Patent Application
20180219209
The present invention relates to the field of lithium electrochemical generators, which operate according to the principle of insertion or extraction, or in other words intercalation/deintercalation, of lithium in at least one electrode.
It relates more particularly to an electrochemical lithium battery having at least one electrochemical cell consisting of an anode and of a cathode on either side of a separator impregnated with electrolyte, two current collectors, of which one is linked to the anode and the other to the cathode, and a casing of a shape that is elongate along a longitudinal axis (X), the casing being designed to house the electrochemical cell in a sealtight manner while being passed through by a portion of the current collectors forming the output terminals, also referred to as poles.
The separator may consist of one or more films.
The casing may have a cover and a container, usually referred to as a can, or have a cover, a bottom and a lateral jacket joined both to the bottom and to the cover.
The present invention aims to produce a short-circuit safety device integrated into the battery.
“Abnormal operation of a battery” is understood to mean, in the context of the invention, battery usage conditions within an extreme current and/or temperature range that goes beyond the range of environmental conditions specified by the battery designer or manufacturer.
This may typically involve a case of overcharging a battery with currents greater than the specified nominal charging current, to a voltage threshold that is more often than not greater than the specified nominal voltage threshold. By way of example, this occurs above a voltage value of 3.6 V for an electrochemical pair of lithium iron phosphate LiFePO4 (LFP) and graphite electrode materials, or above 4.2 V for the electrochemical pairs lithium cobalt oxide LiCoO2/graphite and LiNi0.33Mn0.33Co0.33O2 (NMC)/graphite.
This may also involve a case of deterioration of the performance of the battery due to the advanced ageing thereof beyond the limit recommended by the manufacturer and/or due to abnormal usage conditions, such as the exceedance of upper and lower voltage threshold limits, excessively high charging or discharging currents, extreme usage temperatures that are incompatible with the characteristics of the battery, etc.
As illustrated schematically in
The architecture of conventional lithium-ion batteries is an architecture that may be qualified as monopolar, as it has a single electrochemical cell having an anode, a cathode and an electrolyte. Several types of monopolar architecture geometry are known:
The electrolyte constituent may be in solid, liquid or gel form. In the latter form, the constituent may comprise a separator made of a polymer or of a microporous composite imbibed with organic or liquid-ionic electrolyte(s) that enable(s) lithium ion to move from the cathode to the anode for a charge and vice versa for a discharge, thereby generating the current. The electrolyte is generally a mixture of organic solvents, for example of carbonates, to which a lithium salt, typically LiPF6, is added.
The positive electrode or cathode consists of lithium cation insertion materials that are generally composites, such as lithium iron phosphate LiFePO4, lithium cobalt oxide LiCoO2, optionally substituted lithium manganese oxide LiMn2O4 or a material based on LiNixMnyCozO2 where x+y+z=1, such as LiNi0.33Mn0.33 Co0.33O2, or a material based on LiNixCoyAlzO2 where x+y+z=1, LiMn2O4, LiNiMnCoO2 or lithium nickel cobalt aluminum oxide LiNiCoAlO2.
The negative electrode or anode very often consists of carbon, graphite or is made of Li4TiO5O12 (titanate material), and also possibly based on silicon or based on lithium, or based on tin and alloys thereof, or of a silicon-based composite.
The anode and the cathode made of lithium insertion material may be deposited, using a conventional technique, in the form of an active layer on a metal sheet forming a current collector.
The current collector connected to the positive electrode is generally made of aluminum.
The current collector connected to the negative electrode is generally made of copper, of nickel-plated copper or of aluminum.
Conventionally, a Li-ion accumulator or battery uses a pair of materials at the anode and at the cathode that enable it to operate at a high voltage level, typically around 3.6 volts.
A Li-ion accumulator or battery has a rigid packaging or casing when the targeted applications pose a constraint whereby a long lifetime is sought, with for example very high pressures to be withstood and a requirement for a stricter level of sealtightness, typically less than 10−6 mbar·l/s helium, or in highly constrictive environments such as in the spatial or aeronautic field. The main advantage of rigid packagings is thus their high sealtightness, which is maintained over time on account of the fact that the casings are closed by welding, generally by laser welding.
The geometry of the majority of rigid casings for Li-ion battery packagings is cylindrical, as the majority of the electrochemical cells of the batteries are wound in a cylindrical geometry by spooling. Prism-shaped casings have also already been produced.
One of the types of cylinder-shaped rigid casing, usually manufactured for a high-capacity Li-ion battery with a lifetime of more than 10 years, is illustrated in
The casing 6, with a central axis X, has a cylindrical lateral jacket 7, a bottom 8 at one end, and a cover 9 at the other end. The cover 9 bears the poles or terminals 40, 50 through which the current is output. One of the output terminals (poles), for example the positive terminal 40, is welded to the cover 9, whereas the other output terminal, for example the negative terminal 50, passes through the cover 9 with interposition of a seal (not shown) that electrically insulates the negative terminal 50 from the cover.
“Uncoated bands” is understood to mean, here and in the context of the invention, the portions of the metal sheets, also referred to as foils, forming the current collectors, that are not covered with a lithium insertion material.
Once the bundle has been produced, it is necessary to connect it to the two output terminals, of different polarity, of the battery.
The uncoated bands 20, 30 of the cathode 2 and of the anode 3 are thus bent, folded and/or compacted in order to obtain, at each end of the bundle, a base intended to be welded to a current collector.
One particularly advantageous method is disclosed in patent application WO 2015/044820 in the name of the applicant: this method combines folding with plastic deformation of at least some of the uncoated bands and axial compacting of these bands.
Once a base has been obtained at each end of the bundle, said base is mechanically and electrically linked to a current collector by welding.
Thus, as illustrated in
At the other of the lateral ends 10 of the bundle, the base 31 formed by the compacted part 30T of the anode (negative sides) is welded in the same way to a conventional current-collector part 13 in the form of a solid disk pierced at the center thereof and a tab 130 projecting laterally from the disk 13 (
To complete the final production of the battery, the bundle with the collector 13 is inserted into a rigid container made of aluminum forming only the lateral jacket 7 of the casing 6 or forming a can consisting both of the lateral jacket 7 and of the bottom 8 of the casing 6. It is ensured in particular during this step that the tab 130 does not impede the insertion. To this end, said tab is advantageously folded upward.
The collector 14 is then welded to the bottom 8 of the casing 6.
The collector 13 is welded to a negative pole 50 forming a passage for a casing 6 cover 9.
The cover 9 is then welded to the rigid metal container 7.
Next, a step is performed of filling the casing 6 with an electrolyte, through a through-aperture (not shown) that is formed in the cover 9. The production of the Li-ion battery ends with the filling aperture being plugged.
One aim of battery manufacturers is to increase the autonomy of a cell forming the battery, or the ability thereof to be able to operate in high-power regimes while improving their lifetime, i.e. the number of times they are able to be cycled, their lightness and the costs of manufacturing these components.
Approaches to improving Li-ion batteries relate mainly to the nature of the materials and the methods for producing electrochemical-cell components.
Another possible approach for improvement relates to the safety of Li-ion batteries, it possibly being all the more important to ensure this safety for batteries with a high energy density.
The inventors have thus been prompted to design Li-ion batteries with a high capacity, typically a capacity of greater than 70 Ah, for a high specific energy, typically greater than or equal to 135 Wh/Kg, and an energy density that is also high, typically greater than or equal to 265 Wh/L.
A first approach could consist in replicating the rules for designing safety devices that are present in batteries with a lower capacity and/or a lower energy density, such as a safety seal enabling venting in the event of the internal pressure of the battery rising above a high threshold value, typically 15 bar.
Now, the inventors thought that the potentially high reactivity of a battery with a higher capacity, typically greater than 30 Ah, would require the implantation of additional protective means of the type that operate according to the principle of a short circuit that makes it possible to stop the flow of current in the event of internal overpressure that could possibly lead to a critical event, such as a battery explosion or fire.
The inventors therefore performed an inventory of all of the existing short-circuit safety devices in order to find out whether some of them could be reproduced or adjusted to their high-capacity and high-energy-density batteries.
Patent FR 2 977 379 discloses a battery the bottom of the container (casing) of which comprises a weakened part that breaks in the event of an overpressure brought about by internal gases in the battery. This patent indicates that the breakage of this weakened part interrupts the electrical connection between a deformable conductive membrane that is arranged against the bottom of the container. Now, there is a risk of the solution disclosed not working. Specifically, it is firstly necessary to ensure complete breakage of the weakened part over the entire periphery in order to effectively create the short circuit. Secondly, it is necessary to ensure that no contact (or restoration of contact) is possible between the deformable membrane and the bottom wall of the container at the weakened part. Now, that appears to be strongly questionable between these two conductive parts that are disclosed in this patent.
Patent EP 2270899 B1 and patent application US 2012/007062 each disclose a Li-ion battery the bottom of the container (casing) of which comprises a circular part breaks in the event of internal overpressure in the battery, the breakage of this circular part causing the venting of the gases and then the electrical disconnection between the bottom of the casing and the end of the electrochemical bundle opposite.
Patent application EP 1626456 also describes a Li-ion battery the wall of the bottom of the casing of which has a thinned zone, assumed to have a value of substantially equal to 20% of the thickness of the wall as stipulated in claim 11, so as to break in the event of internal overpressure in the battery and thus interrupt any electrical connection between said wall and the end of the electrochemical bundle opposite. Upon reading this document, one might question the reality of the disclosed short circuit operating, given the difficulty in obtaining the desired thinning and the difficulty in obtaining reproducibility.
All of the patent applications/patents above furthermore have major drawbacks that may be listed as follows:
There is therefore a need to improve the implementation of a short-circuit safety device for a lithium battery in the event of an overpressure of internal gases in the battery, in particular for the purpose of preventing, once the safety device has been tripped, any restoration of electrical contact (current) from outside the battery, and of avoiding venting from inside the battery at least until a certain internal overpressure value.
The aim of the invention is to at least partially meet this need.
To this end, the invention relates, in one of its aspects, to an electrochemical lithium battery (A) having an electrochemical bundle (F) comprising at least one electrochemical cell (C) consisting of at least one anode and one cathode on either side of an electrolyte impregnated into a separator, two current collectors, of which one is linked to the anode and the other to the cathode, a casing of a shape that is elongate along a central axis (X), the casing having a cover, a bottom, a lateral jacket joined both to the bottom and to the cover, the casing being designed to contain the bundle in a sealtight manner while being passed through by a portion of the current collectors forming the poles.
According to the invention, the battery furthermore has:
According to the invention, the thickness of the bottom and the breakage initiation line are dimensioned such that, beyond a predetermined threshold value for the pressure prevailing inside the casing, the bottom undergoes a plastic deformation, preserving the weld between the central portion of the plate and that of the bottom, while generating a breakage of the initiation line, and thereby an irreversible electrical disconnection between the bundle and the bottom of the casing.
In other words, the invention consists in defining a short-circuit device with a casing bottom wall that deforms in the plastic range and at the same time a breakage initiation line, produced around the central portion of the collector, that is able to tear in order to electrically separate this central portion linked to the bottom of the casing from the main outer surface of the current collector, which surface is linked to an end of the electrochemical bundle.
The wall of the bottom of the casing thus deforms to an increasing extent as the internal pressure increases.
Once the disconnection has occurred, the wall of the bottom remains curved on account of its plastic deformation, and the current collector remains connected to the bottom as they are welded together via their central parts. By contrast, the bottom is detached and therefore electrically disconnected from the lower end of the electrochemical bundle F, the electrical insulation element(s) around the central portions moreover guaranteeing electrical insulation.
Once the short circuit according to the invention has been tripped, the safety of the battery is guaranteed, as it is no longer possible for a user to pass current between the two positive and negative terminals of the battery.
Furthermore, when the electrical circuit is broken, the battery remains sealed since, in contrast to the devices from the prior art, the bottom of the casing remains physically intact even though it has been plastically deformed.
According to one variant implementation, the battery comprises, as electrical insulation elements:
The plastic film is advantageously made of a material chosen from polyimide (PI), polyetherimide (PEI), polypropylene (PP) or polyethylene (PE).
Alternatively, it is possible to provide an electrical insulation component covering both the peripheral edge of the electrode foil and the end of the bundle.
The remaining thickness in the breakage initiation line is preferably between 30 μm and 150 μm.
The thickness E of the wall of the bottom is again preferably between 0.5 and 2.0 mm for a bottom made of aluminum-based material.
The predetermined threshold value for the pressure beyond which the bottom of the casing undergoes a plastic deformation is advantageously greater than or equal to 5 bar.
The battery furthermore advantageously comprises a venting seal the opening of which is triggered as soon as the internal pressure reaches another predetermined threshold value, preferably greater than 15 bar.
The battery may be in the shape of a cylinder with a current collector in the shape of a disk with a thickness of preferably between 0.3 and 0.8 mm.
According to one advantageous feature, the casing and the current collector are made of an aluminum-based material.
According to one advantageous embodiment, the battery is configured to remain sealed after electrical disconnection between the current collector linked to the bundle and the bottom of the casing for an internal pressure level not exceeding a predetermined threshold value, preferably less than or equal to 12 bar.
According to one advantageous embodiment, the electrochemical bundle consists of a single electrochemical cell wound upon itself by spooling.
Preferably:
Other advantages and features of the invention will become more clearly apparent on reading the detailed description of examples of implementation of the invention, given by way of non-limiting illustration with reference to the following figures, in which:
For the sake of clarity, the same references denoting the same elements of a lithium-ion battery according to the prior art and according to the invention are used for all of
It will be noted that the various elements according to the invention are shown only for the sake of clarity and that they are not to scale.
The metal foils bearing the electrode materials may have a thickness of between 5 and 50 μm. For an anode foil 3, this may advantageously be a foil made of copper with a thickness of the order of 12 μm. For a cathode foil 2, this may advantageously be a foil made of aluminum with a thickness of the order of 20 μm.
In order to improve the operating safety of a Li-ion battery, in particular a battery with a high energy density and a high capacity, the inventors propose to integrate a new type of short-circuit device at the bottom of the battery casing.
One exemplary implementation of such a short circuit according to the invention integrated into a battery is shown in
The bundle F according to the invention is therefore like the one shown in
The electrical connection steps and the sequence of these steps, between the electrochemical bundle F prepared with a base 21, 31 at each of its ends and the current collectors are performed essentially as described with reference to
In particular, the current collector 13 at the anode 3 and its electrical and mechanical connection and welding to the end 31 of the bundle F may be identical to what is shown in
The Li-ion battery according to the invention is distinguished from those of the prior art by a short-circuit device integrated on the bottom side 8 of the casing 6 that makes it possible to prevent, once it has been tripped, any restoration of electrical contact (current) from outside the battery, and to avoid venting from inside the battery at least until a certain internal overpressure value.
A description is given in relation to
In contrast to the current collectors at the bottom of the battery casing according to the prior art, the current collector according to the invention in the form of a solid metal disk 14 is welded to the bottom 8 of the casing 6 only by its central portion 140. More precisely, this central portion 140 is welded to a central portion 80 of the bottom 8 about the central axis X of the battery. The disk 14 is preferably made of aluminum.
The collector 14 bearing against the end 21 of the electrochemical bundle.
The peripheral portion 141 around the central portion 140 of the collector furthermore has a zone of smaller thickness, forming a breakage initiation line 142.
The battery furthermore comprises at least one electrical insulation element 15, 16 arranged between the disk forming the collector 14 and the bottom of the casing opposite, around the central portions 80, 140 thereof, in order to electrically insulate said portions from one another.
Advantageously, as shown in
Another envisioned embodiment would consist in replacing the two elements, spacer 15 and insulating film 16, with a single insulation component so as to cover both the peripheral edge 20 of the cathode foil 2 and the end 21 of the bundle F.
According to the invention, both the thickness of the bottom 8 and the breakage initiation line 142 are dimensioned such that, beyond a predetermined threshold value for the pressure prevailing inside the casing, the bottom 8 undergoes a plastic deformation, preserving the weld between the central portion 140 of the plate and that 80 of the bottom, while generating a breakage of the initiation line, and thereby an electrical disconnection between the bundle and the bottom of the casing.
Thus, as described hereinafter, an electrical disconnection is obtained between the electrochemical bundle F and the bottom 8, and the electrical circuit of the battery is therefore cut.
The predetermined threshold value for the internal pressure of the battery may advantageously be equal to or greater than 5 bar, in particular between 5 and 10 bar.
When the casing 6, and therefore the bottom 8, is made of aluminum, the thickness E of the wall of the bottom 8 is preferably between 0.5 and 2.0 mm, so as to obtain therefrom a significant deformation in the plastic range, typically at least equal to 1 mm, within an internal pressure range of between 5 and 10 bar.
This bottom wall thickness range is advantageous as it is compatible with the customary methods for obtaining this component that is made of standard aluminum material, typically 1000 series, for example 1050, or 3000 series, for example 3003, by punching or using an impact extrusion technique.
The wall thickness of the collector 14 may be between 0.3 and 0.8 mm.
Preferably, and depending on the format of the battery, the mechanical resistance of the breakage initiation line 142 under a force applied to the center thereof will be between 30 N, which corresponds to a small-diameter battery format, typically of the order of 14 mm, and 140 N or more for battery formats with a diameter of greater than 60 mm.
The thickness of the breakage initiation line 1425 is preferably between 30 μm and 150 μm, depending on the area of the components under consideration.
The insulation component 15 is advantageously made from standard insulation material.
The insulating film 16 is advantageously made of plastic, such as of polyimide (PI), polyetherimide (PEI), polypropylene (PP) or polyethylene (PE).
A description is now given of the various modes of operation of a Li-ion battery according to the invention, depending on the tripping, or lack thereof, of the short-circuit device that has just been described.
During normal operation of the battery, the internal pressure remains low, at a pressure level of usually between 1 and 5 bar, throughout the entire usage lifetime of the battery.
The design of the casing 8 of the battery, and in particular of the thickness E of the bottom and of the breakage initiation line 142, takes into account a possible overpressure, typically of up to 4 bar, which may occur:
During abnormal operation of the battery, generation of internal gas exceeding 5 bar, and possibly reaching a level that is sometimes greater than 15 bar or more, may occur.
In the event of a high internal overpressure in the battery, typically an overpressure of greater than 5 bar, the wall of the bottom 8 of the container (casing) 6 deforms in the plastic range and significantly so, typically to a value of greater than 1 mm.
The desired plastic deformation is defined so as to obtain a breakage of the existing electrical link between the end 21 of the electrochemical bundle F, which is positive in the example illustrated, and the bottom 8.
In other words, the wall of the bottom 8 deforms in the plastic range, and at the same time the breakage initiation line is torn (broken).
As illustrated in
By contrast, the bottom 8 is detached and therefore electrically disconnected from the lower end 21 of the electrochemical bundle F.
Electrical insulation is still guaranteed by the annular spacer 15 and/or the insulating plastic film 16.
As detailed hereinafter, the inside of the casing 8 may or may not remain under pressure. When it remains under pressure, this also contributes to maintaining the maximum deformation of the bottom 8.
Numerous variants may be provided for producing the breakage initiation line 142 in a circular manner 360° around the central portion 140. This initiation may be produced by machining or by an impact tool. The possible profiles may be a V-shaped profile (
Regardless of the profile selected, the weakening of the line 142 is dimensioned so as to guarantee 360° circular tearing when the wall of the bottom 8 deforms plastically and exerts a tractive force on the central portion 140 of the current collector 14.
According to one advantageous embodiment, it is possible to provide a Li-ion battery with a safety ventilation seal in addition to the short-circuit device according to the invention that has just been described.
This ventilation seal is preferably produced on the cover 9 of the battery.
The safety seal is advantageously able to be opened within an internal overpressure range of 16+/−3 bar.
A description is now given of the various modes of operation of such a Li-ion battery according to the invention, depending on the tripping, or lack thereof, of the short-circuit device according to the invention and depending on the tripping, or lack thereof, of the additional ventilation seal.
During normal operation of the battery, that is to say when the internal pressure is lower than a predetermined threshold value typically equal to 5 bar, the wall of the bottom 8 of the casing undergoes little or no deformation.
Electrical conduction is guaranteed between the lower end 21 of the electrochemical bundle F and the bottom 8 connected by welding between the respective central parts 80, 140 of the bottom and of the connector 14.
In the event of a rise in internal pressure in a relatively short time, typically a rise in pressure over several minutes of operation, typically of the order of 15 to 20 minutes, the pressure is within a range of between 10 and 15 bar.
The plastic deformation of the bottom 8 of the casing 6 of the battery then occurs to an extent that is sufficient to enable the breakage of the line 142 and therefore the cutoff of electrical conduction between the end 21 of the electrochemical bundle F and the bottom 14, which moreover remains connected to the center of the bottom through the weld to the central part 140 of the latter.
The short-circuit device according to the invention is thus tripped. The safety of the battery is guaranteed, as it is no longer possible for a user to pass current between the two positive and negative terminals of the battery.
Furthermore, when the electrical circuit is broken, the battery remains sealed since, in contrast to the devices from the prior art, the bottom 8 of the casing remains intact even though it has been plastically deformed.
In the event of a rapid rise in internal pressure, typically an internal overpressure of 15 bar being reached in less than 1 minute, it is possible for the dynamics of the pressure rise to lead to an effect of simultaneous operation both of the short-circuit device according to the invention and of the safety ventilation seal.
In other words, in this case, tripping of the short circuit according to the invention at the bottom 8 of the casing 6 and opening of the safety ventilation seal occur simultaneously.
The safety of the battery is also guaranteed in this case, as it is no longer possible for a user to pass current between the two positive and negative terminals of the battery.
Furthermore, by opening the safety seal, the internal pressure of the battery falls back to atmospheric pressure.
The inventors performed deformation tests on a casing bottom 8 of a cylindrical Li-ion battery according to the invention, under the effect of the pressure exerted inside the casing.
The dimensions of the battery casing on which the tests were performed are as follows: external diameter 65 mm and height 230 mm.
The results are shown in
It is seen from these curves that, for an internal pressure of the order of 12 bar, the deformation of the bottom 8 of the casing reaches around 3.4 mm.
The inventors performed other tests to validate the short-circuit device according to the invention that has just been described.
First of all, they placed the battery under pressure through the negative terminal, and then they applied a pressure rise ramp of 5 bar/min.
The electrical continuity signal between the two terminals is then recorded: if it is 1, current is flowing, if it is 0, the current is no longer flowing.
The results obtained are summarized in the table below:
From this table, it is seen that the components (M7 and M8) having the smallest weakening thickness lead to the lowest short-circuit tripping pressure result, and vice versa for the components (M1 and M2) with the greatest weakening thickness.
Other variants and improvements may be made without however departing from the scope of the invention.
Lastly, although the casing 6 in the illustrated embodiments that have just been described is made of aluminum, it may also be made of steel, or of nickel-plated steel. In such a variant, a casing made of steel or of nickel-plated steel forms the negative potential, the current collector 14 incorporating the short-circuit device according to the invention then forming a connection to the negative pole.
The invention is not limited to the examples that have just been described; it is in particular possible to combine features of the illustrated examples with one another in variants that have not been illustrated.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1557191 | Jul 2015 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2016/067683 | 7/25/2016 | WO | 00 |
