Method for manufacturing a polyacrylonitrile-sulfur composite material

Abstract

A method is provided for manufacturing a polyacrylonitrile-sulfur composite material, the polyacrylonitrile-sulfur composite material having an sp.sup.2 hybrid proportion, with respect to the total carbon atoms included in the composite material, of greater than or equal to 85% including the method steps: a) reaction of polyacrylonitrile with sulfur at a temperature of greater than or equal to 450 C., in particular greater than or equal to 550 C.; b) immediate purification of the product obtained in method step a); and c) drying the purified product, if necessary. A composite material manufactured in this way may be used in particular in an active material of a cathode of a lithium-ion battery and offers a particularly high rate capacity. In addition, methods are provided for manufacturing an active material for an electrode, a polyacrylonitrile-sulfur composite material and an energy store.

Claims

1. A method for manufacturing a polyacrylonitrile-sulfur composite material, wherein the polyacrylonitrile-sulfur composite material has an sp.sup.2 hybrid proportion greater than or equal to 85% with respect to the total carbon atoms included in the composite material, the method comprising: a) reacting polyacrylonitrile with sulfur at a first temperature of greater than or equal to 550 C., wherein the sulfur is used in excess; b) immediately removing the excess sulfur from a product obtained in method step a) by purifying the product; and c) drying the purified product.

2. The method as recited in claim 1, wherein the purification according to method step b) is carried out by a Soxhlet extraction.

3. The method as recited in claim 2, wherein the Soxhlet extraction is carried out using an organic solvent.

4. The method as recited in claim 1, wherein at least method step a) is carried out under an inert gas atmosphere.

5. The method as recited in claim 1, wherein during method step a) a cyclized polyacrylonitrile reacts with sulfur to form a polyacrylonitrile-sulfur composite material having an sp.sup.2 hybrid proportion of greater than or equal to 85%, the cyclized polyacrylonitrile being obtained through a reaction of polyacrylonitrile to cyclized polyacrylonitrile.

6. The method as recited in claim 1, wherein polyacrylonitrile is reacted with sulfur in the presence of a catalyst.

7. The method as recited in claim 1, wherein a weight ratio of sulfur to polyacrylonitrile is greater than or equal to 2:1.

8. A method for manufacturing an active material for an electrode, including a method for manufacturing a polyacrylonitrile-sulfur composite material, wherein the polyacrylonitrile-sulfur composite material has an sp.sup.2 hybrid proportion greater than or equal to 85% with respect to the total carbon atoms included in the composite material, the method comprising: a) reacting polyacrylonitrile with sulfur at a temperature of greater than or equal to 550 C., wherein the sulfur is used in excess; b) immediately removing the excess sulfur from a product obtained in method step a) by purifying the product; and c) drying the purified product.

9. The method as recited in claim 8, wherein the electrode is a cathode of a lithium-sulfur battery.

10. The method as recited in claim 8, wherein the method furthermore includes the following method step: d) admixing at least one electrically conductive additive to the polyacrylonitrile-sulfur composite material.

11. The method as recited in claim 10, wherein the additive includes one of carbon black, graphite, carbon fibers, carbon nanotubes, and mixtures thereof.

12. The method as recited in claim 10, wherein the method furthermore includes the following method step: e) admixing at least one binder to the polyacrylonitrile composite material.

13. The method as recited in claim 12 wherein the binder includes at least one of polyvinylidene fluoride and polytetrafluoroethylene.

14. The method as recited in claim 12, wherein in method step d) and in method step e), greater than or equal to 60 wt.-% to less than or equal to 90 wt.-% polyacrylonitrile-sulfur composite material is used, in method step d), greater than or equal to 0.1 wt.-% to less than or equal to 30 wt.-% electrically conductive additives are admixed, and in method step e), greater than or equal to 0.1 wt.-% to less than or equal to 30 wt.-% binders are admixed.

15. A polyacrylonitrile-sulfur composite material, manufactured using a method for manufacturing a polyacrylonitrile-sulfur composite material, wherein the polyacrylonitrile-sulfur composite material has an sp.sup.2 hybrid proportion greater than or equal to 85% with respect to the total carbon atoms included in the composite material, the method comprising: a) reacting polyacrylonitrile with sulfur at a temperature of greater than or equal to 550 C., wherein the sulfur is used in excess; b) immediately removing the excess sulfur from a product obtained in method step a) by purifying the product; and c) drying the purified product.

16. A method of using a polyacrylonitrile-sulfur composite material manufactured using a method for manufacturing a polyacrylonitrile-sulfur composite material, wherein the polyacrylonitrile-sulfur composite material has an sp.sup.2 hybrid proportion greater than or equal to 85% with respect to the total carbon atoms included in the composite material, the manufacturing method comprising: a) reacting polyacrylonitrile with sulfur at a temperature of greater than or equal to 550 C., wherein the sulfur is used in excess; b) immediately removing the excess sulfur from a product obtained in method step a) by purifying the product; and c) drying the purified product, wherein the polyacrylonitrile-sulfur composite material is used as an active material in an electrode.

17. The method as recited in claim 16, wherein the electrode is a cathode of a lithium-ion battery.

18. An energy store, comprising an electrode with an active material which includes a polyacrylonitrile-sulfur composite material manufactured using a method for manufacturing a polyacrylonitrile-sulfur composite material, wherein the polyacrylonitrile-sulfur composite material has an sp.sup.2 hybrid proportion greater than or equal to 85% with respect to the total carbon atoms included in the composite material, the method comprising: a) reacting polyacrylonitrile with sulfur at a temperature of greater than or equal to 550 C., wherein the sulfur is used in excess; b) immediately removing the excess sulfur from a product obtained in method step a) by purifying the product; and c) drying the purified product.

19. The energy store as recited in claim 18, wherein the energy store includes a lithium-sulfur battery.

20. The method as recited in claim 12, wherein in method step d) and in method step e), greater than or equal to 65 wt.-% to less than or equal to 75 wt.-% polyacrylonitrile-sulfur composite material is used, in method step d), greater than or equal to 5 wt.-% to less than or equal to 20 wt.-% electrically conductive additives are admixed, and in method step e), greater than or equal to 5 wt.-% to less than or equal to 20 wt.-% binders are admixed.

21. The method as recited in claim 1, wherein the method furthermore includes a method step, after method step a) and before method step b), of further reacting polyacrylonitrile with sulfur at a second temperature that is higher than the first temperature.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a graph, which shows the capacity curve of a composite material according to the present invention used in an energy store, compared to a comparison sample;

(2) FIG. 2 shows a graph, which shows the voltage curve within a rate test of a composite material according to the present invention used in an energy store, compared to a comparison sample;

(3) FIG. 3 shows a detail of a Raman spectrum of a composite material according to the present invention and a comparison sample and a sulfur-free, cyclized polyacrylonitrile sample.

DETAILED DESCRIPTION

(4) An example is shown hereafter of manufacturing a polyacrylonitrile-sulfur composite material according to the present invention or an active material based thereon or an electrode according to the present invention for a lithium-sulfur battery with a subsequent electrochemical characterization. In particular, a polyacrylonitrile-sulfur composite material is manufactured as described hereafter, the polyacrylonitrile-sulfur composite material having an sp.sup.2 hybrid proportion, with respect to the total carbon atoms included in the composite material, of greater than or equal to 85%.

(5) For this purpose, polyacrylonitrile (PAN) is mixed with sulfur in a first method step at a ratio of 1:3 (wt.-%). The mixture is heated in an inert gas atmosphere to a temperature of 330 C. (comparison sample) or 550 C. (sample according to the present invention) for six hours. The thus obtained product is then immediately freed from excess elemental sulfur with toluene using a Soxhlet extraction for six hours and dried. The finished composite has a sulfur content of 41 wt.-% (330 C.) or 31 wt.-% (550 C.).

(6) During this process, particularly a cyclized polyacrylonitrile reacts with sulfur to a polyacrylonitrile-sulfur composite material having an sp.sup.2 hybrid proportion of greater than or equal to 95%, the cyclized polyacrylonitrile being obtained through a reaction of polyacrylonitrile to cyclized polyacrylonitrile.

(7) In a next step, the sulfurous, cyclized polyacrylonitrile, i.e., the finished composite, is processed to form a cathode slurry to implement a cathode-active material. For this purpose, the active material (SPAN), carbon black (for example, carbon black available under the trade name Super P Li) as an electrically conductive additive, and polyvinylidene fluoride (PVDF) as a binder are mixed and homogenized in a ratio of 70:15:15 (in wt.-%) in N-methyl-2-pyrrolidone (NMP) as a solvent. The slurry is spread by a doctor blade onto an aluminum foil and dried. After complete drying, a cathode is stamped out and installed in a test cell against a lithium metal anode. Various cyclic and linear carbonates (DEC, DMC, EC) and mixtures thereof with a lithium-containing conducting salt (for example, LiPF.sub.6, lithium-bis (trifluoromethane sulfonyl) imide (LiTFSI)) are used as the electrolyte.

(8) The electrochemical test of the electrode, manufactured as previously described, takes place according to the following test plan: five complete cycles (discharging and charging) at C/10; 1 discharging at C/5, then charging at C/10; 1 discharging at C/2, then charging at C/10; 1 discharging at 1/C, then charging at C/10; and 1 discharging at C/10, then charging at C/10.

(9) Here, discharging at C/5 means, for example, a (complete) discharging over a period of 5 hours at constant power; similarly, charging at C/10 means, for example, a (complete) charge over a period of 10 hours at constant power.

(10) The charging or discharging behavior is shown in FIG. 1, which shows a diagram, where the number of cycles N is plotted against the specific capacity C.sub.S [mAh/g] in relation to the amount of produced active material. FIG. 1 shows that the sample produced according to the present invention at a reaction temperature of 550 C. initially has a lower specific capacity than the comparison sample, which has been manufactured at 330 C. Generally, the loss in capacity over several charging cycles or discharging cycles is similar, and in particular the example according to the present invention is generally stable as of the second cycle. FIG. 1 shows, however, that the loss in capacity has a greater impact at higher current rates (1C) on the comparison sample, which has been manufactured at 330 C. Essentially, 20% of the capacity which this cell achieves at C/10 may still be achieved. However, the sample according to the present invention shows a significantly lower drop in capacity at such high rates.

(11) Furthermore, FIG. 2 shows a diagram in which relative capacity C.sub.r [n % of Q.sub.C/10] is plotted against the voltage U (against an Li/Li+ electrode in [v]). It shows four curves, which show discharging rates for discharging at 10 hours (curve A), 5 hours (curve B), 2 hours (curve C) and 1 hour (curve D), FIG. 2a showing the sample according to the present invention and FIG. 2b showing the comparison sample. In FIG. 2 it is apparent that the voltage has considerably decreased. Besides the loss in capacity associated with this, this also results in another greater reduction of the energy density, which is calculated from the product of the cell capacity and the cell voltage. It is again apparent in FIG. 2 that the capacity of the comparison sample may be reduced to up to 20% at high rates (see curves D) (FIG. 2b), whereas the capacity of the sample according to the present invention (FIG. 2a) is considerably more stable.

(12) FIG. 3 furthermore shows a Raman spectrum, in which the wave number [cm.sup.1] is plotted against the intensity [arbitrary unit]. Here, curve A corresponds to a sulfur-free polyacrylonitrile cyclized in air, curve B corresponds to a composite material according to the present invention, and curve C corresponds to the comparison sample. The higher proportion of the sp.sup.2 hybridized C atoms in curve B (according to the present invention) is clearly evident, since the intensity ratio of D-bands (1332 cm.sup.1) to G-bands (1551 cm.sup.1) is higher at 1.68 than in the reference sample. Furthermore, the position of G-bands is higher at 1551 cm.sup.1 than in the reference sample (1546 cm.sup.1). The SPAN according to the present invention (curve B) thus shows an sp.sup.2 hybrid proportion of greater than or equal to 95%.