DE102017218588A1 - Detection of critical operating states in lithium-ion cells - Google Patents

Abstract

The present invention relates to a method for detecting critical operating conditions in lithium-ion cells by measuring the impedance, an apparatus for carrying out the method and a lithium-ion battery equipped with the apparatus.

Description

Technical area

The present invention relates to a method for detecting critical operating conditions in lithium-ion cells, in particular in cells with shutdown separator, by measuring the impedance, a device for carrying out the method and a lithium-ion battery equipped with the device.

Technical background

Separators in lithium-ion cells

At present, lithium and lithium ion batteries with liquid electrolyte, which essentially comprise a negative electrode (anode), a positive electrode (cathode) and an intermediate separator impregnated with a nonaqueous liquid electrolyte, are used in many mobile and stationary energy storage systems.

On the one hand, the separator has the function of preventing direct contact of the electrodes so as to prevent an internal short circuit and, on the other hand, to enable the exchange of lithium ions between the electrodes. The separator is typically a polyethylene or polypropylene based porous polymeric film or a nonwoven material, optionally also in combination with a ceramic material. Separators based on polymer films are produced by melt extrusion in the so-called dry process or in the wet process. In the dry process, the pore structure is obtained by annealing and stretching the film. In the wet process, on the other hand, a soluble wax is added to the polymer material, which acts as a template and, after the film is made, is extracted with a solvent to form the pores. The polymer material used is usually polyethylene or polypropylene or a combination thereof, but other materials such as polyvinylidene fluoride (PVdF) or copolymers thereof with hexafluoropropylene (PVdF-HFP), polyethylene oxide or polyacrylonitrile are also suitable.

In view of the energy density and the provision of short diffusion paths, the separator should be as thin as possible and have a high porosity. On the other hand, the separator also requires good mechanical strength in order to be able to maintain its structural integrity under the temperature variations and volume changes of the electrodes due to the intercalation or deintercalation of lithium, which requires a certain minimum thickness. Thus, there is a conflict with respect to the requirements of the separator.

As with large format lithium ion cells, as e.g. used for vehicles, heat dissipation is more difficult, and the current loads occurring can be higher than for small-sized lithium-ion cells, such as those used for portable electronic devices, the separator quality requirements are particularly relevant here.

If the integrity of the separator is damaged, for example mechanically or as a result of partial melting, electrically conductive contact between the electrodes may occur. Such an internal short circuit causes an undesirable high temperature rise (thermal runaway).

As a precaution against such thermal runaway due to reflow, separators for large format lithium ion cells may be configured as so-called shutdown separators. A "shutdown separator" is made up of (at least) three layers, a low melting inner layer (e.g., PE) and two higher melting outer layers (e.g., PP). At an abnormal temperature rise, the inner layer would first melt and close the pores of the outer layers so that the ion flow stops.

Ideally, this takes place over the entire area of the separator, which is equivalent to a decommissioning of the cell and prevents a further increase in the temperature. The outer layers remain intact and prevent the internal short circuit.

For further details of the prior art summarized above, reference is made to the relevant specialist literature, for example R. Korthauer (ed.), "Handbuch LithiumIonen-Batterien", Springer Vieweg 2013 (especially Chapter 7: Separators).

Impedance measurement in electrochemical cells

The principle of impedance measurement is basically known. In particular, electrochemical impedance spectroscopy (EIS) is an established method of characterizing electrochemical systems, in particular galvanic cells. The prior art also teaches to use impedance measurements to diagnose the state of lithium-ion cells in the automotive field.

This is for example on the US 2009/0096459 referenced, which is a procedure for Measuring the frequency dependence of the impedance describes to determine the mobility of the charges on the surface of the negative and the positive electrode of a lithium-ion cell. In doing so, special methods are used to create equivalent circuit diagrams and determine the parameters for modeling cell behavior. Intended use is in particular the quality control during battery production.

DE 10 2009 038 663 describes a motor vehicle with a plurality of batteries, each separated individually or in blocks from the electrical system and can be connected to a diagnostic device for performing a model-based battery diagnostic method. The diagnostic method may also include a measurement of the impedance.

DE 10 2013 103 921 relates to cell temperature measurement and degradation measurement in lithium battery systems of electrically powered vehicles by determining the cell impedance based on an AC signal given by an inverter. The method is based on the observation that the course of the application of impedance versus signal frequency is temperature-dependent.

task

In the case of triggering a shutdown separator, in the real case, there may be the difficulty that the temperature distribution across the separator surface is uneven, for example because of better heat dissipation of the section closer to the housing wall, or due to local inhomogeneities, e.g. due to non-uniform mechanical load.

Therefore, in the event of a fault, it can not be ruled out that the shutdown effect occurs locally at a hotter location, while the separator is still intact at colder places and allows further lithium transport. Thus, there is a risk that the temperature further increases at the hot spot, so that in the worst case, the outer layers of the shutdown separator melt and an internal short circuit occurs.

In view of this problem, it would be useful to be able to detect a partial local triggering of the shutdown effect, as it can occur under real conditions, so that the cell can be deactivated in time and a further increase in temperature is prevented.

According to the invention, it has been found that the impedance is proportional to the intact separator area. Thus, by impedance measurement, a reduction of the intact separator area can be determined by locally triggering the shutdown effect. One aspect of the present invention thus relates to a method for detecting the incipient melting of a shutdown separator in a lithium-ion cell by means of impedance measurement.

Furthermore, even temperatures below the shutdown temperature lead to accelerated aging and partial decomposition of the electrolyte. Such temperatures can also occur only locally limited at individual locations in the cell interior and therefore can be difficult to detect.

According to the invention, it has been found that such effects, which occur below the shutdown temperature, are detected by impedance measurement. Therefore, another aspect of the invention relates to the detection of such critical operating conditions by impedance measurement.

In particular, the methods according to the invention can be carried out "online" (i.e., during operation, if necessary under load), thus making it possible to monitor the cell continuously and to switch it off in good time in the event of a fault.

Summary of the invention

The present invention relates to a method for detecting critical operating conditions in lithium-ion cells by measuring the impedance, an apparatus for carrying out the method and a lithium-ion battery equipped with the apparatus.

• - Kontinuierliche oder intermittierende Ermittlung des Betrags und/oder des Realteils der Impedanz Z der Zelle(n), um den Wert Z_t zum Zeitpunkt t zu erhalten;
• - Vergleich des Werts Z_t mit einem oder mehreren Referenzwerten Z_ref, wobei Z_ref ausgewählt ist aus:
  1. (a) einem zu einem vorherigen Zeitpunkt t-x bestimmten Wert Z_t-x, wobei x ein Zeitintervall >0 darstellt;
  2. (b) einem für die Zelle bei der jeweiligen Zelltemperatur T erwarteten Wert Z_T;
  3. (c) einem für den jeweiligen Zeitpunkt t erwarteten Wert Z_erw,t, der für die Zelle anhand eines oder mehrerer Betriebsparameter, ausgewählt aus Zelltemperatur, Zellspannung, Laststrom, Ladezustand und Alterungszustand, berechnet wird;
• - Feststellung eines kritischen Betriebszustands, wenn der Wert Z_t gegenüber einem, mehreren oder allen Z_ref angestiegen ist und der prozentuale Anstieg einen vorbestimmten Schwellwert überschreitet, wobei der Schwellwert 5% oder mehr beträgt.

In particular, the invention relates to a method for detecting critical operating conditions in one or more lithium ion cells, comprising:

• - Continuously or intermittently determining the magnitude and / or the real part of the impedance Z of the cell (s) to obtain the value Z _t at time t;
• Comparison of the value Z _t with one or more reference values Z _ref , where Z _{ref is} selected from:
  1. (a) one at a previous time tx certain value Z _tx, where x is a time period> 0 represents;
  2. (b) a value Z _T expected for the cell at the respective cell temperature _T ;
  3. (c) a value Z _erw, t expected for the respective time t, calculated for the cell from one or more operating parameters selected from cell temperature, cell voltage, load current, state of charge and state of aging;
• - Determining a critical operating condition when the value Z _{t has} risen against one, several or all Z _ref and the percentage increase exceeds a predetermined threshold, the threshold being 5% or more.

Specifically, the cells may have a shutdown separator, and the critical operating condition corresponds to incipient local melting of the shutdown separator.

Typically, when a critical operating condition has been detected, a corresponding signal is passed to the battery controller, which may then take appropriate countermeasures to prevent further damage or thermal runaway, such as disconnecting the cell or the entire battery pack from the grid, initiating cooling measures, etc.

list of figures

• 1 shows possible arrangements of the measuring device in a battery pack.
• 2 shows the plot of case temperature (T _can ), valve temperature (T _vent ), and impedance for a cell in the oven test.
• 3 shows the plot of housing temperature (T _can ), valve temperature (T _vent ) and impedance for a cell in the heater test.
• 4 shows the application of case temperature impedance for every three cells during the oven test or hot plate test. The reversal of the impedance / temperature behavior can be clearly seen.

Detailed description

Basics

The basic operating principle of the impedance measurement for lithium-ion cells is known in the prior art. In particular, this is to the above-cited US 2009/0096459 directed. The methods described therein may be adapted accordingly for the practice of the invention. In the following, the basics are briefly summarized to introduce terminology.

The impedance measurement can be potentiostatic or galvanostatic. In the potentiostatic case, an AC signal is modulated onto the DC component, which in turn corresponds to the cell voltage. The amplitude of the AC signal is typically much lower than the DC voltage, so as not to affect the behavior of the cell, and may for example be between 1 and 50 mV, in particular about 5 to 10 mV. Then measured is the oscillating component of the current caused by the alternating voltage signal, as well as its phase shift φ with respect to the applied voltage.

The galvanostatic case corresponds to the reverse procedure, in which a predetermined alternating current signal I (t) is modulated and the voltage U (t) is measured. In the following, the modulated voltage or current signal is also referred to as the excitation signal, and the corresponding current or voltage signal which is measured as the response signal.

By convention, φ is defined as the difference between the zero-angle of voltage and current (φ = φ _U - φ _I ).

$$\text{I(t)}={\text{I}}_{\text{0}}*{\text{e}}^{{\text{i(}}^{\mathrm{\omega }}\text{t}-\mathrm{\varphi }}{}^{\text{)}}$$

$$\begin{array}{c}\text{Z}=\text{U(t)/I(t)}={\text{(U}}_0{\text{/I}}_0\text{)}*{\text{e}}^{\text{i}\mathrm{\varphi }}\\ ={\text{Z}}_0*\text{(cos}\mathrm{\varphi +}\text{i}*\text{sin}\mathrm{\varphi }\text{)}\\ \mathrm{=}\text{R}+\text{iX}\end{array}$$

If the oscillation of the alternating voltage U (t) is expressed as e ^iωt , the following results for the oscillating current component I (t) and for the impedance (alternating current resistance) Z: $$\text{U (t)}={\text{<figure-callout id="0" label="U" filenames="DE102017218588A1\_0005.png,DE102017218588A1\_0006.png" state="{{state}}">U</figure-callout>}}_0*{\text{e}}^{{\text{i}}^{\mathrm{\omega }}}{}^{\text{t}}$$

$$\text{I (t)}={\text{I}}_{\text{0}}*{\text{e}}^{{\text{i (}}^{\mathrm{\omega }}\text{t}-\mathrm{\phi }}{}^{\text{)}}$$

$$\begin{array}{c}\text{Z}=\text{U (t) / I (t)}={\text{(<figure-callout id="0" label="U" filenames="DE102017218588A1\_0005.png,DE102017218588A1\_0006.png" state="{{state}}">U</figure-callout>}}_0{\text{/ I}}_0\text{)}*{\text{e}}^{\text{i}\mathrm{\phi }}\\ ={\text{<figure-callout id="0" label="Z" filenames="DE102017218588A1\_0005.png,DE102017218588A1\_0006.png" state="{{state}}">Z</figure-callout>}}_0*\text{(cos}\mathrm{\phi \; +}\text{i}*\text{sin}\mathrm{\phi }\text{)}\\ \mathrm{=}\text{R}+\text{iX}\end{array}$$

Thus, the impedance is generally complex and has a real part Re (Z) = R, which corresponds to the ohmic resistance and is also called effective resistance, and an imaginary part Im (Z) = X, which is also called reactance.

The amount of impedance | Z | is the ratio of the effective current and voltage amplitudes and is also referred to as impedance. It contains both the effective resistance and the reactive resistance and is typically frequency-dependent. However, power is only dissipated due to the resistance.

For an ideal ohmic resistance, the phase shift is zero and the impedance corresponds to the effective resistance R. For an ideal capacitor (capacitance), the phase shift is -90 °, and the impedance is purely imaginary and decreases with increasing frequency (Z = - i * ( 1 / ωC); C: capacity). For an ideal inductive component, the phase shift is + 90 °. The impedance is also purely imaginary and Z = + iωL (L: inductance), ie the reactance increases with increasing frequency.

The behavior of a real component such as a galvanic cell is much more complex, but can generally be modeled by combining a plurality of the three aforementioned ideal components (ohmic, capacitive and inductive resistors) in parallel and / or series connection. Corresponding further explanations can also be found in US 2009/0096459 ,

Most processes in the cell that rely on landing transport, including ionic conduction in the electrolyte and the kinetics of intercalation and deintercalation at the electrodes, can be described by ohmic resistances. Capacitive resistors occur, for example, on the electrical double layers on the electrodes. Inductors are usually less relevant in the galvanic cells themselves. However, they may, for example, occur due to the magnetic fields caused by the current flow in the current collectors and the leads and play an increasing role at higher frequencies.

As long as the cell is operated in the normal temperature range, a temperature increase causes an increase in the ionic conductivity, which leads to a decrease in the real part and the amount of the impedance.

According to the invention, it has been found that this behavior occurs when a first undesirably high, i. critical, temperature reversed. Although the temperature increase causes a further increase in ion mobility, but at the same time occur accelerated aging and incipient decomposition of the electrolyte, which lead to an overall higher resistance. A second critical temperature occurs in cells with a shutdown separator and marks the beginning of the shutdown effect. The closing of the pores and the concomitant loss of permeable separator surface cause a large increase in the resistance.

Thus, the inventive method is based on the determination of the occurrence of an abnormal increase in impedance, in particular by reversing the temperature / resistance behavior. The underlying mechanisms are essentially due to changes in the ionic conductivity and can therefore be detected in good approximation by the real part of the impedance or the amount of impedance. The imaginary part plays only a subordinate role for these processes.

The practical advantage of the impedance measurement is that the impedance measurement only affects the high frequency components of current and voltage and does not interact with the DC or DC components. Thus, the measurement can also be done online during operation under load.

For the method according to the invention, it is basically sufficient to detect the absolute value or the real part of the impedance. Unless the context clearly indicates otherwise, in the following discussion, the term "impedance" may refer to the magnitude of the impedance | Z |, to its real part Re (Z), and to the impedance Z as a complex variable of real part and imaginary part or amount and phase relate.

impedance measurement

As described above, the impedance Z is generally measured by applying an oscillating voltage or current signal to the cell as the excitation signal and measuring the corresponding voltage signal in response to the excitation signal.

The excitation signal may comprise a single frequency or a superposition of multiple frequencies, and may be applied to the cell continuously or in a pulsed manner. The frequencies are not particularly limited and may be in the range of 10 Hz to 10 kHz, for example.

In general, the frequency has an impact on the processes in the cell that contribute to the response signal. At high frequencies (e.g., 1 kHz), the impedance is mainly due to the ionic and electronic resistance components in the electrolyte as well as the electrodes and arresters, while at low frequencies additional contributions are added due to relatively slow time scale processes such as solid state diffusion or charge transfer reactions.

At low frequencies, the dependence of impedance on temperature is generally stronger, which can increase the sensitivity of the method of the invention. However, at low frequencies, the dependence on other factors such as in particular the state of charge (SOC) and the aging state of the cell increases. In addition, at low frequencies, interference with the normal traction current may occur.

The measurement duration for a single impedance measurement also depends on the signal frequency, as whole periods must always be recorded. In view of a short measurement period, therefore, higher frequencies are preferable.

As a compromise with respect to the reduction of the influence of interfering factors with sufficient temperature sensitivity and a short measurement period, frequencies in the range of 50 Hz to 10 kHz, preferably 100 Hz to 5 kHz, in particular 500 Hz to 2 kHz are preferred.

Basically, it is sufficient to use a single excitation frequency. Alternatively, two or more excitation frequencies may be alternately or superimposed, or a predetermined bandwidth of excitation frequencies may be traversed to accommodate a spectrum. As a further possibility, the excitation can be pulsed, for example in the form of a pulse representing a superposition of many frequencies, and the measured signal is analyzed by means of Fourier transformation. The spectrum thus obtained is then correlated with the spectrum of the excitation pulse to also obtain an impedance spectrum.

In order to keep the apparatus cost low, the use of a single frequency may be preferred in certain embodiments. However, the use of multiple frequencies or the acquisition of spectra increases the reliability of the measurement, and further allows to obtain additional information about the state of the cell, for example the temperature or the state of charge of the cell (using appropriate calibration data, if applicable) corresponding impedance model of the cell, reference is made to the cited literature). In other embodiments, it is therefore preferable to use a plurality of frequencies and / or to record spectra in order to increase the precision and additionally obtain further diagnostic data on the cell state.

The excitation signal causes in response to a likewise oscillating response signal, which is measured to determine the impedance. If only the amount of impedance is to be determined, it may be sufficient to determine only the amplitude of the excitation and response signal.

However, this has the disadvantage that it is not possible to distinguish between the actual internal resistance of the cell and inductive or capacitive contributions. Especially at higher frequencies, the inductance of the measuring arrangement itself can play an increasing role, so that the accuracy of the measurement may suffer. Therefore, it is preferable to also detect the phase difference (or to detect the real and imaginary parts) in order to increase the accuracy of the measurement. For this purpose, the synchronization of excitation and measurement signal must be ensured, i. It must be ensured that there is no offset of the time zero point in voltage and current signal measurement, which falsifies the phase shift to be determined, or the offset must be at least numerically determined in order to correct the value accordingly.

According to the invention, the impedance measurement is carried out continuously or clocked in order to determine the impedance Z _t at the respective time t and, in the end, to detect the time profile of the impedance.

In the clocked case, the sampling interval, i. the time interval between two impedance measurements, for example 2 minutes or shorter, preferably 1 minute or shorter, more preferably 30 s or shorter, in particular 10 s or shorter. In a preferred embodiment, the sampling interval may be selected depending on the operating condition. For example, if the cell temperature is low, about 20 to 30 ° C, and the likelihood of a critical operating condition is low, longer sampling intervals may be selected, such as e.g. 10-20s. At higher cell temperatures of e.g. 40 ° C or more may require shorter sampling intervals, e.g. 1s or shorter.

Detection of critical operating conditions

Typically, the impedance changes that normally occur during operation are small in magnitude and run continuously over an extended period, i. the plot of impedance versus time is not steep. Thus, a large increase in impedance in a short period of time indicates an abnormal operating condition.

1. (a) einem zu einem vorherigen Zeitpunkt t-x bestimmten Wert Z_t-x, wobei x ein Zeitintervall >0 darstellt;
2. (b) einem für die Zelle bei der jeweiligen Zelltemperatur T erwarteten Wert Z_T; oder
3. (c) einem für den jeweiligen Zeitpunkt t erwarteten Wert Z_erw,t, der für die Zelle anhand eines oder mehrerer Betriebsparameter, ausgewählt aus Zelltemperatur, Zellspannung, Laststrom, Ladezustand und Alterungszustand, berechnet wird;

In the method according to the invention, such an increase is determined by comparing the value Z _t with one or more reference values Z _ref , where Z _{ref is} selected from:

1. (a) one at a previous time tx certain value Z _tx, where x is a time period> 0 represents;
2. (b) a value Z _T expected for the cell at the respective cell temperature _T ; or
3. (c) a value Z _erw, t expected for the respective time t, calculated for the cell from one or more operating parameters selected from cell temperature, cell voltage, load current, state of charge and state of aging;

The determination of a critical operating condition occurs when the value Z _{t has increased} against one, several or all Z _ref by a value exceeding a predetermined threshold value of 5% or more, for example 5%, 7%, 10%, 15%, 25% or 50%. The higher the rise, the stronger the indication of a critical condition. Shortly before passing through the cell, increases of 100% or more can be observed.

Therefore, for each reference value, it is also possible to use a plurality of threshold values which correspond to different warning levels for a critical state. Depending on the warning level, the battery management system may react differently. For example, at a low warning level, the cell can only be temporarily disconnected from the electrical system, while at a high warning level, the entire cell pack is shut down.

If multiple reference values are used, the thresholds for the different reference values may be different. The choice of the threshold value may also depend on the type of the respective reference value Z _ref .

A possible reference value is the value of the impedance at an earlier time x, Z _tx . For example, x may be 10 minutes or less. It is also possible to use several Z _tx with different x. Typically, the shorter the time interval in which the increase occurs, the more indicative of a critical operating condition is an impedance increase. Therefore, lower thresholds can be set for shorter time intervals x.

Conversely, such an increase in impedance in a short period of time may have other causes, such as a rapid drop in temperature. Preferably, it is therefore additionally checked in the method according to the invention whether the excess of the threshold value is accompanied by a simultaneous drop in the cell temperature in order to avoid false-positive messages. For example, this can be done by selecting the threshold value as a function of the temperature change taking place in the time interval t-x. With a temperature decrease higher threshold values can be set and with a temperature increase lower threshold values.

If an abnormal increase in impedance is detected, for example by 10% or more in 10 minutes or less, in particular in 5 minutes or less, especially in 3 minutes or less, this indicates a critical operating condition, in particular the occurrence of a localized defect, which leads to the electrolyte decomposition and thus reduction of the electrolyte conductivity. A localized response of the shutdown separator is typically associated with even higher impedance increases, for example, more than 50% in 5 minutes or less.

Such an abnormal increase in impedance may be detected early with the onset of the critical condition, typically well before the occurrence of a critical condition of the temperature rise of the cell case, an abnormal increase in pressure or the triggering of a safety valve. Thus, the corresponding cell can be detected early and switched off if necessary, and thermal runaway is prevented.

Another possible reference value is the value Z _T expected for the cell at the respective cell temperature T measured at time t. For this purpose, a previously known calibration curve can be recorded for the cell (or for the respective cell type), which reflects the impedance as a function of the cell temperature.

In carrying out the method, the cell temperature T is simultaneously measured, for example by a sensor attached to the housing or inside the cell. The reference value Z _T is then taken from the previously known calibration curve. In this way, the impedance-temperature behavior can be determined, and critical operating conditions can be detected as a reversal of normal behavior.

To improve the reliability and avoid false-positive results, the time course of the impedance is preferably correlated with one or more other operating parameters, such as temperature, load current, load voltage, state of charge or state of aging. These parameters can either be measured directly or they can be determined by the battery management system based on the measurements and the data collected during operation according to known methods.

Preferably, the effects of the operating parameters to be correlated on the impedance are known, so that an abnormal behavior can also be determined as a deviation from the value expected based on the parameter (Z out _{, t} ). In particular, it is also possible to record impedance characteristics or impedance spectra for a parameter space spanned by the operating conditions of the cell (eg charge state, load and temperature) and to predetermine them in the measuring device or to predict them using an impedance model, and abnormal deviations or Detect increases in the impedance actually measured under the current conditions.

Interconnection of cells and measuring arrangement

Preferably, the method is used for cells with shutdown separator to a beginning, possibly locally limited triggering of the shutdown effect to be detected as an impedance increase. However, the method can also be used in cells without a shutdown separator to detect critical operating conditions, such as those caused by thermal decomposition of the electrolyte.

The interconnection of lithium-ion cells and measuring arrangement is not subject to any fundamental restrictions. Typically, the individual cells are connected in series and / or parallel to battery packs. According to the invention, the impedance measurement and detection of a critical operating state can be carried out individually for each individual cell of a battery pack, for several cells together or also for all cells of the battery pack.

Although a simultaneous measurement for several or all cells, the expenditure on equipment is lower, but also reduces the precision because the impedance is averaged over several cells and the impedance increase can be detected less accurately. Also, in this case, the defective cell can not be easily located. Therefore, it is preferable to measure the impedance individually for each cell.

The cells are expediently connected in four-point contacting (Kelvin contacting), with separate terminals for current and voltage. In conventional battery packs of the prior art, such contacting can be provided in any case with regard to voltage monitoring and balancing of the state of charge (balancing). Therefore, the inventive method can be combined with existing battery packs without much effort.

1 shows four circuit options for carrying out the method according to the invention in a series connection of cells, as used in battery packs, wherein the impedance for each cell is measured individually.

In the in 1A As shown arrangement is provided a measuring device for a plurality of cells. The excitation of the cells (ie impressing an alternating current signal) takes place locally by the measuring device. The measurement of the response signal is carried out for each cell in a parallel reference circuit. For this purpose, the measuring device can have its own measuring unit for each cell, for example in the form of a plurality of input channels, or it can have a single current measuring unit which can be connected to each cell alternately in order to carry out the measurement in time-division multiplexing.

Alternatively, the excitation can not be done by the measuring device itself, but globally from the outside. For this purpose, the excitation signal can be modulated onto the load current. Such an arrangement is in 1B shown. The measurement of the response signal is carried out in the same way as in 1A , This allows the saving of separate excitation circuits, however, the effort for the synchronization of excitation and measurement signal could increase.

In the arrangement according to 1C Each cell is provided with a single measuring device. The excitation and measurement is thus for each cell individually. Alternatively, the excitation can again be global, as in 1D shown.

With respect to the measuring device itself, there are no particular restrictions as long as it is suitable for measuring an AC or voltage signal in the corresponding frequency. For example, a battery monitoring unit, as used anyway for the monitoring of current, voltage and other operating parameters such as temperature, can be used for this purpose. If it is not already able to measure the alternating current in any case, the battery monitoring unit would possibly be provided with an additional alternating current measuring unit, for example a high-pass filter and A / D converter. If the excitation signal is to be generated in the measuring device itself, it may be necessary to provide a corresponding signal generator.

The determination of the impedance from the signals and / or the detection of the critical operating state by comparison of the impedance measured value with the reference values or can take place optionally already in the measuring device itself, it can be integrated into the control unit for battery management, or it can help a separate controller are performed.

Examples

material

For the tests, lithium-ion cells in PHEV1 format with a nominal capacity of 21 Ah were used (cathode: NMC, anode: graphite), which were equipped with a shutdown separator (Celgard 2325 Trilayer PP / PE / PP) were equipped.

To measure the temperature, the cells were provided with two temperature sensors, the first of which was attached directly to the housing. The second sensor was suspended above the pressure relief valve to detect the temperature increase due to exiting decomposition gases. The measured temperatures are called "T _Can " or "T _Vent ".

The cells were equipped with an impedance meter (Hioki HiTester) in four-point contact. The excitation frequency was 1kHz. The tests were performed with fully charged cells in the unloaded state.

test 1 : Oven test

In a first series of tests, the cell was placed in an oven and the temperature was raised to 200 ° C at a rate of 5 ° C / min. In each case the temperatures T _Can or T _Vent (° C) and the amount of the impedance (mΩ) were measured. The test was carried out with a total of three cells.

2 shows the result for the first cell. First, the impedance drops from about 2.4 to 2.0 mΩ with increasing temperature. After about 30 minutes at a housing temperature of about 80 ° C, a reversal of the impedance / temperature behavior occurs and the impedance begins to increase significantly (to about 4-5 mΩ) (point ( 1 ) in the figure). It is believed that this increase is due to the onset of decomposition of the electrolyte. After about 50 minutes, a very large increase in the impedance is detected. This is due to the triggering of the shutdown effect.

Until then, the appearance of the cell has not revealed any anomalies. Triggering of the pressure relief valve was not detected until after about 63 minutes (point ( 2 ) in the figure, recognizable by the discontinuity in the curve of T _Vent ). A thermal run through destruction of the cell occurred after 88 minutes (point ( 3 ) in the figure).

test 2 : Hotplate test

In order to investigate the effect of inhomogeneous heating, the cells were placed on a hotplate in a second series of tests and heated with maximum heating power. The temperature increase was on average about 10 ° C / min or more. The test was again carried out with a total of three cells.

3 shows the result for the first cell. As can be seen, the qualitative course is similar to that 2 but the critical conditions are reached much earlier.

The reversal point ( 1 ) of the impedance / temperature behavior followed by a sharp increase in the impedance already occurs after about 7 minutes. The triggering of the pressure relief valve ( 2 ) occurs after about 13 minutes and the thermal run ( 3 ) takes place after about 18 minutes.

Correlation of temperature and impedance

4 is a plot of "T _Can " against the impedance for the three oven tests (solid lines) and the three hot plate tests (dashed lines). All curves show the characteristic reversal point of the temperature / impedance behavior, which occurs at a housing temperature of 60 to 80 ° C.

discussion

The tests carried out show that already well before the overpressure valve ( 2 ) and the thermal runaway of the cell ( 3 ) Anomalies in the impedance can be detected. Particularly characteristic is the reversal point of the temperature / impedance behavior ( 1 ), and for cells with shutdown separator, the subsequent large increase in impedance due to the start of the shutdown effect. These anomalies make it possible to detect a critical state of the cell early, so that the cell can be separated in good time from the composite and possibly cooled to prevent thermal runaway.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been 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.

Cited patent literature

• US 2009/0096459 [0011, 0025, 0033]
• DE 102009038663 [0012]
• DE 102013103921 [0013]

Claims (15)

A method of detecting critical operating conditions in one or more lithium-ion cells, comprising: continuously or intermittently determining the magnitude and / or the real part of the impedance Z of the cell (s) to obtain the value Z _t at time t; Comparing the value Z _t with one or more reference values Z _ref , where Z _{ref is} selected from: (a) a value Z _tx determined at a previous time _tx , where x represents a time interval>0; (b) a value Z _T expected for the cell at the respective cell temperature _T ; (c) a value Z _erw, t expected for the respective time t, calculated for the cell from one or more operating parameters selected from cell temperature, cell voltage, load current, state of charge and state of aging; - Determining a critical operating condition when the value Z _t against one, several or all Z _{ref has} increased and the percentage increase exceeds a predetermined threshold, the threshold is 5% or more. Method according to Claim 1 wherein Z _{tx is used} as the reference value, the change in the cell temperature is determined in the interval of tx, and the threshold value is set in accordance with the cell temperature change. Method according to Claim 1 or 2 wherein a plurality of reference values Z _ref are used, and the threshold values for different Z _{ref may be the} same or different. Method according to at least one of Claims 1 to 3 in which, in addition to the amount of impedance, the phase or, in addition to the real part, also the imaginary part is determined. Method according to at least one of Claims 1 to 4 wherein the impedance measurement is performed with an excitation signal having a frequency or a superposition of a plurality of frequencies in the range of 10 Hz to 10 kHz. Method according to at least one of Claims 1 to 5 wherein Z _{t is determined} intermittently at a sampling interval of 1 minute or shorter, wherein the sampling interval may be variable. Method according to at least one of Claims 1 to 6 wherein the cell (s) comprise a shutdown separator. Method according to at least one of Claims 1 to 7 which is performed for a plurality of lithium-ion cells grouped in a battery pack. Method according to Claim 8 where Z _t and Z _ref are independently determined for each cell and the presence of a critical operating condition is independently detected for each cell. Method according to Claim 9 in which, for several or all cells of the battery pack, a measuring device is provided which monitors the impedance Z _{t of} each cell assigned to it. Method according to Claim 9 in which a measuring device is provided for each cell of the battery pack, each of which determines the impedance Z _t of its associated cell. Method according to Claim 10 or 11 wherein Z _{t is} measured using an excitation signal generated by the measuring device itself. Method according to Claim 10 or 11 wherein Z _{t is} measured using an excitation signal externally applied to the battery pack. Device adapted for carrying out the method according to at least one of Claims 1 to 13 comprising: at least one measuring device comprising at least one measuring unit for an alternating current measuring signal; - A signal generator for generating the excitation signal, if the device is not intended for use with external excitation signal; and - one or more control devices configured to determine at least the magnitude and / or the real part of the impedance Z at the instant t (Z _t ) from the excitation signal and the measurement signal, the provision of the reference value (s) Z _ref , the comparison of Z _t and Z _ref and the determination of the critical operating state. Battery system, the device according to Claim 14 and one or more lithium-ion cells.

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