Argyrodite Structure Research Articles

The improvement of electrochemical performance by mixing InF3 in Li5.3PS4.3Cl1.7 solid electrolyte

All-solid-state batteries (ASSBs) employing solid electrolytes (SE) are currently gaining attention as next-generation energy storage devices. Solid electrolytes exhibit higher energy densities and operation across a wide temperature range than liquid electrolytes. The solid electrolyte acts as a separator, providing higher mechanical strength than polymer separators. This helps prevent cell deterioration and short circuit issues caused by lithium dendrite growth during charge – discharge cycles. Consequently, lithium metal, previously challenging to apply in conventional lithium-ion batteries, can be utilized as an anode electrode material. Sulfide-based solid electrolytes offer advantageous mechanical properties and high ionic conductivity. However, their narrow chemical/electrochemical stability window poses challenges. During the charging-discharging process of a battery with a lithium metal anode, a reduction decomposition reaction of the solid electrolyte occurs. On the other hand, Lithium Fluoride (LiF) exhibits low electronic conductivity due to a wide band gap but has a broad electrochemical stability window. In this study, we prepared a mixture of Li5.3PS4.3Cl1.7 with InF3 to generate an artificial solid electrolyte interphase (SEI) containing LiF. We confirmed the structural properties using powder X-ray diffraction (XRD). The XRD pattern showed the maintenance of the argyrodite structure, with only an increase in the peak intensity corresponding to InF3. The optimized solid electrolyte-based ASSB demonstrated a higher initial discharge capacity of 172.8 mAh/g, a coulombic efficiency of 78.54 %, and a capacity retention rate of 93.6 %, outperforming ASSBs based on the bare solid electrolyte (Li5.3PS4.3Cl1.7).

STRUCTURAL AND ELECTRICAL PROPERTIES OF MONOCRYSTALLINE Ag7+xGexP1-xS6 (x = 0.75; 0.5) SOLID SOLUTIONS

Compounds of the argyrodite structure are considered to be promising functional materials due to their peculiarities. The functional parameters of these compounds are provided by the peculiarities of the crystal structure, namely the simultaneous coexistence of a mobile cationic sublattice within a rigid anionic framework. Such a crystal structure motif determines the formation of wide regions of solid solutions based on argyrodites and makes it possible to increase the values of the functional parameters of these phases within the homogeneity regions. This work presents the results of the study of the structural and electrical properties of Ag7+xGexP1-xS6 (x = 0.75; 0.5) solid solutions with an argyridite structure. Single-crystal samples of solid solutions of Ag7.75Ge0.75P0.25S6 and Ag7.5Ge0.5P0.5S6 were obtained by the method of directional crystallization from the melt in vacuum-sealed quartz ampoules. It was found that the solid solutions of Ag7+xGexP1-xS6 (x = 0.75; 0.5) crystallize in orthorhombic crystal system: SG Pna21, with lattice parameters: a = 15.093 Å, b = 7.462 Å, c = 10.571 Å (Ag7.75Ge0.75P0.25S6) and a = 15.023 Å, b = 7.450 Å, c = 10.545 Å (Ag7.5Ge0.5P0.5S6). The frequency and temperature dependence of the total electrical conductivity was studied by impedance spectroscopy. The total electrical conductivity of the investigated single crystals is Ag7.75Ge0.75P0.25S6 5.43×10–3 S/cm (0.199 eV) and Ag7.5Ge0.5P0.5S6 2.22×10–2 S/cm (0.152 eV) at a temperature of 25 °C.
 Keywords: argyrodites; single crystals; electrical conductivity; phase analysis.

ELECTRICAL PROPERTIES INVESTIGATION OF Ag8GeS6 SINGLE CRYSTAL

The compounds of the argyrodite structure have high conductivity values in the solid state due to the simultaneous coexistence of a "rigid" anionic framework and disordered cationic sublattice. In this regard, argyrodites are considered to be promising superionic materials for use as a working element of solid-state batteries. This work presents the results of the study of the electrophysical parameters of Ag8GeS6 single crystal grown by directional crystallization from the melt technique. It was found by the XRD method that the grown Ag8GeS6 single crystal crystallizes in a low-temperature orthorhombic modification with unit cell parameters: a = 15.147 Å, b = 7.469 Å, c = 10.584 Å. The frequency (10 Hz - 0.3 MHz) and temperature (20 - 110°C) dependence of the electrical conductivity was studied on a single crystal plate of Ag8GeS6 oriented in the (011) plane by impedance spectroscopy. The frequency dependence of the total electrical conductivity of Ag8GeS6 shows an increase in the total electrical conductivity with increasing frequency, which is typical for mixed ionic-electronic conductors. It was determined that the total conductivity of the Ag8GeS6 single crystal is 3.82 × 10-5 S/cm, and the activation energy is 0.728 eV.
 Keywords: argyrodites; single crystals; electrical conductivity; phase analysis.

SINGLE CRYSTALS GROWTH IN THE HOMOGENEITY REGION OF LOW-TEMPERATURE MODIFICATION OF Ag8GeS6

Cationic ion-conductors have found wide practical application as electric batteries. Among them the lithium-ion batteries are the most common commercially used. However, research is being actively carried out to improve the safety of these batteries. Among the possible compounds, silver-containing phases are attractive, since silver-ions, along with high values of ionic conductivity in solid-state, haven’t high chemical activity like the same alkali metal-ions. Realization of the crystal structure necessary for effective ion transport is observed in compounds of the argyrodite structure - a group of ternary chalcogenides and quaternary chalcohalogenides of multi-charged cations (Ge4+, Si4+, P5+). In this work, a study of heterovalent cationic P5+→Ge4+ substitution in the Ag8GeS6 enriched region was carried out. Single crystals of Ag8-x(Ge1-xPx)S6 (x = 0.25; 0.5) solid solutions were grown by the method of directional crystallization using a solution-melt technique. The grown single crystals are characterized by the absence of macrodefects, a dark gray color with a metallic luster, and dimensions of l = ~3 cm and d = 1.2 cm. Using the Rietveld refinement method, the crystal structure was established and the lattice parameters of Ag8-x(Ge1-xPx)S6 (x = 0.25; 0.5) solid solutions were determined. Established, that single crystals of Ag8-x(Ge1-xPx)S6 (x = 0.25; 0.5) solid solutions crystallize in the orthorhombic crystal system in a primitive cell with space group Pna21.
 Keywords: argyrodites; single crystals; directional crystallization; phase analysis.

Microscopic Degradation Mechanism of Argyrodite-Type Sulfide at the Solid Electrolyte-Cathode Interface.

Interfacial engineering of sulfide-based solid electrolyte/lithium-transition-metal oxide active materials in all-solid-state battery cathodes is vital for cell performance parameters, such as high-rate charge/discharge, long lifetime, and wide temperature range. A typical interfacial engineering method is the surface coating of the cathode active material with a buffer layer, such as LiNbO3. However, cell performance reportedly degrades under harsh environments even with a LiNbO3 coating, such as high temperatures and high cathode potentials. Therefore, we investigated the interfacial degradation mechanism focusing on the solid electrolyte side for half cells employing the cathode mixture of argyrodite-type Li6PS5Cl/LiNbO3-coated LiNi0.5Co0.2Mn0.3O2 exposed at 60 °C and 4.25 and 4.55 V vs Li/Li+ using transmission electron microscopy/electron diffraction (TEM/ED) and X-ray absorption spectroscopy (XAS). The TEM/ED results indicated that the ED pattern of the argyrodite structure disappeared and changed to an amorphous phase as the cells degraded. Moreover, the crystal phases of LiCl and Li2S appeared simultaneously. Finally, XAS analysis confirmed the decrease in the PS4 units of the argyrodite structure and the increase in local P-S-P domains with delithiation from the interfacial solid electrolyte, corresponding to the TEM/ED results. In addition, the formation of P-O bonds was confirmed during degradation at higher cathode potentials, such as 4.55 V vs Li/Li+. These results indicate that the degradation of this interfacial region determines the cell performance.

Sn-Substituted Argyrodite Li6PS5Cl Solid Electrolyte for Improving Interfacial and Atmospheric Stability.

Sulfide-based solid electrolytes exhibit good formability and superior ionic conductivity. However, these electrolytes can react with atmospheric moisture to generate H2S gas, resulting in performance degradation. In this study, we attempted to improve the stability of the interface between Li metal and an argyrodite Li6Ps5Cl solid electrolyte by partially substituting P with Sn to form an Sn-S bond. The solid electrolyte was synthesized via liquid synthesis instead of the conventional mechanical milling method. X-ray diffraction analyses confirmed that solid electrolytes have an argyrodite structure and peak shift occurs as substitution increases. Scanning electron microscopy and energy-dispersive X-ray spectroscopy analyses confirmed that the particle size gradually increased, and the components were evenly distributed. Moreover, electrochemical impedance spectroscopy and DC cycling confirmed that the ionic conductivity decreased slightly but that the cycling behavior was stable for about 500 h at X = 0.05. The amount of H2S gas generated when the solid electrolyte is exposed to moisture was measured using a gas sensor. Stability against atmospheric moisture was improved. In conclusion, liquid-phase synthesis could be applied for the large-scale production of argyrodite-based Li6PS5Cl solid electrolytes. Moreover, Sn substitution improved the electrochemical stability of the solid electrolyte.

Blending a Li3N/Li3YCl6 solid electrolyte with Li6PS5Cl argyrodite structure to improve interface stability and electrochemical performance in Lithium solid-state batteries

Lithium dendrite growth and incompatibility with high cell voltage cathode materials is one of the serious issues behind solid-state batteries because it deteriorates the electrochemical performances. To avert these issues, we emphasize the blending of Li3N/Li3YCl6 with the Li6PS5Cl (LPSCl) system to tackle the aforementioned issues. The (100-x) LPSCl:x(Li3N/Li3YCl6) (x = 0, 2, 5 and 10) solid electrolyte composition is synthesized by using the ball milling process. The phase purity analysis reveals that prepared composition still exists in argyrodite structure after milling of two solid electrolytes. The electrochemical impedance measurement indicates that the prepared composition exhibits an ionic conductivity values over 1 mS cm−1 at lower concentration of Li3N/Li3YCl6 (x = 2 and 5). The critical current density measurement confirms the 98LPSCl: 2Li3N composition exhibits a higher critical current density of 0.55 mA cm−2 than the other compositions. Moreover, the assembled battery using 98LPSCl: 2Li3N delivers the discharge capacity of 179.1 mAh g−1 with a capacity retention of 92.3% over 100 cycles and also maintains a high reversible capacity of 131 mAh g−1 at 2 C rate. Finally, the coulombic efficiency of the solid-state battery increases to 74.9% by using 95LPSCl:5Li3YCl6 composition as an interlayer between the electrode and the electrolyte interface.

Cl- and Al-Doped Argyrodite Solid Electrolyte Li6PS5Cl for All-Solid-State Lithium Batteries with Improved Ionic Conductivity.

Argyrodite solid electrolytes such as lithium phosphorus sulfur chloride (Li6PS5Cl) have recently attracted great attention due to their excellent lithium-ion transport properties, which are applicable to all-solid-state lithium batteries. In this study, we report the improved ionic conductivity of an argyrodite solid electrolyte, Li6PS5Cl, in all-solid-state lithium batteries via the co-doping of chlorine (Cl) and aluminum (Al) elements. Electrochemical analysis was conducted on the doped argyrodite structure of Li6PS5Cl, which revealed that the substitution of cations and anions greatly improved the ionic conductivity of solid electrolytes. The ionic conductivity of the Cl- and Al-doped Li6PS5Cl (Li5.4Al0.1PS4.7Cl1.3) electrolyte was 7.29 × 10-3 S cm-1 at room temperature, which is 4.7 times higher than that of Li6PS5Cl. The Arrhenius plot of the Li5.4Al0.1PS4.7Cl1.3 electrolyte further elucidated its low activation energy at 0.09 eV.

Synthesis of Br-rich argyrodite electrolytes enables all-solid-state batteries with superior battery performances at different operating temperatures

Solid-state batteries have attracted significant attention owing to their high energy density and high safety. Nevertheless, highly conductive solid electrolytes with ultrafast Li-ion mobility are greatly demanded. In this work, bromine-rich argyrodite electrolytes have been synthesized by the typical high-rotation milling followed by an annealing method. The precursor with pure argyrodite structure is first obtained and a subsequent sintering process is applied to enhance the Li-ion conductivity. Br dopant is tailored to obtain the optimum composition with the highest conductivity. Li5.6PS4.6Br1.4 electrolyte delivers a high conductivity of 6.91 mS/cm at room temperature. The solid-state battery using the pristine LiNi0.6Mn0.2Co0.2O2 cathode and Li-In anode shows a high discharge capacity of 143.4 mAh g−1 and excellent capacity retention of 92.3% after 100 cycles at 0.2C under room temperature. Moreover, the assembled all-solid-state battery also displays high discharge capacities of 156.6 mAh g−1 at 0.2C under 60 °C and 97.1 mAh g−1 at 0.02C under -20 °C. Due to the good lithium metal compatibility of Li5.6PS4.6Br1.4 electrolyte, LiNi0.6Mn0.2Co0.2O2/Li5.6PS4.6Br1.4/Li all-solid-state Li metal is successfully constructed and displays excellent performances.

Enhanced ionic conductivity and lack of paddle-wheel effect in pseudohalogen-substituted Li argyrodites

<h2>Summary</h2> Superionic conductors are key to the development of safe and high-energy-density, all-solid-state batteries. Using a combined theoretical and experimental approach, we explore the feasibility of increasing the ionic conductivity through pseudohalogen substitution in the Li argyrodite structure. Under the guidance of calculated thermodynamic stability, BH₄-substituted Li argyrodite, Li_5.91PS_4.91(BH₄)_1.09, was successfully synthesized via a mechanochemical method. As-synthesized BH₄-substituted Li argyrodite displays an ionic conductivity of 4.8 mS/cm at 25°C. <i>Ab initio</i> molecular dynamics simulation trajectory analysis was used to investigate how BH₄ facilitates Li-ion diffusion and indicates only a weak correlation with the B–H bond motion. We find that the enhanced conductivity mainly originates from the weak interaction between Li and BH₄ and find no evidence of a paddle-wheel effect from the polyanion. This work provides insight into how cluster ions enhance Li diffusion and systematically describes how to explore superionic conductors with pseudohalogen substitution.

DETERMINATION OF THE OPTICAL PSEUDOGAP IN DISORDERED PHASES WITH ARGYRODITE STRUCTURE

This paper is aimed to the determination of the optical pseudogap values in phases with argyrodite structure by a simple method of diffuse reflectance spectroscopy. Samples of (Cu1-xAgx)7PSe6 (x = 0, 0.2, 0.3, 0.4, 0.6, 0.7, 0.9, 1.0) solid solutions were obtained in the form of microcrystalline powders by grinding in agate mortar. The diffuse reflectance spectra of the obtained (Cu1-xAgx)7PSe6 samples were studied in the spectral range 220–1400 nm (293 K) and analyzed using the Kubelka-Munk function. It was found that for (Cu1-xAgx)7PSe6 solid solutions the increase in the copper amount causes a shift of the beginning of the absorption edge to the region of higher energies. The optical pseudogap of (Cu1-xAgx)7PSe6 solid solutions was estimated by combining the Kubelka-Munch theory and the Tauc equation. It is established that the isovalent cationic substitution of Ag+ → Cu+ leads to a monotonic decrease in the values of the optical pseudogap, as a result, the compositional dependence of which is close to linear. This behavior of the optical pseudogap can only be associated with a decrease in the lattice parameters and characterizes the formation of solid solutions.

CRYSTAL GROWTH OF Ag7PS6 BY DIRECTIONAL CRYSTALLIZATION METHOD

Argyrodites are a class of compounds united by a similar structural motif. Typical for argyrodites is the coexistence of rigid anionic and disordered cationic sublattices. Due to this, argyrodites exhibit a wide range of properties. This work is devoted to the development of the single crystal growth regime of high-quality single-crystalline samples of Ag7PS6, which belongs to the compounds with argyrodite structure. The initial Ag7PS6 alloy is obtained from high purity elementary components by a two-step and one-temperature method. The maximum synthesis temperature was 850°C. The obtained alloy was investigated by DTA and XRD methods. The congruent melting of Ag7PS6 (Tmelt = 801°C) and the structural phase transition P213 → F-43m at a temperature of 271°C was confirmed. The study of the phase composition established the synthesized alloy is single-phase. The growth of Ag7PS6 was carried out by the directional crystallization method from the melt. The growth regime of Ag7PS6 was selected taking into account the nature of the thermal behavior of the compound based on DTA data. As a result, the grown Ag7PS6 single crystal of ~ 40 mm long and 12 mm in diameter was obtained. According to the results of XRD, it has been established that the grown Ag7PS6 single crystal is single-phase and crystallizes in a primitive cubic cell with lattice parameters of 10.3917 Å.

Influence of crystal structure disordering on ionic conductivity of Ag7+x(P1−xGex)S6 single crystals

We present a detailed study of Ag7+x(P1−xGex)S6 (x = 0.1; 0.25; 0.33; 0.5; 0.75) single crystals of novel superionic conductors with argyrodite structure. The single crystals on superionic conductors Ag7+x(P1−xGex)S6 were grown by directional crystallization technique. The crystal structure of Ag7+x(P1−xGex)S6 solid solutions was studied by the Rietveld refinement method. It was found that solid solutions crystallize in three SGs: P213 (x = 0.1), F-43m (x = 0.25; 0.33) and Pna21 (x = 0.5; 0.75). Ag7+x(P1−xGex)S6 solid solutions are characterized by the presence of Ag positions with partial occupancy, which is different from the initial ternary compounds. The frequency and temperature dependences of electrical conductivity were studied by electrochemical impedance spectroscopy method. The structural disordering caused by P+5↔Ge+4 heterovalent cationic substitution induces increasing ionic conductivity of the solid solutions by 5 orders of magnitude. The occurrence of favorable conditions for ion transport is explained using simple structural descriptors.

Argyrodite-type advanced lithium conductors and transport mechanisms beyond paddle-wheel effect

Development of next-generation solid-state Li-ion batteries requires not only electrolytes with high room-temperature (RT) ionic conductivities but also a fundamental understanding of the ionic transport in solids. In spite of considerable work, only a few lithium conductors are known with the highest RT ionic conductivities ~ 0.01 S/cm and the lowest activation energies ~0.2 eV. New design strategy and novel ionic conduction mechanism are needed to expand the pool of high-performance lithium conductors as well as achieve even higher RT ionic conductivities. Here, we theoretically show that lithium conductors with RT ionic conductivity over 0.1 S/cm and low activation energies ~ 0.1 eV can be achieved by incorporating cluster-dynamics into an argyrodite structure. The extraordinary superionic metrics are supported by conduction mechanism characterized as a relay between local and long-range ionic diffusions, as well as correlational dynamics beyond the paddle-wheel effect.

Unsupervised machine learning to classify crystal structures according to their structural distortion: A case study on Li-argyrodite solid-state electrolytes

High-throughput approaches in computational materials discovery often yield a combinatorial explosion that makes the exhaustive rendering of complete structural and chemical spaces impractical. A common bottleneck when screening new compounds with archetypal crystal structures is the lack of fast and reliable decision-making schemes to quantitatively classify the computed candidates as inliers or outliers (too distorted structures). Machine learning-aided workflows can solve this problem and make geometrical optimization procedures more efficient. However, for this to occur, there is still a lack of appropriate combinations of suitable geometrical descriptors and accurate unsupervised models which are capable of accurately differentiating between systems with subtle structural changes. Here, considering as a case study the compositional screening of cubic Li-argyrodites solid electrolytes, we tackle this problem head on. We find that Steinhardt order parameters are very accurate descriptors of the cubic argyrodite structure to train a range of common unsupervised outlier detection models. And, most importantly, the approach enables us to automatically classify crystal structures with uncertainty control. The resulting models can then be used to screen computed structures with respect to an user-defined error threshold and discard too distorted structures during geometrical optimization procedures. Implemented as a decision node in computer-aided materials discovery workflows, this approach can be employed to perform autonomous high-throughput screening methods and make the use of computational and data storage resources more efficient.

High-Entropy Polyanionic Lithium Superionic Conductors

High-entropy ceramics are attracting large interest because of their unique materials properties. Nevertheless, the effect of entropy on the lithium transport remains largely elusive. Here, we report, for the first time, about medium- and high-entropy polyanionic lithium superionic conductors crystallizing in the F–43m space group and adopting the so-called argyrodite structure. The Li6PS5[Cl0.33Br0.33I0.33], Li6P[S2.5Se2.5][Cl0.33Br0.33I0.33], and Li6.5[Ge0.5P0.5][S2.5Se2.5][Cl0.33Br0.33I0.33] materials were structurally characterized using complementary synchrotron and neutron scattering techniques in combination with 31P magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. We show that, in contrast to other high-entropy ceramics, an unequal distribution of elements over the respective crystallographic sites occurs in these materials. Using electrochemical impedance spectroscopy (EIS) and 7Li pulsed field gradient (PFG) NMR spectroscopy, we demonstrate that introducing entropy (compositional disorder) marginally affects the room-temperature ionic conductivity (∼10–3 S cm–1) but instead lowers the activation energy for conduction to 0.22 eV. Our results emphasize the possibility of increasing entropy in polyanionic materials, thereby opening up compositional space for the search of Li-ion conductors with unprecedented properties.

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

Abstract 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.

Electron, phonon and thermoelectric properties of Cu7PS6 crystal calculated at DFT level

The promising class of the environment-friendly thermoelectrics is the copper-based argyrodite-type ion-conducting crystals exhibiting just extraordinary low thermal conductivity below the glass limit associated with the molten copper sublattice leading to a softening of phonon modes. To explain why the argyrodite structure containing copper ions favors the low thermal conductivity, we have utilized the ab initio calculations of the electron, phonon, and thermoelectric properties of Cu7PS6 crystal in the framework of the density functional and Boltzmann transport theories. To obtain the reliable thermoelectric properties of Cu7PS6, we take into account the dependence of the electron effective mass m* on the redundant carrier concentration n. We propose to use the Burstein–Moss effect for the calculation of the electron effective mass m* of a semiconductor. We have found the strong nonlinear character of copper atom vibrations in Cu7PS6 which exceeds substantially the similar values for phosphorous and sulfur atoms. The large vibration nonlinearity of the copper atoms found in Cu7PS6 explains the diffusion-like heat transfer and the relatively low coefficient of the lattice thermal conductivity (κ = 0.7 W/(m K)), which is favorable to achieve the large thermoelectric figure of merit.

Enhanced Electrochemical Stability and Moisture Reactivity of Al2S3 Doped Argyrodite Solid Electrolyte

All-solid-state lithium-ion batteries are considered a promising next-generation lithium-ion batteries for their safety and high energy density. Among solid electrolytes, sulfide-based solid electrolytes have received attention from academia for their good contact properties such as mechanical deformability and high ionic conductivity. Specifically, argyrodite, which has Li6PS5X (X = Cl, Br, I) structure, shows a high lithium ionic conductivity (>10−3 S cm−1) at room temperature. However, sulfide-based solid electrolytes are chemically unstable in reaction to lithium metal and moisture because of the high reactivity of sulfur. Also, sulfide-based solid electrolytes react with active materials on the interface in composite cathodes. Moreover, this insufficient electrochemical stability of sulfide-based solid electrolytes can deteriorate the cell performance such as discharge capacity in prolonged cycles. We investigated the effects of partial substitution of Al2S3 for P2S5 to form the Al-S bonding instead of the P-S bonding in the PS4 3− main unit of the Li6PS5X structure. The Al-S bonding is electrochemically more stable than the P-S bonding due to a larger electronegativity difference between the two atoms. We show that the Al2S3 substitution in an argyrodite structure used for the composite cathode significantly improves the electrochemical stability.

PECULIARITIES OF SINGLE CRYSTAL GROWTH OF SOLID SOLUTION IN SYSTEMS Ag6PS5I‒Ag7GeS5I AND Ag7SiS5I‒Ag7GeS5I

The application of innovative resolutions that are technically grounded and economically acceptable with the use of environmentally friendly components is one of the important points at the present stage of human development to promote the ideas of efficient use of energy resources. Superionic conductors take a leading role in solving the problems of energy-saving technologies because electrochemical sensors and solid energy sources based on them can be created. Quaternary silver-containing compounds of argyrodite structure Ag6PS5I, Ag7SiS5I, and Ag7GeS5I have many advantages, in particular chemical stability, high values of ionic conductivity, the ability to form wide series of solid solutions due to the structural features. This work is devoted to the development and optimization of technology for obtaining high-quality single-crystal samples of solid solutions based on compounds of argyrodite structure formed in systems Ag6PS5I‒Ag7GeS5I та Ag7SiS5I‒Ag7GeS5I.&#x0D; The synthesis of solid solutions of the investigated systems was carried out by a direct single-temperature method from previously obtained quaternary compounds Ag6PS5I, Ag7SiS5I, and Ag7GeS5I. The maximal temperature of synthesis was 1273 K (exposure 72 h). Cooling to the experimentally selected annealing temperatures of 733 K (Ag6PS5I‒Ag7GeS5I) and 873 K (Ag7SiS5I‒Ag7GeS5I) was performed at a rate of 100 K/h. The annealing was performed for 120 hours.&#x0D; Crystal growth of solid solutions based on Ag6PS5I, Ag7SiS5I, and Ag7GeS5I phases was carried out by the directional crystallization from solution-melt by the Bridgman technique. The growth of single crystals was carried out in a three-zone furnace in a quartz container with a conical bottom. The optimal technological regimes for crystal growth of solid solutions of Ag6PS5I‒Ag7GeS5I and Ag7SiS5I‒Ag7GeS5I systems have been established. Grown single crystals of solid solutions Ag6+x(P1-xGex)S5I (х=0.25, 0.5, 0.75) and Ag7(Si1-xGex)S5I (х=0.2, 0.4, 0.6, 0.8) were dark gray color with metallic luster 30-40 mm long and 10-12 mm in diameter. Obtained crystals were investigated by X-ray diffraction analysis. Calculated lattice parameters change according to Vegard’s rule and confirm the formation of solid solutions in studied systems.