WO2017074109A1 - Cathode for secondary battery, method for preparing same, and lithium secondary battery comprising same - Google Patents

Abstract

The present invention relates to a cathode for a secondary battery, a method for preparing the same, and a lithium secondary battery comprising the same, the cathode for a secondary battery comprising: a cathode current collector; and a cathode mixture layer coated on at least one side of the cathode current collector, wherein the cathode mixture layer comprises a cathode active material, a conductive material, a binder, and bimodal-type pores composed of a first pore and a second pore which are different in the maximum diameter, wherein the conductive material and the cathode active material are contained at a volume ratio (K₁) of 0.08 to 0.32 : 1, the volume ratio of the conductive material to the total pores is 0.1 to 0.33 : 1, the porosity is 30 volume% to 45 volume%, the maximum diameter of the first pore and the second pore is less than 1 ㎛, and the average diameter ratio (K) of the first pore to the second pore is 0.13 to 0.27 : 1.

Description

Anode for a secondary battery, a manufacturing method thereof, and a lithium secondary battery including the same

Cross Citation with Related Application (s)

This application claims the benefit of priority based on Korean Patent Application No. 10-2015-0151483 filed October 30, 2015 and Korean Patent Application No. 10-2016-0141349 filed October 27, 2016. All content disclosed in the literature is included as part of this specification.

Technical Field

The present invention relates to a secondary battery positive electrode that controls the pore size in the positive electrode mixture layer to improve the output, a manufacturing method thereof, and a lithium secondary battery with improved output by including the same.

As technology development and demand for mobile devices increase, the demand for secondary batteries as a source of energy is rapidly increasing. Among secondary batteries, lithium secondary batteries that exhibit high energy density and operating potential, have a long cycle life, and have a low self-discharge rate are commercially used.

The lithium secondary battery has a structure in which an electrolyte is impregnated into an electrode assembly in which a cathode including a lithium transition metal oxide, a cathode including a carbon-based active material, and a porous separator are sequentially stacked as an electrode active material.

The positive electrode is prepared by coating a positive electrode mixture containing a lithium transition metal oxide on an aluminum foil, the negative electrode is prepared by coating a negative electrode mixture containing a carbon-based active material on a copper foil.

A conductive material is added to the positive electrode mixture and the negative electrode mixture in order to improve the electrical conductivity of the active material. In particular, since the lithium transition metal oxide used as the positive electrode active material is inherently low in electrical conductivity, a conductive material is essentially added to the positive electrode mixture.

However, due to the difference in particle size of the active material and the conductive material, it is difficult to expect a uniform mixing of the active material and the conductive material. As a result, agglomeration between the conductive materials occurs, or segregation of particles due to the particle size difference between the active material and the conductive material occurs, and a large amount of voids of several μm are generated in the electrode.

Since these voids cause an increase in the resistance of the battery or a decrease in the output, there is a need for a method capable of improving them.

Prior art literature

Korean Patent Publication No. 2015-0016339

Korean Patent Publication No. 2015-0016581

In order to solve the above problems, a first object of the present invention is to provide a positive electrode for secondary batteries, the maximum diameter of the internal void is controlled to less than 1 ㎛.

Another object of the present invention is to provide a method for producing the positive electrode.

In addition, a third object of the present invention is to provide a lithium secondary battery with improved room temperature and low temperature output by including the positive electrode.

In one embodiment of the present invention for achieving the above object,

A positive electrode current collector, and

A positive electrode mixture layer coated on at least one surface of the positive electrode current collector,

The positive electrode mixture layer includes a positive electrode active material, a conductive material, a binder, and a bimodal type void consisting of first and second voids having different maximum diameters,

The conductive material: the positive electrode active material is included in a volume ratio (K ₁ ) of 0.08 to 0.32: 1,

The conductive material: the volume ratio of the entire void is 0.1 to 0.33: 1,

Porosity is 30% to 45% by volume,

The maximum diameter of the first and second pores is less than 1 ㎛,

The average pore ratio k of the first pore: the second pore is 0.13 to 0.27: 1 to provide a positive electrode for a secondary battery.

In addition, in one embodiment of the present invention

Conductive material: mixing the positive electrode active material in a volume ratio of 0.08 to 0.32: 1 to prepare a positive electrode active material slurry having a solid content of 60 wt% to 90 wt%;

Coating the cathode active material slurry on a cathode current collector, followed by drying to prepare a cathode including a cathode mixture layer having a porosity of 55% by volume to 65% by volume;

First rolling the anode to prepare a cathode including a cathode mixture layer having a porosity of 53% by volume to 57% by volume;

Preparing a positive electrode including a positive electrode mixture layer having a porosity of 30% by volume to 45% by volume by rolling the positive electrode prepared after the first rolling; And

It provides a method for manufacturing a positive electrode for a secondary battery comprising a; by manufacturing the positive electrode comprising a positive electrode mixture layer having a porosity of 30% by volume to 45% by volume of the positive electrode prepared after the secondary rolling.

At this time, the positive electrode active material slurry may be coated with a loading amount of 2 mg / ㎠ to 15 mg / ㎠.

In addition, the manufacturing method of the positive electrode may further include the step of leaving for 30 minutes to 2 hours before the secondary rolling of the positive electrode prepared after the first rolling. In addition, the method may further include the step of leaving for 30 minutes to 2 hours before the third rolling of the positive electrode prepared after the secondary rolling.

In this case, the first rolling is carried out under the condition that the gap (gap) between the two upper rolls and the lower roll at room temperature is (the total thickness of the anode before the first rolling + the total thickness of the anode manufactured after the third rolling) / 2. can do.

In addition, the secondary and tertiary rolling step may be carried out under the same conditions in the gap (gap) between the two upper rolls and the lower roll at room temperature equal to the total thickness of the target anode after the third rolling.

In addition, in one embodiment of the present invention

A positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and

A secondary battery comprising a non-aqueous electrolyte containing a lithium salt,

It provides a lithium secondary battery comprising the positive electrode for a secondary battery of the present invention as the positive electrode.

According to the present invention, by providing a positive electrode for a lithium secondary battery in which the maximum diameter of the internal void is controlled to less than 1 μm, an improved lithium secondary battery having improved room temperature / low temperature output characteristics can be manufactured.

1 is a scanning electron micrograph showing a cross-sectional structure of the anode prepared according to Example 1 of the present invention.

2 is a scanning electron micrograph showing a cross-sectional structure of a positive electrode prepared according to Comparative Example 3.

3 is a scanning electron micrograph showing a cross-sectional structure of a positive electrode prepared according to Comparative Example 4.

4 is a graph comparing room temperature output results of a lithium secondary battery according to Experimental Example 1 of the present invention.

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

The terms or words used in this specification and claims are not to be construed as limiting in their usual or dictionary meanings, and the inventors may appropriately define the concept of terms in order to best explain their invention in the best way possible. It should be interpreted as meaning and concept corresponding to the technical idea of the present invention based on the principle that the present invention.

In order to achieve the above object, the present invention provides a secondary battery positive electrode and a manufacturing method thereof in which the maximum diameter of the internal void is controlled to less than 1 ㎛.

In addition, the present invention provides a lithium secondary battery having improved output at room temperature and low temperature by including the positive electrode.

Specifically, in one embodiment of the present invention

A positive electrode current collector, and

A positive electrode mixture layer coated on at least one surface of the positive electrode current collector,

The positive electrode mixture layer includes a positive electrode active material, a conductive material, a binder, and a bimodal type void consisting of first and second voids having different maximum diameters,

The conductive material: the positive electrode active material is included in a volume ratio (K ₁ ) of 0.08 to 0.32: 1,

The conductive material: the volume ratio of the entire void is 0.1 to 0.33: 1,

Porosity is 30% to 45% by volume,

The maximum diameter of the first and second pores is less than 1 ㎛,

The average pore ratio k of the first pore: the second pore is 0.13 to 0.27: 1 to provide a positive electrode for a secondary battery.

First of all, in the positive electrode of the present invention, the positive electrode current collector is not particularly limited so long as it has conductivity without causing chemical change in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum The surface treated with carbon, nickel, titanium, silver, etc. on the stainless steel surface can be used.

The current collector may be used having a thickness of 3 to 500㎛, specifically 3 to 100㎛, in the present invention, it is preferable to use a current collector having a thickness of 5 to 20㎛. Fine irregularities may be formed on the surface of the current collector to increase the adhesion of the positive electrode active material. For example, it can be used in various forms, such as a film, a sheet, a foil, a net, a porous body, a foam, a nonwoven body.

In the positive electrode of the present invention, the positive electrode active material is not particularly limited as long as it is made of a transition metal compound capable of intercalating / deintercalating lithium, and a representative particle having an average particle diameter (D50) of 3 to 20 μm is a representative example. A cathode active material used in the cathode according to an embodiment of the present invention is typically lithium transition metal oxide particles in which lithium ions may be occluded and released, and the lithium composite metal oxide is a lithium-manganese oxide (eg, LiMnO ₂ , LiMn ₂ O Etc.), lithium-cobalt-based oxides (e.g., LiCoO _2, etc.), lithium-nickel-based oxides (e.g., LiNiO _2, etc.), lithium-nickel-manganese-based oxides (e.g., LiNi _1-x Mn _x2 O ₂ (where, 0 <x <1), LiMn _2-y Ni _y O ₄ (here, 0 <y <2) and the like), lithium-nickel-cobalt-based oxide (for example, LiNi _{1- z} Co _z O ₂ (here, 0 <z <1) and the like, lithium-manganese-cobalt-based oxides (eg, LiCo _{1 - a} Mn _a O ₂ (here 0 <a <1), LiMn _{2 - b} Co _b O ₄ (where 0 <b <2) and the like, lithium-nickel-manganese-cobalt-based oxides (eg, Li (Ni _p Co _q Mn _r ) O ₂ (here, 0 <P <1, 0 <q <1, 0 <r <1, p + q + r = 1) or Li (Ni _c Co _d Mn _e ) O ₄ (where 0 <c <2, 0 <d <2, 0 <e <2, c + d + e = 2), or lithium-nickel-cobalt-transition metal (M) oxide (for example, Li (Ni _f Co _g Mn _h M _i ) O ₂ (wherein M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg and Mo, and f, g, h and i are each atoms of independent elements) As a fraction, 0 <f <1, 0 <g <1, 0 <h <1, 0 <i <1, f + g + h + i = 1) etc.) etc. are mentioned, Any one of these is mentioned. Or two or more compounds. Among the lithium composite metal oxides, LiCoO ₂ , LiMnO ₂ , LiNiO ₂ , and lithium nickel manganese cobalt oxides (eg, Li (Ni _0.6 Mn _0.2 Co _0.2 ) O ₂ may be improved in capacity and stability of the battery. , Li (Ni _0.5 Mn _0.3 Co _0.2 ) O ₂ , Li (Ni _0.7 Mn _0.15 Co _0.15 ) O ₂ or Li (Ni _0.8 Mn _0.1 Co _0.1 ) O _2, etc.), or lithium nickel cobalt aluminum oxide (eg, Li (Ni _0.8 Co _0.15 Al _0.05 ) O _2, etc.), and the lithium composite metal oxide may be Li (Ni) in consideration of the remarkable improvement effect by controlling the type and content ratio of the constituent elements forming the lithium composite metal oxide. _0.6 Mn _0.2 Co _0.2 ) O ₂ , _{Li (Ni 0.5 Mn 0.3 Co 0.2} ) O 2, Li (Ni 0. 7 Mn 0. 15 Co 0. 15 O 2 or _{Li (Ni 0.8 Mn 0.1 Co 0.1} ) O may be made of ₂ or the like, either of which Or mixtures of two or more may be used.

The cathode active material may be included in an amount of 80 wt% to 98 wt% based on the total weight of the cathode mixture layer.

In the positive electrode of the present invention, the conductive material is not particularly limited as long as it has conductivity without causing chemical change in the battery, and as a representative example, natural graphite or artificial graphite having an average particle diameter (D50) of 5 to 30000 nm, Single material selected from the group consisting of carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, conductive fiber, carbon fluoride, aluminum, nickel powder, conductive whiskey, conductive metal oxide and polyphenylene derivative Or a mixture of two or more thereof.

In addition, the conductive material: the positive electrode active material may include a volume ratio (K ₁ ) of 0.08 to 0.32: 1, the volume ratio of the total space including the conductive material: the first and second pores is 0.1 to 0.33: 1 In this case, the porosity is 30% by volume to 45% by volume.

At this time, the volume ratio of the conductive material: positive electrode active material and the volume ratio of the conductive material: the entire void in the positive electrode can be calculated through the following process.

First, the total volume (Vt), which is the sum of the solid content and the pore volume including the active material, the binder, and the conductive material, is multiplied by multiplying the thickness of the positive electrode material mixture layer by the area x length (5 x 5 cm 2) of the positive electrode material mixture layer. Obtain

Subsequently, the volume Vp of all the pores can be calculated by the following formula (1).

[Equation 1]

Vp = Vt x Porosity (%)

In addition, the value obtained by subtracting the volume (Vp) of the entire void from the total volume becomes the volume of the solid content (Vs = Vt -Vp).

In addition, the volume of each component is divided by dividing the mass of the active material (Va.m.), the conductive material (Vcon), and the binder (Vbin) used in the area of the width x length (5 x 5 cm 2) by their true density. You can get it.

In addition, the method of measuring the porosity and the pore size of the inside of the anode is not particularly limited, and the size (micro) and mesopore volume (meso) are measured using a Brunauer-Emmett-Teller (BET) measurement method using an adsorption gas such as nitrogen, which is generally used. pore volume) or the like, or may be measured using a commonly used mercury permeation method (Hg porosimeter).

If the volume ratio (K ₁ ) of the conductive material to the positive electrode active material is less than 0.08, or if the volume ratio of the conductive material to the total void is less than 0.1, that is, if the content of the conductive material is low (porosity exceeds 45% by volume), between the active material Insufficient amount of the conductive material to fill the separation distance prevents the conductive material from sufficiently surrounding the active material. As a result, the size of the first pore and the second pore increases, which makes it difficult to transfer the electrons generated by the reaction on the surface of the active material, and the electrical resistance in the electrode may increase because the conductive network is not smoothly connected in the anode. Can be. On the other hand, if the volume ratio (K ₁ ) of the conductive material to the positive electrode active material exceeds 0.32, or if the volume ratio of the conductive material to the entire void exceeds 0.33 (porosity of 30% by volume or less), an excessive amount of conductive material is present on the surface of the active material. Therefore, the resistance according to the decrease in the size of the first pore increases, the reaction area of the positive electrode active material and the electrolyte decreases, and the battery output can be reduced.

Specifically, as shown in FIG. 2, when the volume ratio of the conductive material / anode active material is less than 0.08, and the volume ratio of the conductive material / pore is less than 0.1, that is, when the conductive material is contained in a small amount compared to the voids, the first gap of 1 μm or more And it can be confirmed that a plurality of second voids are formed. In addition, as shown in FIG. 3, even when the volume ratio of the conductive material / anode active material exceeds 0.32 and the volume ratio of the conductive material / total voids exceeds 0.33, the conductive materials coagulate due to excessive use of the conductive material, and the surface of the active material. Since a phenomenon that does not sufficiently cover the occurrence of the second voids of 1 μm or more may be formed.

On the other hand, as shown in Figure 1, containing the conductive material: the positive electrode active material in a volume ratio (K ₁ ) of 0.08 to 0.32: 1, the conductive material: the present containing the voids in a volume ratio of 0.1 to 0.33: 1 In the case of the anode of the invention, the maximum diameters of the first pore 301 and the second pore 302 may be controlled to a level of less than 1 μm, specifically several hundred nm, respectively.

As such, in order for the maximum diameter of the first pore 301 / the second pore 302 to be less than 1 μm in the positive electrode of the present invention, the volume ratio of the conductive material 200 / the positive electrode active material 100 is 0.08 to 0.32. The volume ratio of the entire conductive material / pore may be 0.1 to 0.33, and the porosity may be implemented only when all of them satisfy 30 vol% to 45 vol% (see FIG. 1).

In this case, the first gap includes a gap between the conductive material and the conductive material formed by the adjacent conductive material particles, and the second gap is formed between the conductive material and the active material surrounded by the adjacent conductive material and the cathode active materials. Contains voids.

The outer circumferential surfaces of the first and second voids may be nonlinearly formed along surfaces of the plurality of adjacent conductive material and cathode active material particles, respectively.

As described above, the maximum diameter of the first pore and the second pore is less than 1㎛, specifically, the average diameter of the first pore is 1nm to 100nm, the average diameter of the second pore is 100nm to 500nm, Specifically, it is 200 nm to 400 nm. At this time, it is preferable that the average diameter ratio k of the said 1st voids: 2nd voids is 0.13-0.27: 1.

If the average diameter ratio (k) of the first pore to the second pore is less than 0.13, the first pore size is very small due to the aggregation of the conductive material particles, or the second pore between the conductive material particles and the active material particles. It means that the size is relatively large. When the first pore is small, it is difficult for Li ions in the electrolyte to smoothly move to the surface of the active material by the conductive material-conductor clusters, and when the second pore is large, the contact between the active material and the conductive material is difficult and thus occurs on the surface of the active material. It can be difficult to transfer the electrons generated by it.

On the other hand, when the average diameter ratio (k) of the first pore to the second pore exceeds 0.27, the first pore size is large, so that the conductive material-conductive material contact is not smooth, thereby increasing the electrical resistance in the electrode, Alternatively, since the second pore size is relatively small, the reaction area between the surface of the active material and the electrolyte is reduced, so that the output may be reduced.

Method for measuring the diameter of the first pore and the second pore is not particularly limited, but is represented by the size of the two main peaks appearing by using the Hg porosimeter generally used in the art, electron microscopy (SEM) The image confirmed the position for the size represented by each peak.

In the positive electrode of the present invention, the binder is a component that assists the bonding of the active material and the conductive material and the bonding of the active material and the current collector, and representative examples thereof include polyvinylidene fluoride, polyvinyl alcohol, and carboxymethyl cellulose (CMC). ), Starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene Rubber, fluororubber, various copolymers, and the like.

The binder may be included in an amount of about 1 to 10% by weight, specifically, 2 to 9% by weight, based on the total weight of the positive electrode mixture. The amount may be reduced to decrease the capacity, and when the binder is less than 10% by weight, the electrode may be peeled off, thereby degrading the lifespan performance of the battery.

The positive electrode of the present invention may have an energy of 0.8 mAh / cm 2 to 1.8 mAh / cm 2 per unit area.

In addition, in one embodiment of the present invention

Conductive material: mixing the positive electrode active material in a volume ratio of 0.08 to 0.32: 1 to prepare a positive electrode active material slurry having a solid content of 60 wt% to 90 wt%;

Coating the cathode active material slurry on a cathode current collector and then drying to prepare a cathode including a cathode mixture layer having a porosity of 50% by volume to 60% by volume;

First rolling the anode to prepare a cathode including a cathode mixture layer having a porosity of 53% by volume to 57% by volume;

Preparing a positive electrode including a positive electrode mixture layer having a porosity of 30% by volume to 45% by volume by rolling the positive electrode prepared after the first rolling; And

It provides a method for manufacturing a positive electrode for a secondary battery comprising a; by manufacturing the positive electrode comprising a positive electrode mixture layer having a porosity of 30% by volume to 45% by volume of the positive electrode prepared after the secondary rolling.

In the method of the present invention, the preparing of the positive electrode active material slurry may be performed by dissolving a binder in an organic solvent to prepare a binder solution, and then adding a conductive material to prepare a conductive material-binder mixed solution. It can be prepared by adding a positive electrode active material while stirring.

In this case, the conductive material-binder mixed solution may be prepared by stirring for about 1 minute to 10 minutes at a speed of about 1000rpm to 2000rpm, specifically 1500rpm at room temperature.

In addition, after the active material and an organic solvent is further added to the mixed solution, if necessary, a positive electrode active material slurry may be prepared while stirring for about 5 minutes to 30 minutes at a speed of about 1000rpm to 2000rpm, specifically 1500rpm at room temperature.

In the method of the present invention, when the volume ratio (K ₁ ) of the conductive material / the positive electrode active material is less than 0.08, that is, the content of the conductive material is small, the amount of the conductive material to fill the separation distance between the active material is insufficient, the conductive material is Since the size of the first pore and the second pore increases, it is not easy to transfer the electrons generated by the reaction, and the conductive network is not connected smoothly due to the lack of the conductive material in the electrode. This can increase.

On the other hand, if the volume ratio of the conductive material / anode active material exceeds 0.32, the excess conductive material is present on the surface of the active material, and the size of the first and second pores is relatively reduced, thereby reducing the reaction area between the surface of the active material and the electrolyte. As a result, battery output can be reduced.

In addition, the solid content of the positive electrode active material slurry may be from 60% by weight to 90% by weight, when included in this range can implement the desired porosity. In this case, the solid content means a conductive material and a positive electrode active material.

In the method of the present invention, the positive electrode active material slurry may be coated with a loading amount of 2 mg / cm 2 to 15 mg / cm 2. At this time, the coating method of the active material slurry may be used without limitation the coating method commonly used in the art, non-limiting examples of dip (Dip) coating, die coating, roll coating It may include a variety of methods, such as comma coating or a mixture thereof.

After coating the cathode active material slurry, the drying drying step is preferably performed at a temperature of room temperature to 300 ° C. for 1 to 24 hours. If the drying temperature is lower than room temperature, there is a problem in that the drying of the solvent is not made, and if the drying temperature is higher than 300 ° C., it is not a meaning as a drying step because it corresponds to a heat treatment step. If the drying time is less than 1 hour, there is a problem in that the drying of the solvent is not performed. If the drying time is more than 24 hours, the process time is too large, which is not preferable.

In addition, in the method of the present invention, the primary rolling is a gap (gap) between the two upper rolls and the lower rolls at room temperature (the total thickness of the anode before the first rolling + the total thickness of the target anode after the third rolling) Can be carried out under the condition of) / 2.

By the primary rolling it is possible to obtain the effect of forming the second pore size more uniformly.

In addition, the secondary and tertiary rolling step may be carried out under the same conditions in the gap (gap) between the two upper rolls and the lower roll at room temperature equal to the total thickness of the target anode after the third rolling.

At this time, after the secondary rolling, the positive electrode mixture layer may be formed to be higher than the target thickness by partially recovering the thickness after a predetermined time due to elasticity. Therefore, in the present invention, by further rolling (third rolling), it is possible to manufacture a positive electrode having a positive electrode mixture layer capable of maintaining a target thickness. In addition, the porosity in the positive electrode is 30% by volume to 45% by volume, the volume ratio of the conductive material: the total pore including the first and second pores is 0.1 to 0.33: 1, and the maximum of the first and second pores The diameter can be controlled to be less than 1 μm.

In conclusion, the present invention provides a positive electrode in which the maximum diameters of the first and second voids of the bimodal form are controlled to be less than 1 μm, so that the electrolyte is sufficiently contained in the electrode to contact the positive electrode active material and the electrolyte. As the capillary force can be improved as much as possible, the packing density of the electrode can be improved while reducing the internal plate resistance. Therefore, as compared with the conventional anode manufactured without considering the size of the existing pores, it is possible to realize a secondary battery having improved output at room temperature and low temperature in a short time by securing excellent high-rate charge and discharge characteristics and life characteristics according to an increase in electrical conductivity and binding force. Can be.

In addition, the method may further include the step of leaving for 30 minutes to 2 hours before the secondary rolling of the positive electrode prepared after the primary rolling.

In addition, the method may further include the step of leaving for 30 minutes to 2 hours before the secondary rolling of the positive electrode prepared after secondary rolling.

In addition, in one embodiment of the present invention

A positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and

As a secondary battery containing a non-aqueous electrolyte,

The positive electrode provides a secondary battery including the positive electrode of the present invention.

In this case, since it has been described above with respect to the anode, a detailed description thereof will be omitted.

The negative electrode may be prepared by applying a negative electrode active material slurry composition on at least one surface of the negative electrode current collector, followed by drying and rolling.

In this case, the negative electrode current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery. For example, the negative electrode current collector may be formed on a surface of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper, or stainless steel. Surface-treated with carbon, nickel, titanium, silver and the like, aluminum-cadmium alloy and the like can be used. In addition, the negative electrode current collector, like the positive electrode current collector, may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, a nonwoven fabric having fine irregularities formed on its surface.

The negative electrode active material slurry composition may include at least one or more of a negative electrode active material, a solvent, and optionally a binder and a conductive material.

In this case, the negative electrode active material is a compound capable of reversible intercalation and deintercalation of lithium, and specific examples thereof include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; Metallic or semimetallic compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys or Al alloys; Metal oxides or metalloids which can dope and undo lithium such as SiO _x1 (0 <x1 <2), SnO ₂ , vanadium oxide, lithium vanadium oxide; Or a composite including the inorganic compound and a carbonaceous material, such as a Si-C composite or a Sn-C composite, and any one or a mixture of two or more thereof may be used. In addition, a metal lithium thin film may be used as the anode active material. As the carbon material, both low crystalline carbon and high crystalline carbon can be used. Soft crystalline carbon and hard carbon are typical low crystalline carbon, and high crystalline carbon is amorphous, plate, scaly, spherical or fibrous natural graphite or artificial graphite, Kish graphite (Kish) graphite, pyrolytic carbon, mesophase pitch based carbon fiber, meso-carbon microbeads, mesophase pitches and petroleum or coal tar pitch High-temperature calcined carbon such as derived cokes is typical.

The negative electrode active material may be included in an amount of 80 wt% to 99 wt% based on the total weight of the negative electrode active material slurry composition.

The binder is a component that assists the bonding between the conductive material, the active material and the current collector, and is typically added in an amount of 1 to 30 wt% based on the total weight of the negative electrode active material slurry. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoro Low ethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber, fluorine rubber, various copolymers thereof, and the like.

The conductive material is a component for further improving the conductivity of the negative electrode active material, and may be added in an amount of 1 to 20 wt% based on the total weight of the negative electrode active material slurry. Such a conductive material is not particularly limited as long as it has conductivity without causing chemical change in the battery, and examples thereof include graphite such as natural graphite and artificial graphite; Carbon blacks such as acetylene black, Ketjen black, channel black, furnace black, lamp black and thermal black; Conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride powder, aluminum powder and nickel powder; Conductive whiskeys 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.

The solvent may include an organic solvent such as water or NMP (N-methyl-2-pyrrolidone), and may be used in an amount that becomes a desirable viscosity when including the negative electrode active material, and optionally a binder and a conductive material. . For example, the concentration of the positive electrode active material and, optionally, the solid content including the binder and the conductive material may be included in an amount of 50 wt% to 95 wt%, preferably 70 wt% to 90 wt%.

The separator is a conventional porous polymer film conventionally used as a separator, for example, a polyolefin-based polymer such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer and ethylene / methacrylate copolymer The porous polymer film prepared by using a single or a lamination thereof may be used, or a conventional porous nonwoven fabric, for example, a non-woven fabric made of glass fibers, polyethylene terephthalate fibers of high melting point, etc. may be used, but is not limited thereto. .

In addition, the non-aqueous electrolyte solution consists of an electrolyte solution and a lithium salt, and a non-aqueous organic solvent or an organic solid electrolyte is used as the electrolyte solution.

As the non-aqueous organic solvent, for example, N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dime Methoxyethane, tetrahydroxyfuran, 2-methyl tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolon, formamide, dimethylformamide, dioxolon, acetonitrile, nitromethane, methyl formate, methyl acetate, Phosphate triester, trimethoxy methane, dioxoron derivative, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ether, methyl propionate, ethyl propionate An aprotic organic solvent such as may be used.

Examples of the organic solid electrolytes include polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate ester polymers, polyedgetion lysine, polyester sulfides, polyvinyl alcohols, polyvinylidene fluorides, Polymerizers containing ionic dissociating groups and the like can be used.

The lithium salt may be used, without limitation, those which are commonly used in a lithium secondary battery electrolyte, such as Li ^{+ cations, F -, Cl -, Br} -, I -, NO 3 -, N (CN) 2 - _{^{, BF 4 -, ClO 4 -}} , PF 6 -, (CF 3) 2 PF 4 -, (CF 3) 3 PF 3 -, (CF 3) 4 PF 2 -, (CF 3) 5 PF -, (CF _{^{3) 6 P -, CF 3}} SO 3 -, CF 3 CF 2 SO 3 -, N (CF 3 SO 2) 2 -, N (SO 2 F) 2 -, CF 3 CF 2 (CF 3) 2 CO - _{, (CF 3 SO 2) 2} CH -, (SF 5) 3 C -, (CF 3 SO 2) 3 C -, CF 3 (CF 2) 7 SO 3 -, CF 3 CO 2 -, CH 3 CO 2 ^- , SCN ^- and N (CF ₃ CF ₂ SO ₂ ) ₂ ^- One of the anions selected from the group consisting of.

In addition, in the electrolyte solution, for the purpose of improving the charge and discharge characteristics, flame retardancy, etc., for example, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphate triamide, nitro Benzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-methoxy ethanol, aluminum trichloride and the like may be added. . In some cases, in order to impart nonflammability, halogen-containing solvents such as carbon tetrachloride and ethylene trifluoride may be further included, and carbon dioxide gas may be further included to improve high temperature storage characteristics.

Example 2

A positive electrode and a secondary battery including the same were prepared in the same manner as in Example 1 except that the conductive material: positive electrode active material was included in a volume ratio of 0.28: 1 in the preparation of the positive electrode in Example 1.

In this case, the porosity of the positive electrode is 40% by volume, the volume ratio of the entire conductive material / voids in the positive electrode is 0.31, the diameter of the first pore is 45nm, the diameter of the second pore is 320nm, the first pore / second The average diameter ratio k of the voids is 0.14. The energy density per unit area of the positive electrode was 1.5 mAh / cm 2.

Example 3

A positive electrode and a secondary battery including the same were prepared in the same manner as in Example 1 except that the conductive material: the positive electrode active material was included in a volume ratio of 0.08: 1 in the preparation of the positive electrode in Example 1.

In this case, the porosity of the positive electrode is 40% by volume, the volume ratio of the entire conductive material / voids in the positive electrode is 0.1, the diameter of the first pore is 80nm, the diameter of the second pore is 300nm, the first pore / second The average diameter ratio k of the voids is 0.27. The energy density per unit area of the positive electrode was 1.5 mAh / cm 2.

Example 4

A positive electrode and a secondary battery including the same were manufactured in the same manner as in Example 1 except that the conductive material: positive electrode active material was included in a volume ratio of 0.32: 1 in the preparation of the positive electrode in Example 1.

In this case, the porosity of the positive electrode is 40% by volume, the volume ratio of the entire conductive material / voids in the positive electrode is 0.33, the diameter of the first pore is 45nm, the diameter of the second pore is 335nm, the first pore / second The average diameter ratio k of voids is 0.13. The energy density per unit area of the positive electrode was 1.5 mAh / cm 2.

Comparative Example 1

Except for preparing a positive electrode (total thickness of 60 μm) including the positive electrode mixture layer by rolling once, including the conductive material: positive electrode active material in a volume ratio of 0.051: 1 in the preparation of the positive electrode in Example 1, In the same manner as in Example 1, a cathode and a secondary battery including the same were prepared.

At this time, the volume ratio of the conductive material / total voids in the anode is 0.07, the diameter of the first pore is 150 nm, the diameter of the second pore is 500 nm, the average diameter ratio of the first pore / second pore ( k) is 0.3. The energy density per unit area of the positive electrode was 1.5 mAh / cm 2.

Comparative Example 2

Except for preparing a positive electrode (total thickness 89 μm) including the positive electrode mixture layer by rolling twice, including the conductive material: positive electrode active material in a volume ratio of 0.45: 1 in the preparation of the positive electrode in Example 1, A positive electrode and a secondary battery including the same were manufactured in the same manner as in Example 1.

At this time, the volume ratio of the entire conductive material / pore in the anode is 0.37, the diameter of the first pore is 30 nm, the diameter of the second pore is 1000 nm, the average diameter ratio of the first pore / second pore ( k) is 0.03.

Comparative Example 3

A positive electrode and a secondary battery including the same were prepared in the same manner as in Example 1, except that the conductive material: the positive electrode active material was included in a volume ratio of 0.07: 1 in the preparation of the positive electrode in Example 1.

The cross section of the prepared anode was observed with an electron microscope, and the results are shown in FIG. 2. As shown in FIG. 2, it can be seen that the conductive material does not sufficiently wrap the active material, so that a second gap of several μm is formed.

In this case, the volume ratio of the entire conductive material / pore in the anode is 0.09, the porosity is 40% by volume, the diameter of the first pore is 110nm, the diameter of the second pore is 400nm, the first pore / second pore The average diameter ratio (k) of was 0.28.

Comparative Example 4

A positive electrode and a secondary battery including the same were prepared in the same manner as in Example 1 except that the conductive material: positive electrode active material was included in a volume ratio of 0.34: 1 in the preparation of the positive electrode in Example 1.

The cross section of the prepared anode was observed with an electron microscope, and the results are shown in FIG. 3. As shown in FIG. 3, while the first pore size is reduced, it can be seen that a plurality of second pores of 1 μm or more are formed.

At this time, the volume ratio of the entire conductive material / pore in the anode is 0.34, the porosity is 40% by volume, the diameter of the first pore is 40nm, the diameter of the second pore is 400nm, the first pore / second pore The average diameter ratio k is 0.1. The energy density per unit area of the positive electrode was 1.5 mAh / cm 2.

Experimental Example

Experimental Example 1.

After the cells obtained in Examples 1 to 4 and Comparative Examples 1 to 4 were formed at 4.2 V, the change in output at room temperature (25 ° C., 10 seconds resistance, @SOC 50%) was measured at 50% before SOC. The results are shown in FIG. 4.

Referring to FIG. 4, it can be seen that the secondary batteries using the positive electrodes of Examples 1 to 4 have superior output characteristics at room temperature than the batteries using the positive electrodes of Comparative Examples 1 to 4.

That is, as in Comparative Examples 1 and 3, the volume ratio of the conductive material / anode active material is less than 0.08, and the volume ratio of the conductive material / total voids is less than 0.1 (the average diameter ratio k of the first and second pores is more than 0.27). In the case of the conductive material surrounding the active material is not enough, there is a disadvantage that the output is reduced due to the increase in the electrical resistance in the electrode.

In addition, as in Comparative Examples 2 and 4, the volume ratio of the conductive material / anode active material is more than 0.32, and the volume ratio of the conductive material / total voids is more than 0.33 (average diameter ratio k of the first and second pores is less than 0.13). In the case of), an excessive amount of the conductive material is present on the surface of the active material, so that the reaction area between the surface of the active material and the electrolyte decreases and thus the output is reduced.

At this time, the data shown in Figure 4 is only one example, the detailed output value according to the SOC may vary depending on the type of battery cell, it can be expected that there is no difference in the output characteristics (trend).

Claims (16)

1. A positive electrode current collector, and

   A positive electrode mixture layer coated on at least one surface of the positive electrode current collector,

   The positive electrode mixture layer includes a positive electrode active material, a conductive material, a binder, and a bimodal type air gap having a first pore and a second pore having different average diameters.

   The conductive material: the positive electrode active material is included in a volume ratio (K ₁ ) of 0.08 to 0.32: 1,

   The conductive material: the volume ratio of the entire void is 0.1 to 0.33: 1,

   Porosity is 30% to 45% by volume,

   The maximum diameter of the first and second pores is less than 1 ㎛,

   The average pore ratio (k) of the first pore: the second pore is 0.13 to 0.27: 1 positive electrode for secondary batteries.
2. The method according to claim 1,

   The average particle diameter (D50) of the cathode active material is a secondary battery positive electrode of 3㎛ 20㎛.
3. The method according to claim 1,

   The positive electrode active material may be lithium-manganese oxide, lithium-cobalt oxide, lithium-nickel oxide, lithium-nickel-manganese oxide, lithium-nickel-cobalt oxide, lithium-manganese-cobalt oxide, lithium-nickel- A positive electrode comprising a single material selected from the group consisting of manganese-cobalt-based oxide, and lithium-nickel-cobalt-transition metal oxide or a mixture of two or more thereof.
4. The method according to claim 1,

   The average particle diameter (D50) of the conductive material is a secondary battery positive electrode of 5nm to 30000nm.
5. The method according to claim 1,

   The conductive material may be natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, conductive fiber, carbon fluoride, aluminum, nickel powder, conductive whiskey, conductive metal oxide and poly A positive electrode which is a single substance or a mixture of 2 or more types selected from the group which consists of phenylene derivatives.
6. The method according to claim 1,

   The first gap is a gap between the conductive material and the conductive material formed by the adjacent conductive material particles,

   The second gap is a gap between the conductive material and the cathode active material formed by the adjacent conductive material and the cathode active materials,

   An outer circumferential surface of the first and second voids is a non-linear along the surface of the plurality of adjacent conductive material and the positive electrode active material particles, respectively.
7. The method according to claim 1,

   The average diameter of the first gap is 1nm to 100nm,

   The average diameter of the second voids is 100nm to 500nm.
8. The method according to claim 7,

   The average diameter of the second void is 200nm to 400nm.
9. The method according to claim 1,

   The anode has an energy density of 0.8 mAh / cm 2 to 1.8 mAh / cm 2 per unit area.
10. Conductive material: mixing the positive electrode active material in a volume ratio of 0.08 to 0.32: 1 to prepare a positive electrode active material slurry having a solid content of 60 wt% to 90 wt%;

    Coating the cathode active material slurry on a cathode current collector, followed by drying to prepare a cathode including a cathode mixture layer having a porosity of 55% by volume to 65% by volume;

    First rolling the anode to prepare a cathode including a cathode mixture layer having a porosity of 53% by volume to 57% by volume;

    Preparing a positive electrode including a positive electrode mixture layer having a porosity of 30% by volume to 45% by volume by rolling the positive electrode prepared after the first rolling; And

    Manufacturing a positive electrode including a positive electrode mixture layer having a porosity of 30% by volume to 45% by volume by rolling the positive electrode prepared after the second rolling in a third direction.
11. The method according to claim 10,

    The cathode active material slurry is coated with a loading of 2 mg / ㎠ to 15 mg / ㎠ method for manufacturing a positive electrode for a secondary battery.
12. The method according to claim 10,

    The primary rolling is carried out under the condition that the gap between the two upper rolls and the lower roll at room temperature is (the total thickness of the anode before the first rolling plus the total thickness of the target anode after the third rolling) / 2. Method for producing a positive electrode for a secondary battery.
13. The method according to claim 10,

    The secondary and tertiary rolling step is a method of manufacturing a positive electrode for a secondary battery that is carried out under the same conditions as the gap (gap) between the two upper rolls and the lower roll at room temperature to the total thickness of the target positive electrode after the third rolling.
14. The method according to claim 10,

    The method further comprises the step of leaving for 30 minutes to 2 hours before the secondary rolling of the positive electrode prepared after the first rolling.
15. The method according to claim 10,

    The method further comprises the step of leaving for 30 minutes to 2 hours before the third rolling of the positive electrode prepared after secondary rolling.
16. A positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and

    A secondary battery comprising a non-aqueous electrolyte, wherein the positive electrode comprises the positive electrode of claim 1.

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