Rechargeable battery cell having a separator

Abstract

This disclosure relates to a rechargeable battery cell having a positive electrode, a negative electrode, an electrolyte, which comprises a conducting salt, and a separator, which is arranged between the positive electrode and the negative electrode. The negative electrode and the positive electrode are each an insertion electrode. The electrolyte is based on SO.sub.2. The separator comprises a separator layer which is an organic polymer separator layer. The thickness of the organic polymer separator layer, relative to the loading of the positive insertion electrode with active material per unit area, is less than 0.25 mm.sup.3/mg.

Claims

1. A rechargeable battery cell, comprising: a positive insertion electrode loaded with active material; a negative insertion electrode; an electrolyte based on SO.sub.2, the electrolyte comprising a conductive salt; and a separator between the positive electrode and the negative electrode, wherein the separator comprises an organic polymer separator layer having a thickness, and wherein the ratio of the thickness of the organic polymer separator layer to the loading of the positive electrode with active material per unit area is less than 0.25 mm.sup.3/mg.

2. The battery cell according to claim 1, wherein the ratio of the thickness of the organic polymer layer to the loading of the positive electrode with active material per unit area is less than 0.20 mm.sup.3/mg.

3. The battery cell according to claim 2, wherein the ratio of the thickness of the organic polymer layer to the loading of the positive electrode with active material per unit area is less than 0.10 mm.sup.3/mg.

4. The battery cell according to claim 1, wherein the loading of the positive insertion electrode with active material is at least 30 mg/cm.sup.2.

5. The battery cell according to claim 1, wherein the thickness of the organic polymer separator layer is no more than 0.2 mm.

6. The battery cell according to claim 1, wherein the positive electrode is at least 0.25 mm thick.

7. The battery cell according to claim 1, wherein the polymer separator layer is made of a polyolefin, a partially to completely halogen-substituted polyolefin, a polyester, a polyamide or a polysulfone.

8. The battery cell of claim 7, wherein the polymer separator layer is made of polypropylene.

9. The battery cell according to claim 1, wherein the polymer separator layer has a surface energy that is at least 5 mN/m greater than the surface energy of the electrolyte.

10. The battery cell according to claim 9, wherein the polymer separator layer contains functional molecules which influence the surface energy of the polymer separator layer.

11. The battery cell according to claim 1, wherein the polymer separator layer is substantially devoid of fluorine compounds.

12. The battery cell according to claim 1, wherein the polymer separator layer comprises an organic polymer membrane and the separator has an additional layer, wherein: the additional layer has a first layer surface and a second layer surface and is: (i) formed from a polymer nonwoven consisting of polyolefin, and/or (ii) formed from a layer containing an inorganic ceramic material; and the organic polymer membrane has a first surface and a second surface and one of the first and second surfaces of the organic polymer membrane contacts the first or second layer surface of the additional layer.

13. The battery cell according to claim 12, wherein the separator is a composite material and the organic polymer membrane and the additional layer are permanently bonded to one another whereby their contacting surfaces are inseparably joined.

14. The battery cell according to claim 1, wherein the polymer separator layer is an organic polymer nonwoven.

15. The battery cell according to claim 14, wherein the organic polymer nonwoven contains a ceramic material.

16. The battery cell according to claim 15, wherein the ceramic material is one or more of aluminum oxide, silicon oxide or titanium dioxide.

17. The battery cell according to claim 16, wherein the organic polymer nonwoven contains a binder, wherein both the ceramic material and the binder are present as a layer on the surface and/or are incorporated into a porous layer structure of the polymer nonwoven.

18. The battery cell according to claim 1, wherein the positive insertion electrode comprises an active material with the composition:
A.sub.aM.sub.b(X.sub.cY.sub.d).sub.eZ.sub.f where A is an alkali metal, an alkaline earth metal, a metal of group 12 of the periodic table; M is a metal or several metals selected from the group consisting of the transition metals and/or the non-transition metals and/or the metals; (X.sub.cY.sub.d).sub.e is at least one first anion; Z is at least one second anion; where a≥0; b>0; c>0; d≥0; e>0 and f≥0, and where a, b, c, d, e and f are selected so that electroneutrality is obtained.

19. The battery cell according to claim 18, wherein the M is the metal Fe and wherein (X.sub.cY.sub.d).sub.e is PO.sub.4 or P.sub.2O.sub.7 and f is 0.

20. The battery cell according to claim 1, wherein the negative insertion electrode contains carbon as active material to receive ions.

21. The battery cell according to claim 1, wherein the positive electrode and/or the negative electrode has/have a diverter element with a three-dimensional porous metal structure in the form of a metal foam, wherein the porous metal structure preferably extends essentially over the entire thickness of the positive electrode and/or the negative electrode.

22. The battery cell according to claim 1, wherein the electrolyte contains at least 1.5 mol SO.sub.2 per mol conductive salt.

23. The battery cell according to claim 22, wherein the electrolyte contains at least 2.5 mol SO.sub.2 per mol conductive salt.

24. The battery cell according to claim 1, wherein SO.sub.2 content in the electrolyte is greater than 20% by weight of the weight of the electrolyte.

25. The battery cell according to claim 1, wherein the electrolyte contains organic solvents in an amount less than 50% by weight of the weight of the electrolyte.

26. The battery cell according to claim 25, wherein the electrolyte contains organic solvents in an amount less than 5% by weight of the weight of the electrolyte.

27. The battery cell according to claim 1, wherein the surface energy of the polymer separator layer is equal to or greater than the surface energy of the electrolyte.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above-mentioned aspects of exemplary embodiments will become more apparent and will be better understood by reference to the following description of the embodiments taken in conjunction with the accompanying drawings, wherein:

(2) FIG. 1 shows a cross-sectional diagram of a battery cell according to this disclosure;

(3) FIG. 2 shows a perspective diagram of one positive electrode and two negative electrodes of an electrode stack in a preferred embodiment;

(4) FIG. 3 shows the discharge capacity of the electrical resistance as a function of the number of cycles of experimental cells A1 and B1;

(5) FIG. 4a shows the discharge capacity of the electrical resistance as a function of the number of cycles of experimental cells A2 and B1;

(6) FIG. 4b shows the voltage as a function of the capacity of the negative electrode of the experimental cells A2 and B1;

(7) FIG. 5 shows the discharge capacity of the electrical resistance as a function of the number of cycles of experimental cells A3 and B1;

(8) FIG. 6 shows the discharge capacity of the electrical resistance as a function of the number of cycles of experimental cells A4 and B1; and

(9) FIG. 7 shows the electrical resistance as a function of the number of cycles of experimental cells A1-A5.

DESCRIPTION

(10) The embodiments described below are not intended to be exhaustive or to limit the invention to the precise forms disclosed in the following detailed description. Rather, the embodiments are chosen and described so that others skilled in the art may appreciate and understand the principles and practices of this disclosure.

(11) The details of the description of the experimental cells A1 to A5 and B1 (reference cell) are given in Table 1.

(12) The housing 1 of the rechargeable battery cell 2 shown in FIG. 1 encloses an electrode array which includes several (three in the case shown here) positive electrodes 4 and several (four in the case shown here) negative electrodes 5. The electrodes are each separated from one another by separators 11. The electrodes 4, 5 are connected as usual by electrode terminals 6, 7 to corresponding terminal contacts 9, 10 on the battery. The cell is filled with an SO.sub.2-based electrolyte (not shown in the drawing) in such a way that the electrolyte penetrates as thoroughly as possible into all the pores, in particular within electrodes 4, 5 and within separators 11. The electrolyte may be liquid or gelatinous.

(13) The electrodes 4, 5 are usually designed to be flat, i.e., as layers with a small thickness in relation to their area extent. The electrodes 4, 5 have a diverter element made of metal, as is customary, which is used to enable the required electronically conducting connection of the active material of the respective electrode. The diverter element is in contact with the active material involved in the electrode reaction of the respective electrode. The housing 1 of the prismatic cell shown here is essentially cube-shaped with the electrodes and the walls illustrated in cross section in FIG. 1 extending perpendicular to the plane of the drawing and running essentially in a straight and planar form. However, the cell according to this disclosure may also be designed as a spiral cell.

(14) The positive and negative electrodes 4 and 5 are insertion electrodes. They are made of an electrode material into which the ions of the active metal are incorporated while the cell is being charged and from which they are withdrawn while the cell is being discharged.

(15) FIG. 2 shows two negative electrodes 5 and one positive electrode 4. Each one of the electrodes has a connecting wire 6 and/or 7 for connection to the corresponding electrical connecting contacts 9 and/or 10 of the cell, respectively.

(16) In the preferred embodiment illustrated here, the positive electrode 4 (preferably all the positive electrodes in the cell) is sheathed by a sheath 13 of an organic polymer separator layer (polymer separator bag). The area extent of the sheath here is greater than the area extent of the electrode, the border 14 of which is shown as a dashed line in FIG. 2. Two layers 15 and 16 of the organic polymer separator layer covering the electrode on both sides are joined together at the edge of the electrode 11 by an edge connection 17. The edge connection 17 preferably extends along at least two, more preferably three of the four edges of the polymer separator bag 13 and/or the electrode 4 situated therein, namely its lower edge and its side edges running upward. The edge connection is preferably uninterrupted, i.e., continuous, for example, at the edges where it is present. Depending on the application case, however, it may also be interrupted. The edge connection may be formed by welding or fusing the layers 15 and 16 of the organic polymer separator layer covering the electrode on both sides. It would also be possible to apply suitable adhesive materials in solid or liquid form.

(17) The electrodes 4, 5 are arranged in alternation in the electrode stack. In general, it is preferable if the number of unsheathed electrodes (preferably the negative electrodes) is greater by one than the number of electrodes having a polymer separator bag (preferably the positive electrodes). The result is that the outer end faces of the electrode stack are formed by the unsheathed electrode surfaces (preferably the negative electrodes).

(18) Experiments

(19) All experiments were conducted in battery cells constructed from 12 negative electrodes with an active material comprised of carbon, an electrolyte based on 6×SO.sub.2 (cells A1, A2, A4 and A5) or with an electrolyte based on 4.5 SO.sub.2 (cell A3) with LiAlCl.sub.4 as the conductive salt (LiAlCl.sub.4×6 SO.sub.2, and/or LiAlCl.sub.4×4.5 SO.sub.2) and of 11 positive electrodes with lithium ferrophosphate (LFP) as the active material.

(20) The positive electrodes had a loading with active material, i.e., with LFP, of approximately 100 mg/cm.sup.2. The rated capacity of the cells was approximately 1,000 mAh.

(21) The cells differ in the type of separator used. Cells with an organic polymer separator are labeled with letters A and a respective number. Reference cells are labeled as B1.

(22) TABLE-US-00001 TABLE 1 Description of the Experimental Cells Experimental Reference Cell Cell A1 Cell with polypropylene B1 Cell with woven membrane separator glass separator 25 μm no pretreatment 120 μm A2 Cell with polypropylene B1 Cell with woven membrane separator glass separator 25 μm cooled before 120 μm operation A3 Cell with polypropylene B1 Cell with woven nonwoven separator glass separator 75 μm nonwoven 120 μm calendered A4 Cell with polypropylene B1 Cell with woven membrane separator glass separator 25 μm separator treated 120 μm with plasma A5 Cell with polypropylene membrane separator 25 μm separator coated with surfactant

(23) Several cycle experiments were conducted. In doing so, the cells were charged at 1 C corresponding to a current of 1 A up to a final charge voltage of 3.6 volts and a drop in the charge current to 200 mA. Then, the cells were discharged at the same amperage until reaching a potential of 2.5V. A pause of 10 minutes was inserted between charging and discharging.

(24) The resulting discharge capacity Q.sub.D is expressed as the percentage of the rated capacity Q.sub.N. The rated capacity Q.sub.N is obtained by subtracting the capacity consumed in the first cycles to form the cover layer on the negative electrode from the theoretical capacity Q.sub.th of the positive electrode.

(25) Two types of cells that differed in the separator used were tested in each measurement. Each curve represents an average of four identical measurements.

(26) Experiment 1

(27) FIG. 3 shows the results of experiment 1, in which cells (B1) containing a state-of-the-art separator made of woven glass fiber (GG) are compared with cells (A1) containing a polymer membrane separator made of polypropylene (PP). The usual commercial separator of PP was used in the condition as delivered. The cells having a PP separator were not subjected to any further treatment after being filled with electrolyte.

(28) The cells were charged and discharged over several cycles as described above. FIG. 3 shows the discharge capacity Q.sub.D in percentage of the rated capacity Q.sub.N and the internal resistance R.sub.i for both cells, plotted as a function of the number of cycles # (charge/discharge cycles).

(29) The rated capacity Q.sub.N of the cells B1 with the woven glass fiber separator runs from approximately 87% at first to a value of approximately 75% in the 45.sup.th cycle.

(30) The cells A1 with a PP separator achieve only approximately 60% of the rated capacity Q.sub.N and drop to approximately 50% in the final cycles.

(31) The resistance of the cell B1 with the woven glass fiber separator remains constant at approximately 0.025 ohms after an initial value of 0.04 ohms. The comparative cell A1 with a PP separator has a much higher internal resistance R.sub.i. The values (R.sub.i) are initially almost 10 times higher, amounting to 0.19 ohms. Only after 10 cycles does a linear course begin, starting at approximately 0.04 ohms and dropping to 0.035 ohms by the 38.sup.th cycle, but then increase again irregularly to 0.04 ohms.

(32) These results reflect the poor compatibility of polymer membrane separators with an electrolyte solution based on SO.sub.2. The wetting of the separator with SO.sub.2-based electrolyte solution is presumably so poor that even the permeation of the separator and the filling of the pores of the porous negative and positive electrodes with electrolyte are inhibited. The result is high internal resistance values R.sub.i. Ion transport is inhibited and the discharge capacity Q.sub.D is very low.

(33) Within the context of this disclosure, however, it has surprisingly been found that the negative properties of a polymer separator can be overcome by suitable treatment of the polymer separators (as described above), and these separators can be used advantageously in combination with positive electrodes with a high load of active material in a cell with SO.sub.2-based electrolyte.

(34) Experiment 2

(35) FIGS. 4a and 4b show the results of another experiment, using reference cells B1 identical to those in experiment 1. However, in contrast with experiment 1, the cells A2 with the polymer separator were stored for about 4 hours at −25° C. after being filled with the electrolyte.

(36) FIG. 4a shows the discharge capacity Q.sub.D in percentage of the rated capacity Q.sub.N and the internal resistance R.sub.i for both cells A2, B1, plotted as a function of the number of cycles (charge/discharge cycles).

(37) The capacity curves for the two cells A2 and B1 are almost identical. From approximately 80% of the rated capacity Q.sub.N at the beginning, the discharge capacity Q.sub.D drops to a level of approximately 48% in the 1,000.sup.th cycle.

(38) The cell A2 with the polymer separator has an excellent long-term behavior, achieving 1,000 cycles without an unusual drop in capacity.

(39) The resistance R.sub.i of the cell B1 with the woven glass fiber separator is constant at approximately 0.022 ohm after an initial value of 0.027 ohm. Surprisingly, the comparative cell A2 with the PP separator has a lower internal resistance R.sub.i. The R.sub.i values are initially 0.024 ohms. Then a linear (constant) curve is established at approximately 0.018 ohms.

(40) With the first charging of the lithium ion battery cell A2 described here, a cover layer is formed on the negative electrode. Lithium ions are consumed to form the cover layer and then are no longer available to the cell A2 as capacity in further operation.

(41) FIG. 4b shows the first charge cycle of a battery cell. The voltage curves U (V) of the two battery cells A2 and B1 are plotted as a function of the capacity Q in % based on the theoretical capacity Q.sub.th of the negative electrode. Intercalation of lithium ions begins at approximately 3.2 volts, but first the lithium ions are consumed to form the cover layer.

(42) In the case presented here, the capacity of the cover layer amounts to approximately 16% of the theoretical capacity Q.sub.th of the negative electrode. There is no difference between the cells B1 with the woven glass fiber separator and the cells A2 with the PP separator.

(43) Battery cell A2 according to this disclosure surprisingly has a cover layer capacity comparable to that of a cell B1 using a glass fiber separator. Those skilled in the art would expect that, when organic constituents are present in cell A2, as is the case with a polymer separator, they would be reduced electrochemically and would form additional cover layers with the lithium ions, which would result in an increased cover layer capacity in comparison with the state of the art.

(44) Experiments 3 and 4

(45) Additional polymer separators were tested in these experiments. Cells with a polymer nonwoven separator (A3, FIG. 5) and cells with a polymer membrane separator (A4, FIG. 6) previously treated with plasma were compared with state-of-the-art reference cells B1 containing glass fiber separators.

(46) FIGS. 5 and 6 show the discharge capacity Q.sub.D in % of the rated capacity Q.sub.N and the internal resistance R.sub.i for the respective cells plotted as a function of the number of cycles (charge/discharge cycles).

(47) Both measurements show a similar picture for the experimental cells A3 and A4 with the polymer separator. The discharge capacity Q.sub.D is higher in comparison with that of the state-of-the-art cells B1. At the same time, the resistance values Ri are reduced. These results yield a battery cell that is improved in comparison with the state of the art.

(48) Experiment 5

(49) FIG. 7 shows the resistance values R.sub.i of the cells A1-A5 in the first five cycles. For a better comparison, the resistance values R.sub.i were standardized to an electrode area of 1 cm.sup.2. It can be seen clearly here that the cell A1, which has a commercial PP separator, has a resistance value R.sub.i that is higher by a factor of 4 to 5 in the first cycle. In the following cycles, the resistance R.sub.i is also still twice as high as that in the other cells A2-A5. The separators in cells A2-A5 were modified in various ways. Table 1 summarizes the details. All the measurement curves showed resistance values from 0.3-0.6 ohms/cm.sup.2 in the first cycle to 0.2-0.4 ohms/cm.sup.2 in the additional cycles.

(50) Experiment 6

(51) For this experiment, 1 cm.sup.2 of the positive electrode/separator unit having an electrode thickness of the positive electrode of 600 μm and a loading with active material LiFePO.sub.4 of 100 mg/cm.sup.2 is considered. For this unit, the gravimetric energy density and the volumetric energy density were calculated with an average discharge voltage of 3.2 volts in one case when using a woven glass fiber separator with a thickness of 120 μm, and in another case, when using an organic polymer membrane according to this disclosure with a thickness of 20 μm. Table 2 summarizes the calculated values.

(52) TABLE-US-00002 TABLE 2 Energy Densities Gravimetric Improvement Volumetric Improvement energy over the cell energy over the cell density with glass density with glass Separator (Wh/kg) fiber (Wh/L) fiber Woven glass 332.8 761.6 fiber 120 μm state of the art Organic 358.5 +8% 873.6 +15% polymer membrane 20 μm

(53) Use of an organic polymer membrane in the case in question here increases the gravimetric energy density by 8% and increases the volumetric energy density by 15%.

(54) While exemplary embodiments have been disclosed hereinabove, the present invention is not limited to the disclosed embodiments. Instead, this application is intended to cover any variations, uses, or adaptations of this disclosure using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.