Article pubs.acs.org/cm
Superionic Conductors: Li10+δ[SnySi1−y]1+δP2−δS12 with a Li10GeP2S12type Structure in the Li3PS4−Li4SnS4−Li4SiS4 Quasi-ternary System Yulong Sun,† Kota Suzuki,†,‡ Satoshi Hori,‡ Masaaki Hirayama,†,‡ and Ryoji Kanno*,†,‡ †
Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, and ‡Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan S Supporting Information *
ABSTRACT: Solid solutions of Sn−Si derivatives with an LGPS (Li10GeP2S12)-type structure are synthesized by a solidstate reaction in the Li3PS4−Li4SnS4−Li4SiS4 quasi-ternary system. The monophasic region of the LGPS-type structure deviates from the tie line between Li10SiP2S12 and Li10SnP2S12, and the composition of the solid solution is determined to be −0.1 ≤ δ ≤ 0.5 and 0 ≤ y ≤ 1.0 in Li10+δ[SnySi1−y]1+δP2−δS12 (0.50 ≤ x ≤ 0.7 and 0 ≤ y ≤ 1.0 in Li4−x[SnySi1−y]1−xPxS4). The solid solution is formed by a double substitution that changes the Sn/Si ratio and the M4+ (Sn4+ and Si4+)/P5+ ratio, which adjusts the sizes of the lithium conduction tunnels and the lithium concentration, and contributes to the optimal conductivity value. The highest ionic conductivity value of 1.1 × 10−2 S cm−1 is achieved for the composition of Li10.35[Sn0.27Si1.08]P1.65S12 (Li3.45[Sn0.09Si0.36]P0.55S4) at 298 K, which is close to the value for the original LGPS compound (1.2 × 10−2 S cm−1). The Ge-free solid electrolyte could be suitable for practical applications in all-solid-state batteries.
1. INTRODUCTION All-solid-state systems with solid electrolytes (SEs) are potential candidates for next-generation batteries and are expected to provide a high power and energy density with reliable and improved safety characteristics.1,2 Sulfide-based lithium-ion conductors have the advantages of high conductivities together with suitable electrochemical windows and mechanical properties;3−5 thus, they are intensively studied as promising SEs.6−8 Li10GeP2S12 (LGPS) is a new member in the crystalline sulfide electrolyte family and exhibits an ionic conductivity of 1.2 × 10−2 S cm−1, which is comparable to that of organic liquid electrolytes. The all-solid-state battery, LiCoO2/LGPS/In−Li, showed excellent charge−discharge characteristics using the LGPS electrolyte.2,9 However, Ge is a relatively expensive element and could limit the widespread use of LGPS materials. The type of crystal structure is an important component in the design of ionic conductors. Materials with similar types of structures and high ionic conducting solids might provide high conducting characteristics. The LGPS-type structure is suitable for high ionic diffusion along the one-dimensional tunnels and/ or two-dimensional planes that participate in high ionic diffusion.2,10,11 Si- and Sn-based analogous with Ge-free materials might be promising as SEs for practical applications. The substituents belonging to the LGPS-family have been studied by first-principles calculations12 and materials synthesis experiments, and several LGPS-type derivatives (Li11Si2PS12, © 2017 American Chemical Society
Li10SiP2S12, and Li10SnP2S12) and solid solutions of Li10(Ge− Sn)P 2 S 12 and Li 10 (Ge−Si)P 2 S 12 were synthesized. 13−16 Although a high ionic conductivity was expected for these substituted systems, these derivatives showed lower conductivities than the original LGPS; for example, Sn-based analogues had a conductivity value of 5 × 10−3 S cm−1 at room temperature, which was explained by less Li in the unit cell.15 The present study focuses on Sn and Si substitution for Ge in the LGPS structure, which contains cheap and earth-abundant elements. The LGPS-type Si, Sn, and Ge systems have shown a range of solid solutions described by the composition, Li10+δM1+δP2−δS12, with different Li/M/P ratios (M = Si, Ge, Sn); the monophasic region of the phase is dependent on the M element.10,15 A complete materials search for the Li3PS4− Li4SnS4−Li4SiS4 quasi-ternary system is necessary to determine the compositional region of the LGPS-type phase and improve the ionic conductivity of these solid solutions. This might also adjust the sizes of the lithium conduction tunnels and the lithium concentration contribution to the optimal ionic conduction. Both Sn/Si and M4+ (Sn4+ and Si4+)/P5+ ratios (the ratio of three kinds of atoms, Sn/Si/P, at P(4d) positions in the structure) were adjusted, and the formation region of the LGPS phase was determined according to the composition of Received: March 3, 2017 Revised: June 27, 2017 Published: July 10, 2017 5858
DOI: 10.1021/acs.chemmater.7b00886 Chem. Mater. 2017, 29, 5858−5864
Article
Chemistry of Materials
Al mesh and Al foil current collectors were pressed onto opposite sides of the SE pellet. The electrochemical properties of the cells were evaluated using a TOSCAT-3100 (Toyo System). A cycling test was performed between 1.9 and 3.6 V vs an In−Li anode (≈ 0.6 V vs Li/ Li+) at an applied current of 7 mA g−1 at 298 K.
Li10+δ[SnySi1−y]1+δP2−δS12. Their crystal structures and ionic conductivities were investigated, and the relationship between the lattice parameters, lithium concentration in the structure, and the ionic conductivity is discussed. For comparison to the original Li10GeP2S12, the formula could be written as Li10+δ[SnySi1−y]1+δP2−δS12; in this formula, the original Li10GeP2S12 phase is represented by δ = 0 in Li 10+δM 1+δ P 2−δ S 12 , which corresponds to x = 0.67 in Li4−xM1−xPxS4.10 However, to consider the phase diagrams of the Li2S−MS2−P2S5 system, the latter formula (Li4−xM1−xPxS4), which indicates the tie line between Li4MS4−Li3PS4, is appropriate for a materials search.3 Therefore, the compositions are also represented by Li4−x[SnySi1−y]1−xPxS4 to describe the Sn, Si, and P ratio along with the LGPS formula of Li10+δ[SnySi1−y]1+δP2−δS12. In addition, the relations in these formulas are summarized in Table S1.
3. RESULTS AND DISCUSSION Materials Synthesis. The formation region of the LGPStype phase was examined in the Li4SnS4−Li4SiS4−Li3PS4 quasiternary system. The synthesized composition area is summarized in Figure 1. These compositions could be
2. EXPERIMENTAL SECTION The starting materials to synthesize Li 10+δ [Sn y Si 1−y ] 1+δ P 2−δ S 12 (Li4−x[SnySi1−y]1−xPxS4) were Li2S (Nihon Kagaku Kogyo, >99.9% purity), SiS2 (Mitsuwa Chemical Co., Ltd., 99.9% purity), SnS2 (Kojundo Chemical Laboratory Co., Ltd., >99.9% purity), and P2S5 (Aldrich, >99.9% purity). These starting materials were weighed in stoichiometric ratios in an Ar-filled glovebox and then mechanically mixed in a ZrO2 pot with ZrO2 balls (ϕ = 10 mm). The mixing process was conducted using a planetary ball milling apparatus at 6.3 Hz for 20 h. The product mixture was pelletized at 120 MPa, sealed in a quartz tube at 10 Pa, followed by heating at 823 K for 24 h. Then, the samples were slowly cooled to room temperature. X-ray diffraction (XRD) patterns of the synthesized powders were obtained from an X-ray diffractometer (Rigaku SmartLab) with Cu Kα1 radiation. The diffraction data were collected at a 0.01° step width over a 2θ range from 10 to 50°. Synchrotron XRD measurements were performed using the BL02B2 beamline at SPring-8 with an X-ray wavelength of 0.5 Å. The specimens were sealed in an Ar atmosphere in Lindemann glass capillaries (∼0.3 mm inner diameter), and a Debye−Scherrer diffraction camera was employed for the measurement. Diffraction data were collected in 0.01° steps from 3 to 70° at 298 K. Refinements for structural parameters were conducted using the RIETAN-FP program.17 The ionic conductivity was measured using an AC impedance method with an applied voltage of 20 mV and a frequency range from 10 Hz to 15 MHz using a frequency response analyzer (National Instruments, NF) under an Ar gas atmosphere from 298−400 K. The SE pellets (diameter ≈ 10 mm, thickness = 1−2 mm) used in the measurements were pelletized at 169 MPa and then sintered at 823 K for 12 h. No sintering process was used for the ionic conductivity of the cold-pressed powders at room temperature. The surfaces of the pellets were covered with Au powders to form electrodes. The electrochemical stability was evaluated by cyclic voltammetry. The measurement was conducted on a Li/SE/Au cell at a scan rate of 1 mV s−1 between −0.5 and 5 V using a Solartron 1287 electrochemical interface. The two-electrode all-solid-state batteries were fabricated using the prepared SEs under an Ar atmosphere. The composite electrode incorporating LiCoO2 powder (Nippon Kagaku) with a 10 nm-thick LiNbO3 coating (Powrex, MP-01)18,19 and Li10.35[Sn0.27Si1.08]P1.65S12 (Li3.45[Sn0.09Si0.36]P0.55S4) [δ = 0.35, x = 0.55, y = 0.2] was used as a cathode. The LiCoO2 and Li10.35[Sn0.27Si1.08]P1.65S12 (Li3.45[Sn0.09Si0.36]P0.55S4) powders were weighed in a weight ratio of 70:30, respectively, and then mixed using a pot mill rotator (Nitto, ANZ-10) for 10 min. The In−Li metal and Li10.35[Sn0.27Si1.08]P1.65S12 (Li3.45[Sn0.09Si0.36]P0.55S4) pellet were used as the anode and SE, respectively. To make the SE layer, 100 mg of the Li10.35[Sn0.27Si1.08]P1.65S12 (Li3.45[Sn0.09Si0.36]P0.55S4) powder was pressed into a pellet with a diameter of 10 mm. The lithium foil (thickness = 0.1 mm, diameter = 5 mm) and indium foil (thickness = 0.1 mm, diameter = 10 mm) with a Cu mesh current collector and a cathode composite with
Figure 1. Quasi-ternary diagram of the Li3PS4−Li4SnS4−Li4SiS4 system. The reported solid solution ranges of Li10+δSn1+δP2−δS12 (Li4−xSn1−xPxS4) and Li10+δSi1+δP2−δS12 (Li4−xSi1−xPxS4) are represented by the green lines.15 The compositions of the synthesized samples are indicated by the black and red points in the diagram. These compositions are described by the formula, Li10+δ[SnySi1−y]1+δP2−δS12 (Li4−x[SnySi1−y]1−xPxS4), and the composition variations are indicated by the red lines along the tie lines between Li3PS4 and Li4[SnySi1−y]S4 (the ratio, such as 8/2, represents the ratio of Sn/Si (y/1 − y)). The monophasic region is marked between the green dotted lines, and the red points within the region represent the synthesized LGPS-type single phases.
represented by the formula, Li 10+δ [Sn y Si 1−y ] 1+δ P 2−δ S 12 (Li4−x[SnySi1−y]1−xPxS4), in which the 1+δ/2− δ (1−x/x) and y/1−y ratios correspond to the M4+ (Sn4+ and Si4+)/P5+ and Sn4+/Si4+ molar ratios, respectively. Solid Solutions in a Quasi-ternary System. Along the tie lines between Li4[SnySi1−y]S4 and Li3PS4, a complete compositional region was examined in the Li3PS4−Li4SnS4−Li4SiS4 system with Sn/Si molar ratios from 1/0 to 0/1. For the synthesis, the Sn/Si ratio was fixed at a certain value, and the P to M (Si and Sn) ratio was varied to determine the monophasic region of the LGPS phase. This strategy might also adjust the sizes of the lithium conduction tunnels and lithium concentration that contributed to the ionic conduction to the optimal values, and then improve the ionic conductivity. The XRD patterns for the samples synthesized along each tie line in the quasi-ternary diagram are shown in Figure 2. The solid solution range was determined for each line with a fixed Sn/Si ratio. The diffraction peaks of the LGPS-type phase shift gradually to higher angles with increasing P content within the solid solution range. In contrast, the samples with compositions outside of the solid solution range were indexed to be an 5859
DOI: 10.1021/acs.chemmater.7b00886 Chem. Mater. 2017, 29, 5858−5864
Article
Chemistry of Materials
Diffraction patterns for the LGPS-type phase are observed, and the monophasic character is in the range of δ = 0.50−0.20 (x = 0.50−0.60). The peaks gradually shift to higher angles as δ decreases from 0.50 to 0.20 (an increase of x from 0.50 to 0.60), indicating the formation of solid solutions. The samples with low P ratios (δ > 0.5, x < 0.5) were characterized by the LGPS-type phase and Li4SiS4, while samples with a high P ratio (δ < 0.2, x > 0.6) showed small additional diffraction peaks due to β-Li3PS4. The 203 peak position around 29.5° for all samples is plotted as a function of δ in Figure S1. The peak position clearly indicates that the sample with a higher Sn/Si ratio shows a smaller 2θ value, indicating a larger lattice volume because of the high content of large Sn4+ cations in the structure. In addition, a continuous peak shift in the solid-solution region is confirmed for each Sn/Si ratio; a larger lattice with a higher M4+ content is indicated. The unique behavior of the sample with Sn/Si = 8/2 with δ = −0.2 (x = 0.75) may be attributed to Li3PS4 impurity formation. P5+ is incorporated in the impurity phase, resulting in a smaller ratio of P5+ in the LGPS phase; thus, a relatively expanded LGPS phase formed with this composition. All of the XRD patterns for Sn/Si ratios of 8/2, 6/4, 5/5, 4/ 6, and 2/8 were analyzed based on the aforementioned phase identifications, and the ranges of the solid solutions are −0.10 ≤ δ ≤ 0.20 (0.60 ≤ x ≤ 0.70), −0.10 ≤ δ ≤ 0.35 (0.55 ≤ x ≤ 0.70), 0.05 ≤ δ ≤ 0.35 (0.55 ≤ x ≤ 0.65), 0.05 ≤ δ ≤ 0.50 (0.50 ≤ x ≤ 0.65), and 0.20 ≤ δ ≤ 0.50 (0.50 ≤ x ≤ 0.60), respectively. The solid solution region for the LGPS-type phase in the Li 4SnS4 −Li4 SiS4−Li3PS4 quasi-ternary system is indicated by the green dotted lines in Figure 1. The P content (δ, x value) of the single-phase region in each tie line varies with respect to the Sn/Si ratio, and the solid solution ranges gradually shift to a higher P content area from Sn/Si = 0/1 to Sn/Si = 1/0. This corresponds to an increase of the average radius of M4+. Similar trends for the solid solution ranges corresponding to the radius of M4+ were observed for Si, Ge, and Sn analogues.15 The solid solution ranges varied with respect to the higher P content area with the M4+ cation in the order of Si, Ge, and Sn. Therefore, the single phase region for the LGPS-type Li10+δM1+δP2−δS12 (Li4−xM1−xPxS4) is dependent on the size of M4+ in (M/P)S4. In contrast, in the Si−Sn substitution system, varying the Sn/Si ratio could further finely adjust the average radius of M4+ in the (M/P)S4 tetrahedra and then extend the range of the solid solution. Ionic Conductivity. The composition dependence of the conductivities for the various compositions (monophases) was determined for the samples in pressed powder form. Figure 3a shows the representative impedance plots of the solid solutions for the LGPS-type phase in Li 10+δ [Sn y Si 1−y ] 1+δ P 2−δ S 12 (Li4−x[SnySi1−y]1−xPxS4). The spectra contain a spike, which corresponds to the electrode contributions. Although the low temperature measurements (
