LITHIUM TRANSITION METAL HALIDES

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 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*xM.sub.1−xM′.sub.xX.sub.y  (I) wherein M is Er; M′ is one or more selected from the group consisting of Ti, Zr, Hf, Nb and Ta; X is one or more selected from the group consisting of halides and pseudohalides; 0.12≤x≤0.42; 5.8≤y≤6.2; n is the difference between the valences of M′ and M.

2. The solid material according to claim 1, wherein 5.85≤y≤6.15, preferably 5.9≤y≤6.1.

3. The solid material according to claim 1, wherein M′ is one or more of Ti, Zr and Hf; and X is one or more selected from the group consisting of Cl, Br and I; and 0.12≤x≤0.42, preferably 0.2≤x≤0.4.

4. The solid material according to claim 3, wherein M′ is Zr, and X is Cl.

5. The solid material according to claim 1, wherein M′ is one or both of Nb and Ta; and X is one or more selected from the group consisting of Cl, Br and I; and 0.15≤x≤0.35.

6. The solid material according to claim 1, wherein the solid material is crystalline and has an orthorhombic structure in space group Pnma, or is a glass, or is a glass-ceramics.

7. A composite comprising the solid material according to claim 1, and a cathode active material, wherein the cathode active material preferably comprises one or more compounds of formula (II):
Li.sub.1+tA.sub.1−tO.sub.2  (II), wherein A comprises nickel and one or both members of the group consisting of cobalt and manganese, and optionally one or more further transition metals not selected from the group consisting of nickel, cobalt and manganese, wherein said further transition metals are preferably selected from the group consisting of molybdenum, titanium, tungsten, zirconium, one or more elements selected from the group consisting of aluminum, barium, boron and magnesium, wherein at least 50 mole-% of the transition metal of A is nickel; t is a number in the range of from −0.05 to 0.2.

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

9. A process for preparing a solid material according to claim 1, the process comprising a) providing a reaction mixture comprising the precursors (1) one or more compounds selected from the group consisting of halides and pseudohalides of lithium; and (2) one or more compounds selected from the group consisting of halides and pseudohalides of Er; and (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, Nb and Ta; wherein in said reaction mixture the molar ratio of Li, M, M′, halides and pseudohalides matches general formula (I); b) reacting the reaction mixture 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; and (2) one or more compounds MX.sub.3 wherein M is Er; and (3) one or more compounds from the group consisting of compounds M′X.sub.4 wherein M′ is one or more of Ti, Zr and Hf, and compounds M′X.sub.5 wherein M′ is one or both of Nb and Ta; wherein in each of precursors (1) to (3), independently from the other precursors, X is one or more selected from the group consisting of Cl, Br and I, preferably Cl; wherein the molar ratio of Li, M, M′ and X matches general formula (I).

11. The process according to claim 9, further comprising a) preparing or providing a solid reaction mixture comprising the precursors (1), (2) and (3) b) heat-treating the reaction mixture in a temperature range of 150° C. to 850° C. for a total duration of 1 hour 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.

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

13. A solid structure for an electrochemical cell, wherein the solid structure is a cathode, wherein the cathode comprises a composite according to claim 7.

14. The electrochemical cell comprising a solid material according to claim 6.

15. The electrochemical cell according to claim 14, wherein the solid material is a component of a solid structure as defined in claim 12.

Description

EXAMPLES

[0154] 1. Preparation of Solid Materials

[0155] Reaction mixtures consisting of the precursors

(1) LiCl

[0156] (2) YCl.sub.3 resp. ErCl.sub.3

(3) ZrCl.SUB.4

[0157] in the proportions to obtain the compositions indicated in table 1 resp. 2 were prepared by uniformly mixing the precursors (1), (2) and (3) using a mortar and pestle in an argon filled glovebox (step a)). Each reaction mixture was heated-treated at 450° C. in a vacuum sealed quartz tube for 12 hours to react the reaction mixture (step b)), and in each case the obtained reaction product was cooled at a rate of 2 K/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 resp. 2.

[0158] Materials A1, A2, A7 (cf. table 1) and B1, B2, B8 (cf. table 2) are not according to the invention and were prepared and analysed for comparison.

[0159] 2. Structure Analysis

[0160] 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.

[0161] 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 FIGS. 1 and 2.

[0162] FIG. 1 shows the X-ray diffraction (XRD) patterns of Li.sub.3−xZr.sub.xEr.sub.1−xCl.sub.6 materials B1-B8 over the range from x=0 to x=0.8 (cf. table 2 below). The XRD pattern of Li.sub.3ErCl.sub.6 (x=0, material B1) matches well with its reported trigonal structure (space group: P-3m1, see e.g. US 2009/0088995 A1). This structure (denoted as phase 1) is preserved at x=0.1 (material B2, not according to the invention). When more Zr.sup.4+ ions are introduced, a new XRD pattern is obtained at x=0.2 (material B3), indicating formation of a new phase having orthorhombic structure which is denoted as phase II. Phase II is isostructural to Li.sub.3LuCl.sub.6 and Li.sub.3YbCl.sub.6 (both crystallizing in the Pnma space group). The weight average of the crystal ionic radii of the transition metal ions Er.sup.3+ and Zr.sup.4+ of Li.sub.2.8Zr.sub.0.2Er.sub.0.8Cl.sub.6 is r=99.6 pm which is close to the crystal ionic radius of Lu.sup.3+ (r=100.1 pm) and of Yb.sup.3+ (r=100.8 pm), which is likely responsible for formation of a phase II which is isostructural to Li.sub.3LuCl.sub.6 and Li.sub.3YbCl.sub.6. As more Er.sup.3+ ions are substituted by Zr.sup.4+ ions, another phase having orthorhombic structure in the Pnma space group (phase III) is observed when 0.367≤x≤0.4 (materials B6 and B7) that exhibits a distinctly different and unique XRD pattern, following an excursion in a short two-phase region (0.2<x≤0.3, materials B4 and B5) where phase II and phase Ill are present.

[0163] FIG. 2 shows the X-ray diffraction (XRD) patterns of Li.sub.3−xZr.sub.xY.sub.1−xCl.sub.6 materials A1-A7 over the range from x=0 to x=0.6 (cf. table 1 below). The XRD pattern of Li.sub.3YCl.sub.6 (x=0, material A1) indicates a trigonal structure (space group: P-3m1). When x is in the range up to about 0.2 (materials A2 and A3) the XRD patterns indicate an orthorhombic structure (Pnma space group) almost identical to phase II of the Li.sub.3−xZr.sub.xEr.sub.1−xCl.sub.6 materials described above. As more Y.sup.3+ ions are substituted by Zr.sup.4+ ions, another phase having orthorhombic structure in the Pnma space group almost identical to phase III of the Li.sub.3−xZr.sub.xEr.sub.1−xCl.sub.6 materials described above is observed when 0.367≤x≤0.6 (materials A6 and A7), following an excursion in a short two-phase region (0.2<x≤0.3, materials A4 and A5) where both orthorhombic phases are present.

[0164] 3. Ionic Conductivity

[0165] Ionic conductivities were measured by electrochemical impedance spectroscopy (EIS) at different temperatures from 25° C. to 100° C. Typically, 150-200 mg of powder of the material was placed between two stainless steel rods and pressed into a 10 mm diameter pellet by a hydraulic press at 3 metric tons for 3 min in an Argon-filled glovebox. EIS experiments were performed with 100 mV amplitude within a frequency range of 1 MHz-10 mHz using a VMP3 potentiostat/galvanostat (Bio-Logic). The solid electrolyte (SE) pellet was placed between electronically blocking titanium electrodes (cell configuration Ti|SE|Ti).

[0166] The lithium ion conductivity measured 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 tables 1 and 2 below.

TABLE-US-00001 TABLE 1 Ionic conductivity Activation Material x in Li.sub.3−xY.sub.1−xZr.sub.xCl.sub.6 at 25° C. (S/cm) Energy (eV) A1 0 6.7 × 10.sup.−5 0.50 A2 0.100 3.2 × 10.sup.−4 0.43 A3 0.200 9.7 × 10.sup.−4 0.37 A4 0.250 1.0 × 10.sup.−3 0.37 A5 0.300 1.2 × 10.sup.−3 0.36 A6 0.367 1.3 × 10.sup.−3 0.34 A7 0.600 9.6 × 10.sup.−4 0.34

TABLE-US-00002 TABLE 2 Ionic conductivity Activation Material x in Li.sub.3−xEr.sub.1−xZr.sub.xCl.sub.6 at 25° C. (S/cm) Energy (eV) B1 0 8.7 × 10.sup.−5 0.52 B2 0.100 3.6 × 10.sup.−4 0.44 B3 0.200 7.7 × 10.sup.−4 0.39 B4 0.250 7.9 × 10.sup.−4 0.39 B5 0.300 1.1 × 10.sup.−3 0.38 B6 0.367 1.1 × 10.sup.−3 0.35 B7 0.400 1.0 × 10.sup.−3 0.36 B8 0.800 4.1 × 10.sup.−4 0.38

[0167] Tables 1 and 2 show that the ionic conductivity increases when Y resp. Er is partly substituted by Zr while after passing a maximum of the ionic conductivity further substitution of Y resp. Er by Zr does not result in a further increase of the ionic conductivity.

[0168] 4. Electrochemical Tests

[0169] Cyclic voltammograms of all-solid-state cells having the configuration (SE/carbon black mixture)|Li.sub.3PS.sub.4|Li.sub.11Sn.sub.6 are shown in FIG. 3. A mixture of a solid electrolyte (SE) and carbon black (weight ratio 95:5) was the working electrode, wherein the solid electrolyte SE is either Li.sub.3PS.sub.4 (not according to the invention) or Li.sub.3ErCl.sub.6 (material B1 not according to the invention) resp. Li.sub.2.633Er.sub.0.633Zr.sub.0.367Cl.sub.6 (material B6 according to the invention) or Li.sub.2.633Y.sub.0.633Zr.sub.0.367Cl.sub.6 (material A6 according to the invention). L.sub.11Sn.sub.6 (+0.49 V vs. Li/Li.sup.+) was used as the counter electrode in each case. The scan rate was 1 mV s.sup.−1.

[0170] When the solid electrolyte in the working electrode is Li.sub.3PS.sub.4 (not according to the invention), in the first scan (solid line) an oxidation current of Li.sub.3PS.sub.4 arises after 2.5 V (vs. Li/Li.sup.+) and continues to increase up to 3.8 V. This oxidation current is assigned to the oxidation of sulfide ions. The following scan (dashed line) exhibits a lower oxidation current, reflecting the ion-blocking nature of resulting carbon/Li.sub.3PS.sub.4 interface.

[0171] In contrast, no oxidation current is observed before 4.3 V when the solid electrolyte in the working electrode is Li.sub.3ErCl.sub.6 (material B1 not according to the invention) resp. Li.sub.2-633Er.sub.0.633Zr.sub.0.367Cl.sub.6 (material B6 according to the invention) or Li.sub.2.633Y.sub.0.633Zr.sub.0.367Cl.sub.6 (material A6 according to the invention). A small redox process above 4.40 V is observed on the first scan (solid line) that decreases substantially on the second scan (dashed line).

[0172] The difference in the voltammograms is in accordance with the difference in standard reduction potentials of chlorine (Cl.sub.2, gaseous) (+4.40 V vs. Li/Li.sup.+) and sulfur (S, solid) (+2.56 V vs. Li/Li.sup.+). This observation directly visualizes the superior electrochemical oxidation stability of chlorides compared to sulfides.

[0173] The first (solid lines) and second (dashed lines) charge-discharge profiles (current density 0.1 mA cm.sup.−2) of all-solid-state cells having the configuration (SE/LiCoO.sub.2 mixture)/Li.sub.3PS.sub.4/Li.sub.11Sn.sub.6 are shown in FIG. 4. The inset shows the initial charging behavior. In the cathode, the solid electrolyte SE admixed to the cathode active material LiCoO.sub.2 (weight ratio LiCoO.sub.2:SE of 70:30) is either Li.sub.3PS.sub.4 (not according to the invention) or Li.sub.2.633Er.sub.0.633Zr.sub.0.367Cl.sub.6 (material B6 according to the invention) so that a composite cathode is obtained. In the anode, solid electrolyte Li.sub.3PS.sub.4 powder is admixed to the anode active material Li.sub.11Sn.sub.6 (weight ratio Li.sub.3PS.sub.4:Li.sub.11Sn.sub.6 of 20:80) in each case to enhance Li.sup.+ diffusion.

[0174] When the cathode contains Li.sub.3PS.sub.4 as the solid electrolyte, the discharge capacity was only 93 mAh g.sup.−1, and poor initial coulombic efficiency of 62.7% was obtained. A gradual increase of the voltage that is attributed to sulfide oxidation was observed at the early stage of charging (FIG. 4, bottom part, and lower graph in the inset).

[0175] In contrast, when the cathode contains Li.sub.2.633Er.sub.0.633Zr.sub.0.367Cl.sub.6 as the solid electrolyte, the cell exhibits more than 110 mAh g.sup.−1 discharge capacity with high initial coulombic efficiency of 96.4%. No oxidative side reaction occurred prior to Li.sup.+ de-intercalation from LiCoO.sub.2 when the cathode contains Li.sub.2.633Er.sub.0.633Zr.sub.0.367Cl.sub.6 as the solid electrolyte (FIG. 4, upper part). A steep increase of the voltage occurs during initial discharge (FIG. 4, inset). The absence of undesirable decomposition of the solid electrolyte remarkably improves the cell performance.

[0176] A slightly higher capacity was obtained when a composite cathode having a ratio of LiCoO.sub.2:SE of 85:15 instead of 70:30 was used.

[0177] A diagnostic electrochemical analysis of the cells having the above-indicated configuration was conducted by using electrochemical impedance spectroscopy (EIS). The Nyquist plots measured after the end of 6th charging process are shown in FIG. 5.

[0178] The Nyquist plot in each case exhibits two semi-circles followed by a low-frequency Warburg tail. The semi-circle in the high-frequency region is attributed to the resistance of the solid electrolyte layer, and the semi-circle in the low-frequency region originates from the interfacial charge transport phenomena at the LiCoO.sub.2/solid electrolyte interface in the LiCoO.sub.2/solid electrolyte composite electrode (charge transfer resistance). The charge transfer resistance of the composite electrode LiCoO.sub.2/Li.sub.3PS.sub.4 (˜950Ω) is almost twenty-five fold higher than that of the composite cathode LiCoO.sub.2/Li.sub.2.633Er.sub.0.633Zr.sub.0.367Cl.sub.6 (˜40Ω, see inset of FIG. 5). The high charge transfer resistance of the composite electrode LiCoO.sub.2/Li.sub.3PS.sub.4 may be attributed to oxidative side reactions/decomposition of Li.sub.3PS.sub.4.

[0179] FIG. 6a displays the room temperature cycling performance of the cell using the LiCoO.sub.2/Li.sub.2.633Er.sub.0.633Zr.sub.0.367Cl.sub.6 composite cathode with a current density of 0.1 mA cm.sup.−2, C-rate 0.1 C. The cell exhibits highly reliable cycling performance not only with a 4.3 V cut-off but also with a 4.5 V cut-off in spite of the small oxidation current above 4.4 V observed by cyclic voltammetry on the first sweep (cf. FIG. 3). Highly reliable cycling performance (4.3 V cut-off) was also observed upon cycling with a C-rate of 0.5 C over more than 80 cycles, see FIG. 6b.