KR20230067018A - Cathode and lithium ion secondary battery comprising same - Google Patents

Abstract

Disclosed is a positive electrode which comprises: a current collector; and a positive electrode active material layer located on at least one surface of the current collector and including a positive electrode active material, a conductive material, and a fluorine-based binder polymer. The glossiness of a surface of the positive electrode active material layer measured at an incident light angle of 60° is 10 to 20. The glossiness means the intensity of light when reflected light is received at the same angle as incident light.

Description

A cathode and a lithium ion secondary battery including the same {CATHODE AND LITHIUM ION SECONDARY BATTERY COMPRISING SAME}

The present invention relates to a positive electrode that can be used in an electrochemical device such as a lithium secondary battery and a lithium ion secondary battery including the same.

Recently, interest in energy storage technology is increasing. Efforts in the research and development of electrochemical devices are becoming more and more specific as the fields of application are expanded to mobile phones, camcorders, notebook PCs, and even the energy of electric vehicles.

The electrochemical device is the field that is attracting the most attention in this respect, and among them, the development of a secondary battery capable of charging and discharging has become the focus of attention. In recent years, research and development on the design of new electrodes and batteries have been actively conducted in order to improve capacity density and specific energy in the development of secondary batteries.

Among the currently applied secondary batteries, the lithium secondary battery developed in the early 1990s has the advantage of higher operating voltage and significantly higher energy density than conventional batteries such as Ni-MH, Ni-Cd, and sulfuric acid-lead batteries using aqueous electrolytes. is gaining popularity as

These secondary batteries can be broadly classified into positive electrodes, negative electrodes, separators, and electrolytes. Lithium ions from the positive electrode active material are inserted into the negative electrode active material by the first charge and then desorbed again during discharging to reciprocate between the two electrodes. By transferring energy through this process, charging and discharging are possible.

For example, a lithium secondary battery has a structure in which an electrode assembly composed of a positive electrode including a lithium transition metal oxide as an electrode active material, a negative electrode including a carbon-based active material, and a separator including a porous polymer substrate is impregnated with an electrolyte solution. there is.

In such a battery, the distribution of the binder polymer in the battery electrode active material layer is one factor that determines the performance of the electrode. As the content of the binder polymer in the electrode active material layer increases, it can affect the improvement of the adhesion between the active material layer and the current collector. However, since the content of the binder and the battery capacity are inversely proportional, the distribution of the polymer binder within the electrode active material layer at the same binder content. is important. The distribution of the binder polymer generally tends to decrease from the surface of the electrode active material layer to the inside. As the position of the minimum content of the binder polymer is further away from the current collector, the adhesive strength of the electrode layer is lowered and there is a problem in that it is peeled off. Therefore, it is important in terms of improving electrode adhesion to control the position where the content of the binder polymer is minimum in the electrode layer as close to the current collector as possible and to lower the content of the binder polymer on the surface of the electrode layer.

The present invention has been devised to solve the problems of the prior art as described above, and is intended to provide an anode that simultaneously satisfies high adhesion and low resistance.

However, the technical problem to be achieved by the present invention is not limited to the above-mentioned problem, and other problems not mentioned will be clearly understood by those skilled in the art from the description of the invention described below.

In order to solve the above object of the present invention, the following embodiments of the anode and its manufacturing method are provided.

According to the first embodiment,

current collector;

A cathode active material layer located on at least one surface of the current collector and including a cathode active material, a conductive material, and a fluorine-based binder polymer;

The glossiness of the surface of the positive electrode active material layer measured at an incident light angle of 60 ° is 10 to 20,

The glossiness is provided to the anode, characterized in that it means the intensity of light when the reflected light is received at the same angle as the incident light.

According to the second embodiment, in the first embodiment,

The glossiness may be a relative ratio when the surface gloss of glass having a refractive index of 1.567 is 100.

According to the third embodiment, in the first embodiment or the second embodiment,

The glossiness of the surface of the positive electrode active material layer may be 10.2 to 19.8.

According to the fourth embodiment, in any one of the first to third embodiments,

The content of the fluorine-based binder polymer may be 1 to 4 parts by weight based on 100 parts by weight of the positive electrode active material layer.

According to the fifth embodiment, in any one of the first to fourth embodiments,

The content of the fluorine-based binder polymer may be 1.5 parts by weight to 3.5 parts by weight based on 100 parts by weight of the positive electrode active material layer.

According to the sixth embodiment, in any one of the first to fifth embodiments,

The fluorine-based binder polymer is polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE), polyvinylidene fluoride polytetrafluoroethylene (PVDF-TFE), polyvinylidenefluoride-chlorofluoroethylene (PVDF-CTFE), polytetrafluoroethylene (PTFE), or two or more of these.

According to the seventh embodiment,

Preparing a slurry for a positive electrode by mixing a positive electrode active material, a conductive material, and a fluorine-based binder polymer in a solvent; and

A method for manufacturing a positive electrode according to the first embodiment is provided, including applying the slurry for the positive electrode to at least one surface of the positive electrode current collector and then drying the slurry.

According to the eighth embodiment, in the seventh embodiment,

The drying step may be performed a plurality of times.

According to the ninth embodiment, in the seventh or eighth embodiment,

When the drying step is performed a plurality of times, it may be performed at a different drying temperature compared to the previous step.

According to the tenth embodiment, in any one of the seventh to ninth embodiments,

The drying step is performed three times, and among the three drying steps, the first drying is performed at a temperature of 65 to 80 ° C, the second drying is performed at a temperature of 130 to 160 ° C, and the third drying is performed at a temperature of 80 to 100 ° C. It can be carried out at a temperature of ° C.

According to the eleventh embodiment,

It includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode,

The positive electrode is a lithium secondary battery, characterized in that the positive electrode according to any one of the first to sixth embodiments is provided.

The positive electrode according to one embodiment of the present invention includes a predetermined amount of the fluorine-based binder polymer in the positive electrode active material layer, and controls the position where the content of the fluorine-based binder polymer is minimum in the positive electrode active material layer as close to the current collector as possible, By lowering the content of the fluorine-based binder polymer on the surface of the positive electrode active material layer, the fluorine-based binder polymer in the positive electrode active material layer can be relatively uniformly distributed without being biased on the surface. Therefore, in the positive electrode according to one embodiment of the present invention, the fluorine-based binder polymer having a relatively low refractive index is not excessively distributed on the surface of the positive electrode active material layer and is distributed at an appropriate level, thereby reducing the amount of light reflected from the surface of the positive electrode active material layer. It can be maintained without becoming. As a result, the glossiness of the surface of the positive electrode active material layer measured at an incident light angle of 60° satisfies the range of 2 to 10, and the adhesive force between the positive electrode active material layer and the current collector was higher than 10 gf/20 mm, so that the positive electrode active material layer was a current collector. It is possible to prevent the problem of peeling from the surface in advance, thereby providing a stable anode.

In addition, the positive electrode according to one embodiment of the present invention can provide a positive electrode with relatively low resistance by controlling the distribution and content of the fluorine-based binder polymer in the positive electrode active material layer.

1 is a schematic diagram schematically showing a gloss measurement method according to an embodiment of the present invention.

Hereinafter, the present invention will be described in detail with reference to the drawings. The terms or words used in this specification and claims should not be construed as being limited to ordinary or dictionary meanings, and the inventors may appropriately define the concept of terms in order to explain their invention in the best way. It should be interpreted as a meaning and concept consistent with the technical idea of the present invention based on the principle that there is.

Hereinafter, the embodiments described in this specification and the configurations shown in the drawings are only one of the most preferred embodiments of the present invention and do not represent all of the technical ideas of the present invention, so various equivalents that can replace them at the time of the present application. It should be understood that there may be waters and variations.

A positive electrode of a secondary battery may be manufactured by applying a slurry obtained by mixing a positive electrode active material, a conductive material, and a fluorine-based binder polymer in a solvent to a current collector. At this time, in the process of evaporating the solvent, the fluorine-based binder polymer may move to the surface of the active material layer together with the solvent, so that the distribution of the fluorine-based binder polymer according to the thickness of the positive electrode may not be uniform. However, in a lithium secondary battery, the distribution of the fluorine-based binder polymer in the positive electrode active material layer for a battery is a factor determining the performance of the positive electrode.

The more uniform the distribution of the fluorine-based binder polymer is over the entire thickness of the positive electrode active material layer, the better the adhesive force. The distribution of the fluorine-based binder polymer generally tends to decrease as it enters the inside of the surface of the positive electrode active material layer. As the position, which is the minimum content of the fluorine-based binder polymer, is further away from the current collector, the adhesive strength between the positive electrode active material layer and the current collector is lowered, resulting in peeling. Therefore, it is important to control the position where the content of the fluorine-based binder polymer is minimized in the positive electrode active material layer as close to the current collector as possible and to lower the content of the fluorine-based binder polymer on the surface of the positive electrode active material layer in terms of improving the positive electrode adhesion.

Since the positive electrode active material layer can be formed by coating and drying the positive electrode slurry on the positive electrode current collector, when the positive electrode slurry containing the same amount of the fluorine-based binder polymer is applied, the content of the fluorine-based binder polymer on the surface of the positive electrode active material layer is The smaller the number, the more uniform the distribution of the fluorine-based binder polymer inside the positive electrode active material layer. A positive electrode according to one aspect of the present invention is conceived from the above trends.

The positive electrode according to one aspect of the present invention,

current collector;

Located on at least one surface of the current collector, a cathode active material layer including a cathode active material, a conductive material, and a fluorine-based binder polymer;

The glossiness of the surface of the positive electrode active material layer measured at an incident light angle of 60 ° is 10 to 20,

It is characterized in that the gloss means the intensity of light when the reflected light is received at the same angle as the incident light.

Considering that the adhesion of the positive electrode varies depending on the distribution of the fluorine-based binder polymer in the positive electrode active material layer, the present inventors confirmed the distribution of the fluorine-based binder polymer through the glossiness of the surface of the positive electrode active material layer.

In particular, the present inventors confirmed that the adhesive strength is improved when the glossiness of the surface of the positive electrode active material layer measured at an incident light angle of 60 ° is 10 to 20.

In the present invention, 'glossiness' is a measure of the glossiness of a surface, and means the intensity of light when reflected light is received at the same angle as incident light. This can be confirmed in Figure 1.

The gloss unit (GU) means a relative ratio when the gloss of a glass surface having a refractive index of 1.567 is 100.

The gloss measurement method is not particularly limited, and may be measured using a gloss measurement method used in the art. However, in the gloss measurement method used in one embodiment of the present invention, the reference material is fixed as an absolute value, and the reference value does not change. Accordingly, there is no hassle of setting a new reference according to the material to be measured, and highly reliable results can be obtained.

For example, the gloss measurement method may be measured according to ASTM D528 at a predetermined angle with a gloss meter.

Referring to FIG. 1 , the gloss of the electrode 10 having the positive electrode active material layer 12 located on one surface of the electrode current collector 11 is the angle of incident light of the light irradiation unit 20 in the gloss measuring device (θ). _in ) and the angle (θ _re ) of the reflected light of the light sensing unit 30 is measured by setting it to 60°. In addition, in the step (S2), the glossiness of the electrode is selected by randomly selecting the glossiness measurement part of the positive electrode active material layer, measuring the glossiness a predetermined number of times, and obtaining the average of the measured glossiness. figure can be determined.

In general, glossiness is affected by the refractive index of materials constituting the surface of an object. The smaller the refractive index of the material constituting the surface, the less scattering of light occurs, and the smaller the amount of reflected light, the lower the gloss. As the amount of is increased, the glossiness appears high.

The inventors of the present invention, a fluorine-based binder (1.4 or less, polyvinylidene fluoride, polyvinylidene fluoride having a refractive index smaller than the refractive index of the positive electrode active material (1.8 or more, about 2 in the case of Li[Ni _0.6 Mn _0.2 Co _0.2 ] O ₂ ) -Hexafluoropropylene has excellent adhesive strength by adjusting the gloss within an appropriate range of the surface of the positive electrode active material layer by using the fact that the above-described gloss characteristics change depending on the amount of about 1.4) distributed on the surface of the positive electrode active material layer. A cathode active material layer could be implemented.

As the content of the fluorine-based binder polymer having a low refractive index distributed on the surface of the positive electrode active material layer is small and there is a large amount of positive electrode active material having a relatively high refractive index, the refractive index of the material constituting the surface portion does not decrease, so that the amount of reflected light increases. It can be maintained and the glossiness can be increased.

When the glossiness of the surface of the positive active material layer, measured at an incident light angle of 60 °, satisfies 10 to 20, the adhesive strength of the positive active material layer is higher than 10 gf / 20 mm to prevent the problem of peeling of the positive active material layer in advance. can When the gloss exceeds 20, too much of the fluorine-based binder polymer is located on the surface of the positive electrode active material layer, and the stability of the positive active material layer may deteriorate due to a decrease in adhesive strength.

According to one embodiment of the present invention, the glossiness of the surface of the positive electrode active material layer measured at an incident light angle of 60 ° may be 10.2 to 19.8, or 10.5 to 19.4, or 13.5 to 19.4, or 17.9 to 19.4.

At this time, the positive electrode may be manufactured by applying a positive electrode slurry prepared by mixing a positive electrode mixture including a positive electrode active material, a conductive material, and a fluorine-based binder polymer with an organic solvent on a positive electrode current collector, followed by drying and rolling.

The positive current collector is not particularly limited as long as it does not cause chemical change in the battery and has high conductivity. For example, it is formed on the surface of copper, stainless steel, aluminum, nickel, titanium, fired carbon, copper or stainless steel. A surface treated with carbon, nickel, titanium, silver, or the like, an aluminum-cadmium alloy, or the like may be used. In addition, the cathode current collector may have a thickness of typically 3 to 500 μm, and like the cathode current collector, fine irregularities may be formed on the surface of the current collector to enhance bonding strength of the cathode active material. For example, it may be used in various forms such as films, sheets, foils, nets, porous materials, foams, and non-woven fabrics.

x<0.5, 0≤y<0.5, 0≤z<0.5, 0<x+y+z≤1임)로 이루어진 군으로부터 선택된 어느 하나의 활물질 입자 또는 이들 중 2종 이상의 혼합물을 포함할 수 있다.Examples of the cathode active material include LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiCoPO ₄ , LiFePO ₄ and LiNi _1-xyz Co _x M1 _y M2 _z O ₂ (M1 and M2 are independently Al, Ni, Co, Fe, It is any one selected from the group consisting of Mn, V, Cr, Ti, W, Ta, Mg, and Mo, and x, y, and z are independently the atomic fractions of the oxide composition elements, and are 0

x<0.5, 0≤y<0.5, 0≤z<0.5, 0<x+y+z≤1), and may include any one active material particle selected from the group consisting of or a mixture of two or more of them.

The conductive material is not particularly limited as long as it has conductivity without causing chemical change in the battery, and examples include graphite such as natural graphite or artificial graphite; carbon black such as carbon black, acetylene black, ketjen black, channel black, farnes black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; conductive tubes such as carbon nanotubes; metal powders such as fluorocarbon, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; polyphenylene derivatives; graphene; A conductive material such as may be used.

According to one embodiment of the present invention, the fluorine-based binder polymer is polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE), polyvinylidenefluoride-tetrafluoroethylene (PVDF-TFE), polyvinylidenefluoride-chlorofluoroethylene (PVDF-CTFE), polytetrafluoroethylene (PTFE), or any of these 2 or more may be included. Furthermore, in order to further improve the positive electrode adhesion by smoothly coagulating with the positive electrode active material and the conductive material through the application of high shear force, the fluorine-based binder polymer is made of polyvinylidene fluoride-hexafluoropropylene and polytetrafluorocarbon. It may be at least one of ethylene.

According to one embodiment of the present invention, the content of the fluorine-based binder polymer is 1 part by weight to 4 parts by weight, 1.5 parts by weight to 3.5 parts by weight, or 1.5 parts by weight to 3 parts by weight based on 100 parts by weight of the positive electrode active material layer, or 1 to 2.5 parts by weight, or 3 to 4 parts by weight, or 2.5 parts to 4 parts by weight. When the content of the fluorine-based binder polymer satisfies this range, the binding property of the electrode active material, that is, the adhesion between the current collector and the active material layer is improved, and the movement of electrons and lithium ions in the battery is smooth, resulting in increased cell resistance. can be prevented.

According to one embodiment of the present invention, the anode may have a structure of a single layer or a plurality of layers (double layer, triple layer, etc.) of two or more layers.

The positive electrode may be manufactured by the following method, but is not limited thereto.

First, a slurry for a positive electrode is prepared by mixing a positive electrode active material, a conductive material, and a fluorine-based binder polymer in a solvent. The solvent may be a solvent for the fluorine-based binder polymer and a dispersion medium for the cathode active material and the conductive material. At this time, the content of the fluorine-based binder polymer may be 1 to 4 parts by weight based on 100 parts by weight of the solid content of the coating layer slurry for the positive electrode active material. A method of mixing the materials is not particularly limited, and a method commonly used in the art may be used.

Next, the prepared positive electrode slurry is applied to at least one surface of the positive electrode current collector, and then dried.

The method of coating the positive electrode slurry on the positive electrode current collector is not particularly limited, but examples include bar coating method, knife coating method, roll coating method, blade coating method, die coating method, micro gravure coating method, comma coating method method, a slot die coating method, a lip coating method, or a solution casting method may be used.

At this time, the drying step may be performed a plurality of times. For example, it may be performed 2 to 5 times, or 2 to 3 times. In addition, when the drying step is performed a plurality of times, it may be performed at a different drying temperature compared to the previous step.

According to one embodiment of the present invention, the drying step is performed three times, and among the three drying steps, the first drying may be performed at a temperature of 65 to 80 ° C or 70 to 80 ° C, and the second drying Drying may be performed at a temperature of 130 to 160 °C, or 140 to 160 °C, and tertiary drying may be performed at a temperature of 80 to 100 °C, 80 to 90 °C, or 90 to 100 °C. More specifically, among the three drying steps, the primary drying may be performed at a temperature of 65 to 80 ° C for 10 to 60 seconds, and the secondary drying may be performed at a temperature of 130 to 160 ° C for 60 to 150 seconds, , The tertiary drying may be performed at a temperature of 80 to 100 ° C for 60 to 120 seconds.

Compared to the case where the drying step continues at a specific temperature throughout the drying time, when the drying step proceeds at a different drying temperature in stages compared to the previous step, for example, the drying starts at a relatively low temperature in the initial stage, and then When drying is performed at a much higher temperature and then the drying temperature is lowered, the speed at which the fluorine-based binder polymer moves to the surface of the electrode layer and the speed at which the solvent evaporates are balanced, so that the location of the fluorine-based binder polymer on the surface is minimized. Therefore, the fluorine-based binder polymer can be uniformly distributed as a whole from the vicinity of the current collector to the surface.

Another embodiment of the present invention relates to a lithium secondary battery including the positive electrode prepared as described above. Specifically, the lithium secondary battery may be manufactured by injecting a lithium salt-containing electrolyte into an electrode assembly including a positive electrode, a negative electrode, and a separator interposed therebetween.

The negative electrode is prepared by mixing a negative electrode active material, a conductive material, a binder and a solvent to prepare a slurry, and then directly coating the slurry on a metal current collector, or casting it on a separate support and laminating the negative electrode active material film peeled from the support on the metal current collector Thus, a negative electrode can be prepared.

As the anode active material, a carbon material, lithium metal, silicon, tin, or the like, which can occlude and release lithium ions, may be used. The negative electrode active material may be used by mixing at least one of graphite, non-graphite, and silicon oxide. The graphite-based material may be, for example, artificial graphite or natural graphite. Representative examples of the non-graphite-based material include soft carbon and hard carbon. As the silicon oxide, silicon dioxide or the like can be used.

Meanwhile, the conductive material, binder, and solvent may be used the same as those used in preparing the positive electrode.

The separator is a conventional porous polymer film used as a conventional separator, for example, a polyolefin polymer such as ethylene homopolymer, propylene homopolymer, ethylene/butene copolymer, ethylene/hexene copolymer, and ethylene/methacrylate copolymer. The prepared porous polymer film may be used alone or in a laminated manner. In addition, an insulating thin film having high ion permeability and mechanical strength may be used. The separator may include a safety reinforced separator (SRS) in which a ceramic material is thinly coated on a surface of the separator. In addition, conventional porous non-woven fabrics, for example, non-woven fabrics made of high melting point glass fibers, polyethylene terephthalate fibers, etc. may be used, but are not limited thereto.

The electrolyte solution includes a lithium salt as an electrolyte and an organic solvent for dissolving the lithium salt.

The lithium salt may be used without limitation as long as it is commonly used in an electrolyte solution for a secondary battery. For example, the anion of the lithium salt is F ^- , Cl ^- , 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 ^- , (CF ₃ SO ₂ ) ₂ N ^- , (FSO ₂ ) ₂ N ^- , 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 ^- , One selected from the group consisting of SCN ^- and (CF ₃ CF ₂ SO ₂ ) ₂ N ^- may be used.

Any organic solvent included in the electrolyte may be used without limitation as long as it is commonly used, and representative examples include propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, dipropyl carbonate, and dimethyl sulfoxide. At least one selected from the group consisting of side, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, gamma-butyrolactone, propylene sulfite, and tetrahydrofuran may be used.

In particular, among the carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, are high-viscosity organic solvents and have a high dielectric constant, so they can be preferably used because they dissociate lithium salts in the electrolyte well. When a low-viscosity, low-dielectric constant linear carbonate such as carbonate is mixed and used in an appropriate ratio, an electrolyte solution having high electrical conductivity can be prepared and can be more preferably used.

Optionally, the electrolyte solution stored according to the present invention may further include additives such as an overcharge inhibitor included in conventional electrolyte solutions.

In the lithium secondary battery according to one embodiment of the present invention, an electrode assembly is formed by disposing a separator between a positive electrode and a negative electrode, the electrode assembly is placed in, for example, a pouch, a cylindrical battery case, or a prismatic battery case, and then the electrolyte is After injection, the secondary battery can be completed. Alternatively, a lithium secondary battery may be completed by laminating the electrode assembly, impregnating it with an electrolyte, and sealing the resulting product in a battery case.

According to one embodiment of the present invention, the lithium secondary battery may be a stack type, a winding type, a stack and folding type, or a cable type.

The lithium secondary battery according to the present invention can be used not only as a battery cell used as a power source for a small device, but also can be preferably used as a unit cell in a medium-large battery module including a plurality of battery cells. Preferable examples of the medium-to-large-sized device include an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, and a power storage system. In particular, it is useful for hybrid electric vehicles and batteries for renewable energy storage, which are areas where high power is required. can be used

Hereinafter, examples will be described in detail to aid understanding of the present invention. However, the embodiments according to the present invention can be modified in many different forms, and the scope of the present invention should not be construed as being limited to the following examples. Embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

Example 1

<Manufacture of positive electrode>

96.5wt% of Li[Ni _0.6 Mn _0.2 Co _0.2 ]O ₂ as a cathode active material, 1wt% of carbon nanotubes (bundled type) with a diameter of 10 nm and a length of 50 μm as a conductive material, polyvinylidene fluorine as a fluorine-based binder polymer A slurry was prepared by mixing 2.5 wt% of polyvinylidene fluoride in a N-Methyl-2-Pyrrolidinone (NMP) solvent to have a solid content of 45 wt%. Mixing was performed at 1800 rpm for 25 minutes using a nobilta machine (Hosokawa Co.). Thereafter, a shear force of 250 N was applied to the mixture (Nobilta (Hosokawa micron)) to prepare a slurry (positive electrode slurry) for forming a positive electrode active material layer.

After coating the slurry for forming the positive electrode active material layer on one surface of an aluminum current collector having a thickness of 15 μm, a drying temperature and drying time were controlled as shown in Table 1 below to prepare a positive electrode. That is, after primary drying at 70° C. for 20 seconds, secondary drying at 130° C. for 100 seconds, and third drying at 100° C. for 60 seconds, a positive electrode was prepared by pressing.

<Manufacture of cathode>

Anode slurry by mixing artificial graphite, natural graphite, carbon black, Carboxy Methyl Cellulose (CMC), and Styrene-Butadiene Rubber (SBR) with water in a weight ratio of 30 : 66.5 : 1.0 : 1.0 : 1.5 was manufactured.

The anode slurry was coated on copper foil (Cu-foil) with a capacity of 3.5 mAh/cm ² to form a thin electrode plate, followed by primary drying at 40 ° C. for 30 seconds, secondary drying at 130 ° C. for 120 seconds, and drying at 150 ° C. After tertiary drying for 150 seconds, a negative electrode was prepared by pressing.

<Manufacture of lithium secondary battery>

LiPF ₆ was dissolved in an organic solvent in which ethylene carbonate (EC), dimethyl carbonate (DMC) and diethyl carbonate (DEC) were mixed in a composition of 1:2:1 (volume ratio) to a concentration of 1.0 M to form a non-aqueous electrolyte solution. manufactured.

A polyethylene porous film was interposed as a separator between the positive electrode and the negative electrode prepared above, and the electrolyte was injected after inserting the film into a pouch cell to prepare a lithium secondary battery.

Examples 2 to 4

A positive electrode and a secondary battery were manufactured in the same manner as in Example 1, except that the drying temperature and drying time of the positive electrode slurry were controlled as shown in Table 2 below.

Comparative Examples 1 to 3

A positive electrode and a secondary battery were manufactured in the same manner as in Example 1, except that the drying temperature and drying time of the positive electrode slurry were controlled as shown in Table 2 below.

Experimental Example 1 - Gloss measurement

In the present invention, the glossiness is a measure of the glossiness of the surface, and means the intensity of light when reflected light is received at the same angle as incident light.

The gloss unit (GU) means a relative ratio when the surface gloss of a reference glass (a glass having a refractive index of 1.567) is set to 100.

The surface gloss of the positive electrodes prepared in Examples 1 to 4 and Comparative Examples 1 to 3 was measured using Dark Calibration of BYK Gardner's micro-TRI-gloss equipment in the following manner. The results are shown in Table 2.

Specifically, Dark Calibration made the surface gloss of 92 at 20°, 95 at 60°, and 99 at 85° using a reference glass. Then, the surface gloss of the active material layer of the negative electrode prepared in Examples 1 to 4 and Comparative Examples 1 to 3 was measured. The glossiness was measured 5 times at 60° by randomly selecting measurement parts, and then the average was obtained.

Experimental Example 2 - Adhesion (Peel Test)

The negative electrodes prepared in Examples 1 to 4 and Comparative Examples 1 to 3 were adhered to the slide glass using a double-sided tape for removal. After this, TA. A 90° Peel Test was performed at a speed of 5 mm/sec using XT plusC (Stable Micro Systems) equipment. The results are shown in Table 2.

Experimental Example 3 - Resistance measurement of anode

The interface resistance between the electrode layer and the current collector is measured using a multi-probe tester. The interfacial resistance of the positive electrodes prepared in Examples 1 to 4 and Comparative Examples 1 to 3 was cut into a size of 5 cm x 5 cm using a punching machine, and the interfacial resistance was measured under the following conditions. The results are shown in Table 2.

Current: 100μA (positive), 10mA (negative)

Speed: low

Voltage range: 0.5V

Current collector resistivity: 8.28*10 ^-6 Ωㆍcm (positive), 1.68*10 ^-6 Ωㆍcm (negative)

Experimental Example 4 - Calculation of the content of the fluorine-based binder polymer on the surface of the negative electrode active material layer

Anodes prepared in Examples 1 to 4 and Comparative Examples 1 to 3 were prepared in a size of 1 cm X 1 cm, and the content of the fluorine-based binder polymer on the surface of the prepared anode was measured using energy dispersive X-ray spectroscopy. was analyzed. The atomic ratio of fluorine on the surface of the positive electrode active material layer was measured under the conditions shown in Table 1 below using an X-Max80 and an Extreme detector (Oxford). The results are shown in Table 2.

item condition Secondary Electron Microscopy Equipment JEOL JSM-7200F Acceleration voltage (kV) 5 Magnification (X) 1000 Dwell Time (㎲) 100

Example 1 Example 2 Example 3 Example 4 Comparative Example 1 Comparative Example 2 Comparative Example 3 dry
condition 1st drying 70℃ 20 seconds 65℃ 30 seconds 80℃ 10 seconds 70℃ 25 seconds 45℃ 30 seconds 130℃
120 seconds 70℃ 60 seconds 2nd drying 130℃
100 seconds 160℃ 60 seconds 140℃
150 seconds 140℃
120 seconds 130℃
100 seconds 100℃
100 seconds 3rd drying 100℃ 60 seconds 90℃ 100 seconds 80℃ 120 seconds 90℃ 120 seconds 160℃ 60 seconds 80℃ 60 seconds 60° gloss 19.4 17.9 13.5 10.5 23.6 9.8 20.3 Adhesion (gf/20mm) 30 21 18 15 7 5 7 bipolar resistance
(Ωㆍcm2) 0.476 0.48 0.513 0.565 0.64 0.511 0.71 F content on the surface of the positive electrode active material layer (wt%) 5.8 6.2 6.5 6.6 5.5 7.8 5.4

Referring to Table 2, the anodes of Examples 1 to 4, in which the glossiness of the surface of the cathode active material layer measured at an incident light angle of 60 ° satisfies the range of 10 to 20, compared to the anodes of Comparative Examples 1 to 3. It was confirmed that it exhibited excellent adhesive strength.

This is because, in the positive electrodes of Examples 1 to 4, the fluorine-based binder polymer is relatively uniformly distributed in the positive electrode active material layer without being concentrated on the surface, so that the adhesive strength between the positive electrode active material layer and the current collector is high.

Since the fluorine-based binder polymer has a small refractive index compared to the positive electrode active material, when a large amount of the fluorine-based binder polymer having such a small refractive index is distributed on the surface of the positive electrode active material layer, the amount of reflected light is reduced, resulting in a low 60° gloss. value is displayed (see Comparative Example 2).

However, in the positive electrodes of Examples 1 to 4, since the fluorine-based binder polymer having such a small refractive index is not concentrated on the surface of the positive electrode active material layer and is uniformly distributed throughout the positive electrode active material layer, it is smaller than that of Comparative Examples 1 and 3 described later. , indicating a greater 60 ° gloss value than that of Comparative Example 2.

Meanwhile, in the positive electrodes of Comparative Example 1 and Comparative Example 3, the fluorine-based binder polymer was distributed in a large amount inside the positive electrode active material layer and too little was distributed on the surface of the positive electrode active material layer, so that the 60° glossiness had a high value, but in the adhesion test The positive electrode active material layer is easily peeled off, indicating a small value of adhesive strength. In the positive electrode of Comparative Example 2, as a result of drying the slurry from the beginning at a high temperature of 130 ° C, the fluorine-based binder polymer is distributed the most on the surface of the positive electrode active material layer and has the lowest 60 ° glossiness. As a result, the positive electrode active material layer Since the content of the fluorine-based binder polymer is small in the lower portion, the current collector and the cathode active material layer are easily separated.

Although the present invention has been described above with limited examples and drawings, the present invention is not limited thereto and will be described below along with the technical spirit of the present invention by those skilled in the art to which the present invention belongs. Of course, various modifications and variations are possible within the scope of the claims.

Claims (11)

current collector;
A cathode active material layer located on at least one surface of the current collector and including a cathode active material, a conductive material, and a fluorine-based binder polymer;
The glossiness of the surface of the positive electrode active material layer measured at an incident light angle of 60 ° is 10 to 20,
The anode, characterized in that the gloss means the intensity of light when the reflected light is received at the same angle as the incident light.
According to claim 1,
The anode, characterized in that the gloss is a relative ratio when the surface gloss of glass having a refractive index of 1.567 is 100.
According to claim 1,
A positive electrode, characterized in that the glossiness of the surface of the positive electrode active material layer is 10.2 to 19.8.
According to claim 1,
A positive electrode, characterized in that the content of the fluorine-based binder polymer is 1 part by weight to 4 parts by weight based on 100 parts by weight of the positive electrode active material layer.
According to claim 4,
A positive electrode, characterized in that the content of the fluorine-based binder polymer is 1.5 parts by weight to 3.5 parts by weight based on 100 parts by weight of the positive electrode active material layer.
According to claim 1,
The fluorine-based binder polymer is polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE), polyvinylidene fluoride Ride-tetrafluoroethylene (PVDF-TFE), polyvinylidene fluoride-chlorofluoroethylene (PVDF-CTFE), polytetrafluoroethylene (PTFE), or a cathode characterized in that it comprises two or more of them .
Preparing a slurry for a positive electrode by mixing a positive electrode active material, a conductive material, and a fluorine-based binder polymer in a solvent; and
The manufacturing method of claim 1 including the step of drying the slurry for the positive electrode on at least one surface of the positive electrode current collector.
According to claim 7,
The method of manufacturing a positive electrode, characterized in that the drying step is performed a plurality of times.
According to claim 8,
A method for producing a positive electrode, characterized in that when the drying step is performed a plurality of times, it is performed at a different drying temperature compared to the previous step.
According to claim 9,
The drying step is performed three times, and among the three drying steps, the first drying is performed at a temperature of 65 to 80 ° C, the second drying is performed at a temperature of 130 to 160 ° C, and the third drying is performed at a temperature of 80 to 100 ° C. Method for producing an anode, characterized in that carried out at a temperature of ℃.
It includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode,
The positive electrode is a lithium secondary battery, characterized in that the positive electrode according to any one of claims 1 to 6.

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