KR101839754B1 - Multi-Layered Electrode of Different Porosity and Method for Preparation of the Same - Google Patents

Abstract

The present invention relates to an electrode assembly comprising an electrode collector for transferring electrons between an external conductor and an electrode active material and three or more electrode mixture layers sequentially coated on the electrode current collector, Wherein a porosity of the electrode material mixture layer located relatively closer to the current collector among the mutually adjacent electrode material mixture layers with respect to the forming direction of the electrode material mixture layers is relatively far from the current collector Wherein the electrode mixture layer has a porosity lower than that of the electrode mixture layer and a method of manufacturing the same.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-

The present invention relates to a multi-layered electrode having different porosities, and more particularly to a multi-layered electrode characterized by having a lower porosity as the electrode material mixture layer nearer the current collector and a method of manufacturing the same will be.

In recent years, the demand for environmentally friendly alternative energy sources has become an indispensable factor for the future, as the increase in the price of energy sources due to depletion of fossil fuels and the interest in environmental pollution are amplified. Various researches on power generation technologies such as nuclear power, solar power, wind power, and tidal power have been continuing, and electric power storage devices for more efficient use of such generated energy have also been attracting much attention.

Particularly, as technology development and demand for mobile devices are increasing, the demand for batteries as energy sources is rapidly increasing, and accordingly, a lot of researches on batteries that can meet various demands have been conducted.

Typically, in terms of the shape of a battery, there is a high demand for a prismatic secondary battery and a pouch-type secondary battery which can be applied to products such as mobile phones with a small thickness, and has advantages such as high energy density, discharge voltage, There is a high demand for lithium secondary batteries such as lithium ion batteries and lithium ion polymer batteries.

The lithium secondary battery has a structure in which an electrolyte including a lithium salt is impregnated in an electrode assembly having a porous separator interposed between a positive electrode and a negative electrode coated with an active material on an electrode current collector.

The electrode is prepared by applying a slurry in which an active material, a binder, and a conductive material are dispersed in a solvent to a current collector, followed by drying and pressing. Generally, the above process is a single process which is not repeated. The electrode manufactured by pressing only once has the highest porosity at the side closest to the current collector due to the pressure, the specific gravity of the binder and the conductive material, Has the lowest configuration.

Specifically, when the slurry is dried, the solvent is blown off from the binder in which the solvent is placed, whereby the binder floats toward the surface of the electrode in the current collector. When the binder is floated, the lightweight conductive material combined with the binder also floats together, and the distribution of the conductive material and the binder in the electrode increases in the thickness direction. That is, the electric conductivity increases with distance from the current collector, but the electric conductivity of the electrode is lowered as the current collector approaches the collector, thereby deteriorating the battery characteristics.

In addition, due to the above-mentioned reason, the distribution of the binder is decreased at the bottom portion of the electrode, and the adhesive force is decreased. As a result, the problem of the fairness of dropping of the electrode after electrolyte impregnation and performance deterioration occur. These phenomena are maximized as the C-rate becomes higher, which is more problematic when a middle- or large-sized secondary battery pack is manufactured.

Therefore, there is a great need for a technology that can fundamentally solve these problems.

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

The inventors of the present application have conducted intensive research and various experiments, and have found that, as will be described later, an electrode collector for transferring electrons between an outer conductor and an electrode active material, and an electrode collector Wherein the electrode assembly layers include electrode active materials, and the electrode assembly layers are formed on the electrode assembly layers adjacent to each other with respect to a direction in which the electrode assembly layers are formed, In the case of a multi-layered electrode in which the porosity of the mixed layer is relatively smaller than the porosity of the electrode mixture layer located farther from the current collector, the conductive material and the binder in the electrode layer close to the current collector are densely distributed, Thus, it was confirmed that the electrode did not desorb even in the volume expansion contraction due to charge / discharge, Leading to the completion of the person.

The present invention relates to an electrode collector for transferring electrons between an external conductor and an electrode active material, and electrode mixture layers of three or more layers sequentially coated on the electrode current collector,

Wherein the electrode mixture layers each comprise an electrode active material,

The electrode assembly layer having a porosity of the electrode assembly layer positioned relatively closer to the current collector than that of the electrode assembly layers adjacent to the current collector, And the porosity is smaller than the porosity.

Generally, the electrodes made of one layer by pressing are most likely to have the highest porosity of the electrode mixture layer nearest to the current collector and the lowest porosity of the electrode mixture layer farthest from the current collector, Has a lower electrical conductivity and adhesion force.

This phenomenon is particularly problematic in a high-capacity battery because the conductivity of the active material is lowered and the conductive material is less used. Therefore, in the case of a multi-layer structure having a low porosity of the electrode material mixture layer close to the current collector as described above, A high binding force can be secured.

That is, in the case of the electrode material mixture layer near the current collector, the porosity is small and the density of the active material or the conductive material is high, and therefore, the electric conductivity is excellent.

However, the electrode mixture layer having a low porosity may have a large difference in volume during expansion and contraction during charging and discharging, and there is a risk of short-circuiting and explosion thereof. As a result, the electrode mixture needs to have a predetermined porosity on average.

Accordingly, it is possible to secure a high electrical conductivity by lowering the porosity of the electrode material mixture layer located near the current collector and to increase the porosity of the electrode mixture layer located farther away, thereby reducing the volume expansion and shrinkage difference during charging and discharging, .

Further, the electrode mixture layer far from the current collector has a large porosity and thus has a large surface area in contact with the electrolyte. As a result, the ion conductivity can be improved, and the gas generated inside the electrode can escape to the outside owing to the relatively coarse structure Bar and battery characteristics can be improved.

The three-layered electrode mix layer is a structure in which layers having different porosity are discontinuously contacted. In one specific example, the multi-layered electrode may be three or more layers or five layers or less.

If the electrode mix layer of the multi-layer structure electrode is out of the above range, the process becomes complicated and the production amount per unit time decreases, resulting in an increase in manufacturing cost. When the electrode assembly layer of the multi-layer structure electrode has one layer, it is a single layer structure and is not different from the prior art. In the electrode structure composed of two layers, the electrode assembly layer located near the current collector and the electrode mixture layer located farther It is difficult to achieve the desired effect of the present invention unless the difference in porosity between the two electrode mix layers is large. Problems that arise when the difference in porosity of the electrode mixture layers adjacent to each other is great will be described in detail below.

In one specific example, the difference in porosity of the mutually adjacent electrode material mixture layers is preferably in the range of 0.5% to 15%, more preferably in the range of 1 to 12%, more particularly in the range of 2% to 10% Do.

If the difference in the porosity of the electrode mixture layers adjacent to each other is out of the above range, the difference between the electrode mixture layers is insignificant, which is almost the same as that of the single layer electrode.

On the other hand, if the difference in the porosity of the adjacent electrode mixture layers is out of the above range, the difference in conductivity between the adjacent layers is large, resulting in interlayer interfacial disorder, and only the electrode mixture layer having a relatively high conductivity The current is integrated and the electrode electrode material mixture layer having a relatively small conductivity is not utilized or the cell characteristics are deteriorated by the bottleneck phenomenon.

In one specific example, the porosity of the innermost layer electrode material mixture layer directly contacting the electrode current collector is preferably within a range of 10% to 50% in a range smaller than the porosity of the electrode material mixture layer adjacent to the innermost layer electrode material mixture layer , More preferably from 15% to 45%.

If the porosity is less than 10%, it is structurally very dense and it is difficult to impregnate the electrolyte into the inside of the electrode, so that the charge / discharge capacity and other electrochemical characteristics may be deteriorated, and the porosity reaches to less than 10% It is difficult to do. On the other hand, when the porosity exceeds 50%, the electronic conductivity is lowered and the electron path can not be performed smoothly, and the adhesive force with the current collector is lowered.

In another specific example, the porosity of the outermost electrode composite material layer located farthest from the electrode collector in the electrode material mixture layers is 15% or more in a range larger than the porosity of the electrode material mixture layer adjacent to the outermost electrode composite material layer, To 55%, and more preferably 20% to 50%.

If the porosity of the outermost electrode composite material layer is less than 15%, the impregnation property of the electrolyte deteriorates and the charge / discharge capacity and other electrochemical characteristics may deteriorate. On the other hand, the electrode mixture layer prior to the pressing process generally has a porosity of not more than 60%, and when the porosity of the outermost layer electrode mixture layer exceeds 55%, the electron conductivity lowers, And the thickness of the electrode is increased as a result of securing a desired capacitance, which is not preferable.

The mutually adjacent electrode material mixture layers may be partially or wholly mixed with each other or may form an interface without being mixed with each other at the interface, but it is preferable that the electrode mixture layers form an interface with each other so as to maintain the interlayer porosity difference. However, even when the interface is formed in this way, it is preferable that the difference in porosity between mutually adjacent layers does not exceed the range described above in order to prevent bottleneck phenomenon and interfacial disorder.

The thickness of the three or more electrode mixture layers may be the same or different from each other. At this time, the thickness means the thickness after pressing. When the thicknesses of the electrode mixture layers are equal to each other, it is easy to design the press time and pressure. When the thicknesses of the electrode mixture layers are different from each other, the electrode mixture layers are close to the current collector or depending on the kind of the electrode active material, It is possible to design the thickness of the electrode material mixture layer located at a distant portion to be relatively thick or thin.

In one specific example, the multi-layered electrode may be a cathode including a cathode active material, a cathode including a cathode active material, and both the anode and the cathode may be a multi-layered electrode. Specifically, since the electric conductivity of the cathode active material is generally lower than the electric conductivity of the anode active material, the multi-layered electrode may be an anode so as to maximize the effects of the present invention.

The positive electrode is prepared by applying a mixture of a positive electrode active material, a conductive material and a binder on a positive electrode current collector, followed by drying and pressing. If necessary, a filler may be further added to the mixture.

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 changes in the battery. Examples of the positive electrode current collector include stainless steel, aluminum, nickel, titanium, sintered carbon, aluminum or stainless steel A surface 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 cathode active material may be a layered compound such as lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), or a compound substituted with one or more transition metals; Lithium manganese oxides such as Li _{1 + x} Mn _{2 -x} O ₄ (where x is 0 to 0.33), LiMnO ₃ , LiMn ₂ O ₃ , LiMnO ₂ and the like; Lithium copper oxide (Li ₂ CuO ₂ ); Vanadium oxides such as LiV ₃ O ₈ , LiFe ₃ O ₄ , V ₂ O ₅ and Cu ₂ V ₂ O ₇ ; A Ni-site type lithium nickel oxide expressed by the formula LiNi _1-x M _x O ₂ (where M = Co, Mn, Al, Cu, Fe, Mg, B or Ga and x = 0.01 to 0.3); Formula _{LiMn 2-x M x O 2} ( where, M = Co, Ni, Fe , Cr, and Zn, or Ta, x = 0.01 ~ 0.1 Im) or Li ₂ Mn ₃ MO ₈ (where, M = Fe, Co, Ni, Cu, or Zn); A lithium manganese composite oxide having a spinel structure represented by LiNi _x Mn _2-x O ₄ ; LiMn ₂ O _{4 in} which a part of Li in the formula is substituted with an alkaline earth metal ion; Disulfide compounds; Fe ₂ (MoO ₄ ) ₃ , and the like. However, the present invention is not limited to these.

On the other hand, the negative electrode is manufactured by repeating the process of applying, drying and pressing the negative electrode active material on the negative electrode current collector, and may optionally further include a conductive material, a binder, a filler, and the like.

The negative electrode current 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, a 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.

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

The types of the electrode active materials included in the three or more electrode mixture layers may be the same. That is, the same kind of positive electrode active material and / or negative electrode active material can be used so as to reduce manufacturing process and manufacturing cost of the multi-layered positive electrode and / or the multi-layered negative electrode.

Conversely, among the electrode mixture layers of the multi-layered electrode, the types of the electrode active materials included in the two or more electrode mixture layers may be different from each other. The multi-layer structure electrode according to the present invention is a method for differentiating porosity, and as described below, a method of pressing at different pressures is proposed, but the present invention is not limited thereto, and different types of cathode active materials and / Alternatively, the porosity can be varied by using the negative electrode active material.

The electrode material mixture layers may or may not include a conductive material, but in general, the anode including the cathode active material is poor in conductivity, and therefore, it is preferable to include a conductive material.

The conductive material is usually added in an amount of 1 to 50% by weight based on the total weight of the mixture including the cathode active material. Such a conductive material is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery, for example, graphite such as natural graphite or 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 may be used, but the present invention is not limited thereto.

In addition, the electrode mixture layers may include a binder for assisting the electrode active material, the conductive material, and the like.

The binder is usually added in an amount of 1 to 50% by weight based on the total weight of the mixture including the cathode active material. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene , Polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butylene rubber, fluorine rubber, various copolymers, and the like.

In one specific example, the binder contained in each of the mutually adjacent electrode material mixture layers may be densely distributed as they approach the current collector. The electrode mixture layer having a relatively low porosity and a low porosity as the electrode mixture layer closer to the current collector is more densely distributed in the thickness direction, so that the adhesion with the current collector is improved. Accordingly, the electrode mixture layer located near the current collector can be prevented from being separated from the current collector by the densely distributed binder, and safety can be ensured.

In addition, the electrode material mixture layers may further include a filler that inhibits the expansion of the electrode, and is not particularly limited as long as it is a fibrous material without causing chemical change in the battery. For example, polyethylene, polypropylene , Fiberglass materials such as glass fibers and carbon fibers are used.

The present invention provides a secondary battery having a structure in which a lithium salt-containing electrolyte is impregnated in an electrode assembly having a separator interposed between the positive and negative electrodes.

The separation membrane is interposed between the anode and the cathode, and an insulating thin film having high ion permeability and mechanical strength is used. The pore diameter of the separator is generally 0.01 to 30 μm and the thickness is generally 5 to 300 μm. Such separation membranes include, for example, olefinic polymers such as polypropylene, which are chemically resistant and hydrophobic; A sheet or nonwoven fabric made of glass fiber, polyethylene or the like is used. When a solid electrolyte such as a polymer is used as an electrolyte, the solid electrolyte may also serve as a separation membrane.

The electrolyte solution containing the lithium salt is composed of an electrolyte solution and a lithium salt. The electrolyte solution may be a non-aqueous organic solvent, an organic solid electrolyte, or an inorganic solid electrolyte, but is not limited thereto.

Examples of the non-aqueous organic solvent include N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma -Butyrolactone, 1,2-dimethoxyethane, tetrahydroxyfuran, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane , Acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate Nonionic organic solvents such as tetrahydrofuran derivatives, ethers, methyl pyrophosphate, ethyl propionate and the like can be used.

Examples of the organic solid electrolyte include a polymer electrolyte such as a polyethylene derivative, a polyethylene oxide derivative, a polypropylene oxide derivative, a phosphate ester polymer, an agitation lysine, a polyester sulfide, a polyvinyl alcohol, a polyvinylidene fluoride, A polymer containing an ionic dissociation group and the like may be used.

Examples of the inorganic solid electrolyte include Li ₃ N, LiI, Li ₅ NI ₂ , Li ₃ N-LiI-LiOH, LiSiO ₄ , LiSiO ₄ -LiI-LiOH, Li ₂ SiS ₃ , Li ₄ SiO ₄ , Nitrides, halides and sulfates of Li such as Li ₄ SiO ₄ -LiI-LiOH and Li ₃ PO ₄ -Li ₂ S-SiS ₂ can be used.

The lithium salt is a material that is readily soluble in the non-aqueous electrolyte, for example, LiCl, LiBr, LiI, LiClO 4, LiBF 4, LiB 10 Cl 10, LiPF 6, LiCF 3 SO 3, LiCF 3 CO 2, LiAsF _{6, LiSbF 6, LiAlCl 4,} CH 3 SO 3 Li, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenyl borate and imide.

For the purpose of improving the charge / discharge characteristics and the flame retardancy, the electrolytic solution is preferably mixed with an organic solvent such as pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, glyme, Benzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, . In some cases, halogen-containing solvents such as carbon tetrachloride and ethylene trifluoride may be further added to impart nonflammability. In order to improve the high-temperature storage characteristics, carbon dioxide gas may be further added. FEC (Fluoro-Ethylene Carbonate, PRS (Propene sultone), and the like.

In a preferred _{embodiment, LiPF 6, LiClO 4, LiBF} 4, LiN (SO 2 CF 3) 2 , such as a lithium salt, a highly dielectric solvent of DEC, DMC or EMC Fig solvent cyclic carbonate and a low viscosity of the EC or PC of And then adding it to a mixed solvent of linear carbonate to prepare a lithium salt-containing non-aqueous electrolyte.

The present invention also provides a battery module including the secondary battery as a unit battery, and provides the device using the battery module as a power source.

Preferable examples of the device include, but are not limited to, an electric car, a hybrid electric vehicle, a notebook, a smart phone, a tablet, a wearable device, a plug-in hybrid electric vehicle, or a power storage system.

Since such devices are well known in the art, a detailed description thereof will be omitted herein.

The present invention relates to an electrode assembly comprising an electrode collector for transferring electrons between an external conductor and an electrode active material and three or more electrode mixture layers sequentially coated on the electrode current collector, The electrode mixture layer having a porosity of the electrode material mixture layer located relatively closer to the current collector is located farther from the current collector than the electrode mixture layer adjacent to the current collector, A method for producing a multi-layered electrode having a porosity lower than that of a mixed layer,

(a) preparing an electrode slurry;

(b) Electrode on the electrode current collector The first step of coating the electrode slurry prepared in the step (a), drying and pressing the electrode slurry;

(c) secondarily coating the electrode slurry on the electrode prepared in the step (b), drying and pressing the electrode slurry with a pressure smaller than the first pressure; And

(d) thirdly coating the electrode slurry on the electrode prepared in the step (c), drying and pressing the electrode slurry with a pressure smaller than that of the second electrode;

The present invention also provides a method of manufacturing an electrode.

In general, when the electrode assembly is coated and dried and pressed, the porosity of the electrode assembly layer near the current collector decreases due to the pressure decrease due to the pressure of the surface portion of the electrode, An electrode which is larger than the porosity of the electrode mixture layer far from the current collector is produced.

When the multi-layer coating is performed by increasing the pressure during the first coating as described above, the porosity of the electrode mixture layer close to the current collector is lowered and the porosity of the electrode surface, that is, Can be greatly increased.

In the case of the primary coating, it is preferable that the thickness is in the range of 5% to 40% of the total electrode thickness. If the total thickness of the electrode mixture is less than 5%, the thickness of the secondary coating may be too large to cause problems such as a single coating, and it may be difficult to form a three-layer structure. Impregnation of the electrolyte solution into the electrode is not easy, and it is difficult to secure a space capable of accommodating the expansion and contraction of the electrode mixture, which is not preferable. For the above reasons, it is more preferable that the thickness of the primary coating is in the range of 10% to 35% of the total electrode thickness.

The magnitude of the pressure for applying the pressing may be appropriately selected within a range for realizing a desired porosity and may be different depending on materials such as active material and conductive material, and is not defined separately.

As described above, the electrode for a secondary battery according to the present invention is characterized in that the electrode mix layer has a porosity of the electrode mix layer near the current collector with respect to the thickness direction, The electrolyte impregnability, ionic conductivity, electronic conductivity and the like are improved, and the electrochemical performance of the secondary battery including the same is improved.

Hereinafter, the present invention will be described in more detail with reference to some embodiments thereof, but the scope of the present invention is not limited thereto.

Preparation of positive electrode mixture and lithium secondary battery

≪ Example 1 >

LiNi _0.8 Mn _0.1 Co _0.1 O ₂ as a positive electrode active material, Super-P 65 as a conductive material and polyvinylidene fluoride (PVdF: KF700) as a binder were mixed at a weight ratio of 94: 3: 3, Methylpyrrolidone (NMP) was added dropwise thereto to prepare a positive electrode mixture. The positive electrode material mixture prepared above was applied to an aluminum foil so that the loading amount was 9 mg / cm ² , dried at 120 ° C for 10 minutes, and then roll-pressed to have a porosity of 25% The positive electrode material mixture was applied to an aluminum foil so that the loading amount was 6 mg / cm ² , dried at 120 ° C for 10 minutes, and then subjected to roll coating so as to have a porosity of 30% The positive electrode mixture was applied to an aluminum foil so that the loading amount was 9 mg / cm ² , dried at 120 ° C for 10 minutes, and then subjected to roll coating with a porosity of 35% to prepare a positive electrode for a secondary battery .

Artificial graphite 90.25% by weight as a negative active material, SiO 4.75% by weight, the conductive material (Super-C65) 1.5% by weight, a binder (SBR) 2.3% by weight, and a thickener (CMC) into a 1.2% by weight in a solvent of H ₂ O mix followed by mixing to prepare a negative electrode mixture. The negative electrode mix prepared above was applied to a copper foil so that the loading amount was 13.5 mg / cm ² , dried at 100 ° C for 30 minutes, and rolled to 30% porosity to prepare a negative electrode for a secondary battery.

After the porous polyethylene separator was sandwiched between the positive electrode and the negative electrode, 1% VC of the additive VC, 1.5% by weight of PS and 1 M of LiPF ₆ dissolved in a carbonate solvent of EC: EMC = 1: cm sheet-type lithium secondary battery.

≪ Comparative Example 1 &

A positive electrode mixture was applied on an aluminum foil so that the loading amount was 27 mg / cm < ² >, and roll pressing was performed so as to have a porosity of 30% to perform only one coating. A positive electrode and a secondary battery were produced.

≪ Comparative Example 2 &

The positive electrode mixture was applied on the aluminum foil so that the loading amount was 13.5 mg / cm ² , and the primary coating was performed by roll pressing so as to have a porosity of 25%, and the coating amount was applied so that the loading amount was 13.5 mg / cm ^2. Except that the secondary coating was carried out by roll pressing so as to have a thickness of 35%, thereby preparing a positive electrode and a secondary battery for a secondary battery.

Manufacture of anode mixture and secondary battery

≪ Example 2 >

CNT as a conductive material, SBR as a binder, and CMC as a thickener were mixed at a weight ratio of 93.5 (7/3): 2: 3: 1.5, and H ₂ O was added dropwise to the mixture, A mixture was prepared. The negative electrode mixture prepared above was applied to a copper foil with a loading amount of 3 mg / cm ² , dried at 100 ° C for 10 minutes, and then roll-pressed to have a porosity of 25% The negative electrode material mixture was applied to a copper foil so that the loading amount was 3 mg / cm ² , dried at 100 ° C for 10 minutes, and then subjected to secondary coating by roll pressing so as to have a porosity of 35% The negative electrode mixture was applied to a copper foil so that the loading amount was 3 mg / cm ² , dried at 100 ° C for 10 minutes, and then subjected to roll coating and roll coating so as to have a porosity of 45% to prepare a negative electrode for a secondary battery .

94 wt% of LiNi _0.8 Mn _0.1 Co _0.1 O ₂ as a positive electrode active material, 3 wt% of a conductive material (Super-C65) and 3 wt% of a binder (PVdF) were mixed in NMP as a solvent and mixed to prepare a positive electrode material mixture. The positive electrode mixture prepared above was applied to an aluminum foil so that the loading amount was 30 mg / cm ² , dried at 120 ° C for 30 minutes, and rolled to 30% porosity to prepare a positive electrode for a secondary battery.

After the porous polyethylene separator was sandwiched between the positive electrode and the negative electrode, 1% VC of the additive VC, 1.5% by weight of PS and 1 M of LiPF ₆ dissolved in a carbonate solvent of EC: EMC = 1: cm sheet-type lithium secondary battery.

≪ Comparative Example 3 &

Except that the coating amount of the negative electrode mixture was adjusted to 9 mg / cm ² on the copper foil and roll-pressing was performed so that the porosity became 35%, thereby performing only one coating. A secondary battery was manufactured.

≪ Comparative Example 4 &

The negative electrode mixture was coated on a copper foil so that the loading amount was 4.5 mg / cm ² , and the coating was roll-pressed so as to have a porosity of 25% to perform a primary coating and a loading amount of 4.5 mg / cm ^2. And 45%, respectively, to prepare a negative electrode and a secondary battery in the same manner as in Example 2, except that the secondary coating was performed.

<Experimental Example 1>

1.1 Evaluation of capacity characteristics according to charge / discharge rate (C-rate)

Example 1 and Comparative Examples 1 and 2; And discharge capacities and rate characteristics of each pouch type secondary battery manufactured in Example 2 and Comparative Examples 3 and 4 were measured at 4.2 V full charge state to 2.5 V region at 1 C and 3 C- The retention ratios are shown in Table 1 below.

1.2 Evaluation of capacity characteristics according to charging / discharging cycle

Example 1 and Comparative Examples 1 and 2 at a room temperature of 25 deg. And each of the pouch type secondary batteries manufactured in Example 2 and Comparative Examples 3 and 4 were charged and discharged in the range of 4.2 V to 2.5 V with 1 C / 1 C, and the batteries were charged and discharged in a cycle of 20 minutes 300 cycles of charging and discharging were performed with a rest time. The results of comparison between the initial discharge capacity and the discharge capacity after 300 cycles of charging / discharging are shown in Table 2 below.

0.1 C-rate,
Discharge capacity (mAh) 1C-rate vs. 0.1 C-rate discharge capacity (%) 3C-rate vs. 0.1 C-rate discharge capacity (%) Example 1 56.9 89.4 74.1 Comparative Example 1 57.1 86.2 60.7 Comparative Example 2 50.3 73.5 34.9 Example 2 56.9 87.8 68.2 Comparative Example 3 50.7 68.1 8.2 Comparative Example 4 50.1 70.9 21.4

1C-rate, after 300 cycles of charging / discharging, capacity (%) Example 1 75 Comparative Example 1 60 Comparative Example 2 53 Example 2 69 Comparative Example 3 28 Comparative Example 4 42

According to the results shown in Table 1, the electrodes of Examples 1 and 2 have a large surface area in which the surface of the electrode mixture is in contact with the electrolyte relative to the electrode, and the inside of the electrode is easily impregnated. 3C-rate. In addition, it exhibited the highest capacity% even after 300 cycles of charging / discharging at 1C-rate, and it was confirmed that the battery exhibited excellent effects in life characteristics.

On the other hand, Comparative Examples 1 and 3 were the secondary batteries using the positive electrode and the negative electrode, which were subjected to only one coating, and showed the lowest capacity percentages in the evaluation of the battery characteristics.

In Comparative Examples 2 and 4, the porosity of the innermost electrode composite material layer and the porosity of the outermost electrode material mixture layer were the same as in Examples 1 and 2, respectively, Lt; RTI ID = 0.0 &gt; 1 &lt; / RTI &gt; This may be due to the fact that when the difference in the porosity of the adjacent electrode mixture layers is large, the battery characteristics may be deteriorated.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims (20)

An electrode current collector for transferring electrons between the external conductor and the electrode active material, and three electrode mixture layers sequentially coated on the electrode current collector,
Wherein the electrode mixture layers each comprise an electrode active material,
The electrode assembly layer having a porosity of the electrode assembly layer positioned relatively closer to the current collector than that of the electrode assembly layers adjacent to the current collector, And the difference in porosity between the adjacent electrode mixture layers is 5% to 10%. delete delete 2. The electrode assembly according to claim 1, wherein the porosity of the innermost layer electrode material mixture layer directly contacting the electrode current collector is within a range of 10% to 50% in a range smaller than the porosity of the electrode material mixture layer adjacent to the innermost layer electrode material mixture layer Layer electrode. 2. The electrode assembly according to claim 1, wherein the porosity of the outermost electrode composite material layer located farthest from the electrode current collector among the electrode material mixture layers is 15 % &Lt; / RTI &gt; to &lt; RTI ID = 0.0 &gt; 55%. &Lt; / RTI &gt; The multi-layered electrode of claim 1, wherein the mutually adjacent electrode material mixture layers are not mixed with each other at an interface and form an interface. The multi-layered electrode according to claim 1, wherein the three electrode mixture layers have the same thickness. The multi-layered electrode according to claim 1, wherein the thicknesses of the three electrode mixture layers are different from each other. The multi-layered electrode of claim 1, wherein the multi-layered electrode is a cathode. The multi-layer electrode according to claim 1, wherein the electrode active material layers included in the three electrode mixture layers are the same. The multi-layered electrode according to claim 1, wherein the electrode active materials included in the two or more electrode mixture layers of the three electrode mixture layers are different from each other. The multilayer structure electrode according to claim 1, wherein the three electrode mixture layers include a conductive material. The multi-layered electrode according to claim 1, wherein the three electrode mixture layers include a binder. 14. The multilayer structure electrode according to claim 13, wherein the binder contained in each of the mutually adjacent electrode material mixture layers is densely distributed as the collector is closer to the current collector. A lithium secondary battery comprising the multi-layered electrode according to claim 1. delete delete delete delete delete