KR20210050024A - Positive electrode material slurry for lithium secondary battery comprising at least two types of conductive materials - Google Patents

Abstract

Slurry for a positive electrode according to the present invention includes a positive electrode active material, a conductive material, and a binder, wherein the conductive material can be a mixture of a spherical first conductive material and a linear second conductive material. The conductivity and high rate characteristics of the positive electrode can be improved by adding at least two different types of conductive materials.

Description

Positive electrode material slurry for lithium secondary battery comprising at least two types of conductive materials

The present invention relates to a secondary battery, and more particularly, to a cathode material slurry for a secondary battery comprising a conductive material.

As the market for electric vehicles, batteries for power storage, or mobile smart devices grows rapidly, there is a demand for the development of batteries such as lithium secondary batteries that exhibit higher capacity and output characteristics than previously known.

A secondary battery refers to a battery that can be used repeatedly because it can be charged as well as discharged. In a typical lithium secondary battery among secondary batteries, lithium ions contained in the positive electrode active material are transferred to the negative electrode through the electrolyte, and then inserted into the layered structure of the negative electrode active material (charging), and then the lithium ions inserted into the layered structure of the negative electrode active material are regenerated. It works through the principle of returning to the anode (discharging). These lithium secondary batteries are currently commercialized and used as small power sources such as mobile phones and notebook computers, and are predicted to be usable as large power sources such as hybrid vehicles, and their demand is expected to increase.

In order to develop such a high-capacity battery, it is generally necessary to increase the thickness of the electrode, and it is necessary to smoothly transfer electrons from the thickened electrode to the current collector. However, in the case of carbon black, which has a zero-dimensional structure that has been previously applied as a conductive material in a secondary battery, there is a disadvantage in that it does not properly meet the above technical requirements because it cannot create an effective conductive path. For this reason, in recent years, attempts have been made to further improve the characteristics of the conductive material and the electrode by using two or more types of conductive carbon-based materials as the conductive material. For example, when two or more of conductive carbon-based materials such as graphene, carbon nanotubes, or carbon black are used together, point contact, line contact, and/or surface between conductive carbon-based materials having different structural characteristics Contact can be formed together to form a three-dimensional network structure. As a representative example, when graphene and carbon nanotubes or carbon black are used together, carbon nanotubes or carbon black are adsorbed on the graphene surface, while contact occurs between carbon nanotubes or carbon black, resulting in three-dimensional Network structure can be formed. As another example, even when carbon nanotubes and carbon black are used together, carbon black is adsorbed on the surface of carbon nanotubes, while carbon black or carbon nanotubes contact each other to form a three-dimensional network structure.

On the other hand, two or more types of conductive carbon-based materials must be uniformly dispersed, so that there is no variation in electrical conductivity within the applied cathode material, and uniformity of performance between cells can be displayed, so that two or more conductive carbon-based materials are uniformly distributed. There is a need for a technology regarding the cathode material slurry contained in a dispersed state.

Accordingly, an object to be solved by the present invention is to provide a cathode material slurry for a secondary battery capable of improving the conductivity and high rate characteristics of a cathode by adding at least two different conductive materials.

The technical problems of the present invention are not limited to the technical problems mentioned above, and other technical problems that are not mentioned will be clearly understood by those skilled in the art from the following description.

In order to achieve the above technical problem, an embodiment of the present invention provides a slurry for a positive electrode. The positive electrode slurry includes a positive electrode active material, a conductive material, and a binder, and the conductive material may be a mixture of a spherical first conductive material and a linear second conductive material.

The conductive material may have a shape in which linear second conductive materials are arranged in a net shape between spherical first conductive materials.

The spherical first conductive material may include carbon black, acetylene black, Ketjen black, or superfi. The average particle diameter (D ₅₀ ) of the linear first conductive material may be 50 nm to 110 nm.

The linear second conductive material may include graphene nanoribbons. The graphene nanoribbon may include a single layer or multiple layers of graphene having an aspect ratio of 10 to 150. The graphene nanoribbons may be formed by opening carbon nanotubes along a long axis direction.

The weight ratio of the spherical first conductive material and the linear second conductive material may be 95:5 to 85:15.

The positive electrode active material is LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , Li(Ni _a Co _b Mn _c )O ₂ (here, 0<a<1, 0<b<1, 0<c<1 , a+b+c=1), LiNi _1-Y CoYO ₂ , LiCo _1-Y Mn _Y O ₂ , LiNi _1-Y MnYO ₂ (here, 0≤Y<1), Li(Ni _a Co _b Mn _c )O ₄ (0<a<2, 0<b<2, 0<c<2, a+b+c=2), LiMn _2-z Ni _z O ₄ , LiMn _2-z Co _z O ₄ ( Here, it may include any one selected from the group consisting of 0<Z<2) or a mixture of two or more of them.

The binder may include fluorinated polyvinylidene (PVdF).

Another embodiment of the present invention provides a positive electrode for a secondary battery. The secondary battery positive electrode may include a positive electrode active material, a conductive material, and a binder, and the conductive material may include a mixture of a spherical first conductive material and a linear second conductive material.

Another embodiment of the present invention provides a secondary battery. The secondary battery may include a positive electrode of the secondary battery, an electrolyte and a negative electrode disposed between the positive electrode and the negative electrode.

As described above, since the cathode material slurry according to the embodiments of the present invention includes at least two types of conductive materials, it is evenly dispersed in the cathode active material to improve the conductivity of the cathode, and also provides high rate characteristics of secondary batteries including the same. It can be improved.

However, the effects of the present invention are not limited to the above-mentioned effects, and other effects not mentioned will be clearly understood by those skilled in the art from the following description.

1 is a schematic diagram showing a secondary battery according to an embodiment of the present invention.
Figure 2a is a schematic diagram of the structure of a conductive material according to an embodiment of the present invention.
2B is a schematic diagram of a carbon nanotube and a graphene nanoribbon formed by opening from the carbon nanotube according to an embodiment of the present invention.
2C is a graph showing an FT-IR spectrum of carbon nanotubes and graphene nanoribbons formed by opening from the carbon nanotubes according to an embodiment of the present invention.
3 is an image showing a multi-walled carbon nanotube (MWCNT) and a graphene nanoribbon (GNR) dispersed in an N-methyl-2-pyrrolidone (NMP) solvent, respectively.
4A and 4B are graphs showing the measurement results of the damping factor immediately after formation of the cathode material slurry of the cathode material slurry according to Preparation Example, Comparative Example 1 and Comparative Example 2 and after 3 days (aging) to be.
5A, 5B, and 5C are field emission scanning electron microscope (FE-SEM) images of the positive electrode obtained by casting and drying the positive electrode material slurry according to Preparation Example, Comparative Example 1 and Comparative Example 2, respectively.
6A, 6B, 6C, and 6D are graphs showing the characteristics of the cathode material slurry according to Preparation Example and Comparative Examples 1 and 2 according to the present invention.
7A, 7B, and 7C are graphs showing measurement results of cyclic voltammetry of secondary batteries according to Preparation Example, Comparative Example 1, and Comparative Example 2, respectively.
8 are graphs showing the capacity retention rate as the cycle of the secondary battery according to Preparation Example, Comparative Example 1, and Comparative Example 2 progresses.
9A and 9B are graphs showing capacity according to C-rate characteristics of secondary batteries according to Preparation Examples and Comparative Examples.

In the present invention, various modifications may be made and various embodiments may be provided, and specific embodiments will be illustrated in the drawings and described in detail. However, this is not intended to limit the present invention to a specific embodiment, it should be understood to include all changes, equivalents, and substitutes included in the spirit and scope of the present invention.

Terms such as first and second may be used to describe various elements, but the elements should not be limited by the terms. The above terms are used only for the purpose of distinguishing one component from another component. For example, without departing from the scope of the present invention, a first element may be referred to as a second element, and similarly, a second element may be referred to as a first element. The term and/or includes a combination of a plurality of related listed items or any of a plurality of related listed items.

The terms used in the present application are only used to describe specific embodiments, and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In the present application, terms such as "comprise" or "have" are intended to designate the presence of features, numbers, steps, actions, components, parts, or combinations thereof described in the specification, but one or more other features. It is to be understood that the presence or addition of elements or numbers, steps, actions, components, parts, or combinations thereof does not preclude in advance.

Unless otherwise defined, all terms used herein including technical or scientific terms have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs. Terms as defined in a commonly used dictionary should be interpreted as having a meaning consistent with the meaning in the context of the related technology, and should not be interpreted as an ideal or excessively formal meaning unless explicitly defined in this application. Does not.

Hereinafter, preferred embodiments of the present invention will be described in more detail with reference to the accompanying drawings. In describing the present invention, in order to facilitate an overall understanding, the same reference numerals are used for the same elements in the drawings, and duplicate descriptions for the same elements are omitted.

1 is a schematic diagram showing a secondary battery according to an embodiment of the present invention.

Referring to FIG. 1, the secondary battery 100 includes a negative electrode including a negative electrode active material into which lithium can be inserted, a positive electrode containing the positive electrode active material described above, and an electrolyte 160 disposed between the negative electrode and the positive electrode. I can. The negative active material layer 120 and the negative current collector 110 may be referred to as negative electrodes, and the positive active material layer 140 and the positive current collector 150 may be referred to as positive electrodes. Specifically, the negative electrode active material layer 120, the positive electrode active material layer 140, and including the separator 130 interposed therebetween, between the negative electrode active material layer 120 and the separator 130, and the positive electrode active material layer 140 ) Between the separator 130 and the electrolyte 160 may be disposed or filled. In addition, the negative active material layer 120 may be disposed on the negative current collector 110, and the positive active material layer 140 may be disposed on the positive current collector 150.

<Anode>

In order to form the positive electrode active material layer 140, the present invention may be to provide a positive electrode slurry by mixing a positive electrode active material, a conductive material, and a binder.

Figure 2a is a schematic diagram of the structure of a conductive material according to an embodiment of the present invention.

In the present invention, in order to provide a positive electrode material having improved conductivity, the conductive material in the positive electrode material slurry may include at least two types of conductive materials. Specifically, the conductive material may have a spherical or linear shape.

Referring to FIG. 2A, in an embodiment, the conductive material may be a mixture of a spherical first conductive material and a linear second conductive material. That is, since the linear second conductive materials are arranged in a net shape between the spherical first conductive materials, structural stability may be increased by forming a network structure. In other words, spherical materials are densely arranged on a net structure made of linear materials, and thus may have high density.

The first conductive material has a spherical shape, and may include carbon black, acetylene black (Denka black), Ketjen black, or Super-P, but is not limited thereto. The average particle diameter (D ₅₀ ) of the first conductive material may be 50 nm to 110 nm, preferably 60 nm to 100 nm. In this case, when the average particle diameter of the first conductive material satisfies the above range, in consideration of the size of the particles and the porosity of the positive electrode active material, the conductive materials may be uniformly dispersed without being aggregated or aggregated.

The second conductive material has a linear shape and may include graphene nanoribbons. 2B is a schematic diagram of a carbon nanotube and a graphene nanoribbon formed by opening from the carbon nanotube according to an embodiment of the present invention.

Referring to FIG. 2B, the graphene nanoribbon may be formed by opening a carbon nanotube. In detail, it may be that the carbon nanotubes are opened along the long axis direction. Opening of the carbon nanotubes may be performed by an oxidizing agent.

Graphene nanoribbons formed by treatment of carbon nanotubes with an oxidizing agent _{may further include functional groups such as a hydroxy group (-OH) and a carboxy group (-CO 2} H), and the graphene surface may be modified. . That is, since the graphene nanoribbons opened from the carbon nanotubes contain polar functional groups on the surface, the graphene nanoribbons may have polarity unlike the carbon nanoribbons. As a result, the carbon nanoribbons may exhibit excellent dispersibility in a polar organic solvent, which is a solvent used to form a cathode material slurry.

2C is a graph showing an FT-IR spectrum of carbon nanotubes and graphene nanoribbons formed by opening from the carbon nanotubes according to an embodiment of the present invention. Referring to FIG. 2C, it can be seen that a hydroxyl group (-OH) and a carboxy group (-CO ₂ H) functional group are included.

3 is an image showing a multi-walled carbon nanotube (MWCNT) and a graphene nanoribbon (GNR) dispersed in an N-methyl-2-pyrrolidone (NMP) solvent, respectively.

The image on the left of FIG. 3 shows that the multi-walled carbon nanotubes are mixed with a solvent, and it can be seen that the multi-walled carbon nanotubes are hardly dispersed in the solvent and are aggregated. In contrast, the image on the right of FIG. 3 shows that graphene nanoribbons are mixed with a solvent, and it can be seen that the graphene nanoribbons are sufficiently dispersed in a solvent to form a solution. It is understood that N-methyl-2-pyrrolidone is a polar solvent, so as the graphene nanoribbons exhibit more polarity compared to the multi-walled carbon nanotubes, the dispersibility of the graphene nanoribbons in the polar solvent is excellent. I can.

On the other hand, functional groups on the surface formed by opening the carbon nanotubes using an oxidizing agent can act as resistance, so in order to improve the conductivity of the conductive material, the functional groups are removed by performing a rinsing process using a separate reducing agent. It is common to include the process of doing.

However, the conductive material according to the present invention includes at least two kinds of conductive materials, and does not undergo a washing process using a separate reducing agent, etc., in order to increase the dispersibility of both conductive materials, and more It may be to have a polar functional group. That is, the conductive material according to the present invention may be sufficiently improved in dispersibility in the slurry for the positive electrode by the property of the conductive material, even without a separate dispersant or a separate physicochemical treatment to increase dispersibility.

In addition, the aspect ratio, which is the ratio of the length of the long axis and the length of the short axis of the graphene nanoribbon (that is, the long axis/short axis) may be 10 to 150, and in detail, graphene of 25 to 120 It may mean a single layer or multiple layers of.

The graphene nanoribbons have an aspect ratio of 10 to 150, and may have linearity due to morphological characteristics. As the conductive material has a linear characteristic, a conductive path may be formed, and thus, it may have more excellent conductivity.

Meanwhile, in the cathode material slurry, a weight ratio of the spherical first conductive material and the linear second conductive material may be 95:5 to 85:15, and more specifically, 93:7 to 87:13.

When the weight ratio is satisfied, in particular, the conductive materials have excellent dispersibility, and the conductive material can be distributed evenly without being biased to either side in the cathode material slurry. It may be a characteristic.

The positive electrode active material is capable of de-inserting lithium ions or causing a conversion reaction, and a known positive electrode active material for a secondary battery may be used. Specifically, the positive electrode active material is LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , Li(Ni _a Co _b Mn _c )O ₂ (here, 0<a<1, 0<b<1, 0<c <1, a+b+c=1), LiNi _1-Y CoYO ₂ , LiCo _1-Y Mn _Y O ₂ , LiNi _1-Y MnYO ₂ (here, 0≤Y<1), Li(Ni _a Co _b Mn _c )O ₄ (0<a<2, 0<b<2, 0<c<2, a+b+c=2), LiMn _2-z Ni _z O ₄ , LiMn _2-z Co _z O ₄ (here, 0<Z<2) may include any one selected from the group consisting of or a mixture of two or more of them, but is not limited thereto.

The binder is a thermoplastic resin, for example, polyvinylidene fluoride (PVdF), polytetrafluoroethylene, ethylene tetrafluoride, vinylidene fluoride-based copolymer, fluorine resin such as propylene hexafluoride, and/or polyethylene, polypropylene, and the like. It may include a polyolefin resin.

The positive electrode slurry obtained from the above may be applied on a positive electrode current collector to form a positive electrode. The positive electrode current collector may be a conductor such as Al, Ni, or stainless steel. Applying the positive electrode slurry on the positive electrode current collector may be performed by pressure molding, or a method of making a paste using an organic solvent, etc., and then applying the paste to the current collector and pressing to fix it. Applying the paste on the positive electrode current collector may be performed using, for example, a gravure coating method, a slit die coating method, a knife coating method, or a spray coating method.

Examples of the organic solvent include amines such as N,N-dimethylaminopropylamine and diethyltriamine; Ethers such as ethylene oxide and tetrahydrofuran; Ketone systems such as methyl ethyl ketone; Esters such as methyl acetate; It may be an aprotic polar solvent such as dimethylacetamide and N-methyl-2-pyrrolidone.

<cathode>

In order to form the negative electrode active material layer 120, a negative electrode active material, a conductive material, and a binder may be mixed to form a negative electrode slurry.

The negative electrode active material is a metal, metal alloy, metal oxide, metal fluoride, metal sulfide, and natural graphite, artificial graphite, coke, carbon black, carbon nanotubes, graphite that can de-insert lithium ions or cause a conversion reaction. It can also be formed using carbon materials such as fins.

The conductive material may be a carbon material such as natural graphite, artificial graphite, coke, carbon black, carbon nanotubes, and graphene. The binder is a thermoplastic resin such as polyvinylidene fluoride, polytetrafluoroethylene, tetrafluoroethylene, vinylidene fluoride copolymer, fluorine resin such as propylene hexafluoride, and/or polyolefin resin such as polyethylene and polypropylene. Can include.

The negative electrode may be formed by applying the negative electrode slurry on the negative electrode current collector. The negative electrode current collector may be a conductor such as Al, Ni, or stainless steel. Applying the negative electrode slurry on the negative electrode current collector may be performed by pressure molding or a method of making a paste using an organic solvent, etc., and then applying the paste on the current collector and pressing to fix it.

Examples of the organic solvent include amines such as N,N-dimethylaminopropylamine and diethyltriamine; Ethers such as ethylene oxide and tetrahydrofuran; Ketone systems such as methyl ethyl ketone; Esters such as methyl acetate; It may be an aprotic polar solvent such as dimethylacetamide and N-methyl-2-pyrrolidone. Applying the paste on the negative electrode current collector may be performed using, for example, a gravure coating method, a slit die coating method, a knife coating method, or a spray coating method.

<Electrolyte>

The electrolyte may be a lithium salt. Lithium salts are lithium perchloroate (LiClO ₄ ), lithium tetrafluoroborate (LiBF ₄ ), lithium hexafluorophosphate (LiPF ₆ ), lithium trifluoromethane selfonate (LiCF ₃ SO ₃ ), lithium hexafluoroacetate (LiAsF ₆ ), lithium trifluoromethanesulfonylimide (Li(CF ₃ SO ₂ ) ₂ N), or a mixture of two or more thereof. Among these, an electrolyte containing fluorine can be used. Further, the electrolyte can be dissolved in an organic solvent and used as a non-aqueous electrolyte. As an organic solvent, for example, propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, isopropyl methyl carbonate, vinylene carbonate, 4- Carbonates such as trifluoromethyl-1,3-dioxolan-2-one and 1,2-di(methoxycarbonyloxy)ethane; 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoropropylmethylether, 2,2,3,3-tetrafluoropropyldifluoromethylether, tetrahydrofuran, 2-methyltetrahydro Ethers such as furan; Esters such as methyl formate, methyl acetate, and γ-butyrolactone; Nitriles such as acetonitrile and butyronitrile; Amides such as N,N-dimethylformamide and N,N-dimethylacetamide; Carbamates such as 3-methyl-2-oxazolidone; Sulfur-containing compounds such as sulfolane, dimethyl sulfoxide, and 1,3-propane sultone; Alternatively, a fluorine substituent may be further introduced into the organic solvent described above.

Alternatively, a solid electrolyte may be used. The solid electrolyte may be an organic solid electrolyte such as a polyethylene oxide polymer compound, a polymer compound including at least one of a polyorganosiloxane chain, or a polyoxyalkylene chain. In addition, a so-called gel-type electrolyte in which a non-aqueous electrolyte is supported on a polymer compound can also be used. On the other hand, an inorganic solid electrolyte can also be used. In some cases, the safety of the sodium secondary battery can be further improved by using these solid electrolytes. In addition, the solid electrolyte may serve as a separator to be described later, and in that case, a separator may not be required.

<Separator>

A separator may be disposed between the anode and the cathode. Such a separator may be a material having a form such as a porous film, nonwoven fabric, or woven fabric made of a material such as polyolefin resin such as polyethylene or polypropylene, fluorine resin, and nitrogen-containing aromatic polymer. The thickness of the separator is preferably thinner as long as the mechanical strength is maintained from the viewpoint that the bulk energy density of the battery is increased and the internal resistance is decreased. The thickness of the separator may be generally 5 to 200 μm, and more specifically 5 to 40 μm.

<Method of manufacturing an alkali metal secondary battery>

After forming an electrode group by sequentially stacking a positive electrode, a separator, and a negative electrode, if necessary, the electrode group is rolled up and stored in a battery can, and a secondary battery can be manufactured by impregnating the electrode group with a non-aqueous electrolyte. In contrast, after forming an electrode group by stacking a positive electrode, a solid electrolyte, and a negative electrode, if necessary, the electrode group may be rolled up and stored in a battery can to manufacture a secondary battery.

Hereinafter, a preferred experimental example is presented to aid in understanding of the present invention. However, the following experimental examples are only intended to aid understanding of the present invention, and the present invention is not limited by the following experimental examples.

[Experimental examples; Examples]

Graphene Nano Ribbon Preparation Example

1 g of hydrogen peroxide was added to 20 ml of concentrated sulfuric acid (H ₂ SO ₄ ) solution (95%) and sonicated for 30 minutes to prepare an unzipping solution. 0.2 g of multi-walled carbon nanotubes were added to the obtained unziping solution, and the mixture was stirred at 80°C for 12 hours. Accordingly, a dark red suspension was formed, which was diluted by putting it in 1 L of deionized water. Thereafter, graphene nanoribbons were obtained by vacuum filtration through a cellulose acetate membrane having a pore size of 0.2 μm.

Example of cathode material slurry preparation

2% by weight of the conductive material Super P and 2% by weight of the binder polyvinylidene fluoride (PVdF) were dissolved in a solvent of N-methyl-2-pyrrolidone (NMP), and then LiNi _0.84 Co as a positive electrode active material. _0.12 Mn _0.04 O ₂ 96% by weight was added and mixed.

Then, the graphene nanoribbons are dissolved in an N-methyl-2-pyrrolidone (NMP) solvent, and at this time, the graphene nanoribbons are added to the mixture so as to have a weight ratio of 10:90 with the superpie and stirred to form a cathode material. A slurry was obtained. At this time, by adjusting the amount of the solvent, the solid content in the cathode material slurry was adjusted to be about 64%.

Cathode Slurry Comparative Example 1

2% by weight of the conductive material Super P and 2% by weight of the binder polyvinylidene fluoride (PVdF) were dissolved in a solvent of N-methyl-2-pyrrolidone (NMP), and then LiNi _0.84 Co as a positive electrode active material. _0.12 Mn _0.04 O ₂ 96 wt% was added and mixed to obtain a cathode material slurry.

Cathode Slurry Comparative Example 2

In the cathode material slurry preparation example, except that the multi-walled carbon nanotubes instead of the graphene nanoribbons were added to the mixture in a weight ratio of 10:90 to the superpie. A cathode material slurry was obtained using the same method as in the cathode material slurry preparation example.

Cathode Slurry Comparative Example 3

In the cathode material slurry preparation example, except that the graphene nanoribbon was added to the mixture so as to have a 20:80 weight ratio with the superpie. A cathode material slurry was obtained using the same method as in the cathode material slurry preparation example.

Cathode Slurry Comparative Example 4

In the cathode material slurry preparation example, except that the graphene nanoribbon was added to the mixture so as to have a 50:50 weight ratio with the superpie. A cathode material slurry was obtained using the same method as in the cathode material slurry preparation example.

Cathode Slurry Comparative Example 5

After dissolving 2% by weight of graphene nanoribbon as a conductive material and 2% by weight of polyvinylidene fluoride (PVdF) as a binder in N-methyl-2-pyrrolidone (NMP) solvent, LiNi _0.84 Co _0.12 Mn as a positive electrode active material _{96% by weight of 0.04} O ₂ was added and mixed to obtain a positive electrode material slurry.

Table 1 below is a table showing the weight ratio of conductive materials contained in the cathode material slurry according to the above embodiments.

Graphene
Nano Ribbon Multi-wall
Carbon nanotubes Super blood Comparative Example 1 - - 100 Manufacturing example 10 90 Comparative Example 2 - 10 90 Comparative Example 3 20 - 80 Comparative Example 4 50 - 50 Comparative Example 5 100 - -

4A and 4B are graphs showing the measurement results of the damping factor immediately after formation of the cathode material slurry of the cathode material slurry according to Preparation Example, Comparative Example 1 and Comparative Example 2 and after 3 days (aging) to be.

This is a measurement of a damping factor while providing a frequency. The damping factor is the ratio of the loss modulus (G") to the storage modulus (G'). If the coefficient of the damping factor is low, the slurry Is a solid-like, and if the coefficient is high, the slurry may represent a solid-liquid. The cathode material slurry is a similar liquid when casting, and the more similar solid is when drying to form an anode after casting, The electrode formed accordingly may have a minimized defect in the secondary battery.

In particular, referring to FIG. 4A, in the case of Comparative Example 2, it can be seen that the damping factor is particularly low. That is, since carbon nanotubes are included as a conductive material, the cathode material slurry according to Comparative Example 2 exhibits a similar liquid, so it is particularly difficult to sufficiently disperse the conductive material, and a secondary battery including the same is somewhat deteriorated. It can be expected to represent.

5A, 5B, and 5C show a field emission scanning electron microscope (FE-SEM) of a positive electrode obtained by casting and drying the positive electrode slurry according to the positive electrode slurry Preparation Example, the positive electrode slurry Comparative Example 1, and the positive electrode slurry Comparative Example 2, respectively. ) Is an image. In FIGS. 5A to 5C, CB denotes carbon black, NCM denotes LiNi _0.84 Co _0.12 Mn _0.04 O ₂ , MWCNT denotes multi-walled carbon nanotubes, and GNR denotes griffin nanoribbons. Referring to FIG. 5A, it can be seen that NCM is distributed on the surface of carbon black (CB), and griffin nanoribbons (GNR) are present between the distributed NCMs. Referring to FIG. 5B, it can be seen that NCM is distributed on the surface of the carbon black (CB). Referring to FIG. 5C, it can be seen that the multi-walled carbon nanotubes (MWCNTs) are connected.

6A, 6B, 6C, and 6D are graphs showing the characteristics of the cathode material slurry according to Preparation Example and Comparative Examples 1 and 2 according to the present invention.

This was measured by pelletizing the cathode material slurry according to Preparation Example, Comparative Example 1 and Comparative Example 2. Specifically, each of the cathode material slurry was pelletized to have a pellet density of 3.4 g/cc, a thickness of 3.3 mm, and a weight of 2 g, and a scratching test was performed according to FIG. 6E.

6A, 6B, 6C, and 6D, it can be seen that the positive electrode pellet according to the preparation example has a higher coefficient of friction (COF) than the positive electrode pellet according to the comparative examples. This is believed to be a result of the positive electrode pellet according to the preparation example _{forming a greater resistance as the strength of the normal force (F n} ) increases, that is, having excellent bonding strength between electrode materials.

Secondary battery manufacturing example

A positive electrode was manufactured by casting and drying the positive electrode material slurry according to Preparation Example on an aluminum thin film as a positive electrode current collector.

Lithium metal was used as the negative electrode.

_{1M lithium hexafluorophosphate (LiPF 6} ) is dissolved in a mixed solvent of ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) in a 50:100:50 volume ratio, and a flow of 10 weight ratio to the total weight of the electrolyte An electrolyte containing oroethylene carbonate (FEC) was obtained.

The positive electrode, negative electrode, electrolyte, and separator obtained from the above were prepared to prepare a secondary battery according to a conventional manufacturing process for a secondary battery.

Secondary Battery Comparative Examples 1 to 5

In the secondary battery manufacturing example, each comparative example using the same method as the secondary battery manufacturing example, except that a cathode was formed using the cathode material slurry according to Comparative Examples 1 to 5 instead of the cathode material slurry according to the manufacturing example. Secondary batteries of 1 to 5 were prepared.

7 to 9 are graphs showing electrochemical characteristics of secondary batteries according to Preparation Examples to Comparative Examples according to the present invention.

7A, 7B, and 7C are graphs showing measurement results of cyclic voltammetry of secondary batteries according to Preparation Example, Comparative Example 1, and Comparative Example 2, respectively. Specifically, the current density of the first, second and third cycles was measured at a potential scanning speed of 0.2 mV/s at 3.0 to 4.4 V. Referring to FIG. 7, it can be seen that the secondary battery according to the manufacturing example does not shift significantly and the curve almost coincides with the secondary battery according to Comparative Examples 1 and 2, even if the cycle proceeds, so excellent electricity It can be seen that it shows chemical properties.

8 are graphs showing the capacity retention rate as the cycle of the secondary battery according to Preparation Example, Comparative Example 1, and Comparative Example 2 progresses. This was measured at 1C-rate, and referring to FIG. 8, it can be seen that the secondary battery according to the Preparation Example compared to the secondary battery according to the Comparative Examples has the best capacity retention rate even when the cycle proceeds.

9A and 9B are graphs showing capacity according to C-rate characteristics of secondary batteries according to Preparation Examples and Comparative Examples. This was measured in 1C-2C-4C-1C.

Referring to FIG. 9A, it can be seen that the secondary battery according to Preparation Example compared to the secondary battery according to Comparative Examples 1 and 2 has a better capacity at a high rate.

That is, compared to the case of including the spherical conductive material, when the conductive material having linearity as well as the spherical conductive material is further included, it can be seen that the secondary battery including the same has more excellent electrochemical properties at a high rate. Furthermore, when the dispersibility of the conductive material in the slurry is further improved, it can be seen that the secondary battery has more excellent properties.

Table 2 summarizes the capacity shown in each C-rate of the secondary batteries according to Preparation Examples and Comparative Examples 1, 3 to 5.

1C-rate 2C-rate 4C-rate 1C-rate 4C/1C Comparative Example 1 182.6 174.5 152.9 180.3 84.8 Manufacturing example 191.3 183.3 170.2 186.6 91.2 Comparative Example 3 189.1 179.6 108.6 185.0 58.7 Comparative Example 4 189.2 174.6 94.1 186.9 50.4 Comparative Example 5 145.3 93.0 3.4 120.4 2.8

In particular, referring to Table 2 and FIG. 9B, it can be seen that the secondary battery according to Preparation Example compared to the secondary battery according to Comparative Examples 3, 4 and 5 also has better capacity at a high rate.

In other words, when the spherical conductive material and the graphene nanoribbon satisfy a specific weight ratio, it can be confirmed that the secondary battery including the same has further improved electrochemical properties.

Although described with reference to the above embodiments, those skilled in the art will understand that various modifications and changes can be made to the present invention without departing from the spirit and scope of the present invention described in the following claims. I will be able to.

Claims (12)

A positive electrode active material, a conductive material, and a binder, wherein the conductive material is a mixture of a spherical first conductive material and a linear second conductive material. The method according to claim 1,
The conductive material has a shape in which linear second conductive materials are arranged in a net shape between spherical first conductive materials. The method according to claim 1,
The spherical first conductive material is carbon black, acetylene black, Ketjen black, or the slurry for a positive electrode containing Superpy. The method according to claim 1,
The average particle diameter (D ₅₀ ) of the linear first conductive material is 50 ㎚ to 110 ㎚, the positive electrode slurry. The method according to claim 1,
The linear second conductive material is that of graphene nanoribbons, positive electrode slurry. The method of claim 5,
The positive electrode slurry comprising a single layer or multiple layers of graphene having an aspect ratio of 10 to 150 of the graphene nanoribbons. The method of claim 5,
The graphene nanoribbon is to open the carbon nanotubes along the long axis direction, the slurry for the positive electrode. The method according to claim 1,
The weight ratio of the spherical first conductive material and the linear second conductive material is 95:5 to 85:15, the positive electrode slurry The method according to claim 1,
The positive electrode active material is LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , Li(Ni _a Co _b Mn _c )O ₂ (here, 0<a<1, 0<b<1, 0<c<1 , a+b+c=1), LiNi _1-Y CoYO ₂ , LiCo _1-Y Mn _Y O ₂ , LiNi _1-Y MnYO ₂ (here, 0≤Y<1), Li(Ni _a Co _b Mn _c )O ₄ (0<a<2, 0<b<2, 0<c<2, a+b+c=2), LiMn _2-z Ni _z O ₄ , LiMn _2-z Co _z O ₄ ( Here, the slurry for a positive electrode containing any one selected from the group consisting of 0<Z<2) or a mixture of two or more of them. The method according to claim 1,
The binder is fluorinated polyvinylidene (PVdF) containing a slurry for the positive electrode. A secondary battery positive electrode comprising a positive electrode active material, a conductive material, and a binder, wherein the conductive material is a mixture of a spherical first conductive material and a linear second conductive material. The secondary battery positive electrode of claim 11;
An electrolyte disposed between the positive and negative electrodes; And
Secondary battery comprising a negative electrode.

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