KR100778975B1 - Lithium rechargeable battery - Google Patents

Abstract

The present invention relates to a lithium secondary battery comprising a separator having excellent mechanical strength, expandability, shrinkage resistance and heat resistance, wherein the lithium secondary battery of the present invention is a positive electrode, a negative electrode, a separator for separating the two electrodes, and a nonaqueous electrolyte solution. The separator is formed by coating a porous film formed by bonding a ceramic material and a binder to at least one of a positive electrode and a negative electrode, and the binder has an elongation of 200 to 300%.

Description

Lithium secondary battery {Lithium rechargeable battery}

The present invention relates to a lithium secondary battery, and more particularly, to a lithium secondary battery including a separator excellent in mechanical strength, expandability, shrinkage resistance, and heat resistance.

As miniaturization and light weight of portable electronic devices have recently advanced, the necessity for miniaturization and high capacity of batteries used as driving power sources thereof is increasing. In particular, the lithium secondary battery has an operating voltage of 3.6 V or more, which is three times higher than that of a nickel-cadmium battery or a nickel-hydrogen battery, which is widely used as a power source for portable electronic devices, and rapidly expands in terms of high energy density per unit weight. There is a trend.

Lithium secondary batteries generate electrical energy by oxidation and reduction reactions when lithium ions are intercalated / deintercalated at the positive and negative electrodes. A lithium secondary battery is prepared by using a material capable of reversibly intercalating / deintercalating lithium ions as an active material of a positive electrode and a negative electrode, and filling an organic electrolyte or a polymer electrolyte between the positive electrode and the negative electrode.

A lithium secondary battery includes an electrode assembly in which a negative electrode plate and a positive electrode plate are wound in a form such as a jelly-roll with a separator interposed therebetween, a can containing the electrode assembly and an electrolyte, and a can It consists of a cap assembly assembled on the top. The positive electrode plate of the electrode assembly is electrically connected to the cap assembly through the positive electrode lead, and the negative electrode plate of the electrode assembly is electrically connected to the can through the negative electrode lead.

The basic function of the separator of the lithium secondary battery is to prevent the short circuit by separating the positive electrode and the negative electrode, and furthermore, it is important to suck the electrolyte required for the battery reaction and maintain high ion conductivity. In particular, in the case of a lithium secondary battery, an additional function is required to prevent the movement of substances that inhibit battery reaction or to ensure safety when an abnormality occurs. As the material of the separator, polyolefin-based microporous polymer membranes such as polypropylene and polyethylene or multiple membranes thereof are usually used. In the conventional separator, since the porous membrane layer is in the form of a sheet or a film, there is a drawback that the sheet-like separator shrinks together with the pore blocking of the porous membrane due to heat generation due to internal short circuit or overcharge. Therefore, when the sheet-like separator collapses due to the internal heat generation of the battery, the separator is reduced and the missing part is directly in contact with the positive electrode and the negative electrode, which leads to ignition, rupture, and explosion.

Such a conventional film-like separator can secure stability by a shutdown function in which a polypropylene or polyethylene resin softens during short-circuit heat generation and blocks pores to block the movement of lithium ions, that is, current flow. It is a fragile structure. For example, in the nail test (penetration), which is a surrogate evaluation that simulates an internal short circuit, the exothermic temperature during internal short circuits locally exceeds several hundred degrees Celsius depending on the test conditions. Accompanying the deformation of the porous membrane layer, abnormal overheating may occur because the nail penetrates the anode and the cathode. Therefore, the means using the resin's shutdown effect are not an absolute safety mechanism against internal short circuits.

In the film-like separator, full charge lithium dendrite is formed during overcharging. This is because the film is formed in the excitation space between the negative electrode and the film, and lithium ions that cannot enter the inside of the negative electrode accumulate on the surface of the negative electrode, that is, the excitable space between the negative electrode and the film, and precipitate as a lithium metal phase. When lithium is deposited on the entire surface, the deposited lithium dendrites may penetrate through the separator on the film to contact the positive electrode and the negative electrode, and at the same time, side reaction between lithium metal and the electrolyte proceeds, and the battery ignites due to heat generation and gas generation. There is a problem, exploding.

Furthermore, the film-like separator is a polyolefin-based film separator, in addition to the portion damaged by the initial heat generation during internal short-circuit, the peripheral film is continuously contracted or melted, so that the portion where the film separator burns out becomes wider. Can be generated.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a lithium secondary battery comprising a separator excellent in mechanical strength, expandability, shrinkage resistance and heat resistance.

In order to achieve the above object, the present invention comprises a positive electrode, a negative electrode, a separator for separating the two electrodes and a non-aqueous electrolyte, the separator comprises a porous membrane formed by a ceramic material and a binder, The binder provides a lithium secondary battery having an elongation of 200 to 300%.

Hereinafter, the present invention will be described in more detail.

The separator of the present invention comprises a porous film formed by bonding a ceramic material and a binder. The paste is prepared by mixing the ceramic material and the binder in a solvent, and then using the paste to make pores on the positive electrode, the negative electrode, or both electrodes. A film can be formed. The porous membrane serves as a film-like separator of conventional polyethylene (PE), polypropylene (PP) and the like.

Mechanical properties required for the binder for forming the porous membrane have flexibility, so that the two electrodes are laminated and wound in a jelly-roll form with the porous membrane formed on the anode or cathode or both sides. At this time, elastic strength is required so that there is no crack or detachment of the porous membrane. Such effectiveness can be judged by measuring the elongation of the porous membrane. The binder of the present invention has an elongation in the range of 200 to 300%, and the binder having an elongation in the above range can be wound without uniformity or detachment of the porous membrane.

In addition, since the binder should not swell when the electrolyte solution is moistened, it should have expansion resistance. When the porous film is formed on the electrode by using a paste in which the ceramic material and the binder are mixed in a solvent, the binder may be dried or overheated. When the sclera is reduced by heat, a hard short occurs, which requires shrinkage resistance. The expansion resistance of the binder can be known by measuring the volume change of the binder, that is, the degree of swelling in the electrolyte solution, and the shrinkage resistance can be determined by measuring the degree of decrease after leaving the binder at a high temperature.

It is preferable that the binder of this invention is 200% or less in swelling degree with respect to electrolyte solution. When the binder has a swelling degree outside the above range, there is a problem that the swelling is severe and the porous membrane is detached from the electrode surface or wrinkles occur.

Moreover, it is preferable that the thermal contraction rate of the binder of this invention is 0.5% or less at 150 degreeC. If the binder has a heat shrinkage outside the above range, it is undesirably reduced during abnormal overheating of the battery, which is not preferable in terms of stability.

On the other hand, the porous membrane may be formed on the electrode active material layer already formed on the electrode current collector, the electrode active material layer and the porous membrane should have excellent adhesion. When the adhesive strength is low, the interface between the electrode active material layer and the porous membrane is peeled off when the volume of the electrode active material layer changes, thereby losing the function of the porous membrane. Adhesiveness can be confirmed by peeling strength. When the peeling strength is measured by forming a porous film on the substrate to clearly see the peeling strength, the peeling strength of the electrode active material layer and the porous film must be 3gf / mm or more to provide excellent adhesion without cracking and detachment.

As the binder of the present invention, an acrylic rubber having a crosslinked structure can be used. The acrylic rubber having the crosslinked structure may be formed by a crosslinking reaction of a polymer or copolymer of an acrylic main monomer with a crosslinkable comonomer. If only one type of polymer or copolymer of acrylic main monomer is used, the bonding structure is weak and easy to break. However, if a crosslinkable monomer is added to the polymer or copolymer of acrylic main monomer, the crosslinkable monomer is a polymer or copolymer structure of acrylic main monomer. It can be combined with to create a more rigid net structure. The polymer having such a net structure is less likely to swell in a solvent as the degree of crosslinking increases. The acrylic rubber binder having the crosslinking structure may have a three-dimensional crosslinking structure having 2 to 10 crosslinking points, preferably 4 to 5 crosslinking points, for 10,000 molecular weight units of the main chain molecule. Therefore, the acrylic rubber having a crosslinked structure of the present invention may have expansion resistance that does not swell when the electrolyte solution is moistened.

Due to the inherent characteristics of ceramic materials, acrylic rubber binders having a crosslinked structure having a decomposition temperature of 1000 ° C. or higher and a decomposition temperature of 250 ° C. or higher are used, thereby obtaining a battery having high heat resistance and increasing stability against internal short circuits. .

The acrylic main monomers are methoxymethyl acrylate, methoxyethyl acrylate, methoxyethyl acrylate, ethoxyethyl acrylate, butoxyethyl acrylate, methoxyethoxy Alkoxyalkyl acrylates selected from ethylmethoxy acrylate and dicyclopentenyloxyethyl acrylate; Vinyl methacrylate, vinyl acrylate, allyl methacrylate, 1,1-dimethylpropenyl methacrylate, 1,1-dimethyl Among propenyl acrylate (1,1-dimethylpropenyl acrylate), 3,3-dimethylbutenyl methacrylate (3,3-dimethylbutenyl methacrylate), 3,3-dimethylbutenyl acrylate (3,3-dimethylbutenyl acrylate) Alkenyl acrylate or alkenyl methacrylate selected; Unsaturated dicarboxylic acid esters selected from divinyl itaconate and divinyl maleate; Vinyl group-containing ethers selected from vinyl 1,1-dimethylpropenyl ether and vinyl 3,3-dimethylbutenyl ether; 1-acryloyloxy-1-phenylethene; And methyl methacrylate (methyl methacrylate) may be used one or more selected from the group consisting of.

The crosslinkable comonomers include 2-ethylhexyl acrylate, 2-methylhexyl acrylate, methyl acrylate, ethyl acrylate, propyl acrylate, and butyl acrylate. ), Alkyl acrylates selected from octyl acrylate and isooctyl acrylate; Alkenyl chloroacetate selected from vinyl chloroacetate and acryl chloroacetate; Glycidyl group-containing esters or ethers selected from glycidyl acrylate, vinyl glycidyl ether, and acryl glycidyl ether; Unsaturated carboxylic acids selected from acrylic acid, methacrylic acid and maleic acid; 2-chloroethyl vinyl ether; Chloromethyl styrene; And one or more selected from the group consisting of acrylonitrile can be used.

The polymer or copolymer of the acrylic main monomer and the crosslinkable comonomer may be used in a 90:10 to 60:40 weight ratio, preferably in a 70:30 weight ratio. As the content of the polymer or copolymer of the acrylic main monomer increases, when the porous membrane of the present invention is formed on the positive electrode or the negative electrode, the binder becomes soft because of the elasticity and ductility of the rubber, so that the winding property is improved, but the expandability with respect to the electrolyte is increased. Because of this, when exceeding a certain content, battery characteristics are deteriorated. When the content of the crosslinkable comonomer added to improve the crosslinkability is increased, the binder becomes hard and the ductility is poor, thereby deteriorating the winding characteristics.

The polymer or copolymer of the acrylic main monomer may be used having a weight average molecular weight in the range of 10,000 to 5 million.

When some of the side chain groups of the polymer or copolymer of the acrylic main monomer are capped with an alkali metal, for example, Na or K, and blended with the crosslinkable comonomer, the alkali metal is liberated at a high temperature and then crosslinked. An acrylic rubber binder having a crosslinked structure is formed.

For example, when polymethyl methacrylate (PMMA) is used as the polymer or copolymer of the acrylic main monomer and capped it with an alkali metal, some of the ester side chain groups are present in the carboxylate side chain group.

As an example of the present invention, 10% by weight of 2-ethylhexyl acrylate and 90% by weight of PMMA having a molecular weight of about 1 million are used. Among them, PMMA has four crosslinking points in 10,000 molecular weight units in order to improve life and safety. Alternatively, a binder having a structure in which five parts of PMMA are capped with an alkali metal as a basis capable of causing the crosslinking reaction, and which is crosslinked after the alkali metal is free at 160 ° C. can be used. Polymerization of the acrylic rubber can be carried out by conventional suspension polymerization, emulsion polymerization, bulk polymerization, solution polymerization, and the like. In addition, the polymerization operation can be performed in either a continuous system or a batch system.

Elongation, swelling and heat shrinkage of the binder can be adjusted by changing the crosslinking point and molecular length of the binder. Alternatively, the size of the molecule surrounding the main chain of the polymer or copolymer of the main monomer can be adjusted. For example, the shorter the molecular length and the larger the crosslinking point, the more the swelling degree is suppressed. The effectiveness of the binder is also improved as the molecular length is shortened. In addition, the swelling degree can be suppressed by equipping the side chain part with a molecule having a large molecular radius. Swelling degree can be suppressed only by changing a side chain molecule into a bulky molecule like a butylene group. The change of the side chain molecule may be accomplished by changing the acrylic main monomer from methyl methacrylate to 3,3-dimethylbutenyl methacrylate or by reacting with the butylene compound in the emulsifying step during the synthesis of the binder. On the other hand, a binder having a predetermined swelling degree, heat resistance, electrolysis resistance, dislocation resistance and potency can be obtained by adjusting the amount of 2-ethylhexyl acrylate mixed in the PMMA material capped with Na at the crosslinking point. In addition, in the case of emulsion polymerization of the binder, Na-capped modified PMMA can be made in a mixed solution of NaOH, whereby the capping amount can be adjusted by the concentration of NaOH and the treatment temperature, and the molecular length of the PMMA and the crosslinking point are I can adjust it.

In order to improve the crosslinkability of the binder, the porous membrane may be subjected to a two-step heat treatment process of heat drying and high temperature polymerization. A paste prepared by mixing a ceramic material, a binder, and a solvent is formed on the positive electrode or the negative electrode, or both, and then heat-dried at 100 to 120 ° C. for 2 to 5 hours to pre-heat the binder while evaporating the solvent, and then 150 to 200 High-temperature polymerization is performed again at 5 degreeC for 5 to 20 hours. After the solvent volatilization, the residual solvent content is maintained at 300 ppm or less. In this way, the two-step heat treatment process can prevent cracking of the porous membrane, and polymerize the binder sufficiently, thereby improving crosslinkability.

In the present invention, secondary particles obtained by partial sintering or recrystallization of primary particles of the ceramic material constituting the separator may be combined with a binder to form a porous membrane. It is preferable that the secondary particle of the said porous film is a particle shape (grape-like shape) or a layered particle group. Further, in the present invention, the primary particles themselves may be a layered particle group or a layered particle group in which scale particles are bound in a stack. It is preferable that the individual particle | grains which comprise a square particle group or a secondary particle are 0.01-0.3 micrometer, and the scale-shaped individual flake which forms a layered particle group has a width of 100 nm-1 micrometer. This particle size can be confirmed by observing a scanning electron microscope (SEM) photograph after forming a material having good properties.

Examples of the ceramic material may include silica (SiO ₂ ), alumina (Al ₂ O ₃ ), zirconium oxide (ZrO ₂ ), titanium oxide (TiO ₂ ), ion-conductive derivatives, and mixtures thereof. In particular, zirconium Preference is given to using oxides.

In addition, as the ceramic material forming the porous membrane, an insulating nitride, a hydroxide, a ketone, or a mixture of these compounds may be used, respectively, of zirconium, aluminum, silicon, and titanium. Here, the limitation of insulating nitride is mentioned because titanium nitride (TiN) or the like has conductivity and is not suitable for the ceramic material of the present invention.

Zirconium oxide is easily dispersed from the relation of zeta (Z) potential in mixing and stirring with an acrylic rubber binder having a crosslinked structure, which is advantageous in productivity, chemically stable, and advantageous in terms of unit cost. do. Moreover, zirconium oxide is excellent in heat dissipation and can have diode properties by making a good p / n junction with a lithium compound at high temperature. In addition, it is possible to effectively prevent excessive lithium ions from being inserted into the negative electrode.

The method of making agglomerated secondary particles includes various chemical methods such as dissolving and recrystallizing all or a part of the surface of the primary particles using chemical substances, and physical methods such as applying an external pressure. have. One of the easy methods is a method of raising the temperature of the material by heating to near the melting temperature of the particle material, and necking (necking).

How much of the ceramic material is melted when the particles are partially melted or partially sintered to be agglomerated is determined in the process of stirring the material when a binder or a solvent is mixed with the ceramic material to form a paste or dispersed porous membrane solution. It is preferable that the characteristic particle shape of the present invention can be maintained to a certain degree and determined so that the density of the formed porous membrane is low.

As an example, heating a ceramic material, such as zirconium oxide, at 900 ° C. for about 10 minutes may yield some sintered particle structure. The ceramic material may be re-precipitated by dissolving all of the ceramic material using a solvent having high solubility in the ceramic material, or by mixing some solvent in the primary particle powder to dissolve the solvent and then removing the solvent.

The ceramic material preferably has a coefficient of thermal expansion of less than 0.2% at a temperature of 200 ° C and a coefficient of thermal expansion of 0.1 to 0.4% at a temperature of 400 ° C. Ceramic materials having a thermal expansion rate higher than the thermal expansion rate have a problem of deforming the shape of the battery itself due to an increase in pressure inside the battery.

In the present invention, it is preferable that the porous film made of acrylic rubber having a crosslinked structure with a ceramic material has a porosity of 50% or more. In this case, the porosity is the ratio occupied by the area corresponding to the empty space in the entire cross section of the cut object, and indicates the degree of porosity in the target material together with the porosity based on the volume. Porosity can be determined by electron microscopy (SEM) photography of the cut surface of the material.

The use of a material in which the particles are in some sintered state causes the agglomerated particles to be irregularly present in one another, making it difficult to fill the space regularly as in crystals. That is, it becomes difficult for the filler which consists of these ceramic materials to fill densely in the space which forms a separator. In addition, it is possible to prevent the resin from filling in the spaces between the particles by lowering the proportion of the resin. In this way, the public rate of the separator can be increased.

The thermal conductivity of the ceramic material of the present invention is preferably about 10 W / (m × K) in a wide temperature range of 500 ° C. or more and 1000 ° C. or less.

Moreover, it is preferable that the dielectric constant of the ceramic material of this invention shall be 1-20. If the relative dielectric constant is more than 20, the capacity tends to be insufficient. If the relative dielectric constant is lower than 1, a material having a band gap cannot be formed.

On the other hand, the electrostatic loss is preferably made of 10 ^-5 to 10 ^-2 at 1MHz frequency. If this value is lower than ^10-5 reproducibility of the band gap to any value falling, this becomes difficult to produce a stable, it tends not to be sufficiently higher capacity than ^10-2.

In the present invention, the porous membrane may be formed in a form attached to at least one surface of at least one electrode of the positive electrode and the negative electrode of the lithium secondary battery. A positive electrode and a negative electrode are manufactured by apply | coating the electrode slurry composition which disperse | distributed the active material, the binder, and the conductive agent to the solvent to the electrode collector. As described above, the porous membrane solution may be coated on the electrode to which the active material is applied, and the porous membrane may be formed using a method of removing the solvent from the porous membrane solution applied by baking.

The porous membrane forms a porous membrane liquid in which a dispersed phase of ceramic material particles is formed in a mixed liquid of an acrylic rubber binder and a solvent having a crosslinked structure, and dipping an electrode plate coated with an active material on the electrode current collector into the porous membrane liquid It can be made to surround the entire electrode plate in a way. In addition, the porous membrane may be formed by spraying the porous membrane liquid on the electrode plate in the form of a spray.

In order to allow the porous membrane to exist on at least one of the electrode faces of the positive electrode and the negative electrode to be opposed to each other, for example, in a jelly roll electrode assembly formed by stacking and winding two electrodes, a porous membrane is formed on the outer surface of each of the two electrodes. . Alternatively, the porous membrane may be formed on the inner side of each of the two electrodes, or the porous membrane may be formed on both the inner side and the outer side of one of the two electrodes.

When the electrode is immersed in the porous membrane solution in which the secondary particles of the ceramic material are evenly dispersed in the liquid material composed of the binder and the solvent, the porous membrane is also covered on the inner and outer surfaces of the electrode and the narrow upper and lower surfaces thereof. . Therefore, a short circuit between the electrode covered with the porous membrane and another electrode can be prevented more effectively.

The porous membrane adjusts the thickness in consideration of ion conductivity and energy density, and may be 1 to 40 μm, preferably 5 to 20 μm. When the thickness of the porous membrane is thinner than 1 μm, the strength may be lowered, and when the thickness of the porous membrane is larger than 40 μm, it is disadvantageous in terms of energy density.

The acrylic rubber binder having a crosslinked structure according to the present invention is preferably used in a small amount in the slurry for forming the porous film. In the porous membrane of the present invention, the ratio of the ceramic material and the binder may be used in a weight ratio of 95: 5 to 80:20. When used within the above range, it is possible to prevent the ceramic material from being completely covered by the binder. That is, the problem of the binder covering the ceramic material to limit ion conduction into the ceramic material can be avoided.

Two electrodes are laminated and wound in a state in which the porous membrane of the present invention is formed on the anode or cathode or both. Since the porous membrane itself may serve as a separator, it may be omitted to install a separate separator between the two electrodes. Conventional film type separators have a problem of shrinkage at high temperatures, but the porous membrane is not likely to shrink or melt. Conventional polyolefin-based film separators generate harder shorts because the peripheral film is continuously shrunk or melted in addition to the parts damaged by initial heat generation during internal short-circuit, and thus the film separator burns out. The electrode on which the porous membrane is formed has only a small damage in a portion where an internal short circuit occurs, and does not lead to a widening of the short circuit region. In addition, the electrode formed with the porous membrane generates a very small soft short rather than a hard short when overcharging, and thus continuously consumes the overcharge current, thereby maintaining a constant voltage between 5V and 6V and a battery temperature of 100 ° C. or less. Stability can also be improved.

However, an insulating film made of a separate polyolefin resin may exist between the two electrodes in the wound state, as in the conventional lithium secondary battery. In this case, the porous membrane and the polyolefin resin insulating film together act as separators in the secondary battery. As the polyolefin resin film, a polyethylene single layer film, a polypropylene single layer film, a polypropylene-polyethylene-polypropylene multilayer film, or the like may be used.

The positive electrode of the lithium secondary battery includes a positive electrode active material capable of inserting and detaching lithium ions, and the positive electrode active material is preferably at least one selected from cobalt, manganese, nickel, and at least one of a composite oxide with lithium. As the representative examples thereof, the lithium-containing compound described below can be preferably used.

Li _x Mn _1-y M _y A ₂ (1)

Li _x Mn _1-y MyO _2-z X _z (2)

Li _x Mn ₂ O _4-z X _z (3)

Li _x Mn _2-y M _y M ' _z A ₄ (4)

Li _x Co _1-y M _y A ₂ (5)

Li _x Co _1-y M _y O _2-z X _z (6)

Li _x Ni _1-y M _y A ₂ (7)

Li _x Ni _1-y M _y O _2-z X _z (8)

Li _x Ni _1-y Co _y O _2-z X _z (9)

Li _x Ni _1-yz Co _y M _z A _α (10)

Li _x Ni _1-yz Co _y M _z O _2-α X _α (11)

Li _x Ni _1-yz Mn _y M _z A _α (12)

Li _x Ni _1-yz Mn _y M _z O _2-α X _α (13)

(Wherein 0.9≤x≤1.1, 0≤y≤0.5, 0≤z≤0.5, 0≤α≤2, M and M 'are the same or different, Mg, Al, Co, K, Na, Ca, Si, Ti, Sn, V, Ge, Ga, B, As, Zr, Mn, Cr, Fe, Sr, V and rare earth elements, A is selected from the group consisting of O, F, S and P And X is selected from the group consisting of F, S and P.)

The negative electrode of the lithium secondary battery includes a negative electrode active material capable of inserting and detaching lithium ions, and the negative electrode active material includes carbon materials such as crystalline carbon, amorphous carbon, carbon composite, carbon fiber, lithium metal, and lithium alloy. Can be. For example, amorphous carbon includes hard carbon, coke, mesocarbon microbeads (MCMB) calcined at 1500 ° C. or lower, mesophase pitch-based carbon fibers (MPCF), and the like. The crystalline carbon includes a graphite material, and specific examples thereof include natural graphite, graphitized coke, graphitized MCMB, graphitized MPCF, and the like. The carbonaceous material is preferably a material having an d002 interplanar distance of 3.35 to 3.38 kPa and an Lc (crystallite size) of at least 20 nm by X-ray diffraction. As the lithium alloy, an alloy of lithium with aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium or indium may be used.

Aluminum and an aluminum alloy may be used as the positive electrode current collector, and copper and a copper alloy may be used as the negative electrode current collector. Examples of the positive electrode current collector and the negative electrode current collector include a foil, a film, a sheet, a punched one, a porous body, a foam, and the like.

The positive electrode and the negative electrode may further include a conductive agent to improve electrical conductivity. As the conductive agent, at least one selected from the group consisting of a graphite-based conductive agent, a carbon black-based conductive agent, and a metal or metal compound-based conductive agent may be used. Examples of the graphite conductive agent include artificial graphite and natural graphite, and examples of the carbon black conductive agent include acetylene black, ketjen black, denka black, thermal black, and channel. And black or the like, and examples of the metal or metal compound conductive agent include perovskite such as tin, tin oxide, tin phosphate (SnPO ₄ ), titanium oxide, potassium titanate, LaSrCoO ₃ , and LaSrMnO _3. ) There is a substance. However, it is not limited to the conductive agents listed above. The content of the conductive agent is preferably 0.1 to 10% by weight based on the positive electrode active material. When the content of the conductive agent is less than 0.1% by weight, the electrochemical properties are lowered, and when the content of the conductive agent is greater than 10% by weight, the energy density per weight decreases.

The binder for the electrode active material is a material that plays a role of pasting the active material, mutually bonding the active material, bonding with the current collector, and buffering effect on the expansion and contraction of the active material, for example, polyvinylidene fluoride, polyhexafluoro Copolymers of propylene-polyvinylidene fluoride (P (VdF / HFP)), poly (vinylacetate), polyvinyl alcohol, polyethylene oxide, polyvinylpyrrolidone, alkylated polyethylene oxide, polyvinyl ether, Poly (methyl methacrylate), poly (ethylacrylate), polytetrafluoroethylene, polyvinylchloride, polyacrylonitrile, polyvinylpyridine, styrene-butadiene rubber, acrylonitrile-butadiene rubber and the like. The content of the binder is 0.1 to 30% by weight, preferably 1 to 10% by weight based on the electrode active material. When the content of the binder is too small, the adhesion between the electrode active material and the current collector is insufficient, and when the content of the binder is too high, the adhesion is improved, but the content of the electrode active material decreases by that amount, which is disadvantageous in increasing battery capacity.

A nonaqueous solvent or an aqueous solvent can be used as a solvent used when dispersing an electrode active material, a binder, and a conductive agent. Examples of the non-aqueous solvent include N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, N, N-dimethylaminopropylamine, ethylene oxide and tetrahydrofuran.

When the porous membrane of the present invention is formed on the negative electrode, if the binder of the negative electrode active material is water-based, for example, styrene-butadiene rubber, the binder used for the porous membrane is organic, and the binder of the negative electrode active material is organic, for example poly If it is vinylidene fluoride, it is preferable to use an aqueous system for the binder used for a porous film. When the same organic or aqueous binder is used in combination with the negative electrode active material layer and the porous membrane, the dispersion solvent must also use the same organic or aqueous solvent. The problem of re-melting in the solvent of the porous membrane solution may occur.

The solvent of the porous membrane solution may be NMP / cyclohexanone when used in an organic binder in a volume ratio of 0: 100 to 50:50, preferably 30:70, or isopropyl alcohol, toluene, or xylene (xylene) instead of NMP. ) Can be used. If the binder of the negative electrode active material is an organic type, the binder of the porous membrane liquid may be water-based, and water may be used as the solvent.

The non-aqueous electrolyte of the lithium secondary battery includes a lithium salt and a non-aqueous organic solvent, and may further include additives for improving charge and discharge characteristics, preventing overcharge, and the like. The lithium salt acts as a source of lithium ions in the battery to enable operation of the basic lithium battery, and the non-aqueous organic solvent serves as a medium through which ions involved in the electrochemical reaction of the battery can move.

Examples of the lithium salt include LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N, LiC ₄ F ₉ SO ₃ , LiAlO ₄ , LiAlCl ₄ , LiN ( C _x F _{2x + 1} SO ₂ ) (C _y F _{2y + 1} SO ₂ ) (where x and y are natural numbers), LiCl, and LiI, or a mixture of one or two or more selected from the group consisting of Can be. The concentration of the lithium salt is preferably used in the range of 0.6 to 2.0M, more preferably in the range of 0.7 to 1.6M. If the concentration of the lithium salt is less than 0.6M, the conductivity of the electrolyte is lowered, the performance of the electrolyte is lowered, and if it exceeds 2.0M, the viscosity of the electrolyte is increased, there is a problem that the mobility of lithium ions decreases.

As the non-aqueous organic solvent, carbonate, ester, ether or ketone may be used alone or in combination. Organic solvents should be used to have high dielectric constant (polarity) and low viscosity to increase ion dissociation and smooth ion conduction. Generally, solvents having high dielectric constant, high viscosity and low dielectric constant and low viscosity are used. It is preferable to use two or more mixed solvents configured.

In the case of the carbonate-based solvent of the non-aqueous organic solvent, it is preferable to use a mixture of cyclic carbonate and chain carbonate. The cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinyl Ethylene carbonate (VC) and the like may be used. Ethylene carbonate and propylene carbonate having high dielectric constants are preferred, and ethylene carbonate is preferred when artificial graphite is used as the negative electrode active material. As the linear carbonate, dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylmethyl carbonate (EMC), ethylpropyl carbonate (EPC), etc. may be used. . Low viscosity dimethyl carbonate, ethylmethyl carbonate and diethyl carbonate are preferred.

The ester is methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone (GBL), γ-valerolactone, γ-caprolactone, δ-valerolactone, ε- Caprolactone, and the like, and tetraetherfuran, 2-methyltetrahydrofuran, dibutyl ether, and the like may be used. As the ketone, polymethylvinyl ketone may be used.

Hereinafter, examples and comparative examples of the present invention are described. However, the following examples are only preferred examples of the present invention, and the present invention is not limited to the following examples.

Comparative Example 1

10% by weight of 2-ethylhexyl acrylate and 90% by weight of 1 million molecular weight polymethylmethacrylate (PMMA) were mixed with sodium hydroxide (5 to 10% by weight based on mixed acrylates) and crosslinked at 160 ° C. to PMMA. An acrylic rubber binder mainly composed of was synthesized. 96 wt% zirconium oxide (ZrO ₂ ) secondary particle powder and 4 wt% of the acrylic rubber binder are mixed with a ceramic material, and the N-methyl-2-pyrrolidone / cyclohexanone mixed solvent is used to obtain a viscosity of about 3000 cps. Diluted to 3: 7 weight ratio) to form a mixed solution. The copper foil was immersed in the mixed solution, the film thickness was controlled by a gravure roller, and then heat-treated at 100 ° C. for 10 hours to volatilize the solvent.

Comparative Examples 2 to 4

It was carried out in the same manner as in Comparative Example 1 except for changing the heat treatment temperature as shown in Table 1.

Examples 1-6

As described in Table 1, the first heat treatment was performed at 100 ° C. to 120 ° C. for 5 hours, and the second heat treatment was performed at 150 ° C. to 200 ° C. for 5 hours.

<Peel strength>

The porous film was formed on the cathode or the anode, but in order to clearly see the peel strength of the porous film, the porous film was formed on the copper foil, and the adhesion to the copper foil was measured by the peel strength (180 degrees).

The copper foils prepared in Comparative Examples 1 to 4 and Examples 1 to 6 were cut out using a steel ruler to a width of 25.4 mm and a length of 100 mm. The cut sample was bitten by a jig and measured in a 180 degree peeling type using a tensile strength tester (Hounsfield test equipment, serial No. 0050 medel 100 RC). Tensile velocity was 100 mm / min, and extending | stretching length was performed on 25-50 mm conditions.

Primary heat treatment (℃) Secondary heat treatment (℃) Crack occurrence Peel Test Peel Strength (gf / mm) Comparative Example 1 100 - none 1.02 Comparative Example 2 120 - none 1.15 Comparative Example 3 150 - has exist 1.87 Comparative Example 4 200 - has exist 1.76 Example 1 100 150 none 2.88 Example 2 100 170 none 3.09 Example 3 100 200 none 3.28 Example 4 120 150 none 3.17 Example 5 120 170 none 3.39 Example 6 120 200 none 3.28

As shown in Comparative Examples 1 to 2, when the first heat treatment was performed at 100 to 120 ° C. and the second heat treatment was not performed, the porous membrane was not cracked but the peel strength was low because the binder did not reach a temperature sufficient to polymerize. It can be seen that. As in Comparative Examples 3 to 4, the peel strength was improved at a high temperature of about 150 to 200 ° C. at which the binder could polymerize, but the solvent was volatilized in a moment to cause cracking of the porous membrane.

As shown in Examples 1 to 6, volatilization of the solvent at a relatively low temperature of 100 to 120 ° C. and polymerization at a high temperature of 150 to 200 ° C. also prevent cracking of the porous membrane and polymerization of the binder. It becomes possible enough, and it can confirm that crosslinkability improves.

Comparative Examples 5 to 7 and Examples 10 to 13

Synthetic graphite as a negative electrode active material, styrene-butadiene rubber as a binder, and carboxymethyl cellulose as a thickener were mixed in a weight ratio of 96: 2: 2, and then dispersed in water to prepare a negative electrode active material slurry. The slurry was coated on a copper foil having a thickness of 15 μm, dried, and rolled to prepare a negative electrode.

96% by weight of zirconium oxide (ZrO ₂ ) secondary particle powder as a ceramic material and 4% by weight of acrylic rubber mainly composed of polymethyl methacrylate (PMMA) having elongation shown in Table 2 as binders, Dilute with an N-methyl-2-pyrrolidone / cyclohexanone mixed solvent (3: 7 weight ratio) so as to be about 1000 to 3000 cps. After dipping the negative electrode plate into the mixed solution and controlling the film thickness with a gravure roller, the first heat treatment was performed at 120 ° C. for 5 hours, and the second heat treatment was performed at 150 ° C. for 5 hours. Formed.

<Winding characteristics according to the elongation of the binder>

Elongation is a measure of the extent to which the binder itself is polymerized and then stretched and stretched. Elongation = (Δl / l _o ) * 100%. Here, Δl represents an extended length and l _o represents an initial length.

The negative electrode on which the porous film prepared in Comparative Examples 5-7 and Examples 10-13 was formed was wound up in the form of a jelly roll.

Elongation of binder Plate state Winding Comparative Example 5 125% Crack impossible Comparative Example 6 150% Crack impossible Comparative Example 7 175% Crack impossible Example 9 200% No cracks possible Example 10 225% No cracks possible Example 11 250% No cracks possible Example 12 275% No cracks possible Example 13 300% No cracks possible

As can be seen in Table 2, the winding of the negative electrode was possible when the elongation was about 200 to 300%. When the elongation was less than 200%, when the cathode with the porous membrane formed was wound, cracks were generated in the electrode plate of the folded portion, and thus winding was impossible.

Examples 14-17 and Comparative Examples 10-12

LiCoO ₂ as a positive electrode active material, polyvinylidene fluoride (PVDF) as a binder and carbon as a conductive agent were mixed in a weight ratio of 92: 4: 4, and then dispersed in N-methyl-2-pyrrolidone to prepare a positive electrode slurry. It was. The slurry was coated on an aluminum foil having a thickness of 20 μm, dried, and rolled to prepare a positive electrode. Synthetic graphite as a negative electrode active material, styrene-butadiene rubber as a binder, and carboxymethyl cellulose as a thickener were mixed in a weight ratio of 96: 2: 2, and then dispersed in water to prepare a negative electrode active material slurry. The slurry was coated on a copper foil having a thickness of 15 μm, dried, and rolled to prepare a negative electrode.

96% by weight of zirconium oxide (ZrO ₂ ) secondary particle powder as a ceramic material and 4% by weight of acrylic rubber mainly composed of polymethyl methacrylate (PMMA) having a swelling degree shown in Table 3 as binders, respectively, Dilute with an N-methyl-2-pyrrolidone / cyclohexanone mixed solvent (3: 7 weight ratio) so as to be about 1000 to 3000 cps. After dipping the negative electrode plate into the mixed solution and controlling the film thickness with a gravure roller, the first heat treatment was performed at 120 ° C. for 5 hours, and the second heat treatment was performed at 150 ° C. for 5 hours. Formed.

The prepared electrodes were wound and compressed and inserted into a square can. An electrolyte was injected into the can to prepare a lithium secondary battery. In this case, an ethylene carbonate / ethyl methyl carbonate mixed solution (3: 7 volume ratio) in which 1.3 M LiPF ₆ was dissolved was used.

<Battery Thickness According to Swelling Degree of Binder>

After the binder was formed in a rectangular shape on the aluminum foil, the initial volume was calculated by measuring the width, length, and thickness, and then, in an electrolyte solution composed of an ethylene carbonate / ethylmethyl carbonate mixed solution (50:50 by volume) in which 1.3 M LiPF ₆ was dissolved. The solution was placed in a 60 ° C. chamber for 1 day to check the volume change after swelling with the electrolyte solution. Swelling degree = (extended volume / initial volume) * 100%.

The thicknesses before and after full charge of the lithium secondary batteries of Examples 14 to 17 and Comparative Examples 10 to 12 were measured. The thickness of the battery using a binder having a degree of swelling of 125% was 100%, and the cell thickness was expressed in% for each degree of swelling.

Swelling Degree of Binder Cell thickness Example 14 125% 100% Example 15 150% 103% Example 16 175% 107% Example 17 200% 111% Comparative Example 10 225% 140% Comparative Example 11 250% 146% Comparative Example 12 275% 155%

In view of the thickness specification that is generally acceptable to battery companies, it can be seen that the binder can be used when the swelling degree is 200% or less.

Examples 18-22 and Comparative Examples 13-16

LiCoO ₂ as a positive electrode active material, polyvinylidene fluoride (PVDF) as a binder and carbon as a conductive agent were mixed in a weight ratio of 92: 4: 4, and then dispersed in N-methyl-2-pyrrolidone to prepare a positive electrode slurry. It was. The slurry was coated on an aluminum foil having a thickness of 20 μm, dried, and rolled to prepare a positive electrode. Synthetic graphite as a negative electrode active material, styrene-butadiene rubber as a binder, and carboxymethyl cellulose as a thickener were mixed in a weight ratio of 96: 2: 2, and then dispersed in water to prepare a negative electrode active material slurry. The slurry was coated on a copper foil having a thickness of 15 μm, dried, and rolled to prepare a negative electrode.

96% by weight of zirconium oxide (ZrO ₂ ) secondary particle powder as a ceramic material and 4% by weight of acrylic rubber mainly composed of polymethyl methacrylate (PMMA) having thermal shrinkage shown in Table 4 as binders, respectively, The mixture was diluted with an N-methyl-2-pyrrolidone / cyclohexanone mixed solvent (3: 7 weight ratio) so as to have a viscosity of about 1000 to 3000 cps. After dipping the negative electrode plate and the positive electrode plate into the mixed solution and controlling the film thickness with a gravure roller, the first heat treatment was performed at 120 ° C. for 5 hours, and the second heat treatment was performed at 150 ° C. for 5 hours, respectively. A microporous membrane was formed.

<Desorption Characteristics of Porous Membrane According to the Heat Shrinkage Rate of Binder>

The thermal contraction rate of the binder is formed in a rectangular shape by pouring the binder itself on an aluminum foil in a constant frame of horizontal and vertical widths and leaving it in an oven at 150 ° C. for 24 hours to confirm the decrease. In other words, the width is 100% and the degree of reduction compared to the initial value is expressed in%.

The detachment of the porous membrane was observed in the positive electrode and the negative electrode prepared according to Examples 18 to 22 and Comparative Examples 13 to 16.

Thermal Shrinkage of Binder (150 ℃) Porous membrane detachment Example 18 0.1% No tally Example 19 0.2% No tally Example 20 0.3% No tally Example 21 0.4% No tally Example 22 0.5% No tally Comparative Example 13 0.6% Has Tally Comparative Example 14 0.7% Has Tally Comparative Example 15 0.8% Has Tally Comparative Example 16 0.9% Has Tally

As can be seen from Table 4, a binder having a low shrinkage property, that is, a binder having an area reduction ratio of ΔS = 0.5% or less, is preferred so that the negative electrode active material and the positive electrode active material adhered to the substrate by the stress of the binder used in the porous membrane do not detach. Do.

 The present invention can provide a lithium secondary battery comprising a separator excellent in mechanical strength, expandability, shrinkage resistance and heat resistance. Therefore, the separator according to the present invention improves the stability of the lithium secondary battery.

Claims (28)

A positive electrode, a negative electrode, a separator for separating the positive electrode and the negative electrode, and a nonaqueous electrolyte solution, The separator is made of a porous film formed by bonding a ceramic material and a binder, The porous membrane is formed by coating on at least one surface of the positive electrode and the negative electrode, A lithium secondary battery, wherein the binder has an elongation of 200 to 300%. The lithium secondary battery of claim 1, wherein the binder has a swelling degree of 200% or less with respect to an electrolyte. The lithium secondary battery according to claim 1, wherein the binder has a heat shrinkage of 0.5% or less at 150 ° C. The lithium secondary battery according to claim 1, wherein the peeling strength of the porous membrane with respect to the base material of the positive electrode and the negative electrode is 3 gf / mm or more. The lithium secondary battery according to claim 1, wherein the binder is an acrylic rubber formed by a crosslinking reaction of a polymer or copolymer of an acrylic main monomer with a crosslinkable comonomer. The method of claim 5, wherein the acrylic main monomer is methoxymethyl acrylate, methoxy ethyl acrylate, ethoxy ethyl acrylate, butoxy ethyl acrylate, methoxy ethoxy ethyl acrylate, dicyclopentenyl oxy ethyl acrylic Alkoxyalkyl acrylate selected from rates; Vinyl methacrylate, vinyl acrylate, allyl methacrylate, 1,1-dimethylpropenyl methacrylate, 1,1-dimethylpropenyl acrylate, 3,3-dimethylbutenyl methacrylate, 3,3- Alkenyl acrylate or methacrylate selected from dimethylbutenyl acrylate; Unsaturated dicarboxylic acid ester selected from divinyl itaconate and divinyl maleate; Vinyl group-containing ethers selected from vinyl 1,1-dimethylpropenyl ether and vinyl 3,3-dimethylbutenyl ether; 1-acryloyloxy-1-phenylethene; A lithium secondary battery, characterized in that one kind selected from the group consisting of methyl methacrylate and mixtures thereof. The method of claim 5, wherein the crosslinkable comonomer is alkyl acrylate selected from 2-ethylhexyl acrylate, methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, octyl acrylate, isooctyl acrylate; Alkenyl chloroacetate selected from vinyl chloroacetate, aryl chloroacetate; Glycidyl group-containing esters or ethers selected from glycidyl acrylate, vinyl glycidyl ether and acryl glycidyl ether; Unsaturated carboxylic acid selected from acrylic acid, methacrylic acid and maleic acid; 2-chloroethyl vinyl ether; Chloromethyl styrene; Acrylonitrile; And a mixture selected from the group consisting of a lithium secondary battery. The lithium secondary battery according to claim 5, wherein the polymer or copolymer of the acrylic main monomer has a weight average molecular weight within the range of 10,000 to 5 million. The lithium secondary battery according to claim 5, wherein the polymer or copolymer of the acrylic main monomer and the crosslinkable comonomer are mixed in a weight ratio of 90:10 to 60:40.  The lithium secondary battery according to claim 9, wherein the polymer or copolymer of the acrylic main monomer and the crosslinkable comonomer are mixed in a 70:30 weight ratio. The lithium secondary battery according to claim 1, wherein secondary particles obtained by partial sintering or recrystallization of primary particles of the ceramic material are bonded by a binder. 12. The lithium secondary battery according to claim 11, wherein the secondary particles consist of a group of grains (grapes-like) or layered grains. 13. The lithium secondary battery according to claim 12, wherein the primary particles forming a spin have a diameter in the range of 0.01 to 0.3 µm. The lithium secondary battery according to claim 12, wherein the layered primary particles are scaly flakes having a width of 100 nm to 1 μm. The method of claim 11, wherein the ceramic material is one selected from the group consisting of silica (SiO ₂ ), alumina (Al ₂ O ₃ ), zirconium oxide (ZrO ₂ ), titanium oxide (TiO ₂ ) and mixtures thereof. A lithium secondary battery characterized by the above. The method of claim 11, wherein the ceramic material is selected from the group consisting of insulating nitrides, hydroxides, alkoxides, ketones, and mixtures thereof of silicon (Si), aluminum (Al), zirconium (Zr), and titanium (Ti). Lithium secondary battery, characterized in that one selected. The lithium secondary battery according to claim 11, wherein the ceramic material has a thermal expansion rate of 0.2% or less at 200 ° C. and a thermal expansion rate of 0.1-0.4% at 400 ° C. 12. The lithium secondary battery according to claim 11, wherein the thermal conductivity of the ceramic material is in a 10% region around 100W / (mxK) in a temperature range of 500 ° C or more and 1000 ° C or less. The method of claim 11, wherein the dielectric loss of the ceramic material is a lithium secondary battery, characterized in that consisting of 10 ^-5 to 10 ^-2 at a frequency of 1MHz. The lithium secondary battery of claim 1, wherein the ceramic material and the binder are used in a weight ratio of 95: 5 to 80:20. The lithium secondary battery according to claim 1, wherein the porous membrane has a porosity of 50% or more. The method of claim 1, wherein the porous membrane is a method of dipping at least one of the positive electrode and the negative electrode in a mixed solution in which the ceramic material and the binder are dispersed in a solvent with respect to one of the positive electrode and the negative electrode. Lithium secondary battery characterized in that it comprises at least one method selected from the method of spraying on the electrode surface, the method of printing the mixed solution on the electrode surface. The lithium secondary battery of claim 22, wherein the porous membrane is subjected to a first heat treatment at 100 to 120 ° C., followed by a second heat treatment at 150 to 200 ° C. 24. The lithium secondary battery of claim 1, wherein the porous membrane has a thickness of 1 to 40 μm. The lithium secondary battery of claim 24, wherein the porous membrane is formed to have a thickness of 5 to 20 μm. The lithium secondary battery of claim 1, wherein each of the cathode and the anode comprises a cathode active material and an anode active material capable of intercalating and deintercalating lithium ions, respectively. The lithium secondary battery of claim 26, wherein the positive electrode active material is a lithium compound selected from the group consisting of the following (1) to (13). Li _x Mn _1-y M _y A ₂ (1) Li _x Mn _1-y MyO _2-z X _z (2) Li _x Mn ₂ O _4-z X _z (3) Li _x Mn _2-y M _y M ' _z A ₄ (4) Li _x Co _1-y M _y A ₂ (5) Li _x Co _1-y M _y O _2-z X _z (6) Li _x Ni _1-y M _y A ₂ (7) Li _x Ni _1-y M _y O _2-z X _z (8) Li _x Ni _1-y Co _y O _2-z X _z (9) Li _x Ni _1-yz Co _y M _z A _α (10) Li _x Ni _1-yz Co _y M _z O _2-α X _α (11) Li _x Ni _1-yz Mn _y M _z A _α (12) Li _x Ni _1-yz Mn _y M _z O _2-α X _α (13) (Wherein 0.9≤x≤1.1, 0≤y≤0.5, 0≤z≤0.5, 0≤α≤2, M and M 'are the same or different, Mg, Al, Co, K, Na, Ca, Si, Ti, Sn, V, Ge, Ga, B, As, Zr, Mn, Cr, Fe, Sr, V and rare earth elements, A is selected from the group consisting of O, F, S and P And X is selected from the group consisting of F, S and P.) The lithium secondary battery of claim 26, wherein the negative active material is selected from the group consisting of crystalline carbon, amorphous carbon, carbon composite, carbon fiber, lithium metal, and lithium alloy.