Synthesis, Structural Characterization, and Lithium Ion Conductivity of

May 9, 2017 - The pair distribution function G(r) was employed for a more accurate structural analysis, and PDFgetX2 software was used to extract G(r)...
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Synthesis, Structural Characterization, and Lithium Ion Conductivity of the Lithium Thiophosphate Li2P2S6 Christian Dietrich,† Dominik A. Weber,† Sean Culver,† Anatoliy Senyshyn,‡ Stefan J. Sedlmaier,§ Sylvio Indris,∥ Jürgen Janek,*,†,§ and Wolfgang G. Zeier*,† †

Institute of Physical Chemistry, Justus-Liebig-University Giessen, Heinrich-Buff-Ring 17, D-35392 Giessen, Germany Heinz Maier-Leibnitz Zentrum, Technische Universität München, 85748 Garching, Germany § BELLA − Batteries and Electrochemistry Laboratory, Institute of Nanotechnology, ∥Institute for Applied Materials, Karlsruhe Institute of Technology, Hermann-von-Helmholtz Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany ‡

S Supporting Information *

ABSTRACT: Inspired by the ongoing search for new superionic lithium thiophosphates for use in solid-state batteries, we present the synthesis and structural characterization of Li2P2S6, a novel crystalline lithium thiophosphate. Whereas M2P2S6 with the different alkaline elements (M = Na, K, Rb, Cs) is known, the lithium counterpart has not been reported yet. Herein, we present a combination of synchrotron pair distribution function analysis and neutron powder diffraction to elucidate the crystal structure and possible Li+ diffusion pathways of Li2P2S6. Additionally, impedance spectroscopy is used to evaluate its ionic conductivity. We show that Li2P2S6 possesses P2S62− polyhedral units with edge-sharing PS4 tetrahedra and only one-dimensional diffusion pathways with localized Li−Li pairs, leading to a low ionic conductivity for lithium.

possible: P2S64− and P2S62−. M4P2S6 (M = Li, Na, K, Rb) consists of P2S64− ethane-like polyhedra with a P−P bond and phosphorus exhibiting a formal oxidation state of +4, whereas M2P2S6 (M = Na, K, Rb, Cs) consists of P2S62− anions formed by edge-sharing tetrahedra with two P−S−P bridges instead of a direct P−P bond (with a formal oxidation state of +5 for phosphorus). In the case of M2P2S6, the analogue with a large Cs is known (as well as Cu2P2S6),23 but the existence of Li2P2S6 has been established only based on nuclear magnetic resonance (NMR) spectroscopy.24 Inspired by the missing composition in the Li−P−S phase diagram (Figure 1b), we synthesized crystalline Li2P2S6 and solved the structure of this new lithium thiophosphate. A combination of synchrotron pair-distribution function (PDF) analysis and neutron Bragg powder diffraction was used to solve the structure and understand the possible diffusion pathways for lithium ions. Similar to the phases with heavier alkaline counterparts, P2S62− polyhedra consisting of edge-sharing tetrahedra are found to be the underlying structural motif. Impedance spectroscopy reveals that Li2P2S6 is a poor ionic conductor because of the presence of only one-dimensional diffusion pathways with largely localized Li−Li pairs, as suggested by using the maximum entropy method (MEM).

1. INTRODUCTION Lithium thiophosphates are increasingly considered for use as potential solid electrolytes in solid-state batteries due to their inherently high ionic conductivity and mechanically soft nature.1−3 In particular, the crystalline compounds Li3PS4, Li7P3S11, and Li10GeP2S12, as well as the thiophosphate glasses, have proven to be very promising in this regard.2,4−7 However, other lithium thiophosphates are known to exhibit very low ionic conductivities, such as Li4P2S6.8 The multitude of different compositions and the wide range of attainable ionic conductivities show the importance of the underlying structure and the influence of different structural motifs on the transport properties of these compounds. In the class of ternary crystalline alkaline metal thiophosphates with isolated (zero-dimensional)9 thiophosphate anions, different compositions and structures are known.8,10−17 The known polyhedral motifs are shown in Figure 1. In addition to the ternary compounds, various polyhedral anions have been reported for quaternary thio- and selenophosphates.18−22 In the case of ternary compounds, ortho-thiophosphates (i.e., M3PS4; M = Li, Na, K, Rb) with tetrahedral PS43− anions (Figure 1a) are known to exhibit high ionic conductivities of up to 1.6 × 10−4 S cm−1 in β-Li3PS4.4 In addition to the tetrahedral PS43− anions, the pyro-thiophosphate P2S74− anion is known to consist of two corner-sharing tetrahedra. However, in the alkaline metal series, only Li7P3S11 is found to be a crystalline material, in which PS43− and P2S74− anions coexist in a 1:1 ratio. Furthermore, for the P2S6 unit, two constitutional isomers are © 2017 American Chemical Society

Received: March 27, 2017 Published: May 9, 2017 6681

DOI: 10.1021/acs.inorgchem.7b00751 Inorg. Chem. 2017, 56, 6681−6687

Article

Inorganic Chemistry

Figure 1. (a) Different known ternary (crystalline) alkaline metal thiophosphates and the corresponding anionic species of PS43−, P2S64−, P2S74−, and P2S62−. P2S74− is known only from lithium thiophosphate glasses and Li7P3S11, which contains a mixture of P2S74− and PS43−. All other anionic species are known to exist with different alkaline metal cations; Li2P2S6 has not been reported so far (green label). The references denote the original publication of the reported crystalline phase and the solved structure. (b) Li−P−S ternary phase diagram. double-resonance probe (ZrO2 rotor was filled in the glovebox) at a spinning speed of 30 kHz. The magnetic field strength was 11.7 T, corresponding to a Larmor frequency of 202.4 MHz. A rotorsynchronized Hahn-echo pulse sequence was used for data acquisition with a π/2 time of 2 μs and recycle delays of 60 s. 2.5. Impedance Spectroscopy Measurements. Electrical conductivities were measured by AC impedance spectroscopy, using a custom-built setup. Powder samples of 100 mg were placed between two stainless steel rods with a 10 mm diameter and pressed at 3 tons for 2 min, obtaining a relative density of 80%. Electrochemical impedance analysis (EIS) was conducted in the temperature range of 25 to 65 °C using a SP300 impedance analyzer (Biologic) at frequencies from 7 MHz to 100 mHz with an amplitude of 20 mV. 2.6. Neutron Powder Diffraction. High-resolution neutron powder diffraction data collection on the Li2P2S6 sample was performed in Debye−Scherrer geometry at Heinz Maier-Leibnitz Zentrum (research reactor FRM II, Garching b. München, Germany) on the high-resolution diffractometer SPODI.27 Data collection using two wavelengths was performed, i.e., monochromatic neutrons (λ = 1.54817(2) and 2.53620(2) Å) were obtained from the thermal neutron beam at a 155° takeoff angle using the 551 and 331 reflections of a vertically focused composite Ge monochromator of 200 mm height. The vertical position-sensitive multidetector (300 mm vertical sensitivity range at 1.117 m sample-to-detector distance) consisting of 80 3He tubes and covering an angular range of 160° 2θ was used for data collection. The Li2P2S6 sample (approximately 2 cm3 in volume) was filled into a thin-wall (0.15 mm) vanadium can of 12 mm in diameter under an argon atmosphere and then metal-sealed using indium wire. The vanadium container was then mounted on a capillary spinner, enabling sample rotation and thus minimizing effects of preferred crystallite orientations. Two-dimensional powder diffraction data of the continuously rotated sample were collected and corrected for geometrical aberrations and detector nonlinearities.28 Simultaneous (dual wavelengths) Rietveld and crystal structure independent (Le Bail) refinements were carried out using the software package FullProf.29 The peak profile shape was described by a pseudo-Voigt function using the modified Thomson−Cox−Hastings setting. The instrumental resolution was determined using Na2Ca3Al2F14 as reference material. On the basis of the obtained structure factors in Li2P2S6, the nuclear densities were analyzed using the MEM,30 enabling more accurate determination than with differential Fourier analysis.31 In Li2P2S6, only lithium possesses a negative scattering length (bLi = −1.9 fm), thus limiting the consideration of nuclear

2. EXPERIMENTAL METHODS 2.1. Synthesis. All syntheses were carried out in an argon-filled glovebox (MBraun). A lithium thiophosphate glass of 5 g was synthesized by combining stoichiometric amounts of Li2S (50 mol %, Sigma-Aldrich; 99.98%) and P2S5 (50 mol %, Sigma-Aldrich; 99%). The mixture was ball-milled in a Pulverisette 7 Premium Line (Fritsch) planetary ball mill at a rotation speed of 510 rpm using a ZrO2 grinding bowl of 45 mL volume and ca. 110 g of ZrO2 balls with a diameter of 3 mm until glass formation was completed and no reflections could be observed via powder X-ray diffraction. Over 300 milling cycles (5 min milling and a 15 min natural cooling period to prevent overheating and sulfur evaporation) were necessary for complete amorphization. Crystallization of the glass was performed using a custom-built setup without any overhead volume to avoid evaporation of sulfur. A powdered sample of 150 mg of the 50:50 Li2S:P2S5 glass was placed between two stainless steel rods with a 10 mm diameter and pressed at 3 tons for 2 min.25 The cell was then transferred to a thermocouple-controlled furnace (Nabertherm) and held at 270 °C for 20 h. After thoroughly grinding the obtained pellet in an agate mortar, Li2P2S6 was obtained as a yellowish powder that is sensitive to moist air. The crystalline phases Li4P2S6, Li7P3S11, and Li3PS4 are used for comparisons with Li2P2S6; their syntheses have previously been reported.8,26 2.2. Synthesis of Li2P2S6 for Neutron Powder Diffraction. To obtain a sufficient sample volume for neutron diffraction, a large scale 10 g batch of the above-synthesized glass was ball-milled in a Pulverisette 5 (Fritsch) planetary mill at a rotation speed of 240 rpm using a ZrO2 grinding bowl of 500 mL volume and 2000 ZrO2 balls with a diameter of 4 mm until amorphization was completed after 80 milling cycles (15 min milling and 30 min natural cooling period) and no reflections could be observed via X-ray powder diffraction. The LPS glass was transferred in a borosilicate glass jar in a PTFE-sealed gastight steel vessel. The vessel was transferred to a thermocouplecontrolled furnace (Nabertherm) and held at 270 °C for 20 h. The obtained chunk was thoroughly ground in an agate mortar afterward. 2.3. Raman Spectroscopy. A Senterra Raman spectrometer (Bruker, USA) with an excitation wavelength of 532 nm was used to collect Raman spectra from 55 to 1555 cm−1 using a 20× objective and 0.2 mW of power. Samples were placed on glass substrates in the glovebox and sealed airtight by a cover glass and silicon vacuum grease. 2.4. Solid-State NMR Spectroscopy. 31P magic angle spinning (MAS) NMR experiments were carried out on a Bruker Avance 500 MHz spectrometer equipped with a commercial 2.5 mm MAS NMR 6682

DOI: 10.1021/acs.inorgchem.7b00751 Inorg. Chem. 2017, 56, 6681−6687

Article

Inorganic Chemistry density maps to their negative part only. Reconstruction of the negative nuclear density maps using MEM was performed using the program Dysnomia.32 2.7. Synchrotron Powder Diffraction. Mark-tubes made of borosilicate glass (Hilgenberg) with an outer diameter of 0.9 mm were filled in an argon glovebox and flame-sealed. X-ray scattering data suitable for pair distribution function and diffraction analyses were performed at room temperature using the I15 instrument at the Diamond Light Source (UK) beamline. High-energy X-rays (λ = 0.173369 Å, 71.52 keV, bent Laue monochromator) were used in combination with a PerkinElmer 1621 EN area detector. The structure analysis of Li2P2S6 was performed using the X-ray powder diffraction Bragg data and the simulated annealing function implemented in Expo2014 in combination with a rigid model.33 For structural refinements of the Bragg data, the program package FullProf Suite (version February 2016) was used.29 Peak profiles were fit with a pseudo Voigt function; the background was described by linear interpolation between a set of manual points with refinable heights. The fractional atomic coordinates and atomic displacement parameters of all lithium atoms were fixed to the values obtained by the refinement of neutron diffraction data. For all other atoms, these parameters were independently refined, and only the deviating thermal parameters of the sulfur atoms were constrained to each other. The crystallographic data, obtained from the refinements of the Bragg data, were used as starting values for the analysis of the PDF data. The pair distribution function G(r) was employed for a more accurate structural analysis, and PDFgetX2 software was used to extract G(r) from the raw diffraction data.34 The collected data were first corrected for background, sample absorption, and Compton scattering. Then, normalized structure functions S(Q) were obtained. Finally, S(Q) was Fourier-transformed to yield G(r). A maximum scattering vector (Qmax) of 18 Å−1 was employed in the Fourier transform. Structural refinements were carried out using PDFgui software.35 The local crystal structure of the Li2P2S6 crystallites was refined with the monoclinic C2/m space group. The fit of this structural model to the experimental PDF data was performed in the 1.75−35 Å interatomic distance range. The following parameters were refined: (1) scale factor, (2) lattice constants (a, b, and c), (3) fractional atomic coordinates of the phosphorus and sulfur atoms, and (4) atomic anisotropic displacement parameters constrained by the site symmetry (U11, U22, U33, and U13 for P and S). Fractional atomic coordinates and atomic displacement parameters for the lithium atoms were fixed to values determined from the refinement of neutron diffraction data. The Rw indicator was employed to assess the quality of the refined structural models.36

Figure 2. (a) Pair distribution function G(r) of Li2P2S6, obtained from synchrotron diffraction showing the goodness-of-fit of the structural solution and Rietveld refinement. In the low r-region, small discrepancies can be seen, which can be explained by a small fraction of an underlying glassy phase as recently observed for Li4P2S6.8 (b) Neutron powder diffraction data and results of the Rietveld refinement for Li2P2S6 at ambient temperature. Small fractions of impurity phases can be seen in the neutron diffraction data, due to the large-scale synthesis for this experiment. Experimental data are shown as points, the red line denotes the calculated pattern, and the difference profile is shown in blue underneath. Calculated positions of Bragg reflections are shown by green vertical tick marks (phases from top to bottom: Li2P2S6, Li4P2S6, and Li7P3S11). The inset shows the goodness-of-fit for small d-spacings.

seen in Li4P2S6.8 Figure 2b shows the obtained neutron diffraction intensities, which are found to be consistent with the structural solution in space group C2/m as well. The neutron Bragg data is used to identify the positions of Li in the structure more accurately because the X-ray scattering form factor of Li is too small to sufficiently contribute to the X-ray diffraction. By including the impurity phases of Li4P2S6 and Li7P3S11 (9 and 4 wt %, respectively), an excellent Rietveld fit could be obtained. While the samples prepared using the smaller scale synthesis are phase pure, upscaling for neutron powder diffraction led to minor impurity phases. Despite the minor impurities in the neutron Bragg data, the combination of synchrotron and neutron diffraction helped to solve the structure of Li2P2S6 and determine the Li positions unequivocally. The obtained crystallographic data of the neutron and synchrotron diffraction can be found in Table 1. A table of the crystallographic data including anisotropic thermal displacement parameters can be found in the Supporting Information (Table S1). Figure 3 shows the structure of Li2P2S6 as obtained from the neutron and synchrotron diffraction. Li2P2S6 crystallizes in the monoclinic space group C2/m with isolated P2S62− polyhedral units. In contrast to P2S64− (see Figure 1) and its P−P bond, P2S62− consists of two edge-sharing PS4 tetrahedra that form in an eclipsed arrangement along the b and c directions. As Li2P2S6 is the first known lithium thiophosphate with edge-sharing P2S62− units, Raman and 31P NMR spectra are provided in Figure 4. For both methods, the signals from edge-sharing P2S62− are compared with those of other known polyhedral species of PS43−, P2S74−, P2S64− from crystalline Li3PS4, Li7P3S11, and Li4P2S6, respectively. It should be noted, however, that Li4P2S6 is a glass-ceramic and contains features of the glass in the NMR spectra,8 whereas Li7P3S11 contains a 1:1 ratio of PS43− and P2S74− polyhedra. The NMR spectra of Li2P2S6 confirm the phase purity of the material, as no signals from other polyhedral species were found. The most intense peak of the Raman spectra of Li2P2S6 can be assigned to the symmetric stretching mode of the P−S−P−S ring, which is shifted to higher wavenumbers due to the replacement of counterions by

3. RESULTS AND DISCUSSION 3.1. Structure Analysis. The structure solution was performed on the basis of synchrotron Bragg data (see Supporting Information Figure S1). The structure can be indexed to space group C2/m, which is different from the Na2P2S6 (space group P21/m) and (K,Rb,Cs)2P2S6 (space group Immm) counterparts. The reason for the structural diversity among the different alkaline materials derives from the ionic radius of the different cations. In (K,Rb,Cs)2P2S6, the alkaline cations exhibit 10-fold coordination in the first coordination sphere, representing a bicapped cubic polyhedral environment. As the size of the ionic radius of the alkaline cations decreases, the large coordination number is no longer favorable and a structural change needs to occur. For a determination of atomic positions, displacement parameters, and occupancies, the PDF and the neutron Bragg data have been refined (see Figure 2). Figure 2a shows the PDF fit (Rw = 14.5%) corroborating excellent phase purity and a good description of the structural data. Only a minor discrepancy can be found in the low r-range. This may be due to the presence of a glass phase with small volume fraction, as recently 6683

DOI: 10.1021/acs.inorgchem.7b00751 Inorg. Chem. 2017, 56, 6681−6687

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Inorganic Chemistry

Table 1. Crystallographic Data (Atomic Coordinates, Uiso, and Occupancy) of Li2P2S6 at Room Temperature Obtained from Rietveld Refinements against Neutron Powder Data (λ = 1.54817 Å) and X-ray Pair Distribution Function Analysis (λ = 0.173369 Å) Li2P2S6 structure from neutron powder diffractiona atom

Wyckoff site

Li1 P1 S1 S2 S3

4g 4i 4h 4i 4i

x

y

z

0 0.2407(8) 0 0.6539(2) 0.5 0.5806(4) 0.5 0.2788(4) 0.5 0.6978(3) 0.5 0.3261(5) 0.3192(4) 0 0.9385(6) Li2P2S6 structure from X-ray pair distribution function analysisb

occupancy

Uiso/Å2

1.0 1.0 1.0 1.0 1.0

0.047(4) 0.016(1) 0.025(2) 0.023(2) 0.026(2)

atom

Wyckoff site

x

y

z

occupancy

Uiso/Å2

Li1 P1 S1 S2 S3

4g 4i 4h 4i 4i

0 0.653(1) 0.5 0.698(2) 0.319(2)

0.241 0.5 0.278(2) 0.5 0

0 0.580(2) 0.5 0.325(3) 0.943(4)

1.0 1.0 1.0 1.0 1.0

0.047 0.021 0.021 0.017 0.020

Space group C2/m, No. 12, origin choice 2; a = 11.1156(9) Å; b = 7.0070(6) Å; c = 6.5251(5) Å; β = 125.5717(14)°. Fit residuals (Rp, Rwp, Rexp, χ2): 1.47, 1.76, 1.66, 1.12, respectively. bSpace group C2/m; a = 11.109(5) Å; b = 7.012(4) Å; c = 6.526(3) Å; β = 125.572(8)°.

a

Figure 3. (a) Crystal structure of Li2P2S6 in the monoclinic C2/m system with edge-sharing PS4 tetrahedra, leading to a P2S62− polyhedral unit (b). (c) View along the b axis showing the eclipsed arrangement of the P2S62− polyhedral unit and chains of Li+. (d) View along [001]. All Li atoms are shown in yellow, P, in purple, and S, in orange.

the c direction. This preferable direction of thermal motion could be attributed to the possible diffusion along the c axis, as is often seen in ionic conductors.37 However, Figure 3 shows that P2S62− polyhedra block possible Li diffusion in the c direction, and it is likely that no Li−Li jump connects along [001]. Indeed, ionic hopping between edge-sharing octahedra is expected to occur via a tetrahedral interstitial site,38 and a diffusion pathway along the b axis is expected. Employing the MEM to analyze the neutron diffraction data provides the reconstructed nuclear density maps seen in Figure 5. MEM analysis reveals a 1D pathway along a Li chain in the b direction as the most probable pathway for lithium diffusion. However,

lighter alkaline metal ions (for Cs2P2S6, K2P2S6, and Li2P2S6, the stretching modes are at 412, 419, and 421 cm−1, respectively).15−17 The findings confirm the observations by Eckert et al.,24 who initially predicted the existence of Li2P2S6/LiPS3 by NMR measurements. The structure in Figure 3 shows that the Li atoms seemingly form chains along the b axis, with two distinct Li−Li distances of 3.382(8) and 3.626(8) Å. The first coordination sphere of lithium is represented by a basal-distorted octahedron formed by atoms S1 in the apical positions and S2 and S3 in the basal planes (Figure 5). Ellipsoids representing the anisotropic displacement parameter for lithium are found elongated along 6684

DOI: 10.1021/acs.inorgchem.7b00751 Inorg. Chem. 2017, 56, 6681−6687

Article

Inorganic Chemistry

Li+ chains being only one-dimensional, the basal distorted octahedral coordination and the different S−S distances affect the Li+ jump distance and jump probabilities, as the low symmetry of lithium site causes separation of Li−Li distance into shorter and longer ones. In Li2P2S6, this combination of several crystallographic factors is expected to severely limit the Li+ diffusion.

Figure 4. (a) Raman spectra and (b) 31P-MAS NMR spectra of the different crystalline lithium thiophosphates Li3PS4, Li7P3S11, Li4P2S6, and Li2P2S6 with the corresponding polyhedral units PS43−, P2S74−, P2S64−, and P2S62−, respectively.

Figure 6. (a) Impedance response at three selected temperatures, the employed equivalent circuit with parameters, and the fitting to the data (red solid line). The inset table gives the apex frequencies and capacitances at the shown temperatures, showing this impedance response to be a bulk property. (b) Arrhenius plot of the total lithium ion conductivity of Li2P2S6, exhibiting an activation barrier of 0.48 eV and a low room temperature conductivity of 7.8 × 10−11 S cm−1.

the pathway is characterized by strong irregularities severely hampering the lithium diffusion in this material. Separated “clouds” of negative nuclear densities are located at lithium pairs based on shorter Li−Li distances and are separated by longer Li−Li distances, pointing to possible “dimer-like” exchange character of the lithium diffusion in Li2P2S6. The observed dimer-like behavior is mediated by the sulfur framework, in which the Li1−Li1 jump distance is defined by the distances of Li1−S2−Li1 and Li1−S3−Li1. A shorter Li1− Li1 distance (characterized by pronounced lithium exchange) is built on longer S2−S2 distances (dS2−S2 = 3.957(4) Å), whereas the separation between clouds is correlated to a shorter S3−S3 distance (dS3−S3 = 3.630(7) Å). The longer S−S distances lead to shorter Li1−Li1 distances and vice versa to maintain the Li− S bond length (see Figure 5c). In addition to the nature of the

3.2. Electrical Conductivity Measurements. Figure 6 shows the Arrhenius plot for electric transport for the synthesized Li2P2S6 and measured impedance response in a Nyquist plot. The impedance data were fitted with a single RQ element in the frequency range of 7 MHz to 100 mHz. The contribution of the blocking electrodes is visible only at elevated temperatures (>50 °C), and the grain boundary contributions cannot be separated from the intragrain conductivity. Figure 6 gives the apex frequencies and

Figure 5. (a) LiS6 distorted octahedron with the different bond lengths. (b) Edge-sharing polyhedra along the b axis. The thermal displacement parameters indicate a rattling behavior in the large LiS6 octahedra, suggesting a more localized Li+ motion between the apexes (atoms S1). All atoms are shown as ellipsoids according to refined anisotropic thermal displacement parameters (with 95% probability). (c) Li−S chains along the b axis with shorter (3.382(8) Å) and longer (3.626(8) Å) Li−Li bond lengths. (d) Reconstructed negative nuclear density clouds by MEM and slices in the (100) plane (e) of Li2P2S6 (surface threshold −0.003 fm/Å3). The Li nuclear density maps show pairs of Li positions connected, and no lone-range distribution of the nuclear density is visible, suggesting a more localized Li distribution. Panels (a)−(c) share the crystal axes shown in the top part, and the axes in (d) and (e) correspond to the respective figure part. 6685

DOI: 10.1021/acs.inorgchem.7b00751 Inorg. Chem. 2017, 56, 6681−6687

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Inorganic Chemistry

Further details of the crystal structure investigation may be obtained from FIZ Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: (+49)7247-808-666; e-mail: crysdata@fizkarlsruhe.de), on quoting deposition number CSD-432586.

capacitances, showing the process to be predominantly a bulk response. The capacity of 0.03 nF fits well with the geometric capacity of the SE pellet. Crystalline phase-pure Li2P2S6 shows a room temperature ionic conductivity of 7.8 × 10−11 S cm−1 with an activation barrier of 0.48 eV. The low observed ionic conductivity may be explained by the crystallographic data shown above: (1) The structure of Li2P2S6 does not allow for three-dimensional isotropic diffusion, and because conductivity represents a scalar average, the observed conductivity is low. (2) The pairs of Li−Li distances likely lead to high activation barriers between the different jumps along the chain. (3) The Li+ positions are fully filled, leading to a small number of available sites and hence a low concentration of mobile carriers.8 (4) One-dimensional conducting chains are easily blocked by zero-dimensional point defects, preventing longrange transport.39

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ACKNOWLEDGMENTS The authors acknowledge financial support by BASF SE within the International Network for Electrochemistry and Batteries. This work was supported by Diamond Light Source (beamtime award EE13560) with beamtime proposal SP13560. W.G.Z. furthermore gratefully acknowledges the financial support through start-up funding provided by the Justus-LiebigUniversity Giessen.

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4. CONCLUSIONS We have synthesized the missing lithium thiophosphate Li2P2S6 and solved its crystal structure using synchrotron and neutron diffraction. Li2P2S6 exhibits edge-sharing PS4 tetrahedra, similar to other alkaline metal counterparts, but crystallizes in a different monoclinic space group, C2/m. A combination of 31PMAS NMR and Raman spectroscopy was used to compare the different polyhedral species of PS43−, P2S74−, and P2S64− to the here presented P2S62− polyhedral unit. In addition to the synthesis and structural solution, impedance spectroscopy shows Li2P2S6 to be a poor ionic conductor with high activation barriers due to the rather low structural symmetry of the lithium atom positions, leading to their spatial separation in a highly distorted lithium coordination polyhedron.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00751. Synchrotron Bragg data and neutron Bragg data (λ = 2.53620 Å) of Li2P2S6, as well as crystallographic tables with the anisotropic thermal displacement parameters (PDF) Accession Codes

CCDC 1546874 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.J.). *E-mail: [email protected] (W.G.Z.). ORCID

Stefan J. Sedlmaier: 0000-0002-5337-3076 Sylvio Indris: 0000-0002-5100-113X Wolfgang G. Zeier: 0000-0001-7749-5089 Notes

The authors declare no competing financial interest. 6686

DOI: 10.1021/acs.inorgchem.7b00751 Inorg. Chem. 2017, 56, 6681−6687

Article

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DOI: 10.1021/acs.inorgchem.7b00751 Inorg. Chem. 2017, 56, 6681−6687