Current collector, electrode sheet, secondary battery, electric device, copper foil, and preparation method for copper foil

Abstract

A current collector, an electrode sheet, a secondary battery, an electric device, a copper foil, and a preparation method for the copper foil. The current collector comprises the copper foil, the average particle size of the crystal grains in the copper foil ranges from 50 nm to 400 nm, and based on the total number of crystal grains in the copper foil, the proportion of the crystal grains of nano twin crystals is greater than or equal to 60%. The combination of the high proportion nano twin crystal structure and the small crystal grain size functions to improve the mechanical strength of the current collector while maintaining the high plasticity of the current collector, thereby providing a material basis for further improvement of the energy density and safety of the battery.

Description

Current collector, pole piece, secondary battery, electric device, copper foil and method for preparing the copper foil

[0001] Cross-references

[0002] This application refers to Chinese patent application No. 202410146761.1 filed on February 1, 2024, entitled “Current Collector, Pole Sheet, Secondary Battery, Electrical Device, Copper Foil and Method for Preparing the Copper Foil”, which is incorporated into this application in its entirety by reference. Technical Field

[0003] The present application belongs to the field of battery technology, and specifically relates to a current collector, a pole piece, a secondary battery, an electrical device, a copper foil, and a method for preparing the copper foil. Background Art

[0004] In recent years, secondary batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power stations, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace and other fields.

[0005] The current collector is an important component of secondary batteries. As the market demands for the energy density and safety performance of secondary batteries increase, there is an urgent need to develop lightweight and high-strength current collectors to meet the needs of the new generation of secondary batteries.

[0006] Summary of the Invention

[0007] The present application is made in view of the above-mentioned problems, and its purpose is to provide an extremely thin current collector with excellent mechanical properties to meet the requirements of the new generation of high-performance batteries for current collectors.

[0008] In order to achieve the above-mentioned objectives, the present application provides a current collector, a pole piece, a secondary battery, an electrical device, a copper foil, and a method for preparing the copper foil.

[0009] In a first aspect, the present application provides a current collector comprising copper foil, wherein the average particle size of the grains in the copper foil is 50 nm to 400 nm, and based on the total number of grains in the copper foil, the number of nanotwin grains accounts for no less than 60%.

[0010] Twin structures are an effective means of improving the strength and plasticity of metals. Twin boundaries are coherent interfaces with neatly arranged atoms. They can improve the strength of the material by increasing the resistance to dislocation movement while maintaining the mobility of dislocations, thereby strengthening the material while maintaining its plasticity. Small grain size can further increase the interface structure, increase the external stress required for dislocations to pass through the grain boundary, and achieve material strengthening. The combined effect of a high proportion of nano-twin structures and a small grain size is conducive to maintaining the high plasticity of copper foil while improving its mechanical strength, providing a material basis for further improvements in battery energy density and safety.

[0011] In any embodiment, the average grain size of the copper foil is 50 nm to 300 nm.

[0012] Copper foil with an average particle size within the above range can further improve its mechanical strength while maintaining its high plasticity, meeting the use requirements of the new generation of high energy density secondary batteries.

[0013] In any embodiment, based on the total number of grains in the copper foil, the number of grains with a particle size less than 200 nm accounts for 20%-40%, and the number of grains with a particle size between 200 nm and 500 nm accounts for 50%-80%.

[0014] In any embodiment, based on the total number of grains in the copper foil, the number of grains with a particle size greater than 500 nm accounts for less than or equal to 5%.

[0015] In the prior art, copper foil reinforced by twinning often introduces micron-sized twins during the preparation process, hindering further improvement in the copper foil's strength. The copper foil provided in the embodiments of the present application has a high proportion of nano-twins, significantly improving its strength. Furthermore, the grains have a small average size and a skewed distribution of particle sizes, with a very small proportion of grains larger than 500nm. The fine grains reduce the occurrence of concentrated stress during copper foil deformation and are more conducive to coordinated grain slippage, thereby improving both the strength of the copper foil and its elongation at break.

[0016] In any embodiment, based on the total number of grains in the copper foil, the number of equiaxed grains or quasi-equiaxed grains accounts for greater than or equal to 80%, and the ratio a1 of the major axis to the minor axis length of the equiaxed grains or quasi-equiaxed grains satisfies: 1≤a1<2.

[0017] A large number of equiaxed grains or quasi-equiaxed grains are beneficial to achieving isotropy of the copper foil in all directions, and are beneficial to comprehensively improving the strength and plasticity of the copper foil.

[0018] In any embodiment, based on the total number of grains in the copper foil, the number of columnar grains accounts for less than 10%, and the ratio a2 of the length of the major axis to the minor axis of the columnar grains satisfies: a2>2.

[0019] The formation of columnar grains can easily lead to heterogeneity in the mechanical properties of copper foil. Reducing the proportion of columnar grains can help improve the mechanical strength and plasticity of copper foil and reduce the probability of stress concentration points.

[0020] In any embodiment, the crystal plane diffraction intensity of the (111) texture in the copper foil accounts for 60%-85% of the total crystal plane diffraction intensity of the (111) texture, (200) texture, (220) texture, (311) texture, and (222) texture in the copper foil.

[0021] For pure copper materials, common textures include (111), (200), (220), (311), and (222). According to crystallographic properties, the (111) crystal plane has a high atomic density and high mechanical strength, which can enhance the twin strengthening effect. Copper foils with a ratio of the (111) texture's crystal plane diffraction intensity to the sum of the crystal plane diffraction intensities of the (111) texture, (200) texture, (220) texture, (311) texture, and (222) texture in the copper foil within the above range are beneficial for improving the mechanical strength and plasticity of the copper foil.

[0022] In any embodiment, the copper foil has a first surface and a second surface relative to each other, the thickness of the copper foil is denoted as H, the area from the first surface of the copper foil to the thickness range of 0.2H-0.5H is denoted as the first area of ​​the copper foil, and the area from the second surface of the copper foil to the thickness range of 0.2H-0.5H is denoted as the second area of ​​the copper foil, and the absolute value of the difference between the average particle size of the grains in the first area and the average particle size of the grains in the second area is less than or equal to 150nm.

[0023] The grain size and structure of the copper foil are also highly uniform along the thickness direction of the copper foil, providing a basis for it to maintain high strength and high plasticity while having an ultra-thin thickness.

[0024] In any embodiment, based on the total number of grains in the copper foil, the number of nanotwins accounts for 65% to 90%.

[0025] In any embodiment, the maximum surface roughness of the copper foil is less than or equal to 3 μm.

[0026] Copper foil with a maximum surface roughness not exceeding 3μm has a smooth and uniform surface, and is not prone to forming stress concentration points during cyclic stress, which is beneficial to improving the mechanical strength and plasticity of the copper foil.

[0027] In any embodiment, the thickness H of the copper foil satisfies: 3 μm≤H≤15 μm.

[0028] The copper foil has a low thickness, which can effectively reduce the weight of the battery and is conducive to further improving the energy density of the battery.

[0029] In any embodiment, under the test conditions of room temperature, sample thickness of 6±0.2 μm, and tensile speed of 50±0.5 mm / min, the tensile strength of the copper foil is greater than or equal to 600 MPa and the elongation at break of the copper foil is greater than or equal to 4%.

[0030] In any embodiment, under the test conditions of room temperature, sample thickness of 6±0.2 μm, and tensile speed of 50±0.5 mm / min, the tensile strength of the copper foil is 700 MPa-1500 MPa; and the elongation at break of the copper foil is 4.5%-10%.

[0031] The copper foil has both high strength and high plasticity, which can meet the use requirements of high-energy-density batteries, improving the safety of the battery while increasing the energy density of the battery.

[0032] A second aspect of the present application provides a pole piece, comprising a current collector in any embodiment.

[0033] A third aspect of the present application provides a secondary battery, comprising the electrode according to the second aspect.

[0034] In any embodiment, the secondary battery is any one of a battery cell, a battery module, and a battery pack.

[0035] In any embodiment, the maximum expansion force of the secondary battery cell is greater than or equal to 1000 kgf.

[0036] In any embodiment, the maximum expansion force of the secondary battery cell is greater than or equal to 2500 kgf.

[0037] In any embodiment, the maximum expansion force of the secondary battery cell is greater than or equal to 4000 kgf. Current collectors in existing technologies are prone to fracture under high expansion forces in secondary batteries, causing safety incidents and limiting further improvement in the electrochemical performance of secondary batteries. The current collectors provided in the embodiments of the present application have both excellent tensile strength and elongation at break, making them suitable for secondary batteries with high expansion forces and conducive to further improving the energy density of secondary batteries.

[0038] A fourth aspect of the present application provides an electrical device comprising the secondary battery of the third aspect.

[0039] The fifth aspect of the present application provides a method for preparing copper foil, which is prepared by electroplating. The electroplating method specifically includes periodically applying current to the electroplating solution to reduce and deposit copper ions in the electroplating solution to form copper foil. The average particle size of the grains in the copper foil is 50nm-400nm, and based on the total number of grains in the copper foil, the number of nanotwin grains accounts for greater than or equal to 60%.

[0040] In any embodiment, the current is a pulse current, and the pulse current includes one or more of a square wave pulse current, a sine wave pulse current, a triangle wave pulse current, and a sawtooth wave pulse current.

[0041] Applying a pulsed current to the plating solution can control the nucleation and grain growth of the reduced copper ions by turning the current on and off, thereby inhibiting grain growth and forming uniform nano-grains, thereby improving the tensile strength and elongation at break of the copper foil. Compared to the DC current deposition commonly used in existing technologies, pulsed current deposition can achieve higher peak current densities, which facilitates the formation of large amounts of copper foil nucleation, thereby increasing the density of the copper foil and reducing the grain size of the copper foil to improve its mechanical strength.

[0042] In any embodiment, the electroplating solution includes additives, and the additives include one or more of a leveler, a wetting agent, and a brightener.

[0043] In any embodiment, the additives include levelers, wetting agents, and brighteners.

[0044] Levelers adhere to the tips of copper foil, where deposition rates are high, inhibiting grain growth and balancing the growth rates of pits and tips, thereby improving the flatness of the foil. Wetting agents enhance the wettability of the plating solution and the substrate. The plating solution's wettability on the cathode is sufficient to enable rapid electrodeposition with high currents, increasing the nucleation rate of the copper foil and reducing the average grain size. A certain amount of brightener can further refine the grain size of the copper foil, reduce surface roughness, and improve surface smoothness.

[0045] In any embodiment, the concentration of the leveler in the electroplating solution is 20 mg / L-300 mg / L.

[0046] In any embodiment, the concentration of the leveler in the electroplating solution is 50 mg / L-150 mg / L.

[0047] In any embodiment, the concentration of the wetting agent in the electroplating solution is 10 mg / L-200 mg / L.

[0048] In any embodiment, the concentration of the wetting agent in the electroplating solution is 30 mg / L-100 mg / L.

[0049] In any embodiment, the concentration of the brightener in the electroplating solution is 10 mg / L-200 mg / L.

[0050] In any embodiment, the concentration of copper ions in the electroplating solution is 30 g / L to 100 g / L.

[0051] In any embodiment, the concentration of copper ions in the electroplating solution is 45 g / L to 75 g / L.

[0052] The research results show that a combination of low copper ion concentration and high additive concentration is more conducive to comprehensively improving the strength and elongation at break of copper foil than a combination of high copper ion concentration and low additive concentration. Furthermore, low copper ion concentration helps reduce concentration polarization, control grain growth and deposition rates, facilitate the preparation of small grains, and improve the mechanical strength of copper foil.

[0053] In any embodiment, the concentration of chloride ions in the electroplating solution is 10 mg / L to 80 mg / L.

[0054] In any embodiment, the pH of the plating solution is 2.5-4.4.

[0055] In any embodiment, the leveling agent includes one or more of gelatin and Janus Green; the wetting agent includes one or more of hydroxyethyl cellulose and polyethylene glycol; the brightening agent includes one or more of sodium polydisulfide dipropane sulfonate, sodium 3-mercapto-1-propane sulfonate, and thiourea.

[0056] In any embodiment, the wetting agent comprises hydroxyethylcellulose and polyethylene glycol.

[0057] The results show that adding two different wetting agents simultaneously can more effectively improve the mechanical strength of copper foil than adding a single wetting agent. Although the mechanism is not yet clear, it is speculated that it may be related to the competitive adsorption of the two different additives, which helps to improve the consistency of grains during the deposition process, reduce the difference in grain size through the thickness direction, and improve the uniformity of the copper foil.

[0058] In any embodiment, the mass concentration ratio of the copper ion to the leveling agent is 500:1-4500:1; and / or the mass concentration ratio of the copper ion to the brightener is 300:1-6000:1.

[0059] In some embodiments, the electroplating solution includes gelatin at a concentration of 20 mg / L-150 mg / L, polyethylene glycol at a concentration of 15 mg / L-100 mg / L, hydroxyethyl cellulose at a concentration of 10 mg / L-80 mg / L, sodium polydisulfide bis(propylene sulfonate) at a concentration of 15 mg / L-150 mg / L, copper ions at a concentration of 45 g / L-90 g / L, and chloride ions at a concentration of 10 mg / L-80 mg / L.

[0060] In any embodiment, the peak current density of the pulse current is I, in A / dm 2 The current duty cycle of the pulse current is s, and the peak current density I and the duty cycle s meet: 2A / dm 2 ≤I×s≤18A / dm2 .

[0061] The product of the peak current density I and the duty cycle s is equivalent to the average density of the current during the pulse period. The pulse deposition method allows the average density of the current during the pulse period to be higher than the maximum current density achievable during DC deposition, because too high a DC deposition density will cause hydrogen evolution and concentration polarization in the plating solution, reducing the uniformity of the copper foil during the electrodeposition process. The product of the peak current density I and the duty cycle s within the above range can not only effectively reduce the grain size, but also improve the uniformity of the grain size in the thickness direction of the prepared copper foil. The product of the peak current density I and the duty cycle s within the above range, combined with a low copper ion concentration in the plating solution, can further improve the concentration polarization phenomenon of the plating solution, achieve the preparation of small-sized grains, and improve the tensile strength and elongation at break of the copper foil. In any embodiment, the peak current density I of the pulse current satisfies: 3.3A / dm 2 ≤I≤333A / dm 2 .

[0062] In any embodiment, the duty cycle s of the pulse current satisfies: 2%≤s≤50%.

[0063] In any embodiment, the pulse width of the pulse current is 1 ms to 50 ms.

[0064] In any embodiment, the distance between the cathode electrode and the anode electrode is 15 mm to 20 mm.

[0065] In any embodiment, the deposition temperature is 45°C-60°C.

[0066] In any embodiment, the deposition time is greater than or equal to 80 seconds.

[0067] In any embodiment, the electroplating method specifically includes periodically applying a pulse current to the electroplating solution to reduce and deposit copper ions in the electroplating solution; wherein the peak current density I of the pulse current satisfies 100A / dm 2 ≤I≤180A / dm 2 The duty cycle s of the pulse current satisfies: 2%≤s≤10%, and the deposition time is 80s-250s.

[0068] In any embodiment, the preparation method is a continuous production method.

[0069] The sixth aspect of the present application provides a copper foil, which includes the same features as the copper foil in the aforementioned current collector, and will not be repeated here.

[0070] In any embodiment, the copper foil is prepared by the preparation method of the fifth aspect.

[0071] In any embodiment, the maximum width of the copper foil is greater than or equal to 1.5 meters, and / or the maximum length of the copper foil is greater than or equal to 10,000 meters.

[0072] This copper foil can be manufactured in large sizes and has prospects for industrial application. BRIEF DESCRIPTION OF THE DRAWINGS

[0073] To more clearly illustrate the technical solutions of the embodiments of the present application, the following briefly introduces the drawings required for use in the embodiments of the present application. Obviously, the drawings described below are only some embodiments of the present application. For those skilled in the art, other drawings can be obtained based on the drawings without inventive effort.

[0074] FIG1 is a schematic diagram of the grain size distribution of the copper foil of Example 1 of the present application;

[0075] FIG2 is a distribution diagram of the inverse pole figure of the electron backscatter diffraction image of the copper foil of Example 1 of the present application;

[0076] FIG3 is a tensile curve diagram of the copper foil of Example 1 of the present application;

[0077] FIG4a is an X-ray diffraction pattern of the copper foil of Example 2 of the present application; FIG4b is an X-ray diffraction pattern of the copper foil of Comparative Example 1 of the present application;

[0078] FIG5 is a schematic diagram of an embodiment of a secondary battery of the present application;

[0079] FIG6 is an exploded schematic diagram of an embodiment of a secondary battery of the present application;

[0080] FIG7 is a schematic diagram of an embodiment of a battery module of the present application;

[0081] FIG8 is a schematic diagram of an embodiment of a battery pack of the present application;

[0082] FIG9 is an exploded schematic diagram of the embodiment of the battery pack shown in FIG8 ;

[0083] FIG10 is a schematic diagram of an embodiment of an electric device including the secondary battery of the present application as a power source;

[0084] In the accompanying drawings, which are not necessarily drawn to scale, reference numerals are as follows: 1 battery pack, 2 upper housing, 3 lower housing, 4 battery module, 5 secondary battery, 51 housing, 52 electrode assembly, 53 cover plate. DETAILED DESCRIPTION

[0085] Below, with appropriate reference to the accompanying drawings, the embodiments of the current collector, electrode, secondary battery, electrical device, copper foil, and method for preparing the copper foil of the present application are described in detail. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0086] " range " disclosed in the present application is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of special range. The scope limited in this way can be to include end value or not include end value, and can be arbitrarily combined, that is, any lower limit can form a range with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 are listed, and if the maximum range value 3,4 and 5 are listed, then the following range can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise specified, the numerical range " ab " represents the abbreviation of any real number combination between a and b, wherein a and b are all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0087] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of the present application.

[0088] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form new technical solutions, and such technical solutions should be deemed to be included in the disclosure of this application.

[0089] Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0090] Unless otherwise specified, the terms "include" and "comprising" used in this application may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0091] Unless otherwise specified, the term "or" is used in this application to be inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: 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).

[0092] Unless otherwise specified, the terms used in this application have the common meanings generally understood by those skilled in the art.

[0093] Unless otherwise specified, the values ​​of the parameters mentioned in this application can be measured using various test methods commonly used in the art, for example, they can be measured according to the test methods provided in this application. Specifically, during the test, the sampling area is used to characterize the microstructure of the entire copper foil.

[0094] Unless otherwise specified, in this application, the term "active ions" refers to ions that can be intercalated and extracted between the positive electrode and the negative electrode of a secondary battery, including but not limited to lithium ions.

[0095] In this application, the terms "plurality" and "multiple" refer to two or more.

[0096] With the increase in the energy density of secondary batteries, there is a demand for thinner and lighter electrode current collectors. As the current collector becomes thinner, the maximum load that the current collector can bear decreases sharply, and the thickness of the current collector 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 current collector. As a result, current collectors with conventional strength in the prior art are no longer able to meet the needs of secondary batteries. After thinning, current collectors with ordinary strength are prone to fatigue fracture in the later stages of the secondary battery cycle, which in turn leads to thermal runaway of the battery cell and safety accidents. Therefore, it is necessary to effectively improve the tensile strength and plasticity (elongation at break) of the current collector while making it thinner and lighter.

[0097] Existing metal strengthening techniques often utilize conventional incoherent grain boundaries or phase boundaries to hinder dislocation motion to increase strength. While the introduction of numerous incoherent grain boundaries significantly improves strength, as the number of these "obstructions" to dislocation motion increases, lattice dislocation motion is severely hindered or even completely suppressed, preventing coordinated plastic deformation and causing the material to become brittle. This often results in enhanced mechanical properties of metals at the expense of plasticity, and materials with excellent plasticity often have low strength.

[0098] Based on this, the present application provides a current collector, which includes copper foil, the average particle size of the grains in the copper foil is 50nm-400nm, and based on the total number of grains in the copper foil, the number of nanotwin grains accounts for greater than or equal to 60%.

[0099] In this article, the term "twin" refers to two crystals (or two parts of a crystal) that are mirror-symmetrical along a common crystal plane (i.e., a specific orientation relationship). These two crystals are called "twins," and this common crystal plane is called the twin plane. Nanotwins are twins with dimensions less than 1000 nanometers in all dimensions.

[0100] The average particle size of the grains in the copper foil can be tested by methods known in the art. As an example, the cross-section of the copper foil is measured by electron backscatter diffractometer (EBSD) and scanning electron microscope. A copper foil range of not less than 5 mm × 5 mm and not more than 8 mm × 8 mm is selected to obtain the inverse pole figure distribution diagram of the copper foil. The average diameter of the equivalent circle of the grains is used as the grain size. The particle size of the grains in this range is counted to make a quantity distribution diagram, as shown in Figure 1. The skewed distribution is used for fitting, and the length corresponding to the peak value is used as the average particle size of the grains.

[0101] In some embodiments, the average grain size of the copper foil can be selected to be 50 nm, 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, or any range therebetween.

[0102] In some embodiments, the average grain size of the copper foil is 50 nm to 300 nm.

[0103] Copper foil with an average particle size within the above range can further improve its mechanical strength while maintaining its high plasticity, meeting the use requirements of the new generation of high energy density secondary batteries.

[0104] In some embodiments, based on the total number of grains in the copper foil, the number of grains with a particle size less than 200 nm accounts for 20%-40%, and the number of grains with a particle size between 200 nm and 500 nm accounts for 50%-80%.

[0105] In some embodiments, based on the total number of grains in the copper foil, the proportion of grains with a particle size less than 200 nm can be selected to be 20%, 24%, 28%, 32%, 36%, 40%, or any range therebetween.

[0106] In some embodiments, based on the total number of grains in the copper foil, the proportion of grains with a particle size between 200 nm and 500 nm can be selected as 50%, 55%, 60%, 65%, 70%, 75%, 80% or any numerical range therebetween.

[0107] In some embodiments, based on the total number of grains in the copper foil, the proportion of grains with a particle size greater than 500 nm is less than or equal to 5%, and may be less than or equal to 2%.

[0108] In some embodiments, based on the total number of grains in the copper foil, the proportion of grains with a particle size greater than 500 nm can be selected as 0.5%, 1%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5% or any numerical range therebetween.

[0109] The copper foil grains provided in the embodiment of the present application have a small average particle size, a skewed particle size distribution, and a very small proportion of grains with a particle size greater than 500 nm. The fine grains reduce the occurrence of concentrated stress during the deformation of the copper foil and are more conducive to the coordinated sliding of the grains. Therefore, the strength of the copper foil can be improved while the elongation at break can be increased.

[0110] In this application, the proportion of the number of nano-twin grains can be tested by methods known in the art. As an example, electron backscatter diffraction (EBSD) is combined with a scanning electron microscope to observe the cross-section of the copper foil, and a copper foil range of not less than 5mm×5mm and not more than 8mm×8mm is selected to obtain the inverse pole figure distribution map of the copper foil, where different colors represent different grain orientations. Taking Figure 2 as an example, the red lines inside the grains are twin boundaries, indicating that they have twin structures. The size and number of twins can be counted by the analysis software supporting the electron backscatter diffractometer, and then the proportion of nano-twin grains can be calculated. As an example, the grain characteristics are analyzed by the Oxford C-Nano+ electron backscatter diffractometer and its supporting software.

[0111] In some embodiments, based on the total number of grains in the copper foil, the proportion of the number of nanotwin grains can be selected as 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 100% or any numerical range therebetween.

[0112] Twin structure is an effective means to improve the strength and plasticity of metals. The twin boundary is a coherent interface with neatly arranged atoms, which can increase the strength of the material by increasing the resistance to dislocation movement while maintaining the mobility of dislocations, thereby strengthening the material while maintaining the plasticity of the material. However, copper foils reinforced by twin strengthening in the prior art often introduce micron-level twins during the preparation process, which hinders further improvement of the strength of the copper foil. The copper foil provided in the embodiment of the present application has a high proportion of nano twins, which significantly improves the strength of the copper foil while improving the plasticity of the copper foil. The small grain size can further increase the interface structure, increase the external stress required for dislocations to pass through the grain boundary, and achieve material strengthening. The high proportion of nano twin structure and small grain size work together to improve the mechanical strength of the copper foil while maintaining its high plasticity, providing a material basis for further improvement of battery energy density and safety.

[0113] Herein, the term "equiaxed grains or quasi-equiaxed grains" refers to grains whose major axis and minor axis are equal or have a small difference. In some embodiments, the major axis to minor axis length ratio a1 of the equiaxed grains or quasi-equiaxed grains satisfies: 1≤a1<2.

[0114] In some embodiments, based on the total number of grains in the copper foil, the proportion of equiaxed grains or quasi-equiaxed grains is greater than or equal to 80%, and can be optionally greater than or equal to 90%, and the ratio of the major axis to the minor axis length a1 of the equiaxed grains or quasi-equiaxed grains satisfies: 1≤a1<2.

[0115] In the present application, the proportion of the number of equiaxed grains or quasi-equiaxed grains can be tested by methods known in the art. As an example, electron backscatter diffraction (EBSD) combined with a scanning electron microscope is used to observe the cross-section of the copper foil, and a copper foil range of not less than 5mm×5mm and not more than 8mm×8mm is selected to obtain the inverse pole figure distribution map of the copper foil. The proportion of the number of equiaxed grains or quasi-equiaxed grains is statistically analyzed by analysis software.

[0116] A large number of equiaxed grains or quasi-equiaxed grains are conducive to achieving fine and uniform grains in the copper foil, and the properties of the equiaxed grains or quasi-equiaxed grains in all directions are the same or similar, which is conducive to comprehensively improving the strength and plasticity of the copper foil.

[0117] In some embodiments, based on the total number of grains in the copper foil, the proportion of equiaxed grains or quasi-equiaxed grains is 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, 100% or any range therebetween.

[0118] In some embodiments, based on the total number of grains in the copper foil, the number of columnar grains accounts for less than 10%, and the ratio a2 of the length of the major axis to the minor axis of the columnar grains satisfies: a2>2.

[0119] The formation of columnar grains can easily lead to heterogeneity in the mechanical properties of copper foil. Reducing the proportion of columnar grains can help improve the mechanical strength and plasticity of copper foil and reduce the probability of stress concentration points.

[0120] In some embodiments, the long axis direction of the columnar grains is the thickness direction of the copper foil.

[0121] In some embodiments, the crystal plane diffraction intensity of the (111) texture in the copper foil accounts for greater than or equal to 60% of the total crystal plane diffraction intensity of the (111) texture, (200) texture, (220) texture, (311) texture, and (222) texture in the copper foil, and can be optionally 60%-85%.

[0122] In this article, the term "texture" refers to the phenomenon that during the formation of crystals, the grains in a polycrystal are arranged in an orderly manner along certain directions, showing a more or less statistically uneven distribution, that is, they appear to be clustered and arranged in certain directions, and thus the probability of orientation in these directions is significantly increased, which is also called preferred orientation or preferential orientation.

[0123] In the present application, the crystal plane diffraction intensity of the texture in the copper foil can be tested by methods known in the art. As an example, the copper foil is tested using an X-ray diffractometer, and the X-ray spectrum of the reference copper powder (PDF No. 04-0836) is used as the crystal plane diffraction intensity of the (111) texture. The peak area of ​​the diffraction peak at a peak position of 40°-45° is used as the crystal plane diffraction intensity of the (200) texture. The peak area of ​​the diffraction peak at a peak position of 47°-52° is used as the crystal plane diffraction intensity of the (220) texture. The peak area of ​​the diffraction peak at a peak position of 70°-75° is used as the crystal plane diffraction intensity of the (220) texture. The peak area of ​​the diffraction peak at a peak position of 85°-95° ​​is used as the crystal plane diffraction intensity of the (311) texture. The peak area of ​​the diffraction peak at a peak position of 103°-107° is used as the crystal plane diffraction intensity of the (222) texture.

[0124] In some embodiments, the ratio of the crystal plane diffraction intensity of the (111) texture in the copper foil to the total crystal plane diffraction intensity of the (111) texture, (200) texture, (220) texture, (311) texture, and (222) texture in the copper foil can be selected to be 60%, 65%, 70%, 75%, 80%, 85% or any numerical range therebetween.

[0125] For pure copper materials, common textures include (111), (200), (220), (311), and (222). According to crystallographic properties, the (111) crystal plane has a high atomic density and high mechanical strength, which can enhance the twinning strengthening effect. A high proportion of (111) texture in copper foil is beneficial for improving the mechanical strengthening and plastic enhancement effects of copper foil.

[0126] In some embodiments, the copper foil has a first surface and a second surface relative to each other, the thickness of the copper foil is denoted as H, the area from the first surface of the copper foil to the thickness range of 0.2H-0.5H is denoted as the first area of ​​the copper foil, and the area from the second surface of the copper foil to the thickness range of 0.2H-0.5H is denoted as the second area of ​​the copper foil, and the absolute value of the difference between the average particle size of the grains in the first area and the average particle size of the grains in the second area is less than or equal to 150nm.

[0127] In some embodiments, the absolute value of the difference between the average grain size of the grains in the first region and the average grain size of the grains in the second region is 1 nm, 20 nm, 40 nm, 60 nm, 80 nm, 100 nm, 120 nm, 140 nm, 150 nm or any numerical range therebetween.

[0128] The grain size and structure of the copper foil are also highly uniform along the thickness direction of the copper foil, indicating that it has achieved uniform deposition at different deposition time periods, and the electroplating solution has not undergone obvious polarization during the deposition process, providing a basis for it to maintain high strength and high plasticity while having an ultra-thin thickness.

[0129] In some embodiments, based on the total number of grains in the copper foil, the number of nanotwin grains accounts for 65% to 90%.

[0130] In some embodiments, the maximum surface roughness of the copper foil is less than or equal to 3 μm.

[0131] The maximum surface roughness of a copper foil can reflect its surface uniformity and smoothness. This can be measured using methods known in the art, such as a stylus roughness tester. Wipe the copper foil surface with alcohol, place the foil flat on a horizontal surface, and place the stylus roughness tester on the surface. Place the stylus in contact with the surface and gently run the stylus across it. The instrument reads the roughness of the copper foil. Five tests are performed on different locations of the copper foil, and the maximum reading is taken as the maximum surface roughness of the copper foil.

[0132] In some embodiments, the maximum surface roughness of the copper foil may be 0.1 μm, 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, or any range therebetween.

[0133] Copper foil with a maximum surface roughness of less than or equal to 3μm has a smooth and uniform surface, which is not easy to form stress concentration points during cyclic stress, which is beneficial to improving the mechanical strength and plasticity of the copper foil.

[0134] In some embodiments, the thickness H of the copper foil satisfies: 3 μm≤H≤15 μm, and may be 3 μm≤H≤8 μm.

[0135] In some embodiments, the thickness H of the copper foil may be 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, or any range therebetween.

[0136] In this application, the thickness of the copper foil can be tested using methods known in the art. For example, a 20×15 cm 2 The sample was cut and weighed on an electronic balance to obtain the weight of the sample. The density of the copper foil was ρ, which was 8.96 g / cm 3 Calculate the volume of the spline. Given the length and width of the spline, the thickness of the spline can be calculated.

[0137] The copper foil has a low thickness, which can effectively reduce the weight of the battery and is conducive to further improving the energy density of the battery.

[0138] In some embodiments, under the test conditions of room temperature, sample thickness of 6±0.2 μm, and tensile speed of 50±0.5 mm / min, the tensile strength of the copper foil is greater than or equal to 600 MPa and the elongation at break of the copper foil is greater than or equal to 4%; optionally, the tensile strength of the copper foil is 700 MPa-1500 MPa, and further optionally 800 MPa-1000 MPa; the elongation at break of the copper foil is 4.5%-10%, and further optionally 5%-8%.

[0139] Herein, the term "room temperature" refers to 20±10°C.

[0140] As used herein, the term "tensile strength" refers to the maximum load-bearing strength of a specimen when continuous loading is applied to the specimen until it breaks.

[0141] In this article, the term "elongation at break" refers to the ratio of the change in length of a material when it undergoes plastic deformation until it breaks after being subjected to stress to its original length. It is usually expressed as a percentage and is an important parameter for measuring the material's ability to withstand stress during stretching.

[0142] In this application, the tensile strength and elongation at break of the copper foil can be tested using methods known in the art, such as testing according to the GB / T 5230-1995 "Electrolytic Copper Foil" standard. As an example, cut at least four specimens with a length of 200±0.5mm, a width of 15±0.25mm, and a thickness of 6±0.2μm. The specimens are continuously loaded at a tensile speed of 50±0.5mm / min at room temperature until they break. The maximum load divided by the cross-sectional area of ​​the specimen is used as the tensile strength of the specimen. The cross-sectional area of ​​the specimen can be calculated by dividing the mass of the specimen by the product of the length and density of the specimen. The density of the copper foil specimen can be taken as 8.9g / cm 3 .

[0143] In some embodiments, under the test conditions of room temperature, sample thickness of 6um±0.2μm, and tensile speed of 50±0.5mm / min, the tensile strength of the copper foil may be selected as 600MPa, 700MPa, 800MPa, 900MPa, 1000MPa, 1100MPa, 1200MPa, 1300MPa, 1400MPa, 1500MPa or any numerical range therebetween; the elongation at break of the copper foil may be selected as 4%, 4.5%, 5%, 6%, 7%, 8%, 9%, 10% or any numerical range therebetween.

[0144] The copper foil has both high strength and high plasticity, which can meet the use requirements of high-energy-density batteries, improving the safety of the battery while increasing the energy density of the battery.

[0145] The present application also provides a method for preparing copper foil, which is prepared by electroplating. The electroplating method specifically includes periodically applying an electric current to an electroplating solution so that copper ions in the electroplating solution are reduced and deposited to form copper foil. The average particle size of the grains in the copper foil is 50nm-400nm, and based on the total number of grains in the copper foil, the number of nanotwin grains accounts for greater than or equal to 60%, optionally, greater than or equal to 65%.

[0146] In this article, the term "electroplating" refers to a method of depositing metal or alloy on the surface of a workpiece using the principle of electrolysis to form a metal layer.

[0147] In some embodiments, the current is a pulse current, and the pulse current includes one or more of a square wave pulse current, a sine wave pulse current, a triangle wave pulse current, and a sawtooth wave pulse current.

[0148] In some embodiments, the pulsed current comprises a square wave pulsed current.

[0149] As used herein, the term "pulsed current" refers to current or voltage pulses that occur repeatedly in a cycle.

[0150] Applying a pulsed current to the plating solution can control the nucleation and grain growth of the reduced copper ions by turning the current on and off, thereby inhibiting grain growth and forming uniform nano-grains, thereby improving the tensile strength and elongation at break of the copper foil. Compared to the DC current deposition commonly used in existing technologies, pulsed current deposition can achieve higher peak current densities, which facilitates the formation of large amounts of copper foil nucleation, thereby increasing the density of the copper foil and reducing the grain size of the copper foil to improve its mechanical strength.

[0151] In some embodiments, the electroplating solution includes copper ions and chloride ions.

[0152] In some embodiments, the concentration of copper ions in the electroplating solution is 30 g / L-100 g / L, optionally 45 g / L-75 g / L.

[0153] In some embodiments, the concentration of copper ions in the electroplating solution 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 therebetween.

[0154] The research results show that a combination of low copper ion concentration and high additive concentration is more conducive to improving the strength and elongation at break of copper foil than a combination of high copper ion concentration and low additive concentration. In addition, low copper ion concentration helps control the growth and deposition rate of grains, facilitates the preparation of small grains, and improves the mechanical strength of copper foil.

[0155] In some embodiments, the concentration of chloride ions is 10 mg / L-80 mg / L.

[0156] In some embodiments, the concentration of chloride ions is 10 mg / L, 20 mg / L, 30 mg / L, 40 mg / L, 50 mg / L, 60 mg / L, 70 mg / L, 80 mg / L, or any range therebetween.

[0157] In some embodiments, the electroplating solution further includes additives, wherein the additives include one or more of a leveler, a wetting agent, and a brightener.

[0158] As used herein, the term "leveler" refers to a substance added to an electroplating bath to improve the flatness of the plated layer.

[0159] In this article, the term "wetting agent" refers to a substance used to reduce the interfacial tension between the plating solution and the electrode, thereby improving the adhesion of the plated layer to the substrate.

[0160] In this article, the term "brightener" refers to a substance that improves the smoothness of the coating and reduces the surface roughness.

[0161] Levelers adhere to the tips of copper foil, where deposition rates are high, inhibiting grain growth and balancing the growth rates of pits and tips, thereby improving the flatness of the foil. Wetting agents enhance the wettability of the plating solution and the substrate. The plating solution's wettability on the cathode is sufficient to enable rapid electrodeposition with high currents, increasing the nucleation rate of the copper foil and reducing the average grain size. Brighteners can further refine the grain size of the copper foil and reduce surface roughness, improving surface smoothness.

[0162] In some embodiments, the additives include a leveler, a wetting agent, and a brightener.

[0163] The research results show that the simultaneous addition of multiple additives can produce a synergistic effect and effectively improve the strength and plasticity of copper foil.

[0164] In some embodiments, the leveling agent includes one or more of gelatin and Janus Green; the wetting agent includes one or more of hydroxyethyl cellulose and polyethylene glycol; the brightening agent includes one or more of sodium polydisulfide dipropane sulfonate, sodium 3-mercapto-1-propane sulfonate, and thiourea.

[0165] In some embodiments, the wetting agent includes hydroxyethylcellulose and polyethylene glycol.

[0166] The results show that adding two different wetting agents simultaneously can more effectively improve the mechanical strength of copper foil than adding a single wetting agent. Although the mechanism is not yet clear, it is speculated that it may be related to the competitive adsorption of the two different additives, which helps to improve the consistency of grains during the deposition process, reduce the difference in grain size through the thickness direction, and improve the uniformity of the copper foil.

[0167] In some embodiments, the concentration of the leveler in the electroplating solution is 20 mg / L-300 mg / L, optionally 50 mg / L-150 mg / L.

[0168] In some embodiments, the concentration of the leveler in the plating solution is 20 mg / L, 50 mg / L, 100 mg / L, 150 mg / L, 200 mg / L, 250 g / L, 300 mg / L, or any range therebetween.

[0169] In some embodiments, the concentration of the wetting agent in the electroplating solution is 10 mg / L-200 mg / L, optionally 30 mg / L-100 mg / L.

[0170] In some embodiments, the concentration of the wetting agent in the plating solution is 10 mg / L, 30 mg / L, 50 mg / L, 80 mg / L, 100 mg / L, 130 mg / L, 150 mg / L, 180 mg / L, 200 mg / L, or any range therebetween.

[0171] In some embodiments, the concentration of the brightener in the electroplating solution is 10 mg / L-200 mg / L.

[0172] In some embodiments, the concentration of the brightener in the plating solution is 10 mg / L, 20 mg / L, 30 mg / L, 40 mg / L, 50 mg / L, 60 mg / L, 70 mg / L, 80 mg / L, 90 mg / L, 100 mg / L, 110 mg / L, 120 mg / L, 130 mg / L, 140 mg / L, 150 mg / L, 160 mg / L, 170 mg / L, 180 mg / L, 190 mg / L, 200 mg / L or any range therebetween.

[0173] In some embodiments, the pH of the plating solution is 2.5-4.4.

[0174] In some embodiments, the pH of the plating solution is 2.5, 2.7, 2.9, 3.1, 3.3, 3.5, 3.7, 3.9, 4.1, 4.4, or any range therebetween.

[0175] In some embodiments, the mass concentration ratio of the copper ion to the leveler is 500:1-4500:1; and / or the mass concentration ratio of the copper ion to the brightener is 300:1-6000:1.

[0176] In some embodiments, the mass concentration ratio of the copper ion to the leveler can be 500:1, 1000:1, 1500:1, 2000:1, 2500:1, 3000:1, 3500:1, 4000:1, 4500:1, or any range therebetween.

[0177] In some embodiments, the mass concentration ratio of the copper ion to the brightener can be selected as 300:1, 1000:1, 2000:1, 3000:1, 4000:1, 5000:1, 6000:1 or any numerical range therebetween.

[0178] In some embodiments, the electroplating solution includes gelatin at a concentration of 20 mg / L-150 mg / L, polyethylene glycol at a concentration of 15 mg / L-100 mg / L, hydroxyethyl cellulose at a concentration of 10 mg / L-80 mg / L, sodium polydisulfide bis(propylene sulfonate) at a concentration of 15 mg / L-150 mg / L, copper ions at a concentration of 45 g / L-90 g / L, and chloride ions at a concentration of 10 mg / L-80 mg / L.

[0179] In some embodiments, the peak current density of the pulse current is I, in A / dm 2 The current duty cycle of the pulse current is s, and the peak current density I and the duty cycle s meet: 2A / dm 2 ≤I×s≤18A / dm 2 .

[0180] In this document, the term “current duty cycle of a pulse current” refers to the proportion of the current conduction time in the pulse current to the entire pulse period.

[0181] In some embodiments, the product of the peak current density I and the duty cycle s is 2A / dm 2 , 4A / dm 2 , 6A / dm 2 , 8A / dm 2 、10A / dm 2 , 12A / dm 2 、14A / dm 2 、16A / dm 2 、18A / dm 2 or any range of values ​​between them.

[0182] The product of the peak current density I and the duty cycle s is equivalent to the average density of the current during the pulse period. The pulse current deposition method allows the average current density during the pulse period to be higher than the maximum current density achievable during DC deposition, because excessively high DC deposition density can cause hydrogen evolution and concentration polarization in the electroplating solution, reducing the uniformity of the copper foil during the electroplating process. The product of the peak current density I and the duty cycle s within the above range enables a dynamic balance of copper ion consumption and replenishment in the electroplating solution to be achieved by controlling the opening and closing of the current, reducing the concentration difference between the copper ion deposition site and other parts of the electroplating solution, so that the electroplating solution does not undergo 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 within the above range can control the nucleation and growth rate of the grains, thereby achieving the preparation of small-size grains to improve the mechanical strength of the copper foil.

[0183] In some embodiments, the peak current density I of the pulse current satisfies: 3.3 A / dm 2 ≤I≤333A / dm 2 2 .

[0184] In some embodiments, the peak current density I of the pulse current is 3.3 A / dm 2 、8.35A / dm 2 、10A / dm 2 、100A / dm 2 、150A / dm 2 、167A / dm 2 、180A / dm 2 、200A / dm 2 、300A / dm 2 、333A / dm 2 or any range of values ​​between them.

[0185] In some embodiments, the duty cycle s of the pulse current is 2% to 50%.

[0186] In some embodiments, the duty cycle s of the pulse current is 2%, 3.30%, 5%, 10%, 20%, 30%, 40%, 50%, or any range therebetween.

[0187] In some embodiments, the pulse width of the pulse current is 1 ms to 50 ms.

[0188] In this article, the term "pulse width" refers to the duration of the current within a pulse period.

[0189] In some embodiments, the pulse width of the pulse current is 1 ms, 5 ms, 10 ms, 15 ms, 20 ms, 25 ms, 30 ms, 35 ms, 40 ms, 45 ms, 50 ms, or any range therebetween.

[0190] In some embodiments, the distance between the cathode electrode and the anode electrode is 15 mm to 20 mm.

[0191] In some embodiments, the distance between the cathode electrode and the anode electrode is 15 mm, 16 mm, 17 mm, 18 mm, 19 mm, 20 mm, or any range therebetween.

[0192] In some embodiments, the deposition temperature is 45°C-60°C.

[0193] In some embodiments, the deposition temperature may be 45° C., 50° C., 55° C., 60° C., or any range therebetween.

[0194] In some embodiments, the cathode electrode is a titanium roller or plate.

[0195] In some embodiments, the anode electrode is a titanium substrate plate.

[0196] In some embodiments, the deposition time is greater than or equal to 80 seconds.

[0197] In some embodiments, the deposition time may be 80 s, 100 s, 200 s, 220 s, 240 s, 280 s, 300 s, 400 s, 500 s, 800 s, 1000 s, or any range therebetween.

[0198] In some embodiments, the electroplating method specifically includes periodically applying a pulse current to the electroplating solution to reduce and deposit copper ions in the electroplating solution; wherein the peak current density I of the pulse current satisfies 100A / dm 2 ≤I≤180A / dm 2 The duty cycle s of the pulse current satisfies: 2%≤s≤10%, and the deposition time is 80s-250s.

[0199] In some embodiments, the preparation method is a continuous production method.

[0200] In some embodiments, the preparation method is a roller deposition method. Its operating principle is that a cathode roller is connected to the negative pole of a power supply, and an anode cell is connected to the positive pole of a power supply. When a plating solution containing copper ions enters the anode cell, an electric field is formed between the positive and negative electrodes. Under the influence of this electric field, the copper ions migrate to the surface of the cathode roller and deposit. The deposited copper foil is then peeled off the cathode roller and wound onto another roller. The plating solution is continuously added and circulated. Under the influence of the electric field, copper ions are continuously deposited on the cathode roller, continuously peeled off, and wound onto a reel. This preparation method can achieve continuous production of large-scale copper foil, paving the way for industrial applications.

[0201] The present application also provides a copper foil, which includes the same features as the copper foil in the aforementioned current collector, and will not be repeated here.

[0202] In any embodiment, the copper foil is prepared by the preparation method in any of the above embodiments.

[0203] In some embodiments, the maximum width of the copper foil is greater than or equal to 1.5 meters, and / or the maximum length of the copper foil is greater than or equal to 10,000 meters.

[0204] In some embodiments, the maximum width of the copper foil is 1.5 meters, 2 meters, 2.5 meters, 3 meters, 3.5 meters, 4 meters, 6 meters, 8 meters, 10 meters, or any range therebetween.

[0205] In some embodiments, the maximum length of the copper foil is 10,000 meters, 15,000 meters, 20,000 meters, 25,000 meters, 30,000 meters, 60,000 meters, 100,000 meters, or any range therebetween.

[0206] The copper foil provided in the embodiments of the present application can be manufactured in large sizes and has prospects for industrial application.

[0207] Pole

[0208] The present application also provides a pole piece, which includes the current collector in any embodiment.

[0209] In some embodiments, the electrode sheet is a negative electrode sheet, and the current collector is a negative electrode current collector.

[0210] In some embodiments, the negative electrode plate includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector. For example, the negative electrode current collector has two surfaces that are opposite to each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposing surfaces of the negative electrode current collector.

[0211] In some embodiments, the negative electrode film layer includes a negative electrode active material. In some embodiments, the negative electrode active material includes, but is not limited to, one or more of conventional natural graphite, artificial graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. The silicon-based material may include one or more of elemental silicon, silicon oxide, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy material. The tin-based material may include one or more of elemental tin, tin oxide, and tin alloy material.

[0212] In some embodiments, the electrode sheet is a negative electrode sheet, which includes a current collector and a negative electrode film layer arranged on at least one side of the negative electrode current collector, the negative electrode film layer contains a negative electrode active material, and the negative electrode active material contains a silicon-based material. The tensile elongation at break of the negative electrode sheet at room temperature is greater than or equal to 2%, and can be optionally 3%-5%.

[0213] Herein, the term "room temperature" refers to 20±10°C.

[0214] In this article, the term "tensile elongation at break" refers to the ratio of the change in length of a material when it deforms after being subjected to force until it breaks to its original length. It is usually expressed as a percentage and is an important parameter for measuring the material's ability to withstand stress deformation during stretching.

[0215] The tensile elongation at break of the negative electrode sheet can be tested using methods known in the art. As an example, at least four samples with a length of 200±0.5mm and a width of 15±0.25mm are cut from the negative electrode sheet. Using a pneumatic chuck with the air pressure set to 0.2MPa, the sheet is passed through the upper and lower chucks. The air valve is pressed to clamp the upper chuck first, maintaining a weak connection state (i.e., keeping the sample in a naturally bent state, and no external force is applied). Then, the lower chuck is clamped. The sample is continuously loaded at a tensile speed of 2mm / min until it breaks. The gauge length is 50mm. The tensile elongation at break of the negative electrode sheet is calculated as the extended length between the gauge lengths at the time of sample tensile fracture divided by the length between the gauge lengths before the sample is stretched.

[0216] In some embodiments, the tensile elongation at break of the negative electrode sheet at room temperature may be 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, or any range therebetween.

[0217] In some embodiments, the thickness of the negative electrode sheet is 100-150 μm, which is directly measured using a micrometer.

[0218] The negative electrode plate of the secondary battery has a high tensile elongation at break, which allows it to have sufficient expansion space during the later cycle of the secondary battery and can withstand the reciprocating expansion of the secondary battery during the cycle, so that the secondary battery with silicon-based materials has high energy density while improving safety performance and cycle life simultaneously.

[0219] In some embodiments, the compaction density of the negative electrode sheet is greater than or equal to 1.5 g / cm 3 , and the cold pressing elongation of the negative electrode plate is less than or equal to 0.1%, and can be optionally 0.06%-0.1%.

[0220] In the present application, the compaction density of the negative electrode plate has a meaning well known in the art and can be tested by methods known in the art. The compaction density of the negative electrode plate = the surface density of the negative electrode film layer / the thickness of the negative electrode film layer. The thickness of the negative electrode film layer has a meaning well known in the art and can be tested by methods known in the art, such as using a micrometer (such as Mitutoyo 293-100, with an accuracy of 0.1 μm). The surface density of the negative electrode film layer can be tested by methods known in the art. For example, a negative electrode plate after cold pressing or a negative electrode plate obtained after disassembly of the battery (if it is a double-sided coated negative electrode plate, the negative electrode film layer on one side can be wiped off first) can be punched into small discs with an area of ​​S1, and the weight is measured and recorded as M1. Then wipe off the negative electrode film layer of the weighed negative electrode plate, weigh the weight of the negative electrode current collector, and record it as M0. The surface density of the negative electrode plate = (M1-M0) / S1.

[0221] In some embodiments, the compaction density of the negative electrode sheet can be 1.5 g / cm 3 , 1.55g / cm 3 , 1.6g / cm 3 , 1.65g / cm 3 , 1.7g / cm 3 , 1.75g / cm 3 , 1.8g / cm 3 , 1.85g / cm 3 , 1.9g / cm 3 , 1.95g / cm 3 , 2.0g / cm 3 or any range of values ​​between them.

[0222] The cold pressing elongation of the negative electrode sheet can be tested by methods known in the art. As an example, two points are taken on the surface of the negative electrode sheet before cold pressing along the (MD) direction perpendicular to the cold pressing roller, and the distance L1 is measured; the distance L2 between the two points is measured on the negative electrode sheet after cold pressing, and (L2-L1) / L1 is the cold pressing elongation of the negative electrode sheet. Measure at least five different locations and take the average value as the cold pressing elongation of the negative electrode sheet. It should be understood that the cold pressing elongation of the negative electrode sheet can also be obtained by characterizing the disassembled secondary battery. As an example, the secondary battery is disassembled to obtain the negative electrode sheet, and the difference between the fracture elongation of the current collector in the area covered by the negative electrode film layer and the fracture elongation of the current collector in the empty foil area is tested. This difference can also be used to characterize the cold pressing elongation of the negative electrode sheet.

[0223] In some embodiments, the cold pressing elongation of the negative electrode sheet can be selected as 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1% or any range therebetween.

[0224] The negative electrode of this secondary battery achieves a high compaction density while maintaining a low elongation during the cold pressing process. This allows for more space for expansion during the battery's cycling, further improving the battery's safety and cycle life. Furthermore, the low cold pressing elongation reduces the risk of wrinkling of the negative electrode tab during the cold pressing process, potentially eliminating the need for the pre-stretching process currently used to reduce the likelihood of wrinkling during cold pressing, thereby improving production efficiency.

[0225] In some embodiments, the compaction density of the negative electrode sheet is 1.5 g / cm 3 -2.0g / cm 3 .

[0226] Compared to carbon-based active materials commonly used in negative electrodes, silicon-based materials have higher hardness and relatively poorer slip properties, requiring greater cold pressing pressure to achieve the same compaction density. This secondary battery's silicon-containing negative electrode exhibits high ductility and can achieve a high compaction density, further increasing the battery's energy density while improving safety and lifespan.

[0227] In some embodiments, based on the total mass of the negative electrode film layer, the mass content of the silicon-based material is 5%-100%.

[0228] In some embodiments, based on the total mass of the negative electrode film layer, the mass content of the silicon-based material can be selected to be 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 100% or any numerical range therebetween.

[0229] In some embodiments, based on the total mass of the negative electrode film layer, the mass content of the silicon-based material is 10%-60%.

[0230] The silicon-based material within the above content range enables the secondary battery to have good energy density, safety and cycle stability.

[0231] In some embodiments, the current collector has a thickness of 2-10 μm.

[0232] In some embodiments, the thickness of the current collector may be selected to be 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm or any range of values ​​therebetween. The thickness of the current collector may be measured by any known method in the art. As an example, the thickness of the current collector may be measured by a thickness gauge. The current collector may also be punched into small discs, weighed, and the sample mass divided by the sample surface area is used as the surface density of the sample, and the thickness of the current collector is calculated by dividing the surface density of the sample by the density of the current collector material (for example, if the current collector is copper foil, the density of the current collector is 8.9 g / cm 3 ). The density of the current collector material can be characterized by a composition tester.

[0233] In some embodiments, the negative electrode film layer may further optionally include a negative electrode conductive agent. The present application does not particularly limit the type of the negative electrode conductive agent. As examples, the negative electrode conductive agent may include one or more of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0234] In some embodiments, the negative electrode film layer may further optionally include a negative electrode binder. The present application does not particularly limit the type of the negative electrode binder. As examples, the negative electrode binder may include one or more of styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, water-based acrylic resin (e.g., polyacrylic acid PAA, polymethacrylic acid PMAA, sodium polyacrylate PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethyl chitosan (CMCS).

[0235] In some embodiments, the negative electrode film layer may further include other additives. For example, the other additives may include a thickener, such as sodium carboxymethyl cellulose (CMC), a PTC thermistor material, and the like.

[0236] The negative electrode film layer is typically formed by coating a negative electrode slurry onto a negative electrode current collector, drying, and cold pressing. The negative electrode slurry is typically formed by dispersing the negative electrode active material, an optional conductive agent, an optional binder, and other optional additives in a solvent and stirring until uniformly mixed. The solvent may be, but is not limited to, N-methylpyrrolidone (NMP) or deionized water.

[0237] The negative electrode plate does not exclude other additional functional layers in addition to the negative electrode film layer. For example, in some embodiments, the negative electrode plate described in the present application further includes a conductive primer layer (e.g., composed of a conductive agent and a binder) sandwiched between the negative electrode current collector and the negative electrode film layer and disposed on the surface of the negative electrode current collector; in some embodiments, the negative electrode plate described in the present application further includes a protective layer covering the surface of the negative electrode film layer.

[0238] secondary batteries

[0239] The present application also provides a secondary battery, which can be in the form of a battery cell, a battery module or a battery pack.

[0240] In some embodiments, the maximum expansion force of the secondary battery cell is greater than or equal to 1000 kgf.

[0241] In some embodiments, the maximum expansion force of the secondary battery cell is greater than or equal to 2500 kgf.

[0242] In some embodiments, the maximum expansion force of the secondary battery cell is greater than or equal to 4000 kgf.

[0243] 1 kgf refers to the gravitational force exerted on a 1-kilogram object at sea level at 45 degrees north latitude. 1 kgf is approximately 9.8 Newtons. The expansion force of the secondary battery cell is sensed by pressure sensors installed in fixtures on both sides of the large surface of the secondary battery cell electrode.

[0244] Current collectors in existing technologies are prone to breaking under the high expansion forces of secondary batteries, causing safety accidents and limiting further improvements in the electrochemical performance of secondary batteries. The current collectors provided in the embodiments of the present application have both excellent tensile strength and elongation at break, making them suitable for secondary batteries with high expansion forces and conducive to further improving the energy density of secondary batteries.

[0245] The present application has no particular restrictions on the type of secondary battery. For example, the secondary battery can be a lithium-ion battery, etc. In general, a secondary battery includes a positive electrode plate, a negative electrode plate, and an electrolyte, etc. During the charge and discharge process of the secondary battery, active ions are embedded and released back and forth between the positive electrode plate and the negative electrode plate, and the electrolyte plays a role in conducting active ions between the positive electrode plate and the negative electrode plate. The present application has no particular restrictions on the type of the electrolyte, and it can be selected according to actual needs. For example, the electrolyte can be selected from at least one of a solid electrolyte and a liquid electrolyte (i.e., an electrolyte). Secondary batteries using electrolytes and some secondary batteries using solid electrolytes can also include an isolation membrane, which is arranged between the positive electrode plate and the negative electrode plate to play an isolation role.

[0246] [Positive electrode]

[0247] In some embodiments, the positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector. For example, the positive electrode current collector has two surfaces that are opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposing surfaces of the positive electrode current collector.

[0248] The positive electrode current collector may be a metal foil or a composite current collector. As an example of a metal foil, copper foil may be used. The composite current collector may include a polymer material base layer and a metal material layer formed on at least one surface of the polymer material base layer. As an example, the metal material may include one or more of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy. As an example, the polymer material base layer may include one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS) and polyethylene (PE).

[0249] The positive electrode film layer generally includes a positive electrode active material, an optional binder, and an optional conductive agent. The positive electrode film layer is usually formed by coating a positive electrode slurry on the positive electrode current collector and then drying and cold pressing. The positive electrode slurry is usually formed by dispersing a positive electrode active material, an optional conductive agent, an optional binder, and any other components in a solvent and stirring evenly. The solvent can be N-methylpyrrolidone (NMP), but is not limited thereto. As an example, the binder used for the positive electrode film layer may include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin. As an example, the conductive agent used for the positive electrode film layer includes one or more of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0250] The positive electrode active material can adopt the positive electrode active materials for secondary batteries well-known in the art.

[0251] When the secondary battery of the present application is a lithium-ion battery, the positive electrode active material may include, but is not limited to, one or more of lithium-containing transition metal oxides, lithium-containing phosphates, and their respective modified compounds. Examples of the lithium transition metal oxides may include, but are not limited to, one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and their respective modified compounds. Examples of the lithium-containing phosphates may include, but are not limited to, lithium iron phosphate, a composite material of lithium iron phosphate and carbon, lithium manganese phosphate, a composite material of lithium manganese phosphate and carbon, lithium manganese iron phosphate, a composite material of lithium manganese iron phosphate and carbon, and their respective modified compounds.

[0252] In some embodiments, in order to further improve the energy density of the secondary battery, the positive electrode active material for the lithium-ion battery may include a lithium transition metal oxide having the general formula Li a Ni b Co c M d O e A f and its modified compounds. 0.8 ≤ a ≤ 1.2, 0.5 ≤ b < 1, 0 < c < 1, 0 < d < 1, 1 ≤ e ≤ 2, 0 ≤ f ≤ 1, M is selected from one or more of Mn, Al, Zr, Zn, Cu, Cr, Mg, Fe, V, Ti, and B, and A is selected from one or more of N, F, S, and Cl.

[0253] In some embodiments, as examples, the positive electrode active material for lithium ion batteries may include LiCoO2, LiNiO2, LiMnO2, LiMn2O4, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2(NCM333),LiNi 0.5 Co 0.2 Mn 0.3 O2(NCM523), LiNi 0.6 Co 0.2 Mn 0.2 O2(NCM622),LiNi 0.8 Co 0.1 Mn 0.1 O2(NCM811), LiNi 0.85 Co 0.15 Al 0.05 One or more of O2, LiFePO4 and LiMnPO4.

[0254] In the present application, the modified compounds of the above-mentioned positive electrode active materials may be the ones subjected to doping modification and / or surface coating modification on the positive electrode active materials.

[0255] [Electrolytes]

[0256] In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0257] The type of the electrolyte salt is not particularly limited and can be selected according to actual needs.

[0258] When the secondary battery of the present application is a lithium ion battery, as an example, the electrolyte salt may include one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bisfluorosulfonyl imide (LiFSI), lithium bistrifluoromethanesulfonyl imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalatoborate (LiDFOB), lithium dioxalatoborate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorobisoxalatophosphate (LiDFOP) and lithium tetrafluorooxalatophosphate (LiTFOP).

[0259] The type of the solvent is not specifically limited and can be selected according to actual needs. In some embodiments, for example, the solvent may include ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), ethyl methyl sulfone (EMS) and diethyl sulfone (ESE). One or more.

[0260] In some embodiments, the electrolyte may further optionally include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, or additives capable of improving certain properties of the secondary battery, such as additives that improve the overcharge performance of the secondary battery, additives that improve the high-temperature performance of the secondary battery, and additives that improve the low-temperature power performance of the secondary battery.

[0261] [Isolation film]

[0262] The present application has no particular limitation on the type of the isolation membrane, and any known porous isolation membrane with good chemical stability and mechanical stability can be selected.

[0263] In some embodiments, the material of the isolation membrane may include one or more of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The isolation membrane may be a single-layer film or a multi-layer composite film. When the isolation membrane is a multi-layer composite film, the materials of each layer may be the same or different.

[0264] In some embodiments, the positive electrode sheet, the separator, and the negative electrode sheet can be formed into an electrode assembly through a winding process or a lamination process.

[0265] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0266] In some embodiments, the outer packaging can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging can also be a soft package, such as a bag-type soft package. The material of the soft package can be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0267] The present application has no particular limitation on the shape of the secondary battery, which can be cylindrical, square, or any other shape. FIG5 shows a secondary battery 5 with a square structure as an example.

[0268] In some embodiments, as shown in FIG6 , the outer packaging may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 is used to cover the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation film can be formed into an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated 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 adjusted according to demand.

[0269] The preparation method of the secondary battery of the present application is well known. In some embodiments, a positive electrode sheet, a separator, a negative electrode sheet, and an electrolyte can be assembled to form a secondary battery. As an example, the positive electrode sheet, separator, and negative electrode sheet can be formed into an electrode assembly through a winding process or a lamination process. The electrode assembly is placed in an outer packaging, dried, and then injected with electrolyte. The secondary battery is then vacuum packaged, allowed to stand, formed, and shaped.

[0270] In some embodiments of the present application, the secondary batteries according to the present application can be assembled into a battery module. The battery module can contain multiple secondary batteries, and the specific number can be adjusted according to the application and capacity of the battery module.

[0271] Figure 7 is a schematic diagram of an exemplary battery module 4. As shown in Figure 7 , within the battery module 4, multiple secondary batteries 5 may be arranged sequentially along the length of the battery module 4. Of course, any other arrangement is also possible. Furthermore, the multiple secondary batteries 5 may be secured together using fasteners.

[0272] Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0273] In some embodiments, the battery modules described above may also be assembled into a battery pack, and the number of battery modules contained in the battery pack may be adjusted according to the application and capacity of the battery pack.

[0274] Figures 8 and 9 are schematic diagrams of an exemplary battery pack 1. As shown in Figures 8 and 9, the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the box. The battery box comprises an upper case 2 and a lower case 3. The upper case 2 covers the lower case 3 and forms an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 may be arranged in any manner within the battery box.

[0275] Electrical devices

[0276] The present application also provides an electric device, which includes at least one of the secondary battery, battery module, or battery pack of the present application. The secondary battery, battery module, or battery pack can be used as a power source for the electric device, and can also be used as an energy storage unit for the electric device. The electric device can be, but is not limited to, a mobile device (such as a mobile phone, a tablet computer, a laptop 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, a satellite, an energy storage system, etc.

[0277] The electrical device may select a secondary battery, a battery module or a battery pack according to its usage requirements.

[0278] Figure 10 is a schematic diagram of an exemplary electric device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of this device, a battery pack or battery module may be used.

[0279] As another example, the electric device may be a mobile phone, a tablet computer, a laptop computer, etc. Such an electric device is usually required to be lightweight and thin, and may use a secondary battery as a power source.

[0280] Example

[0281] The following examples describe the present disclosure in more detail. These examples are intended to be illustrative only, as various modifications and variations within the scope of the present disclosure will be apparent to those skilled in the art. Unless otherwise stated, all parts, percentages, and ratios reported in the following examples are by mass, and all reagents used in the examples are commercially available or synthesized according to conventional methods and can be used directly without further processing. The instruments used in the examples are commercially available.

[0282] Example 1

[0283] (1) Preparation of copper foil

[0284] Copper sulfate pentahydrate is prepared by dissolving a copper plate or copper wire with a purity of at least 99.9% in a sulfuric acid solution. This solution is used as the copper source. Additives and oxalic acid are added to prepare the electroplating solution at 60°C. The additives include gelatin, polyethylene glycol, hydroxyethyl cellulose, and sodium chloride. The electroplating solution has a pH of 3.5 and contains the following components: 45g / L copper ion, 90mg / L gelatin, 50mg / L polyethylene glycol, 30mg / L hydroxyethyl cellulose, 80mg / L chloride ion, and 75mg / L sodium polydisulfide. The remainder is deionized water.

[0285] A square wave pulse current waveform is used to periodically apply current to the polished cathode titanium roller. The area of ​​the titanium roller in the plating solution is 0.3dm 2 The rotation speed of the titanium roller is 2m / min, and a pulse current is applied with a current density of 167A / dm 2 , duty cycle 5%, pulse width 1ms, cathode-anode distance 20mm, deposition temperature 60℃, electroplating time 224s, copper foil deposited on the titanium roller, copper foil thickness 6μm.

[0286] (2) Preparation of batteries

[0287] The positive electrode active material LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811), conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) are dissolved in solvent N-methylpyrrolidone (NMP) in a weight ratio of 90:5:5, and are fully stirred and mixed to obtain a positive electrode slurry; the positive electrode slurry is then evenly coated on the positive electrode collector, and then dried, cold pressed, and cut to obtain a positive electrode sheet.

[0288] The negative electrode active material artificial graphite, the conductive agent acetylene black, the binder styrene-butadiene rubber (SBR), and the thickener sodium carboxymethyl cellulose (CMC) are dissolved in the solvent deionized water in a weight ratio of 90:4:4:2, and are evenly mixed to prepare a negative electrode slurry; the negative electrode slurry is then evenly coated on the negative electrode current collector copper foil once or multiple times, and the negative electrode film is obtained after drying, and then the negative electrode sheet is obtained by cold pressing and slitting.

[0289] In an argon atmosphere glove box (H2O <0.1ppm, O2 <0.1ppm), the organic solvents fluoroethylene carbonate (FEC), ethylene carbonate (EC), and ethyl methyl carbonate (EMC) were mixed in a mass ratio of 1:2:7, and 12.5wt% LiPF6 lithium salt was added and dissolved in the organic solvent. Then, 2.0wt% fluoroethylene carbonate, 0.5wt% 1,3-propane sultone, and 0.5wt% succinic anhydride were added as additives to the organic solvent and mixed and stirred to obtain an electrolyte.

[0290] Polypropylene film is used as the isolation film.

[0291] The positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator placed between the positive and negative electrode sheets to serve as an isolation, and then wound to obtain an electrode assembly; the electrode assembly is placed in a battery casing, dried, and then injected with electrolyte, and then undergoes formation, static and other processes to produce a lithium-ion battery.

[0292] Examples 2-8

[0293] The preparation methods of Examples 2-8 are basically the same as those of Example 1, except that the components or concentrations of the electrolyte, or the parameters of the pulse deposition are changed, as shown in Table 1.

[0294] Example 9

[0295] The preparation method of Example 9 is basically the same as that of Example 1, except that the ratio of the negative electrode sheet is changed. Specifically, the negative electrode active material silicon carbon (silicon content 20wt%-50wt%), artificial graphite, conductive agent acetylene black, binder styrene butadiene rubber (SBR), and thickener sodium carboxymethyl cellulose (CMC) are dissolved in solvent deionized water in a mass ratio of 10:86:1:1.5:1.5, and are evenly mixed to prepare a negative electrode slurry; then the negative electrode slurry is evenly coated on the negative electrode collector copper foil once or multiple times, and the negative electrode film is obtained after drying, and then the negative electrode sheet is obtained by cold pressing and slitting.

[0296] Example 10

[0297] Example 10 is basically the same as Example 9, except that the components in the negative electrode film layer are kept unchanged and the ratio of the components in the negative electrode film layer is adjusted. The specific ratio of the negative electrode film layer is 30:66:1:1.5:1.5 of negative electrode active material silicon carbon, artificial graphite, conductive agent acetylene black, binder styrene butadiene rubber (SBR), and thickener sodium carboxymethyl cellulose (CMC).

[0298] Example 11

[0299] Example 11 is basically the same as Example 10, except that the compaction density of the negative electrode film layer is adjusted, as shown in Table 4.

[0300] Comparative Example 1

[0301] The preparation method of Comparative Example 1 is basically the same as that of Example 1, except that the electrodeposition method is changed to use direct current for deposition, and the deposition current density is 8.35A / dm 2 , as shown in Table 1.

[0302] Comparative Example 2

[0303] The preparation method of Comparative Example 2 is basically the same as that of Example 9, except that conventional copper foil in the prior art is used as the current collector with a thickness of 6 μm. Under the test conditions of room temperature, sample thickness of 6±0.2 μm, and tensile speed of 50±0.5 mm / min, the tensile strength of the copper foil is 350 MPa and the elongation at break is 6.2%.

[0304] Comparative Example 3

[0305] The preparation method for Comparative Example 3 is essentially the same as that for Comparative Example 2, except that a pre-stretching process is added to the negative electrode sheet before cold pressing. Specifically, a pre-stretching roller with a tension of 400-700N is added before cold pressing to pre-stretch the blank area of ​​the coated copper foil. This prevents wrinkling caused by stretching the film area while the blank area does not stretch during the cold pressing process.

[0306] Table 1

[0307] Performance Testing

[0308] (1) Thickness test

[0309] Take 20×15cm 2 The thickness of the sample is calculated by weighing method, which is as follows: the cut sample is placed on an electronic balance and weighed to obtain the weight of the sample. Then, according to the density of copper foil ρ of 8.96g / cm 3 Calculate the volume of the spline. Given the length and width of the spline, the thickness of the spline can be calculated.

[0310] (2) Mechanical properties test

[0311] According to GB / T 5230-1995 "Electrolytic Copper Foil," cut tensile specimens with a length L0 of 200 mm and a width of 15 mm. Place the specimen on a balance and weigh it, recording the mass in m. Use a universal testing machine to test tensile properties at 25°C, with a tensile rate set to 50 mm / min.

[0312] The cross-sectional area of ​​the stretched spline is where ρ is 8.96 g / cm 3 , the unit of m is gram, and the unit of L0 is centimeter.

[0313] Continuously apply load to the sample until it breaks, read the maximum load F from the force gauge or tensile curve, and calculate the tensile strength σ according to formula I b .

[0314] The distance between the two lines after the specimen is broken is L1, which can be measured on the specimen or read from the tensile curve. L1 can be measured using the linear method or the displacement method, and the elongation at break δ can be calculated using Formula II.

[0315] (3) Grain characteristic test of copper foil

[0316] Electron backscatter diffraction (EBSD) combined with scanning electron microscopy was used to observe the cross-section of the copper foil, where the electron backscatter diffractometer model was Oxford C-Nano+. The pole figure distribution map was obtained, and the particle size of each grain was measured. The diameter of the equivalent circle of the grain was used as the particle size of the grain, and the distribution statistics of the grain size were performed. The skewed distribution was used for fitting, and the particle size corresponding to the peak was used as the average particle size of the grains in the copper foil. The number of twins was counted by the inverse pole figure. Different colors represent different grain orientations. The red lines inside the grains are twin boundaries, indicating that they have twin structures. The analysis software supporting the Oxford C-Nano+ electron backscatter diffractometer was used to count the percentage of nano twin grains. A similar method was used to measure the percentage of equiaxed grains or quasi-equiaxed grains.

[0317] (4) Maximum surface roughness test

[0318] Wipe the copper foil surface with alcohol. Place the foil firmly on a level surface. Place a stylus roughness tester on the surface of the foil, bringing the stylus into contact with it. Gently run the stylus across the surface, and read the instrument's readings to determine the copper foil's roughness. Test five different locations on the foil, and take the highest reading as the maximum surface roughness.

[0319] (5) State of Health (SOH) corresponding to crack failure

[0320] At 25°C, the battery is charged at a constant current of 1C to a voltage of 3.8V, then charged at a constant voltage of 3.8V to a current ≤ 0.05C, and then the battery is discharged at a constant current of 1C to a voltage of 2.5V. This is a charge and discharge process, and the charge and discharge cycle is repeated. Then, electronic computed tomography (CT) is used to determine whether cracks have occurred inside the battery. If cracks have occurred, the battery that has failed after cracks have occurred during the cycle is disassembled, the negative electrode of the battery is disassembled, and samples are taken at the place where no cracks appear. The fracture elongation of the sample is measured according to the fracture elongation measurement method described above, and the SOH value of the battery at this time is obtained by simulation calculation.

[0321] (6) Cycle simulation calculation:

[0322] A fracture model was established based on finite element analysis, and the battery cell was modeled using the rolled core (JR) after hot pressing. The pole piece tensile curve was then used as the mechanical parameter input. Finally, the expansion force data of the battery cell was used to benchmark the free rebound of the JR size as the input of the crack model. The criterion was that the pole piece elongation of the battery cell under the action of the expansion force was greater than the pole piece fracture elongation. The corresponding state of charge (SOH) when the pole piece of the battery cell broke during a long cycle was simulated and analyzed.

[0323] (7) Observation of the tab area after cold pressing:

[0324] If wrinkles appear in the blank area of ​​the electrode, the tab is determined to be wrinkled; if the blank area of ​​the electrode remains flat and has no abnormalities, the tab is determined to be not wrinkled.

[0325] Test results

[0326] Table 2

[0327] Table 3

[0328] The test results of the embodiments and comparative examples are shown in Tables 1-3.

[0329] From the comparison between the embodiments and the comparative examples, it can be seen that the average particle size of the grains in the copper foil provided in the present application is 50nm-400nm, and based on the total number of grains in the copper foil, the number of twin grains accounts for not less than 60%. It has excellent tensile strength and elongation at break, can reduce the charge (SOH) value corresponding to battery crack failure, and help improve the safety and cycle life of the battery.

[0330] Figure 1 is a schematic diagram of the grain size distribution of the copper foil of Example 1; Figure 2 is a plot of the inverse pole figure distribution of the electron backscatter diffraction image of the copper foil of Example 1; and Figure 3 is a tensile curve plot of the copper foil of Example 1. Combining Figures 1 and 2, it can be seen that the average grain size of the copper foil of Example 1 is 200 nm; the minimum grain size is approximately 10 nm; based on the total number of grains in the copper foil, grains with a diameter below 200 nm account for 34%, grains with a diameter between 200 and 500 nm account for approximately 65%, and grains with a diameter greater than 500 nm account for no more than 2%. The maximum surface roughness of the copper foil is 0.12 μm. Figure 4a is an X-ray diffraction pattern of the copper foil of Example 2. The crystal plane diffraction intensity of the (111) texture in the copper foil accounts for 80.06% of the total crystal plane diffraction intensity of the (111) texture, (200) texture, (220) texture, (311) texture, and (222) texture in the copper foil. Compared to the copper foil of Comparative Example 1 shown in Figure 4b, the proportion of the crystal plane diffraction intensity of the (111) texture in the copper foil to the total crystal plane diffraction intensity of the (111) texture, (200) texture, (220) texture, (311) texture, and (222) texture in the copper foil is significantly increased.

[0331] As can be seen from Table 3, when the additives in the electroplating solution simultaneously contain a certain amount of leveling agent, wetting agent, and brightener, the maximum roughness of the copper foil surface can be reduced, and the tensile strength and elongation at break of the copper foil can be further improved simultaneously.

[0332] From the comparison of Examples 1, 2, and 8, it can be seen that compared with the electroplating solution containing high concentration of copper ions and low concentration of additives, the electroplating solution containing low concentration of copper ions and high concentration of additives can further improve the tensile strength and elongation at break of the copper foil.

[0333] From the comparison between Examples 1, 4, and 6 and Example 5, it can be seen that the product of high peak current density and duty cycle increases the average grain size of the copper foil and decreases the mechanical properties, but is still better than Comparative Example 1.

[0334] Table 4

[0335] From the comparison of Examples 9-11 and Comparative Example 3, it can be seen that the secondary battery includes a negative electrode plate, the negative electrode plate includes a negative electrode current collector and a negative electrode film layer arranged on at least one side of the negative electrode current collector, the negative electrode film layer contains a negative electrode active material, the negative electrode active material contains a silicon-based material, and the tensile elongation at break of the negative electrode plate at room temperature is greater than or equal to 2%, which can reduce the corresponding SOH when the battery cell plate breaks, thereby improving the service life of the battery.

[0336] From the comparison of Examples 9-11 and Comparative Example 2, it can be seen that when the current collector of this embodiment is used in a secondary battery containing silicon-based materials, the tabs will not be wrinkled after the pre-extension process is eliminated during the preparation of the negative electrode sheets, the battery production process can be optimized, and the production efficiency can be improved.

[0337] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A current collector, characterized in that: The current collector includes copper foil, the average particle size of the grains in the copper foil is 50nm-400nm, and based on the total number of grains in the copper foil, the number of nano twin grains accounts for greater than or equal to 60%.

2. The current collector according to claim 1, characterized in that The grains in the copper foil meet one or more of the following characteristics: (1) The average particle size of the grains in the copper foil is 50nm-300nm; (2) Based on the total number of grains in the copper foil, the number of grains with a particle size of less than 200 nm accounts for 20%-40%, and the number of grains with a particle size between 200 nm and 500 nm accounts for 50%-80%; (3) Based on the total number of grains in the copper foil, the number of grains with a particle size greater than 500 nm accounts for less than or equal to 5%; (4) Based on the total number of grains in the copper foil, the number of equiaxed grains or quasi-equiaxed grains accounts for greater than or equal to 80%, and the ratio a1 of the length of the major axis to the minor axis of the equiaxed grains or quasi-equiaxed grains satisfies: 1≤a1<2; (5) Based on the total number of grains in the copper foil, the number of columnar grains is less than 10%, and the ratio a2 of the length of the major axis to the minor axis of the columnar grains satisfies: a2>2; (6) The crystal plane diffraction intensity of the (111) texture in the copper foil accounts for 60% to 85% of the total crystal plane diffraction intensity of the (111) texture, (200) texture, (220) texture, (311) texture, and (222) texture in the copper foil; (7) The copper foil has a first surface and a second surface facing each other, the thickness of the copper foil is denoted as H, the area from the first surface of the copper foil to a thickness range of 0.2H-0.5H is denoted as the first area of the copper foil, and the area from the second surface of the copper foil to a thickness range of 0.2H-0.5H is denoted as the second area of the copper foil, and the absolute value of the difference between the average particle size of the grains in the first area and the average particle size of the grains in the second area is less than or equal to 150 nm; (8) Based on the total number of grains in the copper foil, the number of nanotwin grains accounts for 65% to 90%.

3. The current collector according to claim 1 or 2, characterized in that: The copper foil meets one or more of the following characteristics: (1) The maximum surface roughness of the copper foil is less than or equal to 3 μm; (2) The thickness H of the copper foil satisfies: 3 μm ≤ H ≤ 15 μm; (3) Under the test conditions of room temperature, sample thickness of 6±0.2 μm, and tensile speed of 50±0.5 mm / min, the tensile strength of the copper foil is greater than or equal to 600 MPa and the elongation at break of the copper foil is greater than or equal to 4%; (4) Under the test conditions of room temperature, sample thickness of 6±0.2 μm, and tensile speed of 50±0.5 mm / min, the tensile strength of the copper foil is 700 MPa-1500 MPa; the elongation at break of the copper foil is 4.5%-10%.

4. A pole piece, characterized in that: The pole piece includes the current collector according to any one of claims 1 to 3.

5. A secondary battery, characterized in that: Including the pole piece described in claim 4.

6. The secondary battery according to claim 5, characterized in that The maximum expansion force of the secondary battery cell is greater than or equal to 1000 kgf.

7. The secondary battery according to claim 5, characterized in that The maximum expansion force of the secondary battery cell is greater than or equal to 2500 kgf.

8. The secondary battery according to claim 5, wherein The maximum expansion force of the secondary battery cell is greater than or equal to 4000 kgf.

9. The secondary battery according to any one of claims 5 to 8, characterized in that The electrode sheet is a negative electrode sheet, which includes the current collector and a negative electrode film layer arranged on at least one side of the current collector. The negative electrode film layer contains a negative electrode active material, and the negative electrode active material contains a silicon-based material. The tensile elongation at break of the negative electrode sheet at room temperature is greater than or equal to 2%.

10. The secondary battery according to claim 9, wherein The tensile breaking elongation of the negative electrode plate at room temperature is 3%-5%.

11. The secondary battery according to claim 9, wherein The compaction density of the negative electrode plate is greater than or equal to 1.5g / cm 3 , and the cold pressing elongation of the negative electrode plate is less than or equal to 0.1%.

12. The secondary battery according to claim 11, wherein The cold pressing elongation of the negative electrode plate is 0.06%-0.1%.

13. The secondary battery according to claim 9 or 10, characterized in that: The compaction density of the negative electrode sheet is 1.5 g / cm 3 -2.0g / cm 3 .

14. The secondary battery according to any one of claims 9 to 13, characterized in that Based on the total mass of the negative electrode film layer, the mass content of the silicon-based material is 5%-100%.

15. The secondary battery according to any one of claims 9 to 14, characterized in that Based on the total mass of the negative electrode film layer, the mass content of the silicon-based material is 10%-60%.

16. The secondary battery according to any one of claims 9 to 15, characterized in that: The thickness of the current collector is 2 μm-10 μm.

17. An electrical device, characterized in that: A secondary battery comprising the secondary battery according to any one of claims 5 to 16.

18. A method for preparing copper foil, characterized in that: The copper foil is prepared by electroplating, which specifically includes periodically applying current to the electroplating solution so that the copper ions in the electroplating solution are reduced and deposited to form copper foil. The average particle size of the grains in the copper foil is 50nm-400nm, and based on the total number of grains in the copper foil, the number of nanotwin grains accounts for greater than or equal to 60%.

19. The preparation method according to claim 18, characterized in that The current is a pulse current, and the pulse current includes one or more of a square wave pulse current, a sine wave pulse current, a triangle wave pulse current, and a sawtooth wave pulse current.

20. The preparation method according to claim 18 or 19, characterized in that: The electroplating solution includes additives, and the additives include one or more of a leveler, a wetting agent, and a brightener.

21. The preparation method according to any one of claims 18 to 20, characterized in that: The additives include leveling agents, wetting agents and brighteners.

22. The preparation method according to claim 20 or 21, characterized in that The electroplating solution satisfies one or more of the following conditions: (1) The concentration of the leveling agent in the electroplating solution is 20 mg / L-300 mg / L; (2) The concentration of the leveling agent in the electroplating solution is 50 mg / L-150 mg / L; (3) The concentration of the wetting agent in the electroplating solution is 10 mg / L-200 mg / L; (4) The concentration of the wetting agent in the electroplating solution is 30 mg / L-100 mg / L; (5) The concentration of the brightener in the electroplating solution is 10 mg / L-200 mg / L; (6) The concentration of copper ions in the electroplating solution is 30 g / L-100 g / L; (7) The concentration of copper ions in the electroplating solution is 45 g / L-75 g / L; (8) The concentration of chloride ions in the plating solution is 10 mg / L-80 mg / L; (9) The pH of the electroplating solution is 2.5-4.

4.

23. The preparation method according to any one of claims 20 to 22, characterized in that The leveling agent includes one or more of gelatin and Janus Green; the wetting agent includes one or more of hydroxyethyl cellulose and polyethylene glycol; the brightening agent includes one or more of sodium polydisulfide dipropane sulfonate, sodium 3-mercapto-1-propane sulfonate, and thiourea.

24. The preparation method according to any one of claims 20 to 23, characterized in that Such wetting agents include hydroxyethylcellulose and polyethylene glycol.

25. The preparation method according to any one of claims 22 to 24, characterized in that The mass concentration ratio of the copper ion to the leveling agent is 500:1-4500:1; and / or, The mass concentration ratio of the copper ion to the brightener is 300:1-6000:

1.

26. The preparation method according to any one of claims 20 to 25, characterized in that The electroplating solution includes gelatin with a concentration of 20 mg / L-150 mg / L, polyethylene glycol with a concentration of 15 mg / L-100 mg / L, hydroxyethyl cellulose with a concentration of 10 mg / L-80 mg / L, sodium polydisulfide bis(propylene sulfonate) with a concentration of 15 mg / L-150 mg / L, copper ions with a concentration of 45 g / L-90 g / L, and chloride ions with a concentration of 10 mg / L-80 mg / L.

27. The preparation method according to any one of claims 19 to 26, characterized in that 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 .

28. The preparation method according to any one of claims 19 to 27, characterized in that The electroplating method satisfies one or more of the following conditions: (1) The peak current density I of the pulse current satisfies: 3.3A / dm 2 ≤I≤333A / dm 2 ; (2) The duty cycle s of the pulse current satisfies: 2%≤s≤50%; (3) The pulse width of the pulse current is 1 ms to 50 ms; (4) The distance between the cathode electrode and the anode electrode is 15 mm to 20 mm; (5) The deposition temperature is 45°C-60°C; (6) The deposition time is greater than or equal to 80 seconds.

29. The preparation method according to any one of claims 19 to 28, characterized in that The electroplating method specifically includes periodically applying a pulse current to the electroplating solution to reduce and deposit copper ions in the electroplating solution; wherein the peak current density I of the pulse current satisfies 100A / dm 2 ≤I≤180A / dm 2 The duty cycle s of the pulse current satisfies: 2%≤s≤10%, and the deposition time is 80s-250s.

30. The preparation method according to any one of claims 18 to 29, characterized in that The preparation method is a continuous production method.

31. A copper foil, characterized in that The copper foil is prepared by the preparation method according to any one of claims 18 to 30.

32. The copper foil according to claim 31, wherein The maximum width of the copper foil is greater than or equal to 1.5 meters, and / or the maximum length of the copper foil is greater than or equal to 10,000 meters.

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