Energy storage device provided with an improved fire barrier assembly

Abstract

There is described an electrical energy storage device (1) comprising a housing (2), a plurality of electrical energy storage cells (3) within the housing (2), each energy storage cell (3) including electrical poles (4) and a safety venting port (5), and an insulator assembly (6) arranged within the housing (2) between the energy storage cells (3) and an internal wall (2a) of the housing (2), the insulator assembly (6) comprises: a covering mat (7) resting onto the energy storage cells (3); a support member (8) resting onto the electrical poles (4) of each energy storage cell (3) so as to cover the electrical poles (4), and maintaining the covering mat (7) in abutment against at least at an area (5a) of each energy storage cell (3) around the respective venting port (5); the covering mat (7) includes a plurality of slits (10), each slit (10) being arranged over one respective venting port (5) and being fluidically connectable with the respective venting port (5).

Description

[0001] ENERGY STORAGE DEVICE PROVIDED WITH AN IMPROVED FIRE

[0002] BARRIER ASSEMBLY

[0003] TECHNICAL FIELD

[0004] The present invention relates to an energy storage device , in particular a battery pack having a housing and a plurality of battery cells enclosed in the housing and preferably designed for the use in a full-electric or hybrid vehicle .

[0005] BACKGROUND ART

[0006] Battery cells are known, which are used as electrical energy accumulators .

[0007] In particular, so-called primary batteries are known, which cannot be easily recharged and are therefore destined to single-use , and so-called secondary batteries are known, which can be repeatedly charged after each cycle of usage .

[0008] Single-cell batteries of the secondary type are known which are used for various portable small-si zed electronic devices , such as cellular phones , watches , smartwatches , smart bands , etc .

[0009] Also , battery packs which include a plurality o f battery cells connected in series or in parallel are known, for example as used in the automotive industry .

[0010] The present invention will refer to a battery pack for the use in a hybrid or full-electric vehicle , for example an electric car, without however loss of generality .

[0011] A single battery cell typically comprises a hollow casing, for example in the shape of a hollow parallelepiped and usually made of metallic material , generally open on at least one side ( for example on the top ) and a lid for closing the casing .

[0012] The casing internally defines an inner compartment of the cell and houses the cell electrodes ( cathode and anode ) , which are provided for example in the form of sheets , and a separator material electrically interposed between the electrodes .

[0013] Within the compartment , the electrodes and the separator are immersed in an electrolytic material , usually liquid or gelatinous .

[0014] The lid is coupled to the housing at an opening thereof for closing the inner compartment in a fluid- tight manner, thereby maintaining electrodes and separator in place and preventing the outflow of the electrolytic material .

[0015] Typically, the lid carries the electrical poles , i . e . the positive and negative terminals , of the respective battery cells .

[0016] A typical battery pack essentially comprises a housing and a plurality of such battery cells enclosed within the housing .

[0017] As it is known, the unlikely event of a so called "thermal runway" may occur for one or more battery cells of the battery pack .

[0018] During such a thermal runway event , gas developed within the battery cell may abruptly outflow from the casing thereof .

[0019] Furthermore , a so called "cold degassing" event may occur for one or more battery cells of the battery pack . During such cold degassing, high pressuri zed gases stream out of the battery cell , in order to achieve a pressure equali zation between the inner compartment of the cell and the housing .

[0020] In order to avoid a violent outflow of such gases during the thermal runway event or the cold degassing event , which may cause an explosion, each battery cell is provided with a safety venting port usually obtained at the lid thereof .

[0021] The venting port is defined by a through hole in the lid which is sealed, during normal operation of the battery cell , by a thin membrane, such as an aluminum f oil .

[0022] The membrane defines a rupturable element which during a thermal runway event or a cold degassing event ruptures thereby allowing the gas to outflow from the casing of the battery cell .

[0023] In some cases , the thermal runway event may lead to a development of a fire within the battery pack housing .

[0024] Therefore , it must be ensured that both the flames and the gas are at least temporarily contained within the housing, for example for a reasonable amount of time necessary for the fire rescue reaching the battery pack or for a user to exit from the vehicle .

[0025] To this end, a typical battery pack is provided with an insulator member, for example a fire barrier element interposed between the lids of the battery cells and the housing .

[0026] A thermal runway event may also occur without a development of a fire ( cold degassing event ) , and go unnoticed .

[0027] During such type of thermal runway event ( cold degassing event ) , the gas outflowing from a battery cell through its venting port may damage the adj acent battery cells of the battery pack even without an ignition, or may even lead to a thermal runway chain ef fect i f it reaches the other venting ports . Moreover, such gas usually contains electrically charged and / or conductive particles which may reach the electrical poles ( terminals ) of the other battery cells , thereby causing short circuits and disrupting the correct functioning of the battery pack .

[0028] DISCLOSURE OF INVENTION

[0029] It is therefore an obj ect of the present invention to provide an energy storage device which is designed to overcome at least one of the above-mentioned drawbacks in a straightforward and low-cost manner .

[0030] This obj ect is achieved by an energy storage device as claimed in the appended independent claim 1 . Preferred embodiments of the present invention are laid down in the appended dependent claims .

[0031] BRIEF DESCRIPTION OF THE DRAWINGS

[0032] A non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings , in which :

[0033] Figure 1 is an exploded perspective view, with parts removed for clarity, of a battery pack according to the present invention;

[0034] Figure 2 is a larger-scale transversal section, with part removed for clarity, of the battery pack of Figure 1 in an assembled configuration;

[0035] Figure 3 is a larger-scale transversal section, with part removed for clarity, of an alternative embodiment of the battery pack according to the present invention, in an assembled configuration thereof ; Figure 4 is a larger-scale longitudinal section, with parts removed for clarity, of the battery pack of Figure 1 in an assembled configuration;

[0036] Figure 5 is a sectioned view of a detail of the battery pack according to an alternative embodiment of the present invention; and

[0037] Figure 6 is a sectioned view of a detail of the battery pack according to a further alternative embodiment of the present invention .

[0038] BEST MODE FOR CARRYING OUT THE INVENTION

[0039] With reference to Figure 1 , number 1 indicates as a whole an electrical energy storage device , in particular a battery pack, to which the present description will refer without loss of generality .

[0040] Battery pack 1 includes a housing ( or outer casing) 2 and a plurality of energy storage cells 3 , in particular battery cells , arranged within housing 2 , and in particular enclosed in housing 2 .

[0041] Without loss of generality, battery pack 1 is preferably destined for the use in a vehicle , in particular a full-electric or hybrid vehicle , such as a car, a van, or the like .

[0042] In Figure 1 only a portion of an upper wall o f housing 2 is shown .

[0043] Each battery cell 3 is of the known type and includes a casing which internally defines a compartment containing electrodes ( cathode and anode ) and a separator . The electrodes and the separator are immersed in an electrolytic solution .

[0044] In the preferred embodiment shown, each battery cells 3 is a prismatic battery cell .

[0045] Each battery cell 3 includes two electrical poles 4 , i . e . the positive and negative terminals .

[0046] In particular, each battery cell 3 has a lid 3a coupled to the respective casing at an opening thereof for closing the inner compartment in a fluid-tight manner, thereby maintaining electrodes and separator in place and preventing the outflow of the electrolytic material .

[0047] More in particular, electrical poles 4 are carried by lid 3a .

[0048] Each battery cell 3 further includes a safety venting port 5 , which is defined by a through hole obtained in lid 3a and is closed and sealed, during normal operation of the battery cell 3 , by a thin membrane , such as an aluminum foil .

[0049] The membrane defines a rupturable element which during a so-called thermal runway event ruptures thereby allowing the gas to outflow from the casing of battery cell 3 .

[0050] Each venting port 5 has a central axis X ( only one axis X shown in Figure 1 and 4 ) .

[0051] In a preferred embodiment , each venting port 5 has a substantially oval or elliptical shape .

[0052] However, venting port 5 may have a rectangular, circular or otherwise polygonal shape or elongated shape .

[0053] According to the non-limiting embodiment shown, electrical poles 4 are arranged at opposite longitudinal ends of lid 3a, and venting port 5 is arranged on lid 3a in between the electrical poles 4 .

[0054] Hence , each battery cell 3 has a lid 3a carrying the respective electrical poles 4 and on which the respective venting port 5 is obtained .

[0055] Lid 3a is fixedly coupled to the casing of the battery cell 3 , and preferably is welded thereto .

[0056] In light of the above , each battery cell 3 has an operative wall , which in this embodiment is defined by lid 3a . Such operative wall ( i . e . lid 3a ) carries the respective electrical poles 4 . Moreover, the respective venting port 5 is obtained on such operative wall .

[0057] As visible in Figure 1 , battery pack 1 further includes an insulator assembly 6 arranged within housing 2 between battery cells 3 and an internal wall 2a of the housing 2 itsel f .

[0058] More precisely, insulator assembly 6 is arranged between lids 3a and internal wall 2a . Preferably, insulator assembly 6 is configured to thermally insulate battery cells 3 from an outside environment .

[0059] Preferably, insulator assembly 6 also defines a fire barrier between battery cells 3 and the outside environment .

[0060] According to a first aspect of the present invention, insulator assembly 6 comprises :

[0061] - a covering mat 7 resting onto battery cells 3 ;

[0062] - a support member 8 resting onto the electrical poles 4 of each battery cell 3 so as to cover electrical poles 4 , and maintaining covering mat 7 in abutment against at least at an area 5a of each battery cell 3 around the respective venting port 5 .

[0063] In detail , support member 8 cooperates in contact with covering mat 7 for maintaining covering mat 7 in abutment against areas 5a .

[0064] In greater detail , support member 8 is mechanically coupled to covering mat 7 for maintaining covering mat 7 in abutment against areas 5a .

[0065] It is speci fied that each area 5a is an area of the respective lid 3a extending around, i . e . surrounding, the respective venting port 5 , as shown in Figure 1 .

[0066] Covering mat 7 has a first surface 7a facing lids 3a and a second surface 7b opposite the first surface 7a and facing internal wall 2a .

[0067] In detail , covering mat 7 rests onto lids 3a with first surface 7a .

[0068] In greater detail , covering mat 7 rests onto lids 3a so that first surface 7a contacts at least areas 5a .

[0069] Preferably, first surface 7a contacts the whole surface of lids 3a and not only areas 5a, with the exception of electrical poles 4 , as better explained below .

[0070] As visible in Figures 1 to 3 , support member 8 directly rests onto electrical poles 4 and covers electrical poles 4 , and in particular an electrically conductive surface 4a of electrical poles 4 .

[0071] According to an important aspect of the present invention, covering mat 7 includes a plurality of slits 10 , each slit 10 being arranged over one respective venting port 5 and being fluidically connectable with the respective venting port 5 .

[0072] In particular, covering mat 7 is positioned, i . e . it is maintained by support member 8 in a position, so that each slit 10 is arranged over one respective venting port 5 .

[0073] More precisely, each slit 10 extends through at least part of the thickness of covering mat 7 and defines a flow channel 11 for guiding a gas outflowing from the respective venting port 5 towards internal wall 2a and away from the other venting ports 5.

[0074] In use, each slit 10 is fluidically connected with the respective venting port 5 by means of the rupture (breakage) of the relative membrane due to the gas bursting out of the venting port 5 itself.

[0075] Preferably, covering mat 7 has a substantially flat and planar configuration.

[0076] More specifically, covering mat 7 has a sheet-like or plate-like configuration.

[0077] It is specified that with the term "slit" it is intended, in the present description and in the appended claims, a long narrow cut or opening.

[0078] Hence, according to one preferred embodiment each slit 10 is defined by an opening obtained through at least part of the thickness of covering mat 7, i.e. by a region of covering mat 7 with a removed portion of material.

[0079] According to an alternative embodiment, each slit 10 may be defined by a cut through at least part of the thickness of covering mat 7, delimited by two lateral walls that, in nominal operative conditions, are in contact with one another and that during venting of gases from the respective venting port 5 separate from one another (i.e. are deformed by the venting of the gases) for defining the aforementioned flow channel 11. Preferably, the cut is obtained through the entire thickness of covering mat 7 , thereby defining a local weakening of the material thereof . The aforementioned lateral walls touch one another in nominal operative conditions and define a small clearance / distance due to a bending and / or deformation of the material due to venting of gases .

[0080] In the preferred embodiment shown, slits 10 are defined by linear cuts or openings extending transversally, and in particular orthogonally, with respect to a longitudinal extension of covering mat 7 .

[0081] In an alternative embodiment , each slit 10 may have an X-shape or a cross shape , it may have a curved shape , such as an S-shape .

[0082] In another embodiment , each slit 10 may be discontinuous along the transversal direction .

[0083] In another embodiment , each slit 10 may be defined by a C-shaped cut . In this case , the Applicant has observed that each slit 10 may act as a sort of door which rotates about a hinge portion while opening under the action of the gases bursting out from the relative venting port 5 . This configuration ensures the sealing of the flow channel 11 during nominal operation of battery pack 1 while allowing the passage of the gases during thermal runway or cold degassing . In light of the above , with the term " slit" 10 it is intended, in the present description and in the appended claims , an elongated cut or an elongated opening, As explained above .

[0084] According to a further aspect of the present invention, covering mat 7 seals in a fluid-tight manner each venting port 5 from the adj acent venting port 5 or venting ports 5 , and seals in a fluid-tight manner each venting port 5 from electrical poles 4 .

[0085] In practice , covering mat 7 seals in a fluid-tight manner the venting ports 5 from one another .

[0086] In detail , support member 8 presses covering mat 7 against battery cells 3 , in particular against l ids 3a, more in particular against at least areas 5a around venting ports 5 , so that covering mat 7 seals in a fluid- tight manner venting ports 5 from one another and from electric poles 4 .

[0087] In greater detail , covering mat 7 comprises sealing regions 12 extending around slits 10 which are respectively pressed in abutment against areas 5a by support member 8 , in such a way that each venting port 5 is sealed in a fluid-tight manner from the adj acent venting port 5 or venting ports 5 and from electrical poles 4 , with respect to a direction transversal to the respective central axis X . More speci fically, each sealing region 12 is arranged over one respective area 5a, and therefore over one respective venting port 5 , and is maintained in abutment against such area 5a by pressing action of support member 8 .

[0088] This configuration is visible in particular in Figure 4 , wherein it is clear how each venting port 5 is sealed from the adj acent venting ports 5 by a respective sealing region 12 , which is pressed against the respective area 5a ( i . e . on the respective lid 3a ) .

[0089] Thanks to the presence of covering mat 7 having slits 10 , and in particular thanks also to the fact that covering mat 7 seals the venting ports 5 from one another and from electrical poles 4 , it is possible to avoid a thermal runway chain ef fect amongst adj acent battery cells 3 .

[0090] In fact , in case a thermal runway event occurs in one of the battery cells 3 , whereby gases outburst from the respective venting port 5 , such gases are guided away from the other venting ports 5 and away from electrical poles 4 by means of the relative slit 10 .

[0091] Furthermore , the high-pressure gas bursting out of venting port 5 and the electrically charged particles that it may carry ( such as electrolytic material particles ) are inhibited to reach the other venting ports 5 and, most importantly, the electrical poles 4 , thanks to the sealing function of covering mat 7 , thereby preserving the electrical integrity of the poles 4 .

[0092] Hence , the li fespan of battery pack 1 is signi ficantly improved .

[0093] Advantageously, covering mat 7 i s made of silicone foam .

[0094] The Applicant has observed that such material is particularly advantageous , since s ilicone foam has a high flame resistance . Hence , even in the case that gases bursting out from a venting port 5 are very hot or catch fire , covering may 7 succeeds in ef fectively guiding them .

[0095] Moreover, a covering mat 7 made of silicone foam is elastomeric, which improves its aforementioned sealing function .

[0096] Furthermore , silicone foam is electrically non- conductive , thereby ef fectively acting as an electric insulator against electrically charged and / or conductive particles which may be emitted during a thermal runway event , and which may lead to short circuits i f coming into contact with electrical poles 4 .

[0097] Hence , preferably, covering mat 7 is made of a highly compressible , flame resistant and electrically non- conductive silicone foam .

[0098] Conveniently, covering mat 7 rests onto each lid 3a in such a way that the aforementioned electrically conductive surface 4a of each electrical pole 4 is uncovered from covering mat 7 .

[0099] In other words , covering mat 7 does not extend over electrical poles 4 , i . e . does not cover electrically conductive surface 4a of each electrical pole 4 .

[0100] In particular, electrical poles 4 are only covered by support member 8 .

[0101] This configuration is shown in Figures 2 and 3 .

[0102] Advantageously, support member 8 is made of a rigid and electrically insulating material .

[0103] For example , support member 8 may be made of rigid plastic material .

[0104] In this way, the protection of electrical poles 4 is further ensured and improved, while the function of sealing is demanded to covering mat 7 .

[0105] Moreover, a rigid support member 8 increases the stability in position of covering mat 7 .

[0106] In other words , insulator assembly 6 has the double function of :

[0107] - protection of electrical poles 4 , by means of the rigid and electrically insulating support member 8 covering the electrical poles 4 ; and of

[0108] - sealing, by means of the elastomeric covering mat 7 . Moreover, as it will be explained in detail hereinafter, support member 8 has also the function of protection for covering mat 7 .

[0109] Opportunely, support member 8 is coupled to covering mat 7 in such a way that slits 10 are uncovered by support member 8 .

[0110] In practice , support member 8 does not cover slits 10 .

[0111] In detail , according to a first preferred embodiment of support member 8 shown in Figure 2 , support member 8 completely covers the second surface 7b of covering mat 7 while leaving slits 10 uncovered .

[0112] In greater detail , according to this embodiment , support member 8 has openings 13 corresponding to slits 10 . That is , openings 13 are arranged over slits 10 , respectively .

[0113] In other words , openings 13 , slits 10 and venting ports 5 are respectively aligned .

[0114] More precisely, each opening 13 is aligned with a respective slit 10 and with a respective venting port 5 along the relative axis X .

[0115] This configuration is shown in Figure 4 .

[0116] Thanks to this configuration, covering mat 7 is ef ficiently protected from external agents , such as water or other liquids , dirt , dust , etc . In fact , support member 8 provides a complete protection of second surface 7b .

[0117] As visible in Figure 1 , battery cells 3 are arranged in housing 2 laterally adj acent to one another .

[0118] In detail , the electrical poles 4 form two rows of electrical poles 4 .

[0119] In greater detai l , each row of electrical poles 4 extends along a respective axis Y . That is , the elongated poles 4 of one row are aligned along an axis Y .

[0120] Moreover, each venting port 5 is interposed between a pair of electrical poles 4 , with respect to a direction transversal ( orthogonal ) to the axes Y .

[0121] Advantageously, covering mat 7 is arranged in between the two rows of electrical poles 4 and longitudinally extends parallel to axes Y .

[0122] In one preferred embodiment , covering mat 7 laterally abuts against lateral , electrically non- conductive walls of electric poles 4 .

[0123] This configuration is shown in Figures 2 and 3 .

[0124] In this way, a more ef ficient protection and sealing of electric poles 4 and can be obtained, since the gas and / or electrically charged particles it carries are even more inhibited to reach the electrical poles 4 .

[0125] According to an alternative embodiment of the present invention shown in Figure 3 , support member 8 covers only a portion of second surface 7b .

[0126] In particular, according to this alternative embodiment , support member 8 comprises two elongated elements 8a, each elongated element longitudinally extending along one of said axes Y and covering the electrical poles 4 of one respective said row .

[0127] Each elongated element 8a has a retaining portion 14 extending onto second surface 7b only at a longitudinal edge portion 7c thereof , leaving the rest of the covering mat 7 uncovered from support member 8 .

[0128] Furthermore , each retaining portion 14 maintains covering mat 7 in abutment against lids 3a ( and therefore areas 5a ) .

[0129] Thanks to this configuration, an insulator assembly 6 ( and therefore a battery pack 1 ) involving less material and having less weight can be obtained, while ensuring at the same time an ef fective protection of electrical poles 4 , a suf ficient stability in position of covering mat 7 even in the event of a violent outburst of gases through one venting port 5 , and a suf ficient sealing of the venting ports 5 from one another and from the electrical poles 4 .

[0130] In one preferred embodiment shown in Figure 4 , each slit 10 extends through the entire thickness of covering mat 7 , thereby defining a through hole . In detail , each s lit 10 extends completely from first surface 7a to second surface 7b .

[0131] In this way, the venting and guiding of gases from the respective venting port 5 is improved, since the gases encounter no obstacle or hindering whatsoever along flow channel 11 .

[0132] According to an alternative embodiment shown in Figures 5 and 6 , each slit 10 extends through only part of the thickness of covering mat 7 , thereby defining a blind hole having a plug portion 15 closing the blind hole and arranged over the respective venting port 5 .

[0133] In detail , each plug portion 15 seals the respective venting port 5 in a fluid-tight manner ( relatively to a direction parallel to its central axis X ) and is configured to be ruptured by the gas outflowing from such respective venting port 5 , thereby allowing fluidical connection of venting port 5 with slit 10 .

[0134] This embodiment allows for an improved sealing during nominal operation of battery pack 1 , while still ensuring a satis factory venting and guiding of the gases through flow channel 11 .

[0135] In one embodiment , shown in Figure 5 , each plug portion 15 is arranged at the first surface 7a, so as to be proximal to the respective venting port 5 .

[0136] In this case , an even better sealing of venting ports 5 is obtained .

[0137] In another embodiment , shown in Figure 6 , each plug portion 15 is arranged at the second surface 7a, so as to face internal wall 2a of housing 2 , and in particular so as to be distal from the respective venting port 5 ( and therefore more proximal to inner wall 2a ) .

[0138] In this case , a better behavior of covering mat 7 under deformation stresses is obtained . More speci fically, the Applicant has observed how for this last configuration of covering mat 7 the deformation needed for rupturing each plug portion 15 is reduced, compared to the case in which plug portions 15 are proximal to venting ports 5 .

[0139] Hence , less space for this deformation can be provided within housing 2 , which ultimately leads to si ze reduction of battery pack 1 .

[0140] In light of the above , depending on final application and requirements , plug portions 15 can be arranged according to one of the two embodiments described above .

[0141] Preferably, but not necessarily, i . e . according to one non-limiting embodiment , covering mat 7 is provided with a pressure sensitive adhesive (not shown) on first surface 7a for the fixing with battery cells 3 ( and in particular with lids 3a ) , and on second surface 7b for the fixing with support member 8 . Conveniently, the pressure sensitive adhesive is provided with further slits aligned to slits 10 of covering mat 7 .

[0142] The Applicant has observed that the presence of the pressure sensitive adhesive improves the assembling of insulator assembly 6 and the stability and robustness of its components ( especially of covering mat 7 ) .

[0143] Preferably, insulator assembly 6 further comprises an insulating mat 16 interposed between support member 8 and internal wall 2a of housing 2 .

[0144] In particular, insulating mat 16 has a flat configuration, i . e . is defined by a sheet-like layer of insulating material , and is preferably made of s ilicone or polyurethane or NBR elastomers (nitrile butadiene rubber ) or Polymide or Polyketonebased thermoplastics . Each of these materials has a high flame resistance .

[0145] Insulating mat 16 defines a fire barrier , thereby further enhancing the thermal-and- f ire insulating properties of insulator assembly 6 .

[0146] The advantages o f battery pack 1 according to the present invention will be clear from the foregoing description .

[0147] In particular, thanks to the presence of a covering mat 7 having slits 10 , and in particular thanks also to the fact that covering mat 7 seals the venting ports 5 from one another and from electrical poles 4 , it is possible to avoid a chain ef fect amongst adj acent battery cells 3 during a thermal runway event or a cold degassing event .

[0148] In fact , in case a thermal runway event or a cold degassing event occurs in one of the battery cells 3 , whereby gases outburst from the respective venting port 5 , such gases are guided away from the other venting ports 5 and away from electrical poles 4 by means of the relative slit 10 .

[0149] Furthermore , the high-pressure gas bursting out of venting port 5 and the electrically charged particles that it may carry ( such as electrolytic material particles ) are inhibited to reach the other venting ports 5 and, most importantly, the electrical poles 4 , thanks to the sealing function of covering mat 7 , thereby preserving the electrical integrity of the poles 4 .

[0150] Hence , the li fespan of battery pack 1 is signi ficantly improved .

[0151] Clearly, changes may be made to battery pack 1 as described herein without , however, departing from the scope of protection as defined in the accompanying claims .

Claims

CLAIMS1.- Electrical energy storage device (1) comprising a housing (2) , a plurality of electrical energy storage cells (3) within the housing (2) , each energy storage cell (3) including electrical poles (4) and a safety venting port (5) , and an insulator assembly (6) arranged within the housing (2) between the energy storage cells (3) and an internal wall (2a) of the housing (2) , the insulator assembly (6) comprising:- a covering mat (7) resting onto the energy storage cells ( 3 ) ;- a support member (8) resting onto the electrical poles (4) of each energy storage cell (3) so as to cover the electrical poles (4) , and maintaining the covering mat (7) in abutment against at least at an area (5a) of each energy storage cell (3) around the respective venting port ( 5 ) ; wherein the covering mat (7) includes a plurality of slits (10) , each slit (10) being arranged over one respective venting port (5) and being fluidically connectable with the respective venting port (5) .2.- Electrical energy storage device as claimed in claim 1, wherein each slit (10) extends through at least part of the thickness of the covering mat (7) and defines a flow channel (11) for guiding a gas outflowing from therespective venting port (5) towards the internal wall (2a) of the housing (2) and away from the other venting ports ( 5 ) .3.- Electrical energy storage device as claimed in claim 1 or 2, wherein the covering mat (7) seals in a fluid-tight manner each venting port (5) from the adjacent venting port (5) or venting ports (5) and from the electrical poles (4) .4.- Electrical energy storage device as claimed in claim 3, wherein each venting port (5) has a central axis (X) , wherein the covering mat (7) comprises sealing regions (12) around said slits (10) which are respectively pressed in abutment against said areas (5a) of the energy storage cells (3) by the support member (8) , in such a way that each venting port (5) is sealed in a fluid-tight manner from the adjacent venting port (5) or venting ports (5) and from the electrical poles (4) , with respect to a direction transversal to the respective central axis (X) .5.- Electrical energy storage device as claimed in any one of the foregoing claims, wherein the support member (8) is coupled to the covering mat (7) in such a way that the slits (10) are uncovered by the support member ( 8 ) .6.- Electrical energy storage device as claimed in any one of the foregoing claims, wherein each energystorage cell (3) has an operative wall (3a) carrying the respective electrical poles (4) and on which the respective venting port (5) is obtained; wherein the covering mat (7) rests onto each operative wall (3a) in such a way that at least an electrically conductive surface (4a) of the electrical poles (4) is uncovered from the covering mat (7) .7.- Electrical energy storage device as claimed in claim 6, wherein the energy storage cells (3) are arranged in the housing (2) laterally adjacent to one another, so that the electrical poles (4) thereof form two rows of electrical poles (4) , each row of electrical poles (4) extending along a respective axis (Y) , each venting port (5) being interposed between a pair of electrical poles (4) , with respect to a direction transversal to the axes (Y) , wherein the covering mat (7) is arranged in between the two rows of electrical poles (4) and longitudinally extends parallel to the axes (Y) .8.- Electrical energy storage device as claimed in claim 7, wherein the covering mat (7) has a first surface (7a) facing the operative walls (3a) of the energy storage cells (3) , and a second surface (7b) opposite to the first surface (7a) and facing the internal wall (2a) of the housing ( 2 ) ; wherein the support member (8) comprises twoelongated elements (8a) , each elongated element (8a) longitudinally extending along one of said axes (Y) and covering the electrical poles (4) of one respective said row of electrical poles (4) , each elongated element (8a) having a retaining portion (14) extending onto the second surface (7b) of the covering mat (7) only at a longitudinal edge portion (7c) thereof, leaving the rest of the covering mat (7) uncovered from the support member (8) , each retaining portion (14) maintaining the covering mat (7) in abutment against the operative walls (3a) .9.- Electrical energy storage device as claimed in claim 6, wherein the covering mat (7) has a first surface (7a) facing the operative walls (3a) of the energy storage cells (3) , and a second surface (7b) opposite to the first surface (7a) and facing the internal wall (2a) of the housing ( 2 ) ; wherein the support member (8) completely covers the second surface (7b) leaving the slits (10) uncovered.10.- Electrical energy storage device as claimed in any one of the foregoing claims, wherein each slit (10) extends through the entire thickness of the covering mat (7) , thereby defining a through hole.11.- Electrical energy storage device as claimed in any one of the claims 1 to 9, wherein each slit (10) extends through only part of the thickness of the coveringmat (7) , thereby defining a blind hole having a plug portion (15) closing the blind hole and arranged over the respective venting port (5) , the plug portion (15) sealing the respective venting port (5) in a fluid-tight manner and being configured to be ruptured by gas outflowing from the respective venting port (5) , thereby allowing fluidical connection of the venting port (5) with the slit (10) .12.- Electrical energy storage device as claimed in claim 11, wherein the covering mat (7) has a first surface (7a) facing the energy storage cells (3) , and a second surface (7b) opposite to the first surface (7a) and facing the internal wall (2a) of the housing (2) ; wherein each plug portion (15) is arranged at the first surface (7a) , so as to be proximal to the respective venting port (5) .13.- Electrical energy storage device as claimed in claim 11, wherein the covering mat (7) has a first surface (7a) facing the energy storage cells (3) , and a second surface (7b) opposite to the first surface (7a) and facing the internal wall (2a) of the housing (2) ; wherein each plug portion (15) is arranged at the second surface (7b) , so as to face the internal wall (2a) of the housing (2) .14.- Electrical energy storage device as claimed inany one of the foregoing claims, wherein the covering mat (7) is made of silicone foam.15.- Electrical energy storage device as claimed in any one of the foregoing claims, wherein the support member (8) is made of a rigid and electrically insulating material .16.- Electrical energy storage device as claimed in any one of the foregoing claims, wherein the covering mat(7) has a first surface (7a) facing the energy storage cells (3) , and a second surface (7b) opposite to the first surface (7a) and facing the internal wall (2a) of the housing ( 2 ) ; wherein the covering mat (7) is provided with a pressure sensitive adhesive on the first surface (7a) for the fixing with the energy storage cells (3) , and on the second surface (7b) for the fixing with the support member(8) , the pressure sensitive adhesive being provided with further slits aligned to the slits (10) of the covering mat ( 7 ) .

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