KR101883535B1 - A separator for a secondary battery with enhanced safety - Google Patents

Abstract

The present invention relates to an electrode assembly having improved safety and a secondary battery including the electrode assembly. More particularly, the present invention relates to a jelly-roll type electrode assembly having improved safety, such as preventing a thermal runaway caused by a local short circuit by inducing a large-sized short circuit to an external impact, and a secondary battery including the same.
The electrode assembly according to the present invention has a low strength of the separator portion surrounding the outer periphery and is easily broken by an external impact to induce a short circuit to be generated in a large area so that thermal congestion due to a short circuit or firing can be prevented from concentrating on a local portion . In addition, the lithium secondary battery including the electrode assembly according to the present invention has an effect of improving heat and safety.

Description

[0001] The present invention relates to a separator for a secondary battery,

The present invention relates to an electrode assembly having improved safety and a secondary battery including the electrode assembly. More particularly, the present invention relates to a jelly-roll type electrode assembly having improved safety, such as preventing a thermal runaway caused by a local short circuit by inducing a large-sized short circuit to an external impact, and a secondary battery including the same.

Recently, interest in energy storage technology is increasing. Cell phones, camcorders, tablet PCs and notebook PCs, as well as the energy of electric vehicles, has been expanding its efforts to research and develop electrochemical devices. The electrochemical device is one of the most popular fields in this respect. Of these, the development of a rechargeable secondary battery has become a focus of attention. In recent years, in order to improve capacity density and energy efficiency, Research and development on the design of electrodes and batteries are underway.

Among the currently applied secondary batteries, the lithium secondary battery developed in the early 1990s has advantages such as higher operating voltage and higher energy density than conventional batteries such as Ni-MH, Ni-Cd and sulfuric acid-lead batteries using an aqueous electrolyte solution .

The lithium secondary battery is manufactured by laminating an anode, a separator, and a cathode sequentially to produce a laminate, and then winding the laminate several times to charge a jelly-roll type electrode assembly into the case. Here, a porous thin sheet made of a polyolefin-based polymer material is used as the separation membrane. Also, the lithium secondary battery can be classified into a can-type secondary battery in which an electrode assembly is embedded in a metal can, and a pouch-type secondary battery in which an electrode assembly is embedded in a pouch of an aluminum laminate sheet, depending on the shape of the battery case. The can-type secondary battery can be classified into a cylindrical battery and a prismatic battery depending on the shape of the metal can.

1 schematically shows a secondary battery according to the prior art. 1, a cylindrical rechargeable battery generally includes a cylindrical battery can 10, a jelly-roll type electrode assembly 30 housed inside the battery can 10, A beading portion 11 provided at the tip of the battery can 10 for mounting the cap assembly 20 and a crimping portion 12 for sealing the battery.

The electrode assembly 30 has a structure in which a separator is interposed between a positive electrode and a negative electrode and is wound in a jelly-roll form. The positive electrode lead 31 is attached to the positive electrode and connected to the cap assembly 20, And a lead 32 is attached to the lower end of the battery can 10.

In such an electrode assembly of the secondary battery, breakage of the separator may occur due to external impact. In this case, a short circuit may occur due to the direct contact between the anode and the cathode, and the separator shrinks momentarily due to the short- As the range expands rapidly, the heat of short-circuit reaction sharply increases and leads to abnormal superheat. If overheating occurs, the anode and anode active material layers may be ignited by overheating and cause serious damage.

Therefore, there is a need to develop a separator for preventing abnormal overheating or ignition caused by breakage of the separator.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the problems of the prior art, and it is an object of the present invention to provide an electrode assembly that induces a large-area corrugated membrane of the separator when an impact is applied to the electrode assembly, And an object of the present invention is to provide a lithium secondary battery including the electrode assembly. Other objects and advantages of the present invention will become apparent from the following description. It is also to be easily understood that the objects and advantages of the present invention can be realized by the means or method described in the claims, and the combination thereof.

SUMMARY OF THE INVENTION The present invention provides a jelly-roll type electrode assembly for a secondary battery.

The jelly-roll type electrode assembly includes a negative electrode including a current collector for a negative electrode of a metal material and a negative electrode active material layer formed on a surface of the current collector for the negative electrode; A positive electrode comprising a positive electrode current collector made of a metal material and a positive electrode active material layer formed on one surface of the positive electrode current collector; And a separation membrane sandwiched between the cathode and the anode, wherein the separation membrane includes a low strength region, wherein the low strength region occupies more than one winding unit from the outer end of the electrode assembly And the low strength region may have a lower strength than the other regions of the separation membrane except for the low strength region.

Here, the low strength region may have a breaking strength of 70% or less as compared with the other regions.

Further, the low strength region may be formed by a corona discharge treatment.

The low strength region may be a part of the separation membrane occupying not more than (1) winding units or less than (2) winding units from the outer end of the electrode assembly.

In addition, the separation membrane may include a porous substrate, and further, the separation membrane may further include an organic / inorganic composite porous coating layer formed on all or a part of at least one surface of the porous substrate. Here, the separation membrane may not be formed with the organic / inorganic composite porous coating layer in a low strength region.

The negative electrode and / or positive electrode portion facing the low strength region of the separator may not be formed with the negative electrode active material layer and / or the positive electrode active material layer.

The present invention also provides a secondary battery including an electrode assembly having the above-described characteristics.

The electrode assembly according to the present invention has a low strength of the separator portion surrounding the outer periphery and is easily broken by an external impact to induce a short circuit to be generated in a large area so that thermal congestion due to a short circuit or firing can be prevented from concentrating on a local portion . In addition, the lithium secondary battery including the electrode assembly according to the present invention has an effect of improving heat and safety.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. On the other hand, the shape, size, scale or ratio of the elements in the drawings incorporated herein can be exaggerated to emphasize a clearer description.
1 is a schematic view of a conventional cylindrical secondary battery.
2 schematically shows a cross-section of a cylindrical battery in which local damage is caused by an external impact.
3 and 4 show an exploded view of an electrode assembly according to a specific embodiment of the present invention.
5 is a cross-sectional view of a jelly-roll type electrode assembly according to a specific embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art can easily carry out the present invention. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The singular expressions include plural expressions unless the context clearly dictates otherwise. In the present invention, terms such as "comprises" or "having" are used to designate the presence of stated features, integers, steps, operations, elements, components, or combinations thereof, But do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, or combinations thereof.

The present invention relates to an electrode assembly for a secondary battery and a secondary battery including the same, wherein the electrode assembly comprises a cathode, an anode, and a separator interposed between the cathode and the anode. In the present invention, the electrode assembly may be a stack / folding type electrode assembly or a jelly-roll type electrode assembly according to the shape of the laminate. In an aspect, the electrode assembly is a jelly-roll type electrode assembly.

In one specific embodiment of the present invention, the separation membrane includes a low-strength region LS having a lower breaking strength than other regions. If the shape of the electrode assembly is deformed due to external impact, it may lead to breakage of the separator, which causes a reaction such as overcurrent or heat runaway due to a short circuit between the anode and the cathode. If such an intense reaction is concentrated on a localized site where breakage occurs for a relatively short period of time, it may lead to explosion of the battery. According to the present invention, when a deformation of an electrode assembly is caused by forming a low strength region having a reduced strength in a certain region of a separation membrane, it is possible to induce a wide short in a low strength region, thereby improving safety in use.

Figure 3 illustrates an electrode assembly 100 in accordance with one specific embodiment of the present invention. Referring to FIG. 3, an electrode assembly according to the present invention includes a cathode 130 and an anode 140 stacked with a separator 110 and 120 interposed therebetween. FIG. 5 is a jelly roll type electrode assembly prepared by winding the electrode assembly several times. Referring to FIG. 1, the separators 110 and 120 have a low strength region LS 110a extending from an outer end OE of the electrode assembly to a predetermined position in an inner end portion IE direction.

In one specific embodiment of the present invention, the length of the low strength region LS 110a is set so as to cover the outer peripheral surface of the electrode assembly at least once, starting from the outer end OE of the electrode assembly, Can be set to occupy more than one (1) winding unit.

According to a preferred embodiment of the present invention, the length of the low strength region is one (1) winding unit to (2) winding unit from the outer end of the electrode assembly. When the length of the low strength region is less than one winding unit, the length of the low strength region is shorter than the outer circumferential face of the electrode assembly, so that the large area fracture effect is not sufficiently exhibited. Further, when the length of the low strength region exceeds (2) the winding unit, the breaking strength of the entire separator may be excessively lowered and the safety in use of the battery may be rather lowered.

(1) A winding unit of the present invention is a unit winding unit in which an electrode / separator laminate including a separator or the like is wound from a predetermined unit winding starting point (RS) one turn of the electrode assembly and a unit winding end point RE).

In the present invention, the low strength region has a relatively low breaking strength as compared with the other region NS excluding the low strength region. In the conventional secondary battery in a conventional breaking strength of the membrane is approximately the TD direction 1000kgf / cm ² to 1700kgf / cm ^2, the MD direction 1000kgf / cm ² to belong to 1600kgf / cm ² range. In the present invention, the breaking strength of the low strength region is set to be lower than the breaking strength of a conventional separator. As a specific example, the breaking strength of the low-strength region is preferably 50% or less, or 60% or less, or 70% or less of the breaking strength of the other regions, so that when the electrode assembly is deformed, So that breakage is formed in a relatively wide range. When the breaking strength of the low strength region exceeds 70% of the other regions, it is difficult to exert a predetermined large area breaking effect when deformation due to external impact is applied. Also, when the breaking strength of the low strength region is too low to be less than 50% of the other regions, the breaking film is too easily generated and the durability of the battery is lowered, which makes it difficult to commercialize the product.

FIG. 2 is a diagram illustrating a deformed shape of the electrode assembly due to an external impact. As shown in FIG. 2, when the electrode assembly is deformed, the low strength region of the separator has a lower breaking strength than other regions, so that breakage occurs preferentially in comparison with other regions. Further, the area Lp of the fractured portion formed in the low-strength region is formed to be larger than the area of the fractured portion formed in the other region Hp having a relatively high fracture strength due to the low fracture strength.

As described above, since the breakdown area of the separator is widened when the shape of the electrode assembly is deformed due to an external impact, the short circuit current is widened, so that the overcurrent and the overheat due to the overcurrent can be prevented from being locally concentrated , There is an effect of remarkably lowering the risk of explosion of the battery.

In one specific embodiment of the invention, the separation membrane comprises a porous substrate of a polymeric material. The porous substrate may be a porous film substrate made of a polymer material or a porous nonwoven substrate integrated by melting and spinning a polymer resin.

The porous film substrate is a porous polymer film made of a polyolefin such as polyethylene or polypropylene as is well known. Such a polyolefin porous polymer film substrate exhibits a shutdown function at a temperature of, for example, 80 to 130 ° C. Such a polyolefin porous film base material is a polymer in which polyolefin-based polymers such as polyethylene, polypropylene, polybutylene, and polypentene, such as high density polyethylene, linear low density polyethylene, low density polyethylene and ultra high molecular weight polyethylene, . In addition, the porous film substrate may be formed of a porous polymer film in the form of a sheet using various polymers such as polyesters in addition to polyolefins. In addition, the porous polymeric film substrate may have a structure in which two or more film layers are laminated, and each film layer may be formed of a polymer such as polyolefin or polyester described above alone or in combination of two or more thereof It is possible.

The porous nonwoven substrate may be made of a fiber using a polymer such as the above-mentioned polyolefin-based polymer or polyester such as polyethylene terephthalate (PET) having higher heat resistance. The porous nonwoven substrate may be produced by mixing single fibers or two or more kinds of fibers.

The material and shape of the porous substrate may be variously selected depending on the purpose. The thickness of the porous substrate is not particularly limited, but is preferably 1 to 100 占 퐉, more preferably 5 to 50 占 퐉, and the pore size and porosity present in the porous substrate are also not particularly limited, but 0.01 to 50 占 퐉 and 10 To 95%.

According to a specific embodiment of the present invention, the separation membrane may further include an organic / inorganic composite porous coating layer on at least one side of the porous substrate (see FIG. 4).

The porous coating layer includes inorganic particles and a polymeric binder resin. The inorganic particles form an interstitial volume between inorganic particles to provide micropores. It also serves as a kind of spacer capable of maintaining the physical form. In addition, since the inorganic particles generally have a characteristic that their physical properties do not change even at a high temperature of 200 DEG C or more, the separator has excellent heat resistance by the formed inorganic coating layer.

The inorganic particles are not particularly limited as long as they are electrochemically stable. That is, the inorganic particles usable in the present invention are not particularly limited as long as the oxidation and / or reduction reaction does not occur in the operating voltage range of the applied battery (for example, 0 to 5 V based on Li / Li +). Particularly, when inorganic particles having an ion-transporting ability are used, the ion conductivity in the electrochemical device can be increased and the performance can be improved. Therefore, it is preferable that the ionic conductivity is as high as possible. In addition, when the inorganic particles have a high density, it is difficult to disperse the particles at the time of coating. Also, in the case of an inorganic substance having a high dielectric constant, the dissociation of an electrolyte salt, for example, a lithium salt, in the liquid electrolyte can be increased, and the ion conductivity of the electrolyte can be improved. For the above-mentioned reasons, the inorganic particles are preferably high-permittivity inorganic particles having a dielectric constant of 5 or more, preferably 10 or more.

Specifically, as the dielectric constant is about five or more inorganic particles is _{BaTiO 3, Pb (Zr x,} Ti 1-x) O 3 (PZT, 0 <x <1), Pb 1 - x La x Zr 1 -y Ti y (PMN-PT, 0 < x < 1) O ₃ (PLZT, 0 <x <1, 0 <y <1), (1-x) Pb (Mg _1/3 Nb _2/3 ) O ₃ -xPbTiO ₃ , Hafnia (HfO ₂ ), SrTiO ₃ , SnO ₂ , CeO ₂ , MgO, NiO, CaO, ZnO, ZrO ₂ , SiO ₂ , Y ₂ O ₃ , Al ₂ O ₃ , SiC and TiO ₂ . But is not limited thereto. As the inorganic particles having lithium ion transferring ability, lithium phosphate (Li ₃ PO ₄ ), lithium titanium phosphate (Li _x Ti _y (PO ₄ ) ₃ , 0 <x <2, 0 <y < (Li _x Al _y Ti _z (PO ₄ ) ₃ , 0 <x <2, 0 <y <1, 0 <z <3), (LiAlTiP) _x O _y series glass 0 <y <13), lithium lanthanum titanate (Li _x La _y TiO _{3 ,} 0 <x <2, 0 <y <3), lithium germanium thiophosphate (Li _x Ge _y P _z S _w , x <4, 0 <y < 1, 0 <z <1, 0 <w <5), lithium nitrides _{(Li x N y, 0 <} x <4, 0 <y <2), SiS 2 (Li x _{Si y S z, 0 <x} <3, 0 <y <2, 0 <z <4) based glass, and _{P 2 S 5 (Li x P} y S z, 0 <x <3, 0 <y <3, 0 < z < 7) series glass or the like is preferably used, but is not limited thereto.

The size of the inorganic particles is not limited. However, it is preferable that the size of the inorganic particles is in the range of 0.001 to 10 mu m for the formation of the film having a uniform thickness and the appropriate porosity, and the porosity of the substrate (porous film or nonwoven web) Can be adjusted appropriately.

When the particle size is less than 0.001 탆, the dispersibility is lowered and the physical properties of the inorganic coating layer are difficult to control. When the particle size exceeds 10 탆, the thickness of the inorganic coating layer is increased and the mechanical properties are deteriorated. The probability of an internal short circuit occurring during charging and discharging increases.

The content of the inorganic particles is preferably in the range of 50% by weight to 99% by weight, more preferably 60% by weight to 95% by weight, based on 100% by weight of the mixture of the inorganic particles constituting the inorganic coating layer and the binder polymer resin.

The binder polymer resin may be a polymer resin conventionally used in the art. Particularly, it is possible to use a glass transition temperature (Tg) as low as possible, preferably in the range of -200 to 200 ° C. This is because the mechanical properties such as flexibility and elasticity of the final film can be improved. The polymer resin faithfully performs a binder function to connect and stably fix the inorganic particles and the particles, thereby contributing to prevention of deterioration of the mechanical properties of the finally formed inorganic coating layer.

In one specific embodiment of the present invention, the low strength region of the separator may be formed by a corona discharge treatment. Preferably, one surface or both surfaces of the porous polymer substrate is subjected to surface treatment by corona discharge. When the separator includes the porous coating layer on one side or both sides of the polymer substrate, it is preferable that a corona discharge treatment is performed in a predetermined region as described above to form a low strength region before forming the porous coating layer.

When the corona discharge is conducted by using a conductor as an electrode and an opposite electrode by a metal plate, the electrodes are purple and current flows when the DC power is increased. In the case of corona discharge treatment of the porous polymer substrate of the present invention, And the surface strength is lowered.

The corona discharge treatment may be carried out without limitation according to a conventional method. The discharge amount may be, for example, in the range of 30 to 300 Wmin / m ² , or in the range of 50 to 120 Wmin / m ² . But is not limited thereto.

However, in the electrode assembly of the present invention, the method of forming the low-strength region in the separator is not limited to the corona discharge treatment described above. If the method can reduce the strength of a certain portion of the separator to exhibit a predetermined strength range, Any of these can be used within the scope of the objective of. For example, a sulfonation treatment using fuming sulfuric acid, a fluorination treatment using fluorine gas, an acrylate grafting treatment, a plasma treatment method, or the like can be used as the low strength region forming method.

According to a specific embodiment of the present invention, the organic / inorganic composite porous layer (112, 122) is not formed in the portion of the porous substrate where the low strength region is formed (see FIG. 4).

According to a specific embodiment of the present invention, the negative electrode active material layer and / or the positive electrode active material layer may not be formed on portions of the negative electrode and / or the positive electrode facing the low strength region of the separator. That is, in the present invention, the electrode assembly may be prepared such that the low strength region of the separator faces the negative electrode collector on one side and the positive electrode collector on the other side.

In the present invention, the positive electrode and the negative electrode are not particularly limited, and the electrode active material may be bound to an electrode current collector according to a conventional method known in the art. Examples of the cathode active material include, but are not limited to, lithium manganese oxide, lithium cobalt oxide, lithium nickel oxide, lithium iron oxide, or a combination thereof. The cathode active material may be a conventional cathode active material that can be used for a cathode of a conventional electrochemical device. It is preferable to use a lithium composite oxide. As a non-limiting example of the negative electrode active material, a conventional negative electrode active material that can be used for a negative electrode of an electrochemical device can be used. In particular, lithium metal or a lithium alloy, carbon, petroleum coke, activated carbon, Lithium-adsorbing materials such as graphite or other carbon-based materials and the like are preferable.

Non-limiting examples of the positive electrode current collector include aluminum, nickel, or a foil produced by a combination of these. Non-limiting examples of the negative electrode current collector include copper, gold, nickel, or a copper alloy or a combination thereof Foil and so on.

In addition, the present invention provides an electrode assembly having the above-described characteristics and a secondary battery including the electrolyte. The secondary battery is preferably a lithium secondary battery including a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery.

The electrolyte solution that can be used in the secondary battery of the present invention is a salt having a structure such as A ⁺ B ^- , wherein A ⁺ is Li ⁺ , Na ⁺ , K ⁺ , or combinations thereof, and B ^- is an ion selected from the group consisting of PF ₆ ^- , BF ₄ ^- , Cl ^- , Br ^- , I ^- , ClO ₄ ^- , AsF ₆ ^- , CH ₃ A salt containing an ion such as an anion such as CO ₂ ^- , CF ₃ SO ₃ ^- , N (CF ₃ SO ₂ ) ₂ ^- , C (CF ₂ SO ₂ ) ₃ ^- (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl- But are not limited to, those dissolved or dissociated in an organic solvent composed of pyrrolidone (NMP), ethylmethyl carbonate (EMC), gamma-butyrolactone (g-butyrolactone), or a mixture thereof.

The electrolyte injection may be performed at an appropriate stage of the battery manufacturing process, depending on the manufacturing process and required properties of the final product. That is, it can be applied before assembling the cell or at the final stage of assembling the cell.

The present invention also provides a battery pack including the secondary battery as a unit battery, and the battery pack can be used in a device such as a mobile phone, a PDA, a digital camera, a portable navigator, and a portable game machine.

However, since the battery pack can be easily manufactured by varying the number of the unit cells to provide the desired output and capacity of the device, it can be applied not only to the portable electronic device but also to various devices requiring variable battery capacity But is not limited to.

Since the structure of the battery pack and the method of manufacturing the battery pack are well known in the art, a detailed description thereof will be omitted herein.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. It will be apparent to those skilled in the art that various modifications may be made without departing from the scope of the present invention. The scope of the present invention is defined by the following claims. The scope of the present invention is not limited to the description of the specification, and all variations and modifications falling within the scope of the claims are included in the scope of the present invention.

110, 120 separator
110a, 120a, 111a, 121a,
112, 122 Porous coating layer
130 cathode
140 anode
131 negative active material layer
132 cathode collector
141 cathode active material layer
142 anode collector
133 Cathode tab
143 Bipolar tab

Claims (9)

A first separation membrane; A positive electrode comprising a positive electrode current collector made of a metal material and a positive electrode active material layer formed on one surface of the positive electrode current collector; And a negative electrode comprising a negative electrode active material layer formed on one surface of the negative electrode current collector and a negative electrode active material layer of a second separator and a metal material, Assembly,
Wherein the separator comprises a low strength region, wherein the low strength region is a portion of a separator that occupies more than one (1) winding unit from an outer end of the electrode assembly, The strength is lower than 70% as compared with the other regions except the region,
The low strength region is a portion of the separation membrane occupying not more than one (1) winding unit from the outer end of the electrode assembly and not more than (2) winding units, and the low strength region is a corona discharge treatment with a discharge amount of 50 to 120 Wmin / , And the outermost surface of the electrode assembly is surrounded by a separating membrane having the above characteristics.
The method according to claim 1,
Wherein the separation membrane comprises a porous substrate.
The method according to claim 1,
Wherein the separator has an organic / inorganic composite porous coating layer formed on at least one side of at least one surface of the porous substrate.
The method of claim 3,
Inorganic composite porous coating layer is not formed in the low strength region of the separator.
The method according to claim 1,
Wherein the negative electrode active material layer and / or the positive electrode active material layer are not formed on the negative electrode and / or the positive electrode portion facing the low strength region of the separator.
A secondary battery comprising an electrode assembly and a battery case according to any one of claims 1 to 5, wherein the battery case is a metal can.
A first separation membrane; A positive electrode comprising a positive electrode current collector made of a metal material and a positive electrode active material layer formed on one surface of the positive electrode current collector; A second separation membrane; And a negative electrode comprising a current collector for a negative electrode of a metal material and a negative electrode active material layer formed on a surface of the negative electrode current collector, and winding the first separator film so as to be disposed at an outermost periphery,
Wherein the first and second separation membranes are prepared to include a low strength region and the low strength region has a strength of 70% or less as compared with the other regions excluding the low strength region of the separation membrane, The strength region is formed by corona discharge treatment at a discharge amount of 50 to 120 Wmin / m on a part of the separator occupying not more than (1) winding units and not more than (2) winding units from the outer end of the electrode assembly. A method for manufacturing a jelly-roll type electrode assembly for a battery. delete delete

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