Method for determining an aging state of a cell of a battery and method for operating a lithium-ion battery

Abstract

Method for determining an aging state of at least one cell of a battery of an energy storage device, in particular a cell of a lithium-ion battery, comprising the following steps: - Discharging the cell below a certain charge level; - Charging the cell above a certain charge level; - detecting a cell voltage (U) as well as a charging current and / or a flowing charge quantity (Q) during charging of the cell; and - Determining the ageing state of the cell from the measured variables; characterized in that the cell is discharged below a charge level at which an electrode of the cell exhibits a two-phase reaction.

Description

[0001] The invention relates to a method for determining an aging state of at least one cell of a battery of an energy storage device, in particular a cell of a lithium-ion battery.

[0002] Energy storage systems in motor vehicles typically consist of several lithium-ion cells, which can be interconnected to form cell modules. A cell module therefore consists of several interconnected galvanic cells. In the following, the term "cell" is used synonymously with the terms "galvanic cell" and "module comprising several galvanic cells."

[0003] To operate such energy storage devices optimally, battery management systems must be able to determine their state of health (SOH). However, the algorithms used for this purpose have access to only a few, highly correlated measurement variables, such as current, voltage, and temperature.

[0004] In the current state of the art, the aging state is determined, for example, by measuring the voltage curve in the energy storage device at very low currents to exclude temperature and resistance influences. This requires at least one complete charging or discharging process of the energy storage device, which, due to the low currents, takes a very long time during which the energy storage device cannot be used. Furthermore, identifying defective cells, for example for quality control purposes, is usually time-consuming and costly due to the additional infrastructure required.

[0005] DE 10 2013 220 691 A1 discloses a method according to the preamble of claim 1.

[0006] TORAI, Soichiro; NAKAGOMI, Masaru; YOSHITAKE, Satoshi; YAMAGUCHI, Shuichiro; OYAMA, Noboru: State-of-health estimation of LiFePO4 / graphite batteries based on a model using differential capacity; Journal of Power Sources, 29 February 2016, Vol. 306, pp. 62-69. DOI:10.1016 / j.jpowsour.2015.11.070 discloses a method in which a differential capacity is used to determine the aging state of a battery.

[0007] The object of the invention is therefore to provide a method for determining an aging state of a cell of a battery, in which the disadvantages of the prior art are eliminated.

[0008] The object is achieved according to the invention by a method according to claim 1. In particular, when the charging current intensity is detected, the duration of a period over which the charging current flows is also detected.

[0009] The method according to the invention has the advantage that the steps of discharging and charging below or above a certain charge level only take place for a relatively short period of time, so that the energy storage device can be used again after a short time and / or charged at a normal charging rate. The method can therefore be applied regularly without any particular or noticeable disadvantages for the user, so that the development of the aging state can also be determined. The recorded variables are preferably recorded at a high data rate in order to increase the accuracy of the determined aging state. The cell voltage can be recorded individually for each cell of the energy storage device in a simple manner, whereby defective, weak and / or diseased cells can be easily identified. This enables simple and rapid maintenance of the energy storage device, which also reduces the maintenance costs of the energy storage device.

[0010] Furthermore, an exact assessment of the charge level and thus, if the energy storage device is used in a motor vehicle, for example, of the remaining range is possible. If additional, continuously active devices and / or methods for determining the aging state are implemented in the energy storage device, these can be calibrated using the method according to the invention. The method according to the invention also allows an operating strategy and / or operating parameters (e.g., voltage limits) of the energy storage device to be adapted in order to mitigate the effects of aging and / or counteract it.

[0011] In addition, diagnosing the cell's aging status for subsequent use is facilitated. In particular, the method according to the invention can also be used in the production of the energy storage device, for example, for quality control.

[0012] The recorded quantities are those that are directly measured. The amount of charge flowing can be determined from the measured charging current and the duration of the charging current, so the charging current and duration are the recorded quantities, even if they are only used to determine the amount of charge flowing for further calculations. The amount of charge flowing can therefore be described as a quantity determined from the recorded quantities.

[0013] The ageing state can be determined via the cell voltage and the amount of charge flowing, in particular where the amount of charge flowing is determined directly or indirectly, for example indirectly via the charging current.

[0014] The cell is discharged below a charge level at which one of the cell's electrodes exhibits a two-phase reaction. Specifically, at this charge level, exactly one of the cell's electrodes exhibits a two-phase reaction—i.e., only one anode or only one cathode of the cell, or more precisely, each cell of the energy storage device. The other electrode exhibits a quasi-constant potential curve at this charge level.

[0015] Preferably, the cell is discharged below a lithiation level of an electrode of the cell and / or until a certain limiting current is reached, in particular with constant voltage steps. The two-phase reaction is accordingly the lithiation of the electrode during the lithiation level. For example, the electrode is an anode that is at least partially formed from carbon compounds. In particular, the anode is a graphite anode. The certain limiting current for the discharge current is, for example, approximately C / 10, where C is the well-known C coefficient that serves as a parameter for the available nominal capacity. A discharge current of C / 10 therefore corresponds to the current that transports one-tenth of the theoretical maximum charge of the cell in one hour. Due to the low discharge current, the energy storage device heats up only insignificantly through ohmic losses, so that temperature influences are reduced when determining the aging state.

[0016] More preferably, the cell is discharged below the first lithiation stage of the electrode. The two-phase reaction is accordingly the lithiation of the electrode during the first lithiation stage. The method according to the invention is thus used in a range of the energy storage device's state of charge that corresponds to a relatively deep discharge of the energy storage device. In particular, the cell is discharged to a state of charge that lies outside (or more precisely, below) the operating limits for normal use of the energy storage device.

[0017] One aspect provides for the cell to be charged beyond the charge level at which the electrode exhibits the two-phase reaction. Accordingly, the electrode is formed at least partially from carbon compounds. In particular, it is a graphite anode. The charging current is relatively low compared to a charging current with which the cell is charged during normal operation of the energy storage device. The charging current is preferably less than C / 3, more preferably C / 10, in particular less than C / 20. Due to the low charging current, the energy storage device heats up only insignificantly through ohmic losses, so that temperature influences are reduced when determining the aging state.

[0018] The first lithiation stage can therefore correspond to a certain charge level.

[0019] According to one aspect of the invention, the cell is charged with a constant current. The charging current is preferably less than C / 3, more preferably C / 10, and especially less than C / 20. From the constant, known charging current, the amount of charge flowing can be calculated by (numerical) integration.

[0020] According to one embodiment of the invention, the cell is neither charged nor discharged for a certain period of time between discharging and charging. This certain period of time lasts, for example, more than five minutes, preferably more than ten minutes. This allows the energy storage device to cool down between discharging and charging the cell, further reducing temperature influences when determining the aging state.

[0021] To determine the state of aging, a derivative of the flowing charge with respect to the cell voltage is preferably used. In other words, the differential quotient dQ / dU of the differentials of the flowing charge dQ and the cell voltage dU is calculated, particularly numerically (in this case, it is a difference quotient). The derivative curve has a characteristic shape depending on the state of aging and the cause of aging, which is why the derivative of the flowing charge with respect to the cell voltage is particularly suitable for determining the state of aging. In addition, the causes of aging can also be determined using the derivative curve. This makes it possible to adapt an operating strategy for the energy storage system in order to mitigate the effects of the causes of aging and / or counteract the causes of aging.

[0022] Further preferably, to determine the aging state, a local extremum of the derivative of the flowing charge with respect to the cell voltage is determined. This is, in particular, the local extremum associated with the charge level at which the electrode exhibits a two-phase reaction. The extremum can therefore be associated with a lithiation stage of an electrode, in particular the first lithiation stage. The position of the local extremum, i.e., the abscissa and ordinate, as well as the derivative's course in the vicinity of the extremum, exhibit characteristic values ​​or a characteristic shape depending on the aging state and the causes of aging. Therefore, the aging state and the causes of aging can be easily determined from these values ​​and this shape.

[0023] Another aspect provides that, to determine the aging state, the derivative of the closed charge quantity with respect to the cell voltage in the vicinity of the local extreme is compared with a characteristic curve. In particular, the characteristic curve is stored in a control unit of the energy storage device, so that the derivative can be compared with a characteristic curve particularly quickly.

[0024] In one embodiment of the invention, a peak analysis of the derivative curve is performed in order to determine parameters of at least one peak in the curve. A peak is understood to be the area surrounding a local maximum. In particular, it is the peak associated with the charge level at which the electrode exhibits a two-phase reaction. The peak can therefore, for example, be associated with a lithiation stage of an electrode, in particular the first lithiation stage. The peak analysis can be qualitative and / or quantitative. The parameters can include one or more of the following: abscissa (cell voltage) of the peak, height of the peak, and / or width of the peak (in particular half-width or 1 / e width).

[0025] In particular, at least one of the recorded variables and / or at least one variable determined from the recorded variables is / are smoothed and / or interpolated. The variables can also be processed in another suitable manner. This facilitates further (numerical) processing of the recorded variables or the variable determined from the recorded variables and improves the accuracy of determining the aging state. For example, the accuracy of a qualitative and / or quantitative peak analysis is improved.

[0026] The recorded quantities are, for example, the charging current intensity and the duration of a period over which the charging current flows, whereas the quantity determined from the recorded quantities corresponds to the amount of charge flowed.

[0027] The object is further achieved according to the invention by a method for operating a lithium-ion battery, comprising the following steps: the aging state of at least one cell of the battery is determined using a method according to the invention. Based on the determined aging state, operating parameters of the battery are adjusted. This makes it possible to adapt an operating strategy of the energy storage device in order to mitigate and / or counteract the effects of the causes of aging. In particular, an incorrect operating strategy is avoided, which could result in above-average aging of the energy storage device. For example, depending on the cause of aging, voltage limits are increased in order to continue to be able to draw high power from the energy storage device. Regarding the other advantages, reference is made to the above explanations.

[0028] Further advantages and features of the invention will become apparent from the following description and the drawings, to which reference is made. These show: - Fig. 1 is a schematic flow diagram of the steps of a method according to the invention for determining the aging state of a cell of a battery; and - Fig. 2 a diagram of the derivative of a flowing charge amount according to a cell voltage plotted against the cell voltage.

[0029] The following will be based on Fig. 1 describes a method for determining the aging state of at least one cell of a battery of an energy storage device. In the example considered, the energy storage device consists of one or more cell modules of lithium-ion cells, with the individual galvanic elements each having graphite anodes. Such energy storage devices are used, for example, in motor vehicles.

[0030] In the following, the term cell is used synonymously with the terms “galvanic cell” and “module consisting of several galvanic cells”.

[0031] To determine the aging state of at least one cell, the cell is first discharged below a certain charge level (step S1). This certain charge level is the charge level at which an electrode of the cell exhibits a two-phase reaction. More precisely, in the exemplary embodiment considered, the cell is discharged below the first lithiation stage of the graphite anode, in particular with constant voltage steps. It can be provided that the cell is discharged until the discharge current reaches a certain limiting current, which is, for example, C / 10.

[0032] This is followed by a period of time during which the cell is neither charged nor discharged (step S2), which can also be referred to as a rest period. This period lasts, for example, more than 5 minutes, preferably more than 10 minutes. This rest period normalizes the temperature of the energy storage device and reduces temperature influences on subsequent measurements.

[0033] The cell is now charged above a certain charge level (step S3). The cell is charged with a constant, low current. The current is, for example, less than C / 3, preferably less than C / 10, and more preferably less than C / 20.

[0034] During charging, the cell voltage U of the cell, as well as the charging current and / or the amount of charge Q flowing during charging of the cell, are recorded (step S4). If the charging current is recorded, the duration of the period over which the charging current flows is also recorded. The amount of charge Q flowing is then determined, for example, by (numerical) integration of the charging current and the duration. In this step, the cell is not fully charged. Only a certain fraction of the total charge required to charge the cell is supplied, for example, approximately 15%.

[0035] Alternatively, the flowing charge quantity can also be recorded directly, i.e. as a recorded quantity.

[0036] Now, the derivative of the flowing charge Q with respect to the cell voltage U is determined, in particular by numerical differentiation. Thus, the differential quotient dQ(U) / dU (numerically corresponding to the difference quotient) is determined.

[0037] An exemplary course 10 of such a derivative is shown in Fig. 2 shown.

[0038] A first peak 12 is clearly visible, which is formed by the course of the derivative in the vicinity of a local maximum. This peak 12 is assigned to the first lithiation stage of the graphite anode, which in the example shown is at a cell voltage U of approximately 3.55 V (indicated by the dashed line in Fig. 2) can be considered completed.

[0039] To determine the cell's aging state, a peak analysis of peak 12 in the course 10 of the derivation is performed, during which various parameters of peak 12 are determined. The parameters may include one or more of the following: abscissa (cell voltage U) of the peak, peak height, and / or peak width (in particular, half-width or 1 / e-width).

[0040] In order to improve the peak analysis, it may be provided to smooth, interpolate or process in another suitable manner at least one of the recorded variables and / or variables calculated therefrom.

[0041] Since the peak parameters depend in a characteristic way on the cell's aging state and / or causes of aging, the aging state can be determined from the parameters obtained via peak analysis. Furthermore, causes of aging can also be determined from the determined parameters. In particular, the determined parameters are compared with characteristic parameters that may be stored in a control unit of the energy storage device, for example, in a memory of the control unit. The characteristic parameters therefore contain information about which peak parameter values ​​are characteristic of the various aging phenomena and / or causes of aging in the energy storage device. These can be compared with each other to identify deviations or irregularities.

[0042] In particular, the comparison makes it possible to detect or predict a failure or impairment in good time, so that operational restrictions can be counteracted at an early stage, preferably before they actually occur.

[0043] After the peak has been completely passed through (in Fig. 2 (at approximately 3.55 V in the example shown in Figure 2), the slow charging and the recording of at least one value can be stopped. Accordingly, the energy storage device can be charged at the normal charging rate.

[0044] The method described above for determining the aging state of at least one cell can be used for a method for operating a lithium-ion battery. For this purpose, the operating parameters of the lithium-ion battery are adjusted accordingly based on the determined aging state. For example, depending on the cause of aging, voltage limits are increased in order to continue to draw high power from the energy storage device. In particular, an incorrect operating strategy is avoided, which could cause above-average aging of the energy storage device.

Claims

[1] Method for determining an ageing state of at least one cell of a battery of an energy storage device, in particular a cell of a lithium-ion battery, comprising the following steps: - Discharging the cell below a certain charge level; - Charging the cell above a certain charge level; - detecting a cell voltage (U) as well as a charging current and / or a flowing charge quantity (Q) during charging of the cell; and - Determining the aging state of the cell from the measured values; characterized by that the cell is discharged below a charge level at which one electrode of the cell exhibits a two-phase reaction. [2] Method according to claim 1, characterized by that the cell is discharged below a lithiation level of one electrode of the cell and / or until a certain limit current is reached, in particular with constant voltage steps. [3] Method according to claim 2, characterized by that the cell is discharged below the first lithiation stage of the electrode. [4] Method according to one of the preceding claims, characterized by that the cell is charged beyond the charge level at which the electrode exhibits the two-phase reaction. [5] Method according to one of the preceding claims, characterized by that the cell is charged with a constant current. [6] Method according to one of the preceding claims, characterized by that the cell is neither charged nor discharged for a certain period of time between discharging and charging the cell. [7] Method according to one of the preceding claims, characterized by that a derivative of the flowing charge quantity (Q) with respect to the cell voltage (U) is used to determine the state of aging. [8] Method according to claim 7, characterized bythat in order to determine the state of aging, a local extremum of the curve (10) of the derivative of the flowed charge quantity (Q) with respect to the cell voltage (U) is determined. [9] Method according to claim 8, characterized by that, in order to determine the state of ageing, the curve (10) of the derivative of the flowed charge quantity (Q) with respect to the cell voltage (U) in an environment of the local extremum is compared with a characteristic curve. [10] Method according to one of claims 7 to 9, characterized by that a peak analysis of the course (10) of the derivative is carried out in order to determine parameters of at least one peak (12) in the course. [11] Method according to one of the preceding claims, characterized by that at least one of the recorded variables and / or at least one variable determined from the recorded variables is or is smoothed and / or interpolated. [12] A method for operating a lithium-ion battery, comprising the following steps: - Determining the ageing state of at least one cell of the battery by means of a method according to one of the preceding claims; and - Adjusting battery operating parameters based on the determined aging condition.

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