Secondary battery, method for manufacturing the same, and electric device

Abstract

The application provides a secondary battery, a preparation method thereof and an electric device. The secondary battery comprises a copper foil, the copper foil comprises copper grains with different particle sizes, the copper grains comprise copper grains with a particle size less than or equal to 0.5 microns and copper grains with a particle size greater than 0.5 microns, wherein the proportion of the number of the copper grains with a particle size less than or equal to 0.5 microns in the total number of the copper grains is 70% to 95%, and the proportion of the number of the copper grains with a particle size greater than 0.5 microns in the total number of the copper grains is 5% to 30%. The current collector has excellent mechanical strength and plasticity.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of secondary batteries, in particular to a secondary battery, a preparation method thereof and an electric device. BACKGROUND

[0002] With the secondary batteries being widely applied in energy storage power supply systems such as hydroelectric, thermal, wind and solar power stations, and in many fields such as electric tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, etc., the market has increasingly high requirements for the energy density of secondary batteries.

[0003] The energy density can be improved by using silicon-based negative electrode active materials or increasing the amount of secondary battery active materials, but the silicon-based material has a high thermal expansion coefficient, and the volume of the silicon-based battery expands severely under heat; the volume of the battery cell also increases accordingly after the amount of secondary battery active materials is increased, which all puts higher requirements on the performance of the negative electrode current collector. The negative electrode current collector copper foil is an important component of the secondary battery, which can constrain the expansion of the battery cell and prevent the anode of the secondary battery from breaking during use, and has a great influence on the electrical performance and safety performance of the secondary battery.

[0004] Therefore, there is an urgent need for a secondary battery with improved negative electrode current collector performance. SUMMARY

[0005] The present application provides a secondary battery with a negative electrode current collector with improved tensile properties, which can effectively constrain the expansion of the battery cell, delay or reduce the fracture of the electrode sheet, and prolong the service life of the secondary battery.

[0006] In a first aspect, the present application provides a secondary battery, which comprises a copper foil, the copper foil comprises copper grains with different particle sizes, the copper grains comprise copper grains with a particle size less than or equal to 0.5 μm and copper grains with a particle size greater than 0.5 μm, wherein the number of copper grains with a particle size less than or equal to 0.5 μm accounts for 70%-95% of the total number of copper grains, and the number of copper grains with a particle size greater than 0.5 μm accounts for 5%-30% of the total number of copper grains.

[0007] The copper foil provided by the present application has excellent tensile strength and elongation at break, and has excellent plasticity while improving the mechanical strength, thereby providing a material basis for improving the energy density, capacity and safety of the secondary battery.

[0008] In any embodiment, the particle size of the copper grains with a particle size greater than 0.5 μm is in the range of greater than 0.5 μm and less than or equal to 3 μm, which can adjust or optimize the tensile strength of the copper foil.

[0009] In any embodiment, the number of copper grains with a particle size less than or equal to 0.5 pm accounts for 80-95% of the total number of copper grains, which helps to further improve the tensile strength and improve the mechanical strength of the copper foil.

[0010] In any embodiment, the number of copper grains with a particle size greater than 0.5 pm accounts for 5-20% of the total number of copper grains, which helps to further improve the elongation at break and improve the plasticity of the copper foil.

[0011] In any embodiment, the copper foil satisfies at least one of the following conditions:

[0012] (1) the average particle size of the copper grains is 0.3-1.2 pm;

[0013] (2) the maximum particle size of the copper grains is 1-2.5 pm;

[0014] (3) the minimum particle size of the copper grains is 0.1-0.3 pm;

[0015] (4) the particle size span of the copper grains is 0.8-2.5 pm.

[0016] The particle size distribution of the copper grains helps to adjust the number of copper grains with a particle size less than or equal to 0.5 pm and the number of copper grains with a particle size greater than 0.5 pm, thereby adjusting and improving the tensile strength and elongation at break of the copper foil, so that the copper foil has excellent mechanical properties and plastic properties.

[0017] In any embodiment, the copper foil satisfies at least one of the following conditions:

[0018] (1) the average particle size of the copper grains is 0.3-0.6 pm;

[0019] (2) the maximum particle size of the copper grains is 1.2-2.0 pm;

[0020] (3) the minimum particle size of the copper grains is 0.1-0.3 pm;

[0021] (4) the particle size span of the copper grains is 1-2 pm.

[0022] The particle size distribution of the copper grains helps to further adjust the tensile strength and elongation at break of the copper foil, and improve the mechanical properties and plastic properties.

[0023] In any embodiment, under the test conditions of room temperature (20±10°C), sample length x width of (50±0.25 mm) x (15±0.25 mm), and tensile speed of 50±0.5 mm / min, the tensile strength of the copper foil is 600-1000 MPa, and / or the elongation at break of the copper foil is 4-8%.

[0024] The copper foil has excellent tensile strength and elongation at break, has good mechanical properties and plasticity, can be applied to high-energy-density batteries or high-expansion batteries, and helps to improve the safety of secondary batteries.

[0025] In any embodiment, the tensile strength of the copper foil is 700-1000 MPa, and / or the elongation at break of the copper foil is 4-7% under the test conditions of room temperature (20±10℃), sample length x width of (50±0.25 mm) x (15±0.25 mm), and tensile speed of 50±0.5 mm / min.

[0026] In any embodiment, the tensile strength of the copper foil is 700-800 MPa, and / or the elongation at break of the copper foil is 5-6% under the test conditions of room temperature (20±10℃), sample length x width of (50±0.25 mm) x (15±0.25 mm), and tensile speed of 50±0.5 mm / min.

[0027] The copper foil has good tensile strength, to some extent, can meet the actual demand for high-strength copper foil in the battery field. In addition, the copper foil also has excellent elongation at break, showing good plasticity, which helps to reduce the brittleness defects of the copper foil and reduce the risk of copper foil fracture or slight cracking in the battery cell.

[0028] In any embodiment, the hardness of the copper foil is 55-65 HV. In any embodiment, the hardness of the copper foil is 55-60 HV. Suitable hardness is conducive to the surface treatment of the copper foil and the cold pressing treatment of the secondary battery, reducing the surface damage of the copper foil and reducing the influence on the bonding performance of the negative electrode film layer and the copper foil.

[0029] In any embodiment, the thickness of the copper foil is 4-10 μm. The copper foil can reduce the thickness without affecting the strength, which helps to design the battery to be lightweight, and further improves the energy density or specific capacity.

[0030] In any embodiment, the secondary battery further comprises a negative electrode film layer on at least one side of the copper foil, and the negative electrode active material in the negative electrode film layer comprises at least one of artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based material, tin-based material, and lithium titanate. The secondary battery can be applied to various different battery negative electrode systems, and has a wide range of applications.

[0031] In any embodiment, the negative active material comprises a silicon-based material, which comprises at least one of elemental silicon, silicon oxide, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy. The secondary battery using the silicon-based material as the negative active material has a higher energy density, and meanwhile, the copper foil in the secondary battery can well bind the volume expansion of the negative electrode during the battery cycle.

[0032] The second aspect of the present application provides a method for preparing a secondary battery, comprising preparing a copper foil by an electroplating method, wherein the electroplating method comprises applying a pulse current to an electroplating solution, and reducing and depositing copper ions in the electroplating solution to form a copper foil, wherein the peak value of the pulse current is 40,000 A-100,000 A, the valley value of the pulse current is 100 A-20,000 A, and the change period of the current is 50 ms-5,000 ms.

[0033] Compared with the calendering method, the electroplating method is mature and simple, has low requirements for equipment, and the prepared copper foil has excellent tensile strength and elongation at break, and has excellent mechanical strength and good plasticity.

[0034] In any embodiment, the electroplating method satisfies one or more of the following conditions:

[0035] (1) the peak value of the pulse current is 40,000 A-80,000 A;

[0036] (2) the valley value of the pulse current is 1,000 A-10,000 A;

[0037] (3) the change period of the pulse current is 500 ms-5,000 ms;

[0038] (4) the distance between the cathode electrode and the anode electrode is 8 mm-20 mm;

[0039] (5) the temperature of the electroplating deposition is 45°C-60°C;

[0040] (6) the speed of the cathode roller is 2 m / min-5 m / min.

[0041] In any embodiment, the electroplating method satisfies one or more of the following conditions:

[0042] (1) the peak value of the pulse current is 50,000 A-70,000 A;

[0043] (2) the valley value of the pulse current is 2,000 A-5,500 A;

[0044] (3) the change period of the pulse current is 2,000 ms-4,000 ms;

[0045] (4) the distance between the cathode electrode and the anode electrode is 8mm-12mm;

[0046] (5) the temperature of the electroplating deposition is 50℃-60℃;

[0047] (6) the roller speed of the cathode roller is 2m / min-3m / min.

[0048] Compared with direct current deposition, applying pulse current to the electroplating solution can make copper ions reduce and deposit to form fine grains with small particle size under high current conditions, and form grains with large particle size (for example, columnar grains) under low current conditions. By adjusting the current size and change period, the formation and growth of copper ion crystal nucleus can be adjusted, so that the size and morphology of the grain size, and the proportion of the number of copper grains with a particle size less than or equal to 0.5μm and the number of copper grains with a particle size greater than 0.5μm can be adjusted, and the tensile strength and elongation at break of the copper foil can be improved.

[0049] In any embodiment, the pulse current includes one or more of a square wave pulse current, a sine wave pulse current, a triangular wave pulse current, and a sawtooth wave pulse current. In any embodiment, the pulse current includes a sine wave pulse current. The sine wave pulse current continuously and periodically changes, which is conducive to the continuous and variable growth of the grains.

[0050] In any embodiment, the electroplating solution includes a leveling agent, a wetting agent, and a brightener, the leveling agent includes one or more of collagen, sodium saccharin; the wetting agent includes one or more of hydroxyethyl cellulose, polyethylene glycol; and the brightener includes polydithiobispropyl sulfone sodium.

[0051] The leveling agent can improve the flatness of the copper foil, the wetting agent can improve the wettability of the electroplating solution and the substrate, increase the nucleation rate of the copper foil, and reduce the average grain size of the grains in the copper foil, and the brightener can make the grain size of the copper foil smaller and reduce the surface roughness of the copper foil and improve the smoothness of the surface.

[0052] In any embodiment, the electroplating solution includes collagen with a concentration of 60mg / L-300mg / L, sodium saccharin with a concentration of 0.5g / L-10g / L, polyethylene glycol with a concentration of 50mg / L-200mg / L, hydroxyethyl cellulose with a concentration of 30mg / L-200mg / L, polydithiobispropyl sulfone sodium with a concentration of 500mg / L-2000mg / L, and chloride ions (calculated as chlorine atoms) with a concentration of 20mg / L-80mg / L.

[0053] In any embodiment, the electroplating solution comprises: the electroplating solution comprises: collagen with a concentration of 80 mg / L-150 mg / L, sodium saccharin with a concentration of 0.5 g / L-4 g / L, polyethylene glycol with a concentration of 60 mg / L-150 mg / L, hydroxyethyl cellulose with a concentration of 50 mg / L-150 mg / L, sodium polydithiobis propane sulfonate with a concentration of 500 mg / L-1000 mg / L, chloride ion with a concentration of 40 mg / L-80 mg / L.

[0054] The collagen and sodium saccharin in the electroplating solution help to improve the surface pits and protrusions of the copper foil, improve the flatness of the copper foil, the hydroxyethyl cellulose and the polyethylene glycol are beneficial to reduce the difference in the thickness direction of the copper foil, improve the uniformity of the copper foil, the polydithiobis propane sulfonate sodium can improve the electrochemical reduction rate of copper ions, adjust the grain size, and the prepared copper foil is bright and flat, the number ratio of copper grains with a particle size less than or equal to 0.5 μm and copper grains with a particle size greater than 0.5 μm is appropriate, and has good tensile strength and elongation at break.

[0055] In any embodiment, the pH of the electroplating solution is 2.5-4.5, which is beneficial to the reduction of copper ions.

[0056] The third aspect of the application provides a kind of electric device, including the secondary battery of the first aspect of the application or the secondary battery prepared by the method of the second aspect of the application. BRIEF DESCRIPTION OF DRAWINGS

[0057] In order to more clearly illustrate the technical solutions of the embodiments of the application, the drawings needed to be used in the embodiments of the application will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the application, and other drawings can be obtained by the drawings without paying creative labor for those skilled in the art.

[0058] Figure 1 is the schematic diagram of the secondary battery of an embodiment of the application.

[0059] Figure 2 is Figure 1 the exploded view of the secondary battery of an embodiment of the application shown in the figure.

[0060] Figure 3 is the schematic diagram of the battery module of an embodiment of the application.

[0061] Figure 4 is the schematic diagram of the battery pack of an embodiment of the application.

[0062] Figure 5 is Figure 4 the exploded view of the battery pack of an embodiment of the application shown in the figure.

[0063] Figure 6 is a schematic diagram of a power consuming device using a secondary battery as a power source according to an embodiment of the present application.

[0064] Figure 7 shows a pole figure distribution map of the cross section of the copper foil of Example 1 of the present application, which was subjected to electron backscatter diffraction (EBSD) testing.

[0065] Figure 8 shows a tensile curve of the copper foil of Example 1 of the present application.

[0066] Figure 9 shows a grain size distribution map obtained by EBSD diffraction testing of the cross section of the copper foil of Example 1 of the present application.

[0067] Figure 10 shows a pole figure distribution map of the cross section of the copper foil of Example 2 of the present application, which was subjected to EBSD testing.

[0068] Figure 11 shows a tensile curve of the copper foil of Example 2 of the present application.

[0069] Figure 12 shows a grain size distribution map obtained by EBSD diffraction testing of the cross section of the copper foil of Example 2 of the present application.

[0070] Figure 13 shows a pole figure distribution map of the cross section of the copper foil of Example 3 of the present application, which was subjected to EBSD testing.

[0071] Figure 14 shows a tensile curve of the copper foil of Example 3 of the present application.

[0072] Figure 15 shows a grain size distribution map obtained by EBSD diffraction testing of the cross section of the copper foil of Example 3 of the present application.

[0073] Figure 16 shows a tensile curve of the copper foil of Example 3 of the present application.

[0074] Figure 17 shows a pole figure distribution map of the cross section of the copper foil of Comparative Example 2 of the present application, which was subjected to EBSD testing.

[0075] Figure 18 shows a tensile curve of the copper foil of Comparative Example 2 of the present application.

[0076] Figure 19 shows a grain size distribution map obtained by EBSD diffraction testing of the cross section of the copper foil of Comparative Example 2 of the present application.

[0077] BRIEF DESCRIPTION OF THE DRAWINGS

[0078] 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Secondary battery; 51 Casing; 52 Electrode assembly; 53 Cover plate. Detailed Implementation

[0079] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the positive electrode active material, its manufacturing method, positive electrode sheet, secondary battery, battery module, battery pack, and electrical device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0080] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0081] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0082] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0083] If not specified otherwise, all steps of the present application can be performed in sequence or randomly, preferably in sequence. For example, the method comprises steps (a) and (b) means that the method can comprise steps (a) and (b) in sequence, or steps (b) and (a) in sequence. For example, the method can further comprise step (c) means that step (c) can be added to the method in any order, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0084] If not specified otherwise, the terms "comprising" and "including" as used in the present application are open terms. For example, the terms "comprising" and "including" can mean that other components can also be included in addition to or in place of those listed.

[0085] If not specified otherwise, the term "or" in the present application is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any of the following conditions fulfill the condition "A or B": A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0086] Generally, a secondary battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator. During the charging and discharging of the battery, active ions (e.g., lithium ions, sodium ions) are inserted and extracted between the positive electrode sheet and the negative electrode sheet. The electrolyte serves to conduct the active ions between the positive electrode sheet and the negative electrode sheet. The separator is disposed between the positive electrode sheet and the negative electrode sheet and mainly serves to prevent short circuiting between the positive and negative electrodes while allowing the active ions to pass through. The negative electrode sheet includes a negative current collector, which serves to carry electrode active materials and collect output current, and can also bind the swelling of the battery cell to prevent the anode from breaking during cycling.

[0087] Increasing the amount of active material can increase the energy density or capacity of the secondary battery, but will cause the volume of the cell to increase; using silicon-based negative electrode material can also increase the energy density or capacity of the secondary battery, but the silicon-based negative electrode material has high expansion, which causes the volume of the electrode sheet to expand and contract seriously during the cycle process. This easily causes the copper foil in the cell to break or produce slight cracks, causing the battery to short circuit, which seriously affects the safety of the secondary battery. At present, the tensile strength of the negative electrode current collector copper foil commonly used is usually 200-500 MPa, which cannot meet the use demand of the new generation of secondary battery with high energy density or high capacity. Refining the copper grains can strengthen the material strength, but can aggravate the brittleness defect of the copper foil, increasing the risk of copper foil fracture or slight crack.

[0088] [Secondary battery]

[0089] Based on this, the application provides a secondary battery, which comprises a negative electrode current collector, the negative electrode current collector comprises a copper foil, the copper foil comprises copper grains with different particle sizes, the copper grains comprise copper grains with a particle size less than or equal to 0.5 μm and copper grains with a particle size greater than 0.5 μm, wherein the number percentage of the copper grains with a particle size less than or equal to 0.5 μm in the total number of copper grains is 70%-95%, and the number percentage of the copper grains with a particle size greater than 0.5 μm in the total number of copper grains is 5%-30%.

[0090] The copper grains with a particle size greater than 0.5 μm tend to be columnar grains or columnar-like grains, which are also referred to as "large grains" herein, and the copper grains with a particle size less than or equal to 0.5 μm are fine grains close to particles, forming a fine grain area around the columnar grains. Therefore, in the application, the copper foil substrate forms a grain morphology with fine grains as the main part and mixed distribution of large grains, which is a heterogeneous grain morphology with a small amount of large grains doped in fine grains. The main reason for the difference in the size of the heterogeneous grain morphology is the adjustment of the grain size during the process of the copper foil. Specifically, as an embodiment, the size of the grain size can be controlled by adjusting the change of the current during the process of the copper foil. After the current is reduced, the grains tend to form larger grains, and after the current is increased, the grains tend to form smaller grains.

[0091] The copper grains with a particle size less than or equal to 0.5 μm help to improve the tensile strength of the copper foil and improve the mechanical properties; the copper grains with a particle size greater than 0.5 μm help to improve the elongation at break and improve the plasticity of the copper foil. The number ratio of the copper grains with a particle size less than or equal to 0.5 μm and the copper grains with a particle size greater than 0.5 μm in the total number of copper grains can have different degrees of influence on the mechanical properties and plasticity of the copper foil, so that the copper foil exhibits different degrees of excellent mechanical properties and plasticity. In the copper foil provided in the present application, the number ratio of the copper grains with a particle size less than or equal to 0.5 μm in the total number of copper grains is 70%-95%, which can significantly improve the tensile strength of the copper foil, and the number ratio of the copper grains with a particle size greater than 0.5 μm in the total number of copper grains is 5%-30%, which can make the copper foil have good elongation at break, and the copper foil has excellent mechanical properties and plasticity, which can reduce the risk of fracture or crack of the copper foil in the high-energy-density battery system or the high-expansion battery system, and provide a material basis for improving the energy density and safety of the secondary battery.

[0092] Without wishing to be bound by any theory, the difference in work hardening ability between large grains and fine grains is large, and the heterogeneous grain morphology of a small amount of large grains doped in fine grains makes the strain partition in the initial stage of plastic deformation of the copper foil more obvious. This may be because the moderate doping of large grains produces a higher density of geometrically necessary dislocations than all fine grains. The bending of the crystal plane of the copper foil during inhomogeneous plastic deformation can produce dislocations, which are called geometrically necessary dislocations (GND). Geometrically necessary dislocations can coordinate the plastic strain caused by deformation and maintain the continuity of the material, which helps to reduce the occurrence of concentrated stress during the deformation of the copper foil, so that the copper foil exhibits stronger ability to inhibit strain localization, and the plasticity of the copper foil is improved. In addition, in the copper foil crystal, the contact interface between the grains is called the grain boundary. It can be understood that in the same area, the smaller the grain size, the higher the total area of the grain boundary, and the higher the grain boundary stress that the copper foil needs to overcome during deformation, that is, the higher the mechanical strength. At the same time, the higher density of geometrically necessary dislocations produced during the deformation of the copper foil helps to reduce the concentrated stress, which can improve the mechanical strength.

[0093] In some embodiments, the number ratio of the copper grains with a particle size less than or equal to 0.5 μm in the total number of copper grains is 75%-95%, 80%-95%, 85%-95%, 83%-93%, 85%-93% or 87%-93%, which helps to further improve the tensile strength and optimize the mechanical properties of the copper foil.

[0094] In some embodiments, the number of copper grains with a particle size of less than or equal to 0.5 μm accounts for 75%, 78%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, or any value between any two of the aforementioned values or any value within the range of the aforementioned values.

[0095] In some embodiments, the particle size of the copper grains with a particle size of greater than 0.5 μm ranges from greater than 0.5 μm to less than or equal to 3 μm, which helps to reduce or optimize the average particle size of the copper foil and improve the tensile strength of the copper foil.

[0096] In some embodiments, the number of copper grains with a particle size of greater than 0.5 μm accounts for 5%-20%, 2%-20%, 2%-15%, 5%-15%, 5%-13%, 6%-12%, or 7%-10% of the total number of copper grains, which helps to further improve the elongation at break and optimize the plasticity of the copper foil.

[0097] In some embodiments, the number of copper grains with a particle size of greater than 0.5 μm accounts for 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 18%, 20%, 25%, 30%, or any value between any two of the aforementioned values or any value within the range of the aforementioned values.

[0098] In some embodiments, the particle size of the copper grains with a particle size of greater than 0.5 μm ranges from greater than 0.5 μm to less than or equal to 3 μm, which helps to reduce or optimize the average particle size of the copper foil and improve the tensile strength of the copper foil.

[0099] The number and particle size of the grains in the copper foil can be tested by methods known in the art. For example, the cross-section of the copper foil is measured by an electron backscatter diffraction instrument (EBSD) and a scanning electron microscope, a pole figure distribution map with a magnification of 3000 is obtained, the number and particle size of the grains are counted by the imageJ analysis software of the Oxford C-Nano+ electron backscatter diffraction instrument, the equivalent circle diameter of the grains is taken as the particle size of the grains, a number distribution map is made, a skew distribution is used for fitting, and the particle size of the grains and the number of grains in different particle size intervals are obtained.

[0100] In some embodiments, the average grain size of the copper grains is 0.3 pm to 1.2 pm. In some embodiments, the average grain size of the copper grains is 0.3 pm to 0.6 pm. In some embodiments, the average grain size of the copper grains is 0.3 pm to 0.5 pm. In some embodiments, the average grain size of the copper grains is 0.3 pm, 0.35 pm, 0.4 pm, 0.45 pm, 0.5 pm, 0.55 pm, 0.6 pm, 0.65 pm, 0.7 pm, 0.75 pm, 0.8 pm, 0.85 pm, 0.9 pm, 1.0 pm, 1.1 pm, 1.2 pm, or any range between any of the foregoing values or any value within the foregoing ranges.

[0101] The suitable grain size range is theoretically favorable for the copper foil to obtain suitable grain boundaries, which can make the copper foil have suitable resistance to grain dislocation movement and deformation resistance, and thus can optimize the mechanical strength of the copper foil.

[0102] The average grain size of the grains can be tested by methods known in the art. For example, the cross section of the copper foil is measured by an electron backscatter diffraction instrument (EBSD) and a scanning electron microscope, a pole figure distribution map is obtained, the number and size of the grains are counted by imageJ analysis software matched with the Oxford C-Nano+ electron backscatter diffraction instrument, the equivalent circle diameter of the grains is taken as the grain size, a number distribution map is made, a skewness distribution is used for fitting, and the average grain size of the grains is obtained.

[0103] In some embodiments, the maximum grain size of the copper grains is 1 pm to 2.5 pm. In some embodiments, the maximum grain size of the copper grains is 1.2 pm to 1.8 pm or 1.2 pm to 2.0 pm. In some embodiments, the maximum grain size of the copper grains is 1 pm, 1.2 pm, 1.4 pm, 1.6 pm, 1.8 pm, 2 pm, or any range between any of the foregoing values or any value within the foregoing ranges.

[0104] The maximum grain size of the copper grains can be tested by methods known in the art. For example, the cross section of the copper foil is measured by an electron backscatter diffraction instrument (EBSD) and a scanning electron microscope, a pole figure distribution map is obtained, the number and size of the grains are counted by imageJ analysis software matched with the Oxford C-Nano+ electron backscatter diffraction instrument, the equivalent circle diameter of the grains is taken as the grain size, a number distribution map is made, and a skewness distribution is used for fitting. The maximum grain size of the copper grains is the maximum grain size in the statistical results.

[0105] In some embodiments, the minimum grain size of the copper grains is 0.1 pm to 0.3 pm. In some embodiments, the minimum grain size of the copper grains is 0.1 pm to 0.3 pm. In some embodiments, the minimum grain size of the copper grains is 0.1 pm, 0.15 pm, 0.2 pm, 0.25 pm, 0.3 pm, or any range between any of the aforementioned values or any value within the aforementioned range.

[0106] The minimum grain size of the copper grains can be tested by methods known in the art, for example, by measuring the cross section of the copper foil using an electron backscatter diffraction instrument (EBSD) and a scanning electron microscope, obtaining a pole figure distribution map, counting the number and size of the grains using the imageJ analysis software provided with the Oxford C-Nano+ electron backscatter diffraction instrument, taking the equivalent circle diameter of the grains as the grain size, making a number distribution map, and fitting the skewness distribution. The minimum grain size in the statistical results is the minimum grain size of the copper grains.

[0107] The grain size span of the copper grains is the difference between the maximum grain size and the minimum grain size in the grain size distribution of the copper foil. In some embodiments, the grain size span of the copper grains is 0.8 pm to 2.5 pm. In some embodiments, the grain size span of the copper grains is 0.8 pm to 2 pm. In some embodiments, the grain size span of the copper grains is 1 pm to 2 pm. In some embodiments, the grain size span of the copper grains is 0.8 pm, 1 pm, 1.2 pm, 1.4 pm, 1.6 pm, 1.8 pm, 2.0 pm, 2.2 pm, 2.4 pm, 2.5 pm, or any range between any of the aforementioned values or any value within the aforementioned range.

[0108] The grain size span of the copper grains within the range can make the dispersion degree of the grain size distribution of the copper grains within a suitable range, which can help to balance the grain boundary area and the geometrically necessary dislocation density of the copper grains, improve the grain dislocation movement resistance, and reduce the occurrence of concentrated stress during the deformation of the copper foil, which is beneficial to the copper foil to have excellent plasticity while improving the mechanical strength.

[0109] In some embodiments, the tensile strength of the copper foil is 600 MPa to 1000 MPa under the test conditions of room temperature (20±10°C), sample length x width of (50±0.25 mm) x (15±0.25 mm), and tensile speed of 50±0.5 mm / min. In some embodiments, the tensile strength of the copper foil is 700 MPa to 1000 MPa. In some embodiments, the tensile strength of the copper foil is 700 MPa to 800 MPa.

[0110] In some embodiments, the copper foil has an elongation at break of 4-8% under the test condition of room temperature (20±10°C), sample length x width of (50±0.25mm) x (15±0.25mm), and a tensile speed of 50±0.5mm / min. In some embodiments, the copper foil has an elongation at break of 4-7%. In some embodiments, the copper foil has an elongation at break of 5-6%.

[0111] In the present context, the term "tensile strength" refers to the maximum load-bearing strength per unit area of a test sample when the sample is continuously loaded until it breaks.

[0112] In the present context, the term "elongation at break" refers to the ratio of the length change of a material after being stressed until it breaks due to plastic deformation, usually expressed in percentage, and is an important parameter to measure the deformation ability of a material under stress during tensile process.

[0113] In the present application, the tensile strength and elongation at break of the copper foil can be tested by methods known in the art, such as according to the standard GB / T 5230-1995 "Electrolytic Copper Foil". As an example, at least 4 test samples with a length of 50±0.25mm and a width of 15±0.25mm are cut, and the test samples are continuously loaded until they break at room temperature (20±10°C) with a tensile speed of 50±0.5mm / min, and the maximum load is divided by the cross-sectional area of the test sample to obtain the tensile strength of the test sample. The cross-sectional area of the test sample can be calculated by dividing the mass of the test sample by the product of the length of the test sample and the density. The elongation at break can be calculated according to the displacement method after the above test. The test area refers to the detection area during instrument testing, and the length and width of the test sample can be greater than those of the test area, considering that the test sample may need to be fixed by a clamp during testing.

[0114] In some embodiments, the copper foil has a tensile strength of 600MPa, 650MPa, 700MPa, 750MPa, 800MPa, 850MPa, 900MPa, 950MPa, 1000MPa, or a range between any two of the above values or any value within the range, under the test condition of room temperature (20±10°C), sample length x width of (50±0.25mm) x (15±0.25mm), and a tensile speed of 50±0.5mm / min.

[0115] In some embodiments, the copper foil has an elongation at break of 4.0%, 4.5%, 5.0%, 5.5%, 6.0%, 6.5%, 7.0%, 7.5%, 8.0%, or a range between any two of the above values or any value within the range, under the test conditions of room temperature (20±10°C), sample length x width of (50±0.25 mm) x (15±0.25 mm), and a tensile speed of 50±0.5 mm / min.

[0116] The copper foil with the elongation strength and the elongation at break has excellent mechanical strength and plasticity, and can be suitable for high-energy-density batteries or high-swelling batteries, and helps to improve the safety of secondary batteries.

[0117] In some embodiments, the copper foil has a hardness of 55HV-65HV. In some embodiments, the copper foil has a hardness of 55HV-60HV. In some embodiments, the copper foil has a hardness of 55HV, 56HV, 57HV, 58HV, 59HV, 60HV, 61HV, 62HV, 63HV, 64HV, 65HV, or a range between any two of the above values or any value within the range.

[0118] The hardness can reflect the pressure deformation or puncture resistance of the copper foil. In the preparation process of the secondary battery, the surface quality of the copper foil can affect the bonding performance of the negative electrode film layer and the copper foil, especially in the cold pressing process step of the secondary battery, the negative active material particles press the copper foil under external pressure, and a suitable hardness is conducive to reducing the surface damage of the copper foil and reducing the influence on the bonding performance of the negative electrode film layer and the copper foil.

[0119] In some embodiments, the copper foil has a thickness of 4μm-10μm. In some embodiments, the copper foil has a thickness of 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, or a range between any two of the above values or any value within the range.

[0120] In this application, the thickness of the copper foil can be tested by methods known in the art. As an example, a 20x15cm 2 sample is cut, the cut sample is placed on an electronic balance to weigh, the weight of the sample is obtained, and the volume of the sample is calculated according to the density p of the copper foil of 8.96g / cm 3 The length and width of the sample are known, and thus the thickness of the sample can be calculated.

[0121] The maximum load that can be carried by the ordinary strength copper foil after thinning is sharply attenuated, the thickness of the copper foil that can be used for plastic deformation is severely reduced, resulting in a substantial decrease in the tensile strength and elongation at break of the copper foil, and fatigue fracture is prone to occur in the later stage of the secondary battery cycle, which may cause safety accidents. The copper foil provided in the application still has excellent tensile strength and elongation at break in the thickness range of 4-10 μm, and has good mechanical properties and plasticity. The copper foil can be thinned without affecting the strength, which is helpful for lightweight design of the battery and further improves the energy density or specific capacity of the secondary battery.

[0122] The secondary battery provided in the application can be prepared by a method comprising the following steps: preparing a copper foil by an electroplating method, wherein the electroplating method comprises applying a pulse current to an electroplating solution to reduce and deposit copper ions in the electroplating solution to form a copper foil, the peak value of the pulse current is 40,000-100,000 A, the valley value of the pulse current is 100-20,000 A, and the change period of the current is 50-5,000 ms.

[0123] In this context, the term "electroplating method" refers to a method of depositing a metal or alloy on the surface of a workpiece to form a metal layer by using the principle of electroplating.

[0124] In this context, the term "pulse current" refers to a current or voltage pulse that appears periodically.

[0125] Compared with the calendering method in the prior art, the electroplating method is mature and simple, has low requirements for equipment, and has low manufacturing cost. The copper foil prepared by the electroplating method has excellent tensile strength and elongation at break, and not only has excellent mechanical strength and plasticity, but also helps to reduce the manufacturing cost of the secondary battery.

[0126] Some schemes use high direct current to prepare copper foil by electroplating method in order to improve the mechanical strength of the copper foil. However, due to the small difference in work hardening ability between the grains, the ability to inhibit strain localization is reduced, and the copper foil is prone to concentrated stress during deformation, which reduces the plasticity of the copper foil and easily causes brittle fracture. In the preparation method provided in the application, a pulse current is used to continuously change the size of the current to adjust the nucleation and growth rate of the copper grains, thereby adjusting the size and morphology of the copper grains, and adjusting the number ratio of copper grains with a particle size of less than or equal to 0.5 μm and copper grains with a particle size of greater than 0.5 μm, and improving the tensile strength and elongation at break of the copper foil, so that the copper foil has excellent mechanical properties and plasticity.

[0127] In some embodiments, the peak value of the pulsed current is 40,000 A - 80,000 A. In some embodiments, the peak value of the pulsed current is 50,000 A - 70,000 A. In some embodiments, the peak value of the pulsed current is 55,000 A - 70,000 A, 50,000 A - 80,000 A, 55,000 A - 80,000 A, 50,000 A - 60,000 A.

[0128] In some embodiments, the peak value of the pulsed current is 45,000 A, 50,000 A, 55,000 A, 60,000 A, 65,000 A, 70,000 A, 75,000 A, 80,000 A, 85,000 A, 90,000 A, 95,000 A, 100,000 A, or a range between any two of the above values or any value between the ranges.

[0129] In some embodiments, the valley value of the pulsed current is 1,000 A - 10,000 A. In some embodiments, the valley value of the pulsed current is 2,000 A - 5,500 A. In some embodiments, the valley value of the pulsed current is 2,500 A - 5,000 A, 2,000 A - 8,000 A, 2,000 A - 5,000 A.

[0130] In some embodiments, the valley value of the pulsed current is 1,000 A, 2,000 A, 2,500 A, 3,500 A, 4,000 A, 4,500 A, 5,000 A, 8,000 A, 10,000 A, 15,000 A, 20,000 A, or a range between any two of the above values or any value between the ranges.

[0131] In some embodiments, the period of the pulsed current is 500 ms - 5,000 ms. In some embodiments, the period of the pulsed current is 2,000 ms - 4,000 ms. In some embodiments, the period of the pulsed current is 50 ms, 100 ms, 200 ms, 500 ms, 1,000 ms, 1,200 ms, 1,500 ms, 1,800 ms, 2,000 ms, 2,500 ms, 2,750 ms, 3,000 ms, 3,500 ms, 4,000 ms, 4,500 ms, 5,000 ms, or a range between any two of the above values or any value between the ranges.

[0132] Herein, the term "peak value" refers to the maximum current value of the pulsed current, typically the value at the peak of the pulsed current waveform. Similarly, the term "valley value" refers to the minimum current value of the pulsed current, typically the value at the valley of the pulsed current waveform.

[0133] In the present disclosure, the term "variation period" refers to the time between two adjacent peaks or troughs in the pulsed current waveform, in ms.

[0134] During the deposition of copper ions, increasing the current can accelerate the deposition rate of copper ions, which helps to form fine grains with small particle size; reducing the current can reduce the deposition rate of copper ions, which helps to form relatively regular and orderly grains with large particle size, such as columnar crystals. Adjusting the variation parameters of the current, i.e. the peak value, the trough value and the variation period, helps to adjust the formation of copper grain nuclei, the growth rate of grains and the deposition time of copper ions, to produce a heterogeneous grain morphology with mixed distribution of large and small grains, and to adjust the particle size and quantity ratio of the grains, thereby improving the tensile strength and elongation of the copper foil, so that the copper foil has excellent mechanical strength and plasticity.

[0135] In some embodiments, the pulsed current comprises one or more of a square wave pulsed current, a sinusoidal pulsed current, a triangular pulsed current, and a sawtooth pulsed current.

[0136] In some embodiments, the pulsed current comprises a sinusoidal pulsed current. The sinusoidal pulsed current continuously and periodically varies, which is conducive to the continuous and variable growth of the grains.

[0137] In some embodiments, the preparation method is a continuous production method.

[0138] In some embodiments, the preparation method is a roller deposition method. The working principle is that the cathode roller is connected to the negative electrode of the power supply, and the anode groove is connected to the positive electrode of the power supply. When the electroplating solution containing copper ions enters the anode groove, an electric field is formed between the positive and negative electrodes. Under the action of the electric field, copper ions migrate to the surface of the cathode roller and deposit. The deposited copper foil is peeled off from the cathode roller and wound on another roller. The electroplating solution is continuously added and circulated, and copper ions are continuously deposited on the cathode roller under the action of the electric field, continuously peeled off and wound on the winding shaft. This preparation method can realize the continuous production of large-scale copper foil and provide the possibility for industrial application.

[0139] In some embodiments, the cathode electrode is a titanium roller or a titanium plate.

[0140] In some embodiments, the anode electrode is a titanium substrate plate.

[0141] In some embodiments, the distance between the cathode electrode and the anode electrode is 8mm-20mm. In some embodiments, the distance between the cathode electrode and the anode electrode is 8mm-12mm. In some embodiments, the distance between the cathode electrode and the anode electrode is 8mm, 9mm, 10mm, 11mm, 12mm, 13mm, 14mm, 15mm, 16mm, 17mm, 18mm, 19mm, 20mm, or a range between any two of the aforementioned values or any value between the aforementioned ranges.

[0142] In some embodiments, the temperature of the electroplating deposition is 45°C-60°C. In some embodiments, the temperature of the electroplating deposition is 50°C-60°C. In some embodiments, the temperature of the deposition can be selected from 45°C, 50°C, 55°C, 60°C, a range between any two of the aforementioned values or any value between the aforementioned ranges.

[0143] In some embodiments, the speed of the cathode roller is 2m / min-5m / min. In some embodiments, the speed of the cathode roller is 2m / min-3m / min. In some embodiments, the speed of the cathode roller is 2m / min, 2.5m / min, 3m / min, 3.5m / min, 4m / min, 4.5m / min, 5m / min, or a range between any two of the aforementioned values or any value between the aforementioned ranges.

[0144] The cathode roller can be any roller suitable for the preparation of copper foil in the art, for example, a titanium roller.

[0145] In some embodiments, the electroplating solution comprises a leveling agent, a wetting agent, a brightener.

[0146] In the present context, the term "leveling agent" refers to a substance added to the electroplating solution that can improve the flatness of the plated layer, which can adhere to the tip of the copper foil with a fast deposition rate, inhibit the growth of the crystal grains, balance the growth rate of the pits and the tip, and improve the flatness of the copper foil.

[0147] In the present context, the term "wetting agent" refers to a substance used to reduce the interfacial tension between the electroplating solution and the electrode, and to improve the adhesion of the plated layer to the substrate. The wetting agent can improve the wettability of the electroplating solution to the substrate, and the wetting of the electroplating solution on the cathode is sufficient to allow it to be combined with a large current to achieve fast electrodeposition, to improve the nucleation rate of the copper foil, and to reduce the grain size of the copper foil.

[0148] In the present context, the term "brightener" refers to a substance that improves the smoothness of the plated layer and reduces the surface roughness. The brightener can make the grain size of the copper foil smaller, and reduce the surface roughness of the copper foil and improve the smoothness of the surface.

[0149] In some embodiments, the leveling agent comprises one or more of collagen, sodium saccharin. In some embodiments, the leveling agent comprises collagen and sodium saccharin.

[0150] Without being bound by any theory, collagen can inhibit the deposition of copper ions, balancing the growth rate of the pits and the tips. The introduction of sodium saccharin can attract the deposition of copper ions at the surface depressions of the copper foil, reducing the microscopic defects or unevenness inside the copper foil, reducing these warpage defects caused by defects. Two different leveling agents help to further improve the surface pits and protrusions of the copper foil, improving the flatness of the copper foil.

[0151] In some embodiments, the concentration of the collagen in the electroplating solution is 60 mg / L-300 mg / L. In some embodiments, the concentration of the collagen in the electroplating solution is 80 mg / L-150 mg / L. In some embodiments, the concentration of the collagen in the electroplating solution is 60 mg / L, 80 mg / L, 100 mg / L, 150 mg / L, 200 mg / L, 250 mg / L, 300 mg / L, or any range between any two of the aforementioned values or any value between the aforementioned ranges.

[0152] The collagen can be selected from proteins commonly used in the copper foil field with a molecular weight size, for example, collagen with a relative molecular weight of 8000-12000.

[0153] In some embodiments, the concentration of the sodium saccharin in the electroplating solution is 0.5 g / L-10 g / L. In some embodiments, the concentration of the sodium saccharin in the electroplating solution is 0.5 g / L-4 g / L. In some embodiments, the concentration of the sodium saccharin in the electroplating solution is 0.5 g / L, 0.8 g / L, 1 g / L, 1.5 g / L, 2.0 g / L, 2.5 g / L, 3.0 g / L, 3.5 g / L, 4.0 g / L, 4.5 g / L, 5.0 g / L, 5.5 g / L, 6.0 g / L, 6.5 g / L, 7.0 g / L, 7.5 g / L, 8.0 g / L, 8.5 g / L, 9.0 g / L, 9.5 g / L, 10 g / L, or any range between any two of the aforementioned values or any value between the aforementioned ranges. In some embodiments, the wetting agent comprises one or more of hydroxyethyl cellulose, polyethylene glycol. In some embodiments, the wetting agent comprises hydroxyethyl cellulose and polyethylene glycol.

[0154] Without being bound by any theory, hydroxyethyl cellulose has good water solubility and thickening performance, and can form a uniform solution in water, increase the viscosity of the electroplating solution, and adhere to the surface of the copper foil substrate. Polyethylene glycol, as a lubricant and wetting agent, can reduce the surface tension of the liquid and enhance the wetting ability of the liquid to the solid surface. The combination of the two can improve the adhesion performance of the electroplating solution, increase and assist the adhesion of copper ions to the substrate surface and deposition, improve the consistency of the crystal grains during the deposition process, reduce the difference in crystal grain size in the thickness direction, improve the uniformity of the copper foil, and thus improve the mechanical properties of the copper foil.

[0155] In some embodiments, the concentration of the polyethylene glycol in the electroplating solution is 50 mg / L-200 mg / L. In some embodiments, the concentration of the polyethylene glycol in the electroplating solution is 60 mg / L-150 mg / L. In some embodiments, the concentration of the polyethylene glycol in the electroplating solution is 50 mg / L, 60 mg / L, 70 mg / L, 80 mg / L, 100 mg / L, 150 mg / L, 200 mg / L, or any range between any two of the above values or any value between the ranges.

[0156] The polyethylene glycol can be selected from commonly used molecular weights in the field of copper foil, for example, polyethylene glycol with a relative molecular weight of 4000.

[0157] In some embodiments, the concentration of the hydroxyethyl cellulose in the electroplating solution is 30 mg / L-200 mg / L. In some embodiments, the concentration of the hydroxyethyl cellulose in the electroplating solution is 50 mg / L-150 mg / L. In some embodiments, the concentration of the hydroxyethyl cellulose in the electroplating solution is 30 mg / L, 40 mg / L, 50 mg / L, 60 mg / L, 70 mg / L, 80 mg / L, 90 mg / L, 100 mg / L, 120 mg / L, 140 mg / L, 160 mg / L, 180 mg / L, 200 mg / L, or any range between any two of the above values or any value between the ranges.

[0158] The polyethylene glycol can be selected from commonly used molecular weights in the field of copper foil, and in some embodiments, the relative molecular mass of the hydroxyethyl cellulose is 120000.

[0159] In some embodiments, the mass ratio of the polyethylene glycol and the hydroxyethyl cellulose in the electroplating solution is (1-1.5): 1. In some embodiments, the mass ratio of the polyethylene glycol and the hydroxyethyl cellulose in the electroplating solution is (1.2-1.5): 1. As an example, the mass ratio of the polyethylene glycol and the hydroxyethyl cellulose is 1:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, or a range between any two of the aforementioned values or any value between the aforementioned ranges.

[0160] In some embodiments, the brightener comprises sodium poly(dithio)propane sulfonate.

[0161] Without being bound by any theory, sodium poly(dithio)propane sulfonate is adsorbed on the cathode copper surface through the thiol functional group or disulfide bond, the terminal sulfonate anion captures the hydrated copper ions in the electroplating solution to destroy its hydration, and interacts with the chloride ions adsorbed on the cathode surface to transfer electrons to the captured copper ions through the chloride ions, thereby greatly improving the electrochemical reduction rate of copper ions, refining the grain and achieving material strengthening.

[0162] In some embodiments, the concentration of the sodium poly(dithio)propane sulfonate in the electroplating solution is 500 mg / L-2000 mg / L. In some embodiments, the concentration of the sodium poly(dithio)propane sulfonate in the electroplating solution is 500-1000 mg / L. In some embodiments, the concentration of the sodium poly(dithio)propane sulfonate in the electroplating solution is 500 mg / L, 600 mg / L, 700 mg / L, 800 mg / L, 900 mg / L, 1000 mg / L, 1500 mg / L, 2000 mg / L, or a range between any two of the aforementioned values or any value between the aforementioned ranges.

[0163] In some embodiments, the electroplating solution comprises chloride ions, and the concentration of the chloride ions (in terms of chlorine atoms) is 20 mg / L-80 mg / L. In some embodiments, the concentration of the sodium poly(dithio)propane sulfonate in the electroplating solution is 40 mg / L-80 mg / L. In some embodiments, the concentration of the chloride ions (in terms of chlorine atoms) is 20 mg / L, 30 mg / L, 40 mg / L, 50 mg / L, 60 mg / L, 70 mg / L, 80 mg / L, or a range between any two of the aforementioned values or any value between the aforementioned ranges. The use of chloride ions in combination with wetting agents can further improve the electrodeposition process.

[0164] In some embodiments, the electroplating solution comprises: collagen with a concentration of 60 mg / L-300 mg / L, sodium saccharin with a concentration of 0.5 g / L-10 g / L, polyethylene glycol with a concentration of 50-200 mg / L, hydroxyethyl cellulose with a concentration of 30 mg / L-200 mg / L, sodium poly(dithio-dipropyl sulfone) with a concentration of 500 mg / L-2000 mg / L, chloride ion with a concentration of 20 mg / L-80 mg / L.

[0165] In some embodiments, the electroplating solution comprises: collagen with a concentration of 80 mg / L-150 mg / L, sodium saccharin with a concentration of 0.5 g / L-4 g / L, polyethylene glycol with a concentration of 60 mg / L-150 mg / L, hydroxyethyl cellulose with a concentration of 50 mg / L-150 mg / L, sodium poly(dithio-dipropyl sulfone) with a concentration of 500 mg / L-1000 mg / L, chloride ion with a concentration of 40 mg / L-80 mg / L.

[0166] The electroplating solution also comprises a copper source to provide copper ions for the electroplating solution. The electroplating solution also comprises sulfuric acid to provide an acidic environment for the reduction of copper ions.

[0167] In some embodiments, the concentration of copper ions (in terms of copper atoms) is 60 g / L-100 g / L. In some embodiments, the concentration of copper ions (in terms of copper atoms) is 80 g / L-100 g / L. In some embodiments, the concentration of copper ions (in terms of copper atoms) is 60 g / L, 70 g / L, 80 g / L, 90 g / L, 100 g / L, a range between any two of the above values, or any value between the ranges.

[0168] In some embodiments, the concentration of sulfuric acid is 60 g / L-110 g / L. In some embodiments, the concentration of sulfuric acid is 80 g / L-110 g / L. In some embodiments, the concentration of sulfuric acid is 60 g / L, 70 g / L, 80 g / L, 90 g / L, 100 g / L, 110 g / L, a range between any two of the above values, or any value between the ranges.

[0169] In some embodiments, the pH of the electroplating solution is 2.5-4.5, for example 2.5, 3.0, 3.5, 4.0, 4.5, or a range between any two of the above values, or any value between the ranges.

[0170] The synergistic effect of the electroplating solution and the electroplating parameters helps to form a copper foil with heterogeneous grain morphology with different grain sizes, which has excellent tensile strength and elongation at break, and is conducive to improving the safety performance of high-energy density or high-expansion batteries. At the same time, the preparation method can realize large-size manufacturing and has the prospect of industrial application.

[0171] [negative electrode sheet]

[0172] As an example of the negative electrode sheet, the negative current collector has two surfaces opposite in the thickness direction thereof, and the negative film layer is provided on either one or both of the two surfaces of the negative current collector.

[0173] In some embodiments, the negative current collector can be the current collector described in the first aspect of the present application, thereby providing a material basis for improving the energy density of the secondary battery and helping to improve the safety of the secondary battery.

[0174] In some embodiments, the other negative active material includes, but is not limited to, one or more of conventional natural graphite, artificial graphite, soft carbon, hard carbon, silicon-based material, tin-based material, and lithium titanate. The tin-based material can include one or more of elemental tin, tin oxide, and tin alloy material.

[0175] In some embodiments, the negative active material includes a silicon-based material, which includes one or more of elemental silicon, silicon oxide, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy material.

[0176] In some embodiments, the negative film layer can also optionally include a binder. The binder can be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0177] In some embodiments, the negative electrode sheet also includes a conductive agent. The conductive agent includes one or more of super-conductive carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0178] In some embodiments, the conductive agent includes carbon black. In some embodiments, the conductive agent includes carbon nanotubes. In some embodiments, the conductive agent includes carbon black and carbon nanotubes. The conductive agent is widely available and has excellent conductivity, which is conducive to controlling the manufacturing cost of the secondary battery and improving the conductivity of the negative electrode sheet.

[0179] In some embodiments, the negative film layer can also optionally include other auxiliary agents, such as thickening agents (e.g., sodium carboxymethyl cellulose (CMC-Na)) and the like.

[0180] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder, and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry on a negative electrode current collector; and subjecting the negative electrode current collector to drying, cold pressing, and the like to obtain the negative electrode sheet.

[0181] [Positive electrode sheet]

[0182] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector, the positive electrode film layer including a positive electrode active material.

[0183] As an example, the positive electrode current collector has two surfaces opposite in the thickness direction thereof, and the positive electrode film layer is disposed on either one or both of the two opposite surfaces of the positive electrode current collector.

[0184] In some embodiments, the positive electrode current collector can be a metal foil or a composite current collector. As a metal foil, for example, an aluminum foil can be used. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer material base material (e.g., a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0185] In some embodiments, the positive electrode active material can be a positive electrode active material for a battery known in the art. As an example, the positive electrode active material can include at least one of a lithium-containing phosphate having an olivine structure, a lithium transition metal oxide, and a modified compound of each thereof. However, the present application is not limited to these materials, and other conventional materials that can be used as a positive electrode active material for a battery can also be used. These positive electrode active materials can be used alone or in combination of two or more. Examples of the lithium transition metal oxide can include, but are not limited to, lithium cobalt oxide (e.g., LiCoO2), lithium nickel oxide (e.g., LiNiO2), lithium manganese oxide (e.g., LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (e.g., LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (which can also be referred to as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2 (which can also be referred to as NCM 523 ), LiNi 0.5 Co0.25 Mn 0.25 O2(also can be referred to as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2(also can be referred to as NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1 O2(also can be referred to as NCM 811 ), lithium nickel cobalt aluminum oxide (such as LiNi 0.85 Co 0.15 Al 0.05 O2), and modified compounds thereof. Examples of lithium-containing phosphates of olivine structure can include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4(also can be referred to as LFP)), a composite of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite of lithium manganese phosphate and carbon, lithium manganese iron phosphate, a composite of lithium manganese iron phosphate and carbon.

[0186] In some embodiments, the positive electrode film layer further optionally includes a binder. As an example, the binder can include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene-fluorotetrafluoroethylene-propylene terpolymer, a vinylidene-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin.

[0187] In some embodiments, the positive electrode film layer further optionally includes a conductive agent. As an example, the conductive agent can include at least one of super P, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0188] In some embodiments, the positive electrode tab can be prepared by dispersing the above-mentioned components for preparing the positive electrode tab, such as the positive electrode active material, the conductive agent, the binder, and any other components, in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry on a positive electrode current collector, and after processes such as drying, cold pressing, etc., the positive electrode tab can be obtained.

[0189] [Electrolyte]

[0190] The electrolyte plays a role of conducting ions between the positive electrode tab and the negative electrode tab. The type of the electrolyte is not specifically limited in the present application and can be selected as needed. For example, the electrolyte can be liquid, gel, or all-solid.

[0191] In some embodiments, the electrolyte adopts an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0192] In some embodiments, the electrolyte salt can be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonylimide, lithium bis-trifluoromethanesulfonylimide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorobisoxalate borate, lithium bisoxalate borate, lithium difluorobisoxalate phosphate, and lithium tetrafluorobisoxalate phosphate.

[0193] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclobutane sulfone, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0194] In some embodiments, the electrolyte solution can further optionally include an additive. For example, the additive can include a negative electrode film-forming additive, a positive electrode film-forming additive, and can further include an additive capable of improving certain performance of the battery, such as an additive for improving overcharge performance of the battery, an additive for improving high-temperature or low-temperature performance of the battery, etc.

[0195] [Separator]

[0196] In some embodiments, the secondary battery further includes a separator. The type of the separator is not particularly limited in the present application, and any known porous structure separator having good chemical stability and mechanical stability can be used.

[0197] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single layer film or a multi-layer composite film, and is not particularly limited. When the separator is a multi-layer composite film, the materials of the respective layers can be the same or different, and are not particularly limited.

[0198] In some embodiments, the positive electrode tab, the negative electrode tab, and the separator can be made into an electrode assembly through a winding process or a stacking process.

[0199] In some embodiments, the secondary battery can include an outer package. The outer package can be used to package the above-described electrode assembly and the electrolyte.

[0200] In some embodiments, the outer package of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the secondary battery can also be a soft package, such as a pouch-type soft package. The material of the soft package can be plastic, and as the plastic, polypropylene, polybutylene terephthalate, polybutylene succinate, etc. can be listed.

[0201] Further, the secondary battery, the battery module, the battery pack, and the power consuming device of the present application will be described below with appropriate reference to the accompanying drawings.

[0202] In one embodiment of the present application, a secondary battery is provided.

[0203] The shape of the secondary battery of the present application is not particularly limited, and can be cylindrical, square, or any other shape. For example, Figure 1 is a square structure secondary battery 5 as an example.

[0204] In some embodiments, with reference to Figure 2 , the outer package can include a housing 51 and a cover plate 53. The housing 51 can include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be provided on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet, and the separator can form an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is packaged in the receiving cavity. The electrolyte is impregnated in the electrode assembly 52. The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, and the skilled person in the art can select according to the specific actual needs.

[0205] In some embodiments, the secondary battery can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be one or more, and the specific number can be selected by the skilled person in the art according to the application and capacity of the battery module.

[0206] Figure 3 is a battery module 4 as an example. With reference to Figure 3 , in the battery module 4, a plurality of secondary batteries 5 can be arranged in sequence along the length direction of the battery module 4. Of course, it can also be arranged in any other arbitrary manner. Further, the plurality of secondary batteries 5 can be fixed by fasteners.

[0207] Optionally, the battery module 4 can further include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0208] In some embodiments, the above-mentioned battery module can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, and the specific number can be selected by the skilled person in the art according to the application and capacity of the battery pack.

[0209] Figure 4 and Figure 5 is a battery pack 1 as an example. With reference to Figure 4 and Figure 5In the battery pack 1, a battery case and a plurality of battery modules 4 disposed in the battery case can be included. The battery case includes an upper case 2 and a lower case 3, and the upper case 2 can be disposed on the lower case 3 to form an enclosed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery case in any manner.

[0210] In addition, the application also provides a power utilization device, which includes at least one of the secondary battery, the battery module, or the battery pack provided by the application. The secondary battery, the battery module, or the battery pack can be used as a power supply of the power utilization device, and can also be used as an energy storage unit of the power utilization device. The power utilization device can include a mobile device (such as a mobile phone, a notebook computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship and a satellite, an energy storage system, etc., but is not limited thereto.

[0211] As the power utilization device, the secondary battery, the battery module, or the battery pack can be selected according to the use requirement thereof.

[0212] Figure 6 The power utilization device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the requirement of the power utilization device for high power and high energy density of the secondary battery, the battery pack or the battery module can be used.

[0213] The power utilization device is a mobile phone, a tablet computer, a notebook computer, etc. The device usually requires thinning, and the secondary battery can be used as a power supply.

[0214] Embodiment

[0215] Hereinafter, the embodiments of the application are described. The embodiments described below are exemplary and are only used to explain the application, and cannot be understood as a limitation of the application. If the specific technology or condition is not indicated in the embodiments, the technology or condition described in the literature in the art or according to the product instruction is used. If the reagent or instrument is not indicated by the manufacturer, it is a conventional product that can be obtained by market purchase.

[0216] I. Performance test

[0217] (1) Test of grain characteristics of copper foil

[0218] The cross section of the copper foil is observed by electron backscatter diffraction (EBSD) combined with scanning electron microscopy, wherein the electron backscatter diffraction instrument is Oxford C-Nano+. The inverse pole figure map (magnification 3000) is obtained, the particle size of each grain is measured, the diameter of the equivalent circle of the grain is taken as the particle size of the grain, the grain particle size is statistically distributed, the skewness distribution is fitted, and the total number of copper grains, the average particle size, the maximum particle size, the minimum particle size, the particle size span, the number ratio of copper grains with a particle size less than or equal to 0.5 μm, and the number ratio of copper grains with a particle size greater than 0.5 μm are obtained.

[0219] (2) Mechanical property test

[0220] According to GB / T 5230-1995 "Electrolytic Copper Foil", the copper foil prepared in the examples is cut into a tensile sample with a length L0 of 50 mm and a width of 15 mm. The tensile properties are tested by a universal testing machine at 25°C, and the tensile rate is set to 50 mm / min.

[0221] The cross-sectional area of the tensile sample is wherein ρ is 8.96 g / cm 3 , the unit of m is gram, and the unit of L0 is centimeter.

[0222] The sample is continuously loaded until it is pulled apart, the maximum load F is read from the force dial or the tensile curve, and the tensile strength σ is calculated according to Formula I. b

[0223]

[0224] The distance between the two lines after the sample is pulled apart is L1, which is measured on the sample or read from the tensile curve. L1 can be measured by a straight line method or a displacement method, and the elongation at break δ is calculated according to Formula II.

[0225]

[0226] (3) Hardness test

[0227] The copper foil sample is placed in a metallographic hot-embedding machine, wood powder is poured in, and then heated at a rate of 150°C / 10 min, a Vickers hardness tester is used to press the copper foil sample (50 g weight), the lengths of two diagonal lines are optically measured, and the corresponding Vickers hardness is obtained according to the following Vickers hardness calculation formula.

[0228]

[0229] HV represents the Vickers hardness;

[0230] F represents the load of the indenter (Newton force);

[0231] α represents the included angle between the opposite faces of the indenter (136°).​

[0232] d represents the average value of the diagonal length of the bar (mm).

[0233] (4) Crack failure corresponding to the state of health (SOH)

[0234] At 25°C, the battery is charged at 1C constant current to a voltage of 3.8V, then charged at 3.8V constant voltage to a current ≤0.05C, and then the battery is discharged at 1C constant current to a voltage of 2.5V, which is one charge and discharge process. Repeat the cycle charge and discharge, monitor the SOH of the battery throughout the process, then every 1% SOH, the battery is subjected to computer tomography (CT) to determine whether cracks have occurred inside the battery. If cracks occur, the battery after crack failure during the cycle is disassembled to observe whether the negative electrode sheet is broken, and the SOH corresponding to the crack failure is obtained.

[0235] II. Preparation method

[0236] Example 1

[0237] (1) Preparation of copper foil

[0238] A copper plate or copper wire with a purity of 99.9% or more is dissolved in 98% sulfuric acid to obtain a copper sulfate solution, which is used as a copper source. An additive and hydrochloric acid are added to configure an electroplating solution at 55°C. The concentration of collagen (relative molecular weight 8000-12000) is 120 mg / L, the concentration of polyethylene glycol (relative molecular weight 4000) is 80 mg / L, the concentration of hydroxyethyl cellulose (relative molecular weight about 120000) is 60 mg / L, the concentration of chloride ions is 40 mg / L, the concentration of polydithiodipropyl sulfonic acid sodium is 600 mg / L, the concentration of sodium saccharin is 2 g / L, the concentration of copper ions (calculated as copper atoms) is 90 g / L, and the rest is deionized water. The pH of the electroplating solution is 3.5.

[0239] A sinusoidal pulse current is used to periodically apply current to the cathode titanium roller after polishing treatment, and the anode electrode is a titanium substrate plate. The titanium roller in the electroplating solution has an area of 8.67 m 2 The rotation speed (roller speed) of the titanium roller is 2.4 m / min, the sinusoidal pulse current is applied, the peak current is 55000 A, the peak-to-valley current is 2500 A, the period is 3000 ms, the distance between the cathode and the anode is 10 mm, the deposition temperature is 55°C, the copper foil is deposited on the titanium roller, and the thickness of the copper foil is 6 μm.

[0240] (2) Preparation of the battery

[0241] The positive active material LiNi 0.8 Co 0.1Mn 0.1 O2(NCM811), conductive agent acetylene black, binder polyvinylidene fluoride (PVDF) were dissolved in solvent N-methyl pyrrolidone (NMP) in a weight ratio of 90:5:5, and after being fully stirred and mixed uniformly, a positive electrode slurry was obtained; then the positive electrode slurry was uniformly coated on the positive electrode current collector, and after drying, cold pressing and slitting, a positive electrode tab was obtained.

[0242] The negative electrode active material silicon-carbon (silicon content 20wt%-50wt%), artificial graphite, conductive agent acetylene black, binder styrene-butadiene rubber (SBR), thickening agent sodium carboxymethyl cellulose (CMC-Na) were dissolved in deionized water in a weight ratio of 20:76:1:1.5:1.5, and after being uniformly mixed, a negative electrode slurry was prepared; then the negative electrode slurry was uniformly coated on the negative electrode current collector copper foil one or more times, and after drying, a negative electrode film was obtained, and then cold pressing and slitting were performed to obtain a negative electrode tab.

[0243] In an argon atmosphere glove box (H2O <0.1 ppm, O2<0.1 ppm), ethylene carbonate, diethyl carbonate, dimethyl carbonate were mixed in a volume ratio of 1:1:1, and LiPF6 was dissolved in the above solution to obtain an electrolyte. In the electrolyte, the concentration of LiPF6 was 1 mol / L. Then, 2.0wt% of fluoroethylene carbonate, 0.5wt% of 1,3-propane sultone and 0.5wt% of succinic anhydride were added as additives to the above organic solvent, and mixed and stirred uniformly to obtain an electrolyte.

[0244] A polypropylene film was used as a separator film.

[0245] The positive electrode tab, the separator film and the negative electrode tab were stacked in order, with the separator film between the positive electrode tab and the negative electrode tab to play a separating role, and then wound to obtain an electrode assembly; the electrode assembly was placed in a battery case, dried, and then injected with an electrolyte, and then subjected to processes such as formation and standing to obtain a lithium ion battery.

[0246] Example 2-3

[0247] The preparation method of Example 2-3 was basically the same as that of Example 1, but the composition of the electroplating solution (see Table 1) and the sinusoidal pulse current parameters (see Table 2) were adjusted; the deposited copper foil had a thickness of 6 μm.

[0248] Comparative Example 1-2

[0249] The preparation method of Comparative Example 1 was basically the same as that of Example 1, but direct current was used for deposition, and the deposition current was 55,000 A. The copper foil had a thickness of 6 μm.

[0250] The preparation method of Comparative Example 2 was basically the same as that of Example 1, and a direct current of 30,000 A was used for deposition. The copper foil had a thickness of 6 μm.

[0251] Table 1: Electroplating solution components

[0252]

[0253] Table 2: Sinusoidal pulsed current parameters

[0254]

[0255] NA means not applicable.

[0256] The grain characteristics and mechanical characteristics of the copper foils prepared in Test Examples 1-3 and Comparative Examples 1-2, and the results of the secondary batteries are shown in Table 3 below.

[0257] Table 3

[0258]

[0259]

[0260] Figure 7 、 Figure 10 and Figure 13 respectively show the inverse pole figure maps of the cross-section of the copper foils of Example 1-3, which presents a heterogeneous grain structure with a distribution of fine and coarse grains. In comparison with the inverse pole figure maps of the copper foils of Comparative Example 2 ( Figure 17 ), it can be seen that there are more fine grains and the grain size is smaller in the copper foils of Example 1-3.

[0261] Figure 9 、 Figure 12 、 Figure 15 and Figure 19 respectively show the grain size distribution maps of the copper foils of Example 1-3 and Comparative Example 2. In combination with Table 3, it can be seen that in the copper foils of Example 1-3, the proportion of the number of copper grains with a grain size of 0.5 μm or less is between 70% and 95%, and is concentrated in the range of 80% to 95%, while in Comparative Example 2, the proportion of the number of copper grains with a grain size of 0.5 μm or less is less than 50%, indicating that the copper grains of Example 1-3 are smaller in general and the copper foils have higher mechanical strength. In addition, in the copper foils of Example 1-3, the proportion of the number of copper grains with a grain size of more than 0.5 μm is in the range of 5% to 30%, while in Comparative Example 2, the proportion of the number of copper grains with a grain size of more than 0.5 μm is 50.5%, and the copper foils of Example 1-3 have a plasticity comparable to that of the copper foils of Comparative Example 2. This indicates that the sinusoidal pulsed current for preparing copper foils helps to adjust the grain size distribution of copper grains, thereby improving the mechanical properties and plasticity of the copper foils.

[0262] In the copper foils of Examples 1-3, the average particle size is in the range of 0.3-0.6 μm, the maximum particle size is in the range of 1-2 μm, the minimum particle size is in the range of 0.1-0.3 μm, and the particle size span is 0.8-2 μm. In Comparative Example 2, the span of the copper grain particle size is larger, and the average copper grain particle size is larger. It can be understood that the copper foils of Examples 1-3 have more cumulative crystal interface density, and have high mechanical strength. The copper foil of Comparative Example 2 has less cumulative crystal interface density, and has low mechanical strength. This also corresponds to the tensile strength of Examples 1-3 and Comparative Example 2.

[0263] Figure 8 , Figure 11 and Figure 14 respectively show the tensile curves of the copper foils of Examples 1-3. It can be seen that the tensile strength of the prepared copper foils is in the range of 600-1000 MPa, and the elongation at break of the copper foils is in the range of 4-8% (Table 3), indicating that the copper foils have excellent strength and plasticity. Figure 16 shows the tensile curve of the copper foil of Comparative Example 1. The tensile strength of the copper foil is similar to that of Example 1, but the elongation at break is only 2.7%, indicating poor plasticity. Figure 18 shows the tensile curve of the copper foil of Comparative Example 1. The elongation at break of the copper foil is similar to that of Example 1, but the tensile strength is poor.

[0264] The hardness of the copper foils of Examples 1-3 is in the range of 55-65 HV, indicating that the copper foils have good pressure deformation or puncture resistance.

[0265] Comparative Example 1 uses the same direct current as Example 1 to prepare the copper foil. Compared with Example 1, it can be seen that the copper foil of Comparative Example 1 has similar tensile strength and hardness to the copper foil of Example 1, but the elongation at break is significantly lower than that of Example 1. This indicates that the particle size distribution of the copper grains can be adjusted by the sinusoidal pulse current and the current parameters, so that the copper foil has good mechanical strength and excellent plasticity.

[0266] Compared with Comparative Examples 1-2, the crack failure of Examples 1-3 corresponds to a significantly reduced SOH, indicating that the copper foils prepared by Examples 1-3 significantly improve the service life of the secondary battery and improve the safety of the secondary battery. Although the copper foil of Comparative Example 2 has excellent elongation at break, the tensile strength is low, resulting in a relatively high SOH value corresponding to crack failure. Relatively speaking, the tensile strength of the copper foil prepared by Examples 1-3 using the sinusoidal pulse current with varying current size is significantly improved, and the plasticity is maintained, and the SOH value corresponding to crack failure is significantly reduced, which can improve the service life of the battery by more than 25% SOH.

[0267] Note that the present application is not limited to the above-described embodiments. The above-described embodiments are merely examples, and embodiments having substantially the same configuration, function, and effect as the technical idea of the present application are included in the technical scope of the present application. Furthermore, other modes constructed by applying various modifications to the embodiments, or by combining part of the configurations of the embodiments, which can be conceived by those skilled in the art without departing from the spirit of the present application, are also included in the scope of the present application.

Claims

1. A secondary battery, characterized in that, The invention includes copper foil, which comprises copper grains of different sizes. The copper grains include copper grains with a diameter of less than or equal to 0.5 μm and copper grains with a diameter greater than 0.5 μm. The proportion of copper grains with a diameter of less than or equal to 0.5 μm in the total number of copper grains is 70%-95%, and the proportion of copper grains with a diameter greater than 0.5 μm in the total number of copper grains is 5%-30%.

2. The secondary battery according to claim 1, characterized in that, The particle size range of the copper grains with a particle size greater than 0.5 μm is: greater than 0.5 μm and less than or equal to 3 μm.

3. The secondary battery according to claim 1, characterized in that, Copper grains with a diameter of 0.5 μm or less account for 80%-95% of the total number of copper grains, and / or copper grains with a diameter greater than 0.5 μm account for 5%-20% of the total number of copper grains.

4. The secondary battery according to claim 1, characterized in that, The copper foil satisfies at least one of the following conditions: (1) The average grain size of copper is 0.3 μm-1.2 μm; (2) The maximum grain size of copper is 1μm-2.5μm; (3) The minimum grain size of copper is 0.1μm-0.3μm; (4) The grain size range of copper grains is 0.8μm-2.5μm.

5. The secondary battery according to claim 1, characterized in that, The copper foil satisfies at least one of the following conditions: (1) The average grain size of copper is 0.3 μm-0.6 μm; (2) The maximum grain size of copper is 1.2 μm-2.0 μm; (3) The minimum grain size of copper is 0.1μm-0.3μm; (4) The grain size range of copper grains is 1μm-2μm.

6. The secondary battery according to any one of claims 1 to 5, characterized in that, Under test conditions of room temperature (20±10℃), sample length × width of (50±0.25mm) × (15±0.25mm), and tensile speed of 50±0.5mm / min, the tensile strength of the copper foil is 600MPa-1000MPa, and / or the elongation at break of the copper foil is 4%-8%.

7. The secondary battery according to any one of claims 1 to 5, characterized in that, Under the test conditions of room temperature (20±10℃), sample length×width (50±0.25mm)×(15±0.25mm), and tensile speed of 50±0.5mm / min, the copper foil has a tensile strength of 700MPa-1000 MPa and / or an elongation at break of 4%-7%.

8. The secondary battery according to any one of claims 1 to 5, characterized in that, Under the test conditions of room temperature (20±10℃), sample length × width of (50±0.25mm) × (15±0.25mm), and tensile speed of 50±0.5mm / min, the copper foil has a tensile strength of 700MPa-800 MPa and / or an elongation at break of 5%-6%.

9. The secondary battery according to any one of claims 1 to 5, characterized in that, The hardness of the copper foil is 55HV-65HV.

10. The secondary battery according to any one of claims 1 to 5, characterized in that, The hardness of the copper foil is 55HV-60HV.

11. The secondary battery according to any one of claims 1 to 5, characterized in that, The thickness of the copper foil is 4μm-10μm.

12. The secondary battery according to any one of claims 1 to 5, characterized in that, The secondary battery further includes a negative electrode film layer located on at least one side of the copper foil, wherein the negative electrode active material in the negative electrode film layer includes at least one of artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate.

13. The secondary battery according to claim 12, characterized in that, The negative electrode active material includes silicon-based materials, which include at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys.

14. A method for preparing a secondary battery, the method comprising preparing an electrode using copper foil as a current collector, characterized in that, The preparation of the copper foil includes an electroplating method, wherein the electroplating method includes applying a periodic pulsed current to an electroplating solution to reduce and deposit copper ions in the electroplating solution to form copper foil; the peak value of the pulsed current is 40000A-100000A, the valley value of the pulsed current is 100A-20000A, and the period of the current change is 50ms-5000ms.

15. The preparation method according to claim 14, characterized in that, The electroplating method satisfies one or more of the following conditions: (1) The peak value of the pulse current is 40000A-80000A; (2) The valley value of the pulse current is 1000A-10000A; (3) The period of the pulse current is 500ms-5000ms; (4) The distance between the cathode electrode and the anode electrode is 8mm~20mm; (5) The electroplating deposition temperature is 45℃-60℃; (6) The speed of the cathode roller is 2m / min - 5m / min.

16. The preparation method according to claim 14, characterized in that, The electroplating method satisfies one or more of the following conditions: (1) The peak value of the pulse current is 50000A-70000A; (2) The valley value of the pulse current is 2000A-5500A; (3) The period of the pulse current is 2000ms-4000ms; (4) The distance between the cathode electrode and the anode electrode is 8mm-12mm; (5) The electroplating deposition temperature is 50℃-60℃; (6) The speed of the cathode roller is 2m / min-3m / min.

17. The preparation method according to any one of claims 14 to 16, characterized in that, The pulse current includes one or more of square wave pulse current, sine wave pulse current, triangular wave pulse current, and sawtooth wave pulse current. Optionally, the pulse current includes sine wave pulse current.

18. The preparation method according to any one of claims 14 to 16, characterized in that, The electroplating solution includes a leveling agent, a wetting agent, and a brightening agent. The leveling agent includes one or more of collagen and sodium saccharin; The wetting agent includes one or more of hydroxyethyl cellulose and polyethylene glycol; The brightener includes sodium polydithiopropane sulfonate.

19. The preparation method according to claim 18, characterized in that, The electroplating solution comprises: collagen at a concentration of 60 mg / L to 300 mg / L, sodium saccharin at a concentration of 0.5 g / L to 10 g / L, polyethylene glycol at a concentration of 50 mg / L to 200 mg / L, hydroxyethyl cellulose at a concentration of 30 mg / L to 200 mg / L, sodium polydithiopropane sulfonate at a concentration of 500 mg / L to 2000 mg / L, and chloride ions (calculated as chloride atoms) at a concentration of 20 mg / L to 80 mg / L.

20. The preparation method according to claim 18, characterized in that, The electroplating solution comprises: collagen with a concentration of 80 mg / L to 150 mg / L, sodium saccharin with a concentration of 0.5 g / L to 4 g / L, polyethylene glycol with a concentration of 60 mg / L to 150 mg / L, hydroxyethyl cellulose with a concentration of 50 mg / L to 150 mg / L, sodium polydithiopropane sulfonate with a concentration of 500 mg / L to 1000 mg / L, and chloride ions with a concentration of 40 mg / L to 80 mg / L.

21. The preparation method according to any one of claims 14 to 16, characterized in that, The pH of the electroplating solution is 2.5-4.

5.

22. An electrical appliance, characterized in that, This includes the secondary battery as described in claims 1 to 13 or the secondary battery prepared by the preparation method described in claims 14 to 21.

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