DE8810976U1 - Electrochemical cell - Google Patents

Description

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The invention relates to an electrochemical cell in which the electrodes (anode, cathode) are arranged alternately in layers from the inside to the outside in a circular arrangement with a separator in between.

The electrochemical cell of the invention relates in particular to a lithium cell with a lithium anode for high-performance applications, wherein the cathode is preferably a MnO" cathode, but other cathode materials are also conceivable.

The "round arrangement" of the cell refers to either the bobbin design or the winding design. In the bobbin design, the electrodes are designed as cylindrical tubes and are inserted into each other vertically, separated by a separator that is also cylindrical. In the winding design, the components (electrodes and separator) are laid on top of each other as long strips and twisted into a solid spiral coil. The winding design enables the highest electrical outputs to be achieved.

The use of lithium as an anode material has the advantage that this element has the greatest negative potential of all elements and the highest weight-specific capacity of all anode materials. For this reason, lithium cells form high-performance cells. The lithium anode can be combined with a cathode made of manganese dioxide (MnO ₂ ), for example, i.e. with a solid cathode. Liquid cathodes are also conceivable.

One problem with electrochemical cells of the type mentioned above is the outermost electrode layer. For example, in lithium cells, particularly in the winding design, the outermost lithium layer offers more lithium than can be discharged using the adjacent counter cathode. The capacity of the outermost lithium layer cannot therefore be fully extracted because there is no sufficiently dimensioned counter cathode available. However, optimal use of the available volume, particularly when using solid cathodes, is only possible if the capacity of the anode and cathode in the outer layer of an electrode winding are matched to one another. The same naturally applies to the bobbin design.

An excess of free lithium in a discharged cell represents an acute safety risk if the cell is mistreated, for example by reversing the polarity or charging. This can cause the cell to burst, which of course represents a very high risk of accident. This risk is particularly present when several rows are connected together in a battery. The cell with the lowest capacity is reversed by the other cells at the end of the discharge. In order to rule out the described risk of accidents, the fault current caused by the polarity reversal in batteries with high-performance lithium cells is normally partially absorbed by a diode connected in parallel. However, this protective measure in the form of an eleventh diode is technically complex.

From DE-OS 33 01 297, an electrochemical cell in a winding design with a lithium anode is known, which consists of two anode strips lying on top of each other. A copper strip is embedded between the anode strips, which prevents damage.

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to prevent damage to the cell during irregular operation.

Electrochemical cells are also known in which a separator made of micro glass fibers is arranged between the electrodes and which have an electrolyte. The separator made of micro glass fibers makes use of the positive properties in terms of chemical resistance and electrical properties that these offer. However, the well-known nonwovens (papers) made of micro glass fibers, as used for separators, also have disadvantages.

Firstly, such separators have only low mechanical stability and strength and can therefore only bear a small load. This can lead to the separators being damaged during discharge in wound cells with cathodes that increase in volume during the discharge of the cell, as is the case with Mn0 ₂ cathodes, for example. This can lead to uncontrollable consequences, so that safety is endangered because the necessary strength values of the separators are not achieved. Secondly, the lack of tightness of the known separators means that there is a risk that the anode material will push through the existing defects. This is particularly the case with soft lithium as the anode material.

For this reason, for example in the case of lead-acid batteries, it has already been proposed to use a binding agent to strengthen the micro glass fibres of the separators, which, unfortunately ^, is insoluble in organic solvents.

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Furthermore, in DE-OS 33 23 333, a separator made of micro glass fibers was proposed, which are solidified using a binding agent, for example in the form of polyvinyl alcohol. However, the cells there have liquid positive electrodes.

Another problem with electrochemical cells, and in particular with lithium cells, is the high-performance range, for example in drives. Lithium batteries with organic electrolytes can only be used to a limited extent at low temperatures of up to -30 ⁰ C, as the C voltage level and the available capacity decrease sharply at temperatures below 0 ⁰ C, especially under high loads. As a result, the advantages of lithium cells with non-toxic, organic electrolytes cannot be used in some areas of application.

Another problem is the creation of a stable separator binder/electrolyte combination. It has been shown that certain separator binders are often not usable in cells with organic electrolytes.

From DE-PS 23 04 424 an electrochemical cell with an electrolyte is known which provides a mixture of dioxolane with ethylene carbonate.

Starting from an electrochemical cell, in particular a lithium cell, of the type specified at the outset, the invention is based on the task of providing, without major technical

Efforts to increase the safety of the cell.

As a technical solution, the invention proposes that the outermost electrode layer of the cell has a lower material thickness, in particular essentially half as thick, than the corresponding electrode layers inside the cell.

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An electrochemical cell designed according to this technical theory has the advantage that its safety is significantly improved without requiring any special technical effort. The basic idea is to reduce the supply of electrode material in the outermost electrode layer, for example the lithium supply in a lithium anode. In addition to the advantageous extraction of a minimum of volume, there is no free lithium left in an empty lithium cell, since the cell does not contain more active mass than can actually be discharged. The capacity of the cell must be determined very precisely in terms of design and adhered to very precisely in terms of production technology. This is a decisive advantage in terms of safety if the cell is mistreated by reversing the polarity or charging. In particular, the cell is safe even without special protective measures in the form of diodes.

The inventive concept can be applied both to electrochemical cells in the so-called bobbin design and to cells in the wound design.

Starting from the last-mentioned electrochemical cell in a wound construction, where the anode, in particular lithium anode, consists of two anode strips lying on top of one another, it is proposed in a further development that the anode in the cell is wound on the outside and that one anode strip is shortened in relation to the other anode strip in such a way that the outermost anode layer of the cell consists of only a single anode strip.

Such a cell, especially a lithium cell in the so-called winding construction, is very easy to manufacture. The anode foils are wound with the corresponding counter electrode, whereby one anode strip of the anode is of normal length, while the other anode strip is

over shortened. The outermost anode layer is half as thick as the anode layer inside the cell, so that the outermost anode layer is completely used up over the life of the cell. In a lithium cell, a solid cathode is used as the cathode, preferably made of manganese dioxide.

Advantageously, the outer anode strip is shortened relative to the inner anode strip of the outermost anode layer of the cell.

In a further development, it is proposed that a discharge strip made of electrically conductive material, in particular copper, is arranged between the two anode strips. This inserted discharge foil has the advantage that the corresponding pole of the cell can be led outwards in a technically simple manner.

Preferably, the two anode strips are firmly connected to one another, in particular they are pressed together. This is particularly advantageous for a lithium anode, since lithium is a very soft metal, so that the anode strips can be permanently pressed together by simply applying pressure.

In a further development of the separator, it is proposed that it consists of microfibers, in particular microglass fibers, which are solidified by means of a binding agent. The binding agent of the separator is preferably a butadiene rubber.

The advantage of using a binding agent for the separator is that it can be made very thin, thus ensuring high cell performance. The nonwoven fabric has very good mechanical stability and is break-resistant, making it suitable for many manufacturing techniques.

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sufficiently tensile so that it can be used, for example, on winding machines for the production of wound cells. At the same time, the nonwoven material has the necessary tightness to prevent the soft lithium anode in particular from being pushed through, so that a high level of safety of the cell is guaranteed. This also applies in the event that the cell is incorrectly charged. Despite the butadiene rubber as a binding agent for the separator, a high porosity of approx. 90 % is guaranteed, so that the corresponding electrochemical cell is suitable as a high-performance cell. For example, it is possible to create a prismatic lithium MnO ₂ - j high-performance cell of 2.8 V, 60 Ah and 120 A, which can be operated at temperatures of up to 150 ⁰ C at the end of the discharge.

The butadiene rubber preferably contains acrylonitrile, preferably 40% to 95%, methacrylic acid, preferably 5 % , and butadiene, preferably 55% to 56%. This substance is known as "Perbunan NKA 8250" from BASF.

The proportion of binding agent is between 2 % and 10 %, preferably 7 % of the weight of the microfibers.

Furthermore, the basis weight of the separator is preferably

approximately 50 g/m .

In a further development of the electrolyte of the electrochemical cell, it is proposed that this is an organic electrolyte.

It has been shown that a separator with butadiene rubber as a binding agent can surprisingly be used in cells, especially lithium cells with organic electrolytes. This creates a chemically stable separator binding agent/electrolyte combination. In addition to the chemical resistance of the butadiene rubber in organic electrolytes,

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It is also thermally resistant. The chemical resistance of the butadiene rubber with regard to the organic electrolyte is a further safety benefit for the electrochemical cell, as the separator can reliably meet the requirements placed on it over the entire service life of the cell.

In a further development of the organic electrolyte of the electrochemical cell, it is proposed that this be an organic solvent mixture of ethylene carbonate and dioxolane in which a conducting salt in the form of the salt of perchloric acid is dissolved. By using an electrolyte of this type in combination with butadiene rubber as a binding agent for the separator, a high-performance lithium cell is created, in particular with an Mn0 _o cathode, which is suitable for low temperatures down to -30 ⁰ C and high loads, as the electrolyte increases the voltage level, particularly at these low temperatures of -30 ⁰ C. In this way, approximately the same performance characteristics are achieved at -30 ⁰ C as with conventional electrolytes at 0 ⁰ C. The electrolytes are made up of inexpensive components which are above all environmentally friendly and, in comparison to other electrolytes, are free of arsenic, fluorine, nitriles, acids and chlorinated hydrocarbons.

The essential characteristic of an electrolyte suitable for low temperatures and high loads is its ability to dissolve the highest possible concentrations of a conducting salt under these two conditions. These properties are achieved by using an organic substance with a very high dielectric constant, which is a solid under normal conditions and can dissolve a very large amount of a conducting salt in the molten state. The favorable low-temperature properties are achieved by diluting this base electrolyte with a suitable solvent, which preferably consists of five-membered heterocycles.

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All of these conditions are fulfilled by the organic solvent mixture according to the invention with the conductive salt dissolved therein in the form of the salt of perchloric acid, the cation of this salt being formed by the anode element.

In an advantageous development of the solvent mixture of the electrolyte, this is a solution of 1 mol ethylene carbonate in approximately 11 mol 1.3 oxolane. This corresponds to 1.2 mol/l ethylene carbonate or 13.2 mol/l ethylene carbonate.

8 vol.-SS ethylene carbonate and 92 vol.-SK dioxolane or 9.8 wt.-Si ethylene carbonate and 90.2 wt.% dioxolane.

The conducting salt is dissolved in the solvent mixture at 1 to 2.5 mol/l, especially 1.75 to 2 mol/l. This applies in particular to LiClO as the conducting salt.

An embodiment of an electrochemical cell according to the invention is described below with reference to the drawings. In the drawings:

Fig. 1 is a perspective view of an electrochemical cell cut out in sectors to reveal the interior;

■ Fig. 2 a sector-shaped section

from the electrochemical cell in Fig. 1 on an enlarged scale;

Fig. 3 a side view of an anode package of the

electrochemical cell before mounting;

Fig. 4 is a section along the line IV-IV in Fig.

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Fig. 5 is a front view of the anode package of Fig.

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Fig. 6 a side view of an electrode package

the electrochemical cell before winding;

Fig. 7 a bottom view of the electrode package in

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Fig. 8 is a graph showing the electrolyte used for the electrochemical cell,

Fig. 1 shows an electrochemical cell 1 in a winding design. In order to see the interior of the cell 1, it is cut out in a sector-like manner, whereby the outer layers in particular can be seen.

The cell 1 consists of a cylindrical housing 2, in particular made of stainless steel. The two end faces of this cylindrical housing 2 are each welded to a cover 3, which is each filled with a casting compound 4. A glass/metal pole feedthrough 5 is arranged in the upper cover 3. A connection lug 6 for one pole of the row 1 leads up through this. In the lower area of the row ¹ , the housing 2 has a corresponding connection lug 6' for the other pole of the cell 1.

A spirally wound electrode package 7 is arranged inside the housing 2. This consists of a lithium anode 8 and a cathode 9, in particular a solid cathode made of MnO ₃ , for example. A separator IO is arranged between the anode 8 and the cathode 9. The special structure of the electrode package 7 is described below and is shown in Figs. 3 to 7 before winding and in Fig. 2 after winding in a sector-like section of the cell 1.

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The anode package II of the electrode package 7 is shown in Fig. 3 to 5. The anode 8 consists of 2w«l anode strips 8', ^8M in the form of coils, between which a discharge strip 12 is arranged. The discharge strip 12 is embedded like a sandwich between the two superimposed anode strips ⁸¹ , ^8'1 . The discharge strip 12 can consist of copper, for example. At one protruding end it is angled by 90° and thus forms the connection for the anode pole of the cell 1. This can be seen in particular in Fig. 4.

In Fig. 3 and 4 it can be clearly seen that one anode strip ^8'11 is shortened in relation to the other anode strip 8', in such a way that when the electrode package 7 is wound up, the outermost layer of the anode 8 has only a single anode strip, namely the normal length anode strip 8 ^' . The outermost anode layer is therefore only about half as thick as the inner anode layers, since in the interior of the cell 1 the anode layers are each formed by both the anode strip ^8'1 and the anode strip ^8'1 . This can be seen in particular in the sector-like representation in Fig. 2. This ensures that the cathode layer adjacent to the outermost anode layer can react with the entire anode material of the outermost anode layer without any anode material remaining after the cell has been discharged, since the outermost anode layer has no counter-cathode on its outside.

In Fig. 3 it can also be seen that a separator 10 is arranged on both sides of the anode 8 from the two anode strips 8', 8". This consists of microfibers, in particular micro glass fibers. These fibers essentially have diameters between 0.5 and 4 micrometers, whereby 0?^C ^: - and smaller diameters are not a problem. They are used in the wet flow process.

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to form a dense paper. In order to increase the mechanical stability and the density of the nonwoven fabric, and in the latter case to prevent the anode strips 8 ¹ , 8'* made of soft lithium from being pressed through, the fibers are bonded together using an organic binder in the form of butadiene rubber, which is present in concentrations between 2 and 10 % of the weight of the microfibers. The binder is added to the pulp as a dispersion and is flaked with charged polyelectrolytes immediately before the nonwoven fabric is laid. The binder can consist, for example, of a mixture of 40 _% acrylonitrile, 5% methacrylic acid and 55 % butadiene.

The finished electrode package 7 is then shown in Fig. 6 and 7. The cathode 9 is applied to the anode package 11 from Fig. 3 to 5. This initially consists of a metal conductor in the form of an expanded metal grid 13, into which the cathode material, for example MnO ₂ for a solid cathode, is fitted. Correspondingly, in the conductor strip 12 of the anode 8, the expanded metal grid 13 also has connections.

The electrode package 7 thus formed is wound spirally in the winding direction W, with the reference line B indicated in Fig. 6 before the start of the winding. The electrode package 7 thus wound is then inserted into the housing 2 of the cell 1, as shown in Fig. 2 and previously described.

The electrochemical cell 1 also has an organic electrolyte which is chemically compatible with the binding agent of the separator 10. This is an organic solvent mixture of ethylene carbonate and dioxolane, in which a conductive agent in the form of the salt of perchloric acid is present.

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This electrolyte exhibits significantly better performance at low temperatures than conventional electrolytes. This is shown in the graph in Fig. 8, where the total capacity Q of the electrochemical cell 1 based on 100 % Ah is plotted against temperature. This is a lithium cell with a surface current of 15 mA/cm and a COV of 2 V.

The temperature behavior of a conventional electrolyte is shown with dashed lines, while the temperature behavior of the electrolyte according to the invention is shown with solid lines. It is clearly visible that the temperature behavior of the new electrolyte below ^{0 0} C is much better than the temperature behavior of a conventional electrolyte.

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List of reference symbols

1 electrochemical cell 2 Housing 3 Lid 4 Casting compound 5 Pole feedthrough 6 Connection flag 6 ¹ Connection lug > 7 Electrode package 8th anode 8th' Anode strip 8th" Anode strip 9 cathode 10 . separator 11 Anode package 12 Conductive strips 13 Expanded metal mesh W Winding direction B Reference line

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Claims (14)

Electrochemical cell Expectations in which the electrodes (anode 8, cathode 9) are arranged in a circular arrangement in alternating layers from the inside to the outside, with a separator (10) in between, characterized in that the outermost electrode layer of the cell (1) has a lower material thickness, in particular substantially half as thick as the corresponding electrode layers inside the cell (U). 2. Electrochemical cell (1) according to claim 1 in wound construction, wherein the anode (8), in particular lithium anode, consists of two superimposed anode strips (β ¹ , 8"), &bull;^characterized in that the anode (8) in the cell (1) is wound on the outside and that one anode strip (8··) is shortened relative to the other anode strip (8·) in such a way that the outermost anode system of the cell (1) consists of only a single anode strip (8·). 3. Cell according to claim 2, characterized in that the outer anode strip (8··) is arranged with respect to the inner anode -z- OJU) if HH T«Wk.iiH4VpM«4.TM#fc» o;U/Ji Jj 13 ?« 10-JdJ , ·· ·■· ■· Il Il ■■ strip <8·) of the outermost anode layer of the cell (1) is shortened . 4. Cell according to claim 2 or 3, characterized in that a discharge strip (12) made of electrically conductive material, in particular copper, is arranged between the two anode strips (8 ¹ , 8"). 5. Cell according to one of claims 2 to 4, characterized in that the two anode strips (8 ¹ , 8··) are firmly connected to one another, in particular pressed together. 6. Cell according to one of claims 1 to 5, characterized in that the separator (10) consists of microfibers, in particular microglass fibers, which are solidified by means of a binding agent. 7. Cell according to claim 6, characterized in that the binding agent of the separator (10) is a butadiene rubber. 8. Cell according to claim 7, characterized in that the butadiene rubber contains acrylonitrile, preferably 40 %, methacrylic acid, preferably 5 % , and butadiene, preferably 55 % . 9. Cell according to one of claims 6 to 8, characterized in that the proportion of the binder is between 2 % and 10 %, preferably 7 % of the weight of the microfibers. 10. Cell according to one of claims 6 to 9, characterized in that the basis weight of the separator (10) is approximately 50 g/m ² . &bull; · &igr; ill t< .. &bull; I Il Il III If t ''« &igr;&igr;·&igr;&igr;««; 11« Cell according to one of claims 1 to 13, characterized in that the electrolyte of the electrochemical cell (1) is an organic electrolyte. 12. Cell according to claim 11, characterized in that the organic electrolyte is an organic solvent mixture of ethylene carbonate and dioxolane in which a conductive salt in the form of the salt of perchloric acid is dissolved. 13. Cell according to claim 11 or 12, characterized in that the solvent mixture of the electrolyte contains a solution of 1 mol of ethylene carbonate and approximately 11 mol of 1,3 dioxolane. 14. Cell according to claim 12 or 13, characterized in that the conducting salt is dissolved in the solvent mixture at 1 to 2.5 mol/l, in particular 1.75 to 2 mol/l.