KR102293207B1 - Positive electrode material slurry for lithium secondary battery comprising at least two types of conductive materials - Google Patents

Abstract

The 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 may be a mixture of a spherical first conductive material and a linear second conductive material. By adding at least two or more different kinds of conductive materials, the conductivity and high rate characteristics of the anode can be improved.

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, power storage batteries, or mobile smart devices rapidly grows, the development of batteries such as lithium secondary batteries exhibiting higher capacity and output characteristics than previously known is required.

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 move to the negative electrode through the electrolyte, and then are inserted (charging) into the layered structure of the negative electrode active material, and then lithium ions inserted into the layered structure of the negative electrode active material are again It works on the principle of returning to the positive pole (discharge). These lithium secondary batteries are currently commercialized and used as small power sources for mobile phones and notebook computers, and are expected to be used as large power sources for hybrid vehicles, and the 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 is a zero-dimensional structure previously applied as a conductive material in secondary batteries, an effective conductive path cannot be created, and thus the above technical requirements cannot be properly met. For this reason, recently, attempts have been made to further improve the properties of the conductive material and the electrode by using two or more types of conductive carbon-based materials together as the conductive material. For example, when two or more of the conductive carbon-based materials such as graphene, carbon nanotubes, or carbon black are used together, the conductive carbon-based materials having different structural properties are in point contact, line contact and/or surface Contacts 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 to the graphene surface, while contact occurs between carbon nanotubes or carbon black, resulting in three-dimensional A network structure can be formed. As another example, even when carbon nanotubes and carbon black are used together, carbon black may be 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 kinds of conductive carbon-based materials must be uniformly dispersed, so that there is no deviation in electrical conductivity within the applied cathode material and uniformity of performance between cells can be exhibited, so that two or more kinds of conductive carbon-based materials are uniformly distributed There is a need for a technique for a cathode material slurry contained in a dispersed state.

Accordingly, the problem 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 the cathode by adding at least two or more different kinds of conductive materials.

The technical problems of the present invention are not limited to the technical problems mentioned above, and other technical problems 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 slurry for the positive electrode may include 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 disposed in a net shape between spherical first conductive materials.

The spherical first conductive material may include carbon black, acetylene black, Ketjen black, or superfi. An 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 nanoribbon may be one in which carbon nanotubes are opened along a long axis direction.

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

The positive active material is LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , Li(Ni _a Co _b Mn _c )O ₂ (herein, 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, any one selected from the group consisting of 0 < Z < 2) or a mixture of two or more thereof may be included.

The binder may include polyvinylidene fluoride (PVdF).

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

As described above, since the cathode material slurry according to the embodiments of the present invention contains at least two kinds of conductive materials, it is possible to improve the conductivity of the cathode by evenly dispersing it in the cathode active material, and also to improve the high rate characteristics of the secondary battery including the same. 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 view of the structure of the conductive material according to an embodiment of the present invention.
Figure 2b is a schematic diagram of carbon nanotubes and graphene nanoribbons formed by opening from the carbon nanotubes according to an embodiment of the present invention.
2C is a graph showing FT-IR spectra 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 dispersing multi-walled carbon nanotubes (MWCNT) and graphene nanoribbons (GNR) in an N-methyl-2-pyrrolidone (NMP) solvent, respectively.
4A and 4B are graphs showing the results of measuring the damping factor immediately after forming the cathode material slurry of the cathode material slurry according to Preparation Example, Comparative Example 1, and Comparative Example 2, respectively, and after 3 days (aging); am.
5A, 5B, and 5C are field emission scanning electron microscope (FE-SEM) images of positive electrodes obtained by casting and drying positive electrode material slurries according to Preparation Example, Comparative Example 1, and Comparative Example 2, respectively.
6a, 6b, 6c and 6d are graphs showing the characteristics of cathode material slurries according to Preparation Examples and Comparative Examples 1 and 2 according to the present invention.
7A, 7B, and 7C are graphs showing cyclic voltammetry measurement results of secondary batteries according to Preparation Example, Comparative Example 1, and Comparative Example 2, respectively.
8 is a graph showing the capacity retention rate as the cycle of the secondary batteries according to Preparation Example, Comparative Example 1, and Comparative Example 2 proceeds.
9A and 9B are graphs showing capacities according to C-rate characteristics of secondary batteries according to Preparation Examples and Comparative Examples.

Since the present invention can have various changes and can have various embodiments, specific embodiments are illustrated in the drawings and described in detail. However, this is not intended to limit the present invention to specific embodiments, and it should be understood to include all modifications, equivalents and substitutes included in the spirit and scope of the present invention.

Terms such as first, second, etc. 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. For example, without departing from the scope of the present invention, a first component may be referred to as a second component, and similarly, a second component may also be referred to as a first component. 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. The singular expression includes the plural expression unless the context clearly dictates otherwise. In the present application, terms such as “comprise” or “have” are intended to designate that a feature, number, step, operation, component, part, or combination thereof described in the specification exists, but one or more other features It should be understood that this does not preclude the existence or addition of numbers, steps, operations, components, parts, or combinations thereof.

Unless defined otherwise, all terms used herein, including technical and scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Terms such as those defined in a commonly used dictionary should be interpreted as having a meaning consistent with the meaning in the context of the related art, and should not be interpreted in an ideal or excessively formal meaning unless explicitly defined in the present 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 the overall understanding, the same reference numerals are used for the same components in the drawings, and duplicate descriptions of the same components 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 having a negative active material into which lithium can be inserted and removed, a positive electrode containing the above-described positive electrode active material, and an electrolyte 160 disposed between the negative electrode and the positive electrode. can The anode active material layer 120 and the anode current collector 110 may be referred to as an anode, and the cathode active material layer 140 and the cathode current collector 150 may be referred to as a cathode. Specifically, it includes a negative active material layer 120 , a positive active material layer 140 , and a separator 130 interposed therebetween, and between the negative active material layer 120 and the separator 130 , and the positive active material layer 140 . ) and the separator 130 , the electrolyte 160 may be disposed or filled. Also, the anode active material layer 120 may be disposed on the anode current collector 110 , and the cathode active material layer 140 may be disposed on the cathode current collector 150 .

<Anode>

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

Figure 2a is a schematic view of the structure of the 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, the linear second conductive materials may have a network shape disposed between the spherical first conductive materials, thereby forming a network structure to increase structural stability. In other words, the spherical materials may be densely arranged on the net structure of the linear materials to have a 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, the conductive materials may be uniformly dispersed without agglomeration or agglomeration in consideration of the particle size and the porosity of the positive electrode active material.

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

Referring to FIG. 2B , the graphene nanoribbon may be formed by opening carbon nanotubes. Specifically, the carbon nanotubes may be opened along the long axis direction. The opening of the carbon nanotubes may be performed by an oxidizing agent.

The graphene nanoribbon formed by treating the carbon nanotubes with an oxidizing agent _{may further include functional groups such as a hydroxyl group (-OH) and a carboxyl group (-CO 2} H), and the graphene surface may be modified. . That is, since the graphene nanoribbon opened from the carbon nanotube includes a polar functional group on the surface, the graphene nanoribbon may have a polarity unlike the carbon nanoribbon. As a result, the carbon nanoribbon 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 FT-IR spectra 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 the dispersion of multi-walled carbon nanotubes (MWCNT) and graphene nanoribbons (GNR) in 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 agglomerated with little dispersion in the solvent. In contrast, the right image of FIG. 3 shows that the graphene nanoribbons are mixed with a solvent, and it can be confirmed that the graphene nanoribbons are sufficiently dispersed in the solvent to form a solution. It can be seen that, since N-methyl-2-pyrrolidone is a polar solvent, as the graphene nanoribbons are more polar compared to the multi-walled carbon nanotubes, the dispersibility of the graphene nanoribbons in the polar solvent is excellent. can

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

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

In addition, the ratio of the length of the long axis to the length of the short axis of the graphene nanoribbon (ie, the long axis/short axis) may have an aspect ratio of 10 to 150, and specifically, 25 to 120 graphene. may mean a single layer or multiple layers of

The graphene nanoribbon has an aspect ratio of 10 to 150, and may have linearity due to its morphological characteristics. The conductive material may form a conductive path as it has a linear characteristic, and thus may have superior conductivity.

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

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

The positive electrode active material is one capable of deintercalating lithium ions or causing a conversion reaction, and a known positive electrode active material for a secondary battery may be used. Specifically, the positive 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 (herein, it may include any one selected from the group consisting of 0<Z<2) or a mixture of two or more thereof, but is not limited thereto.

The binder is a thermoplastic resin, for example, polyvinylidene fluoride (PVdF), polytetrafluoroethylene, ethylene tetrafluoride, vinylidene fluoride-based copolymer, fluororesin such as propylene hexafluoride, and/or polyethylene, polypropylene, etc. of polyolefin resin.

The positive electrode slurry obtained above may be applied on the 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 slurry for the positive electrode on the positive electrode current collector may use a method of press molding or a method of making a paste using an organic solvent, then applying the paste on the current collector and pressing the paste to be fixed. 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.

The organic solvent is an amine-based solvent such as N,N-dimethylaminopropylamine and diethyltriamine; ethers such as ethylene oxide and tetrahydrofuran; ketones such as methyl ethyl ketone; esters such as methyl acetate; and an aprotic polar solvent such as dimethylacetamide or N-methyl-2-pyrrolidone.

<Cathode>

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

Anode active materials include metals, metal alloys, metal oxides, metal fluorides, metal sulfides, and natural graphite, artificial graphite, cokes, carbon black, carbon nanotubes, and graphite that can cause lithium ion deintercalation or conversion reaction. It can also be formed using carbon materials, such as a pin.

The conductive material may be a carbon material such as natural graphite, artificial graphite, coke, carbon black, carbon nanotubes, or graphene. The binder is a thermoplastic resin, for example, polyvinylidene fluoride, polytetrafluoroethylene, ethylene tetrafluoride, vinylidene fluoride-based copolymer, fluororesin such as propylene hexafluoride, and/or polyolefin resin such as polyethylene or polypropylene. may 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 slurry for the negative electrode on the negative electrode current collector may be performed by pressure molding or a method of making a paste using an organic solvent and then applying the paste on the current collector and pressing the paste to fix it.

The organic solvent is an amine-based solvent such as N,N-dimethylaminopropylamine and diethyltriamine; ethers such as ethylene oxide and tetrahydrofuran; ketones such as methyl ethyl ketone; esters such as methyl acetate; and an aprotic polar solvent such as dimethylacetamide or 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 salt is lithium perchlorate (LiClO ₄ ), lithium tetrafluoroborate (LiBF ₄ ), lithium hexafluorophosphate (LiPF ₆ ), lithium trifluoromethane cellphonate (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. In addition, the electrolyte can be dissolved in an organic solvent and used as a non-aqueous electrolyte. Examples of the organic solvent include 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, pentafluoropropylmethyl ether, 2,2,3,3-tetrafluoropropyldifluoromethyl ether, 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 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-based polymer compound or a polymer compound containing at least one of polyorganosiloxane chains or polyoxyalkylene chains. Also, a so-called gel-type electrolyte in which a non-aqueous electrolyte is supported on a polymer compound may be used. On the other hand, an inorganic solid electrolyte may be used. There are cases where 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. The separator may be a material having the form of a porous film, a nonwoven fabric, or a woven fabric made of a material such as a polyolefin resin such as polyethylene or polypropylene, a fluororesin, or a nitrogen-containing aromatic polymer. The thickness of the separator is preferably thinner as long as the mechanical strength is maintained from the viewpoint of increasing the bulk energy density of the battery and decreasing the internal resistance. The thickness of the separator may be generally about 5 to 200 μm, and more specifically, 5 to 40 μm.

<Method for manufacturing alkali metal secondary battery>

After the positive electrode, the separator, and the negative electrode are sequentially stacked to form an electrode group, if necessary, the electrode group is rolled up and stored in a battery can, and the secondary battery can be manufactured by impregnating the electrode group with a non-aqueous electrolyte. Alternatively, after forming an electrode group by laminating 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 (example) is presented to help the understanding of the present invention. However, the following experimental examples are only for helping 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 resulting unzipped solution, followed by stirring at 80° C. for 12 hours. As a result, a dark red suspension was formed, which was diluted in 1 L of deionized water. Thereafter, the 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 wt% of Super P as a conductive material and 2 wt% of polyvinylidene fluoride (PVdF) as a binder were dissolved in N-methyl-2-pyrrolidone (NMP) solvent, and then LiNi _0.84 Co as a cathode active material _0.12 Mn _0.04 O ₂ 96 wt% was added and mixed.

Then, the graphene nanoribbons are dissolved in 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 to the superfibre, and stirred to form a cathode material. A slurry was obtained. At this time, the amount of the solvent was adjusted so that the solid content in the cathode material slurry was about 64%.

Cathode Material Slurry Comparative Example 1

2 wt% of Super P as a conductive material and 2 wt% of polyvinylidene fluoride (PVdF) as a binder were dissolved in N-methyl-2-pyrrolidone (NMP) solvent, and then LiNi _0.84 Co as a cathode active material _{96 wt% of 0.12} Mn _0.04 O ₂ was added and mixed to obtain a cathode material slurry.

Cathode Material Slurry Comparative Example 2

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

Cathode Material Slurry Comparative Example 3

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

Cathode Material Slurry Comparative Example 4

In the positive electrode material slurry preparation example, the graphene nanoribbon was added to the mixture in a weight ratio of 50:50 to the superpy except that it was added. A cathode material slurry was obtained using the same method as in the cathode material slurry preparation example.

Cathode Material Slurry Comparative Example 5

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

Table 1 below is a table summarizing the weight ratio of the conductive material included in the cathode material slurry according to the embodiments.

graphene
nano ribbon multiwall
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 results of measuring the damping factor immediately after forming the cathode material slurry of the cathode material slurry according to Preparation Example, Comparative Example 1, and Comparative Example 2, respectively, and after 3 days (aging); am.

This is a measure of the damping factor while providing the frequency, which is the ratio of the loss modulus (G") to the storage modulus (G'), and when the damping factor's coefficient is low, the slurry indicates solid-like, and if the coefficient is high, the slurry may indicate a solid-liquid.The more similar the cathode material slurry is when casting, the more solid it is when drying to form a positive electrode after casting. The electrode thus formed may have a minimized defect in the secondary battery.

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

5A, 5B and 5C are field emission scanning electron microscopy (FE-SEM) of a positive electrode obtained by casting and drying the positive electrode material slurry according to the positive electrode slurry preparation example, positive electrode slurry comparative example 1, and positive electrode slurry comparative example 2, respectively. ) is an image. 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 NCMs are 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 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 cathode material slurries according to Preparation Examples and Comparative Examples 1 and 2 according to the present invention.

This was measured by pelletizing the positive electrode material slurry according to Preparation Example, Comparative Example 1 and Comparative Example 2. In detail, each 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 .

Referring to FIGS. 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, the positive electrode pellet according to the preparation example _{forms a greater resistance as the strength of the normal force (Normal Force, F n} ) increases, that is, it is considered as a result of having excellent bonding strength between the electrode materials.

Secondary battery manufacturing example

A positive electrode was prepared by casting and drying the positive electrode material slurry according to Preparation Example on an aluminum thin film, which is 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 volume ratio of 50:100:50, and a flow rate of 10% by weight relative to the total weight of the electrolyte An electrolyte containing oroethylene carbonate (FEC) was obtained.

A secondary battery was manufactured according to a conventional manufacturing process of a secondary battery by preparing the positive electrode, the negative electrode, the electrolyte, and the separator obtained above.

Secondary battery Comparative Examples 1 to 5

In the secondary battery Preparation Example, each Comparative Example using the same method as in the secondary battery Preparation Example, except that the positive electrode was formed by using the positive electrode material slurry according to Comparative Examples 1 to 5 instead of the positive electrode material slurry according to Preparation Example Secondary batteries 1 to 5 were prepared.

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

7A, 7B, and 7C are graphs showing cyclic voltammetry measurement results of secondary batteries according to Preparation Example, Comparative Example 1, and Comparative Example 2, respectively. In detail, the current densities of the first, second and third cycles were measured at a potential scanning rate of 0.2 mV/s at 3.0 to 4.4V. Referring to FIG. 7 , it can be seen that the peak of the secondary battery according to Preparation Example is not significantly shifted and the curve is almost identical to that of the secondary battery according to Comparative Examples 1 and 2, even when the cycle progresses, so that the excellent electricity It can be confirmed that the chemical properties are shown.

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

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

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

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

Table 2 summarizes the capacities shown at each C-rate of the secondary batteries according to Preparation Examples and Comparative Examples 1 and 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 batteries according to Preparation Examples also have better capacity at high rates compared to the secondary batteries according to Comparative Examples 3, 4 and 5.

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 more improved electrochemical properties.

Although it has been described with reference to the above embodiments, it will be understood by those skilled in the art that various modifications and changes can be made to the present invention without departing from the spirit and scope of the present invention as set forth in the claims below. will be able

Claims (12)

A cathode active material, a conductive material, a polar solvent, and a binder,
The conductive material is a mixture of a spherical first conductive material and a graphene nanoribbon that is a linear second conductive material,
The graphene nanoribbons are disposed between the spherical first conductive materials,
The graphene nanoribbon is a positive electrode slurry comprising a hydroxyl group (-OH) and a carboxyl group (-COOH) on the surface. delete The method according to claim 1,
The spherical first conductive material may include carbon black, acetylene black, Ketjen black, or superpy. The method according to claim 1,
The spherical first conductive material has an average particle diameter (D ₅₀ ) of 50 nm to 110 nm, a slurry for a positive electrode. delete The method according to claim 1,
The positive electrode slurry comprising a single layer or multiple layers of graphene having an aspect ratio of the graphene nanoribbons of 10 to 150. The method according to claim 1,
The graphene nanoribbon is that the carbon nanotubes are opened 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 to the linear second conductive material is 95:5 to 85:15, the positive electrode slurry The method according to claim 1,
The positive active material is LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , Li(Ni _a Co _b Mn _c )O ₂ (herein, 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, any one selected from the group consisting of 0 < Z < 2) or a mixture of two or more of them, the positive electrode slurry. The method according to claim 1,
The binder is a positive electrode slurry comprising polyvinylidene fluoride (PVdF). Including a cathode active material, a conductive material and a binder,
The conductive material is a mixture of a spherical first conductive material and a graphene nanoribbon that is a linear second conductive material,
The graphene nanoribbons are disposed between the spherical first conductive materials,
The graphene nanoribbon is a secondary battery positive electrode comprising a hydroxyl group (-OH) and a carboxyl group (-COOH) on the surface. The secondary battery positive electrode of claim 11;
an electrolyte disposed between the positive electrode and the negative electrode; and
A secondary battery comprising an anode.

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