DE102021006585A1 - Method and device for preventing or reducing the risk of a short circuit in a lithium-ion battery caused by dendrites - Google Patents

Abstract

Method for influencing the growth of lithium dendrites in a lithium-ion accumulator which has a cathode (4) in a cell (1) and an anode (3) lying opposite it, which consists of lithium or has at least one surface containing lithium , An anhydrous electrolyte (5) being located in a space between the cathode (4) and the anode (3) and a separator (6) permeable to lithium ions being arranged. During an electrical charging process, longitudinal ultrasonic waves with variable frequency are generated in the cell (1) or) transmitted into the cell (1), the longitudinal direction of the ultrasonic waves in the electrolyte (5) being transverse to the normal (16) on the anode (3 ) and the cathode (4), and the frequency of the ultrasonic waves is controlled so that it repeatedly sweeps a range of frequencies.

Description

The invention relates to a method having the features specified in the preamble of claim 1 and a device having the features specified in the preamble of claim 18 .

It has long been known that when lithium-ion storage batteries are repeatedly charged, their anodes, which are made of metallic lithium, grow lithium dendrites, which can reach the cathode opposite the anode and then cause an electrical short circuit, leading to a fire and can even cause the battery to explode. Out of necessity, one has therefore searched for alternatives for anodes made of metallic lithium and z. B. found in anodes made of graphite or other layered materials, between the layers of which lithium ions can be stored when charging the accumulator, referred to as "intercalation" in English-language technical literature. Irrespective of this, there is still great interest in anodes made of metallic lithium, because they would make particularly powerful accumulators possible. However, it is important for their acceptance if one could eliminate or reduce the danger posed by the lithium dendrites. There has therefore been no lack of attempts to counteract the formation of dendrites in lithium-ion accumulators.

the EP 3 033 797 B1 names dendrites as "the most common failure mode for cells with Li metal anodes" and suggests providing a large number of pressure sensors along the anode, which should record the extent of a change in the shape of the anode associated with the charging process. If the measured pressure exceeds a specified limit value, the accumulator should be switched off or partially discharged to be on the safe side, which does not solve the problem. Alternatively, the EP 3 033 797 B1 proposes changing the spatial distribution of the charging current after a pressure limit has been exceeded in such a way that the change in the shape of the anode caused by a previous charging process is partially reversed. However, it is not disclosed how this could be achieved and what effort would be associated with it.

the U.S. 8,354,824 B2 even suggests completely discharging the accumulator from time to time in order to thereby reduce the roughness of the surface of the anode and to reform already formed dendrites. This proposal also does not really solve the problem of dendrite growth and is in particular not suitable for applications in accumulators for electrically or partially electrically driven automobiles.

• Die jüngsten Erkenntnisse über die Ursachen der Bildung von Lithium-Dendriten finden sich in dem Forschungsartikel von Elizabeth Santos und Wolfgang Schlicker mit dem Titel „Die entscheidende Rolle von lokalen Ladungsfluktuationen beim Wachstum von Dendriten auf Lithium-Elektroden“, veröffentlicht in der Zeitschrift für Angew. Chem., 2021, 135, 5940-5945. Danach zeigen Modellrechnungen, dass bereits aus wenigen Atomen bestehende Spitzen kleiner Unebenheiten der Anodenoberfläche die beim Ladevorgang im Elektrolyten des Akkumulators vorhandenen Lithium-Ionen anziehen und zu Dendriten heranwachsen lassen können. Die Autoren ziehen in dem Forschungsartikel das Resume, dass ihr Modell zwar einen Grund für die Bildung von Dendriten angibt, aber kein Rezept zur Vermeidung ihrer Bildung.

the U.S. 5,728,482 A proposes reducing the growth of dendrites on a lithium battery anode by generating a magnetic field in front of the surface of the anode, the field lines of which are transverse to the electric field between the cathode and the anode of the battery. The magnetic field is intended to shield protrusions on the surface of the anode, on which the electric field between cathode and anode can concentrate during the charging process and which can be starting points for lithium dendrites. However, this can only partially succeed, especially since even the smallest projections can be starting points for dendrites, which results from the following reference:

• The most recent insight into the origins of lithium dendrite formation can be found in the research article by Elizabeth Santos and Wolfgang Schlicker entitled “The Crucial Role of Local Charge Fluctuations in Dendrite Growth on Lithium Electrodes”, published in the Journal of Angew. Chem., 2021, 135, 5940-5945. According to this, model calculations show that small bumps on the anode surface, consisting of just a few atoms, can attract the lithium ions present in the battery electrolyte during the charging process and allow them to grow into dendrites. In the research article, the authors draw the conclusion that their model gives a reason for the formation of dendrites, but no recipe for avoiding their formation.

The invention has for its object to demonstrate a practicable and promising way how in a lithium-ion battery that has a liquid anhydrous electrolyte and an anode whose surface facing a cathode consists predominantly or entirely of lithium, the occurrence of a Lithium dendrites, which could form when charging the battery, caused electrical short circuit in the battery can be avoided or delayed.

This object is achieved by a method with the features specified in claim 1. Advantageous developments of the invention are the subject matter of the dependent claims.

The method according to the invention for reducing the risk of short circuits emanating from lithium dendrites in a lithium-ion battery, which has a cathode in a cell and an anode opposite it, which consists of lithium or has at least one lithium-containing surface, with an intermediate space between the cathode and the anode a river iger anhydrous electrolyte is located and a separator permeable to lithium ions is arranged, is characterized in that during a charging process, longitudinal ultrasonic waves, the longitudinal direction of which in the electrolyte runs transversely to the normals on the anode and on the cathode, with variable frequency in the cell generated or transmitted into the cell, and that the frequency of the ultrasonic waves is controlled so that it repeatedly sweeps a range of frequencies. The longitudinal ultrasonic waves therefore run transversely to the direction in which lithium ions migrate from the cathode to the anode when the accumulator is charged.

Longitudinal ultrasonic waves oriented according to the invention are simply referred to below as longitudinal ultrasonic waves.

The claims are to be understood in such a way that the accumulator can have one or more cells which—in particular as for use in electromobility—can be combined into one or more packages. The cells can be electrically connected in series to achieve a higher output voltage. The cells can be electrically connected in parallel to allow higher output current and hence higher output power. Groups of cells connected in series may be connected in parallel. Groups of cells connected in parallel may be connected in series. In all of these cases, the ultrasound can be generated in each individual cell or generated on an outer wall or in a recess in an outer wall of the respective cell and transmitted from there into the cell.

The generation of ultrasonic waves "during a charging process" is to be understood in such a way that the charging process can involve both the initial charging and the recharging of the accumulator.

The fact that the longitudinal direction of the ultrasonic waves is "transverse" to the normals on the anode and on the cathode does not mean that the longitudinal direction of the ultrasonic waves must intersect the normals at a right angle. It is sufficient that the longitudinal direction of the ultrasonic waves crosses the normal at any angle, with a right angle or an approximately right angle being preferred.

The method according to the invention can be carried out during each charging process or during selected charging processes, for example in every second or third charging process or only when the charging process takes place at a stationary charging station. The method according to the invention is preferably carried out for each charging process which takes place at a stationary charging station. In the case of vehicles that are hybrid driven, i.e. that have both an electric motor drive and an internal combustion engine, it may be more expedient in view of frequent and irregular changes between charging and discharging processes not to use the method according to the invention while driving or not in each phase of a Carry out a journey in which the internal combustion engine is working. The method according to the invention is preferably carried out in vehicles, but also while driving, in order to avoid the occurrence of short circuits while driving as far as possible. The decision to start charging can be made dependent on the state of charge of the accumulator falling below a predetermined threshold value. It is known to automatically monitor the state of charge of the accumulator while driving, so that the internal combustion engine and the charging process can be started automatically when the charge falls below the threshold value.

The method according to the invention can be carried out during the entire duration of the charging process or only during part of the duration of the charging process. If the procedure is carried out for all cells of an accumulator during the entire charging process, then the risk of an electrical short circuit caused by a lithium dendrite is lowest. If the method is not carried out during the entire duration of the charging process of a cell, then there is the possibility, in the case of an accumulator composed of several or many cells, of the method being carried out on individual cells or groups of cells at different times which can overlap, but not each other have to overlap. This variant of the method according to the invention would have the advantage that the power requirement of the accumulator for generating the ultrasonic waves would thereby be reduced.

The teaching of generating longitudinal ultrasonic waves oriented according to the invention during a charging process does not rule out the possibility that longitudinal ultrasonic waves oriented according to the invention, in particular shock waves, may also be generated in the electrolyte or transmitted into the electrolyte outside of a charging process. Lithium dendrites, which are not long enough to cause a short circuit, can be broken by longitudinal ultrasonic waves generated with variable frequency and by ultrasonic shock waves. Those of dendrites that move when charging the accumulator tors have formed, the risk of a short circuit can be eliminated or reduced as a result.

Ultrasonic shock waves, which are also referred to as ultrasonic shock waves, are ultrasonic impulses that are characterized by a rapid increase in pressure and a short impulse duration. They may be generated from time to time while no frequency sweep is occurring. They are preferably generated towards the end of a charging process or after a charging process has ended, because then any dendrite that has formed has had the longest opportunity to grow.

When charging the accumulator, the longitudinal ultrasonic waves generated according to the invention in the electrolyte worsen the conditions for the formation and growth of lithium dendrites. Lithium ions, which migrate to the anode when the accumulator is charged, should be prevented as far as possible from sticking to pointed projections on the anode. The method according to the invention counteracts the tendency of the lithium ions to preferentially deposit on sharp projections of the anode in that it disrupts the migration of the lithium ions in a targeted manner. The longitudinally oscillating ultrasonic waves can superimpose a sideways movement on the migration movement of the lithium ions from the cathode to the anode in the electrolyte, which deflects a lithium ion from its path towards a sharp projection of the anode and causes it not to be "at rest". ' deposits on the sharp protrusion, but instead deposits on another part of the anode, which is adjacent to the sharp protrusion. At the same time, this means that the projection is less prominent afterwards. As a result, the deposition of lithium ions on the anode under the influence of the longitudinal ultrasonic waves is evened out, which makes it difficult for dendrites to form on the anode.

The longitudinal ultrasonic waves hit the side of any dendrite that may have formed in the electrolyte and can excite it to transversal vibrations (bending vibrations), which can break it, especially when resonance occurs or when impacted by shock waves. It is therefore also an advantage of the invention that the frequency of the ultrasound is controlled in such a way that it repeatedly runs through a frequency range. By repeatedly running through a frequency range, it is possible to ensure that useful resonances are run through, even without more precise knowledge of the frequencies at which resonances can occur in the electrolyte and in any dendrites that may have formed, if the frequency range has been chosen to be sufficiently broad, which - as already mentioned - can be ensured by preliminary tests. In addition, it is advantageous that frequency generators and ultrasonic generators for large frequency ranges from 20 kHz to the megahertz range are commercially available.

The targeted disturbances in the migration of the lithium ions on their way from the cathode to the anode, which are desired when charging the accumulator, are particularly effective if they are amplified by the occurrence of resonances. The frequencies at which resonances can occur depend on a number of influencing factors, including the structure of the cell, the materials used, the temperature, the viscosity of the electrolyte, the strength of the charging current, the state of charge of the battery and the Age of the accumulator. It is therefore a particular advantage of the invention that the frequency of the ultrasound is changed and repeatedly runs through a frequency range.

For this purpose, a frequency generator is expediently provided, which feeds one or more ultrasonic transmitters (“transducers”) and on which the frequency range to be run through, the duration of a complete run through the selected frequency range, and the power with which the frequency generator sends one or more ultrasonic transmitters can be set feeds, whereby the power can be selected depending on the frequency. Because a frequency range is repeatedly run through, it can be ensured that in any case one or more resonances can be used for the purposes of the invention, regardless of whether the exact position of the resonance frequencies is known, which - as noted above - can change continuously. The frequency of the ultrasound is preferably changed cyclically, so that resonant frequencies in the selected frequency range are passed through again and again, thereby increasing the effectiveness of the method according to the invention.

Repeatedly running through a frequency range is also advantageous because shorter lithium dendrites are more likely to be influenced by higher ultrasonic frequencies within the meaning of the invention than longer lithium dendrites, which are more likely to be influenced by lower ultrasonic frequencies within the meaning of the invention. Both longer dendrites and shorter dendrites therefore have a chance of being “decapitated” by exposure to longitudinal ultrasonic waves of variable frequency, for example. This chance increases if additional ultrasonic shock waves are generated from time to time.

Resonances that occur can have an effect on the ultrasonic transmitter or transmitters and further on a frequency generator that feeds them. The frequency generator is preferably set up to detect such repercussions when the strength of the repercussion exceeds a threshold that depends on the design of the frequency generator. The frequency generator can be programmed so that each time it detects a reaction caused by a resonance, it stops changing the frequency for a preset duration in order to increase the dwell time at the resonance and thus limit the formation and growth of Enhance lithium dendrites inhibitory effect of longitudinal ultrasonic waves. Of course, care must be taken to limit the intensity of the ultrasound from the outset, particularly if resonances occur and shock waves are generated, in such a way that damage to the accumulator and its components is avoided. Preliminary tests can be used to determine which ultrasound intensities a specific accumulator can tolerate without being damaged.

Influencing the migration of the lithium ions in such a way that they deposit less frequently on sharp projections of the anode requires less energy than breaking dendrites that have already formed. The power with which the ultrasonic transducers are fed is therefore preferably adjustable.

The frequency range in which useful resonances lie can be determined experimentally in advance for each specific design of an accumulator. The frequency generator can then be tuned or set up to a frequency range in which one or more experimentally determined resonance points lie. In this case, low resonant frequencies are preferred when determining the frequency range which is to be run through repeatedly.

When discharging the accumulator, it is expedient not to be exposed to ultrasound in order not to disturb the discharging process and because no lithium ions are deposited on the anode during discharging, on the contrary: Lithium dendrites formed during charging of the accumulator can even partially reform again during discharging will.

The longitudinal ultrasonic waves are preferably generated in the electrolyte with a piezoelectric ultrasonic generator. Piezoelectric ultrasonic generators are known for cleaning purposes, for metrological purposes, for welding plastics and for diagnostic purposes. They are available in numerous designs that are adapted to the respective application. They can also be adapted for purposes of the invention. Piezoelectric ultrasonic generators with a flat design are particularly suitable for this. Ultrasonic transducers in a flat design are available in different sizes and shapes, including rectangular formats whose dimensions can be adapted to the dimensions of the cells in lithium-ion batteries.

In the present case, the ultrasonic generator is preferably arranged on an outer wall of the cell delimiting the space between the cathode and the anode. Depending on the given spatial conditions, the ultrasonic generator can be close to the anode in order to counteract the formation of lithium dendrites. In addition, another ultrasonic generator can be located close to the cathode. Alternatively, it is possible to provide an ultrasonic transducer extending across the position of the separator to near that of the anode and to near that of the cathode.

An arrangement of the ultrasonic generator or generators on the inside of the outer wall of the cell has the advantage that the ultrasonic generator can emit the longitudinal ultrasonic waves directly into the electrolyte. An arrangement of the ultrasonic generator on the outside of the outer wall of the cell has the advantage, firstly, that its arrangement there is simpler, and secondly, has the advantage that it is possible in an accumulator in which several cells are arranged next to one another to place an ultrasonic generator between two To arrange cells and in this way to use them twice by emitting ultrasound in opposite directions. In this way, a rear shielding of the piezoelectrically oscillating element, which is otherwise customary, is no longer necessary in the ultrasonic generator.

A further advantageous possibility is an integration of the ultrasonic generator in the outer wall of the cell or in a recess provided in the outer wall of the cell. With this type of attachment, the ultrasonic generator can emit the longitudinal ultrasonic waves directly into the electrolyte without disturbing the internal structure of the cell.

Piezoelectric ultrasonic generators are compact and can easily be coupled to flat surfaces, as they are or can be realized with accumulators. They are available in a wide frequency range from 20 KHz to the megahertz range, are easy to control and can obtain the electrical power for their operation from the accumulator itself.

The longitudinal ultrasonic waves not only impede the formation of lithium dendrites, but also their growth in the direction of the cathode. The chance that a lithium dendrite will be broken by its interaction with the longitudinal ultrasonic waves increases with increasing length of the dendrite, with increasing power transmitted by the ultrasonic transducers and with decreasing frequency of the ultrasonic waves.

If the frequency of the ultrasound remains the same, standing waves could form in one cell of the accumulator, as a result of which the lithium could be deposited unevenly on the anode. The invention advantageously avoids this disadvantage in that the frequencies of the ultrasound are variable and repeatedly run through a frequency range.

If a lithium dendrite has formed on the anode despite the impact of ultrasonic waves, then the question arises as to whether it can overcome the separator. Since the separator is permeable to lithium ions, a lithium ion coming from the cathode can pass through the separator and be deposited on a behind tip of a dendrite. However, the longitudinal ultrasonic waves in the adjacent electrolyte prevent the dendrite from growing into the separator by exciting the tip of the dendrite to vibrate. Firstly, there is a chance that the oscillating tip of the dendrite interacting with the separator will break off. Secondly, the ultrasonic vibrations in the electrolyte, as explained using the example of the anode, disrupt the deposition of further lithium ions on the tip of the dendrite, which means that its growth is impeded or inhibited compared to migration of the lithium ions that is not disrupted by ultrasound . Although the ultrasonic vibrations in the electrolyte could excite the separator to ultrasonic vibrations, the vibrations excited in the separator could not nullify either the effect of the vibrations in the electrolyte or the effect of the vibrations of the tip of the dendrite on the growth of the dendrite because they interact with each other different time delays and because both longitudinal and transversal oscillations are possible in the separator. Rather, an excitation of ultrasonic vibrations in the separator can also make it more difficult for dendrites to grow into the separator.

Should a lithium dendrite nevertheless manage to grow in and through the separator and reach the cathode, there is another chance that the tip of the dendrite will be broken off by its interaction with the stationary or otherwise vibrating surface of the cathode and the current carrying capacity of an initially minimal contact between the dendrite and the cathode is limited to an uncritical level. It is therefore advantageous to dimension and/or position the ultrasonic generators in such a way that the longitudinal ultrasonic waves, possibly also shock waves, also run in front of the surface of the cathode in the electrolyte transverse to the longitudinal direction of any dendrites in order to be able to have a maximum effect on them. For example, an ultrasonic transducer can be arranged on or in the outer wall of the cell of the accumulator on either side of the separator.

Lithium dendrites near the cathode reduce the electrical impedance of a cell. The impedance can be measured, for example, using the method of electrochemical impedance spectroscopy (EIS). Examples of the measuring method, which also relate to lithium-ion accumulators, are in the publications DE 10 2009 000 336 A1 , DE 10 2009 000 337 A1 and DE 10 2013 214 821 A1 disclosed, to which reference is made for the details of the measurement method. The EIS includes the possibility of carrying out the measurement not only for an accumulator as a whole, but also for individual cells of an accumulator. For very low frequencies, the impedance of lithium-ion cells often shows an almost purely capacitive behavior. To take advantage of this, one conveniently measures the impedance at frequencies no higher than 10 Hz, preferably no higher than 1 Hz.

Measuring impedance opens up the possibility of detecting an impending short circuit caused by growth of lithium dendrites before it occurs, because while the impedance increases as a cell ages, it decreases as dendrites approach the cathode of the impedance to be expected. A decrease in impedance can be determined by differentiating the measured impedance curve. How clearly this can be determined in a specific cell design can be clarified by preliminary tests. Since the impedance depends on the temperature and the frequency with which it is measured, it should always be measured at the same temperature and the same frequency.

Impedance can be measured in vehicles both "in situ", i.e. while driving (see DE 10 2013 214 821 A1 ) as well as standing still, while the accumulator is neither charged nor discharged (see DE 10 2009 000 336 A1 ). If by measuring the impedance of a cell of the accumulator a decrease in the impedance instead of an increase in the If impedance is observed, this can indicate an impending short circuit. In this case, an attempt can be made to eliminate the risk of a short circuit by - without charging the battery at the same time - generating longitudinal ultrasonic waves, in particular shock waves, in the cell from which the danger emanates, or in all cells of the battery, in order to to break the dendrite or dendrites from which the danger emanates. In order to achieve this, the power of the ultrasonic generator(s) involved is preferably briefly increased once or several times and the impedance is repeatedly measured in between. If no increase in impedance can be detected by repeated measurements of the impedance, it is advisable to replace the accumulator or at least the affected cell(s).

Anhydrous electrolytes suitable for lithium-ion accumulators are known to the person skilled in the art. This is often a lithium-containing salt in an anhydrous organic solvent, for example a 1 molar solution of lithium borate tetrafluoride (LiBF ₄ ) in propylene carbonate or in a cyclic ether such as tetrahydrofuran (THF). A 1 molar solution of zinc chloride (ZnCl ₂ ) in ethyl methyl carbonate (EMC) or in tetrahydrofuran (THF) is also known as an anhydrous electrolyte.

Separators which are suitable for lithium-ion accumulators and are permeable to lithium-ions are also known to the person skilled in the art. Examples are microporous plastics, in particular polyolefins, including polyethylene and polypropylene, and glass fiber mats.

• 1 zeigt schematisch einen senkrecht durch die Kathode und Anode gelegten Schnitt durch eine Zelle eines Lithium-Ionen-Akkumulators.

An embodiment of the invention is shown in the accompanying drawing.

• 1 shows schematically a section through a cell of a lithium-ion accumulator, taken perpendicularly through the cathode and anode.

The cell 1 has a housing 2 made of an electrically insulating material, in particular a plastic. In the housing 2 there is an anode 3 and a cathode 4, between which an anhydrous electrolyte 5 and a separator 6 are provided. The anode 3 consists of metallic lithium and is connected to an anode current collector 7 on the back, which can consist of copper. The cathode 4 consists of a lithium transition metal oxide, e.g. B. from LiCoO ₂ , and is connected to the rear with a cathode current collector 8, which may be made of aluminum. The anode current collector 7 and the cathode current collector 8 are led out of the housing 2 and can be connected in a manner known per se to a direct current source for charging the cell 1 and to a load, for example a direct current motor, for discharging. The separator 6 consists, for example, of a microporous polypropylene. The electrolyte 5 can be a 1 molar solution of LiBF ₄ in tetrahydrofuran (THF).

The housing 2 has a first recess 9 in a first outer wall 10 of the housing 2 at a point which lies laterally between the anode 3 and the separator 6 . In a second outer wall 11 of the housing 2, opposite the first outer wall 10, there is a second recess 12 at a point which is laterally offset from the first recess 9 between the cathode 4 and the separator 6, so that the two recesses 9 and 12 lie diagonally opposite each other. The recesses 9 and 12 are open to the outside and to the inside. A piezoelectric ultrasonic generator 13 or 14 is inserted in the recesses 9 and 12 in a liquid-tight manner, e.g. by gluing or welding. A frequency generator 15 assigned to cell 1, the frequency of which can be changed under program control, simultaneously feeds both ultrasonic generators 13 and 14 and draws the power for its operation from the lithium-ion battery of which cell 1 is a part.

The frequency generator 15 is programmed in such a way that the frequency of its signal feeding the ultrasonic transmitters 13 and 14 cyclically runs through a frequency range. The limits of the frequency range and the amplitude of the signal feeding the ultrasonic transmitters 13 and 14 can preferably be set at the frequency generator 14 .

A straight line 16 is drawn in the drawing, which represents a normal on the front side of the cathode 4 and the anode 3 opposite it, i.e. it runs orthogonally to the front side of the cathode 4 and the anode 3. The direction of the longitudinal ultrasonic waves is symbolically indicated by Arrows 17 shown.

Reference List

1

cell

2

Housing

3

anode

4

cathode

5

electrolyte

6

separator

7

anode current collector

8th

cathode current collector

9

first recess

10

first wall

11

second wall

12

second recess

13

ultrasonic transducer

14

ultrasonic transducer

15

frequency generator

16

normal

17

arrows

QUOTES INCLUDED IN DESCRIPTION

This list of the 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

• EP 3033797 B1 [0003]
• US8354824B2 [0004]
• US5728482A [0005]
• DE 102009000336 A1 [0035, 0037]
• DE 102009000337 A1 [0035]
• DE 102013214821 A1 [0035, 0037]

Claims (10)

Method for detecting an impending short circuit caused by the growth of lithium dendrites when electrically charging a lithium-ion accumulator which has a cathode (4) in a cell (1) and an anode (3) lying opposite this, which consists of lithium or has at least one surface containing lithium, an anhydrous electrolyte (5) being located in a space between the cathode (4) and the anode (3) and a separator (6) permeable to lithium ions being arranged, characterized in that the impedance of the accumulator is measured at least from time to time in order to determine whether a decrease in impedance over time is observed in measurements carried out at the same temperature and at the same frequency. procedure after claim 1 , characterized in that the impedance is measured to determine whether a decrease in impedance is observed instead of an increase in impedance. procedure after claim 1 , characterized in that the impedance is measured in order to determine whether the time course of the impedance changes its sign from "increasing" to "decreasing". Method according to one of the preceding claims, characterized in that the course of the impedance is differentiated. Method according to one of the preceding claims, characterized in that the measurements of the impedance in the case of a battery composed of a plurality of cells (1) are carried out for the battery as a whole or for individual cells (1) of the battery. Method according to one of the preceding claims, characterized in that after a decrease in impedance has been detected, an attempt is made to eliminate the risk of a short circuit. procedure after claim 6 , characterized in that the attempt is made to eliminate the risk of a short circuit when the measured impedance falls below a selected limit value. procedure after claim 6 or 7 , characterized in that to eliminate the risk of a short circuit, the charging process is stopped. Procedure according to one of Claims 6 until 8th , characterized in that to eliminate the risk of a short circuit is to replace the accumulator. Procedure according to one of Claims 6 until 8th , characterized in that in the case of an accumulator composed of several cells (1), the risk of a short circuit is eliminated in that the cell (1) or the cells (1) which is or are the cause of the observed decrease in impedance, is or will be determined and exchanged.

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