SOLID LITHIUM ION CONDUCTING MATERIAL CONTAINING YTTERBIUM AND PROCESS FOR PREPARATION THEREOF

Abstract

Described are a solid material which has ionic conductivity for lithium ions, a composite comprising said solid material and a cathode active material, 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 comprising the solid material, and an electrochemical cell comprising such solid structure.

Claims

1. A solid material having a composition according to general formula (I)
Li.sub.3−n*xYb.sub.1−xM.sub.xX.sub.y  (I) wherein 0.05≤x≤0.95; 5.8≤y≤6.2; n is the difference between the valencies of M and Yb; M is one or more selected from the group consisting of Ti, Zr, Hf, V, Nb and Ta; X is one or more selected from the group consisting of halides and pseudohalides.

2. The solid material according to claim wherein 0.08≤x≤0.85, and/or wherein 5.85≤y≤6.15.

3. The solid material according to claim 1, wherein the solid material is crystalline and comprises one or more crystalline phases having a structure selected from an orthorhombic structure in space group Pnma, and a trigonal structure in the space group P-3m1.

4. The solid material according to claim 1, wherein M is one or more of Ti, Zr, or Hf, and/or X is one or more selected from the group consisting of Cl, Br and I.

5. The solid material according to claim 1, wherein M is Zr and X is Cl.

6. A composite comprising a solid material according to claim 1 and and a cathode active material, wherein the cathode active material comprises one or more compounds of formula (II):
Li.sub.1−t[Co.sub.xMn.sub.yNi.sub.zM.sub.u].sub.1−tO.sub.2  (II) wherein 0≤x≤1 0≤y≤1 0≤z≤1 0≤u≤0.15 M if present is one or more elements selected from the group consisting of Al, Mg, Ba, B, and transition metals other than Ni, Co, and Mn, x+y+z>0 x+y+z+u=1 −0.05≤t≤0.2.

7. The composite according to claim 6, wherein the solid material according to claim 1 and the cathode active material are admixed with each other.

8. The composite according to claim wherein the solid material according to claim 1 is present in the form of a coating on the cathode active material.

9. The process for preparing a solid material as defined in claim 1, the process comprising: (a) providing the precursors (1) one or more compounds selected from the group consisting of halides and pseudohalides of Li; (2) one or more compounds selected from the group consisting of halides and pseudohalides of Yb; (3) one or more compounds selected from the group consisting of halides and pseudohalides of elements M selected from the group consisting of Ti, Zr, Hf, V, Nb and Ta; wherein in the reaction mixture the molar ratio of Li, Yb, M, halides and pseudohalides matches general formula (I) (b) reacting the precursors to obtain a solid material having a composition according to general formula (I).

10. The process according to claim 9, wherein the precursors are (1) one or more compounds LiX (2) one or more compounds YbX.sub.3 (3) one or more compounds MX.sub.4 wherein M is selected from the group consisting of Ti, Zr, and Hf, and/or X is selected from the group consisting of Cl, Br and I.

11. The process according to claim 9, comprising (a1) preparing or providing a solid reaction mixture comprising the precursors (1), (2) and (3) (b1) heat-treating the reaction mixture in a temperature range of from 300° C. to 650° C. for a total duration of 5 hours or more so that a reaction product is formed and cooling the reaction product so that a solid material having a composition according to general formula (I) is obtained.

12. The process according to claim 9, comprising (a2) preparing or providing a liquid reaction mixture by dissolving the precursors (1), (2) and (3) in a solvent selected from the group consisting of ethers, H.sub.2O, alcohols C.sub.nH.sub.2n+1OH wherein 1≤n≤20, formic acid, acetic acid, dimethylformamide, N-methylformamide, pyridine, nitriles, N-methylpyrrolidinone, dimethyl sulfoxide, acetone, ethyl acetate, dimethoxyethane, 1,3-dioxolane, and alkylene carbonates; (b2) removing the solvents from the liquid reaction mixture, so that a solid residue is obtained, and heat-treating the solid residue in a temperature range of from 100° C. to 300° C. for a total duration of 4 hours to 24 hours so that a reaction product is formed and cooling the reaction product so that a solid material having a composition according to general formula (I) is obtained.

13. A solid structure for an electrochemical cell, comprising a solid material according to claim 1 or a composite according to claim 6, wherein the solid structure is selected from the group consisting of cathode, anode and separator.

14. An electrochemical cell comprising a solid material according to claim 1 or a composite according to claim 6.

15. The electrochemical cell according to claim 14, wherein the solid material according to claim 1 or a composite according to claim 6 is a component of a solid structure as defined in claim 13.

Description

EXAMPLES

1 Preparation of Solid Materials

[0220] Solid materials according to the first aspect of the invention having a composition according to general formula (Ia) were prepared by a thermochemical solid-state process as described above. Reaction mixtures consisting of the precursors [0221] (1) LiCl [0222] (2) YbCl.sub.3 [0223] (3) ZrCl.sub.4
in the proportions to obtain the compositions indicated in table 1 were prepared by uniformly mixing the precursors (1), (2) and (3) using a mortar and pestle in an argon filled glovebox (step a1)). Each reaction mixture was heated-treated at 450° C. in a vacuum sealed quartz tube for 36 hours at 350° C. resp. 650° C. to react the reaction mixture (step 131)), and in each case the obtained reaction product was cooled at a rate of 2° C./min to obtain a solid material in the form of a powder having a composition according to general formula (I) as indicated in table 1.

2. Structure Analysis

[0224] Powder X-ray diffraction (XRD) measurements of the solid materials obtained as described above were conducted at room temperature using a PANalytical Empyrean diffractometer with Cu-Kα radiation equipped with a PIXcel bidimensional detector. XRD patterns for phase identification were obtained in Debye-Scherrer geometry, with samples sealed in sealed in 0.3 mm glass capillaries under argon.

[0225] The solid materials obtained as described above were polycrystalline and had little to no impurities as can be derived from the XRD patterns shown in FIG. 1.

[0226] FIG. 1 shows the X-ray diffraction (XRD) patterns of Li.sub.3−xZr.sub.xYb.sub.1−xCl.sub.6 materials over the range from x=0 to x=0.5 (cf. table 1 below) obtained by heat treatment at 350° C. The XRD pattern of Li.sub.3YbCl.sub.6 (x=0) corresponds to a previously unreported trigonal structure (space group: P-3m1). The same structure is expected to prevail when 0 <x <0.1. As more Yb.sup.3+ ions are substituted by Zr.sup.4+ ions (0.1≤x<0.3) a second crystalline phase emerges which has orthorhombic symmetry (Pnma space group), and the mixture is biphasic. When x≥0.3 said second crystalline phase with orthorhombic symmetry (Pnma space group) is present as the only phase.

3. Ionic Conductivity

[0227] Ionic conductivities of Li.sub.3−xYb.sub.1−xZr.sub.xCl.sub.6 (0≤x≤0.8) samples in the form of pellets were determined by the AC impedance technique using a cell configuration having ionically blocking Ti electrodes: Ti|Li.sub.3−xYb.sub.1−xZr.sub.xCl.sub.6|Ti. The pellets were pressed at 374 MPa. Nyquist plots were recorded at a frequency range of 1 MHz-1 Hz using a MTZ-35 impedance analyzer (Bio-logic). The lithium ion conductivity at 25° C. and the activation energy determined in the usual manner from the conductivity as a function of the temperature according to the Arrhenius equation

σ.sub.T=A.sub.T exp(−E.sub.a/k.sub.BT)

(where σ.sub.T is the ionic conductivity at the temperature T, T is the temperature in K, A.sub.T the pre-exponential factor, E.sub.a the activation energy and k.sub.B the Boltzmann constant) of all materials is given in table 1 below.

TABLE-US-00001 TABLE 1 Temperature of Ionic Activation x in heat treatment conductivity at Energy Li.sub.3−xYb.sub.1−xZr.sub.xCl.sub.6 in step (b1)/[° C.] 25° C. (S/cm) (eV) 0 350  1.0 × 10.sup.−4 0.50 0 650 6.28 × 10.sup.−4 0.61 0.1 350  6.1 × 10.sup.−4 0.39 0.2 350 1.08 × 10.sup.−3 0.32 0.3 350 1.04 × 10.sup.−3 0.30 0.3 650 8.98 × 10.sup.−4 0.35 0.4 350 1.08 × 10.sup.−3 0.31 0.5 350  1.1 × 10.sup.−3 0.32 0.5 650 8.38 × 10.sup.−4 0.31 0.8 350  3.8 × 10.sup.−4 0.35

[0228] Table 1 shows that the ionic conductivity increases when Yb is partly substituted by Zr while after passing a plateau around 0.2≤x≤0.5. Further substitution of Yb by Zr does not result in a further increase of the ionic conductivity.

4. Electrochemical Tests

[0229] For the electrochemical tests, composites were prepared by mixing LiCoO.sub.2 (a common cathode active material) and Li.sub.3−xYb.sub.1−xZr.sub.xCl.sub.6 (x=0.3) in a weight ratio of 80:20. The working electrode was formed by spreading the as-prepared electrode composite (10 mg) on a layer of Li.sub.3PS.sub.4 pellet (50 mg). A Li—In alloy was used as a counter electrode. All assembly was carried out in a poly(aryl-ether-ether-ketone (PEEK) mold (diameter: 10 mm) with two Ti rods as the current collectors pressed at 374 MPa before conducting electrochemical tests. The resulting all solid state cell has the configuration

(mixture of LiCoO.sub.2 Li.sub.2.7Yb.sub.2.7Zr.sub.0.3Cl.sub.6)|Li.sub.3PS.sub.4|Li—In alloy.

[0230] A cyclic voltammogram of an all-solid-state cell having the above-defined configuration is shown in FIG. 2. The scan rate was 1 mV s.sup.−1. In the first scan a broad oxidation peak occurs in the voltage range ≥3.5 V vs Li/Li.sup.+ while in the second scan there is no significant anodic current in the potential range of from about 3.3 V vs Li/Li.sup.+ up to about 4.7 V vs Li/Li.sup.+. It is apparent that in the first cycle the interface between Li.sub.3PS.sub.4 and the working electrode has stabilized, likely owing to formation of a passivation layer, and the Li.sub.3−xYb.sub.1−xZr.sub.xCl.sub.6 (x=0.3) present in the working electrode virtually exhibits electrochemical oxidation stability in contact with a cathode active material LiCoO.sub.2.

[0231] FIG. 3 shows the first (solid lines) and second (dashed lines) charge-discharge profiles (0.1 C) of the above-defined all-solid-state cells having the configuration

(mixture of LiCoO.sub.2 and Li.sub.2.7Yb.sub.0.7Zr.sub.0.3Cl.sub.6)|Li.sub.3PS.sub.4|Li—In alloy.

[0232] The cell exhibits a discharge capacity more than 120 mAh g.sup.−1. No oxidative side reaction occurred prior to Li.sup.+ de-intercalation. The second profile does not significantly differ from the first profile, i.e. charge/discharge occurs almost reversible.

[0233] FIG. 4 shows the charge-discharge capacity as a function of cycle number at different C-rates. The cell exhibits high coulombic efficiency and good capacity retention.