DE102014219451A1 - Galvanic element - Google Patents

Abstract

The invention relates to a galvanic element (10) comprising an anode (18), a cathode 14) and a separator (16) which mechanically and electrically separates the anode (18) and the cathode (14), characterized in that the separator (16) is arranged to block an ion current between the anode (18) and the cathode (14) when the temperature of the galvanic element (10) exceeds a critical temperature, the separator (16) being arranged to release the ion current when the temperature of the galvanic element (10) falls below the critical temperature again. Furthermore, the invention relates to a battery cell comprising such a galvanic element (10) and a battery comprising a plurality of such battery cells.

Description

The invention relates to a galvanic element comprising an anode, a cathode and a separator. Furthermore, the invention relates to a battery cell comprising such a galvanic element and a battery comprising a plurality of such battery cells.

State of the art

Among other things, lithium-ion batteries are characterized by a very high specific energy and extremely low self-discharge. Lithium-ion cells have at least one positive and at least one negative electrode (cathode or anode), wherein during charging and discharging of the battery, lithium ions migrate from one electrode to the other electrode. For the transport of lithium ions, a so-called lithium-ion conductor is necessary. In the lithium-ion cells currently used, which are used for example in the consumer sector (mobile phone, MP3 player, etc.) or as energy storage in electric or hybrid vehicles, the lithium-ion conductor is a liquid electrolyte, which often contains the lithium-conducting salt lithium hexa-fluorophosphate (LiPF ₆ ) dissolved in organic solvents. A lithium-ion cell includes the electrodes, the lithium-ion conductor, a separator that separates the two electrodes electrically and mechanically, and current conductors that make the electrical connections.

The lithium-ion cells may be enclosed in a package. For example, aluminum composite films are used as packaging. So packaged cells are also referred to as a pouch or soft pack because of their soft packaging. In addition to softpack packaging design, solid metal housings are also used as packaging, for example in the form of deep-drawn or extruded housing parts. In this case we speak of a solid housing or hardcase.

Decisive for the safety of lithium-ion cells is that an internal short circuit between the electrodes of different polarity is excluded, at the same time an unimpeded flow of ions must be possible. For this purpose, separators are used which insulate both the anode and the cathode both mechanically and electrically from one another, with ionic conduction through the separator being possible. The materials are mainly fine-pored plastic films and nonwovens made of glass fibers. Also ceramic films are now used as a separator, which are particularly temperature resistant.

Furthermore, it is important that the lithium-ion cells are always operated within the permissible voltage range. A maximum charging voltage of 4.2 volts and a minimum discharge voltage of 2.5 volts should not be exceeded or fallen below. If there is an overload of the lithium-ion cell, d. H. the maximum charging voltage of 4.2 volts is exceeded, metallic lithium can deposit on the anode. The cathode material then becomes the oxidizing element and loses its stability. Furthermore, the metallic lithium deposited on the anode can grow in the form of dendrites at the anode, which can damage the separator. As a result, there is an internal short circuit in the lithium-ion cell. Due to the flowing short-circuit current and the internal resistance in the lithium-ion cell, the lithium-ion cell heats up further, which can lead to a fire and in the worst case to an explosion. This is called thermal runaway of the cell.

Out US 2013 / 0017431A1 is a separator known for lithium-ion batteries. The separator comprises a membrane consisting of a fine mesh with pores having a size between 0.1 to 5 μm. Furthermore, the separator comprises a coating with thermoplastic particles. Via the pores in the separator, ionic conduction is possible. When the separator is heated above a predetermined limit temperature, the particles will melt and at least partially block the pores so that the ionic current resistance through the separator increases by at least 50%.

Out DE 102 38 945 B4 is a separator with shutdown function for lithium batteries known. The separator comprises a porous carrier and a shut-off layer of particles which melt at a given temperature and close the pores of the separator.

A disadvantage of the prior art is that even after a normalization of the temperature conditions, the ion conductivity of the separator remains reduced. The battery is thus unusable and must be replaced in any case.

Disclosure of the invention

It is proposed a galvanic element comprising an anode, a cathode and a separator which mechanically and electrically separates the anode and the cathode from each other, wherein the separator is arranged, when exceeding a critical temperature of the galvanic element, an ion current between the anode and cathode and the separator is arranged to release the ionic current when the temperature falls below the critical temperature again.

Such a galvanic element comprises a layer stack comprising, in this order, a current collector associated with the anode, the anode, the separator, the cathode and a current collector associated with the cathode.

The current conductors are typically designed as metal foils, for example copper foils with thicknesses between 6 .mu.m and 12 .mu.m being used for the current conductor associated with the anode. For the current collector associated with the cathode, a metal foil is also usually used, wherein the cathode used is, for example, an aluminum foil with a thickness of between 13 μm and 15 μm. Alternatively, it is conceivable to provide a carrier material such as a plastic film with a metal coating instead of metal foils.

The electrodes comprise an active material mixture which, for example, in addition to an active material and other additives, in particular for improving the conductivity comprises.

For example, in the case of the cathode, the cathode active material is selected from a lithiated transition metal oxide such as Li (NiCoMn) O ₂ , LiMn ₂ O ₄ (or higher lithium content), Li ₂ MO ₃ -LiMO ₂ (where M is Ni, Co, Mn, Mo , Cr, Fe, Ro or V), LiMPO ₄ (where M is, for example, Fe, Ni, Co, Mn), Li (Ni _0.5 Mn _1.5 ) O ₄ (or higher lithium content), Li _X V ₂ O ₅ , Li _X V ₃ O ₈ or further cathode materials known to the person skilled in the art, such as borates, phosphates, fluorophosphates, silicates or composite materials.

A suitable composite material comprises, for example, a mixture of sulfur particles as active material, graphite and Leitruß to increase the electrical conductivity and optionally a binder such. B. PVdF (polyvinylidene fluoride). In a further embodiment, the composite material comprises a mixture of optionally carbon and nanoparticles of LiF and a metal, such as Fe, Cu, Ni. In a further embodiment, the composite material comprises a mixture of optionally carbon and nanoparticles of Li ₂ S and a metal, such as Fe, Cu, Ni. In another embodiment, the prelialization of the metal has already taken place and the composite material consists of carbon and a Li-containing metal hydride, sulfide, fluoride or nitride.

In the case of the anode, the active material is for example selected from a graphite, silicon or a conversion material.

As additives, for example, electrically conductive or ion-conductive materials may be added to the active materials. For example, the electrically conductive material may be selected from carbon nanotubes, a Leitruß, graphene, graphite or a combination of at least two of these materials.

The separator is preferably designed in the form of a membrane of an electrically insulating and ion-conducting material, wherein the separator is designed to be as thin as possible in order to reduce the resistance in the ionic line. Typical thicknesses of the separator are from 1 μm to 10 μm. Particularly suitable materials for the separator are fine-pored films made of plastic, for example polyethylene, polypropylene or non-woven materials made of glass fibers.

In further embodiments, the separator may also comprise a ceramic membrane, for example, the material used here is lithium garnet. Further possible materials are, for example, polymers such as polyvinylidene fluoride (PVDF). In order to achieve a temperature-dependent resistance during the ion conduction through the separator, the separator also comprises a functional material whose ionic conductivity, in particular the conductivity for lithium ions, is dependent on the temperature. In this case, the functional material is selected so that its ionic conductivity below a critical temperature is good and above a critical temperature is poor. Additionally or alternatively, the functional material may be selected such that, conversely, the ionic conduction above a further critical temperature is good and below the further critical temperature is poor. In this case, ionic conductivities greater than 10 ^-3 S / cm ^-1 as good conductivity and ionic conductivities smaller than 10 ^-3 S / cm ^-1 , in particular smaller than 10 ^-5 S / cm ^-1 , are regarded as poor conductivity. Preference is given to functional materials whose ionic conductivity above a critical temperature even completely disappears, which means a conductivity below 10 ^-6 S / cm ^-1 .

In one embodiment of the invention, the critical temperature is chosen to be between 45 ° C and 80 ° C. It is particularly preferred if the critical temperature is between 50 ° C and 80 ° C. It makes sense, however, that the critical temperature be chosen so that it is smaller than the melting point of the separator or a part of the separator. This ensures that an interruption of the ion current and thus a shutdown of the galvanic element takes place before the temperatures have risen so far that the galvanic element is irreparably damaged. In particular, it will be a Damage to the separator avoided, so that the separator can fulfill its function at any time.

Additionally or alternatively, it is conceivable in further embodiments to select the functional material such that the ion conductivity does not disappear above a critical temperature but below a further critical temperature. This could prevent the use of the battery below a permissible temperature range and, consequently, damage to the battery can be avoided. In this case, the further critical temperature is chosen to be between -40 ° C and 0 ° C, preferably between -20 ° C and -5 ° C.

In one embodiment of the galvanic element, the separator comprises a first carrier layer and a switchable barrier layer, which conducts ions below the critical temperature and is impermeable to ions above the critical temperature. The switchable barrier layer consists of the functional material. The carrier layer is made of the electrically insulating and ion-conducting material.

In a further embodiment of the galvanic element, the separator additionally comprises a second carrier layer, wherein the switchable barrier layer is arranged between the first carrier layer and the second carrier layer. The switchable barrier layer is thus protected inside the separator.

The material of the switchable barrier layer, ie the functional material, is selected from hydrogels, nanocomposites, polyacrylamides, polymethacrylates, or a crosslinked polymer.

To allow ionic conduction between the cathode and the anode, an electrolyte is used. In this case, in particular, the separator may be impregnated with the electrolyte. In particular, the electrolyte may be a liquid electrolyte such as lithium hexafluorophosphate (LiPF ₆ ) dissolved in an organic solvent.

When the galvanic element is charged, an ion current formed from positively charged lithium ions flows from the cathode through the separator to the anode. In this case, lithium ions are dissolved out of the cathode active material of the cathode and re-accumulate in the anode active material of the anode. The flow of lithium ions is driven by a corresponding electric current, in which electrons flow from the cathode towards the anode. The electric current is driven when charging the galvanic element by means of an electric power source.

When discharging the galvanic element, conversely, the lithium ions flow from the anode through the separator to the cathode. The corresponding electrical current can thereby flow electrons from the anode in the direction of the cathode, wherein the current flow is passed through an external circuit and can be used to drive electrical loads.

Furthermore, a battery cell is proposed comprising a cell packaging and at least one of the galvanic elements described herein. Cell packaging may be a soft pack packaging design or a solid housing.

In addition, a battery comprising one or more such battery cells is proposed.

In the context of this description, the term battery or battery cell is used as is customary in the vernacular, ie. H. The term battery includes both a primary battery and a secondary battery (accumulator). Likewise, the term battery cell includes both a primary cell and a secondary cell.

Advantages of the invention

The galvanic element according to the invention comprises a separator whose ionic conductivity is dependent on the temperature of the galvanic element. It is provided that the ionic line is suppressed through the separator, when the temperature of the galvanic element exceeds a critical temperature. Alternatively or additionally, the ionic line can be suppressed below a critical temperature. If the ion current is suppressed, then the corresponding electric current is interrupted at the same time. Since a heating is always connected to a flowing electric current due to ohmic resistances, a further heating of the galvanic element is thus prevented by the interruption of the ion current and the associated interruption of the electrical current. At a permissible temperature, the galvanic element can be used normally.

No external fuses or monitoring devices are required for this interruption of the ion current, so this fuse function can be easily implemented without modification of other components such as the battery case or the battery control device. Furthermore, this independence from external components also contributes to safety, as it does not circumvent the security mechanism by manipulating, for example, an external controller.

In addition, the blockage of the ion current is reversible, so that the galvanic element can be reused after a normalization of the temperature.

Brief description of the drawings

Show it

1a and 1b the charging of a galvanic element according to a first embodiment of the invention and

2a and 2 B the charging of a galvanic element according to a second embodiment of the invention.

In the following description of the embodiments of the invention, the same or similar components or elements are designated by the same reference numerals, wherein a repeated description of these components or elements is dispensed with in individual cases. The figures illustrate the subject matter of the invention only schematically.

In 1a and 1b is the charging of a galvanic element 10 shown.

The galvanic element 10 includes one of a cathode in this order 14 associated current conductor 12 , the cathode 14 , a separator 16 , an anode 18 and one of the anode 18 associated current conductor 20 forming a pile.

The separator 16 of the galvanic element 10 is designed so that lithium ions 26 through the separator 16 through from the cathode 14 to the anode 18 can wander and vice versa. This is the separator 16 carried out porous and impregnated with an electrolyte.

In the in the 1a and 1b illustrated embodiment is the galvanic element 10 designed as a lithium-ion battery cell. In this case, the cathode comprises 14 as the cathode active material LiCoO ₂ , wherein in the figures cobalt atoms by the reference numeral 40 are provided, oxygen atoms by the reference numeral 42 and lithium atoms by the reference numeral 27 , The anode 18 includes as anode active material graphite, which is denoted by the reference numeral 44 is provided.

During charging, the cathode active material becomes the cathode 14 delithiated, ie lithium ions 26 come out of the cathode 14 off, walk in the direction of the reference number 24 marked arrows through the separator 16 through and store in the graphite 44 the anode 18 as lithium atoms 27 one. The ion current is driven by an electric current, which serves as charging current. In doing so, electrons flow from the cathode 14 via an external circuit 50 to the anode 18 , The flow direction of the electrons is indicated by arrows with the reference numerals 22 marked.

1a represents a situation in which the temperature T of the galvanic element 10 is less than a critical temperature T _K. Is the temperature T of the galvanic element 10 below this critical temperature T _K , so is the separator 16 for the lithium ions 26 permeable.

The separator 16 includes in the in the 1a and 1b illustrated embodiment, a first carrier layer 30 and a switchable barrier layer 32 , The first carrier layer 30 It is constructed like an ordinary separator membrane and consists of an electrically insulating material that allows ionic conduction. For this purpose, for example, porous materials are suitable, wherein the separator 16 is impregnated with the electrolyte to improve ionic conduction. As materials for the first carrier layer 30 For example, fine-pored plastic films, such as polyethylene or polypropylene in question. Also, nonwovens made of glass fibers and ceramic membranes, for example of lithium garnet, are for the first carrier layer 30 suitable.

The switchable barrier 32 is designed to be below the critical temperature T _K for the lithium ions 26 is permeable, and at a temperature T ≥ T _K for the lithium ions 26 is impermeable. This includes the switchable barrier layer 32 a functional material selected, for example, from hydrogels, nanocomposites, polyacrylamides, polymethacrylates, or a crosslinked polymer. Additionally or alternatively, the functional material may be selected to have another critical temperature and the functional material to be permeable to ions above this second critical temperature and to be impermeable to ions at a temperature below the second critical temperature. If the functional material has both critical temperatures, ionic conduction is possible only at an interval between the two critical temperatures.

In the in 1a The situation shown is the temperature T of the galvanic element 10 smaller than the critical temperature T _K. Therefore, the switchable barrier layer 32 for the lithium ions 26 permeable, so along the with reference numerals 24 provided arrows lithium ions 26 through the separator 16 can wander through it. Charging the galvanic element 10 is thus possible.

In 1b the temperature T is greater than or equal to the critical temperature T _K. The switchable barrier 32 changes at exceeding or reaching the critical temperature T _K their properties, which thereby characterized for the lithium ions 26 becomes impermeable. An ionic line through the separator 16 through is no longer possible. Along the switchable barrier 32 it comes to a blockade 33 containing the lithium ions 26 can not overcome. The ion current flowing from the lithium ions 26 was worn, comes to a halt. At the same time, the electrical current corresponding to the ion current also comes through the external circuit 50 has flowed, also to stop, so that no more electric current from the cathode 14 to the anode 18 can flow. Another charging of the galvanic element 10 is not possible anymore.

By cooling the galvanic element 10 can be back in 1a illustrated situation, in which the switchable barrier layer 32 again for the lithium ions 26 is permeable, so again charging the galvanic element 10 as in 1a shown is possible.

Even if this is in the 1a and in the 1b is not explicitly shown, it goes without saying that even a discharge of the galvanic element 10 only as long as possible, as the temperature T of the galvanic element 10 is below the critical temperature T _K. When unloading the galvanic element 10 the directions of the arrows are reversed 24 and 22 um, leaving lithium ions 26 from the anode 18 through the separator 16 through to the cathode 14 flow and at the same time a corresponding electric current from the anode 18 over the anode 18 associated current conductor 20 through the external circuit 50 to the cathode 14 associated current conductor 12 and finally into the cathode 14 flows. Exceeds when discharging the galvanic element 10 the temperature T of the galvanic element 10 the critical temperature T _K changes the switchable barrier layer 32 of the separator 16 turn their properties, so this for the lithium ions 26 becomes impermeable. As a result, the electricity comes from lithium ions 26 from the anode 18 in the direction of the cathode 14 to stop, which also causes the corresponding electrical current through the external circuit 50 comes to a halt.

In both cases, both for charging and for discharging the galvanic element 10 Accordingly, a cessation of the ion current means that also a corresponding electric current comes to a standstill. Since an electric current leads to a heating due to the ohmic resistances, therefore, a further heating of the galvanic element becomes above a critical temperature T _K 10 prevented due to a flowing electric current.

2a and 2 B show a further embodiment of the galvanic element according to the invention 10 ,

That in the 2a and 2 B illustrated galvanic element 10 includes again as before to the 1a and 2 B described a layer structure in this order with that of the cathode 14 associated current conductor 12 , the cathode 14 , the separator 16 , the anode 18 and that of the anode 18 associated current conductor 20 , Unlike the related to the 1a and 1b described embodiment, the separator comprises 16 now three layers. The separator 16 comprises a first carrier layer 30 , a switchable barrier 32 and a second carrier layer 34 , wherein the first carrier layer 30 and the second carrier layer 34 the switchable barrier 32 between them. In this way, the switchable barrier layer 32 through the two carrier layers 30 . 34 protected against mechanical effects.

The galvanic element 10 of the 2a and 2 B behaves like to the 1a and 1b described, ie as long as the temperature T of the galvanic element 10 is less than a critical temperature T _K , is the switchable barrier layer 32 ion-conducting, allowing lithium ions 26 through the separator 16 can move through. If the temperature T is greater than or equal to the critical temperature T _K , then the properties of the switchable barrier layer change 32 in that they are impermeable to the lithium ions 26 becomes. Charging the galvanic element 10 where lithium ions 26 in the direction of with reference numerals 24 provided arrows from the cathode 14 to the anode 18 wander, is then no longer possible because on both sides of the separator 16 a blockade 33 formed. A corresponding electrical current through the external circuit 50 also comes to a halt here.

The invention is not limited to the embodiments described herein and the aspects highlighted therein. Rather, within the scope given by the claims a variety of modifications are possible, which are within the scope of expert action.

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 2013/0017431 A1 [0006]
• DE 10238945 B4 [0007]

Claims (10)

Galvanic element ( 10 ) comprising an anode ( 18 ), a cathode ( 14 ) and a separator ( 16 ), which is the anode ( 18 ) and the cathode ( 14 ) separates mechanically and electrically, characterized in that the separator ( 16 ) is arranged, an ion current between the anode ( 18 ) and cathode ( 14 ) when the temperature of the galvanic element ( 10 ) exceeds a critical temperature, the separator ( 16 ) is set to release the ion current again when the temperature of the galvanic element ( 10 ) falls below the critical temperature again. Galvanic element ( 10 ) according to claim 1, characterized in that the critical temperature is between 80 and 150 ° C. Galvanic element ( 10 ) according to claim 1 or 2, characterized in that the separator ( 16 ) a first carrier layer ( 30 ) and a switchable barrier layer ( 32 ), which is ionically conductive below the critical temperature and impermeable to ions above the critical temperature. Galvanic element ( 10 ) according to claim 3, characterized in that the separator ( 16 ) additionally a second carrier layer ( 34 ), wherein the switchable barrier layer ( 32 ) between the first carrier layer ( 30 ) and the second carrier layer ( 34 ) is arranged. Galvanic element ( 10 ) according to one of claims 3 to 4, characterized in that the material of the first carrier layer ( 30 ) and / or the second carrier layer ( 34 ) is selected from polypropylene, polyethylene or at least two of these materials. Galvanic element ( 10 ) according to one of claims 3 to 5, characterized in that the material of the switchable barrier layer ( 32 ) is selected from hydrogels, nanocomposites, polyacrylamides, polymethacrylates or a crosslinked polymer. Galvanic element ( 10 ) according to one of claims 1 to 6, characterized in that the cathode ( 14 ) comprises a cathode active material selected from a composite material containing LiF and a metal, a lithiated transition metal oxide, a litiiertem sulfur or a mixture of at least two of these materials. Galvanic element ( 10 ) according to one of claims 1 to 7, characterized in that the anode ( 18 ) contains an anode active material selected from a graphite. Battery cell comprising a galvanic element ( 10 ) according to one of claims 1 to 8 and a cell housing. Battery comprising at least one battery cell according to claim 9.

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