Influence of the Lithium Substructure on the Diffusion Pathways and

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Influence of the Lithium Substructure on the Diffusion Pathways and Transport Properties of the Thio-LISICON LiGe SnS 4

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Nicolo Minafra, Sean P. Culver, Cheng Li, Anatoliy Senyshyn, and Wolfgang G. Zeier Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b01059 • Publication Date (Web): 25 Apr 2019 Downloaded from http://pubs.acs.org on April 26, 2019

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Chemistry of Materials

Influence of the Lithium Substructure on the Diffusion Pathways and Transport Properties of the Thio-LISICON Li4Ge1-xSnxS4 Nicolò Minafra,a,b Sean P. Culver,a,b Cheng Li,c Anatoliy Senyshyn,d Wolfgang G. Zeier*,a,b aInstitute

of Physical Chemistry, Justus-Liebig-University Giessen, Heinrich-BuffRing 17, D-35392 Giessen, Germany.

bCenter

for Materials Research (LaMa), Justus-Liebig-University Giessen, Heinrich-Buff-Ring 16, D-35392 Giessen, Germany.

cJülich

Centre for Neutron Science JCNS, Forschungszentrum Jülich GmbH, Outstation at SNS, 1 Bethel Valley Road, Oak Ridge, Tennessee 37831-6473, United States. dHeinz

Maier-Leibnitz Zentrum, Technische Universität München, 85748 Garching, Germany.

Abstract Inorganic lithium-ion conductors have garnered considerable attention as separators for allsolid-state lithium-ion battery applications, given their potential to solve the safety issues and improve the energy and power densities of conventional devices possessing liquid electrolytes. However, achieving this transition requires the optimization of solid electrolyte materials and thus, developing a better understanding of the structure-property relationships, that govern ionic transport, is of crucial importance. Herein, inspired by the growing technological interest, a systematic study on the correlations between structural modifications and transport properties in the thio-LISICON family resulting from the substitution of Ge4+ by Sn4+ within Li4Ge1-xSnxS4 has been conducted. Using Rietveld refinements against neutron diffraction data coupled with maximum-entropy method (MEM) analyses of nuclear densities, a rigorous investigation into the Li+ diffusion pathways was performed. The substitution of Ge4+ by Sn4+ is shown to broaden the diffusion bottleneck, modify the lithium distribution and enhance the connectivity between the conduction channels, thereby leading to an increase of the ionic conductivity for Li4SnS4. The correlations between composition, structure and transport behavior found in this work provide insights into design strategies for new electrolytes belonging to the thio-LISICON family.

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1. Introduction Inorganic solid electrolytes, acting as separators in Li-ion batteries, have recently attracted great interest due to their potential to offer unique enhancements, as compared with their liquid electrolyte counterparts. The employment of an inorganic solid-state ionic conductor as the electrolyte would mitigate the flammability issues associated with liquid-based electrolytes, and could improve the energy and power densities when coupled with a lithium metal anode.1–4 In this regard, the solid electrolyte serves as a promising focal point for improving lithium battery performance.5,6 In the last decades, many different classes of solid electrolytes have been investigated, such as Li10GeP2S12,7–14 Li6PS5X argyrodites,15–19 as well as the Li-NASICONs20–22 and garnets23–25. However, one particular class that has received attention is the oxide-based LISICON family Li4-2xZnxGeO4.26–28 In order to enhance the ionic transport, a wide range of substitutions have been performed,28–34 however the intrinsically low ionic conductivity of oxides, as well as the need for sintering, limits their viability for solid-state battery (SSB) technologies. More recently, major improvements in the ionic conductivities have been achieved by replacing the hard oxide with a larger and more polarizable sulfide sublattice, giving rise to the thio-LISICON family with the general formula LixM1-δMδ’S4, where M = Si, Ge, Sn and M’ = P, Ga, Al, Zn.34–38 It should be noted however that while such substitution strategies have proven to be effective toward enhancing diffusion in highly crystalline conductors, similar approaches may not be applicable for the optimization of their glassy counterparts, given the high level of disorder present in these materials.

Figure 1: Polyhedral representation of the crystal structure (space group Pnma) of (a) Li4GeS4 and (b) Li4SnS4. In both (a) and (b), the sulfur anions form a distorted hcp lattice with M and Li(2) occupying tetrahedral sites (Wyckoff 4c and 8d, respectively) for the thio-germanate and thio-stannate


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motif, respectively. Li(3) is located in an octahedral pocket (Wyckoff 8d), but displaced toward the apices. Li(1) occupies tetrahedral sites in both structures (Wyckoff 4c) with the apex pointing in opposite direction. In (b) lithium cations occupy an additional Li(4) octahedral site (Wyckoff 4c).

Figure 1a shows the crystal structure of Li4GeS4. The lithium thiogermanate,whose structure was reported by Kanno and coworkers,35 crystallizes in the orthorhombic Pnma space group (No. 62). The crystallographic framework is related to the γ-Li3PO4 structure, in which the anion arrangement may be regarded as a distorted hexagonal close-packed structure, with packing planes perpendicular to the c-axis.39,40 Within the anionic framework, the Ge4+ cations occupy tetrahedral interstices (Wyckoff 4c), forming isolated GeS4 polyhedra. Meanwhile, the Li+ occupy three different crystallographic positions, i.e. the Li(1) and Li(2) sites reside in tetrahedral voids (Wyckoff 4c and 8d, respectively) that share edges and corners with GeS4 polyhedra, accordingly, while the Li(3) site occupies octahedral intersticies (Wyckoff 8d) that are arranged in linear, edge-sharing chains extending parallel to the baxis.41–43 Importantly, the structural motif formed by the Li(3)S6 octahedral chain linked with the adjacent LiS4 tetrahedra is believed to facilitate the lithium-ion conduction pathway,41,44 however, experimental evidence for this has not yet been reported. In contrast to Li4GeS4, the analogous lithium thiostannate has been synthesized only recently (Figure 1b),45–49 with the underlying crystal structure being investigated by single crystal Xray diffraction measured at 100 K and room temperature. The unit cell shows a marked compression along the a-axis, with a decrease of approximately 4% when going from room temperature to 100 K, however both modifications have been found to be isostructural with the thiogermanate, crystallizing in the Pnma space group (No. 62). The difference between the low temperature and the room temperature phase can be attributed to significant changes in the Li substructure, as suggested by solid-state NMR studies.45,46 In the low temperature modification, the Li(1) position is displaced from an edge-sharing toward a corner-sharing configuration with SnS4, thereby occupying tetrahedral interstices between the Li(3)S6 octahedra that form chains along the b-axis. Moreover, in Li4SnS4, the lithium ions occupy another octahedral Li(4) position (Wyckoff 4c), resulting in a reduced occupancy of Li(3). The Li(3) position is displaced from the center of the octahedron, generating two different positions with a coordination number closer to five. While this feature is clear for the thiostannate,45,46 conflicting reports are found within the literature on the splitting of the Li(3) position for the thiogermanate.41,42 Despite belonging to the same space group, Li4SnS4 is known to possess a higher ionic conductivity than Li4GeS4,35,45 however the reasons for the better ionic transport are not yet ACS Paragon Plus Environment

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clear. In order to resolve the differences in the transport behavior, in the present study, we explore a step-wise substitution of Ge4+ with Sn4+ along the Li4Ge1-xSnxS4 solid solution series. The changes in the underlying crystal structure have been monitored by means of Rietveld refinements against neutron powder diffraction data and maximum entropy method analysis, while the ionic transport behavior was analyzed by means of electrochemical impedance spectroscopy. The results show that the different lithium configuration in the thiostannate imparts a higher connectivity among the lithium jumps, leading to greater ionic percolation throughout the structure. This evidence has been corroborated by MEM analysis, which clearly shows strong differences in the diffusion pathways for Li4SnS4, as compared to Li4GeS4. Ultimately, the work presented herein improves our understanding of the structural properties and the importance of the Li substructure in influencing the ionic conductivity in the thio-LISICON family, thereby providing insights for the optimization of Li-ion diffusion in future electrolytes. 2. Experimental Methods Synthesis. All preparations and sample treatments were carried out under Ar atmosphere. Lithium sulfide (Li2S, Sigma Aldrich, 99.98%), germanium (Ge, ChemPur, 99.995%), tin (Sn, ChemPur, 99.999%) and sulfur (S, Acros Organics, 99.999%) were mixed in the appropriate stoichiometric ratios using an agate mortar. All reagents were used as received, with the exception of sulfur, which was sublimated prior to use. The precursor mixtures were pressed into pellets and sealed in quartz ampoules (10 mm inner diameter and 10-12 cm in length) under vacuum. All ampoules were carbon-coated and pre-heated at 800 °C under dynamic vacuum for two hours to avoid all traces of water in the reaction atmosphere. The reaction mixtures were treated at 700 °C for 8 h to ensure reaction completeness and the phase purity of the products. In order to obtain standard deviations on each data set, reflecting changes in microstructure and small compositional changes, each sample was synthesized three times. X-ray powder diffraction. Powder X-ray diffraction measurements were carried out using an Empyrean diffractometer (PANalytical, Netherlands) with Cu Kα radiation (λ1 = 1.54051 Å, λ2 = 1.54433 Å) in Bragg-Brentano θ–θ geometry using a PIXcel3D area detector with 255 measuring channels. Samples were pulverized and placed on (911)-oriented silicon zero diffraction holders and sealed with Kapton foil. Measurements have been performed within a 2θ range of 10–90° with a step size of 0.026° and 170 s per step.


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Neutron powder diffraction. Neutron powder diffraction data were collected using the Spallation Neutron Source (SNS) POWGEN diffractometer at Oak Ridge National Laboratory. Approximately 3 g of sample were loaded into an 8 mm diameter cylindrical vanadium sample can. Using a center wavelength of 0.8 Å with a d-spacing from 0.2 to 6 Å, data were collected for approximately 3 h in the high resolution mode. Maximum Entropy Method analysis. Experimental lithium diffusion pathways were studied by the examination of negative nuclear density maps extracted from the measured structure factors by the maximum-entropy method. The method is based on the estimation of 3D scattering densities from a limited amount of information by maximizing information entropy under restraints, consistent with experimental observations. Compared to a Fouirier analysis, MEM is often better suited for the determination of nuclear density maps from the powder diffraction data sets characterized by “limited” statistics, i.e. reduction of artifacts and termination effects. Since lithium is the only negative scatterer in the studied compounds, the negative nuclear density maps for Li4GeS4 and Li4SnS4 were reconstructed from experimental structure factors using the program Dysnomia.50 Rietveld analysis. Rietveld refinements were carried out using the TOPAS software package,51 while bond lengths and polyhedral volumes were extracted from the Vesta software package (Version 3).52 The structure of Li4GeS4 form Murayama et al.41 was used as a starting model for the refinements and the peak profile shape was described by a pseudoVoigt function. The fit indicators of Rwp and GoF were used to assess the quality of the refined structural models. The following parameters were initially refined: (1) scale factor, (2) background coefficients, (3) peak shape, (4) lattice constants, (5) fractional atomic coordinates and (6) isotropic atomic displacement parameters. A strong improvement of the fit was achieved by allowing the Li(3) special position to be refined. Upon refining the Li(3) position, two new distincts sites are generated that are displaced from the center towards the apex of the octahedron. An additional lithium site was refined for Li4SnS4, i.e. Li(4). Moreover, the Ge/Sn ratios were refined for all compositions, with the exception of the end members, within the solid solution series. Beyond the lithium positions reported by Murayama et al.,41 the lithium occupancy on alternative crystallographic sites (e.g. Wyckoff 4c and 8d, proposed by Abrahams et al.26 for Li3.5Zn0.25GeO4) was also explored, however an additional site was only found within Li4SnS4. Therein, the occupancy on the newly formed Li(4) position was constrained to the Li(3) occupancy and allowed to refine.


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Electrochemical impedance spectroscopy. Electrical conductivities were measured by AC impedance spectroscopy, using isostatically pressed pellets (325 MPa, geometric density of all samples > 80%) that were subsequently coated via thermal evaporation with thin gold (200 nm) electrodes. Electrochemical impedance analysis was conducted in the temperature range of 10 °C to 60 °C using a SP300 impedance analyzer (Biologic) at frequencies from 7 MHz to 50 mHz with an amplitude of 10 mV. For the samples analyzed here, all compositions have been synthesized and characterized three times in order to obtain standard deviations and reflect changes in both the composition and the microstructure. All values shown reflect the average values with the corresponding standard deviations.

3. Results Structural characterization. Solid solutions within Li4Ge1-xSnxS4 have been synthesized in order to study the relationship between the structural changes and the ionic conductivity in the thio-LISICON family. For the initial structural characterization, laboratory X-ray powder diffraction was first performed to confirm the purity of the samples (Figure S1). Although Xray diffraction can be used to confirm the purity of the materials, the Li sublattice cannot be probed due to the low X-ray form factor of lithium. Therefore, neutron powder diffraction was performed for all compositions in order to fully characterize the underlying crystal structure. Representative Rietveld refinements against the neutron diffraction data for Li4GeS4 and Li4SnS4 can be found in Figure 2. As can be seen from a visual inspection of the fits, as well as the low values for the quality-of-fit indicators, all compounds along the solid solution series are well described by the Pnma space group (No. 62), showing no sign of additional impurity phases. All relevant structural parameters and refinements against the neutron data for the explored compositions are provided in the supporting information (Tables S1 – S5 and Figures S2 – S4).


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Figure 2: Representative Rietveld refinements against neutron powder diffraction data for Li4GeS4 and Li4SnS4. Experimental data are shown in black and the red line denotes the calculated pattern, while the difference profile is shown in blue. Calculated positions of the Bragg reflections are shown as green vertical ticks. The low values for the GoF and Rwp confirm both the structure and purity of the Li4Ge1-xSnxS4 series.

The evolution of the extracted unit cell volumes against the Sn composition in Li4Ge1-xSnxS4 is shown in Figure 3a. The unit cell volume increases linearly with increasing Sn content, driven by the larger ionic radius of Sn4+ (0.55 Å) relative to Ge4+ (0.39 Å) in the tetrahedral coordination environment,53 thereby obeying Vegard’s law and confirming the successful synthesis of stable solid solutions. It should be noted, however, that Li4SnS4 shows a slight compression of the unit cell relative to the expected value from the linear trend. This behavior is driven by an anomalous contraction of the lattice parameter a, along with the concurrent expansion of b and c (Figure S5). The offset in the lattice volume suggests that, with the employed synthetic procedure, the low temperature modification of Li4SnS4 was likely obtained. This evidence is consistent with previous theoretical studies, predicting that the low ACS Paragon Plus Environment

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temperature structure form is more accessible for Li4SnS4 when compared to Li4GeS4.44 Nevertheless, the full incorporation of Sn throughout the Li4Ge1-xSnxS4 series is corroborated by the linear expansion of the MS4 tetrahedra (Figure 3b). Moreover, the refinements show a linear increase in the occupancy of Sn on the Wyckoff 4c site (Figure 3c), corresponding well with the nominal stoichiometry. All of which further suggests that the slight deviation of the unit cell volume for the thiostannate is likely due to a redistribution of lithium within the lattice.


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Figure 3: (a) Increasing lattice volume arising from the incorporation of larger Sn4+ atoms, extracted from Rietveld refinements against neutron diffraction data. The unit cell expansion obeys Vergard’s law, thereby confirming the successful synthesis of stable solid solutions. (b) (Ge1-xSnx)S4 polyhedral volumes, which exhibit a linear expansion with increasing Sn content. (c) Nominal Sn occupancy on the Wyckoff 4c position. All data (a)-(c) are plotted against the refined Sn occupancy.

Upon shifting focus to the Li polyhedra, it can be seen that the step-wise incorporation of Sn into Li4Ge1-xSnxS4 does not affect all of the polyhedra in the same fashion (Figure 4). With increasing Sn substitution, the expanding lattice leads to an expansion of both the Li(1)S4 and Li(3)S6 polyhedra. This behavior strongly deviates at higher Sn contents, which is possibly due to the reorganization of the lithium distribution in Li4SnS4. Furthermore, the volume of the Li(2)S4 tetrahedra are largely unaffected by the Sn substitution up to x = 0.75 and decreases substantially for x = 1. Notably, both the Li(3)S6 polyhedral volume and the trigonal surface, that represents the bottleneck for the ionic jumps between two adjacent octahedra along the b-direction, expand only slightly up to x = 0.75 and increase substantially for x = 1 (Figure 4d).


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Figure 4: Changing Li polyhedral volumes and planar areas against the refined Sn occupancy extracted from Rietveld refinements of neutron diffraction data. (a) Increasing Li(1)S4 volumes with x in Li4Ge1-xSnxS4. (b) The Li(2)S4 volumes remain largely unaffected by the Sn content for x ≤ 0.75, but exhibit a significant decrease for x = 1. (c) Increasing of the Li(3)S6 volumes with Sn content. (d) Trigonal planar surface area formed by two adjacent Li(3)S6 octahedra, which increase with increasing Sn content. Both the Li(3)S6 volume and the trigonal planar surface area show a dramatic increase in Li4SnS4.

Besides the coordination polyhedra, neutron diffraction analysis allows for the lithium positions to be mapped, thus permitting an accurate assessment of the Li – Li distances, all of which are of crucial importance for determining the conduction pathways (Figure 5). Substitution of Ge by Sn does not alter the main lithium structural motifs for 0 ≤ x ≤ 0.75, but significantly modifies the framework for x = 1. For Li4SnS4, a greater Li(3) – Li(3) distance was observed (dLi(3)-Li(3) = 1.12(7) Å), as compared to Li4GeS4 (dLi(3)-Li(3) = 0.499(14) Å). Such ACS Paragon Plus Environment

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a rearrangement results in shorter Li(2) – Li(3) distances for Li4SnS4 (dLi(2)-Li(3) = 2.12(4) and 2.85(3) Å) in contrast with Li4GeS4 (dLi(2)-Li(3) = 2.598(5) and 2.921(6) Å). Moreover, the repositioning of Li(1) in the thiostannate results in much shorter Li(1) – Li(3) distances for Li4SnS4 (dLi(1)-Li(3) = 1.98(4) and 2.85(4) Å), relative to the thiogermanate (dLi(1)-Li(3) = 3.067(8) and 3.411(9) Å), as well as a Li(1) – Li(2) distance of 3.152(13) Å. These changing distances may now allow for a direct exchange between the sites. Additionally, the appearance of Li(4) in Li4SnS4 enhances the connectivity of the lithium framework, due to the close proximity of this site with Li(1) and Li(2) (dLi(1)-Li(4) = 2.88(3), dLi(2)-Li(4) = 2.81(2) and 3.16(3) Å). A visual inspection of the lithium substructure in Figure 5 already suggests a better connectivity in all directions for the different Li positions for Li4SnS4.

Figure 5: Lithium substructure of Li4GeS4 and Li4SnS4 projected on (a, b) the ab-plane and (c, d) the ac-plane respectively. The interatomic distances are shown in red for d ≤ 3 Å and blue for 3 < d ≤ 3.5 Å.

Ionic transport. The ionic transport properties of the Li4Ge1-xSnxS4 series were analyzed by means of temperature-dependent impedance spectroscopy. The impedance responses within the explored temperature range are similar throughout the solid solution series and consist of a well-resolved semicircle possessing an apex frequency of ⁓103 Hz or ⁓104 Hz for Li4Ge1ACS Paragon Plus Environment

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≤ x ≤ 0.75) and Li4SnS4, respectively (Figure 6a). The obtained data were fitted with

an equivalent circuit consisting of one parallel CPE/resistor in series with a CPE representing the behavior of electrolyte and the electrodes, respectively. The capacitance of the CPE used to fit the semicircle is in the range of 10-11 F/cm2, and all impedance spectra exhibit α-values ⁓ 0.9, all of which indicate a predominant bulk contribution to the impedance response.54 Arrhenius behavior was noted for all samples in the Li4Ge1-xSnxS4 series within the temperature range of 10 °C to 60 °C (Figure 6b). Further, the room-temperature ionic conductivity values for the solid solution series are reported in Figure 6c. Here, the ionic conductivity does not significantly change with increasing Sn content in Li4Ge1-xSnxS4 for 0 ≤ x ≤ 0.75 within the measurement uncertainties, but dramatically increases for x = 1. It should be noted, however, that the herein observed ionic conductivity for Li4SnS4 is one order of magnitude lower, i.e. 1.4  10-6 Scm-1, than the value reported by Kaib et al. of 7  10-5 Scm1.45

As the value reported in this study reflects the average of three separately synthesized

samples, we believe that the discrepancy is likely due to the difference in the employed synthesis procedures. The activation energy for ionic diffusion, as determined from a linear fit of the Arrhenius plots, is reported in Figure 6d. Within the uncertainty, no major changes to the activation energy can be seen for 0 ≤ x ≤ 0.75, given that the changes in the lithium substructure are relatively small. However, for Li4SnS4 the activation energy strongly decreases, corresponding with the observed increase in the room temperature ionic conductivity.


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Figure 6: (a) Nyquist plot of Li4Ge1-xSnxS4 at room temperature, showing the impedance response (open circles) and the fit (black line) with the provided equivalent circuit for the representative compositions of x = 0, 0.5 and 1. The apex frequencies of the semicirlces are ⁓103 Hz for 0 ≤ x ≤ 0.75 and ⁓104 Hz for x = 1, which corresponds to a bulk capacitance of ⁓10-11 F/cm2. (b) Representative Arrhenius plots of the conductivity values for Li4Ge1-xSnxS4 (x = 0, 0.5, 1) obtained from temperaturedependent impedance spectroscopy. (c) Room-temperature ionic conductivity as a function of x in Li4Ge1-xSnxS4. (d) Activation energy EA for Li4Ge1-xSnxS4 as a function of the Sn content. Errors in the room-temperature ionic conductivity values and the activation energies have been obtained from the measurement of three separately synthesized samples per composition.

Lithium diffusion pathways. Maximum entropy method (MEM) analysis is a highly useful approach for obtaining accurate depictions of the lithium-ion diffusion pathways in ionic conductors.9,55–57 Nevertheless, while the MEM method provides a good estimation of the conduction pathways, it gives only a static probability of the Li distribution. Therefore, via ACS Paragon Plus Environment

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additional dynamic methods (e.g. NMR spectroscopy) would also be helpful in corroborating the herein presented conduction mechanisms. Spin lattice relaxation NMR measurements provide information on the activation energies for the local ionic motion, that coupled with crystallographic data may help in revealing the conduction mechanisms in this class of materials.16,55 However, the conduction mechanisms and diffusion pathways in the structure have already been explored computationally,44 and the presented work corroborates the theoretical results. Probability maps of selected slices from the negative nuclear density data for Li4GeS4 and Li4SnS4 have been provided in Figure 7 and illustrate the key lithium diffusion behavior. In Li4GeS4 (Figure 7a and c), the lithium distribution has a more localized character, reflecting the poorer lithium-ion conduction. The nuclear density maps suggest lithium jumps within the linear Li(2) – Li(3) – Li(2) units, whereas long-range transport likely occurs via the octahedral Li(3) units along the b-direction, bridged by the Li(1) sites. However, upon transitioning from Li4GeS4 to Li4SnS4, a continuous 3-dimensional diffusion pathway can be clearly seen, involving all of the surrounding lithium sites (Figure 7b and d). Here, the Li(4) site plays a crucial role in the diffusion, opening up the linear Li(2) – Li(3) – Li(2) and Li(1) – Li(3) – Li(1) units via Li(2) – Li(4) and Li(1) – Li(4) exchange, respectively.


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Figure 7. Selected slices from the negative nuclear density maps represented in the form of lithium probabilities for Li4GeS4 (a and c) and Li4SnS4 (b and d). Atoms are shown with atomic radii.

4. Discussion The influence of Sn4+ substitution for Ge4+ along the Li4Ge1-xSnxS4 solid solution series on the ionic conducitivty can be explained by considering the structural modifications in the Li substructure and the resulting variations in the lithium diffusion pathways. A schematic representation of the changing Li substructure and polyhedral connectivity that directly affects the percolation pathways for Li can be found in Figure 8.

Figure 8. Schematic representation of the lithium substructure that constitutes the diffusion network for Li4Ge1-xSnxS4 projected on (a, c) the bc-plane and (b, d) the ac-plane respectively. Different lithium sites are defined by color, while the coordination is represented using distinct shapes. It is clear that the redistribution of Li sites within Li4SnS4 leads to well connected three-dimensional diffusion pathways. For clarity Li(2) is omitted in (a) and (c), while Li(1) is omitted in (b) and (d).

The occurring diffusion in the solid solutions can be described as follows: (1) Diffusion in Li4Ge1-xSnxS4: Looking at the lithium substructure of Li4Ge1-xSnxS4 with 0 ≤ x ≤ 0.75, the shortest Li – Li distance occurs between the Li(2) and Li(3) positions (d < 3 Å). ACS Paragon Plus Environment

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The short distance and face-sharing configuration of the coordination polyhedra suggest a localized jump motion along the Li(2) – Li(3) – Li(2) unit. Meanwhile, the long-range diffusion must involve either direct jumps between the aforementioned unit and the Li(1) sites (d > 3 Å) or along the octahedral Li(3) chain along the b-direction, with a transition site that is bridged by the Li(1) tetrahedra. MEM analysis of the negative nuclear densities as well as DFT calculations44 are in agreement with the aforementioned behavior, thus suggesting 1D diffusion along the b-axis with no continuous pathway in the ac-plane (Figure 5a and c). However, the edge-sharing octahedral arrangement will make jumps with low energy barriers difficult, severely restricting the ionic transport.58 With 0 ≤ x ≤ 0.75, the volume of the unit cell increases due to the larger ionic radius of Sn4+, thus slightly broadening the area of the trigonal surface between adjacent Li(3)S6 octahedra that represents the bottleneck for the ionc jumps along the b-axis. Additionally, the larger polarizability of Sn compared to Ge, which has been proven to influence the transport properties in the L6PS5X (X = Cl, Br, I) argyrodites and Li10Ge1-xSnxP2S12,8,15 may also play a role in the enhancement of the ionic conductivity in this range. However, all of these influences seem to be minor, and consequently, the ionic conductivity as well as the activation barriers show negligible changes up to x = 0.75, within the measurement uncertainties. (2) Diffusion in Li4SnS4: For Li4SnS4, the structural modifications and redistribution of lithium lead to a strong decrease in the activation energy, reflectiong a much higher ionic conductivity relative to Li4Ge1-xSnxS4 with 0 ≤ x ≤ 0.75 (Figure 6d). The low value of EA is associated with two principal features of the structure: (i) the relatively compressed unit cell, with respect to the expected lattice expansion, exhibits a much broader planar area for the trigonal bottleneck (Figure 4d), thereby enabling faster diffusion through the linear chain along the b-axis. Additionally, the repositioning of Li(1) into the chain, switching from an edge-sharing to a face-sharing configuration with the Li(3)S6 octahedra, improves the connectivity of the polyhedra and further enhances the ionic diffusion along the onedimensional chains (Figure 8a and c). Moreover, the relocation of Li(1) and Li(3) in Li4SnS4 shortens the distance between these two sites, resulting in a shorter jump length along the baxis. Furthermore, (ii) the partial occupancy of lithium on the Li(4) position facilitates diffusion by bridging the linear chains along the b-direction (Figure 8d), which enables the 3D conduction. The MEM analysis of negative nuclear scattering densities confirms that the redistribution of lithium in Li4SnS4 leads to 3D diffusion character, in which the partially occupied Li(4) sites serve as interconnections between the existing diffusion channels.


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5. Conclusion In this study, the lithium diffusion pathways in Li4Ge1-xSnxS4 have been investigated using Rietveld refinements against neutron powder diffraction data and MEM analysis of negative nuclear density maps. The results reveal that the lithium substructure in Li4GeS4 forms isolated linear chains along the b-axis, only allowing for diffusion with a high energy barrier in one direction. For 0 ≤ x ≤ 0.75, the step-wise inclusion of Sn4+ in Li4Ge1-xSnxS4 does not modify the conduction pathways and, despite the widening of the channels for lithium transport and the inclusion of a larger, more polarizable element, only results in negligible changes of the ionic conductivity and activation barriers. Contrastingly, for Li4SnS4, the lithium substructure is significantly changed. The repositioning of Li(1) into the previously poorly connected 1D chains improves the connectivity of the polyhedra, which enhances the ionic transport. Moreover, the partial occupancy of a newly formed Li(4) position bridges the formerly 1D chains, thereby activating a 3D conduction behavior. This work demonstrates that elemental substitutions can be exploited to alter the lithium distribution and substructure, and with it the diffusion behavior in solid electrolytes. This work shows that a cross-linked lithium network is crucial for achieving good ionic conduction. Moreover, the data suggest that the ionic conductivity in the thio-LISICON family can be further optimized through compositional modifications that influence the underlying lithium substructure and affect the diffusion pathways.

ASSOCIATED CONTENT All structural data as obtained from Rietveld refinements against neutron powder diffraction are tabulated here. Additionally, X-ray powder diffraction data for all synthesized compounds are provided. The Supporting Information further contains the crystallographic information files obtained from the Rietveld refinements.

AUTHOR INFORMATION Corresponding Authors *[email protected]; Notes The authors declare no competing financial interests. ACS Paragon Plus Environment

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Acknowledgements The research was supported by the Deutsche Forschungsgemeinschaft (DFG) under grant number ZE 1010/4-1. This research used resources at the Spallation Neutron Source, as appropriate, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. S.C. gratefully acknowledges the Alexander von Humboldt Foundation for financial support through a Postdoctoral Fellowship. The authors thank Ashfia Huq (Oak Ridge National Laboratory) for the support during the acquisition of the neutron diffraction data. References (1)

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