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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. Throughmore » 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.« less

The advantageous properties of (pseudo)halide argyrodite ion conductors of the formula Li₆PS₅X (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–xBr_x, in which the dipolar cyanide anionsmore » 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.« less

This work elucidates the mechanism by which lithium borohydride (LiBH₄) doping into argyrodite-type Li₆PS₅Cl (LBH-LPSCl) solid-state electrolyte (SSE) enhances electrochemical stability. State-of-the-art electrochemical performance is achieved with 5 wt% borohydride. Symmetric cells achieve critical current density (CCD) of 7.3 mA cm⁻², versus 2.6 mA cm⁻² for baseline-LPSCl. All solid-state batteries (ASSBs) employing lithium metal and NMC811 cathode are stable over 400 cycles at 0.5C, with capacity retention of 83%. An anode-free ASSB (AF-ASSB) is stable over 600 cycles, with capacity loss of 0.04% per cycle. 5LBH-LPSCl allows for enhanced low temperature operation, down to −14 °C. Yet the difference inmore » electrolytes’ bulk microstructures and hardnesses are minimal, while ionic conductivity is incrementally improved (≈50%). Theoretical modeling indicates limited effect of substitution on thermodynamic stability of PS₄^3- units, which decompose when contacting Li. Instead, enhanced electrochemical stability is site-specific kinetic effect: In situ electrodeposition experiments using X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) reveal tri-layer SEI based predominately on Li₃P/LiBH₄/Li₂S that blocks electrons while facilitating ion transport. This SEI manifests reduced interface resistance and accelerated nucleation and growth of metallic Li. With baseline-LPSCl the SEI based on Li₃P/Li₂S is substantially thicker, generating localized stresses that promote interfacial cracking while cycling.« less

All-solid-state batteries (ASSBs) have the potential to provide greater energy density than conventional batteries based on liquid electrolytes. Here, an operando ASSB cell setup for tender X-ray photoelectron spectroscopy (XPS) was developed, and the interface of a Ni-rich layered transition metal oxide cathode active material (CAM) and an Li₆PS₅Cl (LPSCl) solid electrolyte (SE) was evaluated during initial charge/discharge cycles. After validating the cell performance against a conventional pouch cell operated at high compression, intermittent galvanostatic cycling was performed, and XPS data were recorded as a function of state of charge (SOC). Upon the initial charge of the cell to ≈3.3more » V_Li, the LPSCl appears to decompose into LiCl, Li₃PS₄, and polysulfides, whose amount gradually increases with potential. Upon further charge, at a potential higher than ≈3.8 V_Li, initially, present sulfate and sulfite impurities decompose, and at ≈74% SOC (corresponding to a cathode potential of ≈4.10 V_Li), surface reconstruction of the CAM particles due to lattice oxygen release is detected. In addition, at potentials beyond ≈4.6 V_Li, a decrease of the S 1s counts of the sum of the LPSCl, the thiophosphate, and polysulfide species suggests the formation of elemental sulfur that is lost via sublimation into the vacuum chamber.« less

This study demonstrates a rapid, dry, microwave-assisted (MW) synthesis method that enables preparation of halide argyrodites Li₆PS₅X (X = Cl^-, Br^-, I^-) in less than 20 min. The structures and ion transport properties of the resulting materials are compared with those synthesized by conventional solid-state synthesis methods. The microwave-assisted method leads to increased 4a/4d site disorder and elevated Arrhenius prefactors (σ₀), which lead to an order of magnitude improvement in the 30 ° C ionic conductivity of MW-Li₆PS₅I. X-ray pair distribution function analysis (XPDF) reveals significant rotational disorder of the PS^3-₄ units, which is impacted by the synthesis method, choicemore » of halide, and presence of S^2-/X^- site disorder. These rotational displacements are strongly correlated with ion transport, specifically and entropy of migration (ΔS_m). Overall, this study demonstrates a rapid synthesis route for preparing high-quality halide argyrodite solid-state electrolytes in less than 20 min, and further unravels atomistic insights into the interplay of structural disorder, rotational dynamics, and ion transport mechanisms.« less

Sulfide solid state electrolytes (SSE) are among the most promising materials in the effort to replace liquid electrolytes, largely due to their comparable ionic conductivities. Among the sulfide SSEs, Argyrodites (Li₆PS₅X, X=Cl, Br, I) further stand out due to their high theoretical ionic conductivity (~1×10^–2 S cm^–1) and interfacial stability against reactive metal anodes such as lithium. Generally, solid state electrolyte pellets are pressed from powder feedstock at room temperature, however, pellets fabricated by cold pressing consistently result in low bulk density and high porosity, facilitating interfacial degradation reactions and allowing dendrites to propagate through the pores and grain boundaries.more » Here, we demonstrate the mechanical and electrochemical implications of hot-pressing standalone LPSCl SSE pellets with near-theoretical ionic conductivity, superior cycling performance, and enhanced mechanical stability. X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and x-ray diffraction spectroscopy (XRD) analysis reveal no chemical changes to the Argyrodite surface after hot pressing up to 250°C. Furthermore, we use electrochemical impedance spectroscopy (EIS) to understand mechanical stability of Argyrodite SSE pellets as a function of externally applied pressure, demonstrating for the first time pressed standalone Argyrodite pellets with near-theoretical conductivities at external pressures below 14 MPa.« less

Microstructure of argyrodite solid-state electrolyte (SSE) critically affects lithium metal electrodeposition/dissolution. While the stability of unmodified SSE is mediocre, once optimized state-of-the-art electrochemical performance is achieved (symmetric cells, full cells with NMC811) without secondary interlayers or functionalized current collectors. Planetary mechanical milling in wet media (m-xylene) is employed to alter commercial Li₆PS₅Cl (LPSCl) powder. Quantitative stereology demonstrates how milling progressively refines grain and pore size/distribution in the SSE compact, increases its density, and geometrically smoothens the SSE-Li interface. Mechanical indentation demonstrates that these changes lead to reduced site-to-site variation in the compact's hardness. Milled microstructures promote uniform early-stage electrodeposition onmore » foil collectors and stabilize solid electrolyte interphase (SEI) reactivity. Analysis of half-cells with bilayer electrolytes demonstrates the importance of microstructure directly contacting current collector, with interface roughness due to pore and grain size distribution being key. For the first time, short-circuiting Li metal dendrite is directly identified, employing 1.5 mm diameter “mini” symmetrical cell and cryogenic focused ion beam (cryo-FIB) electron microscopy. The branching sheet-like dendrite traverses intergranularly, filling the interparticle voids and forming an SEI around it. Importantly, mesoscale modeling reveals the relationship between Li-SSE interface morphology and the onset of electrochemical instability, based on underlying reaction current distribution.« less

Thin intermetallic Li₂Te–LiTe₃ bilayer (0.75 µm) derived from 2D tellurene stabilizes the solid electrolyte interphase (SEI) of lithium metal and argyrodite (LPSCl, Li₆PS₅Cl) solid-state electrolyte (SSE). Tellurene is loaded onto a standard battery separator and reacted with lithium through single-pass mechanical rolling or transferred directly to SSE surface by pressing. State-of-the-art electrochemical performance is achieved, e.g., symmetric cell stable for 300 cycles (1800 h) at 1 mA cm^-2 and 3 mAh cm^-2 (25% DOD, 60 µm foil). Cryo-stage focused ion beam (Cryo-FIB) sectioning and Raman mapping demonstrate that the Li₂Te–LiTe₃ bilayer impedes SSE decomposition. The unmodified Li–LPSCl interphase is electrochemicallymore » unstable with a geometrically heterogeneous reduction decomposition reaction front that extends deep into the SSE. Decomposition drives voiding in Li metal due to its high flux to the reaction front, as well as voiding in the SSE due to the associated volume changes. Analysis of cycled SSE found no evidence for pristine (unreacted) lithium metal filaments/dendrites, implying failure driven by decomposition phases with sufficient electrical conductivity that span electrolyte thickness. In conclusion, DFT calculations clarify thermodynamic stability, interfacial adhesion, and electronic transport properties of interphases, while mesoscale modeling examines interrelations between reaction front heterogeneity (SEI heterogeneity), current distribution, and localized chemo-mechanical stresses.« less

Interfacial mechanics are a significant contributor to the performance and degradation of solid-state batteries. Spatially resolved measurements of interfacial properties are extremely important to effectively model and understand the electrochemical behavior. Herein, we report the interfacial properties of thiophosphate (Li₃PS₄)- and argyrodite (Li₆PS₅Cl)-type solid electrolytes. Using atomic force microscopy, we showcase the differences in the surface morphology as well as adhesion of these materials. Additionally we investigate solvent-less processing of hybrid electrolytes using UV-assisted curing. Physical, chemical, and structural characterizations of the materials highlight the differences in the surface morphology, chemical makeup, and distribution of the inorganic phases between themore » argyrodite and thiophosphate solid electrolytes.« less