Secondary battery and electronic apparatus

Abstract

Disclosed in the present application are a secondary battery and an electronic apparatus. The secondary battery comprises an electrode assembly. The electrode assembly comprises a positive electrode sheet, a separator and a negative electrode sheet, which are alternately stacked in the thickness direction of the electrode assembly. The negative electrode sheet comprises a first negative electrode sheet and a second negative electrode sheet, wherein the first negative electrode sheet comprises a first current collector and a first negative-electrode active substance layer arranged on at least one surface of the first current collector, the second negative electrode sheet comprises a second current collector and a second negative-electrode active substance layer arranged on at least one surface of the second current collector, the first negative-electrode active substance layer comprises a first negative-electrode active material, which includes a silicon-based material, and the second negative-electrode active substance layer comprises a second negative-electrode active material, which is graphite.

Description

Secondary batteries and electronic devices Technical Field

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

[0002] With the widespread use of electrochemical devices (e.g., lithium-ion batteries) in various electronic products, users have also put forward higher and higher requirements for the energy density of electrochemical devices. For this reason, silicon-based materials are usually used as negative electrode active materials because silicon-based materials have a gram capacity of up to 4200mAh / g. However, as negative electrode active materials, silicon-based materials produce large volume changes accompanied by the insertion and extraction of lithium ions, which leads to increased overall expansion and deformation of lithium-ion batteries. In addition, silicon has poor electronic conductivity and ion conductivity, which causes the kinetic performance of lithium-ion batteries to deteriorate, making lithium precipitation prone to occur, affecting cycle life.

[0003] Summary of the Invention

[0004] An object of the present application is to provide a secondary battery having good balance between energy density, cycle life and cycle expansion performance, and an electronic device including the secondary battery.

[0005] In a first aspect, the present application provides a secondary battery comprising an electrode assembly. The electrode assembly comprises positive electrode sheets, separators, and negative electrode sheets alternately stacked in the thickness direction of the electrode assembly. The negative electrode sheets comprise a first negative electrode sheet and a second negative electrode sheet, the first negative electrode sheet comprising a first current collector and a first negative electrode active material layer disposed on at least one surface of the first current collector, the second negative electrode sheet comprising a second current collector and a second negative electrode active material layer disposed on at least one surface of the second current collector, the first negative electrode active material layer comprising a first negative electrode active material comprising a silicon-based material, the second negative electrode active material layer comprising a second negative electrode active material, and the second negative electrode active material being graphite.

[0006] Compared to the case where the negative electrode active materials of the negative electrode plates all contain silicon-based materials and graphite, the negative electrode plates of the present application are divided into a first negative electrode plate whose negative electrode active material contains silicon-based materials and a second negative electrode plate whose negative electrode active material is graphite. The impact of the silicon-based material on the graphite is reduced, and the silicon-based material of the first negative electrode plate does not cause expansion, cycle degradation, lithium deposition, etc. on the second negative electrode plate. At the same time, the electrode assembly is configured as a stacked structure, so that the first negative electrode plate containing silicon-based materials can be placed in different positions of the electrode assembly. This allows the secondary battery to maintain a high energy density while achieving a long cycle life and improving cycle expansion performance. In other words, the secondary battery of the present application takes into account energy density, cycle life, and cycle expansion performance.

[0007] According to some embodiments of the present application, the content of silicon in the first negative electrode active material layer is 5 to 50 wt %.

[0008] According to some embodiments of the present application, the content of silicon in the first negative electrode active material layer is 5 to 30 wt %.

[0009] According to some embodiments of the present application, the coating weight of the first negative electrode active material layer is 2 to 6 mg / cm 2 .

[0010] According to some embodiments of the present application, the compaction density of the first negative electrode active material layer is 1.1 to 1.4 g / cm 3 .

[0011] According to some embodiments of the present application, the coating weight of the second negative electrode active material layer is 8-9 mg / cm 2 .

[0012] According to some embodiments of the present application, in the thickness direction, the first negative electrode sheet is located on at least one side of the electrode assembly.

[0013] According to some embodiments of the present application, in the thickness direction, the first negative electrode sheets and the second negative electrode sheets are arranged alternately.

[0014] According to some embodiments of the present application, in the thickness direction, the first negative electrode sheet is located in the middle of the electrode assembly.

[0015] According to some embodiments of the present application, the first negative electrode active material layer includes the following components: 90-98 wt % of the first negative electrode active material, 2-6 wt % of polyacrylic acid, 1-2 wt % of sodium carboxymethyl cellulose, and 0.5-2 wt % of carbon nanotubes.

[0016] According to some embodiments of the present application, the second negative electrode active material layer includes the following components: 96-99 wt % of the second negative electrode active material, 0.5-2 wt % of polyacrylic acid or styrene-butadiene rubber, and 0.5-2 wt % of sodium carboxymethyl cellulose.

[0017] A second aspect of the present application further provides an electronic device comprising the above-mentioned secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The above and / or additional aspects and advantages of the present application will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0019] FIG1 is a schematic structural diagram of an electrode assembly provided in one embodiment of the present application.

[0020] FIG2 is a schematic cross-sectional view of the electrode assembly shown in FIG1 along line II-II.

[0021] FIG3 is a schematic structural diagram of an electrode assembly provided in another embodiment of the present application.

[0022] FIG4 is a schematic structural diagram of an electrode assembly provided in yet another embodiment of the present application.

[0023] FIG5 is a schematic structural diagram of an electrode assembly provided in yet another embodiment of the present application.

[0024] FIG6 is a schematic structural diagram of an electrode assembly provided in yet another embodiment of the present application.

[0025] FIG7 is a schematic structural diagram of an electrode assembly provided in yet another embodiment of the present application.

[0026] FIG8 is a schematic structural diagram of an electrode assembly provided in yet another embodiment of the present application.

[0027] Description of main component symbols

[0028] Electrode assembly 100;

[0029] Negative electrode sheet 10;

[0030] Positive electrode sheet 20;

[0031] a first negative electrode plate 11;

[0032] a second negative electrode sheet 12;

[0033] a first current collector 11a;

[0034] First negative electrode active material layer 11b;

[0035] Second current collector 12a;

[0036] Second negative electrode active material layer 12b;

[0037] positive electrode current collector 21;

[0038] Positive electrode active material layer 22 . DETAILED DESCRIPTION

[0039] The technical solutions in the embodiments of the present application are described clearly and in detail below. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present application. The terms used in the specification of the present application are only for the purpose of describing specific embodiments and are not intended to limit the present application.

[0040] In addition, for the sake of brevity and clarity, the size or thickness of various components or layers may be exaggerated in the accompanying drawings. Throughout the text, the same numerical value refers to the same element. As used herein, the terms "and / or" and "and / or" include any and all combinations of one or more related enumerated items. In addition, it should be understood that when element A is referred to as "connecting" element B, element A can be directly connected to element B, or there may be an intermediate element C and element A and element B can be indirectly connected to each other.

[0041] Further, when describing embodiments of the present application, the use of “may” refers to “one or more embodiments of the present application.”

[0042] The technical terms used herein are for the purpose of describing specific embodiments and are not intended to limit this application. As used herein, the singular is intended to include the plural, unless the context clearly indicates otherwise. It should be further understood that the term "comprising", when used in this specification, refers to the presence of the described features, values, steps, operations, elements and / or components, but does not exclude the presence or addition of one or more other features, values, steps, operations, elements, components and / or combinations thereof.

[0043] It should be understood that although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and / or parts, these elements, components, regions, layers and / or parts should not be limited by these terms. These terms are used to distinguish one element, component, region, layer or part from another element, component, region, layer or part. Therefore, the first element, component, region, layer or part discussed below may be referred to as the second element, component, region, layer or part without departing from the teachings of the exemplary embodiments.

[0044] An embodiment of the present application provides a secondary battery, including a housing, and an electrode assembly and an electrolyte contained in the housing.

[0045] Referring to Figure 1 , an electrode assembly 100 includes a negative electrode sheet 10, a positive electrode sheet 20, and a separator (not shown) disposed between the negative electrode sheet 10 and the positive electrode sheet 20. The electrode assembly 100 has a laminate structure. Specifically, the negative electrode sheet 10, the separator, and the positive electrode sheet 20 are alternately stacked in the thickness direction T of the electrode assembly 100 to form the electrode assembly 100.

[0046] The negative electrode sheet 10 includes a first negative electrode sheet 11 and a second negative electrode sheet 12. The negative electrode active material of the first negative electrode sheet 11 is different from the negative electrode active material of the second negative electrode sheet 12. Referring to Figure 2, the first negative electrode sheet 11 includes a first current collector 11a and a first negative electrode active material layer 11b disposed on two opposing surfaces of the first current collector 11a. The second negative electrode sheet 12 includes a second current collector 12a and a second negative electrode active material layer 12b disposed on two opposing surfaces of the second current collector 12a. The first negative electrode active material layer 11b includes a first negative electrode active material, which includes a silicon-based material. The second negative electrode active material layer 12b includes a second negative electrode active material, which includes graphite. The silicon-based material may include one or more of pure silicon, silicon-carbon, silicon-oxygen, silicon-oxygen-carbon, and a silicon alloy.

[0047] When silicon-based materials are added to graphite as negative electrode active materials, the cycle life of the secondary battery can be drastically reduced and the cycle expansion can be drastically increased. This is due to the generation of byproducts from the silicon-based materials themselves, and the impact of these side reactions on the graphite cycle. In the present application, the negative electrode plate 10 is divided into a first negative electrode plate 11 whose negative electrode active material includes a silicon-based material, and a second negative electrode plate 12 whose negative electrode active material includes graphite. Compared to the case where the negative electrode active materials of the negative electrode plates both include silicon-based materials and graphite, the effect of the silicon-based material on the graphite is reduced, and the silicon-based material of the first negative electrode plate 11 does not affect the second negative electrode plate 12 by causing expansion, cycle degradation, or lithium deposition. At the same time, the electrode assembly 100 is configured as a stacked structure, so that the first negative electrode plate 11 including the silicon-based material can be placed at different positions in the electrode assembly 100. This allows the secondary battery to maintain a high energy density while achieving a high cycle life and improving cycle expansion performance. In other words, the secondary battery of the present application takes into account energy density, cycle life, and cycle expansion performance.

[0048] In some embodiments, the first negative electrode active material is a silicon-based material and graphite, which is beneficial for improving energy density, cycle life and cycle expansion performance.

[0049] The first negative electrode active material layer 11b and the second negative electrode active material layer 12b may further include a conductive agent and a binder. The conductive agent may include at least one of conductive carbon black, carbon nanotubes, carbon fibers or graphene, and the binder may include at least one of styrene-butadiene rubber, polyvinyl alcohol, polytetrafluoroethylene, polyvinylidene fluoride, polyacrylic acid or sodium carboxymethyl cellulose.

[0050] In some embodiments, the first negative electrode active material layer 11b comprises the following components: 90-98 wt% of a first negative electrode active material, 2-8 wt% of a binder, and 0.5-2 wt% of a conductive agent. In some embodiments, the binder of the first negative electrode active material layer 11b comprises polyacrylic acid and sodium carboxymethylcellulose, wherein the polyacrylic acid content in the first negative electrode active material layer 11b is 2-6 wt%, and the sodium carboxymethylcellulose content in the first negative electrode active material layer 11b is 1-2 wt%. In some embodiments, the conductive agent is carbon nanotubes.

[0051] In some embodiments, the second negative electrode active material layer 12b includes 96-99 wt% of the second negative electrode active material and 1-4 wt% of the binder. In some embodiments, the binder of the second negative electrode active material layer 12b includes polyacrylic acid and sodium carboxymethyl cellulose or styrene-butadiene rubber and sodium carboxymethyl cellulose, wherein the content of the polyacrylic acid or styrene-butadiene rubber in the second negative electrode active material layer 12b is 0.5-2 wt%, and the content of the sodium carboxymethyl cellulose in the second negative electrode active material layer 12b is 0.5-2 wt%.

[0052] In some embodiments, the silicon content in the first negative electrode active material layer 11b is 5-50 wt%, which is beneficial for improving energy density, cycle life and cycle expansion performance. Preferably, the silicon content in the first negative electrode active material layer 11b is 5-30 wt%.

[0053] In some embodiments, the coating weight of the first negative electrode active material layer 11b is 2 to 6 mg / cm 2 .

[0054] In some embodiments, the coating weight of the second negative electrode active material layer 12b is 8-9 mg / cm 2 , which is beneficial to improving energy density, cycle life and cycle expansion performance.

[0055] In some embodiments, the compaction density of the first negative electrode active material layer 11b is 1.1 to 1.4 g / cm 3 , which is beneficial to improving energy density, cycle life and cycle expansion performance.

[0056] In some embodiments, the compaction density of the second negative electrode active material layer 12b is 1.4 to 1.7 g / cm 3 , which is beneficial to improving energy density, cycle life and cycle expansion performance.

[0057] Because the second negative electrode active material in the second negative electrode sheet 12 is graphite and lacks a silicon substrate, a higher compaction density of the second negative electrode sheet 12 can improve energy density. The first negative electrode active material in the first negative electrode sheet 11 is a silicon-based material and graphite. Since the silicon substrate provides more capacity, its compaction density is slightly lower than that of the second negative electrode active material layer 12b to match the capacity of the second negative electrode active material layer 12b. Compared to a situation where all negative electrode sheets are mixed with silicon, the weighted average compaction density of the first negative electrode sheet 11 and the second negative electrode sheet 12 is higher, thereby improving the energy density of the entire battery.

[0058] The first current collector 11 a and the second current collector 12 a may use negative electrode current collectors known in the art, such as copper foil, copper alloy foil, or a composite current collector.

[0059] Referring to Figure 2 , the positive electrode sheet 20 includes a positive electrode current collector 21 and a positive electrode active material layer 22 disposed on at least one surface of the positive electrode current collector 21. The positive electrode current collector 21 can be any known in the art, such as copper foil, copper alloy foil, or a composite current collector. The positive electrode active material layer 22 includes a positive electrode active material, a conductive agent, and a binder. The positive electrode active material includes one or more of lithium cobalt oxide, lithium nickel cobalt manganese oxide, lithium manganese oxide, lithium iron phosphate, or lithium manganese iron phosphate. The conductive agent can be any known in the art, such as doxen black, conductive carbon black, acetylene black, graphene, carbon nanotubes, carbon fibers, and the like. The binder can be any known in the art, such as one or more of polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, or sodium carboxymethylcellulose.

[0060] The separator can be any separator known in the art. For example, the separator can be a film made of one or more materials selected from polyethylene, polypropylene, non-woven fabric, and polyfiber.

[0061] The electrolyte can be any electrolyte known in the art. For example, the electrolyte can be selected from an electronically insulating and ionically conductive solution containing one or more organic carbonates such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, or ethyl methyl carbonate as a solvent, and one or more lithium salts selected from the group consisting of LiPF6, LiBF4, LiBOB, LiAsF6, Li(CF3SO2)2N, LiCF3SO3, and LiCTO4 as a solute.

[0062] The housing may be any known housing in the art. For example, the housing may be a packaging bag encapsulated with an encapsulation film, such as an aluminum-plastic film or a steel-plastic film; or the housing may be a metal housing, such as a steel shell or an aluminum shell.

[0063] Please refer to Figures 1 and 2. In the thickness direction T, the positive electrode sheet 20 is located at the outermost layer on opposite sides of the electrode assembly 100, and the positive electrode sheet 20 located at the outermost layer is a single-sided coated sheet. In this application, a single-sided coated sheet refers to a sheet with active material coated on only one surface. It can be understood that the surface of the single-sided coated sheet that is not coated with active material can be coated with other functional layers, such as an insulating layer. The outermost layer of the electrode assembly 100 in the thickness direction T is set as a positive electrode sheet, so that the negative active material of the adjacent negative electrode sheet can be fully utilized, thereby improving the energy density. The positive electrode sheet located at the outermost layer is set as a single-sided coated sheet, so that the positive active material of the positive electrode sheet can be fully utilized, thereby improving the energy density.

[0064] In some embodiments, the first negative electrode sheet 11 is located on at least one side of the electrode assembly 100 in the thickness direction T. In this application, "the first negative electrode sheet 11 is located on at least one side of the electrode assembly 100 in the thickness direction T" means that, regardless of the position of the separator and the positive electrode sheet, the first negative electrode sheet 11 is located on one or both sides of the electrode assembly 100. Referring to Figures 1 and 2 , in the thickness direction T, considering the position of the positive electrode sheet 20, the first negative electrode sheet 11 is located on the secondary outermost layer of one side of the electrode assembly 100. Regardless of the position of the positive electrode sheet 20 and the separator, the first negative electrode sheet 11 is located on one side of the electrode assembly 100. The first negative electrode sheet 11 located on the secondary outermost layer is a double-sided coated sheet. Referring to Figure 3 , in another embodiment, considering the position of the positive electrode sheet 20, the first negative electrode sheet 11 is located on the secondary outermost layers of opposite sides of the electrode assembly 100 in the thickness direction T. Regardless of the positive electrode sheet 20 and the separator, the first negative electrode sheet 11 is located on opposite sides of the electrode assembly 100. Referring to Figure 4 , in another embodiment, the first negative electrode tab 11 is located at the outermost layers on opposite sides of the electrode assembly 100 in the thickness direction T. In this application, "the first negative electrode tab 11 is located in the middle of the electrode assembly" means that, without considering the positions of the separator and positive electrode tabs, the first negative electrode tab 11 is not stacked on either side of the electrode assembly, but rather in the middle region of the electrode assembly. Referring to Figure 5 , in another embodiment, the first negative electrode tab 11 is located in the middle of the electrode assembly 100 in the thickness direction T.

[0065] For the convenience of manufacturing, when the energy density and expansion meet the requirements, it is preferred to set the first negative electrode plate 11 on at least one side of the electrode assembly (regardless of the position of the separator and the positive electrode plate 20), as shown in Figures 1, 3, 6 and 7.

[0066] In some embodiments, the number of first negative electrode sheets 11 is N1, and the number of second negative electrode sheets 12 is N2, where N1:N2=1 / X, 1≤X≤50, and X is an integer. When the number of first negative electrode sheets 11 and second negative electrode sheets 12 is greater than 1, multiple first negative electrode sheets 11 can be stacked sequentially on at least one side of the electrode assembly 100 in the thickness direction T, and corresponding multiple second negative electrode sheets 12 can be stacked sequentially on the other side or in the middle of the electrode assembly 100 in the thickness direction T. As shown in Figure 6, two first negative electrode sheets 11 are stacked sequentially on one side of the electrode assembly 100 in the thickness direction T, and four second negative electrode sheets 12 are stacked sequentially and distributed on the other side of the electrode assembly 100 in the thickness direction T. As shown in Figure 3, two first negative electrode sheets 11 are respectively disposed on opposite sides of the electrode assembly 100 in the thickness direction T, and three second negative electrode sheets 12 are stacked sequentially in the middle of the electrode assembly 100 in the thickness direction T. In other embodiments, the number of first negative electrode sheets 11 disposed on both sides of the stacked plurality of second negative electrode sheets 12 is greater than one, and the plurality of first negative electrode sheets 11 on each side are stacked sequentially. In another embodiment, the plurality of first negative electrode sheets 11 are stacked sequentially in the thickness direction T in the middle of the electrode assembly 100, and the corresponding plurality of second negative electrode sheets 12 are stacked sequentially in the thickness direction T on at least one side of the electrode assembly 100. As shown in FIG5 , two second negative electrode sheets 12 are stacked sequentially in the thickness direction T on one side of the electrode assembly 100, another two second negative electrode sheets 12 are stacked sequentially in the thickness direction T on the other side of the electrode assembly, and two first negative electrode sheets 11 are stacked sequentially in the thickness direction T in the middle of the electrode assembly 100. As shown in FIG7 , two second negative electrode sheets 12 are stacked sequentially in the thickness direction T on one side of the electrode assembly 100, and four first negative electrode sheets 11 are stacked sequentially and distributed on the other side of the electrode assembly 100 in the thickness direction T. In another embodiment, a plurality of first negative electrode sheets 11 and a plurality of second negative electrode sheets 12 are alternately stacked in the thickness direction T. As shown in FIG8 , every two second negative electrode sheets 12 and one first negative electrode sheet 11 are alternately stacked in the thickness direction T, that is, two second negative electrode sheets 12 and one first negative electrode sheet 11 form a minimum unit and are then stacked in sequence.

[0067] One embodiment of the present application further provides an electronic device comprising the secondary battery of the present application. The electronic device can be any electrical device that uses a secondary battery, for example, a mobile phone, a portable device, a laptop computer, a battery-powered vehicle, an electric vehicle, a ship, a spacecraft, an electric toy, or an electric tool.

[0068] Some specific embodiments and comparative examples are listed below to better illustrate the present application, wherein a lithium-ion battery is used as an example.

[0069] Comparative Example 1

[0070] Preparation of the positive electrode: Lithium cobalt oxide, conductive carbon black, and polyvinylidene fluoride were mixed in a weight ratio of 97:1.4:1.6, and N-methylpyrrolidone was added as a solvent. The mixture was stirred evenly to obtain a slurry with a solid content of 72 wt%. The slurry was evenly coated on aluminum foil to a thickness of 80 μm, dried at 85°C, and then cold pressed, cut, and slit. The resulting positive electrode was then dried under vacuum at 85°C for 4 hours to obtain the positive electrode.

[0071] Preparation of negative electrode sheet: 97.5wt% negative electrode active material, 1.5wt% polyacrylic acid, 0.5wt% sodium carboxymethyl cellulose and 0.5wt% carbon nanotubes are mixed and coated on copper foil to form a negative electrode active material layer to obtain a negative electrode sheet. The negative electrode active material includes 5wt% silicon-based material (silicon-based material is silicon carbon) and 95wt% graphite (silicon accounts for 45% of the silicon-based material, that is, the mass of silicon in a single negative electrode sheet accounts for 2.25% of the mass of the negative electrode active material layer). The coating weight of the negative electrode active material layer is 7mg / cm 2 The compaction density of the negative electrode active material layer is 1.45 g / cm 3 In lithium-ion batteries, the percentage content of silicon in the entire negative electrode active material layer is 19% (that is, the percentage of the total mass of silicon in all negative electrode sheets to the total mass of the active material layers of all negative electrode sheets is 19%).

[0072] Preparation of diaphragm: A polyethylene film with a thickness of 7 μm was selected as the diaphragm.

[0073] Preparation of electrolyte: In a dry argon atmosphere glove box, ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) were mixed in a mass ratio of EC:PC:DEC = 1:1:1, dissolved and stirred thoroughly, and then lithium salt LiPF6 was added. After mixing evenly, an electrolyte was obtained, in which the mass of lithium salt LiPF6 accounted for 12% of the total mass of the electrolyte.

[0074] Preparation of a lithium-ion battery: The positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator positioned between the positive and negative electrodes to provide isolation. The electrode assembly is formed by stacking the sheets. The number of negative electrode sheets is 60. The electrode assembly is then placed in an aluminum-plastic film package. After dehydration at 80°C, the electrolyte is injected and the battery is sealed. The battery undergoes formation, degassing, and trimming to produce a lithium-ion battery.

[0075] Examples 1-8

[0076] The difference from Comparative Example 1 is that the negative electrode sheets are divided into a first negative electrode sheet whose negative electrode active material comprises a silicon-based material and a second negative electrode sheet whose negative electrode active material is graphite, the content of each component in the first negative electrode sheet is different, the content of each component in the second negative electrode sheet is different, the coating weight or compaction density of the first negative electrode active material layer of the first negative electrode sheet is different, the coating weight or compaction density of the second negative electrode active material layer of the second negative electrode sheet is different, the number of the first negative electrode sheets and the ratio N1 / N2 of the second negative electrode sheets are different, or the position of the first negative electrode sheet in the electrode assembly is different. In each embodiment and comparative example, the content of silicon element in the lithium-ion battery is the same, that is, the ratio of the total mass of silicon element in all negative electrode sheets (the first negative electrode sheet and the second negative electrode sheet) to the total mass of the active material layer of all negative electrode sheets (the first negative electrode sheet and the second negative electrode sheet) is the same. The parameters of Comparative Example 1 and Examples 1-8 are shown in Table 1. It should be noted that the components of the first negative electrode sheet in Examples 1-8 are the same as those of the negative electrode sheet in Comparative Example 1, so the first negative electrode sheet in Table 1 also refers to the negative electrode sheet in Comparative Example 1.

[0077] The lithium-ion batteries of the comparative examples and embodiments were subjected to energy density tests, cycle performance tests, cycle thickness expansion tests, etc. The test results are shown in Table 1.

[0078] Energy density test: At 25°C, charge the lithium-ion battery at a constant voltage of 0.2C to 4.5V. Then, maintain constant voltage until the battery is fully charged. After standing for 30 minutes, discharge at a constant voltage of 0.2C until the battery voltage reaches 3.0V. The discharge energy at this time is recorded. The ratio of the discharge energy of the lithium-ion battery to the battery volume is the volumetric energy density of the lithium-ion battery. Using the volumetric energy density of the lithium-ion battery in Comparative Example 1 as the baseline value, the improvement ratio of the volumetric energy density of the lithium-ion battery of each example compared to the volumetric energy density of the lithium-ion battery in Comparative Example 1 is calculated.

[0079] Cycling performance test: Use a battery voltage internal resistance tester or multimeter to test the charge and discharge cut-off voltage of the lithium-ion battery. The measured charge and discharge cut-off voltage is 3.0V to 4.5V. At 25°C, charge the lithium-ion battery at a constant current of 3C to 4.5V, then charge at a constant voltage until the battery is fully charged. After standing for 30 minutes, discharge at a rate of 0.5C until the battery voltage reaches 3.0V. This charge and discharge process is considered one cycle. The capacity at this time is recorded as the initial capacity. Record the cycle number corresponding to the first decay of the capacity to 80% of the initial capacity (or less than 80% of the initial capacity).

[0080] Cyclic Thickness Expansion Test: At 25°C, a lithium-ion battery is charged at a constant current of 3C to 4.5V, then charged at a constant voltage until fully charged. Using a flat-plate thickness gauge, the thickness of the lithium-ion battery is recorded as the initial thickness. The battery is then discharged at a rate of 0.5C until the voltage drops to 3.0V. This constitutes one charge-discharge cycle. The thickness of the lithium-ion battery after 800 cycles is recorded as the post-cycle thickness. The increase in thickness after the cycle compared to the initial thickness is calculated as the thickness expansion rate of the lithium-ion battery.

[0081] Testing for silicon content: The lithium-ion battery was disassembled to obtain the negative electrode, which was soaked in dimethyl carbonate (DMC) solution for 15 minutes and then dried for later use. The negative electrode active material layer was scraped off to make a powder, and the Si content was tested using inductively coupled plasma (ICP) technology.

[0082] Coating weight and compaction density test: disassemble the lithium-ion battery to obtain the negative electrode sheet, retain only the negative electrode active material layer on one side of the negative electrode sheet, punch out → weigh → measure thickness; punch out to obtain the area S of the negative electrode sheet, weigh and deduct the mass of the negative electrode current collector to obtain the mass m of the negative electrode active material layer, measure the thickness and deduct the thickness of the negative electrode current collector to obtain the thickness h of the negative electrode active material layer; the coating weight G = m / S formula can be used to calculate the coating weight of the negative electrode active material layer on this side; the compaction density of the negative electrode active material layer on this side can be calculated by the compaction PD = m / S / h formula.

[0083] Table 1

[0084] Comparative Example 1 and Examples 1-8 demonstrate that, with the same silicon content, replacing some of the negative electrode sheets in Comparative Example 1 with a second negative electrode sheet containing graphite as the negative electrode active material improves energy density, increases cycle life, and reduces thickness expansion. When the silicon content in the first negative electrode active material layer of a single first negative electrode sheet is 5-30 wt%, even better energy density, cycle life, and thickness expansion are achieved.

[0085] Comparing Examples 1-5, it can be seen that, with the same silicon content, as the N1 / N2 ratio increases, the energy density improvement ratio first increases and then decreases, the cycle number first increases and then decreases, and the thickness expansion rate first decreases and then increases. In Example 2, when N1:N2 = 1:2, the energy density, cycle number, and thickness expansion rate are optimized.

[0086] Comparing Example 2 with Examples 6-8, it can be seen that when the first negative electrode sheets and the second negative electrode sheets are alternately stacked in the thickness direction of the electrode assembly, the cycle expansion rate is lower.

[0087] The above disclosure is only a preferred embodiment of the present application and certainly cannot be used to limit the present application. Therefore, equivalent changes made based on the present application are still within the scope covered by the present application.

Claims

1. A secondary battery, comprising an electrode assembly, characterized in that: The electrode assembly includes positive electrode sheets, separators and negative electrode sheets alternately stacked in the thickness direction of the electrode assembly, the negative electrode sheets include a first negative electrode sheet and a second negative electrode sheet, the first negative electrode sheet includes a first current collector and a first negative electrode active material layer arranged on at least one surface of the first current collector, the second negative electrode sheet includes a second current collector and a second negative electrode active material layer arranged on at least one surface of the second current collector, the first negative electrode active material layer includes a first negative electrode active material, the first negative electrode active material includes a silicon-based material, the second negative electrode active material layer includes a second negative electrode active material, and the second negative electrode active material is graphite.

2. The secondary battery according to claim 1, wherein: The content of silicon in the first negative electrode active material layer is 5-50 wt %.

3. The secondary battery according to claim 2, characterized in that: The content of silicon in the first negative electrode active material layer is 5-30 wt %.

4. The secondary battery according to claim 1, wherein: The coating weight of the first negative electrode active material layer is 2 to 6 mg / cm 2 .

5. The secondary battery according to claim 1, wherein: The compaction density of the first negative electrode active material layer is 1.1 to 1.4 g / cm 3 .

6. The secondary battery according to claim 1, wherein: The coating weight of the second negative electrode active material layer is 8 to 9 mg / cm 2 .

7. The secondary battery according to claim 1, wherein: In the thickness direction, the first negative electrode sheet is located on at least one side of the electrode assembly.

8. The secondary battery according to claim 1, wherein: In the thickness direction, the first negative electrode sheets and the second negative electrode sheets are arranged alternately.

9. The secondary battery according to claim 1, wherein: In the thickness direction, the first negative electrode sheet is located in the middle of the electrode assembly.

10. The secondary battery according to claim 1, wherein: The first negative electrode active material layer includes the following components: 90-98 wt % of a first negative electrode active material, 2-6 wt % of polyacrylic acid, 1-2 wt % of sodium carboxymethyl cellulose, and 0.5-2 wt % of carbon nanotubes.

11. The secondary battery according to claim 1, wherein The second negative electrode active material layer comprises the following components: 96-99 wt % of a second negative electrode active material, 0.5-2 wt % of polyacrylic acid or styrene-butadiene rubber, and 0.5-2 wt % of sodium carboxymethyl cellulose.

12. An electronic device, characterized in that: The invention comprises the secondary battery according to any one of claims 1 to 11.

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