Method for determining the ageing of an individual battery cell

Abstract

The invention relates to a method for determining the ageing of an individual battery cell in a lithium-ion battery made up of a plurality of such individual battery cells, to which end a cell thickness change (x) of the individual battery cell over the state of charge (SOC) is detected in order to draw conclusions herefrom about the ageing of the individual battery cell. The method according to the invention is characterised in that a curve of the cell thickness change (x) over the state of charge (SOC) during charging and discharging of the respective individual battery cell is evaluated such that based on characteristic properties of the curve, a conclusion is drawn about the involvement of at least one of the active materials (Si, Gr, NMC) in the cell thickness change (x) and thus about the ageing.

Description

[0001] Method for determining the aging of a single battery cell

[0002] The invention relates to a method for determining the aging in a single battery cell according to the type defined in more detail in the preamble of claim 1.

[0003] It is generally known from the state of the art that individual battery cells, especially lithium-ion cells, experience a volume increase or decrease with increasing charge and / or discharge. Furthermore, the volume of an individual battery cell also changes with age.

[0004] DE 102021 005418 A1 now takes up this idea to determine the aging state of a single battery cell using a method based on cell thickness. The change in cell thickness of a single battery cell is recorded over the state of charge and then divided into the portion caused by charging or discharging, on the one hand, and the portion caused by the aging of the single battery cell, on the other. This allows conclusions to be drawn about the aging of the single battery cell.

[0005] However, the aging-related change in cell thickness has different effects on different materials. The publication "Volume and thickness change of NMC811|SiO x-graphite large-format lithium-ion cells: from pouch cell to active material level” by Hendrik Pegel et al. in the Journal of Power Sources 537 (2022) 231443 shows the relationships that occur here. Essentially, the result is that the total cell thickness change over a charge and discharge cycle of the individual battery cell is partly made up of the cell thickness change caused by the silicon and graphite (anode side) on the one hand, and the change caused by the manganese-nickel-cobalt material known as NMC (cathode side) on the other. The sum of the cell thickness change is therefore the sum of the cathode aging on the one hand and the sum of the anode aging on the other.

[0006] The object of the present invention is to provide a method for determining the aging of a single battery cell that is improved compared to the prior art and in which conclusions can be drawn about the aging caused by the respective active material.

[0007] According to the invention, this object is now achieved by the method having the features in claim 1, and in particular in the characterizing part of claim 1. Advantageous embodiments and further developments emerge from the dependent claims.

[0008] Similar to the prior art mentioned above, the cell thickness change of the individual battery cell as a whole is recorded here. This cell thickness change over the state of charge, in particular over a charge and discharge cycle of the individual battery cell, then results in a corresponding cell thickness change curve. This curve is then evaluated for each individual battery cell to determine, based on characteristic properties, the contribution of at least one of the active materials to the cell thickness change and thus ultimately the aging of this specific active material. The characteristic properties that the individual active materials have on the cell thickness change are known in principle from the aforementioned scientific publication.These characteristic properties are now used to infer the contribution of the active material used to a cell thickness change from a recorded age-related change. This ultimately makes it possible to determine the aging of this specific active material, for example, the removal of graphite or silicon in the anode area or the loss of active material in the cathode area, for example, through plating processes or similar.

[0009] This makes it possible to specifically exclude or adjust the battery control for those operating conditions that have a particularly strong impact on the aging of one or another active material. This makes it possible to efficiently halt the progression of aging processes. Furthermore, the method allows for the breakdown of the influence of individual active materials on this cell thickness change by simply measuring the change in cell thickness. This makes it relatively easy to collect data under real operating conditions in a fleet of electric or hybrid vehicles, which can be used to further develop the understanding of aging processes in such individual battery cells.

[0010] In principle, the analysis of the curve according to the invention based on characteristic properties of at least one active material can be used to determine its contribution to the cell thickness change occurring over the lifetime of the individual battery cell. According to a very advantageous development, this is done accordingly for all active materials, for example, silicon, graphite, NMC, lithium metal, and lithium iron phosphate, assuming a linear relationship between the volumetric proportion of the respective active material and its influence on the cell thickness change.

[0011] A particularly advantageous embodiment of the method according to the invention can provide that, to determine silicon aging, a plateau hysteresis of the cell thickness change between a charging plateau and a discharging plateau is evaluated. The curve of the cell thickness change of the entire battery single cell will always have a characteristic plateau during charging on the one hand and a characteristic plateau above it during discharging on the other. The distance between these two plateaus is referred to here as the plateau hysteresis. This plateau hysteresis can then be evaluated in order to draw conclusions about the proportion of this cell thickness change caused by the silicon from the measured cell thickness change of the entire battery single cell. In particular, it has been shown that the plateau hysteresis is significantly greater at the beginning of the life of a battery single cell than when this battery single cell has already aged.The silicon aging can therefore be derived from the magnitude of the plateau hysteresis. Studies have shown that the silicon aging, which is determined based on the plateau hysteresis in the characteristic cell thickness change curve for silicon, corresponds to that determined based on the cell thickness change curve of the full cell. Silicon aging can therefore be determined accordingly based on the two thickness values ​​of the first and second plateaus, which are set in relation to each other.

[0012] Another very advantageous embodiment further provides for the evaluation of the height of the discharge plateau of the cell thickness change to determine graphite aging. Here, too, the height of the discharge plateau corresponds to the cell thickness change caused by graphite aging. In particular, if silicon aging is already known and thus the silicon share in the cell thickness change of the discharge plateau is known, the graphite-related share in the height of the discharge plateau can be easily deduced from the difference, so that graphite aging can be calculated accordingly. If silicon aging is known, NMC aging plays virtually no role in the area of ​​the second discharge plateau, so its share can be neglected here without any loss of accuracy.

[0013] To determine NMC aging, the maximum cell thickness change over the discharge state can be evaluated. Typically, this maximum will be in the range of a fully charged cell, i.e., a state of charge of 100%. A maximum occurring in this range then allows inferences to be drawn about the characteristic minimum in the cell thickness change caused by NMC that typically occurs in this range. With known values ​​for silicon and graphite aging, NMC aging can be easily determined in this way.

[0014] According to a very advantageous development of the invention, it is therefore provided that silicon aging is determined first, then graphite aging, and finally NMC aging based on the cell thickness changes characteristic of the respective active material. This has the advantage that the known plateau height in the cell thickness change caused by the silicon can already be included in the determination of graphite aging, and that the other two values ​​can already be taken into account in determining the absolute value of the minimum in the NMC-induced cell thickness change.

[0015] In order to be able to determine this across relevant parts and in particular the entire state of charge, a very advantageous development of the method according to the invention proposes that a value of the silicon-induced cell thickness change over the state of charge, determined from the plateau hysteresis, be used to compress a known initial curve of the silicon-induced cell thickness change at the beginning of the life cycle over the state of charge in the direction of the cell thickness change, in order to obtain an age-related curve of the silicon-induced cell thickness change over the entire state of charge during charging and discharging. The measured value of the plateau hysteresis also allows conclusions to be drawn about the cell thickness changes that occurred in the respective plateau and thus makes it possible to determine the height of the plateau in the curve accordingly.From this, a current curve of the silicon-induced cell thickness change can be calculated starting from a known curve of the silicon-induced cell thickness change at the beginning of a cell's life by compressing this curve using the determined values. This current curve thus indicates the value of the silicon-induced cell thickness change at the current aging state of the individual battery cell over the entire charging and discharging range. This curve can then be subtracted from the measured overall curve to obtain the proportion of graphite- and NMC-induced cell thickness change at the current aging state. To determine this, characteristic ranges for the respective material are used, in particular the height of the final charge plateau in the sense described above for determining the graphite-induced cell thickness change and thus ultimately the graphite aging.

[0016] Analogous to the use of the value recorded from the plateau hysteresis for the silicon-related cell thickness change, a known curve at the beginning of the life of the single battery cell can now be compressed accordingly for the proportion of the graphite-related cell thickness change from the height of the discharge plateau for the graphite-related cell thickness change, in order to obtain not only the age-current silicon-related cell thickness change but also the age-current graphite-related cell thickness change as a new curve in the current aging state of the single battery cell.

[0017] In particular, these two curves can be used to determine the proportion of the cell thickness change of the entire cell caused exclusively by the NMC. This allows the peak at the minimum of the NMC curve to be easily and efficiently identified and its magnitude used to determine the NMC-related cell thickness change and thus the aging state of the NMC, essentially the cathode. Specifically, according to an advantageous embodiment, the cell thickness difference between a charging plateau of the cell thickness change versus state of charge curve and a discharge plateau of this curve can be evaluated to determine silicon aging. A decreasing distance between the plateaus, i.e. a decreasing plateau hysteresis, indicates greater silicon aging.

[0018] According to an advantageous development, to determine graphite aging, a graphite-related height of the discharge plateau can be determined from the difference between the recorded height from the curve of the cell thickness change of the full battery single cell and the previously determined silicon-related height change of the discharge plateau. As the size or height of the graphite-related cell thickness change of the discharge plateau decreases, greater graphite aging is then inferred. Furthermore, using the two agings already determined, it can then be provided that the respective maxima of the cell thickness change, which are caused by silicon and graphite, are determined during full charge, that the minimum of the cell thickness change due to NMC occurring during full charge is determined from the difference between the overall maximum and the graphite- and silicon-related maxima, whereby as the absolute value of the minimum decreases, increasing NMC aging is inferred.

[0019] Simply by evaluating the corresponding characteristics in the curve, it is possible to easily and efficiently determine the breakdown of the aging-related cell thickness change among the various active materials involved, i.e., their influence on the respective amount of cell thickness change. As already mentioned above, this can be used to improve the control or operating strategy for such a battery, as well as for comprehensive data collection to further analyze and understand aging processes in such individual battery cells.

[0020] The method according to the invention is explained in detail below using an exemplary embodiment, for which reference is made to the following figures.

[0021] Showing:

[0022] Fig. 1 is a diagram of the methodology applied with the process according to the invention; Fig. 2 is a schematic representation of the percentage expansion of the individual battery cell in percent over the state of charge, with curves for the entire cell, the anode, essentially made of silicon and graphite, and the cathode made of NMC being plotted here;

[0023] Fig. 3 two curves of the percentage expansion in percent of the cell thickness, i.e. the cell thickness change over the state of charge, once at the beginning of the life of a single battery cell (a) and in an aged state (b);

[0024] Fig. 4 shows the silicon-related contribution to the cell thickness change according to Fig. 3;

[0025] Fig. 5 shows the graphite-related portion of the cell thickness change analogous to Fig. 3 and 4; and

[0026] Fig. 6 shows the NMC-related contribution to the cell thickness change analogous to or in addition to Figures 3, 4 and 5.

[0027] Figure 1 depicts the methodology of the procedure presented here in a flow chart. The first block, labeled 1 on the far left, describes the in-operando measurement in a vehicle. This measurement records the change in the expansion of a single battery cell as a function of the state of charge. This is also referred to below as cell thickness change and essentially represents the percentage change in the thickness of the single battery cell over the state of charge, starting from an empty single battery cell to full charge and then to the next discharge.

[0028] The middle block of the diagram in Figure 2, designated here by 2, essentially describes a battery control unit 2. Based on the in-operando measurement 1 in the vehicle, the measured cell thickness change over the state of charge is then transmitted to this battery control unit 2. In a first block 2a, the theoretical knowledge about the distribution of the cell thickness change among its components determined by the respective active material is then applied in order to establish a relationship between the cell thickness change and the aging of the active materials over the aging of the individual battery cell. This is symbolized by the block designated 2b. The corresponding target information 3 is then further used, either in the battery control unit itself to adapt the control system to minimize the aging effects, or to collect data that helps to better understand the aging process.

[0029] Figure 2 shows the percentage expansion of the cell thickness, referred to below as the cell thickness change, as a percentage change over the state of charge. This curve represents the initial state of a single battery cell at the beginning of its life. The cell thickness change varies between -1% and approximately +5.5%. The state of charge has full charge in the center, so that discharging is shown to the left of the center and charging is shown to the right of the center. In other words, charging takes place to the left of the center, hence on the x-axis from 0% to 100% SOC is in the center, and then discharging to the right of the center takes place, from 100% SOC to 0% SOC. The in-operando measured cell thickness change is indicated by the solid bold curve. It is broken down into the individual components firstly into the cathode component, which is shown with a dash-dotted line, and the anode component, which is shown with a dash-two-dotted line.The proportion of the cathode-related cell thickness change x over the state of charge (SOC) essentially corresponds directly to the NMC-related aging, as this is the main material of the cathode. The anode-related cell thickness change is in turn divided into the two active materials of the anode: silicon on the one hand and graphite on the other. The silicon-related cell thickness change is shown with a dashed line, the graphite-related cell thickness change with a dotted line. This is also maintained in the following figures, although a repeated representation of the anode-related cell thickness change, which is in fact only the sum of the silicon-related and graphite-related cell thickness changes, will be omitted below.

[0030] As already mentioned, during in-operando measurement 1 in the vehicle, in the block labeled 1, the cell thickness change x is measured over the state of charge SOC. In the illustration in Figure 3a, this is shown again in the diagram alone, analogous to the illustration in Figure 2. The state in the diagram in Figure 3a represents the state of the cell thickness change at the beginning of the life of the individual battery cell. Next to it, in Figure 3b, essentially the same curve, i.e. the cell thickness change x of the entire cell, is shown at a later point in time with increased aging of the individual battery cell. The respective curve indicates an unknown superposition of the proportions of the individual active materials in the respective aging state (a), b)).This curve also already shows some characteristic elements such as a discharge plateau designated by 4, a charging plateau designated by 5 and a maximum designated by 6, which can be seen as characteristic points or areas of the curve both in Figure 3a, at the beginning of the life of the single battery cell, and in Figure 3b, i.e. in an already aged single battery cell.

[0031] The difference between the discharge plateau 4 and the charging plateau 5 is indicated in Figure 3a as AXP,Z,I. For the cell thickness change x of the aged battery single cell, this difference between the charging plateau 5 and the discharge plateau 4 is also shown and labeled AXP,Z,2. The plateau hysteresis, i.e., the distance between the discharge plateau 4 and the charging plateau 5 in the direction of the cell thickness change, has noticeably decreased.

[0032] In Figures 4a and 4b, the curve for the proportion of silicon-related aging is plotted at the start of the life cycle of the single battery cell in Figure 4a and in the aged single battery cell in Figure 4b. The silicon-related aging itself, which is often expressed as £si, can therefore be determined from the ratio of the current plateau hysteresis AXP,Z,2 and the plateau hysteresis of the new single battery cell AXP,Z,I. This value, multiplied by 100, yields the percentage value for silicon aging or a degree of silicon aging. Figures 4a and b show that the plateau hysteresis values ​​AXP,Z,I , AXP,Z,2 determined based on the cell thickness change of the single battery cell correspond to those in the silicon curve Axp,su , AXP,SI,2. This gives:

[0033] This correspondence now makes it possible to derive the entire curve from the silicon contribution to the cell thickness change known from Figure 2 by compression in the direction of the cell thickness change.

[0034] Analogous to the previous illustration, Figures 5a and 5b now address graphite-induced aging. Characteristic of graphite-induced aging or the graphite-induced change in cell thickness x is the level of the discharge plateaus 4 above the zero line of cell thickness change. The height xp,Ent,2 of the discharge plateau 4 of the solid overall curve is thus composed of the graphite-induced height xp,Gr,2 of the discharge plateaus 4 and the silicon-induced height XP.G of the discharge plateau 4.

[0035] Theoretically, the NMC-induced cell thickness change XP,NMC,2 of the cathode also plays a minimal role. However, in practice, this change is so small that it can be completely neglected in the area of ​​the discharge plateau 4.

[0036] With the silicon-related height of the discharge plateau 4 known from the relationship described in Figure 4, the graphite-related plateau height xp,Gr,2 can now be mathematically determined. ,Gr,2 — Xpßnt,!, ~ X P,Si,2

[0037] The graphite aging fr can then be derived from the plateau height xp,Gr,2 of the aged cell in relation to the plateau height XP.G of the cell at the beginning of its life and multiplied by 100 as a percentage.

[0038] Similar to the curve for the corresponding ageing state of the individual battery cell for silicon, the initial curve known from Figure 2 can now also be compressed for graphite according to the recorded values ​​of the final discharge plateau, so that ultimately, in addition to the curve of the cell thickness change of the individual battery cell, the silicon and graphite values ​​shown here again as dashed and dotted lines are shown.

[0039] Graphite-related cell thickness changes can be included.

[0040] As also evident from the illustration in Figure 2, a characteristic point for determining the NMC-induced cell thickness change x and thus the NMC aging $NMC is the minimum in the range of 100% SOG. The known maxima of the silicon-induced cell thickness changes and, above all, the graphite-induced cell thickness changes can be offset accordingly. The total maximum XM,Z,2 in Figure 6b is therefore composed of the sum together, so that XM,NMC,2 can ultimately be calculated by determining the difference between the maximum in the full cell and the two silicon or graphite-related maxima.

[0041] X MJiMC,2 — X M,Z,Z ~ X M1Si12 ~ X M,GT,2

[0042] For NMC-related aging, the following applies:

[0043] Here, too, the decreasing value of the minimum indicates increasing aging of the NMC or cathode.

Claims

Patent claims 1 . Method for determining the ageing of a single battery cell in a lithium-ion battery made up of several such single battery cells, for which purpose a cell thickness change (x) of the single battery cell is recorded over the state of charge (SOC) in order to draw conclusions about the ageing of the single battery cell, characterized in that a curve of the cell thickness change (x) over the state of charge (SOC) during charging and discharging of the respective single battery cell is evaluated in such a way that, on the basis of characteristic switching on of the curve, a conclusion is drawn as to the participation of at least one of the active materials (Si, Gr, NMC) in the cell thickness change (x) and thus in the ageing.

2. Method according to claim 1, characterized in that the aging of at least silicon (Si), graphite (Gr) and NMC is closed, whereby a linear relationship between the volumetric proportion of the respective active material (Si, Gr, NMC) and its influence on the cell thickness change (x) is taken as a basis.

3. Method according to claim 1 or 2, characterized in that to determine the silicon aging (Osi) a plateau hysteresis (AXP) of the cell thickness change (x) between a charging plateau (5) and a discharge plateau (4) is evaluated.

4. Method according to claim 1, 2 or 3, characterized in that to determine the graphite aging (OGF) a height (XP) of the discharge plateau (4) of the cell thickness change (x) is evaluated.

5. Method according to one of claims 1 to 4, characterized in that to determine the NMC aging (ONMC) the maximum (XM) of the cell thickness change (x) over the state of charge (SOC) is evaluated.

6. Method according to claim 3, 4 and 5, characterized in that first the silicon ageing (f ), then the graphite ageing (OGF) and finally the NMC ageing (ONMC) are evaluated on the basis of the recorded proportions of the cell thickness change (x).

7. Method according to one of claims 3 to 6, characterized in that at least one value determined from the plateau hysteresis (AXP) is used to compress a known initial curve of the silicon-related cell thickness change (x) over the state of charge (SOC) in the direction of the cell thickness change (x) in order to obtain a curve of the cell thickness change (x) corresponding to the current aging over the entire state of charge (SOC) during charging and discharging.

8. Method according to one of claims 4 to 7, characterized in that at least one value determined from the height (XP) of the discharge plateau (4) is used to compress a known initial curve of the graphite-related cell thickness change (x) over the state of charge (SOC) in the direction of the cell thickness change (x) in order to obtain a curve of the cell thickness change (x) corresponding to the current aging over the entire state of charge (SOC) during charging and discharging.

9. Method according to one of claims 3 to 8, characterized in that for determining the silicon aging (Osi) the plateau hysteresis (AXP) of the cell thickness change (x) between the charging plateau (5) and the discharge plateau (4) is evaluated, whereby a reduction in the plateau hysteresis (AXP) is concluded to be an indication of higher silicon aging (Osi).

10. The method according to claim 9, characterized in that, to determine the graphite aging (OGF), a graphite-related height (xp.Gr) of the discharge plateau (4) is determined from the difference between the recorded height from the curve of the full cell and the previously determined silicon-related height (xp,si) of the discharge plateau (4), wherein a higher graphite aging (OGF) is inferred as the graphite-related height (XP.GF) decreases.

11. Method according to claim 9 and 10, characterized in that, with known silicon and graphite aging (-9si, $Gr), the respective maxima (XM) of the cell thickness change (x), which are caused by silicon (Si) and graphite (Gr), are determined at full charge, so that the minimum (XM.NMC) of the cell thickness change (x) occurring at full charge is determined by NMX from the difference between the total maxima and the Gr- and Si-related maximum (XM), whereby with a decreasing amount of the minimum (XM.NMC) an increasing NMC aging (ONMC) is concluded.

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