Electrochemical accumulator whose separator impregnated with liquid electrolyte has zones with differentiated thermal stability properties.

Abstract

Electrochemical accumulator whose separator impregnated with liquid electrolyte has zones with differentiated thermal stability properties. The invention relates to a metal-ion electrochemical accumulator (A) comprising at least one electrochemical cell consisting of a cathode (2), an anode (3) and a separator (1) impregnated with a liquid electrolyte arranged between the cathode (2) and the anode (3), the cell(s) defining an electrochemical bundle (F) of elongated shape along a longitudinal axis (X), the separator having at least one zone (12), arranged at at least one of the outer lateral ends of the electrochemical bundle (F), the melting temperature of which is lower than the rest (12) of the separator. Figure for abstract: Fig. 9

Description

Title of the invention: Electrochemical accumulator whose separator impregnated with liquid electrolyte has zones with differentiated thermal stability properties. Technical field

[0001] The present invention relates to the field of electrochemical accumulators, and more particularly metal-ion accumulators.

[0002] More particularly, the invention relates to the evaluation of the safety of batteries.

[0003] The invention mainly aims to force a metal-ion accumulator to go into thermal runaway in order to study safety devices or the risks of propagation within a battery pack.

[0004] Although described with reference to a Lithium-ion accumulator, the invention applies to any metal-ion electrochemical accumulator, i.e. also Sodium-ion, Magnesium-ion, Aluminum-ion...or more generally to any electrochemical accumulator. The invention applies to any chemistry of metal-ion accumulators, such as for example NMC / Graphite, NCA / Graphite, NMC / G-Si, LFP / Graphite, Na-ion with liquid electrolyte.

[0005] An accumulator according to the invention may be in an on-board or stationary battery module or pack. For example, the fields of electric and hybrid transport and grid-connected storage systems may be envisaged within the scope of the invention. Prior art

[0006] As illustrated schematically in Figures 1 and 2, a lithium-ion battery or accumulator usually comprises at least one electrochemical cell consisting of an electrolyte constituent 1 between a positive electrode or cathode 2 and a negative electrode or anode 3, a current collector 4 connected to the cathode 2, a current collector 5 connected to the anode 3 and finally, a packaging 6 arranged to contain the electrochemical cell with sealing while being crossed by a part of the current collectors 4, 5.

[0007] The architecture of conventional lithium-ion batteries comprises an anode, a cathode and an electrolyte. Several types of conventional architecture geometry are known:

[0008] - a cylindrical geometry as disclosed in the patent application US 2006 / 0121348,

[0009] - a prismatic geometry as disclosed in US patents 7348098, US 7338733;

[0010] - a stacking geometry as disclosed in US patent applications 2008 / 060189, US 2008 / 0057392, and US patent 7335448.

[0011] The electrolyte component 1 may be in solid, liquid or gel form. In gel or liquid form, the component may comprise a polymer, ceramic or microporous composite separator soaked in organic or ionic liquid electrolyte(s) which allows the movement of the Lithium ion from the cathode to the anode for charging and vice versa for discharging, which generates the current. The electrolyte is generally a mixture of organic solvents, for example carbonates to which a lithium salt, typically LiPF6, is added.

[0012] The positive electrode or cathode is made of lithium cation insertion materials which are generally composite, such as lithium iron phosphate LiFePO4, lithium cobalt oxide LiCoO2, lithium manganese oxide, possibly substituted, LiMn2O4 or transition metal oxide, such as lamellar materials for example, a material based on LiNixMnyCozO2 with x+y+z = 1, such as LiNio.33Mno.33 Co0.33O2, or a material based on nickel cobalt aluminum oxide type Li-NixCoyAlzO2 with x+y+z = 1, such as LiNi0.sCo0.15Al0.05O3.

[0013] The negative electrode or anode is very often made of carbon, graphite or Li4TiO5Oi2 (titanate material), possibly perhaps based on silicon or based on lithium, or based on tin and their alloys or a composite formed from silicon. This negative electrode, like the positive electrode, may also contain electronically conductive additives as well as polymer additives which give it mechanical properties and electrochemical performances appropriate to the lithium-ion battery application or to its implementation method.

[0014] The anode and the cathode made of lithium insertion material can be continuously deposited using a standard technique in the form of an active layer on a metal sheet or foil constituting a current collector.

[0015] The current collector 4 connected to the positive electrode is generally made of aluminum.

[0016] The current collector 5 connected to the negative electrode is generally made of copper, nickel-plated copper or aluminum.

[0017] More specifically, aluminum is used for current collectors common to positive and negative electrodes of titanate Li4Ti5O12. Copper is rather for negative electrodes of graphite (Cgr), silicon (Si) or silicon composite (Si-C).

[0018] Traditionally, a Li-ion battery or accumulator uses a pair of materials at the anode and cathode allowing it to operate at a voltage level, typically between 1.5 and 4.2 Volts.

[0019] A lithium-ion battery or accumulator can obviously include a plurality of electrochemical cells.

[0020] Depending on the type of application targeted, the aim is to produce either a thin and flexible lithium-ion accumulator or a rigid accumulator: the packaging is then either flexible or rigid and in the latter case constitutes a sort of case.

[0021] Flexible packaging is usually manufactured from a multi-layer composite material, consisting of a stack of aluminum layers covered by one or more polymer films laminated by bonding.

[0022] Rigid packaging is used when the intended applications are restrictive where a long service life is required, for example with much higher pressures to be supported and a stricter required level of sealing, typically less than 10 8mbar.l / s, or in environments with high constraints such as the aeronautical or space sector.

[0023] Rigid packaging (cases) is usually made from a metallic material, typically an aluminum alloy or stainless steel or a rigid polymer such as acrylonitrile butadiene styrene (ABS).

[0024] The geometry of most rigid Li-ion battery packaging cases is cylindrical, because most electrochemical cells in batteries are wound by winding in a cylindrical geometry around a cylindrical mandrel. Prismatic shapes of cases have also already been produced by winding around a prismatic mandrel.

[0025] One of the types of rigid cylindrical shaped case, usually manufactured for a high capacity Li-ion accumulator, is illustrated in [Fig.3].

[0026] [Fig. 4] shows a longitudinal sectional view of such a housing 6 of axisymmetric geometry around the central axis 10 and housing an electrochemical bundle F of elongated shape and comprising a single electrochemical cell consisting of an anode 3 and a cathode 4 on either side of a separator 1 adapted to be impregnated with the electrolyte. [Fig. 5] shows the bundle F obtained, usually by winding around a central winding axis 10 inside the cylindrical housing 6.

[0027] A rigid prismatic shaped case is also shown in [Fig.5].

[0028] The housing 6 comprises a cylindrical lateral casing 7, a base 8 with a end, a cover 9 at the other end, the bottom 8 and the cover 9 being assembled to the casing 7. The cover 9 supports the current output poles or terminals 4, 5. One of the output terminals (poles), for example the negative terminal 5 is welded to the cover 9 while the other output terminal, for example the positive terminal 4, passes through the cover 9 with the interposition of a seal not shown which electrically insulates the positive terminal 4 from the cover.

[0029] The widely manufactured rigid housing type also consists of a stamped cup and a cover, welded together on their periphery. On the other hand, current collectors include a feedthrough with a part protruding from the top of the case and which forms a terminal also called the visible pole of the battery.

[0030] [Fig.6] illustrates in sectional view the interior of a rigid casing accumulator 6 once finalized, with the different alternating layers of anode 2, separator 1 impregnated with electrolyte, cathode 3 constituting the electrochemical bundle F from the central winding axis 10 to its external lateral ends delimited by the casing 6.

[0031] A battery pack P is made up of a variable number of accumulators which can reach several thousand which are electrically connected in series or in parallel with each other and generally by connection bars, usually called busbars.

[0032] An example of a battery pack P is shown in [Fig.7]. This pack consists of two modules M1, M2 of identical Li-ion accumulators A connected together in series, each module M1, M2 consisting of four rows of accumulators connected in parallel, each row consisting of a number equal to six of Li-ion accumulators in series.

[0033] As shown, the mechanical and electrical connection between two Li-ion accumulators of the same row is made by screwing busbars B1, advantageously made of copper, each connecting a positive terminal 4 to a negative terminal 5. The connection between two rows of accumulators in parallel within the same module M1 or M2 is ensured by a busbar B2, also advantageously made of copper. The connection between the two modules M1, M2 is ensured by a busbar B3, also advantageously made of copper.

[0034] In the development and manufacture of lithium-ion batteries, for each profile / new demand, regardless of the market players, this requires precise sizing (series / parallel electrical architectures, mechanical, thermal, etc.) to optimally design a high-performance and safe battery pack.

[0035] In particular, the safety of lithium-ion accumulators must be taken into consideration both at the scale of a single accumulator, a module and a battery pack.

[0036] Different passive or active devices having a safety function can also be integrated at the level of a cell (accumulator), and / or a module and / or the battery pack to prevent problems, when the battery finds itself in so-called abusive operating conditions or in the event of a fault at the level of a cell.

[0037] A lithium electrochemical system, whether at the cell (accumulator), module or pack scale, produces exothermic reactions regardless of the given cycling profile. Thus, at the scale of a unit accumulator, in Depending on the chemistries considered, the optimal operation of lithium ion accumulators is limited within a certain temperature range.

[0038] An electrochemical accumulator must operate within a defined temperature range, typically generally less than 70°C at its outer casing surface, otherwise its performance will be degraded, or even physically degraded to the point of destruction.

[0039] Examples include lithium iron-phosphate accumulators which have an operating range generally between -20°C and +60°C. Above 60°C, the materials and the electrolyte may undergo significant degradation, reducing the performance of the cell. Above a so-called thermal runaway temperature which may be between 70°C and 110°C, exothermic internal chemical reactions begin. When the accumulator is no longer able to dissipate sufficient heat, the temperature of the cell increases until it is destroyed, this phenomenon being usually referred to as thermal runaway.

[0040] In other words, thermal runaway occurs in a cell (accumulator) when the energy released by the exothermic reactions occurring inside it exceeds the capacity to dissipate it to the outside. This runaway may be followed by the generation of gas and explosion and / or fire. For this thermal runaway phenomenon, please refer to publication [1] and the protocol described in this publication. The so-called "self-heating" and "thermal runaway" temperatures are respectively denoted T1 and T2 in this publication.

[0041] The temperature Tl, typically 70°C, in [Fig.2] of the publication, is the temperature from which the accumulator heats up without an external source at a typical rate of 0.02°C / min in adiabatic conditions.

[0042] The temperature T2, typically 150°C, in [Fig.2] of the publication, is the temperature from which the accumulator heats up at a typical heating rate of 10°C / min under adiabatic conditions, which leads to the melting of the separator in the electrochemical beam of the accumulator, to a short circuit and therefore to the collapse of the voltage.

[0043] By "thermal runaway", we can thus understand here and within the framework of the invention, a ratio between the value of the derivative of the heating temperature and that of the time at least equal to 0.02°C per min.

[0044] Also, maintaining a temperature below 70°C makes it possible to increase the lifespan of an accumulator, because the higher the operating temperature of an accumulator, the more its lifespan will be reduced.

[0045] Additionally, some battery chemistries require an operating temperature of operation well above ambient temperature and therefore it is necessary to regulate their temperature level by initial preheating of the accumulators, or even by maintaining the accumulators at permanent temperature.

[0046] At the level of a cell (accumulator), the various known internal protection devices are:

[0047] - a positive temperature coefficient device (PTC anglo-Saxon acronym for "Polymeric positive temperature coefficient"): currently, a large number of cylindrical accumulators already on the market are equipped with this. Such a device is in the form of a polymer ring (polyethylene) laminated with a metal. In the event of an overcharge, this polymer heats up, changes phase, becomes more resistive and thus limits the flow of current;

[0048] - a current interruption device (CID): it interrupts the current if the gas pressure in the cell exceeds the specified limits;

[0049] - a circuit breaker device (in English "shutdown") which prevents the generation of high currents;

[0050] - a vent consisting of a valve or rupture disc, which opens when the internal pressure increases suddenly, and exceeds a determined critical pressure, in order to avoid the cell exploding;

[0051] - a thermal fuse, currently implemented in large-capacity accumulators capacity, which cuts off the current when the temperature in the accumulator is too high.

[0052] These protective devices, also known as cell safety devices (accumulators), play a vital role in mitigating the effects linked to their thermal runaway.

[0053] Furthermore, battery manufacturers aim to constantly increase the energy of their batteries in order to improve their performance. Thus, the use of materials with high energy capacity but with highly exothermic behavior in the event of thermal runaway of an accumulator is increasingly common: [2].

[0054] In this context, there is a need to further improve safety solutions for electrochemical accumulators, in particular metal-ion accumulators and more particularly those with highly exothermic behavior in the event of thermal runaway.

[0055] The aim of the invention is to meet at least part of this need. Statement of the invention

[0056] To this end, the invention relates, in one of its aspects, to a metal-ion electrochemical accumulator comprising at least one electrochemical cell consisting of a cathode, an anode and a separator impregnated with a liquid electrolyte arranged between the cathode and the anode, the cell(s) defining an electrochemical beam of elongated shape along a longitudinal axis, the separator having at least one zone, arranged at at least one of the outer lateral ends of the electrochemical beam, the melting temperature of which is lower than the rest of the separator.

[0057] According to a first embodiment, the zone of the separator with a melting temperature lower than the rest of the separator extends over at least part of the height of the electrochemical bundle between the portion of cathode(s) and the portion of anode(s) delimiting the outer lateral end of the electrochemical bundle.

[0058] According to a second embodiment, the zone of the separator with a melting temperature lower than the rest of the separator extends over at least part of the height of the electrochemical bundle between the portion of cathode(s) or anode(s) and an additional electrode delimiting the outer lateral end of the electrochemical bundle, the additional electrode having an electrical resistance greater than the initial internal resistance of the electrochemical bundle.

[0059] By "initial internal resistance" is meant here and within the framework of the invention, the sum of the electrical resistance of the solid materials (electrodes, connections) and the resistance of the liquid electrolyte once the accumulator is finalized and before its first operation.

[0060] Preferably the electrical resistance of the additional electrode being at least 0.5Ω higher than the initial internal resistance of the electrochemical bundle.

[0061] More preferably, the additional electrode has a thermal resistance at least equal to 30W / mK.

[0062] According to an advantageous embodiment, the additional electrode comprises a mixture of electrical charges and a binder and / or a metal film which has preferably undergone a surface treatment. As charges, carbon particles or fibers, or ground carbon fibers, or metal fibers may be considered. As a binder, polyvinylidene fluoride (PVDF) may be considered. A charge of copper powder may be considered, for example in polyethylene or a copper foil having a passivation layer such as chromium oxide.

[0063] Advantageously, the melting temperature of the zone is at least 20% lower than that of the rest of the separator.

[0064] According to a first advantageous construction, the zone of the separator is made of polypropylene while the rest of the separator is made of polypropylene coated with a coating with a melting temperature higher than that of polypropylene.

[0065] According to a second advantageous constitution, the zone of the separator is made of polyethylene while the rest of the separator is made of polypropylene.

[0066] More generally, the intrinsic properties of the separator can be modified, generally in the form of a polymer and / or create a localized defect in the separator on one / and / or the other lateral end of the electrochemical beam to create the zone(s) with a lower melting temperature.

[0067] As regards the modifications of the intrinsic properties, one or other of the following characteristics may be considered, possibly combined:

[0068] - presence or absence of a ceramic film applied to one or both sides of a film base polymer, except for its end(s);

[0069] - adaptation of the thicknesses of the ceramic film applied on each side of the film base polymer;

[0070] - local modification of the composition of the different constituents of the separator, to know the types of polymer, the binder used for the application of a ceramic and the ceramic itself;

[0071] - adaptation of the ratio between the ceramic and the binder according to the final position considered in the electrochemical beam.

[0072] With regard to the addition of a local defect in the separator, it may be envisaged to locally remove the ceramic film at the outer lateral end of the separator. This removal may comprise local scraping of the ceramic film and / or local chemical treatment and / or local heat treatment.

[0073] According to a first advantageous configuration, the electrochemical bundle consists of a single electrochemical cell, obtained by winding, the zone of the separator with a melting temperature lower than the rest of the separator being that located at the outer end of the winding.

[0074] According to a second advantageous configuration, the electrochemical bundle comprises a stack of elementary electrochemical cells each comprising a cathode, an anode, and a separator impregnated with electrolyte interposed between the anode and the cathode, the separator zone with a melting temperature lower than the rest of the separators being constituted by at least one of the two separators at the outer end of the stack.

[0075] The electrochemical bundle according to the invention can be produced using a so-called Z-fold technique. In such a process, the addition of the separator zone with a lower melting temperature requires stopping the folding itself before the last electrode layer in order to add said lower-temperature melting zone to the lateral ends of the stack of electrochemical cells.

[0076] According to a preferred embodiment of the accumulator, the accumulator comprises:

[0077] - a housing arranged to contain the electrochemical bundle in a sealed manner,

[0078] - two current output terminals each connected to one or the other of the anode(s) and the cathode(s), one of the terminals called the first terminal passing through a wall of the housing and being electrically insulated from it,

[0079] The accumulator according to the invention may be of generally cylindrical or prismatic shape.

[0080] Each accumulator may be a Li-ion accumulator in which:

[0081] - the negative electrode(s) material is chosen from the group comprising the graphite, lithium, titanate oxide Li4TiO5Oi2; of positive electrode(s) material is chosen from the group of intercalation / insertion compounds of the LiM02 type with M representing Co, Ni or Mn; LiM'2O4 with M' representing Ni or Mn; LiM”PO4 with M” representing Fe, Co, Mn or Ni.

[0082] Thus, the invention essentially consists of integrating at least one of the lateral ends of an accumulator one or more separator zones at a lower melting temperature than the rest of the separator, i.e. one or more zones which will be less thermally stable.

[0083] The inventors have identified that the critical point of an accumulator during thermal runaway is the cathode destabilization temperature, typically between 170 and 230°C depending on the chemistry of the insertion material(s).

[0084] Thus, they considered one or more separator zones at the ends of the electrochemical beam with the melting temperature lower than that of the cathode destabilization temperature while that of the rest of the separator is close to the cathode destabilization temperature.

[0085] This or these less thermally stable zones will promote the creation of short circuits following thermal runaway of an adjacent accumulator within a module or battery pack.

[0086] Due to its (their) location at one and / or the other of the lateral ends of the accumulator, they are close to the usual cooling systems, in particular cooling plates, accumulators and / or a battery module or a battery pack with several accumulators, which thus allows for high heat dissipation at these locations.

[0087] Thus, thanks to the invention, the core of the electrochemical bundle of an accumulator is preserved and therefore the effects of thermal runaway originating from an adjacent accumulator are mitigated.

[0088] Indeed, within a battery module or a battery pack with several accumulators, during thermal runaway of an accumulator, the flames and the heat released by the latter, which is therefore defective, contribute to the increase in the temperature of the adjacent accumulators, which leads to a risk of thermal runaway in turn of at least one of these adjacent accumulators.

[0089] For these adjacent accumulators, the temperature begins to increase on the outer layers before diffusing towards the core of the electrochemical bundle.

[0090] A battery module or battery pack with several accumulators conforming to The invention can withstand flames for several tens of seconds, which gives time to the cooling systems, in particular cooling plates, the cooling plates to resume their role as heat sinks.

[0091] In the case of a lithium-ion accumulator, another advantage linked to the short-circuiting on one and / or the other of the lateral ends of the electrochemical bundle, induced by the fusible separator zone(s) at a lower temperature compared to the rest of the separator, is to allow at least part of the positive electrode to be relithified and therefore to be thermally stabilized. More generally in the case of a metal-ion accumulator, the short-circuiting allows the reinsertion of metal ions into an electrode of the electrochemical bundle.

[0092] The inventors went against existing solutions for the design of metal-ion accumulators which have always considered that a melting of the separator marks a point of no return, because the short circuit generated renders the accumulator unusable and causes a sudden rise in temperature which will trigger the chain reactions of thermal runaway.

[0093] Other advantages and characteristics of the invention will become more apparent upon reading the detailed description of examples of implementation of the invention given by way of illustration and not limitation with reference to the following figures. Brief description of the drawings

[0094] [Fig.l] [Fig.l] is an exploded perspective schematic view showing the various elements of a lithium-ion accumulator.

[0095] [Fig.2] [Fig.2] is a front view showing a lithium-ion battery with its flexible packaging according to the state of the art.

[0096] [Fig.3] [Fig.3] is a perspective view of a lithium-ion accumulator according to state of the art with its rigid packaging consisting of a cylindrical shaped box.

[0097] [Fig.4] [Fig.4] is a longitudinal and partially unrolled sectional view of a lithium-ion accumulator according to the state of the art, showing the electrochemical bundle consisting of a single electrochemical cell wound on itself by winding according to a cylindrical geometry inside the casing.

[0098] [Fig.5] [Fig.5] is a perspective view of a lithium-ion accumulator according to state of the art with its rigid packaging consisting of a prismatic shaped box.

[0099] [Fig.6] [Fig.6] is a schematic view in partial longitudinal section of a battery lithium-ion emulator according to [Fig.4] or 5.

[0100] [Fig.7] [Fig.7] is a perspective view of an assembly using busbars of state-of-the-art lithium-ion accumulators, forming a battery pack is a photographic reproduction of a thermal runaway trigger device of a standard 18650 format Li-ion accumulator, according to the invention.

[0101] [Fig.8] [Fig.8] illustrates in the form of curves the decrease in the voltage of an accumulator with a separator A and with a separator B of different composition and construction.

[0102] [Fig.9] [Fig.9] is a longitudinal and partially unrolled sectional view of a lithium-ion accumulator according to the invention, showing the electrochemical bundle consisting of a single electrochemical cell wound on itself by winding according to a cylindrical geometry inside the casing.

[0103] [Fig. 10] [Fig. 10] is a schematic view in partial longitudinal section of a lithium-ion accumulator according to a first embodiment of the invention.

[0104] [Fig. 11] [Fig. 11] is a schematic view in partial longitudinal section of a lithium-ion accumulator according to a second embodiment of the invention. Detailed description

[0105] Figures 1 to 7 relate to different examples of Li-ion accumulator, flexible packaging and accumulator case as well as a battery pack according to the state of the art. These figures 1 to 7 have already been commented on in the preamble and are therefore not further commented on below.

[0106] For the sake of clarity, the same references designating the same elements according to the state of the art and according to the invention are used for all figures 1 to 11.

[0107] [Fig. 8] illustrates the difference in thermal stability between two separators, namely separator A and separator B, of different composition and application method. Thus, it emerges from this [Fig. 8] that the accumulator voltage drops drastically for separator B as soon as it reaches a temperature of the order of 140°C while for separator A the drop occurs rather at a temperature above 170°C.

[0108] After having identified that the critical point of a Li-ion accumulator during thermal runaway is the cathode destabilization temperature, typically between 170 and 230°C depending on the chemistry of the insertion material(s), the inventors thought of taking advantage of the variability of the thermal stability between two different separators to improve the safety of accumulators within a battery module or battery pack during the overheating of at least one of them.

[0109] [Fig.9] shows an example of a Li-ion accumulator A, according to the invention. In this illustrated example, the accumulator A has a cylindrical format case, typically of 18650 format.

[0110] The separator 1 of the electrochemical beam F obtained by winding comprises two portions 11, 12 distinct in their chemical composition but which are both impregnated with the same liquid electrolyte.

[0111] The separator portion 11, which once the electrochemical beam has been formed is arranged at the heart of the latter, is made of a material whose melting temperature is close to the thermal destabilization temperature of the cathode 3, for example of the order of 200°C.

[0112] The separator portion 12, which once the electrochemical beam is arranged at its lateral end, that is to say between the cathode layer 3 and the anode layer 2 furthest from the central winding axis 10, is made of a material whose melting temperature is lower, preferably at least 20°C, than that of the portion IL

[0113] For example, the separator material of the inner portion 11 is made of polypropylene coated with a ceramic coating with a melting temperature higher than that of polypropylene and the outer portion 12 is made of polypropylene only.

[0114] [Fig. 10] illustrates in longitudinal section view of an electrochemical beam F according to the invention with the different layers of electrodes 2, 3 arranged on either side, at the heart of the bundle of a portion of separator 11 made of a material which fuses at a temperature close to the thermal destabilization temperature of cathode 3 and at each lateral end of the bundle, of a portion of separator 12 made of a material which fuses at a lower temperature, preferably at least 20°C lower than that of the portion IL

[0115] [Fig. 11] shows another embodiment in which the portion 12 of the se separator with a melting temperature lower than the portion 12 of the separator 1 is interposed between the portion of external cathode(s) 3 and an additional electrode 100 which delimits the external lateral end of the electrochemical beam F.

[0116] This additional electrode 100 has an electrical resistance greater than the initial internal resistance of the electrochemical bundle.

[0117] Thus, this additional electrode 100 makes it possible to significantly increase the internal resistance of the accumulator once the separator portion 12 has melted. This additional electrical resistance electrode 100 can be produced using the same technique as the anodes 2 and cathodes 3 of the electrochemical bundle F, in particular by application, for example by coating on a metal strip which has, where appropriate, undergone heat treatment.

[0118] The increase in the electrical resistance induced by this additional electrode 100 makes it possible to reduce the short-circuit resistance and therefore to limit the thermal power emitted which would be due to short circuits caused by an increase in temperature.

[0119] This additional electrode 100 may be constituted by a mixture of electrically conductive fillers such as carbon particles or carbon fibers, or ground carbon fibers, or metal fibers and a binder such as PVDF.

[0120] In this mode with additional electrode 100, care is taken to adapt its electrical resistance to the cooling power of the battery module or battery pack in which an accumulator according to the invention is installed.

[0121] Indeed, the short-circuit power Pcc to be dissipated by a cooling plate of an accumulator can be expressed by equation 1:

[0122] [Equation 1]: pcc = = R^U / (Rcc+Rin{)) 2

[0123] in which

[0124] Rint denotes the internal resistance of the electrochemical beam F,

[0125] Rcc denotes the resistance of the additional resistive electrode 100,

[0126] Icc denotes the value of the short-circuit current,

[0127] U the voltage of the electrochemical beam during the short circuit.

[0128] It goes without saying that whatever the arrangement of the separator zone with lower melting temperature, care is taken to electrically connect either the outer electrode layer of the electrochemical bundle or the additional electrode with increased electrical resistance with the rest of the electrodes of the bundle.

[0129] The invention is not limited to the examples which have just been described; it is possible in particular to combine characteristics of the examples illustrated within non-illustrated variants.

[0130] Other variants and improvements may be envisaged without departing from the scope of the invention.

[0131] If in the examples illustrated, the electrode of the electrochemical beam is positive (cathode), the invention also applies with a negative electrode (anode).

[0132] Although the illustrated examples relate to boxed accumulators, the invention applies to flexible packaging accumulators. List of cited references:

[0133] [1] Xuning Feng, et al. « Key Characteristics for Thermal Runaway of Li-ion Batteries » Energy Procedia, 158 (2019) 4684-4689.

[0134] [2] Xuning Feng, et al. “Thermal runaway mechanism of lithium-ion battery for electric vehicles: A review» Energy Storage Materials, Volume 10, January 2018, Pages 246-267.

Claims

Claims

1. Metal-ion electrochemical accumulator (A) comprising at least one electrochemical cell consisting of a cathode (2), an anode (3) and a separator (1) impregnated with a liquid electrolyte arranged between the cathode (2) and the anode (3), the cell(s) defining an electrochemical bundle (F) of elongated shape along a longitudinal axis (X), the separator having at least one zone (12), arranged at at least one of the outer lateral ends of the electrochemical bundle (F), the melting temperature of which is lower than the rest (12) of the separator,the zone (12) of the separator with a melting temperature lower than the rest (12) of the separator extending over at least part of the height of the electrochemical bundle between the portion of cathode(s) and the portion of anode(s) delimiting the outer lateral end of the electrochemical bundle or between the portion of cathode(s) or anode(s) and an additional electrode delimiting the outer lateral end of the electrochemical bundle, the additional electrode having an electrical resistance greater than the initial internal resistance of the electrochemical bundle.,

2. Accumulator according to claim 1, the electrical resistance of the additional electrode being at least 0.5 Q higher than the initial internal resistance of the electrochemical bundle.

3. Accumulator according to claim 1 or 2, the additional electrode having a thermal resistance at least equal to 30W / mK.

4. Accumulator according to one of claims 1 to 3, the additional electrode comprising a mixture of electrical charges and a binder and / or a metal film which has preferably undergone a surface treatment.

5. Accumulator according to one of the preceding claims, the melting temperature of the zone being at least 20% lower than that of the rest of the separator.

6. Accumulator according to one of the preceding claims, the separator area being made of polypropylene while the rest of the separator is made of polypropylene coated with a coating with a melting temperature higher than that of polypropylene.

7. Accumulator according to one of claims 1 to 5, the separator area being made of polyethylene while the rest of the separator is made of polypropylene.

8. Accumulator according to one of the preceding claims, the beam electrochemical (5) consisting of a single electrochemical cell, obtained by winding, the zone of the separator with a melting temperature lower than the rest of the separator being that located at the outer end of the winding.

9. Accumulator according to one of claims 1 to 8, the electrochemical bundle comprising a stack of elementary electrochemical cells each comprising a cathode (2), an anode (3), and a separator (1) impregnated with electrolyte interposed between the anode and the cathode, the separator zone with a melting temperature lower than the rest of the separators being constituted by at least one of the two separators at the outer end of the stack.

10. Accumulator according to one of the preceding claims, comprising: - a housing (6) arranged to contain the electrochemical bundle in a sealed manner, - two current output terminals (4, 5) each connected to one or other of the anode(s) and the cathode(s), one of the terminals, called the first terminal (2), passing through a wall of the housing (6) and being electrically insulated therefrom,

11. Accumulator according to one of the preceding claims, the accumulator being of generally cylindrical or prismatic shape.

12. Accumulator according to one of the preceding claims, constituting a Li-ion accumulator in which: - the anode material(s) is chosen from the group comprising graphite, lithium, titanate oxide Li4TiO5Oi2; the cathode material(s) is chosen from the group of intercalation / insertion compounds of the LiM02 type with M representing Co, Ni or Mn; LiM'2O4 with M' representing Ni or Mn; LiM”PO4 with M” representing Fe, Co, Mn or Ni.

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