LITHIUM ION-CONDUCTING SOLID MATERIALS

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 comprising the solid material, and an electrochemical cell comprising such solid structure.

Claims

1. A solid material having a composition according to formula (IIc)
Li.sub.6+mP.sub.1-3nM1.sup.(z1)+.sub.nM2.sup.(z2)+.sub.nM3.sup.(z3)+.sub.nY.sub.5X(IIc) wherein X is selected from F, Cl, Br and I, Y is selected from O, S, Se and Te, M1.sup.(z1)+ to M3.sup.(z3)+ are selected from the group consisting of Si.sup.4+, Ge.sup.4+, Sn.sup.4+ and Sb.sup.5+, n is a number in the range of from 0.05 to 0.3, and m=5[5*(1x)+n*(z1+z2+z3)].

2. The solid material according to claim 1, wherein X is I or Y is S.

3. The solid material according to claim 1, wherein X is I and Y is S.

4. The solid material according to claim 1, wherein n is a number in the range of from 0.2 to 0.28.

5. The solid material according to claim 1, wherein M1.sup.(z1)+ is Si.sup.4+, M2.sup.(z2)+ is Ge.sup.4+, and M3.sup.(z3)+ is Sb.sup.5+, X is I (iodine) and Y is S.

6. The solid material according to claim 1, wherein the material comprises a crystalline phase having an argyrodite structure.

7. A process for preparing a solid material according to claim 1, comprising: (a) providing a reaction mixture comprising the precursors (1) one or more of oxide, sulfide, selenide and telluride of lithium, (2) one or more of oxide, sulfide, selenide and telluride of phosphorous, (3) one or more compound LiX wherein X is selected from F, Cl, Br and I, (4) compounds selected from the group consisting of oxides, sulfides, selenides and tellurides of three cations selected from the group consisting of, Si.sup.4+, Ge.sup.4+, Sn.sup.4+, and Sb.sup.5+, (5) optionally one or more of S, Se and Te in elemental form, wherein in the reaction mixture the molar ratio of all elements is selected so that it matches formula (IIc); and (b) reacting the precursors to obtain a solid material having a composition according to formula (IIc).

8. The process according to claim 7, wherein in step (a) the precursor (1) is Li.sub.2S, and/or the precursor (2) is P.sub.2S.sub.5, and/or the precursor (3) is LiI, and/or the precursor (4) is selected from the group consisting of SiS.sub.2, GeS.sub.2, SnS.sub.2 and Sb.sub.2S.sub.3, and/or the precursor (5) is elemental sulphur or is not present.

9. The process according to claim 7, wherein in step (a) the precursor (1) consists of Li.sub.2S, the precursor (2) consists of P.sub.2S.sub.5, the precursor (3) consists of LiI, the precursor (4) consists of SiS.sub.2, GeS.sub.2, and Sb.sub.2S.sub.3, the precursor (5) consist of elemental sulfur or is not present.

10. The process according to claim 7, comprising: (a) preparing or providing a solid reaction mixture comprising the precursors as defined in claim 7; and (b1) heat-treating the reaction mixture in a temperature range of from 200 C. to 600 C. for a total duration of 1 to 48 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 formula (IIc) is obtained.

11. The process according to claim 7, comprising: (a) preparing or providing a solid reaction mixture comprising the precursors as defined in claim 7; and (b2) mechanochemical treatment of the solid reaction mixture so that a solid material having a composition according to formula (IIc) is obtained.

12. The process according to claim 11, further comprising (c) annealing the solid material obtained by mechanochemical treatment in a temperature range of from 200 C. to 600 C. for a total duration of 1 to 48 hours and cooling the annealed solid material.

13. 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 a solid material according to claim 1.

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

15. The electrochemical cell according to claim 14, wherein the solid material is a component of a solid structure selected from the group consisting of cathode, anode and separator.

Description

EXAMPLES

1. Material Preparation

[0196] All steps were conducted under Argon atmosphere.

[0197] In step (a), four different reaction mixtures (total amount around 1.5 g) consisting of the precursors Li.sub.2S (99.99%, Sigma Aldrich), P.sub.2S.sub.5 (99%, Sigma Aldrich), GeS.sub.2 (99.9%, GoodFellow), SiS.sub.2 (99.99%, GoodFellow), Sb.sub.2S.sub.3 (99.99%, Alfa Aesar), S.sub.8 (99.99%, Sigma Aldrich) and LiI (99.999%, Sigma Aldrich) was loaded into a 70 mL zirconia milling jar with 10 zirconia milling balls (10 mm diameter). The stoichiometric ratio of the precursors was selected so that it matches a composition selected from Li.sub.6.5P.sub.0.25Si.sub.0.25Ge.sub.0.25Sb.sub.0.25S.sub.5I, Li.sub.6.342P.sub.0.5Si.sub.0.166Ge.sub.0.166Sb.sub.0.166S.sub.5I and Li.sub.6.171P.sub.0.75Si.sub.0.083Ge.sub.0.083Sb.sub.0.083S.sub.5I.

[0198] For comparison, a reaction mixture consisting of the above-mentioned precursors except for P.sub.2S.sub.5 was provided. Here, the stoichiometric ratio of the precursors was selected so that it matches a composition Li.sub.6.7Si.sub.0.33Ge.sub.0.33Sb.sub.0.33S.sub.5I.

[0199] Reacting of the precursors was achieved by mechanochemical processing. Thus, in step (b) the reaction mixture was milled for 1 h at 250 rpm, and then the speed was increased to 450 rpm and milling was continued for another 20 h.

[0200] The recovered powder was pressed into the pellets (300 mg, 10 mm diameter) at 3 t and vacuum sealed (10.sup.3 bar) in quartz ampules. The ampules were pre-dried at 500 C. for 10 min using a heat gun under dynamic vacuum (10.sup.3 mbar) to avoid trace water.

[0201] The samples were subsequent annealed at 400 C. for 24 h (step (c) with a heating and cooling rate of 5 C./min. The cooling rate corresponds to natural cooling.

[0202] The sample having the composition Li.sub.6.7Si.sub.0.33Ge.sub.0.33Sb.sub.0.33S.sub.5I (not according to the invention) and an additional sample of the composition Li.sub.6.5P.sub.0.25Si.sub.0.25Ge.sub.0.25Sb.sub.0.25S.sub.5I were annealed at 500 C. for 24 h (step (c) with a heating and cooling rate of 5 C./min.

[0203] For the composition Li.sub.6.5P.sub.0.25Si.sub.0.25Ge.sub.0.25Sb.sub.0.25S.sub.5I, additional samples were prepared without annealing (step (c)).

[0204] For the composition Li.sub.6.5P.sub.0.25Si.sub.0.25Ge.sub.0.25Sb.sub.0.25S.sub.5I, additional samples were prepared to study the influence of the following alternative cooling conditions in step (c): either slow cooling over 48 hours, or quenching in liquid nitrogen.

2. Structural Analysis

[0205] X-ray powder diffraction (XRD) measurements were conducted on a STADI P diffractometer (STOE) at room temperature with Mo-K1 radiation (=0.70931715 ) and annular collimator equipped Mythen 1 k detector (Dectris). Samples were sealed in borosilicate capillaries (0.48 mm inner diameter and 0.01 mm wall thickness; Hilgenberg) under argon atmosphere.

[0206] The three different materials having a composition according to general formula Li.sub.(6+m)P.sub.(1-3n)Si.sub.nGe.sub.nSb.sub.nS.sub.5I show typical single phase XRD pattern referring to the so called cubic argyrodite structure (FIG. 1a). Moreover, a gradual shift of the reflections to lower 2 angles was observed with decreasing phosphorous content (FIG. 1b), indicating the presence of a solid solution between the different cationic substituents (P, Si, Ge and Sb).

[0207] For the material of the composition Li.sub.6.5P.sub.0.25Si.sub.0.25Ge.sub.0.25Sb.sub.0.25S.sub.5I, XRD patterns of a sample obtained without annealing (no step (c)) and another one obtained with annealing (step (c)) followed by natural cooling were compared. After annealing the reflections in the XRD pattern are more clearly defined, indicating an increased crystallinity than after ball milling without annealing (FIG. 2).

[0208] The cooling conditions (natural cooling, or fast cooling by quenching, or slow cooling over 48 hours) did not show a significant influence on the XRD pattern of Li.sub.6.5P.sub.0.25Si.sub.0.25Ge.sub.0.25Sb.sub.0.25S.sub.5I.

[0209] FIG. 3 shows XRD patterns (cf. FIG. 3) of Li.sub.6.5P.sub.0.25Si.sub.0.25Ge.sub.0.25Sb.sub.0.25S.sub.5I and Li.sub.6.7Si.sub.0.33Ge.sub.0.33Sb.sub.0.33S.sub.5I (both annealed at 500 C.). The XRD pattern of Li.sub.6.5P.sub.0.25Si.sub.0.25Ge.sub.0.25Sb.sub.0.25S.sub.5I having four different elements (P, Si, Ge, Sb) at the lattice sites, which in the parent material L.sub.6PS.sub.5I are occupied by phosphorus, shows a single phase with only traces of the educts LiI and Li.sub.2S. The XRD pattern of Li.sub.6.7Si.sub.0.33Ge.sub.0.33Sb.sub.0.33S.sub.5I having three different elements (Si, Ge, Sb) at the lattice sites, which in the parent material L.sub.6PS.sub.5I are occupied by phosphorus, shows multiple phases including LiSbS.sub.2 and several unknown phases. Thus, it may be assumed that the presence of four different elements (P, Si, Ge, Sb) at the lattice sites, which in the parent material L.sub.6PS.sub.5I are occupied by phosphorus results in entropy stabilization so that a single phase is obtained.

3. Ionic Conductivity

[0210] Electrochemical impedance spectroscopy (EIS) was measured using a custom-made two-electrode cell, including two stainless steel plungers and a PEEK sleeve with an inner diameter of 10 mm. Around 150 mg powder was introduced into the cell and pressed at 3 t for 3 min (i.e. cold-pressed). EIS was measured from 0.1 Hz to 7 MHz with a 20 mV voltage amplitude using a SP-200 potentiostat (BioLogic) at room temperature. An external pressure of 2 t was applied during the measurement.

[0211] The results are given in table 1 below.

TABLE-US-00001 TABLE 1 Preparation Ionic conductivity at Material composition (for details see section 1 above) 25 C./mS/cm.sup.1 Li.sub.6.171P.sub.0.75Si.sub.0.083Ge.sub.0.083Sb.sub.0.083S.sub.5I Mechanochemical processing 0.063 Li.sub.6.345P.sub.0.5Si.sub.0.166Ge.sub.0.166Sb.sub.0.083S.sub.5I with subsequent annealing 1.89 Li.sub.6.5P.sub.0.25Si.sub.0.25Ge.sub.0.25Sb.sub.0.25S.sub.5I and natural cooling 10.7 Li.sub.6.5P.sub.0.25Si.sub.0.25Ge.sub.0.25Sb.sub.0.25S.sub.5I Mechanochemical processing 7.0 with subsequent annealing and quenching Li.sub.6.5P.sub.0.25Si.sub.0.25Ge.sub.0.25Sb.sub.0.25S.sub.5I Mechanochemical processing 5.2 with subsequent annealing and slow cooling

[0212] The following trends may be recognized from the data given in table 1:

[0213] For the three different materials having a composition according to general formula Li.sub.(6+m)P.sub.(1-3n)Si.sub.nGe.sub.nSb.sub.nS.sub.5I, the ionic conductivity increases with decreasing phosphorus content.

[0214] Surprisingly, the cooling conditions (natural cooling, or fast cooling by quenching, or slow cooling over 48 hours) appear to have a significant influence on the ionic conductivity of Li.sub.6.5P.sub.0.25Si.sub.0.25Ge.sub.0.25Sb.sub.0.25S.sub.5. Accordingly, natural cooling (cooling rate approximately 5 C. per minute) appears favorable, compared to both, quenching and slow cooling.