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Conduction mechanism of Li10GeP2S12-type lithium superionic conductors in a Li-Sn-Si-P-S system Makoto Inagaki, Kota Suzuki, Satoshi Hori, Kazuhiro Yoshino, Naoki Matsui, Masao Yonemura, Masaaki Hirayama, and Ryoji Kanno Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00743 • Publication Date (Web): 16 Apr 2019 Downloaded from http://pubs.acs.org on April 17, 2019

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Chemistry of Materials

Conduction mechanism of Li10GeP2S12-type lithium superionic conductors in a Li-Sn-Si-P-S system Makoto Inagaki,a Kota Suzuki,a,b,c,d Satoshi Hori,c Kazuhiro Yoshino,b Naoki Matsui,b Masao Yonemura,e Masaaki Hirayama,a,b,c and Ryoji Kannoa,b,c*

a Department

of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan b Department

of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan All-Solid-State Battery Unit, Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan c

Japan Science and Technology Agency (JST), Precursory Research for Embryonic Science and Technology (PRESTO), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan. d

Neutron Science Laboratory (KENS), Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), 203-1 Shirakata, Tokai, Ibaraki 319-1106, Japan e

ABSTRACT: Crystal structures of Li10GeP2S12 (LGPS)-type Li10+δ[SnySi1−y]1+δP2−δS12 (Li4−x[SnySi1−y]1−xPxS4) solid electrolytes were analyzed by Rietveld refinement using neutron diffraction data. Maximum entropy method (MEM) analysis was performed to visualize the distribution of lithium along the c-axis via the Li1–Li3 sites, which indicated one-dimensional (1D) lithium diffusion for all the examined compositions. The Li10.35Sn0.27Si1.08P1.65S12 (Li3.45Sn0.09Si0.36P0.55S4) ( = 0.35, x = 0.55, y = 0.2) system, which had the highest ionic conductivity in Li-Sn-Si-P-S system, exhibited an additional lithium diffusion pathway in the ab-plane through the Li1 and Li4 sites. High ionic conductivity (>10 mS cm–1) was achieved in the Sn–Si derivatives owing to the formation of three-dimensional (3D) ion diffusion channels. Comparison of the conductivity and crystal structural parameters related revealed the requirements for fast lithium diffusion along the c-axis and 3D lithium diffusion in the LGPS-type crystal structure. Large atomic displacement of the Li1 site, a large S3–S3 distance, a large bottleneck size, and small differences in Li1–Li4 distances are important for 1D and 3D lithium diffusion, respectively.

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1. INTRODUCTION Solids possessing superionic conductivity are required for energy devices with high power characteristics and high stability.1-4 Lithium superionic conductors have attracted much attention in this regard, and all-solid-state lithium batteries are promising next-generation energy devices because of their high energy density5 and high power characteristics.1 All-solid-state lithium batteries contain lithium ion-conducting solids, that is, solid electrolytes instead of flammable liquid organic electrolytes. Thus, non-flammability and reliability of the system are achieved. Sulfide-based materials are also attractive owing to their relatively high ionic conductivity compared to those of oxides.2,6 Notably, Li10GeP2S12 (LGPS)-type materials, which have extremely high conductivity (exceeding 10 mS cm−1 at 300 K)1,2 contribute to the high discharge current drain of the battery. However, such high ionic conductivity in the LGPS family is achieved for limited compositions, for example, Ge-based solid solutions7,8 or Li9.54Si1.74P1.44S11.7Cl0.3.1 The abovementioned systems require an expensive element (Ge) or strict composition control for synthesis, respectively. To expand the LGPS family of superionic conductors, a quasi-ternary Li3PS4– Li4SnS4–Li4SiS4 system with the following composition was investigated: Li10+δ[SnySi1–y]1+δP2–δS12 (Li4–x[SnySi1–y]1– 9 Finally, Li10.35Sn0.27Si1.08P1.65S12 xPxS4). (Li3.45Sn0.09Si0.36P0.55S4), with a Sn/Si ratio of 2/8 (y = 0.2), and δ = 0.35 (x = 0.55), was discovered to have a conductivity of 11 mS cm−1 at 27 °C. Previous studies on the lithium conduction mechanism demonstrated by maximum entropy method (MEM) analysis using neutron diffraction data for LGPS-type materials revealed that one-dimensional (1D) fast lithium diffusion along the c-axis (Li1 and Li3 sites) is an important factor to achieve high conductivity.1,7,10 In addition, lithium diffusion through the ab-plane (Li1 and Li4 sites) is considered to be a key factor for superionic conductivity of >10 mS cm−1 in the LGPS-type structure. Contrary to these highly conducting compositions, the Li-Sn-P-S system, which has relatively low ionic conductivity (5 mS cm–1 at room temperature), shows 1D diffusion behavior even at 800 K.10 Although it is clear that three-dimensional (3D) lithium diffusion contributes to superionic conductivities, the origin (i.e., structural characteristics) of multi-dimensional diffusion has not been elucidated. The lattice parameters and density of the charge carrier (lithium content) may be key aspects here.11 However, these parameters do not directly relate to the lithium diffusion dimension. For example, Li9.81Sn0.81P2.19S1210—with a larger lattice volume and higher lithium content than those of Li9.54Si1.74P1.44S11.7Cl0.31—shows low ionic conductivity with 1D diffusion behavior. Recent systematic structural analysis of the Li-Ge-Sn-P-S system based on Rietveld refinements revealed why Sn substitution suppressed the ionic conductivity.12 Thus, the local structure analysis with respect to factors such as the bond length and bottleneck size could reveal the underlying relationship, contributing to the understanding of the origin of the superionic

conducting characteristics. In this study, MEM analysis of representative compositions of the Li-Sn-Si-P-S system with different ionic conductivities was performed using high-resolution neutron powder diffraction data to elucidate the relationship between the conductivity and crystal structures. Further, the crystal structure parameters are systematically compared with those of the reported LGPS-type materials. Finally, the structural characteristics of the LGPS-type structure that enable 3D lithium diffusion are revealed.

2. EXPERIMENTAL SECTION Three representative materials with the LGPS-type structure belonging to the Li-Sn-Si-P-S system were fabricated based on a previous report:9 (i) a Sn-rich composition with relatively high ionic conductivity, (ii) a Si-rich composition with higher ionic conductivity, and (iii) the Li-Sn-Si-P-S system with the highest ionic conductivity. Hence, the absolute value of the ionic conductivity at room temperature increases from system (i) to (iii). Precise compositions and reported ionic conductivities of the prepared samples are summarized in Table 1. The starting materials, Li2S (Nihon Kagaku Kogyo, >99.9% purity), SiS2 (Mitsuwa Chemical, 99.9% purity), SnS2 (Kojundo Chemical Laboratory Co., Ltd., >99.9% purity), and P2S5 (Aldrich, >99.9% purity), were weighted in stoichiometric ratios in an Ar-filled glove box, and then ground with an agate pestle and mortar. The mixed powder was then mechanically milled for 20 h at 380 rpm using a planetary ball milling apparatus in a ZrO2 pot with ZrO2 balls ( = 10 mm). The product mixture was pelletized under 120 MPa pressure and sealed in a quartz tube under 10 Pa. Then, the quartz tube was heated at 823 K for 24 h, and the sample was slowly cooled to room temperature. The phase of the prepared samples was identified by X-ray diffraction (XRD) of the powdered samples performed using an X-ray diffractometer (Rigaku, SmartLab) with Cu Kα radiation. Crystal structures of the samples were refined using time-of-flight neutron powder diffraction data collected using the BL09 SPICA special environment neutron powder diffractometer at the Material and Life Science Experimental Facility of the Japan Proton Accelerator Research Complex. Ground powder samples were sealed in 6-mm-diameter vanadium cells under Ar using an indium ring. Rietveld refinements of the obtained data were conducted using the Z-Rietveld program.13 Nuclear density distributions were calculated by MEM analysis using crystal structure factors and standard deviations obtained by Rietveld refinement. All the MEM calculations were performed using the Z-MEM algorithm in the Z-Code software package,14 which employs the conventional Sakata–Sato algorithm with zeroth-order single-pixel approximation.15 The Z-three-dimensional algorithm was used to generate nuclear density maps of the structures.16 The MEM analysis data were visualized with the VESTA software package.17


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Table 1. Composition and ionic conductivity of the solid electrolytes synthesized in this study , y, x in Sample No.

: pressed

Li10+δ[SnySi1–y]1+δP2–δS12

pellet

(Li4–x[SnySi1–y]1–xPxS4)

(mS cm−1)

(i)

 = 0.2, y = 0.8, x = 0.6

2.50

(ii)

 = 0.5, y = 0.2, x = 0.5

2.93

(iii)

 = 0.35, y = 0.2, x = 0.55

3.31

measurements and/or the wait time before the measurements (including sample transport from glove box to J-Parc facility). Therefore, a two-phase model including LGPS and -Li3PO4 phases was applied for the Rietveld analysis in this study. Since the main diffraction patterns for all the compositions can be indexed to the space group of P42/nmc (No. 137), the LGPS-type phase formation is confirmed.2

3. RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of the samples. Since the main peaks in all the diffraction patterns can be indexed to the space group of P42/nmc (137),2 LGPS-type isostructural phase formation is confirmed. This is consistent with our previous reports on the Li-Sn-Si-P-S system.9 The obtained samples were then subjected to neutron diffraction.

Fig. 1 XRD patterns of Li10+δ[SnySi1–y]1+δP2–δS12 (Li4–x[SnySi1– y]1–xPxS4): (i) δ (x) = 0.20 (0.6), y = 0.8, (ii) δ (x) = 0.50 (0.5), y = 0.2, and (iii) δ (x) = 0.35 (0.55), y = 0.2 obtained with CuKα radiation. XRD data of the original LGPS (ICSD: #248307) are illustrated at the bottom.

Figure 2 shows the Rietveld refinement results of the neutron diffraction data. Although the XRD patterns obtained using Cu Kα radiation displayed mono-phasic characteristics, an impurity -Li3PO4 phase was observed in the neutron diffraction data of all the samples. In addition, a tiny amount of an unknown impurity phase was observed for samples (i) and (ii). Similar phenomena have been previously reported for LGPS-type Li-Si-P-S and Li-Sn-P-S systems.10 These impurities are likely to arise from reactions of the prepared Li-Sn-Si-P-S samples with water from ambient air, during the neutron diffraction

Fig. 2 Neutron Rietveld refinement results of Li10+δ[SnySi1– y]1+δP2–δS12 (Li4–x[SnySi1–y]1–xPxS4); (i) δ (x) = 0.20 (0.6), y = 0.8, (ii) δ (x) = 0.50 (0.5), y = 0.2, and (iii) δ (x) = 0.35 (0.55), y = 0.2. Observed, calculated, and residual differences are shown as red crosses, light blue lines, and dark blue lines, respectively. Green and blue tick marks indicate the positions of the Bragg reflections for LGPS and -Li3PO4 phases, respectively. The black line indicates the background curves for fitting.



The initial structural parameters from the X-ray Rietveld refinement results were applied for fitting the obtained data.9 The structural model for the fitting is depicted in Figure 3. In the polyhedral framework, Sn and Si atoms share the 4d position with P1 atoms forming the [Sn/Si/P1](4d)S4 tetrahedra. These tetrahedra form 1D-chains with Li2(4d)S6 octahedra by edge-sharing along the c-axis. The P2(2b)S4 tetrahedra are connected to the four Li2(4d)S6 octahedra in the 1D chains by corner sharing, resulting in the formation of the 3D framework. Three partially occupied Li1(16h), Li3(8f), and Li4(4c) positions, which could contribute to ion conduction, are located in the framework. Li1(16h)S4 and Li3(8f)S4 tetrahedra are arranged by edge sharing to form a 1D diffusion pathway (i.e., –Li1–Li1–Li3–Li1–). On the other hand, Li4(4c)S6 distorted octahedra exist between Li1(16h)S4 tetrahedra in the 1D-diffusion pathway. Two-dimensional diffusion pathways in the ab-plane are provided through the Li4 site (i.e., –Li1–Li4–Li1–). The obtained structural parameters are summarized in Table S1-3.

distribution maps for all the samples with a minimum iso-surface level of –0.06 fm Å–3 (the negative portion of the scattering amplitude), continuous lithium distribution through sites Li1 and Li3 was clearly observed, indicating fast lithium diffusion along the c-axis, as previously found both experimentally and computationally.1,7,10,18 The large atomic displacement parameters of the Li1 and Li3 sites could correspond to the lithium distributions. This distinctive characteristic is similar to that observed in superionic conductors such as -AgI and Rb4Cu16I7.2Cl12.8,19-20 which respectively show silver and copper ion conductivities of over 100 mS cm−1. For sample (iii), which showed the highest ionic conductivity, an additional lithium distribution connecting the sites Li1 and Li4 sites was noted, whereas the lithium distributions of the 1D chain and Li4 sites were isolated for samples (i) and (ii). In sample (iii), 3D lithium diffusion may be activated, which contributes to its high conductivity. These tendencies visualized by the MEM analysis basically follow the conductive properties of the samples; higher ionic conductivity is achieved through fast lithium diffusion along c-axis and 3D diffusion involving the ab-plane.

Fig. 4 Nuclear distributions of Li atoms in the LGPS-type Li10+δ[SnySi1–y]1+δP2–δS12 (Li4–x[SnySi1–y]1–xPxS4) unit cell at 300 K. The equi-contour surfaces of the lithium nuclear density distribution appear in yellow. Contour maps for slices are shown for (0 0.75 0) (top) and at (001) (bottom).

Fig. 3 Crystal structure model of the LGPS-type Li10+δ[SnySi1– (Li4–x[SnySi1–y]1–xPxS4) used for the Rietveld analysis. Polyhedral framework and lithium diffusion pathway are illustrated at the top and bottom, respectively. Merged structure of the framework and lithium units are depicted in the middle. Blue dotted lines depict the size of the single unit cell.

y]1+δP2–δS12

All the fittings for this structural model showed good reliability values. MEM analysis was then conducted to visualize the nuclear scattering density distribution in the crystal structure, as illustrated in Figure 4. In the lithium

The MEM analysis using the results of the Rietveld approach confirmed the ionic conductive property. Subsequently, the obtained structural parameters, including the reported structural information for the LGPS-type materials, were analyzed. We separate the structural characteristics into two categories: c-axis lithium diffusion and ab-plane lithium diffusion. Data reported on Li10.05Ge1.05P1.95S12,7 Li9.81Sn0.81P2.19S12,10,11 and 1 Li9.54Si1.74P1.44S11.7Cl0.3 are considered as references for the comparison of the structural parameters. The compositions and ionic conductivities for the compared materials are listed in Table 2.


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Table 2. Composition, ionic conductivity, dimensions of lithium distribution, and temperature of LGPS-type solid electrolytes

Composition

ion: sintered pellet (pressed pellet) [mS cm−1]

Li10.05Ge1.05P1.95S12

100* (–)

3D

750

Li9.81Sn0.81P2.19S12

100* (–)

1D

800

Li9.54Si1.74P1.44S11.7Cl0.3

25.3 (14.0)

3D

300

Li10.2Sn0.96Si0.24P1.8S12 [sample (i)]

– (2.5)

1D

300

Li10.5Sn0.3Si1.2P1.5S12 [sample (ii)]

8.9 (2.9)

1D

300

Li10.35Sn0.27Si1.08P1.65S12 [sample (iii)]

11.0 (3.3)

3D

300

*Values predicted by extrapolation from previous reports

Fig. 5 Crystal structure model of the LGPS related to lithium diffusion: c-axis diffusion through sites Li1 and Li3 (a) and ab-plane diffusion through sites Li1 and Li4 (b). Interatomic distances of Li1–Li1, S3–S3, and Li1–Li4, and triangle bottleneck are indicated.

To consider the c-axis lithium diffusion, interatomic distances of Li1–Li1 and S3–S312 were selected as the parameters (Figure 5(a)). These parameters for various LGPS-type materials are illustrated in Figure 6. Each value of the parameter is plotted as a function of the lattice volume. A decrease in the Li1–Li1 distance is observed for the Li10.05Ge1.05P1.95S12 and Li9.81Sn0.81P2.19S12 systems with increasing temperature (i.e., increase in conductivity) while the lattice volume expands simultaneously.7 Moreover, the highly conductive Li9.54Si1.74P1.44S11.7Cl0.3 has a relatively small Li1–Li1 distance (~1.5 Å). Although the lattice contraction may contribute to this parameter in the Li-Sn-Si-P-S system, similar changes were confirmed; the

Li1–Li1 distance decreased with an increase in the conductivity. This change in the parameter corresponds to the high probability of Li1 being located at the center of two Li1 sites (i.e., not localized at the Li1S4 tetrahedral site). It seems that the coordination of sulfur with Li1 transforms it from a four-coordinated to a six-coordinated state (Figure S1). This significant deviation from the original position for a charge carrier could appear in superionic conductors.19,20 In addition, computational studies support concerted migration in an LGPS crystal containing this kind of unusual (high-energy) lithium location.21 Therefore, this parameter may be a good indicator of high lithium diffusion along the c-axis. The interatomic S3–S3 distance has been proposed as another indicator of fast lithium diffusion.12 This trend is also confirmed by comparison with Figure 6(b). A larger S3–S3 value is confirmed for higher conductivity in Li10.05Ge1.05P1.95S12 and Li9.81Sn0.81P2.19S12, whereas no serious change in this value is observed with increasing conductivity in the Li-Sn-Si-P-S system. The additional lithium diffusivity in the ab-plane, which is a key factor for superionic conductivity greater than 10 mS cm–1, was also verified. One simple parameter is the bottleneck size for diffusion, as indicated in Figure 5(b). The triangle, which provides a diffusion path between the Li1S4 tetrahedra and Li4S6 octahedron (formed by S1–S2– S3), is investigated. The Li10.05Ge1.05P1.95S12 and Li-Sn-Si-P-S systems show that 3D lithium diffusion occurs with bottleneck size expansion. Further, a relatively large size of 6.9 Å2 is confirmed for Li9.54Si1.74P1.44S11.7Cl0.3 with 3D lithium distribution at 300 K. However, Li9.81Sn0.81P2.19S12 at 800 K also shows a large size (over 6.9 Å2), whereas 3D lithium distribution was not confirmed for this composition by MEM analysis.10 To consider this point, an additional parameter, the interatomic distance of Li1–Li4, was examined. In the crystal structure, Li1upper–Li4 is shorter than Li1lower–Li4. Except for Li9.81Sn0.81P2.19S12, the Li1–Li4 values exist in the range of 3.0 to 3.4 Å. A small difference of less than the ionic radius of Li+ (0.6 Å; coordination number (C.N.) = 4) was confirmed. Therefore, these two Li1 sites can be considered as quasi-equivalent sites to the adjacent Li4 site. In contrast, Li9.81Sn0.81P2.19S12 at 800 K shows characteristic values of 2.86 and 3.53 Å, indicating the large difference between the upper and lower Li1 sites. Thus, the two Li1 sites are not quasi-equivalent to the neighboring site. In this case, lithium diffusion through sites Li1 and Li4 could be unfavorable, resulting in the negligible lithium distribution via the ab-plane. In other words, a large bottleneck size of ca. 6.9 Å2 and quasi-equivalent Li1 sites to the adjacent Li4 site are required for 3D lithium distribution. These structural characteristics, bottleneck size, and Li1–Li4 distance may be the indicators of the feasibility of 3D lithium diffusion in the LGPS-type crystal structure.

4. CONCLUSIONS Superionic conductors with the LGPS-type structure were investigated. Rietveld refinements using neutron diffraction data were conducted for three representative



compositions of the Li10+δ[SnySi1–y]1+δP2–δS12 (Li4– x[SnySi1−y]1–xPxS4) system. The obtained materials had different conducting properties and showed a clear relation between the conductivities and dimensions of lithium diffusion pathways. The 3D lithium distribution of the composition with the highest conductivity, Li10.35Sn0.27Si1.08P1.65S12 (Li3.45[Sn0.09Si0.36]P0.55S4), was visualized by MEM analysis. Therefore, multi-dimensional lithium diffusion is suggested to be a key factor for superionic conductivity greater than 10 mS cm–1. The structural analysis revealed that (i) shorter Li1–Li1 and larger S3–S3 distances are important factors for 1D lithium diffusion along the c-axis, and (ii) larger bottleneck size and two quasi-equivalent Li1 sites to the neighboring site could contribute to the appearance of 3D lithium diffusion for superion-conducting LGPS-type materials.

Fig. 6 Crystal structure parameters of LGPS-type materials: interatomic distances of Li1–Li1 (a) and S3–S3 (b), bottleneck size (c), and interatomic distance of Li1–Li4 (d).  Li-Ge-P-S: Li10.05Ge1.05P1.95S12;  Li-Sn-P-S: Li9.81Sn0.81P2.19S12, □ Li-Si-P-S-Cl: Li9.54Si1.74P1.44S11.7Cl0.3, ○ Li-Sn-Si-P-S (i): Li10.2Sn0.96Si0.24P1.8S12, (ii): Li10.5Sn0.3Si1.2P1.5S12, and (iii): Li10.35Sn0.27Si1.08P1.65S12.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Rietveld refinement results and crystal structure model (PDF)


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AUTHOR INFORMATION Corresponding Author *(R.K.). E-mail: [email protected]

Author Contributions All authors have approved the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was partly supported by a Grant-in-Aid for the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST) and a Grant-in-Aid for Scientific Research (S) (No. 17H06145) of the Japan Society for the Promotion of Science. The neutron scattering experiment was approved by the Neutron Scattering Program Advisory Committee of IMSS, KEK (Proposal No. 2014S10).

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Conduction mechanism of Li10GeP2S12-type lithium superionic conductors in a Li-Sn-Si-P-S system


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