KR20150080655A - Sealing film for lead tab of secondary battery and secondary battery including the same - Google Patents

Abstract

The present invention relates to a sealing film for a lead tab of a secondary battery for sealing an external material and the lead tab of the secondary battery. According to one aspect of the present invention, provided is the sealing film of the lead tab of the secondary battery which includes an intermediate layer, an internal layer which is formed on one side of the intermediate layer, and an external layer which is formed on the other side of the intermediate layer. At least the internal layer among the internal layer and the external layer includes ethylene vinyl acetate (EVA) resins and polystyrene.

Description

TECHNICAL FIELD [0001] The present invention relates to a sealing film for a lead tab of a secondary battery, and a secondary battery comprising the same. BACKGROUND ART [0002]

The present disclosure relates to a sealing film for sealing between a lead tab of a secondary battery and a casing, and a secondary battery including the sealing film.

Recently, as technology development and demand for mobile devices have increased, demand for secondary batteries has also increased. Among these secondary batteries, the demand for a lithium secondary battery having a particularly high energy density and a discharge voltage is rapidly increasing.

Generally, a lithium secondary battery is classified into a lithium ion secondary battery, a lithium ion polymer battery, and a lithium polymer battery depending on the type of an electrolytic solution. Depending on the external shape, the battery can be largely divided into a cylindrical secondary battery, a rectangular secondary battery, and a pouch type secondary battery.

Among these, there is an increasing demand for a thin rectangular prismatic secondary battery and a pouch-shaped secondary battery in accordance with the miniaturization trend of mobile devices, and a pouch-type secondary battery which is easy to deform in shape and low in manufacturing cost is getting more attention . In addition, the pouch-type secondary battery is advantageous in weight and miniaturization compared with a cylindrical or prismatic secondary battery, and has attracted attention as a power source for a battery automobile or a hybrid electric automobile.

Most of the pouch-type secondary batteries have lead tabs drawn from the electrode group, and the lead tabs and the casing (pouch) are sealed through a sealing film. This will be described with reference to FIGS. 1 and 2. FIG.

FIGS. 1 and 2 show a configuration of a conventional pouch-type secondary battery. 1 is a perspective view showing a manufacturing process of a pouch type secondary battery. FIG. 2 is a perspective view showing a sealing film for a lead tab of a secondary battery according to a related art, in which a sealing film is thermally bonded to a lead tab. FIG.

1 and 2, a pouch-type secondary battery generally includes an electrode assembly 1 in which an anode, a separator, and a cathode are alternately stacked, Lead tabs 2 and 2 'drawn out from the lead tabs 2 and connected to external terminals, and a casing member 3 for packaging the electrode group 1. Then, the electrode group 1 is impregnated with the electrolytic solution. 1 and 2, most of the pouch-type secondary batteries have sealing films 4 and 4 'for sealing between the lead tabs 2 and 2' and the casing 3, ).

At this time, the electrode group 1 and the electrode tabs 1a 'and 1a' of the positive electrode and the negative electrode are formed in the electrode group 1, and the lead tabs 2 and 2 ' Welded or revetted, and drawn out to the outside.

The outer sheath 3 is a pouch, which generally consists of a flexible film of a multi-layer structure comprising an inner sealant layer, a gas barrier layer and an outer resin layer. At this time, the inner sealant layer is heat-sealed to prevent electrolyte or gas from leaking out, and it is composed of a resin such as polypropylene (PP) or polyethylene (PE) having a low melting point which is mainly heat- The gas barrier layer is for blocking moisture, air, and the like, and it is composed of a metal thin film such as aluminum (Al).

The lead tabs 2 and 2 'mainly have a plate or rod shape, and are made of metal for conductivity. Generally, the lead tabs 2 and 2 'are made of a conductive metal selected from aluminum (Al), nickel (Ni), copper (Cu) and alloys thereof, etc., advantageous in terms of conductivity. The sealing films 4 and 4 'are interposed between the lead tabs 2 and 2' and the joint member 3 and the sealing films 4 and 4 ' (2) and (2 ') and the outer covering material (3). In addition, the sealing films 4 and 4 'provide electrical insulation between the lead tabs 2 and 2' and the case 3.

The sealing films 4 and 4 'are thermally adhered (thermally fused) to the respective contact surfaces of the lead tabs 2 and 2' and the casing member 3 by a heat bonding method using heat and pressure. At this time, the sealing films 4 and 4 'are mainly made of polypropylene (PP) or polyethylene (PE) film for thermal adhesion with the inner sealant layer of the casing 3. For example, Korean Patent Laid-Open No. 10-2009-0076278 discloses a technique related to this.

Further, the sealing films 4 and 4 'may have a multilayer structure of two or three layers. For example, Japanese Patent No. 4498639 and Japanese Patent No. 4508477 disclose a sealing film 4 (4 ') composed of a low-flow polypropylene (PP) layer and a high-flow polypropylene (PP) ). Japanese Patent No. 4993054 discloses a sealing film 4 (4 ') composed of a low-flow polypropylene (PP) layer, a high-flow polypropylene (PP) layer and a low-flow polypropylene .

However, the sealing films 4 and 4 'for the lead tabs 2 and 2' of the secondary battery according to the prior art including the above-mentioned patent documents have the following problems.

The sealing films 4 and 4 'are thermally bonded to the lead tabs 2 and 2' and the casing 3 through heat and pressure as described above.

However, according to the study results of the present inventors, the sealing films 4 and 4 'according to the prior art have a problem in that the thermal bonding strength (thermal bonding strength) between the lead tabs 2 and 2' have. In addition, as described above, the lead tabs 2 and 2 'are made of metal for conductivity, and the sealing films 4 and 4' according to the related art are especially made of the lead tabs 2 and 2 ' (Heat bonding strength) between the adhesive layer and the adhesive layer is low, resulting in poor sealing performance.

Korean Patent Publication No. 10-2009-0076278 Japanese Patent No. 4498639 Japanese Patent No. 4508477 Japanese Patent No. 4993054

Embodiments of the present invention provide a sealing film for a lead taps of a secondary battery having excellent chemical resistance and electrical insulation, as well as having excellent adhesion strength (heat bonding strength) with lead taps, and a secondary battery including the same .

According to embodiments of the present invention, there is provided a sealing film for a lead taps of a secondary battery which seals between a lead tab of the secondary battery and a casing, the sealing film comprising: an intermediate layer; An inner layer formed on one surface of the intermediate layer; And an outer layer formed on the other surface of the intermediate layer, wherein the inner layer or the inner layer and the outer layer are resin layers comprising ethylene vinyl acetate (EVA) resin and polystyrene. Here, the inner layer composed of the mixed resin is overheated with the lead tab of the secondary battery.

In an exemplary embodiment, the content of polystyrene is preferably 10-20 parts by weight based on 100 parts by weight of the ethylene vinyl acetate (EVA) resin.

In an exemplary embodiment, the ethylene vinyl acetate (EVA) resin preferably comprises 23-32 wt% of vinyl acetate (VA) based on the total weight of ethylene vinyl acetate, and has a Melt Flow Index of 10 -30 g / 10 min.

In one exemplary embodiment, the resin layer preferably further comprises an organic peroxide based cross-linking agent.

In an exemplary embodiment, it is preferable that the resin layer further includes a crosslinking auxiliary.

Embodiments of the present invention also provide a secondary battery comprising a leadfil release film of the secondary battery.

Embodiments of the present invention, in one aspect, include an inner layer of a sealing film for a lead tab that adheres to a lead tab includes ethylene vinyl acetate (EVA) and polystyrene (PS) (Thermal bonding strength) to improve the sealing property, and has improved mechanical strength and heat resistance. In addition, it has chemical resistance that is not eroded well in lithium salts and the like, and thus the electrical insulation of the lithium secondary battery can be improved.

Further, in one aspect, by adding polystyrene to ethylene vinyl acetate, the melting point of the film is controlled to be lower than the melting point of a film containing a conventional polypropylene resin, and the melt flow index (MFI) The adhesion to the lead tab metal can be further improved.

1 is a perspective view showing a manufacturing process of a general pouch type secondary battery.
FIG. 2 is a perspective view illustrating a sealing film for a lead tab of a secondary battery according to a related art, wherein the sealing film is thermally bonded to the lead tab.
3 is a schematic cross-sectional view of a sealing film for a lead tab of a secondary battery according to an embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

3 is a schematic cross-sectional view of a sealing film for a lead tab of a secondary battery according to an embodiment of the present invention.

A sealing film for a lead taps (hereinafter abbreviated as "sealing film") of a secondary battery according to an embodiment of the present invention includes an intermediate layer 10, an inner layer 20 formed on one surface of the intermediate layer 10, (3-layer) or more (see FIG. 3), including the outer layer 30 formed on the other surface of the substrate.

In the case of sealing between the lead tabs of the secondary battery and the casing (pouch), the inner layer is brought into contact with the lead tab and thermally adhered (thermally fused). In addition, the outer layer may be thermally adhered to the outer material (pouch).

At least the inner layer 20 of the sealing film according to the embodiments of the present invention is an ethylene vinyl acetate (hereinafter, abbreviated as EVA) capable of maintaining a good thermal adhesion strength with a metal that is not possessed by a sealing film for existing lead taps ) Resin is composed of a mixed resin layer containing polystyrene (hereinafter abbreviated as PS) excellent in electric insulation and the like. Further, not only the inner layer 20 but also the outer layer 30 may be composed of a mixed resin layer including EVA and PS, as the case may be.

The EVA resin is a copolymer of ethylene and vinyl acetate (VA), which is superior to polyethylene (PE) or polypropylene (PP) used as a sealing film for lead taps of a secondary battery in the related art, (Thermal bonding strength). Accordingly, when the secondary battery is sealed and packed, the sealing property between the lead tab and the sealing film is improved, effectively preventing the leakage of the electrolytic solution and gas, and improving the durability. Further, according to the embodiments of the present invention, it is possible to have an improved mechanical strength and heat resistance while having excellent thermal bonding strength (thermal bonding strength).

In a preferred embodiment, the EVA resin may comprise 15-40 wt% of vinyl acetate (VA) based on the total weight of the EVA resin. If the content of vinyl acetate (VA) is less than 15% by weight, flexibility and adhesion may not be good. If the content of vinyl acetate exceeds 40% by weight, blocking resistance may be deteriorated. In another aspect, the EVA resin may comprise 23-32 wt% of vinyl acetate (VA) based on the total weight of the resin.

In one aspect, the EVA resin preferably has a melt flow index (MFI) of 5-50 g / 10 min as measured at 190 ° C under a load of 2.16 kg. If the melt index (MFI) is too low as less than 5 g / 10 min, the mechanical properties of the inner layer may be deteriorated. In addition, if the melt index (MFI) is higher than 50 g / 10 min, the moldability in the molding process such as co-extrusion may be deteriorated. Taking this into consideration, it is preferable that the EVA resin has a melt index (MFI) of 10-30 g / 10 min (measured at a load of 2.16 kg at 190 캜).

Polystyrene is included in the mixed resin layer according to embodiments of the present invention.

For reference, polystyrene is one of thermoplastic resins, for example, a polymer which is polymerized using a metallocene catalyst polymerization method and has a melting point of 270 캜. It is colorless and transparent, has a tensile strength similar to that of glass, and has a high modulus of elasticity but a low impact strength. The electrical properties are excellent. The light resistance is lowered and gradually changes to yellow under direct sunlight and deteriorates. However, a light resistance grade compounded with an ultraviolet absorber and an antioxidant may have sufficient light resistance for use.

Polystyrene is soluble in aromatic or chlorinated hydrocarbons, ketones, esters, etc., but it has excellent chemical resistance such as water resistance, acid resistance and alkali resistance. In addition, since the thermal decomposition temperature is high and the thermal stability and the fluidity at the time of dissolution are excellent, the molding processability is excellent and injection molding is particularly easy.

Examples of the polystyrene include High Impact Polystyrene (HIPS) and General Purpose Polystyrene (GPPS). In a preferred embodiment, the polystyrene may be a general purpose polystyrene. In a non-limiting example, the above-mentioned general-purpose polystyrene may be a product of Styrolution.

In a preferred embodiment, the mixed resin layer of ethylene vinyl acetate (EVA) and polystyrene (PS) is 2 to 40 parts by weight, more preferably 5 to 30 parts by weight, based on 100 parts by weight of ethylene vinyl acetate (EVA) , And most preferably 10 to 20 parts by weight. When the content of polystyrene is included in the above amount, the electrical insulation and adhesion between the lead tapped metal and the sealing film can be increased. In addition, when the polystyrene is out of the above range, the effect of the ethylene vinyl acetate (EVA) resin may be deteriorated.

In preferred embodiments, the mixed resin layer may further comprise a crosslinking agent. The crosslinking agent may be an organic peroxide crosslinking agent. Such an organic peroxide type crosslinking agent can further improve the adhesion of the sealing film. Specifically, the organic peroxide crosslinking agent accelerates the crosslinking reaction of the EVA resin to increase the degree of crosslinking of the EVA resin, while removing the residual radicals in the resin, thereby improving the adhesion of the inner layer. In addition, the organic peroxide-based crosslinking agent induces a uniform distribution of ethylene in the polymerization resin. This improves the thickness uniformity of the inner layer, which in turn increases the surface adhesion with the lead taps and effectively improves the adhesive strength.

The organic peroxide-based crosslinking agent is not limited as long as it is an organic compound having at least one peroxide group (-O-O-) in the molecule. The organic peroxide-based cross-linking agent may be at least one selected from among peroxyketals, peroxycarbonates, and dialkyl peroxides in one aspect.

The peroxyketals include t-butyl 2-ethylhexyl monoperoxycarbonate, 1,1-di (t-amylperoxy) cyclohexane, 1,1-di (t-butylperoxy) , 5-trimethylsyclohexane, 1,1-di (t-butylperoxy) cyclohexane, and the like. As the peroxycarbonate, 2,5-dimethyl-2,5-di- (2-ethylhexanonylperoxy) hexane, t-amylperoxy-2-ethylhexanoate, t- (2-ethylhexyl) monoperoxycarbonate, t-butylisopropyl monoperoxycarbonate, 2,5-dimethyl-2,5-di (benzoylperoxy) hexane, t -Butyl-3,5,5-trimethylhexanoate, t-butyl peroxybenzoate, and the like. Examples of the dialkyl peroxide include dicumyl peroxide, 2,5-dimethyl-2,5-di (t-butylperoxy) hexane, di-t-amyl peroxide, di- , 3-bis (2-t-butylperoxyisopropyl) hexane, t-butylcumylperoxide and di-t-butylperoxide.

In one embodiment, the organic peroxide-based cross-linking agent is preferably selected such that the cross-linking reaction temperature is between 90 and 150 캜. More preferably, the crosslinking reaction temperature is between 135 and 150 占 폚. When the organic peroxide type crosslinking agent has a crosslinking reaction temperature in the high temperature range, it is very preferable for improving the adhesion with the lead tab.

Further, in one embodiment, the organic peroxide type crosslinking agent preferably has a half life of not more than 2 hours, more specifically, 30 minutes to 2 hours. The organic peroxide crosslinking agent causes a crosslinking reaction at a certain time and temperature depending on the kind of crosslinking agent. In the present invention, the term "half-life" refers to the time (initial reaction time) required for the organic peroxide crosslinking agent to take place for the first time after the crosslinking reaction with the EVA resin.

In a preferred embodiment, the organic peroxide cross-linking agent may have a half-life of 1 hour and a temperature (crosslinking reaction temperature) of 135-150 ° C in order to achieve high adhesion with the metal lead tab. Examples of the organic peroxide crosslinking agent include dialkyl peroxides, and more specific examples thereof include dicumyl peroxide, 2,5-dimethyl-2,5-di (t-butylperoxy) -Amyl peroxide, di-t-butyl peroxide, 1,3-bis (2-t-butylperoxyisopropyl) hexane, t-butyl cumyl peroxide and di- One or more selected.

In a preferred embodiment, the mixed resin layer may contain the organic peroxide crosslinking agent in an amount of 7 to 60 parts by weight based on 100 parts by weight of the EVA resin. At this time, if the content of the organic peroxide crosslinking agent is less than 7 parts by weight, the effect of improving the adhesive strength may be insufficient due to its low crosslinking degree depending on its content, and if it exceeds 60 parts by weight, . Considering this point, it is preferable that 20-28 parts by weight of an organic peroxide crosslinking agent is included in 100 parts by weight of the EVA resin in one embodiment. In addition, the EVA resin may have a degree of crosslinking of, for example, 80% or more by controlling the content of the organic peroxide crosslinking agent. Here, the degree of crosslinking refers to the weight ratio of the EVA resin crosslinked by the organic peroxide-based crosslinking agent in the total weight of the EVA resin.

In a preferred embodiment, the mixed resin layer may further comprise a crosslinking auxiliary. The crosslinking aid is not limited as long as it can promote crosslinking of the EVA resin. In one embodiment, the crosslinking aid may preferably be at least one selected from an allyl compound and a (meth) acryloyloxy compound. In one embodiment, the crosslinking aid is more preferably a polyallyl compound having two or more allyl groups in the molecule; And poly (meth) acryloyloxy compounds having two or more (meth) acryloyloxy groups in the molecule, and the like. Specifically, the crosslinking aids may include, in one embodiment, at least one polyallyl compound selected from triallyl isocyanurate, triallyl cyanurate, diallyl phthalate, diallyl fumarate, and diallyl maleate; At least one poly (meth) acryloyloxy compound selected from ethylene glycol diacrylate, ethylene glycol dimethacrylate, trimethylol propane and trimethacrylate; Or < / RTI >

In one embodiment, the crosslinking assistant is included in the mixed resin layer in an amount of 0.1-3.0 parts by weight based on 100 parts by weight of the EVA resin. If the content of the crosslinking aid is less than 0.1 part by weight, the effect of accelerating crosslinking may be insignificant. If the amount of the crosslinking aid is more than 3.0 parts by weight, the synergistic effect may not be so large.

On the other hand, the intermediate layer 10 is not limited as long as it has a supporting force. The middle layer preferably has a lower melt index (MFI) than that of the inner layer and the outer layer, so as to maintain the shape of the sealing film upon thermal bonding. The intermediate layer may be a film having a high heat resistance and may be a polypropylene (PP) film in one embodiment. In another embodiment, a polyethylene terephthalate (PET), a polyethylene naphthalate (PEN), a polybutylene terephthalate Phthalate (PBT), polybutylene naphthalate (PBN), and stretched polypropylene (OPP) film. Such a film is excellent in mechanical strength as well as heat resistance and is useful in the present invention.

The outer layer 30 is not particularly limited as long as it has a thermal adhesive force with the casing (pouch) of the secondary battery. The outer layer preferably has a good thermal adhesion strength with the sealant layer constituting the casing (pouch) of the secondary battery. This outer layer 30 can be composed of a mixed resin layer of EVA and PS as described above. In another embodiment, the outer layer is made of one selected from the group consisting of polyethylene (PE), polypropylene (PP) Or more can be used as the base resin. Further, in another embodiment, the outer layer may be selected from low melting point resin films comprising at least one selected from the group consisting of polyethylene (PE), polypropylene (PP), cast polypropylene (CPP) and ethylene- have.

The sealing film according to the present invention described above can be manufactured by various methods, and the manufacturing method thereof is not limited. For example, the outer layer, the intermediate layer and the inner layer may be laminated together with the formation of each layer through co-extrusion, or each layer 10, 20, 30 may be separately extruded and then laminated via an adhesive or thermal lamination . The adhesive may be selected from resin adhesives such as acrylic, urethane, epoxy, and the like.

In addition, the sealing film according to embodiments of the present invention has at least a three-layer structure as described above, and may further include a separate layer in addition to the layers. For example, a separate functional layer such as an antistatic layer may be further included between the intermediate layer 10 and the inner layer 20, and / or between the intermediate layer 10 and the outer layer 30. [ The antistatic layer may include at least one selected from the group consisting of a conductive polymer, a carbon material, and a surfactant as the antistatic agent. Examples of the conductive polymer include polyethylene dioxythiophene, polythiophene, polyaniline, and polypyrrole.

In addition, the sealing film according to embodiments of the present invention may further include at least one sealing agent selected from a filler, a slip agent, an anti-blocking agent, a UV stabilizer, an antioxidant, a surface smoothing agent, And may further optionally include additives. These additives may be contained as separate layers or may be added in an appropriate amount to at least one selected from the outer layer 30, the intermediate layer 10 and the inner layer 20.

Hereinafter, examples and comparative examples of the present invention will be exemplified. The following examples and comparative examples are provided to illustrate the present invention in order to facilitate understanding of the present invention, and thus the technical scope of the present invention is not limited thereto.

 [Example 1]

A sealing film was prepared using a co-extruder so as to have a three-layer structure of an inner layer (EVA + PS) / an intermediate layer (PP) / an outer layer (PP). An inner layer was prepared by using the EVA (DuPont (DuPont)), polystyrene (polystyrene-Pollution (Styrolution)'s GPPS), peroxy ketal (cross-linking agent, DuPont) and poly allyl compounds (crosslinking auxiliary, DuPont).

In the inner layer, 15 parts by weight of polystyrene (PS) was used per 100 parts by weight of the ethylene vinyl acetate (EVA) resin.

The inner layer used was an EVA resin composition obtained by mixing 25 parts by weight of a crosslinking agent with 100 parts by weight of an ethylene vinyl acetate (EVA) resin. The EVA resin used had a vinyl acetate (VA) content of 25% by weight and a melt index (MFI) of 28 g / 10 min (measured at 190 캜 under a load of 2.16 kg). The crosslinking agent is peroxyketal, and 1,1-di (tert-amylperoxi) cyclohexane having a half-life of 1 hour is in the range of 110 to 120 ° C. Respectively.

The outer layer and the intermediate layer were made of a low melting point polypropylene (PP) (DuPont) having a melt index (MFI) of 25 g / 10 min (measured at 190 캜 under a load of 2.16 kg).

The polyallyl compound was used in an amount of 3 parts by weight based on 100 parts by weight of an ethylene vinyl acetate (EVA) resin.

[Comparative Example 1]

A three-layer structure film which is currently used as a sealing film for lead taps is manufactured. The inner layer was prepared by using 25 parts by weight of peroxyketal (crosslinking agent, manufactured by DuPont) and 3 parts by weight of polyallyl compound (crosslinking assistant, manufactured by DuPont) based on 100 parts by weight of polypropylene and polypropylene.

[Comparative Example 2]

The inner layer was prepared by using 25 parts by weight of a dialkyl peroxide (crosslinking agent, DuPont) and 3 parts by weight of a polyallyl compound (crosslinking assistant, manufactured by DuPont) based on EVA (DuPont) and 100 parts by weight of EVA.

As the crosslinking agent, di-tert-butyl peroxide having a dialkyl peroxide having a half-life temperature (crosslinking reaction temperature) of 135 to 150 ° C was used.

[Experimental Example 1] Evaluation of tensile strength

The tensile strength of the sealing films according to each of the Examples and Comparative Examples was evaluated as follows after heat bonding.

First, an aluminum (Al) electrode plate most widely used as a lead tab of a lithium secondary battery was prepared. Then, the sealing film according to each of the examples and the comparative examples was cut into a width of 5 mm and a length of 5 cm, followed by heat bonding at 165 ° C. and a pressure of 30 kgf for 3 seconds. Thereafter, the film was pressed at a pressure of 30 kgf for 3 seconds while applying heat to the Al plate only at a temperature of 230 占 폚 and thermally adhered.

Each of the thermally adhered specimens was subjected to a rising load at a speed of 200 mm / min using a tensile strength meter (AGS-1kNX model of SHIMADZU, Japan) to measure the thermal adhesion strength between the Al plate and the sealing film. The results are shown in Table 1 below.

Adhesion Example 1
(EVA + polystyrene) Comparative Example 1
(Polypropylene type) Comparative Example 2
(EVA) Tensile strength (kgf / 15mm) 2.0 1.1 1.8

[Experimental Example 2] Evaluation of chemical resistance

On the other hand, the chemical resistance was evaluated as follows. As in Experimental Example 1, an aluminum (Al) electrode plate, which is most often used as a lead tab of a lithium secondary battery, was prepared. Then, the sealing film according to each of the examples and the comparative examples was cut into a width of 5 mm and a length of 5 cm, followed by heat bonding at 165 ° C. and a pressure of 30 kgf for 3 seconds. After the bonding, the Al electrode plates to which the sealing films according to each of the Examples and Comparative Examples were adhered were immersed in a lithium salt solution.

For reference, as time goes by, the lithium salt solution is impregnated between the Al metal and the film, and the adhesion between the two metals is deteriorated. Therefore, it is possible to evaluate the degree of chemical resistance through the degree of tensile strength after a certain period of time in the lithium salt solution. That is, good adhesion between the Al metal and the sealing film after a lapse of a certain time in the lithium salt solution means excellent chemical resistance, and the excellent chemical resistance in the lithium salt solution means that the electrical insulation between the sealing film and the metal is excellent .

After immersing in a lithium salt solution, the adhesive strength between the film and the metal was measured with a tensile strength meter after 240 hours, and the results are shown in Table 2 below.

Chemical Resistance Adhesion (240 hours elapsed) Example 1
(EVA + polystyrene) Comparative Example 1
(Polypropylene type) Comparative Example 2
(EVA) Tensile strength (kgf / 15mm) 1.0 0.4 0.4

As shown in Tables 1 and 2, when the sealing film according to the embodiment of the present invention including EVA and polystyrene as the inner layer was a polypropylene-based (Comparative Example 1) or EVA (Comparative Example 2) (Thermal bonding strength) with the lead tab (Al electrode plate), and thus it has excellent chemical resistance and electrical insulation.

1: electrode group 1a, 1a ': electrode terminal
2, 2 ': Lead tab 3: Outer material
4, 4 ': sealing film 10: middle layer
20: inner layer 30: outer layer

Claims (10)

A sealing film for a lead tab of a secondary battery which seals between a lead tab of the secondary battery and a casing,
Intermediate layer;
An inner layer formed on one surface of the intermediate layer; And
And an outer layer formed on the other surface of the intermediate layer,
Wherein the inner layer or inner layer and the outer layer are a mixed resin layer containing ethylene vinyl acetate (EVA) resin and polystyrene. The method according to claim 1,
Wherein a content of polystyrene in the mixed resin layer is 10-20 parts by weight based on 100 parts by weight of an ethylene vinyl acetate (EVA) resin. The method according to claim 1,
Wherein the ethylene vinyl acetate (EVA) resin comprises 23-32 wt% of vinyl acetate (VA) based on the total weight of the ethylene vinyl acetate (EVA) resin. The method according to claim 1,
Wherein the ethylene vinyl acetate (EVA) resin has a melt flow index of 10-30 g / 10 minutes. The method according to claim 1,
Wherein the mixed resin layer further comprises an organic peroxide type crosslinking agent. 6. The method of claim 5,
Wherein the content of the organic peroxide crosslinking agent is 20-28 parts by weight based on 100 parts by weight of an ethylene vinyl acetate (EVA) resin. 6. The method of claim 5,
Wherein the organic peroxide-based crosslinking agent is at least one selected from the group consisting of peroxyketals, peroxycarbonates, and dialkyl peroxides. 6. The method of claim 5,
Wherein the mixed resin layer further comprises a crosslinking auxiliary agent. 9. The method of claim 8,
Wherein the content of the crosslinking aid is 0.1-3.0 parts by weight based on 100 parts by weight of an ethylene vinyl acetate (EVA) resin. 9. The method of claim 8,
Wherein the crosslinking assistant is at least one selected from the group consisting of a polyallyl compound and a poly (meth) acryloyloxy compound.

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