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

Abstract

The application provides a secondary battery, a preparation method thereof and an electric device. The secondary battery comprises a copper foil, the copper foil comprises copper grains with different particle sizes, the copper grains comprise copper grains with a particle size less than or equal to 0.5 microns, the proportion of the number of copper grains with a particle size less than or equal to 0.5 microns accounts for 70% to 95% based on the total number of the copper grains; the maximum particle size of the copper grains is 1.2 microns to 3 microns; the average particle size of the copper grains is 0.04 microns to 0.6 microns; and the proportion of the number of nano-twin crystal grains is greater than or equal to 45% based on the total number of the copper grains.

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 with improved tensile properties, which can effectively constrain the expansion of the battery cell, delay or reduce the fracture of the electrode sheet, and prolong the service life of the secondary battery.

[0009] In a first aspect, the present application provides a secondary battery, the secondary battery comprising a negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a negative electrode film layer arranged on at least one side surface of the negative electrode current collector, the negative electrode current collector comprising a copper foil, the copper foil comprising copper grains with different grain sizes, the copper grains comprising copper grains with a grain size of 0.5 μm or less, the proportion of the number of copper grains with a grain size of 0.5 μm or less in the total number of the copper grains being 70% to 95%, the maximum grain size of the copper grains being 1.2 μm to 3 μm, the average grain size of the copper grains being 0.04 μm to 0.6 μm, and the proportion of the number of grains of nanotwins in the total number of the copper grains being 45% or more.

[0010] The copper grains with a grain size of 0.5 μm or more tend to be columnar grains or columnar-like grains, which are also referred to as "large grains" herein, and the copper grains with a grain size of 0.5 μm or less are fine grains close to particles, forming a fine grain region around the columnar grains. The copper foil of the present application comprises copper grains with a grain size of 0.5 μm or less in a proportion of 70% to 95%, the maximum grain size of the copper grains is 1.2 μm to 3 μm, and the average grain size of the copper grains is 0.04 μm to 0.6 μm, which indicates that the copper foil substrate forms a grain morphology of "fine grains as the main component, mixed with large grains", that is, a heterogeneous grain morphology with a small amount of large grains doped in fine grains, which is beneficial to improving the tensile strength and plasticity of the copper foil, and to improving the mechanical strength and toughness of the copper foil. The copper foil of the present application has a certain proportion of nanotwins, and the proportion of the number of nanotwins is 45% or more, which can form a good match with the fine grains in a proportion of 70% to 95%, and the small grain size grains can further increase the characteristics of the copper foil crystal interface structure, improve the additional stress required for dislocation to cross the grain boundary, and realize material strengthening. In summary, the copper foil provided by the present application is designed by optimizing the crystal structure, so that it has a nanotwin structure and a high proportion of small size grains, and the combined action of the two is beneficial to improving the mechanical strength while maintaining the high plasticity of the copper foil, and provides a material basis for further improving the energy density and safety of the battery.

[0011] In any embodiment, the proportion of the number of copper grains with a grain size of 200 nm or less in the total number of copper grains in the copper foil is 20% to 60%.

[0012] In any embodiment, the proportion of the number of grains of nanotwins in the total number of the copper grains is 50% to 90%. In any embodiment, the proportion of the number of grains of nanotwins in the total number of the copper grains is 65% to 85%.

[0013] In any embodiment, the expansion force of the secondary battery is greater than or equal to 1000 kgf, the secondary battery comprises a negative electrode sheet, the negative electrode sheet comprises a negative electrode current collector and a negative electrode film layer arranged on at least one side surface of the negative electrode current collector, the negative electrode current collector comprises a copper foil, the copper foil comprises copper grains of 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 copper grains with a particle size less than or equal to 0.5 μm in the total number of copper grains is 70%-95%, and the proportion of the number of copper grains with a particle size greater than 0.5 μm in the total number of copper grains is 5%-30%.

[0014] During the charging and discharging cycle operation of the secondary battery, the volume of the electrode sheet expands due to the embedding and extraction of active ions, especially in the self-generated negative electrode battery or new silicon-based or lithium metal negative electrode, and the overall expansion of the secondary battery is high due to the comprehensive influence of the design of high group margin or large size battery cell for the purpose of improving the energy density of the single battery cell. The copper foil current collector of the high expansion secondary battery is obviously stretched and stressed during the operation process, which increases the probability of fracture or crack of the copper foil current collector, and deteriorates the safety and service life of the battery cell. Therefore, the copper foil comprising copper grains of different particle sizes provided in the secondary battery has excellent tensile strength and elongation at break, and has excellent plasticity while improving the mechanical strength, which 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 cracks and fails, and is beneficial to further improve the safety and service life of the secondary battery.

[0015] In any embodiment, the thickness expansion rate of the secondary battery is 4%-10%,

[0016] In any embodiment, the secondary battery is a wound secondary battery, 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 of 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 copper grains with a particle size less than or equal to 0.5 μm in the total number of copper grains is 70%-95%, and the proportion of the number of copper grains with a particle size greater than 0.5 μm in the total number of copper grains is 5%-30%.

[0017] The current collector of the negative electrode tab in the wound secondary battery is prone to crack fracture in the corner area. This is because the wound battery is pressed and shaped after winding, which can cause irreversible damage to the current collector. The probability of crack or fracture in the corner area of the outer circle and the bending area of the inner circle of the negative electrode tab increases significantly under the expansion stress caused by the increase in the internal pressure of the secondary battery and the volume expansion. This aggravates the risk of battery failure and worsens the service life and safety of the secondary battery. The copper foil provided in the present application includes copper grains of different particle sizes as described above, which significantly improves the tensile strength and elongation at break of the copper foil, making the copper foil have excellent mechanical properties and plasticity, reducing the probability of crack or fracture of the copper foil under expansion stress, reducing the probability of crack fracture of the wound battery tab, and improving the service life of the secondary battery when the tab cracks or fails. This improves the safety and service life of the secondary battery.

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

[0019] In any embodiment, the secondary battery includes a negative electrode tab, the negative electrode tab includes a negative electrode current collector and a negative electrode film layer arranged on at least one side surface of the negative electrode current collector, the negative electrode current collector includes a copper foil, the copper foil includes copper grains of different particle sizes, the copper grains include copper grains with a particle size less than or equal to 0.5 μm and copper grains with a particle size greater than 0.5 μm, wherein the number of copper grains with a particle size less than or equal to 0.5 μm accounts for 70%-95% of the total number of copper grains, and the number of copper grains with a particle size greater than 0.5 μm accounts for 5%-30% of the total number of copper grains; and the gram capacity of the negative electrode active material in the negative electrode film layer is 800 mAh / g-1500 mAh / g.

[0020] 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 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. However, the particle volume of the high gram capacity negative electrode active material expands greatly before and after lithium ion intercalation / deintercalation, and the expansion stress acts on the current collector copper foil, increasing the probability of fracture or crack of the copper foil along the thickness direction under expansion stress, and worsening the safety and service life of the secondary battery. The copper foil provided in the present application includes copper grains of different particle sizes as described above, which has excellent tensile strength and elongation at break, and has excellent plasticity while improving the mechanical strength, improves the energy density and capacity of the secondary battery while improving the safety of the secondary battery.

[0021] In any embodiment, the gram capacity of the negative active material in the negative electrode film layer is 800 mAh / g-1500 mAh / g.

[0022] The copper foil provided by the present application is suitable for secondary batteries including negative active materials with a gram capacity of 800 mAh / g-1500 mAh / g. High gram capacity negative materials can embed more active ions, such as silicon-based negative electrodes, alkali metal negative electrodes, etc., which generally have high swelling properties, which can increase the probability of copper foil fracture or crack in the battery cell. The copper foil has excellent mechanical properties and plasticity, which can reduce the risk of copper foil fracture or crack in high energy density battery systems or high swelling battery systems, improve the energy density, capacity and safety of the secondary battery.

[0023] In any embodiment, the particle size of the copper grains with a particle size greater than 0.5 μm is in the range of greater than 0.5 μm and less than or equal to 3 μm, which can adjust or optimize the tensile strength of the copper foil.

[0024] In any embodiment, the number of copper grains with a particle size greater than 0.5 μm and less than or equal to 3 μm accounts for 5%-30% based on the total number of copper grains.

[0025] As mentioned before, 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, further improve the mechanical strength of the copper foil; further controlling the number of copper grains with a particle size greater than 0.5 μm and less than or equal to 3 μm to meet the condition of 5%-30% can make the copper foil have good fracture elongation rate, excellent mechanical properties and plasticity, which can reduce the risk of copper foil fracture or crack in high energy density battery systems or high swelling battery systems, and realize the simultaneous improvement of the energy density and safety of the secondary battery.

[0026] 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.

[0027] 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, which indicates that the internal grain boundary of the copper foil has high 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 to reduce the probability of copper foil fracture along the thickness direction, improve the brittleness of the current collector, further reduce the risk of copper foil fracture or crack, and improve the safety of the secondary battery.

[0028] In any embodiment, the number of copper grains with a particle size less than or equal to 0.5 μm accounts for 80%-95% of the total number of copper grains, which helps to further improve the tensile strength and improve the mechanical strength of the copper foil.

[0029] In any embodiment, the number of copper grains with a particle size greater than 0.5 pm accounts for 5%-20% of the total number of copper grains, which helps to further improve the elongation at break and improve the plasticity of the copper foil.

[0030] In any embodiment, the copper foil satisfies at least one of the following conditions:

[0031] (1) the minimum particle size of the copper grains is 0.1-0.3 pm;

[0032] (2) the particle size span of the copper grains is 0.8-2.5 pm.

[0033] The particle size distribution of the copper grains helps to adjust the number of copper grains with a particle size less than or equal to 0.5 pm and the number of copper grains with a particle size greater than 0.5 pm, thereby adjusting and improving the tensile strength and elongation at break of the copper foil, so that the copper foil has excellent mechanical properties and plasticity.

[0034] In any embodiment, the copper foil satisfies at least one of the following conditions:

[0035] (1) the minimum particle size of the copper grains is 0.1-0.3 pm;

[0036] (2) the particle size span of the copper grains is 1-2 pm.

[0037] The particle size distribution of the copper grains helps to further adjust the tensile strength and elongation at break of the copper foil, and improve the mechanical properties and plasticity.

[0038] In any embodiment, under the test conditions of room temperature (20±10°C), sample length x width of (50±0.25 mm) x (15±0.25 mm), and tensile speed of 50±0.5 mm / min, the tensile strength of the copper foil is 600-1000 MPa, and / or the elongation at break of the copper foil is 4%-8%.

[0039] The copper foil has excellent tensile strength and elongation at break, has good mechanical properties and plasticity, and can be applied to high-energy-density batteries or high-expansion batteries, which helps to improve the safety of secondary batteries.

[0040] In any embodiment, under the test conditions of room temperature (20±10°C), sample length x width of (50±0.25 mm) x (15±0.25 mm), and tensile speed of 50±0.5 mm / min, the tensile strength of the copper foil is 700-1000 MPa, and / or the elongation at break of the copper foil is 4%-7%.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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 the 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.

[0047] 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.

[0048] In any embodiment, the mass fraction of silicon in the silicon-based material is 20-50%.

[0049] In any embodiment, the mass fraction of silicon is 4-10%, based on the total mass of the negative electrode film layer.

[0050] In any embodiment, the expansion force of the secondary battery is greater than or equal to 1000 kgf.

[0051] In any embodiment, the expansion force of the secondary battery is greater than or equal to 2500 kgf.

[0052] In any embodiment, the expansion force of the secondary battery is greater than or equal to 4000 kgf.

[0053] In any embodiment, the expansion force of the secondary battery is 1000 kgf-10000 kgf.

[0054] 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 a secondary battery with high expansion force, which is conducive to further improvement of the energy density of the secondary battery.

[0055] In any embodiment, the thickness expansion rate of the secondary battery is 4%-10%.

[0056] 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 a secondary battery with high thickness expansion rate, which is conducive to further improvement of the energy density and safety of the secondary battery.

[0057] 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. The copper grains comprise copper grains with a particle size of less than or equal to 0.5 μm. The proportion of the number of copper grains with a particle size of less than or equal to 0.5 μm accounts for 70%-95% based on the total number of the copper grains. The maximum particle size of the copper grains is 1.2 μm-3 μm. The average particle size of the copper grains is 0.04 μm-0.6 μm. The proportion of the number of nanotwins in the copper grains is greater than or equal to 45% based on the total number of the copper grains.

[0058] Compared with the calendering method, the electroplating method is mature and simple, has low requirements for equipment, and the prepared copper foil has excellent tensile strength and elongation at break, and has excellent mechanical strength and good plasticity.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] The application provides a secondary battery preparation method, and a copper foil is prepared by the above electroplating method, so that the copper foil has excellent tensile strength and elongation at break, excellent plasticity is achieved while the mechanical strength is improved, the probability of fracture or crack of the copper foil under the expansion stress of the high-capacity negative active material due to the volume change of the particles before and after the lithium ion insertion / desorption is reduced, and the safety and service life of the secondary battery are further improved.

[0065] The applicant finds that, in the process of preparing the copper foil by the electroplating method, the current size is directly related to the nucleation and growth of copper grains, and the smaller the current is, the more conducive to promoting the nucleation and growth of the grains, so that the copper grains with a large particle size are generated in the copper foil; the larger the current is, the more conducive to the rapid reduction and nucleation of copper ions, so that the copper grains with a small particle size exist in the copper foil. Further controlling the peak value and the valley value of the pulse current in the electroplating method to meet the above range is conducive to promoting the formation of the grain morphology of “mainly fine grains mixed with large grains”, and at the same time, a certain proportion of nanotwins exist, so that the tensile strength and plasticity of the material are improved, and a material basis for improving the service life and safety of the secondary battery is provided.

[0066] In any embodiment, the electroplating method meets one or more of the following conditions:

[0067] (1) the peak value of the pulse current is 40000 A-80000 A;

[0068] (2) the valley value of the pulse current is 1000 A-10000 A;

[0069] (3) the change period of the pulse current is 500 ms-5000 ms;

[0070] (4) the distance between the cathode electrode and the anode electrode is 8 mm-20 mm;

[0071] (5) the temperature of the electroplating deposition is 45°C-60°C;

[0072] (6) the roller speed of the cathode roller is 2 m / min-5 m / min.

[0073] In any embodiment, the electroplating method meets one or more of the following conditions:

[0074] (1) the peak value of the pulse current is 50000 A-70000 A;

[0075] (2) the valley value of the pulse current is 2000 A-5500 A;

[0076] (3) the change period of the pulse current is 2000 ms-4000 ms;

[0077] (4) the distance between the cathode electrode and the anode electrode is 8 mm-12 mm;

[0078] (5) The temperature of electroplating deposition is 50-60℃;

[0079] (6) The roller speed of cathode roller is 2-3m / min.

[0080] Compared with direct current deposition, applying pulse current to the electroplating solution can make copper ions reduce and deposit to form fine grains with small particle size under high current conditions, and form grains with large particle size (e.g. columnar grains) under low current conditions. By adjusting the current size and variation period, the formation and growth of copper ion crystal nucleus can be adjusted, so that the size and morphology of the grain size, and the proportion of the number of copper grains with a particle size of less than or equal to 0.5μm and the copper grains with a particle size of greater than 0.5μm can be adjusted, and the tensile strength and elongation at break of the copper foil can be improved.

[0081] In any embodiment, the pulse current includes one or more of square wave pulse current, sinusoidal wave pulse current, triangular wave pulse current, and sawtooth wave pulse current. In any embodiment, the pulse current includes sinusoidal wave pulse current. The sinusoidal wave pulse current continuously and periodically changes, which is conducive to the continuous and variable growth of the grains.

[0082] In any embodiment, the peak current density of the pulse current is I, and the unit is A / dm 2 ; the current duty cycle of the pulse current is s, and the peak current density I and the duty cycle s satisfy: 2A / dm 2 ≤I×s≤18A / dm 2 .

[0083] The product of the peak current density I and the duty cycle s is equivalent to the average density of the current in the pulse period. The mode of pulse current deposition makes the average density of the current in the pulse period higher than the maximum current density that can be achieved in direct current deposition, because too high a direct current deposition density will cause hydrogen evolution and concentration polarization of the electrolyte, reducing the uniformity of the copper foil in the electroplating process. The product of the peak current density I and the duty cycle s in the above range enables the dynamic balance of copper ion consumption and replenishment in the electrolyte by controlling the opening and closing of the current, reduces the concentration difference between the copper ion deposition site and other parts of the electrolyte, so that the electrolyte does not undergo significant concentration polarization, and improves the uniformity of the grain size in the thickness direction of the prepared copper foil. Moreover, the product of the peak current density I and the duty cycle s in the above range can control the nucleation and growth rate of the grains, realize the preparation of small particle size grains, and improve the mechanical strength of the copper foil.

[0084] In any embodiment, the peak current density I of the pulse current satisfies: 3.3A / dm 2 ≤I≤333A / dm2, which can be optionally 100A / dm2 ≤I≤180A / dm 2 .

[0085] In any embodiment, the duty cycle s of the pulse current is 2%-50%, optionally 2%-10%.

[0086] In any embodiment, the electroplating solution comprises a leveling agent, a wetting agent and a brightener, the leveling agent comprises one or more of collagen, sodium saccharin; the wetting agent comprises one or more of hydroxyethyl cellulose, polyethylene glycol; the brightener comprises sodium polydithiopropyl sulfone.

[0087] In any embodiment, the leveling agent further comprises one or more of gelatin, gentian violet; the brightener further comprises one or more of sodium 3-mercapto-1-propane sulfonate, thiourea.

[0088] The leveling agent can improve the flatness of the copper foil, the wetting agent can improve the wettability of the electroplating solution with the substrate, improve the nucleation rate of the copper foil, reduce the average grain size of the copper foil, the brightener can make the grain size of the copper foil more fine, and reduce the surface roughness of the copper foil, improve the smoothness of the surface.

[0089] In any embodiment, the electroplating solution comprises: collagen with a concentration of 60mg / L-300mg / L, sodium saccharin with a concentration of 0.5g / L-10g / L, polyethylene glycol with a concentration of 50mg / L-200mg / L, hydroxyethyl cellulose with a concentration of 30mg / L-200mg / L, polydithiopropyl sulfone sodium with a concentration of 500mg / L-2000mg / L, and chloride ions (calculated as chlorine atoms) with a concentration of 20mg / L-80mg / L.

[0090] In any embodiment, the electroplating solution comprises: collagen with a concentration of 80mg / L-150mg / L, sodium saccharin with a concentration of 0.5g / L-4g / L, polyethylene glycol with a concentration of 60mg / L-150mg / L, hydroxyethyl cellulose with a concentration of 50mg / L-150mg / L, polydithiopropyl sulfone sodium with a concentration of 500mg / L-1000mg / L, and chloride ions with a concentration of 40mg / L-80mg / L.

[0091] 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 sodium polydithiobispropyl sulfone can improve the electrochemical reduction rate of copper ions and adjust the grain size. The prepared copper foil is bright and flat, the number ratio of copper grains with a particle size of less than or equal to 0.5 μm and copper grains with a particle size of greater than 0.5 μm is appropriate, and the copper foil has good tensile strength and elongation at break. The preparation method in the embodiments of the application uses brighteners that meet the above-mentioned concentrations, which is conducive to improving the electrochemical reduction rate of copper ions, realizing grain refinement, increasing the number of grains with a particle size of less than or equal to 0.5 μm, and increasing the number ratio of small grains in the copper foil substrate to 70%-95%, thereby improving the tensile strength of the copper foil substrate. The applicant further found that the brightener concentration meeting the above range is conducive to promoting the co-growth of grains with a particle size of less than or equal to 0.5 μm along the same common crystal face during the electroplating process, forming a twin crystal structure, further increasing the number ratio of nano-twin crystal grains in the copper foil substrate, and helping to improve the dislocation motion resistance, and further improve the strength and plasticity of the material.

[0092] In any embodiment, the concentration of copper ions in the electrolyte is 30 g / L-100 g / L, which can be selected as 45 g / L-75 g / L.

[0093] The research results show that the combination of low copper ion concentration and high additive concentration is more conducive to improving the strength and elongation at break of the copper foil than the combination of high copper ion concentration and low additive concentration. Moreover, the low copper ion concentration helps to control the growth and deposition rate of the grains, is conducive to preparing small-size grains, and improves the mechanical strength of the copper foil.

[0094] In any embodiment, the pH of the electroplating solution is 2.5-4.5, which is conducive to the reduction of copper ions.

[0095] The third aspect of the application provides a pole piece, which comprises the copper foil in the secondary battery provided by the first aspect of the application or the copper foil prepared by the preparation method of the second aspect of the application.

[0096] 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.

[0097] 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.

[0098] In any embodiment, the silicon-based material has a mass fraction of 5% to 100%, optionally 10% to 60%, and more optionally 10% to 30%, based on the total mass of the negative electrode film layer.

[0099] In any embodiment, the silicon-based material has a mass fraction of 20% to 50% of silicon element.

[0100] In any embodiment, the silicon-based material has a mass fraction of 4% to 10% of silicon element, based on the total mass of the negative electrode film layer.

[0101] A fourth aspect of the present application provides a wound secondary battery comprising the secondary battery provided by the first aspect of the present application or prepared by the method provided by the second aspect of the present application or comprising the electrode tab provided by the third aspect of the present application.

[0102] A fifth aspect of the present application provides a power consumption device comprising the secondary battery of the first aspect of the present application or prepared by the method of the second aspect of the present application or the wound secondary battery of the fourth aspect of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0103] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed to be used in the embodiments of the present application will be briefly introduced as follows. Obviously, the drawings described below are only some embodiments of the present application, and other drawings can also be obtained according to the drawings without creative labor for those skilled in the art.

[0104] FIG. 1 is a schematic diagram of a secondary battery according to an embodiment of the present application.

[0105] FIG. 2 is an exploded view of the secondary battery according to an embodiment of the present application shown in FIG. 1.

[0106] FIG. 3 is a schematic diagram of a battery module according to an embodiment of the present application.

[0107] FIG. 4 is a schematic diagram of a battery pack according to an embodiment of the present application.

[0108] FIG. 5 is an exploded view of the battery pack according to an embodiment of the present application shown in FIG. 4.

[0109] FIG. 6 is a schematic diagram of a power consumption device using the secondary battery according to an embodiment of the present application as a power source.

[0110] FIG. 7 shows the inverse pole figure distribution map of the cross section of the copper foil of Example 1 of the present application tested by electron backscatter diffraction (EBSD).

[0111] FIG. 8 shows the tensile curve of the copper foil of Example 1 of the present application.

[0112] Figure 9 shows a grain size distribution diagram obtained by EBSD diffraction testing of the cross section of the copper foil of Example 1 of the present application.

[0113] Figure 10 shows a pole figure distribution diagram obtained by EBSD testing of the cross section of the copper foil of Example 2 of the present application.

[0114] Figure 11 shows a tensile curve of the copper foil of Example 2 of the present application.

[0115] Figure 12 shows a grain size distribution diagram obtained by EBSD diffraction testing of the cross section of the copper foil of Example 2 of the present application.

[0116] Figure 13 shows a pole figure distribution diagram obtained by EBSD testing of the cross section of the copper foil of Example 3 of the present application.

[0117] Figure 14 shows a tensile curve of the copper foil of Example 3 of the present application.

[0118] Figure 15 shows a grain size distribution diagram obtained by EBSD diffraction testing of the cross section of the copper foil of Example 3 of the present application.

[0119] Figure 16 shows a tensile curve of the copper foil of Example 1-1 of the present application.

[0120] Figure 17 shows a pole figure distribution diagram obtained by EBSD testing of the cross section of the copper foil of Comparative Example 1 of the present application.

[0121] Figure 18 shows a tensile curve of the copper foil of Comparative Example 1 of the present application.

[0122] Figure 19 shows a grain size distribution diagram obtained by EBSD diffraction testing of the cross section of the copper foil of Comparative Example 1 of the present application.

[0123] Figure 20 shows a grain size distribution diagram of the copper foil of Example 4 of the present application.

[0124] Figure 21 shows a pole figure distribution diagram of the electron backscatter diffraction image of the copper foil of Example 4 of the present application.

[0125] Figure 22 shows a tensile curve of the copper foil of Example 4 of the present application.

[0126] BRIEF DESCRIPTION OF DRAWINGS

[0127] 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

[0128] Hereinafter, specific embodiments of the secondary battery and the manufacturing method thereof, the electrode sheet, the wound-type secondary battery, and the electric device of the present application will be described in detail with appropriate reference to the accompanying drawings. However, there will be cases where unnecessary detailed description is omitted. For example, there will be cases where detailed description of matters known well, repeated description of substantially identical structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by 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.

[0129] The ranges disclosed herein are defined by the lower and upper limits of the range, given that the range is defined by selecting a lower limit and an upper limit, the selected lower limit and upper limit defining the boundaries of the particular range. Ranges defined by the limits in this manner can be inclusive or exclusive of 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 ranges of 60-120 and 80-110 are listed for a particular parameter, it is understood that ranges of 60-110 and 80-120 are also contemplated. In addition, if a minimum range value of 1 and 2 is listed, and if a maximum range value of 3, 4, and 5 is listed, then 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" indicates a shorthand way of describing each and every integer numeral, real number, and fraction between the upper and lower limits of that range. 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 way of describing those numerical combinations. Additionally, when a parameter is stated to be an integer ≥ 2, it is equivalent to disclose that the parameter is, for example, integer 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0130] If not specifically stated, all embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions.

[0131] 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.

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

[0133] If not specified otherwise, the terms "comprising" and "including" as used in the present application are open terms. For example, the terms "comprising" and "including" can mean that other components can also be included in addition to the recited components, or that only the recited components can be included.

[0134] If not specified otherwise, the term "or" in the present application is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any of the following conditions fulfill the condition "A or B": A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0135] Generally, a secondary battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator. During the charging and discharging of the battery, active ions (e.g., lithium ions, sodium ions) are inserted and extracted between the positive electrode sheet and the negative electrode sheet. The electrolyte serves to conduct the active ions between the positive electrode sheet and the negative electrode sheet. The separator is disposed between the positive electrode sheet and the negative electrode sheet, and mainly serves to prevent short circuiting between the positive and negative electrodes, while allowing the active ions to pass through. The negative electrode sheet includes a negative current collector, which serves to carry electrode active materials and collect output current, and can also bind the swelling of the battery cell to prevent the anode from breaking during cycling.

[0136] Increasing the amount of active material can increase the energy density or capacity of the secondary battery, but will cause the volume of the cell to increase; using silicon-based negative electrode materials, high-capacity graphite, lithium metal, and other negative electrode materials can also increase the energy density or capacity of the secondary battery, but the above-mentioned materials have high expansion, which in turn causes the volume of the pole piece to expand and contract severely during the cycle process. This easily leads to the fracture or slight cracking of the copper foil in the cell, 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 put forward for the bending resistance of the current collector. The cell pressing and shaping will cause irreversible damage to the current collector, and for the use of thinning the current collector substrate, high surface density of active material coating amount, or limit group margin design, the risk of cell cracking is greatly increased, further aggravating the risk of cell failure. At present, the tensile strength of the negative electrode current collector copper foil commonly used is usually 200-500 MPa, which cannot meet the use demand of the new generation of secondary batteries with high energy density or high capacity. Refining the copper grains can strengthen the material strength, but can aggravate the brittleness defect of the copper foil, increase the risk of copper foil fracture or slight cracking. How to improve the strength of the copper foil while considering good plasticity is a problem to be solved at present.

[0137] [Secondary battery]

[0138] 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 in the at least one side surface of negative electrode current collector, negative electrode current collector includes copper foil, copper foil includes copper grain of different particle size, copper grain includes copper grain with particle size less than or equal to 0.5 μm, copper grain with particle size less than or equal to 0.5 μm accounts for 70%-95% in quantity ratio based on total number of the copper grain;The maximum particle size of the copper grain is 1.2 μm-3 μm;The average particle size of the copper grain is 0.04 μm-0.6 μm;And the grain number ratio of nanometer twin crystal is greater than or equal to 45% in quantity ratio based on total number of the copper grain.

[0139] The copper grains with a particle size of 0.5 μm or more tend to present columnar grains or columnar-like grains, also referred to as "large grains" herein, and the copper grains with a particle size of less than or equal to 0.5 μm present fine grains close to granular, forming a fine grain region around the columnar grains. The copper foil in the present application includes copper grains with a particle size of less than or equal to 0.5 μm in a proportion of 70%-95%, the maximum particle size of the copper grains satisfies 1.2 μm-3 μm, and the average particle size of the copper grains is 0.04 μm-0.6 μm, indicating that the copper foil substrate forms a "fine grain-based, large grain mixed distribution" grain morphology, with a heterogeneous grain morphology of a small amount of large grains doped in fine grains, which is beneficial to improving the tensile strength and plasticity of the copper foil, and improving the mechanical strength and toughness of the copper foil. The main reason for the difference in the size of the heterogeneous grain morphology is the adjustment of the grain size in the process of growing the foil. Specifically, as an embodiment, the copper foil can control the size of the grain size by adjusting the change of the current size in the process of growing 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.

[0140] 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; and the large grains present in the copper foil substrate can hinder the expansion of dislocations, improving the toughness and elongation of the copper foil; the applicant found that there are the following growth behaviors in the process of growing and preparing the copper foil: the reduced copper ions nucleate and gradually grow, and part of the growing two copper grains grow along a common crystal face to form a twin structure with mirror symmetry orientation. The inclusion of twin structures in the copper foil can effectively improve the strength and plasticity of the copper foil, because the coherent interface with orderly arranged atoms at the twin grain boundary can improve the strength of the material while maintaining the mobility of dislocations, thereby maintaining the plasticity of the material while strengthening the material. The copper foil in the present application has a certain proportion of nanotwins, and the proportion of nanotwins is greater than or equal to 45%, which can form a good match with the fine grains in a proportion of 70%-95%, and the small grain size grains can further increase the characteristics of the copper foil crystal interface structure, improve the additional stress required for dislocations to pass through the grain boundary, and realize material strengthening. In summary, the copper foil provided in the present application is designed by optimizing the crystal structure, so that it has a nanotwin structure and a high proportion of small size grains, which is beneficial to improving the mechanical strength while maintaining the high plasticity of the copper foil, and provides a material basis for further improving the energy density and safety of the battery.

[0141] In some embodiments, the number of nanotwinned grains is 45%, 48%, 50%, 52%, 55%, 58%, 60%, 62%, 65%, 68%, 70%, 72%, 75%, 78%, 80%, 82%, 85%, 88%, 90%, 92%, 95%, 98%, 99%, or any range between any two of the aforementioned values, based on the total number of copper grains.

[0142] In some embodiments, the number of nanotwinned grains is 50%-90%, based on the total number of copper grains. In some embodiments, the number of nanotwinned grains is 65%-85%, based on the total number of copper grains.

[0143] In this document, the term “twin” refers to two crystals (or two parts of one crystal) that are mirror-symmetrically related along a common crystal face (i.e., a particular orientation relationship), and the two crystals are referred to as “twins” and the common crystal face is referred to as a twin plane. Nanotwinned refers to twins with each dimension less than 1000 nanometers.

[0144] In this application, the number of nanotwinned grains can be tested by methods known in the art. As an example, the cross-section of the copper foil is observed by scanning electron microscopy combined with electron backscatter diffraction (EBSD) to obtain a pole figure map, in which different colors represent different grain orientations. As an example, the red lines inside the grain are twin boundaries, representing that they have twin structures. The size and number of twins can be counted by the analysis software of the electron backscatter diffraction instrument, and then the number of nanotwinned grains can be calculated. As an example, the grain characteristics are analyzed by Oxford C-Nano+ electron backscatter diffraction instrument and its supporting software.

[0145] In some embodiments, the number of copper grains with a grain size less than 200 nm is 20%-60%, based on the total number of copper grains in the copper foil. In some embodiments, the number of copper grains with a grain size less than 200 nm is 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, or any range between any two of the aforementioned values, based on the total number of copper grains in the copper foil.

[0146] In some embodiments, the expansion force of the secondary battery is greater than or equal to 1000 kgf, the secondary battery comprises a negative electrode sheet, the negative electrode sheet comprises a negative electrode current collector and a negative electrode film layer arranged on at least one side surface of the negative electrode current collector, the negative electrode current collector comprises a copper foil, the copper foil comprises copper grains with different particle sizes, the copper grains comprise copper grains with a particle size less than or equal to 0.5 μm and copper grains with a particle size greater than 0.5 μm, wherein the proportion of the number of copper grains with a particle size less than or equal to 0.5 μm in the total number of copper grains is 70%-95%, and the proportion of the number of copper grains with a particle size greater than 0.5 μm in the total number of copper grains is 5%-30%.

[0147] During the charging and discharging cycle operation of the secondary battery, the volume of the electrode sheet expands due to the embedding and extraction of active ions, especially in the self-generated negative electrode battery or the new silicon-based or lithium metal negative electrode, and the overall expansion of the secondary battery is high due to the comprehensive influence of the design of high group margin or large size battery cell for the purpose of improving the energy density of the single battery cell. The copper foil current collector of the high expansion secondary battery is obviously stretched and stressed during the operation process, which increases the probability of fracture or crack of the copper foil current collector, and deteriorates the safety and service life of the battery cell. Therefore, the copper foil comprising the copper grains with different particle sizes as described above is used in the secondary battery provided by the present application, which has excellent tensile strength and elongation at break, and has excellent plasticity while improving the mechanical strength, thereby reducing the probability of fracture or crack of the copper foil current collector in the high expansion system, which is beneficial to improving the service life of the secondary battery when the electrode sheet cracks or fails during operation, and is beneficial to further improving the safety and service life of the secondary battery.

[0148] In some embodiments, the thickness expansion rate of the secondary battery is 4%-10%.

[0149] In some embodiments, the secondary battery is a wound type secondary battery, and the expansion force of the secondary battery is greater than or equal to 1000 kgf; the wound type secondary battery comprises a negative electrode sheet, the negative electrode sheet comprises a negative electrode current collector and a negative electrode film layer arranged on at least one side surface of the negative electrode current collector, the negative electrode current collector comprises a copper foil, the copper foil comprises copper grains with different particle sizes, the copper grains comprise copper grains with a particle size less than or equal to 0.5 μm and copper grains with a particle size greater than 0.5 μm, wherein the proportion of the number of copper grains with a particle size less than or equal to 0.5 μm in the total number of copper grains is 70%-95%, and the proportion of the number of copper grains with a particle size greater than 0.5 μm in the total number of copper grains is 5%-30%.

[0150] The current collector of the negative electrode tab in the wound secondary battery is prone to crack fracture in the corner area. This is because the wound battery is pressed and shaped after winding, which can cause irreversible damage to the current collector. The probability of crack or fracture of the negative electrode tab outer corner area and inner corner area under the expansion stress caused by the increase of the internal pressure of the secondary battery and the volume expansion is greatly increased, which aggravates the risk of battery failure and worsens the service life and safety of the secondary battery. The copper foil provided in the present application includes copper grains of different particle sizes, which significantly improves the tensile strength and elongation at break of the copper foil, making the copper foil have excellent mechanical properties and plasticity, reducing the probability of crack or fracture of the copper foil under expansion stress, reducing the probability of crack fracture of the wound battery tab, and improving the service life of the secondary battery when the tab cracks or fails, thereby further improving the safety and service life of the secondary battery.

[0151] In some embodiments, the thickness expansion rate of the wound secondary battery is 4%-10%. Increasing the amount of active material is one of the effective means to improve the energy density or capacity of the secondary battery, but it will cause the volume of the battery to increase. The applicant found that using high-gram-capacity negative electrode active material can significantly improve the available lithium intercalation capacity of the negative electrode active material, improve the energy density or capacity of the secondary battery, and reduce the impact on the volume of the battery. However, the high-gram-capacity negative electrode active material has a large volume expansion before and after lithium ion intercalation / deintercalation, and the expansion stress acts on the current collector copper foil, increasing the probability of fracture or crack of the copper foil along the thickness direction under the expansion stress, and worsening the safety and service life of the secondary battery.

[0152] Based on this, another embodiment of the present application provides a secondary battery, which includes a negative electrode tab, the negative electrode tab includes a negative electrode current collector and a negative electrode film layer arranged on at least one side surface of the negative electrode current collector, the negative electrode current collector includes a copper foil, the copper foil includes copper grains of different particle sizes, the copper grains include copper grains with a particle size less than or equal to 0.5 μm and copper grains with a particle size greater than 0.5 μm, wherein the number of copper grains with a particle size less than or equal to 0.5 μm accounts for 70%-95% of the total number of copper grains, and the number of copper grains with a particle size greater than 0.5 μm accounts for 5%-30% of the total number of copper grains; and the gram capacity of the negative electrode active material in the negative electrode film layer is 800 mAh / g-1500 mAh / g.

[0153] The copper foil provided in the present application includes copper grains of different particle sizes, which has excellent tensile strength and elongation at break, improves the mechanical strength while having excellent plasticity, and forms a good match with high-gram-capacity negative electrode active material, improves the energy density and capacity of the secondary battery, and improves the safety and service life of the secondary battery.

[0154] In some embodiments, the gravimetric capacity of the negative active material in the negative electrode film layer is 800 mAh / g-1500 mAh / g.

[0155] The copper foil provided by the present application is particularly suitable for a negative active material system with a gravimetric capacity of 800 mAh / g-1500 mAh / g. High gravimetric capacity negative active materials can embed more active ions, such as silicon-based negative electrodes, alkali metal negative electrodes, etc., which generally have high swelling properties, which can increase the probability of fracture or cracking of the copper foil in the battery. The copper foil has excellent mechanical properties and plasticity, which can reduce the risk of fracture or cracking of the copper foil in high-energy density battery systems or high-swelling battery systems.

[0156] Without wishing to be bound by any theory, the difference in work hardening capacity between large grains and fine grains is large, and the heterogeneous grain morphology of a small amount of large grains doped in fine grains makes the strain partition in the initial stage of plastic deformation of the copper foil more obvious. This is because the moderate doping of large grains produces a higher density of geometrically necessary dislocations (GND) than fine grains during inhomogeneous plastic deformation. The bending of the crystal plane of the copper foil during inhomogeneous plastic deformation can produce dislocations, which are called geometrically necessary dislocations (GND). Geometrically necessary dislocations can coordinate the plastic strain caused by deformation and maintain the continuity of the material, which helps to reduce the occurrence of concentrated stress during the deformation of the copper foil, making the copper foil exhibit stronger strain localization inhibition ability and improving the plasticity of the copper foil. In addition, in the copper foil crystal, the contact interface between the grains is called the grain boundary. It can be understood that in the same area, the smaller the grain size 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 deformation, that is, the higher the mechanical strength. At the same time, the production of a higher density of geometrically necessary dislocations during the deformation of the copper foil helps to reduce the concentration of stress, which can improve the mechanical strength.

[0157] 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.

[0158] 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 range between any two of the above values or any value within the range.

[0159] In the present disclosure, the grain size of the copper grains with a grain size of less than or equal to 0.5 μm ranges from greater than or equal to 0.1 μm to less than or equal to 0.5 μm.

[0160] In some embodiments, the number of copper grains with a grain size of greater than 0.5 μm accounts for 5%-20%, 2%-20%, 2%-15%, 5%-15%, 5%-13%, 6%-12% or 7%-10% of the total number of copper grains, which helps to further improve the fracture elongation and optimize the plasticity of the copper foil.

[0161] In some embodiments, the number of copper grains with a grain size of greater than 0.5 μm accounts for 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 18%, 20%, 25%, 30% or any range or value between any two of the above values.

[0162] In the present disclosure, the grain size of the copper grains with a grain size of greater than 0.5 μm ranges from greater than 0.5 μm to less than or equal to 3 μm, which helps to reduce or optimize the average grain size of the copper foil and improve the tensile strength of the copper foil.

[0163] The number and grain 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 grain size of the grains can be counted by the imageJ analysis software of the Oxford C-Nano+ electron backscatter diffraction instrument, the equivalent circle diameter of the grains can be taken as the grain size, a number distribution map can be made, a skew distribution can be used for fitting, and the grain size and the number ratio of the grains in different grain size intervals can be obtained.

[0164] In some embodiments, the number of copper grains with a grain size of greater than 0.5 μm and less than or equal to 3 μm accounts for 5%-30% of the total number of copper grains.

[0165] As described above, the copper grains with a grain size of greater than 0.5 μm and less than or equal to 3 μm can help to reduce the average grain size of the copper foil and further improve the mechanical strength of the copper foil. Further, the number of the copper grains with a grain size of greater than 0.5 μm and less than or equal to 3 μm accounts for 5%-30%, which can make the copper foil have a good fracture elongation, excellent mechanical properties and plasticity, reduce the risk of fracture or crack of the copper foil in a high-energy-density battery system or a high-expansion battery system, and realize the simultaneous improvement of the energy density and safety of the secondary battery.

[0166] In some embodiments, the gravimetric capacity of the negative active material can be selected from 800 mAh / g, 820 mAh / g, 850 mAh / g, 880 mAh / g, 900 mAh / g, 920 mAh / g, 950 mAh / g, 980 mAh / g, 1000 mAh / g, 1020 mAh / g, 1050 mAh / g, 1080 mAh / g, 1100 mAh / g, 1120 mAh / g, 1150 mAh / g, 1180 mAh / g, 1200 mAh / g, 1250 mAh / g, 1300 mAh / g, 1350 mAh / g, 1400 mAh / g, 1450 mAh / g, 1500 mAh / g, or a range between any two of the above values or any value within the range.

[0167] As used herein, the gravimetric capacity of the negative active material can be determined by using instruments and methods known in the art. For example, the gravimetric capacity of the negative active material can be determined by the following method: the negative active material, carbon black, and polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 91.6:1.8:6.6 in an appropriate amount of solvent NMP to form a uniform negative electrode slurry; the negative electrode slurry is uniformly coated on the surface of a negative 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, and an electrolyte is injected, wherein the electrolyte formulation used is as follows: dimethyl carbonate (DMC), methyl ethyl carbonate (EMC), and ethylene carbonate (EC) are mixed in a weight ratio of 1:1:1 to obtain an organic solvent, and then LiPF6 is dissolved in the organic solvent to prepare an electrolyte with a concentration of 1.0 mol / L. A CR2430 button cell is assembled in an argon glove box. At 25°C, charge at a rate of 0.1C to an upper limit cutoff voltage of 3.8V, and then charge at a constant voltage until the current is less than 0.05C; after standing for 30 min, discharge at a rate of 0.1C to a lower limit cutoff voltage of 2.0V, and record the first discharge capacity as Cm, then the gravimetric capacity of the negative active material = discharge capacity Cm / mass of the negative active material.

[0168] It can be understood that the gravimetric capacity of the negative active material can also be obtained by disassembling the battery, obtaining the negative electrode sheet, and then testing the button cell assembled according to the method described above.

[0169] 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.

[0170] The arrangement of the short diameter of at least part of the copper grains with a particle size greater than 0.5 pm along the thickness direction of the current collector indicates that the internal grain boundary of the copper foil is highly tortuous, and the energy required for the copper foil to break through the grain boundary (i.e., the thickness direction of the current collector) is also greater, which helps to reduce the probability of the copper foil breaking along the thickness direction, improve the brittleness of the current collector, further reduce the risk of copper foil breaking or cracking, and improve the safety of the secondary battery.

[0171] It can be understood that the arrangement direction of the short diameter of at least part of the copper grains with a particle size greater than 0.5 pm can be determined by referring to the EBSD and scanning electron microscopy described above for the characterization of the short diameter of the copper grains and the arrangement direction.

[0172] In some embodiments, the average particle size of the copper grains is 0.3 pm to 1.2 pm. In some embodiments, the average particle size of the copper grains is 0.3 pm to 0.6 pm. In some embodiments, the average particle size of the copper grains is 0.3 pm to 0.5 pm. In some embodiments, the average particle size of the copper grains is 0.3 pm, 0.35 pm, 0.4 pm, 0.45 pm, 0.5 pm, 0.55 pm, 0.6 pm, 0.65 pm, 0.7 pm, 0.75 pm, 0.8 pm, 0.85 pm, 0.9 pm, 1.0 pm, 1.1 pm, 1.2 pm, or any value between any two of the above values or any value within the range.

[0173] The suitable particle size range is theoretically beneficial for the copper foil to obtain suitable grain boundaries, which can make the copper foil have suitable resistance to grain dislocation movement and deformation resistance, and thus can optimize the mechanical strength of the copper foil.

[0174] The average particle size of the grains can be tested by methods known in the art. As an example, the cross section of the copper foil is measured by electron backscatter diffraction (EBSD) and scanning electron microscopy, the inverse pole figure distribution map is obtained, the number and particle size of the grains are counted by the imageJ analysis software matched with the Oxford C-Nano+ electron backscatter diffraction instrument, the equivalent circle diameter of the grains is taken as the grain size, the number distribution map is made, the skewness distribution is used for fitting, and the average particle size of the grains is obtained.

[0175] In some embodiments, the maximum particle size of the copper grains is 1 pm to 2.5 pm. In some embodiments, the maximum particle size of the copper grains is 1.2 pm to 1.8 pm or 1.2 pm to 2.0 pm. In some embodiments, the maximum particle size of the copper grains is 1 pm, 1.2 pm, 1.4 pm, 1.6 pm, 1.8 pm, 2 pm, or any value between any two of the above values or any value within the range.

[0176] The maximum grain size of the copper grains can be tested by methods known in the art, for example, by measuring the cross section of the copper foil using an electron backscatter diffraction instrument (EBSD) and a scanning electron microscope, obtaining a pole figure distribution map, counting the number and size of the grains using the imageJ analysis software provided with the Oxford C-Nano+ electron backscatter diffraction instrument, taking the equivalent circle diameter of the grains as the grain size, making a number distribution map, and fitting the map using a skew distribution. The maximum grain size in the statistical results is the maximum grain size of the copper grains.

[0177] In some embodiments, the minimum grain size of the copper grains is 0.1 pm to 0.3 pm. In some embodiments, the minimum grain size of the copper grains is 0.1 pm to 0.3 pm. In some embodiments, the minimum grain size of the copper grains is 0.1 pm, 0.15 pm, 0.2 pm, 0.25 pm, 0.3 pm, or a range between any two of the above values or any value within the range.

[0178] The minimum grain size of the copper grains can be tested by methods known in the art, for example, by measuring the cross section of the copper foil using an electron backscatter diffraction instrument (EBSD) and a scanning electron microscope, obtaining a pole figure distribution map, counting the number and size of the grains using the imageJ analysis software provided with the Oxford C-Nano+ electron backscatter diffraction instrument, taking the equivalent circle diameter of the grains as the grain size, making a number distribution map, and fitting the map using a skew distribution. The minimum grain size in the statistical results is the minimum grain size of the copper grains.

[0179] 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 grains. In some embodiments, the grain size span of the copper grains is 0.8 pm to 2.5 pm. In some embodiments, the grain size span of the copper grains is 0.8 pm to 2 pm. In some embodiments, the grain size span of the copper grains is 1 pm to 2 pm. In some embodiments, the grain size span of the copper grains is 0.8 pm, 1 pm, 1.2 pm, 1.4 pm, 1.6 pm, 1.8 pm, 2.0 pm, 2.2 pm, 2.4 pm, 2.5 pm, or a range between any two of the above values or any value within the range.

[0180] 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.

[0181] In some embodiments, the copper foil has a tensile strength of 600-1000 MPa under the test condition of room temperature (20±10°C), sample length x width of (50±0.25 mm) x (15±0.25 mm), and a tensile speed of 50±0.5 mm / min. In some embodiments, the copper foil has a tensile strength of 700-1000 MPa. In some embodiments, the copper foil has a tensile strength of 700-800 MPa.

[0182] 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±0.25 mm) x (15±0.25 mm), and a tensile speed of 50±0.5 mm / 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%.

[0183] 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.

[0184] 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 to the original length, usually expressed in percentage, which is an important parameter for measuring the deformation ability of a material under stress during stretching.

[0185] 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±0.25 mm and a width of 15±0.25 mm are cut, and the test samples are continuously loaded until they break at room temperature (20±10°C) with a tensile speed of 50±0.5 mm / min, and the maximum load is divided by the cross-sectional area of the test sample to obtain the tensile strength of the test sample. The cross-sectional area of the test sample can be calculated by dividing the mass of the test sample by the product of the length of the test sample and the density. The elongation at break can be calculated according to the displacement method after the above test. The test area refers to the detection area during instrument testing, and the length and width of the test sample can be greater than the length and width of the test area, considering that the test sample may need to be fixed by a clamp during testing.

[0186] In some embodiments, the copper foil has a tensile strength of 600 MPa, 650 MPa, 700 MPa, 750 MPa, 800 MPa, 850 MPa, 900 MPa, 950 MPa, 1000 MPa, or any value between any two of the foregoing values or any value within a range defined by any two of the foregoing values, under the test conditions of room temperature (20 ± 10°C), sample length x width of (50 ± 0.25 mm) x (15 ± 0.25 mm), and a tensile speed of 50 ± 0.5 mm / min.

[0187] 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 any value between any two of the foregoing values or any value within a range defined by any two of the foregoing values, under the test conditions of room temperature (20 ± 10°C), sample length x width of (50 ± 0.25 mm) x (15 ± 0.25 mm), and a tensile speed of 50 ± 0.5 mm / min.

[0188] The copper foil having the tensile 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.

[0189] In some embodiments, the copper foil has a hardness of 55 HV-65 HV. In some embodiments, the copper foil has a hardness of 55 HV-60 HV. In some embodiments, the copper foil has a hardness of 55 HV, 56 HV, 57 HV, 58 HV, 59 HV, 60 HV, 61 HV, 62 HV, 63 HV, 64 HV, 65 HV, or any value between any two of the foregoing values or any value within a range defined by any two of the foregoing values.

[0190] The hardness can reflect the pressure deformation or puncture resistance of the copper foil. In the preparation process of the secondary battery, the surface quality of the copper foil can affect the bonding performance of the negative electrode film layer and the copper foil, especially in the cold pressing process step of the secondary battery, the negative active material particles press the copper foil under external pressure, and a suitable hardness is conducive to reducing the surface damage of the copper foil and reducing the influence on the bonding performance of the negative electrode film layer and the copper foil.

[0191] In some embodiments, the copper foil has a thickness of 4 μm-10 μm. In some embodiments, the copper foil has a thickness of 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, or any value between any two of the foregoing values or any value within a range defined by any two of the foregoing values.

[0192] In the present application, the thickness of the copper foil can be tested by methods known in the art. As an example, a sample of 20x15 cm2is cut, the cut sample is placed on an electronic balance to weigh, and the weight of the sample is obtained. Then, according to the density p of the copper foil of 8.96 g / cm3, the thickness of the sample can be calculated. 3 The volume of the sample is calculated, and the length and width of the sample are known, so the thickness of the sample can be calculated.

[0193] The maximum load that can be carried by the ordinary strength copper foil after thinning decreases sharply, the thickness of the copper foil that can be used for plastic deformation is severely reduced, resulting in a significant decrease in the tensile strength and elongation at break of the copper foil, and fatigue fracture is prone to occur in the later stage of the secondary battery cycle, which may cause safety accidents. The copper foil provided in the present application still has excellent tensile strength and elongation at break in the thickness range of 4-10 pm, and has good mechanical properties and plasticity. The copper foil can thin the thickness without affecting the strength, which is helpful for the lightweight design of the battery, and helps to reduce the weight of the battery and further improve the energy density or specific capacity of the secondary battery.

[0194] In some embodiments, the expansion force of the secondary battery is greater than or equal to 1000 kgf.

[0195] In some embodiments, the expansion force of the secondary battery is greater than or equal to 2500 kgf.

[0196] In some embodiments, the expansion force of the secondary battery is greater than or equal to 4000 kgf.

[0197] In some embodiments, the expansion force of the secondary battery is 1000-10000 kgf.

[0198] 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.

[0199] 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.

[0200] 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.

[0201] In some embodiments, the thickness swelling rate of the secondary battery is 4%-10%.

[0202] 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.

[0203] 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.

[0204] 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 leads to 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 secondary batteries with high thickness expansion rate, and is conducive to the further improvement of the energy density and safety of the secondary battery.

[0205] 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, the number proportion of the copper grains with a particle size of less than or equal to 0.5 μm being 70%-95% based on the total number of the copper grains; the maximum particle size of the copper grains being 1.2 μm-3 μm; the average particle size of the copper grains being 0.04 μm-0.6 μm; and the number proportion of the grains of nanotwins being greater than or equal to 45% based on the total number of the copper grains.

[0206] In the present document, 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.

[0207] In the present document, the term "pulse current" refers to a current or voltage pulse that appears repeatedly in a cycle.

[0208] 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.

[0209] 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.

[0210] In some embodiments, the preparation method of the secondary battery comprises preparing a pole piece using a copper foil as a current collector, the preparation of the copper foil comprises preparing the copper foil by electroplating, the electroplating comprises applying a pulse current to an electroplating solution to reduce and deposit copper ions in the electroplating solution to form the copper foil, the peak value of the pulse current is 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.

[0211] 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 stress when the secondary battery is expanded due to the volume expansion of the pole piece, the decomposition of the electrolyte, the increase of the internal pressure, and other factors, and is beneficial to further improve the safety and service life of the secondary battery.

[0212] In some embodiments, the preparation method of the secondary battery comprises preparing a wound-type secondary battery, preparing a pole piece using a copper foil as a current collector, the preparation of the copper foil comprises preparing the copper foil by electroplating, the electroplating comprises applying a pulse current to an electroplating solution to reduce and deposit copper ions in the electroplating solution to form the copper foil, the peak value of the pulse current is 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.

[0213] 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 stress when the secondary battery is expanded due to the volume expansion of the pole piece, the decomposition of the electrolyte, the increase of the internal pressure, and other factors, and is beneficial to further improve the safety and service life of the secondary battery.

[0214] 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.

[0215] 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 aforementioned ranges.

[0216] 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.

[0217] 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 aforementioned ranges.

[0218] The applicant found that in the process of preparing copper foil by electroplating method, the current size is directly related to the nucleation and growth of copper grains, the smaller the current, the more conducive to promoting the nucleation and growth of grains, so that larger copper grains are generated in the copper foil; the larger the current, the more conducive to the rapid reduction and nucleation of copper ions, so that smaller copper grains exist in the copper foil. Further controlling the peak value and valley value of the pulsed current in the electroplating method to meet the above ranges is conducive to promoting the formation of a grain morphology of "mainly fine grains mixed with large grains", while taking into account the existence of a certain proportion of nanotwins, realizing the improvement of the tensile strength and plasticity of the material, and providing a material basis for improving the service life and safety of secondary batteries.

[0219] 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 aforementioned ranges.

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

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

[0222] During the deposition of copper ions, increasing the current can accelerate the deposition rate of copper ions, which helps to form fine grains with small particle size; reducing the current can reduce the deposition rate of copper ions, which helps to form relatively regular and orderly grains with large particle size, such as columnar crystals. Adjusting the change parameters of the current, i.e. the peak value, the valley value and the change period, helps to adjust the formation of copper grain nuclei, the growth rate of grains and the deposition time of copper ions, to produce a heterogeneous grain morphology with mixed distribution of large and small grains, and to adjust the particle size and quantity ratio of the grains, thereby improving the tensile strength and elongation at break of the copper foil, so that the copper foil has excellent mechanical strength and plasticity.

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

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

[0225] In some embodiments, the preparation method is a continuous production method.

[0226] 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.

[0227] In some embodiments, the cathode electrode is a titanium roller or a titanium plate.

[0228] In some embodiments, the anode electrode is a titanium substrate plate.

[0229] In some embodiments, the distance between the cathode electrode and the anode electrode is 8 mm to 20 mm. In some embodiments, the distance between the cathode electrode and the anode electrode is 8 mm to 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 a range between any two of the above values or any value between the ranges.

[0230] 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 above values or any value between the ranges.

[0231] In some embodiments, the speed of the cathode roller is 2 m / min to 5 m / min. In some embodiments, the speed of the cathode roller is 2 m / min to 3 m / min. In some embodiments, the speed of the cathode roller is 2 m / min, 2.5 m / min, 3 m / min, 3.5 m / min, 4 m / min, 4.5 m / min, 5 m / min, or a range between any two of the above values or any value between the ranges.

[0232] The cathode roller can be any roller suitable for the preparation of copper foil in the art, for example, a titanium roller.

[0233] In some embodiments, the peak current density of the pulse current is I, in A / dm 2 ; the duty cycle of the pulse current is s, and the peak current density I and the duty cycle s satisfy: 2 A / dm 2 ≤ I x s ≤ 18 A / dm 2 .

[0234] In this document, the term "duty cycle of the pulse current" refers to the proportion of the current on time in the entire pulse cycle.

[0235] In some embodiments, the product I x s of the peak current density I and the duty cycle s is 2 A / dm 2 , 4 A / dm 2 , 6 A / dm 2 , 8 A / dm 2 , 10 A / dm 2 , 12 A / dm 2 , 14 A / dm 2 , 16 A / dm 2 , 18 A / dm 2or a numerical range between any two of them.

[0236] The product of the peak current density I and the duty cycle s corresponds to the average density of the current over the pulse period. The way the pulse current is deposited is such that the average density of the current over the pulse period is higher than the maximum current density that can be achieved in direct current deposition, because too high a direct current deposition density would result in hydrogen evolution and concentration polarization of the electrolyte, reducing the uniformity of the copper foil in the electro-deposition process. The product of the peak current density I and the duty cycle s in the above range enables a dynamic balance of the consumption and replenishment of copper ions in the electrolyte by controlling the switching on and off of the current, reducing the concentration difference between the site of copper ion deposition and other sites in the electrolyte, so that the electrolyte does not experience significant concentration polarization, and improving the uniformity of the grain size in the thickness direction of the prepared copper foil. Moreover, the product of the peak current density I and the duty cycle s in the above range can control the nucleation and growth rate of the grains, enabling the preparation of small-grained grains to improve the mechanical strength of the copper foil.

[0237] In some embodiments, the peak current density I of the pulse current satisfies: 3.3 A / dm 2 ≤ I ≤ 333 A / dm 2 .

[0238] In some embodiments, the peak current density I of the pulse current is 3.3 A / dm 2 , 8.35 A / dm 2 , 10 A / dm 2 , 100 A / dm 2 , 150 A / dm 2 , 167 A / dm 2 , 180 A / dm 2 , 200 A / dm 2 , 300 A / dm 2 , 333 A / dm 2 or a numerical range between any two of them. In some embodiments, the peak current density I of the pulse current satisfies: 100 A / dm 2 ≤ I ≤ 180 A / dm 2 .

[0239] In some embodiments, the duty cycle s of the pulse current is 2%-50%.

[0240] In some embodiments, the duty cycle s of the pulse current is 2%, 3.30%, 5%, 10%, 20%, 30%, 40%, 50% or a numerical range between any two of them. In some embodiments, the duty cycle s of the pulse current is 2%-10%.

[0241] In some embodiments, the electroplating solution includes a leveling agent, a wetting agent, and a brightener.

[0242] As used herein, the term "leveler" refers to a substance added to the electroplating solution that improves the flatness of the plated layer, which can attach to the tip of the copper foil with a fast deposition rate, inhibit grain growth, balance the growth rate of the pits and the tip, and improve the flatness of the copper foil.

[0243] As used herein, the term "wetting agent" refers to a substance used to reduce the interfacial tension between the electroplating solution and the electrode, and to improve the adhesion of the plated layer to the substrate. The wetting agent can improve the wettability of the electroplating solution to the substrate, and the wetting of the electroplating solution on the cathode is sufficient to enable fast electrodeposition at a large current, to improve the nucleation rate of the copper foil, and to reduce the grain size of the copper foil.

[0244] As used herein, the term "brightener" refers to a substance that improves the smoothness of the plated layer and reduces the surface roughness. The brightener can make the grain size of the copper foil smaller and reduce the surface roughness of the copper foil, and improve the smoothness of the surface.

[0245] In some embodiments, the leveler includes one or more of collagen, sodium saccharin. In some embodiments, the leveler includes collagen and sodium saccharin. In some embodiments, the leveler further includes one or more of gelatin, gentian violet.

[0246] Without being bound by any theory, collagen can inhibit the deposition of copper ions and balance the growth rate of the pits and the tip. The introduction of sodium saccharin can attract copper ions to deposit at the depressions on the surface of the copper foil, reduce the microscopic defects or unevenness inside the copper foil, and reduce the warpage defects caused by the defects. The two different levelers help to further improve the surface pits and protrusions of the copper foil and improve the flatness of the copper foil.

[0247] In some embodiments, the concentration of collagen in the electroplating solution is 60 mg / L to 300 mg / L. In some embodiments, the concentration of collagen in the electroplating solution is 80 mg / L to 150 mg / L. In some embodiments, the concentration of collagen in the electroplating solution is 60 mg / L, 80 mg / L, 100 mg / L, 150 mg / L, 200 mg / L, 250 mg / L, 300 mg / L, or any two of the above values or any value between the ranges.

[0248] The collagen can be a protein with a molecular weight commonly used in the field of copper foil, for example, collagen with a relative molecular weight of 8000-12000.

[0249] In some embodiments, the concentration of sodium saccharin in the electroplating solution is 0.5 g / L to 10 g / L. In some embodiments, the concentration of sodium saccharin in the electroplating solution is 0.5 g / L to 4 g / L. In some embodiments, the concentration of sodium saccharin in the electroplating solution is 0.5 g / L, 0.8 g / L, 1 g / L, 1.5 g / L, 2.0 g / L, 2.5 g / L, 3.0 g / L, 3.5 g / L, 4.0 g / L, 4.5 g / L, 5.0 g / L, 5.5 g / L, 6.0 g / L, 6.5 g / L, 7.0 g / L, 7.5 g / L, 8.0 g / L, 8.5 g / L, 9.0 g / L, 9.5 g / L, 10 g / L, or a range between any two of the foregoing values or any value between the ranges.

[0250] Without being bound by any theory, hydroxyethyl cellulose has good water solubility and thickening properties, and can form a uniform solution in water, increase the viscosity of the electroplating solution, and adhere to the surface of the copper foil substrate. Polyethylene glycol, as a lubricant and wetting agent, can reduce the surface tension of the liquid and enhance the wetting ability of the liquid to the surface of the solid. The combination of the two can improve the adhesion properties of the electroplating solution, increase and assist the adhesion of copper ions to the surface of the substrate and deposition, improve the consistency of the crystal grains during the deposition process, reduce the difference in crystal grain size in the thickness direction, improve the uniformity of the copper foil, and thus improve the mechanical properties of the copper foil.

[0251] In some embodiments, the concentration of polyethylene glycol in the electroplating solution is 50 mg / L to 200 mg / L. In some embodiments, the concentration of polyethylene glycol in the electroplating solution is 60 mg / L to 150 mg / L. In some embodiments, the concentration of polyethylene glycol in the electroplating solution is 50 mg / L, 60 mg / L, 70 mg / L, 80 mg / L, 100 mg / L, 150 mg / L, 200 mg / L, or a range between any two of the foregoing values or any value between the ranges.

[0252] Polyethylene glycol can be selected from commonly used molecular weights in the field of copper foil, for example, polyethylene glycol with a relative molecular weight of 4000.

[0253] 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 values.

[0254] The polyethylene glycol can be selected from those commonly used in the copper foil industry. In some embodiments, the relative molecular mass of the hydroxyethyl cellulose is 120,000.

[0255] 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 examples, 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 values.

[0256] In some embodiments, the brightener includes sodium polydi thio- propane sulfonate. In some embodiments, the brightener further includes one or more of sodium 3-mercapto-1-propane sulfonate, thiourea.

[0257] Without being bound by any theory, the sodium polydi thio- propane sulfonate is adsorbed on the cathode copper surface through the mercapto functional group or disulfide bond, the terminal sulfonate anion captures the hydrated copper ions in the electroplating solution to destroy its hydration, and interacts with the chloride ions adsorbed on the cathode surface, so that the electrons are transferred to the captured copper ions through the chloride ions, thereby greatly improving the electrochemical reduction rate of the copper ions, refining the grain size and achieving material strengthening.

[0258] In some embodiments, the concentration of sodium poly(dithiopropyl sulfone) in the electroplating solution is 500 mg / L-2000 mg / L. In some embodiments, the concentration of sodium poly(dithiopropyl sulfone) in the electroplating solution is 500-1000 mg / L. In some embodiments, the concentration of sodium poly(dithiopropyl sulfone) in the electroplating solution is 500 mg / L, 600 mg / L, 700 mg / L, 800 mg / L, 900 mg / L, 1000 mg / L, 1500 mg / L, 2000 mg / L, or any range between any two of the above values or any value between the ranges. The brightener used in the preparation method of the embodiments of the present application satisfies the above concentration, which is conducive to improving the electrochemical reduction rate of copper ions, achieving grain refinement, increasing the number of grains with a particle size of less than or equal to 0.5 μm, and increasing the proportion of small grains in the copper foil substrate to 70%-95%, thereby improving the tensile strength of the copper foil substrate. The applicant has further found that the concentration of the brightener satisfies the above range, which is conducive to promoting the co-growth of grains with a particle size of less than or equal to 0.5 μm along the same common crystal face to form a twin crystal structure, further increasing the proportion of nanotwin grains in the copper foil substrate, and helping to improve the dislocation motion resistance and further improve the strength and plasticity of the material.

[0259] In some embodiments, the electroplating solution includes chloride ions, and the concentration of chloride ions (in terms of chlorine atoms) is 20 mg / L-80 mg / L. In some embodiments, the concentration of sodium poly(dithiopropyl sulfone) in the electroplating solution is 40 mg / L-80 mg / L. In some embodiments, the concentration of chloride ions (in terms of chlorine atoms) is 20 mg / L, 30 mg / L, 40 mg / L, 50 mg / L, 60 mg / L, 70 mg / L, 80 mg / L, or any range between any two of the above values or any value between the ranges. The use of chloride ions in combination with the wetting agent can further improve the electrodeposition process.

[0260] In some embodiments, the electroplating 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 50-200 mg / L, hydroxyethyl cellulose with a concentration of 30 mg / L-200 mg / L, sodium poly(dithiopropyl sulfone) with a concentration of 500 mg / L-2000 mg / L, and chloride ions with a concentration of 20 mg / L-80 mg / L.

[0261] In some embodiments, the electroplating solution comprises: 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 polydithiobispropyl sulfone at a concentration of 500 mg / L to 1000 mg / L, chloride ions at a concentration of 40 mg / L to 80 mg / L.

[0262] The electroplating solution also comprises a copper source to provide copper ions to the electroplating solution. The electroplating solution also comprises sulfuric acid to provide an acidic environment for reduction of the copper ions.

[0263] In some embodiments, the concentration of copper ions in the electrolyte is 30 g / L to 100 g / L. In some embodiments, the concentration of copper ions in the electrolyte is 45 g / L to 75 g / L.

[0264] In some embodiments, the concentration of copper ions in the electrolyte is 30 g / L, 40 g / L, 50 g / L, 60 g / L, 70 g / L, 80 g / L, 90 g / L, 100 g / L, or any range between any two of these values.

[0265] The results of the study show that a low concentration of copper ions in combination with a high concentration of additives is beneficial to improving the strength and elongation of the copper foil compared to a high concentration of copper ions in combination with a low concentration of additives. Moreover, a low concentration of copper ions is helpful to control the growth of the grains and the deposition rate, which is beneficial to preparing small size grains and improving the mechanical strength of the copper foil.

[0266] 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, or any range between any two of these values, or any value between the ranges.

[0267] 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, or any range between any two of these values, or any value between the ranges.

[0268] In some embodiments, the pH of the electroplating solution is 2.5 to 4.5, such as 2.5, 3.0, 3.5, 4.0, 4.5, or any range between any two of these values, or any value between the ranges.

[0269] 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.

[0270] [Negative electrode sheet]

[0271] As an example of the negative electrode sheet, the negative current collector has two opposite surfaces in the thickness direction of itself, and the negative film layer is provided on any one or both of the two opposite surfaces of the negative current collector.

[0272] 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.

[0273] 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.

[0274] 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.

[0275] In some embodiments, the mass content of the silicon-based material is 5%-100% based on the total mass of the negative film layer.

[0276] In some embodiments, the mass content of the silicon-based material is 10%-80% based on the total mass of the negative film layer.

[0277] In some embodiments, the mass content of the silicon-based material is 10%-30% based on the total mass of the negative film layer.

[0278] In some embodiments, the mass content of the silicon-based material can be selected as 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 100%, or any numerical range between any two of them, based on the total mass of the negative film layer.

[0279] In some embodiments, the mass content of silicon in the silicon-based material is 20%-50%.

[0280] In some embodiments, the silicon element accounts for 20%, 25%, 30%, 35%, 40%, 45%, 50% or any numerical range between any two of the aforementioned percentages by mass in the silicon-based material.

[0281] In some embodiments, the silicon element accounts for 4%-10% by mass based on the total mass of the negative electrode film layer.

[0282] In some embodiments, the silicon element accounts for 4%, 5%, 6%, 7%, 8%, 9%, 10% or any numerical range between any two of the aforementioned percentages by mass based on the total mass of the negative electrode film layer.

[0283] In some embodiments, the negative electrode film layer further optionally comprises a binder. The binder can be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA) and carboxymethyl chitosan (CMCS).

[0284] In some embodiments, the negative electrode tab further comprises a conductive agent. The conductive agent comprises one or more of super conductive carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers.

[0285] In some embodiments, the conductive agent comprises carbon black. In some embodiments, the conductive agent comprises carbon nanotubes. In some embodiments, the conductive agent comprises carbon black and carbon nanotubes. The conductive agent is widely available and has excellent conductivity, which is conducive to controlling the manufacturing cost of the secondary battery and improving the conductivity of the negative electrode tab.

[0286] In some embodiments, the negative electrode film layer further optionally comprises other auxiliary agents, such as thickening agents (e.g., sodium carboxymethyl cellulose (CMC-Na)) and the like.

[0287] In some embodiments, the negative electrode tab can be prepared by dispersing the aforementioned components for preparing the negative electrode tab, such as the negative electrode active material, the conductive agent, the binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry on a negative electrode current collector, and after processes such as drying and cold pressing, the negative electrode tab can be obtained.

[0288] [Positive electrode tab]

[0289] The positive electrode tab comprises a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector, the positive electrode film layer comprising a positive electrode active material.

[0290] By way of example, the positive electrode current collector has two opposite surfaces in the thickness direction of the positive electrode current collector, and the positive electrode film layer is disposed on either one or both of the two opposite surfaces of the positive electrode current collector.

[0291] In some embodiments, the positive electrode current collector can employ a metal foil or a composite current collector. For example, as the metal foil, an aluminum foil can be employed. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, 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.).

[0292] In some embodiments, the positive electrode active material can employ a positive electrode active material for a battery known in the art. As an example, the positive electrode active material can include at least one of a lithium-containing phosphate of an olivine structure, a lithium transition metal oxide, and a modified compound of each thereof. However, the present application is not limited to these materials, and other conventional materials that can be used as a positive electrode active material for a battery can also be used. These positive electrode active materials can be used alone only one 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(also can be simply referred to as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2(also can be simply referred to as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2(also can be simply referred to as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2(also can be simply referred to as NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1 O2(also can be simply referred to as NCM 811 ), lithium nickel cobalt aluminum oxide (such as LiNi 0.85 Co 0.15 Al 0.05O2) and modified compounds thereof, etc. Examples of the lithium-containing phosphate of olivine structure can include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (which can also be referred to simply as LFP)), a composite of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite of lithium manganese phosphate and carbon, lithium manganese iron phosphate, a composite of lithium manganese iron phosphate and carbon.

[0293] In some embodiments, the positive electrode film layer further optionally includes a binder. As an example, the binder can include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene-fluorotetrafluoroethylene-propylene terpolymer, a vinylidene-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin.

[0294] In some embodiments, the positive electrode film layer further optionally includes a conductive agent. As an example, the conductive agent can include at least one of super-P, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0295] In some embodiments, the positive electrode tab can be prepared by dispersing the above-mentioned components for preparing the positive electrode tab, such as the positive electrode active material, the conductive agent, the binder, and any other components, in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry on a positive electrode current collector, and after processes such as drying, cold pressing, etc., obtaining the positive electrode tab.

[0296] [Electrolyte]

[0297] The electrolyte functions to conduct ions between the positive electrode tab and the negative electrode tab. The type of electrolyte is not specifically limited in the present application and can be selected as needed. For example, the electrolyte can be liquid, gel, or all-solid.

[0298] In some embodiments, the electrolyte employs an electrolytic solution. The electrolytic solution includes an electrolyte salt and a solvent.

[0299] In some embodiments, the electrolyte salt can be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonylimide, lithium bistrifluoromethylsulfonylimide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorobisoxalate borate, lithium bisoxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorodioxalate phosphate.

[0300] 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, butanedisulfone, methyl ethyl sulfone, and diethyl sulfone.

[0301] In some embodiments, the electrolyte solution can further optionally include an additive. For example, the additive can include a negative electrode film-forming additive, a positive electrode film-forming additive, and can further include an additive capable of improving certain performance of the battery, such as an additive for improving overcharge performance of the battery, an additive for improving high-temperature or low-temperature performance of the battery, etc.

[0302] [Separator]

[0303] In some embodiments, the secondary battery further includes a separator. The type of the separator is not particularly limited in the present application, and any known porous structure separator having good chemical stability and mechanical stability can be used.

[0304] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single layer film or a multi-layer composite film, and is not particularly limited. When the separator is a multi-layer composite film, the materials of the respective layers can be the same or different, and are not particularly limited.

[0305] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be made into an electrode assembly through a winding process or a stacking process.

[0306] In some embodiments, the secondary battery can include an outer package. The outer package can be used to package the above-described electrode assembly and the electrolyte solution.

[0307] In some embodiments, the outer package of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the secondary battery can also be a soft package, such as a pouch-type soft package. The material of the soft package can be plastic, and as the plastic, polypropylene, polybutylene terephthalate, polybutylene succinate, etc. can be listed.

[0308] In some embodiments, the secondary battery includes a winding-type secondary battery, and the positive electrode sheet, the negative electrode sheet, and the separator are made into an electrode assembly through a winding process.

[0309] In some embodiments, the secondary battery includes a stacking-type secondary battery, and the positive electrode sheet, the negative electrode sheet, and the separator are made into an electrode assembly through a stacking process.

[0310] Further, the secondary battery, the battery module, the battery pack, and the electric device according to the present application will be described below with appropriate reference to the accompanying drawings.

[0311] In one embodiment of the present application, a secondary battery is provided.

[0312] The shape of the secondary battery according to the present application is not particularly limited, and can be cylindrical, square, or any other shape. For example, FIG. 1 is a secondary battery 5 of a square structure as an example.

[0313] In some embodiments, referring to FIG. 2, the outer package can include a housing 51 and a cover plate 53. The housing 51 can include a bottom plate and side plates connected to the bottom plate, which enclose a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be provided on the opening to close the receiving cavity. The positive electrode tab, the negative electrode tab, and the separator can form an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is packaged in the receiving cavity. The electrolyte is impregnated in the electrode assembly 52. The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, which can be selected by those skilled in the art according to specific actual needs.

[0314] 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.

[0315] 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.

[0316] Optionally, the battery module 4 can further include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0317] In some embodiments, the above-mentioned battery module can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0318] FIGS. 4 and 5 are battery packs 1 as examples. Referring to FIGS. 4 and 5, the battery pack 1 can include a battery box and a plurality of battery modules 4 arranged in the battery box. The battery box includes an upper box body 2 and a lower box body 3, and the upper box body 2 can be provided on the lower box body 3 to form a closed space for receiving the battery modules 4. The plurality of battery modules 4 can be arranged in the battery box in any manner.

[0319] In addition, the application also provides a power utilization device, which comprises at least one of the secondary battery, the battery module, or the battery pack provided by the application. The secondary battery, the battery module, or the battery pack can be used as a power supply of the power utilization device, or can be used as an energy storage unit of the power utilization device. The power utilization device can include a mobile device (such as a mobile phone, a notebook computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship and a satellite, an energy storage system, etc., but is not limited thereto.

[0320] As the power utilization device, the secondary battery, the battery module, or the battery pack can be selected according to the use requirements thereof.

[0321] 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.

[0322] The device as another example can be a mobile phone, a tablet computer, a notebook computer, etc. The device usually requires thinning, and the secondary battery can be used as a power supply.

[0323] Implementation

[0324] Hereinafter, the embodiments of the application are described. The embodiments described below are exemplary and are only used to explain the application, and cannot be understood as a limitation of the application. If the specific technology or condition is not indicated in the embodiments, the technology or condition described in the literature in the art or according to the product instruction is used. If the reagent or instrument used is not indicated by the manufacturer, it is a conventional product that can be obtained by purchase.

[0325] I. Performance test

[0326] (1) Grain characteristics test of copper foil

[0327] The cross section of the copper foil is observed by electron backscatter diffraction (EBSD) combined with scanning electron microscopy, wherein the electron backscatter diffraction instrument is Oxford C-Nano+. The inverse pole figure distribution map (magnification 3000) is obtained, the particle size of each grain is measured, the diameter of the equivalent circle of the grain is taken as the particle size of the grain, the grain particle size is statistically distributed, the skewness distribution is fitted, and the total number of copper grains, the average particle size, the maximum particle size, the minimum particle size, the particle size span, the number ratio of copper grains with a particle size less than or equal to 0.5 μm, and the number ratio of copper grains with a particle size greater than 0.5 μm are obtained.

[0328] (2) Mechanical property test

[0329] According to GB / T 5230-1995 "Electrolytic Copper Foil", the copper foil sample prepared in the example was cut into a tensile sample with a length L0 of 50 mm and a width of 15 mm. The tensile properties were tested at 25°C using a universal testing machine with a tensile rate of 50 mm / min.

[0330] The cross-sectional area of the tensile sample is where p is 8.96 g / cm 3 , m is in grams, and L0 is in centimeters.

[0331] The maximum load F was read from the force dial or the tensile curve, and the tensile strength σ was calculated according to Formula I. b .

[0332] The distance between the two lines after the sample was pulled apart was L1, which was measured on the sample or read from the tensile curve. L1 was measured using a straight line method or a shift method, and the elongation at break δ was calculated according to Formula II.

[0333] (3) Hardness Test

[0334] The copper foil sample was placed in a metallographic hot mounting machine, and wood powder was poured in and heated at a rate of 150°C / 10 min. A Vickers hardness tester was used to press the copper foil sample with a 50g weight, and the lengths of two diagonal lines were optically measured to obtain the corresponding Vickers hardness according to the following Vickers hardness calculation formula.

[0335] HV represents the Vickers hardness;

[0336] F represents the load of the indenter (Newton force);

[0337] a represents the included angle between the opposite faces of the indenter (136°);

[0338] d represents the average value of the diagonal line length (mm).

[0339] (4) Crack Failure Corresponding State of Health (SOH)

[0340] At 25°C, the battery was charged at 1C constant current to a voltage of 3.8V, then charged at 3.8V constant voltage until the current was ≤0.05C, then discharged the battery at 1C constant current to a voltage of 2.5V, which was one charge and discharge process, and the cycle was repeated. The SOH of the battery was monitored throughout the process, and then the battery was subjected to computer tomography (CT) every 1% SOH to determine whether cracks were generated inside the battery. If cracks were generated, the battery after crack failure during the cycle was disassembled, and the negative electrode sheet was observed to determine whether it was broken, and the SOH corresponding to the crack failure was obtained.

[0341] (5) Thickness expansion rate and expansion force test of secondary battery at crack failure:

[0342] First, the total thickness H0' of the secondary battery is measured at 25°C, then the battery is charged at 1C constant current to 3.8V, then charged at 3.8V constant voltage to current ≤0.05C, then the battery is discharged at 1C constant current to 2.5V, which is a charge and discharge process, and the cycle charge and discharge is repeated in this way, and the SOH of the battery is monitored throughout the process, then the battery is subjected to computer tomography (CT) every 1% SOH to determine whether cracks are generated inside the battery. If cracks are generated, the battery after crack failure in the cycle process is disassembled to observe whether the negative electrode sheet is broken, the SOH corresponding to crack failure is obtained, the total thickness H1' of the secondary battery at this time is measured, and the thickness expansion rate is calculated 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 arranged on both sides of the large surface of the secondary battery monomer sheet.

[0343] (6) Thickness expansion rate and expansion force test of secondary battery at 60% SOH:

[0344] First, the total thickness of the secondary battery is measured at 25°C, then the battery is charged at 1C constant current to 3.8V, then charged at 3.8V constant voltage to current ≤0.05C, then the battery is discharged at 1C constant current to 2.5V, which is a charge and discharge process, and the cycle charge and discharge is repeated in this way, and the SOH of the battery is monitored throughout the process, until the battery reaches 60% SOH, the expansion force at this time is measured by the pressure sensor in the clamp arranged on both sides of the large surface of the secondary battery monomer sheet; and the total thickness of the secondary battery at this time is measured, and the thickness expansion rate of the secondary battery at 60% SOH is obtained by (the thickness at this time - the total thickness in the initial state) / the total thickness in the initial state.

[0345] II. Preparation method example 1

[0346] (1) Preparation of copper foil

[0347] Copper plate or copper wire with purity of 99.9% or above is dissolved in sulfuric acid with mass content of 98% to obtain a copper sulfate solution, which is used as a copper source to configure an electroplating solution by adding additives and hydrochloric acid at 55°C. In the electroplating solution, the concentrations of collagen (relative molecular weight: 8000-12000), polyethylene glycol (relative molecular weight: 4000), hydroxyethyl cellulose (relative molecular weight: about 120000), chloride ion, polydithiodipropyl sulfonic acid sodium, saccharin sodium, and copper ion (calculated as copper atom) are 120 mg / L, 80 mg / L, 60 mg / L, 40 mg / L, 600 mg / L, 2 g / L, and 90 g / L respectively, and the rest is deionized water. The pH of the electroplating solution is 3.5.

[0348] A sinusoidal pulse current is used to periodically apply a current to a cathode titanium roller subjected to polishing treatment, and a titanium substrate plate is used as an anode electrode. The titanium roller has an area of 8.67 m2 in the electroplating solution, and the rotation speed (roller speed) of the titanium roller is 2.4 m / min. A sinusoidal pulse current is applied, with a peak current of 55,000 A, a peak-to-valley current of 2,500 A, a period of 3,000 ms, a cathode-to-anode distance of 10 mm, and a deposition temperature of 55°C. Copper foil with a thickness of 6 μm is deposited on the titanium roller.

[0349] (2) Preparation of the battery

[0350] A positive electrode active material LiNi 0.8 Co 0.1 Mn 0.1 O2(NCM811), a conductive agent acetylene black, and a binder polyvinylidene fluoride (PVDF) are dissolved in a solvent N-methyl pyrrolidone (NMP) at a weight ratio of 90:5:5, and then uniformly mixed by stirring to obtain a positive electrode slurry. The positive electrode slurry is uniformly coated on a positive electrode current collector, and then subjected to drying, cold pressing, and slitting to obtain a positive electrode sheet.

[0351] A negative electrode active material silicon-carbon (silicon content: 20 wt%-50 wt%), artificial graphite, a conductive agent acetylene black, a binder styrene-butadiene rubber (SBR), and a thickening agent sodium carboxymethyl cellulose (CMC-Na) are dissolved in deionized water at a weight ratio of 20:76:1:1.5:1.5, and then uniformly mixed to prepare a negative electrode slurry. The negative electrode slurry is uniformly coated on a negative electrode current collector copper foil one or more times, and then subjected to drying to obtain a negative electrode film, and then subjected to cold pressing and slitting to obtain a negative electrode sheet. The gram capacity of the negative electrode active material in the negative electrode film layer is 800 mAg / h-1500 mAg / h.

[0352] 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.

[0353] A polypropylene film was used as the separator film.

[0354] 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.

[0355] Example 2-3

[0356] The preparation method of Example 2-3 was basically the same as that of Example 1, but the composition of the plating solution (see Table 1) and the sine wave pulse current parameters (see Table 2) were adjusted; the thickness of the deposited copper foil was 6 μm.

[0357] Example 1-1

[0358] The preparation method of Example 1-1 was basically the same as that of Example 1, but direct current was used for deposition, and the deposition current was 55,000 A. The thickness of the copper foil was 6 μm.

[0359] Comparative Example 1

[0360] The preparation method of Comparative Example 1 was basically the same as that of Example 1, and direct current deposition was used with a current of 30,000 A. The thickness of the copper foil was 6 μm.

[0361] Table 1: Composition of plating solution

[0362] Table 2: Sine wave pulse current parameters

[0363] NA means not applicable.

[0364] 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.

[0365] Table 3

[0366] Wherein " / " represents that it cannot be measured.

[0367] FIG. 7, FIG. 10 and FIG. 13 respectively show the inverse pole figure maps of the cross-section of the copper foil of Example 1-3 tested by EBSD, wherein T is along the thickness direction of the current collector, the cross-section of the copper foil 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 1 (FIG. 17), more fine grains are distributed in the copper foils of Example 1-3, and the grain size is smaller in general.

[0368] FIG. 9, FIG. 12, FIG. 15 and FIG. 19 respectively show the grain size distribution of the copper foils of Example 1-3, Comparative Example 1. As can be seen from Table 3, in the copper foils of Example 1-3, the number of copper grains with a grain size of 0.5 μm or less accounts for 70%-95%, and is concentrated in the range of 80%-95%, while in Comparative Example 1, the number of copper grains with a grain size of 0.5 μm or less accounts for less than 50%, indicating that the copper grains of Example 1-3 are smaller in general, and the copper foils have higher mechanical strength. In addition, in the copper foils of Example 1-3, the number of copper grains with a grain size of more than 0.5 μm accounts for 5%-30%, while in Comparative Example 1, the number of copper grains with a grain size of more than 0.5 μm accounts for 50.5%, and the copper foils of Example 1-3 have a plasticity basically equivalent to that of the copper foil of Comparative Example 1. This indicates that the preparation of the copper foil by the sinusoidal pulse current helps to adjust the grain size distribution of the copper grains, thereby improving the mechanical properties and plasticity of the copper foil.

[0369] In the copper foils of Example 1-3, the average grain size is in the range of 0.3 μm-0.6 μm, the maximum grain size is in the range of 1 μm-2 μm, the minimum grain size is in the range of 0.1 μm-0.3 μm, and the grain size span is 0.8 μm-2 μm. In Comparative Example 1, the span of the copper grain size is larger, and the average grain size 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 foil of Comparative Example 1 has less cumulative crystal interface density, and has low mechanical strength. This also corresponds to the tensile strength of Example 1-3 and Comparative Example 1.

[0370] Further, compared with the copper foil of Comparative Example 1, the copper foils of Example 1-3 have a grain morphology of "mainly fine grains and mixed distribution of large grains", and also have a number of nanotwins greater than or equal to 45%, the nanotwins in the copper foils can improve the material strength by improving the dislocation motion resistance while maintaining the dislocation motion, which is beneficial to realizing the material strength reinforcement while maintaining good plasticity, and further utilizing the small grain size of the grains can increase the crystal interface structure of the copper foils, improve the additional stress required for the dislocation to pass through the grain boundary, and realize the material reinforcement.

[0371] Figures 8, 11 and 14 show the tensile curves of the copper foils of Examples 1-3, respectively, and it can be seen that the tensile strength of the prepared copper foils is in the range of 600 MPa-1000 MPa, and the elongation at break of the copper foils is in the range of 4%-8% (Table 3), indicating that the copper foils have excellent strength and plasticity. Figure 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 an elongation at break of only 2.7%, showing poor plasticity. Figure 18 shows the tensile curve of the copper foil of Comparative Example 1, which has an elongation at break similar to that of Example 1, but a lower tensile strength.

[0372] The hardness of the copper foils of Examples 1-3 is in the range of 55 HV-65 HV, indicating that the copper foils have good pressure deformation or puncture resistance.

[0373] Example 1-1 was prepared using a direct current with the same peak current as Example 1. Compared with Example 1, the copper foil of Example 1-1 has a tensile strength and hardness similar to those of 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 copper grains can be adjusted by the sinusoidal pulse current and current parameters, so that the copper foil has excellent plasticity while having good mechanical strength.

[0374] Compared with Comparative Example 1, the crack failure of the copper foil of Example 1-3 corresponds to a significantly reduced SOH, indicating that the copper foil prepared by Example 1-3 significantly improves the service life of the secondary battery and improves the safety of the secondary battery. Although the copper foil of Comparative Example 1 has excellent elongation at break, the tensile strength is low, resulting in a relatively high SOH value corresponding to crack failure. In comparison, the tensile strength of the copper foil prepared by Example 1-3 using a sinusoidal pulse current with varying current size is significantly improved, and the plasticity is maintained, and the SOH value corresponding to crack failure is significantly reduced, which can improve the service life of the battery cell by more than 25% SOH.

[0375] Compared with Comparative Example 1, the copper foil of Example 1-3 has good mechanical properties and plasticity, which can effectively reduce the probability of crack or fracture of the current collector caused by the expansion of the secondary battery. For secondary batteries with high expansion properties, such as an expansion force greater than or equal to 1000 kgf and a thickness expansion rate of 4%-10%, the copper foil of Example 1-3 can effectively reduce the probability of crack or fracture of the current collector, so that the SOH of the secondary battery decreases when the crack fails (i.e., effectively improving the cycle service life and safety of the secondary battery), and the probability of failure of the secondary battery due to battery expansion and performance diving is reduced.

[0376] Examples 4-6

[0377] The preparation method of Example 4 is basically the same as that of Example 1, and the following method is used to prepare the copper foil:

[0378] A copper plate or copper wire with a purity of 99.9% or above is dissolved in a sulfuric acid solution to prepare a copper sulfate pentahydrate solution, which is used as a copper source. Additives and oxalic acid are added to configure an electrolyte at 60°C. The additives include gelatin, polyethylene glycol, hydroxyethyl cellulose, and sodium chloride. The pH of the electrolyte is 3.5, and the concentrations of the components are as follows: the concentration of copper ions is 45 g / L; the concentration of gelatin is 90 mg / L; the concentration of polyethylene glycol is 50 mg / L; the concentration of hydroxyethyl cellulose is 30 mg / L; the concentration of chloride ions is 80 mg / L; the concentration of sodium polydithiobispropyl sulfonate is 75 mg / L; and the rest is deionized water.

[0379] A square wave pulse current waveform is used to periodically apply a current to a polished cathode titanium roller in an electrolyte. The area of the titanium roller in the electrolyte is 0.3 dm 2 , the rotation speed of the titanium roller is 2 m / min, the pulse current is applied with a current density of 167 A / dm 2 , a duty cycle of 5%, a pulse width of 1 ms, a cathode-anode distance of 20 mm, and a deposition temperature of 60°C. The copper foil is deposited on the titanium roller for 224 s.

[0380] The preparation method of Examples 5-6 is basically the same as that of Example 4, except that the components or concentrations of the electrolyte or the parameters of pulse deposition are changed, as shown in Table 4.

[0381] Table 4

[0382] Table 5

[0383] Figure 20 is a schematic diagram of the grain size distribution of the copper foil of Example 4; Figure 21 is a reverse pole figure distribution map of the electron backscatter diffraction image of the copper foil of Example 4; and Figure 22 is a tensile curve diagram of the copper foil of Example 4. As can be seen from Figures 20 and 21, the average grain size of the copper foil of Example 4 is 200 nm; the minimum grain size is about 10 nm; the number of grains with a grain size of less than 200 nm accounts for 34% based on the total number of grains in the copper foil; the number of grains with a grain size of 200-500 nm accounts for about 65%; the number of grains with a grain size greater than 500 nm accounts for no more than 2%; and the maximum roughness of the surface of the copper foil is 0.12 μm. As can be seen from the results of Examples 4-7, the average grain size of the copper foil provided by the present application is 0.04 μm-0.6 μm, the maximum grain size is 1.2 μm-3 μm, and the number of nanotwin grains accounts for no less than 45% based on the total number of grains in the copper foil. The copper foil has excellent tensile strength and elongation at break, can reduce the state of health (SOH) value corresponding to battery crack failure, and helps to improve the safety and cycle life of the battery.

[0384] Note that the present application is not limited to the above-described embodiments. The above-described embodiments are merely examples, and embodiments having substantially the same configuration, function, and effect as the technical idea of the present application are included in the technical scope of the present application. In addition, other modes constructed by combining part of the configurations of the embodiments in a manner that a person skilled in the art can think of within the scope of the present application are also included in the scope of the present application.

Claims

1. A secondary battery characterized by comprising: The copper foil comprises copper grains with different particle sizes, the copper grains comprise 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 less than or equal to 0.5 μm accounts for 70% to 95% based on the total number of the copper grains; the maximum particle size of the copper grains is 1.2 μm to 3 μm; the average particle size of the copper grains is 0.04 μm to 0.6 μm; and the number of nanotwins accounts for more than or equal to 45% based on the total number of the copper grains. The number of copper grains with a particle size of less than 200 nm accounts for 20% to 60% based on the total number of the copper grains in the copper foil.

2. The secondary battery according to claim 1, characterized by The number of nanotwins accounts for 50% to 90% based on the total number of the copper grains.

3. The secondary battery according to claim 1 or 2, characterized by The number of nanotwins accounts for 65% to 85% based on the total number of the copper grains.

4. The secondary battery according to any one of claims 1 to 3, characterized by, The particle size of the copper grains with a particle size of more than 0.5 μm ranges from more than 0.5 μm to less than or equal to 3 μm.

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

6. The secondary battery according to any one of claims 1 to 5, characterized by, The copper foil satisfies at least one of the following conditions:

7. The secondary battery according to any one of claims 1 to 6, characterized by, (1) the minimum particle size of the copper grains is 0.1 μm to 0.3 μm; and / or (2) the particle size span of the copper grains is 0.8 μm to 2.5 μm. The copper foil satisfies at least one of the following conditions:

8. The secondary battery according to any one of claims 1 to 7, characterized by, (1) the minimum particle size of the copper grains is 0.1 μm to 0.3 μm; and / or (2) the particle size span of the copper grains is 1 μm to 2 μm. The tensile strength of the copper foil is 600 MPa to 1000 MPa, and / or the breaking elongation of the copper foil is 4% to 8% under the test conditions of room temperature (20±10 ℃), a sample length×width of (50±0.25 mm)×(15±0.25 mm), and a tensile speed of 50±0.5 mm / min.

9. The secondary battery according to any one of claims 1 to 8, characterized by, The tensile strength of the copper foil is 700 MPa to 1000 MPa, and / or the breaking elongation of the copper foil is 4% to 7% under the test conditions of room temperature (20±10 ℃), a sample length×width of (50±0.25 mm)×(15±0.25 mm), and a tensile speed of 50±0.5 mm / min.

10. The secondary battery according to any one of claims 1 to 9, characterized by The tensile strength of the copper foil is 700 MPa to 800 MPa, and / or the breaking elongation of the copper foil is 5% to 6% under the test conditions of room temperature (20±10 ℃), a sample length×width of (50±0.25 mm)×(15±0.25 mm), and a tensile speed of 50±0.5 mm / min.

11. The secondary battery according to any one of claims 1 to 10, characterized by The hardness of the copper foil is 55 HV to 65 HV.

12. The secondary battery according to any one of claims 1 to 11, characterized by The hardness of the copper foil is 55 HV to 60 HV.

13. The secondary battery according to any one of claims 1 to 12, characterized by The thickness of the copper foil is 4 μm to 10 μm.

14. The secondary battery according to any one of claims 1 to 13, 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.

15. The secondary battery according to any one of claims 1 to 14, characterized by, ​ 16. The secondary battery according to claim 15, characterized by The negative active material includes a silicon-based material, and the silicon-based material includes at least one of elemental silicon, a silicon oxide compound, a silicon-carbon composite, a silicon-nitrogen composite, and a 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 the copper foil; the peak value of the pulse current is 40,000 A-100,000 A, the valley value of the pulse current is 100 A-20,000 A, and the change period of the current is 50 ms-5,000 ms; the copper foil comprises copper grains with different particle sizes, the copper grains comprise copper grains with a particle size of 0.5 μm or less, and the number of the copper grains with a particle size of 0.5 μm or less accounts for 70%-95% based on the total number of the copper grains; the maximum particle size of the copper grains is 1.2 μm-3 μm; the average particle size of the copper grains is 0.04 μm-0.6 μm; and the number of the grains of nanometer twinning accounts for 45% or more based on the total number of the copper grains.

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 the electroplating deposition is 45°C-60°C; (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 the electroplating deposition is 50°C-60°C; (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 peak current density of the pulse current is I, in A / dm 2 ; The current duty cycle of the pulsed current is s, and the peak current density I and the duty cycle s satisfy: 2A / dm 2 ≤ I x s ≤ 18A / dm 2 .

22. The method of claim 21, wherein, The peak current density I of the pulse current satisfies 3.3 A / dm2≤I≤333 A / dm2, and optionally, 100 A / dm2≤I≤180 A / dm2.

23. The method of claim 21 or 2, wherein, The duty cycle s of the pulse current satisfies 2%≤s≤50%, and optionally, 2%≤s≤10%.

24. The production method according to any one of claims 17 to 23, characterized by, The electroplating solution comprises a leveling agent, a wetting agent, and a brightener, The leveling agent comprises one or more of collagen and sodium saccharin; The wetting agent comprises one or more of hydroxyethyl cellulose and polyethylene glycol; The brightener comprises sodium polydithiopropyl sulfone.

25. The preparation method according to claim 24, wherein The leveling agent further comprises one or more of gelatin and gentian violet; The brightener further comprises one or more of sodium 3-mercapto-1-propane sulfonate and thiourea.

26. The method of manufacturing according to claim 24 or 25, wherein, The electroplating solution comprises: collagen with a concentration of 60 mg / L-300 mg / L, sodium saccharin with a concentration of 0.5 g / L-10 g / L, polyethylene glycol with a concentration of 50 mg / L-200 mg / L, hydroxyethyl cellulose with a concentration of 30 mg / L-200 mg / L, sodium poly(dithiobispropane sulfonate) 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.

27. The production method according to any one of claims 24 to 26, wherein, The electroplating solution comprises: collagen with a concentration of 80 mg / L-150 mg / L, sodium saccharin with a concentration of 0.5 g / L-4 g / L, polyethylene glycol with a concentration of 60 mg / L-150 mg / L, hydroxyethyl cellulose with a concentration of 50 mg / L-150 mg / L, sodium poly(dithiobispropane sulfonate) with a concentration of 500 mg / L-1000 mg / L, and chloride ions with a concentration of 40 mg / L-80 mg / L.

28. The production method according to any one of claims 24 to 27, wherein, The concentration of copper ions in the electrolyte is 30 g / L-100 g / L, and optionally 45 g / L-75 g / L.

29. The production method according to any one of claims 17 to 28, wherein The pH of the electroplating solution is 2.5-4.

5.

30. An electrical device, comprising: A secondary battery comprising the secondary battery of any one of claims 1 to 16 or prepared by the preparation method of any one of claims 17 to 29.

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