Carbon-coated lithium sulphide

Abstract

Process for producing an active material for batteries from lithium sulfide and ionic liquids, corresponding active materials, cathode materials, batteries and corresponding uses.

Claims

1. A process for producing an active material for a battery, wherein the process comprises: (a) providing lithium sulfide particles, (b) optionally drying and/or comminution of the lithium sulfide particles, (c) adding at least one ionic liquid, optionally in an organic solvent, to the lithium sulfide particles and mixing to obtain a mixture, (d) heating the mixture of (c), optionally under protective gas, to a temperature above a decomposition limit of the at least one ionic liquid and below a decomposition temperature of the lithium sulfide particles, as a result of which the at least one ionic liquid decomposes to carbon, which deposits as a homogeneous layer on a surface of the lithium sulfide particles, (e) optionally comminution of the product of (d) to break up agglomerates.

2. The process of claim 1, wherein the lithium sulfide is Li.sub.2S.

3. The process of claim 1, wherein in (b) drying is carried out at temperatures of from 50 to 150 C. for from 5 hours to 2 days and/or comminution of the lithium sulfide particles is carried out by milling.

4. The process of claim 1, wherein the at least one ionic liquid comprises a nitrogen-containing cation and dicyanamide and/or tricyanomethanide as anion.

5. The process of claim 4, wherein the cation is selected from one or more of pyridinium, pyridinium derivatives, pyrrolidinium, pyrrolidinium derivatives, imidazolium, and imidazolium derivatives.

6. The process of claim 4, wherein the cation is selected from 1-butyl-3-methylimidazolium, 1-ethyl-3-methylimidazolium and 1-butyl-1-methylpyrrolidinium.

7. The process of claim 5, wherein the anion is selected from tricyanomethanide.

8. The process of claim 1, wherein the at least one ionic liquid is selected from one or more of 1-butyl-3-methylimidazolium dicyanamide, 1-ethyl-3-methylimidazolium tricyanomethanide, and 1-butyl-1-methylpyrrolidinium tricyanomethanide.

9. The process of claim 1, wherein acetonitrile and/or N-methylpyrrolidone is used as the organic solvent in (c).

10. The process of claim 1, wherein in (d) heating is carried out to a temperature of up to 850 C., and this temperature is maintained for from 2 to 5 hours.

11. The process of claim 1, wherein in (d) heating is firstly carried out to a temperature of from 250 to 350 C. and this temperature is then maintained for from 0.5to 3 hours and heating is then carried out to a temperature of up to 850 C. and this temperature is maintained for from 2 to 5 hours.

12. The process of claim 1, wherein the at least one ionic liquid comprises a nitrogen-containing cation and tricyanomethanide as anion.

13. The process of claim 1, wherein acetonitrile is used as the organic solvent in (c).

14. A process for producing a cathode material, wherein the process comprises: (i) providing an active material produced by the process of claim 1, (ii) adding at least one electronically conductive additive to the active material, (iia) optionally adding one or more further additives, (iii) mixing the the active material, the at least one electronically conductive additive and the optionally added one or more further additives to provide a mixture, (iv) processing the mixture obtained in (iii), (v) drying the processed mixture obtained in (iv).

15. The process of claim 14, wherein a solvent is used in (i) to (iv).

16. The process of claim 15, wherein the solvent is N-methyl-pyrrolidone.

17. The process of claim 14, wherein the process consists of (i), (ii) and (iii) to (v).

18. The process of claim 14, wherein the process consists of (i), (ii), (iia) and (iii) to (v).

19. The process of claim 14, wherein processing the mixture in (iv) comprises drawing and/or casting.

20. The process of claim 14, wherein in (ii) the at least one electronically conductive material is added in combination with a binder.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows TGA measurements on three ILs to be used according to the invention.

(2) FIG. 2 shows corresponding TGA measurements on materials of the prior art (WO 2013/057023), with polyacrylonitrile being shown at left and sucrose being shown at right.

(3) FIG. 3 shows scanning electron micrographs of an active material according to the invention, in the case of which Li.sub.2S was coated with EMIM TCM in a ratio of 5:1.

(4) FIG. 4 shows EDX mapping of the active material according to the invention shown in FIG. 3. At top left, a scanning electron micrograph of the particle; at top right sulfur; at bottom left carbon and at bottom right oxygen.

(5) FIG. 5 shows a cycling test using an active material according to the invention.

(6) FIG. 6 shows scanning electron micrographs of Li.sub.2S coated on the basis of PAN

(7) FIG. 7 shows scanning electron micrographs of Li.sub.2S coated on the basis of EMIM TCM.

(8) FIG. 8 shows a cycling comparison of the electrodes obtained from coatings based on PAN and EMIM TCM.

(9) Abscissa: cycle number

(10) Ordinate: specific capacity [mAh/g of Li.sub.2S]

(11) for cycle No. 0, upper curve (red): PAN

(12) for cycle No. 0, lower curve (green): EMIM-TCM

(13) FIG. 9

(14) Abscissa: capacity [mAh]

(15) Ordinate: voltage [V]

(16) short curves (capacity from about 0 to 60 mAh) (red): PAN

(17) long curves (capacity from about 0 to greater than 600 mAh) (green): EMIM-TCM

EXAMPLES

Example 1

Production of Various Active Materials According to the Invention

(18) An active material was produced by firstly taking 1-2 gram of Li.sub.2S and comminuting it in the presence of acetonitrile in a ball mill in order to break up the secondary particles and then drying it at 120 C. for 24 hours.

(19) The Li.sub.2S was subsequently wetted with liquids shown in Table 1 by grinding the mixture of IL or IL/acetonitrile with Li.sub.2S in the various ratios indicated in a mortar for 30 minutes in each case.

(20) In the next step, the carbonization under a protective gas atmosphere (argon) was carried out by firstly heating to 300 C. and maintaining this temperature for 1 hour. The material was then heated at 2.5 C. per minute to 300 C. and then at 3.3 C. per minute up to 700 C. and the temperature was maintained for 3 hours. After cooling to room temperature, the material was ground in a mortar to break up the agglomerates.

(21) The resulting material was black, which is evidence that the lithium sulfide is completely enclosed. This is confirmed by the scanning electron micrographs (FIG. 3).

(22) TABLE-US-00001 TABLE 1 Pro- Pre- Li.sub.2S:IL Carbon- IL duction treatment ratio ization Appearance C H N Li.sub.2S Pre-drying Pure 300 C. 1 h Powder; white 0.3 0 0 at 120 C. 700 C. 3 h for 24 h Pyr.sub.14 simple Pre-drying 1:1 300 C. 1 h Powder; dark; 16.25 0.06 9.88 TCM mix at 120 C. Pyr.sub.14 700 C. 3 h agglomerates for 24 h TCM EMIM simple Pre-drying 3:2 300 C. 1 h Powder; dark; 12.28 0.06 7.98 TCM mix at 120 C. EMIM 700 C. 3 h agglomerates for 24 h TCM EMIM EMIM Pre-drying 10:1 300 C. 1 h Dark grey, 1.41 0.04 0.77 TCM TCM + at 120 C. EMIM 700 C. 3 h some-times ACN for 24 h TCM still white (1:9) EMIM EMIM Pre-drying 5:1 300 C. 1 h Powder; dark; 3.66 0.05 2.01 TCM TCM + at 120 C. EMIM 700 C. 3 h no ACN for 24 h TCM agglomerates (1:9) BMIM BMIM Pre-drying 5:1 300 C. 1 h Powder; / / / DCA DCA + at 120 C. BMIM 700 C. 3 h light-grey; ACN for 24 h DCA some-what (1:9) white BMIM BMIM Pre-drying 2:1 300 C. 1 h Powder; grey, DCA DCA + at 120 C. BMIM 700 C. 3 h sometimes / / / ACN for 24 h DCA still white (1:9)

(23) The best coating was obtained when Li.sub.2S was coated in a mass ratio of 5:1 with EMIM TCM, with the ionic liquid having been diluted beforehand in a mass ratio of 9:1 with acetonitrile.

(24) It was able to be established by means of EDX mapping that the active materials according to the invention display a uniform distribution of sulfur and carbon. The active materials according to the invention thus have a uniform coating.

(25) The elemental analysis of EMIM TCN (5:1) indicated a proportion of C and N of about 5-6%. This is in agreement with TGA tests in example 1b) which indicated that EMIM TCN loses about one third of its weight up to 700 C.

Examples 1a-1c

Production of the Active Materials

(26) The general procedure of example 1 was repeated using the following ILs:

(27) BMIM DCA from table line 6 (example 1a-ratio of Li.sub.2S:IL=5:1),

(28) EMIM TCM from table line 5 (example 1b -ratio of Li.sub.2S:IL=5:1),

(29) Pyr.sub.14TCM from table line 2 (example 1c-ratio of Li.sub.2S:IL=1:1).

(30) Direct comparison of examples 1a to 1c showed that EMIM TCM and Pyr.sub.14TCM lead to a higher yield of carbon compared to BMIM DCA. Compared to BMIM DCA, EMIM TCM and Pyr.sub.14TCM lose about of their mass up to 700 C., BMIM DCA significantly more. Thermogravimetric measurements (TGA) were carried out for this purpose. Here, the respective sample was heated under inert gas and the weight loss was measured as a function of the temperature. (FIG. 1)

(31) Corresponding TGA tests were carried out on the active materials as described in WO 2013/057023 (FIG. 2).

(32) Scanning electron micrographs of coated Li.sub.2S produced on the basis of EMIM TCM (FIG. 7) or using PAN (FIG. 6) were recorded. It can be seen that more homogeneous, denser coatings on the Li.sub.2S particles were achieved using EMIM TCM.

Example 2

Production of an Electrode

(33) 40 g of the active materials produced in Example 1 were in each case mixed with 40 g of Super P(.sup.R) Li and 20 g of 105 strength PVdF solution in a mixer (ball mill) at 200-400 revolutions per minute for one hour, then left to rest for 10 minutes. This was repeated three times.

(34) The products obtained were dried for 24 hours at room temperature in a dry space and subsequently for another 2 hours at 60 C., for 2 hours at 80 C. and for 2 hours at 100 C.

(35) The resulting mixture was applied to an aluminum foil (layer thickness wet: 130 m) and dried for 24 hours under slightly subatmospheric pressure at room temperature and for 48 hours in vacuo at 100 C.

(36) Electrodes were produced using the dried cathode materials by placing the constituents in a pouch bag. Here, Al/Ni voltage collectors were used, Pyr.sub.14TFSI:LiTFSI (9:1) was used as electrolyte and Celgard(.sup.R) 2500 was used as separator.

Example 3a

Production of an Active Material According to the Prior Art

(37) The procedure of Example 1 of WO 2013/057023 was employed.

Example 3b

Production of an Electrode According to the Prior Art (Comparison)

(38) The procedure of Example 2 was repeated using the active material from example 3a as active material.

(39) Measurements were carried out on the electrodes produced as described in Example 2 and Example 3 (comparison), and the results of these are shown in FIGS. 8 and 9.

(40) A comparison shows that the capacity of the cells according to the invention decreases significantly more slowly.

Example 4

Cyclic Test Using Active Material According to the Invention

(41) A procedure analogous to Example 2 was employed.

(42) Li.sub.2S/C EMIM-TCM:SuperP:PVdF (40:20:20) was used as electrode (PVdF=polyvinylidene fluoride).

(43) The separator was a polyethylene oxide-coated Separion separator, and

(44) Pyr.sub.14TFSI LiTFSI (9:1) was used as electrolyte.

(45) The test cell was a pouch bag at 40 C.

(46) The result is shown in FIG. 5.

(47) It can be seen from this that a high efficiency was achieved; the shuttle effect was suppressed without addition of an additive.