Separator, battery cell and methods for monitoring them

Abstract

Separator (8) for a single battery cell (1) made of a porous polymer material characterized by at least one porous layer (8') extending over the entire surface of a separator surface made of a plastically or elastically deformable material, wherein the porosity of the layer (8') is determined such that at a given pressure value at the level of a critical maximum pressure (pmax) the pore volume in the layer (8') becomes so small that no ion conduction through the pores of the layer (8') takes place.

Description

[0001] The invention relates to a separator for a single battery cell according to the type defined in more detail in the preamble of claim 1. Furthermore, the invention relates to a single battery cell comprising such a separator. The invention further relates to a battery module composed of such single battery cells. Furthermore, the invention relates to a method for monitoring such a single battery cell or such a battery module. Finally, the invention also relates to a vehicle.

[0002] Single battery cells are generally known from the prior art. For example, in lithium-ion cells, a separator, or battery separator, is arranged between adjacent anodes and cathodes of the single battery cell. This separator is typically made of a porous material and allows ion conduction due to the material's porosity.

[0003] DE 10 2011 121 246 A1, for example, describes a battery separator made of porous polymer. This separator is constructed in such a way that it has a variable porosity along its length or surface. Such battery separators can increase the uniformity of the current density in individual battery cells, since higher current densities and higher temperatures typically occur near the terminal lugs. These are compensated for by the variable porosity of the separator, so that, for example, it has a lower porosity in the area of ​​the terminal ends than at the opposite end. This allows the transport of ions, such as lithium ions, through the separator to be more restricted in the range of the normally very high current than in the range of the normally lower current. By varying the porosity across the surface of the separator, a uniform current density can be ensured across the surface of the electrode.

[0004] Furthermore, the pressure dependence of the ionic conductivity of separators was investigated in Sauerteig, D., Hanselmann, N., Arzberger, A., Reinshagen, H., Ivanov, S., Bund, A.: Electrochemical-mechanical coupled modeling and parameterization of swelling and ionic transport in lithium-ion batteries, Journal of Power Sources (2018) 235-247.

[0005] A fundamental problem with individual battery cells, and especially with lithium-ion cells, is that their expansion changes both during charging and discharging, as well as over the battery's lifetime. Such growth behavior over battery aging is particularly difficult to predict in advance. Many unknown factors play a role, such as temperatures, operating mode, but also calendar influences, production fluctuations, and / or tolerances. The mechanical stress in the cell or battery casing therefore leads to an increase in pressure in the casing when the mechanical layer structure of the cell expands against the casing. Furthermore, production tolerances (materials and processes) and rapidly expanding active materials, such as silicon compounds, influence cell thickness growth.

[0006] Nevertheless, the individual battery cells should be designed in such a way that, in the event of such growth, neither the functionality of the cell nor a subsystem is impaired, nor is the periphery affected by mechanical changes in its function.

[0007] To improve the safety of individual battery cells, it would be desirable to be able to address this potentially unknown growth due to aging early on in their design, thus minimizing safety risks. In contrast, to achieve high power density or high power volume, another design goal in the construction of individual battery cells is naturally to keep their volume and mass as small as possible. Maintaining larger volumes is just as disadvantageous as using thicker and heavier, and thus potentially more stable, materials for the housing.

[0008] These opposing efforts give rise to the task of providing a battery separator, a single battery cell, or a battery module constructed from it that can ensure high safety while maintaining a compact design. Furthermore, the task of the present invention is to provide a method for monitoring such a battery for critical conditions.

[0009] According to the invention, this object is achieved by a battery separator having the features of claim 1. Advantageous further developments emerge from the dependent subclaims. A single battery cell with such a separator also achieves the object. A comparable situation applies to a battery module constructed from such single battery cells. The object is further achieved by a method for monitoring such a single battery cell or such a battery module according to claim 6. Advantageous embodiments and further developments thereof emerge from the dependent subclaim. A vehicle according to claim 8 also achieves the object. Here, too, an advantageous further development of the vehicle arises from the dependent subclaim.

[0010] A separator or battery separator according to the invention is constructed from a porous polymer material, similar to what is known from the prior art. Such a porous polymer material provides the necessary porosity for ions in a suitable electrolyte to pass through the separator. At the same time, the polymer material ensures electrical insulation between the opposing electrodes arranged adjacent to it, so that they do not come into direct contact, but are only electrically connected to each other via ion conduction through the separator.

[0011] The separator according to the invention is designed to have at least one porous layer made of a plastically or elastically deformable material extending over the entire surface of the separator. The porosity of this layer is designed such that, at a given pressure value, the pore volume in the layer becomes so small that ion conduction through the pores of the layer no longer occurs. The pores of the layer are thus narrowed or squeezed to such an extent that, upon reaching the given pressure value, which should be the maximum pressure the cell should be able to withstand without damage, ion conduction through the separator is inhibited.

[0012] The separator, and in particular its layer, is compressed under increasing pressure, which occurs particularly due to cell growth due to aging. Preferably, the layer is more elastic than the rest of the separator, so that most of the deformation occurs first in the layer.

[0013] Alternatively, the layer could also form the entire separator, so that its porosity is designed in such a way that at a given pressure value the pore volume in the separator becomes so small that no more ion conduction through the pores of the layer takes place.

[0014] If cell thickness growth occurs due to aging, this typically originates in the active materials of the individual battery cell. Due to its pores, the separator is the only significantly elastic component in a conventional cell structure. Increasing pressure therefore essentially causes deformation of the separator. This deformation acts primarily in the thickness direction; the effects perpendicular to this are typically negligible, so the area of ​​the separator can be assumed to remain constant. The increasing pressure therefore presses the separator and the layer, if independently formed, together in the thickness direction. This reduces the volume of the pores and increases the internal resistance. Above the critical maximum pressure, the pores of the separator or layer are squeezed so tightly that ion conduction is no longer possible.The remaining pore volume of the layer or separator can then approach zero or even be zero. In this case, the affected battery cell is shut down, resulting in an intrinsically safe battery cell.

[0015] The separator according to the invention thus makes it possible to produce intrinsically safe individual battery cells or battery modules in which individual battery cells that reach a critical cell growth level are automatically shut down. Nevertheless, it can be helpful to initiate battery monitoring in parallel in order to predict a potentially impending shutdown of individual battery cells.

[0016] A method according to the invention for monitoring such a single battery cell or a battery module composed of such single battery cells for critical pressure increases can therefore provide for the internal resistance of the single battery cell or each of the single battery cells to be recorded and monitored. The current value of the internal resistance is compared with characteristic- or model-based default values ​​in order to detect an increase in internal resistance caused by a pressure increase.

[0017] The separator according to the invention described at the beginning not only ensures that ion conduction is prevented at the moment the maximum pressure occurs, but rather the pores of the layer or separator are already somewhat reduced in advance due to the increase in pressure, so that while ion conduction can still occur in principle, its efficiency is limited. This leads to an increase in the internal resistance of the respective individual battery cell. This occurs even before the automatic shutdown occurs. By monitoring the internal resistance of the individual battery cell or all individual battery cells in a battery module, it is possible to predict the expected remaining service life of the individual battery cells, up to and including shutdown due to the maximum pressure reached or the corresponding maximum cell growth achieved.

[0018] According to a very advantageous development of this method, it can therefore be provided that, starting at a predefined threshold value or a limit gradient of the internal resistance, an error message is generated and stored and / or output. The generated error message can thus be stored accordingly in an error log, for example, so that it can be read out as needed during maintenance in the workshop. Alternatively or in addition, the error message can also be output directly to inform a person using the individual battery cell or battery module about the impending problem.

[0019] The design is, in principle, suitable for any type of single battery cell, especially lithium-ion cells. The single battery cells or battery modules can be used for a variety of applications. High safety is particularly important for large battery modules or multiple battery modules interconnected to form a large battery. Such batteries, consisting of a large number of battery modules and thus a large number of single battery cells, often play a role in high-performance applications. These applications can include, in particular, the use of such batteries to store electrical drive energy in vehicles.

[0020] It is precisely here that the separator according to the invention demonstrates its particular safety advantages.

[0021] Further advantageous embodiments of the separator according to the invention also emerge from the remaining dependent claims and from the exemplary embodiment which is illustrated in more detail below with reference to the figures.

[0022] Showing: Fig. 1 a schematic cross-section through a single battery cell; Fig. 2 the separator according to Fig. 1 at low pressure load; Fig. 3 the separator from Fig. 2 when subjected to maximum pressure; Fig. 4 a diagram illustrating the systematics of a single battery cell with such a separator; Fig. 5 a diagram of the normalized separator volume and its porosity versus pressure; Fig. 6 a representation of an alternative embodiment of a separator analogous to the representation in Fig. 3; Fig. 7 another diagram illustrating an optional supplementary monitoring procedure; and Fig. 8 a schematically indicated vehicle with a battery, which has individual battery cells according to Fig. 1 includes.

[0023] In the presentation of the Fig. 1 shows a schematic cross-section through part of a single battery cell 1. In the thickness direction D shown here, two parts of a cell housing 2 delimit the materials of the single battery cell 1 in their thickness and height directions. Purely by way of example, in the embodiment shown here, an anode current collector 3, typically made of copper or a copper alloy, is indicated following the lower section of the housing 2. This is followed by an active material 5 of the anode. Starting from the upper section of the housing 2, a cathode current collector 6, typically made of aluminum or an aluminum alloy, is arranged, followed by the cathode active material 7. A separator 8 is arranged between these two electrodes or their active materials 5, 7.This separator 8 is typically made of a porous polymer material and, on the one hand, is electrically insulating between the active materials 5, 7, and, on the other hand, allows ionic conduction due to its porosity (indicated here). Ionic conduction is typically enabled by an electrolyte fluid with which the active materials 5, 7 and separator 8 are equally impregnated. In the exemplary embodiment shown here, the passage of four lithium ions 9, represented here as black dots, through the separator 8 is schematically indicated. The different assumed movement paths of the lithium ions 9 within the separator 8 are indicated.

[0024] The Fig. Figure 2 again depicts the separator 8 with the lithium ions 9 migrating through it due to ion conduction. These lithium ions then ensure an ion current flow within the single battery cell 1, here from the cathode to the anode.

[0025] As already mentioned at the beginning, various mechanisms lead to cell thickness growth as the battery cell 1 ages. This means that the materials within the battery cell grow, particularly in the direction of their thickness. This particularly affects the active materials 5, 7. The more or less rigid housing 2 of the battery cell 1 only allows such thickness growth to a certain extent. Therefore, the cell thickness growth leads to an increase in the pressure p within the housing 2 of the battery cell. Since the active materials 5, 7 themselves grow and typically have comparatively low elasticities, and since the current collectors 4, 6 also have very low elasticities and layer thicknesses, the increase in pressure essentially results in the separator 8 being compressed.This compression typically occurs in the thickness direction D, since, due to the characteristic shape of the separator 8 as a flat or coiled plate, a change in size transverse to the surface plays a negligible role. The pressure p therefore essentially compresses the separator 8 in its thickness direction D.

[0026] However, a pressure increase above a critical level in the housing 2 of the single battery cell can cause the cell housing 2 to rupture or a deformation of the cell housing 2, causing attachments to be critically deflected. If such a scenario occurs, there is an acute risk of a short circuit within the single battery cell 1 and / or in the area of ​​the single battery cell 1, for example, if electrical terminal lugs are displaced or something similar. This safety-critical situation should be prevented if possible.

[0027] In the separator 8 described here, this is achieved by designing its porosity in such a way that at a critical maximum pressure p max the volume V of the separator 8 is reduced to such an extent that the pores are reduced in size or completely closed. This is shown schematically in the illustration of the Fig. 3 indicated.

[0028] The representation of the Fig. 2 still recognizable pores within the separator 8 have been closed accordingly due to the compression of the separator 8 in its thickness direction D shown here from top to bottom, so that the residual volume of the pores within the separator 8 approaches zero. As a result, it is no longer possible for the lithium ions 9 to pass through the separator 8. They are therefore trapped by the separator 8 in the embodiment shown here above the separator 8 and thus analogous to the representation in Fig. 1 is retained in the cathode active material 7. The single battery cell 1 has thus switched itself off by compressing the separator 8 until it no longer allows ion conduction, before the pressure p could cause safety-critical problems.

[0029] In the presentation of the Fig. 4, this concept is explained again using a diagram. The box labeled A symbolizes cell growth, which leads to a pressure development, which is indicated within box A by box B. This results in a compression of the separator 8, which is symbolized here by box C. This could also be understood as a kind of passive "measurement" of aging in the surrounding box D. Ultimately, this mechanism leads to an intrinsically safe single battery cell 1 or a battery module 10 constructed from such single battery cells 1, which will be explained later and is indicated here by another box. Within this intrinsically safe battery module 10, the defined closure of the separator 8 for the ion conduction at a predetermined maximum pressure p maxrepresented by the box labeled E. Ultimately, this leads to a target information labeled Z, within which the shutdown of the respective single battery cell 1 due to a safety risk caused by cell thickness growth is symbolized by the box labeled F.

[0030] When designing the separator 8 to construct such an intrinsically safe single battery cell, the following procedure can be followed: First, a maximum permissible pressure p of the cell casing 2 must be determined. This can be defined, for example, by a maximum permissible deformation of the battery casing that is classified as just non-critical. The maximum permissible pressure p can be determined from mechanical simulation and / or expansion testing. A safety buffer can be added. This then results in the maximum pressure p max in the sense of this description.

[0031] The design of separator 8 is based on the assumption that this analysis considers only the pressure generation due to cell thickness growth perpendicular to the electrode stack and housing wall, i.e., in the thickness direction D, since this is the determining factor. Soft plastic or polymer materials are assumed as materials for separator 8, in accordance with the state of the art for separators 8. The exact manufacturing process of the pores of separator 8 is irrelevant for the proposed design process, since the process is based on the pressure-variable porosity of separator 8.

[0032] Theoretical design depending on the pressure p: Separator volume: Vsep(p)=A⋅t0⋅(1+ε(p)) with the pressure-dependent separator expansion ε(p)=−pEsep

[0033] This means: p Pressure on the cell surface resulting from cell thickness growth A separator surface (electrochemically active) t separator thickness t0 initial separator thickness E sep Compression modulus of the separator

[0034] The following assumptions are made for the interpretation: 1. When compressed by pressure p in the thickness direction, only the separator thickness t is deformed, since the length and width of the separator are >> the separator thickness t. 2. In the quasi-static case (relevant in battery cell), hydrodynamic pressure changes due to fluid movements can be neglected since the separator 8 is porous / open towards the electrodes. 3. The volume change of the separator 8 due to compression is based exclusively on changes in the pore volume V Poren , since the cavities in the plastic offer a much lower mechanical resistance than those of the plastic skeleton and the pore volume V Poren in separator 8 is functionally large (usually > 40%). The third assumption results in: |ΔVsep(p)|=|ΔVPoren(p)| from which the pore volume V Poren depending on the pressure p can be derived: Vpores(p)=Vpores,0−A⋅t0∈(p)

[0035] This means: V separator volume V0 initial separator volume V Poren Pore ​​volume V Poren,0 initial pore volume

[0036] The initial separator volume V0 can be determined, for example, using the mercury porosimetry method.

[0037] Now the separator 8 should be at maximum pressure p max no longer conduct ions. This can be achieved at least if all pores are closed. For this to happen, the following condition must be met: VPore(pmax)=!0 Example calculation

[0038] In the presentation of the Fig. 5 is now represented by a dashed line equation (1) and a solid line equation (4) normalized, whereby a realistic compression modulus (E-modulus) of a separator 8 (e.g. from the above-mentioned article by D. Sauerteig et al.) was used for the calculation. It results in Fig. 5 a maximum pressure of p max = 5 MPa, above which the porosity and thus the ion flow would be zero.

[0039] In practical design, the pressure-dependent effective compression modulus E(p) of separator 8 can be measured, for example, by a force-displacement measurement using a suitable measuring device, such as a press. This is familiar to those skilled in the art.

[0040] After inserting equation (4) with the determined maximum pressure p max , the maximum pressure defined as critical, the following equation solved for t0 can be derived by several transformations: t0=V0,Pores∗EsepA∗pmax

[0041] The result corresponds to the required initial separator thickness t0 so that at the expected maximum pressure p max , which was considered critical, e.g., with regard to the mechanical integrity of the battery casing, separator 8 would regulate the ion flow. Equivalently, the equation can be rearranged for the initial pore volume V0, and separator 8 can be optimized accordingly: V0,Pores=t0∗A∗pmaxEsep

[0042] If the installation space is limited, the focus can be on optimizing the pore volume V0, otherwise the separator thickness t0 can be varied.

[0043] The successful, pressure-dependent pore closure can then be validated using ion flow measurements (effective ionic conductivity) under a defined pressure. The results may need to be fitted to measurement curves and corrected, as the design assumptions may introduce errors. The proportion of separator solid material that may be compressed in addition to the pores due to the expansion of the skeletal webs can be estimated and optimized, for example, using simple tests based on the Archimedes principle.

[0044] As an alternative to the use of a separator 8, which takes these considerations into account in its porosity, it would also be conceivable to design the structure in such a way that the separator 8 is largely designed according to conventional criteria. As shown in the illustration of the Fig. 6, a layer 8' can be provided on one of its surfaces, or in principle on both of its surfaces or embedded in the middle, which ensures the intrinsic safety of the individual battery cell 1. This layer 8' can have a relatively large pore size and typically has a higher elasticity than the rest of the separator 8. This layer 8' is therefore the primary part of the separator 8, which absorbs the resulting pressure p. If the critical maximum pressure p max , then the pores within layer 8' are closed accordingly, forming a barrier for the lithium ions 9, preventing them from penetrating the separator at all, thus preventing ion conduction through the separator 8. The principles regarding the design of the porosity of layer 8' are the same as described above.

[0045] Even a significant reduction in the effective ion flow would result in a significant increase in the cell's internal resistance, which could be detected by a battery control unit 11 equipped with the appropriate device. Optionally, a model-based determination of a characteristic internal resistance profile of the battery can also be determined. If an increase in internal resistance occurs during later operation due to the separator closure, this can be stored in the battery control unit 11 as an early warning.

[0046] In addition to the Fig. Based on the concept of passive “measurement” described in chapter 4, an active measurement could now be established in the system, for example in the vehicle. The diagram of the Fig. 7 takes up this concept and shows again the boxes D to F, 10 and Z from Fig. 4, which are also to be understood analogously here. The target information Z can now also be obtained through the active measurement mentioned above, which is indicated here by box G. Box H within this box G symbolizes the measurement of a pressure-dependent characteristic change in the electrical internal resistance of the individual battery cell and, optionally, the capacity development of the individual battery cell. Via a battery control unit indicated by box 11, suitable measurement curves and / or models can now be used to detect the pressure influence on the performance of the individual battery cell in the block designated I. In the block designated J, a plausibility check of the internal resistance drop can then be carried out as clearly pressure-dependent and resulting from the separator.If this is the case, then an early initiation of a shutdown due to a critical pressure development caused by the cell thickness growth can take place within the battery control unit 11 by the box indicated by K, so that the intrinsically safe functionality of the separator 8 only serves as a safety redundancy, which comes into effect if a warning or error message from the battery control unit 11 has not already been responded to.

[0047] As already mentioned above, several such individual battery cells 1 can be combined to form a battery module 10, as shown in the illustration of the Fig.8 is indicated schematically. One or more such battery modules 10 then form a battery 12, which here, purely by way of example, is intended to comprise two such battery modules 10. This battery 12 has a battery control unit 11 and can be used, for example, in the vehicle 13 indicated schematically here to provide electrical drive energy. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DE 102011121246 A1

[0003] Cited non-patent literature

[0000] Sauerteig, D., Hanselmann, N., Arzberger, A., Reinshagen, H., Ivanov, S., Bund, A.: Electrochemical-mechanical coupled modeling and parameterization of swelling and ionic transport in lithium-ion batteries, Journal of Power Sources (2018) 235-247

[0004]

Claims

[1] Separator (8) for a single battery cell (1) made of a porous polymer material characterized by at least one porous layer (8') extending over the entire surface of a separator surface and made of a plastically or elastically deformable material, wherein the porosity of the layer (8') is predetermined such that at a predetermined pressure value (p max ) the pore volume in the layer (8') becomes so small that ion conduction through the pores of the layer (8') no longer takes place. [2] Separator (8) according to claim 1, characterized by that the layer (8') has a higher elasticity than the remaining part of the separator (8). [3] Separator (8) according to claim 1 characterized by his education solely through the shift (8'). [4] Single battery cell (1) with two electrodes and a separator (8) in between, characterized by that the separator (8) is designed according to one of claims 1 to 3. [5] Battery module (10) with several individual battery cells (1) according to claim 4. [6] Method for monitoring a single battery cell (1) according to claim 4 or a battery module (10) according to claim 5 for critical pressure increases in at least one of the single battery cells (1), for which purpose the internal resistance of the / each of the single battery cells (1) / single battery cells (1) is recorded and compared with characteristic curve- or model-based default values ​​in order to detect an increase in the internal resistance caused by a pressure increase. [7] Method according to claim 6, characterized by that an error message is generated and stored and / or output above a specified threshold value or limit gradient of the internal resistance. [8] Vehicle (13) with at least one battery module (10) according to claim 5. [9] Vehicle (13) according to claim 8, characterized bythat a battery control unit (11) is provided which is designed to carry out the method according to claim 6 or 7.

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