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Weak Anisotropic Lithium-Ion Conductivity in Single Crystals of Li10GeP2S12 Rui Iwasaki, Satoshi Hori, Ryoji Kanno, Takeshi Yajima, Daigorou Hirai, Yuki Kato, and Zenji Hiroi Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00420 • Publication Date (Web): 29 Apr 2019 Downloaded from http://pubs.acs.org on May 1, 2019
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Chemistry of Materials 2019/4/27 to be submitted to Chem. Mater.
Weak Anisotropic Lithium-Ion Conductivity in Single Crystals of Li10GeP2S12 Rui Iwasaki,† Satoshi Hori,‡ Ryoji Kanno,‡ Takeshi Yajima,† Daigorou Hirai,† Yuki Kato, §,‡ and Zenji Hiroi*,† †
Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan Institute of Innovative Research (IIR), All-Solid-State Battery Unit, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama 226-8502, Japan. § Advanced Material Engineering Div. Higashifuji Technical Center, Toyota Motor Corporation. 1200 Mishuku, Susono, Shizuoka, 410-1193, Japan ‡
ABSTRACT: Single crystals of the lithium-ion conductor Li10GeP2S12 have been successfully grown by the self-flux method and are studied by means of X-ray diffraction and impedance spectroscopy. The weak anisotropic ionic conductivity is observed to be 27 and 7 mS cm–1 in the [001] and [110] directions, respectively, at room temperature. Markedly, however, the activation energies are nearly equal, approximately 0.3 eV, in the two directions, which means that common diffusion paths along [110] dominate the long-range diffusion in Li10GeP2S12. 1. INTRODUCTION The all-solid-state lithium-ion battery has been studied extensively as a next-generation secondary battery.1,2 It uses a solid instead of a liquid electrolyte, which enables to achieve greater safety, higher energy density, wider temperature range of operation, and larger output power compared with the conventional battery. To realize this, an efficient Li-ion conductor is required, which exhibits conductivity that is either comparable to or larger than that of commercially-used organic liquid electrolytes. Li10GeP2S12 (LGPS) is a promising candidate for the super ionic conductor; compared to other candidates, it has the highest conductivity of 12 mS cm–1 at room temperature and an outstanding electrochemical performance in Li batteries.1 An even higher conductivity of 25 mS cm–1 is achieved in the related compound Li9.54Si1.74P1.44S11.7Cl0.3.2 Thus, LGPS is a key compound in the materialization of the all-solid-state battery. The high Li-ion conductivity of LGPS has been ascribed to its specific crystal structure shown in Fig. 1a. There are four partially occupied Li sites inside the rigid framework composed of the tetrahedral units of Ge0.5P0.5S4 and PS4.1,3-6 Among them, the Li1, Li3, and Li4 sites are located in the distorted tetrahedra of S, while the Li2 site is in the distorted octahedron of S. It has been shown that the Li diffusion between the former sites is crucial for the high conductivity, while the Li2 ions are almost immobile.4,7 A highly anisotropic ion diffusion has been proposed based on neutron diffraction (ND) measurements1,8 and molecular dynamics calculations:9,10 a one-dimensional (1D) channel via the Li1–Li3 path along the c axis is dominant, and a two-dimensional (2D) hopping via the Li1–Li4 path in the direction is secondary.11 The activation energies of the local hopping processes for the 1D and 2D paths are determined as 0.16 (0.17) and 0.26 (0.28) eV from the NMR experiments3,11 (the molecular dynamics calculations9,10), respectively. Thus, considerable anisotropy can be expected in the atomic-scale diffusion process. In contrast, pulsed-field gradient NMR measurements, which may be sensitive to long-range Li diffusion at the micrometer scale, suggests a nearly isotropic Li diffusion.5 An impedance measurement using a single crystal provides important information with which to clarify the origin of this
difference and the mechanism of ion diffusion in LGPS. However, because of the lack of a sufficiently large single crystal, the Li diffusion in the bulk had not been measured thus far. Previous impedance measurements using polycrystalline samples yielded an average conductivity of ~10 mS cm–1 and activation energy of 0.22–0.28 eV.1,3,5 Understanding of the true conduction mechanism in LGPS is crucial for the improvement of LGPS and for further development of a better solid electrolyte for the all-solid-state battery. In the present study, we obtain large single crystals of LGPS, a few millimeters in size, using the self-flux method, and carry out impedance measurements in the [001] and [110] directions. The conductivity is, in fact, higher along [001] than along [110]; however, the difference is only a factor of four. In addition, the activation energies are nearly equal for the two directions, suggesting a weak anisotropic 3D diffusion in LGPS.
Figure 1. (a) Schematic crystal structure of Li10GeP2S12 reported in the previous studies1 and (b) the thermal ellipsoids of four Li sites from the present structural refinement using the X-ray diffraction data taken at room temperature on a single crystal. The Li3 site is single in (a) and split in (b).
2. EXPERIMENTAL Synthesis. Single crystals of LGPS, such as those shown in Fig. 2(c), were grown in a quartz tube by the self-flux method, the details of which are described in Section 3. In the following
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quenching temperature was chosen to avoid the contamination of the Li3PS4-type β phase, with consideration of the accuracy of the phase diagram. Our practical experimental procedure is as follows. Li2S, Ge, P, and S were mixed and pelletized in a molar ratio of 4.72:0.45:2.55:12 (δ = –0.55) in an Ar-filled grove box; the S content was increased by 0.5%, taking into account the loss due to evaporation or reaction with the quartz tube at elevated temperatures. The pellet was placed in an evacuated quartz ampoule 10 mm in diameter and 100 mm in length, together with glass wool. As depicted in Fig. 2(b), the ampoule was heated at 400 ºC for 8 h and then at 670 ºC for 1 h to obtain a complete melt. Then, the ampoule was cooled slowly to 580 ºC after fast cooling to 650 ºC. The cooling rate was 0.5–7 ºC h–1; a rate of 1 ºC h–1 may be ideal to obtain high-quality crystals. Finally, the ampoule was quenched from 580 ºC: the hot ampoule was quickly placed in a centrifuge (KOKUSAN H-19a) at room temperature, so that only the melt was moved to the other side of the ampoule through the glass wool under centrifugal force of ~1,000 gravity to separate the crystals from the melt. Crystals grew on the wall of ampoule or became lodged in the glass wool.
experiments, special care was taken not to expose the crystals to air, as they are hygroscopic and unstable in moist conditions. X-ray diffraction measurements. For the powder X-ray diffraction (XRD) measurements, an aggregate of crystals was ground into a powder and placed in a sealed capillary tube of 0.3 mm in diameter in an Ar-filled grove box. Data were collected at 300 K in a Rigaku RINT-2000 diffractometer using Cu–Kα radiation. For the single-crystal XRD experiments, a cuboid crystal with a size of 0.19 × 0.20 × 0.22 mm3 was selected and sealed into a capillary 0.4 mm in diameter in the glove box. A Rigaku R-AXIS RAPID diffractometer with graphite-monochromated Mo–Kα radiation was used for data collection and also to determine the orientation of the crystal for impedance measurements. Structural analyses were performed by the direct method using the Jana2006 program.12 Chemical analysis. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was used to decide the chemical composition of the crystals. A total of 1–3 mg of crystals was dissolved into water, and the amounts of Li, Ge, and P were determined. Ionic conductivity. The Li-ion conductivity was measured using the impedance method. A few millimeter-size crystal was selected and shaped into a cuboid with the long axis in the [001] or [110] direction [Figs. 4(a) and (b), respectively]. A gold powder dispersed in hexyl acetate was deposited to the crystal surface as electrodes; commercial gold paste caused a degradation of the crystal. The crystal was then covered with Araldite and placed in a homemade airtight measurement cell. AC impedance measurements were carried out using the auto balancing bridge method with an impedance analyzer (Agilent 4294A) in the wide frequency range of 100 Hz – 110 MHz. The measurements were carried out upon heating after cooling to the liquid nitrogen temperature. 3. RESULTS Growth and characterization of LGPS crystals. LGPS forms a solid solution Li10+δGe1+δP2–δS12 with 0 ≤ δ ≤ 0.7 in the pseudo-binary phase diagram of Li4GeS4–Li3PS4,6 which is reproduced in Fig. 2(a). Upon heating, Li10GeP2S12 at the ideal composition (δ = 0) partially melts at 550 ºC and decomposes incongruently to the Li4GeS4-type β’ phase above 650 ºC. Upon cooling from a high-temperature melt, the β’ phase first precipitates as a primary crystal, and the formation of a LGPS crystal is limited because of the kinematically slow solid-to-solid transformation. Thus, it is difficult to obtain a large crystal of LGPS by slow cooling. In fact, only a small crystal was grown and used for the single-crystal XRD measurement in the previous study.13 In contrast, there is a composition window at –0.85 < δ < –0.4 in the pseudo-binary phase diagram, where LGPS forms as a primary crystal from the melt. Thus, we have a chance to grow a large LGPS crystal in the Li3PS4-rich composition, using the self-flux method. A composition of δ = –0.55 was employed in the present experiment. As depicted schematically in Fig. 2(a), precipitation of LGPS crystals occurs at 630 ºC upon cooling of the melt, and their growth at the expense of melt reaches δ ~ 0.2 at 580 ºC before being quenching to room temperature; the 2
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Max. 2θ
27.46°
Index range
–11 ≤ h ≤ 10 –11 ≤ k ≤ 10 –16 ≤ l ≤ 16
The obtained LGPS crystals are yellow in color and are a maximum of a few millimeters in size with a (001) or (110) habit, as shown in Fig. 2(c); there was no tendency for cleavage. Chemical analyses by ICP-AES yielded an average composition of Li:Ge:P = 10:1:2 (δ = 0), but the accuracy was low; δ = 0.11 was estimated from the structural refinement process mentioned in the next paragraph. In some preparation trials, we obtained brownish or reddish crystals, which may be due to a small deviation in composition that generates impurity states or reduces the effective band gap. We used yellow crystals for the following measurements. Table 1. Crystallographic parameters for Li10.11Ge1.11P1.89S12 determined by means of single-crystal XRD. Li10.11Ge1.11P1.89S12 293 K
Space group
P42/nmc
Lattice constants
a = 8.6640(2) Å c = 12.5830(5) Å
Cell volume
944.54(5) Å3
Calc. density
2.0698 g cm–3
Radiation
Mo Kα
8739
Unique reflections
614
Reflection used (I > 3σ)
526
R1 (>3σ)
2.42%
wR2 (>3σ)
3.52%
Structural refinements. All the diffraction peaks in a powder XRD pattern from crushed crystals were assigned to a LGPS phase with a = 8.6803(15) Å and c = 12.617(3) Å. Thus, there were neither inclusions nor impurity phases in the product. The a-axis length is slightly smaller and the c-axis length is nearly equal compared with those reported for δ = 0.11: a = 8.705 Å and c = 12.62 Å.3 Our single-crystal XRD experiment yields a = 8.6640(2) Å and c = 12.5830(5) Å. The results of the single-crystal XRD analyses are summarized in Tables 1 and 2. The positions of Ge, P, and S are determined in the initial stage using the direct method, and those of Li are determined via difference Fourier syntheses. The refinement has converged to reasonably small R factors: R1 = 2.42% and wR2 = 3.52% (model 1). The overall features are the same as those of the previous structural model; however, a critical difference is found at the Li3 site: there is a splitting of the Li3 site from the 8f to 16h Wyckoff position, as shown in Fig. 1(b). An alternative analysis assuming a single Li3 site (model 2) yields slightly larger R values: R1 = 2.44% and wR2 = 3.58%. It should be noted in the latter analysis that the thermal ellipsoid of Li3 is unusually elongated, suggesting a tendency toward a splitting. Such an elongated thermal ellipsoid of Li3 was observed in previous studies at room temperature or at 700–800 K.3,4,13 To confirm the splitting of the Li3 site, neutron diffraction experiments using a 7 Li-sbstituted crystal are in progress. We focus on the site occupancy g for each Li site. The gs of the P2, S1, S2, and S3 sites are fixed to one as they were converged well to unity in the initial refinement. For the Ge1/P1 site, the occupancies of Ge and P are determined to be 0.555(4) and 0.445(4), respectively, assuming no vacancy. This corresponds to δ = 0.11, which is reasonably close to δ ~ 0.2 expected from the growth condition. Finally, the gs of all the Li sites are refined assuming charge neutrality, which yields gs listed in Table 3 and the chemical formula of Li10.11Ge1.11P1.89S12.
Figure 2. (a) Li4GeS4–Li3PS4 pseudo-binary phase diagram;6 β': Li4(Ge,P)S4, LGPS: Li10+δGe1+δP2–δS12, β: β-Li3(P,Ge)S4, L: liquid. The blue arrows represent the employed cooling process starting from a liquid with δ = –0.55, which is also schematically depicted in (b). The inset of (b) shows a photograph of a quartz tube containing LGPS crystals after the heat treatment. (c) Photograph of typical LGPS crystals of a few millimeters in size.
Temperature
Total reflections
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Table 2. Atomic coordinates and anisotropic displacement parameters (Å2 ) for Li10.11 Ge1.11P1.89S12 Atom
Site
x
y
z
U11
U22
U33
U12
U13
U23
Li1
16h
0.264(2)
0.2661(19)
0.1884(19)
0.080(10)
0.056 (9)
0.27(3)
0.017(8)
0.001(16)
0.0018(14)
Li2
4d
0
1/2
0.9473(8)
0.070(8)
0.078(9)
0.026(6)
0
0
0
Li3
16h
0.227(4)
0.261(3)
0.9653(18)
0.14(2)
0.060(11)
0.15(4)
–0.036(14)
0.09(2)
–0.040(16)
Li4
4c
1/2
1/2
0.2455(14)
0.061(11)
0.20(3)
0.056(12)
0
0
0
Ge1/P1
4d
0
1/2
0.69091(5)
0.0334(4)
0.0275(3)
0.0198(3)
0
0
0
P2
2a
0
0
1/2
0.0263(4)
0.0263(4)
0.0226(7)
0
0
0
S1
8g
0
0.18810(10)
0.40780(7)
0.0644(6)
0.0314(4)
0.0427(5)
0
0
0.0057(4)
S2
8g
0
0.29477(10)
0.09678(7)
0.0381(4)
0.0416(5)
0.0304(4)
0
0
–0.0021(3)
S3
8g
0
0.69978(9)
0.79128(6)
0.0428(5)
0.0314(4)
0.0315(4)
0
0
–0.0002(3)
Table 3. Occupancies g of the four Li sites and the composition of the Ge1/P1 site obtained in the present study assuming two kinds of model and from the previous studies by means of powder neutron diffraction (PND) and single-crystal XRD (SXRD). Models 1 and 2 assume split sites and a single site for Li3, respectively: the former model is likely more realistic. δ is the composition parameter in the general formula Li10+δGe1+δP2–δS12 . Model 1 Model 2 Kwon3 Kuhn13 Weber4 Atom
Site
g
Site
g
g
g
g
Li1
16h
0.479(18)
16h
0.464(18)
0.473(11)
0.466(18)
0.50(1)
Li2
4d
0.84(4)
4d
0.86(4)
1
0.86(6)
0.54(1)
Li3
16h
0.399(16)
8f
0.84(3)
0.75(2)
0.74(5)
0.62(3)
Li4
4c
0.71(10)
4c
0.67(11)
0.77(2)
0.81(7)
0.68(3)
Ge1/P1
4d
0.555(4)/0.445(4)
4d
0.559(4)/0.441(4)
0.675(12)/0.325(12)
0.5/0.5
0.5/0.5
Note
split Li3 site, δ = 0.11
PND, δ = 0.35
SXRD, δ = 0 fixed
PND, δ = 0 fixed
single Li3 site, δ = 0.12
capacitances obtained from the fits are 0.8–1 pF and 0.2–1.6 nF for the bulk and electrode, respectively.
The Li1 and Li3 sites have small g values of less than half, whereas the Li2 and Li4 sites have 20–30% vacancies. Thus, the presence the 1D Li1–Li3 path is likely, as reported previously. The small occupancies of Li1 and Li3 indicate that the nearby 16h sites are not occupied simultaneously for each case, which is also evidenced by the short distances of 1.59(3) Å and 0.97(3) Å for the L1 and Li3 sites, respectively. These split sites may cause a cooperative hopping as proposed in the concerted migration mechanism14 and can enhance the ion conductivity along the c axis. Table 3 shows comparisons with previous results by means of powder ND for δ = 0.35 by Kwon et al.,3 single-crystal XRD assuming δ = 0 by Kuhn et al.13, and powder ND assuming δ = 0 by Weber et al.4 They are basically similar to each other, but some differences are found particularly for the Li2 and Li3 sites. Ionic conductivity. In general, the ion transport in an ionic conductor contains contributions from the bulk, grain boundary and electrode, which can be distinguished by impedance spectroscopy over a range of frequencies. Figure 3 shows a typical Cole–Cole plot measured at 214 K in the [001] direction. The semicircle at high frequencies expected from a bulk contribution is partly observed, and it is followed by a sharp rise at low frequencies, which may be due to the contribution of the electrodes. Thus, the data are fitted into an equivalent circuit consisting of a constant phase element (CPE) in series with two parallel CPE/resistor components shown in the inset. The
Figure 3. Typical impedance response from a LGPS crystal measured at 214 K with the electric field in the [001] direction. The black line on the data points shows a fit to an equivalent circuit consisting of a constant phase element (CPE) in series with the two parallel CPE/resistor components shown in the inset. The blue and green broken lines show
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separate components for the bulk and electrode, respectively.
Figure 4 shows the temperature evolution of the Cole-Cole plots measured in the [001] and [110] directions. In each case, the semicircle shrinks and moves to a high frequency upon heating, and finally disappears near room temperature. Ionic conductivities at 293 K are approximately estimated by extrapolating the data linearly to zero impedance [Fig. 4(c)],15 which yields σ = 27 and 7 mS cm–1 along [001] and [110], respectively. These values are comparable to those from polycrystalline samples; 14 and 5 mS cm–1,3,4 which may include contributions from grain boundaries. The high conductivity along [001] is consistent with the presence of the 1D conduction path, while the not-too-small one along [110] suggests a significant 2D conduction. The observed anisotropy in the bulk ionic conductivity is approximately four, which is considerably smaller than the values of 1.2 and 0.01 mS cm–1 for the typical 2D Li-ion conductor Li3N with parallel and perpendicular to the plane, respectively.16 Figure 4. Temperature evolution of the impedance response for the electric fields parallel to (a) [001] and (b) [110]. The insets in (a) and (b) show the LGPS crystals used for the measurements. The impedance curves at room temperature are shown in (c). The ionic conductivities at room temperature are determined as 27 and 7 mS cm–1 for the [001] and [110] directions, respectively.
Figure 5 shows Arrhenius plots for the two sets of data in Fig. 4. The data for both [001] and [110] show linear behavior at approximately 273 K with similar slopes, maintaining differences of factors of ~4, independent of temperature. Then, at low temperatures below ~250 K, both slopes increase to similar larger values. Note that a similar temperature dependence has been observed for polycrystalline samples, which is suggested to be due to a blocking grain boundary effectively working at low temperatures.5 However, because our crystal must have no grain boundaries, or at least a far lower contribution from them, there may be an alternative, intrinsic reason for the change in the slope. It is likely that a substantial change in the Li diffusion process occurs at this temperature. The activation energies Ea are determined from linear fits to the curves above and below 250 K: 0.29 and 0.39 eV for [001] and 0.30 and 0.40 eV for [110], respectively. Thus, the Ea values are nearly equal in the two directions. The room-temperature values are close to those of the polycrystalline samples, which are scattered at 0.22 eV,5 0.25 eV,1 0.28 eV,3 and 0.35 eV.4 These value are typical for superionic conductors.14
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may be from the singularity of the 1D diffusion, as pointed out by Kuhn et al:5 inevitable in actual crystals are crystalline defects which can generate a large potential barrier in the 1D diffusion path; thus, long-range diffusion occurs by making a lateral detour. Note that the frequency of ionic conductivity in LGPS is ca. 105 Hz: the measured ionic current is the sum of contributions of charge displacements along electric field in Δt < 10–5 sec. In contrast, the time constant of jump events in LGPS is 3 × 10–6 ~ 7 × 10–10 sec,11 which is much shorter than Δt: 30 to 10,000 jump events are included. Thus, it is natural that the long-range migration path includes Li1–Li3 jumps as well as many Li1–Li4 jumps even at E // [001], when there are critical defects in the Li1–Li3 path. To increase the practical conductivity, it is required to decrease the density of the defects and to reduce the bottleneck potential barrier around the Li4 site. The source of the observed weak anisotropy in σ by a factor of four may be ascribed to the small differences in Ea and carrier density: for [001], Ea is slightly smaller, and the carrier density may be slightly larger, judging from the occupancies of g ~0.5 for Li1, 0.6–0.8 for Li3, and 0.7–0.8 for Li4.3,4,13 In addition, a difference in the plane density of the paths is taken into account. As sketched in Fig. 6, there are four [001] paths in the unit cell and four [110] paths perpendicular to the √2ac rectangle: the plane densities are 5.328 and 2.594 nm–2, respectively. Thus, the plane density of [001] is larger by a factor of two.
Figure 5. Arrhenius plots for the ionic conductivities in the [001] and [110] directions.
4. DISCUSSION In the classical case, the basic step in ionic diffusion is the hopping migration of the ion between stable sites through inter-connected diffusion channels with small energy barriers.7,14,15,17 For long-range diffusion, which contributes to the practical ionic conductivity, a mobile ion migrates through the energy landscape, and the highest energy of the energy landscape along the path determines the activation energy Ea. Another factor is the concentration n of mobile ion carriers such as vacancies or interstitials. To achieve high ionic conductivity, a low Ea and a large n are required: σ = neµ0exp(–Ea/kBT) at temperature T. As depicted schematically in Fig. 6, two kinds of diffusion path exist in LGPS: the 1D path composed of Li1 and Li3 along [001], and the 2D path composed of Li1 and Li4 along ; the 2D path via Li2 may be inactive.4,7,9-11 The Ea values in the two paths have been estimated to be 0.16–0.19 and 0.26–0.30 eV for [001] and [110], respectively, from molecular dynamics calculations and the NMR experiments.9-11 Note that these values correspond to local jumps or short-range diffusions. In contrast, our impedance measurements on single crystals show similar Ea values of ~0.3 eV for the two directions, which are close to the local Ea value for [110]. This strongly suggests that a long-range diffusion that governs the bulk conductivity is determined by a bottleneck along [110]. As illustrated in Fig. 6, a meandering conduction involving the Li1–Li4 paths may occur even in the [001] direction, realizing almost 3D diffusion. This is consistent with the nearly isotropic Li diffusion in the medium range of the micrometer scale observed by pulsed-field gradient NMR measurements.5 Another interpretation of Ea ~ 0.3 eV along [001] assumes that there are crystalline defects with a barrier of this Ea along the 1D path: it happens to coincide with the Ea of the 2D path. In either case, both the 1D and 2D paths contribute to the nearly isotropic lithium-ion conductivity in LGPS. The microscopic origin of the meandering path is not clear, but
Figure 6. Schematic drawing illustrating the possible conducting paths of Li ions in LGPS. The bottom-left picture shows the arrangement of the Li1 (red balls), Li3 (blue), and Li4 (green) ions in the unit cell. The vertical right-blue and horizontal yellow stripes represent the Li1–Li3 paths in the [001] direction and the Li1–Li4 paths in the [110] direction, respectively. The meandering green and orange arrows show possible conducting paths for actual long-range conductions along [001] and [110], respectively.
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5. CONCLUSION
(12)
We successfully prepared single crystals, a few millimeters in size, of the Li-ion conductor LGPS. The crystal structure at room temperature was refined by XRD, and shows a tendency toward a splitting of the Li3 site. The impedance measurements reveal that the ionic conductivities at room temperature are 27 and 7 mS cm– 1 in the [001] and [110] directions, respectively, while the activation energies are nearly equal at ~0.3 eV for these directions. Thus, the long-range diffusion in LGPS is weakly anisotropic and may be dominated by the common lateral paths via the Li4 site.
(13) (14) (15) (16) (17)
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AUTHOR INFORMATION Corresponding Authors *(Z.H.) E-mail:
[email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We thank Tohru Kawamoto and Akira Takahashi for their help in the impedance measurements. R. K. acknowledges support by Grant-in-Aid for Scientific Research (S, No. 17H06145). This work was partly supported by the Core-to-Core Program for Advanced Research Networks given by the Japan Society for the Promotion of Science (JSPS). REFERENCES (1)
Kamaya, N.; Homma, K.; Yamakawa, Y.; Hirayama, M.; Kanno, R.; Yonemura, M.; Kamiyama, T.; Kato, Y.; Hama, S.; Kawamoto, K.; Mitsui, A. A lithium superionic conductor. Nat. Mater. 2011, 10, 682–686. (2) Kato, Y.; Hori, S.; Saito, T.; Suzuki, K.; Hirayama, M.; Mitsui, A.; Yonemura, M.; Iba, H.; Kanno, R. High-power all-solid-state batteries using sulfide superionic conductors. Nat. Energy 2016, 1, 16030. (3) Kwon, O.; Hirayama, M.; Suzuki, K.; Kato, Y.; Saito, T.; Yonemura, M.; Kamiyama, T.; Kanno, R. Synthesis, structure, and conduction mechanism of the lithium superionic conductor Li10+dGe1+dP2–dS12. J. Mater. Chem. A 2015, 3, 438–446. (4) Weber, D. A.; Senyshyn, A.; Weldert, K. S.; Wenzel, S.; Zhang, W.; Kaiser, R.; Berendts, S.; Janek, J.; Zeier, W. G. Structural Insights and 3D Diffusion Pathways within the Lithium Superionic Conductor Li10GeP2S12. Chem. Mater. 2016, 28, 5905–5915. (5) Kuhn, A.; Duppel, V.; Lotsch, B. V. Tetragonal Li10GeP2S12 and Li7GePS8 - exploring the Li ion dynamics in LGPS Li electrolytes. Energy Environ. Sci. 2013, 6, 3548–3552. (6) Hori, S.; Kato, M.; Suzuki, K.; Hirayama, M.; Kato, Y.; Kanno, R.; Sprenkle, V. Phase Diagram of the Li4GeS4–Li3PS4 Quasi‐Binary System Containing the Superionic Conductor Li10GeP2S12. J. Am. Ceram. Soc. 2015, 98, 3352–3360. (7) Wang, Y.; Richards, W. D.; Ong, S. P.; Miara, L. J.; Kim, J. C.; Mo, Y.; Ceder, G. Design principles for solid-state lithium superionic conductors. Nat. Mater. 2015, 14, 1026. (8) Hori, S.; Taminato, S.; Suzuki, K.; Hirayama, M.; Kato, Y.; Kanno, R. Structure–property relationships in lithium superionic conductors having a Li10GeP2S12-type structure. Acta Cryst. 2015, B71, 727–736. (9) Mo, Y.; Ong, S. P.; Ceder, G. First Principles Study of the Li10GeP2S12 Lithium Super Ionic Conductor Material. Chem. Mater. 2012, 24, 15-17. (10) Adams, S.; Prasada Rao, R. Structural requirements for fast lithium ion migration in Li10GeP2S12. J. Mater. Chem. 2012, 22, 7687– 7691. (11) Liang, X.; Wang, L.; Jiang, Y.; Wang, J.; Luo, H.; Liu, C.; Feng, J. In-Channel and In-Plane Li Ion Diffusions in the Superionic
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