Secondary battery and electronic device

Abstract

The application relates to a secondary battery and an electronic device, the secondary battery comprising an electrode assembly, the electrode assembly comprising a positive electrode sheet, a separator and a negative electrode sheet stacked together, 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 at least one surface of the negative electrode current collector in the thickness direction being provided with the negative electrode active material layer. In the thickness direction of the negative electrode current collector, the negative electrode 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, the first metal layer comprises 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 >= 5 mu m. The secondary battery and the electronic device can improve the problems of deformation and fracture of the negative electrode sheet in the cycle process.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a secondary battery and an electronic device. BACKGROUND

[0002] As a power supply of an electronic device, a secondary battery is a key to ensure normal use of the electronic device. In order to improve the energy density, silicon-based materials are widely concerned and applied in battery negative active materials due to their high specific capacity. However, the silicon-based materials expand rapidly in volume during the charging and discharging process, which causes the negative plate to easily deform and break during the cycle process. SUMMARY

[0003] The present application inventors have found that the silicon-based material expands rapidly in volume during the charging and discharging process, and the negative plate containing the silicon-based material easily deforms and breaks during the cycle process. The current collector of the negative plate is usually a copper foil, which has good elongation but low strength. Therefore, the negative plate using the copper foil as the current collector is not easy to break but easy to deform during the expansion of the silicon-based material. By setting a high-strength metal plating layer 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 plate using the composite current collector can improve the problem of deformation caused by the expansion of the silicon-based material, and the negative plate is not easy to break during the expansion of the silicon-based material.

[0004] The present application aims to provide a secondary battery and an electronic device, and aims to improve the problem of deformation and breakage of the negative plate during the cycle process.

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

[0006] In the technical solution, the negative active material includes a silicon-based material, the silicon-based material has a high gram capacity, and the energy density of the secondary battery can be improved. However, the silicon-based material is prone to rapid expansion in the charging and discharging process, and the negative plate is prone to deformation and fracture. The negative 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, and the first metal layer includes copper. Copper has a good elongation rate, which can make the negative current collector have a good elongation rate, reduce the possibility of fracture of the negative plate during the expansion of the silicon-based material, and the negative current collector also has good electrical conductivity. The tensile strength of the negative current collector is not less than 700 MPa, which can reduce the possibility of deformation of the negative plate during the expansion of the silicon-based material. The elongation rate of the negative current collector is not less than 8%, which can reduce the possibility of fracture of the negative plate during the expansion of the silicon-based material, and can reduce the possibility of short circuit of the secondary battery during the piercing process. The thickness of the negative current collector is H1, and H1≥5 μm, which is conducive to obtaining better processing stability of the negative current collector in the process of coating the negative active material and rolling.

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

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

[0009] In some preferred embodiments, the tensile strength of the negative current collector is not higher than 1200 MPa. The greater the tensile strength of the negative current collector, the greater the thickness of the first metal layer and the second metal layer that needs to be set. By setting the tensile strength of the negative current collector to be not higher than 1200 MPa, the thickness of the negative current collector can be reduced, and the energy density of the secondary battery can be improved.

[0010] In some preferred embodiments, the tensile strength of the negative current collector is not higher than 1100 MPa, which is conducive to further reducing the thickness of the negative current collector, and further improving the energy density of the secondary battery.

[0011] In some preferred embodiments, the elongation rate of the negative current collector is not higher than 10%. The greater the elongation rate of the negative 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 rate of the negative current collector to be not higher than 10%, the thickness of the first metal layer can be reduced, and the energy density of the secondary battery can be improved.

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

[0013] In some preferred embodiments, the second metal layer has a thickness of 0.5 μm to 3 μm, which is conducive to improving the uniformity and integrity of the second metal layer adhering to the surface of the first metal layer, and is conducive to improving the tensile strength of the negative current collector and the energy density of the secondary battery.

[0014] In some preferred embodiments, the second metal layer is arranged on both surfaces of the first metal layer in the thickness direction of the negative current collector. By arranging the second metal layer on both surfaces of the first metal layer, the negative current collector can have a better tensile strength compared to arranging the second metal layer on one surface of the first metal layer.

[0015] In some preferred embodiments, the second metal layer on a single surface of the first metal layer has a thickness of 0.5 μm to 2 μm, which is conducive to improving the uniformity and integrity of the second metal layer adhering to the surface of the first metal layer, and is conducive to improving the tensile strength of the negative current collector and 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 conducive to improving the tensile strength and elongation of the negative current collector, and is conducive to reducing the cost of the negative current collector.

[0017] In some preferred embodiments, the electrode assembly is in a winding structure, and the electrode assembly is formed by winding the positive electrode sheet, the separator and the negative electrode sheet, and the second metal layer is arranged on the surface of the first metal layer away from the winding center. For the winding electrode assembly, the bending curvature of the inner layer of the winding is larger than that 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, the possibility of breaking of the second metal layer is reduced, which is conducive to improving the anti-breaking ability of the negative 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 plating layer, and the second metal layer is arranged on the surface of the first metal layer by electroplating, evaporation or vapor deposition, which is conducive 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, a silicon oxide compound, a silicon carbide compound or a silicon alloy, which is conducive to improving the specific capacity of the negative active material.

[0021] In a second aspect, the application further provides an electronic device comprising the secondary battery according to any one of the embodiments of the first aspect.

[0022] Additional aspects and advantages of the embodiments will be described in the following description, will be apparent from the description, or will be learned from the practice of the embodiments. BRIEF DESCRIPTION OF DRAWINGS

[0023] One or more embodiments are illustrated by way of example in the accompanying drawings, which are not necessarily drawn to scale, and which, if simple, are not limited to the illustrated embodiment(s). Like reference numerals refer to like elements throughout the drawings. Dimensions of components and / or shapes of components in the drawings are chosen for simplicity and clarity of discussion.

[0024] Figure 1 Structure diagram of a secondary battery according to some embodiments of the present application;

[0025] Figure 2 Structure diagram of an electrode assembly according to some embodiments of the present application;

[0026] Figure 3 Partial structure diagram of an electrode assembly according to some embodiments of the present application;

[0027] Figure 4 Structure diagram of a negative current collector according to some embodiments of the present application;

[0028] Figure 5 Structure diagram of a negative current collector according to some embodiments of the present application;

[0029] Figure 6 Structure diagram of a negative current collector according to some embodiments of the present application.

[0030] BRIEF DESCRIPTION OF DRAWINGS

[0031] 100, secondary battery;

[0032] 10, case;

[0033] 20, electrode assembly; 21, positive electrode sheet; 211, positive current collector; 212, positive active material layer; 22, negative electrode sheet; 221, negative current collector; 2211, first metal layer; 2212, second metal layer; 222, negative active material layer; 23, separator;

[0034] X, first direction. DETAILED DESCRIPTION

[0035] So that the purposes, technical solutions and advantages of the embodiments of the present application are more apparent, the technical solutions in the embodiments of the present application will be described clearly below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are some embodiments of the present application, but not all the embodiments of the present application.

[0036] The term "embodiment" is mentioned in the present application means that the specific features, structures or properties described in combination with the embodiment can be included in at least one embodiment of the present application. The phrase appears at various places in the specification does not necessarily refer to the same embodiment, nor is it necessarily independent or alternative to 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 explicitly specified.

[0038] In the description of the embodiments of the present application, the term "and / or" is only used to describe the association relationship of the associated objects, which means that there can be three relationships, for example, A and / or B, which can represent the three cases of A alone, A and B together, and B alone. In addition, the character " / " in this paper generally represents that the front and rear associated objects have an "or" relationship.

[0039] The term "vertical" is used to describe the ideal state between two components. In the actual production or use state, there can be an approximate vertical state between the two components. For example, in combination with numerical description, vertical can refer to the included angle between two straight lines within the range of 90±10°, vertical can also refer to the dihedral angle between two planes within the range of 90±10°, and vertical can also refer to the included angle between a straight line and a plane within the range of 90±10°. The two components described as "vertical" can not be absolute straight lines or planes, and can be approximately straight lines or planes, and as a whole, the overall extension direction is a straight line or a plane. The components can be considered as "straight lines" or "planes".

[0040] The technical features involved in 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 a first aspect, embodiments of the present application provide a secondary battery 100, please refer to Figure 1 The secondary battery 100 includes a housing 10 and an electrode assembly 20, the housing 10 can accommodate the electrode assembly 20 and an electrolyte (not marked in the figure), and the electrolyte infiltrates the electrode assembly 20 in the housing 10.

[0042] For the above-mentioned electrode assembly 20, please refer to Figure 2 , wherein Figure 2The winding structure of the electrode assembly 20 is shown. The electrode assembly 20 includes the positive electrode sheet 21, the separator 23, and the negative electrode sheet 22, which are stacked and wound, and the separator 23 is arranged between adjacent positive electrode sheet 21 and negative electrode sheet 22. In the embodiments of the present application, the electrode assembly 20 is taken as an example for description in the winding structure, and in some other embodiments, the electrode assembly 20 can also be in 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, referring to Figure 3 The positive electrode sheet 21 includes a positive electrode current collector 211 and a positive electrode active material layer 212, and the positive electrode current collector 211 is provided with the positive electrode active material layer 212 on at least one surface thereof. In some embodiments, the positive electrode current collector 211 can be an aluminum foil.

[0044] In some embodiments, the positive electrode active material layer 212 is soaked in the electrolyte in the shell 10 to generate an electrochemical reaction. The positive electrode active material layer 212 includes a positive electrode active material, a conductive agent, a binder, and the like, which are uniformly mixed and stirred and coated on at least one surface of the positive electrode current collector 211, thereby obtaining the positive electrode active material layer 212. The positive electrode active material can 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 manganese iron 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 current collector 221 is provided with the negative electrode active material layer 222 on at least one surface thereof.

[0046] In some embodiments, the negative electrode active material layer 222 is soaked in the electrolyte in the shell 10 to generate an electrochemical reaction. The negative electrode active material layer 222 includes a negative electrode active material, a conductive agent, a binder, and the like, which are uniformly mixed and stirred and coated on at least one surface of the negative electrode current collector 221, thereby obtaining the negative electrode active material layer 222. The negative electrode active material can include at least one of graphite, silicon, hard carbon, and carbon fiber.

[0047] In some embodiments, the negative active material includes a silicon-based material, which has a higher capacity per gram and can improve the energy density of the secondary battery 100. However, the silicon-based material is prone to rapid expansion in volume during charging and discharging, and the negative electrode sheet 22 is prone to deformation and breakage. The negative current collector 221 is usually a copper foil, which has a good elongation but low strength. Therefore, the negative electrode sheet 22 using the copper foil as the negative current collector 221 is not prone to breakage but prone to deformation during the expansion of the silicon-based material. Although increasing the thickness of the negative current collector 221 can reduce the possibility of deformation of the negative current collector 221, it will reduce the energy density of the secondary battery 100 and increase the weight of the secondary battery 100. Although using a rolled copper foil as the negative current collector 221 can reduce the possibility of deformation of the negative current collector 221, the rolled copper foil is difficult to process and has a complex process, high cost, and poor elongation. Reducing the thickness and cold-pressing density of the negative active material layer 222 can reduce the possibility of expansion of the negative active material layer 222 and thus reduce the possibility of deformation and breakage of the negative electrode sheet 22. However, reducing the thickness and cold-pressing density of the negative active material layer 222 will increase the thickness of the electrode assembly 20 and thus reduce the energy density of the secondary battery 100. Although reducing the content of the silicon-based material in the negative active material layer 222 can reduce the possibility of expansion of the negative active material layer 222 and thus reduce the possibility of deformation and breakage of the negative electrode sheet 22, the capacity per gram of the negative electrode sheet 22 will be reduced and thus the energy density of the secondary battery 100 will be reduced.

[0048] To improve the above problems, in the embodiments of the present application, please refer to Figure 3 and Figure 4The negative current collector 221 includes a first metal layer 2211 and a second metal layer 2212 attached to a surface of the first metal layer 2211. The first metal layer 2211 is a metal foil, and the first metal layer 2211 includes copper. The tensile strength of the negative current collector 221 is not less than 700 MPa. The elongation of the negative current collector 221 is not less than 8%. The thickness of the negative current collector 221 is H1, and H1 is greater than or equal to 5 micrometers. The first metal layer 2211 includes copper, which has good elongation, so that the negative current collector 221 has good elongation, and the possibility of fracture of the negative plate 22 during expansion of the silicon-based material is reduced. Meanwhile, the negative current collector 221 also has good electrical conductivity. The tensile strength of the negative current collector 221 is not less than 700 MPa, which can reduce the possibility of deformation of the negative plate 22 during expansion of the silicon-based material. The elongation of the negative current collector 221 is not less than 8%, which can reduce the possibility of fracture of the negative plate 22 during expansion of the silicon-based material, and can reduce the possibility of short circuit of the secondary battery 100 during the puncture process. The thickness of the negative current collector 221 is H1, and H1 is greater than or equal to 5 micrometers, which is conducive to ensuring that the negative current collector 221 has good safety performance, and is also conducive to obtaining better processing stability of the negative current collector 221 during processes such as coating of the negative active material and rolling.

[0049] In some embodiments, H1 is less than or equal to 11 micrometers, which is conducive to improving the energy density of the secondary battery 100.

[0050] In some embodiments, H1 is less than or equal to 8 micrometers, which is conducive to balancing the energy density of the secondary battery 100 on the premise of 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 current collector 221 is not greater than 1200 MPa. The greater the tensile strength of the negative 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 current collector 221 to be not greater than 1200 MPa is conducive to reducing the thickness of the negative current collector 221, and thus is conducive to improving the energy density of the secondary battery 100.

[0052] In some embodiments, the tensile strength of the negative current collector 221 is not greater than 1100 MPa. When the tensile strength of the negative current collector 221 reaches 1100 MPa, the negative current collector 221 can have good expansion inhibition capability. Continuing to increase the tensile strength requires setting a second metal layer 2212 with a greater thickness, which will affect the energy density and gradually reduce the cost performance. Therefore, setting the tensile strength of the negative current collector 221 to be not greater than 1100 MPa is conducive to further reducing the thickness of the negative current collector 221, and thus is conducive to further improving the energy density of the secondary battery 100.

[0053] In some embodiments, the elongation of the negative current collector 221 is not higher than 10%. The greater the elongation of the negative 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 current collector 221 to be not higher than 10%, the thickness of the first metal layer 2211 is reduced, and the energy density of the secondary battery 100 is improved.

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

[0055] In some embodiments, the thickness of the second metal layer 2212 is 0.5-3 μm, which is conducive to improving the uniformity and integrity of the second metal layer 2212 attached to the surface of the first metal layer 2211, improving the tensile strength of the negative current collector 221, and 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 current collector 221, the opposite surfaces of the first metal layer 2211 are provided with the second metal layer 2212. By providing the second metal layer 2212 on the opposite surfaces of the first metal layer 2211, compared with providing the second metal layer 2212 on one surface of the first metal layer 2211, the negative current collector 221 can have better tensile strength.

[0057] In some embodiments, the thickness of the second metal layer 2212 on a single surface of the first metal layer 2211 is 0.5-2 μm, which is conducive to improving the uniformity and integrity of the second metal layer 2212 attached to the surface of the first metal layer 2211, improving the tensile strength of the negative current collector 221, and 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 conducive to improving the tensile strength and elongation of the negative current collector 221 and reducing the cost of the negative current collector 221.

[0059] In some embodiments, please refer to Figure 2 and Figure 6, the electrode assembly 20 is in a wound structure, the electrode assembly 20 is wound after being stacked by 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 that of the outer layer of the winding, so the metal layer of the inner layer of the winding is more prone to breakage. The elongation of the copper foil is greater than that of the nickel layer, so the anti-breakage ability 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 located on the surface of the first metal layer 2211 toward the winding center, it is beneficial to reduce the possibility of breakage of the second metal layer 2212, thereby improving the anti-breakage ability of the negative electrode current collector 221.

[0060] In some embodiments, the mass percentage content 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 plating layer, and the second metal layer 2212 is arranged 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 plating layer, 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, a silicon oxide compound, a silicon carbon compound or a silicon alloy, which is beneficial to improve the specific capacity of the negative active material.

[0063] The second aspect of the present application also provides an electronic device comprising the secondary battery 100 according to any one of the embodiments of the first aspect. The electronic device according to the embodiments of the present application is not particularly limited and can be any electronic device known in the prior art. For example, the electronic device includes but is not limited to a Bluetooth headset, a mobile phone, a tablet, a notebook computer, an electric toy, an electric tool, an electric vehicle, an electric car, a ship, a spacecraft, etc. Among them, the electric toy can include a fixed or mobile electric toy, for example, a game console, an electric car toy, an electric ship toy and an electric plane toy, etc., and the spacecraft can include an airplane, a rocket, a space shuttle and a spacecraft, etc.

[0064] Test section:

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

[0066] The negative current collector in the present scheme embodiment is tested according to the actual test situation with reference to the national standard GB / T 29847-2013 "Test Method of Copper Foil for Printed Circuit Board". A universal material testing machine is used as the tensile strength testing equipment. First, the current collector without the active material layer or the active material layer on the surface of the current collector is removed, such as being scraped off with a scraper, and then cut into a sample with a length of L1 = 100 ± 0.5 mm (less than 100 mm, the original length is taken) 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 during stretching. Next, the sample is fixed on the clamps of the testing machine, and the force applied to the sample is recorded using a load sensor, and the deformation of the sample is recorded by a displacement sensor. The tensile speed is set to 50 mm / min, and other test parameters such as the length of the sample and the distance between the clamps of the testing machine are determined according to the test standard and the actual situation. The test is stopped when the sample is stretched to break, and the maximum tensile force F when the sample is pulled to break is recorded. The tensile strength T of the sample is calculated 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. Five parallel samples are tested, and the average value is taken as the test result. The length direction of the sample is parallel to the axis of the clamp during testing, and the sample is kept straight. The experimental temperature is 20 ± 5 °C.

[0067] 2. Elongation of the current collector:

[0068] The negative current collector in the present scheme embodiment is tested according to the actual test situation with reference to the national standard GB / T 29847-2013 "Test Method of Copper Foil for Printed Circuit Board". A universal material testing machine is used as the tensile strength testing equipment. First, the current collector without the active material layer or the active material layer on the surface of the current collector is removed, such as being scraped off with a scraper, and then cut into a sample with a length of L1 = 100 ± 0.5 mm (less than 100 mm, the original length is taken) 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 during stretching. Next, the sample is fixed on the clamps of the testing machine, and the force applied to the sample is recorded using a load sensor, and the deformation of the sample is recorded by a displacement sensor. The tensile speed is set to 50 mm / min, and other test parameters such as the length of the sample and the distance between the clamps of the testing machine are determined according to the test standard and the actual situation. The test is stopped when the sample is stretched to break, and the maximum tensile force F when the sample is pulled to break is recorded. The tensile strength T of the sample is calculated 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. Five parallel samples are tested, and the average value is taken as the test result. The length direction of the sample is parallel to the axis of the clamp during testing, and the sample is kept straight. The experimental temperature is 20 ± 5 °C.

[0069] 3. Energy density of the secondary battery:

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

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

[0072] 4. Needle penetration test of the secondary battery:

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

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

[0075] (2) left for 10 min;

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

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

[0078] (5) left for 10 min;

[0079] (6) Place the sample on the test bed, with one half of the positive and negative facing up, use a blunt needle with a diameter of 6mm, a pressing force of 1600N, a falling speed of 300N / min, test from the center of the sample, monitor the cell voltage and surface temperature rise during the test. Judgment criteria: no explosion, no fire; record the pass rate.

[0080] End

[0081] Needle penetration pass rate: record the number of non-fire / test number.

[0082] 5. Cycle test of the secondary battery:

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

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

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

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

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

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

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

[0090] (7) Rest for 5 min;

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

[0092] (9) Rest for 5 min;

[0093] (10) Repeat steps (1) to (9) for 1000 cycles;

[0094] End

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

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

[0097] Corner fracture ratio: the number of corner fractures of the battery cell after 1000 cycles / the number of tests.

[0098] 6. Negative electrode sheet gram capacity test:

[0099] First, the negative electrode sheet is removed from the secondary battery in the full discharge state, and the electrolyte in the negative electrode sheet is dried in an electric oven at a temperature of 60°C; after the positive active material layer is scraped off the negative electrode sheet with a knife, the negative active material layer powder is scraped off with a knife, and the weight m1 = 100mg is weighed by a balance, which is immersed in a hydrochloric acid solution (mass fraction 10%) for 3h; then, the remaining solid material is filtered out, and the solid material is baked in an electric oven at a temperature of 60°C for 24h, and the weight of the solid material m2 is weighed; the mass ratio x of the negative active material in the original electrode sheet formula is calculated as (m1-m2) / m1. Again, the negative active material layer powder is scraped off the negative electrode sheet with a knife, and the weight m3 = 100mg is weighed by a balance, which is mixed with conductive carbon black (Super P), polyvinylidene fluoride (PVDF) according to the mass ratio 95:2.5:2.5, and N-methyl pyrrolidone (NMP) is added as a solvent to prepare a slurry with a solid content of 65wt%, and the slurry is stirred uniformly. The slurry is uniformly coated on the copper foil of the negative electrode current collector, and after the electrode sheet is dried, the electrode sheet is assembled into a button cell with a lithium sheet, a separator and an electrolyte. Finally, the effective capacity of the button cell is obtained by charging at a rate of 0.1C to 4.5V, charging at a constant voltage of 4.5V to 0.02C, and discharging at a rate of 0.1C to 2.8V, and the gram capacity of the negative active material is obtained by dividing the effective capacity by the weight of the effective active material powder m4 (m4 = m3 x x).

[0100] Silicon-based material mass percentage calculation: assuming the silicon-based material mass percentage is X, X*1750 + (1-X)*358 = the gram capacity of the negative active material, from which the silicon-based material mass percentage X can be calculated.

[0101] Example 1

[0102] <Preparation of the positive electrode sheet>:

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

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

[0105] <Preparation of the negative electrode sheet>:

[0106] The negative active material graphite, silicon carbon, the binder styrene butadiene rubber (SBR), and the thickening agent sodium carboxymethyl cellulose (CMC) were mixed in a weight ratio of 87:10:1.5:1.5, deionized water was added as a solvent, and the negative electrode slurry was stirred in a vacuum stirrer until the solid content was 70wt% and the system was uniform.

[0107] A 4μm high-purity copper foil was selected as the substrate, and the surface of the copper foil was cleaned with an alkaline solution to remove grease and impurities; then the surface of the copper foil was treated with an acidic solution (such as nitric acid) to remove oxides and activate the surface. A plating solution was prepared by mixing nickel sulfate, nickel chloride, and boric acid, and the temperature was maintained at 40-60°C. The copper substrate was immersed in the plating bath, and a uniform nickel layer was deposited by electrolysis. The thickness of the nickel layer was 1μm, and the mass percentage of nickel in the nickel layer was 99.9%. After the double-side plating of the copper foil was completed, a 6μm copper-nickel-plated negative current collector was obtained. The above negative electrode slurry was coated on one surface of the negative current collector and a negative empty foil section was reserved, and the negative electrode slurry was dried to obtain a single-side negative electrode sheet with a single-side negative active material layer. Then, the above steps were repeated on the other surface of the negative current collector to obtain a double-side negative electrode sheet with a double-side negative active material layer. The mass percentage of silicon carbon in the negative active material was 10%.

[0108] <Preparation of the separator>:

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

[0110] <Preparation of electrolyte>:

[0111] In a dry argon atmosphere, ethylene carbonate, methyl ethyl carbonate and diethyl carbonate were mixed in a mass ratio of 30:50:20 to obtain an organic solution, then lithium salt lithium hexafluorophosphate was dissolved and mixed uniformly in the organic solvent 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 above were subjected to cold pressing and slitting to obtain electrode sheets with the desired length and width, and were wound together with the separator into an electrode assembly by cutting the sheets to a fixed shape. The electrode assembly was subjected to packaging, liquid injection and formation to obtain a finished secondary battery.

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

[0115] The negative current collector used in Comparative Example 1 and Comparative Example 2 was an electrolytic copper foil with a thickness of 6 μm, but the crystal lattice structures were different. The copper foil in Comparative Example 2 had a more refined crystal lattice structure. By refining the crystal lattice, the movement of dislocations can be effectively hindered, thereby improving the strength. However, the plastic deformation ability of the current collector is weakened after the crystal lattice is refined, i.e. the elongation is deteriorated. The crystal lattice refinement can be achieved by cold rolling, heat treatment, etc. The negative current collector in Comparative Example 3 and Examples 1 to 14 was a nickel-plated copper foil. The nickel layer of the negative current collector in Comparative Example 3 and Examples 1 to 10 was two layers, and the nickel layer of Examples 11 to 14 was a single layer, and the nickel layer was located on the surface of the copper foil away from the winding center.

[0116] Table 1

[0117]

[0118] Note: In Table 1, “\” indicates that the parameter is not included.

[0119] According to Table 1 above, in combination with Comparative Examples 1 to 3 and Examples 1 to 14, it can be seen that when the negative current collector is an electrolytic copper foil, the elongation of the negative current collector is relatively high, but the tensile strength is relatively low, and the negative current collector is prone to deformation during the silicon expansion process, resulting in a relatively low capacity retention rate and a relatively high expansion rate of the battery. The use of an electrolytic copper foil with a small lattice can increase the tensile strength of the negative current collector, reduce the possibility of deformation of the negative current collector during the silicon expansion process, and 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 relatively low blunt spike pass rate and a relatively large corner fracture rate of the battery. By setting the negative current collector as a nickel-plated copper foil, compared with the electrolytic copper foil, the tensile strength of the negative current collector can be not less than 700 MPa, which can reduce the possibility of deformation of the negative current collector during the silicon expansion process, thereby improving the capacity retention rate of the battery and reducing the expansion rate of the battery, and the elongation of the negative current collector is not too poor, and the elongation can be not less than 8%, so that the battery does not have a relatively low blunt spike pass rate and a relatively large corner fracture rate. In addition, if the thickness H1 of the negative current collector is less than 5 μm, the process is difficult to prepare, and the safety performance is poor, and when H1 is greater than or equal to 5 μm, it is beneficial to obtain better processing stability of the negative current collector in the process operations such as coating of active material and compaction.

[0120] In combination with Examples 1 to 14, it can be seen that the thicker the negative current collector, the lower the energy density of the secondary battery, and when the thickness H1 of the negative current collector is greater than 11 μm, a relatively large amount of energy density of the secondary battery is easily lost, so it is preferred that H1 is less than or equal to 11 μm. It is further preferred that H1 is less than or equal to 8 μm, which is beneficial to balance the energy density of the secondary battery under the premise of 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 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 current collector is not higher than 1200 MPa, which is beneficial to reduce the thickness of the negative current collector, and thereby beneficial to improve the energy density of the secondary battery. It is further preferred that the tensile strength of the negative current collector is not higher than 1100 MPa, and when the tensile strength of the negative current collector reaches 1100 MPa, it can have a relatively good expansion inhibition ability, and continuing to increase the tensile strength requires a thicker second metal layer, and the cost performance gradually decreases. Therefore, it is further preferred that the tensile strength of the negative current collector is not higher than 1100 MPa, which is beneficial to further reduce the thickness of the negative current collector, and thereby beneficial to further improve the energy density of the secondary battery.

[0122] As can be seen from Examples 1 to 4 and Example 6, the greater the elongation of the negative 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 current collector be no greater than 10%, which is conducive to reducing the thickness of the first metal layer and thus improving the energy density of the secondary battery.

[0123] As can be seen from Example 5 and Example 14, when the total thickness of the first metal layer (copper foil) and the total thickness of the second metal layer (nickel layer) are both unchanged, the negative current collector has better tensile strength, the battery has higher capacity retention rate and lower expansion rate by setting the second metal layer on the opposite surfaces of the first metal layer than by setting the second metal layer on one surface of the first metal layer. Therefore, the two-layer second metal layer is better than the single-layer second metal layer.

[0124] As can be seen from Examples 1 to 4 and Example 6, when the thickness of the second metal layer is unchanged, the greater the thickness of the first metal layer, the greater the elongation of the negative current collector. When the thickness of the first metal layer is ≥ 3 μm, the negative current collector has a relatively large elongation. In addition, it is difficult to prepare a first metal layer with a thickness less than 3 μm. When the thickness of the first metal layer is increased to 6 μm, the elongation of the negative current collector does not increase significantly, but the energy density of the secondary battery decreases. Therefore, it is preferred that the thickness of the first metal layer be ≤ 6 μm.

[0125] As can be seen from Examples 1, 8 to 10 and Examples 11 to 13, when the thickness of the first metal layer is unchanged, the greater the thickness of the second metal layer, the greater the tensile strength of the negative current collector. When the second metal layer is a plating layer, the thickness thereof needs to be no less than 0.5 μm, which is conducive to improving the uniformity and integrity of the plating layer and reducing the possibility of affecting product performance due to uneven coverage caused by the plating layer being too thin.

[0126] Since the second metal layer is a plating layer and the first metal layer is a base layer, in actual production, the base layer serves as a support layer and needs to satisfy the condition that the thickness of the plating layer on a single surface of the base layer be ≤ 75% of the thickness of the base layer. Otherwise, the uniformity of the plating layer is difficult to control, the processing difficulty is high, and the cost is increased. As can be seen from Examples 13 and 14, when the second metal layer is a single layer, if the thickness of the second metal layer exceeds 3 μm, the thickness of the first metal layer also needs to correspondingly exceed 4 μm. At this time, the thickness of the negative current collector is likely to exceed 8 μm, which in turn reduces the energy density of the secondary battery. As can be seen from Examples 9 and 10, when the second metal layer is two layers, if the thickness of the second metal layer on a single surface of the first metal layer exceeds 2 μm, the thickness of the first metal layer also needs to correspondingly exceed 3 μm. At this time, the thickness of the negative current collector is likely to exceed 8 μm, which in turn reduces 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, in combination with Examples 1, 15 and 16, it can be seen that when the negative current collector is a 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 current collector, the higher the capacity retention rate of the battery, and the lower the expansion rate. When the mass percentage of nickel in the second metal layer is not less than 99.5%, the negative current collector has a good tensile strength, the battery has a high capacity retention rate and a low expansion rate.

[0132] The above only describes the embodiments of the present application, and does not limit the patent scope of the present application. Any equivalent structure or equivalent process transformation using the content of the present application specification and drawings, or direct or indirect application 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 positive electrode sheet, a separator, and a negative electrode sheet stacked together, 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, at least one surface of the negative electrode current collector in a thickness direction of the negative electrode current collector being provided with the negative electrode active material layer; characterized in that in the thickness direction of the negative electrode current collector, the negative electrode current collector comprises a first metal layer and a second metal layer attached to a surface of the first metal layer; the first metal layer is a metal foil, the first metal layer comprises copper, a tensile strength of the negative electrode current collector is not less than 700 MPa, an elongation of the negative electrode current collector is not less than 8%, and a thickness of the negative electrode current collector is H1, H1≥5 μm.

2. The secondary battery according to claim 1, characterized by H1≤11 μm.

3. The secondary battery according to claim 2, characterized by H1≤8 μm.

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

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

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

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

8. The secondary battery according to claim 7, characterized by The thickness of the second metal layer is 0.5 μm to 3 μm.

9. The secondary battery according to claim 7, characterized by In the thickness direction of the negative electrode current collector, both surfaces of the first metal layer are provided with the second metal layer.

10. The secondary battery according to claim 9, characterized by 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 by 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 by The electrode assembly is in a wound structure, and the second metal layer is provided on a surface of the first metal layer facing away from a winding center.

13. The secondary battery according to claim 11, characterized by 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 by The second metal layer is a plating layer, and the second metal layer is provided on a surface of the first metal layer by electroplating, evaporation plating, or vapor deposition.

15. The secondary battery according to claim 1, characterized by The silicon-based material comprises at least one of elemental silicon, a silicon oxide compound, a silicon carbide compound, or a silicon alloy.

16. An electronic device, comprising: The secondary battery as claimed in any one of claims 1 to 15.

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