Lid assembly for a prismatic battery cell

Abstract

There is described a lid assembly (1) for a prismatic battery cell having a casing (2) which internally defines an inner compartment (3), the lid assembly (1) comprises: a base plate (4) for closing the inner compartment (3) and provided with a vent port (10); at least one current collector (5) and at least one terminal member (6); a rupturable foil (11) closing the vent port (10) in a sealed manner and configured to be ruptured by a pressure action applied from the inner compartment (3) outwardly for allowing a degassing therefrom; and a cover membrane (12) fixed to the base plate (4) and covering the rupturable foil (11) for protecting and separating it from the outer environment; the cover membrane (12) is provided with a plurality of elongated slits (13) arranged in an organized manner.

Description

[0001] LID ASSEMBLY FOR A PRISMATIC BATTERY CELL

[0002] Technical Field

[0003] The present invention relates to a lid assembly for a prismatic battery cell , in particular a lid assembly for closing and sealing in a fluid-tight manner an inner compartment of a prismatic battery cell housing electrodes and electrolytic material .

[0004] Background Art

[0005] Battery cells are known, which are used as electrical energy accumulators .

[0006] 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 .

[0007] 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 .

[0008] Also , batteries which include a plurality of cells connected in series or in parallel are known, for example as used in the automotive industry .

[0009] The present invention will refer to a battery cell for the use in a battery of a hybrid or full electric vehicle , without however losing generality .

[0010] Secondary battery cells can be manufactured as various types , such as cylindrical or prismatic cells .

[0011] The present invention will refer to prismatic battery cells , without however losing generality .

[0012] A prismatic battery cell typically comprises a hollow prismatic casing, for example in the shape of a parallelepiped and usually made of metallic material , and two electrodes ( cathode and anode ) housed within the casing .

[0013] 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 .

[0014] Within the compartment , the electrodes and the separator are immersed in an electrolytic solution, usually liquid or gelatinous .

[0015] The battery cell also comprises at least one lid assembly which is coupled to the casing for closing the compartment .

[0016] More speci fically, the casing usually has at least one opening ( for example rectangular ) , normally located on its top, through which the electrodes , separator and electrolytic solution are inserted into the compartment . The lid assembly is coupled to the casing for closing such opening .

[0017] The lid assembly typically comprises :

[0018] - a base plate , typically metallic, arranged to match and engage the opening of the casing for the closing thereof ;

[0019] - a current collector for each electrode , which is electrically connected to the relative electrode and is configured to collect the electric energy form such electrode and feed it from the inner compartment towards the outside environment ; and

[0020] - a terminal member for each electrode , which is electrically connected with the respective collector for receiving the electric energy therefrom, and which is configured to receive a connector of a consumer device which has to be powered by the battery cell .

[0021] The terminal member and the collector are usually coupled ( electrically and mechanically) with one another via a terminal pin, which engages a respective through hole obtained in the base plate .

[0022] The lid assembly further comprises a seal element for each collector, which is made of resilient and electrically insulating material and which serves the twofold function of : electrically insulating the collector from the base plate and f luid-tightly sealing the inner compartment of the casing from the outside environment .

[0023] To this end, the seal element is mechanically interposed between the respective collector and the base plate , so that the collector member and the base plate are not arranged in direct contact with one another, but still being mutually coupled only via the seal element itsel f .

[0024] In some configurations , the casing has two openings , one for each electrode , for example two lateral openings . In this case , the battery has two lid assemblies , one for each electrode and each having its respective base plate , a single terminal member, a single collector, and a single seal element .

[0025] In light of the above , the lid assembly seals the inner compartment in a fluid-tight manner .

[0026] 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 .

[0027] During such a thermal runway event , gas developed within the battery cell may abruptly outflow from the casing thereof .

[0028] 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 .

[0029] 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 assembly thereof .

[0030] More speci fically, the venting port is typically defined by a through hole obtained in the base plate , which allows a fluidic communication between the inner compartment and the outside environment .

[0031] During normal operation of the battery cell , the venting port is sealed by a thin foil , such as an aluminum f oil .

[0032] This thin foil defines a rupturable element which during a thermal runway event or a cold degassing event is configured to rupture or break apart , thereby al lowing the gas to outflow from the casing of the battery cell .

[0033] For this reason, the thin foil is also known as "burst disk" , even i f its shape is not properly discoidal .

[0034] In light of the above , the burst disk is a very important component of the lid assembly, since it allows the nominal operation of the battery cell , due to its sealing of the venting port .

[0035] However, the burst disk is also a very delicate component , given that it is usually defined by a thin foil of aluminum .

[0036] This means that there is a risk of damaging or even of pre-rupturing the burst disk during handling of the battery cell or already during assembling of the battery cell . However, a proper gas release in the event of thermal runway or cold degassing it must also be ensured for safety purposes , in order to avoid increased internal pressure and uncontrolled rupture or explosion of the cell casing . Therefore , a thicker or more robust burst disk may be detrimental for the safety of the battery cell .

[0037] To this end, the Applicant has observed how the known lid assemblies are still open to further improvement , in particular as per the protection of the burst disk .

[0038] Disclosure Of Invention

[0039] It is therefore an obj ect of the present invention to provide a lid assembly for a prismatic battery cell which is designed to overcome at least one of the above- mentioned drawbacks in a straightforward and low-cost manner .

[0040] This obj ect is achieved by a lid assembly as claimed in the appended independent claim 1 . Preferred embodiments of the present invention are laid down in the appended dependent claims .

[0041] Brief Description of the Drawings

[0042] A non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings , in which :

[0043] Figure 1 is an exploded perspective view, with parts removed for clarity, of a lid assembly for a prismatic battery cell according to the present invention;

[0044] Figures 2 to 6 are larger-scale top views , with parts removed for clarity, of a component of the lid assembly of Figure 1 according to respective alternative embodiments of the present invention; and

[0045] Figure 7 is a top view, with parts removed for clarity, of the lid assembly .

[0046] Best Mode for Carrying Out the Invention

[0047] With reference to Figure 1 , number 1 indicates as a whole a lid assembly for a prismatic battery cell .

[0048] In particular, the battery cell is of the prismatic type and has a hollow prismatic casing 2 which internally defines an inner compartment 3 . Casing 2 has an (upper ) opening 3a to access inner compartment 3 .

[0049] According to the embodiment shown, casing 2 has a parallelepiped shape , i . e . prismatic with a rectangular base .

[0050] Alternatively, casing 2 may have any other prismatic shape , such as cubic, triangular-based, cylindrical etc .

[0051] The present invention will refer to a battery cell for use in the automotive industry, for example in a battery module of a hybrid or full-electric vehicle , without however losing generality .

[0052] The battery cell further comprises electrodes ( cathode and anode , not shown) , which are housed in inner compartment 3 together with separator material (not shown) and an electrolytic material (not shown) .

[0053] The electrolytic material may be liquid or gelatinous .

[0054] Lid assembly 1 is configured to be coupled with casing 2 at opening 3a for f luid-tightly closing inner compartment 3 , as required during the normal use of the battery cell .

[0055] Lid assembly 1 comprises :

[0056] - a base plate 4 configured to be coupled to casing 2 for closing opening 3a of inner compartment 3 , namely arranged to match the shape of , and engage , opening 3a, thereby closing inner compartment 3 ; at least one , and in particular two current collectors 5 , each configured to be electrically connected to a respective electrode of the battery cell within inner compartment 3 for collecting electrical energy therefrom and trans ferring it from inner compartment 3 outwards ; at least one , and in particular two terminal members 6 , each electrically connected with one respective collector 5 for receiving the electrical energy therefrom and each configured to be electrically coupled with a connector (not shown) of a consumer device (not shown) which has to be powered by the battery cell .

[0057] Each terminal 6 is configured to be electrically connected to one respective electrode , namely one terminal 6 is connectable with the anode and the other terminal 6 is connectable with the cathode .

[0058] In particular, the terminal 6 is arranged on an outer side of the base plate 4 , so as to be connected to the consumer device , and the collector 5 is arranged, at least partially, on an inner side of the base plate 4 ( i . e . the side facing the inner compartment ) , so as to be connected with the respective electrode .

[0059] Each current collector 5 gathers electrical energy generated by the cell and channels it towards the respective terminal member 6 .

[0060] Lid assembly 1 conveniently further comprise at least one , and in particular two insulator members 7 , each interposed between one respective terminal member 6 and the base plate 4 for electrically insulating such terminal member 6 from base plate 4 .

[0061] Thanks to the presence of the insulator member 7 , a direct electric connection between each terminal member 6 and the base plate 4 , which would be dangerous and detrimental to the correct functioning of the battery cell , is avoided .

[0062] Conveniently, lid assembly 1 further comprises a stop frame 8 made of an electrically insulating material ( such as rigid plastic ) , the function of which is known per se and not described in detail .

[0063] Base plate 4 is a critical component of lid assembly 1 which ensures that the battery cell , and in particular its inner compartment 3 , remains sealed during normal operation .

[0064] As shown in Figure 1 , base plate 4 is provided with a vent port 10 defined by a through hole obtained in the base plate 4 itsel f .

[0065] Vent port 10 allows fluidic communication between inner compartment 3 and the outer environment .

[0066] Vent port 10 is essential for releasing excess pressure that may build up inside inner compartment 3 .

[0067] More precisely, a so called "thermal runway" or "cold degassing" event may occur . During such an event , gas developed within the sealed inner compartment 3 may abruptly outflow from casing 2 thereof , in order to achieve a pressure equali zation between inner compartment 3 and the outer environment .

[0068] The presence of vent port 10 avoids a violent outflow of such gases during the thermal runway event or the cold degassing event , which may cause an explosion .

[0069] Opportunely, a corresponding through hole 8a is obtained also in stop frame 8 .

[0070] During normal operation of the battery cell , vent port 10 must be sealed .

[0071] To this end, lid assembly 1 comprises a rupturable foil 11 closing vent port 10 in a sealed manner .

[0072] Rupturable foil 11 is configured to be ruptured by the pressure action applied from the inner compartment 3 outwardly ( during a thermal runway or cold degassing event ) for allowing a degassing therefrom .

[0073] In other words , rupturable foil 11 is designed to break under speci fic pressure conditions from within the inner compartment 3 , allowing gases to escape and preventing potential damage or explosion .

[0074] Hence , it must be ensured that rupturable foil 11 ef fectively ruptures when the predetermined pressure conditions are reached within inner compartment 3 .

[0075] For this reason, rupturable foil 11 is expediently defined by a thin foil of malleable material , such as an aluminum foil , which is relatively resistant but also easily breakable .

[0076] In light of the above , rupturable foil 11 defines the sole barrier sealing vent port 10 , and thus separating inner compartment 3 from the outside environment during use .

[0077] To this end, a protection of rupturable foil 11 must be ensured during normal operation of the battery cell .

[0078] That is , it must be ensured that rupturable foil 11 stays intact during normal operation of the battery cell .

[0079] Accordingly, lid assembly 1 comprises a cover membrane 12 fixed to base plate 4 and covering rupturable foil 11 for protecting and separating this latter from the outer environment .

[0080] In particular, cover membrane 12 is defined by a thin film which is attached, for example glued, to an external surface 4a of base plate 4 facing the outer environment .

[0081] More in particular, cover membrane 12 is attached at a region of base plate 4 surrounding vent port 10 .

[0082] In use , rupturable foil 11 is interposed ( and in particular stacked) between base plate 4 and stop frame 8 . Hence , rupturable foil 11 rests in contact with an internal surface 4b of base plate 4 which is opposite to external surface 4a and faces in use inner compartment 3 .

[0083] That is , rupturable foil 11 and cover membrane 12 are arranged on opposite sides of base plate 4 .

[0084] In detail , cover membrane 12 is arranged at one ( external ) side of vent port 10 while rupturable foil 11 is arranged at the other ( outer ) side of vent port 10 .

[0085] In light of the above , cover membrane 12 rests on base plate 4 without contacting rupturable foil 11 .

[0086] In practice , cover membrane 12 serves to protect the rupturable foil 11 from external environmental factors ( impurities , corrosion) and physical damage ( deformation, rupturing) .

[0087] However, cover membrane 12 must not hinder the aforementioned escaping of gases during thermal runway or cold degassing events .

[0088] According to an aspect of the present invention, cover membrane 12 is provided with a plurality of elongated slits 13 arranged in an organi zed manner .

[0089] It is stated that the wording "arranged in an organi zed manner" means herein that the slits 13 are distributed on cover membrane 12 according to a precise geometric scheme and not according to a casual distribution .

[0090] In detail , each slit 13 is defined by an elongated cut delimited by lateral walls (which are not visible in the appended figures ) .

[0091] Such lateral wall s are in contact against one another during nominal operation of lid assembly 1 , so that the slit 13 is closed, and are configured to be pushed away from one another under said pressure action ( i . e . when the pressure from inner compartment 3 reaches a predetermined level ) , for determining an opening of the slit 13 and aiding a breakage of cover membrane 12 .

[0092] In particular, slits 13 represent a sort of "die cutting" obtained on cover membrane 12 .

[0093] In other words , when the pressure from the inner compartment 3 reaches a predetermined level and rupturable foil 11 ruptures allowing a degassing, the lateral walls delimiting each sl it 13 are pushed apart , creating an opening that facilitates the breakage of cover membrane 12 . That is , each slit 13 acts as a starting point for the breakage of cover membrane 12 when the pressure from inner compartment 3 reaches the predetermined level , while during normal operation each slit 13 remains closed and sealed . In this way, each slit 13 defines a sort of "preopening" or "pre-breakage" or "pre- f allure" point of cover membrane 12 .

[0094] Therefore , the presence of slits 13 improves the breakability of cover membrane 12 , with respect to the case in which cover membrane 12 has no slits ( i . e . is defined by a whole , intact thin film) , thereby signi ficantly limiting the risk of violent degassing ( explosion) and at the same time providing an ef fective protection for rupturable foil 11 in an easy and cost- ef fective manner .

[0095] Cover membrane 12 has an inner surface facing the rupturable foil 11 and an outer surface opposite to the inner surface and facing the outer environment .

[0096] The slits 13 are obtained at least on the inner surface , so as to initiate the opening action and assist in breaking the cover membrane 12 under pressure .

[0097] In a preferred embodiment , each slit 13 cuts through the entire thickness of the cover membrane 12 , so as to extend from the inner surface to the outer surface .

[0098] That is , each one of the lateral walls delimiting the slit 13 extends from the inner surface to the outer surface .

[0099] The Applicant has observed that this configuration further reduces the risks of cover membrane 12 not breaking under the predetermined pressure .

[0100] Conveniently, cover membrane 12 is defined by a thin film made of PET .

[0101] PET is known for its excellent strength-to-weight ratio , meaning it provides substantial mechanical strength without adding signi ficant weight to the lid assembly 1 . This contributes to the overall durability and robustness of the battery cell while keeping the weight manageable ; moreover, PET exhibits strong resistance to chemicals and solvents , which is crucial for battery applications where exposure to electrolytes and other chemicals can occur ; furthermore , PET has good thermal stability, meaning it can withstand the temperature fluctuations that may occur during battery operation; also , PET is an ef fective barrier against gases and moisture ; moreover, although not always necessary for all applications , the natural transparency of PET can be advantageous for an easy inspection and quality control of rupturable foil 11 ; finally, PET can be easily fabricated into thin films with precise dimensions and patterns , including the elongated slits required for the venting mechanism; this ease of fabrication allows for consistent production quality and precise control over the membrane ' s properties .

[0102] According to a preferred embodiment of the present invention, cover membrane 12 has a first symmetry line 14a and a second symmetry line 14b orthogonal to first symmetry line 14a .

[0103] More precisely, cover membrane 12 is shaped so as to have a first symmetry line 14a and second symmetry line 14b orthogonal to one another .

[0104] For example , in one embodiment cover membrane 12 has a quadrilateral , and in particular rectangular, shape so that first and second symmetry lines 14a, 14b intersect the midpoints of opposite sides of the cover membrane 12 itsel f , respectively .

[0105] In an alternative embodiment not shown, cover membrane 12 has an oval , and in particular substantially elliptical , shape so that the first and second symmetry lines 14a, 14b define respective main axes of the cover membrane 12 itsel f .

[0106] In a further alternative embodiment , cover membrane 12 may have a squared shape or a circular shape .

[0107] Figure 2 shows a first preferred embodiment of cover membrane 12 .

[0108] According to this embodiment , cover membrane 12 has a substantially rectangular shape and its slits 13 are organi zed so that each slit 13 extends along the first symmetry line 14a or along the second symmetry line 14b .

[0109] Preferably, the slits 13 are advantageously distributed both along the first symmetry line 14a and along the second symmetry line 14b .

[0110] In detail , the slits 13 which are distributed along the first symmetry line 14a extend along the first symmetry line 14a, and the slits 13 which are distributed along the second symmetry line 14b extend along the second symmetry line 14b, as shown in Figure 2 .

[0111] In greater detail , slits 13 are equally spaced from one another along first symmetry line 14a and along second symmetry line 14b, respectively .

[0112] Preferably, cover membrane 12 has slits 13 of the first symmetry line 14a and of the second symmetry line 14b which intersect one another in the middle point ( center point ) of the symmetry lines 14a, 14b .

[0113] The Applicant has observed that the configuration according to the first embodiment is particularly advantageous for certain pressure conditions which may be established within inner compartment 3 , for facilitating breakage of cover membrane 12 under such pressure conditions . In fact , the organization and spatial distribution of slits 13 along symmetry lines 14a and 14b ensures uni formity and ef fectiveness of the breakage in such pressure conditions . Furthermore , distributing the slits 13 along symmetry lines 14a and 14b ensures that the pressure exerted on the cover membrane 12 is evenly distributed . This uni form di stribution helps in preventing localized stress points , thereby reducing the risk of uneven rupturing or tearing of the cover membrane 12 , which could ultimately hinder a correct degassing from vent port 10 . Hence , the symmetry in the organi zation of the slits 13 allows for a predictable breaking behavior .

[0114] Figure 3 shows a second preferred embodiment of cover membrane 12 .

[0115] According to this embodiment , cover membrane 12 includes a center region 15 arranged at a central portion of the cover membrane 12 itsel f and at which center region 15 the first and second symmetry lines 14a, 14b intersect .

[0116] According to this embodiment , the center region 15 is advantageously devoid of slits 13 .

[0117] Such configuration allows for an improved protection of rupturable foil 11 while maintaining an ef fective level of breakability under the desired pressure conditions within inner compartment 3 . In fact , a center region 15 devoid of slits 13 acts as a reinforcement point that prevents the entire cover membrane 12 from rupturing uni formly all at once . This allows for a more controlled and gradual venting process . More speci fically, when the pressure inside the inner compartment 13 increases and rupturable foils 11 ruptures , the slits 13 located around the center region 15 are more likely to open first , enabling a controlled release of gases . Moreover, by maintaining a central area without slits 13 , the structural integrity of cover membrane 12 is improved, which may be preferable in certain operative conditions . Furthermore , the center region 15 without slits 13 helps to evenly distribute the pressure across the cover membrane 12 ; thi s can prevent the membrane from tearing unevenly and ensures that the slits 13 function as intended by opening up in response to speci fic pressure thresholds . Finally, the center region 15 devoid of slits 13 provides an additional sealing barrier ; this can be crucial for maintaining the integrity of the rupturable foil 11 ( during nominal operation) and preventing minor external factors from triggering the venting mechanism prematurely .

[0118] Figure 4 shows a third preferred embodiment of cover membrane 12 .

[0119] According to this embodiment , cover membrane 12 has a substantially rectangular shape and includes a first diagonal line 16a and a second diagonal line 16b transversal ( and in particular inclined with respect ) to the first diagonal line 16a .

[0120] The first diagonal line 16a and the second diagonal line 16b are inclined with respect to symmetry lines of the cover membrane 12 ( and in particular to symmetry lines 14a, 14b of Figure 2 ) .

[0121] According to this embodiment , each slit 13 extends along the first diagonal line 16a or along the second diagonal line 16b . Preferably, the slits 13 are advantageously distributed both along the first diagonal line 16a and along the second diagonal line 16b .

[0122] In detail , the slits 13 which are distributed along the first diagonal line 16a extend along the first diagonal line 16a, and the slits 13 which are distributed along the second diagonal line 16b extend along the second diagonal line 16b .

[0123] In greater detail , slits 13 are equally spaced from one another along first diagonal line 16a and along second diagonal line 16b .

[0124] Preferably, cover membrane 12 has slits 13 of the first diagonal line 16a and slits 13 of the second diagonal line 16b which intersect one another in the middle point ( center point ) of the diagonal lines 16a, 16b .

[0125] The Applicant has observed that the configuration according to the third embodiment is particularly advantageous for certain pressure conditions which may be established within inner compartment 3 , for facilitating breakage of cover membrane 12 under such pressure conditions . In fact , the organization and spatial distribution of slits 13 along diagonal lines 16a and 16b ensures uni formity and ef fectiveness of the breakage in such pressure conditions . Furthermore , distributing the slits 13 along diagonal lines 16a, 16b enhances , under certain pressure conditions , the distribution of pressure across the cover membrane 12 . Diagonal lines 16a, 16b intersect more areas of the membrane , potentially providing a more uniform stress di stribution and reducing the likelihood of locali zed pressure points . The diagonal arrangement of slits 13 can improve the overall structural integrity of the membrane . This configuration can help to maintain the strength of the membrane by dispersing stress more evenly, thereby reducing the risk of undesired failure , under certain pressure conditions .

[0126] Figure 5 shows a fourth preferred embodiment of cover membrane 12 , simi lar to second embodiment shown in Figure 3 and with the di f ference that slits 13 are arranged along diagonal lines 16a, 16b .

[0127] According to this embodiment , cover membrane 12 includes center region 15 at which the first and second diagonal lines 16a, 16b intersect .

[0128] According to this embodiment , the center region 15 is advantageously devoid of slits 13 .

[0129] The technical ef fects which stem from such configuration are comparable to the one indicated in connection with the second embodiment shown in Figure 3 .

[0130] Referring now to Figure 6 , a fi fth preferred embodiment of the present invention will be described .

[0131] In particular, according to this embodiment , slits 13 are organi zed so as to be distributed both along the first and second symmetry lines 14a, 14b and along the first and second diagonal lines 16a, 16b .

[0132] In detail , cover membrane 12 is provided with slits 13 distributed all along first and second symmetry lines 14a, 14b and all along first and second diagonal lines 16a, 16b .

[0133] Preferably, cover membrane 12 has slits 13 which intersect in the middle point ( center point ) of the symmetry lines 14a, 14b and of the diagonal lines 16a, 16b . In particular, the Applicant has found that the distribution of slits 13 according to the fi fth embodiment represent the optimal spatial configuration of slits 13 , as it is adaptable to various operative conditions and pressure conditions which may be established within inner compartment 3 .

[0134] More in particular, the configuration according to the fi fth embodiment provides for a redundant safety mechanism relative to the breakabi lity of cover membrane : i f the slits 13 along one set of lines fail to open as intended, the other set can act as a backup, ensuring the intended breakage of cover membrane 12 and enhancing the reliability of the venting system .

[0135] Furthermore , the cover membrane 12 achieves a balanced stress distribution . This balanced distribution helps to evenly distribute the forces exerted on the membrane during breakage , minimi zing the risk of locali zed stress concentrations .

[0136] Also , slits 13 distributed along both diagonal lines 16a, 16b and symmetry lines 14a, 14b maximi ze the flexibility and responsiveness of the cover membrane 12 ; this allows the membrane to adapt ef fectively to di f ferent pressure conditions , facilitating a rapid and ef ficient breakage response when needed .

[0137] Preferably, diagonal lines 16a, 16b are inclined with respect to the symmetry lines 14a, 14b, respectively, of an angle comprised between 30 ° and 60 ° , preferably between 40 ° and 50 ° , more preferably by an angle equal to 45 ° .

[0138] The Applicant has observed that this represents the optimal spatial configuration regarding the organi zation of the slits 13 on cover membrane 12 .

[0139] Cover membrane 12 has a perimetral edge 17 which laterally delimits the cover membrane 12 itself .

[0140] In the preferred embodiment shown in Figure 6 , perimetral edge 17 is substantially rectangular, for example rectangular with rounded vertexes .

[0141] Advantageously, cover membrane 12 is provided with an elongated through opening 18 extending from the perimetral edge 17 inward, towards center region 15 .

[0142] In detail , opening 18 is defined by a lack of material on cover membrane 12 .

[0143] In greater detail , opening 18 extends along one of the first and second symmetry lines 14a, 14b .

[0144] Opening 18 di f ferentiates from slits 13 , since opening 18 is always in an open condition, while the lateral walls delimiting each slit 13 open under the pressure action as explained above .

[0145] Preferably, opening 18 extends along first symmetry line 14a, which is the symmetry line along the breadth of the cover membrane 12 (when cover membrane 12 is oval or elliptical , first symmetry line 14a corresponds to the minor axis thereof ) .

[0146] The presence of a through opening 18 allows for immediate venting of gases once the internal pressure reaches a critical threshold and rupturable foil 11 ruptures , without waiting for cover membrane 12 to fully break, thereby ensuring an immediate release of pressure . This could be preferable for some operative configurations . Moreover, through opening 18 adds an additional layer of reliability to the venting system : it ensures that even i f slits 13 fail to operate as intended ( and cover membrane 12 does not break or partially breaks hindering gases ) , there is a guaranteed path for pressure relief , enhancing the overall safety of the battery cell . In other words , through opening 18 provides a direct and unobstructed pathway for gas expulsion, without the need for a breakage of cover membrane 12 .

[0147] To this end, through opening 18 conveniently, but not necessarily, extends at least partially above rupturable foil 11 , as shown in Figure 7 .

[0148] In this way, it is ensured that through opening 18 is in fluidic communication with vent port 10 via a ruptured rupturable foil 11 , when needed .

[0149] Given that the primary function of cover membrane 12 is to protect rupturable foil 11 , through opening 18 must preferably be small enough so as to ensure a suf ficient level of protection .

[0150] For example , through opening 18 may extend from perimetral edge 17 for about 1 / 15 to 1 / 8 of the length of first symmetry line 14a, preferably for 1 / 10 of the length of first symmetry line 14a .

[0151] In one embodiment not shown, also the cover membrane 12 according to the fi fth embodiment may be provided with a center region 15 devoid of slits 13 , with the technical ef fects listed above .

[0152] The advantages o f lid assembly 1 according to the present invention will be clear from the foregoing description .

[0153] In particular, lid assembly 1 features a protective cover membrane 12 with strategically arranged sl its 13 . This configuration allows for an ef fective breakage of the cover membrane 12 when needed, thereby minimi zing the risk of hindering the venting of gases when internal pressure exceeds a predetermined threshold value and rupturable foil 11 ruptures, thus enhancing the safety and reliability of the battery cell while at the same time effectively protecting the rupturable foil 11.

[0154] In other words, the presence of slits 13 as described above improves the breakability of cover membrane 12, with respect to the case in which cover membrane 12 has no slits (i.e. is defined by a whole, intact thin film) , thereby significantly limiting the risk of violent degassing (explosion) and at the same time providing an effective protection for rupturable foil 11 in an easy and cost-effective manner.

[0155] Furthermore, corrosion of rupturable foil 11 is limited and in particular avoided: rupturable foil 11 is effectively protected by cover membrane 12 form possible corrosive action that may occur during use.

[0156] Clearly, changes may be made to lid assembly 1 as described herein without, however, departing from the scope of protection as defined in the accompanying claims.

Claims

CLAIMS1.- Lid assembly (1) for a prismatic battery cell having a casing (2) which internally defines an inner compartment (3) for housing electrodes and an electrolytic material, the lid assembly (1) being configured to f luid-tightly close the inner compartment (3) and comprising:- a base plate (4) configured to be coupled to the casing (2) for closing the inner compartment (3) and provided with a vent port (10) defined by a through hole obtained in the base plate (4) which allows fluidic communication between the inner compartment (3) and the outer environment;- at least one current collector (5) configured to be electrically connected to a respective said electrode for collecting electrical energy therefrom; at least one terminal member (6) electrically connected with the current collector (5) for receiving the electrical energy therefrom;- a rupturable foil (11) closing the vent port (10) in a sealed manner and configured to be ruptured by a pressure action applied from the inner compartment (3) outwardly for allowing a degassing therefrom; and- a cover membrane (12) fixed to the base plate (4) and covering the rupturable foil (11) for protecting and separating it from the outer environment; wherein the cover membrane (12) is provided with a plurality of elongated slits (13) arranged in an organized manner .2.- Lid assembly as claimed in claim 1, wherein each slit (13) is defined by an elongated cut delimited bylateral walls which are in contact against one another during nominal operation of lid assembly (1) so that the slit (13) is closed, and that are configured to be pushed away from one another under said pressure action for determining an opening of the slit (13) and aiding a breakage of the cover membrane (12) itself.3.- Lid assembly as claimed in claim 1 or 2, wherein the cover membrane (12) has an inner surface facing the rupturable foil (11) and an outer surface opposite to the inner surface and facing the outer environment; wherein the slits (13) are obtained at least on the inner surface.4.- Lid assembly as claimed in claim 3, wherein each slit (13) cuts through the entire thickness of the cover membrane (12) , so as to extend from the inner surface to the outer surface.5.- Lid assembly as claimed in any one of the foregoing claims, wherein the cover membrane (12) has a first symmetry line (14a) and a second symmetry line (14b) orthogonal to the first symmetry line (14a) , wherein each slit (13) extends along the first symmetry line (14a) or along the second symmetry line (14b) .6.- Lid assembly as claimed in claim 5, wherein the slits (13) are distributed both along the first symmetry line (14a) and along the second symmetry line (14b) , the slits (13) which are distributed along the first symmetry line (14a) extending along the first symmetry line (14a) , the slits (13) which are distributed along the second symmetry line (14b) extending along the second symmetry line (14b) .7 Lid assembly as claimed in claim 6, wherein the cover membrane (12) includes a center region (15) arranged at a central portion of the cover membrane (12) itself and at which center region (15) the first and second symmetry lines (14a, 14b) intersect, said center region (15) being devoid of slits (13) .8.- Lid assembly as claimed in any one of the claims 5 to 7, wherein the cover membrane (12) has a quadrilateral shape, preferably substantially rectangular, so that the first and second symmetry lines (14a, 14b) intersect the midpoints of opposite sides of the cover membrane (12) , respectively; or wherein the cover membrane (12) has an oval shape, preferably substantially elliptical, so that the first and second symmetry lines (14a, 14b) define respective main axes of the cover membrane (12) .9.- Lid assembly as claimed in any one of the foregoing claims, wherein the cover membrane (12) has a first diagonal line (16a) and a second diagonal line (16b) transversal to the first diagonal line (16a) , the first diagonal line (16a) and the second diagonal line (16b) being inclined with respect to symmetry lines (14a, 14b) of the cover membrane (12) ; wherein each slit (13) extends along the first diagonal line (16a) or along the second diagonal line (16b) .10.- Lid assembly as claimed in claim 9, wherein the slits (13) are distributed both along the first diagonal line (16a) and along the second diagonal line (16b) , the slits (13) which are distributed along the first diagonal line (16a) extending along the first diagonal line (16a) ,the slits (13) which are distributed along the second diagonal line (16b) extending along the second diagonal line (16b) .11.- Lid assembly as claimed in claim 10, wherein the cover membrane (12) includes a center region (15) arranged at a central portion of the cover membrane (12) itself and at which center region (15) the first and second diagonal lines (16a, 16b) intersect, said center region (15) being devoid of slits (13) .12.- Lid assembly as claimed in claims 5 and 9, wherein the slits (13) are organized so as to be distributed both along the first and second symmetry lines (14a, 14b) and along the first and second diagonal lines (16a, 16b) .13.- Lid assembly as claimed in any one of the foregoing claims, wherein the cover membrane (12) has a perimetral edge (17) , the cover membrane (12) being provided with an elongated through opening (18) extending from the perimetral edge (179 inward towards a center region (15) of the cover membrane (12) .14.- Lid assembly as claimed in claims 5 and 13, wherein the through opening (18) extends along one of the first and second symmetry lines (14a, 14b) and / or extends at least in part above the rupturable foil (11) .15.- Lid assembly as claimed in any one of the foregoing claims, wherein the cover membrane (12) is defined by a thin film made of PET.

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