Secondary battery and electronic device

Abstract

The invention relates to a secondary battery and electronic equipment, the secondary battery comprises an electrode assembly, the electrode assembly comprises a positive electrode plate, a diaphragm and a negative electrode plate which are stacked, the negative electrode plate comprises a negative electrode current collector and a negative electrode active material layer, the negative electrode active material layer comprises a negative electrode active material, the negative electrode active material comprises a silicon-based material, and a negative active material layer is arranged on at least one surface of the negative current collector in the thickness direction. In the thickness direction of the negative current collector, the negative current collector comprises a first metal layer and a second metal layer attached to the surface of the first metal layer. The first metal layer is a metal foil and comprises copper, the tensile strength of the negative current collector is not lower than 700 MPa, the elongation of the negative current collector is not lower than 8%, the thickness of the negative current collector is H1, and H1 is larger than or equal to 5 microns. According to the secondary battery and the electronic equipment, the problems of deformation and fracture of the negative plate in the circulation process can be improved.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a secondary battery and an electronic device. Background Art

[0002] As the power source of electronic devices, secondary batteries are the key to ensure the normal use of electronic devices. In order to improve energy density, silicon-based materials have attracted widespread attention and are used in battery negative electrode active materials due to their high gram capacity. However, silicon-based materials expand rapidly in volume during the charging and discharging process, causing the negative electrode sheet to easily deform and break during the cycle process. Summary of the invention

[0003] The inventor of the present application has found that the volume of silicon-based materials expands rapidly during the charge and discharge process, and the negative electrode sheet containing silicon-based materials is prone to deformation and fracture during the cycle process. The current collector of the negative electrode sheet is usually copper foil, which has good elongation but low strength. Therefore, the negative electrode sheet using copper foil as the current collector is not easy to break during the expansion of the silicon-based material, but is easy to deform. By providing a high-strength metal coating on the surface of the copper foil to form a composite current collector, the requirements of the current collector for strength, elongation and conductivity can be met. The negative electrode sheet using the composite current collector can improve the problem of deformation caused by the expansion of the silicon-based material, and the negative electrode sheet is not easy to break during the expansion of the silicon-based material.

[0004] The purpose of the present application is to provide a secondary battery and an electronic device, aiming to improve the problem of deformation and breakage of negative electrode sheets during the cycle process.

[0005] According to the first aspect of the present application, a secondary battery is provided, comprising an electrode assembly, the electrode assembly comprising a stacked positive electrode sheet, a separator and a negative electrode sheet, the negative electrode sheet comprising a negative electrode collector and a negative electrode active material layer, the negative electrode active material layer comprising a negative electrode active material, the negative electrode active material comprising a silicon-based material, and at least one surface of the negative electrode collector in the thickness direction is provided with a negative electrode active material layer. In the thickness direction of the negative electrode collector, the negative electrode collector comprises a first metal layer and a second metal layer attached to the surface of the first metal layer. The first metal layer is a metal foil, the first metal layer comprises copper, the tensile strength of the negative electrode collector is not less than 700 MPa, the elongation of the negative electrode collector is not less than 8%, and the thickness of the negative electrode collector is H1, H1≥5μm.

[0006] In the above technical solution, the negative electrode active material includes a silicon-based material. The silicon-based material has a high gram capacity and can improve the energy density of the secondary battery. However, the volume of the silicon-based material is prone to rapid expansion during the charge and discharge process, and the negative electrode sheet is prone to deformation and fracture. By setting the negative electrode collector to include a first metal layer and a second metal layer attached to the surface of the first metal layer, the first metal layer is a metal foil, and the first metal layer includes copper. The copper has a good elongation, which can make the negative electrode collector have a good elongation, reduce the possibility of the negative electrode sheet breaking during the expansion of the silicon-based material, and the negative electrode collector also has good conductivity. The tensile strength of the negative electrode collector is not less than 700MPa, which can reduce the possibility of deformation of the negative electrode sheet during the expansion of the silicon-based material. The elongation of the negative electrode collector is not less than 8%, which can reduce the possibility of the negative electrode sheet breaking during the expansion of the silicon-based material, and can reduce the possibility of short circuit of the secondary battery during the blunt puncture process. The thickness of the negative electrode collector is H1, H1≥5μm, which is conducive to the negative electrode collector in the coating of negative active materials and rolling and other process operations to obtain better processing stability.

[0007] In some preferred embodiments, H1≤11 μm, which is beneficial to improving the energy density of the secondary battery.

[0008] In some preferred embodiments, H1≤8 μm, which is beneficial to further improve the energy density of the secondary battery.

[0009] In some preferred embodiments, the tensile strength of the negative electrode current collector is not higher than 1200 MPa. The greater the tensile strength of the negative electrode current collector, the greater the thickness of the first metal layer and the second metal layer that need to be set. Setting the tensile strength of the negative electrode current collector to be no higher than 1200 MPa is beneficial to reducing the thickness of the negative electrode current collector, thereby facilitating improving the energy density of the secondary battery.

[0010] In some preferred embodiments, the tensile strength of the negative electrode current collector is not higher than 1100 MPa, which is beneficial to further reduce the thickness of the negative electrode current collector, and further help to further improve the energy density of the secondary battery.

[0011] In some preferred embodiments, the elongation of the negative electrode current collector is not higher than 10%. The greater the elongation of the negative electrode current collector, the greater the thickness of the first metal layer that needs to be set, and the lower the energy density of the secondary battery. By setting the elongation of the negative electrode current collector to be not higher than 10%, it is beneficial to reduce the thickness of the first metal layer, thereby facilitating the improvement of the energy density of the secondary battery.

[0012] In some preferred embodiments, the thickness of the first metal layer is 3 μm to 6 μm, which is beneficial to improving the elongation of the negative electrode current collector and the energy density of the secondary battery.

[0013] In some preferred embodiments, the thickness of the second metal layer is 0.5 μm to 3 μm, which is beneficial to improving the uniformity and integrity of the second metal layer attached to the surface of the first metal layer, and is beneficial to improving the tensile strength of the negative electrode current collector, while also being beneficial to improving the energy density of the secondary battery.

[0014] In some preferred embodiments, the second metal layer is disposed on both opposite surfaces of the first metal layer in the thickness direction of the negative electrode current collector. By disposing the second metal layer on the opposite surfaces of the first metal layer, the negative electrode current collector can have better tensile strength than disposing the second metal layer on one surface of the first metal layer.

[0015] In some preferred embodiments, the thickness of the second metal layer on a single surface of the first metal layer is 0.5 μm to 2 μm, which is beneficial to improving the uniformity and integrity of the second metal layer attached to the surface of the first metal layer, and is beneficial to improving the tensile strength of the negative electrode current collector, while also being beneficial to improving the energy density of the secondary battery.

[0016] In some preferred embodiments, the first metal layer is a copper foil and the second metal layer is a nickel layer, which is beneficial to improving the tensile strength and elongation of the negative electrode current collector and reducing the cost of the negative electrode current collector.

[0017] In some preferred embodiments, the electrode assembly is a winding structure, the electrode assembly is formed by stacking the positive electrode sheet, the separator and the negative electrode sheet and then winding them, and the second metal layer is arranged on the surface of the first metal layer away from the winding center. For the wound electrode assembly, the bending curvature of the inner layer of the winding is greater than the bending curvature of the outer layer of the winding, so the metal layer of the inner layer of the winding is more likely to break. By arranging the second metal layer on the surface of the first metal layer away from the winding center, it is helpful to reduce the possibility of the second metal layer breaking, and thus it is helpful to improve the anti-fracture ability of the negative electrode current collector.

[0018] In some preferred embodiments, the mass percentage of nickel in the second metal layer is not less than 99.5%, so that the second metal layer has excellent electrical conductivity and excellent mechanical strength.

[0019] In some preferred embodiments, the second metal layer is a coating, and the second metal layer is disposed on the surface of the first metal layer by electroplating, evaporation or vapor deposition, which is beneficial to improving the stability between the first metal layer and the second metal layer.

[0020] In some preferred embodiments, the silicon-based material includes at least one of elemental silicon, silicon oxide compounds, silicon carbon compounds or silicon alloys, which is beneficial to improving the gram capacity of the negative electrode active material.

[0021] In a second aspect, the present application further proposes an electronic device, comprising a secondary battery as in any embodiment of the first aspect above.

[0022] Additional aspects and advantages of the embodiments of the present application will be described, shown, or explained in part in the subsequent description through implementation of the embodiments of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] One or more embodiments are exemplarily described by corresponding drawings, which do not constitute limitations on the embodiments. Elements with the same reference numerals in the drawings represent similar elements, and unless otherwise stated, the dimensions in the drawings do not constitute proportional limitations.

[0024] Figure 1 A schematic diagram of the structure of a secondary battery in some embodiments of the present application;

[0025] Figure 2 This is a schematic diagram of the structure of the electrode assembly of some embodiments of the present application;

[0026] Figure 3 A partial schematic diagram of an electrode assembly according to some embodiments of the present application;

[0027] Figure 4 This is a schematic diagram of the structure of the negative electrode current collector of some embodiments of the present application;

[0028] Figure 5 This is a schematic diagram of the structure of the negative electrode current collector of some embodiments of the present application;

[0029] Figure 6 Schematic diagram of the structure of the negative electrode current collector of some embodiments of the present application.

[0030] Description of reference numerals:

[0031] 100. Secondary battery;

[0032] 10. Shell;

[0033] 20. Electrode assembly; 21. Positive electrode sheet; 211. Positive electrode current collector; 212. Positive electrode active material layer; 22. Negative electrode sheet; 221. Negative electrode current collector; 2211. First metal layer; 2212. Second metal layer; 222. Negative electrode active material layer; 23. Separator;

[0034] X, first direction. DETAILED DESCRIPTION

[0035] In order to make the purpose, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments.

[0036] Reference to "embodiment" in this application means that a particular feature, structure, or characteristic described in conjunction with the embodiment may be included in at least one embodiment of the present application. The appearance of the phrase in various places in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments.

[0037] In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "multiple" is more than two, unless otherwise clearly and specifically defined.

[0038] In the description of the embodiments of the present application, the term "and / or" is only a description of the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / " in this article generally indicates that the associated objects before and after are in an "or" relationship.

[0039] The term "perpendicular" is used to describe the ideal state between two components. In actual production or use, there may be a state that is approximately perpendicular between the two components. For example, in combination with numerical descriptions, perpendicularity can refer to the angle between two straight lines being between 90±10°, perpendicularity can also refer to the dihedral angle between two planes being between 90±10°, and perpendicularity can also refer to the angle between a straight line and a plane being between 90±10°. The two components described as "perpendicular" may not be absolute straight lines or planes, but may be roughly straight lines or planes. From a macroscopic perspective, a component can be considered a "straight line" or a "plane" if the overall extension direction is a straight line or a plane.

[0040] The technical features involved in the different embodiments of the present application described below can be combined with each other as long as they do not conflict with each other.

[0041] In the first aspect, the embodiment of the present application provides a secondary battery 100, please refer to Figure 1 The secondary battery 100 includes a shell 10 and an electrode assembly 20 . The shell 10 can accommodate the electrode assembly 20 and an electrolyte (not shown in the figure). The electrolyte infiltrates the electrode assembly 20 in the shell 10 .

[0042] For the electrode assembly 20, please refer to Figure 2 ,in Figure 2The winding structure of the electrode assembly 20 is shown. The electrode assembly 20 includes a positive electrode sheet 21, a separator 23 and a negative electrode sheet 22. The positive electrode sheet 21, the separator 23 and the negative electrode sheet 22 are stacked and wound, and a separator 23 is provided between adjacent positive electrode sheets 21 and negative electrode sheets 22. In the embodiment of the present application, the electrode assembly 20 is described as a winding structure as an example. In some other embodiments, the electrode assembly 20 may also be a laminated structure, for example, the positive electrode sheet 21, the separator 23 and the negative electrode sheet 22 are sequentially stacked to form a laminated electrode assembly 20.

[0043] In some embodiments, please refer to Figure 3 The positive electrode sheet 21 includes a positive electrode current collector 211 and a positive electrode active material layer 212. The positive electrode active material layer 212 is disposed on at least one surface of the positive electrode current collector 211. In some embodiments, the positive electrode current collector 211 may be an aluminum foil.

[0044] In some embodiments, the positive electrode active material layer 212 is infiltrated by the electrolyte in the housing 10 to generate an electrochemical reaction. The positive electrode active material layer 212 includes a positive electrode active material, a conductive agent, a binder, etc. The above materials are mixed and stirred evenly and coated on at least one surface of the positive electrode current collector 211 to obtain the positive electrode active material layer 212. The positive electrode active material may include at least one of lithium nickel cobalt manganese oxide, lithium cobalt oxide, lithium iron phosphate, lithium nickel cobalt aluminum oxide, lithium manganese oxide, and lithium iron manganese phosphate.

[0045] In some embodiments, the negative electrode sheet 22 includes a negative electrode current collector 221 and a negative electrode active material layer 222 , and the negative electrode active material layer 222 is disposed on at least one surface of the negative electrode current collector 221 .

[0046] In some embodiments, the negative electrode active material layer 222 is infiltrated by the electrolyte in the housing 10 to generate an electrochemical reaction. The negative electrode active material layer 222 includes a negative electrode active material, a conductive agent, a binder, etc. The above materials are mixed and stirred evenly and coated on at least one surface of the negative electrode current collector 221 to obtain the negative electrode active material layer 222. The negative electrode active material may include at least one of graphite, silicon, hard carbon, and carbon fiber.

[0047] In some embodiments, the negative electrode active material includes a silicon-based material. The silicon-based material has a high gram capacity and can improve the energy density of the secondary battery 100. However, the volume of the silicon-based material is easy to expand rapidly during the charge and discharge process, and the negative electrode sheet 22 is easy to deform and break. The negative electrode collector 221 is usually a copper foil. The copper foil has a good elongation, but the strength is low. Therefore, the negative electrode sheet 22 using copper foil as the negative electrode collector 221 is not easy to break during the expansion of the silicon-based material, but it is easy to deform. Although the possibility of deformation of the negative electrode collector 221 can be reduced by increasing the thickness of the negative electrode collector 221, the energy density of the secondary battery 100 will be reduced, and the weight of the secondary battery 100 will be increased. Although the possibility of deformation of the negative electrode collector 221 can be reduced by using rolled copper foil as the negative electrode collector 221, the processing of rolled copper foil is difficult, the process is complicated, the cost is high, and the elongation is poor. By reducing the thickness and cold pressing density of the negative electrode active material layer 222, the possibility of expansion of the negative electrode active material layer 222 can be reduced, thereby reducing the possibility of deformation and fracture of the negative electrode sheet 22. However, reducing the thickness and cold pressing density of the negative electrode active material layer 222 will increase the thickness of the electrode assembly 20, thereby reducing the energy density of the secondary battery 100. Although reducing the silicon-based material content in the negative electrode active material layer 222 can reduce the possibility of expansion of the negative electrode active material layer 222, thereby reducing the possibility of deformation and fracture of the negative electrode sheet 22, the gram capacity of the negative electrode sheet 22 will be reduced, thereby reducing the energy density of the secondary battery 100.

[0048] To improve the above problems, in the embodiments of this application, please refer to Figure 3 and Figure 4The negative electrode current collector 221 includes a first metal layer 2211 and a second metal layer 2212 attached to the surface of the first metal layer 2211. The first metal layer 2211 is a metal foil. The first metal layer 2211 includes copper. The tensile strength of the negative electrode current collector 221 is not less than 700 MPa. The elongation of the negative electrode current collector 221 is not less than 8%. The thickness of the negative electrode current collector 221 is H1, and H1 ≥ 5 μm. The first metal layer 2211 includes copper. Copper has a good elongation, which can make the negative electrode current collector 221 have a good elongation, reduce the possibility of the negative electrode sheet 22 breaking during the expansion of the silicon-based material, and the negative electrode current collector 221 also has good conductivity. The tensile strength of the negative electrode current collector 221 is not less than 700 MPa, which can reduce the possibility of the negative electrode sheet 22 deforming during the expansion of the silicon-based material. The elongation of the negative electrode current collector 221 is not less than 8%, which can reduce the possibility of the negative electrode sheet 22 breaking during the expansion of the silicon-based material, and can reduce the possibility of the secondary battery 100 short-circuiting during the blunt puncture process. The thickness of the negative electrode current collector 221 is H1, H1 ≥ 5μm, which is conducive to ensuring that the negative electrode current collector 221 has good safety performance and is also conducive to the negative electrode current collector 221 in the coating of negative active materials and rolling and other process operations to obtain better processing stability.

[0049] In some embodiments, H1 ≤ 11 μm, which is beneficial to improving the energy density of the secondary battery 100 .

[0050] In some embodiments, H1≤8 μm is beneficial to balancing the energy density of the secondary battery 100 while ensuring that the current collector has good safety performance, so as to obtain better comprehensive benefits.

[0051] In some embodiments, the tensile strength of the negative electrode current collector 221 is not higher than 1200 MPa. The greater the tensile strength of the negative electrode current collector 221, the greater the thickness of the first metal layer 2211 and the second metal layer 2212 that need to be set. Setting the tensile strength of the negative electrode current collector 221 to be not higher than 1200 MPa is beneficial to reducing the thickness of the negative electrode current collector 221, thereby facilitating improving the energy density of the secondary battery 100.

[0052] In some embodiments, the tensile strength of the negative electrode current collector 221 is not higher than 1100 MPa. When the tensile strength of the negative electrode current collector 221 reaches 1100 MPa, it can have a good ability to inhibit expansion. To further improve the tensile strength, a thicker second metal layer 2212 needs to be set, which will affect the energy density and the cost performance gradually decreases. Therefore, setting the tensile strength of the negative electrode current collector 221 to not higher than 1100 MPa is conducive to further reducing the thickness of the negative electrode current collector 221, and thus further improving the energy density of the secondary battery 100.

[0053] In some embodiments, the elongation of the negative electrode current collector 221 is not higher than 10%. The greater the elongation of the negative electrode current collector 221, the greater the thickness of the first metal layer 2211 that needs to be set, and the lower the energy density of the secondary battery 100. By setting the elongation of the negative electrode current collector 221 to be not higher than 10%, it is beneficial to reduce the thickness of the first metal layer 2211, thereby facilitating the improvement of the energy density of the secondary battery 100.

[0054] In some embodiments, the thickness of the first metal layer 2211 is 3 μm to 6 μm, which is beneficial to improving the elongation of the negative electrode current collector 221 and the energy density of the secondary battery 100 .

[0055] In some embodiments, the thickness of the second metal layer 2212 is 0.5 μm to 3 μm, which is beneficial to improving the uniformity and integrity of the second metal layer 2212 attached to the surface of the first metal layer 2211 , and is beneficial to improving the tensile strength of the negative electrode current collector 221 , while also being beneficial to improving the energy density of the secondary battery 100 .

[0056] In some embodiments, please refer to Figure 5 In the thickness direction (first direction X) of the negative electrode current collector 221, the second metal layer 2212 is disposed on both opposite surfaces of the first metal layer 2211. By disposing the second metal layer 2212 on both opposite surfaces of the first metal layer 2211, the negative electrode current collector 221 can have better tensile strength than disposing the second metal layer 2212 on one surface of the first metal layer 2211.

[0057] In some embodiments, the thickness of the second metal layer 2212 on a single surface of the first metal layer 2211 is 0.5 μm to 2 μm, which is beneficial to improving the uniformity and integrity of the second metal layer 2212 attached to the surface of the first metal layer 2211, and is beneficial to improving the tensile strength of the negative electrode current collector 221, while also being beneficial to improving the energy density of the secondary battery 100.

[0058] In some embodiments, the first metal layer 2211 is a copper foil, and the second metal layer 2212 is a nickel layer, which is beneficial to improving the tensile strength and elongation of the negative electrode current collector 221 and reducing the cost of the negative electrode current collector 221 .

[0059] In some embodiments, please refer to Figure 2 and Figure 6The electrode assembly 20 is a winding structure. The electrode assembly 20 is formed by stacking the positive electrode sheet 21, the separator 23 and the negative electrode sheet 22, and the second metal layer 2212 is arranged on the surface of the first metal layer 2211 away from the winding center. For the wound electrode assembly 20, the bending curvature of the inner layer of the winding is greater than the bending curvature of the outer layer of the winding, so the metal layer of the inner layer of the winding is more likely to break. The elongation of the copper foil is greater than that of the nickel layer, so the fracture resistance of the copper foil is greater than that of the nickel layer. By arranging the second metal layer 2212 on the surface of the first metal layer 2211 away from the winding center, compared with the second metal layer 2212 being located on the surface of the first metal layer 2211 toward the winding center, it is helpful to reduce the possibility of the second metal layer 2212 breaking, and thus it is helpful to improve the fracture resistance of the negative electrode current collector 221.

[0060] In some embodiments, the mass percentage of nickel in the second metal layer 2212 is not less than 99.5%, so that the second metal layer 2212 has excellent electrical conductivity and excellent mechanical strength.

[0061] In some embodiments, the second metal layer 2212 is a coating, and the second metal layer 2212 is disposed on the surface of the first metal layer 2211 by electroplating, evaporation or vapor deposition, so that the first metal layer 2211 serves as a base layer, and the second metal layer 2212 is plated on the surface of the first metal layer 2211 as a coating, thereby improving the stability between the first metal layer 2211 and the second metal layer 2212.

[0062] In some embodiments, the silicon-based material includes at least one of elemental silicon, silicon oxides, silicon carbon compounds, or silicon alloys, which is beneficial for increasing the gram capacity of the negative electrode active material.

[0063] In the second aspect of the present application, an electronic device is also proposed, including a secondary battery 100 as in any embodiment of the first aspect described above. The electronic device of the embodiment of the present application is not particularly limited, and it can be any electronic device known in the prior art. For example, electronic devices include but are not limited to Bluetooth headsets, mobile phones, tablets, laptops, electric toys, electric tools, battery cars, electric cars, ships, spacecraft, and the like. Among them, electric toys can include fixed or mobile electric toys, for example, game consoles, electric car toys, electric ship toys, electric airplane toys, and the like, and spacecraft can include airplanes, rockets, space shuttles, spacecraft, and the like.

[0064] Test part:

[0065] 1. Tensile strength test of current collector:

[0066] With reference to the national standard GB / T 29847-2013 "Test Method for Copper Foil for Printed Circuit Boards", the negative electrode current collector in the embodiment of this scheme is tested according to the actual test situation. A universal material testing machine is used as a tensile strength testing device. First, select a current collector that is not coated with an active material layer or remove the active material layer on the surface of the current collector coated with an active material layer, such as by scraping it with a scraper, and then cut it into a sample with a length of L1 = 100 ± 0.5 mm (the original length is taken if it is less than 100 mm) and a width of 20 ± 0.25 mm. Then, clamps are installed at both ends of the sample to ensure that the sample will not slip when stretched. Next, fix the sample on the clamp of the test machine, use a load sensor to record the force applied to the sample, and use a displacement sensor to record the deformation of the sample. Set a tensile speed of 50 mm / min, and other test parameters such as the measurement length and the distance between the chucks of the test machine are determined according to the actual situation with reference to the test standard. Stop the test when the sample is stretched to break, record the maximum tensile force F that the sample bears when it breaks, and calculate the tensile strength T of the sample according to T=F / S, where S is the initial cross-sectional area of ​​the sample, which is equal to the product of the width of the sample and the thickness of the sample. Test 5 parallel samples and take the average value as the test result. During the test, the length direction of the sample is parallel to the axis of the fixture, and the sample is kept straight. The experimental temperature is 20±5℃.

[0067] 2. Elongation test of current collector:

[0068] With reference to the national standard GB / T 29847-2013 "Test Method for Copper Foil for Printed Circuit Boards", the negative electrode current collector in the embodiment of this scheme is tested according to the actual test situation. A universal material testing machine is used as a tensile strength testing device. First, select a current collector that is not coated with an active material layer or remove the active material layer on the surface of the current collector, such as scraping it with a scraper, and then cut it into a sample with a length of L1 = 100 ± 0.5 mm (the original length is taken if it is less than 100 mm) and a width of 20 ± 0.25 mm. Then, clamps are installed at both ends of the sample to ensure that the sample will not slip when stretched. Next, fix the sample on the clamp of the test machine, use a load sensor to record the force applied to the sample, and use a displacement sensor to record the deformation of the sample. Set a stretching speed of 50 mm / min, and other test parameters such as the measurement length and the distance between the chucks of the test machine are determined according to the actual situation with reference to the test standard. Stop the test when the sample is stretched to break, record the maximum deformation L that the sample withstands when it is pulled apart, and calculate the elongation of the sample = (L-L1) / L1. Test 5 parallel samples and take the average value as the test result. During the test, the length direction of the sample is parallel to the axis of the fixture and the sample is kept straight. The experimental temperature is 20±5℃.

[0069] 3. Energy density test of secondary batteries:

[0070] The secondary battery was placed in a 25°C thermostat for 30 minutes to allow the secondary battery to reach a constant temperature. The secondary battery that reached a constant temperature was charged at a constant current of 0.5C to a full charge voltage of 4.5V, then charged at a constant voltage of 4.5V to a current of 0.05C, and then discharged at 0.5C to a voltage of 3.0V, and the discharge energy was recorded.

[0071] Energy density = discharge energy / (length×width×thickness of secondary battery).

[0072] 4. Blunt puncture test of secondary batteries:

[0073] The test temperature was adjusted to a constant temperature of 25°C, and the following steps were performed on the secondary battery sample:

[0074] (1) 0.5C constant current discharge to 3.0V;

[0075] (2) Let stand for 10 minutes;

[0076] (3) 0.5C constant current charging to 4.5V;

[0077] (4) 4.5V constant voltage charging to 0.05C;

[0078] (5) Let stand for 10 minutes;

[0079] (6) Place the sample on the test table with the front and back halves facing up. Use a blunt nail with a diameter of 6 mm, an extrusion force of 1600 N, and a drop speed of 300 N / min. Test from the center of the sample. Monitor the cell voltage and surface temperature during the test. Judgment criteria: no explosion or fire; record the pass rate.

[0080] Finish

[0081] Blunt puncture pass rate: record the number of non-ignition / test number.

[0082] 5. Cycle test of secondary batteries:

[0083] The test temperature was adjusted to a constant temperature of 25°C, and the following steps were performed on the secondary battery sample:

[0084] (1) 3C constant current charging to 4.3V;

[0085] (2) 4.3V constant voltage charging to 2C;

[0086] (3) 2C constant current charging to 4.4V;

[0087] (4) 4.4V constant voltage charging to 1C;

[0088] (5) 1C constant current charging to 4.5V;

[0089] (6) 4.5V constant voltage charging to 0.1C;

[0090] (7) Let stand for 5 minutes;

[0091] (8) 1C constant current discharge to 3.0V

[0092] (9) Let stand for 5 minutes;

[0093] (10) Steps (1) to (9) are repeated 1000 times;

[0094] Finish

[0095] Capacity retention rate: the ratio of the discharge capacity after 1000 cycles to the discharge capacity in the first cycle;

[0096] Thickness expansion rate: (cell thickness after 1000 cycles - initial cell thickness) / initial cell thickness;

[0097] Corner fracture ratio: number of broken corners in 1000 cycles of battery cells / number of tests.

[0098] 6. Negative electrode gram capacity test:

[0099] First, the negative electrode sheet is removed from the fully discharged secondary battery, and the electrolyte in the negative electrode sheet is dried in an electric oven at a temperature of 60°C; after scraping off the positive electrode active material layer from the negative electrode sheet with a knife, the negative electrode active material layer powder is scraped off with a knife, and the weight m1=100 mg is weighed on a balance, and it is immersed in a hydrochloric acid solution (mass fraction 10%) for 3 hours; then, the remaining solid matter is filtered out, and the solid matter is baked in an electric oven at a temperature of 60°C for 24 hours, and the solid weight m2 is weighed; (m1-m2) / m1 is calculated, which is the mass proportion x of the negative electrode active material in the original electrode sheet formula. Scrape the negative electrode active material layer powder from the negative electrode sheet with a knife again, weigh the weight m3 = 100 mg on a balance, mix it with conductive carbon black (Super P) and polyvinylidene fluoride (PVDF) at a mass ratio of 95:2.5:2.5, add N-methylpyrrolidone (NMP) as a solvent, and prepare a slurry with a solid content of 65wt%, and stir evenly. The slurry is evenly coated on the copper foil on the negative electrode current collector. After the electrode sheet is dried, the electrode sheet is assembled with the metal lithium sheet, the diaphragm, and the electrolyte into a button battery. Finally, it is charged to 4.5V at a constant current rate of 0.1C, charged to 0.02C at a constant voltage of 4.5V, and then discharged to 2.8V at a constant current rate of 0.1C to obtain the effective capacity of the button battery. The effective capacity is divided by the weight of the effective active material powder m4 (m4 = m3×x) to obtain the gram capacity of the negative electrode active material.

[0100] Calculation of silicon-based material mass percentage: Assuming the silicon-based material mass percentage is X, X×1750+(1-X)×358=gram capacity of the negative electrode active material, the silicon-based material mass percentage X can be calculated.

[0101] Example 1

[0102] <Preparation of positive electrode sheet>:

[0103] The positive electrode active material is lithium cobalt oxide, the positive electrode conductive agent is acetylene black, and the positive electrode binder is polyvinylidene fluoride (PVDF, with a weight average molecular weight of 5×10 5 ) were mixed in a mass ratio of 94:3:3, N-methylpyrrolidone (NMP) was added as a solvent, and the mixture was stirred under a vacuum stirrer until a positive electrode slurry with a solid content of 75 wt% and a uniform system was obtained.

[0104] The positive electrode slurry was evenly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 7 μm, and the positive electrode slurry was dried to obtain a positive electrode sheet with a single-sided positive electrode active material layer coated. Thereafter, the above steps were repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided positive electrode active material layer coated.

[0105] <Preparation of negative electrode sheet>:

[0106] The negative electrode active materials graphite, silicon carbon, binder styrene butadiene rubber (SBR) and thickener sodium carboxymethyl cellulose (CMC) are mixed in a weight ratio of 87:10:1.5:1.5, deionized water is added as a solvent, and stirred under the action of a vacuum mixer to obtain a negative electrode slurry with a solid content of 70wt% and a uniform system.

[0107] A 4μm high-purity copper foil is selected as the substrate, and an alkaline solution is used to clean the surface of the copper foil to remove grease and impurities; then an acidic solution (such as nitric acid) is used to treat the surface of the copper foil to remove oxides and activate the surface. Nickel sulfate, nickel chloride and boric acid are prepared into a plating solution, and the copper substrate is immersed in a plating tank at a temperature of 40°C to 60°C. A uniform nickel layer is deposited by electrolysis, and the nickel layer thickness is 1μm. The mass percentage of nickel in the nickel layer is 99.9%. After the copper foil is double-sidedly plated, a 6μm copper-plated nickel negative electrode current collector is obtained. The above-mentioned negative electrode slurry is coated on one surface of the negative electrode current collector and a negative electrode empty foil section is reserved. The negative electrode slurry is dried to obtain a single-sided negative electrode sheet coated with a negative electrode active material layer on one side. After that, the above steps are repeated on the other surface of the negative electrode current collector to obtain a double-sided negative electrode sheet coated with a negative electrode active material layer on both sides. Among them, the mass percentage of silicon carbon in the negative electrode active material is 10%.

[0108] <Preparation of diaphragm>:

[0109] A polyethylene (PE) porous film with a thickness of 8 μm was used as the separator.

[0110] <Electrolyte Preparation>:

[0111] In a dry argon atmosphere, ethylene carbonate, ethyl methyl carbonate and diethyl carbonate were mixed in a mass ratio of 30:50:20 to obtain an organic solution, and then lithium hexafluorophosphate was added to the organic solvent to dissolve and mix evenly to obtain an electrolyte with a lithium salt concentration of 1.15 mol / L.

[0112] <Preparation of Secondary Battery>:

[0113] The positive and negative electrode sheets prepared as above are subjected to cold pressing and slitting processes to obtain electrode sheets of required length and width, which are then cut into fixed shapes and wound together with separators into electrode assemblies. After packaging, liquid injection and formation, finished secondary batteries are made.

[0114] The relevant parameters in Comparative Examples 1 to 3 and Examples 1 to 11 are shown in Table 1 below.

[0115] The negative electrode current collectors used in Comparative Examples 1 and 2 are both electrolytic copper foils with a thickness of 6 μm, but their lattice structures are different. The copper foil in Comparative Example 2 has a more refined lattice structure. By lattice refinement, dislocation movement can be effectively hindered, thereby improving strength. However, after lattice refinement, the plastic deformation ability of the current collector is weakened, that is, the elongation deteriorates. Lattice refinement can be achieved in technology through cold rolling, heat treatment, etc. The negative electrode current collectors in Comparative Examples 3 and Examples 1 to 14 are nickel-plated copper foils, and the nickel layer of the negative electrode current collectors in Comparative Examples 3 and Examples 1 to 10 is two layers, and the nickel layer of Examples 11 to 14 is a single layer, and the nickel layer is located on the surface of the copper foil away from the winding center.

[0116] Table 1

[0117]

[0118] Note: “\” in Table 1 means that the parameter is not included.

[0119] According to Table 1 above, combined with Comparative Examples 1 to 3 and Examples 1 to 14, it can be seen that the negative electrode current collector uses electrolytic copper foil, the negative electrode current collector has a high elongation, but a low tensile strength, and the negative electrode current collector is easily deformed during the silicon expansion process, resulting in a low capacity retention rate and a high expansion rate of the battery. The negative electrode current collector uses electrolytic copper foil with a small lattice to increase the tensile strength of the negative electrode current collector, reduce the possibility of deformation of the negative electrode current collector during the silicon expansion process, reduce the expansion rate of the battery and improve the capacity retention rate of the battery, but the elongation of the electrolytic copper foil with a small lattice is poor, resulting in a low blunt puncture pass rate of the battery and a large corner fracture ratio. By setting the negative electrode current collector as nickel-plated copper foil, the tensile strength of the negative electrode current collector can be no less than 700MPa compared with the electrolytic copper foil, which can reduce the possibility of deformation of the negative electrode current collector during silicon expansion, thereby improving the capacity retention rate of the battery and reducing the expansion rate of the battery. At the same time, the elongation of the negative electrode current collector will not be too bad, and the elongation can be no less than 8%, so that the battery will not have a low blunt puncture pass rate and a large corner fracture ratio. In addition, if the thickness H1 of the negative electrode current collector is less than 5μm, the process is difficult to prepare and the safety performance is poor. When H1≥5μm, it is beneficial for the negative electrode current collector to obtain better processing stability in process operations such as coating active materials and compaction.

[0120] It can be seen from Examples 1 to 14 that the thicker the negative electrode current collector, the lower the energy density of the secondary battery. When the thickness H1 of the negative electrode current collector is greater than 11 μm, the energy density of the secondary battery is easily lost. Therefore, H1≤11 μm is preferred. H1≤8 μm is further preferred, which is conducive to balancing the energy density of the secondary battery while ensuring that the current collector has good safety performance, so as to obtain better comprehensive benefits.

[0121] In combination with Examples 1 to 14, it can be seen that the greater the tensile strength of the negative electrode current collector, the greater the thickness of the first metal layer and the second metal layer that need to be set, so it is preferred that the tensile strength of the negative electrode current collector is not higher than 1200MPa, which is conducive to reducing the thickness of the negative electrode current collector, and thus is conducive to improving the energy density of the secondary battery. It is further preferred that the tensile strength of the negative electrode current collector is not higher than 1100MPa. When the tensile strength of the negative electrode current collector reaches 1100MPa, it can have a good ability to inhibit expansion. To continue to improve the tensile strength, a thicker second metal layer needs to be set, and the cost performance gradually decreases. Therefore, it is further preferred that the tensile strength of the negative electrode current collector is not higher than 1100MPa, which is conducive to further reducing the thickness of the negative electrode current collector, and thus is conducive to further improving the energy density of the secondary battery.

[0122] Combining Examples 1 to 4 and Example 6, it can be seen that the greater the elongation of the negative electrode current collector, the greater the thickness of the first metal layer that needs to be set, and the lower the energy density of the secondary battery. Therefore, it is preferred that the elongation of the negative electrode current collector is not higher than 10%, which is beneficial to reducing the thickness of the first metal layer, and further beneficial to improving the energy density of the secondary battery.

[0123] Combining Example 5 and Example 14, it can be seen that when the total thickness of the first metal layer (copper foil) and the total thickness of the second metal layer (nickel layer) remain unchanged, by setting the second metal layer on two opposite surfaces of the first metal layer, compared with setting the second metal layer on one surface of the first metal layer, the negative electrode current collector has better tensile strength, the battery has a higher capacity retention rate and a lower expansion rate, so the two-layer second metal layer is better than the single-layer second metal layer.

[0124] Combining Examples 1 to 4 and Example 6, it can be seen that when the thickness of the second metal layer remains unchanged, the greater the thickness of the first metal layer, the greater the elongation of the negative electrode current collector. The thickness of the first metal layer is ≥3μm, which can make the negative electrode current collector have a larger elongation. In addition, the first metal layer is difficult to prepare when the thickness is less than 3μm. When the thickness of the first metal layer increases to 6μm, the elongation of the negative electrode current collector does not increase significantly if the thickness of the first metal layer continues to increase, but the energy density of the secondary battery will decrease, so the thickness of the first metal layer is preferably ≤6μm.

[0125] Combining Examples 1, 8 to 10 and Examples 11 to 13, it can be seen that when the thickness of the first metal layer remains unchanged, the greater the thickness of the second metal layer, the greater the tensile strength of the negative electrode current collector. When the second metal layer is used as a coating, its thickness must be no less than 0.5 μm, which is beneficial to improving the uniformity and integrity of the coating and reducing the possibility of uneven coverage due to too thin a coating, thereby affecting product performance.

[0126] Since the second metal layer is used as a coating and the first metal layer is used as a base layer, in actual production, the base layer is used as a supporting layer, and the coating thickness on a single surface of the base layer must be ≤ the thickness of the base layer × 75%, otherwise the coating is too thick and the uniformity is difficult to control, the processing is difficult, and the cost will increase. In combination with Examples 13 and 14, it can be seen that when the second metal layer is a single layer, when the thickness of the second metal layer exceeds 3μm, the thickness of the first metal layer also needs to be correspondingly more than 4μm. At this time, the thickness of the negative electrode collector is likely to exceed 8μm, which will further reduce the energy density of the secondary battery. In combination with Examples 9 and 10, it can be seen that when the second metal layer is two layers, when the thickness of the second metal layer on a single surface of the first metal layer exceeds 2μm, the thickness of the first metal layer also needs to be correspondingly more than 3μm. At this time, the thickness of the negative electrode collector is likely to exceed 8μm, which will further reduce the energy density of the secondary battery.

[0127] The relevant parameters in Examples 15 and 16 are shown in Table 2 below.

[0128] The mass percentage of nickel in the second metal layer in Examples 1, 15 and 16 is different.

[0129] Table 2

[0130]

[0131] According to Table 2 above, combined with Examples 1, 15 and 16, it can be seen that when the negative electrode current collector is nickel-plated copper foil, the greater the mass percentage of nickel in the second metal layer (nickel layer), the greater the tensile strength of the negative electrode current collector, the higher the capacity retention rate of the battery, and the lower the expansion rate. The mass percentage of nickel in the second metal layer is not less than 99.5%, the negative electrode current collector has good tensile strength, and the battery has a higher capacity retention rate and a lower expansion rate.

[0132] The above descriptions are merely embodiments of the present application and are not intended to limit the patent scope of the present application. Any equivalent structure or equivalent process transformation made using the contents of the present application specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present application.

Claims

1. A secondary battery, comprising an electrode assembly, the electrode assembly comprising a stacked positive electrode sheet, a separator and a negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a negative electrode active material layer, the negative electrode active material layer comprising a negative electrode active material, the negative electrode active material comprising a silicon-based material, and the negative electrode active material layer being disposed on at least one surface of the negative electrode current collector in a thickness direction; It is characterized in that In the thickness direction of the negative electrode current collector, the negative electrode current collector includes a first metal layer and a second metal layer attached to the surface of the first metal layer; the first metal layer is a metal foil, the first metal layer includes copper, the tensile strength of the negative electrode current collector is not less than 700 MPa, the elongation of the negative electrode current collector is not less than 8%, and the thickness of the negative electrode current collector is H1, H1≥5μm.

2. The secondary battery according to claim 1, characterized in that: H1≤11μm.

3. The secondary battery according to claim 2, characterized in that: H1≤8μm.

4. The secondary battery according to claim 1, characterized in that: The tensile strength of the negative electrode current collector is not higher than 1200 MPa.

5. The secondary battery according to claim 4, characterized in that: The tensile strength of the negative electrode current collector is not higher than 1100 MPa.

6. The secondary battery according to claim 1, characterized in that: The elongation of the negative electrode current collector is not higher than 10%.

7. The secondary battery according to claim 1, characterized in that: The thickness of the first metal layer is 3 μm to 6 μm.

8. The secondary battery according to claim 7, characterized in that: The second metal layer has a thickness of 0.5 μm to 3 μm.

9. The secondary battery according to claim 7, characterized in that: In the thickness direction of the negative electrode current collector, the second metal layer is disposed on two opposite surfaces of the first metal layer.

10. The secondary battery according to claim 9, characterized in that: The thickness of the second metal layer on a single surface of the first metal layer is 0.5 μm to 2 μm.

11. The secondary battery according to any one of claims 1 to 10, characterized in that: The first metal layer is a copper foil, and the second metal layer is a nickel layer.

12. The secondary battery according to claim 11, characterized in that: The electrode assembly is a winding structure, and the second metal layer is arranged on a surface of the first metal layer away from the winding center.

13. The secondary battery according to claim 11, characterized in that: The mass percentage of nickel in the second metal layer is not less than 99.5%.

14. The secondary battery according to claim 11, characterized in that: The second metal layer is a plated layer, and the second metal layer is disposed on the surface of the first metal layer by electroplating, evaporation or vapor deposition.

15. The secondary battery according to claim 1, characterized in that: The silicon-based material includes at least one of elemental silicon, silicon-oxygen compounds, silicon-carbon compounds or silicon alloys.

16. An electronic device, characterized in that: The invention comprises the secondary battery according to any one of claims 1 to 15.

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