DE102021103949A1 - Method for detecting lithium plating in a lithium ion cell and lithium ion battery - Google Patents

Abstract

A method for detecting lithium plating in a lithium ion cell includes the following steps: First, an impedance of the lithium ion cell is measured at a single test frequency. A first temperature is determined based on the real part of the measured impedance and a second temperature is determined based on the imaginary part of the impedance. A temperature difference between the first and second temperatures is then calculated and the temperature difference is compared with a threshold value, the threshold value being a temperature difference that is characteristic of the occurrence of lithium plating. A lithium-ion battery is also specified.

Description

The invention relates to a method for detecting lithium plating in a lithium ion cell and a lithium ion battery with at least one lithium ion cell.

In the following, the term “lithium ion cell” is used synonymously for all designations commonly used in the prior art for galvanic elements and cells containing lithium. In particular, rechargeable galvanic elements (secondary cells) are included.

Due to their high energy density, lithium-ion cells have found acceptance in a large number of applications, including traction batteries for vehicles. Such traction batteries have a high capacity, which is crucial for the achievable range of the vehicle between two charging processes. The charging duration of a charging process depends not only on the capacity but also on the level of the charging current, with a higher charging current leading to a shorter charging process.

The disadvantage of lithium-ion cells is that so-called "lithium plating" can occur. This is understood to mean the deposition of metallic lithium on the anode during the charging process of the lithium-ion cell, which can have a disadvantageous effect on the performance and/or the service life of the lithium-ion cell. It is known that lithium plating is favored by high charging currents, as a result of which the minimum charging time that can be achieved is limited. However, other environmental conditions and mechanisms also contribute to lithium plating.

From the DE 10 2017 218 713 A1 It is known to evaluate lithium plating using a current/voltage response of a battery pack obtained after applying a test signal to the battery pack, and to adjust a charging process based on the evaluation. The evaluation uses a Fourier analysis to detect harmonics in the current/voltage response and use them as an indicator for lithium plating.

The evaluation is correspondingly complex, so that there is a need for a simplified method for detecting lithium plating.

In Koleti et al.: "A new on-line method for lithium plating detection in lithium-ion batteries" (Journal of Power Sources, 451 (2020), 227798) is a method for detecting lithium plating based on a continuous impedance measurement of the lithium-ion cell is described, with the dependence of the impedance measurement on the state of charge, the cell temperature and the charging rate being discussed. The occurrence of lithium plating is determined via a discontinuity in the time profile of the impedance during the charging process, as a result of which the beginning of the onset of lithium plating can only be reliably determined with a time delay. On the other hand, it is desirable to detect lithium plating with as little delay as possible in order to be able to reliably prevent its occurrence.

the DE 10 2017 209 448 A1 shows a method for monitoring electrochemical energy stores, in which the temperature of the electrochemical energy store is determined based on an impedance measurement at two different frequencies in order to determine spatial temperature inhomogeneities present in the energy store. The influence of lithium plating is not taken into account.

The object of the invention is to provide a simple, inexpensive and reliable way of detecting lithium plating in a lithium ion cell. In particular, an in operando detection should be made possible during a charging process of the lithium-ion cell.

The object of the invention is achieved by a method for detecting lithium plating in a lithium ion cell, comprising the following steps: First, an impedance of the lithium ion cell is measured at a single test frequency. A first temperature is determined based on the real part of the measured impedance and a second temperature is determined based on the imaginary part of the impedance. A temperature difference between the first temperature and the second temperature is then calculated and the temperature difference is compared to a threshold value, the threshold value being a temperature difference that is characteristic of the occurrence of lithium plating.

It has been recognized that lithium plating affects the real and imaginary impedance of a lithium ion cell to different degrees at the same frequency. In particular, it has been found that lithium plating affects the real part more than the imaginary part.

By converting the measured impedance into temperature values, a simple and reliable comparison with a threshold value that is characteristic of the occurrence of lithium plating is possible.

The method according to the invention also offers the advantage that the respectively collected measurement data can be obtained from a single measurement.

According to the invention, not several but only a single test frequency are used for impedance measurement, so that effects of the lithium plating, which affect the impedance to different extents at different frequencies, do not impair the detection. In this way, the evaluation can be simplified and the reliability of the method can be increased.

At the same time, it can be prevented or at least reduced that the measured impedance is a mixed signal from spatially separated components of the lithium-ion cell, the effects of which could weaken or compensate one another, whereby the sensitivity of the method according to the invention for detecting lithium plating can be increased.

The simple evaluation enables a quick and reliable detection of lithium plating also in operando during charging processes of the lithium ion cell.

The first temperature and the second temperature reflect the internal temperature of the lithium ion cell, so that particularly precise information about the state inside the lithium ion cell can be obtained. In particular, additional sources of error can be avoided, which can occur when using temperatures measured only outside the lithium-ion cell.

The threshold value can be determined in advance depending on the cell type and the cell chemistry of the lithium ion cell and stored as a predefined threshold value, for example in an evaluation module which is connected to the lithium ion cell for data exchange.

The test frequency can be in the range from 1 to 10,000 Hz, preferably in the range from 1000 to 6000 Hz, particularly preferably in the range from 2000 to 5000 Hz.

In principle, a test frequency is preferred which is in the so-called “charge transfer” range of the electrochemical impedance spectrum of the lithium-ion cell in order to make the kinetics of the processes occurring in this range decisive for the measurement results obtained in the method according to the invention.

In the "charge transfer" region, greater changes in impedance values are to be expected when lithium plating occurs than in the remaining regions of the impedance spectrum, since this region is characteristic of the charge transport through the interface between the electrode and the electrolyte of the lithium-ion cell. This interface changes to a particular extent as a result of precipitating lithium when lithium plating occurs.

The frequency range of the "Charge Transfer" range depends on the state-of-charge (SoC) of the lithium-ion cell. Therefore, the test frequency can be adjusted as a function of the SoC of the lithium-ion cell in order to enable an optimal impedance measurement and thus an optimal temperature determination at any time.

The first and the second temperature can be determined using a calibration function from the real part or from the imaginary part of the measured impedance.

In particular, the calibration function can be created during a charging process that is carried out with a charging current in which it is known that no lithium plating occurs. This can be a low current charge, for example a charge with a C rate in the range of C/5 to C/3 or lower.

The temperature difference is in particular the difference between the first temperature and the second temperature. In other words, in particular the second temperature based on the imaginary part of the measured impedance is subtracted from the first temperature based on the real part of the measured impedance.

It has been found that when lithium plating occurs, the second temperature based on the imaginary part can be expected to be lower than the first temperature based on the real part. Since the difference between the first temperature and the second temperature is used to calculate the temperature difference, a positive temperature difference can generally be assumed.

In a variant, the absolute value of the calculated temperature difference is compared to the threshold value.

The threshold value is in particular in the range from 0.1 to 5 K, preferably in the range from 0.5 to 3 K, particularly preferably in the range from 1.0 to 2.5 K.

The impedance measurement must be carried out with sufficient measurement accuracy in order to be able to reliably determine temperature differences that are in the range of the threshold value. This is possible, for example, using an impedance measuring device integrated in the lithium-ion cell in the form of a measuring chip, which has a resolution in the range of a few μΩ and is commercially available.

In order to enable a reliable detection of lithium plating over the entire lifetime of the lithium ion cell, the threshold value can be adjusted depending on the age of the lithium ion cell.

In particular, larger temperature differences are to be expected for older lithium-ion cells than for younger lithium-ion cells, so that the threshold value can be raised as the lithium-ion cell ages. This effect is due in particular to lithium previously deposited by lithium plating.

In one variant, the calculated temperature difference and the threshold value are used to control a charging process for the lithium-ion cell, in particular a rapid charging process for the lithium-ion cell.

The method according to the invention for detecting lithium plating can be carried out during a charging process, in particular a rapid charging process, so that the charging current used can be adjusted immediately if necessary.

In particular, the charging current is reduced if the threshold is reached or exceeded. However, it is also possible to increase the charging current to shorten the necessary charging time if the threshold value is not reached, in particular if the calculated temperature difference is only 0.6 times the threshold value or less. In this way, the method according to the invention makes it possible to determine an optimal charging current for a charging process of the lithium-ion battery in operando.

The method steps are preferably repeated at regular time intervals, there being in particular a waiting time of 120 seconds or less, preferably 60 seconds or less, particularly preferably 5 seconds or less, between two repetitions.

The process steps are particularly preferably repeated continuously, with the time interval between two repetitions being defined only by the time required to carry out the individual process steps.

By repeating the method steps, lithium plating, which can only start during the charging process, can be detected even more reliably and, in particular, the charging current can be adjusted in a timely manner. In addition, a change in the difference between the calculated temperature difference and the threshold value that increases or decreases over time can be determined, from which it can be estimated whether the threshold value will be reached with the currently used charging current and the expected remaining charging time or not.

The object of the invention is also achieved by a lithium-ion battery with at least one lithium-ion cell, with each lithium-ion cell being assigned an impedance measuring device, and an evaluation module which is connected to the at least one lithium-ion cell and the assigned impedance measuring device for data exchange, the lithium-ion battery being set up to carry out the procedures described above.

The impedance measuring device is in particular a measuring chip integrated in or on the at least one lithium ion cell.

In one variant, the lithium-ion battery comprises at least two lithium-ion cells that are electrically connected to form a parallel connection of lithium-ion cells, with the impedance measuring device being assigned to the parallel connection of lithium-ion cells.

In other words, the impedance measuring device is assigned to one of the lithium ion cells of the parallel connection of lithium ion cells, so that the entire parallel connection of lithium ion cells can be monitored using the assigned impedance measuring device.

The values and/or functions required to carry out the method according to the invention can be stored in the evaluation module, for example the threshold value, a function for adapting the threshold value depending on the age of the at least one lithium-ion cell, the calibration line for determining the first temperature and the second temperature and/or or a function to select the test frequency.

In particular, the evaluation module includes a control unit that is set up to adapt a charging current when charging the lithium-ion battery. In particular, the control unit is set up to adapt the charging current for each of the lithium-ion cells independently of one another.

The lithium ion battery is in particular a traction battery for a vehicle.

• - 1 eine schematische Querschnittsansicht einer erfindungsgemäßen Lithiumionenbatterie,
• - 2 ein Blockschema eines erfindungsgemäßen Verfahrens zum Detektieren von Lithium-Plating in einer Lithiumionenzelle,
• - 3 Spannungskurven während eines Ladevorgangs mehrerer erfindungsgemäßer Lithiumionenbatterien nach 1, und
• - 4 den Verlauf der mittels Impedanz ermittelten Temperaturdifferenzen während der Ladevorgänge aus 3.

Further advantages and properties of the invention result from the following description of exemplary embodiments, which should not be understood in a limiting sense, and from the drawings. In these show:

• - 1 a schematic cross-sectional view of a lithium-ion battery according to the invention,
• - 2 a block diagram of a method according to the invention for detecting lithium plating in a lithium ion cell,
• - 3 Voltage curves during charging of several lithium-ion batteries according to the invention 1 , and
• - 4 the course of the temperature differences determined by means of impedance during the charging process 3 .

In 1 1 is a schematic representation of a lithium ion battery 10 according to the invention, the lithium ion battery 10 comprising a housing 12 and a lithium ion cell 14 arranged within the housing 12 .

The lithium ion cell 14 has an associated impedance measuring device 16 which is set up to determine the impedance of the lithium ion cell 14 . The impedance measuring device 16 is a measuring chip which has a resolution in the range of a few μΩ.

The lithium-ion battery 10 also includes an evaluation module 18, which is connected to the lithium-ion cell 14 and the impedance measuring device 16 assigned to the lithium-ion cell 14 for data exchange.

The lithium ion battery 10 has electrical contacts (not shown) in order to be able to connect the lithium ion battery 10 to a charging device (not shown) for charging the lithium ion battery 10, specifically the lithium ion cell 14.

The evaluation module 18 has a control unit 20 which is set up to adapt a charging current of the charging device for charging the lithium-ion battery 10 .

in the in 1 shown view is only a lithium ion cell 14 visible. Of course, the lithium-ion battery 10 can have any number of lithium-ion cells 14 connected in parallel and/or in series.

In this case, each parallel circuit of lithium-ion cells 14 is assigned an impedance measuring device 16, which is connected to the evaluation module 18 for data exchange.

In 2 14 is a block diagram of a method for detecting lithium plating in a lithium ion cell 14, such as that found in the lithium ion battery 10, in accordance with the present invention 1 is used, shown.

The method according to the invention comprises the following steps.

First, an impedance of the lithium-ion cell 14 is measured at a single test frequency (see step S1 in 2 ).

The impedance measuring device 16 assigned to the lithium-ion cell 14 is used for this purpose.

The impedance data obtained from the impedance measuring device 16 is sent to the evaluation module 18, in which a first temperature T ₁ is determined based on the real part of the measured impedance and a second temperature T ₂ is determined based on the imaginary part of the impedance (cf. step S2 in 2 ) becomes.

The evaluation module 18 determines the first temperature T ₁ and the second temperature T ₂ in particular using a calibration function stored in the evaluation module 18 .

A temperature difference ΔT _Im,Re between the first temperature T ₁ and the second temperature T ₂ is then calculated (cf. step S3 in 2 ).

Finally, the calculated temperature difference ΔT _Im,Re is compared with a threshold value, the threshold value being a temperature difference that is characteristic of the occurrence of lithium plating and is also stored in the evaluation module 18 .

The threshold value is in particular in the range from 0.1 to 5 K, preferably in the range from 0.5 to 3 K, particularly preferably in the range from 1.0 to 2.5 K.

Method steps S1 to S4 are preferably repeated at regular time intervals, with a waiting time of 120 seconds or less, preferably 60 seconds or less, particularly preferably 5 seconds or less, between two repetitions.

The process steps are particularly preferably repeated continuously, with the time interval between two repetitions being defined only by the time required to carry out the individual process steps.

In 3 exemplary experimental voltage curves of the charging process of a lithium-ion battery 10 according to the invention are shown.

The lithium ion battery 10 used in the measurements is a lithium ion battery for use in automotive applications.

The lithium ion battery was charged using four different charge profiles as indicated in Table 1, with the lithium ion battery being completely discharged between each charge. Table 1: Charge profiles of the experimental measurements. test designation loading procedure charging current loading process 1 Constant Current (CC) 2.5c loading process 2 Constant Current (CC) 2.0c loading process 3 Constant Current (CC) 1.5c loading process 4 Constant Current (CC) - multi-level 1.6C - 1.0C - 0.8C - 0.6C

In the 1 The measurement series shown were recorded for 60 minutes each. The charging time was in the range of about 15 to 40 minutes.

A multi-stage constant current charging process was used for the “Charging process 4” measurement, with the charging current being adjusted at 15, 22, 28 and 35 minutes.

During each of the experimental charging operations, the method for detecting lithium plating according to the present invention was carried out, continuously repeating steps S1 to S4.

A frequency of about 2600 Hz was used as the test frequency for the impedance measurement.

In 4 are the resulting curves of the determined temperature differences ΔT _Im,Re during the charging process 3 shown, the temperature difference ΔT _Im,Re being calculated as the difference between the first temperature T ₁ and the second temperature T ₂ .

It can be seen that for the test series "Charging process 1", "Charging process 2" and "Charging process 3" a strong increase in the temperature difference ΔT _Im,Re can be observed from about minute 15, 17 or 25, while the temperature difference ΔT _{Im, Re} for the test series "Charging process 4", which used a lower charging current for at least a large part of the charging period, never exceeded a value of 0.5 K.

In 4 the dash-double-dot line S indicates the predetermined threshold value characteristic of the occurrence of lithium plating, which was 1.5 K in the experiments performed.

Each of the test series "Charging process 1", "Charging process 2" and "Charging process 3" exceeds this value at at least one point in time, with the points in time at which the threshold value is reached for the first time in 3 are marked with the points P ₁ , P ₂ and P ₃ .

While no influence of the occurrence of lithium plating can be seen in the course of the voltage curve, the differences in the temperature differences ΔT _Im,Re determined using the method according to the invention are clearly apparent. Thus, a simple and reliable detection of lithium plating in operando during the charging process of the lithium ion battery 10 is possible.

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• DE 102017218713 A1 [0005]
• DE 102017209448 A1 [0008]

Claims (10)

A method for detecting lithium plating in a lithium ion cell (14), comprising the steps of: - measuring an impedance of the lithium ion cell (14) at a single test frequency, - determining a first temperature T ₁ based on the real part of the measured impedance and a second temperature T ₂ based on the imaginary part of the measured impedance, - calculating a temperature difference ΔT _Im,Re between the first temperature T ₁ and the second temperature T ₂ , - comparing the temperature difference ΔT _Im,Re with a threshold value, the threshold value being one for the occurrence of lithium -Plating is characteristic temperature difference. procedure after claim 1 , characterized in that the test frequency is in the range from 1 to 10,000 Hz, preferably in the range from 1000 to 6000 Hz, particularly preferably in the range from 2000 to 5000 Hz. procedure after claim 1 or 2 , characterized in that the first temperature T ₁ and the second temperature T ₂ are determined using a calibration function from the real part or the imaginary part of the measured impedance. Method according to one of the preceding claims, characterized in that the threshold value is in the range from 0.1 to 5 K, preferably in the range from 0.5 to 3 K, particularly preferably in the range from 1.0 to 2.5 K. Method according to one of the preceding claims, characterized in that the threshold value is adjusted as a function of the age of the lithium-ion cell. Method according to one of the preceding claims, characterized in that a charging process of the lithium-ion cell (14), in particular a rapid charging process of the lithium-ion cell (14), is controlled on the basis of the calculated temperature difference and the threshold value. Method according to one of the preceding claims, characterized in that the method steps are repeated at regular time intervals, there being in particular a waiting time of 120 seconds or less, preferably 60 seconds or less, particularly preferably 5 seconds or less, between two repetitions. Lithium-ion battery with at least one lithium-ion cell (14), wherein the at least one lithium-ion cell (14) is associated with an impedance measuring device (16), and an evaluation module (18) which is connected to the at least one lithium-ion cell (14) and the associated impedance measuring device (16) for data exchange, wherein the lithium ion battery (10) is set up to carry out the method according to any one of the preceding claims. Lithium ion battery after claim 8 , characterized in that the lithium ion battery comprises at least two lithium ion cells (14) which are electrically connected to form a parallel connection of lithium ion cells (14), the impedance measuring device (16) being associated with the parallel connection of lithium ion cells (14). Lithium ion battery after claim 8 or 9 , characterized in that the evaluation module (18) comprises a control unit (20) which is set up to adjust a charging current when charging the lithium-ion battery (10), in particular the charging current for each of the lithium-ion cells (14) and / or parallel connection of lithium-ion cells ( 14) adjust independently.

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