Method for analysing a battery layer type for a battery by means of a compressive stress-compression path ratio

Abstract

A method for analysing a battery layer type for a battery is characterised in that a battery layer of the battery layer type is compressed and at least one compressive stress-compression path ratio is determined. On the basis thereof, a swelling factor can be determined by determining a layer thickness in the compressed state from the determined compressive stress-compression path ratio and setting said layer thickness in relation to the layer thickness of the battery layer in an initial state. Such a swelling factor can be used to design the battery.

Description

[0001] Description

[0002] METHOD FOR ANALYSTING A BATTERY LAYER TYPE FOR A BATTERY BY A COMPRESSION VOLTAGE-COMPRESSION PATH RATIO

[0003] The invention relates to a method for analyzing a battery layer type for a battery and a method for designing a battery comprising a casing and a plurality of battery layers of different battery layer types.

[0004] A battery is an electrochemical storage device for electrical energy, in which stored chemical energy is converted into electrical energy through an electrochemical redox reaction during discharge. In the context of the invention, the term "battery" refers to both primary batteries, which are designed for a single discharge and not for recharging, and secondary batteries or accumulators, which are designed and intended for multiple charging and discharging cycles. Charging a secondary battery represents the electrolytic reversal of the electrochemical redox reaction that occurs during discharge, achieved by applying an electrical voltage.

[0005] A battery comprises one or, more commonly, several galvanic cells or battery elements arranged within an enclosure, usually in the form of a foil pouch or casing. Each battery element includes two electrodes, a separator positioned between the electrodes for electrical separation, and an electrolyte acting as an ion conductor between the electrodes. The two electrodes of a battery element differ with respect to an enclosed active material, making one electrode anodic and the other cathodic (with respect to a discharge of the battery element).

[0006] Furthermore, a battery cell typically comprises two battery cell terminals integrated into the casing and electrically connected to the electrodes on the inside of the casing via current collectors. All anodically active electrodes are connected to one of the battery cell terminals, and the cathodically active electrodes to the other. A battery design with a casing can offer the advantage of higher structural strength compared to batteries with a foil casing.

[0007] Typically, a variety of battery elements, for example in the form of a stacking or winding of electrodes and separators, are combined in a battery cell to achieve a sufficiently high electrical performance. The various components present as a stacking or winding (hereinafter referred to as "layering") within a battery casing, i.e., in particular the electrodes and separators, are referred to below as "battery layers".

[0008] When designing a battery, and especially a lithium-ion battery, significant volume changes (so-called "swelling") of the battery layers must be taken into account. Such volume changes can occur both once during battery manufacturing, particularly due to impregnation of the battery layers with a liquid electrolyte, and regularly during each charging or discharging process as a result of lithiation / delithiation of the electrodes. These volume changes lead to varying pressures exerted on the battery layers and thus to varying mechanical stresses within the materials from which the battery layers are composed. Volume changes with respect to pressure in the layering direction, i.e., perpendicular to the adjacent large surface areas of the individual battery layers, can be particularly significant.It should be taken into account that different voltages significantly influence the electrochemical performance of the battery elements formed from the battery layers.

[0009] When designing a battery, attempts are typically made to account for the volume changes or swelling that occur during later use. This is usually achieved by appropriately oversizing the available space for arranging the battery layers compared to the geometric dimensions of the layers in their initial state. Such oversizing applies not only to the space within the battery's casing but also to the space required for integrating the battery into a product, such as an electric vehicle, which typically contains a large number of such batteries in a densely packed arrangement.

[0010] Regarding the swelling behavior of lithium-ion batteries and the effects of different pressures applied to them, reference is made to the publication by A. Aufschläger et al., “High precision measurement of reversible swelling and electrochemical performance of flexibly compressed 5 Ah NMC622 / graphite lithium-ion pouch cells,” in Journal of Energy Storage 59 (2023) 106483. This publication also discloses an investigation of pouch and foil-encased batteries with respect to their swelling behavior.

[0011] The invention is based on the objective of enabling a battery design that better takes into account the swelling behavior.

[0012] A solution to this problem is achieved by a method for analyzing a battery layer type of a battery according to claim 1 and by a method for designing a battery according to claim 10.

[0013] According to the invention, a method for analyzing a battery layer type for a battery, in particular a lithium-ion battery, is provided, characterized in that at least one, and preferably exactly one, battery layer of the battery layer type is compressed and at least one compression stress-compression distance ratio is determined. The compression stress is a mechanical stress that results from the compression of the battery layer. The compression distance is the distance over which the battery layer is compressed.

[0014] The term "battery layer type" refers to a specific type of battery layer among several different types found in a battery. These battery layer types include, in particular, the electrodes (i.e., the anodes and cathodes) and the separators, although other battery layer types (e.g., deformation layers) may also be present.

[0015] The method according to the invention makes it possible to determine the deformation and, specifically, the compression of the battery layer to be analyzed as a function of the mechanical compression stress that develops in the material or materials of the battery layer as a result of such compression. From this, relevant information regarding the design of a battery comprising the corresponding type of battery layer can be derived. In particular, a specific layer thickness of a single battery layer of this specific battery layer type can be derived as a function of a predetermined compression stress, wherein the predetermined compression stress results, in particular, from an external force acting on a layering of several battery layers comprising the battery layer and can be calculated accordingly.Furthermore, if this layer thickness is compared to the layer thickness of this battery layer or battery layer type in an initial state, a compression stress-dependent swelling factor for the battery layer and also the battery layer type can be determined. Accordingly, according to a preferred embodiment of a method according to the invention, a compression stress-dependent swelling factor can be determined by calculating a layer thickness in the loaded state from the determined voltage-to-compression displacement ratio and comparing it to the layer thickness of the battery layer in an initial state.

[0016] The "initial state" of a battery layer, or of a battery layer of the corresponding battery layer type, is defined as the state in which the battery layer is prepared for combination with the other battery layers to form a layered structure for a battery. Accordingly, the initial state is characterized in particular by the fact that the battery layer has not yet been brought into contact with an electrolyte (if provided for) and has not yet been charged, and thus no change in its state, especially its lithiation state, has occurred through charging or discharging as part of the battery.

[0017] Since the electrodes in particular can exhibit pronounced swelling behavior, it can be advantageous according to the invention to analyze at least or exclusively one of the electrode types, i.e., anodes or cathodes, preferably both electrode types. This can be particularly relevant for electrodes intended for a lithium-ion battery. It is especially preferred that the electrode (in each case) to be analyzed is compressed in a defined lithiated state, thereby enabling the determination of the dependence of the swelling behavior, and thus also of the compression voltage-compression displacement ratio, on the lithiation state. This allows for a particularly efficient battery design.

[0018] The term "lithiation state" refers to the extent of lithium ion incorporation into the electrode, and this extent can take any value between a minimum and maximum lithium incorporation.

[0019] To prepare a battery layer to be analyzed for carrying out a method according to the invention, for example, to establish a defined lithiation state, it is preferably provided that the battery layer, preferably all battery layers to be analyzed, is first assembled as part of a dummy battery and the dummy battery is then prepared so that it is functional as a battery. This preparation can include introducing a layer of battery layers into a casing, at least partially impregnating the battery layers with an electrolyte, and at least an initial (partial or complete) charging of the dummy battery. After such preparation of the dummy battery, it can be disassembled again, thereby isolating the battery layer(s) to be analyzed in order to subsequently analyze them within the framework of a method according to the invention.

[0020] Furthermore, to prepare a battery layer for analysis in accordance with the invention, it may be possible to disassemble a previously used battery and isolate at least one of its layers for subsequent analysis. This makes it possible, in particular, to determine irreversible changes in layer thickness, which may have been caused by cyclic charging and discharging. A swelling factor can also be determined by comparison with a corresponding battery layer in an initial state. This is also feasible for different lithiation states.

[0021] A determined swelling factor, dependent on the compression stress, can advantageously be used in a method according to the invention for designing a battery, in particular a lithium-ion battery, wherein the battery comprises a casing, preferably in the form of a housing, and a plurality of battery layers of different battery layer types, which are incorporated as layers within the casing. It is provided that a layer height (i.e., the dimension in one of the layering directions) is calculated as a function of a pressure acting on the layer in that layering direction. The layer thickness of the at least one battery layer for which the swelling factor has been determined and taken into account, and thus for all battery layers of the same type, which contributes to this layer height, can be directly derived from the corresponding swelling factor.

[0022] The housing of a battery designed according to the invention can preferably be dimensionally stable. A housing is considered "dimensionally stable" if its three-dimensional shape does not collapse under its own weight without external load. Preferably, such a housing can be designed to be dimensionally stable in such a way that it does not collapse under load from external forces occurring during normal use and, particularly preferably, is also not deformed to a relevant extent ("rigid" housing). The housing can further preferably be made entirely or partially of metal, for example, aluminum, which allows for a relatively simple and cost-effective realization of a dimensionally stable and also thermally conductive housing.In order to determine the layer height as precisely as possible within the framework of a design method according to the invention, it is preferably provided that a swelling factor is determined and taken into account for each of the existing battery layer types, or at least for the (different) electrodes, i.e., the anodes and the cathodes, using an analysis method according to the invention, because the electrodes usually experience by far the greatest change in volume and thus layer thickness during battery use. However, since the separators can also exhibit at least relevant swelling behavior, it is also possible to determine at least one swelling factor for the separators, as one of the battery layer types, using a method according to the invention and to take this into account in the battery design.

[0023] Provided that at least one swelling factor is determined and taken into account for at least the two electrode types, it is preferably possible that the lithiation states at which the swelling factors are determined are adapted with respect to the interaction when the electrode types are used together in the battery. For example, it is possible that maximum lithiation of the anodes in the battery only occurs in combination with minimal lithiation of the cathodes – and vice versa.

[0024] According to a preferred embodiment of an analysis method according to the invention, the battery layer can be compressed over a load range and several compression stress-compression displacement ratios can be determined. This allows particularly useful design data to be obtained, which can have a beneficial effect on the optimal design of the battery. The load range can be defined by a predetermined compression displacement range or a predetermined pressure range through which the battery layer is compressed.

[0025] According to a preferred embodiment of such a method according to the invention, a continuous profile of the compression stress-compression displacement ratios can be determined within the load range, so that, in principle, a corresponding compression stress is determined for every compression displacement within the load range, and a corresponding compression displacement can be determined for every compression displacement within the load range. This allows a corresponding number of swelling factors to be determined, which are then advantageously available for the design of a battery. Alternatively, instead of determining a continuous profile of the compression stress-compression displacement ratios, only individual, specifically defined compression stress-compression displacement ratios can be determined.

[0026] To enable an advantageous battery design by determining as many meaningful compression stress-compression displacement ratios as possible, it can be provided that, within the framework of an analysis method according to the invention, several battery layers of the (same) battery layer type are analyzed, wherein the several battery layers differ with respect to their lithiation state. This allows the dependence of the compression stress-compression displacement ratios, and thus the swelling factors, on the variability with respect to the lithiation state of the battery layer type to be determined and taken into account. This dependence can be proportional, i.e., an increasing or decreasing lithiation state leads to a constant increasing or decreasing change in the compression stress-compression displacement ratios according to a proportionality factor.However, the dependence can also be non-proportional, at least in some areas, so that a maximum layer thickness dependent on the lithiation state can result in only partial (i.e., not complete and not minimal) lithiation.

[0027] Particularly relevant for the design of a battery can be a maximum compression voltage-compression distance ratio of the battery layer or the corresponding battery layer type, so that such a maximum compression voltage-compression distance ratio for the battery layer should be determined within the framework of a method according to the invention.

[0028] Particularly relevant for battery design can be the compression stress-compression displacement ratio of the battery layer or the corresponding battery layer type, which exists at a compression stress derived from a defined maximum load of the battery layer within the battery. Accordingly, such a compression stress-compression displacement ratio should preferably be determined using a method according to the invention.

[0029] According to a preferred embodiment of a method according to the invention, the battery layer can be compressed between pressure plates, in particular plane-parallel pressure plates (i.e., whose surfaces contacting the battery layer are planar or flat and parallel to each other). This allows for the most uniform possible loading of the battery layer and thus a particularly precise determination of the at least one compression stress-compression displacement ratio.

[0030] The same advantage can be realized if, as is preferably provided in a method according to the invention, the battery layer to be analyzed has a circular shape. The circular shape refers to the surface shapes of the large areas of the battery layer. This helps to avoid, as far as possible, a non-uniform voltage distribution in a circumferential edge region of the battery layer, such as could occur, for example, with a square battery layer.

[0031] If a method according to the invention is carried out using a battery layer to be analyzed that has a shape differing from the shape of the battery layer of the same type as intended for use in the battery, it may be provided that the battery layer to be analyzed is a corresponding blank made from a battery layer with the shape intended for use in the battery or from, for example, a ribbon-shaped semi-finished product having a material structure corresponding to the type of battery layer. Such a blank can be produced, for example, by stamping.

[0032] The invention is explained in more detail below with reference to exemplary embodiments illustrated in the drawings. The drawings show, in simplified representations:

[0033] Fig. 1: a battery layer arranged between two press plates in an uncompressed state,

[0034] Fig. 2: the battery layer arranged between the two press plates in a compressed state,

[0035] Fig. 3: the course of a compression voltage-compression distance ratio resulting from a defined compression of the battery layer according to Figs. 1 and 2,

[0036] Fig. 4: a battery and

[0037] Fig. 5: a longitudinal section through the battery.

[0038] Figures 1 and 2 show an arrangement of a battery layer 1 between two plane-parallel pressing plates 2 of an analysis device. The battery layer 1 is an electrode 3. This comprises a planar, film-like substrate 4 coated with an active material 5.

[0039] In carrying out a method according to the invention for analyzing a type of battery layer, from which the illustrated battery layer 1 originates, it is provided that the battery layer 1 is compressed over a defined load range by means of the pressure plates 2 and that a course of the compression stress-compression distance ratios according to Fig. 3 is determined in order to determine swelling factors which can be used for the design of a battery 6 comprising, among other things, several battery layers of the type of battery layer as shown in an exemplary embodiment in Figs. 4 and 5.

[0040] To determine the compression stress-compression displacement ratio, the force with which an upper of the pressure plates 2 is pressed against the battery layer 1 is measured, with the battery layer 1 being supported by a rigid or immobile lower of the pressure plates 2. This force is continuously increased according to the defined load range to achieve increasing compression of the battery layer 1. The load range can be defined (starting from a zero value for the force or displacement) up to a predetermined maximum value for the force or displacement. Knowing the dimensions of the large areas of the battery layer 1, the force can be converted into a mechanical compression stress o, which is present as a result of the compression within the materials of the battery layer 1. In this way, a compression stress-compression displacement ratio, as shown in Fig.The qualitative representation in section 3 can be determined. This trend can be particularly exponential.

[0041] The analysis device can include, for carrying out such an analysis procedure, an actuating device 7 for controlled movement of the upper pressure plate 2, a displacement sensor 8 for determining the travel distance of the upper pressure plate 2 and thus also the distance over which the battery layer is compressed (i.e. the compression distance s), a force sensor 9 for determining the force required for the compression of the battery layer 1, and a control device 10 for controlling the actuating device 7 and for evaluating the data transmitted by the sensors 8, 9.

[0042] For each compression stress-displacement ratio, a swelling factor dependent on the compression stress o can be determined by calculating the layer thickness of battery layer 1 in the compressed state from this compression stress-displacement ratio and relating it to the layer thickness in an initial state of battery layer 1. Based on the determined profile of the compression stress-displacement ratios, a corresponding number of swelling factors, and thus also a profile of the swelling factors versus the compression stress o, can be determined.This makes it possible to determine and utilize in the design of the battery 6 what layer thickness the battery layer 1 in the battery 6 will have at a predetermined compression stress o, which can be calculated from the space available within a casing 11 of the battery 6 for accommodating all battery layers and, if necessary, from a predetermined clamping force acting on the casing 11 from the outside.

[0043] Particularly when the battery layer 1 to be analyzed is intended as the electrode 3 of a lithium-ion battery 6 and comprises a corresponding active material 5, a change in volume, and thus a change in the layer thickness of the battery layer 1, depends to a considerable extent on the lithiation state of the battery layer 1 or the active material 5. In order to take this dependency into account during the design of the battery 6, the analysis method is performed on a plurality of battery layers 1 of the same type, with these multiple battery layers 1 differing only in their lithiation state. Accordingly, for each of the considered lithiation states of the battery layer type, a curve of the compression stress-compression displacement ratios according to Fig. 3, as well as a curve of the derived swelling factors versus the compression stress o, is determined.This makes it possible, in particular, to take into account the maximum layer thickness achieved at a predetermined compression voltage o for the battery layers 1 of the corresponding battery layer type when designing the battery 6, taking into account all possible lithiation states.

[0044] The battery 6 shown in Figures 4 and 5 comprises a casing 11 in the form of a cuboid housing, preferably made entirely of a metal (e.g., aluminum), and several battery elements arranged within the casing 11. Specifically, the battery elements are stacked in the form of an electrode-separator assembly (ESA). This assembly comprises, in an alternating arrangement, a plurality of first electrodes 3a, which function as anodes during discharge of the battery 6, and a plurality of second electrodes 3b, which function as cathodes during discharge of the battery 6. Due to the alternating arrangement of the electrodes 3, with the exception of the electrodes 3 located on the outside of the stack or the ESA, a first electrode 3a is always positioned between two second electrodes 3b, and a second electrode 3b is always positioned between two first electrodes 3a.Adjacent electrodes 3 are spatially separated by a separator 12 and thus electrically isolated from each other. A first electrode 3a and a second electrode 3b, along with a separator 12 arranged between them, are components of a battery cell. Due to the stacking arrangement, each of the electrodes 3, with the exception of the two electrodes 3 located on the outside of the stack or the ESV, is functionally a component of two battery cells.

[0045] The battery elements further comprise an electrolyte, which is arranged in liquid form within the casing 11. At least the separators 12 are impregnated with the electrolyte, so that conduction of ions between adjacent electrodes 3 via the intervening separator 12 is possible.

[0046] Each of the electrodes 3 comprises a planar, foil-like substrate 4, which, for example, can be made of copper for the first electrodes 3a, intended as anodes, and of aluminum for the second electrodes 3b, intended as cathodes. Within a square section of this substrate, the two large surfaces of each electrode 3, located in the stacking direction of the ESV, are coated with an active material 5 to enable the different electrodes 3a and 3b to function as anodes or cathodes during battery 6 operation. Within these square sections of the substrates 4, and thus of the electrodes 3, the substrates and the corresponding square-shaped separators 12 are stacked, resulting in a cuboid shape for the ESV.

[0047] On one transverse side of the rectangular section of each electrode 3, a region of the substrate 4 is provided in which it is not coated with the respective active material 5. This region of the electrodes 3 serves as a current collector 13, through which the individual electrodes 3 are electrically connected to a corresponding battery terminal 14 of the battery 6. The current collectors 13a of all first electrodes 3a are connected to a first (14a) of the battery terminals 14, and the current collectors 13b of all second electrodes 3b are connected to a second (14b) of the battery terminals 14.

[0048] The battery terminals 14 are integrated into the casing 11 in such a way that a connection section is accessible from the outside to integrate the battery 6 into a circuit. Reference numeral list: Battery layer, Press plate, Electrode a, First electrode b, Second electrode, Electrode substrate, Electrode active material, Battery, Actuating device, Displacement sensor, Force sensor 0, Control device 1, Battery casing 2, Separator 3, Current collector 3a, First current collector 3b, Second current collector 4, Battery terminal 4a, First battery terminal 4b, Second battery terminal, Compression voltage, Compression travel

Claims

Patent claims 1. Method for analyzing a battery layer type for a battery (6), characterized in that a battery layer (1) of the battery layer type is compressed and at least one compression voltage-compression distance ratio is determined.

2. Method according to claim 1, characterized in that the battery layer (1) is an electrode (3).

3. Method according to claim 2, characterized in that the electrode (3) is compressed in a defined lithiated state.

4. Method according to one of the preceding claims, characterized in that a swelling factor is determined by determining a layer thickness in the compressed state from the determined compression stress-compression distance ratio and relating it to the layer thickness of the battery layer (1) in an initial state.

5. Method according to one of the preceding claims, characterized in that the battery layer (1) is compressed over a load range and several compression stress-compression distance ratios are determined.

6. Method according to claim 5, characterized in that a continuous course of the compression stress-to-compression distance ratios is determined in the load range.

7. Method according to one of the preceding claims, characterized by application to several battery layers (1) of the battery layer type, wherein the several battery layers (1) differ with respect to the state of lithiation.

8. Method according to one of the preceding claims, characterized in that the battery layer (1) is compressed between plane-parallel press plates (2).

9. Method according to one of the preceding claims, characterized in that the battery layer (1) has a circular shape.

0. Method for laying out a battery (6) comprising a casing (11) and a plurality of battery layers (3) of different battery layer types, which are incorporated as layering within the casing (11), characterized in that a height of the layering is calculated as a function of a defined pressure acting on the layering in the layering direction, using at least one swelling factor determined by a method according to claim 4 or one of the claims dependent on claim 4.

Images