KR101655278B1 - Lithium secondary battery with improved safety - Google Patents

Abstract

In the present invention, the separation membrane having two kinds of cathode active materials having different average particle diameters included in the cathode active material and having a porous coating layer formed of a mixture of inorganic particles and organic binder polymer is used as the separation membrane, A lithium secondary battery is provided.

Description

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

The present invention relates to a lithium secondary battery, and more particularly, to a lithium secondary battery having improved safety.

Recently, interest in energy storage technology is increasing. As the application fields of cell phones, camcorders, notebook PCs, and electric vehicles further expand, there is a growing demand for higher energy density of batteries used as power sources for such electronic devices. Lithium secondary batteries are the ones that can best meet these demands, and researches on them are actively under way.

The lithium secondary battery includes a cathode including a cathode active material, a cathode including a cathode active material, a separator interposed between the cathode and the anode, and an electrolyte. The cathode active material includes lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel Manganese cobalt composite oxide and the like have conventionally been used.

Among them, lithium-nickel-manganese-cobalt composite oxide has various physical properties such as improved energy density and lifetime characteristics compared to a battery produced by using nickel, cobalt or manganese separately, but the necessity of complicated manufacturing process and improvement of high- In addition, there is a disadvantage that thermal stability is insufficient compared with lithium manganese oxide. This is also confirmed in FIG. 1 in which the exothermic peak is measured for a cell using lithium nickel manganese cobalt composite oxide or lithium manganese oxide alone as a cathode active material Do. That is, a cylindrical lithium secondary battery was fabricated using the above-mentioned cathode active material, and the battery was charged up to 4.3 V at 0.1 C, and its exothermic position and strength were measured through a differential scanning calorimeter (DSC). As a result, the exothermic peak of the lithium nickel manganese cobalt composite oxide The intensity of the exothermic peak of the lithium manganese oxide was small (Fig. 1).

Lithium manganese oxide is easy to synthesize, has low cost, has good electrochemical discharge characteristics, and has low environmental pollution. However, lithium manganese oxide has a high possibility of application to active material, but has a problem that electrolyte is decomposed due to low conductivity and capacity and high operating voltage .

In order to solve the above problems, there has been proposed a method of mixing a lithium manganese composite oxide with a lithium nickel manganese cobalt composite oxide. However, since the particle diameter difference between the lithium manganese composite oxide and the lithium nickel manganese cobalt composite oxide is not large, There is a problem that the cathode active material layer has a low tap density and low leveling or leveling property of the cathode active material layer.

Particularly, when a separation membrane having an organic / inorganic porous coating layer including a mixture of inorganic particles and a binder polymer for a separation membrane is used as the separation membrane, the porous active material layer having low smoothness or leveling property and the porous The coating layer has a poor interfacial property, and thus it is particularly difficult to realize a high capacity.

Accordingly, in the present invention, by increasing the tap density of the lithium manganese oxide and the lithium nickel manganese cobalt composite oxide, the leveling property or the smoothness of the positive electrode active material layer is improved by improving the rolling property, The present invention provides a lithium secondary battery which can realize a high capacity and has excellent thermal stability.

According to an aspect of the present invention, there is provided a lithium secondary battery comprising a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, wherein the positive electrode is a lithium manganese oxide represented by LiMn ₂ O ₄ , A cathode active material made of lithium nickel manganese cobalt composite oxide, wherein the two cathode active materials have different average particle diameters (D50), the separator has a porous substrate having pores, and a porous substrate formed on at least one surface of the porous substrate And a composite separator comprising an organic / inorganic porous coating layer comprising a mixture of inorganic particles and a binder polymer for a separator.

[Chemical Formula 1]

Li _{1 + a} (Ni _x Mn _y Co _z ) _1-a O ₂

In this formula,

0.05? A? 0.10,

x + y + z = 1,

0.4? X? 0.6,

0.2? Y? 0.4,

0.1? Z? 0.3.

(D50) corresponding to 0.2 to 0.7 times the average particle diameter (D50) of one kind of the cathode active material.

The average particle diameter (D50) of the lithium manganese oxide may be 15 to 30 mu m.

The average particle diameter (D50) of the lithium nickel manganese cobalt composite oxide may be 5 to 20 mu m.

The difference in particle diameter between the lithium manganese oxide and the lithium nickel manganese cobalt composite oxide may be 10 to 25 mu m.

The lithium manganese oxide and the lithium nickel manganese cobalt composite oxide may be contained in a weight ratio of 10:90 to 90:10 based on the total cathode active material.

The lithium manganese oxide and the lithium nickel manganese cobalt composite oxide may be contained in a weight ratio of 30:70 to 70:30 based on the total cathode active material.

The cathode active material may be applied to an electrode collector in an amount of 700 mg / 25 cm 2 or more.

The porous substrate may be selected from the group consisting of high density polyethylene, low density polyethylene, linear low density polyethylene, ultra high molecular weight polyethylene, polypropylene polyethylene terephthalate, polybutyleneterephthalate, polyester, polyacetal, polyamide, polyamide, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenylene oxide, polyphenylenesulfroid and polyethylene naphthalene (polyethylenenaphthalene), and the like.

The inorganic particles may be at least one selected from the group consisting of inorganic particles having a dielectric constant of 5 or more, inorganic particles having a lithium ion transporting ability, and mixtures thereof.

The inorganic particles having a dielectric constant of 5 or more include BaTiO ₃ , Pb (Zr _x , Ti _{1 -x} ) O ₃ (PZT, 0 <x <1), Pb _{1 -x} La _x Zr _{1 -y} Ti _y O ₃ , 0 <x <1, 0 <y <1), (1-x) Pb (Mg _1/3 Nb _2/3 ) O _3-x PbTiO ₃ At least one selected from the group consisting of HfO ₂ , SrTiO ₃ , SnO ₂ , CeO ₂ , MgO, NiO, CaO, ZnO, ZrO ₂ , SiO ₂ , Y ₂ O ₃ , Al ₂ O ₃ , SiC and TiO ₂ .

The inorganic particles having lithium ion transferring ability include lithium phosphate (Li ₃ PO ₄ ), lithium titanium phosphate (Li _x Ti _y (PO ₄ ) ₃ , 0 <x <2, 0 <y <3), lithium aluminum titanium phosphate _{(Li x Al y Ti z (} PO 4) 3, 0 <x <2, 0 <y <1, 0 <z <3), (LiAlTiP) x O y series glass (0 <x <4, 0 <y <13), lithium lanthanum titanate (Li _x La _y TiO ₃ , 0 <x <2, 0 <y <3), lithium germanium thiophosphate (Li _x Ge _y P _z S _w , , 0 <y <1, 0 <z <1, 0 <w <5), lithium nitrides _{(Li x N y, 0 <} x <4, 0 <y <2), SiS 2 (Li x Si y S _{z, 0 <x <3,} 0 <y <2, 0 <z <4) based glass, and _{P 2 S 5 (Li x P} y S z, 0 <x <3, 0 <y <3, 0 <z &Lt; 7) series glass.

The binder polymer for the separator may be selected from the group consisting of polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichlorethylene, polymethylmethacrylate ), Polybutylacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinyl acetate, polyethylene oxide but are not limited to, polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan, No ethyl polyvinyl alcohol (cyano ethylpolyvinylalcohol, cyanoethylcellulose, cyanoethylsucrose, pullulan, and carboxyl methyl cellulose. In the present invention, it is also possible to use at least one selected from the group consisting of hydroxypropylcellulose, ethylpolyvinylalcohol, cyanoethylcellulose, cyanoethylsucrose, pullulan and carboxyl methylcellulose.

The content of the binder polymer for separator may be 1 to 50 parts by weight based on 100 parts by weight of the inorganic particles.

The negative electrode may include a graphite-based active material.

The electrolyte may include a LiPF ₆ electrolyte and a cyclic carbonate organic solvent.

According to one aspect of the present invention, as the two types of cathode active materials having different average particle diameters form the cathode active material layer, the tap density is increased as compared with the conventional one, and a high capacity secondary battery can be manufactured.

In addition, since the cathode active material layer exhibits high smoothness or leveling property, when a separator having a porous coating layer formed of a mixture of inorganic particles and a binder polymer for a separator is used as a separator, the separator exhibits desirable interface characteristics with the porous coating layer of the separator . Accordingly, not only the advantages of thermal and mechanical stability of the separation membrane having the porous coating layer can be enjoyed, but also the performance of the battery can be improved.

FIG. 1 is a graph showing an exothermic peak measured by a differential scanning calorimeter (DSC) for each electrode including a lithium nickel manganese cobalt composite oxide or a lithium manganese oxide alone as a cathode active material.
Fig. 2 is a graph showing discharge energy (unit: Wh) of the lithium secondary battery manufactured in Example 1 and Comparative Example 1. Fig.

The terms and words used in the present specification and claims should not be construed as limited to ordinary or dictionary terms and the inventor may appropriately define the concept of the term in order to best describe its invention It should be construed as meaning and concept consistent with the technical idea of the present invention.

In addition, the embodiments described in the present specification and the configurations shown in the drawings are merely the most preferred embodiments of the present invention and do not represent all the technical ideas of the present invention. Therefore, various equivalents It should be understood that water and variations may be present.

The positive electrode active material of the present invention includes a mixture of lithium manganese oxide represented by LiMn ₂ O ₄ and lithium manganese composite oxide of the following formula 1, and the two positive electrode active materials have different average particle sizes (D50) Features:

[Chemical Formula 1]

Li _{1 + a} (Ni _x Mn _y Co _z ) _1-a O ₂

In this formula,

0.05? A? 0.10,

x + y + z = 1,

0.4? X? 0.6,

0.2? Y? 0.4,

0.1? Z? 0.3.

The lithium manganese oxide represented by LiMn ₂ O ₄ and the lithium nickel cobalt manganese composite oxide represented by the following formula 1 are doped or coated with trace amounts of transition metals such as Fe, Cr, Ti, Zn, V, Al, Mg and Zr .

The positive electrode active material layer of the present invention may optionally further include components such as a binder, a conductive material, and a filler, if necessary.

According to the present invention, there is provided a lithium secondary battery including a positive electrode including the above-mentioned positive electrode active material, a negative electrode, a separator interposed between the positive and negative electrodes, and an electrolyte.

As used herein, the term "average particle diameter" refers to the cumulative 50% (D50) of the particle distribution obtained by a laser diffraction particle analyzer, and unless otherwise stated herein, the secondary particles Means the average particle size of the particles. This is because the primary particles generally have a size on the order of nanometers and have a disadvantage in that they are difficult to be uniformly dispersed in a solvent. Therefore, the secondary particles having a size of about micrometers, Because it is used in particle form.

The term 'tap density' or 'tapped density' of the active material particles in the present specification means that the particles (powders) in the container are tapped a predetermined number of times at a predetermined height, Powder) bulk density. In one embodiment of the present invention, one kind of the cathode active material may have an average particle diameter (D50) corresponding to 0.2 to 0.7 times the average particle diameter (D50) of the other kind of the cathode active material. In the case where the two kinds of cathode active materials have the above-mentioned difference in average particle diameter, they can be made into a desired rolled active material layer.

According to one aspect of the present invention, the lithium manganese oxide may have an average particle diameter (D50) of 15 to 30 mu m. For example, it may have an average particle diameter (D50) of about 20 mu m. If the lithium manganese oxide is smaller than the lower limit value, electrode rolling becomes difficult, and production of lithium manganese oxide larger than the upper limit value described above is difficult in the active material production process, and battery efficiency is lowered by weight.

The lithium nickel manganese cobalt composite oxide may simultaneously contain nickel, manganese and cobalt as shown in Formula 1, and may have an average particle size (D50) of 5 to 20 占 퐉.

For the rolling process of the present invention, the difference in particle diameter between the lithium manganese oxide and the lithium nickel manganese cobalt composite oxide may be 10 占 퐉 or more, and the maximum value of the particle diameter difference may be 25 占 퐉. For example, the particle size difference may be about 10 [mu] m. In one embodiment of the present invention, the use of two kinds of positive electrode active material having such a difference in particle diameter in the positive electrode active material layer can improve the hot rolling property of the positive electrode active material.

In the present invention, the lithium manganese oxide and the lithium nickel manganese cobalt composite oxide are preferably mixed and contained in a weight ratio of 90:10 to 10:90, more preferably 70:30 to 30:70. The mixing ratio is determined so as to maximize the thermal stability of the battery, which may result from the lithium manganese oxide, and the battery performance that can be derived from the lithium nickel manganese cobalt composite oxide. When the content of lithium manganese oxide in the mixture is too small The thermal stability of the battery deteriorates. When the content of the lithium nickel manganese cobalt composite oxide is too small, desired battery performance can not be obtained, which is not preferable.

The mixing of the lithium manganese oxide and the lithium nickel manganese cobalt composite oxide can be carried out without any particular limitation as long as it can be carried out industrially, and it is also possible to use a container rotary mixer such as horizontal cylindrical type, V type, double conical type, , A single-shaft rotor, a flow-jet mixer, or the like.

The particle shape of the lithium manganese oxide and lithium nickel manganese cobalt composite oxide according to the present invention is not particularly limited as long as the object of the present invention is satisfied, but spherical shape is preferable in view of the increase of the reaction surface area due to the increase of porosity and packing ratio between the active materials .

The mixture of lithium manganese oxide and lithium nickel manganese cobalt composite oxide according to the present invention can be applied to the positive electrode collector in an amount of 700 mg / 25 cm 2 or more.

In the present invention, the lithium manganese oxide can be produced by a method of wet milling mixed lithium source, manganese source and water well known in the art, spray drying and then calcining, pulverizing and sieving.

The lithium nickel manganese cobalt composite oxide according to the present invention can be produced by a known method such as coprecipitation or solid phase method using a nickel compound, a manganese compound, and a cobalt compound as raw materials.

Examples of free nickel compounds are _{Ni (OH) 2, NiO,} NiOOH, NiCO 3 · 2Ni (OH) 2 · 4H 2 O, NiC 2 O 4 · 2H 2 O, Ni (NO 3) 2 · 6H 2 O, NiSO ₄ , NiSO ₄ .6H ₂ O, fatty acid nickel salts, and nickel halides. Among them, a compound containing no nitrogen atom or sulfur atom such as Ni (OH) ₂ , NiO, NiOOH, NiCO ₃ .2Ni (OH) ₂ .4H ₂ O and NiC ₂ O ₄ .2H ₂ O, NO _X and SO _X, and the like. Ni (OH) ₂ , NiO and NiOOH are particularly preferable because they are inexpensively available as industrial raw materials and have high reactivity. These nickel compounds may be used singly or in combination of two or more.

Examples of manganese compounds that can be used include manganese oxides such as Mn ₂ O ₃ , MnO ₂ and Mn ₃ O ₄ , MnCO ₃ , Mn (NO ₃ ) ₂ , MnSO ₄ , manganese acetate, manganese dicarboxylate, Manganese salts such as fatty acid manganese salts, oxyhydroxides, and halides such as manganese chloride. Of these, MnO ₂ , Mn ₂ O ₃ and Mn ₃ O ₄ are preferable, because gases such as NO _x , SO _x and CO ₂ are not generated at the time of the calcination treatment and can be obtained at low cost as an industrial raw material . These manganese compounds may be used singly or in combination of two or more.

Examples of usable cobalt compounds include Co (OH) ₂ , CoOOH, CoO, Co ₂ O ₃ , Co ₃ O ₄ , Co (OCOCH ₃ ) ₂ .4H ₂ O, CoCl ₂ , Co (NO) ₂ .6H ₂ O and Co (SO _{4) 2} · 7H ₂ O may be mentioned. Of these, Co (OH) ₂ , CoOOH, CoO, Co ₂ O ₃ and Co ₃ O ₄ are preferable because they do not generate harmful substances such as NO _x and SO _x during the baking treatment. Co (OH) ₂ and CoOOH are more industrially inexpensive and more preferable from the viewpoint of high reactivity. These cobalt compounds may be used singly or in combination of two or more.

The mixing method of the nickel, manganese and cobalt compounds is not particularly limited and may be mixed by a wet or dry process. For example, a method using a device such as a ball mill, a vibration mill, or a bead mill may be used. Wet mixing is preferable because it enables more uniform mixing and can increase the reactivity of the mixture in the firing step.

A method of manufacturing a positive electrode using the positive electrode active material according to the present invention will now be described.

First, the cathode active material, the binder and the conductive material obtained by mixing the lithium manganese oxide and the lithium nickel manganese cobalt composite oxide according to the present invention in the range of 10:90 to 90:10 by weight are added to the dispersion and stirred to prepare a paste Then, it is coated on a metal plate for a positive electrode current collector, rolled and dried to produce a laminate-shaped electrode.

The cathode current collector generally has a thickness of 3 to 500 mu m. Such a positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical change in the battery, and may be formed of a material such as stainless steel, aluminum, nickel, titanium, sintered carbon, or a surface of aluminum or stainless steel Treated with carbon, nickel, titanium, silver or the like may 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.

The method of applying the paste of the electrode material to the metal material in a uniform manner can be selected from known methods in consideration of the characteristics of the material and the like or can be carried out by a new suitable method. For example, it is preferable that the paste is uniformly dispersed on a current collector by using a doctor blade or the like. In some cases, a method of performing the distribution and dispersion processes in a single process may be used. In addition, a method such as die casting, comma coating, screen printing, or the like may be employed, or may be formed on a separate substrate, and then pressed or laminated by a pressing or laminating method. . Drying of the paste applied on the metal plate is preferably performed in a vacuum oven at 50 to 200 DEG C for 1 to 3 days.

The negative electrode to be used in the present invention may be formed, for example, by coating and drying an anode active material on an anode current collector, and if necessary, may further include components such as the conductive material, the binder and the filler have.

As the negative electrode active material, a graphite-based active material capable of absorbing and desorbing lithium ions can be used. For example, natural graphite or synthetic graphite can be used.

The negative electrode collector is generally made to have a thickness of 3 to 500 mu m. Such an anode current collector is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery, and examples of the anode current collector include copper, stainless steel, aluminum, nickel, titanium, sintered carbon, surface of copper or stainless steel A surface treated with carbon, nickel, titanium, silver or the like, an aluminum-cadmium alloy, or the like 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.

Examples of the binder for the positive and negative electrode active materials include vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate polymethylmethacrylate, polytetrafluoroethylene (PTFE), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, Such as polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR) copolymers, SBR-CMC copolymers, modified SBR copolymers, A binder polymer may be used.

The conductive material is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery, and examples thereof include graphite such as natural graphite and artificial graphite; Carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; Conductive fibers such as carbon fiber and metal fiber; Metal powders such as carbon fluoride, aluminum, and nickel powder; Conductive whiskey such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used. Concrete examples of commercially available conductive materials include acetylene black series such as Chevron Chemical Company, Denka Singapore Private Limited, Gulf Oil Company, etc.), Ketjenblack, EC (Armak Company), Vulcan XC-72 (Cabot Company), and Super P (Timcal).

If necessary, a filler may be selectively added as a component inhibiting electrode expansion. Such a filler is not particularly limited as long as it is a fibrous material that does not cause a chemical change in the battery, and examples thereof include olefin polymers such as polyethylene and polypropylene; Fibrous materials such as glass fibers and carbon fibers are used.

As the dispersion liquid for dispersing the positive electrode and the negative electrode active material, isopropyl alcohol, N-methylpyrrolidone (NMP), acetone and the like can be used.

The separator is a thin insulating thin film having a high ion permeability and mechanical strength interposed between the anode and the cathode. In the present invention, a porous substrate having pores; And a porous organic-inorganic composite coating layer formed on at least one surface of the porous substrate, the porous organic-inorganic coating layer including a mixture of inorganic particles and a binder polymer for a separation membrane.

In the present invention, the term 'separator' means an organic-inorganic complex porous separator unless otherwise specified.

In the present invention, the term 'porous coating layer' means an organic / inorganic porous coating layer unless otherwise specified.

In the present invention, the porous substrate can be used in any conventional porous substrate used in the separation membrane of an electrochemical device. Any porous substrate such as a porous membrane or a nonwoven fabric formed of various polymers can be used for a planar porous substrate commonly used in an electrochemical device. Do. For example, a polyolefin porous film or a nonwoven fabric used as a separator of an electrochemical device, particularly, a lithium secondary battery can be used, and the material and the shape thereof can be variously selected according to the purpose. For example, the polyolefin-based porous membrane may be made of a polyolefin-based polymer such as polyethylene, polypropylene, polybutylene, or polypentene, such as high-density polyethylene, linear low-density polyethylene, low-density polyethylene, ultra-high molecular weight polyethylene, And may be formed of a polymer. The nonwoven fabric may be a conventional porous nonwoven fabric used as a separator of a secondary battery such as a high melting point polyethylene terephthalate, polyacrylonitrile, polyimide, polyamide, cellulose, aramid, nylon, polyester, polyparaphenylene A nonwoven fabric sheet formed by mixing fibers of two or more of these fibers may be used, but the present invention is not limited thereto.

The thickness of the porous substrate is not particularly limited, but is 1 to 100 占 퐉 or 5 to 50 占 퐉.

The size and porosity of the pores existing in the porous substrate are also not particularly limited, but may be 0.001 to 50 탆 and 10 to 95%, respectively.

The average particle size of the inorganic particles is not particularly limited, but may be in the range of 0.001 to 10 mu m for the formation of a porous coating layer of uniform thickness and adequate porosity. When the average particle diameter of the inorganic particles satisfies the above range, the dispersibility of the inorganic particles can be prevented from decreasing, and the porous coating layer can be adjusted to an appropriate thickness.

The inorganic particles may be inorganic particles having a dielectric constant of 5 or more, or inorganic particles having a lithium ion transporting ability, either singly or in combination.

The inorganic particles having a dielectric constant of 5 or more may be BaTiO ₃ , Pb (Zr _x , Ti _1-x ) O ₃ (PZT, 0 <x <1), Pb _{1 -x} La _x Zr _{1 -y} Ti _y O ₃ 1, 0 <y <1), (1-x) Pb (Mg _1/3 Nb _2/3 ) O _3-x PbTiO ₃ (PMN- Wherein the metal oxide is selected from the group consisting of HfO ₂ , SrTiO ₃ , SnO ₂ , CeO ₂ , MgO, NiO, CaO, ZnO, ZrO ₂ , SiO ₂ , Y ₂ O ₃ , Al ₂ O ₃ , SiC and TiO ₂ Or a mixture of two or more thereof.

Wherein the inorganic particles having lithium ion transferring ability are at least one selected from the group consisting of lithium phosphate (Li ₃ PO ₄ ), lithium titanium phosphate (Li _x Ti _y (PO ₄ ) ₃ , 0 <x < phosphate _{(Li x Al y Ti z (} PO 4) 3, 0 <x <2, 0 <y <1, 0 <z <3), (LiAlTiP) x O y series glass (glass) (0 <x < 4 , 0 <y <13), lithium lanthanum titanate _{(Li x La y TiO 3,} 0 <x <2,0 <y <3), lithium germanium thiophosphate Mani Titanium (Li _x Ge _y P _z S _w, 0 (Li _x N _y , 0 <x <4, 0 <y <2), SiS ₂ (Li _x N _y , 0 <x <4, 0 <y <1, 0 <z < _x Si _y S _z , 0 <x <3, 0 <y <2, 0 <z <4) series glass and P ₂ S ₅ (Li _x P _y S _z , , 0 < z < 7) series glass, or a mixture of two or more thereof.

The thickness of the porous coating layer formed on the porous substrate by the inorganic particles and the binder polymer for the separation membrane may be 0.01 to 20 탆, but is not limited thereto.

The content of the binder polymer for separator may be 1 to 50 parts by weight based on 100 parts by weight of the inorganic particles. When the content of the binder polymer for a separation membrane satisfies the above range, the thermal stability of the separation membrane is improved, the interstitial volume formed between the inorganic particles is sufficiently ensured, the performance of the final cell is increased, The peeling resistance of the coating layer can be improved. As used herein, the term "interstitial volume" refers to a space defined by inorganic particles that are substantially interfaced in a closed packed or densely packed structure, . &Lt; / RTI &gt;

The binder polymer for a separator may be independently selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene, polyvinylidene fluoride-trichlorethylene, polymethyl methacrylate, polyacrylonitrile, polyvinyl pyrrolidone, polyvinyl Cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan, cyanoethylpolyvinyl alcohol, cyanoethylcellulose, cyanoethyl sucrose, pullulan, hydroxypropylcellulose, hydroxypropylcellulose, hydroxypropylmethylcellulose, Acrylonitrile styrene-butadiene copolymer, and polyimide, or a mixture of two or more thereof.

The method for preparing a separation membrane according to the present invention comprises the steps of: preparing a planar porous substrate having pores; And coating a slurry containing inorganic particles, a binder polymer for a separator, and a solvent on at least one surface of the porous substrate to form a porous coating layer.

According to the separation membrane producing method of the present invention, first, a porous substrate having pores is prepared.

Then, at least one surface of the porous substrate is coated with a slurry containing inorganic particles, a binder polymer for a separation membrane, and a solvent to form a porous coating layer.

The coating apparatus is not particularly limited as long as it is a conventional coating apparatus that can be used in the art and includes, for example, a dip coating apparatus, a die coating apparatus, a roll coating apparatus, a comma coating apparatus, And is preferably a slot die coating apparatus.

The solvent contained in the coated slurry may be dried according to a method commonly used in the art.

The drying method of the solvent may be carried out batchwise or continuously using an oven or a heating chamber at a temperature range condition considering the vapor pressure of the solvent used.

The solvent used in the present invention is not particularly limited as long as it can disperse the inorganic particles while dissolving the binder polymer, but it may be more advantageous that the boiling point is low. This is to facilitate subsequent solvent removal. Non-limiting examples of usable solvents include acetone, tetrahydrofuran, methylene chloride, chloroform, dimethylformamide, N-methyl-2-pyrrolidone, cyclohexane, water or mixtures thereof.

Although a method of forming a coating layer on only one side of the porous substrate has been described above, the present invention is not limited thereto, and a porous organic-inorganic hybrid membrane can be manufactured by forming a coating layer on both sides of the porous substrate.

The nonaqueous electrolyte solution injected into the electrode assembly according to the present invention includes a lithium salt and an organic solvent.

There, the lithium salt is a lithium salt that is commonly used in the non-aqueous electrolyte may be used, without limitation, non-limiting examples include _{LiPF 6, LiBF 4, LiSbF 6} , LiAsF 6, LiClO 4, LiN (C 2 F 5 SO 2) 2 , LiN (CF ₃ SO ₂₎ and the like _{2, CF 3 SO 3 Li,} LiC (CF 3 SO 2) 3, LiPF ₆ is preferred.

As the organic solvent, cyclic carbonates, linear carbonates, esters and ethers commonly used in the art may be used, and organic solvents composed of cyclic carbonates including ethylene carbonate and propylene carbonate are preferable.

The electrolytic solution may further contain additive compounds such as lactone, ether, ester, acetonitrile, lactam, ketone and the like insofar as the object of the present invention is not impaired.

The assembly of the positive electrode and the negative electrode can be manufactured by being housed in a metal can such as a square type or a cylindrical type which is a typical exterior member. Of these, a cylindrical metal can is preferable because it is made of a lithium secondary battery having a relatively large capacity and structurally stable. But is not limited thereto.

Hereinafter, the present invention will be described in further detail with reference to the following examples. However, the following examples are for illustrative purposes only and are not intended to limit the scope of the present invention.

Example 1

The lithium source, manganese source and water were mixed and wet milled, followed by spray drying, calcination, pulverization and sieving to obtain LiMn ₂ O ₄ having an average particle diameter (D50) of 20 탆 _. The obtained LiMn ₂ O ₄ was mixed with LiNi _0.5 Mn _0.3 Co _0.2 O _{2 at a} weight ratio of 1: 1 to prepare a cathode active material. The cathode active material was mixed with 5 wt% of carbon black and 5 wt% of PVdF, stirred with a solvent of N-methyl-2-pyrrolidone (NMP), coated on the aluminum current collector to a thickness of about 200 μm, And dried once again under vacuum and at a temperature of 110 ° C to prepare a positive electrode. Finally, the positive electrode was rolled by a roll press into a sheet form to prepare a positive electrode.

The copper foil was coated with a negative electrode mixture prepared by mixing natural graphite, a styrene-butadiene rubber (SBR) binder, and carboxymethyl cellulose (CMC).

For the preparation of the separator, Al ₂ O ₃ as inorganic particles, PVDF-HFP as a binder polymer and acetone as a solvent were mixed at a weight ratio of 16: 4: 80 to prepare a slurry. The slurry was coated on a 16 μm thick polyolefin membrane (Celgard, C210) to prepare a composite separator having an organic / inorganic porous coating layer.

An electrode assembly was fabricated by interposing a composite separator between the anode and the cathode. The electrode assembly was placed in a pouch and lead wires were connected to each other. After the leads were connected, a 1 M LiPF ₆ salt was dissolved in an organic solvent of ethylene carbonate (EC), propylene carbonate (PC) or propyl propionate (PP) in a volume ratio of 3: 2: After the electrolyte solution was injected, the lithium secondary battery was assembled by sealing.

Comparative Example 1

A lithium secondary battery was fabricated in the same manner as in Example 1, except that LiNi _0.5 Mn _0.3 Co _0.2 O ₂ alone was used to produce a cathode active material.

Experimental Example 1: High-temperature nail penetration test

Five batteries prepared according to Example 1 and five batteries prepared according to Comparative Example 1 were fully charged at a voltage of 4.2 V and stored at 100 ° C for 30 minutes. Then, using a nail having a diameter of 2.5 mm, At a rate of 5 to 12 m / min, and the ignition was observed. The results are shown in Table 1 below.

Example 1 Comparative Example 1 Number of ignition batteries 0 5 Explosion Battery Number 0 5

Experimental Example 2: Discharge test

Discharge capacity test was performed on the cylindrical lithium secondary battery manufactured in Example 1 and Comparative Example 1 at room temperature (25 캜) by 1 C charging / 1 C discharging cycling in a voltage range of 3 to 4.4 V, Respectively.

From the graph of FIG. 2, it can be seen that the energy density of the secondary battery of Example 1 is higher than that of the secondary battery of Comparative Example 1.

Claims (16)

A lithium secondary battery comprising a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte,
Wherein the positive electrode comprises a lithium manganese oxide represented by LiMn ₂ O ₄ and a lithium nickel manganese cobalt composite oxide represented by the following formula 1,
The average particle diameter (D50) of the lithium manganese oxide is 20 to 30 mu m,
The average particle diameter (D50) of the lithium nickel manganese cobalt composite oxide is 5 to 20 占 퐉,
Wherein the lithium manganese oxide and the lithium nickel manganese cobalt composite oxide have a particle diameter difference of 10 to 25 mu m,
The lithium manganese oxide and the lithium nickel manganese cobalt composite oxide are contained in a weight ratio of 30:70 to 70:30 based on the total cathode active material,
The positive electrode active material is applied to the electrode collector in an amount of 700 mg / 25 cm &lt; 2 &gt;
Wherein the separator is a composite separator comprising a porous substrate having pores and an organic / inorganic porous coating layer formed on at least one surface of the porous substrate and including a mixture of inorganic particles and a binder polymer for a separator. battery:
[Chemical Formula 1]
Li _{1 + a} (Ni _x Mn _y Co _z ) _1-a O ₂
In this formula,
x + y + z = 1,
0.05? A? 0.10,
0.4? X? 0.6,
0.2? Y? 0.4,
0.1? Z? 0.3.
The method according to claim 1,
Wherein the one positive electrode active material has an average particle diameter (D50) corresponding to 0.2 to 0.7 times the average particle diameter (D50) of the other one positive electrode active material.
delete delete delete The method according to claim 1,
Wherein the lithium manganese oxide and the lithium nickel manganese cobalt composite oxide are contained in a weight ratio of 10:90 to 90:10 based on the total cathode active material.
delete delete The method according to claim 1,
The porous substrate may be selected from the group consisting of high density polyethylene, low density polyethylene, linear low density polyethylene, ultra high molecular weight polyethylene, polypropylene polyethylene terephthalate, polybutyleneterephthalate, polyester, polyacetal, polyamide, polyamide, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenylene oxide, polyphenylenesulfroid and polyethylene naphthalene (polyethylenenaphthalene). &lt; / RTI &gt;
The method according to claim 1,
Wherein the inorganic particles are at least one selected from the group consisting of inorganic particles having a dielectric constant of 5 or more, inorganic particles having a lithium ion transporting ability, and mixtures thereof.
11. The method of claim 10,
The inorganic particles having a dielectric constant of 5 or more include BaTiO ₃ , Pb (Zr _x , Ti _{1 -x} ) O ₃ (PZT, 0 <x <1), Pb _{1 -x} La _x Zr _{1 -y} Ti _y O ₃ , 0 <x <1, 0 <y <1), (1-x) Pb (Mg _1/3 Nb _2/3 ) O _3-x PbTiO ₃ At least one selected from the group consisting of HfO ₂ , SrTiO ₃ , SnO ₂ , CeO ₂ , MgO, NiO, CaO, ZnO, ZrO ₂ , SiO ₂ , Y ₂ O ₃ , Al ₂ O ₃ , SiC and TiO ₂ Lt; RTI ID = 0.0 &gt; rechargeable < / RTI &gt;
11. The method of claim 10,
The inorganic particles having lithium ion transferring ability include lithium phosphate (Li ₃ PO ₄ ), lithium titanium phosphate (Li _x Ti _y (PO ₄ ) ₃ , 0 <x <2, 0 <y <3), lithium aluminum titanium phosphate _{(Li x Al y Ti z (} PO 4) 3, 0 <x <2, 0 <y <1, 0 <z <3), (LiAlTiP) x O y series glass (0 <x <4, 0 <y <13), lithium lanthanum titanate (Li _x La _y TiO ₃ , 0 <x <2, 0 <y <3), lithium germanium thiophosphate (Li _x Ge _y P _z S _w , , 0 <y <1, 0 <z <1, 0 <w <5), lithium nitrides _{(Li x N y, 0 <} x <4, 0 <y <2), SiS 2 (Li x Si y S _{z, 0 <x <3,} 0 <y <2, 0 <z <4) based glass, and _{P 2 S 5 (Li x P} y S z, 0 <x <3, 0 <y <3, 0 <z &Lt; 7) -glass. &Lt; / RTI &gt;
The method according to claim 1,
The binder polymer for the separator may be selected from the group consisting of polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichlorethylene, polymethylmethacrylate ), Polybutylacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinyl acetate, polyethylene oxide but are not limited to, polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan, No ethyl polyvinyl alcohol (cyano ethylpolyvinylalcohol, cyanoethylcellulose, cyanoethylsucrose, pullulan, and carboxyl methyl cellulose. The lithium secondary battery according to claim 1,
The method according to claim 1,
Wherein the content of the binder polymer for separation membrane is 1 to 50 parts by weight based on 100 parts by weight of the inorganic particles.
The method according to claim 1,
Wherein the negative electrode comprises a graphite-based active material.
The method according to claim 1,
Wherein the electrolyte solution comprises a LiPF ₆ electrolyte and a cyclic carbonate organic solvent.

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