Rechargeable electrochemical cell

Abstract

A rechargeable electrochemical battery cell with a housing, a positive electrode, a negative electrode and an electrolyte which contains SO.sub.2 and a conducting salt of the active metal of the cell, whereby at least one of the electrodes contains a binder chosen from the group: Binder A, which consists of a polymer, which is made of monomeric structural units of a conjugated carboxylic acid or of the alkali salt, earth alkali salt or ammonium salt of this conjugated carboxylic acid or a combination thereof or binder B which consists of a polymer based on monomeric styrene structural units or butadiene structural units or a mixture of binder A and B.

Claims

1. A rechargeable electrochemical battery cell comprising a housing, a positive electrode, a negative electrode including carbon and an electrolyte comprising SO.sub.2 and a conductive salt of an active metal of the cell, wherein the SO.sub.2 is present in an amount of at least 1.5 mol SO.sub.2 per mol of conductive salt, wherein the SO.sub.2 mobilizes ions of the conductive salt and wherein the negative electrode comprises Li-PAA binder in a concentration of no more than 10 wt %.

2. The battery cell according to claim 1, wherein the negative electrode contains additional binder different from binder A.

3. The battery cell according claim 1, wherein at least one of the electrodes has a current collector element with a three-dimensional porous metal structure.

4. The battery cell according claim 3, wherein the porous metal structure extends essentially over the entire thickness of the electrode.

5. The battery cell according claim 3, wherein the active metal is distributed essentially homogeneously in the positive metal structure.

6. The battery cell according claim 1, wherein the positive electrode has a thickness of at least 0.25 mm.

7. The battery cell according claim 1, wherein the positive electrode has a thickness of at least 0.25 mm and a maximum thickness of 2.0 mm.

8. The battery cell according claim 1, wherein the negative electrode has a thickness of at least 0.2 mm.

9. The battery cell according claim 1, wherein the negative electrode has a thickness of at least 0.2 mm and a maximum thickness of 1.5 mm.

10. The battery cell according claim 1, wherein the active metal is selected from the group consisting of alkali metals, alkaline earth metals, metals in the subgroup 12 of the periodic system and aluminum.

11. The battery cell according claim 8, wherein the active metal is lithium, sodium, calcium, zinc, or aluminum.

12. The battery cell according claim 1, wherein the negative electrode is an insertion electrode.

13. The battery cell according claim 1, wherein the positive electrode contains a metal oxide or a metal halide or a metal phosphate.

14. The battery cell according claim 12, wherein the positive electrode contains an intercalation compound.

15. The battery cell according to claim 1, wherein the positive electrode contains lithium iron phosphate.

16. The battery cell according claim 1, wherein the electrolyte is based on SO2 and wherein the electrolyte contains at least 2.0 mol SO2 per mol conducting salt.

17. The battery cell according claim 1, wherein the electrolyte is based on SO.sub.2 and wherein the electrolyte contains at least 1.5 mol SO.sub.2 per mol conducting salt and the maximum concentration is 22 mol SO.sub.2 per mol conducting salt.

18. The battery cell according claim 1, wherein the electrolyte contains a halide, an oxalate, a borate, a phosphate, an arsenate, or a gallate of the active metal, as the conductive salt.

Description

(1) The invention is explained in more detail by means of the figures hereinafter. The depicted and described special features can be used individually or in combination to gain preferred embodiment of the invention. Key:

(2) FIG. 1 a cross-section model of a battery cell according to the invention;

(3) FIG. 2 Drawing in perspective of an electrode with schematically enlarged image details for the explanation of their inner structure;

(4) FIG. 3 the graph of the electric potential over the charge capacity for four negative electrodes with different binder materials;

(5) FIG. 4 the graph of the electric potential over the charge capacity for three different negative electrodes;

(6) FIG. 5 the graph of the electric potential over the charge capacity for four negative electrodes with a different content of Li-PAA binder;

(7) FIG. 6 the comparison of the absolute level of dischargeable capacity of cells with thermal pretreatment of the negative electrode and a negative electrode, which has been produced with a binder according to the invention.

(8) The housing 1 of the rechargeable battery cell 2 shown in FIG. 1 encloses an electrode arrangement 3 comprising a plurality (three in the case shown) of positive electrodes 4 and a plurality (four in the case shown) of negative electrodes 5. The electrodes 4, 5 are connected in the usual manner with corresponding terminal contacts 9, 10 of the battery by means of electrode leads 6, 7. The cell is filled with an SO.sub.2-based electrolyte, not shown in the figures, in such a manner that the electrolyte preferably penetrates completely into all pores, particularly inside the electrodes 4, 5.

(9) As is common, the electrodes 4, 5 have a planar shape, i.e. they are shaped as layers having a thickness which is small relative to their extension in the other two dimensions. They are separated from each other by separators 11. The housing 1 of the prismatic cell shown is essentially cuboid, the electrodes and the walls shown in cross-section in FIG. 1 extending perpendicularly to the drawing plane and being essentially straight and flat. However, the cell according to the invention can also be designed as a spirally wound cell.

(10) The electrodes 4, 5 comprise in usual manner a current collector element, which is made of metal and serves to provide the required electronically conductive connection of the active material of the respective electrode. The current collector element is in contact with the active material involved in the electrode reaction of the respective electrode. Preferably, the current collector element of the positive electrode, most preferably also the current collector element of the negative electrode, is provided in the form of a three-dimensional porous metal structure, particularly in the form of a metal foam. The term “three-dimensional porous metal structure” designates in this context every structure made of metal that does not just extend like a thin sheet only over the length and width of the planar electrode, but also extends over its thickness dimension, and which is porous in such a manner that the active material of the electrode can be incorporated into the pores.

(11) FIG. 4 shows, by means of two schematic enlarged excerpts, the internal structure of a preferred positive electrode. It has a conducting element 30 with a three-dimensional porous metal structure. The conducting element is preferably formed by a metal foam, wherein it is particularly advantageous if the porous metal structure extends substantially over the entire thickness d of the positive electrode. The active material 33 of the electrode, for example for a positive electrode lithium-iron phosphate, is located in the pores of the porous metal structure and is preferably distributed homogeneously therein. Further details can be obtained from the above-mentioned document WO 2011/098233 A2 . In the context of the invention particularly advantageous results have been found to be obtained with the combination of the electrode types described there and a binder A or B or a mixture of A and B as described here.

(12) During manufacture of the electrode, the active material and binder A or B or a mixture of A and B are incorporated into the porous structure of the current collector element such that it fills the pores of the current collector element uniformly over the whole thickness of the metal structure. The material is then pressed under high pressure, the thickness after the pressing operation being preferably no more than 80%, particularly preferably no more than 60% and more particularly preferably no more than 40% of the initial thickness.

(13) The three-dimensional porous metal structure 30 of the current collector element extends essentially over the whole thickness d of the current collector element and the active material and binder A or B or a mixture of A and B are distributed essentially homogeneously therein. With respect to the two stated conditions, “essentially” is to be construed such that the cell function is only slightly impaired by any deviations. In any case, the porous metal structure should extend over at least 70%, preferably at least approximately 80%, of the thickness of the electrode.

(14) A preferred embodiment of the battery cell according to the invention has positive and negative electrodes of which at least either the positive or the negative electrodes contain only binder A.

(15) Another preferred embodiment of the battery cell according to the invention has positive and negative electrodes of which at least either the positive or the negative electrodes contain only binder B.

(16) Another preferred embodiment of the battery cell according to the invention has positive and negative electrodes of which at least either the positive or the negative electrodes contain a mixture of binder A and binder B.

(17) Another preferred embodiment of the battery cell according to the invention has positive and negative electrodes of which at least either the positive or the negative electrodes contain a binder chosen from the group:

(18) Binder A which consists of a polymer which is made of monomeric structural units of a conjugated carboxylic acid or of the alkali salt or earth alkali salt or ammonium salt of this conjugated carboxylic acid or of a combination of these

(19) or binder B which consists of a polymer based on monomeric styrene structural units or butadiene structural units

(20) or a mixture of binder A and binder B.

(21) and which in addition contain another binder which is different from binder A and binder B.

(22) The electrodes described in WO 2011/098233 A2 are remarkably thick. Due to the thickness and, in addition due to the pore structure of the used porous metal structure of the current collector, there were additional problems expected in combination with a binder A or binder B or a mixture of A and B.

(23) In order to receive a high percentage of solid material in the electrode, the paste used in production must have optimal characteristics, the paste is made of active material, binder, possibly further components and solvent. Only then the pores of the current collector consisting of a porous metallic structure can be filled almost completely with solid material and so an electrode with high capacity can be manufactured.

(24) The mechanical and chemical stability of the negative and the positive electrode are important criteria for the quality of a rechargeable battery cell. This stability can be obtained by the use of a binder in the electrodes. A binder must meet the following requirements for an electrode: Simple processibility upon production of the electrode: homogeneous mixing with the other parts of the electrode such as the active material to obtain an electrode with sufficient loading. good solubility or dispersibility in the solvent, which should be water, because of its easy handling suitable melting range to avoid both melting away (e.g. stability during thermal drying of the cell) in the further process and being too firm (e.g. process temperature during activation of the binder). No release of harmful gases (e.g. gases containing fluorine) upon thermal stress. High shelf life (e.g. storage temperature and storage period). Positive characteristics upon operation of the battery cell No degradation of the binder by the electrolyte. Chemical compatibility with all cell components. Enabling the formation of a thinner surface layer on the electrodes especially on the negative electrode Mechanical stabilization of the electrode over a long period of time or during many charge and discharge cycles. This mainly serves the purpose to compensate for the volume changes during intercalation and de-intercalation. Good wettability of the surface of the electrode with electrolyte Thermal stability within the operating temperature of the cell

(25) As the cell according to the invention preferably has three dimensional current collectors and a SO.sub.2 containing electrolyte, the selection of a binder meeting all of the above mentioned requirements is particularly difficult.

EXAMPLE 1

(26) In the state of the art, as for instance described in WO 2011/098233 A2, the filling of the metal structure of the positive electrode was achieved by means of an organic solvent containing a soluble fluorine containing binder. The obtained capacities were typically around 15 mAh/cm.sup.2 of electrode surface area.

(27) During the invention, it was tried to simplify the production of the positive electrodes through the substitution of the organic solvent by water. Therefore the fluorine containing binder was dispersed in water. It was found, that the achievable loadings were reduced by around 7% to approx. 14 mAh/cm.sup.2.

(28) Surprisingly, the filling of the metal structure could in fact be optimized with one of the binders A or B according to the invention or a mixture of A and B and water as solvent, so that a similarly high level of filling of active material can be reached as previously only possible with the of use organic solvents.

(29) As an example, for a preferred embodiment of the battery cell according to the invention positive electrodes were produced from the following components: 94 wt % lithium iron phosphate (active material of the positive electrode) 2 wt % carbon black (conductivity enhancer) 4 wt % binder A

(30) With stirring, these components were used to produce a paste using water. The finished paste was introduced homogeneously into a metal foam with an initial porosity of more than 90% and dried at 50° C. for one hour. This step is necessary in order to obtain solvent free electrodes. Through calandering technology, after cooling the electrode was compressed from an initial thickness of approx. 1.00 mm to 0.56 mm, followed by another drying process at 120° C. in vacuum. The achieved capacities of these positive electrodes are typically again at 15 mAh/cm.sup.2 of electrode surface area.

(31) With a negative electrode a similar filling level was achieved through the optimization of the production parameters with water as solvent, as previously only achieved with the use of organic solvents.

(32) As an example, for the production of a preferred embodiment of the battery cell according to the invention, the following components for the negative electrode were used: 96 wt % graphite (active material of the negative electrode) 4 wt % binder A

(33) Whilst stirring, a paste was produced from the components and water as solvent. The paste was introduced homogeneously into a metal-foam with an initial porosity of more than 90% and dried at 50° C. for one hour. This step is necessary to obtain solvent free electrodes. Through calandering technology, after cooling the electrode was compressed from an initial thickness of 1.00 mm to a thickness of 0.40-0.42 mm, followed by another drying process at 120° C. in vacuum.

(34) The achieved capacities of the produced negative electrodes according to the invention are around 13 mAh/cm.sup.2 electrode surface are.

EXAMPLE 2

(35) The battery cell as described in WO 2011/098233 A2 contains a negative electrode, which is free of binder. This is due to the fact that many common binder materials, which can exclusively be used with organic and flammable solvents are not stable towards the used inorganic electrolyte.

(36) The absence of a binder adds complexity to the production process of the negative electrode and leads to complex solutions.

(37) Another reason for not using binder is that the addition of the therein described fluorine containing binder leads to a significant rise in the amount of lithium ions, which, during the first charge cycle, are irreversibly used by the formation of the surface layer on the negative electrode.

(38) The impact of various binders on the irreversibly used capacity due to the formation of the surface layer on the negative electrode in the first cycle has been examined. To this end, various negative electrodes were produced with graphite as active material, the corresponding binder and a three dimensional current collector, as described in experiment 1 . A binder-free reference electrode was produced the same way. The content of binder was adjusted to the different binder characteristics of the individual binder.

(39) It should be noted that a high percentage of binder has a negative impact on the energy density respectively on the electrical energy per weight and volume unit of the battery cell.

(40) In addition, a binder-free reference electrode was produced the same way. Five different sets of experimental electrodes have been manufactured this way. Table 1 describes the used binders:

(41) TABLE-US-00001 TABLE 1 Complete description of the binders used in example 2 electrodes electrodes with binder of state of the art binder free electrodes number in FIG. 3 A B C wt % of binder 0, 1 1 0 solvent (production) Aceton Isopropanol Isopropanol polymer THV PTFE ,- Terpolymer of Tetrafluorethylene, Polytetra-fluorethylene Hexafluorpropylene und Vinylidene fluoride chemical structure (—CF.sub.2—CF.sub.2—CF.sub.2—C.sub.2F.sub.4—CH.sub.2—CF.sub.2—).sub.n (—CF.sub.2—CF.sub.2—).sub.n ,- electrodes electrodes with binders according the invention number in FIG. 3 D E wt % of binder 2 4 solvent (production) Wasser Wasser polymer SBR LiPAA Styrene-Butadiene Lithium-Polyacrylate chemical structure [—CH.sub.2—CH═CH—CH.sub.2—].sub.m[—CHPh—CH.sub.2—].sub.n [—CH.sub.2—CH(COOX)—].sub.n

(42) The five experimental electrodes were examined via a three-electrode arrangement, where, during the charging of the electrode, the course of the electrical potential U expressed in volts was shown over the state of charge Q in relation to the rated capacity Q.sub.N of the electrode. The measurements were run in an electrolyte consisting of LiAlCl.sub.4×1.5 SO.sub.2.

(43) The five graphs show the results of several experiments with the above described electrodes. In FIG. 3 the abscissa of the intercept between graph and 400 mV line corresponds to the used cell capacity due to formation of surface layer. It can be seen that the loss of capacity of the electrode with fluorine containing binders THV and PTFE is highest, followed by the reference electrode not containing a binder.

(44) The curve of the electrode with the SBR binder according to the invention shows a significantly better relationship than the previous electrodes.

(45) The lowest loss of capacity of just 6% has the electrode according to the invention with Li-PAA binder.

(46) It is clearly visible that the electrode without binder also has a high loss of capacity during formation of the surface layer.

(47) Table 2 summarizes the results:

(48) TABLE-US-00002 TABLE 2 Used cell capacity due to formation of a surface layer for electrodes with different binders. electrodes electrodes with binder electrodes with binder of free binders according state of the art electrodes the invention number in FIG. 3 A B C D E Polymer THV PTFE — SBR LiPAA Used cell capacity 17% 18% 14% 11% 6% due to formation of a surface layer

EXAMPLE 3

(49) In order to reduce the very high capacity of surface layer of a binder-free electrode WO 2011/098233 A2 suggests elaborate measures such as e.g. the temperature treatment of electrodes at 900° C. minimum for at least 10 h or the coating of the surface of the electrode. After the production of the electrode, both measures require time-consuming and costly production steps, such as described in example 1.

(50) Electrodes produces according to example 1 can be used in a battery cell according to the invention without further treatment.

(51) The test was performed analogous to example 2 . In FIG. 4 as well the abscissa of the intercept between curve an 400 mV line corresponds to the used cell capacity due to formation of surface layer. The following electrodes were used: A Electrode with temperature treatment according to WO 2011/098233 A2 B Electrode with coating according to WO 2011/098233 A2 C Electrode with 4% Li-PAA

(52) It is clearly visible that the two electrodes A and B have a substantially higher irreversible capacity loss than electrode C.

(53) Table 3 summarizes the results:

(54) TABLE-US-00003 TABLE 3 Irreversible capacity loss due to formation of a surface layer for electrodes treated differently. electrodes electrode with electrode electrode with temperature treatment with coating 4% Li-PAA number in FIG. 3 A B C Irreversible capacity 12% 8% 6% loss due to formation of a surface layer

(55) It is clearly visible that the irreversible capacity loss due to formation of a surface layer for electrodes, which are manufactured according to procedure described in WO 2011/098233 A2, has still high values.

(56) Surprisingly, an electrode according to the invention shows outstanding behavior even without work-intensive post-treatment.

EXAMPLE 4

(57) In example 4 the influence of the content of binder on the irreversible capacity loss due to formation of a surface layer was examined.

(58) In order to compare electrodes according to the invention with the current state of the art, negative electrodes with different levels of THV binder were produced according to the procedure with acetone as solvent as described in example 1.

(59) Here, the surface layer was determined through charging and discharging at 1 C rate two times. The cycle efficiency of the first two cycles was converted into the irreversible capacity loss due to formation of a surface layer (in % of the theoretical discharge capacity). Table 4 shows the results:

(60) TABLE-US-00004 TABLE 4 Irreversible capacity loss due to formation of a surface layer for electrodes with different content of THV binder. wt % of THV-binder 1 2 4 used cell capacity (in %) of the theoretical 24.1% 26.4% 33.5% capacity, due to formation of a surface layer

(61) It can clearly be seen that a higher level of binder has a negative impact on the characteristics of the electrodes. Whereas the irreversible capacity loss due to formation of a surface layer is still at 24.1% at a binder level of 1 wt % THV, it rises by approx. 10% points to 33.5% at a binder level of 4 wt %.

(62) FIG. 5 shows negative electrodes according to the invention with different content of Li-PAA binder. The capacity of the surface layer can be seen in the graph.

(63) Here, surprisingly, better results can be achieved with a higher level of binder. The performance of the electrodes with binder according to the invention is contrary to the performance of the electrodes with THV binder.

(64) The best results are achieved with a level of Li-PAA of 8 wt %. But also the electrode with a content of Li-PAA of 2 wt % shows a significant improvement of the electrode.

(65) However, the electrodes with a high level of Li-PAA show a lower energy density as the electrodes contain less active material. The increase of binder content within an electrode must not exceed the decrease of irreversible capacity loss due to the replacement of active material, otherwise there is no increase in the further cycleable capacity. Best results are achieved with a binder content of approx. 4 wt %.

EXAMPLE 5

(66) For this experiment, two prismatic full cells with two negative and a positive electrode were produced. The electrodes were stacked together with a separator arranged in between and introduced in a prismatic housing.

(67) Both full cells contained positive electrodes which were produced from the following components according to the procedure in Example 1, yet with acetone as solvent: 94 wt % lithium iron phosphate 2 wt % carbon black (conductivity enhancerr) 4 wt % THV as binder

(68) Positive electrodes with a capacity of 15 mAh/cm.sup.2 were produced.

(69) Full cell B contained binder-free negative electrodes, which were treated with a temperature treatment according to WO 2011/098233 A2.

(70) For the production of complete cell A negative electrodes according to the invention were produced from the following components according to the procedure in Example 1: 96 wt % graphite (active material of the negative electrode) 4 wt % binder A

(71) Negative electrodes with a capacity of 14mAH/cm.sup.2 electrode surface were produced respectively.

(72) The complete cells were filled with an electrolyte consisting of LiAlCl.sub.4×6SO.sub.2.

(73) First, the cell was gradually conditioned at a charge and discharge rate of approx. 0.05 C (equals 10 mA). In doing so, the surface layer is formed and the irreversible capacity loss due to the formation of the surface layer can be determined.

(74) During the following 100 cycles the cells were cycled at a charge and discharge rate of approx. 0.5 C (equals 100 mA). After 100 cycles the charge rate and discharge rate is increased to approx 1 C (equals 200 mA).

(75) The charge was carried out in an IU process. The cell is being charged with a constant current up to a voltage level of 3.6 V. At a cell voltage of 3.6 V the current is reduced. When the current first falls below a minimum current of 40 mA the charge process is stopped.

(76) The discharge was carried out with a constant current and stopped at a cell voltage of 2.5 V.

(77) FIG. 6 shows the discharge capacity over the number of cycles. The two graphs shown indicate average values of various experiments with the electrodes described above.

(78) It is surprising that despite the addition of 4% binder a higher absolute discharge capacity can be reached.

(79) The cycle life of a cell is determined by the retaining a certain discharge capacity, e.g. 70% of the nominal capacity. Due to the higher initial capacity the cycle life of cell A is substantially higher than the one of cell B. Cell A reaches the final discharge capacity later than cell B.