Negative electrode for secondary battery, method for manufacturing the negative electrode, and lithium secondary battery including the negative electrode

Abstract

The present disclosure relates to a negative electrode for a secondary battery, a method for manufacturing the negative electrode, and a lithium secondary battery including the negative electrode. The negative electrode has an active material layer with improved peeling resistance and adhesion, thereby improving the lifespan characteristics of the negative electrode and the lithium secondary battery. The negative electrode for a secondary battery includes: a metal current collector; and an active material layer formed on the metal current collector and comprising a negative electrode active material, a binder, and a conductive material. The active material layer has an average shear strength of 1.6 MPa or greater, measured at a predetermined depth.

Description

Technical Field

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of Korean Patent Application No. 10-2021-0026804 filed in the Korean Intellectual Property Office on February 26, 2021, the contents of which are incorporated herein by reference in their entirety.

[0003] The present disclosure relates to a negative electrode for a secondary battery having improved peeling resistance and adhesiveness of an active material layer, thereby improving life characteristics of the negative electrode and a lithium secondary battery; a method for manufacturing the negative electrode; and a lithium secondary battery including the negative electrode. Background Art

[0004] With the technological development and increasing demand for mobile devices, electric vehicles, etc., the demand for secondary batteries as energy sources has also increased dramatically. Among such secondary batteries, lithium secondary batteries, which exhibit high energy density and voltage, long cycle life, and low discharge rate, are now commercialized and widely used.

[0005] Generally, a lithium secondary battery has a structure in which an electrolyte containing a lithium salt is impregnated into an electrode assembly in which a porous separator is inserted between a positive electrode and a negative electrode, the positive electrode and the negative electrode are each coated with an active material on a metal current collector, and each electrode is manufactured by applying a slurry composition in which the active material, a binder, and a conductive material are dispersed in a solvent to the metal current collector, pressing, and drying the coated slurry composition.

[0006] Therefore, the lifespan characteristics of lithium secondary batteries can be mainly determined by how long the electrochemical characteristics of the electrodes, especially the active material layer, are maintained. However, in the case of existing lithium secondary batteries, as the use time passes, the active material layer peels off from the metal current collector in the negative electrode, and the lifespan characteristics of lithium secondary batteries often deteriorate.

[0007] This appears to be because the negative electrode active material layer contains a graphite-based negative electrode active material as its main component, which has properties different from those of metals. Therefore, it is difficult to ensure sufficient adhesion, tightness, and peeling resistance to the metal current collector. Due to this poor adhesion of the negative electrode active material layer, there is a disadvantage that the active material layer easily peels off from the metal current collector after the lithium secondary battery has expired. The peeled active material layer can no longer serve as the active area of ​​the lithium secondary battery, causing a sharp deterioration in the charge / discharge cycle characteristics of the lithium secondary battery.

[0008] Due to these problems, there is still a need to continue developing a technology that can further improve the binding properties and the like of the active material layer contained in the negative electrode and improve the life characteristics of the lithium secondary battery. Summary of the Invention

[0009] Technical issues

[0010] An object of the present disclosure is to provide a negative electrode for a secondary battery and a method for manufacturing the negative electrode, wherein the peeling resistance and adhesion between the active material layer and the metal current collector are improved, thereby improving the life characteristics of the negative electrode and the lithium secondary battery.

[0011] Another object of the present disclosure is to provide a lithium secondary battery including the negative electrode for a secondary battery and thereby exhibiting improved lifespan characteristics.

[0012] Technical Solution

[0013] According to one embodiment of the present disclosure, there is provided a negative electrode for a secondary battery, comprising:

[0014] a metallic current collector; and

[0015] an active material layer formed on the metal current collector and comprising a negative electrode active material, a binder, and a conductive material,

[0016] Wherein, when the active material layer is cut with a microblade at a constant cutting speed using a SAICAS instrument, when the shear strength of each cutting depth is calculated according to the following equation 1, the average value of the shear strength of the active material layer measured at a cutting depth of 10 μm to 40 μm is 1.6 MPa or more.

[0017] [Equation 1]

[0018]

[0019] In Equation 1, τs represents the shear strength at each cutting depth, b represents the width of the microblade, t0 represents the cutting depth, represents the shear angle,

[0020] Fh and Fv represent the measurements of the horizontal and vertical forces, respectively, applied to the microblade to maintain a constant cutting speed.

[0021] According to another embodiment of the present disclosure, there is provided a method for manufacturing the negative electrode for a secondary battery, the method comprising the following steps:

[0022] applying a slurry composition comprising a negative electrode active material, a binder, a conductive material, and a solvent onto a metal current collector; and

[0023] The slurry composition is rolled through the first step and the second step,

[0024] Here, when the rolling ratio defined by the following equation 2 is calculated, the first rolling and the second rolling are performed so that the first rolling ratio / the second rolling ratio is 5 or more.

[0025] [Equation 2]

[0026] Rolling rate (%) = [thickness reduction of the slurry composition after rolling (μm) / thickness of the slurry composition before rolling (μm)] * 100

[0027] According to yet another embodiment of the present disclosure, a lithium secondary battery is provided, including the negative electrode for a secondary battery.

[0028] Beneficial effects

[0029] According to the present disclosure, a negative electrode for a secondary battery having greatly improved peeling resistance and adhesion between an active material layer and a metal current collector can be manufactured and provided by a simple method of controlling the conditions for performing the rolling step during the negative electrode manufacturing process.

[0030] In this negative electrode for a secondary battery, even when the lithium secondary battery is used for a long time, the phenomenon of peeling of the active material layer from the metal current collector can be greatly reduced, and thus the excellent charge / discharge cycle characteristics of the lithium secondary battery can be maintained for a long time.

[0031] Therefore, a lithium secondary battery including the negative electrode for a secondary battery exhibits greatly improved lifespan characteristics and thus can be very preferably used as a power source device such as mobile devices and electric vehicles. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 is a schematic diagram showing an example of measuring the shear strength at various cutting depths when cutting an active material layer with a microblade using a SAICAS instrument;

[0033] Figure 2 is a graph showing the measurement results of the shear strength of the active material layer at each cutting depth of the negative electrodes prepared in Comparative Examples 1 to 5 (sample numbers: 1 and 5 to 8) and Examples 1 to 3 (sample numbers: 2 to 4), respectively. DETAILED DESCRIPTION

[0034] Hereinafter, a negative electrode for a secondary battery, a method of manufacturing the negative electrode, and a lithium secondary battery including the negative electrode according to one embodiment of the present disclosure will be described in more detail with reference to the accompanying drawings.

[0035] According to one embodiment of the present disclosure, there is provided a negative electrode for a secondary battery, comprising:

[0036] a metallic current collector; and

[0037] an active material layer formed on the metal current collector and comprising a negative electrode active material, a binder, and a conductive material,

[0038] Wherein, when the active material layer is cut with a microblade at a constant cutting speed using a SAICAS instrument, when the shear strength of each cutting depth is calculated according to the following equation 1, the average value of the shear strength of the active material layer measured at a cutting depth of 10 μm to 40 μm is 1.6 MPa or more.

[0039] [Equation 1]

[0040]

[0041] In Equation 1, τs represents the shear strength at each cutting depth, b represents the width of the microblade, t0 represents the cutting depth, represents the shear angle,

[0042] Fh and Fv represent the measurements of the horizontal and vertical forces, respectively, applied to the microblade to maintain a constant cutting speed.

[0043] The present inventors have continued their research to improve the adhesion and peeling resistance of the active material layer of the negative electrode to the metal current collector by a simple method that does not significantly change the existing negative electrode manufacturing process.

[0044] As a result of this continuous research, the inventors have found that in addition to the surface portion of the active material layer containing the negative electrode active material, the binder and the conductive material, and the interface portion with the metal current collector, the anti-cutting shear strength measured at the center portion of the active material layer (i.e., the cutting depth of the active material layer is 10 μm to 40 μm) can also reach a certain level or above, thereby greatly improving the adhesion and peeling resistance of the active material layer to the metal current collector. This seems to be because the binder, the conductive material and the negative electrode active material are physically more tightly bonded, and therefore, the active material layer with high anti-cutting shear strength at the center portion can be tightly adhered to the surface of the metal current collector with higher adhesion.

[0045] Furthermore, based on the above findings, the present inventors continued their research into a method for manufacturing a negative electrode capable of forming an active material layer having a higher shear strength against cutting in the central portion. As a result of this continued research, the present inventors discovered that, in the negative electrode manufacturing process, multiple rolling steps, including a first step and a second step, are performed, but the multiple rolling steps are performed so that the first rolling rate range and the second rolling rate range are within a certain range. Thus, it is possible to produce an active material layer having a higher shear strength and a negative electrode including the active material layer, thereby completing the present disclosure.

[0046] As described above, according to the embodiments of the present disclosure, a negative electrode for a secondary battery having greatly improved peeling resistance and adhesion between an active material layer and a metal current collector can be manufactured and provided by a relatively simple method. In this negative electrode for a secondary battery, even when the lithium secondary battery is used for a long time, the phenomenon of the active material layer peeling off from the metal current collector can be greatly reduced, thereby greatly improving the life characteristics of the lithium secondary battery.

[0047] Now, a negative electrode for a secondary battery according to one embodiment will be described in more detail.

[0048] A negative electrode for a secondary battery according to one embodiment basically includes: a metal current collector; and an active material layer formed on the metal current collector.

[0049] As the metal current collector, any metal current collector that has been used as an electrode current collector such as a lithium secondary battery can be used, for example, a metal current collector that has conductivity without causing chemical changes in the battery, without particular limitation. Specific examples of the metal current collector include: a current collector comprising at least one metal selected from copper, stainless steel, aluminum, nickel, and titanium. Among these metal current collectors, a copper current collector can be used in view of its excellent conductivity for the negative electrode of the secondary battery and its excellent adhesion to the active material layer.

[0050] The thickness of the metal current collector is not particularly limited, but may be 3 μm to 500 μm, or 5 μm to 100 μm, or 7 μm to 50 μm, which are generally used thicknesses.

[0051] In addition, the active material layer may include, for example, a negative electrode active material including a graphite-based active material, a binder, and a conductive material.

[0052] Specific examples of the negative electrode active material include: at least one graphite-based active material selected from natural graphite, artificial graphite, fibrous artificial graphite, graphitized carbon black and graphitized nanofibers, and in addition to the graphite-based active material, additional active materials such as silicon-based active materials may also be included.

[0053] In addition, as the binder, various polymer binders can be used, such as polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, styrene-butadiene rubber (SBR) or various other copolymers. Among them, considering the excellent adhesion to the metal current collector and the excellent dispersibility to the negative electrode active material and the conductive material, polyvinylidene fluoride polymers or copolymers can be preferably used.

[0054] There is no particular limitation on the conductive material as long as it has high conductivity and does not cause chemical changes in the corresponding battery. For example, graphite such as natural graphite and artificial graphite; carbon black-based conductive materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black and thermal black; conductive fibers such as carbon fibers and metal fibers; metal powders such as carbon fluoride powder, aluminum powder and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0055] However, when a graphite-based active material is mainly used as a negative electrode active material, a carbon black-based conductive material may be appropriately used in consideration of its excellent dispersibility and electrical properties.

[0056] On the other hand, the active material layer may contain: 80 weight percent to 98 weight percent, or 85 weight percent to 98 weight percent, or 90 weight percent to 97 weight percent of the negative electrode active material; 0.5 weight percent to 15 weight percent, or 0.7 weight percent to 10 weight percent, or 1 weight percent to 5 weight percent of the binder; and 0.1 weight percent to 10 weight percent, or 0.2 weight percent to 5 weight percent, or 0.3 weight percent to 2 weight percent of the conductive material.

[0057] Since the active material layer contains the negative electrode active material, the binder and the conductive material in this content range, uniform dispersion of the negative electrode active material and the conductive material can be achieved, while the adhesion of the active material layer to the metal current collector can be further improved, and excellent electrochemical performance of the negative electrode for the secondary battery can be achieved.

[0058] On the other hand, a negative electrode for a secondary battery of one embodiment is formed by a process comprising: coating a slurry composition comprising the above-mentioned negative electrode active material, a binder and a conductive material and a solvent for dispersing them on a metal current collector, and then rolling and drying the coated slurry composition through a plurality of rolling steps described later.

[0059] Through this process, when the active material layer is cut with a microblade at a constant cutting speed using a SAICAS instrument, when the shear strength at each cutting depth is calculated according to Equation 1, the active material layer is formed so that the average value of the shear strength measured at a cutting depth of 10 μm to 40 μm is 1.6 MPa or more, or 1.6 MPa to 3 MPa, or 1.65 MPa to 2 MPa.

[0060] As described above, since the active material layer is formed to have such an average shear strength at each cutting depth, the active material layer can exhibit excellent peeling resistance and adhesion to the metal current collector, from which it can be confirmed that the active material layer maintains electrochemical properties for a long time, so that the lithium secondary battery including the active material layer can exhibit excellent life characteristics.

[0061] Figure 1 Schematic diagram showing an example of measuring the shear strength at various cutting depths when cutting an active material layer with a microblade using a SAICAS instrument.

[0062] like Figure 1 As shown in , in order to measure the shear strength of each cutting depth of the active material layer, a micro blade provided in the SAICAS instrument can be used by setting a constant shear angle ( ) to cut the active material layer at a constant cutting speed. While performing this cutting, the horizontal force (Fh) and vertical force (Fv) applied to the microblade were measured to maintain a constant cutting speed, and from these measurement results, the shear strength (τs) at each cutting depth was calculated according to Equation 1.

[0063] From this calculation result, for example, Figure 2 The relationship of the shear strength of each cutting depth is derived from the form shown in . This measurement is repeated many times, for example, 3 to 5 times, and the average shear strength of each cutting depth can be derived.

[0064] As a result of continuous experiments by the present inventors, it was confirmed that in the portion with a cutting depth of 10 μm or less, the reliability of the data decreased due to the compression and cutting resistance generated before cutting. In addition, as a result of continuous experiments by the present inventors, it was confirmed that the active material layer had the best peeling resistance and adhesion to the metal current collector because the average shear strength value derived in the central portion of the active material layer, that is, in the portion with a thickness (cutting depth) of 10 μm to 40 μm, was higher.

[0065] Therefore, the metal current collector of the negative electrode of one embodiment including an active material layer having a high average shear strength value derived in a thickness (cut depth) portion of 10 μm to 40 μm has excellent adhesion and excellent peeling resistance between the active material layer, thereby significantly improving the life characteristics of the lithium secondary battery. However, when the average shear strength value is too high, it means that the rolling step is excessively performed, and it can be confirmed that the electrochemical performance of the active material layer itself deteriorates, and the effect of further improving the adhesion is basically not observed.

[0066] On the other hand, the detailed measurement method of the above average shear strength is also described in "2019 Korea Society of Automotive Engineers, Gwangju Honam Branch, Spring Meeting, SAICAS Polymer Film Measurement Method" and the like, and can be measured using general SAICAS equipment commercially available from Daipla and other companies.

[0067] As described above, since the negative electrode of one embodiment includes the active material layer satisfying the average shear strength range for each predetermined cutting depth, the active material layer may exhibit excellent adhesion to the metal current collector and peeling resistance.

[0068] For example, the active material layer is bonded to the metal current collector with an adhesion force of 30 gf / 20 mm to 50 gf / 20 mm, or 31 gf / 20 mm to 40 gf / 20 mm, and can maintain excellent electrochemical performance for a long time, and can ensure excellent long-life characteristics of the lithium secondary battery.

[0069] The negative electrode of the above embodiment can be manufactured by a manufacturing method including predetermined rolling conditions. Therefore, according to another embodiment of the present disclosure, there is provided a method for manufacturing the negative electrode for a secondary battery of the above embodiment, the method comprising the following steps:

[0070] applying a slurry composition comprising a negative electrode active material, a binder, a conductive material, and a solvent onto a metal current collector; and

[0071] The slurry composition is rolled through the first step and the second step,

[0072] Here, when the rolling ratio defined by the following equation 2 is calculated, the first rolling and the second rolling are performed so that the first rolling ratio / the second rolling ratio is 5 or more.

[0073] [Equation 2]

[0074] Rolling rate (%) = [thickness reduction of the slurry composition after rolling (μm) / thickness of the slurry composition before rolling (μm)] * 100

[0075] According to another embodiment of the present disclosure, a slurry composition for forming an active material layer is coated on a metal current collector, and then a rolling step including a first step and a second step is performed multiple times, wherein the multiple rolling steps are performed so that the ratio of the first and second rolling rates defined by Equation 2 ranges from 5 or more, or from 5 to 30 or from 5 to 25, thereby forming a negative electrode for a secondary battery.

[0076] Since the rolling steps are performed multiple times under these conditions, it can be confirmed that the active material layer is dense while suppressing damage to the negative electrode active material and the conductive material and maintaining excellent electrochemical performance. As a result, an active material layer and a negative electrode having an average shear strength range of one embodiment and thus exhibiting excellent adhesion and peeling resistance can be prepared. However, when the first and second rolling rate ranges are too small, the excellent adhesion due to the negative electrode of one embodiment cannot be properly obtained. Conversely, when the first and second rolling rate ranges are too large, the negative electrode active material may be damaged and the electrochemical performance of the negative electrode may deteriorate.

[0077] On the other hand, in another embodiment of the manufacturing method, first, the above-mentioned negative electrode active material, conductive material, and binder are mixed with a solvent for dispersing them to form a slurry composition. In this case, the types of the negative electrode active material, conductive material, and binder are the same as those described above, and their content ranges are also the same as the content ranges of the respective components contained in the active material layer finally formed above, and therefore, their additional description will be omitted.

[0078] In addition, as the solvent, conventional solvents for negative electrode slurry compositions previously used, for example, N-methylpyrrolidone, acetone, water, etc., can be used, and the slurry composition can be formed by mixing and stirring such solvents so that the solid content concentration is 30 wt % to 70 wt %, or 40 wt % to 60 wt %.

[0079] After forming the slurry composition, the slurry composition can be applied to the negative electrode current collector by a conventional coating method. There is no particular limitation on the coating method. For example, a coating method using a slit die can be applied, or other methods such as a Meyer bar coating method, a gravure coating method, a dip coating method, a spray coating method, etc. can be applied without any particular limitation.

[0080] In addition, the slurry composition can be coated onto the metal current collector to a thickness of the final formed active material layer, for example, a thickness ranging from 50 μm to 400 μm, or 100 μm to 300 μm, and considering the above-mentioned first and second rolling rate ranges, to a thickness of 100 μm to 500 μm or 150 μm to 400 μm.

[0081] After this coating step, for example, the slurry composition is subjected to a plurality of rolling steps including a first rolling step and a second rolling step using a rolling device such as a roller press. Specifically, in another embodiment of the manufacturing method, when the rolling ratio of the ratio of the thickness reduction after the rolling step to the initial thickness before the rolling step as defined in Equation 2 is calculated for each of the first rolling step and the second rolling step, the first rolling step and the second rolling step are performed so that the first rolling ratio / the second rolling ratio is 5 or more, or 5 or more, or 5 to 30, or 5 to 25.

[0082] In order to achieve the ratio of these first and second rolling rates, the pressure applied to the slurry composition in the first rolling step and the second rolling step can be controlled, for example, the respective rolling steps can be performed so that the first rolling rate is 20% to 40%, or 22% to 35%, or 25% to 30%, and the second rolling rate is 0.5% to 6%, or 0.7% to 5.5%, or 0.9% to 5.2%.

[0083] In order to achieve these first and second rolling rates, for example, in the first rolling step, a pressure of 0.5 MPa to 50 MPa or 1 MPa to 20 MPa can be applied to the slurry composition. In addition, taking into account the rolling rate ranges of the first rolling step and the second rolling step, in the second rolling step, pressure can be applied at a ratio reduced than that of the first rolling step, for example, at a pressure of less than 1 / 3 or less than 1 / 5 of the pressure applied in the first rolling step. However, the pressure ranges applied in each of these rolling steps can be controlled differently, taking into account the composition of each slurry composition, the characteristics of the rolling device, etc. This can be adjusted in a manner obvious to those skilled in the art.

[0084] In addition, the plurality of rolling steps including the first rolling step and the second rolling step may be performed at a temperature of 15°C to 30°C.

[0085] After the above-mentioned rolling step is performed, a step of drying the slurry composition to remove the solvent may be further performed, and this drying step may be performed by a conventional method using, for example, an infrared drying device or the like.

[0086] In the negative electrode for a secondary battery manufactured by the above method, the active material layer may exhibit excellent adhesion and peeling resistance with a metal current collector, and thus, a lithium secondary battery including the negative electrode may exhibit significantly improved lifespan characteristics.

[0087] On the other hand, according to another embodiment of the present disclosure, a lithium secondary battery is provided, comprising the negative electrode for a secondary battery described above. This lithium secondary battery can be manufactured and provided by injecting an electrolyte containing a lithium salt into an electrode assembly comprising a positive electrode, the negative electrode described above, and a separator interposed therebetween.

[0088] The positive electrode can be manufactured by mixing a positive electrode active material, a conductive material, a binder, and a solvent to prepare a slurry composition, and then directly coating it onto a metal current collector, or casting it onto a separate carrier and laminating a positive electrode active material film peeled off from the carrier onto the metal current collector.

[0089] The active material for the positive electrode can include any one selected from LiCoO2, LiNiO2, LiMn2O4, LiCoPO4, LiFePO4, and LiNi 1-x-y-z Co x M1 y M2 z O2 (where M1 and M2 are each independently any one selected from Al, Ni, Co, Fe, Mn, V, Cr, Ti, W, Ta, Mg, and Mo, and x, y, and z are each independently the atomic fractions of the elements constituting the oxide, that is, 0 ≤ x < 0.5, 0 ≤ y < 0.5, 0 ≤ z < 0.5, 0 < x + y + z ≤ 1) of any one active material particle, or a mixture of two or more of them.

[0090] On the other hand, the conductive material, the binder, and the solvent can be used in the same manner as those used for manufacturing the negative electrode.

[0091] As the separator, a conventional porous polymer film used as a separator can be used alone, for example, a porous polymer film made of polyolefin polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or these substances can be laminated and used. In addition, an insulating film with high ion permeability and mechanical strength can be used. The separator can include a safety-enhanced separator (SRS) in which a ceramic material is thinly coated on the surface of the separator. In addition, conventional porous non-woven fabrics can be used, for example, non-woven fabrics made of high melting point glass fibers, polyethylene terephthalate fibers, etc., but not limited thereto.

[0092] The electrolyte can contain a lithium salt and an organic solvent for dissolving the lithium salt.

[0093] The lithium salt can be used without limitation as long as it is commonly used in the electrolyte of secondary batteries. For example, as the anion of the lithium salt, any one selected from F - 、Cl - 、I - 、NO3 - 、N(CN)2 - 、BF4 - 、ClO4 - 、PF6 - 、(CF3)2PF4 -、(CF3)3PF3 - 、(CF3)4PF2 - 、(CF3)5PF - 、(CF3)6P - CF3SO3 - CF3CF2SO3 - 、(CF3SO2)2N - 、(FSO2)2N - CF3CF2(CF3)2CO - 、(CF3SO2)2CH - 、(SF5)3C - 、(CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - 、CH3CO2 - 、SCN - and (CF3CF2SO2)2N - One of them.

[0094] The organic solvent included in the electrolyte can be used without restriction as long as it is commonly used. Typically, at least one selected from propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, methylpropyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, cyclopentane, gamma-butyrolactone, propylene sulfite and tetrahydrofuran can be used.

[0095] Particularly, in carbonate organic solvent, ethylene carbonate and propylene carbonate as cyclic carbonate are organic solvents with high viscosity, and have high dielectric constant, thus lithium salts are dissociated well in the electrolyte, therefore can preferably use them.When this cyclic carbonate is mixed with low viscosity, the linear carbonate of low dielectric constant such as dimethyl carbonate and diethyl carbonate with suitable ratio, can manufacture the electrolyte with high conductivity, therefore can more preferably use.

[0096] Optionally, the electrolyte may further include additives, such as overcharge inhibitors contained in conventional electrolytes.

[0097] The lithium secondary battery can be manufactured by placing a separator between the positive electrode and the negative electrode to form an electrode assembly, placing the electrode assembly in, for example, a pouch, a cylindrical battery case, or a prismatic battery case, and then injecting an electrolyte. Alternatively, the lithium secondary battery can be completed by laminating the electrode assembly, then impregnating it with an electrolyte, placing the resultant in a battery case, and sealing the case.

[0098] The lithium secondary battery may be a stack type, a winding type, a stack-folding type, or a cable type.

[0099] The lithium secondary battery can be used not only in battery cells used as power sources for small devices, but can also be preferably used as a unit cell in a medium or large battery module comprising a plurality of battery cells. Preferred examples of medium or large devices include electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, power storage systems, and the like. In particular, it can be effectively used in hybrid electric vehicles and new and renewable energy storage batteries in fields requiring high output.

[0100] Hereinafter, preferred embodiments are provided to help understand the present disclosure. However, the following examples are for illustrative purposes only and are not intended to limit the present disclosure.

[0101] Example 1 to Example 3 (sample numbers: 2 to 4) and Comparative Example 1 to Comparative Example 5 (sample numbers: 1 and 5 to 8): Manufacturing negative electrodes for secondary batteries

[0102] A graphite-based active material (QCG-X2; graphite), carbon black (Super C65) as a negative electrode conductive material, a thickener (Daicel2200: carboxymethyl cellulose (CMC)), and a binder (ADB22D; styrene-butadiene rubber (SBR)-based polymer) were mixed in a ratio of 96.5:0.5:1.1:2.8 for 85 minutes, and then a solvent (water) for forming a slurry was added thereto to form a negative electrode slurry composition with a solid content of 55.5% by weight.

[0103] A copper current collector (8 μm thick and 260 mm wide) was used as the negative electrode current collector, and the negative electrode slurry composition was diluted with 15 mg / cm 2 A loading of 1000 μm was coated on one surface thereof (the thickness after coating was 300 μm).

[0104] The slurry composition coated on the copper current collector in this manner was subjected to the first rolling step and the second rolling step at room temperature under the conditions summarized in Table 1 below (Comparative Example 1 was subjected to only the first rolling step). For reference, the initial coating thickness of the slurry composition, the thickness of the slurry composition after the first rolling step and the second rolling step, and the respective rolling rate ranges calculated according to Equation 2 are shown in Table 1 below.

[0105] After this rolling step, mid-infrared radiation drying was performed at 50° C. to 100° C. at an evaporation rate of 605 g / min for 15 to 100 minutes. Thus, negative electrodes of Examples 1 to 3 (Sample Nos. 2 to 4) and Comparative Examples 1 to 5 (Sample Nos. 1 and 5 to 8) were manufactured.

[0106] [Table 1]

[0107]

[0108] Test Example 1: Derivation of Shear Strength and Average Shear Strength at Each Cutting Depth

[0109] In the negative electrodes manufactured in Examples and Comparative Examples, the shear strength at each cutting depth and the average shear strength of the active material layer were measured and calculated by the following method.

[0110] First, a SAICAS (Daipla, Japan) device is used. Each negative electrode sample is placed on the sample stage of this SAICAS instrument. Then, while the edge of a microblade with a width of 1 mm is in contact with the surface of the active material layer at a back angle of 10 ° and a cutting angle of 20 °, the active material layer is cut while moving the microblade at a constant vertical speed (0.05 μm / sec) and horizontal speed (0.5 μm / sec). At this time, the definitions of the margin angle and the cutting angle are known by “2019 Korea Society of Automotive Engineers, Gwangju Hunan Branch, Spring Meeting, SAICAS Polymer Film Measurement Method” and the like, and the shear angle in the cutting step can be calculated from the margin angle and the cutting angle, as known from the above literature.

[0111] In order to perform such cutting while keeping the horizontal and vertical speeds for cutting constant, the vertical force (FV) and the horizontal force (FH) applied to the microblade were measured, respectively.

[0112] From these vertical and horizontal force measurements, the shear strength at each cutting depth was calculated according to Equation 1 below.

[0113] [Equation 1]

[0114]

[0115] In Equation 1, τs represents the shear strength at each cutting depth, b represents the width of the microblade, t0 represents the cutting depth, represents the shear angle,

[0116] Fh and Fv represent the measurements of the horizontal and vertical forces, respectively, applied to the microblade to maintain a constant cutting speed.

[0117] For each sample, the shear strength at each cutting depth was measured and calculated three times, and the calculation results are shown in FIG. Figure 2 For reference, in Figure 2 The negative electrodes of various embodiments and comparative examples are summarized by sample numbers in Tables 1 and 2.

[0118] From the calculation results of the shear strength at each cutting depth, except for the depth of the cutting (thickness) area where the correlation between the quantitative data and the adhesion of the active material layer is poor, the shear strengths at cutting depths of 10 μm to 40 μm were measured and calculated as an average, and the average value of the shear strength of the active material layer of each embodiment and comparative example was calculated and summarized in Table 2 below.

[0119] [Table 2]

[0120]

[0121]

[0122] Referring to Table 2, it can be confirmed that the range of the first rolling rate / second rolling rate is above 5, or 5 to 30, and in Examples 1 to 3 in which the first rolling step and the second rolling step were performed, the average shear strength finally reached above 1.6 MPa.

[0123] Test Example 2: Measurement of Adhesion of Active Material Layer

[0124] In the negative electrodes manufactured in Examples and Comparative Examples, the adhesive force between the active material layer and the metal current collector was measured by the following method.

[0125] First, each negative electrode sample was cut into a predetermined size (20 mm × 100 mm) and mounted on a glass slide. The 180° adhesion between the copper current collector and the negative active material layer was then measured. The adhesion of each sample was measured three times, and the average value was calculated. The calculated adhesion is summarized in Table 3 below.

[0126] [Table 3]

[0127]

[0128] Referring to Table 3, it can be confirmed that the active material layer of the Example exhibits superior adhesion to the copper current collector compared to the Comparative Example. This suggests that while the fracture resistance and mechanical durability of the active material layer due to external forces are improved, the external forces transmitted to the active material layer / metal current collector interface are reduced, and the peeling resistance of the active material layer is improved.

Claims

1. A negative electrode for a secondary battery, comprising: Metal current collector; and an active material layer formed on the metal current collector and comprising a negative electrode active material, a binder, and a conductive material, wherein, when the active material layer is cut with a microblade at a constant cutting speed using a SAICAS instrument, when the shear strength at each cutting depth is calculated according to the following equation 1, the average value of the shear strength of the active material layer measured at a cutting depth of 10 μm to 40 μm is 1.6 MPa or more, The negative electrode active material comprises at least one graphite active material selected from natural graphite, artificial graphite, graphitized carbon black and graphitized nanofibers, The conductive material is selected from carbon black, The binder comprises at least one selected from polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid and styrene-butadiene rubber (SBR), The active material layer comprises: 80 wt % to 98 wt % of the negative electrode active material, 0.5 wt % to 15 wt % of the binder, and 0.1 wt % to 10 wt % of the conductive material, [Equation 1] In Equation 1, τs represents the shear strength at each cutting depth, b represents the width of the microblade, t0 represents the cutting depth, represents the shear angle, Fh and Fv represent the measured values ​​of the horizontal force and vertical force respectively applied to the microblade to maintain the constant cutting speed, The negative electrode for the secondary battery is manufactured by a method comprising the following steps: applying a slurry composition comprising a negative electrode active material, a binder, a conductive material, and a solvent onto a metal current collector; and The slurry composition is rolled through the first step and the second step, Wherein, when the rolling rate defined by the following equation 2 is calculated, the first rolling and the second rolling are performed so that the first rolling rate / the second rolling rate is 5 or more, and The first rolling ratio is 22% to 35%, the second rolling ratio is 0.5% to 6%, [Equation 2] Rolling reduction (%)=[reduction in thickness of the slurry composition after rolling (μm) / thickness of the slurry composition before rolling (μm)]*100.

2. The negative electrode for a secondary battery according to claim 1, wherein The active material layer has a thickness of 50 μm to 400 μm.

3. The negative electrode for a secondary battery according to claim 1, wherein The metal current collector includes at least one selected from copper, stainless steel, aluminum, nickel and titanium.

4. A method for manufacturing the negative electrode for a secondary battery according to claim 1, the method comprising the following steps: applying a slurry composition comprising a negative electrode active material, a binder, a conductive material, and a solvent onto a metal current collector; and The slurry composition is rolled through the first step and the second step, Wherein, when the rolling rate defined by the following equation 2 is calculated, the first rolling and the second rolling are performed so that the first rolling rate / the second rolling rate is 5 or more: and The first rolling ratio is 22% to 35%, the second rolling ratio is 0.5% to 6%, [Equation 2] Rolling reduction (%)=[reduction in thickness of the slurry composition after rolling (μm) / thickness of the slurry composition before rolling (μm)]*100.

5. The method for manufacturing a negative electrode for a secondary battery according to claim 4, wherein The slurry composition is coated on a metal current collector to a thickness of 100 μm to 500 μm.

6. The method for manufacturing a negative electrode for a secondary battery according to claim 4, wherein After the first rolling step and the second rolling step, the method further includes drying the slurry composition to remove the solvent.

7. A lithium secondary battery comprising: positive electrode; The negative electrode according to claim 1; and A separator is interposed between the positive electrode and the negative electrode.

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