. The [Formula: see text] then carry out intercage jumps via interstitial and interstitialcy mechanisms. With that, one single [Formula: see text] defect enables [Formula: see text] diffusion over an extended spatial area explaining why low degrees of site-exchange are sufficient to trigger superionic conduction. The vacant sites of the Frenkel pairs, namely [Formula: see text], are mostly immobile and bound to the [Formula: see text] defect. Because [Formula: see text] defects on [Formula: see text] sites act as sinks for [Formula: see text] they seem to be beneficial only for the local [Formula: see text] transport. In their vicinity T4 tetrahedral sites start to get occupied. Because the [Formula: see text] transport was found to be rather confined if [Formula: see text] and [Formula: see text] defects are direct neighbours, their relative arrangement seems to be crucial for effective long-range transport. This article is part of the Theo Murphy meeting issue 'Understanding fast-ion conduction in solid electrolytes'.

Site preferences and ion dynamics in lithium chalcohalide solid solutions with argyrodite structure: II. Multinuclear solid state NMR of the systems Li6PS5−x Se x Cl and Li6PS5−x Se x Br

A comprehensive multinuclear (7Li, 31P, 35Cl, 77Se, 79Br) nuclear magnetic resonance (NMR) study has been conducted to characterize local structural configurations and atomic distributions in the crystallographically disordered solid solutions of composition Li6PS5−x Se x X (0 ≤ x ≤ 1, X = Cl, Br) with the Argyrodite structure. In contrast to the situation with the corresponding iodide homologs, there is no structural ordering between the 4a and 4c sites, with the halide ions occupying both of them with close to statistical probabilities. Nevertheless, throughout the composition range, the 16e Wyckoff sites of the Argyrodite structure are exclusively occupied by the chalcogen atoms, forming PY4 3− (Y = S, Se) tetrahedra, indicating the absence of P-halogen bonds. 31P magic-angle spinning (MAS)-NMR can serve to differentiate between the various possible PS4−n Se n 3− tetrahedral units in a quantitative fashion. Compared to the case of the anion-ordered Li6PS5−x Se x I solid solutions, the preference of P–S over P–Se bonding is significantly stronger, but it is weaker than in the halide free solid solutions Li7PS6−x Se x . Each individual PS4−n Se n 3− tetrahedron is represented by a peak cluster of up to five resonances, representing the five different configurations in which the PY4 3− ions are surrounded by the four closest chalcogenide and halide anions occupying the 4c sites; this distribution is close to statistical and can be used to deduce deviations of sample compositions from ideal stoichiometry. Non-linear 7Li chemical shift trends as a function of x are interpreted to indicate that the Coulombic traps created by sulfur-rich PS4−n Se n 3− ions (n ≤ 2) within the energy landscape of the lithium ions are deeper than those of the other anionic species present (i.e., selenium-richer PY4 3− tetrahedra, isolated chalcogenide or iodide ions), causing the Li+ ions to spend on average more time near them. Temperature dependent static 7Li NMR linewidths indicate higher mobility in the present systems than in the previously studied Li6PS5−x Se x I solid solutions. Unlike the situation in Li6PS5−x Se x I no rate distinction between intra-cage and inter-cage ionic motion is evident. Lithium ionic mobility increases with increasing selenium content. This effect can be attributed to the influences of higher anionic polarizability and a widening of the lithium ion migration pathways caused by lattice expansion. The results offer interesting new insights into the structure/ionic mobility correlations in this new class of compounds.

Paradigms of frustration in superionic solid electrolytes

Superionic solid electrolytes have widespread use in energy devices, but the fundamental motivations for fast ion conduction are often elusive. In this Perspective, we draw upon atomistic simulations of a wide range of superionic conductors to illustrate some ways frustration can lower diffusion cation barriers in solids. Based on our studies of halides, oxides, sulfides and hydroborates and a survey of published reports, we classify three types of frustration that create competition between different local atomic preferences, thereby flattening the diffusive energy landscape. These include chemical frustration, which derives from competing factors in the anion-cation interaction; structural frustration, which arises from lattice arrangements that induce site distortion or prevent cation ordering; and dynamical frustration, which is associated with temporary fluctuations in the energy landscape due to anion reorientation or cation reconfiguration. For each class of frustration, we provide detailed simulation analyses of various materials to show how ion mobility is facilitated, resulting in stabilizing factors that are both entropic and enthalpic in origin. We propose the use of these categories as a general construct for classifying frustration in superionic conductors and discuss implications for future development of suitable descriptors and improvement strategies. This article is part of the Theo Murphy meeting issue 'Understanding fast-ion conduction in solid electrolytes'.

Paradigms of frustration in superionic solid electrolytes

Superionic solid electrolytes have widespread use in energy devices, but the fundamental motivations for fast ion conduction are often elusive. In this Perspective, we draw upon atomistic simulations of a wide range of superionic conductors to illustrate some ways frustration can lower diffusion cation barriers in solids. Based on our studies of halides, oxides, sulfides and hydroborates and a survey of published reports, we classify three types of frustration that create competition between different local atomic preferences, thereby flattening the diffusive energy landscape. These include chemical frustration, which derives from competing factors in the anion-cation interaction; structural frustration, which arises from lattice arrangements that induce site distortion or prevent cation ordering; and dynamical frustration, which is associated with temporary fluctuations in the energy landscape due to anion reorientation or cation reconfiguration. For each class of frustration, we provide detailed simulation analyses of various materials to show how ion mobility is facilitated, resulting in stabilizing factors that are both entropic and enthalpic in origin. We propose the use of these categories as a general construct for classifying frustration in superionic conductors and discuss implications for future development of suitable descriptors and improvement strategies. This article is part of the Theo Murphy meeting issue 'Understanding fast-ion conduction in solid electrolytes'.

Understanding fast-ion conduction in solid electrolytes

The ability of some solid materials to exhibit exceptionally high ionic conductivities has been known since the observations of Michael Faraday in the nineteenth century (Faraday M. 1838 Phil. Trans. R. Soc. 90), yet a detailed understanding of the atomic-scale physics that gives rise to this behaviour remains an open scientific question. This theme issue collects articles from researchers working on this question of understanding fast-ion conduction in solid electrolytes. The issue opens with two perspectives, both of which discuss concepts that have been proposed as schema for understanding fast-ion conduction. The first perspective presents an overview of a series of experimental NMR studies, and uses this to frame discussion of the roles of ion-ion interactions, crystallographic disorder, low-dimensionality of crystal structures, and fast interfacial diffusion in nanocomposite materials. The second perspective reviews computational studies of halides, oxides, sulfides and hydroborates, focussing on the concept of frustration and how this can manifest in different forms in various fast-ion conductors. The issue also includes five primary research articles, each of which presents a detailed analysis of the factors that affect microscopic ion-diffusion in specific fast-ion conducting solid electrolytes, including oxide-ion conductors [Formula: see text] and [Formula: see text], lithium-ion conductors [Formula: see text] and [Formula: see text], and the prototypical fluoride-ion conductor [Formula: see text]-[Formula: see text]. This article is part of the Theo Murphy meeting issue 'Understanding fast-ion conduction in solid electrolytes'.

Fuzzy logic: about the origins of fast ion dynamics in crystalline solids

Nuclear magnetic resonance offers a wide range of tools to analyse ionic jump processes in crystalline and amorphous solids. Both high-resolution and time-domain [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] NMR helps throw light on the origins of rapid self-diffusion in materials being relevant for energy storage. It is well accepted that [Formula: see text] ions are subjected to extremely slow exchange processes in compounds with strong site preferences. The loss of this site preference may lead to rapid cation diffusion, as is also well known for glassy materials. Further examples that benefit from this effect include, e.g. cation-mixed, high-entropy fluorides [Formula: see text], Li-bearing garnets ([Formula: see text]) and thiophosphates such as [Formula: see text]. In non-equilibrium phases site disorder, polyhedra distortions, strain and the various types of defects will affect both the activation energy and the corresponding attempt frequencies. Whereas in [Formula: see text] ([Formula: see text]) cation mixing influences F anion dynamics, in [Formula: see text] ([Formula: see text]) the potential landscape can be manipulated by anion site disorder. On the other hand, in the mixed conductor [Formula: see text] cation-cation repulsions immediately lead to a boost in [Formula: see text] diffusivity at the early stages of chemical lithiation. Finally, rapid diffusion is also expected for materials that are able to guide the ions along (macroscopic) pathways with confined (or low-dimensional) dimensions, as is the case in layer-structured [Formula: see text] or [Formula: see text]. Diffusion on fractal systems complements this type of diffusion. This article is part of the Theo Murphy meeting issue 'Understanding fast-ion conduction in solid electrolytes'.

Site preferences and ion dynamics in lithium chalcohalide solid solutions with argyrodite structure: I. A multinuclear solid state NMR study of the system Li6PS5-xSexI and of Li6AsS5I

A comprehensive multinuclear (7Li, 31P, 75As, 77Se, 127I) NMR study has been conducted to characterize local structural configurations and atomic distributions in the crystallographically ordered solid solutions of composition Li6PS5-x Se x I (0 ≤ x ≤ 1) and in Li6AsS5I. Throughout the composition range, structural ordering between the atoms on the Wyckoff sites 4a and 4c is maintained, with the I− ions exclusively occupying the 4a sites. 31P magic-angle spinning nuclear magnetic resonance (MAS NMR) can serve to differentiate between the various possible PS4-n Se n 3− tetrahedral units in a quantitative fashion, indicating a preference of P-S relative to P-Se bonding. Each individual PS4-n Se n 3− tetrahedron is represented by a peak cluster containing up to five resonances, representing the five different configurations in which the PCh4 3− units are surrounded by the four closest chalcogenide anions occupying the 4c sites; the distribution of S2− and Se2− over these sites is close to statistical. Non-linear 7Li chemical shift trends as a function of x are interpreted to indicate that the Coulombic traps created by sulfur-rich PS4-n Se n 3− ions (n ≥ 2) within the energy landscape of the lithium ions are deeper than those of the other anionic species present (i.e. selenium-richer PCh4 3− tetrahedra, isolated chalcogenide or iodide ions), causing the Li+ ions to spend on average more time near them. Temperature dependent static 7Li NMR linewidths measured on Li6PS5I and Li6AsS5I indicate a two-step motional narrowing process characterized by a clear dynamic distinction between a more rapid localized intra-cage process and a slower, long-range inter-cage process. In the solid solutions this differentiation gradually disappears, leading to an overall increase of lithium ionic mobility with increasing selenium content, which can be attributed to the influences of higher anionic polarizability and a widening of the lithium migration pathways caused by lattice expansion. Furthermore, the low-temperature phase transition in Li6PS5I, which tends to immobilize the lithium ions below 170 K, is suppressed in the solid solutions. The results offer interesting new insights into the -structure/ionic mobility correlations in this new class of compounds.

New insights into Li distribution in the superionic argyrodite Li6PS5Cl

By using temperature-dependent neutron powder diffraction combined with maximum entropy method analysis, a previously unreported Li lattice site was discovered in the argyrodite Li₆PS₅Cl solid-state electrolyte. This new finding enables a more complete description of the Li diffusion model in argyrodites, providing structural guidance for designing novel high-conductivity solid-state electrolytes.

Innovative Approaches to Li-Argyrodite Solid Electrolytes for All-Solid-State Lithium Batteries

ConspectusAs the world transitions away from fossil energy to green and renewable energy, electrochemical energy storage increasingly becomes a vital component of the mix to conduct this transition. The central goal in developing next-generation batteries is to maximize the gravimetric and volumetric energy density and battery cycle life and improve safety. All solid-state batteries using a solid electrolyte and a lithium metal anode represent one of the most promising technologies that can achieve this goal. Highly conductive solid electrolytes (>10 mS·cm^-1) are the key component to remove the safety concerns inherent with flammable organic liquid electrolytes and achieve high energy density by enabling high active material loading. Considering a range of inorganic solid electrolytes that have been developed to date, sulfide solid electrolytes exhibit the highest ionic conductivities, which even surpass those of conventional organic liquid electrolytes. Argyrodite-structured sulfide solid electrolytes are among the most promising materials in this class and are currently the dominantly used solid electrolytes for all-solid-state battery fabrication. Argyrodite solid electrolytes are particularly appealing because of their ultrahigh Li-ion conductivity, quasi-stable solid-electrolyte interphase (SEI) formed with Li metal, and ability to be prepared via scalable solution-assisted synthesis approaches. These factors are all vital for commercial applications.In this Account, we afford an overview of our recent development of several argyrodite superionic conductors, including Li_6.6Si_0.6Sb_0.5S₅I (24 mS·cm^-1), Li_6.6Ge_0.6P_0.4S₅I (18 mS·cm^-1), and Li_5.5PS_4.5Cl_1.5 (12 mS·cm^-1), and a comprehensive understanding of the origin of the underlying high conductivity, namely, sulfide/halide anion site disorder and Li cation site disorder. A high degree of sulfide/halide anion site disorder (changes in anion distribution) modifies the anionic charge, which in turn strongly influences the lithium distribution. A more inhomogeneous charge distribution in anion-disordered systems generates a spatially diffuse and delocalized lithium density, resulting in faster ionic transport. Lithium cation site disorder generated by increasing Li carrier concentration through aliovalent substitution creates high-energy interstitial sites for Li ion diffusion, which activate concerted ion migration and flatten the energy landscape for Li ion diffusion. This enables high conductivity in Li-rich argyrodite superionic conductors. These concepts are also expected to promote the design of rational new solid electrolytes and fundamental understanding of the structure-ion transport relationships in inorganic ionic conductors.Collectively, a comprehensive and deep understanding of the interphase formation between argyrodite solid electrolytes and cathode active materials/Li metal and the failure mechanism of all-solid-state batteries with argyrodite solid electrolytes will lead to the bottom-up engineering of the cathode/anode-solid electrolyte interfaces, which will accelerate the development of safe, high-energy-density all-solid-state lithium batteries.