Secondary batteries and electronic devices
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- NINGDE AMPEREX TECHNOLOGY LTD
- Filing Date
- 2024-06-27
- Publication Date
- 2026-08-03
Smart Images
Figure 2026525741000001_ABST
Abstract
Description
[Technical Field]
[0001] This application relates to the field of energy storage devices, and more specifically to secondary batteries and electronic devices. [Background technology]
[0002] As electrochemical devices (e.g., lithium-ion batteries) are widely applied to various electronic products, users are increasingly demanding higher energy density from these devices. For this reason, silicon-based materials, with their high weight capacity of 4200 mAh / g, are typically used as negative electrode active materials. However, when silicon-based materials are used as negative electrode active materials, significant volume changes occur due to the intercalation and release of lithium ions, increasing the degree of expansion and deformation of the entire lithium-ion battery. Furthermore, because silicon has poor electronic and ionic conductivity, it degrades the power performance of lithium-ion batteries, makes lithium deposition more likely, and affects the cycle life. [Overview of the Initiative]
[0003] One objective of this application is to provide a secondary battery with improved energy density, cycle life, and cycle expansion performance, and an electronic device equipped with said secondary battery.
[0004] A first aspect of this application provides a secondary battery comprising 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. The first negative electrode sheet comprises 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 comprises 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 comprises a first negative electrode active material, the first negative electrode active material comprising a silicon-based material. The second negative electrode active material layer comprises a second negative electrode active material, the second negative electrode active material comprising graphite.
[0005] Compared to a case where the negative electrode active material contains both silicon-based material and graphite, the negative electrode sheet of this application comprises a first negative electrode sheet containing silicon-based material and a second negative electrode sheet containing graphite, thereby reducing the influence of silicon-based material on graphite. The silicon-based material in the first negative electrode sheet does not cause expansion, cycle decay, lithium deposition, or other adverse effects on the second negative electrode sheet. On the other hand, the electrode assembly is configured as a laminated structure, making it possible to place the first negative electrode sheet containing silicon-based material at different positions in the electrode assembly. As a result, the secondary battery achieves a long cycle life while maintaining a high energy density and improving cycle expansion performance. In other words, the secondary battery of this application can balance energy density, cycle life, and cycle expansion performance.
[0006] According to some embodiments of this application, the silicon content in the first negative electrode active material layer is 5 wt% to 50 wt%.
[0007] According to some embodiments of this application, the silicon content in the first negative electrode active material layer is 5 wt% to 30 wt%.
[0008] According to some embodiments of this application, the coating weight of the first negative electrode active material layer is 2 to 6 mg / cm³. 2 That is the case.
[0009] According to some embodiments of this application, the compressive density of the first negative electrode active material layer is 1.1 to 1.4 g / cm³. 3 That is the case.
[0010] According to some embodiments of this application, the coating weight of the second negative electrode active material layer is 8-9 mg / cm². 2 That is the case.
[0011] According to some embodiments of this application, in the thickness direction, the first negative electrode sheet is located on at least one side of the electrode assembly.
[0012] According to some embodiments of this application, the first negative electrode sheet and the second negative electrode sheet are arranged alternately in the thickness direction.
[0013] According to some embodiments of this application, in the thickness direction, the first negative electrode sheet is located in the central part of the electrode assembly.
[0014] According to some embodiments of this application, the first negative electrode active material layer contains 90 wt% to 98 wt% of the first negative electrode active material, 2 wt% to 6 wt% of polyacrylic acid, 1 wt% to 2 wt% of carboxymethylcellulose sodium, and 0.5 wt% to 2 wt% of carbon nanotubes.
[0015] According to some embodiments of this application, the second negative electrode active material layer contains 96 wt% to 99 wt% of the second negative electrode active material, 0.5 wt% to 2 wt% of polyacrylic acid or styrene-butadiene rubber, and 0.5 wt% to 2 wt% of carboxymethylcellulose sodium.
[0016] A second aspect of this application further provides an electronic device comprising the above-mentioned secondary battery.
[0017] The above and / or additional aspects and advantages of this application will become apparent and readily apparent from the description of the embodiments in combination with the following drawings. [Brief explanation of the drawing]
[0018] [Figure 1] This is a schematic diagram of the electrode assembly according to an embodiment of the present invention. [Figure 2] Figure 1 is a schematic cross-sectional view of the electrode assembly along line II-II. [Figure 3] This is a schematic diagram of an electrode assembly according to another embodiment of the present invention. [Figure 4] This is a schematic diagram of an electrode assembly according to yet another embodiment of the present invention. [Figure 5] This is a schematic diagram of an electrode assembly according to yet another embodiment of the present invention. [Figure 6] It is a schematic structural diagram of an electrode assembly according to still another embodiment of the present invention. [Figure 7] It is a schematic structural diagram of an electrode assembly according to still another embodiment of the present invention. [Figure 8] It is a schematic structural diagram of an electrode assembly according to still another embodiment of the present invention.
Mode for Carrying Out the Invention
[0019] Hereinafter, the technical solution means in the embodiments of the present application will be clearly and detailedly described. It is clear that each described embodiment is only a part of the embodiments of the present application and not all embodiments. All technical terms and scientific terms used in this specification have the same meaning as generally understood by those skilled in the technical field to which the present application belongs, unless otherwise defined. The terms used in the specification of the present application are for specifically explaining the embodiments and do not limit the present application.
[0020] Furthermore, for the sake of brevity and clarity, in the drawings, the sizes or thicknesses of various components and layers may be enlarged. However, throughout the specification, the same numerical values refer to the same elements. The term "or / and" used in this specification includes any and all combinations of one or more related items. It should be understood that when element A is "connected to" element B, element A may be directly connected to element B, or an intermediate element C may exist, and element A and element B may be indirectly connected to each other.
[0021] Furthermore, when describing the embodiments of the present application, the use of "possible" and "may" refers to "one or more embodiments of the present application".
[0022] 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 explicitly specified by the context. Furthermore, as used herein, the terms “includes” and “compris” should be understood to indicate the presence of the described features, values, processes, operations, elements, and / or components, but not to exclude the presence or addition of one or more other features, values, processes, operations, elements, components, and / or combinations thereof.
[0023] The terms “first,” “second,” “third,” etc., may be used herein to describe various elements, components, regions, layers, and / or parts, but it should be understood that these elements, components, regions, layers, and / or parts should not be limited by these terms. These terms are used solely to distinguish one element, component, region, layer, or part from another element, component, region, layer, or part. Thus, a first element, component, region, layer, or part discussed herein may be referred to as a second element, component, region, layer, or part without departing from the art.
[0024] One embodiment of this application provides a secondary battery comprising a housing, and an electrode assembly and an electrolyte housed within the housing.
[0025] As shown in Figure 1, the electrode assembly 100 includes a negative electrode sheet 10, a positive electrode sheet 20, and a separator (not shown) placed between the negative electrode sheet 10 and the positive electrode sheet 20. The electrode assembly 100 has a laminated structure. Specifically, the negative electrode sheet 10, the separator, and the positive electrode sheet 20 are alternately laminated in the thickness direction T of the electrode assembly 100 to form the electrode assembly 100.
[0026] The negative electrode sheet 10 includes a first negative electrode sheet 11 and a second negative electrode sheet 12, the negative electrode active material materials of the first negative electrode sheet 11 and the second negative electrode sheet 12 being different. 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, as shown in Figure 2. 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, the first negative electrode active material includes a silicon-based material. The second negative electrode active material layer 12b includes a second negative electrode active material, the second negative electrode active material is graphite. Silicon-based materials may include one or more selected from pure silicon, silicon carbide, silicon oxide, silicon oxycarbide, and silicon alloys.
[0027] When silicon-based materials are added to graphite and used as negative electrode active materials, the cycle life of secondary batteries tends to decrease sharply and cycle expansion tends to increase sharply. One reason for this is the generation of by-products from the silicon-based material itself. Another reason is that these side reactions affect the cycle performance of graphite. The negative electrode sheet 10 of this application consists of a first negative electrode sheet 11 in which the negative electrode active material material contains a silicon-based material, and a second negative electrode sheet 12 in which the negative electrode active material material is graphite. Compared to the case where the negative electrode sheet's negative electrode active material material contains both a silicon-based material and graphite, the influence of the silicon-based material on graphite is reduced. The silicon-based material in the first negative electrode sheet 11 does not cause expansion, cycle decay, lithium deposition, or other adverse effects on the second negative electrode sheet 12. On the other hand, the electrode assembly 100 is configured as a laminated structure, making it possible to place the first negative electrode sheet 11 containing the silicon-based material at different positions in the electrode assembly 100. As a result, the secondary battery achieves a long cycle life while maintaining a high energy density, and its cycle expansion performance is improved. In other words, the secondary battery of this application can balance energy density, cycle life, and cycle expansion performance.
[0028] In some embodiments, the first negative electrode active material is a combination of a silicon-based material and graphite, which is advantageous for improving energy density, cycle life, and cycle expansion performance.
[0029] The first negative electrode active material layer 11b and the second negative electrode active material layer 12b may further contain a conductive agent and a binder. The conductive agent may include at least one selected from conductive carbon black, carbon nanotubes, carbon fibers, graphene, etc. The binder may include at least one selected from styrene-butadiene rubber, polyvinyl alcohol, polytetrafluoroethylene, polyvinylidene fluoride, polyacrylic acid, sodium carboxymethylcellulose, etc.
[0030] In some embodiments, the first negative electrode active material layer 11b contains 90 wt% to 98 wt% of the first negative electrode active material, 2 wt% to 8 wt% of a binder, and 0.5 wt% to 2 wt% of a conductive agent. In some embodiments, the binder of the first negative electrode active material layer 11b contains polyacrylic acid and sodium carboxymethylcellulose, where the polyacrylic acid content in the first negative electrode active material layer 11b is 2 wt% to 6 wt%, and the sodium carboxymethylcellulose content in the first negative electrode active material layer 11b is 1 wt% to 2 wt%. In some embodiments, the conductive agent is carbon nanotubes.
[0031] In some embodiments, the second negative electrode active material layer 12b comprises 96 wt% to 99 wt% of the second negative electrode active material and 1 wt% to 4 wt% of the binder. In some embodiments, the binder of the second negative electrode active material layer 12b comprises polyacrylic acid and sodium carboxymethylcellulose, or styrene-butadiene rubber and sodium carboxymethylcellulose. The content of polyacrylic acid or styrene-butadiene rubber in the second negative electrode active material layer 12b is 0.5 wt% to 2 wt%. The content of sodium carboxymethylcellulose in the second negative electrode active material layer 12b is 0.5 wt% to 2 wt%.
[0032] In some embodiments, the silicon content in the first negative electrode active material layer 11b is 5 wt% to 50 wt%, which is advantageous 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 wt% to 30 wt%.
[0033] In some embodiments, the coating weight of the first negative electrode active material layer 11b is 2 to 6 mg / cm². 2 That is the case.
[0034] In some embodiments, the coating weight of the second negative electrode active material layer 12b is 8-9 mg / cm². 2 This is advantageous for improving energy density, cycle life, and cycle expansion performance.
[0035] In some embodiments, the compressed density of the first negative electrode active material layer 11b is 1.1 to 1.4 g / cm³. 3 This is advantageous for improving energy density, cycle life, and cycle expansion performance.
[0036] In some embodiments, the compressed density of the second negative electrode active material layer 12b is 1.4 to 1.7 g / cm³. 3 This is advantageous for improving energy density, cycle life, and cycle expansion performance.
[0037] The second negative electrode active material in the second negative electrode sheet 12 is graphite and does not contain silicon-based material, which allows for a higher compressive density of the second negative electrode sheet 12 and thus improves energy density. The first negative electrode active material in the first negative electrode sheet 11 is a combination of silicon-based material and graphite, and since the silicon-based material provides more capacity, the first negative electrode sheet 11 has a compressive density that is slightly lower than that of the second negative electrode active material layer 12b in order to match its capacity with that of the second negative electrode active material layer 12b. Compared to the case where all negative electrode sheets contain silicon, the weighted average compressive densities of the first negative electrode sheet 11 and the second negative electrode sheet 12 are higher, thereby improving the overall energy density of the battery.
[0038] The first current collector 11a and the second current collector 12a may use conventional negative electrode current collectors, such as copper foil, copper alloy foil, or composite current collectors.
[0039] Each 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, as shown in Figure 2. The positive electrode current collector 21 may be a conventional positive electrode current collector, such as aluminum foil, aluminum 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 selected from lithium cobalt oxide, nickel-cobalt-manganate lithium, lithium manganese oxide, lithium iron phosphate, or lithium manganese iron phosphate. The conductive agent may be a conventional conductive agent, such as one or more selected from Ketjenblack, conductive carbon black, acetylene black, graphene, carbon nanotubes, carbon fibers, etc. The binder may be a conventional binder, and may include one or more selected from, for example, polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate salt, sodium carboxymethylcellulose, etc.
[0040] The separator may be a conventional separator. For example, the separator may be a thin film made of one or more materials selected from polyethylene, polypropylene, nonwoven fabric, and polymer fibers.
[0041] The electrolyte may be a conventional electrolyte. For example, the electrolyte may use one or more organic carbonate esters selected from ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethylmethyl carbonate as the solvent, which have electronic insulating and ionic conductive properties. The solute may use one or more lithium salts selected from LiPF6, LiBF4, LiBOB, LiAsF6, Li(CF3SO2)2N, LiCF3SO3, and LiClO4.
[0042] The enclosure may be a conventional enclosure. For example, the enclosure may be a packaging bag obtained by sealing with a packaging film such as an aluminum laminated film or a steel laminated film, or the enclosure may be a metal enclosure such as a steel can or an aluminum can.
[0043] In the thickness direction T, the positive electrode sheet 20 is located on the outermost layers on both opposing sides of the electrode assembly 100, as shown in Figures 1 and 2, and the positive electrode sheet 20 located on the outermost layer is a single-sided coated electrode sheet. In this application, a single-sided coated electrode sheet refers to an electrode sheet on which the active material is coated on only one side. It is understood that other functional layers, such as an insulating layer, may be coated on the surface of the single-sided coated electrode sheet where the active material is not coated. By arranging the outermost layer of the electrode assembly 100 in the thickness direction T as the positive electrode sheet, the negative electrode active material of the adjacent negative electrode sheet can be fully utilized, and the energy density is improved. By arranging the positive electrode sheet located on the outermost layer as a single-sided coated electrode sheet, the positive electrode active material of the positive electrode sheet can be fully utilized, and the energy density is improved.
[0044] 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 the first negative electrode sheet 11 is located on one side or both sides of the electrode assembly 100, regardless of the positions of the separator and the positive electrode sheet. Considering the position of the positive electrode sheet 20, the first negative electrode sheet 11 is located in the outermost adjacent layer on one side of the electrode assembly 100, as shown in Figures 1 and 2. Regardless of the positions 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 in the outermost adjacent layer is a double-sided coated electrode sheet. In other embodiments, considering the position of the positive electrode sheet 20, the first negative electrode sheet 11 is located in the outermost adjacent layers on both opposing sides in the thickness direction T of the electrode assembly 100, as shown in Figure 3. Regardless of the positions 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. In other embodiments, as shown in Figure 4, the first negative electrode sheet 11 is located in the outermost layer on opposite sides of the electrode assembly 100 in the thickness direction T. In this application, "the first negative electrode sheet 11 is located in the central part of the electrode assembly" means that, regardless of the positions of the separator and the positive electrode sheet, the first negative electrode sheet 11 is not laminated on either side of the electrode assembly, but rather laminated in the central region of the electrode assembly. In other embodiments, as shown in Figure 5, the first negative electrode sheet 11 is located in the central part of the electrode assembly 100 in the thickness direction T.
[0045] If the desired energy density and expansion characteristics can be obtained, from the viewpoint of ease of manufacture, the first negative electrode sheet 11 is preferably placed on at least one side of the electrode assembly (regardless of the positions of the separator and positive electrode sheet 20), as shown in Figures 1, 3, 6, and 7.
[0046] 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. If both the number of first negative electrode sheets 11 and second negative electrode sheets 12 are greater than 1, multiple first negative electrode sheets 11 may be sequentially laminated on at least one side of the electrode assembly 100 in the thickness direction T, and the corresponding multiple second negative electrode sheets 12 may be sequentially laminated on the other side or in the center of the electrode assembly 100 in the thickness direction T. As shown in Figure 6, two first negative electrode sheets 11 are sequentially laminated in the thickness direction T on one side of the electrode assembly 100, and four second negative electrode sheets 12 are sequentially laminated in the thickness direction T on the other side of the electrode assembly 100. As shown in Figure 3, two first negative electrode sheets 11 are arranged in the outermost adjacent layers on opposite sides in the thickness direction T of the electrode assembly 100, and three second negative electrode sheets 12 are sequentially laminated in the central part in the thickness direction T of the electrode assembly 100. In other embodiments, the number of first negative electrode sheets 11 arranged on both sides of the laminated second negative electrode sheets 12 is greater than one, and multiple first negative electrode sheets 11 are sequentially laminated on each side. In other embodiments, multiple first negative electrode sheets 11 are sequentially laminated in the central part in the thickness direction T of the electrode assembly 100, and the corresponding multiple second negative electrode sheets 12 are sequentially laminated on at least one side in the thickness direction T of the electrode assembly 100. As shown in Figure 5, two second negative electrode sheets 12 are sequentially stacked in the thickness direction T on one side of the electrode assembly 100, two other second negative electrode sheets 12 are sequentially stacked in the thickness direction T on the other side of the electrode assembly 100, and two first negative electrode sheets 11 are sequentially stacked in the central part of the electrode assembly 100 in the thickness direction T. As shown in Figure 7, two second negative electrode sheets 12 are sequentially stacked in the thickness direction T on one side of the electrode assembly 100, and four first negative electrode sheets 11 are sequentially stacked in the thickness direction T on the other side of the electrode assembly 100. In other embodiments, multiple first negative electrode sheets 11 and multiple second negative electrode sheets 12 are alternately stacked in the thickness direction T. As shown in Figure 8, two second negative electrode sheets 12 and one first negative electrode sheet 11 are stacked alternately in the thickness direction T, that is, two second negative electrode sheets 12 and one first negative electrode sheet 11 form the smallest unit, and then they are stacked sequentially.
[0047] One embodiment of the present application further provides an electronic device including the secondary battery of the present application. The electronic device may be any electrical device that uses a secondary battery, such as a mobile phone, a portable device, a notebook computer, an electric bicycle, an electric vehicle, a ship, a spacecraft, an electric toy, an electric tool, etc.
[0048] To better explain the present application, several specific examples and comparative examples are listed below. Here, a lithium-ion battery is taken as an example.
[0049] <Comparative Example 1> (Fabrication of the positive electrode sheet) Lithium cobaltate, conductive carbon black, and polyvinylidene fluoride were mixed at a weight ratio of 97:1.4:1.6, and N-methylpyrrolidone was added as a solvent and stirred uniformly to obtain a slurry with a solid content of 72 wt%. The slurry was uniformly coated on an aluminum foil, dried at a coating thickness of 80 μm and 85 °C, then cold-rolled, cut, slit, and vacuum dried at 85 °C for 4 hours to obtain a positive electrode sheet.
[0050] (Fabrication of the negative electrode sheet) 97.5 wt% of the negative electrode active material, 1.5 wt% of polyacrylic acid, 0.5 wt% of sodium carboxymethyl cellulose, and 0.5 wt% of carbon nanotubes were mixed and coated on a copper foil to form a negative electrode active material layer, thereby obtaining a negative electrode sheet. The negative electrode active material contained a silicon-based material (silicon carbide) of 5 wt% and graphite of 95 wt% (the proportion of silicon in the silicon-based material was 45%, that is, the mass proportion of silicon in a single negative electrode sheet was 2.25% with respect to the negative electrode active material layer). The coating weight of the negative electrode active material layer was 7 mg / cm 2 and the compression density of the negative electrode active material layer was 1.45 g / cm 3 In the lithium-ion battery, the percentage content of silicon in the whole negative electrode active material layer was 19% (that is, the ratio of the total mass of silicon in all the negative electrode sheets to the total mass of the active material layers of all the negative electrode sheets was 19%).
[0051] (Separator fabrication) A 7μm thick polyethylene film was selected as the separator.
[0052] (Preparation of electrolyte solution) In a dry argon 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 thoroughly stirred. Then, lithium salt LiPF6 was added and mixed uniformly to obtain the electrolyte. Here, the mass ratio of lithium salt LiPF6 to the total mass of the electrolyte was 12%.
[0053] (Manufacturing of lithium-ion batteries) A positive electrode sheet, a separator, and a negative electrode sheet were laminated in order, with a separator placed between the positive and negative electrode sheets to provide insulation, and an electrode assembly was obtained through lamination. The negative electrode sheet had 60 layers. The electrode assembly was placed inside an aluminum laminated film for exterior use, dehydrated at 80°C, the electrolyte was injected and sealed, and a lithium-ion battery was obtained through processes such as chemical conversion, degassing, and trimming.
[0054] <Examples 1-8> The differences compared to Comparative Example 1 are that the negative electrode sheet consists of a first negative electrode sheet containing a silicon-based negative electrode active 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 differs, the content of each component in the second negative electrode sheet differs, the coating weight or compressed density of the first negative electrode active material layer of the first negative electrode sheet differs, the coating weight or compressed density of the second negative electrode active material layer of the second negative electrode sheet differs, the ratio N1 / N2 of the number of first negative electrode sheets N1 to the number of second negative electrode sheets N2 differs, or the position of the first negative electrode sheet within the electrode assembly differs. In each example and comparative example, the silicon content in the lithium-ion battery is the same, that is, the ratio of the total mass of silicon in all negative electrode sheets (first negative electrode sheet and second negative electrode sheet) to the total mass of the active material layer of all negative electrode sheets (first negative electrode sheet and second negative electrode sheet) is the same. The parameters for Comparative Example 1 and Examples 1 to 8 are shown in Table 1. Since the components of the first negative electrode sheet in Examples 1 to 8 are the same as those of the negative electrode sheet in Comparative Example 1, the first negative electrode sheet shown in Table 1 also refers to the negative electrode sheet of Comparative Example 1.
[0055] For each comparative example and example of lithium-ion battery, energy density measurements, cycle performance measurements, and cycle thickness expansion measurements were performed. The measurement results are shown in Table 1.
[0056] (Measurement of energy density) At 25°C, a lithium-ion battery was charged to 4.5V with a constant current of 0.2C, then charged to full capacity with a constant voltage, left for 30 minutes, and then discharged with a constant current of 0.2C until the battery voltage reached 3.0V. The discharge energy at this time was recorded. The ratio of the discharge energy to the battery volume of a lithium-ion battery is the volumetric energy density of the lithium-ion battery. The volumetric energy density of the lithium-ion battery of Comparative Example 1 was used as the reference value, and the increase rate of the volumetric energy density of the lithium-ion battery of each example compared to Comparative Example 1 was calculated.
[0057] (Measurement of cycle performance) The charge / discharge termination voltage of the lithium-ion battery was measured using a battery voltage / internal resistance tester or multimeter, and the measured charge / discharge termination voltage was 3.0V to 4.5V. At 25°C, the lithium-ion battery was charged to 4.5V with a constant current of 3C, then charged to full capacity with a constant voltage, left for 30 minutes, and then discharged with a constant current of 0.5C until the battery voltage reached 3.0V. This charge / discharge process was considered one cycle, and the capacity at this time was recorded as the initial capacity. The number of cycles at which the capacity first decreased to 80% (or less than 80% of the initial capacity) was recorded.
[0058] (Measurement of cycle thickness expansion) At 25°C, a lithium-ion battery was charged to 4.5V with a constant current of 3C, then charged to a constant voltage until fully charged. The thickness of the lithium-ion battery was measured with a flat plate thickness gauge and recorded as the initial thickness. Subsequently, the battery was discharged with a constant current of 0.5C until the battery voltage reached 3.0V. This constituted one charge-discharge cycle. The thickness of the lithium-ion battery after 800 cycles was measured and recorded as the post-cycle thickness. The rate of increase in the post-cycle thickness relative to the initial thickness was calculated and defined as the thickness expansion rate of the lithium-ion battery.
[0059] (Measurement of silicon content) Lithium-ion batteries were disassembled to obtain the negative electrode sheet, which was then immersed in a dimethyl carbonate (DMC) solution for 15 minutes, dried, and tested. The negative electrode active material layer was scraped off to obtain a powder, and the Si element content was measured using inductively coupled plasma (ICP) technology.
[0060] (Measurement of coating weight and compression density) The lithium-ion battery was disassembled to obtain the negative electrode sheet, and the negative electrode active material layer was left on only one side of the negative electrode sheet. The process was as follows: punching → weight measurement → thickness measurement. The area S of the negative electrode sheet was obtained by punching, the mass m of the negative electrode active material layer was obtained by subtracting the mass of the negative electrode current collector from the weighed mass, and the thickness h of the negative electrode active material layer was obtained by subtracting the thickness of the negative electrode current collector from the measured thickness. The coating weight G of the negative electrode active material layer on this side could be calculated using the formula G = m / S. The compressed density PD of the negative electrode active material layer on this side could be calculated using the formula PD = m / (S × h).
[0061] [Table 1] JPEG2026525741000002.jpg208165 JPEG2026525741000003.jpg225165 JPEG2026525741000004.jpg152165
[0062] From Comparative Example 1 and Examples 1-8, it can be seen that when a portion of the negative electrode sheet of Comparative Example 1 is replaced with a second negative electrode sheet whose negative electrode active material is graphite, while maintaining the same silicon content, the energy density is improved, the number of cycles is increased, and the thickness expansion rate is reduced. When the silicon content in the first negative electrode active material layer of a single first negative electrode sheet is 5wt% to 30wt%, a better balance of energy density, number of cycles, and thickness expansion rate is achieved.
[0063] Comparing Examples 1-5, it can be seen that, with the same silicon content, as the N1 / N2 ratio increases, the rate of increase in energy density increases first and then decreases, the number of cycles increases first and then decreases, and the rate of thickness expansion decreases first and then increases. In Example 2, the effects of energy density, number of cycles, and rate of thickness expansion are optimal when N1:N2 = 1:2.
[0064] Comparing Example 2 with Examples 6-8, the cycle expansion rate is lower when the first negative electrode sheet and the second negative electrode sheet are alternately stacked in the thickness direction of the electrode assembly.
[0065] The embodiments disclosed herein are merely preferred embodiments and, of course, are not intended to limit this application. Accordingly, equivalent modifications made in accordance with this application remain within the scope of this application. [Explanation of symbols]
[0066] 100 electrode assembly 10 Negative electrode sheets 20 Positive electrode sheets 11. First negative electrode sheet 12. Second negative electrode sheet 11a First current collector 11b First negative electrode active material layer 12a Second current collector 12b Second negative electrode active material layer 21 Positive electrode current collector 22 Cathode active material layer
Claims
1. A secondary battery having an electrode assembly, The electrode assembly comprises a positive electrode sheet, a separator, and a negative electrode sheet that 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. The first negative electrode sheet comprises a first current collector and a first negative electrode active material layer disposed on at least one surface of the first current collector, and the second negative electrode sheet comprises 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 comprises a first negative electrode active material, and the first negative electrode active material comprises a silicon-based material. The aforementioned second negative electrode active material layer contains a second negative electrode active material, and the aforementioned second negative electrode active material is graphite. A secondary battery characterized by the following features.
2. The silicon content in the first negative electrode active material layer is 5 wt% to 50 wt%. The secondary battery according to claim 1.
3. The silicon content in the first negative electrode active material layer is 5 wt% to 30 wt%. The secondary battery according to feature 2.
4. The coating weight of the first negative electrode active material layer is 2 to 6 mg / cm². ２ That is The secondary battery according to claim 1.
5. The compressive density of the first negative electrode active material layer is 1.1 to 1.4 g / cm³. 3 That is The secondary battery according to claim 1.
6. The coating weight of the second negative electrode active material layer is 8-9 mg / cm². ２ That is The secondary battery according to claim 1.
7. In the thickness direction, the first negative electrode sheet is located on at least one side of the electrode assembly. The secondary battery according to claim 1.
8. In the thickness direction, the first negative electrode sheet and the second negative electrode sheet are arranged alternately. The secondary battery according to claim 1.
9. In the thickness direction, the first negative electrode sheet is located in the central part of the electrode assembly. The secondary battery according to claim 1.
10. The first negative electrode active material layer contains 90 wt% to 98 wt% of the first negative electrode active material, 2 wt% to 6 wt% of polyacrylic acid, 1 wt% to 2 wt% of carboxymethylcellulose sodium, and 0.5 wt% to 2 wt% of carbon nanotubes. The secondary battery according to claim 1.
11. The second negative electrode active material layer contains 96 wt% to 99 wt% of the second negative electrode active material, 0.5 wt% to 2 wt% of polyacrylic acid or styrene-butadiene rubber, and 0.5 wt% to 2 wt% of carboxymethylcellulose sodium. The secondary battery according to claim 1.
12. The secondary battery comprises the battery described in any one of claims 1 to 11. An electronic device characterized by the following features.