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

Abstract

Disclosed are a negative electrode (100) and a secondary battery including the same. The negative electrode (100) according to one embodiment includes: a negative electrode current collector (10); and a negative electrode mixture layer (20) disposed on at least one side of the negative electrode current collector, wherein the negative electrode mixture layer (20) contains a silicon-based active material, the negative electrode current collector includes a tab connecting portion (S X ), a shoulder portion (S Y ), and a rounded portion (111) between the tab connecting portion and the shoulder portion, the rounded portion (111) having a radius of curvature that satisfies an R value measured in mm within a range according to the following Formula 1.[Formula 1](p×A+q)−r<R≤(p×A+q)+rIn the Formula 1, A is a value corresponding to a weight ratio of the silicon-based active material with respect to the negative electrode mixture layer measured in weight%, p is a value of 0.05 to 0.2, q is a value of 0.1 to 1, and r is a value of 0.1 to 1.

Description

Technical Field

[0001] This invention 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. Background Technology

[0002] In recent years, with increasing attention to environmental issues, the demand for secondary batteries, which are mainly used as power sources for electric vehicles (EVs), portable electronic devices, and the like, is growing. Typically, a secondary battery includes a negative electrode, a positive electrode, and a separator between the negative and positive electrodes, with the negative electrode containing carbon-based active materials such as graphite.

[0003] In addition, in order to manufacture high-capacity negative electrodes, research has been actively underway in recent years to use silicon-based active materials, which have a higher discharge capacity than carbon-based active materials, as negative electrode active materials. However, silicon-based active materials have a larger volume expansion rate than carbon-based active materials, and the electrode may swollen or expand during continuous charging / discharging of lithium secondary batteries containing silicon-based active materials as the negative electrode.

[0004] When the electrode bulges or expands as described above, electrode deformation may occur not only in the thickness direction (Z-axis) of the negative electrode stacking, but also in the planar direction (X-axis and Y-axis) of the negative electrode mixture layer 20 on the negative electrode current collector 10 (see reference). Figure 1 In particular, in the case of the uncoated portion 11 where the negative electrode mixture layer 20 is not formed on the surface of the negative electrode tab 110 and the negative electrode current collector 10 (see [reference]). Figure 1 and Figures 2a to 2d When the negative electrode active material in the negative electrode mixture layer 20 expands, strong stress is generated at the interface between the part where the negative electrode mixture layer is formed and the part where the negative electrode mixture layer is not formed, which may cause the negative electrode current collector 10 to break.

[0005] In some specific implementations, this breakage can be reduced or prevented by increasing the thickness of the negative electrode current collector. However, in this case, the unit price increases with the increase in the thickness of the negative electrode current collector, and there are substantial difficulties in ensuring the energy density of the negative electrode due to the increase in battery weight and volume. Summary of the Invention

[0006] Technical problems to be solved

[0007] One specific embodiment aims to provide a negative electrode for a secondary battery that contains silicon-based active materials, thereby exhibiting excellent capacity characteristics and excellent lifespan characteristics, as the current collector does not break.

[0008] Another object of a specific embodiment is to provide a method for manufacturing a negative electrode, which can maintain the thin thickness of the negative electrode current collector and can effectively suppress the occurrence of problems such as breakage.

[0009] Technical solution

[0010] The negative electrode according to a specific embodiment includes: a negative electrode current collector; and a negative electrode mixture layer provided on at least one surface of the negative electrode current collector, wherein the negative electrode mixture layer contains a silicon-based active material, and the negative electrode current collector includes an ear connection part, a shoulder, and a circular part located between the ear connection part and the shoulder, and the circular part has a curvature radius satisfying the R value measured in mm within the range according to the following formula 1.

[0011] [Formula 1]

[0012] (p×A + q) - r < R ≤ (p×A + q) + r

[0013] In the formula 1, A is the value corresponding to the weight ratio of the silicon-based active material relative to the negative electrode mixture layer measured in wt%, p is a value from 0.05 to 0.2, q is a value from 0.1 to 1, and r is a value from 0.1 to 1.

[0014] In the formula 1, the p value may include 0.1.

[0015] In the formula 1, the q value may include 0.5.

[0016] In the formula 1, the r value may include 0.5.

[0017] The thickness of the negative electrode current collector may be 1 - 10 μm.

[0018] The content of the silicon-based active material in the negative electrode mixture layer may be 3 - 50 wt%.

[0019] The silicon-based active material may include at least one selected from Si, SiO x , SiO doped with Q or coated with Q x , Si-Q alloy and Si-C composite, where 0 < x < 2. At this time, Q may be an element selected from alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, Group 15 elements, Group 16 elements, transition metals, rare earth elements, and combinations thereof and not Si.

[0020] A method for manufacturing a negative electrode for a secondary battery according to a specific embodiment includes the following steps: preparing a first negative electrode and a second negative electrode, the first negative electrode including a first negative electrode current collector that breaks under the same charge / discharge conditions, and the second negative electrode including a second negative electrode current collector that does not break under the same charge / discharge conditions; and providing a third negative electrode, the third negative electrode including a third negative electrode current collector and having a lower PS value than the second negative electrode. The first, second, and third negative electrodes each include a negative electrode mixture layer disposed on at least one side of the first, second, and third negative electrode current collectors, the negative electrode mixture layer comprising a silicon-based active material, the first, second, and third negative electrode current collectors each including a tab connection portion, a shoulder portion, and a circular portion located between the tab connection portion and the shoulder portion, the PS value of the second and third negative electrodes being the maximum plastic strain value of the circular portion.

[0021] The thickness of the first negative electrode current collector can be less than the thickness of the second negative electrode current collector.

[0022] The thickness of the third negative electrode current collector can be less than the thickness of the second negative electrode current collector.

[0023] The third negative electrode may include a circular portion having a radius of curvature of R measured in mm within a range satisfying Equation 1 below.

[0024] [Formula 1]

[0025] (p×A+q)-r<R≤(p×A+q)+r

[0026] In Formula 1, A is the weight ratio of the silicon-based active material to the negative electrode mixture layer, measured in weight percent, p is a value of 0.05 to 0.2, q is a value of 0.1 to 1, and r is a value of 0.1 to 1.

[0027] The secondary battery according to one specific embodiment includes the negative electrode described in any of the above specific embodiments.

[0028] Beneficial effects

[0029] According to a specific implementation plan, by designing the shape of the negative electrode current collector that takes into account the content of silicon-based active materials, even if the content of silicon-based active materials in the negative electrode increases, problems such as current collector breakage will not actually occur during battery charging / discharging. Therefore, a negative electrode for secondary batteries with excellent capacity characteristics, durability, and lifespan characteristics can be provided.

[0030] According to another specific embodiment, a method for manufacturing a negative electrode for a secondary battery can be provided, wherein the method maintains a thin thickness of the negative electrode current collector containing silicon-based active material, thereby giving the negative electrode excellent energy density and ensuring economic efficiency. Attached Figure Description

[0031] Figure 1 This is a schematic cross-sectional view showing the shape of the negative electrode according to a specific implementation scheme, viewed from the side.

[0032] Figures 2a to 2d This is a schematic top view showing the shape of the negative electrode according to each specific implementation plan.

[0033] Figure 3 It is shown Figure 2a An enlarged view of the shape of the circular part in the image.

[0034] Figure 4 It is a flowchart of a method for manufacturing a negative electrode according to a specific implementation plan.

[0035] Figure 5 This is a top view of the shape of the negative electrode according to one implementation scheme.

[0036] Explanation of reference numerals in the attached figures

[0037] 100: Negative electrode for secondary batteries

[0038] 10: Negative electrode current collector

[0039] 11: Uncoated area

[0040] 12: Coating Section

[0041] 20: Negative electrode mixture layer

[0042] 110: Negative electrode tab

[0043] 111: Circular part

[0044] D: Thickness of the negative electrode current collector

[0045] L: Length of the coating section

[0046] S x : Electrode connection part

[0047] S y Shoulders

[0048] R: Radius of curvature

[0049] X_exp: First expansion direction

[0050] Y_exp: Second expansion direction Detailed Implementation

[0051] Hereinafter, preferred embodiments of the present invention will be described with reference to various specific embodiments. However, the embodiments can be modified in various different forms, and the scope of the present invention is not limited to the embodiments described below.

[0052] As mentioned above, increasing the thickness of the negative electrode current collector in a negative electrode containing silicon-based active materials to prevent breakage may make it difficult to ensure that the energy density and economic efficiency of the negative electrode reach excellent levels. According to a specific embodiment, a negative electrode for secondary batteries and its manufacturing method can be provided that substantially solves the problems described above even without increasing the thickness of the negative electrode current collector. The following refers to... Figures 1 to 5 The specific implementation plan will be explained.

[0053] Figure 1 This is a schematic cross-sectional view showing the shape of the negative electrode according to a specific implementation scheme, viewed from the side.

[0054] Figures 2a to 2d This is a schematic top view showing the shape of the negative electrode according to each specific implementation plan.

[0055] Figure 3 It is shown Figure 2a An enlarged view of the shape of the circular part in the image.

[0056] Figure 4 It is a flowchart of a method for manufacturing a negative electrode according to a specific implementation plan.

[0057] Figure 5 This is a top view of the shape of the negative electrode according to one implementation scheme.

[0058] Secondary batteries use negative terminals

[0059] According to one specific embodiment, the negative electrode 100 includes: a negative electrode current collector 10; and a negative electrode mixture layer 20, the negative electrode mixture layer being disposed on at least one side of the negative electrode current collector, wherein the negative electrode mixture layer 20 comprises a silicon-based active material, and the negative electrode current collector includes a tab connection portion S. X Shoulder S Y And a circular portion 111 located between the tab connection and the shoulder, the circular portion 111 having a radius of curvature satisfying an R value measured in mm within the range according to Formula 1 below.

[0060] [Formula 1]

[0061] (p×A+q)-r<R≤(p×A+q)+r

[0062] In Equation 1, all parameters p, A, q, r, and R are unitless values. The value of R represents the radius of curvature of the circular portion measured in millimeters (mm), the value of A is the weight ratio of the silicon-based active material to the negative electrode mixture layer measured in weight percent, p is a value of 0.05 to 0.2, q is a value of 0.1 to 1, and r is a value of 0.1 to 1.

[0063] The negative electrode 100 for the secondary battery includes: a negative electrode current collector 10; and a negative electrode mixture layer 20, the negative electrode mixture layer being disposed on at least one side of the negative electrode current collector. In this case, the negative electrode current collector may include: an uncoated portion 11 on its surface where no negative electrode mixture layer is formed; and a coated portion 12 on at least one side where a negative electrode mixture layer is formed.

[0064] As described above, in order to prevent the breakage of the negative electrode current collector 10 during continuous charging / discharging of the secondary battery, the thickness D of the negative electrode current collector can be increased. However, in a negative electrode including a relatively thick negative electrode current collector, it is difficult to ensure high energy density due to its volume and weight, and considering the increase in the unit price of the negative electrode current collector, it may be difficult to ensure economic efficiency.

[0065] On the other hand, in a lithium secondary battery according to a specific embodiment, the negative electrode current collector includes a circular portion 111 with an appropriate radius of curvature R, thus suppressing electrode deformation and breakage of the negative electrode current collector even without increasing its thickness. (See below for reference.) Figures 2a to 2d The radius of curvature R and the circular portion 111 will be described in more detail.

[0066] The negative electrode current collector 10 includes an electrode tab connection part S. X Shoulder S Y And a circular portion 111 located between the tab connection and the shoulder, the circular portion 111 having a radius of curvature that satisfies the R value according to Equation 1.

[0067] The negative electrode 100 for the secondary battery can have the following structure: a structure formed in which the negative electrode mixture layer 20 contacts the boundary of the circular portion 111 (see reference). Figure 2a The structure formed in which the negative electrode mixture layer 20 includes a portion of the circular portion 111 (not shown); or the structure formed in which the negative electrode mixture layer 20 includes all of the circular portion 111 (see reference). Figure 2b ).

[0068] To form a negative electrode tab 110 on the uncoated portion 11 of the negative electrode mixture layer 20 in the negative electrode current collector 10, a portion of the negative electrode current collector 10 can be cut using methods such as laser cutting. In this case, the negative electrode tab 110 can be formed to include: a tab connection portion S. X The electrode connecting part S X This refers to the part connected to the electrode leads via subsequent welding, etc.; and the shoulder S. Y The shoulder S Y It is the boundary portion between the negative electrode mixture layer 20 and the outside.

[0069] When the negative electrode mixture layer 20 and the negative electrode tab 110 are respectively set as the lower part and the upper part, the tab connection part S X It can be a straight section located on one side of the negative electrode tab 110, with reference to the direction from the negative electrode mixture layer 20 toward the negative electrode tab 110 (for example, see reference). Figure 2a ).

[0070] The shoulder S Y It can be located at the boundary between the negative electrode mixture layer 20 and the outside, with the surface where the negative electrode tab 110 is located in the negative electrode current collector 10 as the reference, and connected to the tab portion S. X Adjacent straight sections (refer to) Figure 2a wait).

[0071] The electrode connection part S X and shoulder S Y It can be connected via the circular portion 111. Specifically, in the electrode connecting portion S X There may be a point X between the circular portion 111 and the point where the shape changes from a straight line to a curved line, i.e., a first point X, and in the shoulder portion S Y Between the circular part 111 and the point where the shape changes from a straight line to a curved line, i.e., the second point Y (see reference). Figure 3 ).

[0072] The negative electrode current collector 10 may include circular portions 111 on one side and the other side respectively. That is, according to a specific embodiment, the negative electrode for a secondary battery may have the following structure: one side of the negative electrode current collector 10 includes a first tab connection portion S. X First shoulder S Y And a first circular portion 111 located between the first electrode tab connection portion and the first shoulder portion, and a second electrode tab connection portion S on the other side of the negative electrode current collector 10. X '、Second shoulder S Y 'and the second circular portion 111 located between the second pole ear connection and the second shoulder' (see reference) Figure 2c wait).

[0073] As described above, during the formation of the negative electrode current collector 10 including a circular portion 111, the circular portion can be formed such that its radius of curvature R satisfies the condition of Equation 1. In this case, the radius of curvature R of the circular portion can be measured with reference to the circular portion between the first point X and the second point Y. Furthermore, when the negative electrode current collector 10 has a structure including a circular portion 111 on one or the other side (see...), Figure 2c The radius of curvature R of the circular portion can be measured with reference to either the first circular portion 111 between the first point X and the second point Y, or the second circular portion 111' between the first point X' and the second point Y'.

[0074] When the radius of curvature R of the circular portion 111 satisfies the condition of Equation 1, even if the silicon-based active material contained in the negative electrode mixture layer 20 expands and causes the negative electrode to bulge, damage to the negative electrode can be prevented by dispersing the stress on the negative electrode current collector. When the radius of curvature R of the circular portion 111 is too small, it is practically difficult to disperse the stress applied to the negative electrode current collector and prevent breakage. On the other hand, when the radius of curvature R of the circular portion 111 is too large, it is relatively difficult to design and manufacture the shape of the uncoated portion with an excessively large curved portion.

[0075] Specifically, the conditions in Formula 1 are set considering the weight ratio (A) of the silicon-based active material to the negative electrode mixture layer. Therefore, when adjusting the radius of curvature R of the circular portion 111, the shape of the negative electrode current collector 10 can be appropriately controlled according to the weight ratio of the silicon-based active material. Thus, even if the negative electrode mixture layer 20 contains a high content of silicon-based active material, damage to the negative electrode can be prevented without substantially increasing the thickness of the negative electrode current collector 10.

[0076] In Equation 1, p is the silicon-based active material content coefficient, and p has a value from 0.05 to 0.2; q is the silicon-based active material content constant, and q has a value from 0.1 to 1; and r is a correction value, and r has a value from 0.1 to 1. Specifically, the value of p can include 0.1, the value of q can include 0.5, and the value of r can include 0.5. When the values ​​of p, q, and r are adjusted to the above ranges, the radius of curvature R of the circular portion 111 can be appropriately controlled, taking into account the weight ratio of the silicon-based active material to the negative electrode mixture layer 20. Therefore, when designing a negative electrode containing silicon-based active material, excellent levels of energy density, lifetime characteristics, etc., can be ensured.

[0077] Exemplarily, when the weight ratio of the silicon-based active material with respect to the negative electrode mixture layer 20 is 10 wt% (i.e., A is 10), p is 0.1, q is 0.5, and r is 0.5, the value of the radius of curvature R of the circular portion 111 can exceed 1.0, be more than 1.5, and can be 2.0 or less.

[0078] In the negative electrode 100 for the secondary battery, considering the weight ratio of the silicon-based active material with respect to the negative electrode mixture layer 20, the negative electrode current collector 10 having an appropriate shape is formed. Therefore, when charging / discharging the lithium secondary battery including the negative electrode, the stress applied to the negative electrode current collector 10 caused by the expansion of the silicon-based active material is dispersed, so that its influence can be minimized. Therefore, even if the secondary battery undergoes a continuous charging / discharging process, no substantial breakage occurs in the negative electrode current collector, and the life characteristics of the battery, etc., can be improved to an excellent level.

[0079] The thickness of the negative electrode current collector 10 can be 1 - 10 μm. Specifically, the thickness of the negative electrode current collector 10 can be 2 μm or more, 4 μm or more, and can be 8 μm or less, 6 μm or less. When the thickness of the negative electrode current collector 10 is too thin, problems may occur in terms of durability, etc. When the thickness of the negative electrode current collector 10 is too thick, it may be difficult to ensure that the energy density of the negative electrode, etc., reaches an excellent level. Therefore, when the thickness of the negative electrode current collector 10 is within the above range, both durability and energy density, etc., can be improved to an excellent level.

[0080] The negative electrode current collector 10 can be at least one selected from copper foil (Foil), nickel foil, stainless steel foil, titanium foil, foam nickel, foam copper, a polymer substrate coated with a conductive metal, and combinations thereof. Specifically, the negative electrode current collector 10 can be copper foil.

[0081] The weight ratio of the silicon-based active material with respect to the negative electrode mixture layer 20 can be 3 - 50 wt%. The weight ratio of the silicon-based active material can be a value calculated based on the solid content. Specifically, the weight ratio of the silicon-based active material with respect to the negative electrode mixture layer 20 can be 5 wt% or more, 10 wt% or more, and can be 40 wt% or less, 30 wt% or less. When the weight ratio of the silicon-based active material with respect to the negative electrode mixture layer 20 is within the above range, the influence caused by the expansion of the negative electrode during charging / discharging can be minimized, and an excellent level of energy density can be ensured.

[0082] The silicon-based active material can include Si, SiO x (0 < x < 2), SiO doped with Q or coated with Q xAt least one of (0 < x < 2), Si-Q alloy, and Si-C composite. At this time, Q is an element selected from alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, Group 15 elements, Group 16 elements, transition metals, rare earth elements, and combinations thereof and is not Si.

[0083] The negative electrode mixture layer 20 may further include a carbon-based active material. Exemplarily, the carbon-based active material may be one or more selected from artificial graphite, natural graphite, hard carbon, soft carbon, carbon black, acetylene black, Ketjen black, Super P, graphene, and fibrous carbon, but is not limited thereto. When the negative electrode mixture layer 20 further includes a carbon-based active material, the weight ratio of the carbon-based active material may be 50-97 wt%. The weight ratio of the carbon-based active material may be a value calculated based on the solid content.

[0084] The negative electrode mixture layer 20 may further include a conductive material. The conductive material is used to impart conductivity to the electrode and maintain the structure of the electrode, etc., and a material that does not cause side reactions with other elements of the secondary battery and has conductivity may be used. Exemplarily, the conductive material may be one or more selected from the following: graphite such as natural graphite or artificial graphite; carbon-based substances such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking carbon black, and carbon fiber; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive polymers such as polyphenyl derivatives, but is not limited thereto. When the negative electrode mixture layer 20 further includes a conductive material, the weight ratio of the conductive material may be 0.1-10 wt%. The weight ratio of the conductive material may be a value calculated based on the solid content.

[0085] The negative electrode mixture layer 20 may further comprise an adhesive. The adhesive is not particularly limited, as long as it is a compound that serves to ensure good adhesion between the structures in the negative electrode mixture layer and good adhesion of the negative electrode mixture layer to the current collector. Exemplarily, the adhesive may be at least one rubber-based adhesive selected from styrene-butadiene rubber (SBR), fluoropolymer, ethylene propylene rubber, butyl acrylate rubber, butadiene rubber, isoprene rubber, acrylonitrile rubber, acrylic rubber, and silane-based rubber; cellulose-based adhesives such as carboxymethyl cellulose (CMC), hydroxypropyl methylcellulose, methylcellulose, or their alkali metal salts; water-soluble polymer-based adhesives such as polyacrylic acid (PAA)-based adhesives, polyvinyl alcohol (PVA)-based adhesives, and polyvinyl alcohol-polyacrylic acid copolymer (PVA-PAA Copolymer)-based adhesives; and combinations thereof. When the negative electrode mixture layer 20 further comprises an adhesive, the weight percentage of the adhesive may be 0.1-10% by weight. The weight percentage of the adhesive may be a value calculated based on the solids content.

[0086] Method for manufacturing negative electrodes for secondary batteries

[0087] A method for manufacturing a negative electrode according to one specific embodiment includes the following steps: preparing a first negative electrode and a second negative electrode, the first negative electrode including a first negative electrode current collector that breaks under the same charge / discharge conditions, and the second negative electrode including a second negative electrode current collector that does not break under the same charge / discharge conditions; and setting a third negative electrode, the third negative electrode including a third negative electrode current collector and having a lower PS value than the second negative electrode.

[0088] The PS value is a parameter used to predict and prevent current collector fracture by measuring the degree of deformation experienced by the current collector during the volume expansion of the negative electrode mixture layer, and it can represent the maximum plastic strain value of the circular portion. Therefore, when the PS value is large, the impact applied to the current collector is large, thus the probability of fracture is high; when the PS value is small, the probability of fracture is low. Therefore, it is preferable to reduce the PS value.

[0089] The first negative electrode, the second negative electrode, and the third negative electrode each include a negative electrode mixture layer disposed on at least one side of the first negative electrode current collector, the second negative electrode current collector, and the third negative electrode current collector. The negative electrode mixture layer contains a silicon-based active material. The first negative electrode current collector, the second negative electrode current collector, and the third negative electrode current collector each include a tab connection portion S. X Shoulder S YAnd a circular portion 111 located between the tab connection and the shoulder, wherein the PS values ​​of the second and third negative electrodes are the maximum plastic strain values ​​of the circular portion 111, which have different plastic strain values ​​according to their positions. By comparing and analyzing the PS values ​​of each negative electrode, the degree of deformation of the negative electrode current collector 10 when the negative electrode mixture layer 20 expands can be measured, and the breakage of the negative electrode current collector 10 can be predicted and prevented.

[0090] The detailed descriptions of the negative electrode current collector, negative electrode mixture layer, silicon-based active material, etc. of the first negative electrode, second negative electrode, and third negative electrode are the same as the descriptions of the negative electrode current collector 10, negative electrode mixture layer 20, silicon-based active material, etc. of the negative electrode 100 for secondary batteries described above, so the relevant descriptions are omitted.

[0091] According to a specific implementation, when designing a negative electrode (third negative electrode) that takes into account the characteristics of a negative electrode (first negative electrode) where the negative current collector actually breaks under the same charging / discharging conditions and a negative electrode (second negative electrode) where the negative current collector does not break, a method for manufacturing a negative electrode for secondary batteries with excellent energy density and lifetime characteristics can be provided.

[0092] Specifically, the method for manufacturing the negative electrode for a secondary battery may include: step S1, preparing a first negative electrode and a second negative electrode, wherein the first negative electrode includes a first negative electrode current collector that breaks under the same charge / discharge conditions, and the second negative electrode includes a second negative electrode current collector that does not break under the same charge / discharge conditions. Exemplarily, the charge / discharge conditions may be conditions that repeatedly charge and discharge a lithium secondary battery including the negative electrode 100 at 1 / 3C within a SOC range of 0 to 100% at 25°C more than once. Whether the negative electrode current collector 10 breaks can be evaluated by visually observing whether the edge portion (grid area) of the tab weld of the negative electrode current collector breaks.

[0093] In the method for manufacturing a negative electrode for a secondary battery, after step S1, a step S2 may be included to measure the PS value of the second negative electrode. Specifically, step S2 is a step of measuring the maximum plastic strain (PS) value of the circular portion of the negative electrode current collector in the second negative electrode where the negative electrode current collector has not broken during charging / discharging. Plastic strain is a numerical value expressed as a percentage (%) of the degree of material deformation when plastic deformation occurs. Plastic deformation is a permanent deformation that does not return to its original shape even after the external force is removed from the material to which the external force was applied. The plastic strain value of the circular portion can be obtained by measuring the first point X (located at the tab connection S). XThe point between the circular portion 111 and the point where the shape changes from a straight line to a curved line) and the second point Y (located in the shoulder portion S) Y The uncoated portion 11 is measured and calculated by pulling on the curved portion (between the circular portion 111 and the point where the straight shape changes to a curved shape) after fixing the negative electrode in a tensile strength measuring instrument.

[0094] The stress experienced by the circular portion of the negative electrode when it expands at a certain rate due to the silicon-based active material in the negative electrode mixture layer can be quantitatively evaluated by the maximum plastic strain value of the circular portion. A higher maximum plastic strain value indicates greater stress on the circular portion, and a higher likelihood of current collector breakage during charging / discharging of the negative electrode in a manufactured secondary battery. Therefore, designing the negative electrode to have a value lower than the maximum plastic strain value of the circular portion in a negative electrode where current collector breakage actually occurs during charging / discharging can substantially prevent current collector breakage under the same charging / discharging conditions.

[0095] Relatedly, the method for manufacturing a negative electrode for a secondary battery may include: step S3, providing a third negative electrode having a lower PS value than the second negative electrode. Specifically, step S3 is the step of providing a third negative electrode, wherein the maximum plastic strain (PS) value of the circular portion of the negative electrode current collector of the third negative electrode is relatively low compared to the second negative electrode, which did not break during charging / discharging. The negative electrode manufactured by the method for manufacturing a negative electrode for a secondary battery according to a specific embodiment may be the third negative electrode.

[0096] The third negative electrode can be manufactured under the condition that the expansion rate of the third negative electrode in the planar direction is greater than the expansion rate of the second negative electrode in the planar direction. In this case, the expansion rate in the planar direction can be calculated according to the following formula 2, with the major axis (X-axis) and minor axis (Y-axis) in the planar direction as references respectively.

[0097] [Equation 2]

[0098] Expansion rate (%) = (T2 - T1) / T1 × 100

[0099] In Equation 2, T1 is the thickness of the electrode in the planar direction in the discharged state (SOC is 0%) before charging, and T2 is the thickness of the electrode in the planar direction in the charged state (SOC is 100%).

[0100] The PS value ratio of the third negative electrode to the second negative electrode can be greater than 0.1 and less than 1, and can be between 0.3 and 0.8. Specifically, when the negative electrode mixture layer expands by 1% in both the lateral and longitudinal (X-axis and Y-axis) directions, the PS value ratio of the third negative electrode to the second negative electrode can be greater than 0.1 and less than 1, and can be between 0.5 and 0.8. When the negative electrode mixture layer expands by 1.5% in both the lateral and longitudinal (X-axis and Y-axis) directions, the PS value ratio of the third negative electrode to the second negative electrode can be greater than 0.1 and less than 1, and can be between 0.4 and 0.6.

[0101] The PS value of the third negative electrode can be 0.1-20%. Specifically, when the negative electrode mixture layer expands by 1% in both the lateral and longitudinal directions (X-axis and Y-axis), the PS value of the third negative electrode can be less than 10%, less than 6%, and more than 1%. Furthermore, when the negative electrode mixture layer expands by 1.5% in both the lateral and longitudinal directions (X-axis and Y-axis), the PS value of the third negative electrode can be less than 20%, less than 10%, and more than 1%.

[0102] In step S3, the method of setting the PS value of the third negative electrode to be lower than that of the second negative electrode is not particularly limited, but it can be applied to methods such as appropriately adjusting the radius of curvature R of the circular portion 111 during the process of forming the negative electrode tab 110 by cutting a portion of the uncoated portion 11 of the negative electrode current collector 10. The circular portion 111, etc., will be described in detail below.

[0103] Furthermore, the thickness of the negative current collector of the first negative electrode can be less than the thickness of the negative current collector of the second negative electrode. As mentioned above, the thicker the negative current collector, the better it can suppress stress-induced fracture caused by the expansion of the silicon-based active material. Therefore, under the same charge / discharge conditions, the thickness of the negative current collector of the first negative electrode where the negative current collector fractures can be less than the thickness of the negative current collector of the second negative electrode where the negative current collector does not fracture. However, as the thickness of the negative current collector 10 increases, there are limitations in improving the energy density of the negative electrode 100. Therefore, in the case of a second negative electrode including a relatively thick negative current collector, not only is the energy density relatively poor, but the economic efficiency may also be poor due to issues such as unit price.

[0104] Relatedly, the thickness of the negative electrode current collector of the third negative electrode can be less than the thickness of the negative electrode current collector of the second negative electrode. Specifically, although the third negative electrode has a lower PS value than the second negative electrode (which has a relatively thicker negative electrode current collector than the first negative electrode without breakage in the current collector), the thickness of the negative electrode current collector of the third negative electrode can be less than the thickness of the negative electrode current collector of the second negative electrode. This is because, when the third negative electrode is designed by appropriately shaping the radius of curvature R of the circular portion included in the negative electrode current collector, the PS value of the third negative electrode can be relatively reduced even without increasing the thickness of the negative electrode current collector. Therefore, when the thickness of the negative electrode current collector of the third negative electrode is less than the thickness of the negative electrode current collector of the second negative electrode, the resulting negative electrode can not only have superior durability and lifespan characteristics, but also superior energy density and economy.

[0105] The third negative electrode may include a circular portion having a radius of curvature that satisfies an R value measured in mm within the range according to Formula 1 below.

[0106] [Formula 1]

[0107] (p×A+q)-r<R≤(p×A+q)+r

[0108] In Equation 1, all parameters p, A, q, r, and R are unitless values. The value of R represents the radius of curvature of the circular portion measured in millimeters (mm), the value of A is the weight ratio of the silicon-based active material to the negative electrode mixture layer measured in weight percent, p is a value of 0.05 to 0.2, q is a value of 0.1 to 1, and r is a value of 0.1 to 1.

[0109] By appropriately shaping the circular portion included in the negative electrode current collector, the third negative electrode can have a relatively low PS value, and the circular portion of the third negative electrode can have a radius of curvature R value that satisfies the condition of Equation 1. That is, the above-described negative electrode 100 for a secondary battery can be manufactured by the method for manufacturing a negative electrode for a secondary battery. The detailed description of the radius of curvature R, Equation 1, etc., is repeated in the above description, so related descriptions are omitted.

[0110] The method for manufacturing the negative electrode 100 for a secondary battery may further include step S4 of manufacturing the third negative electrode according to the above design. The manufacturing method is not particularly limited as long as the third negative electrode can be manufactured according to the above design. For example, the third negative electrode can be manufactured by coating a first negative electrode slurry containing a first solvent, a first carbon-based active material, a first silicon-based active material, a first binder, and a first conductive material onto a negative electrode current collector such as copper foil (Cu-Foil) using methods such as rod coating, casting, or spraying, and drying it at 70-100°C to form a negative electrode mixture layer. At this time, the negative electrode mixture layer can be formed into a multilayer structure according to additional designs, etc. In this case, the third negative electrode can be manufactured by coating a second negative electrode slurry containing a second solvent, a second carbon-based active material, a second silicon-based active material, a second binder, and a second conductive material onto the negative electrode mixture layer using methods such as rod coating, casting, or spraying, and drying it at 70-100°C to form a second negative electrode mixture layer.

[0111] The solvent may be, for example, dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, provided that the amount of solvent used is sufficient to dissolve or disperse the active material, conductive material, and binder, and has a viscosity that exhibits excellent thickness uniformity when subsequently used for coating to form the negative electrode mixture layer, taking into account the coating thickness and manufacturing yield of the composition used to form the negative electrode mixture layer.

[0112] Furthermore, the third negative electrode formed on the negative electrode current collector 10 by the above method includes an uncoated portion 11 on the surface where no negative electrode mixture layer is formed, and the uncoated portion can be appropriately formed into a circular portion 111 during the process of forming the negative electrode tab 110 by notching a portion of its area. At this time, the circular portion 111 can be formed to have a radius of curvature R value that satisfies the condition of Equation 1.

[0113] Lithium secondary batteries

[0114] A lithium secondary battery according to one specific embodiment includes the aforementioned negative electrode 100 for a secondary battery. Specifically, the lithium secondary battery may include the aforementioned negative electrode, positive electrode, and a separator between the positive and negative electrodes.

[0115] The active material of the positive electrode is not particularly limited, but may be a lithium transition metal oxide such as lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), lithium nickel oxide (LiNiO2); a lithium transition metal composite oxide in which a part of the transition metal is replaced by other transition metals; and combinations thereof. Specifically, the positive electrode may contain at least one selected from the following as the active material: lithium nickel manganese oxide (LNMO) represented by chemical formulas such as LiNi x Mn 2-x O4(0 < x < 1), etc.; lithium-rich manganese oxide (LMR) represented by chemical formulas such as Li 1+x Mn 2-x O4(0 < x < 0.1), etc.; nickel cobalt manganese oxide (NCM) represented by chemical formulas such as Li x Ni a Co b Mn c O y (0 < x ≤ 1.1, 2 ≤ y ≤ 2.02, 0 < a < 1, 0 < b < 1, 0 < c < 1, 0 < a + b + c ≤ 1), etc.; and lithium iron phosphate oxide (LFP) represented by chemical formulas such as LiFePO4.

[0116] The separator can be implemented in various ways to be suitable for a lithium secondary battery. Exemplarily, the separator may include a porous substrate, and the porous substrate may be a polyolefin-based porous substrate. The polyolefin-based porous substrate may be a substrate having multiple pores and commonly used in electrochemical devices. The polyolefin-based porous substrate may be exemplarily selected from a polyethylene single layer film, a polypropylene single layer film, a polyethylene / polypropylene bilayer film, a polypropylene / polyethylene / polypropylene trilayer film, and a polyethylene / polypropylene / polyethylene trilayer film, but is not limited thereto.

[0117] When the lithium secondary battery includes the above-described negative electrode 100 for a secondary battery, a lithium secondary battery excellent in energy density, capacity characteristics, life characteristics, etc. can be provided by including a negative electrode that appropriately adjusts the shape of the uncoated portion according to the weight ratio of the silicon-based active material to the electrode.

[0118] Examples

[0119] 1. Design of the negative electrode for secondary batteries

[0120] (1) Comparative Examples and Reference Examples

[0121] A comparative example of a negative electrode (first negative electrode) including a negative electrode current collector (copper foil) was modeled. The negative electrode current collector includes: a coated portion on one surface of which a negative electrode mixture layer is formed. Based on the solid content, the negative electrode mixture layer contains 10 wt% of a silicon-based active material (SiO x ; 0 < x < 2); and an uncoated portion, which is the area other than the coated portion (see Figure 5 ). At this time, the thickness D of the negative electrode current collector of the comparative example is 6 μm. The negative electrode current collector includes a circular portion with a radius of curvature R value of 1.0 mm. For the coated portion, only a part of 95 mm out of the total length of 262 mm was modeled as the length L of the coated portion. In addition, a reference example of a negative electrode (second negative electrode) was modeled with the same design as the negative electrode of the comparative example, except that the thickness D of the negative electrode current collector was set to 8 μm.

[0122] (2) Example 1 and Example 2

[0123] The negative electrodes of Example 1 and Example 2 (third negative electrodes) were modeled with the same design as the negative electrode of the comparative example, except that the radius of curvature R values of the circular portions were set to 1.5 mm and 2.0 mm, respectively.

[0124] (3) Whether Equation 1 is satisfied

[0125] It was evaluated whether the radius of curvature values of the circular portions in the designed negative electrodes satisfy the conditions of the following Equation 1. At this time, in Equation 1, p was set to 0.1, q was set to 0.5, and r was set to 0.5. The value of (p×A + q) ± r was calculated according to the conditions of Equation 1 and shown in Table 1 below. In addition, when the conditions of Equation 1 are satisfied, it is indicated as ○, and when the conditions of Equation 1 are not satisfied, it is indicated as X, and the results are shown in Table 1 below.

[0126] [Equation 1]

[0127] (p×A + q) - r < R ≤ (p×A + q) + r

[0128] In Equation 1, all parameters p, A, q, r, and R are dimensionless values. The value of R represents the radius of curvature value of the circular portion measured in millimeters (mm). The value of A is the value corresponding to the weight ratio of the silicon-based active material to the negative electrode mixture layer measured in wt%. p is a value from 0.05 to 0.2, q is a value from 0.1 to 1, and r is a value from 0.1 to 1.

[0129] (4) Symmetry conditions in different directions and PS values under different expansion rates

[0130] Each modeled negative pole is set to have a symmetry condition in both the horizontal and vertical directions (X-axis and Y-axis) (see reference). Figure 5 At this point, considering that each negative electrode is stacked in a real secondary battery, the thickness direction (Z-axis) is also set to have symmetry.

[0131] Furthermore, considering that the negative electrode mixture layer containing silicon-based active material expands during the charging / discharging process of the battery, an expansion rate value is input to cause the negative electrode mixture layer in each modeled negative electrode to expand by 1% in both the lateral and longitudinal directions (X-axis and Y-axis), and under the same conditions, an expansion rate value is input to cause the negative electrode mixture layer in each modeled negative electrode to expand by 1.5% in both the lateral and longitudinal directions (X-axis and Y-axis). The PS value based on each expansion rate is shown in Table 1 below. In the case of Example 1, the PS value when the expansion rate value is input under the same conditions to cause the negative electrode mixture layer in the negative electrode to expand by 1.3% in both the lateral and longitudinal directions (X-axis and Y-axis) is also shown in Table 1 below.

[0132] 2. Evaluation of negative electrodes for secondary batteries

[0133] (1) Manufacturing of the negative electrode

[0134] To evaluate whether the negative electrode current collector actually fractured and expanded during charging / discharging, negative electrodes for the comparative and reference examples were actually manufactured. In each of the manufactured negative electrodes, based on solid content, the weight ratio of carbon-based active material (artificial graphite) to the negative electrode mixture layer was 87 wt%, the weight ratio of silicon-based active material was 10 wt%, the weight ratio of binders (SBR and CMC) was 1.5 wt%, and the weight ratio of conductive material (CNT) was 1.5 wt%.

[0135] Next, the secondary battery unit is placed in a pouch for secondary batteries, and then an electrolyte containing 1M LiPF6 dissolved in a solvent of mixed ethylene carbonate (EC) and diethyl carbonate (DEC) is injected into the pouch for secondary batteries and sealed to manufacture a pouch-type lithium secondary battery. The secondary battery unit is manufactured by inserting a polyolefin separator between a positive electrode having a layer of positive electrode mixture containing NCM-based positive electrode active material formed on a positive electrode current collector (aluminum foil (Al-Foil)) and a negative electrode manufactured as described above.

[0136] (2) Whether fracture and expansion rate occurred

[0137] The prepared secondary battery was repeatedly charged and discharged at 1 / 3C within a SOC range of 0 to 100% at 25°C more than once. Then, the edge portion (grid area) of the negative electrode current collector was visually evaluated for breakage. When breakage occurred, it was indicated by ○, and when no breakage occurred, it was indicated by X. The results are shown in Table 1 below.

[0138] Furthermore, for the prepared secondary battery, the cycle of charging at 1 / 3C and discharging at 1 / 3C within the range of SOC from 0 to 100% was repeated once or more at 25°C. Then, the expansion rate of the negative electrode in the planar direction was measured according to the following formula 2, and the expansion rates of the major axis (X-axis) and minor axis (Y-axis) in the planar direction were 1%.

[0139] [Equation 2]

[0140] Expansion rate (%) = (T2 - T1) / T1 × 100

[0141] In Equation 2, T1 is the thickness of the electrode in the planar direction in the discharged state (SOC is 0%) before charging, and T2 is the thickness of the electrode in the planar direction in the charged state (SOC is 100%).

[0142] Furthermore, the plastic strain value of the circular portion and the corresponding PS value can be obtained by examining the first point X (located in the tab connection portion S) in the negative electrode manufactured as described above. X The point between the circular part 111 and the point where the shape changes from a straight line to a curved line) and the second point Y (located in the shoulder S) Y The curved portion between the circular portion 111 and the point where the straight shape changes to a curved shape is measured and calculated by pulling the uncoated portion 11 after fixing the negative electrode in the tensile strength measuring instrument.

[0143] [Table 1]

[0144]

[0145]

[0146] Referring to Table 1, in the case of the reference example's negative electrode where the thickness D of the negative electrode current collector was designed to be relatively larger compared to the comparative example, it can be confirmed that even with the same input expansion rate, the maximum plastic strain value of the circular portion is relatively low. Furthermore, in the actual manufacturing of the negative electrodes of the comparative and reference examples and the evaluation of whether fracture occurred during charging / discharging under the same conditions, even with the same actual expansion rate, unlike the negative electrode of the comparative example which fractured, the negative electrode of the reference example did not fracture. Considering this, when the radius of curvature of the circular portion is the same, if the thickness of the negative electrode current collector is designed to be relatively large, it is determined that fracture of the negative electrode current collector can be suppressed by reducing the plastic strain of the circular portion during charging / discharging.

[0147] Furthermore, under the same expansion rate, when the maximum plastic strain value of the circular portion is controlled to be lower than that of Examples 1 and 2 (comparative and reference examples), it can be confirmed that, unlike the comparative and reference examples, the radius of curvature value of the circular portion satisfies the condition of Equation 1. Considering this, even if the thickness of the negative electrode current collector is not designed to be relatively large as in the reference example, when the radius of curvature R is appropriately designed considering the maximum plastic strain value of the circular portion, it is determined that the influence of the stress caused by the expansion of the negative electrode can be minimized, and the fracture of the negative electrode current collector can be effectively suppressed.

[0148] Relatedly, considering the experimental results regarding whether fracture occurred in the negative electrodes actually manufactured according to the designs of the comparative and reference examples, the maximum plastic strain values ​​of the negative electrodes of Examples 1 and 2 were lower than those of the reference example at the same expansion rate (1%). Furthermore, in the case of Example 1, even with an expansion rate of 1.3%, the maximum plastic strain value was relatively low compared to the comparative example using an expansion rate of 1%. In the case of Example 2, even with an expansion rate of 1.5%, the maximum plastic strain value was relatively low compared to both the comparative and reference examples using an expansion rate of 1%. Taking this into account, it was determined that even when an expansion rate with the same or higher value in the planar direction as that of existing negative electrodes is applied during charging / discharging of the negative electrodes manufactured according to the designs of Examples 1 and 2, fracture of the negative electrode current collector would be effectively suppressed.

[0149] Therefore, when the negative electrode current collector is designed as a circular portion having a radius of curvature R value that satisfies the conditions of Equation 1, as shown in Examples 1 and 2, the thickness of the negative electrode current collector can be maintained, and the breakage of the negative electrode current collector can be substantially mitigated during charging / discharging. Therefore, it is judged to have excellent energy density, economy, lifespan characteristics, etc.

[0150] The embodiments have been described in detail above, but the implementation methods are not limited thereto. Various modifications and variations can be made without departing from the technical concept described in the claims, which will be obvious to those skilled in the art.

Claims

1. A negative electrode comprising: a negative electrode current collector; and a negative electrode mixture layer provided on at least one side of the negative electrode current collector, wherein the negative electrode mixture layer contains a silicon-based active material, the negative electrode current collector includes a tab connecting portion, a shoulder portion, and a round portion between the tab connecting portion and the shoulder portion, the round portion has a radius of curvature that satisfies an R value measured in mm in a range according to the following formula 1, [Formula 1] (p x A + q) - r < R ≤ (p x A + q) + r in the formula 1, A is a value corresponding to a weight ratio of the silicon-based active material with respect to the negative electrode mixture layer measured in weight %, p is 0.1, q is 0.5, and r is 0.

5.

2. The negative electrode according to claim 1, wherein the thickness of the negative electrode current collector is 1 to 10 pm.

3. The negative electrode according to claim 1, wherein the content of the silicon-based active material in the negative electrode mixture layer is 3 to 50 weight %.

4. The negative electrode according to claim 1, wherein The silicon-based active substance comprises at least one of Si, SiO x , SiO doped with Q or coated with Q x , Si-Q alloys and Si-C composites, wherein 0 < x < 2, Q is an element selected from the group consisting of alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, Group 15 elements, Group 16 elements, transition metals, rare earth elements, and combinations thereof, and is not Si.

5. A secondary battery comprising a negative electrode, The negative electrode comprises: a negative electrode current collector; and a negative electrode mixture layer provided on at least one side of the negative electrode current collector, the negative electrode mixture layer contains a silicon-based active material, the negative electrode current collector includes a tab connecting portion, a shoulder portion, and a round portion between the tab connecting portion and the shoulder portion, the round portion has a radius of curvature that satisfies an R value measured in mm in a range according to the following formula 1, [Formula 1] (p x A + q) - r < R ≤ (p x A + q) + r in the formula 1, A is a value corresponding to a weight ratio of the silicon-based active material with respect to the negative electrode mixture layer measured in weight %, p is 0.1, q is 0.5, and r is 0.5.

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