Anode for secondary battery, method for producing same, and secondary battery including same

Abstract

The present invention relates to a negative electrode for a secondary battery, which has a negative electrode active material layer formed on at least one surface of a current collector, and which satisfies at least one of the following conditions 1 and 2: [Condition 1] (the continuous adhesive strength in the thickness direction of the negative electrode for a secondary battery is 30 gf / 20 mm or more), and [Condition 2] (the breaking stress is 3.6 N or more), and the breaking stress is a value measured by pressing the negative electrode active material layer at a speed of 10 μm / s with a 2.5 mm wide, straight-edge-shaped tip with one sharp end.

Description

[Technical Field]

[0001] [CROSS-REFERENCE TO RELATED APPLICATIONS] This application claims the benefit of priority based on Korean Patent Application No. 10-2021-0112407 dated August 25, 2021, and all contents disclosed in the documents of that Korean patent application are incorporated herein by reference.

[0002] The present invention relates to a negative electrode for a secondary battery, a method for producing the same, and a secondary battery including the same. [Background technology]

[0003] Due to technological development and increasing demand for mobile devices, the demand for secondary batteries as an energy source is rapidly increasing. Among these secondary batteries, lithium secondary batteries, which exhibit high energy density and working potential, long cycle life, and low self-discharge rate, have been commercialized and are widely used.

[0004] Recently, with growing interest in environmental issues, much research is being conducted into electric vehicles (EVs) and hybrid electric vehicles (HEVs) as alternatives to vehicles that use fossil fuels such as gasoline and diesel, which are one of the main causes of air pollution. Lithium secondary batteries, which have high energy density, high discharge voltage, and stable output, are primarily being researched and used as the power source for such electric vehicles (EVs) and hybrid electric vehicles (HEVs).

[0005] Such lithium secondary batteries are generally manufactured by stacking or winding positive and negative electrodes with a separator interposed therebetween, and then housing the stacked or wound electrodes together with an electrolyte in a secondary battery case.

[0006] In this case, the negative electrode is manufactured by applying an active material slurry containing a negative electrode active material to a metal current collector, drying and rolling it, and cutting it into unit electrodes, etc. However, depending on the physical properties of the negative electrode, there is a problem that cracks and / or detachment of the negative electrode active material layer may occur during the cutting process.

[0007] However, such a defect in the negative electrode causes a sudden drop in battery performance during the subsequent production of secondary batteries, which also poses a safety problem.

[0008] Therefore, there is a need to develop a technology that can solve this problem. Summary of the Invention [Problem to be solved by the invention]

[0009] The present invention aims to provide a negative electrode for a secondary battery that can prevent cracking and detachment of a negative electrode active material layer during cutting of the negative electrode in manufacturing the negative electrode, a method for manufacturing the same, and a secondary battery including the same. [Means for solving the problem]

[0010] The negative electrode for a secondary battery according to one embodiment of the present invention comprises: A negative electrode for a secondary battery, in which a negative electrode active material layer is formed on at least one surface of a current collector, The negative electrode satisfies at least one of the following conditions 1 and 2: [Condition 1] The continuous adhesive strength in the thickness direction of the negative electrode for secondary batteries is 30 gf / 20 mm or more. [Condition 2] The breaking stress is 3.6N or more. The breaking stress is measured by pressing the negative electrode active material layer with a 2.5 mm wide, sharp-ended, straight tip at a speed of 10 μm / s.

[0011] In this case, the continuous adhesive strength in the condition 1 is the adhesive strength between the active material layers per 10 μm in the thickness direction of the negative electrode.

[0012] In addition, the negative electrode active material layer includes a negative electrode active material, a conductive material, and a binder. To satisfy the condition 1 that the continuous adhesive strength is 30 gf / 20 mm or more, the content of the binder contained in each active material layer per 10 μm in the thickness direction of the negative electrode may be 2 wt % or more based on the total weight of each active material layer having a thickness of 10 μm.

[0013] Furthermore, when the negative electrode satisfies the condition 1, the porosity of the negative electrode may be 28% or less, and specifically, may be 5% to 28%.

[0014] More specifically, the negative electrode can satisfy both conditions 1 and 2.

[0015] Meanwhile, the negative electrode has a porosity of 1000 .mu.m or less. According to another embodiment of the present invention, there is provided a method for manufacturing a negative electrode for a secondary battery, comprising: (a) applying a negative electrode active material slurry to at least one surface of a current collector; (b) drying the negative electrode active material slurry to form a negative electrode active material layer; and (c) rolling the negative electrode active material layer to produce a negative electrode for a secondary battery; Including, There is provided a method for manufacturing a negative electrode for a secondary battery, which satisfies at least one of the following conditions 1, 3, and 4, and the deformation not conforming to the following condition 3 or 4 is represented by the following formula 1: [Condition 1] The continuous adhesive strength in the thickness direction of the negative electrode for secondary batteries is 30 gf / 20 mm or more. [Condition 3] Misfit strain ε that occurs during drying in stage (b) mf is 0.1% or less, [Condition 4] Misfit strain ε that occurs during rolling in stage (c) mf is 0.1% or less, [Formula 1]

[0016]

number

[0017] Here, the drying stress (σ) can be calculated by the following formula 2. [Formula 2]

[0018]

number

[0019] In the above formula 2, t s is the thickness of the current collector, t c is the thickness of the dried negative electrode active material layer, Es is the elastic modulus of the current collector, L is the length of the current collector in a direction parallel to the protruding direction of the tab, and ν s is the Poisson's ratio of the current collector, and D is the deflection of the current collector due to the drying.

[0020] The rolling stress (σ) can be the rolling load acting per unit area.

[0021] The electrode elastic modulus (E e ) can be calculated using the following formula 3. [Formula 3]

[0022]

number

[0023] In the formula 3, E tot is the overall elastic modulus of the current collector and the negative electrode active material layer after drying or the negative electrode active material layer after rolling, E f is the elastic modulus of the current collector, T f is the thickness of the current collector, and T e is the thickness of the negative electrode active material layer after drying or after rolling.

[0024] According to yet another embodiment of the present invention, there is also provided a secondary battery including a negative electrode for the secondary battery. [Brief explanation of the drawings]

[0025] [Figure 1] 1 is a graph showing the continuous adhesive strength of a negative electrode prepared according to Comparative Example 1. [Figure 2] 1 is a graph showing the continuous adhesive strength of the negative electrode prepared in Example 1. [Figure 3] 1 is a graph showing the continuous adhesive strength of a negative electrode prepared according to Example 2. [Figure 4] 1 is a graph showing the continuous adhesive strength of a negative electrode prepared in Example 3. [Figure 5] 10 is a micrograph of the negative electrode of Comparative Example 1 according to Experimental Example 5. [Figure 6] 10 is a micrograph of the negative electrode of Example 1 according to Experimental Example 5. [Figure 7] 10 is a micrograph of the negative electrode of Example 2 according to Experimental Example 5. [Figure 8] 10 is a micrograph of the negative electrode of Example 3 according to Experimental Example 5. DETAILED DESCRIPTION OF THE INVENTION

[0026] The terms and words used in this specification and claims should not be interpreted in their ordinary and dictionary sense, but should be interpreted in a sense and concept that is appropriate to the technical idea of ​​the present invention, in accordance with the principle that the inventors can appropriately define the concepts of terms in order to best describe their invention. Therefore, the embodiments described in this specification and the configurations shown in the drawings are merely preferred embodiments of the present invention, and do not represent all of the technical ideas of the present invention. Therefore, various equivalents and modifications may exist as substitutes for these at the time of filing this application, and the scope of the present invention is not limited to the embodiments described below.

[0027] According to one embodiment, the present invention comprises: A negative electrode for a secondary battery, in which a negative electrode active material layer is formed on at least one surface of a current collector, The negative electrode satisfies at least one of the following conditions 1 and 2: [Condition 1] the negative electrode for secondary batteries has a continuous adhesive strength in the thickness direction of 30 gf / 20 mm or more; [Condition 2] The breaking stress is 3.6N or more. The breaking stress is a value measured by pressing the negative electrode active material layer with a sharpened line-shaped tip having a width of 2.5 mm at a speed of 10 μm / s. A negative electrode for a secondary battery is provided.

[0028] Specifically, the continuous adhesive strength in Condition 1 refers to the adhesive strength between active material layers per 10 μm in the thickness direction of the negative electrode. That is, when measuring the adhesive strength between active material layers per 10 μm, if the adhesive strength is less than 30 gf / 20 mm in any section, it is deemed that Condition 1 is not met.

[0029] This continuous adhesive strength is similar to the method used to measure the adhesive strength of a fabricated negative electrode. Specifically, double-sided tape was applied to a glass slide, and a 30 mm x 125 mm punched negative electrode was placed on top of it. A 20 mm wide Scotch Magic Tape was then applied to the top of the negative electrode. The negative electrode, glass slide, and negative electrode were then passed through a laminator at room temperature to ensure uniform adhesion. The Scotch Magic Tape was then pulled at 100 mm / min using a UTM (TA) device to measure the peel force from the negative electrode. The measurement angle between the glass slide and the negative electrode was 90°. The peeled upper active material layer was then removed, and Scotch Magic Tape was attached. The peel force from the negative electrode was measured using a UTM (TA) device at 100 mm / min. The peeled active material layer was then subsequently removed and an adhesive strength test was conducted. This value represents the value measured.

[0030] If the adhesive strength is less than 30 gf / 20 mm in any one range of the measured values, it is determined that the condition 1 is not satisfied.

[0031] That is, the inventors of the present application have confirmed that when the condition 1 is satisfied, cracks and separation of the negative electrode active material layer do not occur during cutting of the negative electrode.

[0032] There are various methods for satisfying Condition 1, and regardless of the method, cracking or detachment of the negative electrode active material layer does not occur when Condition 1 is satisfied. However, as one example, to satisfy Condition 1, the negative electrode active material layer may include a negative electrode active material, a conductive material, and a binder, and the content of the binder in each active material layer per 10 μm in the thickness direction of the negative electrode may be 2 wt % or more based on the total weight of each active material layer with a thickness of 10 μm.

[0033] That is, the binder must be contained in an amount of 2 wt% or more per 10 μm thickness of the active material layer, which is similar to the fact that the binder must be uniformly distributed at 2 wt% or more everywhere. When the binder is contained in an amount of 2 wt% or more in every section, the continuous adhesive strength satisfies the above condition 1.

[0034] When the negative electrode satisfies the condition 1, the porosity of the negative electrode may be 28% or less, specifically 5% to 28% or less, and more specifically 10% to 25%.

[0035] The porosity can be calculated by measuring the surface of the negative electrode at 2,500x magnification using a scanning electron microscope (FE-SEM) (Hitachi S-4800 Scanning Electron Microscope), calculating the area ratio of the surface voids observed in a randomly sampled area (10 μm or more in width and 15 μm or more in length) in the measured photograph to the total area, and converting the ratio into volume.

[0036] If the porosity is too large outside the above range, the adhesive strength of the negative electrode may decrease, which is undesirable.

[0037] On the other hand, even if condition 1 is not satisfied, the inventors of the present application have confirmed that, as explained above, if the breaking stress of the negative electrode in condition 2 is 3.6 N or more, cracking or detachment of the negative electrode active material layer does not occur.

[0038] Specifically, the breaking stress can be 3.6N or more and 5N or less.

[0039] As described above, the breaking stress may be a value measured by pressing the negative electrode active material layer with a 2.5 mm wide, sharp-ended, straight tip at a speed of 10 μm / s. More specifically, the breaking stress is a value measured by pressing the negative electrode vertically from above at a 90° angle using a DHR (TA Co.).

[0040] If the breaking stress satisfies the condition according to the present invention, cracks or separation of the negative electrode active material layer do not occur.

[0041] If the breaking stress is too small and out of the range of the present invention, the negative electrode active material layer may crack or detach, and if it is too large, the flexibility may be excessively reduced, which is undesirable. The breaking stress can be adjusted by adjusting the mismatch deformation during drying and rolling during the manufacturing process of the negative electrode, as will be described below.

[0042] Therefore, more specifically, the negative electrode can satisfy both the condition 1 and the condition 2.

[0043] Meanwhile, according to another embodiment of the present invention, there is provided a method for manufacturing a negative electrode for a secondary battery, comprising: (a) applying a negative electrode active material slurry to at least one surface of a current collector; (b) drying the negative electrode active material slurry to form a negative electrode active material layer; and (c) rolling the negative electrode active material layer to produce a negative electrode for a secondary battery; Including, There is provided a method for producing a negative electrode for a secondary battery, which satisfies at least one of the following conditions 1, 3 and 4, and the deformation not conforming to the following condition 3 or 4 is represented by the following formula 1: [Condition 1] The continuous adhesive strength in the thickness direction of the negative electrode for secondary batteries is 30 gf / 20 mm or more. [Condition 3] The misfit strain occurring during drying in step (b) must be 0.1% or less. [Condition 4] Misfit strain occurring during rolling in stage (c) is 0.1% or less. [Formula 1]

[0044]

number

[0045] As a result of extensive research, the inventors of the present application have confirmed that, as described above, in order for the breaking stress of a negative electrode for a secondary battery to have the above value, the non-conforming deformation occurring during the manufacturing process must satisfy at least one of the above conditions 3 and 4.

[0046] Therefore, when the negative electrode is manufactured so as to satisfy the above conditions 3 and 4, the effects of the present invention can be achieved.

[0047] When the above conditions are satisfied, the intended effect of the present invention is that the negative electrode does not crack or detach.

[0048] Condition 1 is as explained above.

[0049] The mismatch deformation under conditions 3 and 4 can be calculated by Equation 1, and the drying or rolling stress and the elastic modulus of the negative electrode can be calculated as follows.

[0050] Specifically, the drying stress (σ) can be a value calculated by the following formula 2. [Formula 2]

[0051]

number

[0052] In the above formula 2, t s is the thickness of the current collector, t c is the thickness of the dried negative electrode active material layer, E sis the elastic modulus of the current collector, L is the length of the current collector in the direction parallel to the protruding direction of the tab, and ν s is the Poisson's ratio of the current collector, and D is the deflection of the current collector due to the drying.

[0053] Here, the thickness of the current collector and the negative electrode active material layer and the length of the current collector can be determined by the naked eye, and the elastic modulus and Poisson's ratio of the current collector refer to values ​​possessed by the metal constituting the current collector, which are predetermined values. For example, the elastic modulus of Al is 7.19 × 10 2 , the Poisson's ratio can be 0.34, and the elastic modulus of Cu is 1.25 × 10 3 The Poisson's ratio may be 0.34. The degree of deformation of the current collector can be measured by measuring the degree of bending of the negative electrode using a laser and a position sensor. The degree of bending is measured by placing a dried negative electrode on a flat holder and measuring the height of the Z axis at which the negative electrode bends downward or upward when viewed from the side.

[0054] The rolling stress (σ) is different from the drying stress and means the rolling load acting per unit area.

[0055] The rolling load is a rolling force applied to the negative electrode during rolling and can be selected by the operator.

[0056] In addition, the negative electrode elastic modulus (E e ) can be a value calculated by the following formula 3: [Formula 3]

[0057]

number

[0058] In the formula 3, E tot is the overall elastic modulus of the current collector and the negative electrode active material layer after drying or the negative electrode active material layer after rolling, E f is the elastic modulus of the current collector, T f is the thickness of the current collector, and Te is the thickness of the negative electrode active material layer after drying or after rolling.

[0059] The above E f The elastic modulus of the current collector, the thickness of the current collector, and the thickness of the negative electrode active material layer are as described above, and E tot The overall elastic modulus is a value measured using DMA equipment (viscoelasticity measuring equipment).

[0060] When the drying or rolling stress and negative electrode elastic modulus thus obtained are substituted into Equation 1, if the condition that any one of the mismatch deformations is 0.1% or less is satisfied, the breaking stress of the negative electrode becomes 3.6 N or more, and the intended effect of the present invention can be achieved.

[0061] Therefore, it is possible to determine whether the conditions of the present invention are met from the manufacturing stage, quickly detect defects, and prevent cracks and / or detachment of the negative electrode active material layer, thereby preventing deterioration in the performance and safety of the secondary battery including the negative electrode active material layer.

[0062] Meanwhile, according to another embodiment of the present invention, there is also provided a secondary battery including the negative electrode for a secondary battery.

[0063] Other manufacturing methods and components of the secondary battery are known in the art, and therefore, the description thereof will be omitted herein and are included within the scope of the present invention.

[0064] The present invention will be described in detail below based on preferred embodiments of the present invention with reference to examples, comparative examples and the accompanying drawings.

[0065] <Comparative Example 1> (When the negative electrode continuous adhesive strength is 30 gf or less and the drying / rolling misfit strain is 0.1% or more) A slurry for the active material layer was prepared by mixing artificial graphite as the active material, a binder (SBR and CMC mixed in a 2:1 weight ratio), and carbon black as the conductive material in a weight ratio of 96:2.5:1.5, and using water as the dispersant, the mixture and dispersant were mixed in a weight ratio of 1:2.

[0066] The active material layer slurry was coated on one side of a copper (Cu) thin film, which was a 10 μm-thick negative electrode current collector, using a slot die and dried for 1 hour under vacuum at 130°C to form an active material layer. The active material layer thus formed was rolled using a roll pressing method to prepare a negative electrode having an active material layer with a thickness of 80 μm.

[0067] The porosity of the prepared anode was 30%. The porosity was calculated by measuring the anode surface at 2,500x magnification using a scanning electron microscope (FE-SEM) (Hitachi S-4800 Scanning Electron Microscope), calculating the area ratio of surface voids observed in a randomly sampled area (10 μm or more horizontally and 15 μm or more vertically) within the measured image, and converting the ratio to the total area.

[0068] <Example 1> (When the negative electrode continuous adhesive strength is 30 gf or more and the drying / rolling misfit strain is 0.1% or more) A negative electrode was prepared in the same manner as in Comparative Example 1, except that the weight ratio of active material:binder:conductive material was mixed at 96:2.8:1.2 and the active material layer was rolled to a thickness of 75 μm. The porosity of the prepared negative electrode was 25%.

[0069] Example 2 (Negative electrode continuous adhesive strength is 30 gf or less, drying misfit strain is 0.1% or less, and rolling misfit strain is 0.1% or more) A negative electrode was fabricated in the same manner as in Comparative Example 1, except that the active material layer slurry was dried at 100°C under vacuum for 30 minutes, then at 110°C under vacuum for 30 minutes, and then again at 130°C under vacuum for 30 minutes to form an active material layer. The porosity of the fabricated negative electrode was 30%.

[0070] Example 3 (Negative electrode continuous adhesive strength is 30 gf or less, drying misfit strain is 0.1% or more, and rolling misfit strain is 0.1% or less) A negative electrode was prepared in the same manner as in Comparative Example 1, except that the active material layer was first rolled to a thickness of 90 μm and then second rolled to a thickness of 80 μm. The porosity of the prepared negative electrode was 30%.

[0071] <Experimental Example 1> The continuous adhesive strength of the negative electrodes prepared in Comparative Example 1 and Examples 1 to 3 was measured and is shown in FIGS. 1 to 4 below.

[0072] The negative electrode continuous adhesive strength was measured by attaching double-sided tape to a glass slide, placing a 30 mm x 125 mm punched negative electrode on it, and then attaching a 20 mm wide Scotch Magic Tape® to the top of the negative electrode. The negative electrode and glass slide, and the negative electrode and Scotch Magic Tape® were then passed through a laminator (Supernex-syn C325, 9 speed settings) at room temperature to uniformly adhere the negative electrode to the glass slide and Scotch Magic Tape®. The force required to peel the Scotch Magic Tape® from the negative electrode was measured using a UTM (TA) device at 100 mm / min, with the measurement angle between the glass slide and the negative electrode being 90°. The peeled upper active material layer was then removed, and Scotch Magic Tape® was attached. The peeling force was measured using a UTM (TA) device at 100 mm / min. The peeled active material layer was then removed and an adhesion test was conducted.

[0073] 1 to 4 below, it can be seen that the negative electrode continuous adhesive strength of Comparative Example 1 and Examples 2 and 3 is 30 gf / 20 mm or less in some sections, whereas the negative electrode continuous adhesive strength of Example 1 is 30 gf / 20 mm or more in all sections.

[0074] <Experimental Example 2> Misfit strain during the drying and rolling processes of the negative electrodes prepared in Comparative Example 1 and Examples 1 to 3 was calculated and is shown in Table 1 below.

[0075] [Table 1]

[0076] Referring to Table 1, it can be seen that Comparative Example 1 and Example 1 both have misfit strains of 0.1% or more, whereas Examples 2 and 3 have at least one misfit strain of 0.1% or less.

[0077] The stress and elastic modulus are determined by the above-mentioned formula 2, rolling load and formula 3, and the misfit strain is determined from the above-mentioned formula 1 based on the above-mentioned values.

[0078] <Experimental Example 3> The breaking stresses of the negative electrodes prepared in Comparative Example 1 and Examples 1 to 3 were measured and are shown in Table 2 below.

[0079] The breaking stress is measured by pressing the negative electrode active material layer vertically from above at a 90-degree angle at a speed of 10 μm / s using a DHR (TA) instrument with a 2.5 mm wide, sharpened tip.

[0080] [Table 2]

[0081] When comparing Table 2 with Experimental Example 2, it can be seen that when at least one of the misfit strains in the drying process or the rolling process is 0.1% or less, the breaking stress is 3.6N or more.

[0082] <Experimental Example 4> The negative electrodes prepared in Comparative Example 1 and Examples 1 to 3 were notched in half to check for cracks.

[0083] The presence or absence of cracks was confirmed by taking a microscopic photograph of the top of the negative electrode in the vertical direction and visually inspecting it. The results are shown in FIGS. 5 to 8 below.

[0084] 5 to 8, it can be seen that cracks occurred only in Comparative Example 1, which did not satisfy all of Conditions 1 and 2, or Conditions 1, 3, and 4.

[0085] Therefore, it can be seen that when any one of the above conditions 1 to 4 of the present invention is satisfied, the intended effects of the present invention can be obtained.

[0086] Those skilled in the art will appreciate that various applications and modifications within the scope of the present invention can be made based on the above content. [Industrial Applicability]

[0087] The negative electrode for a secondary battery according to the present invention satisfies certain conditions, and therefore does not suffer from cracking or detachment of the negative electrode active material layer during cutting of the negative electrode, thereby effectively eliminating the problems of performance degradation or safety degradation of a secondary battery including such a negative electrode.

Claims

1. A negative electrode for a secondary battery, in which a negative electrode active material layer is formed on at least one surface of a current collector, The negative electrode for a secondary battery satisfies the following condition 1: [Condition 1] The continuous adhesive strength in the thickness direction of the negative electrode for secondary batteries is 125 gf / 20 mm or more, The continuous adhesive strength of Condition 1 is the adhesive strength between the active material layers per 10 μm in the thickness direction of the negative electrode for secondary batteries, the negative electrode active material layer includes a negative electrode active material, a conductive material, and a binder, when the negative electrode for a secondary battery satisfies the condition 1, the content of the binder contained in each active material layer per 10 μm in the thickness direction of the negative electrode for a secondary battery is 2 wt % or more based on the total weight of each active material layer having a thickness of 10 μm.

2. 2. The negative electrode for a secondary battery according to claim 1, wherein when the negative electrode for a secondary battery satisfies the condition 1, the porosity of the negative electrode for a secondary battery is 28% or less.

3. 3. The negative electrode for a secondary battery according to claim 2, wherein when the negative electrode for a secondary battery satisfies the condition 1, the porosity of the negative electrode for a secondary battery is 5% to 28%.

4. A method for producing the negative electrode for a secondary battery according to claim 1, comprising: (a) applying a negative electrode active material slurry to at least one surface of a current collector; (b) drying the negative electrode active material slurry to form a negative electrode active material layer; and (c) rolling the negative electrode active material layer to produce a negative electrode for a secondary battery; Including, A method for producing a negative electrode for a secondary battery that satisfies the following condition 1: [Condition 1] The continuous adhesive strength in the thickness direction of the negative electrode for secondary batteries is 125 gf / 20 mm or more.

5. A secondary battery comprising the negative electrode for secondary batteries according to claim 1.

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