KR20180065167A - Electrode for Secondary Battery Comprising PTC layer with Crosslinking-Polymer and Manufacturing Method thereof - Google Patents

Abstract

The present invention relates to an electrode for a secondary battery and a manufacturing method thereof. The electrode comprises: a current collector; a positive temperature coefficient (PTC) layer applied on the current collector, including a polymer, a conductive material and a binder, and having a property that a conductive path of the conductive material is cut off due to volume expansion of the polymer when the temperature rises, wherein the polymer is crosslinked by E-bean irradiation; and an active material layer applied on the PTC layer. The present invention enhances reliability of the battery safety by suppressing a flow of conductive material particles.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to an electrode for a secondary battery including a polymer-crosslinked PTC layer and a method for manufacturing the same. [0002] Electrode for Secondary Battery Comprising PTC layer with Crosslinking-

The present invention relates to an electrode for a secondary battery comprising a polymer-crosslinked PTC layer and a method for producing the same.

Due to the rapid increase in the use of fossil fuels, the demand for the use of alternative energy or clean energy is increasing. As a part of this, the most active field of research is electric power generation and storage.

At present, a typical example of an electrochemical device utilizing such electrochemical energy is a secondary battery, and the use area thereof is gradually increasing.

Such a secondary battery is classified into a cylindrical battery and a prismatic battery in which an electrode assembly is embedded in a cylindrical or rectangular metal can according to the shape of the battery case, and a pouch type battery in which an electrode assembly is embedded in a pouch-shaped case of an aluminum laminate sheet do.

The electrode assembly of the anode / separator / cathode structure constituting the secondary battery is largely classified into a jelly-roll type (winding type) and a stack type (laminate type) depending on its structure. In the jelly-roll type electrode assembly, an electrode active material or the like is coated on a metal foil used as a current collector, dried and pressed, cut into a band shape having a desired width and length, and a cathode and an anode are diaphragm- . The jelly-roll type electrode assembly is suitable for a cylindrical battery. However, the jelly-roll type electrode assembly has disadvantages such as peeling of an electrode active material layer and low space utilization when applied to a rectangular or pouch type battery. On the other hand, the stacked electrode assembly has a structure in which a plurality of anode and cathode unit members are sequentially stacked, and it is easy to obtain a square shape.

In addition, a full cell or a positive electrode (cathode) / separator / cathode (anode) having an anode / separator / cathode basic structure of a constant unit size, which is an advanced assembly of the jelly- A stack / folding type electrode assembly having a structure in which a bicell having a basic structure of a membrane / separator / anode (cathode) is folded by using a continuous membrane of a long length has been developed and disclosed in Korean Patent Application Laid- -82058, 2001-82059, 2001-82060, and the like.

On the other hand, the lithium secondary battery is at a high level in terms of safety. However, in order to secure high capacity and high output, it is desired to improve the productivity of the battery in terms of safety. For example, when the lithium ion secondary battery is overcharged, there is a possibility of heat generation.

In addition, when the electrode assembly of the secondary battery is penetrated by a sharp needle-like conductor having electrical conductivity like a nail, the anode and the cathode are electrically connected by the needle-shaped conductor, and the current flows to the needle-shaped conductor having low resistance. At this time, deformation of the penetrating electrode occurs, and a high resistance heat is generated by the electric current passing through the contact resistance portion between the positive electrode active material and the negative electrode active material. When the temperature of the electrode assembly rises above a critical value due to the heat, contact between the positive electrode and the negative electrode occurs due to shrinkage of the separator, resulting in a short circuit. This short circuit may lead to thermal runaway, which may be a major cause of ignition or explosion of the electrode assembly and the secondary battery comprising it.

In addition, when the electrode active material or the collector bent by the needle-shaped conductor comes in contact with the opposite electrode facing each other, heat generation higher than the resistance heat is generated, which can further accelerate the above-described thermal runaway phenomenon. May occur more seriously in a bi-cell including electrodes and an electrode assembly containing the same.

As an approach to mitigate these problems, attempts have been made to incorporate a positive temperature coefficient (PTC) material, which exhibits a constant conductivity at the operating temperature of a general battery, but has a characteristic that the resistance rapidly increases when the temperature rises, there was. Japanese Patent Application Laid-Open Nos. 2002- 000568, 2001-1050294, and 2009-176599 propose electrodes having PTC layers formed thereon. Like the PTC element, the PTC layer is a layer having a function of raising the electric resistance (direct current resistance) in accordance with the heat generation of the battery. The electrodes described in JP-A-2002-526897, JP-A-10-50294 and JP-A-2009-176599 each include a positive electrode active material layer or a negative electrode active material layer, a PTC layer and a current collector in this order Stacked body.

In such a PTC layer, a conductive network is formed by the contact between the conductive particles. In this conductive network, when the melting point of the polymer resin is reached, the resin expands, and the conductive particles become non-contact with each other, thereby interrupting the current.

However, when the polymer is melted at a temperature higher than the melting point of the polymer resin, the fluidity of the conductive particles increases, and a new conductive network is generated by the movement of the conductive particles, thereby causing a negative temperature coefficient (NTC) There is a problem that sufficient safety can not be ensured.

Accordingly, there is a need to develop a technique for securing the safety of a battery by improving the reproducibility of the PTC effect while solving such a problem.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned problems of the prior art and the technical problems required from the past.

The inventors of the present application have conducted intensive research and various experiments, and as described later, the E-beam crosslinking of the polymer contained in the PTC layer causes structural stability of the polymer due to the crosslinking chain, The present invention has been accomplished on the basis of confirming that it is possible to increase the reliability of the cell safety by increasing the PTC effect and removing the NTC phenomenon occurring above the melting point of the polymer.

Therefore, in the electrode for a secondary battery according to the present invention,

Collecting house;

Wherein the polymer is coated on the current collector and comprises a polymer, a conductive material and a binder, the polymer is crosslinked by E-beam irradiation, and when the temperature rises, A PTC (Positive Temperature Coefficient) layer having a property of blocking the conductive path of the ash; And

An active material layer coated on the PTC layer;

And is characterized by comprising:

Since the PTC layer includes the polymer and the conductive material, the polymer expands to protect the product or the electronic circuit from being damaged when the specific temperature or the overcurrent flows, and has a thermal and electrical protection property .

However, as described above, the conventional polymer used for the PTC layer is a quasi-crystalline polymer. When the electrode temperature is increased to the melting point, the polymer is melted and the volume increases due to thermal expansion, However, when the temperature is higher than the melting point, the molten polymer has high fluidity so that the conductive particles migrate and re-agglomerate to form a new conductive network. (NTC phenomenon), so that the electrode resistance was lowered again and safety was again questioned.

On the other hand, according to the present invention, in the PTC layer, the polymer is crosslinked by E-beam irradiation, and when the temperature rises above the melting point of the polymer, the polymer expands to break the conductive network of the conductive materials, And the overall shape is maintained in a semi-solid form like a jelly shape, and the NTC phenomenon can be prevented because the conductive material particles do not flow.

Therefore, the polymer of the PTC layer should be crosslinked by E-beam irradiation. At this time, the degree of crosslinking may be changed according to the amount of irradiation of the E-beam, which appears as a gel fraction. Therefore, the dose and the gel fraction of the E-beam are the most important factors for solving the problems of the present invention. The inventors of the present application have conducted intensive studies and found that the E-beam condition and the gel fraction value Respectively.

In detail, in order to achieve the desired effect according to the present invention, the polymer may be crosslinked by irradiating an E-beam in the range of 50 kGy to 250 kGy, more specifically 70 kGy to 200 kGy Specifically, it can be crosslinked by irradiating E-beam in the range of 100 kGy to 200 kGy.

When the dose of the E-beam is less than 50 kGy, the degree of cross-linking of the polymer is low and it is impossible to prevent the occurrence of the NTC phenomenon desired by the present invention. On the other hand, when the dose exceeds 250 kGy, The cleavage of the polymer occurs due to the crosslinking, and since the degree of crosslinking is reduced due to the cleavage phenomenon, the viscosity of the polymer is not improved when the polymer is melted. Therefore, a new conductive network is formed by the polymer flow, Which is undesirable.

As described above, the cross-linking of the polymer can be known from the gel fraction. The gel fraction is defined as the gel fraction (%) = Wf / (Wf) as the weight ratio of the insoluble dry weight (Wf) Specifically, the initial weight (Wi) of the PTC layer was measured and then placed in a 100-mesh net, refluxed in xylene for 10 hours, dried for 6 hours at 100 ° C, The gel fraction can be obtained by measuring the gel permeation constant (Wf). Here, the gelation means that radicals are generated in the polymer chain as the E-beam is irradiated, and crosslinking occurs between the polymer chains at the part where radicals are generated.

Therefore, the increase in the gel fraction indicates that the gelation is caused by cross-linking of the polymer. The gel fraction that is too low is in a state where the crosslinking is hardly performed or the crosslinking is performed, It can be said that it means. Therefore, the gel fraction continuously increases according to the irradiation amount, and when the irradiation amount exceeds a certain amount, the cleavage phenomenon occurs, so that the gel fraction can not be increased beyond a predetermined value.

For this reason, in order to exhibit the effect according to the present invention, in order to reduce the cleavage of the polymer and to sufficiently crosslink the polymer, the gel fraction of the PTC layer obtained by the irradiation is 30 to 70%, more specifically 35 to 65% More specifically, it may be 40% to 60%. In other words, it is more preferable that the gel fraction is within the above-mentioned range, because it means that the cross-linking is sufficiently carried out but the cutting phenomenon is hardly occurred.

When the cross-linking is performed, the polymer swells and the conductive network of the conductive materials is broken even when the temperature rises above the melting point of the polymer. The viscosity of the conductive material is not completely melted and the overall shape can be maintained in a semi- have

The melting point (Tm) of the polymer may be set in the range of 80 to 200 degrees Celsius, and more specifically, in the range of 90 to 140 degrees Celsius. As described above, it is preferable that the melting point of the polymer is set to a temperature at which the PTC phenomenon occurs, that is, a temperature capable of functioning as a fuse for sharply increasing the resistance during the generation of an excessive current or a rise in temperature, , It is possible to shut off the current when an abnormality occurs in the battery itself or various devices equipped with the battery to suppress heat generation and to stop the supply of electric power to various devices from the battery. have. Further, there is no malfunction during normal use, and it is possible to obtain an advantage that the current can be reliably cut off in the event of overcharging or the like.

If the melting point is too low and the temperature is too low, a reaction occurs in the operating temperature range of the battery to cause performance problems. If the melting point is too high, the PTC effect does not appear until the internal temperature of the battery rises to over 140 ° C., Which is not preferable from the viewpoint of safety.

Such a polymer is not particularly limited as long as it is a particle that is a thermoplastic resin, but may be a semi-crystalline material, and more specifically, it may have a crystallinity of 10% or more. Examples thereof include polyolefins, ethylene-vinyl acetate copolymers (EVA), ethylene-acrylic acid copolymers, ethylene-methacrylic acid copolymers, polyvinyl chloride, polyvinylidene chloride, polyvinyl fluoride, polyvinylidene fluoride, (Meth) acrylate, and (meth) acrylate, and an ionomer resin such as polyacrylate, polyamide, polystyrene, polyacrylonitrile, thermoplastic elastomer, polyethylene oxide, polyacetal, thermoplastic modified cellulose, polysulfone, polymethyl The polyolefin may be one selected from the group consisting of high density polyethylene, medium density polyethylene, linear low density polyethylene, low density polyethylene, or polypropylene, in particular, as the polyolefin.

The particle size of the polymer prior to crosslinking is not particularly limited, but may be in the range of from 0.2 to 10 micrometers, and more specifically from 0.3 to 5 micrometers, and more specifically, from 0.5 to 1 micrometer .

This is in accordance with the reason why the slurry must be coated with a thin film when the slurry is coated to form a PTC layer in the current collector.

The thickness of the PTC layer may be 0.1 to 50 micrometers, specifically 1 to 20 micrometers, more specifically 1 to 10 micrometers, and the thickness of the PTC layer may be 0.1 micrometer , The PTC effect is not easily manifested. On the other hand, when the thickness exceeds 50 micrometers, the thickness of the electrode increases and the energy density becomes low, so that the thin film coating is preferably performed in the above range.

However, since the particle size of the polymer affects the thin film coating of the PTC layer, the particle diameter of the polymer should satisfy the above range. If the particle size is less than 0.2 micrometer, it is difficult to disperse the particles and the volume expansion is small during melting. Therefore, when the particle size exceeds 10 micrometers, It is not desirable.

The crosslinked polymer may be contained in an amount of 60% by weight to 90% by weight, specifically 80% by weight to 90% by weight, based on the total weight of the PTC layer. When the amount of the crosslinked polymer is less than 60 wt%, the amount of the crosslinked polymer for expressing the PTC effect is not sufficient. If the amount exceeds 90 wt%, the content of the conductive material and the binder is relatively high The formation of the conductive network between the active material and the current collector at the normal operating temperature of the battery is insufficient or the bonding force with the current collector interface of the PTC layer is lowered.

On the other hand, the conductive material contained in the PTC layer may be graphite, carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black,

Of the carbon particles, metal particles, WC, B4C, such as nickel particles, ZrC, NbC, MoC, TiC, metal carbides, such as TaC, TiN, ZrN, TaN, etc. of the metal nitride, WSi _2, MoSi ₂ and so on and so on of the metal silicide . Among these, in particular, it may be carbon particles, metal particles, or CNTs, and more specifically carbon particles. The conductive material may be used singly or in combination of two or more kinds.

The particle size of the conductive material can be from 50 to 100 nanometers, and more specifically from 70 to 90 nanometers.

The conductive material may include 5% by weight to 30% by weight, more specifically 10% by weight to 25% by weight, and more preferably 10% by weight to 20% by weight, based on the total weight of the PTC layer .

If the amount is less than 5% by weight, the sufficient electrical conductivity network between the active material and the current collector can not be secured at the normal operating temperature of the battery, and the output characteristics are deteriorated. When the amount exceeds 30% by weight, The content of the polymer and the binder is undesirable.

The binder may be at least one selected from the group consisting of polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene , Polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butylene rubber, and fluorine rubber.

The binder may comprise from 1% to 20% by weight based on the total weight of the PTC layer. If the content is less than 1% by weight, the adhesion between the PTC layer and the current collector or the active material layer is deteriorated. If it exceeds 20% by weight, the electrode resistance is increased and the content of the polymer and the conductive material is relatively decreased It is not preferable.

The active material layer includes a conductive material and a binder as described above together with the positive electrode active material or the negative electrode active material, depending on the type of the electrode.

The cathode active material may be, for example, a layered compound such as lithium cobalt oxide (LiCoO ₂ ) or lithium nickel oxide (LiNiO ₂ ) or a compound substituted with one or more transition metals; Lithium manganese oxides such as Li _{1 + x} Mn _{2 -x} O ₄ (where x is 0 to 0.33), LiMnO ₃ , LiMn ₂ O ₃ , LiMnO ₂ and the like; Lithium copper oxide (Li ₂ CuO ₂ ); Vanadium oxides such as LiV ₃ O ₈ , LiV ₃ O ₄ , V ₂ O ₅ and Cu ₂ V ₂ O ₇ ; A Ni-site type lithium nickel oxide expressed by the formula LiNi _1-x M _x O ₂ (where M = Co, Mn, Al, Cu, Fe, Mg, B or Ga and x = 0.01 to 0.3); Formula _{LiMn 2-x M x O 2} ( where, M = Co, Ni, Fe , Cr, and Zn, or Ta, x = 0.01 ~ 0.1 Im) or Li ₂ Mn ₃ MO ₈ (where, M = Fe, Co, Ni, Cu, or Zn); LiMn ₂ O _{4 in} which a part of Li in the formula is substituted with an alkaline earth metal ion; Disulfide compounds; Fe ₂ (MoO ₄ ) ₃ , and the like. However, the present invention is not limited to these.

The negative electrode active material may include, for example, carbon such as non-graphitized carbon or graphite carbon; _{Li x Fe 2 O 3 (0≤x≤1} ), Li x WO 2 (0≤x≤1), Sn x Me 1-x Me 'y O z (Me: Mn, Fe, Pb, Ge; Me' : Metal complex oxides such as Al, B, P, Si, Group 1, Group 2, Group 3 elements of the periodic table, Halogen, 0 < x &lt; Lithium metal; Lithium alloy; Silicon-based alloys; Tin alloy; _{SnO, SnO 2, PbO, PbO} 2, Pb 2 O 3, Pb 3 O 4, Sb 2 O 3, Sb 2 O 4, Sb 2 O 5, GeO, GeO 2, Bi 2 O 3, Bi 2 O 4, and Bi ₂ O ₅ ; Conductive polymers such as polyacetylene; Li-Co-Ni-based materials and the like can be used.

The current collector may also vary depending on the type of electrode.

The current collector is generally formed to a thickness of 3 to 300 탆 and is not particularly limited as long as it has high conductivity without causing chemical change in the battery. If the electrode is a positive electrode, for example, stainless steel , Aluminum, nickel, titanium, and a surface of aluminum or stainless steel surface-treated with carbon, nickel, titanium or silver can be used. Specifically, aluminum can be used. The current collector may have fine irregularities on the surface thereof to increase the adhesive force of the cathode active material, and various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric are possible.

If the electrode is a negative electrode, the surface of the surface of copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel may be surface-treated with carbon, nickel, Can be used. In addition, like the positive electrode collector, fine unevenness can be formed on the surface to enhance the bonding force of the negative electrode active material, and it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams and nonwoven fabrics.

Such an active material layer may be formed in such a range that the PTC layer: active material layer has a thickness ratio of 1:10 to 1:20.

In the case where the PTC layer is too thick, the capacity is difficult to express and the energy density is lowered because the PTC layer is too thick. If the PTC layer is too thick, the PTC layer is too thin to exhibit the PTC effect Not enough.

The present invention also provides a method of manufacturing the electrode for a secondary battery.

Specifically,

(a) a step of applying a first slurry in which a conductive material, a binder, and a polymer are mixed in a solvent on one side or both sides of a current collector, followed by drying to form a pre-PTC layer;

(b) irradiating the pre-PTC layer with an E- beam to crosslink the polymer to form a PTC layer; And

(c) applying and drying a second slurry in which a mixture containing an electrode active material is mixed with a solvent on the PTC layer to form an active material layer;

. &Lt; / RTI &gt;

After the above process, a rolling process may be optionally performed.

As described above, the particle diameter of the polymer contained in the pre-PTC layer may be 0.2 to 10 micrometers, and the E-beam of the process (b) may be in the range of 30 kGy to 250 kGy, 200 kGy, and more particularly in the range of 100 kGy to 200 kGy.

The electrode is used to manufacture an electrode assembly together with a separator, and the electrode assembly is embedded in a battery case together with an electrolyte to produce a secondary battery.

Such a lithium secondary battery, a structure of a battery pack including the same as a unit battery, and a method of manufacturing a device including a battery pack as a power source are well known in the art, and thus a detailed description thereof will be omitted.

As described above, the electrode for a secondary battery according to the present invention includes the PTC layer in which the polymer is E-beam crosslinked, so that the structural stability of the polymer due to the crosslinking chain of the polymer is maintained at a predetermined temperature or higher than the melting point of the polymer. By suppressing the flow of the conductive material particles, NTC phenomenon occurring at a temperature higher than the melting point of the polymer is removed, thereby improving the reproducibility of the PTC effect and consequently improving the reliability of the battery safety.

1 is a photograph showing the fluidity of the PTC layer at a temperature higher than the melting point of the polymer according to Experimental Example 1 of the present invention;
2 is a table showing the interface resistance of the PTC electrode according to Experimental Example 3 of the present invention.

Hereinafter, the present invention will be described in detail with reference to examples. However, the scope of the present invention is not limited to these examples.

&Lt; Example 1 >

Manufacture of PTC layer

A PTC slurry was prepared by mixing polyethylene (particle diameter: 1 mu m, molecular weight: 700,000 to 1,000,000) as a polymer, SBR as a binder, and carbon black mixture as a conductive material in distilled water at a weight ratio of 78: 20: 2, , And dried at an irradiation dose of 100 kGy (irradiated at a rate of 5.7 m / min with a current of 30 mA in a 0.7 MeV accelerator) to form a PTC layer.

Manufacture of anode

LiNi _0.6 Mn _0.2 Co _0.2 O ₂ as a cathode active material, SBR as a binder, and carbon black mixture as a conductive material were mixed in distilled water at a weight ratio of 90: 5: 5 to prepare a cathode active material slurry. This slurry was coated on the aluminum foil The PTC layer thus formed was coated to a thickness of 60 탆 and then dried to form a positive electrode active material layer, thereby preparing a positive electrode.

&Lt; Example 2 >

Manufacture of PTC layer

A PTC slurry was prepared by mixing polyethylene (particle diameter: 1 mu m, molecular weight: 700,000 to 1,000,000) as a polymer, SBR as a binder, and carbon black mixture as a conductive material in distilled water at a weight ratio of 78: 20: 2, To a thickness of 15 탆, dried, and then irradiated with an E-beam at a dose of 100 kGy to form a PTC layer.

Cathode manufacturing

A negative electrode active material slurry was prepared by mixing natural graphite as a negative active material, SBR as a binder, CMC as a thickener, and carbon black mixture as a conductive material in distilled water at a weight ratio of 96: 2: 1: 1. The slurry was applied to a PTC layer formed on the copper foil to a thickness of 70 탆 and dried to form a negative electrode active material layer, thereby preparing a negative electrode.

&Lt; Example 3 >

A positive electrode was prepared in the same manner as in Example 1, except that the dose of the E-beam was changed to 70 kGy in Example 1.

<Example 4>

A positive electrode was prepared in the same manner as in Example 1, except that the dose of the E-beam was changed to 200 kGy in Example 1. [

&Lt; Comparative Example 1 &

A positive electrode was prepared in the same manner as in Example 1, except that the E-beam irradiation was not performed in Example 1.

&Lt; Comparative Example 2 &

A positive electrode was prepared in the same manner as in Example 1, except that the dose of the E-beam was changed to 30 kGy in Example 1.

&Lt; Comparative Example 3 &

A positive electrode was prepared in the same manner as in Example 1, except that the irradiation amount of the E-beam was changed to 300 kGy in Example 1.

&Lt; Comparative Example 4 &

A negative electrode was prepared in the same manner as in Example 2, except that the E-beam irradiation was not carried out in Example 2 above.

&Lt; Comparative Example 5 &

A negative electrode was prepared in the same manner as in Example 2, except that the irradiation amount of the E-beam was changed to 300 kGy in Example 2.

&Lt; Comparative Example 6 >

A positive electrode was prepared in the same manner as in Example 1, except that the particle size of the polymer was 0.1 mu m.

&Lt; Comparative Example 7 &

A negative electrode was prepared in the same manner as in Example 2 except that the polymer particle size was 0.1 탆 in Example 2.

<Experimental Example 1>

In order to confirm the fluidity of the PTC layer of Example 1 and Comparative Example 1 at a temperature higher than the melting point of the polymer, it was heat treated on a hot plate at 140 degrees for 5 minutes, Respectively.

Referring to FIG. 1, the PTC layer of Example 1 is structurally stabilized to maintain the PTC layer because the polymer is crosslinked to increase the viscosity, and the conductive material is present as it is as the conductive network is disconnected. On the other hand, Layer, the polymer is completely melted and the molten polymer migrates to the pores, and only the conductive material remains in the PTC layer, and all of them are spread, and in some cases, the conductive network is formed.

<Experimental Example 2>

The gel fraction of the PTC layers of Examples 1 to 4 and Comparative Examples 1 to 5 was measured and shown in Table 1 below.

The gel fraction was measured by measuring the initial weight (Wi) of the PTC layer and then putting it into a 100-mesh net, refluxing in xylene for 10 hours, drying the insoluble weight (Wf) at 100 ° C. for 6 hours , And the gel fraction (%) = Wf / Wi * 100.

Example 1 Example 3 Example 4 Comparative Example 1 Comparative Example 2 Comparative Example 3 Example 2 Comparative Example 4 Comparative Example 5 Gel fraction (%) 53% 42% 45% 5% 10% 29% 53% 5% 29%

As shown in Table 1, the gel fraction of the PTC layers of Examples 1, 3, 4 and 2 according to the present invention is 40 to 55%, while Comparative Examples 1 to 5 have a gel fraction of less than 30%.

The gel fraction is improved as the gelation is caused by crosslinking of the polymer of the PTC layer. When the E-beam is not irradiated or the amount of irradiation of the E- beam is small, the gel fraction is very low, When the amount of E-beam irradiation is large, the cross-linking mode of the polymer is increased, but also the cleavage phenomenon occurs and the gel fraction is lowered again.

On the other hand, when the irradiation amount of the range according to the present invention is examined, the crosslinking is sufficiently performed, but the gel fraction is close to 50% as long as the cleavage phenomenon does not occur.

These gel fractions lead to differences in physical properties, and as shown below, this difference affects the electrode resistance and also affects the cell safety, so it is most appropriate to have a range of 40 to 60%.

Specifically, when the gel fraction is small, crosslinking of the polymer hardly occurs, or when the polymer is melted at a temperature higher than the melting point of the polymer due to a large number of cleaved polymers, and the molten polymer moves to the pore, A phenomenon occurs.

<Experimental Example 3>

The electrodes of Example 1 and Comparative Example 1 were heat-treated for 5 minutes at 25, 60, 90, 120, and 140 degrees Celsius on a hot plate, and the interfacial resistance in the electrodes was measured using a multi- 2, and the electrodes of Examples 1 to 4 and Comparative Examples 1 to 5 were heat-treated at 140 degrees Celsius for 5 minutes, and the interfacial resistance in the electrodes was measured using a multi-probe resistance meter. The results are shown in Table 2 below .

Example 1 Example 3 Example 4 Comparative Example 1 Comparative Example 2 Comparative Example 3 Example 2 Comparative Example 4 Comparative Example 5 Resistance after 140 ° heat treatment (ohm * cm ² ) 6.9 5.4 6.2 2.6 2.7 4.3 5.8 2.1 3.8

Referring to FIG. 2 and Table 2, it can be seen that the electrode including the crosslinked PTC layer according to the present invention has an electrode resistance continuously increased even when it is increased to 130 ° C. or more. However, in the comparative examples, Although the resistance increases, it decreases again at 140 degrees, indicating that the resistance is less than 4.5 ohm * cm &lt; ² &gt;.

Therefore, at a temperature of 140 degrees Celsius or more, the conductivity becomes rather high, and the risk of ignition and the like may be increased, so that the safety of the battery can not be relied upon.

<Experimental Example 4>

Manufacture of Secondary Battery

Lithium metal as a counter electrode to the positive electrode or negative electrode prepared in Examples 1 to 4 and Comparative Examples 1 to 5, a polyethylene porous film of Asahi as a separator, and a mixture of ethylene carbonate / dimethyl carbonate / diethyl carbonate 3: 3: 4 by volume) the large cell in folding the bi-cell 19 to the electrode size of 100mm * 250 mm size using the dissolved LiPF ₆ was prepared in the.

A large cell activated by SOC20 was charged with SOC100 and then a nail having a diameter of 5 mm made of iron was pierced through the center of the battery cells prepared above using a nail penetration tester to measure the ignition. The results are summarized in Table 3 as PASS / FAIL according to utterance.

Example 1 Example 3 Example 4 Comparative Example 1 Comparative Example 2 Comparative Example 3 Example 2 Comparative Example 4 Comparative Example 5 Whether ignited PASS PASS PASS FAIL FAIL FAIL PASS FAIL FAIL

The results are shown in Table 3. Referring to Table 3, in the case of the electrode including the polymer crosslinked PTC layer according to the present invention, ignition did not occur in the needle penetration test and the test was passed with reliability. In the comparative examples, I can confirm that I did not pass the test.

<Experimental Example 5>

The electrodes of Example 1 and Comparative Examples 6 and 7 were heat-treated on a hot plate at a temperature of 140 degrees Celsius for 5 minutes, and electrode resistance was measured using a 4-probe resistance meter. The results are shown in Table 4, 6, and 7 were used to fabricate a large cell as in the experimental example, the nail penetration test results in SOC100 are shown in Table 5 below.

Example 1 Comparative Example 6 Example 2 Comparative Example 7 Resistance after 140 ° heat treatment (ohm * cm ² ) 6.9 3.1 5.8 2.5

Example 1 Comparative Example 6 Example 2 Comparative Example 7 Whether ignited PASS FAIL PASS FAIL

Referring to Tables 4 and 5, it can be confirmed that the electrode including the PTC layer to which the 1 탆 polymer particle according to the present invention is applied passes the nail penetration test because the electrode resistance is high at 140 캜. However, , It was confirmed that the polymer had a small size of 0.1 ㎛ and the volume expansion was small and the size of the polymer was small during the dissolution. Thus, the dispersion was not well dispersed due to the dispersion problem and the electrode resistance was low and the nail penetration test could not be passed. As the size of the polymer increases, the resistance of the electrode increases and the energy density decreases. Therefore, it is not applicable because the safety is improved.

Therefore, if the size of the polymer is as small as about 0.1 占 퐉, the electrode resistance is lowered, and the risk of ignition may increase, so that the safety of the battery can not be relied upon.

While the invention has been shown and described with reference to certain embodiments thereof, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined by the appended claims.

Claims (19)

Collecting house;
Wherein the polymer is coated on the current collector and comprises a polymer, a conductive material and a binder, the polymer is crosslinked by E-beam irradiation, and when the temperature rises, A PTC (Positive Temperature Coefficient) layer having a property of blocking the conductive path of the ash; And
An active material layer coated on the PTC layer;
And the second electrode is made of a metal. The secondary battery electrode according to claim 1, wherein the polymer of the PTC layer is crosslinked by irradiating an E- beam in a range of 50 kGy to 250 kGy. The electrode for a secondary battery according to claim 1, wherein the gel fraction of the PTC layer is 30 to 70%. The secondary battery electrode according to claim 1, wherein the melting point (Tm) of the polymer is set in a range of 80 to 200 degrees Celsius. The electrode for a secondary battery according to claim 1, wherein the PTC layer expands at a temperature higher than the melting point of the polymer, and maintains the overall shape in a semi-solid form. The method of claim 1, wherein the polymer is selected from the group consisting of polyolefin, ethylene-vinyl acetate copolymer (EVA), ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer, polyvinyl chloride, polyvinylidene chloride, polyvinyl fluoride, poly (Meth) acrylate, vinylidene fluoride, polyamide, polystyrene, polyacrylonitrile, thermoplastic elastomer, polyethylene oxide, polyacetal, thermoplastic modified celluloses, polysulfone, polymethyl And an ionomer resin. &Lt; RTI ID = 0.0 &gt; 11. &lt; / RTI &gt; The secondary battery electrode according to claim 6, wherein the polyolefin is selected from high-density polyethylene, medium-density polyethylene, linear low-density polyethylene, and low-density polyethylene. The electrode for a secondary battery according to claim 1, wherein the polymer has a particle diameter of 0.2 to 10 micrometers prior to crosslinking. The electrode for a secondary battery according to claim 1, wherein the crosslinked polymer is contained in an amount of 40 wt% to 90 wt% based on the total weight of the PTC layer. The conductive material according to claim 1, wherein the conductive material is at least one selected from the group consisting of graphite, carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, metal particles, metal carbides, metal nitrides, metal silicides, And at least one selected from the group consisting of silicon oxide and silicon oxide. The electrode for a secondary battery according to claim 1, wherein the conductive material has a particle size of 50 to 100 nanometers. The electrode for a secondary battery according to claim 1, wherein the conductive material is contained in an amount of 5 to 30% by weight based on the total weight of the PTC layer. The method of claim 1, wherein the binder is selected from the group consisting of polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoro Wherein at least one selected from the group consisting of ethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butylene rubber and fluorine rubber. The electrode for a secondary battery according to claim 1, wherein the binder is contained in an amount of 1 to 20% by weight based on the total weight of the PTC layer. The electrode for a secondary battery according to claim 1, wherein the thickness of the PTC layer is 0.1 to 50 micrometers. The electrode for a secondary battery according to claim 1, wherein the PTC layer: active material layer has a thickness ratio of 1:10 to 1:20. A method of manufacturing an electrode for a secondary battery according to claim 1,
(a) a step of applying a first slurry in which a conductive material, a binder, and a polymer are mixed in a solvent on one side or both sides of a current collector, followed by drying to form a pre-PTC layer;
(b) irradiating the pre-PTC layer with an E- beam to crosslink the polymer to form a PTC layer; And
(c) applying and drying a second slurry in which a mixture containing an electrode active material is mixed with a solvent on the PTC layer to form an active material layer;
Wherein the electrode is made of a metal. 18. The method of claim 17, wherein the polymer of step (a) has a particle diameter of 0.2 to 10 micrometers. 18. The method of claim 17, wherein the E-beam of step (b) is irradiated at a dose ranging from 50 kGy to 250 kGy.

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