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SOLID LITHIUM ION CONDUCTING MATERIAL AND PROCESS FOR PREPARATION THEREOF

20210323824 · 2021-10-21

    Inventors

    Cpc classification

    International classification

    Abstract

    Described are a solid material which has ionic conductivity for lithium ions, a process for preparing said solid material, a use of said solid material as a solid electrolyte for an electrochemical cell, a solid structure selected from the group consisting of a cathode, an anode and a separator for an electrochemical cell, and an electrochemical cell comprising such solid structure.

    Claims

    1. A solid material comprising: Li, P, S, O, and one or more component selected from the group consisting of Cl, Br and I in a molar ratio according to general formula (I):
    UaPSbOcXdYe wherein X and Y are different and are selected from the group consisting of Cl, Br and I, a is in the range of from 4.5 to 7.5, b is in the range of from 3.0 to 5.4, c is in the range of from 0.1 to 2, b+c is in the range of from 4.4 to 6, d is in the range of from 0 to 1.6, e is in the range of from 0 to 1.6, d+e is in the range of from 0.4 to 1.8, and b+c+d+e is in the range of from 4.8 to 7.6.

    2. The solid material according to claim 1, wherein the ratio b/c is in the range of from 1.5 to 40.

    3. The solid material according to claim 1, wherein X is Cl and Y is Br, d+e is in the range of from 0.9 to 1.7, and the ratio of d/e is in the range of from 1:4 to 4:1.

    4. The solid material according to claim 1, wherein the ratio (b+c)/(d+e) is in the range of from 2.8 to 5.2.

    5. The solid material according to claim 1, wherein the solid material comprises a fraction consisting of crystalline phases, wherein one of said crystalline phases has the argyrodite structure.

    6. The solid material according to claim 1, wherein the solid material comprises structural units PS.sub.4.sup.3− and structural units PO.sub.4.sup.3− wherein preferably the ratio between the amount of structural units PS.sub.4.sup.3− and the amount of structural units PO.sub.4.sup.3− is in the range of from von 30:1 to 1.5:1.

    7. The solid material according to claim 1, wherein the solid material has an ionic conductivity of 1 mS/cm or more.

    8. A process for preparing a solid material according to claim 1, said process comprising: a) providing the precursors (1) a compound of formula (II)
    U3PS4  (II) and/or a mixture of U2S and P.sub.2S.sub.5 in a molar ratio in the range of from 2.7:1 to 3.3:1 preferably 2.9:1 to 3.1:1, (2) U2S, (3) one or more compounds selected from the group consisting of LiCl, LiBr and LiI, and (4) one or more solvents selected from the group consisting of alkanol; b) preparing a mixture comprising the precursors and solvents provided in a); and c) converting the mixture prepared in b) to a solid material by removing the solvents to form a residue, and heating the residue at a temperature in the range of from 50° C. up to 600° C. to form the solid material.

    9. The process according to claim 8, wherein in b) the precursors (2) and (3) are dissolved in solvent (4) resp. in a mixture of solvents (4) and (5), then precursor (1) is added and dissolved, and the obtained solution is stirred for 15 min to 24 hours, and/or in c) heating is performed in a closed vessel for a duration of 1 to 12 hours, at a temperature in the range of from 50° C. up to 600° C.

    10. The process according to claim 8, wherein the molar ratio of the total amount of Li in precursor (1) to the total amount of Li in precursors (2) and (3) is in the range of from 3:5 to 3:1, preferably 3:4.7 to 3:1.3, and the molar ratio of Li in precursor (2) to Li in precursor (3) is in the range of from 1:2 to 4:1, more preferably 2:3.5 to 3:1.

    11. The process according to claim 8, the compound of formula (II) is provided in solvated form
    Li.sub.3PS.sub.4*g solv  (If) wherein solv is selected from the group consisting of tetrahydrofuran (THF), acetonitrile, dimethylether (DME), 1,3-dioxolane, 1,4-dioxane g is in the range of from 1 to 4, preferably 2 to 3.5.

    12. The process according to claim 8, wherein precursor (3) consists of the compounds LiCl and LiBr.

    13. A method of using the solid material according to claim 1 as a solid electrolyte for an electrochemical cell, wherein the solid electrolyte is a component of a solid structure for an electrochemical cell selected from the group consisting of a cathode, anode and separator.

    14. A solid structure for an electrochemical cell, wherein said solid structure is selected from the group consisting of cathode, anode and separator, wherein the solid structure for an electrochemical cell comprises a solid material according to claim 1.

    15. An electrochemical cell comprising: a solid material according to claim 1, wherein the solid material is a component of the solid structure of claim 14.

    16. The solid material of claim 1, wherein b+c+d+e is in the range of from 4.8 to 7.6.

    17. The solid material according to claim 16, wherein, b+c+d+e is in the range of 5.5 to 6.7.

    18. The solid material according to claim 1, wherein a=3+2(b+c−4)+d+e.

    19. The solid material according to claim 5, wherein said crystalline phase having the argyrodite phase makes up for 70% or more of the total weight of the fraction comprising crystalline phases.

    20. The method of claim 9, wherein the precursors further comprise: (5) one or more solvents selected from the group consisting of aprotic solvents, wherein said aprotic solvents are selected from the group consisting of ethers, aliphatic hydrocarbons and aromatic hydrocarbons, most preferably one or both of tetrahydrofuran (THF) and toluene.

    Description

    EXAMPLES

    [0215] 1. Preparation of Solid Materials

    [0216] Step a)

    [0217] The following precursors were provided: [0218] (1) Li.sub.3PS.sub.4 x THF (x=2 to 3) obtained in the manner described in WO 2018/054709 A1 with the exception that THF was used as the solvent [0219] (2) Li.sub.2S (Sigma-Aldrich, 99.98%) [0220] (3) Li halide(s), i.e. one or more of LiCl (Sigma-Aldrich, 99%), LiBr (Alfa Aesar, 99%) and LiI.

    [0221] As the solvent (4), anhydrous ethanol (Sigma-Aldrich, anhydrous, dried with 3 Å molecular sieve 3 days before use) was provided.

    [0222] Step b)

    [0223] In an Argon-filled glovebox, (2) Li.sub.2S and (3) Li halide were dissolved in (4) anhydrous ethanol. The molar ratio between Li.sub.2S and Li halide was selected according to the target stoichiometry (see table 1 below). Solid Li.sub.3PS.sub.4 solvated with THF (1) was added to the solution in an amount according to the target stoichiometry (see table 1 below) and the mixture was stirred overnight to give a pale-yellow solution.

    [0224] Step c)

    [0225] Outside of the glovebox, the solvent was removed under reduced pressure while immersing the flask containing the solution prepared in step b) into a 100° C. hot oil bath. The obtained residue was a pale yellow/pink powder. The obtained residue was further dried under reduced pressure at 140° C. for 40 hours. Portions of 200 mg were pressed into pellets with a diameter of 13 mm and sealed into a carbon coated quartz tube under vacuum. The sample was heated to 550° C. at a rate of 5 K/min and kept at 550° C. for 6 hours. After cooling to ambient temperature, the pellet was removed from the quartz tube inside a glovebox and characterized chemically and electrochemically.

    [0226] 2. Structural and Chemical Characterization

    [0227] X-ray diffraction (XRD) measurements were conducted at room temperature on a PANalytical Empyrean diffractometer with Cu-Kα radiation equipped with a PIXcel bidimensional detector. XRD patterns for phase identification were obtained in the Bragg-Brentano geometry, with samples placed on a zero-background sample holder in an Argon-filled glovebox and protected by Kapton film. Standard addition analysis was carried out by mixing the sample with 10 wt. % Si in an Argon-filled glovebox and sealed in glass capillaries (inner diameter 0.3 mm). XRD patterns were collected in the Debye-Scherrer geometry. Rietveld refinement was performed using the FullProf suit. Scale factor, zero point, background, lattice parameters, fraction coordinates, occupancies, and thermal parameters were sequentially reined in the argyrodite structure Li.sub.6PS.sub.5X (X=Cl, Br).

    [0228] The element composition was determined by elemental analysis. The ratio between structural units PS.sub.4.sup.3− and structural units PO.sub.4.sup.3− was determined by means of quantitative solid state .sup.31P MAS NMR.

    [0229] The material morphology was examined using a Zeiss field emission scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectroscopy detector (EDX).

    [0230] 3. Conductivity

    [0231] The ionic conductivity was determined by means of electrochemical impedance spectroscopy (EIS) with a home-built setup. Typically, 100 mg of a powder of the material to be studied was placed between two stainless steel stamps, which closely fit into a tube made of polyether ether ketone (PEEK) with a length of 10 mm, an inner diameter of 10 mm and an outer diameter of approx. 30 mm. The setup is then pressed by a manual press at 375 MPa giving a symmetric cell having the configuration SS/solid lithium-conducting material/SS (SS=stainless steel). The pressure of 375 MPa was maintained during recording of the electrochemical impedance spectrum. EIS was performed with 20 mV amplitude within a frequency range of from 1 MHz to 1 Hz using a VMP3 potentiostat/galvanostat (Bio-logic) at room temperature. The pellet thickness was determined in-situ during the measurement using a digital micrometer, taking into account the compression of the stainless-steel stamps at the respective pressure.

    [0232] Direct-current (DC) polarization curves at applied voltages of 0.25 V, 0.5 V and 0.75 V were recorded using the same cell configuration for 30 min each at room temperature to determine the electronic conductivities of samples.

    [0233] 4. Results

    [0234] 4.1 Overview

    [0235] In table 1, the target stoichiometry, the result of the elemental analysis, the Li ion conductivity and the ratio between structural units PS.sub.4.sup.3− and structural units PO.sub.4.sup.3− are compiled.

    [0236] The last two entries are comparison materials. Empty fields in table 1 mean that the related parameter has not been determined yet.

    [0237] When the stoichiometry determined by elemental analysis as given in table 1 is recalculated so that the stoichiometric coefficient of P is 1, it can be seen that the solid materials according to the invention comprise Li, P, S, O, and one or both of Cl and Br in a molar ratio according to general formula (I).

    [0238] It is observed that the solid materials according to the invention have superior Li ion conductivity.

    TABLE-US-00001 TABLE 1 Li-ion conduc- Target Stoichiometry determined by elemental analysis tivity PS.sub.4/PO.sub.4- Stoichiometry Li P S Cl Br O [mS/cm] ratio Li.sub.6PS.sub.5Cl 6 0.95 4.22 0.99 0 0.85 1.3  8.9:1 Li.sub.6PS.sub.5Cl.sub.0.75Br.sub.0.25 1.8 Li.sub.6PS.sub.5Cl.sub.0.5Br.sub.0.5 6 0.93 4.51 0.72 0.55 0.72 2.2 11.7:1 Li.sub.6PS.sub.5Cl.sub.0.25Br.sub.0.75 1.8 Li.sub.6PS.sub.5Br 6 0.93 4.52 0 1.0 0.77 1.0 13.3:1 Li.sub.5.75PS.sub.4.75Cl.sub.1.25 5.75 1.0 4.15 1.22 0 0.9 1.1 Li.sub.5.5PS.sub.4.5Cl.sub.1.5 5.5 0.95 3.77 1.48 0 1.1 1.4 Li.sub.5.25PS.sub.4.25Cl.sub.1.75 5.25 0.95 2.22 1.7 0 2.4 0.2 Li.sub.5PS.sub.4Cl.sub.2 5 0.92 2.64 2.0 0 1.3 0.3

    [0239] 4.2 Crystal Structure and Morphology

    [0240] For the sake of convenience, herein the samples of the tested materials are referred to by their target stoichiometry (cf. table 1 above), although the stoichiometry determined by elemental analysis is different from the target stoichiometry.

    [0241] FIGS. 1a-c show XRD patterns of solid materials having the target stoichiometries Li.sub.6PS.sub.5Cl (FIG. 1a), Li.sub.6PS.sub.5Br (FIG. 1b) and Li.sub.6PS.sub.5I (FIG. 1c) after heat treatment. All reflections correspond to the respective argyrodite phase except for those which are marked. The argyrodite phase (F-43m) is present as the major crystalline phase (77 wt.-% to 91 wt.-%, see below) in the solid materials having the target stoichiometries Li.sub.6PS.sub.5Cl (FIG. 1a) and

    [0242] Li.sub.6PS.sub.5Br (FIG. 1b), while the remainder of the crystalline fraction detectable by XRD is comprised of minor amounts of Li.sub.3PO.sub.4, LiCl and LiBr. The solid material having the target stoichiometry Li.sub.6PS.sub.5I contains only a trace of Li.sub.3PO.sub.4 (FIG. 1c).

    [0243] The SEM images (insets in FIGS. 1a, 1b and 1c) of well-ground solid materials having the target stoichiometries Li.sub.6PS.sub.5Cl (FIG. 1a), Li.sub.6PS.sub.5Br (FIG. 1b), Li.sub.6PS.sub.5I (FIG. 1c) illustrate the dense nature of the obtained materials which is highly beneficial when the solid materials are processed into all solid-state batteries.

    [0244] The weight fraction of the crystalline argyrodite phase relative to the total weight of crystalline phases detectable by XRD was determined using Si as an external standard (see tables 2 and 3). In the solid materials having the target stoichiometry Li.sub.6PS.sub.5Cl resp. Li.sub.6PS.sub.5Br, the weight percentages of crystalline argyrodite were 77(5)% and 91(6)%, respectively, with crystalline Li.sub.3PO.sub.4, Li.sub.2S, LiCl resp. LiBr accounting for the remainder (Tables 2 and 3). In tables 2 and 3, estimated standard deviations (esd's) are given in parentheses.

    TABLE-US-00002 TABLE 2 Weight fraction of crystalline phases in the solid material having the target stoichiometry Li.sub.6PS.sub.5Cl (−10 wt. % Si added as the reference standard for intensity normalization). Component Refined weight fraction with Si Calculated weight fraction Li.sub.6PS.sub.5Cl  71(2)%  77(5)% Li.sub.3PO.sub.4 9.2(9)%  10(2)% LiCl 5.1(3)% 5.6(5)% Li.sub.2S 4.8(3)% 5.2(5)% Si 10.2(3)%  N/A

    TABLE-US-00003 TABLE 3 Weight fraction of crystalline phases in the solid material having the target stoichiometry Li.sub.6PS.sub.5Br (−10 wt. % Si added as the reference standard for intensity normalization). Component Refined weight fraction with Si Calculated weight fraction Li.sub.6PS.sub.5Br  78(2)%  91(6)% Li.sub.3PO.sub.4   7(2)%   8(3)% LiBr 3.0(2)% 3.5(4)% Li.sub.2S 2.9(3)% 3.3(4)% Si 9.6(3)% N/A

    [0245] Rietveld refinements of the XRD patterns of the solid materials having the target stoichiometry Li.sub.6PS.sub.5Cl (FIG. 2a) resp. Li.sub.6PS.sub.5Br (FIG. 2b) result in lattice and atomic parameters (see tables 4 and 5 below) similar to those values previously reported by Kraft, M. A.; Culver, S. P.; Calderon, M.; Böcher, F.; Krauskopf, T.; Senyshyn, A.; Dietrich, C.; Zevalkink, A.; Janek, J.; Zeier, W. G. in “Influence of lattice polarizability on the ionic conductivity in the lithium superionic argyrodites Li.sub.6PS.sub.5X (X=Cl, Br, I)”, J. Am. Chem. Soc. 2017, 139, 10909-10918.

    [0246] In the following tables 4-7, “occ” means occupancy. Estimated standard deviations (esd's) are given in parentheses.

    TABLE-US-00004 TABLE 4 Atom coordinates, Wyckoff symbols and isotropic displacement parameters B.sub.iso/Å.sup.2 for the atoms in Li.sub.6PS.sub.5Cl (space group = F-43 m, a = 9.8598(3) Å, R.sub.Bragg = 4.83, X.sup.2 = 4.50). Wyckoff B.sub.iso Atom Site x y z Occ. (Å.sup.2) Li1 48h 0.3205 0.0182 0.6798 0.5 2 Cl1 4a 0 0 0 0.385 2.5(2) Cl2 4d 0.75 0.75 0.75 0.615 2.5(2) P1 4b 0 0 0.5 1 1.71(1) S1 16e 0.1195(2) −0.1195(2) 0.6195(2) 1 2.99(5) S2 4a 0 0 0 0.615 2.5(2) S3 4d 0.75 0.75 0.75 0.385 2.5(2)

    TABLE-US-00005 TABLE 5 Atom coordinates, Wyckoff symbols and isotropic displacement parameters B.sub.iso/Å.sup.2 for the atoms in Li.sub.6PS.sub.5Br (space group = F-43 m, a = 9.9855(4) Å, R.sub.Bragg = 3.26, X.sup.2 = 4.71). Wyckoff Atom Site x y z Occ. B.sub.iso (Å.sup.2) Li1 48h 0.3071 0.0251 0.6929 0.441 2 Li2 24g 0.25 0.017 0.75 0.119 2 Br1 4a 0 0 0 0.785(2) 2.9(1) Br2 4d 0.75 0.75 0.75 0.215(2) 1.6(1) P1 4b 0 0 0.5 1 1.3(1) S1 16e 0.1184(2) −0.1184(2) 0.6184(2) 1 1.97(7) S2 4a 0 0 0 0.215(2) 2.9(1) S3 4d 0.75 0.75 0.75 0.785(2) 1.6(1)

    [0247] FIGS. 3a-c show the XRD patterns of solid materials having the target stoichiometries Li.sub.6PS.sub.5Cl.sub.0.25Br.sub.0.75 (FIG. 3a), Li.sub.6PS.sub.5Cl.sub.0.5Br.sub.0.5 (FIG. 3b) resp. Li.sub.6PS.sub.5Cl.sub.0.75Br.sub.0.25 (FIG. 3c) after heat treatment. All reflections correspond to the respective argyrodite phase except for the marked reflections. As for the solid materials of single-halide target stoichiometries (cf. FIGS. 1a-1c above), the argyrodite phase is present as the major crystalline phase in each case, beside minor amounts of Li.sub.3PO.sub.4, Li.sub.2S, LiCl and LiBr. The lattice parameters are given in tables 6-8.

    [0248] FIG. 3d shows that the lattice parameter obtained from Rietveld refinements (see FIGS. 2a, 2b, 4-6) of the materials having target stoichiometries Li.sub.6PS.sub.5Cl.sub.1-xBr.sub.x with 0≤x≤1 increases linearly from x=0 to x=1. This indicates that in the argyrodite phases of the materials having mixed halide target stoichiometries Cl.sup.− ions and Br ions are randomly disordered throughout the structure.

    TABLE-US-00006 TABLE 6 Atom coordinates, Wyckoff symbols and isotropic displacement parameters B.sub.iso/Å.sup.2 for the atoms in Li.sub.6PS.sub.5Cl.sub.0.75Br.sub.0.25 (space group = F-43 m, a = 9.8880(4) Å, R.sub.Bragg = 3.42, X.sup.2 = 2.90). Wyckoff Atom Site x y z Occ. B.sub.iso (Å.sup.2) Li1 48h 0.3166 0.0178 0.6834 0.5 2 Br1 4a 0 0 0 0.22(2) 2.9(2) Cl1 4a 0 0 0 0.26(2) 2.9(2) Br2 4d 0.75 0.75 0.75 0.04(2) 1.3(2) Cl2 4d 0.75 0.75 0.75 0.49(2) 1.3(2) P1 4b 0 0 0.5 1 1.54(8) S1 16e 0.1200(2) −0.1200(2) 0.6200(2) 1 2.99(5) S2 4a 0 0 0 0.53(2) 2.9(2) S3 4d 0.75 0.75 0.75 0.47(2) 1.3(2)

    TABLE-US-00007 TABLE 7 Atom coordinates, Wyckoff symbols and isotropic displacement parameters B.sub.iso/Å.sup.2 for the atoms in Li.sub.6PS.sub.5Cl.sub.0.5Br.sub.0.5 (space group = F-43 m, a = 9.9185(6) Å, R.sub.Bragg = 3.12, X.sup.2 = 3.35). Wyckoff Atom Site x y z Occ. B.sub.iso (Å.sup.2) Li1 48h 0.3132 0.0212 0.6868 0.5 2 Br1 4a 0 0 0 0.39(2) 3.0(2) Cl1 4a 0 0 0 0.20(2) 3.0(2) Br2 4d 0.75 0.75 0.75 0.11(2) 1.4(2) Cl2 4d 0.75 0.75 0.75 0.30(2) 1.4(2) P1 4b 0 0 0.5 1 1.6(1) S1 16e 0.1194(2) −0.1194(2) 0.6194(2) 1 2.86(6) S2 4a 0 0 0 0.41(2) 3.0(2) S3 4d 0.75 0.75 0.75 0.59(2) 1.4(2)

    TABLE-US-00008 TABLE 8 Atom coordinates, Wyckoff symbols and isotropic displacement parameters B.sub.iso/Å.sup.2 for the atoms in Li.sub.6PS.sub.5Cl.sub.0.25Br.sub.0.75 (space group = F-43 m, a = 9.9543(3) Å, R.sub.Bragg = 3.48, X.sup.2 = 3.76). Wyckoff Atom Site x y z Occ. B.sub.iso (Å.sup.2) Li1 48h 0.3138 0.0235 0.6862 0.5 2 Br1 4a 0 0 0 0.61(2) 2.79(9) Cl1 4a 0 0 0 0.10(2) 2.79(9) Br2 4d 0.75 0.75 0.75 0.14(2) 1.4(1) Cl2 4d 0.75 0.75 0.75 0.15(2) 1.4(1) P1 4b 0 0 0.5 1 1.02(7) S1 16e 0.1191(1) −0.1191(1) 0.6191(1) 1 2.05(4) S2 4a 0 0 0 0.29(2) 2.79(9) S3 4d 0.75 0.75 0.75 0.71(2) 1.4(1)

    [0249] FIG. 7 shows the XRD patterns of solid materials having the target stoichiometries Li.sub.5.75PS.sub.4.75Cl.sub.1.25 (upper pattern) and Li.sub.5.5PS.sub.4.5Cl.sub.1.5 (lower pattern). All reflections correspond to the respective argyrodite phase except for those marked. The argyrodite phase is present as the major phase in each case, beside minor amounts of Li.sub.3PO.sub.4—and compared to the solid materials having the target stoichiometry Li.sub.6PS.sub.5Cl (cf. FIG. 1a)—much less Li.sub.2S and slightly more LiCl. The XRD patterns indicate successful substitution of sulfur with chlorine, which introduces lithium vacancies in the argyrodite phase, which may further improve the ionic conductivity.