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

Abstract

A secondary battery (5), a method for manufacturing the same, and an electric device. The secondary battery (5) includes a negative electrode sheet including a negative electrode current collector including a copper foil including copper grains having different grain sizes, the copper grains including copper grains having a grain size of 0.5 μm or less and copper grains having a grain size of 1 μm to 4 μm, wherein the number ratio of the copper grains having a grain size of 0.5 μm or less in the total number of the copper grains is 70% to 95%, and the short diameter of at least some of the copper grains having a grain size of 1 μm to 4 μm is arranged in the thickness direction of the negative electrode current collector.

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 wide application of secondary batteries in energy storage power supply systems such as hydropower, thermal power, wind power 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 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 material has a high thermal expansion coefficient, and the silicon-based battery expands severely in volume when heated; the volume of the secondary battery also increases accordingly after the amount of secondary battery active materials is increased, 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, 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 0.5 μm or less and copper grains with a particle size of 1 μm to 4 μm, wherein the number of the copper grains with a particle size of 0.5 μm or less accounts for 70% to 95% of the total number of the copper grains, and the long diameter of at least part of the copper grains with a particle size of 1 μm to 4 μm is arranged along the thickness direction of the negative electrode current collector.

[0010] The copper grains with a particle size of 0.5 μm or less help to improve the tensile strength of the copper foil and improve the mechanical properties, and the particle size of the copper grains with a particle size of more than 0.5 μm helps to improve the elongation at break and improve the plasticity of the copper foil. The copper foil substrate in the embodiments of the application comprises copper grains with a particle size of 0.5 μm or less, which accounts for 70% to 95% of the total number, is beneficial to increase the internal grain boundary area, effectively hinders the dislocation slip movement, and improves the tensile strength of the copper foil. Meanwhile, the copper foil comprises copper grains with a particle size of 1 μm to 4 μm, which promotes the formation of a heterogeneous grain morphology of fine grains doped with a small amount of large grains, and the long diameter of at least part of the copper grains with a particle size of 1 μm to 4 μm in the copper foil is arranged along the thickness direction of the negative electrode current collector, which increases the grain tortuosity of the copper foil along the thickness direction of the current collector, improves the tensile strength of the copper foil, and at the same time improves the plasticity and bending resistance of the copper foil substrate (i.e., improves the brittleness of the copper foil), which helps to reduce the probability of fracture of the inner circle of the current collector in the cell winding process, and at the same time reduces the probability of cracks and fracture of the current collector caused by the expansion and extrusion of active materials, and provides a material basis for improving the energy density and safety of the secondary battery.

[0011] The copper foil provided by the application has excellent tensile strength and elongation at break, and at the same time has excellent plasticity, which provides a material basis for improving the energy density, capacity and safety of the secondary battery.

[0012] In any embodiment, the grain tortuosity of the copper foil along the thickness direction of the negative electrode current collector is 20 μm to 30 μm.

[0013] In any embodiment, the ratio of the long diameter to the short diameter of the copper grains with a particle size of 1 μm to 4 μm is greater than 1, and can be 2 to 5.

[0014] In any embodiment, the number of the copper grains with a particle size of 1 μm to 4 μm accounts for 2% to 10% of the total number of the copper grains.

[0015] The number ratio of the copper grains with a particle size of 1-4 μm in the copper foil satisfies 2-10%, indicating that there is a certain number of grains with a larger particle size in the copper foil, which is beneficial to improve the brittleness of the copper foil substrate, improve the plasticity of the copper foil, increase the elongation at break of the copper foil, further reduce the probability of fracture of the inner circle of the current collector, and improve the safety, production efficiency and service life of the secondary battery.

[0016] In any embodiment, among the copper grains with a particle size of 1-4 μm, the copper grains with an angle between the short diameter and the thickness direction of the negative electrode current collector less than 20° and an angle between the long diameter and the thickness direction of the negative electrode current collector greater than 75° are recorded as first grains, and the number ratio of the first grains based on the total number of the copper grains with a particle size of 1-4 μm is 60-80%.

[0017] The first grains with an angle between the short diameter and the thickness direction of the current collector less than 20° and an angle between the long diameter and the thickness direction of the current collector greater than 75° are arranged perpendicular to the thickness direction of the current collector in the copper foil. The applicants realize that the first grains with a larger particle size and a long diameter perpendicular to the thickness direction can form a combination with fine grains, increase the internal grain boundary area, improve the tensile strength of the copper foil, and at the same time, increase the degree of grain boundary meandering, so that the copper foil has a suitable tortuosity in the thickness direction of the current collector. This makes the energy required for the copper foil to break when it is subjected to the extrusion stress in the thickness direction of the current collector due to the expansion of the active material higher, thereby helping to reduce the probability of the copper foil breaking in the thickness direction of the current collector, control the number ratio of the first grains to satisfy the above range, improve the tensile strength of the copper foil while improving the brittleness of the copper foil, reduce the probability of cracks and breaking of the current collector due to the extrusion of the active material expansion, and effectively improve the safety and service life of the secondary battery.

[0018] In any embodiment, the expansion force of the secondary battery is greater than or equal to 1000 kgf, the secondary battery includes a negative electrode sheet, the negative electrode sheet 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 with 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 ratio 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%, and the number ratio of the copper grains with a particle size greater than 0.5 μm in the total number of the copper grains is 5-30%.

[0019] During the charge-discharge cycle of the secondary battery, the volume expansion of the active ion insertion / deintercalation causes the volume expansion of the electrode sheet, especially in the self-generated negative electrode battery or the new silicon-based or lithium metal negative electrode, and the high group margin or large size battery designed to pursue the improvement of the energy density of the single cell, etc. Comprehensive effects make the overall expansion of the secondary battery higher. The copper foil current collector of the high expansion secondary battery is obviously stretched and stressed during the working process, which increases the probability of fracture or crack, and worsens the safety and service life of the battery. Therefore, the copper foil including the copper grains with different particle sizes provided in the secondary battery has excellent tensile strength and fracture elongation, 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 improve the service life of the secondary battery when the electrode sheet appears crack and failure, and is beneficial to further improve the safety and service life of the secondary battery.

[0020] In any embodiment, the thickness expansion rate of the secondary battery is 4%-10%,

[0021] In any embodiment, the secondary battery is a wound type secondary battery, and the expansion force of the secondary battery is greater than or equal to 1000kgf; the wound type secondary battery includes a negative electrode sheet, the negative electrode sheet 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 with different particle sizes, the copper grains include copper grains with a particle size less than or equal to 0.5μm and copper grains with a particle size greater than 0.5μm, wherein the number of the copper grains with a particle size less than or equal to 0.5μm accounts for 70%-95% of the total number of the copper grains, and the number of the copper grains with a particle size greater than 0.5μm accounts for 5%-30% of the total number of the copper grains.

[0022] The current collector of the negative electrode sheet in the wound type secondary battery is prone to crack fracture in the corner area, because the cell is pressed and shaped after winding, which can cause irreversible damage to the current collector, so that the outer corner area of the negative electrode sheet and the inner corner area are prone to crack or fracture 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 including the copper grains with different particle sizes provided in the wound type secondary battery significantly improves the tensile strength and fracture elongation 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 the expansion stress, reduces the probability of crack fracture of the electrode sheet of the wound cell, is beneficial to improve the service life of the secondary battery when the electrode sheet appears crack and failure, and further improves the safety and service life of the secondary battery.

[0023] In any embodiment, the thickness expansion rate of the wound secondary battery is 4%-10%.

[0024] In any embodiment, 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 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 copper grains with a particle size of less than or equal to 0.5 μm in the total number of copper grains is 70%-95%, and the proportion of the number of copper grains with a particle size of greater than 0.5 μm in the total number of copper grains is 5%-30%; and the gram capacity of the negative electrode active material in the negative electrode film layer is 800 mAh / g-1500 mAh / g.

[0025] 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 cause the volume of the battery cell 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 while reducing the impact on the volume of the battery cell, however, the particle volume of the high gram capacity negative electrode active material expands greatly 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 comprising copper grains with different particle sizes provided in the secondary battery of the present application 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.

[0026] 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.

[0027] The copper foil provided in the present application is suitable for a secondary battery comprising 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 electrode, alkali metal negative electrode, etc., which usually has high swelling property, 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 the high energy density battery system or the high swelling system battery, and can improve the energy density, capacity and safety of the secondary battery.

[0028] 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.

[0029] 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% of the total number of copper grains.

[0030] As previously described, the copper grain particle size satisfying greater than 0.5 μm and less than or equal to 3 μm can help reduce the average grain size of the copper foil, achieving further improvement of 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, and can reduce the risk of copper foil fracture or crack in a high-energy-density battery system or a high-expansion battery system, achieving simultaneous improvement of the energy density and safety of the secondary battery.

[0031] 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.

[0032] In any embodiment, the short diameter of at least part of the copper grains with a particle size greater than or equal to 1 μm is arranged along the thickness direction of the current collector.

[0033] In any embodiment, the short diameter of at least part of the copper grains with a particle size greater than or equal to 1 μm and less than or equal to 4 μm is arranged along the thickness direction of the current collector.

[0034] In any embodiment, the short diameter of at least part of the copper grains with a particle size greater than or equal to 1.5 μm and less than or equal to 3 μm is arranged along the thickness direction of the current collector.

[0035] The short diameter of at least part of the copper grains with a particle size greater than 0.5 μm arranged along the thickness direction of the current collector indicates that the internal grain boundary of the copper foil has a high degree of tortuosity, and the energy required for the copper foil to fracture through the grain boundary (i.e., the thickness direction of the current collector) is also greater, which helps reduce the probability of fracture of the copper foil along the thickness direction, improves the brittleness of the current collector, further reduces the risk of copper foil fracture or crack, and improves the safety of the secondary battery.

[0036] 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 further improve the tensile strength and improve the mechanical strength of the copper foil.

[0037] 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 further improve the elongation at break and improve the plasticity of the copper foil.

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

[0039] (1) the average grain size of the copper grains is 0.3-1.2 pm;

[0040] (2) the maximum grain size of the copper grains is 1-2.5 pm;

[0041] (3) the minimum grain size of the copper grains is 0.1-0.3 pm;

[0042] (4) the grain size span of the copper grains is 0.8-2.5 pm.

[0043] The grain size distribution of the copper grains helps to adjust the proportion of the number of copper grains with a grain size less than or equal to 0.5 pm and the number of copper grains with a grain size greater than 0.5 pm, thereby adjusting and improving the tensile strength and the elongation at break of the copper foil, so that the copper foil has excellent mechanical properties and plastic properties.

[0044] In any embodiment, the copper foil satisfies at least one of the following conditions:

[0045] (1) the average grain size of the copper grains is 0.3-0.6 pm;

[0046] (2) the maximum grain size of the copper grains is 1.2-2.0 pm;

[0047] (3) the minimum grain size of the copper grains is 0.1-0.3 pm;

[0048] (4) the grain size span of the copper grains is 1-2 pm.

[0049] The grain size distribution of the copper grains helps to further adjust the tensile strength and the elongation at break of the copper foil, and improve the mechanical properties and plastic properties.

[0050] In any embodiment, 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, the tensile strength of the copper foil is 600-1000 MPa, and / or the elongation at break of the copper foil is 4-8%.

[0051] The copper foil has excellent tensile strength and elongation at break, has good mechanical properties and plastic properties, and can be suitable for high-energy-density batteries or high-expansion batteries, which helps to improve the safety of secondary batteries.

[0052] In any embodiment, 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, the tensile strength of the copper foil is 700-1000 MPa, and / or the elongation at break of the copper foil is 4-7%.

[0053] In any embodiment, the tensile strength of the copper foil is 700-800 MPa, and / or the elongation at break of the copper foil is 5-6% under the test conditions of room temperature (20±10℃), sample length x width of (50±0.25 mm) x (15±0.25 mm), and tensile speed of 50±0.5 mm / min.

[0054] The copper foil has good tensile strength, which can meet the practical needs of the battery field for high-strength copper foil to some extent. In addition, the copper foil also has excellent elongation at break, showing 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 cell.

[0055] In any embodiment, the hardness of the copper foil is 55-65 HV. In any embodiment, the hardness of the copper foil is 55-60 HV. Suitable hardness is conducive to the surface treatment of the copper foil and the cold pressing process of the secondary battery, reducing the surface damage of the copper foil and reducing the influence on the bonding performance of the negative electrode film layer and the copper foil.

[0056] In any embodiment, the thickness of the copper foil is 4-10 μm. The copper foil can reduce the thickness without affecting the strength, which helps to design the battery to be light, and further improves the energy density or specific capacity.

[0057] In any embodiment, the secondary battery further comprises a negative electrode film layer on at least one side of the copper foil, and the negative electrode active material in the negative electrode film layer comprises at least one of artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based material, tin-based material, lithium titanate and metal lithium. The secondary battery can be suitable for a variety of different battery negative systems, and has a wide range of applications.

[0058] In any embodiment, the negative electrode active material comprises a silicon-based material, and the silicon-based material comprises at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites and silicon alloys. The secondary battery using a silicon-based material as the negative electrode active material has higher energy density, and at the same time, the copper foil in the secondary battery can well bind the volume expansion of the negative electrode during the battery cycle.

[0059] In any embodiment, the mass fraction 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.

[0060] In any embodiment, the mass fraction of silicon in the silicon-based material is 20-50%.

[0061] In any embodiment, the mass fraction of silicon is 4-10%, based on the total mass of the negative electrode film layer.

[0062] In any embodiment, the expansion force of the secondary battery is greater than or equal to 1000 kgf.

[0063] In any embodiment, the expansion force of the secondary battery is greater than or equal to 2500 kgf.

[0064] In any embodiment, the expansion force of the secondary battery is greater than or equal to 4000 kgf.

[0065] In any embodiment, the expansion force of the secondary battery is 1000 kgf-10000 kgf.

[0066] 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 further improvement of the electrochemical performance of the secondary battery. The current collector provided in the embodiments has excellent tensile strength and elongation at break, and can be applied to the secondary battery with high expansion force, which is conducive to further improvement of the energy density of the secondary battery.

[0067] In any embodiment, the thickness expansion rate of the secondary battery is 4%-10%.

[0068] The current collector in the prior art is prone to breakage under the expansion stress caused by the volume change of the secondary battery, which causes the performance of the secondary battery to plummet and easily causes safety accidents, which limits further improvement of the performance of the secondary battery. The current collector provided in the embodiments has good strength performance, and can be applied to the secondary battery with high thickness expansion rate, which is conducive to further improvement of the energy density and safety of the secondary battery.

[0069] The second aspect of the present application provides a preparation method of a secondary battery, which comprises preparing a copper foil by electroplating method. The electroplating method comprises applying a pulse current to an electroplating solution to reduce and deposit copper ions in the electroplating solution to form a copper foil. The peak value of the pulse current is 40000 A-100000 A, the valley value of the pulse current is 100 A-20000 A, and the change period of the current is 50 ms-5000 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 1 μm-4 μm. The proportion of the number of copper grains with a particle size of less than or equal to 0.5 μm in the total number of copper grains is 70%-95%, and the short diameter of at least part of the copper grains with a particle size of 1 μm-4 μm is arranged along the thickness direction of the negative electrode current collector.

[0070] Compared with the rolling method, the electroplating method is mature and simple, has low requirements on equipment, and the prepared copper foil has excellent tensile strength and elongation at break, and has excellent mechanical strength and good plasticity.

[0071] In any of the embodiments, the preparation method of the secondary battery comprises preparing the pole piece by using the copper foil as the current collector, the preparation of the copper foil comprises preparing the copper foil by electroplating method, the electroplating method comprises applying pulse current to the electroplating solution to reduce and deposit the copper ions in the electroplating solution to form the copper foil, the peak value of the pulse current is 40000 A-100000 A, the valley value of the pulse current is 100 A-20000 A, and the change cycle of the current is 50 ms-5000 ms; and the expansion force of the secondary battery is greater than or equal to 1000 kgf.

[0072] The preparation method of the secondary battery provided in the application adopts the above-mentioned electroplating method to prepare the copper foil, so that the copper foil with excellent tensile strength and elongation at break is obtained, the mechanical strength is improved while excellent plasticity is achieved, the probability of fracture or crack of the copper foil under stress caused by the expansion of the pole piece volume in the secondary battery, the increase of internal pressure caused by the decomposition of the electrolyte, and other factors is reduced, the safety and service life of the secondary battery are further improved.

[0073] In any of the embodiments, the preparation method of the secondary battery comprises preparing the pole piece by using the copper foil as the current collector, the preparation of the copper foil comprises preparing the copper foil by electroplating method, the electroplating method comprises applying pulse current to the electroplating solution to reduce and deposit the copper ions in the electroplating solution to form the copper foil, the peak value of the pulse current is 40000 A-100000 A, the valley value of the pulse current is 100 A-20000 A, and the change cycle of the current is 50 ms-5000 ms; and the expansion force of the secondary battery is greater than or equal to 1000 kgf.

[0074] The preparation method of the secondary battery provided in the application adopts the above-mentioned electroplating method to prepare the copper foil, so that the copper foil with excellent tensile strength and elongation at break is obtained, the mechanical strength is improved while excellent plasticity is achieved, the probability of fracture or crack of the copper foil under stress caused by the expansion of the pole piece volume in the secondary battery, the increase of internal pressure caused by the decomposition of the electrolyte, and other factors is reduced, the safety and service life of the secondary battery are further improved.

[0075] In any of the embodiments, the preparation method comprises preparing the pole piece by using the copper foil as the current collector, the preparation of the copper foil comprises preparing the copper foil by electroplating method, the electroplating method comprises applying pulse current to the electroplating solution to reduce and deposit the copper ions in the electroplating solution to form the copper foil, the peak value of the pulse current is 40000 A-100000 A, the valley value of the pulse current is 100 A-20000 A, and the change cycle of the current is 50 ms-5000 ms; and the gram capacity of the negative active material in the negative electrode film layer of the secondary battery is 800 mAh / g-1500 mAh / g.

[0076] The secondary battery preparation method provided in the present application adopts the above electroplating method to prepare the copper foil, and the copper foil obtained 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 when the copper foil is subjected to the expansion stress of the volume change of the high-capacity negative active material before and after the lithium ion insertion / extraction, and is conducive to further improving the safety and service life of the secondary battery.

[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 40000 A-80000 A;

[0079] (2) the valley value of the pulse current is 1000 A-10000 A;

[0080] (3) the change period of the pulse current is 500 ms-5000 ms;

[0081] (4) the distance between the cathode electrode and the anode electrode is 8 mm-20 mm;

[0082] (5) the temperature of the electroplating deposition is 45°C-60°C;

[0083] (6) the speed of the cathode roller is 2 m / min-5 m / min.

[0084] In any embodiment, the electroplating method satisfies one or more of the following conditions:

[0085] (1) the peak value of the pulse current is 50000 A-70000 A;

[0086] (2) the valley value of the pulse current is 2000 A-5500 A;

[0087] (3) the change period of the pulse current is 2000 ms-4000 ms;

[0088] (4) the distance between the cathode electrode and the anode electrode is 8 mm-12 mm;

[0089] (5) the temperature of the electroplating deposition is 50°C-60°C;

[0090] (6) the speed of the cathode roller is 2 m / min-3 m / min.

[0091] Compared with direct current deposition, applying pulse current to the electroplating solution can make copper ions reduce and deposit under high current conditions to form fine-grained fine grains, and form grains with larger particle size (e.g., columnar grains) under low current conditions. By adjusting the current size and change cycle, the formation and growth of copper ion crystal nucleus can be adjusted, so that the size and morphology of the grain size, and 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 can be adjusted, and the tensile strength and elongation at break of the copper foil can be improved.

[0092] 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.

[0093] In any embodiment, the electroplating solution includes a leveling agent, a wetting agent, and a brightener, the leveling agent includes one or more of collagen, sodium saccharin; the wetting agent includes one or more of hydroxyethyl cellulose, polyethylene glycol; and the brightener includes sodium polydithiopropyl sulfone. In any embodiment, the brightener further includes sodium propane sulfone modified thiourea.

[0094] The leveling agent can improve the flatness of the copper foil, the wetting agent can improve the wettability of the electroplating solution and the substrate, increase the nucleation rate of the copper foil, and reduce the average grain size of the grains in the copper foil, and the brightener can make the grain size of the copper foil finer and reduce the surface roughness of the copper foil and improve the smoothness of the surface.

[0095] The preparation method of the embodiment of the present application uses the above-mentioned kind of brightener, which helps the small part of copper grains to grow and form copper grains with a particle size of 1 μm-4 μm, and promotes the distribution morphology of the short diameter of the copper grains with a particle size of 1 μm-4 μm arranged along the thickness direction of the current collector, so that the copper foil has a suitable tortuosity in the thickness direction of the current collector, refines the grains, improves the tensile strength of the copper foil, and at the same time, improves the brittleness of the copper foil, and improves the toughness and bending resistance of the current collector.

[0096] Thiourea helps to improve the preferential degree and proportion of the (220) crystal plane on the basis of refining the grains, but thiourea can be degraded to produce SH - in the electric field and react with Cu 2+ , Cu + to deposit in the copper foil, and the sulfur atoms entering the grain structure of the copper foil easily cause defects of the copper foil brittleness, which easily causes the risk of copper foil fracture in the outer circle in the pole piece winding process. The sodium propane sulfone modified thiourea molecule has high stability, reduces the production of SH -And the number of sulfur atoms entering the copper foil is improved, and the brittleness defects caused thereby are improved. The sulfonic acid group in sodium propane sulfonate modified thiourea also helps to capture copper ions in the electrolyte and promote copper ion deposition. At the same time, the modified thiourea contains C-S and C-N bonds at the same time, has a higher charge density, and has a stronger interaction with the (220) crystal plane with a lower atomic density, thereby improving the degree of preference and proportion of the (220) crystal plane, selectively adsorbing to promote lateral grain growth, and changing the slip plane during copper foil stretching, which helps to reduce the elastic modulus.

[0097] 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, polydithiobispropane sulfonate sodium 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.

[0098] 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, polydithiobispropane sulfonate sodium with a concentration of 500 mg / L-1000 mg / L, and chloride ions with a concentration of 40 mg / L-80 mg / L.

[0099] The collagen and sodium saccharin in the electroplating solution help to improve the surface pits and protrusions of the copper foil, improve the flatness of the copper foil, the hydroxyethyl cellulose and polyethylene glycol help to reduce the difference in the thickness direction of the copper foil, improve the uniformity of the copper foil, and the polydithiobispropane sulfonate sodium can improve the electrochemical reduction rate of copper ions, adjust the grain size, and the prepared copper foil is bright and flat, the number of copper grains with a particle size of less than or equal to 0.5 μm and the number of copper grains with a particle size of greater than 0.5 μm are appropriate, and has good tensile strength and elongation at break.

[0100] In any embodiment, the electroplating solution further comprises sodium propane sulfonate modified thiourea with a concentration of 2 mg / L-20 mg / L.

[0101] The electroplating solution includes sodium propane sulfonate modified thiourea with a concentration within the above range, which helps to promote grain growth to form copper grains with a particle size of 1 μm-4 μm, and promote the formation of copper grains with a particle size of 1 μm-4 μm, and the short diameter of the distribution morphology along the thickness direction of the current collector, improve the tensile strength of the copper foil while improving the brittleness of the copper foil, and improve the toughness and bending resistance of the current collector.

[0102] In any embodiment, the pH of the electroplating solution is 2.5-4.5, which is conducive to the reduction of copper ions.

[0103] The third aspect of the present application provides a pole piece, which comprises the copper foil in the secondary battery provided by the first aspect of the present application or the copper foil prepared by the preparation method provided by the second aspect of the present application.

[0104] In any embodiment, 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.

[0105] In any embodiment, 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.

[0106] In any embodiment, the mass fraction 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.

[0107] In any embodiment, the mass fraction of silicon element in the silicon-based material is 20%-50%.

[0108] In any embodiment, the mass fraction of silicon element is 4%-10%, based on the total mass of the negative electrode film layer.

[0109] The fourth aspect of the present application provides a wound secondary battery, which comprises the secondary battery provided by the first aspect of the present application or prepared by the preparation method provided by the second aspect of the present application or comprises the pole piece provided by the third aspect of the present application.

[0110] The fifth aspect of the present application provides an electric device, which comprises 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

[0111] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed in the embodiments of the present application will be briefly introduced as follows. Obviously, the drawings described below are only some embodiments of the present application, and other drawings can be obtained by the drawings without creative labor for those skilled in the art.

[0112] FIG. 1 is a schematic diagram of a secondary battery according to an embodiment of the present application.

[0113] FIG. 2 is an exploded view of the secondary battery of the embodiment of the application shown in FIG. 1.

[0114] FIG. 3 is a schematic view of a battery module of the embodiment of the application.

[0115] FIG. 4 is a schematic view of a battery pack of the embodiment of the application.

[0116] FIG. 5 is an exploded view of the battery pack of the embodiment of the application shown in FIG. 4.

[0117] FIG. 6 is a schematic view of an electric device using the secondary battery of the embodiment of the application as a power source.

[0118] FIG. 7 shows a reverse pole figure distribution map of the cross section of the copper foil of Example 1 of the application subjected to electron backscatter diffraction (EBSD) testing.

[0119] FIG. 8 shows a tensile curve of the copper foil of Example 1 of the application.

[0120] FIG. 9 shows a grain size distribution map of the cross section of the copper foil of Example 1 of the application obtained by EBSD diffraction testing.

[0121] FIG. 10 shows a reverse pole figure distribution map of the cross section of the copper foil of Example 2 of the application subjected to EBSD testing.

[0122] FIG. 11 shows a tensile curve of the copper foil of Example 2 of the application.

[0123] FIG. 12 shows a grain size distribution map of the cross section of the copper foil of Example 2 of the application obtained by EBSD diffraction testing.

[0124] FIG. 13 shows a reverse pole figure distribution map of the cross section of the copper foil of Example 3 of the application subjected to EBSD testing.

[0125] FIG. 14 shows a tensile curve of the copper foil of Example 3 of the application.

[0126] FIG. 15 shows a grain size distribution map of the cross section of the copper foil of Example 3 of the application obtained by EBSD diffraction testing.

[0127] FIG. 16 shows a tensile curve of the copper foil of Example 1-1 of the application.

[0128] FIG. 17 shows a reverse pole figure distribution map of the cross section of the copper foil of Comparative Example 1 of the application subjected to EBSD testing.

[0129] FIG. 18 shows a tensile curve of the copper foil of Comparative Example 1 of the application.

[0130] FIG. 19 shows a grain size distribution map of the cross section of the copper foil of Comparative Example 1 of the application obtained by EBSD diffraction testing.

[0131] Explanation of Reference Numerals: 1 battery pack; 2 upper case; 3 lower case; 4 battery module; 5 secondary battery; 51 case; 52 electrode assembly; 53 cover plate; T thickness direction. DETAILED DESCRIPTION

[0132] Hereinafter, embodiments of the secondary battery and the method of manufacturing the same, the electrode sheet, the wound-type secondary battery, and the electric device according to the present application are specifically disclosed with appropriate reference to the accompanying drawings. However, there can be cases where unnecessary detailed descriptions are omitted. For example, there can be cases where detailed descriptions of matters well known in the art, repetitive descriptions of substantially identical structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand the present application, and are not intended to limit the subject matter recited in the claims.

[0133] The ranges disclosed in the present application are defined in the form of lower and upper limits, and a given range is defined by selecting one lower limit and one upper limit, and the selected lower limit and upper limit define the boundaries of a particular range. The ranges defined in this way can include or exclude the end values, and can be arbitrarily combined, i.e., any lower limit can be combined with any upper limit to form a range. For example, if the ranges of 60-120 and 80-110 are listed for a particular parameter, it is understood that the ranges of 60-110 and 80-120 are also contemplated. In addition, if the minimum range values of 1 and 2 are listed, and if the maximum range values of 3, 4, and 5 are listed, the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In the present application, unless otherwise stated, a numerical range "a-b" represents a shorthand manner of describing each and every numerical value that is contained within the range, wherein a and b are both real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed herein, and "0-5" is merely a shorthand manner of describing those numerical combinations. In addition, when it is stated that a certain parameter is an integer ≥ 2, it is equivalent to disclose that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0134] If not specifically stated, all embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions.

[0135] If not specifically stated, all technical features and optional technical features of the present application can be combined with each other to form new technical solutions.

[0136] If not specifically stated, all steps of the present application can be performed in sequence or randomly, preferably in sequence. For example, the method comprises steps (a) and (b) means that the method can comprise steps (a) and (b) in sequence, or steps (b) and (a) in sequence. For example, the method can further comprise step (c) means that step (c) can be added to the method in any order. For example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0137] If not specifically stated, the terms "comprise" and "comprising" mentioned in the present application are open-ended, and can also be closed-ended. For example, the terms "comprise" and "comprising" can mean that other components not listed can also be included, or can mean that only the listed components are included.

[0138] If not specifically stated, 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, any one 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).

[0139] 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.

[0140] 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 pole piece to expand and contract severely during the cycle process. This easily leads to 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 square shell winding type battery, since the cell is pressed and shaped after winding, 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 limit group margin is designed, 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 breaking or producing slight cracks. How to improve the strength of the copper foil while considering good plasticity is a problem that needs to be solved at present.

[0141] [Secondary battery]

[0142] Based on this, the application provides a kind of secondary battery, secondary battery includes negative pole piece, negative pole piece includes negative electrode current collector and the negative electrode film layer of being arranged on at least one side surface of negative electrode current collector, negative electrode current collector includes copper foil, copper foil includes copper grain with different particle size, copper grain includes copper grain with particle size less than or equal to 0.5 μm and copper grain with particle size of 1 μm-4 μm, wherein, the number ratio of copper grain with particle size less than or equal to 0.5 μm in total number of copper grain is 70%-95%, and the short diameter of at least part of copper grain with particle size of 1 μm-4 μm is arranged along the thickness direction of the negative electrode current collector.

[0143] Copper grains with particle size above 0.5 μm tend to be columnar grains or columnar-like grains, also referred to as "large grains" herein, and copper grains with particle size less than or equal to 0.5 μm are fine grains in the form of particles, forming a fine grain region around the columnar grains. Therefore, in the present application, the copper foil substrate forms a grain morphology with fine grains as the main component and large grains mixedly distributed, which is a heterogeneous grain morphology with a small amount of large grains doped in fine grains. The main reason for the difference in the size of the heterogeneous grain morphology is the adjustment of the grain size during the process of producing the foil. Specifically, as an embodiment, the size of the grain can be controlled by adjusting the change of the current during the process of producing the foil. After the current is reduced, the grains tend to form larger grains, and after the current is increased, the grains tend to form smaller grains.

[0144] The copper grains with a particle size of less than or equal to 0.5 μm help to improve the tensile strength of the copper foil and improve the mechanical properties; the copper grains with a particle size of greater than 0.5 μm help to improve the elongation at break and improve the plasticity of the copper foil. The copper foil substrate in the embodiments of the present application includes copper grains with a particle size of less than or equal to 0.5 μm, which accounts for 70%-95% in quantity, is beneficial to increase the internal grain boundary area, effectively hinders the dislocation slip movement, and improves the tensile strength of the copper foil; meanwhile, the copper foil includes copper grains with a particle size of 1 μm-4 μm, promotes the formation of a heterogeneous grain morphology of fine grains doped with a small amount of large grains, and makes the short diameter of the copper grains with a particle size of 1 μm-4 μm in the copper foil arranged along the thickness direction of the negative current collector, increases the grain tortuosity of the copper foil along the thickness direction of the current collector, improves the tensile strength of the copper foil, and at the same time improves the plasticity and bending resistance of the copper foil substrate (i.e., improves the brittleness of the copper foil), which helps to reduce the probability of fracture of the current collector at the inner circle in the cell winding process, and at the same time reduces the probability of cracks and fracture of the current collector due to the expansion and extrusion of active substances, and provides a material basis for improving the energy density and safety of the secondary battery.

[0145] In some embodiments, the grain tortuosity of the copper foil in the thickness direction of the negative current collector is 20 μm-30 μm.

[0146] In some embodiments, the grain tortuosity of the copper foil in the thickness direction of the negative current collector can be selected as 20 μm, 21 μm, 22 μm, 23 μm, 24 μm, 25 μm, 26 μm, 27 μm, 28 μm, 29 μm, 30 μm, or any numerical range between any two of them.

[0147] Herein, the grain tortuosity refers to the actual length of the grain boundary in a certain direction of the grain structure of the material cross section. It can be understood that the greater the tortuosity, the more tortuous the grain boundary, and the higher the required energy for the material to break in this direction, and the more difficult it is to break.

[0148] The grain tortuosity can be tested by instruments and methods known in the art. As an example, the following method can be used: a sample to be tested with a thickness of 6 μm is prepared, or the current collector substrate is disassembled from the secondary battery, the cross section of the copper foil is tested by electron backscatter diffraction (EBSD) and scanning electron microscopy, the inverse pole figure distribution map is obtained, the grain boundaries formed by the grains are identified and counted, and the actual path of the grain boundary along the thickness direction of the copper foil is measured by the software, so that the grain tortuosity of the copper foil in the thickness direction of the negative current collector is obtained. In order to eliminate the test error, the actual path of the grain boundary in multiple non-overlapping regions of the same copper foil can be counted, and the average value is taken as the grain tortuosity of the copper foil.

[0149] In some embodiments, the ratio of the long diameter to the short diameter of the copper grains with a size of 1-4 μm is greater than 1. In some embodiments, the ratio of the long diameter to the short diameter of the copper grains with a size of 1-4 μm is 2-5. In some embodiments, the ratio of the long diameter to the short diameter of the copper grains with a size of 1-4 μm can be 1.1, 1.2, 1.5, 1.8, 2, 2.2, 2.5, 2.8, 3, 3.2, 3.5, 3.8, 4, 4.2, 4.5, 4.8, 5, or any numerical range between any two of them.

[0150] In this context, the long diameter and the short diameter of the grains have the meanings known in the art and can be tested using instruments and 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, and counting the long diameter and the short diameter of the grains using the imageJ analysis software provided with the Oxford C-Nano+ electron backscatter diffraction instrument, so as to calculate the ratio of the long diameter to the short diameter of the copper grains.

[0151] In some embodiments, the number of the copper grains with a size of 1-4 μm accounts for 2-10% of the total number of the copper grains.

[0152] In some embodiments, the number of the copper grains with a size of 1-4 μm accounts for 2-10% of the total number of the copper grains.

[0153] The number of the copper grains with a size of 1-4 μm accounting for 2-10% of the total number of the copper grains in the copper foil indicates that there are a certain number of grains with a larger size in the copper foil, which is beneficial to improve the brittleness of the copper foil substrate, improve the plasticity of the copper foil, increase the elongation at break of the copper foil, further reduce the probability of fracture of the inner circle of the current collector, and improve the safety, production efficiency and service life of the secondary battery.

[0154] In some embodiments, among the copper grains with a size of 1-4 μm, the copper grains with a short diameter and a long diameter having an angle less than 20° with the thickness direction of the negative electrode current collector and an angle greater than 75° with the thickness direction of the negative electrode current collector are referred to as first grains, and the number of the first grains accounts for 60-80% of the total number of the copper grains with a size of 1-4 μm.

[0155] The first crystal grains with an angle less than 20° between the short diameter and the thickness direction of the current collector and an angle greater than 75° between the long diameter and the thickness direction of the current collector present an arrangement pattern perpendicular to the thickness direction of the current collector in the copper foil. The applicant realizes that the first crystal grains with a larger particle size and a long diameter inclined to the thickness direction can form a combination with fine crystal grains, increase the internal grain boundary area, improve the tensile strength of the copper foil, and at the same time, the degree of winding of the grain boundary is improved, so that the copper foil has a suitable tortuosity in the thickness direction of the current collector. When the copper foil is subjected to the extrusion stress along the thickness direction of the current collector from the expansion of the active material, the energy required for the copper foil to break is higher, thereby helping to reduce the probability of the copper foil breaking along the thickness direction of the current collector, and the number ratio of the first crystal grains meeting the above range is beneficial to improving the tensile strength of the copper foil while taking into account the improvement of the brittleness of the copper foil, reducing the probability of the current collector being cracked and broken due to the extrusion of the active material expansion, and effectively improving the safety and service life of the secondary battery.

[0156] 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 crystal grains with different particle sizes, the copper crystal grains comprise copper crystal grains with a particle size less than or equal to 0.5 μm and copper crystal grains with a particle size greater than 0.5 μm, wherein the number ratio of the copper crystal grains with a particle size less than or equal to 0.5 μm in the total number of copper crystal grains is 70%-95%, and the number ratio of the copper crystal grains with a particle size greater than 0.5 μm in the total number of copper crystal grains is 5%-30%.

[0157] During the charging and discharging cycle process of the secondary battery, the volume expansion of the electrode sheet caused by the insertion and extraction of active ions, especially the self-generated negative electrode battery or new silicon-based or lithium metal negative electrode, and the design of high group margin or large size battery for the purpose of improving the energy density of single cell, etc. comprehensively affect the high overall expansion of the secondary battery. The copper foil current collector of the high expansion secondary battery is obviously stretched and stressed during the working process, which increases the probability of breaking or cracking of the copper foil current collector, and deteriorates the safety and service life of the battery monomer. Therefore, the copper foil comprising the above-mentioned copper crystal grains with different particle sizes provided in the secondary battery has excellent tensile strength and elongation at break, which improves the mechanical strength while having excellent plasticity, reduces the probability of breaking or cracking 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 and fails during the working of the secondary battery, and is beneficial to further improving the safety and service life of the secondary battery.

[0158] In some embodiments, the thickness expansion rate of the secondary battery is 4%-10%.

[0159] In some 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%-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%-30%.

[0160] The current collector of the negative electrode sheet in the wound secondary battery is prone to crack and fracture at the corner area, because the wound battery 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 outer corner area of the negative electrode sheet and the bending area of the inner circle 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 battery 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 and fracture of the wound battery is reduced, which is beneficial to improving the service life of the secondary battery when the electrode sheet cracks and fails, and the safety and service life of the secondary battery are further improved.

[0161] 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 this will cause the volume of the battery to increase; the applicant finds 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 while reducing 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 copper foil of the current collector, which increases the probability of fracture or crack of the copper foil along the thickness direction under the expansion stress, and worsens the safety and service life of the secondary battery.

[0162] Based on this, another embodiment of the present 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%, and the proportion of the number of the copper grains with a particle size of 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.

[0163] The copper foil comprising the copper grains with different particle sizes provided by the present application has excellent tensile strength and elongation at break, and has excellent plasticity while improving the mechanical strength, and is well matched with high gram capacity negative electrode active materials, thereby improving the energy density and capacity of the secondary battery and improving the safety and service life of the secondary battery.

[0164] In some embodiments, the gram capacity of the negative electrode active material in the negative electrode film layer is 800 mAh / g to 1500 mAh / g.

[0165] The copper foil provided by the present application is particularly suitable for a negative electrode active material system with a gram capacity of 800 mAh / g to 1500 mAh / g. The high gram capacity negative electrode material can embed more active ions, such as silicon-based negative electrodes, alkali metal negative electrodes, etc., and generally has high swelling, which can increase the probability of fracture or cracking of the copper foil in the battery cell. The copper foil has excellent mechanical properties and plasticity, and can reduce the risk of fracture or cracking of the copper foil in a high energy density battery system or a high swelling battery system.

[0166] Without wishing to be bound by any theory, the difference in work hardening ability between the large grains and the fine grains is large, and the heterogeneous grain morphology of doping a small amount of large grains in the fine grains makes the strain partition in the initial plastic deformation of the copper foil more obvious. This can be due to the fact that the moderate doping of large grains compared to all fine grains produces a higher density of geometrically necessary dislocations (GND) during the process of inhomogeneous plastic deformation. The bending of the crystal plane of the copper foil during the process of inhomogeneous plastic deformation can produce dislocations, which are called geometrically necessary dislocations (GND). Geometrically necessary dislocations (GND) 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, so that the copper foil exhibits stronger ability to inhibit strain localization, and the plasticity of the copper foil is improved. In addition, in the copper foil crystal, the contact interface between the grains is called the grain boundary. It can be understood that in the same area, the smaller the grain size of the grain, the higher the total area of the grain boundary, and the higher the grain boundary stress that the copper foil needs to overcome during the deformation process, that is, the higher the mechanical strength. At the same time, the generation of a higher density of geometrically necessary dislocations (GND) during the deformation process of the copper foil helps to reduce the concentrated stress, which can improve the mechanical strength.

[0167] 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.

[0168] 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 within the range between any two of the above values.

[0169] In this paper, the particle size of the copper grains with a particle size of less than or equal to 0.5 μm is in the range of greater than or equal to 0.1 μm and less than or equal to 0.5 μm.

[0170] In some embodiments, the number of copper grains with a particle size of greater than 0.5 μm accounts for 5%-20%, 2%-20%, 2%-15%, 5%-15%, 5%-13%, 6%-12%, or 7%-10% of the total number of copper grains, which helps to further improve the elongation at break and optimize the plasticity of the copper foil.

[0171] In some embodiments, the number of copper grains having a particle size greater than 0.5 μm accounts for 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 18%, 20%, 25%, 30%, or a range between any two of the above values or any value within the range, of the total number of copper grains.

[0172] In this context, the particle size of the copper grains having a particle size greater than 0.5 μm ranges from greater than 0.5 μm to less than or equal to 3 μm, which helps to reduce or optimize the average particle size of the copper foil and improve the tensile strength of the copper foil.

[0173] The number and particle size of the grains in the copper foil can be tested by methods known in the art. For example, the cross section of the copper foil 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 particle 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 taken as the particle size of the grains, a number distribution map can be made, a skew distribution can be used for fitting, and the particle size and number of the grains in different particle size intervals can be obtained.

[0174] In some embodiments, the number of copper grains having a particle size greater than 0.5 μm and less than or equal to 3 μm accounts for 5%-30% of the total number of the copper grains.

[0175] As mentioned above, the copper grain particle size meeting the condition 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 controlling the number of the copper grains having a particle size greater than 0.5 μm and less than or equal to 3 μm to meet the condition of 5%-30% can make the copper foil have a good elongation at break, have excellent mechanical properties and plasticity, and reduce the risk of fracture or crack of the copper foil in a high-energy-density battery system or a high-expansion battery system, thereby achieving a simultaneous improvement in the energy density and safety of the secondary battery.

[0176] In some embodiments, the specific capacity of the negative active material can be selected as 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.

[0177] As used herein, the gram capacity of the negative active material can be determined using instruments and methods known in the art, for example, the following method can be used for testing: the negative active material is mixed with conductive agent carbon black, polyvinylidene fluoride (PVDF) 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 the negative electrode 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), ethyl methyl 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 above organic solvent to prepare an electrolyte with a concentration of 1.0 mol / L. A CR2430 type button cell is assembled in a glove box under argon protection. At 25°C, charge to the upper limit cutoff voltage of 3.8V at a rate of 0.1C, and then charge at constant voltage until the current is less than 0.05C; after standing for 30 min, discharge to the lower limit cutoff voltage of 2.0V at a rate of 0.1C, and record the first discharge capacity as Cm, then the gram capacity of the negative active material = discharge capacity Cm / mass of the negative active material.

[0178] It can be understood that the gram capacity of the negative active material can also be obtained by disassembling the battery, obtaining the negative electrode sheet, and then testing after assembling the button cell according to the method described above.

[0179] 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.

[0180] 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 breaking the copper foil 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.

[0181] In some embodiments, the short diameter of at least part of the copper grains with a particle size greater than or equal to 1 μm is arranged along the thickness direction of the current collector.

[0182] In some embodiments, the short diameter of at least part of the copper grains with a particle size greater than or equal to 1 μm and less than or equal to 4 μm is arranged along the thickness direction of the current collector.

[0183] In some embodiments, the short diameter of at least part of the copper grains with a particle size greater than or equal to 1.5 μm and less than or equal to 3 μm is arranged along the thickness direction of the current collector.

[0184] 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 μm can be determined by referring to the EBSD and scanning electron microscope combination described above for the characterization of the short diameter of the copper grains and the arrangement direction.

[0185] In some embodiments, the average particle size of the copper grains is 0.3 μm-1.2 μm. In some embodiments, the average particle size of the copper grains is 0.3 μm-0.6 μm. In some embodiments, the average particle size of the copper grains is 0.3 μm-0.5 μm. In some embodiments, the average particle size of the copper grains is 0.3 μm, 0.35 μm, 0.4 μm, 0.45 μm, 0.5 μm, 0.55 μm, 0.6 μm, 0.65 μm, 0.7 μm, 0.75 μm, 0.8 μm, 0.85 μm, 0.9 μm, 1.0 μm, 1.1 μm, 1.2 μm, or a range between any two of the above values or any value within the range.

[0186] The suitable particle size range is theoretically favorable for the copper foil to obtain suitable grain boundaries, so that the copper foil has suitable resistance to grain dislocation movement and deformation resistance, and thus the mechanical strength of the copper foil can be optimized.

[0187] The average particle size of the grains can be tested by methods known in the art. For example, the cross section of the copper foil is measured by the combination of an electron backscatter diffraction instrument (EBSD) and a scanning electron microscope, a 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 particle size, a number distribution map is made, a skew distribution is used for fitting, and the average particle size of the grains is obtained.

[0188] In some embodiments, the maximum particle size of the copper grains is 1 μm-2.5 μm. In some embodiments, the maximum particle size of the copper grains is 1.2 μm-1.8 μm or 1.2 μm-2.0 μm. In some embodiments, the maximum particle size of the copper grains is 1 μm, 1.2 μm, 1.4 μm, 1.6 μm, 1.8 μm, 2 μm, or a range between any two of the above values or any value within the range.

[0189] The maximum particle size of the copper grains can be tested by methods known in the art. For example, the cross section of the copper foil is measured by the combination of an electron backscatter diffraction instrument (EBSD) and a scanning electron microscope, a 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 particle size, a number distribution map is made, a skew distribution is used for fitting, and the maximum grain particle size in the statistical result is the maximum particle size of the copper grains.

[0190] 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.

[0191] 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 by the imageJ analysis software of the Oxford C-Nano+ electron backscatter diffraction instrument, taking the equivalent circle diameter of the grains as the grain size, making a number distribution map, and fitting the skewness distribution. The minimum grain size in the statistical results is the minimum grain size of the copper grains.

[0192] The grain size span of the copper grains is the difference between the maximum grain size and the minimum grain size in the grain size distribution of the copper foil. In some embodiments, the grain size span of the copper grains is 0.8 pm-2.5 pm. In some embodiments, the grain size span of the copper grains is 0.8 pm-2 pm. In some embodiments, the grain size span of the copper grains is 1 pm-2 pm. In some embodiments, the grain size span of the copper grains is 0.8 pm, 1 pm, 1.2 pm, 1.4 pm, 1.6 pm, 1.8 pm, 2.0 pm, 2.2 pm, 2.4 pm, 2.5 pm, or a range between any two of the above values or any value within the range.

[0193] The grain size span of the copper grains within the range can make the dispersion degree of the grain size distribution of the copper grains within a suitable range, which can help to balance the grain boundary area and the geometrically necessary dislocation density of the copper grains, improve the grain dislocation movement resistance, and reduce the occurrence of concentrated stress during the deformation of the copper foil, which is conducive to improving the mechanical strength of the copper foil while maintaining excellent plasticity.

[0194] 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), sample length x width of (50±0.25 mm) x (15±0.25 mm), and tensile speed of 50±0.5 mm / min. In some embodiments, the tensile strength of the copper foil is 700 MPa-1000 MPa. In some embodiments, the tensile strength of the copper foil is 700 MPa-800 MPa.

[0195] In some embodiments, the copper foil has an elongation at break of 4-8% under the test condition of room temperature (20±10°C), sample length x width of (50±0.25mm) x (15±0.25mm), and a tensile speed of 50±0.5mm / min. In some embodiments, the copper foil has an elongation at break of 4-7%. In some embodiments, the copper foil has an elongation at break of 5-6%.

[0196] In the present context, the term "tensile strength" refers to the maximum load-bearing strength per unit area of a test sample when the sample is continuously loaded until it breaks.

[0197] In the present context, the term "elongation at break" refers to the ratio of the length change of a material after being stressed until it breaks due to plastic deformation, usually expressed in percentage, and is an important parameter for measuring the deformation ability of a material under stress during stretching.

[0198] In the present application, the tensile strength and elongation at break of the copper foil can be tested by methods known in the art, such as according to the standard GB / T 5230-1995 "Electrolytic Copper Foil". As an example, at least 4 test samples with a length of 50±0.25mm and a width of 15±0.25mm are cut, and the test samples are continuously loaded until they break at room temperature (20±10°C) with a tensile speed of 50±0.5mm / min. The tensile strength of the test sample is calculated by dividing the maximum load by the cross-sectional area of the test sample. The cross-sectional area of the test sample can be calculated by dividing the mass of the test sample by the product of the length of the test sample and the density. The elongation at break can be calculated according to the displacement method after the above test. The test area refers to the detection area during instrument testing. Considering that the test sample may need to be fixed by a clamp during testing, the length and width of the test sample can be greater than the length and width of the test area.

[0199] In some embodiments, the copper foil has a tensile strength of 600MPa, 650MPa, 700MPa, 750MPa, 800MPa, 850MPa, 900MPa, 950MPa, 1000MPa, or a range between any two of the above values or any value within the range, under the test condition of room temperature (20±10°C), sample length x width of (50±0.25mm) x (15±0.25mm), and a tensile speed of 50±0.5mm / min.

[0200] In some embodiments, the copper foil has an elongation at break of 4.0%, 4.5%, 5.0%, 5.5%, 6.0%, 6.5%, 7.0%, 7.5%, 8.0%, or a range between any two of the above values or any value within the range, under the test conditions of room temperature (20±10℃), sample length x width of (50±0.25mm) x (15±0.25mm), and a tensile speed of 50±0.5mm / min.

[0201] The copper foil with the elongation strength and the elongation at break has excellent mechanical strength and plasticity, and can be suitable for high-energy-density batteries or high-swelling batteries, and helps to improve the safety of secondary batteries.

[0202] In some embodiments, the copper foil has a hardness of 55HV-65HV. In some embodiments, the copper foil has a hardness of 55HV-60HV. In some embodiments, the copper foil has a hardness of 55HV, 56HV, 57HV, 58HV, 59HV, 60HV, 61HV, 62HV, 63HV, 64HV, 65HV, or a range between any two of the above values or any value within the range.

[0203] 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 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.

[0204] In some embodiments, the copper foil has a thickness of 4μm-10μm. In some embodiments, the copper foil has a thickness of 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, or a range between any two of the above values or any value within the range.

[0205] In this application, the thickness of the copper foil can be tested by methods known in the art. As an example, a 20x15cm 2 sample is cut, the cut sample is placed on an electronic balance to weigh, the weight of the sample is obtained, and the volume of the sample is calculated according to the density p of the copper foil of 8.96g / cm 3 The length and width of the sample are known, and thus the thickness of the sample can be calculated.

[0206] The maximum load that can be carried by the ordinary strength copper foil after thinning is sharply attenuated, the thickness of the copper foil that can be used for plastic deformation is severely reduced, resulting in a substantial decrease in the tensile strength and elongation at break of the copper foil, and fatigue fracture is prone to occur in the later stage of the secondary battery cycle, 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 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 helps to reduce the weight of the battery and further improve the energy density or specific capacity of the secondary battery.

[0207] In some embodiments, the expansion force of the secondary battery is greater than or equal to 1000 kgf.

[0208] In some embodiments, the expansion force of the secondary battery is greater than or equal to 2500 kgf.

[0209] In some embodiments, the expansion force of the secondary battery is greater than or equal to 4000 kgf.

[0210] In some embodiments, the expansion force of the secondary battery is 1000 kgf-10000 kgf.

[0211] 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 above values or any value within the range.

[0212] 1 kgf is the gravity of an object of 1 kg at the sea level at 45 degrees north latitude. 1 kgf is about 9.8 Newtons. The swelling force of the secondary battery cell refers to the swelling force of the secondary battery at 60% SOH (state of charge), which can be sensed by a pressure sensor in the clamps arranged on both sides of the large surface 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 to a current of ≤0.05C, and then discharged at 1C constant current to a voltage of 2.5V, which is one charge and discharge process. This process is repeated for cyclic charge and discharge. The SOH of the battery is monitored throughout the process. When the SOH reaches 90%, the swelling force of the secondary battery at 90% SOH is measured by the pressure sensor in the clamps arranged on both sides of the large surface of the secondary battery cell. Through simulation modeling, the swelling force of the secondary battery at 60% SOH is obtained.

[0213] The current collector in the prior art is prone to breakage under high swelling 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 of the present application has excellent tensile strength and elongation at break, and can be applied to secondary batteries with high swelling force, which is conducive to the further improvement of the energy density of the secondary battery.

[0214] In some embodiments, the thickness swelling rate of the secondary battery is 4%-10%.

[0215] In the present application, the thickness swelling rate of the secondary battery can be tested by methods known in the art. As an example, the thickness swelling rate = (H1-H0) / H0, where 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 used 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 to a current of ≤0.05C, and then discharged at 1C constant current to a voltage of 2.5V, which is one charge and discharge process. This process is repeated for cyclic charge and discharge. The SOH of the battery is monitored throughout the process. When the SOH of the secondary battery reaches 90%, the total thickness H' of the secondary battery at this time is determined. Through simulation modeling, the total thickness H1 of the secondary battery at 60% SOH is obtained, and the thickness swelling rate of the secondary battery is obtained.

[0216] In some embodiments, the thickness swelling rate of the secondary battery is 4%, 5%, 6%, 7%, 8%, 9%, 10%, or any numerical range between any two of them.

[0217] 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, can be applied to a secondary battery with high thickness expansion rate, and is conducive to the further improvement of the energy density and safety of the secondary battery.

[0218] 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 40,000 A-100,000 A, the valley value of the pulse current being 100 A-20,000 A, and the change period of the current being 50 ms-5,000 ms; 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 1 μm-4 μm, wherein the number proportion 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%-95%, and the short diameter of at least part of the copper grains with a particle size of 1 μm-4 μm is arranged along the thickness direction of the negative electrode current collector.

[0219] In the present application, the term "electroplating method" refers to a method of depositing a metal or alloy on the surface of a workpiece to form a metal layer by using the principle of electroplating.

[0220] In the present application, the term "pulse current" refers to a current or voltage pulse that appears repeatedly in a cycle.

[0221] Compared with the calendering method in the prior art, the electroplating method is mature and simple, has low requirements for equipment, and has low manufacturing cost. The copper foil prepared by the electroplating method has excellent tensile strength and elongation at break, and not only has excellent mechanical strength and plasticity, but also helps to reduce the manufacturing cost of the secondary battery.

[0222] Some schemes use high direct current to prepare a copper foil by an electroplating method in order to improve the mechanical strength of the copper foil. However, because the work hardening ability of the grains is very small, the ability to inhibit strain localization is reduced, and the copper foil is prone to concentrated stress during deformation, the plasticity of the copper foil is reduced, and the risk of brittle fracture is easily caused. In the preparation method provided in the present application, a pulse current is used, the nucleation and growth rate of the copper grains are adjusted by the continuous change of the size of the current, so as to adjust the size and morphology of the copper grains, and adjust the number proportion of the copper grains with a particle size of less than or equal to 0.5 μm and the copper grains with a particle size of greater than 0.5 μm, and improve the tensile strength and elongation at break of the copper foil, so that the copper foil has excellent mechanical properties and plasticity.

[0223] In some embodiments, the preparation method of the secondary battery comprises preparing the pole piece by 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 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 40,000 A-100,000 A, the valley value of the pulse current is 100 A-20,000 A, and the change cycle of the current is 50 ms-5,000 ms; and the expansion force of the secondary battery is greater than or equal to 1,000 kgf.

[0224] In the preparation method of the secondary battery provided in the present application, the copper foil is prepared by the above-mentioned electroplating method, and the copper foil has excellent tensile strength and elongation at break, has excellent plasticity while improving the mechanical strength, reduces the probability of cracking or breaking of the copper foil under the stress caused by the expansion of the pole piece volume, the decomposition of the electrolyte, the increase of the internal pressure, and other factors in the secondary battery, and is beneficial to further improving the safety and service life of the secondary battery.

[0225] In some embodiments, the preparation method of the secondary battery comprises preparing a wound type secondary battery, preparing the pole piece by 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 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 40,000 A-100,000 A, the valley value of the pulse current is 100 A-20,000 A, and the change cycle of the current is 50 ms-5,000 ms; and the expansion force of the secondary battery is greater than or equal to 1,000 kgf.

[0226] In the preparation method of the secondary battery provided in the present application, the copper foil is prepared by the above-mentioned electroplating method, and the copper foil has excellent tensile strength and elongation at break, has excellent plasticity while improving the mechanical strength, reduces the probability of cracking or breaking of the copper foil under the stress caused by the expansion of the pole piece volume, the decomposition of the electrolyte, the increase of the internal pressure, and other factors in the secondary battery, and is beneficial to further improving the safety and service life of the secondary battery.

[0227] In some embodiments, the peak value of the pulse current is 40,000 A-80,000 A. In some embodiments, the peak value of the pulse current is 50,000 A-70,000 A. In some embodiments, the peak value of the pulse current is 55,000 A-70,000 A, 50,000 A-80,000 A, 55,000 A-80,000 A, 50,000 A-60,000 A.

[0228] In some embodiments, the peak value of the pulsed current is 45000 A, 50000 A, 55000 A, 60000 A, 65000 A, 70000 A, 75000 A, 80000 A, 85000 A, 90000 A, 95000 A, 100000 A, or a range between any two of the aforementioned values or any value between the ranges.

[0229] In some embodiments, the valley value of the pulsed current is 1000 A-10000 A. In some embodiments, the valley value of the pulsed current is 2000 A-5500 A. In some embodiments, the valley value of the pulsed current is 2500 A-5000 A, 2000 A-8000 A, 2000 A-5000 A.

[0230] In some embodiments, the valley value of the pulsed current is 100 A, 500 A, 1000 A, 1500 A, 2000 A, 2500 A, 3500 A, 4000 A, 4500 A, 5000 A, 8000 A, 10000 A, 15000 A, 20000 A, or a range between any two of the aforementioned values or any value between the ranges.

[0231] In some embodiments, the change period of the pulsed current is 500 ms-5000 ms. In some embodiments, the change period of the pulsed current is 2000 ms-4000 ms. In some embodiments, the change period of the pulsed current is 50 ms, 100 ms, 200 ms, 500 ms, 1000 ms, 1200 ms, 1500 ms, 1800 ms, 2000 ms, 2500 ms, 2750 ms, 3000 ms, 3500 ms, 4000 ms, 4500 ms, 5000 ms, or a range between any two of the aforementioned values or any value between the ranges.

[0232] Herein, the term "peak value" refers to the maximum current value of the pulsed current, typically the value at the peak of the pulsed current waveform. Similarly, the term "valley value" refers to the minimum current value of the pulsed current, typically the value at the valley of the pulsed current waveform.

[0233] Herein, the term "change period" refers to the time between two adjacent peaks or valleys of the pulsed current waveform, in ms.

[0234] During the copper ion deposition process, increasing the current can accelerate the deposition rate of copper ions, which helps to form fine grains with smaller particle size; reducing the current can reduce the deposition rate of copper ions, which helps to form relatively regular and orderly grains with larger particle size, such as columnar crystals. Adjusting the variation parameters of the current, i.e. peak value, valley value and variation period, helps to adjust the formation of copper grain nuclei, the growth rate of grains and the deposition time of copper ions, to produce a heterogeneous grain morphology with mixed distribution of large and small grains, and to adjust the particle size and quantity ratio of the grains, thereby improving the tensile strength and elongation at break of the copper foil, so that the copper foil has excellent mechanical strength and plasticity.

[0235] In some embodiments, the pulsed current includes one or more of a square wave pulsed current, a sinusoidal pulsed current, a triangular pulsed current, and a sawtooth pulsed current.

[0236] In some embodiments, the pulsed current includes a sinusoidal pulsed current. The sinusoidal pulsed current continuously and periodically varies, which is conducive to the continuous and variable growth of the grains.

[0237] In some embodiments, the preparation method is a continuous production method.

[0238] In some embodiments, the preparation method is a roller deposition method. The working principle is that the cathode roller is connected to the negative electrode of the power supply, and the anode tank is connected to the positive electrode of the power supply. When the electroplating solution containing copper ions enters the anode tank, an electric field is formed between the positive and negative electrodes. Under the action of the electric field, copper ions migrate to the surface of the cathode roller and deposit. The deposited copper foil is peeled off from the cathode roller and wound on another roller. The electroplating solution is continuously added and circulated, and copper ions are continuously deposited on the cathode roller under the action of the electric field, continuously peeled off and wound on the winding shaft. This preparation method can realize the continuous production of large-scale copper foil and provide the possibility for industrial application.

[0239] In some embodiments, the cathode electrode is a titanium roller or a titanium plate.

[0240] In some embodiments, the anode electrode is a titanium substrate plate.

[0241] 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.

[0242] In some embodiments, the temperature of the electroplating deposition is 45 °C to 60 °C. In some embodiments, the temperature of the electroplating deposition is 50 °C to 60 °C. In some embodiments, the temperature of the deposition can be selected from 45 °C, 50 °C, 55 °C, 60 °C, a range between any two of the aforementioned values, or any value between the aforementioned ranges.

[0243] In some embodiments, the roller speed of the cathode roller is 2 m / min to 5 m / min. In some embodiments, the roller speed of the cathode roller is 2 m / min to 3 m / min. In some embodiments, the roller speed of the cathode roller can be selected from 2 m / min, 2.5 m / min, 3 m / min, 3.5 m / min, 4 m / min, 4.5 m / min, 5 m / min, a range between any two of the aforementioned values, or any value between the aforementioned ranges.

[0244] The cathode roller can be any roller suitable for the preparation of copper foil in the art, for example, a titanium roller.

[0245] In some embodiments, the electroplating solution includes a leveling agent, a wetting agent, a brightener.

[0246] In this context, the term "leveling agent" refers to a substance added to the electroplating solution that can improve the flatness of the plated layer, which can adhere to the tip of the copper foil with a fast deposition rate, inhibit grain growth, balance the growth rate of pits and tips, and improve the flatness of the copper foil.

[0247] In this context, the term "wetting agent" refers to a substance used to reduce the interfacial tension between the electroplating solution and the electrode, allowing the plated layer to better adhere to the substrate. The wetting agent can improve the wettability of the electroplating solution and the substrate, the wetting of the cathode is sufficient to allow it to be combined with a large current to achieve fast electrodeposition, improve the nucleation rate of the copper foil, and reduce the grain size in the copper foil.

[0248] In this context, the term "brightener" refers to a substance that improves the smoothness of the plated layer and reduces surface roughness. The brightener can make the grain size of the copper foil smaller and reduce the surface roughness of the copper foil, improving the smoothness of the surface.

[0249] In some embodiments, the leveling agent includes one or more of collagen, sodium saccharin. In some embodiments, the leveling agent includes collagen and sodium saccharin.

[0250] Without being bound by any theory, collagen can inhibit the deposition of copper ions and balance the growth rate of pits and tips. 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 these defects. The two different leveling agents help to further improve the surface pits and protrusions of the copper foil and improve the flatness of the copper foil.

[0251] In some embodiments, the concentration of the collagen in the electroplating solution is 60 mg / L to 300 mg / L. In some embodiments, the concentration of the collagen in the electroplating solution is 80 mg / L to 150 mg / L. In some embodiments, the concentration of the collagen in the electroplating solution is 60 mg / L, 80 mg / L, 100 mg / L, 150 mg / L, 200 mg / L, 250 mg / L, 300 mg / L, or any range between any two of the above values or any value between the ranges.

[0252] The collagen can be selected from proteins with a molecular weight commonly used in the field of copper foil, for example, collagen with a relative molecular weight of 8000-12000.

[0253] In some embodiments, the concentration of the sodium saccharin in the electroplating solution is 0.5 g / L to 10 g / L. In some embodiments, the concentration of the sodium saccharin in the electroplating solution is 0.5 g / L to 4 g / L. In some embodiments, the concentration of the sodium saccharin in the electroplating solution is 0.5 g / L, 0.8 g / L, 1 g / L, 1.5 g / L, 2.0 g / L, 2.5 g / L, 3.0 g / L, 3.5 g / L, 4.0 g / L, 4.5 g / L, 5.0 g / L, 5.5 g / L, 6.0 g / L, 6.5 g / L, 7.0 g / L, 7.5 g / L, 8.0 g / L, 8.5 g / L, 9.0 g / L, 9.5 g / L, 10 g / L, or any range between any two of the above values or any value between the ranges. In some embodiments, the wetting agent comprises one or more of hydroxyethyl cellulose, polyethylene glycol. In some embodiments, the wetting agent comprises hydroxyethyl cellulose and polyethylene glycol.

[0254] 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, improve and assist the adhesion of copper ions to the substrate surface and deposition, improve the consistency of the crystal grains during the deposition process, reduce the difference in crystal grain size in the thickness direction, improve the uniformity of the copper foil, and thus improve the mechanical properties of the copper foil.

[0255] 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 any range between any of the foregoing.

[0256] The polyethylene glycol can be of a molecular weight commonly used in the field of copper foil, for example, polyethylene glycol with a relative molecular weight of 4000.

[0257] In some embodiments, the concentration of hydroxyethyl cellulose in the electroplating solution is 30 mg / L to 200 mg / L. In some embodiments, the concentration of hydroxyethyl cellulose in the electroplating solution is 50 mg / L to 150 mg / L. In some embodiments, the concentration of 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.

[0258] The polyethylene glycol can be of a molecular weight commonly used in the field of copper foil, and in some embodiments, the relative molecular weight of the hydroxyethyl cellulose is 120,000.

[0259] In some embodiments, the mass ratio of polyethylene glycol to hydroxyethyl cellulose in the electroplating solution is (1-1.5): 1. In some embodiments, the mass ratio of polyethylene glycol to hydroxyethyl cellulose in the electroplating solution is (1.2-1.5): 1. As an example, the mass ratio of polyethylene glycol to 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.

[0260] In some embodiments, the brightener includes sodium polydithiopropyl sulfone.

[0261] In some embodiments, the brightener further includes sodium propane sulfone modified thiourea.

[0262] Without accepting any theory, sodium polydithiobispropane 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 make the electrons pass through the chloride ions to the captured copper ions, thereby greatly improving the electrochemical reduction rate of the copper ions, refining the crystal grains and realizing material strengthening. The preparation method of the embodiment of the application uses the above-mentioned brightener, so that the copper ions in the electroplating solution form fine crystal grains, while helping a small part of the copper crystal grains to grow and form copper crystal grains with a particle size of 1 μm-4 μm, and promoting the distribution morphology of the short diameter of the copper crystal grains with a particle size of 1 μm-4 μm to be arranged along the thickness direction of the current collector, so that the copper foil has a suitable tortuosity in the thickness direction of the current collector, and the crystal grains are refined, the tensile strength of the copper foil is improved, and the brittleness of the copper foil is improved at the same time, and the toughness and bending resistance of the current collector are improved.

[0263] Thiourea helps to improve the preferential degree and proportion of the (220) crystal plane on the basis of refining the crystal grains, but thiourea can be degraded to produce SH - in the acid electrolyte under the action of the electric field and is deposited in the copper foil by reacting with Cu 2+ , Cu + , and the sulfur atoms enter the copper foil crystal organization, which easily causes the brittleness defects of the copper foil and easily causes the risk of the outer ring copper foil fracture in the pole piece winding process. The stability of the propane sulfonic acid sodium modified thiourea molecule is high, the number of sulfur atoms entering the copper foil is reduced by reducing the SH - produced by the degradation of the molecule under the acid electrolysis condition, and the brittleness defects caused thereby are improved. The sulfonic acid group in the propane sulfonic acid sodium modified thiourea also helps to capture the copper ions in the electrolyte and promote the deposition of the copper ions. At the same time, the modified thiourea contains C-S bond and C-N bond at the same time, has higher charge density, and has stronger interaction with the (220) crystal plane with lower atomic density, thereby improving the preferential degree and proportion of the (220) crystal plane, selectively adsorbing and promoting the lateral grain growth, changing the slip surface during the stretching of the copper foil, and helping to reduce the elastic modulus.

[0264] In some embodiments, the concentration of the sodium polydithiobispropane sulfonate in the electroplating solution is 500 mg / L-2000 mg / L. In some embodiments, the concentration of the sodium polydithiobispropane sulfonate in the electroplating solution is 500-1000 mg / L. In some embodiments, the concentration of the sodium polydithiobispropane 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 two of the above values or any value in the range between the ranges.

[0265] In some embodiments, the electroplating solution further comprises sodium propane sulfonate modified thiourea at a concentration of 2 mg / L to 20 mg / L. In some embodiments, the concentration of sodium propane sulfonate modified thiourea in the electroplating solution can be selected from 2 mg / L, 4 mg / L, 5 mg / L, 6 mg / L, 8 mg / L, 10 mg / L, 12 mg / L, 14 mg / L, 15 mg / L, 16 mg / L, 18 mg / L, 20 mg / L, or any range between any two of the recited values.

[0266] The inclusion of sodium propane sulfonate modified thiourea in the electroplating solution at a concentration within the above range facilitates the formation of copper grains with a grain size of 1 pm to 4 pm and a distribution morphology in which the short diameter of the copper grains with a grain size of 1 pm to 4 pm is arranged along the thickness direction of the current collector, thereby improving the tensile strength of the copper foil while also improving the brittleness of the copper foil, and improving the toughness and bending resistance of the current collector.

[0267] In some embodiments, the electroplating solution includes chloride ions at a concentration of 20 mg / L to 80 mg / L as measured by the number of chlorine atoms. In some embodiments, the concentration of sodium polydithiobispropane sulfonate in the electroplating solution is 40 mg / L to 80 mg / L. In some embodiments, the concentration of chloride ions as measured by the number of chlorine atoms is 20 mg / L, 30 mg / L, 40 mg / L, 50 mg / L, 60 mg / L, 70 mg / L, 80 mg / L, or any range between any two of the recited values or any value within the recited range. The use of chloride ions in combination with the wetting agent can further improve the electrodeposition process.

[0268] 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 polydithiobispropane sulfonate 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.

[0269] 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 polydithiobispropane sulfonate 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.

[0270] The electroplating solution further includes a copper source to provide copper ions to the electroplating solution. The electroplating solution further includes sulfuric acid to provide an acidic environment for the reduction of the copper ions.

[0271] In some embodiments, the concentration of copper ions (in terms of copper atoms) is 60 g / L to 100 g / L. In some embodiments, the concentration of copper ions (in terms of copper atoms) is 80 g / L to 100 g / L. In some embodiments, the concentration of copper ions (in terms of copper atoms) is 60 g / L, 70 g / L, 80 g / L, 90 g / L, 100 g / L, a range between any two of the aforementioned values, or any value between the aforementioned ranges.

[0272] 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 the aforementioned values, or any value between the aforementioned ranges.

[0273] 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.

[0274] 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.

[0275] [Negative electrode tab]

[0276] 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.

[0277] 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.

[0278] 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.

[0279] 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.

[0280] In some embodiments, the mass content of the silicon-based material is 5%-100% based on the total mass of the negative electrode film layer.

[0281] In some embodiments, the mass content of the silicon-based material is 10%-80% based on the total mass of the negative electrode film layer.

[0282] In some embodiments, the mass content of the silicon-based material is 10%-30% based on the total mass of the negative electrode film layer.

[0283] In some embodiments, the mass content of the silicon-based material can be selected from 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 100%, or any numerical range between any two of them, based on the total mass of the negative electrode film layer.

[0284] In some embodiments, the mass content of silicon in the silicon-based material is 20%-50%.

[0285] In some embodiments, the mass content of silicon in the silicon-based material is 20%, 25%, 30%, 35%, 40%, 45%, 50%, or any numerical range between any two of them.

[0286] In some embodiments, the mass content of silicon is 4%-10% based on the total mass of the negative electrode film layer.

[0287] In some embodiments, the mass content of silicon is 4%, 5%, 6%, 7%, 8%, 9%, 10%, or any numerical range between any two of them, based on the total mass of the negative electrode film layer.

[0288] In some embodiments, the negative electrode film layer can also optionally include a binder. The binder can be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0289] In some embodiments, the negative electrode film layer also includes a conductive agent. The conductive agent includes one or more of super conductive carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0290] In some embodiments, the conductive agent includes carbon black. In some embodiments, the conductive agent includes carbon nanotubes. In some embodiments, the conductive agent includes carbon black and carbon nanotubes. The conductive agent is widely available and has excellent conductivity, which is conducive to controlling the preparation cost of the secondary battery and improving the conductivity of the negative electrode film layer.

[0291] In some embodiments, the negative electrode film layer can also optionally include other auxiliary agents, such as thickening agents (e.g., sodium carboxymethyl cellulose (CMC-Na)), etc.

[0292] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder, and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry on a negative electrode current collector, and after processes such as drying, cold pressing, etc., a negative electrode sheet can be obtained.

[0293] [Positive electrode sheet]

[0294] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector, and the positive electrode film layer includes a positive electrode active material.

[0295] As an example, the positive electrode current collector has two surfaces opposite in the thickness direction thereof, and the positive electrode film layer is disposed on either one or both of the two opposite surfaces of the positive electrode current collector.

[0296] In some embodiments, the positive electrode current collector can adopt a metal foil or a composite current collector. For example, as the metal foil, an aluminum foil can be adopted. 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, and silver alloy, etc.) on a polymer material base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0297] In some embodiments, the positive electrode active material can adopt 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 the following materials: lithium-containing phosphate with olivine structure, lithium transition metal oxide, and their respective modified compounds. However, the present application is not limited to these materials, and other conventional materials that can be used as positive electrode active materials for batteries 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 (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (which can also be referred to as NCM 333LiNi 0.5 Co 0.2 Mn 0.3 O2(also can be referred to as NCM 523 LiNi 0.5 Co 0.25 Mn 0.25 O2(also can be referred to as NCM 211 LiNi 0.6 Co 0.2 Mn 0.2 O2(also can be referred to as NCM 622 LiNi 0.8 Co 0.1 Mn 0.1 O2(also can be referred to as NCM 811 lithium nickel cobalt aluminum oxide (such as LiNi 0.85 Co 0.15 Al 0.05 O2) and modified compounds thereof. Examples of lithium-containing olivine phosphates can include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4(also can be referred to as LFP)), a composite of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite of lithium manganese phosphate and carbon, lithium manganese iron phosphate, a composite of lithium manganese iron phosphate and carbon.

[0298] In some embodiments, the positive electrode film layer can further optionally include a binder. As an example, the binder can include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene-fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene-fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin.

[0299] In some embodiments, the positive electrode film layer can further optionally include 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.

[0300] 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 then performing processes such as drying, cold pressing, and the like to obtain the positive electrode tab.

[0301] [Electrolyte]

[0302] The electrolyte functions to conduct ions between the positive electrode sheet and the negative electrode sheet. The type of electrolyte is not particularly limited in the present application and can be selected as desired. For example, the electrolyte can be liquid, gel, or all-solid.

[0303] In some embodiments, the electrolyte employs an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0304] In some embodiments, the electrolyte salt can be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonylimide, lithium bis-trifluoromethanesulfonylimide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluoro oxalate borate, lithium di-oxalate borate, lithium difluoro di-oxalate phosphate, and lithium tetrafluoro oxalate phosphate.

[0305] 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.

[0306] In some embodiments, the electrolyte solution can optionally further 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.

[0307] [Separator]

[0308] In some embodiments, the secondary battery further includes a separator. The type of 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.

[0309] 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 layers can be the same or different, and are not particularly limited.

[0310] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be used to make an electrode assembly through a winding process or a stacking process.

[0311] In some embodiments, the secondary battery can include an outer package. The outer package can be used to package the electrode assembly and the electrolyte described above.

[0312] 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, or the like. 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 plastic, polypropylene, polybutylene terephthalate, polybutylene succinate, or the like can be listed.

[0313] In some embodiments, the secondary battery includes a jelly-roll type secondary battery, and the positive electrode sheet, the negative electrode sheet, and the separator are formed into an electrode assembly through a jelly-roll process.

[0314] In some embodiments, the secondary battery includes a stacked type secondary battery, and the positive electrode sheet, the negative electrode sheet, and the separator are formed into an electrode assembly through a stacking process.

[0315] In addition, the secondary battery, the battery module, the battery pack, and the power consuming device of the present application are described below with appropriate reference to the accompanying drawings.

[0316] In one embodiment of the present application, a secondary battery is provided.

[0317] The shape of the secondary battery of the present application is not particularly limited, and can be cylindrical, square, or any other shape. For example, FIG. 1 is a secondary battery 5 of a square structure as an example.

[0318] 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 a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be provided on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly 52 through a jelly-roll 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.

[0319] 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.

[0320] 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.

[0321] Alternatively, the battery module 4 can also include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0322] In some embodiments, the above battery module can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0323] FIGS. 4 and 5 are a battery pack 1 as an example. Referring to FIGS. 4 and 5, the battery pack 1 can include a battery case and a plurality of battery modules 4 disposed in the battery case. The battery case includes an upper case 2 and a lower case 3, and the upper case 2 can be disposed on the lower case 3 and form an enclosed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery case in any manner.

[0324] In addition, the present application also provides a power utilization device, which includes at least one of the secondary battery, the battery module, or the battery pack provided by the present 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.

[0325] As the power utilization device, the secondary battery, the battery module, or the battery pack can be selected according to the use requirements thereof.

[0326] 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.

[0327] As another example of the device, it 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.

[0328] Embodiments

[0329] Hereinafter, embodiments of the present application will be described. The embodiments described below are exemplary and are intended to explain the present application only, and should not be understood as limiting the present application. If a specific technique or condition is not mentioned in the embodiments, it is performed according to the technique or condition described in the literature in the art or according to the product manual. If the reagent or instrument used is not mentioned by the manufacturer, it is a conventional product that can be obtained by purchase on the market.

[0330] I. Performance test

[0331] (1) Grain characteristics test of copper foil

[0332] The cross section of the copper foil is observed by electron backscatter diffraction (EBSD) combined with scanning electron microscope, wherein the electron backscatter diffraction instrument is Oxford C-Nano+. The inverse pole figure map (magnification 3000) is obtained, the particle size of each grain is measured, the diameter of the equivalent circle of the grain is taken as the particle size of the grain, the grain particle size is statistically distributed, the skew 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.

[0333] (2) Mechanical property test

[0334] According to GB / T 5230-1995 "Electrolytic Copper Foil", the copper foil prepared in the example is cut into a tensile sample with a length L0 of 50 mm and a width of 15 mm. The tensile properties are tested by a universal testing machine at 25°C, and the tensile rate is set to 50 mm / min.

[0335] The cross-sectional area of the tensile sample is wherein ρ is 8.96 g / cm 3 , the unit of m is gram, and the unit of L0 is centimeter.

[0336] The sample is continuously loaded until it is pulled apart, the maximum load F is read from the force dial or the tensile curve, and the tensile strength σ is calculated according to Formula I. b .

[0337] The distance between the two lines after the sample is pulled apart is L1, which is measured on the sample or read from the tensile curve. L1 can be measured by a straight line method or a displacement method, and the elongation at break δ is calculated according to Formula II.

[0338] (3) Hardness test

[0339] The copper foil sample is placed in a metallographic hot mounting machine, wood powder is poured in, and heated at a rate of 150°C / 10 min, a Vickers hardness tester is used to press the copper foil sample (50g weight), the lengths of two diagonal lines are optically measured, and the corresponding Vickers hardness is obtained according to the following Vickers hardness calculation formula.

[0340] HV represents Vickers hardness;

[0341] F represents the load of the indenter (Newton force);

[0342] α represents the included angle between the opposite faces of the indenter (136°);

[0343] d represents the average value of the lengths of the two diagonal lines (mm).

[0344] (4) Crack failure corresponding to the state of health (SOH)

[0345] First, measure the total thickness of the secondary battery at 25°C, then charge the battery at 1C constant current to a voltage of 3.8V, then charge at 3.8V constant voltage to a current ≤0.05C, then discharge the battery at 1C constant current to a voltage of 2.5V, which is a charge and discharge process, so repeat the cycle charge and discharge, monitor the SOH of the battery throughout the process, then every 1% SOH, the battery is disassembled to observe whether the negative electrode plate of the battery is broken, and the SOH corresponding to the crack failure is obtained.

[0346] (5) Thickness expansion rate and expansion force test of secondary battery at crack failure:

[0347] First, measure the total thickness H0' of the secondary battery at 25°C, then charge the battery at 1C constant current to a voltage of 3.8V, then charge at 3.8V constant voltage to a current ≤0.05C, then discharge the battery at 1C constant current to a voltage of 2.5V, which is a charge and discharge process, so repeat the cycle charge and discharge, monitor the SOH of the battery throughout the process, then every 1% SOH, the battery is disassembled to observe whether the negative electrode plate of the battery is broken, and the SOH corresponding to the crack failure is obtained, measure the total thickness H1' 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 plate.

[0348] (6) Thickness expansion rate and expansion force test of secondary battery at 60% SOH:

[0349] First, measure the total thickness of the secondary battery at 25°C, then charge the battery at 1C constant current to a voltage of 3.8V, then charge at 3.8V constant voltage to a current ≤0.05C, then discharge the battery at 1C constant current to a voltage of 2.5V, which is a charge and discharge process, so repeat the cycle charge and discharge, monitor the SOH of the battery throughout the process, until the battery reaches 60% SOH, 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 plate; And measure the total thickness of the secondary battery at this time, and get the thickness expansion rate of the secondary battery at 60% SOH by (thickness at this time-initial total thickness in initial state) / initial total thickness.

[0350] II. Preparation method

[0351] Example 1

[0352] (1) Preparation of copper foil

[0353] A copper plate or copper wire with a purity of 99.9% or more was dissolved in sulfuric acid with a mass content of 98% to obtain a copper sulfate solution, which was 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) was 120 mg / L, the concentration of polyethylene glycol (relative molecular weight 4000) was 80 mg / L, the concentration of hydroxyethyl cellulose (relative molecular weight about 120000) was 60 mg / L, the concentration of chloride ion was 40 mg / L, the concentration of sodium polydithiobispropane sulfonate was 600 mg / L, the concentration of sodium saccharin was 2 g / L, the concentration of copper ions (calculated as copper atoms) was 90 g / L, and the rest was deionized water. The pH of the electroplating solution was 3.5.

[0354] A sinusoidal pulse current was used to periodically apply current to a cathode titanium roller that had been polished, and the anode electrode was a titanium base plate. The titanium roller had an area of 8.67 m 2 in the electroplating solution, the rotation speed of the titanium roller (roll speed) was 2.4 m / min, a sinusoidal pulse current was applied, the peak current was 55000 A, the peak-to-valley current was 2500 A, the period was 3000 ms, the distance between the cathode and the anode was 10 mm, the deposition temperature was 55°C, and a copper foil with a thickness of 6 μm was deposited on the titanium roller.

[0355] (2) Preparation of battery

[0356] 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) were 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 was obtained; then the positive electrode slurry was uniformly coated on the positive electrode current collector, and after drying, cold pressing, and slitting, a positive electrode sheet was obtained.

[0357] 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) were 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 was prepared; then the negative electrode slurry was uniformly coated one or more times on the negative electrode current collector copper foil, and after drying, a negative electrode film was obtained, which was then cold pressed and slitted to obtain a negative electrode sheet, wherein the gram capacity of the negative electrode active material in the negative electrode film layer was 800 mAg / h-1500 mAg / h.

[0358] In an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), ethylene carbonate, diethyl carbonate, dimethyl carbonate were mixed in a volume ratio of 1:1:1, and LiPF6 was dissolved in the above solution to obtain an electrolyte. In the electrolyte, the concentration of LiPF6 was 1 mol / L. Then, 2.0 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, and mixed and stirred uniformly to obtain an electrolyte.

[0359] A polypropylene film was used as a separator film.

[0360] The positive electrode sheet, the separator film and the negative electrode sheet were stacked in order, with the separator film between the positive electrode sheet and the negative electrode sheet to play a separating role, and then wound to obtain an electrode assembly; the electrode assembly was placed in a battery shell, and after drying, an electrolyte was injected, and then a lithium ion battery was prepared through processes such as formation and standing.

[0361] Example 2-4

[0362] The preparation method of Example 2-4 was basically the same as that of Example 1, but the composition of the electroplating 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; wherein the sodium propane sulfonate modified thiourea was purchased from Shenzhen Jihaochang New Material Co., Ltd., and the CAS was 21668-81-5.

[0363] Example 1-1 The preparation method of Example 1-1 was basically the same as that of Example 1, but a direct current was used for deposition, and the deposition current was 55,000 A. The thickness of the copper foil was 6 μm.

[0364] Comparative Example 1

[0365] The preparation method of Comparative Example 1 was basically the same as that of Example 1, and a direct current of 30,000 A was used for deposition. The thickness of the copper foil was 6 μm.

[0366] Table 1: Composition of electroplating solution

[0367] Table 2: Sine wave pulse current parameters

[0368] NA means not applicable.

[0369] The grain characteristics and mechanical characteristics of the copper foils prepared in Example 1-3 and Example 1-1, Comparative Example 1, and the results of the secondary batteries were as shown in Table 3.

[0370] Table 3

[0371] Wherein, “ / ” represents that it cannot be measured.

[0372] Table 4

[0373] Fig. 7, Fig. 10 and Fig. 13 respectively show the inverse pole figure maps of the cross-section of the copper foils of Example 1-3 tested by EBSD, where T is along the thickness direction of the current collector, the cross-section of the copper foils presents a heterogeneous grain structure with a size grain doping distribution. Compared with the inverse pole figure map of the copper foil of Comparative Example 2 (Fig. 17), it can be seen that more fine grains are distributed in the copper foils of Example 1-3, and the grain size is smaller as a whole. In addition, it can be seen from the above figures that the copper foils of Example 1-3 include copper grains with a particle size of 1 μm-4 μm, and the short diameter of the copper grains is arranged along the thickness direction of the current collector.

[0374] Fig. 9, Fig. 12, Fig. 15 and Fig. 19 respectively show the particle size distribution maps of the copper foils of Example 1-3, Comparative Example 1. It can be seen from Table 3 that in the copper foils of Example 1-3, the number of copper grains with a particle size of less than or equal to 0.5 μm accounts for 70%-95%, and is concentrated in the range of 80%-95%, while in Comparative Example 1, the number of copper grains with a particle size of less than or equal to 0.5 μm accounts for less than 50%, indicating that the copper grains of Example 1-3 are smaller as a whole, and the copper foils have higher mechanical strength. In addition, in the copper foils of Example 1-3, the number of copper grains with a particle size greater than 0.5 μm accounts for 5%-30%, while in Comparative Example 1, the number of copper grains with a particle size greater than 0.5 μm accounts for 50.5%, and the copper foils of Example 1-3 have plasticity basically equivalent to that of the copper foils of Comparative Example 1. This indicates that the preparation of the copper foils by the sinusoidal pulse current helps to adjust the particle size distribution of the copper grains, thereby improving the mechanical properties and plasticity of the copper foils.

[0375] In the copper foils of Example 1-3, the average particle size is in the range of 0.3 μm-0.6 μm, the maximum particle size is in the range of 1 μm-2 μm, and the minimum particle size is in the range of 0.1 μm-0.3 μm, and the particle size span is 0.8 μm-2 μm. In Comparative Example 1, the span of the copper grain particle size is larger, and the average particle size of the copper grains is larger. It can be understood that the copper foils of Example 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 Example 1-3 and Comparative Example 1.

[0376] Further, as can be seen from the above figures and Table 3, in the copper foils of Examples 1-3, at least part of the copper grains with a particle size of 1-4 μm have their short diameters arranged along the thickness direction of the current collector, and the number of copper grains with an internal particle size of 0.5 μm or less accounts for 70-95%, forming a heterogeneous grain structure with a size grain doping distribution and arranging the short diameters of the large grains along the thickness direction of the current collector, thereby increasing the grain tortuosity of the copper foil along the thickness direction of the current collector, improving the tensile strength of the copper foil while taking into account the plasticity and bending resistance of the copper foil substrate, helping to reduce the probability of fracture of the inner circle of the current collector in the cell winding process, and helping to improve the safety and service life of the secondary battery.

[0377] As can be seen from the results of Example 1 and Example 1-1 in Table 4, the number of copper grains with a particle size of 1-4 μm in the copper foil accounts for 2-10%, indicating that the copper foil has a certain number of grains with a larger particle size, which is beneficial to improve the brittleness of the copper foil substrate, improve the plasticity of the copper foil, increase the elongation at break of the copper foil, further reduce the probability of fracture of the inner circle of the current collector, and improve the safety, production efficiency and service life of the secondary battery.

[0378] Fig. 8, Fig. 11 and Fig. 14 respectively show the tensile curves of the copper foils of Examples 1-3, and it can be seen that the tensile strength of the prepared copper foils is in the range of 600-1000 MPa, and the elongation at break of the copper foils is in the range of 4-8% (Table 3), indicating that the copper foils have excellent strength and plasticity. 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%, indicating poor plasticity. Fig. 18 shows the tensile curve of the copper foil of Example 1-1, which has an elongation at break similar to that of Example 1, but the tensile strength is poor.

[0379] The hardness of the copper foils of Examples 1-3 is in the range of 55-65 HV, indicating that the copper foils have good pressure deformation or puncture resistance.

[0380] Example 1-1 uses the same direct current as the peak current of Example 1 to prepare the copper foil. 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 particle size distribution of the copper grains can be adjusted by the sine wave pulse current and the current parameters, so that the copper foil has good mechanical strength while taking into account excellent plasticity.

[0381] Compared with Comparative Example 1, the crack failure of the copper foils of Examples 1-3 corresponds to a significant decrease in SOH, which indicates that the copper foils prepared in Examples 1-3 significantly improve the service life of the secondary battery and increase the safety of the secondary battery. Although the copper foil of Comparative Example 1 has excellent fracture elongation, the tensile strength is low, resulting in a relatively high crack failure corresponding SOH value. In contrast, the tensile strength of the copper foils prepared in Examples 1-3 using the current size varying sinusoidal pulse current is significantly improved, and the copper foils maintain good plasticity, the crack failure corresponding SOH value is significantly reduced, and the service life of the battery cell can be improved by more than 25% SOH.

[0382] Compared with Comparative Example 1, the copper foils of Examples 1-3 have 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, the expansion force is greater than or equal to 1000 kgf, and the thickness expansion rate is 4%-10%, the copper foils of Examples 1-3 can effectively reduce the probability of crack fracture of the current collector, so that the SOH of the secondary battery decreases when the crack failure occurs (i.e., the cycle life and safety of the secondary battery are effectively improved), and the probability of secondary battery failure and performance diving caused by battery expansion is reduced.

[0383] Table 5

[0384] As can be seen from the results of Examples 1, 4 and Comparative Example 2 in Table 5, the copper foils of the present application have heterogeneous grain structure and distribution morphology of large grains distributed in the transverse direction perpendicular to the thickness direction of the current collector, which helps to make the copper foil substrate have a suitable grain tortuosity in the thickness direction of the current collector, and further control the grain tortuosity to be 10-30 μm, which can improve the tensile strength of the copper foil while improving the brittleness of the copper foil, improving the toughness and bending resistance of the current collector, helping to reduce the probability of fracture of the inner circle of the current collector in the cell winding process, improving the manufacturing yield, and further improving the safety and service life of the secondary battery. Further, as can be seen from the results of Examples 1 and 4, Example 4 also has good tensile strength, fracture elongation, and higher bending resistance, which is beneficial to further improve the SOH of the secondary battery when the crack failure occurs and prolong the service life of the secondary battery.

[0385] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and embodiments having the same technical idea and playing the same role and effect within the scope of the technical solution 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 of the embodiments that can be thought of by those skilled in the art, and other ways constructed by combining part of the components of the embodiments are also included in the scope of the present application.

Claims

1. A secondary battery characterized by comprising: The negative electrode tab comprises a negative electrode current collector comprising a copper foil, the copper foil comprising copper grains of different particle sizes, the copper grains comprising copper grains with a particle size of 0.5 μm or less and copper grains with a particle size of 1 μm-4 μm, wherein the number of copper grains with a particle size of 0.5 μm or less accounts for 70%-95% of the total number of copper grains; The short diameter of at least part of the copper grains with a particle size of 1 μm-4 μm is arranged along the thickness direction of the negative electrode current collector.

2. The secondary battery according to claim 1, characterized by The grain tortuosity of the copper foil in the thickness direction of the negative electrode current collector is 20 μm-30 μm.

3. The secondary battery according to claim 1 or 2, characterized by The ratio of the long diameter to the short diameter of the copper grains with a particle size of 1 μm-4 μm is greater than 1, and is optionally 2-5.

4. The secondary battery according to any one of claims 1 to 3, characterized by, The number of copper grains with a particle size of 1 μm-4 μm accounts for 2%-10% of the total number of copper grains.

5. The secondary battery according to any one of claims 1 to 4, characterized by, The copper grains with a particle size of 1 μm-4 μm are divided into first grains, which have an angle of less than 20° between the short diameter and the thickness direction of the negative electrode current collector and an angle of greater than 75° between the long diameter and the thickness direction of the negative electrode current collector, and the number of the first grains accounts for 60%-80% of the total number of copper grains with a particle size of 1 μm-4 μm.

6. The secondary battery according to any one of claims 1 to 5, characterized by The number of copper grains with a particle size of 0.5 μm or less 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.

7. The secondary battery according to any one of claims 1 to 6, characterized by, The copper foil satisfies at least one of the following conditions: (1) the average particle size of the copper grains is 0.3 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; (4) the particle size span of the copper grains is 0.8 μm-2.5 μm.

8. The secondary battery according to any one of claims 1 to 7, characterized by, The copper foil satisfies at least one of the following conditions: (1) the average particle size of the copper grains is 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; (4) the particle size span of the copper grains is 1 μm-2 μm.

9. The secondary battery according to any one of claims 1 to 8, characterized by, 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, the tensile strength of the copper foil is 600 MPa-1000 MPa, and / or the breaking elongation of the copper foil is 4%-8%.

10. The secondary battery according to any one of claims 1 to 9, characterized by 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, the tensile strength of the copper foil is 700 MPa-1000 MPa, and / or the breaking elongation of the copper foil is 4%-7%.

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-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×width of (50±0.25 mm)×(15±0.25 mm), and tensile speed of 50±0.5 mm / min.

12. The secondary battery according to any one of claims 1 to 11, characterized by The hardness of the copper foil is 55 HV-65 HV.

13. The secondary battery according to any one of claims 1 to 12, characterized by The hardness of the copper foil is 55 HV-60 HV.

14. The secondary battery according to any one of claims 1 to 13, characterized by The thickness of the copper foil is 4 μm-10 μm.

15. The secondary battery according to any one of claims 1 to 14, 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.

16. The secondary battery according to claim 15, 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.

17. 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 a 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, the copper grains comprise copper grains with a particle size of 0.5 μm or less and copper grains with a particle size of 1 μm-4 μm, wherein the proportion of the number of copper grains with a particle size of 0.5 μm or less in the total number of copper grains is 70%-95%, and the short diameter of at least part of the copper grains with a particle size of 1 μm-4 μm is arranged along the thickness direction of the negative electrode current collector.

18. The method of claim 17, 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 electroplating deposition is 45 ℃-60 ℃; (6) the speed of the cathode roller is 2 m / min-5 m / min.

19. The method of manufacturing according to claim 17 or 18, 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 electroplating deposition is 50 ℃-60 ℃; (6) the speed of the cathode roller is 2 m / min-3 m / min.

20. The production method according to any one of claims 17 to 19, 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.

21. The production method according to any one of claims 17 to 20, 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 leveling agent comprises one or more of collagen and sodium saccharin; The wetting agent includes one or more of hydroxyethyl cellulose, polyethylene glycol; The brightener includes sodium polydithiobispropane sulfonate.

22. The method of claim 21, wherein, The brightener also includes sodium propane sulfonate modified thiourea.

23. The method of manufacturing according to claim 21 or 22, wherein, The plating solution includes: 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 polydithiobispropane sulfonate with a concentration of 500 mg / L-2000 mg / L, and chloride ions (as chlorine atoms) with a concentration of 20 mg / L-80 mg / L.

24. The production method according to any one of claims 21 to 23, characterized by, The plating solution includes: 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 polydithiobispropane sulfonate with a concentration of 500 mg / L-1000 mg / L, and chloride ions with a concentration of 40 mg / L-80 mg / L.

25. The production method according to any one of claims 22 to 24, characterized by, The plating solution also includes sodium propane sulfonate modified thiourea with a concentration of 2 mg / L-20 mg / L.

26. The production method according to any one of claims 17 to 25, wherein, The pH of the plating solution is 2.5-4.

5.

27. An electrical device, comprising: A secondary battery including the secondary battery of any one of claims 1 to 16 or the secondary battery prepared by the preparation method of claims 17 to 26.

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