KR20020015158A - Lithium secondary battery - Google Patents

Abstract

PURPOSE: A lithium secondary cell is provided to prevent deterioration of reliability and safety by effectively preventing electrolyte leakage, while reducing the size and weight of the lithium secondary cell. CONSTITUTION: A lithium secondary cell comprises an electrode assembly(35) for generating a current, and a case for accommodating the electrode assembly. The case has, at the innermost surface thereof contacting the electrode assembly, an electrolyte absorbing layer(30) including a polymer having a superior moisture absorbing ability. A polyethylene acrylic acid(EAA) layer(31) serving as a thermal adhesive layer is formed onto the electrolyte absorbing layer, and a polyethylene(PE) layer(32), an EAA layer(31') and an aluminum layer(34) are sequentially deposited onto the EAA layer(31). A nylon layer(33) is deposited onto the resultant structure. The PE layer increases the stiffness of the armoring member and prevents short caused due to the lead wire. The EAA layer(31') serves as an adhesive, and the aluminum layer prevent moisture penetration and electrolyte leakage. The nylon layer prevents the outer layer from being cracked.

Description

Lithium secondary battery

The present invention relates to a lithium secondary battery, and more specifically, to a lithium secondary battery having an improved electrolyte reliability and safety by preventing an electrolyte from leaking to the outside by forming an electrolyte absorbing layer on the innermost surface in contact with an electrode assembly of a case. It is about.

Lithium secondary batteries generate electricity by lithium reciprocating between the cathode and the anode. Compared to nickel cadmium and nickel hydride batteries, such lithium secondary batteries have a higher energy density and higher voltage / volume than the nickel cadmium and nickel hydride batteries. Suitable for small size, light weight and long time use.

As described above, lithium secondary batteries have received much attention as next generation high performance batteries because they have higher voltage, much more charge / discharge cycles, and do not cause environmental problems than conventional nickel cadmium batteries and nickel hydrogen batteries. However, the lithium secondary battery has a risk of explosion, etc., and ensuring safety is a key issue.

Meanwhile, lithium secondary batteries may be classified into lithium ion batteries using liquid electrolytes and lithium ion polymer batteries using solid electrolytes, depending on the type of electrolyte.

The lithium ion battery generally uses a cylindrical case or a rectangular case as a case for sealing the electrode assembly. Recently, however, the use of the pouch instead of the case has been in the spotlight. The reason is that the use of the pouch as a case not only increases the energy density per unit weight and volume, makes the battery thinner and lighter, but also lowers the case material cost.

1 is an exploded perspective view schematically showing an example of a lithium ion battery using a pouch as a case.

Referring to FIG. 1, a lithium ion battery includes an electrode assembly 10 including a cathode 11, an anode 12, and a separator 13, and a case 11 enclosing and sealing the electrode assembly 10. It is done by At this time, the electrode assembly is formed by inserting a separator between the cathode and the anode and winding it. The cathode tab 12 and the anode tab 12 ′, which serve as an external electrical passage with the electrode assembly 10, are drawn out from the cathode and the anode to form electrode terminals 13 and 13 ′.

2 is an exploded perspective view schematically showing an example of a conventional lithium ion polymer battery.

Referring to this, the lithium ion polymer battery includes an electrode assembly 21 having a cathode, an anode, and a separator, and a case 12 enclosing and sealing the electrode assembly 21. In addition, the electrode terminals (or lead wires) 24 and 24 ′ serving as electrical paths for guiding the current formed in the electrode assembly 21 to the outside are provided at the cathode tabs and the anode tabs 23 and 23 ′. It is connected to and installed to expose a predetermined length out of the case (22).

In the lithium ion battery of FIG. 1 and the lithium ion polymer battery of FIG. 2 as described above, the electrode assemblies in the cases 11 and 22 are exposed while only a portion of the electrode terminals 13 and 13 'and 24 and 24' are exposed. (10, 21) was added thereto, an electrolyte was injected therein, and heat and pressure were applied to bond the heat-adhesive materials between the edge of the upper case and the edge of the lower case to seal the battery. Therefore, a part of the electrolyte is present in the inner space of the case, there is a risk that the electrolyte leaks to some outside. In particular, the lithium ion battery of FIG. 1 using a pouch as a case has a higher possibility of leaking the electrolyte to the outside than in the case of using a conventional cylindrical case or a rectangular case.

SUMMARY OF THE INVENTION The present invention has been made in an effort to provide a lithium secondary battery having improved reliability and safety by preventing the electrolyte from leaking to the outside by solving the above problems.

1 is an exploded perspective view schematically showing an example of a lithium ion battery according to the prior art,

Figure 2 is an exploded perspective view schematically showing an example of a lithium ion polymer battery according to the prior art,

3 is a view showing a laminated cross-sectional structure of the case according to the present invention.

<Explanation of symbols for the main parts of the drawings>

10, 21 ... electrode assembly 11, 22 ... case

13, 13 ', 24, 24' ... electrode terminal 35 ... electrode assembly

30. Electrolyte absorbing layer 31, 31 '... poly (ethylene acrylic acid) layer

32 ... polyethylene layer 33 ... nylon layer

34 ... aluminum layer

In the present invention, to achieve the above technical problem, in the lithium secondary battery having an electrode assembly for generating a current and a case containing the same,

Provided is a lithium secondary battery, characterized in that an electrolyte absorbing layer containing a polymer having excellent electrolyte solution moisture absorption capability is formed on the innermost surface of the case in contact with the electrode assembly.

In particular, the electrode assembly is a wound type, the case is preferably in the form of a pouch.

The present invention is characterized in that the electrolyte absorbing layer is formed on the innermost surface of the case sealing the electrode assembly. The electrolyte absorbing layer contains a polymer having excellent electrolyte-moisturizing ability to absorb the electrolyte present in the inner space of the case and prevent leakage to the outside.

The material constituting the electrolyte absorbing layer is a polymer having excellent electrolyte-moisture capability and a polymer having a polar functional group such as a carbonyl group in itself, and a material which does not denature within the temperature range applied during sealing of the case.

Specific examples of the polymer include vinylidene fluoride-hexafluoropropylene copolymer, poly (ethyleneacrylic acid), polymethyl methacrylate, acrylonitrile-methacrylate-styrene terpolymer, acrylonitrile-paravinylacetate Copolymers and the like.

Looking at the case of the battery according to the invention and the manufacturing method thereof in detail.

The case of the present invention has a laminated cross-sectional structure as shown in FIG. Referring to FIG. 3, an electrolytic solution absorbing layer 30 is formed on the innermost surface of an electrode assembly 35 that generates a current, and a poly (ethylene acrylic acid) layer is a heat adhesive material on the electrolyte absorbing layer 30. [poly (ethylene acrylic acid: EAA) (31) layer, the upper layer, the polyethylene layer (polyethylene: PE) 32, EAA layer 31 'and aluminum layer 34 is sequentially stacked The outermost layer has a structure in which a nylon layer 33 is laminated on top of the resulting product, wherein the PE layer 32 has a function of strengthening the strength of the packaging material and preventing a short circuit caused by the lead wire. 31 'has an adhesive function, and the aluminum layer 34 has moisture intrusion prevention and electrolyte leakage preventing functions, and the nylon layer 33 smoothly forms the inner layer while preventing cracks in the outer layer. Has a function to let

The thickness of each layer of the case having the above-described laminated structure is as follows.

The thickness of the heat-adhesive material layer 31 is 5 to 50 µm, the thickness of the polyethylene layer 32 is 5 to 50 µm, the thickness of the EAA layer 31 'is 5 to 50 µm, and the thickness of the aluminum layer 34. Is 10 to 100 µm, and the thickness of the outermost nylon layer 33 is 5 to 50 µm. In this case, when the thickness of the heat adhesive material layer 31 is less than 5 μm, the adhesive strength is weak. When the thickness of the polyethylen layer 32 is less than 5 µm, the short circuit prevention effect is small. When the thickness of the polyethylen layer 32 exceeds 50 µm, it is not preferable due to weight increase.

In the case where the thickness of the EAA layer 31 'is less than 5 µm, the adhesive strength is weak, and in the case where the thickness of the EAA layer 31' exceeds 50 µm, the weight is not preferable. The shape retention is poor, and when the thickness is greater than 100 µm, the battery weight is increased, which is not preferable. When the thickness of the nylon layer is less than 5 µm, the water resistance is weak, and when the thickness exceeds 50, the adhesion is poor, which is undesirable.

The manufacturing method of the case having the structure as described above will be described.

A case is manufactured by sequentially stacking a poly (ethylene acrylic acid: EAA) layer, a polyethylene (PE) layer, an EAA layer, an aluminum layer, and a nylon layer.

Separately, a polymer having excellent electrolytic solution moisture content is dissolved in a solvent to form a composition for forming an electrolytic solution absorbing layer. In this case, any solvent may be used as long as it is an organic solvent capable of dissolving a polymer having excellent electrolyte-moisture capability. Specific examples thereof include N-methylpyrrolidone, tetrahydrofuran, and cyclohexanone. At this time, the content of the polymer excellent in the electrolyte solution moisture content is 4 to 30% by weight, the content of the solvent is 70 to 96% by weight.

The composition for forming the electrolyte absorbing layer is coated on top of the EAA layer of the case, and then dried by hot air at 50 to 100 ° C. to complete the case in which the electrolyte absorbing layer is formed. In this case, the thickness of the electrolyte absorbing layer is preferably 10 to 50 µm. If the thickness of the electrolyte absorbing layer exceeds 50 µm, the electrolyte is absorbed by an appropriate amount or more to increase the material cost. I can't.

The case according to the invention has a very wide range of applications. That is, when the separator is inserted between the cathode and the anode and the electrode assembly is wound in a jelly roll method (FIG. 1), the separator is stacked between the cathode and the anode to form a bicell electrode. After the assembly is made, it is applicable to the case where it is embedded in the case (Fig. 2). In particular, the case of the present invention is very useful for lithium ion batteries incorporating a jelly roll-type wound electrode assembly having a high possibility of electrolyte leakage.

Hereinafter, the present invention will be described in detail with reference to the following examples, but the present invention is not limited to the following examples.

Example 1

On the electrode assembly, a lithium ion was formed by sequentially laminating a PE layer having a thickness of 30 µm, an EAA layer having a thickness of 30 µm, an aluminum layer having a thickness of 50 µm, and a nylon layer having a thickness of 20 µm, on top of an EAA layer having a thickness of 30 µm. The case of the battery was produced.

10 g of vinylidene fluoride-hexafluoropropylene copolymer was dissolved in 100 g of N-methylpyrrolidone, and then coated on the EAA layer of the case, followed by hot air drying at 80 ° C. to prepare a case having an electrolyte absorbing layer. .

A lithium ion battery was completed by placing a wound electrode assembly in the case, injecting an electrolyte solution, and then sealing an edge portion of the upper outer cover and an edge portion of the lower outer cover.

Example 2

When forming the electrolyte absorbing layer, lithium was carried out in the same manner as in Example 1 except that acrylonitrile-methacrylate-styrene terpolymer was used instead of vinylidene fluoride-hexafluoropropylene copolymer. The ion battery was completed.

Comparative example

A lithium ion battery was completed in the same manner as in Example 1 except that the electrolyte absorbing layer was not formed on the EAA layer of the case.

In the lithium ion battery prepared according to Example 1-2 and Comparative Example, it was examined whether the electrolyte leaked. The leakage of the electrolyte was evaluated by the change in weight by applying a force of 500kgf to the battery.

As a result, it was confirmed that the lithium ion battery according to Example 1-2 hardly leaked the electrolyte solution as compared with the case of the comparative example.

In the lithium secondary battery of the present invention, an electrolyte solution-containing layer including a polymer having excellent electrolyte solution-moisture capability is formed on the innermost surface of the lithium secondary battery in contact with the electrode assembly, thereby effectively preventing the electrolyte from leaking to the outside. Therefore, it is possible to prevent the degradation of the reliability and safety of the battery due to the electrolyte leakage. In particular, in the case of incorporating an odor type electrode assembly, a pouch can be used instead of a conventional rectangular or cylindrical case, thereby obtaining a lighter and thinner lithium ion battery.

Claims (3)

In a lithium secondary battery having an electrode assembly for generating a current and a case therein, A lithium secondary battery, characterized in that an electrolyte absorbing layer containing a polymer having excellent electrolyte solution moisture absorption capability is formed on the innermost surface of the case in contact with the electrode assembly. The polymer of claim 1 wherein the polymer is vinylidene fluoride-hexafluoropropylene copolymer, poly (ethyleneacrylic acid), polymethylmethacrylate, acrylonitrile-methacrylate-styrene terpolymer and acrylonitrile-para. Lithium secondary battery, characterized in that at least one selected from the group consisting of vinyl acetate copolymer. The lithium secondary battery of claim 1, wherein the electrode assembly is wound and the case is a pouch.