Battery cell, battery module and vehicle

Abstract

Battery cell (1) with a stack or winding of active material (3) which is electrically contacted with the cell poles (8, 2), wherein in the contacting of the active materials (3) with at least one of the cell poles (8, 2) a device (14, 15) for increasing the electrical contact resistance at temperatures below a limit temperature is arranged, characterized in that the device (14, 15) has a bimetal (15) which forms a further current path in the area of ​​the contacting above the limit temperature.

Description

[0001] The invention relates to a single battery cell with a stack or winding of active material, as defined in more detail in the preamble of claim 1. The invention further relates to a battery module with several such single battery cells, and to a vehicle with such a battery module, wherein the vehicle is at least partially electrically powered.

[0002] Individual battery cells, for example in lithium-ion technology, which can be used in particular to power vehicles, are known from the state of the art.

[0003] The typical construction involves the use of stacks or windings of electrode material. These windings consist, for example, of metal foils for the anode, metal foils for the cathode, and separators in between. The metal foils for the anode protrude on one end, and those for the cathode on the other. Collector plates are typically welded onto these metal foils of the winding, which are then connected to the ends of the individual battery cell, and thus to its terminals, using various design features. This results in significant electrical resistance in the connection area, which limits the performance of the individual battery cell.The goal of battery manufacturers is therefore to eliminate these collector plates and implement direct contact between the active materials and the cell terminals, resulting in a lower internal resistance in the individual battery cell than in the conventional design mentioned above. In this context, reference can be made to WO 2020 / 096 973 A1.

[0004] Battery cells constructed in this way represent a positive development in many functional areas. However, they have one crucial disadvantage. At very low ambient temperatures, the otherwise undesirable resistances in the contact area are advantageous because they cause the active materials to heat up very quickly. This is because they are located directly in the area of ​​the active materials, allowing the resulting heat to flow directly into them. Therefore, the elimination of contact resistances, which offers significant advantages in most load cases, leads to a deterioration in cold-charge performance.

[0005] DE 10 2018 003 704 A1 shows the state of the art in the generic sense with an NTC resistor between the electrodes of a battery pole and the current collector or terminal of the respective individual battery cell.

[0006] For further information on the state of the art, reference can also be made to DE 10 2012 213 100 A1. This patent describes a battery with a thermal switch which triggers a safety mechanism above a certain temperature threshold to prevent a further temperature increase of the individual battery cell.

[0007] Furthermore, reference can be made to US 2014 / 0342194A1, which describes a device that can be operated with different internal resistances. This device is a thermal switch controlled by a temperature sensor. US 2011 / 0157755A1 describes, in a similar context, a current path running parallel to the electrodes with a resistor and, optionally, a thermal switch.

[0008] The object of the present invention is to provide an improved single battery cell which offers both the advantages of minimizing internal resistance in the contacting and good cold charging performance.

[0009] According to the invention, this problem is solved by a single battery cell having the features of claim 1, and in particular those of the characterizing part of claim 1. Advantageous embodiments and further developments are described in the dependent claims. Furthermore, a battery module with a plurality of such single battery cells solves the problem, as does a vehicle with at least one such battery module.

[0010] In the case of a single battery cell, a device for increasing the electrical contact resistance at temperatures below a certain threshold is arranged at the contact point between the active materials and at least one, preferably both, of the cell terminals. This device thus maintains a comparatively high contact resistance at very low temperatures. In these situations with very low ambient temperatures, this generates heat at the contact points, which directly benefits the adjacent active material. This is far faster and much more energy-efficient than any other type of heating element, such as heating mats, heating the battery's cooling medium, or similar methods.At the same time, the device ensures that above the limit temperature the contact resistance is not affected or not significantly affected, so that the advantages associated with the reduction of the contact resistances in charging and discharging performance at warmer temperatures are still achieved.

[0011] According to the invention, the device also includes a bimetal strip which, above the limiting temperature, forms an additional current path in the contact area. Such a bimetal strip is fundamentally known, for example, from thermostats, thermal switches, or the like. It can be designed such that at a certain limiting temperature, it deforms so significantly that, above the limiting temperature, this bimetal strip creates an additional current path parallel to a resistive current path. Above the limiting temperature, current can then flow with very low resistance through the bimetal strip and / or a contact mechanically closed by it.Below the limit temperature, the bimetal is shaped in such a way that it does not form a parallel current path, so that a resistive current path is the only current path and thus enables the desired resistance for heating the active materials below the limit temperature.

[0012] According to a particularly advantageous embodiment of the battery cell as described in the invention, the device can be designed in the form of a thermistor. Such a thermistor, often also referred to by the English term NTC, has a significantly higher electrical resistance at correspondingly low temperatures than at higher temperatures. The resistance curve is usually non-linear with respect to temperature and can be influenced by a suitable mixture of materials used in the manufacture of the thermistor. For example, a thermistor can be used whose resistance increases sharply at low temperatures and exhibits a very flat curve or remains largely constant at a low level at higher temperatures, such as above freezing.

[0013] Such a thermistor in the contact area can be designed to allow for large-area contact, so that in the case of a sufficiently warm individual battery cell there is virtually no significant contact resistance, while at sufficiently low temperatures, for example at temperatures below 0°C, the resistance increases significantly.

[0014] This design can be implemented in individual battery cells with stacked active materials, typically so-called prismatic cells, which are housed, for example, in a foil bag or a hard case. It is particularly advantageous to implement this design in cylindrical cells arranged in a cup-shaped casing, where the active materials are wound. These cells have the advantage that such a winding can be manufactured much faster in large quantities with sufficient tolerances.

[0015] This allows for the cup-shaped housing to serve as one pole and a lid closing the cup-shaped housing as the other pole. Several such individual battery cells can then be combined to form a battery module. Such a battery module, with good performance at high temperatures and still offering good cold-charge performance, is particularly suitable as a traction battery in a vehicle, since varying conditions are very common in vehicles. For example, in temperate or northern latitudes, sub-zero temperatures are quite typical in winter, so traction batteries for such vehicles can significantly benefit from improved cold-charge performance. The vehicle itself can be fully or partially electrically powered, i.e., it can be a battery-electric vehicle or a hybrid vehicle.

[0016] Further advantageous embodiments of the battery cell according to the invention also result from the exemplary embodiment, which is described in more detail below with reference to the figures.

[0017] This shows: Fig. 1 a schematic sectional view through the structure of a single battery cell according to the state of the art; Fig. 2 a schematic cross-sectional view through another battery cell according to the state of the art; Fig. 3 a schematic sectional view through a possible embodiment of a single battery cell according to the invention; Fig. 4 a diagram of the resistance curve versus temperature for the embodiments according to Fig. 3 and Fig. 5; and Fig. 5 a schematic sectional view of an alternative embodiment of a single battery cell according to the invention in a first state (a) and a second state (b).

[0018] In the presentation of the Fig. Figure 1 shows a schematic cross-sectional view of a single battery cell, designated as 1 in its entirety. The single battery cell 1 is shown here as a cylindrical cell, purely as an example. It has a cup-shaped housing, designated 2, in which a coil of its active material 3 is arranged. Metal foils, designated 4 and 5 respectively, protrude from this coil of active material 3 at the top and bottom, forming the anode and cathode of the active material 3.

[0019] The upper metal foils 4 are connected to a contact plate 6, which is connected via a tab 7 to one terminal 8 of the battery cell 1. This terminal 8 is electrically insulated from the housing 2 by insulation 9. The housing 2 forms the other terminal of the battery cell 1. For this purpose, the downward-facing metal foils 5 of the active material 3 are connected to a contact plate 10, which in turn is connected to the base 12 of the housing 2 via a tab 11, so that the housing 2, which is deep-drawn from an aluminum alloy, for example, forms the other battery terminal. In practice, these so-called collector plates 6, 10 and their connection via the tabs 7, 11 to the respective terminals 8, 2 of the battery cell 1 exhibit a comparatively high electrical resistance.Based on measurements carried out by the inventors, this is in the range of 20 to 25% of the total cell resistance, depending on the state of charge and temperature.

[0020] It is therefore the stated and understandable goal of manufacturers of individual battery cells 1 to constructively improve these connections for contacting, for example by directly connecting the metal foils 4, 5 to the cell ends or poles 8, 2 without additional collector plates 6, 11. The contact resistance should thus be significantly reduced, ideally completely eliminated. A basic design for this purpose, also known from the prior art, is analogous to the representation in Fig. 1 in Fig. Figure 2 shows that in the lower region of the individual battery cell 1, the metal foils 5 are directly connected to the base 12 of the housing 2. In the upper region, the battery terminal 8 has a plate-like extension 13. The metal foils 4 are directly connected to this extension 13 of the battery terminal 8. This results in a significant reduction in the contact resistances.

[0021] The internal resistance of such a cell is thus reduced by approximately 20% through the reduction of contact resistance in the area of ​​the contacts. The individual battery cell 1 therefore reaches its overheating limit during charging or fast charging much later, or ideally not at all, so that optimal performance can be achieved close to the plating limit of the anode.

[0022] This is fundamentally positive in many operating areas. However, the low heat generation and low resistance in the contact area are not desirable in all operating situations. At very low starting temperatures, for example, temperatures of the individual battery cells 1 below or near freezing, rapid heating of the active material 3 is essential to minimize the internal resistance of the individual battery cells 1. A higher temperature then results in lower resistance, which in turn leads to more current and thus a higher temperature. This positive feedback loop must be initiated under such external conditions to generate acceptable charging times even under these challenging circumstances.

[0023] While a battery, and thus its individual battery cells 1, can in principle be heated via an external heating system, the expensive and complex heating mats, for example those installed in the module base, and the heated cooling medium used to regulate the temperature of the individual battery cells, always have the problem that comparatively high heating power is required before the heat even reaches the active material of the individual battery cells 1. In the configuration according to Fig. 1. This problem does not arise because the resistances in the contact area, which here are caused in particular by the contact plates 6, 10 and the tabs 7, 11, ensure that heat is transferred directly into the immediately adjacent active material. However, if, in the future, the in Fig. If the two depicted battery cells 1 with minimized resistance in the area of ​​the contacts are used, the above-mentioned problem will occur in any case.

[0024] To prevent precisely this and to be able to utilize the advantages of both concepts, a single battery cell 1, as shown in the illustration of the Fig. As can be seen in section 3, the structure is analogous to the representation in the Fig. 1 and Fig. 2. In addition to the structure described so far, an electrical thermistor 14 is located between the metal tabs 5 and the base 12 of the housing 2. A similar electrical thermistor 14 is provided in the area between the cell pole 8 and its extension 13 for the metal foils 4 that protrude from the active material 3. This thermistor 14, also known as an NTC resistor, has the property that its resistance increases sharply and usually non-linearly at low temperatures. In the diagram of the Fig. Figure 4 shows a characteristic curve of an NTC material with a solid line. It clearly shows that there is a sharp increase in resistance below approximately 0-10°C. At low temperatures below 10°C, and especially below 0°C, the resistance is correspondingly high. This results in the following at these low temperatures: Fig. 1. A known behavior in which heat is generated directly within the individual battery cell 1 and contributes to the heating of the active material 3. As the temperatures rise, the resistance is so low above approximately 25°C that the operating principle of the individual battery cell 1 according to Fig. 2. The thermistor 14, acting as a device to increase the electrical contact resistance at temperatures below a certain limit, thus achieves good overall performance of the individual battery cell combined with good cold-charge performance.

[0025] The material for the thermistor 14 can be varied as desired during the manufacturing process to achieve the desired characteristic curve. Various oxides can be mixed, metal oxides doped, and the resulting ceramic material further influenced by the oxygen content during the firing process, all in a manner known per se. This allows for virtually any characteristic curve to be achieved. The representation of the Fig. A characteristic curve shown with a solid line would be very good here to achieve the desired properties.

[0026] In the presentation of the Fig. In two separate images 5a) and 5b), an alternative arrangement to the thermistor 14 can be seen. This consists of a bimetallic strip designated 15. This bimetallic strip 15 is positioned between, for example, a ring-shaped contact element 16. In the illustration of the Fig. 5a) The temperature is now, for example, below freezing. In this situation, pole 8 is connected to the metal foils 4 or the plate-like extension 13 only via the contact element 16. The contact element 16 can have a comparatively high electrical resistance in order to achieve a similar structure to that of the single battery cell 1 in the illustration of the Fig. 1. As the current increases, the heat generation in the area of ​​the contact element 16 also increases; the heat is transferred directly into the winding of the active material 3, in the exemplary embodiment of the Fig. 5 only from above, initiated.

[0027] As the temperature increases, for example to 10 to 20°C, the bimetal 15 is heated accordingly and deforms in a manner known per se. In the representation of the Fig. 5b) The bimetal 15 is deformed to such an extent that it establishes direct contact between the extension 13 and the terminal 8 of the battery cell 1. This creates an additional electrical connection parallel to the contact element 16. This electrical connection can be largely free of resistance, so that at correspondingly higher temperatures, unimpeded electrical conduction occurs, analogous to the construction of the battery cell according to Fig. 2 is to be reached.

[0028] Such bimetals 15 have the advantage that they can be designed in such a way that the deformation occurs almost instantaneously. In the representation of the Fig. Figure 4 shows such a "characteristic curve" of a connection via the bimetal 15, indicated by a dashed line. Up to a temperature of approximately 5 to 10°C, the resistance R is virtually infinite. As soon as this temperature is reached and the bimetal 15 deforms accordingly, the resistance R drops to a value of almost zero. The desired performance can also be achieved analogously to the representation in Figure 4. Fig. 1 at low temperatures and analogous to the representation in Fig. 2. Convert accordingly at higher temperatures.

[0029] In addition, other devices could be conceivable to achieve this behavior, for example temperature-dependent spring elements, temperature-dependent mechanical actuators such as in a thermostat or similar.

Claims

[1] Battery cell (1) with a stack or winding of active material (3) which is electrically contacted with the cell poles (8, 2), wherein in the contacting of the active materials (3) with at least one of the cell poles (8, 2) a device (14, 15) for increasing the electrical contact resistance at temperatures below a limit temperature is arranged, characterized by , that the device (14, 15) has a bimetal (15) which forms a further current path in the area of ​​the contact above the limit temperature. [2] Battery cell (1) according to claim 1, characterized by , that in the contacting of the active materials (3) with both cell poles (8, 2) a device (14, 15) is arranged to increase the electrical contact resistance at temperatures below a limit temperature. [3] Battery cell (1) according to claim 1 or 2, characterized by , that the device (14, 15) has a thermistor (14). [4] Battery cell (1) according to any one of claims 1 to 3, characterized by their formation as a round cell with a cup-shaped shell (2). [5] Battery cell (1) according to claim 4, characterized by , that at least a part of the cup-shaped housing (2) forms one cell pole (2) and the other cell pole (8) is arranged in the area of ​​a lid of the cup-shaped housing (2). [6] Battery module comprising a plurality of battery individual cells (1) according to any one of claims 1 to 5. [7] Vehicle with at least a partially electric drive and at least one battery module according to claim 6.

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