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 mu m and copper grains with a particle size greater than 0.5 mu m, wherein the proportion of the number of copper grains with a particle size less than or equal to 0.5 mu m in the total number of copper grains is 70% to 95%, the proportion of the number of copper grains with a particle size greater than 1 mu m in the total number of copper grains is 1% to 10%, and the particle size span of the copper grains is 1.3 mu m to 3.4 mu m.

Description

Secondary battery, preparation method thereof and power utilization device

[0001] Cross-reference to related applications

[0002] This application refers to the Chinese Patent Application No. 202411154807.0, filed on August 21, 2024, for “Secondary battery, preparation method thereof and power utilization device”, which is incorporated by reference in its entirety.

[0003] This application refers to the Patent Application No. PCT / CN2025 / 115989, filed on August 20, 2025, for “Secondary battery, preparation method thereof and power utilization device”, which is incorporated by reference in its entirety. TECHNICAL FIELD

[0004] The present application relates to the technical field of secondary batteries, in particular to a secondary battery, a preparation method thereof and a power utilization device. BACKGROUND

[0005] With the secondary batteries being widely used in energy storage power supply systems such as hydroelectric, thermal, wind and solar power stations, and in multiple fields such as electric tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, etc., the market requires higher and higher energy density of the secondary batteries.

[0006] The energy density can be increased by using silicon-based negative electrode active materials or increasing the amount of secondary battery active materials, but the silicon-based materials have high thermal expansion coefficient, and the silicon-based batteries have serious volume expansion under heat; after the amount of secondary battery active materials is increased, the volume of the battery cell also increases accordingly, which 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.

[0007] Therefore, there is an urgent need for a secondary battery with improved performance of the negative electrode current collector. SUMMARY

[0008] The present application provides a secondary battery with a negative electrode current collector having 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.

[0009] In a first aspect, the application provides a secondary battery, the secondary battery comprising a negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a negative electrode film layer arranged on at least one side surface of the negative electrode current collector, the negative electrode current collector comprising a copper foil, the copper foil comprising copper grains with different grain sizes, the copper grains comprising copper grains with a grain size of 0.5 μm or less and copper grains with a grain size of more than 0.5 μm, wherein the proportion of the number of the copper grains with a grain size of 0.5 μm or less in the total number of the copper grains is 70% to 95%, the proportion of the number of the copper grains with a grain size of more than 1 μm in the total number of the copper grains is 1% to 10%, and the grain size span of the copper grains is 1.3 μm to 3.4 μm.

[0010] The copper grains with a grain size of more 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 grain size of 0.5 μm or less are fine grains close to particles, forming a fine grain region around the columnar grains. The copper foil substrate in the embodiments of the application has a proportion of 70% to 95% of fine grains, which is beneficial to increase the internal grain boundary area of the material, hinder the dislocation slip movement, and improve the tensile strength of the copper foil. Meanwhile, the copper foil has a proportion of 1% to 10% of columnar or columnar-like large grains with a grain size of more than 1 μm, and a copper grain distribution morphology with a grain size span of 1.3 μm to 3.4 μm, forming a grain morphology of "mainly fine grains and mixed distribution of large grains", which is beneficial to improve the tensile strength of the copper foil. Further, the dispersion degree of the copper grain size distribution is within a suitable range, and a small amount of heterogeneous grains of large grains are doped in the fine grains with a large proportion of the number, which can improve the internal grain boundary area while helping to balance the grain boundary area and the geometric necessary dislocation density of the copper grains, improve the grain dislocation movement resistance and reduce the occurrence of concentrated stress in the copper foil deformation process, improve the phenomenon of low grain boundary tortuosity and high brittleness of the copper foil caused by too many fine grains, make the copper foil have a suitable yield ratio, and is beneficial to reduce the probability of fracture along the grain boundary of the copper foil after the copper foil is subjected to external force, and balance the strength and brittleness of the copper foil, providing a material basis for solving the fracture of the current collector in the battery and the failure of the battery, and further improving the energy density and safety of the secondary battery.

[0011] The copper foil with a suitable yield ratio exhibits excellent mechanical strength and plasticity. The heterogeneous grain distribution morphology of the copper foil substrate in the embodiments of the application has a suitable yield ratio, effectively reduces the probability of brittle fracture along the grain boundary of the current collector after the current collector is subjected to external force, and helps to improve the energy density of the secondary battery while making the secondary battery have good safety and service life.

[0012] In any embodiment, the yield ratio of the copper foil is 0.45 to 0.80. In any embodiment, the yield ratio of the copper foil is 0.45 to 0.7.

[0013] The copper foil provided by the 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.

[0014] In any embodiment, the number of copper grains with a particle size greater than 1 μm accounts for 2%-10% of the total number of copper grains.

[0015] In any embodiment, the particle size span of the copper grains is 1.3 μm-3 μm.

[0016] In any embodiment, the expansion force of the secondary battery is greater than or equal to 1000 kgf, the secondary battery comprises a negative electrode sheet, the negative electrode sheet comprises a negative electrode current collector and a negative electrode film layer arranged on at least one side surface of the 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 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.

[0017] During the charging and discharging cycle operation of the secondary battery, the volume of the electrode sheet expands due to the insertion and extraction of active ions, especially in the self-generated negative electrode battery or the new silicon-based or lithium metal negative electrode, and the overall expansion of the secondary battery is high due to the comprehensive influence of the design of high group margin or large-size battery cells for the purpose of improving the energy density of the single battery cell. The copper foil current collector of the high-expansion secondary battery is obviously stretched and stressed during the operation process, which increases the probability of fracture or crack of the copper foil current collector, and deteriorates the safety and service life of the battery cell. Therefore, the copper foil comprising the copper grains with different particle sizes provided in the secondary battery of the application has excellent tensile strength and elongation at break, and has excellent plasticity while improving the mechanical strength, thereby reducing the probability of fracture or crack of the copper foil current collector in the high-expansion system, improving the service life of the secondary battery when the electrode sheet cracks or fails during operation, and further improving the safety and service life of the secondary battery.

[0018] In any embodiment, the thickness expansion rate of the secondary battery is 4%-10%,

[0019] In any of the embodiments, the secondary battery is a wound secondary battery, and the expansion force of the secondary battery is greater than or equal to 1000 kgf; the wound secondary battery comprises a negative electrode sheet, the negative electrode sheet comprises a negative electrode current collector and a negative electrode film layer arranged on at least one side surface of the 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 proportion of the number of the copper grains with a particle size less than or equal to 0.5 μm 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 μm in the total number of the copper grains is 5% to 30%.

[0020] The current collector of the negative electrode sheet in the wound secondary battery is prone to crack fracture in the corner area, because the wound cell is pressed and shaped after winding, which can cause irreversible damage to the current collector, so that the probability of crack or fracture of the corner area of the outer circle and the bending area of the inner circle of the negative electrode sheet increases significantly under the expansion stress caused by the increase of the internal pressure of the secondary battery and the volume expansion, which aggravates the risk of cell failure and worsens the service life and safety of the secondary battery. The copper foil comprising the copper grains with different particle sizes provided in the wound secondary battery of the present application significantly improves the tensile strength and elongation at break of the copper foil, so that the copper foil has excellent mechanical properties and plasticity, the probability of crack or fracture of the copper foil under the expansion stress is reduced, the probability of crack fracture of the wound cell electrode sheet is reduced, which is beneficial to improving the service life of the secondary battery when the electrode sheet cracks and fails, so that the safety and service life of the secondary battery are further improved.

[0021] In any of the embodiments, the thickness expansion rate of the wound secondary battery is 4% to 10%.

[0022] In any of the embodiments, the secondary battery comprises a negative electrode sheet, the negative electrode sheet comprises a negative electrode current collector and a negative electrode film layer arranged on at least one side surface of the 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 proportion of the number of the copper grains with a particle size less than or equal to 0.5 μm 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 μm in the total number of the copper grains is 5% to 30%; and the gram capacity of the negative electrode active material in the negative electrode film layer is 800 mAh / g to 1500 mAh / g.

[0023] Increasing the amount of active material is one of the effective means to improve the energy density or capacity of the secondary battery, but this will lead to an increase in the volume of the battery cell; the applicant found that using high-gram capacity negative electrode active material can significantly improve the available lithium intercalation capacity of the negative electrode active material, increase the energy density or capacity of the secondary battery while reducing the impact on the volume of the battery cell, however, the high-gram capacity negative electrode active material has a large particle volume expansion before and after the lithium ion intercalation / deintercalation, the expansion stress acts on the current collector copper foil, increasing the probability of the copper foil being broken or cracked along the thickness direction after being subjected to the expansion stress, and deteriorating the safety and service life of the secondary battery. The copper foil provided in the present application includes copper grains with different particle sizes as described above, which has excellent tensile strength and elongation at break, has excellent plasticity while improving the mechanical strength, improves the energy density, capacity of the secondary battery while improving the safety of the secondary battery.

[0024] In any embodiment, the gram capacity of the negative electrode active material in the negative electrode film layer is 800 mAh / g-1500 mAh / g.

[0025] The copper foil provided in the present application is suitable for a secondary battery including a negative electrode active material with a gram capacity of 800 mAh / g-1500 mAh / g, high-gram capacity negative electrode material can intercalate more active ions, such as silicon-based negative electrodes, alkali metal negative electrodes, etc., which usually have high swelling properties, which will increase the probability of the copper foil being broken or cracked in the battery cell, and the copper foil has excellent mechanical properties and plasticity, which can reduce the risk of the copper foil being broken or cracked in a high-energy density battery system or a high-swelling battery system, and can improve the energy density, capacity and safety of the secondary battery.

[0026] In any embodiment, 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, which can adjust or optimize the tensile strength of the copper foil.

[0027] In any embodiment, the number of copper grains with a particle size greater than 0.5 μm and less than or equal to 3 μm accounts for 5%-30% based on the total number of the copper grains.

[0028] As described previously, the copper grain particle size satisfying greater than 0.5 μm and less than or equal to 3 μm can help to reduce the average grain size of the copper foil, further improve the mechanical strength of the copper foil; further controlling the number of copper grains with a particle size greater than 0.5 μm and less than or equal to 3 μm to satisfy 5%-30% can make the copper foil have good elongation at break, have excellent mechanical properties and plasticity, can reduce the risk of the copper foil being broken or cracked in a high-energy density battery system or a high-swelling battery system, and realize the simultaneous improvement of the energy density and safety of the secondary battery.

[0029] In any embodiment, the short diameter of at least part of the copper grains with a particle size greater than 0.5 μm is arranged along the thickness direction of the current collector.

[0030] The arrangement of the short diameter of at least part of the copper grains with a particle size greater than 0.5 μm along the thickness direction of the current collector indicates that the internal grain boundary of the copper foil is highly tortuous, and the energy required for the copper foil to break through the grain boundary (i.e., the thickness direction of the current collector) is also greater, which helps to reduce the probability of the copper foil breaking along the thickness direction, improve the brittleness of the current collector, further reduce the risk of copper foil breaking or cracking, and improve the safety of the secondary battery.

[0031] In any embodiment, the number of copper grains with a particle size less than or equal to 0.5 μm 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.

[0032] In any embodiment, the number of copper grains with a particle size greater than 0.5 μm 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.

[0033] In any embodiment, the copper foil satisfies at least one of the following conditions:

[0034] (1) the average particle size of the copper grains is 0.1 μm-1.2 μm;

[0035] (2) the maximum particle size of the copper grains is 1 μm-2.5 μm;

[0036] (3) the minimum particle size of the copper grains is 0.1 μm-0.3 μm.

[0037] 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 μm and the number of copper grains with a particle size greater than 0.5 μm, 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.

[0038] In any embodiment, the copper foil satisfies at least one of the following conditions:

[0039] (1) the average particle size of the copper grains is 0.1 μm-0.7 μm, which can be optionally 0.3 μm-0.6 μm;

[0040] (2) the maximum particle size of the copper grains is 1.2 μm-2.0 μm;

[0041] (3) the minimum particle size of the copper grains is 0.1 μm-0.3 μm.

[0042] 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.

[0043] In any of the embodiments, the tensile strength of the copper foil is 600-1000 MPa, and / or the elongation at break of the copper foil is 4-8% 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.

[0044] The copper foil has excellent tensile strength and elongation at break, good mechanical properties and plasticity, and can be applied to high-energy-density batteries or high-expansion batteries, thereby helping to improve the safety of secondary batteries.

[0045] In any of the embodiments, 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.

[0046] In any of the embodiments, 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.

[0047] The copper foil has good tensile strength, which can meet the practical needs of high-strength copper foil in the battery field to some extent. In addition, the copper foil also has excellent elongation at break and good plasticity, which helps to reduce the brittle defects of the copper foil and reduce the risk of copper foil fracture or slight cracking in the battery.

[0048] In any of the embodiments, the hardness of the copper foil is 55-65 HV. In any of the embodiments, the hardness of the copper foil is 55-60 HV. The appropriate hardness is conducive to the surface treatment of the copper foil and the cold pressing treatment of the secondary battery, and reduces the surface damage of the copper foil and the influence on the bonding performance of the negative film layer and the copper foil.

[0049] In any of the embodiments, 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 light in weight and further improve the energy density or specific capacity.

[0050] In any embodiment, the secondary battery further comprises a negative electrode film layer on at least one side of the copper foil, the negative 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, lithium titanate and metallic lithium, and the secondary battery can be applied to various different battery negative electrode systems, and has a wide application range.

[0051] In any embodiment, the negative active material comprises a silicon-based material, and the silicon-based material comprises at least one of elemental silicon, silicon oxide compound, 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.

[0052] In any embodiment, the mass percentage of the silicon-based material is 5%-100%, optionally 10%-60%, and more optionally 10%-30%, based on the total mass of the negative electrode film layer.

[0053] In any embodiment, the mass percentage of silicon element in the silicon-based material is 20%-50%.

[0054] In any embodiment, the mass percentage of silicon element is 4%-10%, based on the total mass of the negative electrode film layer.

[0055] In any embodiment, the expansion force of the secondary battery is greater than or equal to 1000 kgf.

[0056] In any embodiment, the expansion force of the secondary battery is greater than or equal to 2500 kgf.

[0057] In any embodiment, the expansion force of the secondary battery is greater than or equal to 4000 kgf.

[0058] In any embodiment, the expansion force of the secondary battery is 1000 kgf-10000 kgf.

[0059] The current collector in the prior art is prone to breakage under high expansion force of the secondary battery, which causes safety accidents and limits the further improvement of the electrochemical performance of the secondary battery. The current collector provided in the embodiments has excellent tensile strength and breaking elongation, and can be applied to the secondary battery with high expansion force, which is beneficial to the further improvement of the energy density of the secondary battery.

[0060] In any embodiment, the thickness expansion rate of the secondary battery is 4%-10%.

[0061] The current collector in the prior art is prone to breakage under the expansion stress of the secondary battery volume change, which causes the performance of the secondary battery to plummet and easily causes safety accidents, thereby limiting the further improvement of the performance of the secondary battery. The current collector provided in the embodiments of the present application has good strength performance and can be applied to a secondary battery with high thickness expansion rate, thereby being beneficial to the further improvement of the energy density and safety of the secondary battery.

[0062] The second aspect of the present application provides a preparation method of a secondary battery, including preparing a copper foil by an electroplating method, the electroplating method including applying a pulse current to an electroplating solution to reduce and deposit copper ions in the electroplating solution to form the copper foil, the peak value of the pulse current being 40000 A-100000 A, the valley value of the pulse current being 100 A-20000 A, and the change period of the current being 50 ms-5000 ms; the copper foil including copper grains with different particle sizes, the copper grains including 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 proportion of the number of the copper grains with a particle size less than or equal to 0.5 μm in the total number of the copper grains is 70%-95%, the proportion of the number of the copper grains with a particle size greater than 1 μm in the total number of the copper grains is 1%-10%, and the particle size span of the copper grains is 1.3 μm-3.4 μm.

[0063] Compared with the calendering method, the electroplating method is mature and simple, has low requirements on equipment, and can prepare a copper foil with excellent tensile strength and elongation at break, and has excellent mechanical strength and good plasticity.

[0064] In any embodiment, the preparation method of the secondary battery includes preparing a pole piece by using a copper foil as a current collector, the preparation of the copper foil includes preparing the copper foil by an electroplating method, the electroplating method includes applying a pulse current to an electroplating solution to reduce and deposit copper ions in the electroplating solution to form the copper foil, the peak value of the pulse current being 40000 A-100000 A, the valley value of the pulse current being 100 A-20000 A, and the change period of the current being 50 ms-5000 ms; and the expansion force of the secondary battery is greater than or equal to 1000 kgf.

[0065] The preparation method of the secondary battery provided in the present application uses the above-mentioned electroplating method to prepare a copper foil, thereby obtaining a copper foil with excellent tensile strength and elongation at break, improving the mechanical strength while having excellent plasticity, reducing the probability of breakage or cracks of the copper foil when the copper foil is subjected to stress caused by the expansion of the volume of the pole piece in the secondary battery, the decomposition of the electrolyte to cause the increase of the internal pressure, and other factors, and being beneficial to further improving the safety and service life of the secondary battery.

[0066] In any embodiment, the preparation method of the secondary battery comprises preparing a wound secondary battery, preparing a pole piece using a copper foil as a current collector, the preparation of the copper foil comprises preparing the copper foil by electroplating, the electroplating comprises applying a pulse current to an electroplating solution to reduce and deposit copper ions in the electroplating solution to form the copper foil, the peak value of the pulse current is 40000A-100000A, the valley value of the pulse current is 100A-20000A, and the change cycle of the current is 50ms-5000ms; and the expansion force of the secondary battery is greater than or equal to 1000kgf.

[0067] The preparation method of the secondary battery provided in the present application uses the above-mentioned electroplating method to prepare the copper foil, thereby obtaining a copper foil with excellent tensile strength and elongation at break, which has excellent plasticity while improving the mechanical strength, reduces the probability of cracks or breakage of the copper foil under the expansion stress after the secondary battery is wound, pressed, and shaped, reduces the probability of crack breakage of the wound cell pole piece, and further improves the safety and service life of the secondary battery.

[0068] In any embodiment, the preparation method comprises preparing a pole piece using a copper foil as a current collector, the preparation of the copper foil comprises preparing the copper foil by electroplating, the electroplating comprises applying a pulse current to an electroplating solution to reduce and deposit copper ions in the electroplating solution to form the copper foil, the peak value of the pulse current is 40000A-100000A, the valley value of the pulse current is 100A-20000A, and the change cycle of the current is 50ms-5000ms; and the gram capacity of the negative active material in the negative electrode film layer of the secondary battery is 800mAh / g-1500mAh / g.

[0069] The preparation method of the secondary battery provided in the present application uses the above-mentioned electroplating method to prepare the copper foil, thereby obtaining a copper foil with excellent tensile strength and elongation at break, which has excellent plasticity while improving the mechanical strength, reduces the probability of cracks or breakage of the copper foil under the expansion stress when the high gram capacity negative active material changes in volume before and after lithium ion intercalation / deintercalation, and is beneficial to further improve the safety and service life of the secondary battery.

[0070] In any embodiment, the electroplating method satisfies one or more of the following conditions:

[0071] (1) the peak value of the pulse current is 40000A-80000A;

[0072] (2) the valley value of the pulse current is 1000A-10000A;

[0073] (3) the change cycle of the pulse current is 500ms-5000ms;

[0074] (4) the distance between the cathode electrode and the anode electrode is 8mm-20mm;

[0075] (5) the temperature of the electroplating deposition is 45-60°C;

[0076] (6) the roller speed of the cathode roller is 2-5 m / min.

[0077] In any embodiment, the electroplating method satisfies one or more of the following conditions:

[0078] (1) the peak value of the pulse current is 50,000-70,000 A;

[0079] (2) the valley value of the pulse current is 2,000-5,500 A;

[0080] (3) the change period of the pulse current is 2,000-4,000 ms;

[0081] (4) the distance between the cathode electrode and the anode electrode is 8-12 mm;

[0082] (5) the temperature of the electroplating deposition is 50-60°C;

[0083] (6) the roller speed of the cathode roller is 2-3 m / min.

[0084] Compared with direct current deposition, applying a pulse current to the electroplating solution can make copper ions reduce and deposit to form fine grains with small particle sizes under high current conditions and form grains (e.g., columnar grains) with large particle sizes under low current conditions. By adjusting the current size and change period, the formation and growth of copper ion crystal nuclei can be adjusted, thereby adjusting the size and morphology of the grain size, adjusting the proportion of the number 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.

[0085] 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.

[0086] In any embodiment, the pulse current further includes a linear oscillation current. During the deposition of copper ions, increasing the current can accelerate the deposition rate of copper ions, thereby forming fine copper grains; decreasing the current can reduce the deposition rate of copper ions, which is helpful to the formation of regular and orderly large-size grains. Through the continuous change of the linear oscillation current, the particle size of the deposited copper grains changes in size.

[0087] In any embodiment, the electroplating solution comprises a leveling agent, a wetting agent and a brightener, the leveling agent comprises one or more of collagen, sodium saccharin; the wetting agent comprises one or more of hydroxyethyl cellulose, polyethylene glycol; the brightener comprises sodium poly(dithiopropyl sulfone).

[0088] The leveling agent can improve the flatness of the copper foil, the wetting agent can improve the wettability of the electroplating solution to the substrate, improve the nucleation rate of the copper foil, and reduce the average grain size of the copper foil. 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.

[0089] In any embodiment, 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 mg / L-200 mg / L, hydroxyethyl cellulose with a concentration of 30 mg / L-200 mg / L, sodium poly(dithiopropyl sulfone) with a concentration of 500 mg / L-2000 mg / L, and chloride ions (calculated as chlorine atoms) with a concentration of 20 mg / L-80 mg / L.

[0090] In any embodiment, 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(dithiopropyl sulfone) with a concentration of 500 mg / L-1000 mg / L, and chloride ions with a concentration of 40 mg / L-80 mg / L.

[0091] The collagen and sodium saccharin in the electroplating solution help to improve the surface pits and protrusions of the copper foil and improve the flatness of the copper foil. The hydroxyethyl cellulose and polyethylene glycol help to reduce the difference in the thickness direction of the copper foil and improve the uniformity of the copper foil. The sodium poly(dithiopropyl sulfone) can increase the electrochemical reduction rate of copper ions and adjust the grain size. 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.

[0092] In any embodiment, the electroplating solution further comprises copper ions with a concentration of 60 g / L-100 g / L.

[0093] In any embodiment, the pH of the electroplating solution is 2.5-4.5, which is conducive to the reduction of copper ions.

[0094] The third aspect of the application provides a pole piece, which comprises the copper foil in the secondary battery provided by the first aspect of the application or the copper foil prepared by the preparation method of the second aspect of the application.

[0095] In any of the embodiments, the pole piece 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, lithium titanate and metallic lithium.

[0096] In any of the embodiments, the negative electrode active material comprises a silicon-based material, and the silicon-based material comprises at least one of elemental silicon, silicon oxide compound, silicon-carbon composite, silicon-nitrogen composite and silicon alloy.

[0097] In any of the embodiments, the mass percentage of the silicon-based material is 5%-100%, optionally 10%-60%, and more optionally 10%-30%, based on the total mass of the negative electrode film layer.

[0098] In any of the embodiments, the mass percentage of silicon element in the silicon-based material is 20%-50%.

[0099] In any of the embodiments, the mass percentage of silicon element is 4%-10%, based on the total mass of the negative electrode film layer.

[0100] The fourth aspect of the present application provides a wound secondary battery comprising the secondary battery provided by the first aspect of the present application or prepared by the method provided by the second aspect of the present application or comprising the pole piece provided by the third aspect of the present application.

[0101] The fifth aspect of the present application provides an electric device comprising the secondary battery of the first aspect of the present application or prepared by the method of the second aspect of the present application or the wound secondary battery of the fourth aspect of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0102] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed to be used in the embodiments of the present application will be briefly introduced as follows. Obviously, the drawings described below are only some of the embodiments of the present application, and other drawings can also be obtained according to the drawings without creative labor for those skilled in the art.

[0103] FIG. 1 is a schematic diagram of a secondary battery according to an embodiment of the present application.

[0104] FIG. 2 is an exploded view of the secondary battery according to an embodiment of the present application shown in FIG. 1.

[0105] FIG. 3 is a schematic diagram of a battery module according to an embodiment of the present application.

[0106] FIG. 4 is a schematic diagram of a battery pack according to an embodiment of the present application.

[0107] FIG. 5 is an exploded view of the battery pack of the embodiment of the present application shown in FIG. 4.

[0108] FIG. 6 is a schematic view of a power consuming device using a secondary battery as a power source according to an embodiment of the present application.

[0109] FIG. 7 shows a reverse pole figure distribution map of a cross section of a copper foil according to Example 1 of the present application, which was subjected to electron backscatter diffraction (EBSD) testing.

[0110] FIG. 8 shows a tensile curve of a copper foil according to Example 1 of the present application.

[0111] FIG. 9 shows a grain size distribution map of a cross section of a copper foil according to Example 1 of the present application, which was subjected to EBSD diffraction testing.

[0112] FIG. 10 shows a reverse pole figure distribution map of a cross section of a copper foil according to Example 2 of the present application, which was subjected to EBSD testing.

[0113] FIG. 11 shows a tensile curve of a copper foil according to Example 2 of the present application.

[0114] FIG. 12 shows a grain size distribution map of a cross section of a copper foil according to Example 2 of the present application, which was subjected to EBSD diffraction testing.

[0115] FIG. 13 shows a reverse pole figure distribution map of a cross section of a copper foil according to Example 3 of the present application, which was subjected to EBSD testing.

[0116] FIG. 14 shows a tensile curve of a copper foil according to Example 3 of the present application.

[0117] FIG. 15 shows a grain size distribution map of a cross section of a copper foil according to Example 3 of the present application, which was subjected to EBSD diffraction testing.

[0118] FIG. 16 shows a tensile curve of a copper foil according to Example 1-1 of the present application.

[0119] FIG. 17 shows a reverse pole figure distribution map of a cross section of a copper foil according to Comparative Example 1 of the present application, which was subjected to EBSD testing.

[0120] FIG. 18 shows a tensile curve of a copper foil according to Comparative Example 1 of the present application.

[0121] FIG. 19 shows a grain size distribution map of a cross section of a copper foil according to Comparative Example 1 of the present application, which was subjected to EBSD diffraction testing.

[0122] FIG. 20 shows a reverse pole figure distribution map of a copper foil according to Example 5 of the present application, which was subjected to electron backscatter diffraction (EBSD) testing.

[0123] FIG. 21 shows a grain size distribution map of a copper foil according to Example 5 of the present application, which was subjected to electron backscatter diffraction (EBSD) testing.

[0124] FIG. 22 shows a reverse pole figure distribution map of a copper foil according to Example 6 of the present application, which was subjected to electron backscatter diffraction (EBSD) testing.

[0125] Figure 23 shows the particle size distribution of the copper foil of Embodiment 6 of this application as determined by electron backscatter diffraction (EBSD).

[0126] Figure 24 shows the particle size distribution of the copper foil cross section of Embodiment 7 of this application obtained by EBSD diffraction testing.

[0127] Explanation of reference numerals in the attached drawings: 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Secondary battery; 51 Casing; 52 Electrode assembly; 53 Cover plate; T in the thickness direction. Detailed Implementation

[0128] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the secondary battery, its preparation method, electrodes, wound secondary battery, and power-consuming 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.

[0129] 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.

[0130] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0131] If not particularly specified, all the technical features and optional technical features of the present application can be combined with each other to form new technical solutions.

[0132] If not particularly specified, all the steps of the present application can be performed in sequence or randomly, preferably in sequence. For example, the method comprises steps (a) and (b), which means that the method can comprise steps (a) and (b) performed in sequence, or steps (b) and (a) performed in sequence. For example, the method can further comprise step (c), which 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.

[0133] If not particularly specified, the "comprise" and "include" mentioned in the present application are open-ended, and can also be closed. For example, the "comprise" and "include" can mean that other components not listed can also be included or contained, or only the listed components can be included or contained.

[0134] If not particularly specified, in the present application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, either of the following conditions satisfies 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 A and B are both true (or exist).

[0135] Generally, a secondary battery comprises 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 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 comprises a negative current collector, which serves to carry electrode active materials and collect output current, and can also bind the expansion of the battery cell to prevent the anode from breaking during cycling.

[0136] 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 materials, high-capacity graphite, lithium metal, and other negative electrode materials can also increase the energy density or capacity of the secondary battery, but the above-mentioned materials have high expansion, which in turn causes the volume of the electrode sheet to expand and contract severely 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. In particular, the design of the prismatic wound battery, since the cell is pressed and shaped after being wound, higher requirements are placed on the bending resistance of the current collector. The cell pressing and shaping will cause irreversible damage to the current collector, and for cases where the current collector substrate is thinned, the coating amount of the active material is high, or the design of the limit group margin is used, the risk of cell cracking is greatly increased, further exacerbating the risk of cell failure. Currently, the tensile strength of the negative electrode current collector copper foil commonly used is usually 200-500 MPa, which cannot meet the use requirements of the new generation of secondary batteries with high energy density or high capacity. Refining the copper grains can strengthen the material strength, but can exacerbate the brittleness of the copper foil, increasing the risk of copper foil breakage or slight cracking. How to improve the strength of the copper foil while considering good plasticity is a problem that needs to be solved at present.

[0137] When the active material expands, the expansion of the electrode sheet is blocked in the direction perpendicular to the current collector (i.e., the thickness direction), which causes the expansion force to shift to the in-plane direction parallel to the current collector, and the local stress overload caused by the transverse tensile force easily causes the weak point to break, causing the current collector to break and fail. When the current collector fails, it usually first produces elastic deformation and unidirectional stretching under external force, and as the volume continuously expands, it continuously applies pressure and tension to the current collector, causing the current collector to transition from elastic deformation to plastic deformation (i.e., irreversible deformation), until the irreversible deformation exceeds the maximum deformation that the current collector can bear, causing the current collector to tear, which in turn causes the electrode sheet to break and fail, and the battery performance to plummet. The applicant realizes that after the copper foil is refined, the internal grain boundary area increases, which hinders the dislocation slip movement, thereby improving the strength of the copper foil, but after the grain is refined, the tortuosity of the internal grain boundary decreases, making the copper foil prone to grain boundary fracture under external force, causing the current collector to be highly brittle. How to improve the strength and brittleness of the current collector and improve the bending resistance of the current collector has become a technical problem that needs to be solved in the art.

[0138] [Secondary battery]

[0139] Based on this, the application provides a secondary battery, which comprises a negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a negative electrode film layer arranged on at least one side surface of the negative electrode current collector, the negative electrode current collector comprising a copper foil, the copper foil comprising copper grains with different particle sizes, the copper grains comprising 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, wherein the proportion of the number of the copper grains with a particle size of less than or equal to 0.5 μm in the total number of the copper grains is 70% to 95%, the proportion of the number of the copper grains with a particle size of greater than 1 μm in the total number of the copper grains is 1% to 10%, and the particle size span of the copper grains is 1.3 μm to 3.4 μm.

[0140] The copper grains with a particle size of 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 of less than or equal to 0.5 μm are fine grains close to particles, forming a fine grain area around the columnar grains. The copper foil substrate in the embodiments of the application has a proportion of 70% to 95% of fine grains, which is beneficial to improve the internal grain boundary area of the material, hinder the dislocation slip movement, and improve the tensile strength of the copper foil. Meanwhile, the copper foil has a proportion of 1% to 10% of columnar or columnar-like large grains with a particle size of greater than 1 μm, and a copper grain distribution morphology with a particle size span of 1.3 μm to 3.4 μm, forming a grain morphology of "mainly fine grains and mixed distribution of large grains", which is beneficial to improve the tensile strength of the copper foil. Further, the dispersion degree of the copper grain particle size distribution is within a suitable range, and a small amount of heterogeneous grains of large grains are doped in the fine grains with a large proportion, which can improve the internal grain boundary area while helping to balance the grain boundary area and the geometric necessary dislocation density of the copper grains, improve the grain dislocation movement resistance and reduce the occurrence of concentrated stress in the copper foil deformation process, improve the phenomenon of low grain boundary tortuosity and high brittleness of the copper foil caused by too many fine grains, make the copper foil have a suitable yield ratio, and is beneficial to reduce the probability of fracture along the grain boundary of the copper foil after the copper foil is subjected to external force, balance the strength and brittleness of the copper foil, provide a material basis for solving the fracture of the current collector in the battery cell and the failure of the battery cell, and further improve the energy density and safety of the secondary battery.

[0141] In this paper, the term "yield ratio" refers to the ratio of yield strength to tensile strength, which can reflect the ability of the copper foil to form a certain shape after processing or stress and maintain this shape, i.e. formability.

[0142] In this paper, the term "yield strength" refers to the yield limit when the material exhibits yield phenomenon. In order to measure the yield characteristics of the material, the stress value when the permanent residual plastic deformation is equal to a certain value (generally 0.2% of the original length) is defined as the yield strength.

[0143] In the present document, the term "tensile strength" refers to the maximum load-carrying strength of a test sample until it is pulled apart, i.e. the stress value at which permanent residual plastic deformation occurs.

[0144] It can be understood that the yield ratio refers to the ratio of the yield strength of the metal to the tensile strength, the yield strength is the stress value at which the metal begins to produce plastic deformation during the force process, the tensile strength is the maximum stress value that the metal can withstand during the tensile process, the lower the yield ratio, the more plastic deformation the metal can withstand after yielding, and the better the ductility and toughness. Increasing the yield ratio can reduce the strain hardening capacity of the copper foil grain and reduce the lateral bending resistance of the copper foil. Reducing the yield ratio can improve the strain hardening capacity of the copper foil grain and improve the plasticity of the copper foil. The copper foil with a suitable yield ratio exhibits excellent mechanical strength and plasticity. The heterogeneous grain distribution morphology of the copper foil substrate in the embodiments of the present application through large grains and small grains enables the copper foil to have a suitable range of yield ratio, effectively reduces the probability of brittle fracture along the grain boundary of the current collector under external force, and helps to improve the energy density of the secondary battery while improving the safety and service life of the secondary battery.

[0145] In some embodiments, the yield ratio of the copper foil is 0.45-0.80. In some embodiments, the yield ratio of the copper foil can be selected as 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8 or any numerical range between any two of them. In some embodiments, the yield ratio of the copper foil is 0.45-0.7.

[0146] The particle size span of the maximum particle size and the minimum particle size is the difference between the maximum particle size and the minimum particle size in the copper grain particle size distribution. In some embodiments, the particle size span of the copper grain can be selected as 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, 2.0 μm, 2.1 μm, 2.2 μm, 2.3 μm, 2.4 μm, 2.5 μm, 2.6 μm, 2.7 μm, 2.8 μm, 2.9 μm, 3.0 μm, 3.1 μm, 3.2 μm, 3.3 μm, 3.4 μm or any numerical range between any two of them. In some embodiments, the particle size span of the copper grain is 1.3 μm-3 μm.

[0147] The particle size span of the copper grain within the range can make the dispersion degree of the copper grain particle size distribution within a suitable range, which can help to balance the grain boundary area and the geometric necessary dislocation density of the copper grain, improve the grain dislocation movement resistance and reduce the concentration of stress during the deformation of the copper foil, which is beneficial to the copper foil to have excellent plasticity while improving the mechanical strength.

[0148] In some embodiments, the number of copper grains with a particle size greater than 1 μm accounts for 2%-10% of the total number of copper grains.

[0149] In some embodiments, the expansion force of the secondary battery is greater than or equal to 1000 kgf, the secondary battery comprises a negative electrode sheet, the negative electrode sheet comprises a negative electrode current collector and a negative electrode film layer arranged on at least one side surface of the 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 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.

[0150] During the charging and discharging cycle operation of the secondary battery, the volume of the electrode sheet expands due to the insertion and extraction of active ions, especially in the self-generated negative electrode battery or the new silicon-based or lithium metal negative electrode, and the overall expansion of the secondary battery is high due to the comprehensive influence of the design of high group margin or large size battery cell for the purpose of improving the energy density of the single cell. The copper foil current collector of the high expansion secondary battery is obviously stretched and stressed during the operation, which increases the probability of fracture or crack of the copper foil current collector, and deteriorates the safety and service life of the battery cell. Therefore, the copper foil comprising the copper grains with different particle sizes as described above is used in the secondary battery provided by the present application, which has excellent tensile strength and elongation at break, improves the mechanical strength while having excellent plasticity, reduces the probability of fracture or crack of the copper foil current collector in the high expansion system, is beneficial to improving the service life of the secondary battery when the electrode sheet cracks or fails during operation, and is beneficial to further improving the safety and service life of the secondary battery.

[0151] In some embodiments, the thickness expansion rate of the secondary battery is 4%-10%.

[0152] In some embodiments, the secondary battery is a wound type secondary battery, and the expansion force of the secondary battery is greater than or equal to 1000 kgf; the wound type secondary battery comprises a negative electrode sheet, the negative electrode sheet comprises a negative electrode current collector and a negative electrode film layer arranged on at least one side surface of the 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 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.

[0153] The current collector of the negative electrode tab in the wound secondary battery is prone to crack fracture in the corner area. This is because the wound battery is pressed and shaped after winding, which can cause irreversible damage to the current collector. The probability of crack or fracture of the negative electrode tab outer corner area and inner corner area under the expansion stress caused by the increase of the internal pressure of the secondary battery and the volume expansion is greatly increased, which aggravates the risk of battery failure and worsens the service life and safety of the secondary battery. The copper foil provided in the present application includes copper grains of different particle sizes, which significantly improves the tensile strength and elongation at break of the copper foil, so that the copper foil has excellent mechanical properties and plasticity, reduces the probability of crack or fracture of the copper foil under expansion stress, reduces the probability of crack fracture of the wound battery tab, and is beneficial to improving the service life of the secondary battery when the tab cracks or fails, so that the safety and service life of the secondary battery are further improved.

[0154] In some embodiments, the thickness expansion rate of the wound secondary battery is 4%-10%. Increasing the amount of active material is one of the effective means to improve the energy density or capacity of the secondary battery, but it will cause the volume of the battery to increase. The applicant found that using high-gram-capacity negative electrode active material can significantly improve the available lithium intercalation capacity of the negative electrode active material, improve the energy density or capacity of the secondary battery, and reduce the impact on the volume of the battery. However, the high-gram-capacity negative electrode active material has a large volume expansion before and after lithium ion intercalation / deintercalation, and the expansion stress acts on the current collector copper foil, increasing the probability of fracture or crack of the copper foil along the thickness direction under expansion stress, and worsening the safety and service life of the secondary battery.

[0155] Based on this, another embodiment of the present application provides a secondary battery, which includes a negative electrode tab, the negative electrode tab includes a negative electrode current collector and a negative electrode film layer arranged on at least one side surface of the negative electrode current collector, the negative electrode current collector includes a copper foil, the copper foil includes copper grains of different particle sizes, the copper grains include 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; and the gram capacity of the negative electrode active material in the negative electrode film layer is 800 mAh / g-1500 mAh / g.

[0156] The copper foil provided in the present application includes copper grains of different particle sizes, which has excellent tensile strength and elongation at break, improves the mechanical strength while having excellent plasticity, and forms a good match with high-gram-capacity negative electrode active material, improves the energy density and capacity of the secondary battery, and improves the safety and service life of the secondary battery.

[0157] In some embodiments, the gravimetric capacity of the negative active material in the negative electrode film layer is 800 mAh / g-1500 mAh / g.

[0158] The copper foil provided by the present application is particularly suitable for a negative active material system with a gravimetric capacity of 800 mAh / g-1500 mAh / g. High gravimetric capacity negative materials can embed more active ions, such as silicon-based negative electrodes, alkali metal negative electrodes, etc., which generally have high swelling properties, which can increase the probability of copper foil fracture or cracking in the battery. The copper foil has excellent mechanical properties and plasticity, which can reduce the risk of copper foil fracture or cracking in high-energy density battery systems or high-swelling battery systems.

[0159] Without wishing to be bound by any theory, the difference in work hardening capacity 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 partitioning in the initial stage of plastic deformation of the copper foil more obvious. This is because the moderate doping of large grains produces a higher density of geometrically necessary dislocations (GND) than the case where all grains are fine. During the process of inhomogeneous plastic deformation, the bending of the crystal plane of the copper foil 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 process of the copper foil, making the copper foil exhibit stronger strain localization inhibition ability and improving the plasticity of the copper foil. 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 production of a higher density of geometrically necessary dislocations during the deformation process of the copper foil helps to reduce the concentration of stress, which can improve the mechanical strength.

[0160] In some embodiments, the number of copper grains with a particle size of less than or equal to 0.5 μm accounts for 75%-95%, 80%-95%, 85%-95%, 83%-93%, 85%-93%, or 87%-93% of the total number of copper grains, which helps to further improve the tensile strength and optimize the mechanical properties of the copper foil.

[0161] 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 above values or any value within the range.

[0162] In the present disclosure, the grain size of the copper grains with a grain size of less than or equal to 0.5 μm ranges from greater than or equal to 0.1 μm to less than or equal to 0.5 μm.

[0163] In some embodiments, the number of copper grains with a grain 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 fracture elongation and optimize the plasticity of the copper foil.

[0164] In some embodiments, the number of copper grains with a grain 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 range or value between any two of the above values.

[0165] In the present disclosure, the grain size of the copper grains with a grain 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 grain size of the copper foil and improve the tensile strength of the copper foil.

[0166] The grain size and number ratio 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 can be measured by an electron backscatter diffraction instrument (EBSD) and a scanning electron microscope, a pole figure distribution map with a magnification of 3000 can be obtained, the number and grain size of the grains can be counted by the imageJ analysis software of the Oxford C-Nano+ electron backscatter diffraction instrument, the equivalent circle diameter of the grains can be used as the grain size, a number distribution map can be made, a skew distribution can be used for fitting, and the grain size and number ratio of the grains in different grain size intervals can be obtained.

[0167] In some embodiments, the number of copper grains with a grain size of greater than 0.5 μm and less than or equal to 3 μm accounts for 5%-30% of the total number of copper grains.

[0168] As mentioned above, the copper grains with a grain size of greater than 0.5 μm and less than or equal to 3 μm can help to reduce the average grain size of the copper foil and further improve the mechanical strength of the copper foil. Further, the number ratio of the copper grains with a grain size of greater than 0.5 μm and less than or equal to 3 μm satisfying 5%-30% can make the copper foil have a good fracture elongation, excellent mechanical properties and plasticity, and can reduce the risk of fracture or crack of the copper foil in a high-energy-density battery system or a high-expansion battery system, and achieve a simultaneous improvement in the energy density and safety of the secondary battery.

[0169] In some embodiments, the gravimetric capacity of the negative active material can be selected from 800 mAh / g, 820 mAh / g, 850 mAh / g, 880 mAh / g, 900 mAh / g, 920 mAh / g, 950 mAh / g, 980 mAh / g, 1000 mAh / g, 1020 mAh / g, 1050 mAh / g, 1080 mAh / g, 1100 mAh / g, 1120 mAh / g, 1150 mAh / g, 1180 mAh / g, 1200 mAh / g, 1250 mAh / g, 1300 mAh / g, 1350 mAh / g, 1400 mAh / g, 1450 mAh / g, 1500 mAh / g, or a range between any two of the above values or any value within the range.

[0170] As used herein, the gravimetric capacity of the negative active material can be determined by using instruments and methods known in the art. For example, the gravimetric capacity of the negative active material can be determined by the following method: the negative active material, carbon black, and polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 91.6:1.8:6.6 in an appropriate amount of solvent NMP to form a uniform negative electrode slurry; the negative electrode slurry is uniformly coated on the surface of a negative current collector copper foil and dried and cold-pressed; then a lithium metal sheet is used as the counter electrode, a polypropylene (PP) film is used as the separator film, and an electrolyte is injected, wherein the electrolyte formulation used is as follows: dimethyl carbonate (DMC), methyl ethyl carbonate (EMC), and ethylene carbonate (EC) are mixed in a weight ratio of 1:1:1 to obtain an organic solvent, and then LiPF6 is dissolved in the organic solvent to prepare an electrolyte with a concentration of 1.0 mol / L. A CR2430 button cell is assembled in an argon glove box. At 25°C, charge at a rate of 0.1C to an upper limit cutoff voltage of 3.8V, and then charge at a constant voltage until the current is less than 0.05C; after standing for 30 min, discharge at a rate of 0.1C to a lower limit cutoff voltage of 2.0V, and record the first discharge capacity as Cm, then the gravimetric capacity of the negative active material = discharge capacity Cm / mass of the negative active material.

[0171] It can be understood that the gravimetric capacity of the negative active material can also be obtained by disassembling the battery to obtain the negative electrode sheet, and then testing the negative electrode sheet after assembling it into a button cell according to the method described above.

[0172] In some embodiments, the short diameter of at least part of the copper grains with a particle size greater than 0.5 μm is arranged along the thickness direction of the current collector.

[0173] The arrangement of the short diameter of at least part of the copper grains with a particle size greater than 0.5 pm along the thickness direction of the current collector indicates that the internal grain boundary of the copper foil is highly tortuous, and the energy required for the copper foil to break through the grain boundary (i.e., the thickness direction of the current collector) is also greater, which helps to reduce the probability of the copper foil breaking along the thickness direction, improve the brittleness of the current collector, further reduce the risk of copper foil breaking or cracking, and improve the safety of the secondary battery.

[0174] It can be understood that the arrangement direction of the short diameter of at least part of the copper grains with a particle size greater than 0.5 pm can be determined by the EBSD and scanning electron microscopy described above.

[0175] In some embodiments, the average particle size of the copper grains is 0.1 pm to 1.2 pm. In some embodiments, the average particle size of the copper grains is 0.1 pm to 0.7 pm. In some embodiments, the average particle size of the copper grains is 0.3 pm to 0.6 pm. In some embodiments, the average particle size of the copper grains is 0.3 pm to 0.5 pm. In some embodiments, the average particle size of the copper grains is 0.1 pm, 0.15 pm, 0.2 pm, 0.25 pm, 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 value within the range between any two of the above values.

[0176] The suitable particle size range is theoretically beneficial to 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.

[0177] The average particle size of the grains can be tested by methods known in the art. As an example, the cross section of the copper foil is measured by electron backscatter diffraction (EBSD) and scanning electron microscopy, the inverse pole figure distribution map is obtained, the number and particle size of the grains are counted by the 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, the number distribution map is drawn, the skewness distribution is used for fitting, and the average particle size of the grains is obtained.

[0178] In some embodiments, the maximum particle size of the copper grains is 1 pm to 2.5 pm. In some embodiments, the maximum particle 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 particle size of the copper grains is 1 pm, 1.2 pm, 1.4 pm, 1.6 pm, 1.8 pm, 2 pm, or any value within the range between any two of the above values.

[0179] The maximum 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 a skew distribution. The maximum grain size in the statistical results is the maximum grain size of the copper grains.

[0180] In some embodiments, the minimum grain size of the copper grains is 0.1 pm-0.3 pm. In some embodiments, the minimum grain size of the copper grains is 0.1 pm-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 a range between any two of the above values or any value within the range.

[0181] 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 a skew distribution. The minimum grain size in the statistical results is the minimum grain size of the copper grains.

[0182] In some embodiments, the tensile strength of the copper foil is 600 MPa-1000 MPa under the test conditions of room temperature (20±10°C), a sample length x width of (50±0.25 mm) x (15±0.25 mm), and a tensile speed of 50±0.5 mm / min. In some embodiments, the tensile strength of the copper foil is 700 MPa-1000 MPa. In some embodiments, the tensile strength of the copper foil is 700 MPa-800 MPa.

[0183] In some embodiments, the elongation at break of the copper foil is 4%-8% under the test conditions of room temperature (20±10°C), a sample length x width of (50±0.25 mm) x (15±0.25 mm), and a tensile speed of 50±0.5 mm / min. In some embodiments, the elongation at break of the copper foil is 4%-7%. In some embodiments, the elongation at break of the copper foil is 5%-6%.

[0184] In this document, the term “tensile strength” refers to the maximum load-bearing strength per unit area of a test sample when a continuous load is applied to the test sample until it is pulled apart.

[0185] In the present document, the term "elongation at break" refers to the ratio of the change in length of a material after plastic deformation until breakage to the original length, usually expressed in percentage, and is an important parameter for measuring the deformation ability of a material under stress during stretching.

[0186] In the present application, the yield strength, tensile strength, yield ratio, 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 samples with a test zone length of 50±0.25 mm and a width of 15±0.25 mm are continuously loaded at room temperature (20±10°C) at a stretching speed of 50±0.5 mm / min until breakage, and the maximum load is divided by the cross-sectional area of the sample to obtain the tensile strength of the sample. The cross-sectional area of the sample can be calculated by dividing the mass of the sample by the product of the length of the sample and the density. The elongation at break can be calculated according to the displacement method after the above test. The test zone refers to the detection area during instrument testing, and the length and width of the sample can be greater than those of the test zone, considering that the sample may need to be fixed by a clamp during testing.

[0187] In some embodiments, the tensile strength of the copper foil is 600 MPa, 650 MPa, 700 MPa, 750 MPa, 800 MPa, 850 MPa, 900 MPa, 950 MPa, 1000 MPa, 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 stretching speed of 50±0.5 mm / min.

[0188] In some embodiments, the elongation at break of the copper foil is 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 stretching speed of 50±0.5 mm / min.

[0189] The copper foil with the above elongation strength and elongation at break has excellent mechanical strength and plasticity, and can be suitable for high-energy-density batteries or high-expansion batteries, which helps to improve the safety of secondary batteries.

[0190] In some embodiments, the copper foil has a hardness of 55 HV-65 HV. In some embodiments, the copper foil has a hardness of 55 HV-60 HV. In some embodiments, the copper foil has a hardness of 55 HV, 56 HV, 57 HV, 58 HV, 59 HV, 60 HV, 61 HV, 62 HV, 63 HV, 64 HV, 65 HV, or a range between any two of the foregoing values or any value within the range.

[0191] 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 appropriate 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.

[0192] In some embodiments, the copper foil has a thickness of 4 μm-10 μm. In some embodiments, the copper foil has a thickness of 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, or a range between any two of the foregoing values or any value within the range.

[0193] In the present application, the thickness of the copper foil can be tested by methods known in the art. As an example, a 20x15 cm 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 8.96 g / cm 3 The length and width of the sample are known, and thus the thickness of the sample can be calculated.

[0194] The maximum load that can be carried by the ordinary strength copper foil after thinning decreases sharply, 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, causing safety accidents. The copper foil provided in the present application still has excellent tensile strength and elongation at break in the thickness range of 4 μm-10 μm, and has good mechanical properties and plasticity. The copper foil can thin the thickness without affecting the strength, which is helpful for the lightweight design of the battery, and is helpful for the weight reduction of the battery and further improves the energy density or specific capacity of the secondary battery.

[0195] In some embodiments, the expansion force of the secondary battery is greater than or equal to 1000 kgf.

[0196] In some embodiments, the expansion force of the secondary battery is greater than or equal to 2500 kgf.

[0197] In some embodiments, the expansion force of the secondary battery is greater than or equal to 4000 kgf.

[0198] In some embodiments, the expansion force of the secondary battery is 1000 kgf-10000 kgf.

[0199] In some embodiments, the expansion force of the secondary battery is 1000 kgf, 1200 kgf, 1500 kgf, 1800 kgf, 2000 kgf, 2200 kgf, 2500 kgf, 2800 kgf, 3000 kgf, 3200 kgf, 3500 kgf, 3800 kgf, 4000 kgf, 4200 kgf, 4500 kgf, 4800 kgf, 5000 kgf, 5500 kgf, 6000 kgf, 6500 kgf, 7000 kgf, 7500 kgf, 8000 kgf, 8500 kgf, 9000 kgf, 9500 kgf, 10000 kgf, or a range between any two of the aforementioned values or any value within the range.

[0200] 1 kgf refers to the gravity of an object of 1 kg on the sea level at 45 degrees north latitude. 1 kgf is about 9.8 Newton. The expansion force of the secondary battery cell refers to the expansion force of the secondary battery at 60% SOH (state of charge), which can be sensed by the pressure sensor in the clamp arranged on both sides of the large surface of the pole piece of the secondary battery cell. As an example, the following method is used for testing: at 25°C, the battery is charged at 1C constant current to 3.8V, then charged at 3.8V constant voltage until the current is less than or equal to 0.05C, and then the battery is discharged at 1C constant current to 2.5V, which is one charge and discharge process, and the cycle charging and discharging is repeated in this way, the SOH of the battery is monitored throughout the process, when the SOH reaches 90%, the expansion force of the secondary battery at 90% SOH is measured by the pressure sensor in the clamp arranged on both sides of the large surface of the pole piece of the secondary battery, and the expansion force of the secondary battery at 60% SOH is obtained by simulation model simulation.

[0201] The current collector in the prior art is prone to breakage under high expansion force of the secondary battery, which causes safety accidents and limits the further improvement of the electrochemical performance of the secondary battery. The current collector provided by the embodiments of the present application has excellent tensile strength and elongation at break, and can be applied to secondary batteries with high expansion force, which is conducive to the further improvement of the energy density of the secondary battery.

[0202] In some embodiments, the thickness expansion rate of the secondary battery is 4%-10%.

[0203] In the present application, the thickness expansion rate of the secondary battery can be tested by a method known in the art, as an example, wherein the thickness expansion rate = (H1-H0) / H0, wherein H0 and H1 are the total thickness of the secondary battery at 100% SOH and the total thickness of the secondary battery at 60% SOH, respectively; as an example, the following method can be referred to for determination, at 25°C, the total thickness H0 of the secondary battery is determined, then the battery is charged at 1C constant current to 3.8V, then charged at 3.8V constant voltage until the current is ≤0.05C, then the battery is discharged at 1C constant current to 2.5V, which is one charge and discharge process, and the cycle is repeated in this way, the SOH of the battery is monitored throughout the process, when the secondary battery reaches 90% SOH, the total thickness H' of the secondary battery at this time is determined, and the total thickness H1 of the secondary battery at 60% SOH is obtained by simulation model simulation, and the thickness expansion rate of the secondary battery is obtained.

[0204] In some embodiments, the thickness expansion rate of the secondary battery is 4%, 5%, 6%, 7%, 8%, 9%, 10% or any numerical range between any two of them.

[0205] The current collector in the prior art is prone to breakage under the expansion stress of the volume change of the secondary battery, which causes the performance of the secondary battery to drop sharply and easily causes safety accidents, limiting the further improvement of the performance of the secondary battery. The current collector provided in the embodiments of the present application has good strength performance and can be applied to secondary batteries with high thickness expansion rate, which is conducive to the further improvement of the energy density and safety of the secondary battery.

[0206] The secondary battery provided in the present application can be prepared by a method comprising the following steps: preparing a copper foil by an electroplating method, the electroplating method comprising 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 being 40000A-100000A, the valley value of the pulse current being 100A-20000A, and the change period of the current being 50ms-5000ms; the copper foil comprising copper grains of different particle sizes, the copper grains comprising copper grains with a particle size of less than or equal to 0.5μm and copper grains with a particle size of greater than 0.5μm, wherein the number of copper grains with a particle size of less than or equal to 0.5μm accounts for 70%-95% of the total number of copper grains, the number of copper grains with a particle size of greater than 1μm accounts for 1%-10% of the total number of copper grains, and the particle size span of the copper grains is 1.3μm-3.4μm.

[0207] In the present text, the term "electroplating method" refers to a method of depositing metal or alloy on the surface of a workpiece to form a metal layer by using the principle of electroplating.

[0208] In the present text, the term "pulse current" refers to a current or voltage pulse that appears repeatedly in a cycle.

[0209] 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 prepared copper foil has excellent tensile strength and elongation at break, and has excellent mechanical strength and plasticity, and also helps to reduce the manufacturing cost of the secondary battery.

[0210] 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. The preparation method provided in the present application uses pulse current, adjusts the nucleation and growth rate of copper grains by continuous change of the size of the current, 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.

[0211] In some embodiments, the preparation method of the secondary battery includes preparing a pole piece using a copper foil as a current collector, and the preparation of the copper foil includes electroplating method to prepare the copper foil, and the electroplating method includes applying pulse current to the electroplating solution to reduce and deposit copper ions in the electroplating solution to form the copper foil, the peak value of the pulse current is 40000A-100000A, the valley value of the pulse current is 100A-20000A, and the change period of the current is 50ms-5000ms; and the expansion force of the secondary battery is greater than or equal to 1000kgf.

[0212] The preparation method of the secondary battery provided in the present application uses the above-mentioned electroplating method to prepare the copper foil, and obtains the copper foil with excellent tensile strength and elongation at break, which improves the mechanical strength while having excellent plasticity, reduces the probability of fracture or crack when the copper foil is subjected to stress caused by the volume expansion of the pole piece in the secondary battery, the decomposition of the electrolyte to produce gas, and other factors to cause the expansion of the expansion force and / or the thickness of the secondary battery, which is beneficial to further improve the safety and service life of the secondary battery.

[0213] In some embodiments, the preparation method of the secondary battery includes preparing a roll type secondary battery, using a copper foil as a current collector to prepare a pole piece, and the preparation of the copper foil includes electroplating method to prepare the copper foil, and the electroplating method includes applying pulse current to the electroplating solution to reduce and deposit copper ions in the electroplating solution to form the copper foil, the peak value of the pulse current is 40000A-100000A, the valley value of the pulse current is 100A-20000A, and the change period of the current is 50ms-5000ms; and the expansion force of the secondary battery is greater than or equal to 1000kgf.

[0214] The secondary battery preparation method provided in this application uses the above-mentioned electroplating method to prepare copper foil, which obtains copper foil with excellent tensile strength and elongation at break. While improving mechanical strength, it also has excellent plasticity, reducing the probability of copper foil cracking or breaking under expansion stress after secondary battery winding and pressing. It also reduces the probability of cracking of the wound cell electrode, thereby further improving the safety and service life of the secondary battery.

[0215] In some embodiments, the peak value of the pulse current is 40,000A-80,000A. In some embodiments, the peak value of the pulse current is 50,000A-70,000A. In some embodiments, the peak value of the pulse current is 55,000A-70,000A, 50,000A-80,000A, 55,000A-80,000A, or 50,000A-60,000A.

[0216] In some implementations, the peak value of the pulse current is 45000A, 50000A, 55000A, 60000A, 65000A, 70000A, 75000A, 80000A, 85000A, 90000A, 95000A, 100000A, or any range between any two of the above values ​​or any value between the ranges.

[0217] In some embodiments, the valley value of the pulse current is 1000A-10000A. In some embodiments, the valley value of the pulse current is 2000A-5500A. In some embodiments, the valley value of the pulse current is 2500A-5000A, 2000A-8000A, or 2000A-5000A.

[0218] In some implementations, the valley value of the pulse current is 100A, 500A, 1000A, 1500A, 2000A, 2500A, 3500A, 4000A, 4500A, 5000A, 8000A, 10000A, 15000A, 20000A, or any range between any two of the above values ​​or any value between the ranges.

[0219] In some embodiments, the pulse current variation period is 500ms-5000ms. In some embodiments, the pulse current variation period is 2000ms-4000ms. In some embodiments, the pulse current variation period is 50ms, 100ms, 200ms, 500ms, 1000ms, 1200ms, 1500ms, 1800ms, 2000ms, 2500ms, 2750ms, 3000ms, 3500ms, 4000ms, 4500ms, 5000ms, or any range between any two of the above values, or any value within the range.

[0220] In the present disclosure, the term "peak value" refers to the maximum current value of the pulse current, typically the value at the peak of the pulse current waveform. Similarly, the term "valley value" refers to the minimum current value of the pulse current, typically the value at the valley of the pulse current waveform.

[0221] In the present disclosure, the term "change period" refers to the time between two adjacent peaks or valleys of the pulse current waveform, in ms.

[0222] 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 large-sized grains, such as columnar crystals. Adjusting the change parameters of the current, i.e. the peak value, the valley value and the change 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, and making the copper foil have excellent mechanical strength and plasticity.

[0223] In some embodiments, the pulse current comprises one or more of a square wave pulse current, a sinusoidal wave pulse current, a triangular wave pulse current, and a sawtooth wave pulse current.

[0224] In some embodiments, the pulse current comprises a sinusoidal wave pulse current. The sinusoidal wave pulse current continuously and periodically changes, which is conducive to the continuous and variable growth of the grains.

[0225] In some embodiments, the pulse current further comprises a linear oscillation current.

[0226] In the present disclosure, the term "linear oscillation current" refers to an oscillation current with a linear current waveform that changes linearly and periodically.

[0227] 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 large-sized grains. Through the continuous change of the linear oscillation current, the particle size of the deposited copper grains changes in size.

[0228] In some embodiments, the preparation method is a continuous production method.

[0229] In some embodiments, the preparation method is a roller deposition method. The working principle is that the cathode roller is connected to the negative pole of the power supply, and the anode groove is connected to the positive pole 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 poles. 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. 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.

[0230] In some embodiments, the cathode electrode is a titanium roller or a titanium plate.

[0231] In some embodiments, the anode electrode is a titanium substrate plate.

[0232] In some embodiments, the distance between the cathode electrode and the anode electrode is 8-20 mm. In some embodiments, the distance between the cathode electrode and the anode electrode is 8-12 mm. In some embodiments, the distance between the cathode electrode and the anode electrode is 8 mm, 9 mm, 10 mm, 11 mm, 12 mm, 13 mm, 14 mm, 15 mm, 16 mm, 17 mm, 18 mm, 19 mm, 20 mm, or any range between any two of the above values or any value between the ranges.

[0233] In some embodiments, the temperature of electroplating deposition is 45-60°C. In some embodiments, the temperature of electroplating deposition is 50-60°C. In some embodiments, the deposition temperature can be selected from 45°C, 50°C, 55°C, 60°C, any range between any two of the above values or any value between the ranges.

[0234] In some embodiments, the roller speed of the cathode roller is 2-5 m / min. In some embodiments, the roller speed of the cathode roller is 2-3 m / min. In some embodiments, the roller speed of the cathode roller is 2 m / min, 2.5 m / min, 3 m / min, 3.5 m / min, 4 m / min, 4.5 m / min, 5 m / min, or any range between any two of the above values or any value between the ranges.

[0235] The cathode roller can be any roller suitable for preparing copper foil in the art, for example, a titanium roller.

[0236] In some embodiments, the electroplating solution includes a leveling agent, a wetting agent, and a brightener.

[0237] As used herein, the term "leveler" refers to a substance added to the electroplating solution to improve the flatness of the plated layer, which can attach to the tip of the copper foil with a high deposition rate, inhibit grain growth, balance the growth rate of the pits and the tip, and improve the flatness of the copper foil.

[0238] As used herein, the term "wetting agent" refers to a substance used to reduce the interfacial tension between the electroplating solution and the electrode, 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, the wetting of the electroplating solution on the cathode is sufficient to enable fast electrodeposition with a large current, to improve the nucleation rate of the copper foil, and to reduce the grain size in the copper foil.

[0239] As used herein, 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.

[0240] In some embodiments, the leveler includes one or more of collagen, sodium saccharin. In some embodiments, the leveler includes collagen and sodium saccharin.

[0241] Without being bound by any theory, collagen can inhibit the deposition of copper ions and balance the growth rate of the pits and the tip. The introduction of sodium saccharin can attract copper ions to deposit at the depressions on the surface of the copper foil, reduce the microscopic defects or unevenness inside the copper foil, and reduce the warpage defects caused by the defects. The two different levelers help to further improve the surface pits and protrusions of the copper foil and improve the flatness of the copper foil.

[0242] In some embodiments, the concentration of collagen in the electroplating solution is 60 mg / L to 300 mg / L. In some embodiments, the concentration of collagen in the electroplating solution is 80 mg / L to 150 mg / L. In some embodiments, the concentration of 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 two of the above values or any value between the ranges.

[0243] The collagen can be a protein with a molecular weight commonly used in the field of copper foil, for example, collagen with a relative molecular weight of 8000-12000.

[0244] In some embodiments, the concentration of sodium saccharin in the electroplating solution is 0.5 g / L to 10 g / L. In some embodiments, the concentration of sodium saccharin in the electroplating solution is 0.5 g / L to 4 g / L. In some embodiments, the concentration of 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 a range between any two of the foregoing values or any value between the ranges.

[0245] Without being bound by any theory, hydroxyethyl cellulose has good water solubility and thickening properties, 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 surface of the solid. The combination of the two can improve the adhesion properties of the electroplating solution, increase and assist the adhesion of copper ions to the surface of the substrate 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.

[0246] In some embodiments, the concentration of polyethylene glycol in the electroplating solution is 50 mg / L to 200 mg / L. In some embodiments, the concentration of polyethylene glycol in the electroplating solution is 60 mg / L to 150 mg / L. In some embodiments, the concentration of 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 a range between any two of the foregoing values or any value between the ranges.

[0247] 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.

[0248] In some embodiments, the concentration of the hydroxyethyl cellulose in the electroplating solution is 30 mg / L to 200 mg / L. In some embodiments, the concentration of the hydroxyethyl cellulose in the electroplating solution is 50 mg / L to 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 of the foregoing.

[0249] The polyethylene glycol can be selected from those commonly used in the copper foil industry. In some embodiments, the relative molecular mass of the polyethylene glycol is 120,000.

[0250] In some embodiments, the mass ratio of the polyethylene glycol to the hydroxyethyl cellulose in the electroplating solution is (1-1.5): 1. In some embodiments, the mass ratio of the polyethylene glycol to the hydroxyethyl cellulose in the electroplating solution is (1.2-1.5): 1. As examples, the mass ratio of the polyethylene glycol to the hydroxyethyl cellulose is 1:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, or any range between any of the foregoing.

[0251] In some embodiments, the brightener includes sodium polydi-thio-propane sulfonate.

[0252] Without being bound by any theory, the sodium polydi-thio-propane sulfonate is adsorbed on the cathode copper surface through the mercapto functional group or disulfide bond, the terminal sulfonate anion captures the hydrated copper ions in the electroplating solution to destroy the hydration of the copper ions, and interacts with the chloride ions adsorbed on the cathode surface to transfer the electrons to the captured copper ions through the chloride ions, thereby greatly improving the electrochemical reduction rate of the copper ions, refining the crystal grains, and achieving material strengthening.

[0253] In some embodiments, the concentration of the sodium polydi-thio-propane sulfonate in the electroplating solution is 500 mg / L to 2000 mg / L. In some embodiments, the concentration of the sodium polydi-thio-propane sulfonate in the electroplating solution is 500 mg / L to 1000 mg / L. In some embodiments, the concentration of the sodium polydi-thio-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 any range between any of the foregoing.

[0254] In some embodiments, the electroplating solution includes chloride ions at a concentration of 20 mg / L to 80 mg / L (as chlorine atoms). In some embodiments, the concentration of sodium poly(dithiobispropyl sulfone) in the electroplating solution is 40 mg / L to 80 mg / L. In some embodiments, the concentration of chloride ions (as 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 these values or any value between ranges.

[0255] In some embodiments, the electroplating solution includes 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 poly(dithiobispropyl sulfone) at a concentration of 500 mg / L to 2000 mg / L, and chloride ions at a concentration of 20 mg / L to 80 mg / L.

[0256] In some embodiments, the electroplating solution includes collagen at a concentration of 80 mg / L to 150 mg / L, sodium saccharin at a concentration of 0.5 g / L to 4 g / L, polyethylene glycol at a concentration of 60 mg / L to 150 mg / L, hydroxyethyl cellulose at a concentration of 50 mg / L to 150 mg / L, sodium poly(dithiobispropyl sulfone) at a concentration of 500 mg / L to 1000 mg / L, and chloride ions at a concentration of 40 mg / L to 80 mg / L.

[0257] The electroplating solution also includes a copper source to provide copper ions to the electroplating solution. The electroplating solution also includes sulfuric acid to provide an acidic environment for reduction of the copper ions.

[0258] In some embodiments, the concentration of copper ions (as copper atoms) is 60 g / L to 100 g / L. In some embodiments, the concentration of copper ions (as copper atoms) is 80 g / L to 100 g / L. In some embodiments, the concentration of copper ions (as copper atoms) is 60 g / L, 70 g / L, 80 g / L, 90 g / L, 100 g / L, a range between any two of these values or any value between ranges.

[0259] In some embodiments, the concentration of sulfuric acid is 60 g / L to 110 g / L. In some embodiments, the concentration of sulfuric acid is 80 g / L to 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 these values or any value between ranges.

[0260] 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 aforementioned values or any value between the aforementioned ranges.

[0261] 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.

[0262] [Negative electrode tab]

[0263] As an example of the negative electrode tab, the negative current collector has two opposite surfaces in the thickness direction of itself, and the negative film layer is arranged on any one or both of the two opposite surfaces of the negative current collector.

[0264] In some embodiments, the negative current collector can be the current collector described in the first aspect of the 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.

[0265] 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, lithium titanate, and lithium metal. The tin-based material can include one or more of elemental tin, tin oxide, and tin alloy material.

[0266] In some embodiments, the negative active material includes a silicon-based material, and the silicon-based material includes one or more of elemental silicon, silicon oxide, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy material.

[0267] In some embodiments, the mass content of the silicon-based material is 5%-100% based on the total mass of the negative film layer.

[0268] In some embodiments, the mass content of the silicon-based material is 10%-80% based on the total mass of the negative film layer.

[0269] In some embodiments, the mass content of the silicon-based material is 10%-30% based on the total mass of the negative film layer.

[0270] In some embodiments, the mass content of the silicon-based material can be selected as 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 100%, or a numerical range between any two of the aforementioned values, based on the total mass of the negative film layer.

[0271] In some embodiments, the silicon element accounts for 20-50% of the mass of the silicon-based material.

[0272] In some embodiments, the silicon element accounts for 20%, 25%, 30%, 35%, 40%, 45%, 50%, or any numerical range between any two of the aforementioned values, of the mass of the silicon-based material.

[0273] In some embodiments, the silicon element accounts for 4-10% of the mass of the negative electrode film layer, based on the total mass of the negative electrode film layer.

[0274] In some embodiments, the silicon element accounts for 4%, 5%, 6%, 7%, 8%, 9%, 10%, or any numerical range between any two of the aforementioned values, of the mass of the negative electrode film layer, based on the total mass of the negative electrode film layer.

[0275] In some embodiments, the negative electrode film layer further optionally comprises 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).

[0276] In some embodiments, the negative electrode tab further comprises a conductive agent. The conductive agent comprises one or more of super conductive carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0277] In some embodiments, the conductive agent comprises carbon black. In some embodiments, the conductive agent comprises carbon nanotubes. In some embodiments, the conductive agent comprises 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 tab.

[0278] In some embodiments, the negative electrode film layer further optionally comprises other auxiliary agents, such as thickening agents (e.g., sodium carboxymethyl cellulose (CMC-Na)) and the like.

[0279] In some embodiments, the negative electrode tab can be prepared by dispersing the aforementioned components for preparing the negative electrode tab, 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 after processes such as drying and cold pressing, the negative electrode tab can be obtained.

[0280] [Positive electrode tab]

[0281] The positive electrode tab comprises a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector, and the positive electrode film layer comprises a positive electrode active material.

[0282] As an example, the positive electrode current collector has two surfaces opposite in the thickness direction thereof, and the positive electrode film layer is provided on either one or both of the two opposite surfaces of the positive electrode current collector.

[0283] In some embodiments, the positive electrode current collector can employ a metal foil or a composite current collector. For example, as the metal foil, an aluminum foil can be employed. 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.).

[0284] In some embodiments, the positive electrode active material can employ 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 of 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 only or in combination of two or more. Among them, 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 simply as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2 (which can also be referred to simply as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2 (which can also be referred to simply as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2 (which can also be referred to simply as NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1 O2 (which can also be referred to simply as NCM 811 ), lithium nickel cobalt aluminum oxide (e.g., LiNi 0.85 Co 0.15 Al 0.05O2) and modified compounds thereof, etc. Examples of the lithium-containing phosphate of olivine structure can include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (which can also be referred to simply 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.

[0285] 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.

[0286] 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.

[0287] 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., obtaining the positive electrode tab.

[0288] [Electrolyte]

[0289] The electrolyte functions to conduct ions between the positive electrode tab and the negative electrode tab. The type of 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.

[0290] In some embodiments, the electrolyte employs an electrolytic solution. The electrolytic solution includes an electrolyte salt and a solvent.

[0291] 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 bistrifluoromethylsulfonylimide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorobisoxalate borate, lithium bisoxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorodioxalate phosphate.

[0292] 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.

[0293] 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 properties 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.

[0294] [Separator]

[0295] 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.

[0296] 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.

[0297] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be made into an electrode assembly through a winding process or a stacking process.

[0298] 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 solution.

[0299] 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.

[0300] In some embodiments, the secondary battery includes a winding-type secondary battery, and the positive electrode sheet, the negative electrode sheet, and the separator are made into an electrode assembly through a winding process.

[0301] In some embodiments, the secondary battery includes a stacking-type secondary battery, and the positive electrode sheet, the negative electrode sheet, and the separator are made into an electrode assembly through a stacking process.

[0302] Further, the secondary battery, the battery module, the battery pack, and the electric device according to the present application will be described below with appropriate reference to the accompanying drawings.

[0303] In one embodiment of the present application, a secondary battery is provided.

[0304] The shape of the secondary battery according to the present application is not particularly limited, and can be cylindrical, square, or any other shape. For example, FIG. 1 is a secondary battery 5 of a square structure as an example.

[0305] In some embodiments, referring to FIG. 2, the outer package can include a housing 51 and a cover plate 53. The housing 51 can include a bottom plate and side plates connected to the bottom plate, which 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 tab, the negative electrode tab, 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, which can be selected by those skilled in the art according to specific actual needs.

[0306] 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, which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0307] FIG. 3 is a battery module 4 as an example. Referring to FIG. 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, they can also be arranged in any other manner. Further, the plurality of secondary batteries 5 can be fixed by fasteners.

[0308] 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.

[0309] 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, which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0310] FIGS. 4 and 5 are battery packs 1 as examples. Referring to FIGS. 4 and 5, the battery pack 1 can include a battery box and a plurality of battery modules 4 arranged in the battery box. The battery box includes an upper box body 2 and a lower box body 3, and the upper box body 2 can be provided on the lower box body 3 to form a closed space for receiving the battery modules 4. The plurality of battery modules 4 can be arranged in the battery box in any manner.

[0311] In addition, the application also provides a power utilization device, which comprises 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, or can 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.

[0312] As the power utilization device, the secondary battery, the battery module, or the battery pack can be selected according to the use requirements thereof.

[0313] FIG. 6 is a power utilization device as an example. 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 requirements of high power and high energy density of the secondary battery for the power utilization device, the battery pack or the battery module can be used.

[0314] The device as another example can be 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.

[0315] Embodiment

[0316] 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 used is not indicated by the manufacturer, it is a conventional product that can be obtained by purchase.

[0317] I. Performance test

[0318] (1) Grain characteristics test of copper foil

[0319] 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 distribution 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.

[0320] (2) Mechanical property test

[0321] According to GB / T 5230-1995 "Electrolytic Copper Foil", the copper foil sample prepared in the examples was cut into a tensile sample with a length L0 of 50 mm and a width of 15 mm. The tensile properties were tested at 25°C using a universal testing machine, and the tensile rate was set to 50 mm / min.

[0322] The cross-sectional area of the tensile sample is where p is 8.96 g / cm 3 , m is in grams, and L0 is in centimeters.

[0323] The sample was continuously loaded until it was pulled apart, and the maximum load F was read from the force dial or the tensile curve, and the tensile strength σ b was calculated according to Formula I.

[0324] The distance between the two lines after the sample was pulled apart is L1, which was measured on the sample or read from the tensile curve. L1 can be measured using the straight line method or the displacement method, and the elongation at break δ is calculated according to Formula II.

[0325] (3) Hardness Test

[0326] The copper foil sample was placed in a metallographic hot mounting machine, and after pouring in wood powder, it was heated at a rate of 150°C / 10 min, and a Vickers hardness tester was used to press the copper foil sample with a 50g weight, and the lengths of the two diagonal lines were optically measured, and the corresponding Vickers hardness was obtained according to the following Vickers hardness calculation formula.

[0327] HV represents the Vickers hardness;

[0328] F represents the load of the indenter (Newton force);

[0329] a represents the included angle between the opposite faces of the indenter (136°);

[0330] d represents the average value of the diagonal line length (mm).

[0331] (4) Crack (Crack) failure corresponds to the state of charge (SOH, State of Health)

[0332] Firstly, measure the total thickness of the secondary battery at 25℃, then charge the battery at 1C constant current to 3.8V, then charge at 3.8V constant voltage until the current is ≤0.05C, then discharge the battery at 1C constant current to 2.5V, which is a 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 that has failed due to cracks during the cycle is disassembled to observe whether the negative electrode sheet has been broken, and the SOH corresponding to the crack failure is obtained.

[0333] (5) Thickness expansion rate and expansion force test of secondary battery at crack failure:

[0334] Firstly, measure the total thickness of the secondary battery at 25℃, then charge the battery at 1C constant current to 3.8V, then charge at 3.8V constant voltage until the current is ≤0.05C, then discharge the battery at 1C constant current to 2.5V, which is a 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 that has failed due to cracks during the cycle is disassembled to observe whether the negative electrode sheet has been broken, and the SOH corresponding to the crack failure is obtained, measure the total thickness of the secondary battery at this time, and calculate the thickness expansion rate by the formula (H1'-H0') / H0'; at the same time, the expansion force at this time is measured by the pressure sensor in the clamp set on both sides of the large surface of the secondary battery monomer electrode sheet.

[0335] (6) Thickness expansion rate and expansion force test of secondary battery at 60% SOH:

[0336] Firstly, measure the total thickness of the secondary battery at 25℃, then charge the battery at 1C constant current to 3.8V, then charge at 3.8V constant voltage until the current is ≤0.05C, then discharge the battery at 1C constant current to 2.5V, which is a 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 that has failed due to cracks during the cycle is disassembled to observe whether the negative electrode sheet has been broken, and the SOH corresponding to the crack failure is obtained, measure the total thickness of the secondary battery at this time, and calculate the thickness expansion rate by the formula (H1'-H0') / H0'; at the same time, the expansion force at this time is measured by the pressure sensor in the clamp set on both sides of the large surface of the secondary battery monomer electrode sheet.

[0337] II. Preparation method

[0338] Example 1

[0339] (1) Preparation of copper foil

[0340] Copper plates or copper wires with a purity of 99.9% or above are dissolved in sulfuric acid with a mass content of 98% to obtain a copper sulfate solution, which is used as a copper source to configure an electroplating solution by adding additives and hydrochloric acid at 55°C. Among them, 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 sodium polydithiobispropane sulfonate is 600 mg / L, the concentration of sodium saccharin is 2 g / L, the concentration of copper ions (in terms of copper atoms) is 90 g / L, and the rest is deionized water. The pH of the electroplating solution is 3.5.

[0341] A sinusoidal pulse current is used to periodically apply current to a cathode titanium roller that has been polished, and the anode electrode is a titanium substrate plate. The titanium roller has an area of 8.67 m 2 in the electroplating solution, the rotation speed of the titanium roller (roller speed) 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, and the deposition temperature is 55°C. Copper foil with a thickness of 6 μm is deposited on the titanium roller.

[0342] (2) Preparation of the battery

[0343] The positive electrode active material LiNi 0.8 Co 0.1 Mn 0.1 O2(NCM811), the conductive agent acetylene black, and the binder polyvinylidene fluoride (PVDF) are dissolved in the solvent N-methyl pyrrolidone (NMP) at a weight ratio of 90:5:5, and after being thoroughly stirred and mixed uniformly, a positive electrode slurry is obtained; then the positive electrode slurry is uniformly coated on a positive electrode current collector, and after drying, cold pressing, and slitting, a positive electrode sheet is obtained.

[0344] The negative electrode active material silicon-carbon (silicon content 20wt%-50wt%), artificial graphite, the conductive agent acetylene black, the binder styrene-butadiene rubber (SBR), and the thickening agent sodium carboxymethyl cellulose (CMC-Na) are dissolved in deionized water at a weight ratio of 20:76:1:1.5:1.5, and after being uniformly mixed, a negative electrode slurry is prepared; then the negative electrode slurry is uniformly coated one or more times on a negative electrode current collector copper foil, and after drying, a negative electrode film is obtained, which is then cold-pressed and slit to obtain a negative electrode sheet. The gram capacity of the negative electrode active material in the negative electrode film layer is 800 mAg / h-1500 mAg / h.

[0345] 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, LiPF6 was dissolved in the above solution to obtain an electrolyte. In the electrolyte, the concentration of LiPF6 was 1 mol / L. Then, 2.0 wt% of fluoroethylene carbonate, 0.5 wt% of 1,3-propane sultone and 0.5 wt% of succinic anhydride were added as additives to the above organic solvent, mixed and stirred uniformly to obtain an electrolyte.

[0346] A polypropylene film was used as a separator film.

[0347] The positive electrode sheet, the separator film and the negative electrode sheet were stacked in order, the separator film was between the positive electrode sheet and the negative electrode sheet to play a role of isolation, then the electrode assembly was obtained by winding. The electrode assembly was placed in a battery shell, after drying, the electrolyte was injected, and then the lithium ion battery was prepared through processes such as formation and standing.

[0348] Example 2-4

[0349] The preparation method of Example 2-4 was basically the same as that of Example 1, but the composition of the plating solution (see Table 1) and the sine wave pulse current parameters (see Table 2) were adjusted; the thickness of the deposited copper foil was 6 μm.

[0350] Example 1-1

[0351] The preparation method of Example 1-1 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 thickness of the copper foil was 6 μm.

[0352] Comparative Example 1

[0353] The preparation method of Comparative Example 1 was basically the same as that of Example 1, and direct current deposition current was 30,000 A. The thickness of the copper foil was 6 μm.

[0354] Table 1: Composition of plating solution

[0355] Table 2: Sine wave pulse current parameters

[0356] NA means not applicable.

[0357] The grain characteristics and mechanical characteristics of the copper foils prepared in Test Example 1-4 and Example 1-1, Comparative Example 1, and the results of the secondary batteries were as shown in Table 3.

[0358] Table 3

[0359] FIGS. 7, 10 and 13 respectively show the inverse pole figure maps of the cross-sections of the copper foils of Examples 1-3 tested by EBSD, wherein T is along the thickness direction of the current collector, and the cross-sections of the copper foils present heterogeneous grain organizations with fine grain doping distribution. Compared with the inverse pole figure map of the copper foil of Comparative Example 1 (FIG. 17), it can be seen that there are more fine grains in the copper foils of Examples 1-3, and the grain size is smaller in general.

[0360] FIGS. 9, 12, 15 and 19 respectively show the grain size distribution maps of the copper foils of Examples 1-3 and Comparative Example 1. As can be seen from Table 3, in the copper foils of Examples 1-3, the number of copper grains with a grain size of 0.5 μm or less accounts for 70%-95%, and is concentrated in the range of 80%-95%. In Comparative Example 1, the number of copper grains with a grain size of 0.5 μm or less accounts for less than 50%, indicating that the copper grains of Examples 1-3 are smaller in general, and the copper foils have higher mechanical strength. In addition, in the copper foils of Examples 1-3, the number of copper grains with a grain size of more than 0.5 μm accounts for 5%-30%, while in Comparative Example 1, the number of copper grains with a grain size of more than 0.5 μm accounts for 50.5%, and the copper foils of Examples 1-3 have plasticity basically equivalent to that of the copper foil of Comparative Example 1. This indicates that the preparation of the copper foil by the sinusoidal pulse current helps to adjust the grain size distribution of the copper grains, thereby improving the mechanical properties and plasticity of the copper foil.

[0361] In the copper foils of Examples 1-3, the average grain size is in the range of 0.3 μm-0.6 μm, the maximum grain size is in the range of 1 μm-2 μm, the minimum grain size is in the range of 0.1 μm-0.3 μm, and the grain size span is 0.8 μm-2 μm. In Comparative Example 1, the span of the copper grain size is larger, and the average grain size of the copper grains 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 foils of Comparative Example 1 have less cumulative crystal interface density, and have low mechanical strength. This also corresponds to the tensile strength of Examples 1-3 and Comparative Example 1.

[0362] Further, compared with the copper foils of Comparative Example 1, the copper foils of Examples 1-3 have a grain morphology of "mainly fine grains mixed with large grains", and also have large grains with a grain size of more than 1 μm, a grain size span of 1.3 μm-3.4 μm, and a number percentage of 2%-10%. The dispersion degree of the copper grain size distribution is in a suitable range, and the heterogeneous grains of a small amount of large grains doped in the fine grains with a large number percentage can help to balance the crystal boundary area and the geometric necessary dislocation density of the copper grains, so that the copper foils have a suitable yield ratio, which is beneficial to reduce the probability of fracture along the crystal boundary of the copper foils under external force, and balances the strength and brittleness of the copper foils, thereby reducing the probability of fracture of the current collector in the battery and the failure of the battery.

[0363] Fig. 8, Fig. 11 and Fig. 14 show the tensile curves of the copper foils of Examples 1-3, respectively, and it can be seen that the tensile strength of the prepared copper foils is in the range of 600 MPa-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. Fig. 16 shows the tensile curve of the copper foil of Example 1-1, which has a tensile strength similar to that of Example 1, but the elongation at break is only 2.7%, showing poor plasticity. Fig. 18 shows the tensile curve of the copper foil of Comparative Example 1, which has an elongation at break similar to that of Example 1, but the tensile strength is poor.

[0364] The hardness of the copper foils of Examples 1-3 is in the range of 55 HV-65 HV, indicating that the copper foils have good pressure deformation or puncture resistance.

[0365] Example 1-1 uses the same direct current as the peak current of Example 1 to prepare the copper foil, and compared with Example 1, the copper foil of Example 1-1 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 grain size distribution of copper can be adjusted by the sinusoidal pulse current and the current parameters, so that the copper foil has excellent plasticity on the premise of having good mechanical strength.

[0366] Compared with Comparative Example 1, the crack failure of Example 1-3 corresponds to a significantly reduced SOH, indicating that the copper foil prepared by Example 1-3 significantly improves the service life of the secondary battery and improves the safety of the secondary battery. Although the copper foil of Comparative Example 1 has excellent elongation at break, the tensile strength is low, resulting in a relatively high SOH value corresponding to crack failure. In comparison, the tensile strength of the copper foil prepared by Example 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.

[0367] Compared with Comparative Example 1, the copper foil of Example 1-3 has good mechanical properties and plasticity, which can effectively reduce the probability of crack or fracture of the current collector caused by the expansion of the secondary battery. For secondary batteries with high expansion properties, such as an expansion force greater than or equal to 1000 kgf and a thickness expansion rate of 4%-10%, the copper foil of Example 1-3 can effectively reduce the probability of crack or fracture of the current collector, so that the SOH of the secondary battery decreases when the crack fails (i.e., effectively improving the cycle service life and safety of the secondary battery), and the probability of secondary battery failure and performance diving caused by battery expansion is reduced.

[0368] Example 5-6

[0369] The preparation method of Example 5 is basically similar to that of Example 1, and the difference is that the preparation of the copper foil is adjusted as follows:

[0370] Preparation of copper foil

[0371] A copper plate or copper wire with purity of 99.9% or above is dissolved in 98% mass content sulfuric acid to prepare a copper sulfate solution, which is used as a copper source to configure an electrolyte by adding additives and hydrochloric acid at 60°C. The prepared electrolyte has a pH of 3.5, a collagen concentration of 90 mg / L, a polyethylene glycol concentration of 50 mg / L, a hydroxyethyl cellulose concentration of 30 mg / L, a chloride ion concentration of 30 mg / L, a sodium polydithiobispropane sulfonate concentration of 75 mg / L, a copper ion concentration of 90 g / L, a sulfuric acid concentration of 100 g / L, and the rest is deionized water.

[0372] A linear current waveform is used to periodically apply a current to a cathode titanium roller that has been polished, and a titanium substrate plate is used as an anode electrode. The titanium roller has an area of 0.3 dm 2 in the electrolyte, the rotation speed of the titanium roller is 2 m / min, the applied current has a minimum current of 20 KA and a maximum current of 45 KA, the period is 50 ms, the distance between the cathode and the anode is 20 mm, and the deposition temperature is 60°C. The copper foil is deposited on the titanium roller by electroplating for 224 s, and the thickness of the copper foil is 6 μm.

[0373] Example 6

[0374] The preparation method of Example 6 is basically similar to that of Example 5, except that the minimum current is adjusted to 25 KA.

[0375] Example 7

[0376] The preparation process of Example 7 is basically similar to that of Example 5, but a direct current of 45 KA is used to prepare the copper foil.

[0377] Table 4

[0378] FIGS. 20-21 show the cross-sectional electron backscatter diffraction (EBSD) pattern and the grain size distribution of the copper foil prepared in Example 5, and FIGS. 22-23 show the cross-sectional electron backscatter diffraction (EBSD) pattern and the grain size distribution of the copper foil prepared in Example 6. As can be seen from the above figures and Table 4, the copper foil prepared in the examples has fine grains with a particle size of 0.5 μm or less, accounting for 70%-95% of the total number, grains with a particle size of more than 1 μm, accounting for 2%-10% of the total number, and a grain size span of 1.3 μm-3.4 μm, which improves the ability of the copper foil to enhance and inhibit strain localization, reduces the occurrence of stress concentration, and improves the grain sliding resistance, thereby improving the mechanical strength of the copper foil while reducing the brittleness of the copper foil, and the copper foil has excellent plasticity.

[0379] As can be seen from the results of the examples in Table 4, the copper foil including copper grains with a particle size of less than or equal to 0.5 μm in a proportion of 70%-95% has a higher tensile strength, and as the number of small grains increases, the mechanical strength and tensile strength of the copper foil are effectively improved. Further, as can be seen from the results of Examples 2, 4, and 1-1, when the copper foil includes small grains in a similar proportion, the copper foil has a higher tensile strength. On this basis, a larger span of the particle size of the copper grains makes the copper foil exhibit a lower yield ratio, which is beneficial to improving the brittleness of the copper foil, so that the copper foil has good tensile strength while having excellent bending resistance. As can be seen from the results of Examples 5-7, the reasonable matching of large and small copper grains in the copper foil substrate and the formation of a heterogeneous grain distribution morphology make the ratio of the tensile strength to the yield strength (i.e., the yield ratio) of the copper foil satisfy 0.45-0.8, and further satisfy 0.45-0.7. A lower yield ratio is beneficial to improving the brittleness of the copper foil while maintaining the tensile strength of the copper foil, and is beneficial to improving the brittleness and plasticity of the copper foil, so as to improve the bending resistance.

[0380] It should be noted that the present application is not limited to the above-described embodiments. The above-described embodiments are only examples, and embodiments having substantially the same configuration and playing the same role and effect as the technical idea within the scope of the technical solutions of the present application are all included in the technical scope of the present application. In addition, within the scope of the main idea of the present application, various modifications that can be thought of by those skilled in the art, other modes constructed by combining part of the elements of the embodiments are also included in the scope of the present application.

Claims

1. A secondary battery characterized by comprising: The copper foil comprises copper grains with different particle sizes, and 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, the number of copper grains with a particle size greater than 1 μm accounts for 1%-10% of the total number of copper grains, and the particle size span of the copper grains is 1.3 μm-3.4 μm.

2. The secondary battery according to claim 1, characterized by The particle size of the copper grains with a particle size greater than 0.5 μm ranges from greater than 0.5 μm to less than or equal to 3 μm.

3. The secondary battery according to claim 1 or 2, characterized by The number of copper grains with a particle size greater than 1 μm accounts for 2%-10% of the total number of copper grains.

4. The secondary battery according to any one of claims 1 to 3, characterized by, The number of copper grains with a particle size less than or equal to 0.5 μm accounts for 80%-95% of the total number of copper grains, and / or the number of copper grains with a particle size greater than 0.5 μm accounts for 5%-20% of the total number of copper grains.

5. The secondary battery according to any one of claims 1 to 4, wherein the particle size span of the copper grains is 1.3 μm-3 μm.

6. The secondary battery according to any one of claims 1 to 5, characterized by, The copper foil satisfies at least one of the following conditions: (1) the average particle size of the copper grains is 0.1 μm-1.2 μm; (2) the maximum particle size of the copper grains is 1 μm-2.5 μm; (3) the minimum particle size of the copper grains is 0.1 μm-0.3 μm.

7. The secondary battery according to any one of claims 1 to 6, characterized by The yield ratio of the copper foil is 0.45-0.

80.

8. The secondary battery according to any one of claims 1 to 7, characterized by, The yield ratio of the copper foil is 0.45-0.

7.

9. The secondary battery according to any one of claims 1 to 8, characterized by, The copper foil satisfies at least one of the following conditions: (1) the average particle size of the copper grains is 0.1 μm-0.7 μm, and optionally 0.3 μm-0.6 μm; (2) the maximum particle size of the copper grains is 1.2 μm-2.0 μm; (3) the minimum particle size of the copper grains is 0.1 μm-0.3 μm.

10. The secondary battery according to any one of claims 1 to 9, characterized by The tensile strength of the copper foil is 600 MPa-1000 MPa, and / or the breaking elongation of the copper foil is 4%-8% under the test conditions of room temperature (20±10 ℃), a sample length×width of (50±0.25 mm)×(15±0.25 mm), and a tensile speed of 50±0.5 mm / min.

11. The secondary battery according to any one of claims 1 to 10, characterized by The tensile strength of the copper foil is 700 MPa-1000 MPa, and / or the breaking elongation of the copper foil is 4%-7% under the test conditions of room temperature (20±10 ℃), a sample length×width of (50±0.25 mm)×(15±0.25 mm), and a tensile speed of 50±0.5 mm / min.

12. The secondary battery according to any one of claims 1 to 11, characterized by The tensile strength of the copper foil is 700 MPa-800 MPa, and / or the breaking elongation of the copper foil is 5%-6% under the test conditions of room temperature (20±10 ℃), a sample length×width of (50±0.25 mm)×(15±0.25 mm), and a tensile speed of 50±0.5 mm / min.

13. The secondary battery according to any one of claims 1 to 12, characterized by The hardness of the copper foil is 55 HV-65 HV.

14. The secondary battery according to any one of claims 1 to 13, characterized by The hardness of the copper foil is 55 HV-60 HV.

15. The secondary battery according to any one of claims 1 to 14, characterized by, The thickness of the copper foil is 4 μm-10 μm.

16. The secondary battery according to any one of claims 1 to 15, characterized by 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.

17. The secondary battery according to claim 16, characterized by The negative electrode active material comprises a silicon-based material, and the silicon-based material comprises at least one of elemental silicon, silicon oxide compound, silicon-carbon composite, silicon-nitrogen composite and silicon alloy.

18. A method for producing a secondary battery, characterized by, The copper foil is prepared by an electroplating method, the electroplating method comprises applying a periodic pulse current to an electroplating solution to reduce and deposit copper ions in the electroplating solution to form the copper foil; 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; the copper foil comprises copper grains with different particle sizes, and the copper grains comprise 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, wherein the number of copper grains with a particle size of less than or equal to 0.5 μm accounts for 70%-95% of the total number of copper grains, the number of copper grains with a particle size of greater than 1 μm accounts for 1%-10% of the total number of copper grains, and the particle size span of the copper grains is 1.3 μm-3.4 μm.

19. The method of claim 18, wherein, The electroplating method satisfies one or more of the following conditions: (1) the peak value of the pulse current is 40,000 A-80,000 A; (2) the valley value of the pulse current is 1,000 A-10,000 A; (3) the change period of the pulse current is 500 ms-5,000 ms; (4) the distance between the cathode electrode and the anode electrode is 8 mm-20 mm; (5) the temperature of the electroplating deposition is 45°C-60°C; (6) the speed of the cathode roller is 2 m / min-5 m / min.

20. The method of manufacturing according to claim 18 or 19, wherein, The electroplating method satisfies one or more of the following conditions: (1) the peak value of the pulse current is 50,000 A-70,000 A; (2) the valley value of the pulse current is 2,000 A-5,500 A; (3) the change period of the pulse current is 2,000 ms-4,000 ms; (4) the distance between the cathode electrode and the anode electrode is 8 mm-12 mm; (5) the temperature of the electroplating deposition is 50°C-60°C; (6) the speed of the cathode roller is 2 m / min-3 m / min.

21. The production method according to any one of claims 18 to 20, characterized by, The pulse current comprises 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, and optionally, the pulse current comprises a sine wave pulse current.

22. The production method according to any one of claims 18 to 20, characterized by, The pulse current further comprises a linear oscillation current.

23. The production method according to any one of claims 18 to 22, characterized by, The electroplating solution comprises a leveling agent, a wetting agent and a brightener, The leveling agent comprises one or more of collagen and sodium saccharin; The wetting agent comprises one or more of hydroxyethyl cellulose and polyethylene glycol; The brightener comprises sodium polydithiopropyl sulfone.

24. The method of claim 23, wherein, The electroplating solution comprises: collagen with a concentration of 60 mg / L to 300 mg / L, sodium saccharin with a concentration of 0.5 g / L to 10 g / L, polyethylene glycol with a concentration of 50 mg / L to 200 mg / L, hydroxyethyl cellulose with a concentration of 30 mg / L to 200 mg / L, sodium poly(dithiobispropanesulfonate) with a concentration of 500 mg / L to 2000 mg / L, and chloride ions (as chlorine atoms) with a concentration of 20 mg / L to 80 mg / L.

25. The method of manufacturing according to claim 23 or 24, wherein, 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 poly(dithiobispropanesulfonate) 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.

26. The production method according to any one of claims 23 to 25, wherein, The electroplating solution further comprises copper ions with a concentration of 60 g / L to 100 g / L.

27. The production method according to any one of claims 18 to 26, wherein The electroplating solution has a pH of 2.5 to 4.

5.

28. An electrical device, comprising: A secondary battery comprising the secondary battery of any one of claims 1 to 17 or prepared by the preparation method of any one of claims 18 to 27.

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