Current collector, secondary battery, electric device, and manufacturing method for current collector

Abstract

The present application provides a current collector, a secondary battery, an electric device, and a manufacturing method for a current collector. The current collector comprises surface layers, and an intermediate layer arranged between the surface layers. The intermediate layer comprises a first metal element, and the total mass content of the first metal element is greater than or equal to 80% based on the total mass of elements in the intermediate layer. The surface layers comprise a second metal element, and the total mass content of the second metal element is greater than or equal to 80% based on the total mass of elements in the surface layers. The tensile strength of the intermediate layer is greater than or equal to that of the surface layers, and the bending resistance of the surface layers is greater than or equal to that of the intermediate layer.

Description

Current collector, secondary battery, electric device, and method for manufacturing current collector

[0001] Cross-reference to related applications

[0002] This application is based on the Chinese Patent Application No. 202411124874.8, filed on August 15, 2024, entitled “Current collector, secondary battery, electric device, and method for manufacturing current collector”, which is incorporated by reference in its entirety. TECHNICAL FIELD

[0003] The present application belongs to the technical field of batteries, and specifically relates to a current collector, a secondary battery, an electric device, and a method for manufacturing a current collector. BACKGROUND

[0004] In recent years, secondary batteries are widely used in energy storage power systems such as hydraulic, thermal, wind and solar power stations, and in many fields such as electric tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, etc.

[0005] The current collector is an important component of the secondary battery. The improvement of the energy density of the secondary battery often comes at the cost of the cycle life, and it is urgent to develop a current collector that takes into account both the energy density and the cycle life of the battery to meet the needs of the new generation of secondary batteries. SUMMARY

[0006] The present application is made in view of the above-mentioned problem, and aims to provide a current collector that can take into account both the energy density and the cycle life of the battery.

[0007] The first aspect of the present application provides a current collector, the current collector comprising a surface layer and an intermediate layer disposed between the surface layer, the intermediate layer comprising a first metal element, a total mass content of the first metal element being greater than or equal to 80% based on a total mass of elements of the intermediate layer, the surface layer comprising a second metal element, a total mass content of the second metal element being greater than or equal to 80% based on a total mass of elements of the surface layer; a tensile strength of the intermediate layer being greater than or equal to a tensile strength of the surface layer, a bending resistance of the surface layer being greater than or equal to a bending resistance of the intermediate layer.

[0008] The current collector realizes the balance of strength and bending resistance through multi-layer design, the intermediate layer has relatively high tensile strength, and the surface layer has relatively high bending resistance. The current collector can improve the bending resistance while maintaining high strength, delay the time of crack generation of the pole tab in the cycle process of the secondary battery, improve the cycle life of the secondary battery, and reduce the safety risk.

[0009] In any embodiment, the first metal element comprises one or more of Fe, Sn, Ni, and Zn.

[0010] The metal material with the first metal element as the main component has high strength, which is conducive to improving the tensile strength of the current collector, delaying the time of crack of the inner ring of the pole piece in the cycle process, and improving the cycle life of the battery.

[0011] In any embodiment, the second metal element includes Cu.

[0012] The copper metal material has high ductility, which is conducive to improving the bending resistance of the current collector, delaying the time of crack of the inner ring of the pole piece in the cycle process, and improving the cycle life of the battery.

[0013] In any embodiment, the average grain size of the surface layer is greater than or equal to 1.7 μm.

[0014] The surface layer with the average grain size in the above range can improve the bending resistance of the current collector, delay the time of crack of the inner ring of the battery in the cycle process.

[0015] In any embodiment, the average grain size of the surface layer is 1.7 μm-3.5 μm.

[0016] In any embodiment, the average grain size of the surface layer is 1.9 μm-2.5 μm.

[0017] The surface layer with the average grain size in the above range can better balance the strength and bending resistance of the current collector, delay the time of crack of the battery in the cycle process, improve the cycle life of the secondary battery, and reduce the safety risk.

[0018] In any embodiment, the ratio of the diffraction intensity of the (111) crystal plane in the surface layer to the sum of the diffraction intensities of the (111) crystal plane, the (200) crystal plane, the (220) crystal plane, the (311) crystal plane, and the (222) crystal plane in the surface layer is 20%-40%.

[0019] The embodiments of the present application control the ratio of the diffraction intensity of the (111) crystal plane to be 20%-40% to improve the bending resistance of the current collector, balance the strength and bending resistance of the current collector, improve the cycle life of the secondary battery, and reduce the safety risk.

[0020] In any embodiment, the ratio of the diffraction intensity of the (111) crystal plane in the surface layer to the sum of the diffraction intensities of the (111) crystal plane, the (200) crystal plane, the (220) crystal plane, the (311) crystal plane, and the (222) crystal plane in the surface layer is 25%-35%.

[0021] The ratio of the diffraction intensity of the (111) crystal plane in the surface layer in the above range can further balance the strength and bending resistance of the current collector, comprehensively improve the cycle life of the secondary battery, and reduce the safety risk.

[0022] In any embodiment, the average grain size of the intermediate layer is 0.1-1 μm.

[0023] The current collector with the average grain size of the intermediate layer in the above range has high strength, and can reduce the probability of cracks on the outer ring of the wound cell and the edge of the jelly-roll cell.

[0024] In any embodiment, the thickness of the current collector is 3-8 μm.

[0025] The current collector has low thickness, and can effectively reduce the weight of the battery, and is conducive to further improving the energy density of the battery.

[0026] In any embodiment, the thickness of the intermediate layer is 2-6 μm, and the thickness of the single-side surface layer is 0.5-2 μm.

[0027] The current collector with the thickness of the intermediate layer and the surface layer in the above range can effectively balance the strength and bending resistance of the current collector, improve the cycle life of the secondary battery, and improve the safety of the secondary battery.

[0028] In any embodiment, the tensile strength of the current collector is greater than or equal to 900 MPa under the test conditions of room temperature, sample thickness of 6±0.2 μm, and tensile speed of 50±0.5 mm / min.

[0029] The current collector has high strength, can improve the anti-expansion ability of the cell, improve the cycle life of the battery, and improve the safety performance of the battery.

[0030] In any embodiment, the tensile strength of the current collector is greater than or equal to 1000 MPa under the test conditions of room temperature, sample thickness of 6±0.2 μm, and tensile speed of 50±0.5 mm / min.

[0031] The current collector with the tensile strength greater than or equal to 1000 MPa can reduce the risk of early cracks on the outer ring or edge of the cell under the expansion force during the cycle process, and further improve the safety of the battery.

[0032] In any embodiment, the tensile strength of the current collector is 1000-1250 MPa under the test conditions of room temperature, sample thickness of 6±0.2 μm, and tensile speed of 50±0.5 mm / min.

[0033] The current collector with the tensile strength in the above range can balance the strength and bending resistance, and comprehensively improve the cycle life of the battery.

[0034] In any embodiment, the number of bendable times of the current collector is greater than or equal to 7 under the test conditions of room temperature and sample thickness of 6±0.2 μm.

[0035] The current collector has good bending resistance, can reduce the probability of battery failure caused by early cracks at the bending part of the battery, and improve the cycle life of the battery.

[0036] In any embodiment, the current collector has 7-18 times of bendable times under the test conditions of room temperature and a sample thickness of 6±0.2 μm.

[0037] The current collector with the bendable times in the above range can balance the bending resistance and strength, and comprehensively improve the cycle life of the battery.

[0038] The second aspect of the present application provides a secondary battery including a pole piece, the pole piece including the current collector of the first aspect.

[0039] The third aspect of the present application provides a power consumption device including the secondary battery of the second aspect.

[0040] The fourth aspect of the present application provides a preparation method of a current collector, the preparation method specifically including: obtaining an intermediate layer, the intermediate layer including a first metal element, the mass content of the first metal element being greater than or equal to 80% based on the total mass of the elements of the intermediate layer; depositing a surface layer on the surface of the intermediate layer to obtain the current collector; the surface layer including a second metal element, the mass content of the second metal element being greater than or equal to 80% based on the total mass of the elements of the surface layer; the tensile strength of the intermediate layer being greater than or equal to the tensile strength of the surface layer, and the bending resistance of the surface layer being greater than or equal to the bending resistance of the intermediate layer.

[0041] The current collector realizes the balance of strength and bending resistance through the multi-layer design, the intermediate layer has relatively high tensile strength, and the surface layer has relatively high bending resistance, thereby maintaining the high strength of the current collector while improving the bending resistance of the current collector, delaying the time of crack generation of the pole piece in the cycle process of the secondary battery, improving the cycle life of the secondary battery, and reducing the safety risk.

[0042] In any embodiment, the surface layer is deposited on the surface of the intermediate layer by an electroplating method, and the electroplating method specifically includes applying an electric current to the electroplating solution to reduce and deposit copper ions in the electroplating solution to form the surface layer on the surface of the intermediate layer.

[0043] In any embodiment, the electroplating solution includes an additive, and the additive includes one or more of a leveling agent, a wetting agent, and a brightener.

[0044] In any embodiment, the additive includes the leveling agent, the wetting agent, and the brightener.

[0045] The three additives can cooperate with each other to prepare the surface layer of the current collector with both strength and bending resistance.

[0046] In any embodiment, the concentration of the leveling agent in the electroplating solution is 40 mg / L-80 mg / L.

[0047] In any embodiment, the concentration of the wetting agent in the electroplating solution is 30 mg / L-80 mg / L.

[0048] Wetting agents with concentrations within the above range in the electroplating solution are beneficial for depositing copper grains of appropriate size on the surface layer, balancing the strength and bending resistance of the current collector, and improving the cycle life of the secondary battery.

[0049] In any embodiment, the concentration of chloride ions in the electroplating solution is 30 mg / L-60 mg / L.

[0050] In any embodiment, the concentration of copper ions in the electroplating solution is 50 g / L to 100 g / L.

[0051] In any embodiment, the concentration of brightener in the electroplating solution is 10 mg / L-25 mg / L.

[0052] In any embodiment, the total mass concentration of the leveling agent and brightener in the electroplating solution is less than 100 mg / L.

[0053] Leveling agents and brighteners with concentrations within the above range in the electroplating solution are beneficial for depositing copper grains of appropriate size on the surface layer, taking into account both the strength and bending resistance of the current collector, and improving the cycle life of the secondary battery.

[0054] In any embodiment, the electroplating temperature is 50±5℃.

[0055] In any embodiment, the pH of the electroplating solution is 4-7.

[0056] 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; and the brightening agent includes one or more of sodium dithiopropane sulfonate, sodium 3-mercapto-1-propane sulfonate, and thiourea.

[0057] In any embodiment, the electroplating solution includes gelatin at a concentration of 40 mg / L-80 mg / L, hydroxyethyl cellulose at a concentration of 30 mg / L-80 mg / L, sodium polydisulfide dipropane sulfonate at a concentration of 10 mg / L-25 mg / L, copper ions at a concentration of 50 g / L-100 g / L, and chloride ions at a concentration of 30 mg / L-60 mg / L.

[0058] In any embodiment, the direct current density is 4000 A / m. 2 -8000A / m 2 The electroplating process takes 0.5 to 3 minutes.

[0059] In any embodiment, the preparation method further comprises: heat treating the current collector to obtain the annealed current collector.

[0060] The grain size in the annealed current collector is further grown, and the proportion of the diffraction peak intensity of the (111) crystal plane is further reduced, which is beneficial to further improve the bending resistance of the current collector, realize the balance of the strength and the bending resistance of the current collector, and comprehensively improve the cycle life of the secondary battery. BRIEF DESCRIPTION OF DRAWINGS

[0061] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed to be used in the embodiments of the present application will be briefly introduced as follows. Obviously, the drawings described below are only some embodiments of the present application, and other drawings can be obtained by the drawings without paying creative labor for those skilled in the art.

[0062] Fig. 1 is a structural schematic diagram of a current collector according to an embodiment of the present application;

[0063] Fig. 2 is a reverse pole figure distribution diagram of an electron backscatter diffraction image of a surface of a current collector according to an embodiment of the present application;

[0064] Fig. 3 is an X-ray diffraction diagram of a current collector according to an embodiment of the present application;

[0065] Fig. 4 is a schematic diagram of an embodiment of a secondary battery according to the present application;

[0066] Fig. 5 is an exploded schematic diagram of an embodiment of a secondary battery according to the present application;

[0067] Fig. 6 is a schematic diagram of an embodiment of a battery module according to the present application;

[0068] Fig. 7 is a schematic diagram of an embodiment of a battery pack according to the present application;

[0069] Fig. 8 is an exploded schematic diagram of an embodiment of the battery pack shown in Fig. 7;

[0070] Fig. 9 is a schematic diagram of an embodiment of an electric device comprising a secondary battery according to the present application as a power supply.

[0071] In the drawings, the drawings are not necessarily drawn according to the actual scale. The reference signs are explained as follows: 1 battery pack, 2 upper box body, 3 lower box body, 4 battery module, 5 secondary battery, 51 shell, 52 electrode assembly, 53 cover plate, 6 current collector, 61 intermediate layer, 62 surface layer. DETAILED DESCRIPTION

[0072] Hereinafter, specific embodiments of the current application, which disclose a current collector, a secondary battery, an electric device, and a method for manufacturing a current collector, will be described in detail with appropriate reference to the accompanying drawings. However, there will be cases where unnecessary detailed descriptions are omitted. For example, there will be cases where detailed descriptions of matters known to those skilled in the art, repeated descriptions of substantially identical structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand the current application, and are not intended to limit the subject matter recited in the claims.

[0073] The ranges disclosed herein are defined by their lower and upper limits. Ranges that include both endpoints are inclusive of the endpoints. Ranges that exclude both endpoints are not inclusive of the endpoints. Ranges that include one or both endpoints are inclusive of the endpoint(s) and ranges that exclude one or both endpoints are not inclusive of the endpoint(s). Ranges are defined by their lower and upper limits. Unless specifically stated otherwise, the use of a range of values for a parameter includes each and every value and sub-range within the range. Exemplary values for physical parameters, such as temperature and pressure, are included in ranges unless otherwise stated herein. All ranges and parameters, including those for quantities, are inclusive of the recited endpoint and endpoints, unless expressly stated otherwise. For example, a range of "about 1% to 20%" is inclusive of from 1% to 20% and is also inclusive of from 20% to 1%. All individual values and sub-ranges from the stated ranges for parameters are included and disclosed. When no range or specific value is given, the range of values, for example, useful in the application is contemplated, for example, from 1 to 100. It is specifically intended that at least some minimum or maximum values defining the range be included in the disclosure, even if not explicitly stated. For example, it is intended that at least the minimum value be included in the range even if not specifically and explicitly recited. Likewise, it is intended that at least the maximum value be included in the range even if not specifically and explicitly recited. It is intended that the range include any and all sub-ranges between (and including) the minimum value and the maximum value, even if not specifically and explicitly recited. The use of any and all examples, or exemplary language (e.g. "such as") provided herein, is intended merely to better illuminate the application and does not pose a limitation on the scope of the application unless otherwise claimed. No language is intended to indicate that the application will necessarily encompass one or more specific examples, or that it will necessarily exclude one or more specific examples.

[0074] If not specifically stated otherwise, 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.

[0075] If not specifically stated otherwise, all technical features and optional technical features 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.

[0076] If not specified otherwise, all steps of the present application can be carried out in sequence or randomly, preferably in sequence. For example, the method comprising steps (a) and (b) means that the method can comprise steps (a) and (b) in sequence, or steps (b) and (a) in sequence. For example, the method can further comprise step (c) means that step (c) can be added to the method in any sequence, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0077] If not specified otherwise, the terms "comprising" and "including" as used in the present application are open-ended. For example, the terms "comprising" and "including" can mean that other components not listed can also be included, or only the listed components can be included.

[0078] If not specified otherwise, the term "or" as used in the present application is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any of the following conditions fulfill the condition "A or B": A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0079] If not specified otherwise, the terms used in the present application have the commonly understood meanings by those skilled in the art.

[0080] If not specified otherwise, the values of the parameters mentioned in the present application can be measured by various test methods commonly used in the art, for example, by the test methods given in the present application.

[0081] If not specified otherwise, the term "active ion" as used in the present application means an ion that can be reversibly intercalated and deintercalated between the positive and negative electrodes of a secondary battery, including but not limited to lithium ion.

[0082] In the present application, the terms "a plurality of" and "a plurality of kinds" mean two or more.

[0083] With the improvement of the energy density of the secondary battery, the demand for light and thin electrode tab current collectors is put forward. With the thinning of the thickness direction of the current collector, the maximum tensile load that the current collector can withstand decreases, so that the current collector with conventional strength in the prior art has been difficult to meet the performance requirements of the secondary battery in the long cycle process. With the increase of the cycle number, under the action of the reciprocating expansion of the electrode tab, the wound battery using the current ordinary strength current collector is prone to crack at the outermost part where the expansion force is the largest, which causes the electrode tab to break, resulting in loose battery structure, electrode tab displacement; the electrode tab fracture is easy to pierce the diaphragm, causing the battery to have internal short circuit, generating thermal runaway, and causing safety accidents. Similarly, the stacked battery is prone to crack at the stress concentration position of the edge of the battery, which causes the electrode tab to break, causing battery failure and safety problems.

[0084] Improving the strength of the current collector is beneficial to improve the ability of the battery to resist cycle expansion and delay the time of crack at the outermost part or edge of the battery, which causes the electrode tab to break, however, the improvement of the strength of the current collector often means that the movement of the lattice dislocation in the current collector is seriously hindered, so that the brittleness of the current collector increases and the bending resistance deteriorates. Studies have shown that the wound battery using high-strength current collector is prone to brittle fracture at the innermost part with large bending degree during the cycle process, which also causes battery failure. How to improve the strength of the current collector while considering the bending resistance of the current collector to delay the time of crack in the battery and the failure of the electrode tab and comprehensively improve the cycle life and safety performance of the battery has become a technical problem to be solved.

[0085] Based on this, as shown in FIG. 1, the first aspect of the present application provides a current collector 6, the current collector 6 includes a surface layer 62 and an intermediate layer 61 arranged between the surface layer 62, the intermediate layer includes a first metal element, the total mass content of the first metal element is greater than or equal to 80% based on the total mass of the intermediate layer element, the surface layer includes a second metal element, the total mass content of the second metal element is greater than or equal to 80% based on the total mass of the surface layer element; the tensile strength of the intermediate layer is greater than or equal to the tensile strength of the surface layer, and the bending resistance of the surface layer is greater than or equal to the bending resistance of the intermediate layer.

[0086] The layer structure of the current collector can be tested by any known method in the art. As an example, the layer structure of the current collector can be characterized by metallographic detection; or the cross section of the current collector can be subjected to surface scanning by an energy spectrometer in combination with argon ion polishing technology and scanning electron microscopy, to obtain an element distribution map of the cross section of the current collector, which can show that the components of the current collector provided in the embodiments of the application present a multi-layer structure in the cross section. In the element distribution map of the cross section of the current collector, the surface layers are located on the upper surface or the lower surface of the current collector, and the intermediate layers are arranged between the surface layers in the thickness direction. The element types and mass contents of the respective layers can be obtained by energy spectrum analysis on the cross section or the surface of each layer of the current collector. It should be noted that the surface layers and the intermediate layers can be identified by the obvious composition boundary in the cross section of the current collector, and do not necessarily mean that there is an obvious stratification in the cross section morphology.

[0087] The mass content of the first metal element based on the total mass of the elements in the intermediate layer can be obtained by surface scanning of the intermediate layer by an energy spectrometer. The mass content of the second metal element based on the total mass of the elements in the surface layer can be obtained by surface scanning of the surface layer by an energy spectrometer.

[0088] In some embodiments, the surface layers and the intermediate layers are arranged adjacent to each other in the thickness direction; and in some embodiments, other functional layers are arranged between the intermediate layers and the surface layers. It can be understood that the related features of the surface layers can be obtained by measuring the surface layer region in the cross section of the current collector, or by directly characterizing the surface of the current collector.

[0089] In some embodiments, the total mass content of the first element based on the total mass of the elements in the intermediate layer can be selected from 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, 99.1%, 99.2%, 99.3%, 99.4%, 99.5%, 99.6%, 99.7%, 99.8%, 99.98%, 100%, or any numerical range between any two of them.

[0090] In some embodiments, the total mass content of the second metal element based on the total mass of the elements in the surface layer can be selected from 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, 99.1%, 99.2%, 99.3%, 99.4%, 99.5%, 99.6%, 99.7%, 99.8%, 99.98%, 100%, or any numerical range between any two of them.

[0091] In this document, the term "tensile strength" refers to the maximum load-bearing strength of a test sample when continuously loaded until it is pulled apart.

[0092] In the present document, the term "bending resistance" refers to the ability of a sample to resist bending, which can be characterized by the number of times the sample can be bent.

[0093] The tensile strength of the intermediate layer and the surface layer can be tested by any known method in the art. As an example, the surface layer element of the current collector is etched to obtain the intermediate layer, the tensile strength of the obtained intermediate layer and the current collector are tested under the same test conditions, and according to the mechanics of composite materials, the tensile strength of the intermediate layer is greater than or equal to the tensile strength of the current collector, which indicates that the tensile strength of the intermediate layer is greater than or equal to the tensile strength of the surface layer.

[0094] In the present application, the tensile strength of the intermediate layer and the current collector can be tested by a method known in the art, such as the GB / T 5230-1995 "Electrolytic Copper Foil" standard. As an example, at least 10 samples with a length of 200±0.5mm, a width of 15±0.25mm, and a thickness of 6±0.2μm are cut, and the samples are continuously loaded at a tensile speed of 50±0.5mm / min until they are pulled apart at room temperature, and the maximum load is divided by the cross-sectional area of the sample to obtain the tensile strength of the sample. The cross-sectional area of the sample can be tested by size measurement.

[0095] The bending resistance of the intermediate layer and the surface layer can be tested by any known method in the art. As an example, the surface layer element of the current collector is etched to obtain the intermediate layer, the number of times the obtained intermediate layer and the current collector can be bent are tested under the same test conditions, and according to the mechanics of composite materials, the number of times the current collector can be bent is greater than or equal to the number of times the intermediate layer can be bent, which indicates that the bending resistance of the surface layer is greater than or equal to the bending resistance of the intermediate layer.

[0096] In the present application, the number of times the intermediate layer and the current collector can be bent can be tested by a method known in the art, such as folding the sample 180°, rolling back and forth at the folding point with a 1.5kg roller, and then unfolding to observe whether cracks appear at the folding point, recording the number of times the cracks appear at the folding point, and taking the average of at least ten samples.

[0097] The current collector realizes the balance of strength and bending resistance through a multi-layer design, the intermediate layer has a relatively high tensile strength, and the surface layer has a relatively high bending resistance, which improves the bending resistance of the current collector while maintaining the high strength of the current collector, delays the time of crack generation of the pole piece of the secondary battery during the cycle process, improves the cycle life of the secondary battery, and reduces the safety risk.

[0098] In some embodiments, the first metal element comprises one or more of Fe, Sn, Ni, and Zn.

[0099] The metal material with the first metal element as the main component has high strength, which is conducive to improving the tensile strength of the current collector, delaying the time of crack of the inner ring of the pole piece in the cycle process, and improving the cycle life of the battery.

[0100] In some embodiments, the second metal element includes Cu.

[0101] The copper metal material has high ductility, which is conducive to improving the bending resistance of the current collector, delaying the time of crack of the inner ring of the pole piece in the cycle process, and improving the cycle life of the battery.

[0102] In some embodiments, the average grain size of the surface layer is greater than or equal to 1.7 μm.

[0103] The average grain size of the surface layer can be tested by a method known in the art. For example, the surface layer (e.g., copper layer) of the current collector is measured by a combination of a cross-section electron backscatter diffraction instrument (EBSD) and a scanning electron microscope, and a pole figure distribution map is obtained as shown in FIG. 2. The grain size of the grains in the pole figure distribution map of the surface layer of the current collector is counted, a number distribution map is made, a statistical distribution curve is fitted, and the length corresponding to the peak is taken as the average grain size of the grains. The grain size is expressed as the equivalent circle diameter of the grains.

[0104] In some embodiments, the average grain size of the surface layer can be 1.7 μm, 1.8 μm, 1.9 μm, 2.0 μm, 2.1 μm, 2.2 μm, 2.3 μm, 2.4 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, or any numerical range between any two of them.

[0105] The surface layer with the average grain size in the above range can improve the bending resistance of the current collector, delay the time of crack of the inner ring of the battery in the cycle process.

[0106] In some embodiments, the average grain size of the surface layer is 1.7 μm-3.5 μm, which can be 1.9 μm-2.5 μm.

[0107] The surface layer with the average grain size in the above range can better balance the strength and bending resistance of the current collector, delay the time of crack of the battery in the cycle process, improve the cycle life of the secondary battery, and reduce the safety risk.

[0108] In some embodiments, the ratio of the diffraction intensity of the (111) crystal plane in the surface layer to the total diffraction intensity of the (111) crystal plane, the (200) crystal plane, the (220) crystal plane, the (311) crystal plane, and the (222) crystal plane in the surface layer is 20%-40%.

[0109] In the present application, the diffraction intensity of the crystal face in the surface layer can be tested by methods known in the art. As an example, the surface layer is tested by an X-ray diffractometer, and the X-ray spectrum of the standard copper powder (PDF No. 04-0836) is referred to, as shown in FIG. 3. The peak area of the diffraction peak with a peak position of 40°-45° is taken as the diffraction intensity of the (111) crystal face, the peak area of the diffraction peak with a peak position of 47°-52° is taken as the diffraction intensity of the (200) crystal face, the peak area of the diffraction peak with a peak position of 70°-75° is taken as the diffraction intensity of the (220) crystal face, the peak area of the diffraction peak with a peak position of 85°-95° is taken as the diffraction intensity of the (311) crystal face, and the peak area of the diffraction peak with a peak position of 93°-100° is taken as the diffraction intensity of the (222) crystal face.

[0110] In some embodiments, the diffraction intensity of the (111) crystal face in the surface layer accounts for 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, or any numerical range between any two of the above values, of the total diffraction intensity of the (111) crystal face, the (200) crystal face, the (220) crystal face, the (311) crystal face, and the (222) crystal face in the surface layer.

[0111] For crystalline copper, common crystal faces include (111), (200), (220), (311), and (222). According to crystallographic properties, the (111) crystal face has high atomic density and high mechanical strength, which can improve the strengthening effect. The diffraction peak intensity ratio of the crystal face can be used to characterize the orientation probability of the crystal face, also known as preferred orientation or preferred orientation. In the prior art, the (111) crystal face orientation accounts for about 45%-55% in the common strength current collector copper foil. The embodiments of the present application improve the bending resistance of the current collector by controlling the diffraction intensity ratio of the (111) crystal face to be 20%-40%, realize the balance of the strength and bending resistance of the current collector, improve the cycle life of the secondary battery, and reduce the safety risk.

[0112] In some embodiments, the diffraction intensity of the (111) crystal face in the surface layer accounts for 25%-35% of the total diffraction intensity of the (111) crystal face, the (200) crystal face, the (220) crystal face, the (311) crystal face, and the (222) crystal face in the surface layer.

[0113] The diffraction intensity ratio of the (111) crystal face in the surface layer within the above range can further balance the strength and bending resistance of the current collector, comprehensively improve the cycle life of the secondary battery, and reduce the safety risk.

[0114] In some embodiments, the average grain size of the intermediate layer is 0.1 μm-1 μm.

[0115] The average grain size of the intermediate layer can be tested in any manner known in the art. As an example, the cross-section of the current collector is measured by a cross-section electron backscatter diffraction instrument (EBSD) and a scanning electron microscope, an inverse pole figure map is obtained, the grain size of the intermediate layer part in the cross-section is counted, a number distribution map is made, a statistical distribution is fitted, and the length corresponding to the peak is taken as the average grain size of the grain. Wherein, the diameter of the equivalent circle of the grain is taken as the grain size.

[0116] In some embodiments, the average grain size of the intermediate layer can be selected as 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, or a numerical range between any two of them.

[0117] The current collector with the average grain size of the intermediate layer in the above range has higher strength, which can reduce the probability of cracks in the outer ring of the wound battery cell and the edge of the laminated battery cell.

[0118] In some embodiments, the thickness of the current collector is 3 μm-8 μm.

[0119] In some embodiments, the thickness of the current collector can be selected as 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm, 6.5 μm, 7 μm, 7.5 μm, 8 μm, or a numerical range between any two of them.

[0120] The current collector 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.

[0121] In some embodiments, the thickness of the intermediate layer is 2 μm-6 μm, and the thickness of the single-sided surface layer is 0.5 μm-2 μm.

[0122] In this application, the thickness of the intermediate layer and the single-sided surface layer can be tested by a method known in the art. As an example, the longitudinal face of the current collector is cut by argon ion polishing technology, and the thickness of each layer is tested.

[0123] In some embodiments, the thickness of the intermediate layer can be selected as 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, or a numerical range between any two of them.

[0124] In some embodiments, the thickness of the surface layer on one side can be selected from 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1.0 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, 2.0 μm, or any numerical range between any two of the above values.

[0125] The current collector with the thickness of the intermediate layer and the surface layer within the above range can effectively balance the strength and bending resistance of the current collector, improve the cycle life of the secondary battery, and improve the safety of the secondary battery.

[0126] In some embodiments, the tensile strength of the current collector is greater than or equal to 900 MPa under the test conditions of room temperature, a sample thickness of 6 ± 0.2 μm, and a tensile speed of 50 ± 0.5 mm / min.

[0127] In this document, the term "room temperature" refers to 20 ± 10 °C.

[0128] In some embodiments, the tensile strength of the current collector can be selected from 900 MPa, 950 MPa, 1000 MPa, 1050 MPa, 1100 MPa, 1150 MPa, 1200 MPa, 1250 MPa, 1300 MPa, 1350 MPa, 1400 MPa, 1450 MPa, 1500 MPa, or any numerical range between any two of the above values, under the test conditions of room temperature, a sample thickness of 6 ± 0.2 μm, and a tensile speed of 50 ± 0.5 mm / min.

[0129] The current collector described above has high strength, can improve the anti-expansion ability of the battery cell, improve the cycle life of the battery, and improve the safety performance of the battery.

[0130] In some embodiments, the tensile strength of the current collector is greater than or equal to 1000 MPa under the test conditions of room temperature, a sample thickness of 6 ± 0.2 μm, and a tensile speed of 50 ± 0.5 mm / min.

[0131] The current collector with a tensile strength greater than or equal to 1000 MPa can reduce the risk of early cracking of the battery cell at the outer circle or edge of the battery cell under the expansion force during the cycle process, and further improve the safety of the battery.

[0132] In some embodiments, the tensile strength of the current collector is 1000 MPa-1250 MPa under the test conditions of room temperature, a sample thickness of 6 ± 0.2 μm, and a tensile speed of 50 ± 0.5 mm / min.

[0133] The current collector with a tensile strength within the above range can balance the strength and bending resistance, and comprehensively improve the cycle life of the battery.

[0134] In some embodiments, the number of foldable times of the current collector is greater than or equal to 7 under the test conditions of room temperature and a sample thickness of 6±0.2 μm.

[0135] In the present application, the test method for the number of foldable times is to fold the current collector by 180°, then roll back and forth at the folded part using a roller of 1.5 kg, and then unfold to observe whether cracks appear at the folded part. The number of rolling times that causes cracks to appear at the folded part is recorded. The more the number of foldable times of the current collector, the better the bending resistance of the current collector, and the less likely the brittle fracture.

[0136] In some embodiments, the number of foldable times of the current collector can be 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 or any numerical range between any two of them under the test conditions of room temperature and a sample thickness of 6±0.2 μm.

[0137] The above-mentioned current collector has good bending resistance, which can reduce the probability of premature cracks at the bending part inside the battery cell, thereby improving the cycle life of the battery.

[0138] In some embodiments, the number of foldable times of the current collector is 7-18 under the test conditions of room temperature and a sample thickness of 6±0.2 μm.

[0139] The current collector with the number of foldable times within the above-mentioned range can balance the bending resistance and strength, thereby comprehensively improving the cycle life of the battery.

[0140] On the other hand, the present application provides a preparation method of a current collector, which specifically comprises: obtaining an intermediate layer, the intermediate layer comprising a first metal element, the mass content of the first metal element being greater than or equal to 80% based on the total mass of the elements of the intermediate layer; depositing a surface layer on the surface of the intermediate layer to obtain a current collector; the surface layer comprising a second metal element, the mass content of the second metal element being greater than or equal to 80% based on the total mass of the elements of the surface layer; the tensile strength of the intermediate layer being greater than or equal to the tensile strength of the surface layer, and the bending resistance of the surface layer being greater than or equal to the bending resistance of the intermediate layer.

[0141] The current collector realizes the balance of strength and bending resistance through the multi-layer design. The intermediate layer has a relatively high tensile strength, and the surface layer has a relatively high bending resistance. The current collector has high strength and improved bending resistance, delays the time of cracks in the pole of the secondary battery during the cycle process, improves the cycle life of the secondary battery, and reduces the safety risk.

[0142] In some embodiments, the surface layer is deposited on the surface of the intermediate layer by electroplating, which specifically includes applying an electric current to the electroplating solution so that copper ions in the electroplating solution are reduced and deposited to form a surface layer on the surface of the intermediate layer.

[0143] In this context, the term "electroplating" refers to a method of depositing a metal or alloy on the surface of a workpiece by using the principle of electrolysis to form a metal layer.

[0144] In some embodiments, the intermediate layer is also prepared by electroplating.

[0145] In some embodiments, the electric current is direct current and / or alternating current.

[0146] In some embodiments, the electroplating solution includes an additive, which includes one or more of a leveler, a wetting agent, and a brightener.

[0147] In this context, the term "leveler" refers to a substance added to the electroplating solution to improve the flatness of the plated layer.

[0148] In this context, the term "wetting agent" refers to a substance used to reduce the interfacial tension between the electroplating solution and the electrode, so that the plated layer is better attached to the substrate.

[0149] In this context, the term "brightener" refers to a substance that improves the smoothness of the plated layer and reduces the surface roughness.

[0150] The leveler can be attached to the tip of the copper foil with a high deposition rate, inhibit the growth of crystal grains, balance the growth rate of pits and tips, and improve the flatness of the copper foil. The wetting agent can improve the wettability of the electroplating solution to the substrate, and the wetting of the electroplating solution on the cathode is sufficient to enable fast electrodeposition with a large current, improve the nucleation rate of the copper foil, and control the average grain size of the copper foil. The brightener can cooperate with the leveler to regulate the grain size of the copper foil, and reduce the surface roughness of the copper foil and improve the smoothness of the surface.

[0151] In some embodiments, the additive includes a leveler, a wetting agent, and a brightener.

[0152] The three additives described above can cooperate with each other to prepare a surface layer that enables the current collector to have both strength and bending resistance.

[0153] In some embodiments, the concentration of the leveler in the electroplating solution is 40 mg / L-80 mg / L.

[0154] In some embodiments, the concentration of the leveler in the electroplating solution can be selected as 40 mg / L, 45 mg / L, 50 mg / L, 55 mg / L, 60 mg / L, 65 mg / L, 70 mg / L, 75 mg / L, 80 mg / L, or any numerical range between any two of them.

[0155] In some embodiments, the concentration of the wetting agent in the electroplating solution is 30 mg / L-80 mg / L.

[0156] In some embodiments, the concentration of the wetting agent in the electroplating solution can be selected from 30 mg / L, 35 mg / L, 40 mg / L, 45 mg / L, 50 mg / L, 55 mg / L, 60 mg / L, 65 mg / L, 70 mg / L, 75 mg / L, 80 mg / L, or a numerical range between any two of them.

[0157] The wetting agent in the electroplating solution with the concentration in the above range is conducive to depositing copper grains with appropriate size on the surface layer, taking into account the strength and bending resistance of the current collector, and improving the cycle life of the secondary battery.

[0158] In some embodiments, the concentration of the chloride ion in the electroplating solution is 30 mg / L-60 mg / L.

[0159] In some embodiments, the concentration of the chloride ion in the electroplating solution can be selected from 30 mg / L, 35 mg / L, 40 mg / L, 45 mg / L, 50 mg / L, 55 mg / L, 60 mg / L, or a numerical range between any two of them.

[0160] In some embodiments, the concentration of the copper ion in the electroplating solution is 50 g / L-100 g / L.

[0161] In some embodiments, the concentration of the copper ion in the electroplating solution can be selected from 50 g / L, 55 g / L, 60 g / L, 65 g / L, 70 g / L, 75 g / L, 80 g / L, 85 g / L, 90 g / L, 95 g / L, 100 g / L, or a numerical range between any two of them.

[0162] In some embodiments, the concentration of the brightener in the electroplating solution is 10 mg / L-25 mg / L.

[0163] In some embodiments, the concentration of the brightener in the electroplating solution can be selected from 10 mg / L, 11 mg / L, 12 mg / L, 13 mg / L, 14 mg / L, 15 mg / L, 16 mg / L, 17 mg / L, 18 mg / L, 19 mg / L, 20 mg / L, 21 mg / L, 22 mg / L, 23 mg / L, 24 mg / L, 25 mg / L, or a numerical range between any two of them.

[0164] In some embodiments, the total mass concentration of the leveling agent and the brightener in the electroplating solution is less than 100 mg / L.

[0165] In some embodiments, the total mass concentration of the leveler and the brightener in the electroplating solution can be selected from 70 mg / L, 75 mg / L, 80 mg / L, 85 mg / L, 88 mg / L, 90 mg / L, 95 mg / L, or any numerical range between any two of them.

[0166] The leveler and the brightener in the electroplating solution with the concentration in the above range are beneficial to depositing copper grains with appropriate size on the surface layer, taking into account the strength and bending resistance of the current collector, and improving the cycle life of the secondary battery.

[0167] In some embodiments, the temperature of the electroplating is 50±5℃.

[0168] In some embodiments, the temperature of the electroplating can be selected from 45℃, 46℃, 47℃, 48℃, 49℃, 50℃, 51℃, 52℃, 53℃, 54℃, 55℃, or any numerical range between any two of them.

[0169] In some embodiments, the pH of the electroplating solution is 4-7.

[0170] In some embodiments, the pH of the electroplating solution can be selected from 4, 5, 6, 7, or any numerical range between any two of them.

[0171] In some embodiments, the leveler comprises one or more of gelatin, gentian violet; the wetting agent comprises one or more of hydroxyethyl cellulose, polyethylene glycol; the brightener comprises one or more of sodium polydithiopropyl sulfone, sodium 3-mercapto-1-propane sulfonate, thiourea.

[0172] In some embodiments, the electroplating solution comprises gelatin with a concentration of 40 mg / L-80 mg / L, hydroxyethyl cellulose with a concentration of 30 mg / L-80 mg / L, polydithiopropyl sulfone with a concentration of 10 mg / L-25 mg / L, copper ions with a concentration of 50 g / L-100 g / L, and chloride ions with a concentration of 30 mg / L-60 mg / L.

[0173] In some embodiments, the electroplating solution comprises gelatin at a concentration of 40 mg / L, 45 mg / L, 50 mg / L, 55 mg / L, 60 mg / L, 65 mg / L, 70 mg / L, 75 mg / L, 80 mg / L, or any numerical range between any two of them, hydroxyethyl cellulose at a concentration of 30 mg / L, 35 mg / L, 40 mg / L, 45 mg / L, 50 mg / L, 55 mg / L, 60 mg / L, 65 mg / L, 70 mg / L, 75 mg / L, 80 mg / L, or any numerical range between any two of them, sodium polydithio- dipropanesulfonate at a concentration of 10 mg / L, 15 mg / L, 20 mg / L, 25 mg / L, or any numerical range between any two of them, copper ions at a concentration of 50 g / L, 55 g / L, 60 g / L, 65 g / L, 70 g / L, 75 g / L, 80 g / L, 85 g / L, 90 g / L, 95 g / L, 100 g / L, or any numerical range between any two of them, chloride ions at a concentration of 30 mg / L, 35 mg / L, 40 mg / L, 45 mg / L, 50 mg / L, 55 mg / L, 60 mg / L, or any numerical range between any two of them.

[0174] In some embodiments, the direct current has a current density of 4000 A / m 2 - 8000 A / m 2 and the electroplating process has a time of 0.5 min - 3 min.

[0175] In some embodiments, the direct current has a current density of 4000 A / m 2 , 4500 A / m 2 , 5000 A / m 2 , 5500 A / m 2 , 6000 A / m 2 , 6100 A / m 2 , 6200 A / m 2 , 6300 A / m 2 , 6400 A / m 2 , 6500 A / m 2 , 6600 A / m 2 , 6700 A / m 2 , 6800 A / m 2 , 6900 A / m 2 , 7000 A / m 2 , 7500 A / m 2 , 8000 A / m 2 , or any numerical range between any two of them.

[0176] In some embodiments, the electroplating method can be selected from 0.5 min, 1 min, 1.5 min, 2 min, 2.5 min, 3 min or any numerical range between any two of them.

[0177] In some embodiments, the preparation method further comprises: heat treating the current collector to obtain an annealed current collector.

[0178] In some embodiments, the heat treatment temperature is 60-80℃.

[0179] In some embodiments, the heat treatment temperature can be selected from 60℃, 65℃, 70℃, 75℃, 80℃ or any numerical range between any two of them.

[0180] In some embodiments, the heat treatment time is 12-25h.

[0181] In some embodiments, the heat treatment time can be selected from 12h, 13h, 14h, 15h, 17h, 20h, 23h, 25h or any numerical range between any two of them.

[0182] The grain size in the annealed current collector is further grown, and the diffraction peak intensity ratio of the (111) crystal plane is further reduced, which is beneficial to further improve the bending resistance of the current collector, realize the balance of the strength and bending resistance of the current collector, and comprehensively improve the cycle life of the secondary battery.

[0183] In some embodiments, the preparation method is a continuous production method.

[0184] In another aspect, the application provides a composite foil, which comprises any feature of the current collector of any embodiment of the application or is prepared by the preparation method of any embodiment of the application.

[0185] In some embodiments, the maximum width of the composite foil is greater than or equal to 1.5 meters, and / or the maximum length of the composite foil is greater than or equal to 10,000 meters.

[0186] In some embodiments, the maximum width of the composite 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 numerical range between any two of them.

[0187] In some embodiments, the maximum length of the composite 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 numerical range between any two of them.

[0188] The composite foil provided by the embodiments of the application can realize large-size manufacturing and has the prospect of industrial application.

[0189] In another aspect, the present application provides a secondary battery, which comprises a pole piece, the pole piece comprising the current collector provided by the first aspect of the present application.

[0190] In some embodiments, the pole piece is a positive pole piece.

[0191] In some embodiments, the pole piece is a negative pole piece.

[0192] In some embodiments, the negative pole piece comprises a current collector and a negative film layer disposed on at least one surface of the current collector.

[0193] In some embodiments, the negative film layer comprises a negative active material. In some embodiments, the negative active material comprises, but is not limited to, one or more of conventional natural graphite, artificial graphite, soft carbon, hard carbon, silicon-based material, tin-based material, and lithium titanate. The silicon-based material can comprise one or more of elemental silicon, silicon oxide, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy material. The tin-based material can comprise one or more of elemental tin, tin oxide, and tin alloy material.

[0194] In some embodiments, the negative film layer can further optionally comprise a negative conductive agent. The present application does not have a particular limitation on the type of the negative conductive agent. As an example, the negative conductive agent can comprise one or more of super conductive carbon, conductive graphite, acetylene black, carbon black, ketjen black, carbon dot, carbon nanotube, graphene, and carbon nanofiber.

[0195] In some embodiments, the negative film layer can further optionally comprise a negative binder. The present application does not have a particular limitation on the type of the negative binder. As an example, the negative binder can comprise one or more of styrene butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, water-based acrylic resin (e.g., polyacrylic acid PAA, polymethylacrylic acid PMAA, sodium polyacrylate PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethyl chitosan (CMCS).

[0196] In some embodiments, the negative film layer can further optionally comprise other auxiliary agents. As an example, the other auxiliary agents can comprise thickening agents, such as carboxymethyl cellulose sodium (CMC), PTC thermistor material, and the like.

[0197] The negative film layer is generally formed by coating a negative slurry on a negative current collector, drying, and cold pressing. The negative slurry is generally formed by dispersing a negative active material, an optional conductive agent, an optional binder, and other optional auxiliary agents in a solvent and stirring uniformly. The solvent can be N-methyl pyrrolidone (NMP) or deionized water, but is not limited thereto.

[0198] The negative electrode tab does not exclude other additional functional layers in addition to the negative electrode film layer. For example, in some embodiments, the negative electrode tab 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, disposed on the surface of the negative electrode current collector; in some embodiments, the negative electrode tab described in the present application further includes a protective layer covering the surface of the negative electrode film layer.

[0199] In another aspect, the present application also provides a power utilization device comprising the secondary battery of any embodiment of the present application, which can be in the form of a battery cell, a battery module or a battery pack.

[0200] The secondary battery of the present application is not particularly limited in type, for example, the secondary battery can be a lithium ion battery, etc. Generally, the secondary battery comprises a positive electrode tab, a negative electrode tab and an electrolyte, etc. During the charging and discharging process of the secondary battery, active ions are reversibly inserted and extracted between the positive electrode tab and the negative electrode tab, and the electrolyte plays a role in conducting active ions between the positive electrode tab and the negative electrode tab. The type of the electrolyte is not particularly limited in the present application and can be selected according to actual needs. For example, the electrolyte can be selected from at least one of a solid-state electrolyte and a liquid electrolyte (i.e., electrolyte solution). Some secondary batteries using electrolyte solution and some secondary batteries using solid-state electrolyte can further comprise a separator film, which is disposed between the positive electrode tab and the negative electrode tab and plays a role in isolation.

[0201] [Positive electrode tab]

[0202] In some embodiments, the positive electrode tab comprises a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector. For example, the positive electrode current collector has two opposite surfaces in the thickness direction of itself, and the positive electrode film layer is disposed on any one or both of the two opposite surfaces of the positive electrode current collector.

[0203] The positive electrode current collector can be a metal foil or a composite current collector. As an example of the metal foil, an aluminum foil can be used. The composite current collector can comprise 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 can comprise 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 can comprise one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS) and polyethylene (PE).

[0204] 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 generally formed by coating a positive electrode slurry on the positive electrode current collector, drying, and cold-pressing. The positive electrode slurry is generally formed by dispersing and uniformly stirring the positive electrode active material, the optional conductive agent, the optional binder, and any other components in a solvent. The solvent can be N-methyl pyrrolidone (NMP), but is not limited thereto. As an example, the binder used in the positive electrode film layer can include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin. As an example, the conductive agent used in the positive electrode film layer includes one or more of super P, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0205] The positive electrode active material can be a positive electrode active material for a secondary battery known in the art.

[0206] When the secondary battery of the present application is a lithium ion battery, the positive electrode active material can include, but is not limited to, one or more of a lithium-containing transition metal oxide, a lithium-containing phosphate, and a modified compound of each thereof. Examples of the lithium-containing transition metal oxide can 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 a modified compound of each thereof. Examples of the lithium-containing phosphate can include, but are not limited to, one or more of lithium iron phosphate, a composite of lithium iron phosphate and carbon, lithium manganese phosphate, a composite of lithium manganese phosphate and carbon, lithium manganese iron phosphate, a composite of lithium manganese iron phosphate and carbon, and a modified compound of each thereof.

[0207] In some embodiments, in order to further increase the energy density of the secondary battery, the positive electrode active material for the lithium ion battery can include one or more of a lithium transition metal oxide of the general formula Li a Ni b Co c M d O e A f and a modified compound thereof, 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.

[0208] In some embodiments, as an example, the positive electrode active material for a lithium ion battery can include one or more of 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 O2, LiFePO4, and LiMnPO4.

[0209] In the present application, the modified compound of each of the above positive electrode active materials can be a doping modification and / or a surface coating modification to the positive electrode active material.

[0210] [Electrolyte]

[0211] In some embodiments, the electrolyte employs an electrolytic solution including an electrolyte salt and a solvent.

[0212] The kind of the electrolyte salt is not particularly limited and can be selected according to the actual needs.

[0213] When the secondary battery of the present application is a lithium ion battery, as an example, the electrolyte salt can include one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bisfluorosulfonylimide (LiFSI), lithium bis-trifluoromethanesulfonylimide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalato borate (LiDFOB), lithium bisoxalato borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorodioxalato phosphate (LiDFOP), and lithium tetrafluorooxalato phosphate (LiTFOP).

[0214] The kind of the solvent is not particularly limited, and can be selected according to actual needs. In some embodiments, the solvent can include, by way of example, one or more of ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl 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), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE).

[0215] In some embodiments, the electrolyte solution can also optionally include an additive. For example, the additive can include a negative electrode film-forming additive, a positive electrode film-forming additive, and an additive capable of improving certain performance of the secondary battery, such as an additive capable of improving overcharge performance of the secondary battery, an additive capable of improving high-temperature performance of the secondary battery, an additive capable of improving low-temperature power performance of the secondary battery, and the like.

[0216] [Separator]

[0217] The kind of the separator is not particularly limited, and any known porous structure separator having good chemical stability and mechanical stability can be used.

[0218] In some embodiments, the material of the separator can include one or more of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film. When the separator is a multi-layer composite film, the materials of the layers can be the same or different.

[0219] In some embodiments, the positive electrode tab, the separator, and the negative electrode tab can be made into an electrode assembly through a winding process or a stacking process.

[0220] In some embodiments, the secondary battery can include an outer package. The outer package can be used to package the electrode assembly and the electrolyte solution.

[0221] In some embodiments, the outer package can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, and the like. The outer package 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).

[0222] The shape of the secondary battery according to the present application is not particularly limited, and can be cylindrical, square, or any other shape. FIG. 4 is a square structure of the secondary battery 5 as an example.

[0223] In some embodiments, as shown in FIG. 5, the outer package can include a housing 51 and a cover plate 53. The housing 51 can include a bottom plate and side plates connected to the bottom plate, which enclose a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 is used to cover the opening to seal the receiving cavity. The positive electrode sheet, the negative electrode sheet, and the separator can be wound or stacked to form an electrode assembly 52. The electrode assembly 52 is packaged in the receiving cavity. The electrolyte is impregnated in the electrode assembly 52. The number of electrode assemblies 52 contained in the secondary battery 5 can be one or several, which can be adjusted according to the needs.

[0224] The preparation method of the secondary battery according to the present application is known. In some embodiments, the positive electrode sheet, the separator, the negative electrode sheet, and the electrolyte can be assembled to form a secondary battery. As an example, the positive electrode sheet, the separator, the negative electrode sheet can be wound or stacked to form an electrode assembly, the electrode assembly is placed in the outer package, the electrolyte is injected after drying, and the secondary battery is obtained after vacuum packaging, standing, formation, shaping, and other processes.

[0225] In some embodiments of the present application, the secondary battery according to the present application can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be multiple, and the specific number can be adjusted according to the application and capacity of the battery module.

[0226] FIG. 6 is a schematic diagram of a battery module 4 as an example. As shown in FIG. 6, in the battery module 4, a plurality of secondary batteries 5 can be arranged in sequence along the length direction of the battery module 4. Of course, other arbitrary arrangements can also be used. Further, the plurality of secondary batteries 5 can be fixed by fasteners.

[0227] Optionally, the battery module 4 can further include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0228] In some embodiments, the above-mentioned battery module can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0229] FIGS. 7 and 8 are schematic diagrams of a battery pack 1 as an example. As shown in FIGS. 7 and 8, the battery pack 1 can include a battery box and a plurality of battery modules 4 arranged in the battery box. The battery box includes an upper box body 2 and a lower box body 3, and the upper box body 2 is used to cover the lower box body 3 and form a closed space for receiving the battery modules 4. The plurality of battery modules 4 can be arranged in the battery box in any manner.

[0230] Electric device

[0231] The application also provides an electric device comprising at least one of the secondary battery, the battery module or the battery pack of the application. The secondary battery, the battery module or the battery pack can be used as a power source of the electric device, or can be used as an energy storage unit of 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 notebook computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship and a satellite, an energy storage system, etc.

[0232] The electric device can select a secondary battery, a battery module or a battery pack according to its use requirements.

[0233] FIG. 9 is a schematic diagram of an electric device as an example. The electric device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the requirements of high power and high energy density of the electric device, a battery pack or a battery module can be used.

[0234] The electric device as another example can be a mobile phone, a tablet computer, a notebook computer, etc. The electric device usually requires thin and light, and a secondary battery can be used as a power source.

[0235] Embodiments

[0236] The following examples more specifically describe the disclosure of the application, which are only used for illustrative purposes, because various modifications and changes within the scope of the disclosure of the application are obvious to those skilled in the art. Unless otherwise stated, all parts, percentages, and ratios reported in the following examples are based on mass, and all reagents used in the examples are commercially available or synthesized according to conventional methods, and can be used directly without further treatment, and the instruments used in the examples are commercially available.

[0237] Example 1

[0238] (1) Preparation of composite foil

[0239] First, an iron sulfate solution is prepared, with a concentration of 20 g / L, and the pH value is controlled at 6-7. The solution is poured into a Hull cell to complete the solution preparation. Then, the leveling agent gelatin, the brightener polydithiobispropyl sulfone sodium (SPS), and the wetting agent hydroxyethyl cellulose are added to the solution, wherein the concentration of gelatin is 70 mg / L, the concentration of brightener is 25 mg / L, and the concentration of wetting agent is 50 mg / L. The solution is heated to 50-55°C, and the current density is controlled at 5000 A / m 2The Fe foil with a thickness of 4 μm was formed on the left and right surfaces of the Fe foil by electroplating for about 3 min, and the average size of the crystal grains in the Fe foil was 0.2 μm.

[0240] The Fe foil was taken out of the solution and put into a solution containing copper sulfate, and the intermediate layer was connected to a negative current. The concentration of the copper sulfate was 80 g / L, the concentration of sodium chloride was 37 mg / L, the concentration of the leveling agent gelatin was 70 mg / L, the concentration of the brightener polydithiobispropane sulfonic acid sodium (SPS) was 25 mg / L, the concentration of the wetting agent hydroxyethyl cellulose (HEC) was 50 mg / L, the solution was heated to 50-55 °C, and the current density was controlled at 6000 A / m 2 The Cu foil with a thickness of 1 μm was formed on the upper and lower surfaces of the Fe foil by electroplating for about 2-3 min, and the Cu / Fe / Cu composite foil with a thickness of 6 μm was formed. The main component elements in the surface layer were Cu except for inevitable impurity elements, and the main component elements in the intermediate layer were Fe except for inevitable impurity elements.

[0241] (2) Preparation of the battery

[0242] Preparation of the positive electrode sheet

[0243] The lithium iron phosphate, conductive carbon black and PVDF were mixed in a mass ratio of 8:1:1, and then added into the solvent N-methyl pyrrolidone. The mixture was coated on both sides of the aluminum foil, and then cold-pressed and cut to obtain the positive electrode sheet. The positive electrode sheet was rolled into a film roll, and ceramic slurry was sprayed on the cutting surface of the film roll. In the ceramic slurry, the ceramic material was boehmite, accounting for 39 wt%; the binder was polyacrylate, accounting for 5 wt%; the solvent was N-methyl pyrrolidone, the solid content of the slurry was 10%, and the viscosity of the slurry was 800 mPa·s.

[0244] Preparation of the negative electrode sheet

[0245] The artificial graphite, conductive carbon black, binder carboxymethyl cellulose (CMC) and solvent water were uniformly mixed in a weight ratio of 1.1% SBR+0.9% CMC+0.7% SP+97.3% graphite, and then coated on both sides of the composite foil prepared above. After cold-pressing and cutting, the negative electrode sheet was obtained.

[0246] Preparation of the separator

[0247] A polyethylene film with a thickness of 13 μm was used as the isolation film.

[0248] Preparation of the electrolyte

[0249] Ethylene carbonate, diethyl carbonate and dimethyl carbonate were mixed in a volume ratio of 1:1:1, and LiPF6 was dissolved in the above solution to obtain the electrolyte. In the electrolyte, the concentration of LiPF6 was 1 mol / L.

[0250] Assembly of battery

[0251] The positive electrode sheet, the negative electrode sheet and one end of the two separators are fixed to the discharge roller in the order of "separator-negative electrode sheet-separator-positive electrode sheet", and the other end is fixed to the winding shaft after being stacked together. The winding shaft is rotated by a motor to wind the positive electrode sheet, the negative electrode sheet and the two separators, thereby obtaining a wound battery.

[0252] Example 2-3

[0253] The preparation method of Example 2-3 is basically the same as that of Example 1, except that the composition of the plating solution in the surface layer deposition step is changed, as shown in Table 1.

[0254] Example 4

[0255] The preparation method of Example 4 is basically the same as that of Example 1, except that the current collector prepared in Example 1 is subjected to heat treatment, and the heat treatment temperature is 80℃ and the heat treatment time is 12h.

[0256] Comparative Example 1-2

[0257] The preparation method of Comparative Example 1-2 is basically the same as that of Example 1, except that the composition of the plating solution in the surface layer deposition step is adjusted, as shown in Table 1.

[0258] Comparative Example 3

[0259] The preparation method of Comparative Example 3 is basically the same as that of Example 1, except that the negative current collector uses an iron foil, and the thickness of the iron foil is 6 microns.

[0260] Comparative Example 4

[0261] The preparation method of Comparative Example 4 is basically the same as that of Example 1, except that the negative current collector uses a conventional copper foil, and the thickness of the copper foil is 6 microns.

[0262] Table 1

[0263] Test method

[0264] (1) The state of health (SOH) corresponding to the crack of the battery cell

[0265] The battery was charged at 1C constant current to 3.8V at 25°C, then charged at 3.8V constant voltage until the current was less than or equal to 0.05C, and then discharged at 1C constant current to 2.5V, which was one charge-discharge process, and the cycle charge-discharge was repeated in this way. Then, whether a crack was generated inside the battery was determined by computerized tomography (CT), and the state of health (SOH) of the battery when the crack occurred was recorded. The fresh cell was 100% SOH, and the SOH of the cell decreased continuously with the loss of active ions during the cycle process of the cell. The greater the SOH corresponding to the occurrence of the crack in the cell indicated that the cell was prone to crack in the early cycle, and the cycle life of the battery was low. Conversely, the smaller the SOH corresponding to the occurrence of the crack in the cell indicated that the cell would crack only in the later cycle, and the cycle life of the battery was relatively high.

[0266] Test results

[0267] The test results are shown in Table 2.

[0268] Table 2

[0269] The conventional general-purpose copper foil in Comparative Example 4 had a low tensile strength, and cracks were generated preferentially at the outer circle of the cell with the largest expansion force during the cycle of the battery. The iron foil in Comparative Example 3 had a high tensile strength, but poor bending resistance, and cracks were generated preferentially at the inner circle of the cell with the largest bending deformation during the cycle of the battery. As can be seen from the comparison of the examples and the comparative examples, the surface layer was a copper foil, and the average grain size of the surface layer was greater than or equal to 1.7 μm; the current collector with the intermediate layer being an iron foil could withstand more cycles of expansion, and the SOH corresponding to the occurrence of the crack in the cell was less than or equal to 60%, which was helpful to improve the cycle life and safety performance of the secondary battery, so that the battery tab would not break within the expected service life (e.g., the warranty period), and the use demand of the market for the secondary battery was met.

[0270] As can be seen from the comparison of the examples and Comparative Example 1, the total mass concentration of the leveling agent and the brightener in the electroplating solution was less than 100 mg / L, which was helpful to reduce nucleation, increase the average size of the copper grains of the surface layer, and improve the bending resistance of the current collector. As can be seen from the comparison of the examples and Comparative Example 2, the concentration of the wetting agent in the electroplating solution was 30 mg / L-80 mg / L, which was helpful to reduce nucleation, increase the average size of the copper grains of the surface layer, and improve the bending resistance of the current collector.

[0271] Note that the present application is not limited to the above-described embodiments. The above-described embodiments are merely examples, and embodiments having substantially the same configuration, function, and effect as the technical idea of the present application are included in the technical scope of the present application. Furthermore, other modes constructed by applying various modifications to the embodiments, or by combining part of the configurations of the embodiments, which can be conceived by those skilled in the art, without departing from the spirit of the present application, are also included in the scope of the present application.

Claims

1. A current collector characterized by comprising: The current collector comprises a surface layer and an intermediate layer arranged between the surface layers, the intermediate layer comprises a first metal element, the total mass content of the first metal element is greater than or equal to 80% based on the total mass of the elements in the intermediate layer, the surface layer comprises a second metal element, the total mass content of the second metal element is greater than or equal to 80% based on the total mass of the elements in the surface layer; the tensile strength of the intermediate layer is greater than or equal to the tensile strength of the surface layer, and the bending resistance of the surface layer is greater than or equal to the bending resistance of the intermediate layer.

2. The current collector of claim 1, wherein The first metal element comprises one or more of Fe, Sn, Ni, and Zn.

3. The current collector according to claim 1 or 2, characterized by The second metal element comprises Cu.

4. The current collector according to any one of claims 1 to 3, characterized in that, The average grain size of the surface layer is greater than or equal to 1.7 μm.

5. The current collector according to any one of claims 1 to 4, wherein The average grain size of the surface layer is 1.7 μm to 3.5 μm.

6. The current collector according to any one of claims 1 to 5, wherein The average grain size of the surface layer is 1.9 μm to 2.5 μm.

7. The current collector of any one of claims 1 to 6, wherein, The ratio of the diffraction intensity of the (111) crystal plane in the surface layer to the total diffraction intensity of the (111) crystal plane, the (200) crystal plane, the (220) crystal plane, the (311) crystal plane, and the (222) crystal plane in the surface layer is 20% to 40%.

8. The current collector of any one of claims 1 to 7, wherein, The ratio of the diffraction intensity of the (111) crystal plane in the surface layer to the total diffraction intensity of the (111) crystal plane, the (200) crystal plane, the (220) crystal plane, the (311) crystal plane, and the (222) crystal plane in the surface layer is 25% to 35%.

9. The current collector of any one of claims 1 to 8, wherein, The average grain size of the intermediate layer is 0.1 μm to 1 μm.

10. The current collector of any one of claims 1 to 9, wherein, The thickness of the current collector is 3 μm to 8 μm.

11. The current collector of any one of claims 1 to 10, wherein, The thickness of the intermediate layer is 2 μm to 6 μm, and the thickness of the surface layer on one side is 0.5 μm to 2 μm.

12. The current collector of any one of claims 1 to 11, wherein, The tensile strength of the current collector is greater than or equal to 900 MPa under the test conditions of room temperature, a sample thickness of 6±0.2 μm, and a tensile speed of 50±0.5 mm / min.

13. The current collector of any one of claims 1 to 11, wherein, The tensile strength of the current collector is greater than or equal to 1000 MPa under the test conditions of room temperature, a sample thickness of 6±0.2 μm, and a tensile speed of 50±0.5 mm / min.

14. The current collector of any one of claims 1 to 11, wherein, The tensile strength of the current collector is 1000 MPa to 1250 MPa under the test conditions of room temperature, a sample thickness of 6±0.2 μm, and a tensile speed of 50±0.5 mm / min.

15. The current collector of any one of claims 1 to 14, wherein, The number of bendable times of the current collector is greater than or equal to 7 times under the test conditions of room temperature and a sample thickness of 6±0.2 μm.

16. The current collector of any one of claims 1 to 14, wherein, The number of bendable times of the current collector is 7 times to 18 times under the test conditions of room temperature and a sample thickness of 6±0.2 μm.

17. A secondary battery characterized by comprising: The secondary battery comprises the current collector according to any one of claims 1 to 16.

18. An electrical device, comprising: The secondary battery comprises the current collector according to claim 17.

19. A method of making a current collector, comprising: The preparation method specifically comprises: obtaining an intermediate layer, the intermediate layer comprises a first metal element, the mass content of the first metal element is greater than or equal to 80% based on the total mass of the elements in the intermediate layer; Depositing a surface layer on a surface of the intermediate layer to obtain a current collector; the surface layer comprises a second metal element, a mass content of the second metal element is greater than or equal to 80% based on a total mass of elements of the surface layer; a tensile strength of the intermediate layer is greater than or equal to a tensile strength of the surface layer, and a bending resistance of the surface layer is greater than or equal to a bending resistance of the intermediate layer.

20. The method of claim 19, wherein, The surface layer is deposited on the surface of the intermediate layer by electroplating, which specifically comprises applying an electric current to an electroplating solution so that copper ions in the electroplating solution are reduced and deposited to form the surface layer on the surface of the intermediate layer.

21. The method of claim 20, wherein, The electroplating solution comprises an additive, and the additive comprises one or more of a leveling agent, a wetting agent, and a brightener.

22. The method of claim 21, wherein, The additive comprises the leveling agent, the wetting agent, and the brightener.

23. The production method according to any one of claims 19 to 22, characterized by, The electroplating solution satisfies one or more of the following conditions: (1) a concentration of the leveling agent in the electroplating solution is 40 mg / L-80 mg / L; (2) a concentration of the wetting agent in the electroplating solution is 30 mg / L-80 mg / L; (3) a concentration of chloride ions in the electroplating solution is 30 mg / L-60 mg / L; (4) a concentration of copper ions in the electroplating solution is 50 g / L-100 g / L; (5) a concentration of the brightener in the electroplating solution is 10 mg / L-25 mg / L; (6) a total mass concentration of the leveling agent and the brightener in the electroplating solution is less than 100 mg / L; (7) a temperature of the electroplating is 50±5℃; (8) a pH of the electroplating solution is 4-7.

24. The production method according to any one of claims 19 to 23, characterized by, The leveling agent comprises one or more of gelatin and gentian violet; the wetting agent comprises one or more of hydroxyethyl cellulose and polyethylene glycol; and the brightener comprises one or more of sodium poly(dithiopropyl sulfone), sodium 3-mercapto-1-propane sulfonate, and thiourea.

25. The production method according to any one of claims 19 to 24, characterized by, The electroplating solution comprises gelatin at a concentration of 40 mg / L-80 mg / L, hydroxyethyl cellulose at a concentration of 30 mg / L-80 mg / L, poly(dithiopropyl sulfone) at a concentration of 10 mg / L-25 mg / L, copper ions at a concentration of 50 g / L-100 g / L, and chloride ions at a concentration of 30 mg / L-60 mg / L.

26. The production method according to any one of claims 19 to 25, wherein, The direct current has a current density of 4000 A / m 2 - 8000 A / m 2 The electroplating method has a time of 0.5-3 minutes.

27. The production method according to any one of claims 19 to 26, wherein The preparation method further comprises: Heat treating the current collector to obtain an annealed current collector.

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