Secondary battery and device containing the same

Abstract

The application relates to a secondary battery and a device containing the same. Specifically, the application provides a secondary battery, which comprises a negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a negative electrode film arranged on at least one surface of the negative electrode current collector, the negative electrode film comprising a negative electrode active material, a conductive agent and a binder, characterized in that: the negative electrode active material comprises SiOx (0 < x < 2) and graphite; the average particle size Dv50 of the negative electrode active material is 8-14 mu m; and the conductive agent comprises carbon nanotubes, the aspect ratio of the carbon nanotubes being greater than or equal to 2500:1. The secondary battery can have high energy density, good rate performance and good cycle performance at the same time.

Description

[0001] This application is a divisional application of the invention patent application filed on December 4, 2019, with application number 201911230498.X and invention title "Secondary Battery and Device Containing the Same". Technical Field

[0002] This application belongs to the field of electrochemical technology, and more specifically, this application relates to a secondary battery and an apparatus containing the same. Background Technology

[0003] With the increasing prominence of energy crises and environmental pollution, lithium-ion batteries have attracted much attention as a new type of high-energy green energy storage solution and have been widely used in electric vehicles and hybrid vehicles. As consumers' demands for longer driving range increase, the development of high-capacity lithium-ion batteries has become a focus of industry attention.

[0004] To improve the energy density of lithium-ion batteries, higher energy density positive and negative electrode active materials are needed. For negative electrode materials, traditional graphite is increasingly unable to meet the demands of technological development. Silicon-based materials, due to their high theoretical specific capacity (more than ten times that of graphite) and low equilibrium potential, are considered to be high-energy-density negative electrode materials with great research and development potential. However, silicon-based materials undergo significant volume changes during lithium insertion / extraction, leading to electrode pulverization and peeling, resulting in rapid capacity decay. Furthermore, the inherently low electronic conductivity of silicon-based materials causes significant polarization during charging and discharging, thus affecting the rate performance and cycle performance of the battery cell.

[0005] Therefore, there is still a need to improve the rate performance and cycle performance of existing silicon-based lithium batteries. Summary of the Invention

[0006] One objective of this application is to solve the problems of electrode structure stability, cycle performance, and rate performance degradation caused by the expansion of silicon-containing anode materials in secondary batteries.

[0007] To address the problems of the prior art, a first aspect of this application provides a secondary battery, the secondary battery comprising a negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a negative electrode film disposed on at least one surface of the negative electrode current collector, the negative electrode film comprising a negative electrode active material, a conductive agent and a binder, the negative electrode active material comprising SiOx (0 < x < 2) and graphite; the average particle size Dv50 of the negative electrode active material is 8 μm to 14 μm; the conductive agent comprises carbon nanotubes, the aspect ratio of the carbon nanotubes being ≥2500:1.

[0008] In a second aspect of this application, an apparatus is provided that includes the secondary battery described in the first aspect of this application.

[0009] The technical solution of this application has the following characteristics / technical effects:

[0010] The negative electrode of the secondary battery in this application uses a silicon-containing material of a specific size as the negative electrode active material and carbon nanotubes with a specific aspect ratio as the conductive agent. Under their combined action, the battery can have both high energy density and good cycle performance and rate performance. Attached Figure Description

[0011] Figure 1 This is a schematic diagram of one embodiment of the secondary battery of this application.

[0012] Figure 2 This is a schematic diagram of one embodiment of the battery module.

[0013] Figure 3 This is a schematic diagram of one embodiment of the battery pack.

[0014] Figure 4 yes Figure 3 The exploded diagram.

[0015] Figure 5 This is a schematic diagram of one embodiment of the device for using a secondary battery as a power source according to this application.

[0016] The reference numerals in the attached figures are explained as follows:

[0017] 1 battery pack

[0018] 2 upper box

[0019] 3 lower cabinets

[0020] 4 battery modules

[0021] 5 Secondary batteries Detailed Implementation

[0022] The present application will be further described below with reference to specific embodiments. It should be understood that these specific embodiments are for illustrative purposes only and are not intended to limit the scope of the present application.

[0023] For the sake of brevity, this article only discloses a few specific numerical ranges. However, any lower limit can be combined with any upper limit to form an unspecified range; and any lower limit can be combined with other lower limits to form an unspecified range, just as any upper limit can be combined with any other upper limit to form an unspecified range. Furthermore, each individually disclosed point or single value can itself serve as a lower or upper limit and be combined with any other point or single value or with other lower or upper limits to form an unspecified range.

[0024] In the description of this article, it should be noted that, unless otherwise stated, "above" and "below" include the number itself, and "several" in "one or more" means two or more.

[0025] Unless otherwise stated, the terms used in this application have their common meanings as commonly understood by those skilled in the art. Unless otherwise stated, the values ​​of the parameters mentioned in this application can be measured using various measurement methods commonly used in the art (e.g., they can be tested according to the methods given in the embodiments of this application).

[0026] In a first aspect, this application provides a secondary battery comprising a negative electrode sheet, the negative electrode sheet comprising a negative current collector and a negative electrode film disposed on at least one surface of the negative current collector, the negative electrode film comprising a negative electrode active material, a conductive agent and a binder, characterized in that: the negative electrode active material comprises SiOx (0 < x < 2) and graphite; the average particle size Dv50 of the negative electrode active material is 8 μm to 14 μm; the conductive agent comprises carbon nanotubes, the aspect ratio of the carbon nanotubes being ≥2500:1.

[0027] The negative electrode of the secondary battery in this application uses a mixed negative electrode active material of SiOx (0 < x < 2) and graphite. This ensures high energy density while avoiding excessive volume expansion of pure silicon-based materials. Furthermore, the negative electrode also uses carbon nanotubes with a high aspect ratio (≥2500:1) as a conductive agent, and the particle sizes of SiOx (0 < x < 2) and graphite are matched. In silicon systems, the particle size matching of silicon and graphite mainly affects the cycle performance of the battery. Controlling the particle size within the range of 8μm to 14μm can effectively improve this performance. This is because the volume change of silicon material is significant during charge-discharge cycles. If the particle size matching of silicon and graphite is not controlled, the volume expansion of silicon material during charging will compress the electrolyte in the graphite, while the volume contraction of silicon material during discharging will increase the porosity of the electrode, easily causing untimely electrolyte reflux and obstructing the lithium-ion transport channels, thus affecting the cycle performance of the cell. However, silicon and graphite materials with similar particle sizes have a large specific surface area, which increases the total amount of binder required. A higher proportion of this inactive material not only increases the production cost of the battery cell but also reduces its gravimetric energy density. Controlling the aspect ratio of CNTs to be greater than or equal to 2500:1 can reduce the binder content while ensuring the bonding strength is controlled within a suitable range (10N / m≤F≤90N / m), thus alleviating this problem. Therefore, controlling the average particle size Dv50 of the negative electrode active material to 8μm~14μm and using carbon nanotubes with an aspect ratio ≥2500:1 as a conductive agent allows the battery to simultaneously achieve good cycle performance and energy density.

[0028] The negative electrode of the secondary battery in this application uses carbon nanotubes with a high aspect ratio (≥2500:1). Due to their excellent electrical and thermal conductivity and structural stability, carbon nanotubes (CNTs) are commonly used in lithium-ion battery electrodes to construct a stable conductive network, maintain complete electron conduction during cycling, and improve the conductivity of Li-ion batteries. + The CNT network improves the rate of transmission, reduces electrode resistance, and minimizes polarization, thereby enhancing the cell's rate performance and cycle performance. Furthermore, the CNT network possesses a certain porosity and a large specific surface area, ensuring sufficient contact between the electrolyte and active materials for electrochemical reactions. Additionally, due to the high structural stability of CNTs, their flexibility effectively buffers the mechanical stress caused by the significant volume changes in silicon-based materials during battery cycling, thus mitigating electrode pulverization and peeling. Therefore, in the negative electrode of the secondary battery of this application, CNTs can construct a robust conductive network, enhancing the structural stability of the electrode. This not only reduces the increase in DC resistance (DCR) during cycling but also prevents rapid capacity decay in the early stages of cycling. High aspect ratio CNTs (≥2500:1) can establish more cross-linking nodes in the active material, providing more conductive pathways, mitigating the increase in negative electrode resistance (DCR) and polarization, thereby improving rate performance. Moreover, this conductive network maintains good stability during charge-discharge cycles, effectively improving the cell's cycle performance. Meanwhile, using the selected carbon nanotubes allows for a suitable reduction in binder content (3%–9%), and reducing the proportion of inactive materials effectively improves the energy density of the cell. Furthermore, the selected carbon nanotubes and binder content ensure that the bonding force remains within an appropriate range (10–90 N / m), guaranteeing that the active material does not detach from the current collector surface during cycling, thereby improving the structural stability of the electrode and enhancing the cell's cycle performance. In summary, a cell made using the selected negative electrode can simultaneously achieve good rate performance, high energy density, and superior cycle performance.

[0029] Considering the processing performance of carbon nanotubes in electrode fabrication, the aspect ratio of the carbon nanotubes is preferably ≥2500:1 and ≤20000:1, preferably ≥2800:1 and ≤20000:1, more preferably ≥7000:1 and ≤20000:1, even more preferably ≥10000:1 and ≤20000:1, and even more preferably ≥15000:1 and ≤20000:1. The carbon nanotubes are preferably single-walled carbon nanotubes (SWCNTs). Single-walled carbon nanotubes possess excellent electrical conductivity and mechanical properties. Doping a small amount of carbon nanotubes, preferably with a high aspect ratio, into the negative electrode material can significantly improve the structural stability of the negative electrode sheet while forming a strong and stable conductive network. This reduces the proportion of inactive materials in the negative electrode sheet and prevents the active material from peeling off from the current collector surface and the conductive path from being blocked due to the huge volume expansion of silicon material during cycling. This avoids the rapid decay of the cell capacity, i.e., the performance drop, and thus improves its cycle performance.

[0030] Considering the energy density of the battery, preferably, the carbon nanotubes have a weight content of ≤1% relative to the entire negative electrode film, more preferably 0.3% to 0.6%.

[0031] The conductive agent includes carbon nanotubes, and may also include other conventional conductive agents, such as conductive carbon materials like acetylene black.

[0032] The binder is a non-active component in lithium-ion battery electrodes, and its main function is to ensure the overall connectivity between the active material, conductive agent, and current collector. Various binders commonly used in the lithium-ion battery field (such as SBR) can be used, but preferably, the binder in this application includes polyacrylate binders, especially polyacrylates; more preferably, sodium polyacrylate is used as the binder for the negative electrode. Sodium polyacrylate can not only form strong hydrogen bonds with silicon-based materials but also form a relatively uniform coating film on the material surface, which can alleviate the volume change of the silicon-based material, thereby enhancing the mechanical properties and processability of the electrode to meet actual production needs.

[0033] Preferably, the content x of the binder in the negative electrode film is 3% ≤ x ≤ 9%, and more preferably 4% ≤ x ≤ 6%.

[0034] Using a preferred binder (e.g., sodium polyacrylate) can reduce the binder content to 3%–9% while maintaining the adhesion strength within the range of 10 N / m–90 N / m. In other words, using a preferred binder can reduce the proportion of inactive material in the negative electrode, thereby effectively improving the energy density of the battery cell. In contrast, in existing technologies, the binder content generally needs to be 10%–20% to maintain the adhesion strength between the active material and the current collector surface within a suitable range.

[0035] In this paper, adhesive force is defined as the force required to peel the active material from a substrate per unit width, and is used to characterize the degree of adhesion between the active material and the substrate. If the adhesive force is too low, the active material may detach from the substrate during long-term cycling.

[0036] The negative electrode active material used in the negative electrode film is a mixture of silicon-based material and graphite, and may also contain other negative electrode active materials commonly used in the art. The silicon substrate is SiOx (0 < x < 2), such as SiO. The graphite can be one or more selected from artificial graphite and natural graphite. The average particle size Dv50 of the negative electrode active material is 8 μm to 14 μm, for example, from about 8 μm, 9 μm, 10 μm to about 13 μm, 14 μm. The average particle size Dv50 of the graphite can be 10 μm to 20 μm; preferably 13 μm to 18 μm. The average particle size Dv50 of the SiOx (0 < x < 2) can be 3 μm to 10 μm; preferably 5 μm to 8 μm. The average particle size Dv50 of the graphite is greater than the average particle size Dv50 of the SiOx (0 < x < 2). In some preferred embodiments, the silicon-based material may include a first silicon-based material and a second silicon-based material, wherein the particle size Dv50 of the first silicon-based material is different from that of the second silicon-based material.

[0037] In the negative electrode active material, the mass percentage W of SiOx (0 < x < 2) is preferably 15% ≤ W ≤ 40%, more preferably 20% ≤ W ≤ 40%. For example, W can be from 15%, 16%, 17%, 18%, 19%, 20%, 21%, or 22% to 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, or 40%.

[0038] Materials such as metal foil or porous metal plates can be used as the negative electrode current collector. The material of the negative electrode current collector can be a metallic conductive material selected from copper, nickel, titanium, silver, nickel-copper alloys, aluminum-zirconium alloys, etc., or a polymer composite current collector coated with a metallic conductor coating can also be used. Copper foil or a copper-coated polymer composite current collector is preferred as the negative electrode current collector. Preferably, the thickness of the negative electrode current collector is 4 μm to 10 μm, more preferably 4 μm to 8 μm. Preferably, the roughness Ra of the copper foil is in the range of 1.6 μm ≤ Ra ≤ 3.2 μm, because the surface roughness of the current collector directly affects the adhesion between it and the active material. The adhesion of the negative electrode sheet can be improved by increasing the surface roughness of the current collector, but if the surface roughness of the current collector is too large, it is easily corroded by the electrolyte. Therefore, the surface roughness Ra of the negative electrode current collector is preferably in the range of 1.6μm≤Ra≤3.2μm, which makes it easy to ensure that the adhesion force is in the range of 10N / m~90N / m (more preferably in the range of 30N / m~80N / m).

[0039] The negative electrode sheet in the secondary battery of this application can be prepared using methods known in the art. Typically, the negative electrode active material, along with conductive agents, binders, and other optional additives, are mixed together and dispersed in a solvent (e.g., deionized water). After stirring evenly, the mixture is uniformly coated onto the negative electrode current collector. After post-treatment such as drying and cold pressing, the negative electrode sheet containing the negative electrode film is obtained.

[0040] In some preferred embodiments, the compaction density (PD) of the negative electrode film is in the range of 1.6 g / cm³. 3 ≤PD≤2.0g / cm 3 Between, more preferably 1.65 g / cm 3 ≤PD≤1.8g / cm 3 .

[0041] In some preferred embodiments, the coating weight (CW) of the negative electrode film is 0.045 mg / mm². 2 ≤CW≤0.09mg / mm 2 Preferably, 0.06 mg / mm 2 ≤CW≤0.08mg / mm 2 .

[0042] It is worth noting that when preparing the negative electrode sheet, the current collector can be coated on both sides or on one side.

[0043] It should be noted that the negative electrode membrane parameters given in this application refer to the parameter range of a single-sided membrane. When the negative electrode membrane is disposed on both surfaces of the current collector, if the membrane parameters on either surface meet the requirements of this application, it is considered to fall within the protection scope of this application. Furthermore, the membrane compaction density, areal density, and other ranges mentioned in this application refer to the parameter range after cold pressing and compaction for use in battery assembly.

[0044] The secondary battery of this application can be manufactured according to known methods. For example, the secondary battery can be manufactured and assembled according to the following methods.

[0045] First, the positive electrode sheet for a battery is prepared according to conventional methods in the art. This application does not limit the positive active material used in the positive electrode sheet. Typically, conductive agents (e.g., carbon materials such as carbon black) and binders (e.g., PVDF) are added to the aforementioned positive active material. Other additives, such as PTC thermistor materials, may also be added as needed. These materials are usually mixed together and dispersed in a solvent (e.g., NMP), stirred evenly, and then uniformly coated onto the positive current collector. After drying, the positive electrode sheet is obtained. Metal foil such as aluminum foil or porous metal plates can be used as the positive current collector. Aluminum foil is preferred.

[0046] Then, the negative electrode sheet of the battery is prepared as described above.

[0047] Finally, the positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The cells are then wound to obtain a bare cell. The bare cell is placed in an outer packaging shell, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping, a secondary battery is obtained.

[0048] For the various components of a secondary battery (positive electrode, electrolyte, separator, etc.), various conventional materials in this field can be selected.

[0049] The positive electrode of the secondary battery of this application includes a positive electrode current collector and a positive electrode film disposed on at least one surface of the positive electrode current collector and comprising a positive electrode active material. In this document, the positive electrode active material may be selected from one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and lithium phosphates with an olivine structure; however, this application is not limited to these materials, and other conventionally known materials that can be used as positive electrode active materials for lithium-ion batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Preferably, the positive electrode active material may be selected from LiCoO2, LiNiO2, LiMnO2, LiMn2O4, LiNi 1 / 3 Co 1 / 3Mn 1 / 3 O2(NCM333), LiNi0.5 Co 0.2 Mn 0.3 O2 (NCM523), LiNi 0.6 Co 0.2 Mn 0.2 O2 (NCM622), LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811), LiNi 0.85 Co 0.15 Al 0.05 O2, LiFePO4 (LFP), LiMnPO4, or one or more of these. Most preferably, the positive electrode active material includes Li 1+y Ni a Co b M c O 2-z A z , where -0.2 ≤ y ≤ 0.2, 0.5 ≤ a < 1, 0 < b ≤ 0.3, 0 < c ≤ 0.2, 0 ≤ z < 0.2, M is selected from one or more of Mn and Al, and A is selected from one or more of S, N, F, Cl, Br, and I. Because the positive electrode active material has a relatively high energy density, it can better match the negative electrode active material with a high energy density. The positive electrode membrane usually also includes a conductive agent (such as acetylene black, etc.), a binder (such as PVDF, etc.), and other optional additives (such as PTC thermistor materials, etc.). The positive electrode current collector can be a metal conductive material selected from aluminum, copper, nickel, titanium, silver, nickel - copper alloy, aluminum - zirconium alloy, etc., or a polymer composite current collector coated with a metal conductor coating; preferably, aluminum or a polymer composite current collector coated with aluminum is used.

[0050] In the secondary battery of the present application, the separator is disposed between the positive electrode plate and the negative electrode plate, playing a role of isolation. Among them, the type of the separator is not specifically limited and can be any separator material used in existing batteries, such as polyethylene, polypropylene, polyvinylidene fluoride, and their multilayer composite films, but not limited to these.

[0051] In the secondary battery of the present application, the type of the electrolyte is not specifically limited. The electrolyte may include an electrolyte salt and an organic solvent, and the types of the electrolyte salt and the organic solvent are not specifically limited and can be selected according to actual needs. For example, as a non - aqueous electrolyte, a lithium salt solution dissolved in an organic solvent is usually used. The lithium salt is, for example, an inorganic lithium salt such as LiClO4, LiPF6, LiBF4, LiAsF6, LiSbF6, or LiCF3SO3, LiCF3CO2, Li2C2F4(SO3)2, LiN(CF3SO2)2, LiC(CF3SO2)3, LiCn F 2n+1 Organic lithium salts such as SO3 (n≥2). Organic solvents used in non-aqueous electrolytes include, for example, cyclic carbonates such as ethylene carbonate, propylene carbonate, butene carbonate, and vinylene carbonate; chain carbonates such as dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate; chain esters such as methyl propionate; cyclic esters such as γ-butyrolactone; chain ethers such as dimethoxyethane, diethyl ether, diethylene glycol dimethyl ether, and triethylene glycol dimethyl ether; cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran; nitrile solvents such as acetonitrile and propionitrile; or mixtures of these solvents. The electrolyte may also include additives, and the types of additives are not particularly limited. These can be negative electrode film-forming additives, positive electrode film-forming additives, or additives that can improve certain battery performance, such as additives that improve battery overcharge performance, battery high-temperature performance, and battery low-temperature performance. Preferably, the additives contain fluoroethylene carbonate solvent (FEC).

[0052] Unless otherwise specified, the various parameters mentioned in this specification have meanings known in the art and can be measured using methods known in the art. For example, they can be tested according to the methods given in the embodiments of this application.

[0053] Compared to traditional rechargeable batteries, this application allows for effective improvement in cycle performance and rate performance while maintaining high energy density. Therefore, it is of great significance for fields such as new energy vehicles.

[0054] In some embodiments, the secondary battery may include an outer packaging for encapsulating a positive electrode, a negative electrode, a separator, and an electrolyte. As an example, the positive electrode, negative electrode, and separator may be stacked or wound to form a stacked or wound battery cell, with the cell encapsulated within the outer packaging; the electrolyte is immersed in the cell. The number of cells in the secondary battery can be one or more, adjustable as needed.

[0055] In some embodiments, the outer packaging of the secondary battery can be a soft pack, such as a pouch. The soft pack can be made of plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS). The outer packaging of the secondary battery can also be a hard shell, such as an aluminum shell.

[0056] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. Figure 1 This is an example of a square-structured secondary battery 5.

[0057] In some embodiments, secondary batteries can be assembled into battery modules, and the number of secondary batteries contained in a battery module can be multiple, the specific number of which can be adjusted according to the application and capacity of the battery module.

[0058] Figure 2 This is battery module 4, used as an example. (See reference...) Figure 2 In battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple secondary batteries 5 can be fixed in place using fasteners.

[0059] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0060] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0061] Figure 3 and Figure 4 This is battery pack 1 as an example. (See reference...) Figure 3 and Figure 4 The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3, with the upper body 2 covering the lower body 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0062] A second aspect of this application provides an apparatus comprising a secondary battery as described in the first aspect of this application, the secondary battery providing power to the apparatus. The apparatus may be, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0063] The device can be configured to use a secondary battery (Cell), a battery module (Module), or a battery pack, depending on its usage requirements.

[0064] Figure 5 This is an example device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the device's requirements for high power and high energy density in its secondary batteries, a battery pack or battery module can be used.

[0065] Another example of the device can be a mobile phone, a tablet computer, a laptop computer, etc. This device usually requires to be thin and light, and a secondary battery can be used as the power source.

[0066] The beneficial effects of the present application will be further described below in conjunction with the embodiments.

[0067] Embodiment

[0068] In order to make the invention object, technical solution and beneficial technical effects of the present application clearer, the present application will be further described in detail below in conjunction with the embodiments. However, it should be understood that the embodiments of the present application are only for explaining the present application and are not for limiting the present application, and the embodiments of the present application are not limited to the embodiments given in the specification. The specific experimental conditions or operation conditions not specified in the embodiments are made according to the conventional conditions or according to the conditions recommended by the material suppliers.

[0069] I. Preparation of the battery for testing

[0070] The batteries of each embodiment and comparative example are prepared and tested according to the following method.

[0071] (1) Preparation of the positive electrode plate

[0072] Mix the positive electrode active material NCM811, the conductive agent acetylene black, and the binder PVDF in a mass ratio of 98:2:2, add the solvent NMP, and stir under the action of a vacuum mixer until the system is homogeneous to obtain the positive electrode slurry; uniformly coat the positive electrode slurry on the positive electrode current collector aluminum foil (double-sided coating), dry at room temperature and then transfer to an oven for further drying, and then obtain the positive electrode plate through cold pressing, slitting, and cutting. The compaction density PD of the positive electrode film is 3.5 g / cm 3 , and the single-sided coating weight CW is 0.192 mg / mm 2 .

[0073] (2) Preparation of the negative electrode plate <于

[0074] Mix the first negative electrode active material SiOx (0 < x < 2), the second negative electrode active material graphite, the conductive agent (acetylene black and carbon nanotubes), and the binder (sodium polyacrylate or polyacrylamide) in a certain mass ratio (see Table 1 for details), add the solvent deionized water, and stir under the action of a vacuum mixer until the system is homogeneous to obtain the negative electrode slurry; uniformly coat the negative electrode slurry on the negative electrode current collector copper foil (double-sided coating), dry at room temperature and then transfer to an oven for further drying, and then obtain the negative electrode plate through cold pressing, slitting, and cutting. The compaction density PD of the negative electrode film is 1.7 g / cm 3 , and the single-sided coating weight CW is 0.074 mg / mm 2 .

[0075] (3) Preparation of the electrolyte

[0076] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent. Then, fully dried lithium salt LiPF6 was dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L. Additive FEC was added to the electrolyte, and the mass percentage of additive FEC in the electrolyte was 8%.

[0077] (4) Separating membrane

[0078] Polyethylene film is used as the separation membrane.

[0079] (5) Preparation of lithium-ion batteries

[0080] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. They are then wound to obtain a bare cell. The bare cell is placed in an outer casing containing an SSD, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping processes, a lithium-ion battery is obtained. The casing dimensions of the lithium-ion battery are length * width * height = 28.5mm * 148mm * 97.5mm.

[0081] II. Performance Parameter Testing

[0082] 1. Adhesion test: After vacuum drying the negative electrode sheet prepared in the example for 12 hours, it was adhered to a stainless steel plate. Then, one end of the negative electrode sheet was pulled by a tensile testing machine. The speed of the tensile testing machine was fixed at 50 mm / min, and the displacement was set to 40 mm. After the instrument stabilized, the tensile force that separated the active material from the substrate per unit width was the adhesion force (unit: N / m).

[0083] 2. Cyclic performance test: At 25°C, the lithium-ion battery prepared in the example was charged and discharged at 1C / 1C with a voltage range of 2.5-4.25V on a Newway tester until the capacity of the lithium-ion battery was less than 80% of the initial capacity. The number of cycles of the lithium-ion battery was recorded to characterize the cycle performance of the cell, with the unit being cycles.

[0084] 3. Energy density test: At 25°C, the lithium-ion battery prepared in the example was charged and discharged on the Xinwei tester at a rate of 1C / 1C and a voltage range of 2.5-4.25V. The energy released by the cell during the first charge and discharge cycle was recorded and then divided by the weight of the cell to obtain the weight energy density of the cell, in Wh / kg.

[0085] 4. Rate Performance Test: At 25°C, the lithium-ion batteries prepared in the examples were charged and discharged at rates of 0.33C / 0.33C, 0.5C / 0.33C, 1C / 0.33C, and 2C / 0.33C, with the voltage range always between 2.5-4.25V. The tests were conducted on a Newway tester, and the ratio of the 2C / 0.33C cycle discharge capacity to the initial 0.33C / 0.33C cycle discharge capacity was recorded. This ratio is called the capacity retention rate, which is the rate performance of the cell.

[0086] 5. Average particle size of the negative electrode active material

[0087] The average particle size Dv50 of the negative electrode active material can be determined using a laser diffraction particle size distribution measuring instrument (Mastersizer3000). Specifically, the particle size distribution can be measured according to the particle size distribution laser diffraction method (refer to GB / T19077-2016), and the average particle size is expressed as the median value of the volume distribution, Dv50. The Dv50 of the first negative electrode active material, the Dv50 of the second negative electrode active material, and the total average particle size Dv50 of the negative electrode active material are measured separately.

[0088] III. Test Results of Each Embodiment and Comparative Example

[0089] Batteries for the examples and comparative examples were prepared according to the above method, and various performance parameters were measured. The results are shown in Tables 1 and 2 below.

[0090]

[0091]

[0092] Table 2: Comparative test results of each embodiment

[0093]

[0094] First, a comparison of Comparative Examples 3 and Examples 5-10 shows that in lithium-ion secondary batteries using a mixture of SiOx (0 < x < 2) and graphite as the negative electrode active material, introducing carbon nanotubes to replace conventional conductive agents can significantly improve the battery's cycle performance and kinetic performance. Therefore, the use of carbon nanotubes can offset some of the negative effects caused by the volume expansion of silicon materials during charging. Simultaneously, data on the adhesion strength of the negative electrode sheet indicate that the introduction of carbon nanotubes improves the adhesion strength of the negative electrode sheet at the same binder content.

[0095] Furthermore, a comparison of Comparative Examples 3 and 4 and Examples 5-10 reveals that a relatively high aspect ratio of carbon nanotubes is necessary to better utilize their function. When the aspect ratio of carbon nanotubes is relatively small (Comparative Example 4), although the cycle performance and kinetic performance of the battery are improved to some extent, the improvement is small; while when the aspect ratio is ≥2500:1, the improvement in both the adhesion of the negative electrode sheet and the battery performance is unexpectedly significant.

[0096] Furthermore, a comparison of Comparative Examples 1 and 2 and Examples 1-4 shows that when carbon nanotubes of the same size and content are introduced into the negative electrode film, the average particle size of the negative electrode active material has a significant impact on battery performance. When the Dv50 of the negative electrode active material is too small (Comparative Example 1) or too large (Comparative Example 2), the battery's cycle performance and kinetic performance are both poor. Only when the average particle size Dv50 of the negative electrode active material is between 8 μm and 14 μm can the battery guarantee excellent cycle performance and kinetic performance.

[0097] Data from Examples 11-16 show that as the amount of negative electrode binder added increases, the adhesion of the negative electrode sheet also increases; however, compared with conventional binders such as SBR, polyacrylic binders (polyacrylamide, sodium polyacrylate, etc.) have stronger adhesion; in particular, sodium polyacrylate has the best effect, and can achieve very good battery cycle performance and kinetic performance at a low addition amount.

[0098] Based on the disclosure and guidance provided in the foregoing specification, those skilled in the art can make appropriate changes and modifications to the above embodiments. Therefore, this application is not limited to the specific embodiments disclosed and described above, and some modifications and alterations to this application also fall within the scope of protection of the claims. Furthermore, although some specific terms are used in this specification, these terms are only for convenience of explanation and do not constitute any limitation on this application.

Claims

1. A secondary battery, comprising a negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a negative electrode film disposed on at least one surface of the negative electrode current collector, the negative electrode film comprising a negative electrode active material, a conductive agent, and a binder, characterized in that: The negative electrode active material includes SiO₂ x and graphite, wherein 0 < x < 2; the SiO x The average particle size Dv50 is 5 μm ~ 8 μm; the average particle size Dv50 of the graphite is 13 μm ~ 18 μm; The average particle size Dv50 of the negative electrode active material is 8 μm ~ 14 μm; The conductive agent includes carbon nanotubes, and the aspect ratio of the carbon nanotubes is 2800:1 to 10000:

1.

2. The secondary battery according to claim 1, characterized in that: The carbon nanotubes are single-walled carbon nanotubes.

3. The secondary battery according to claim 1, characterized in that: The carbon nanotubes account for ≤1% of the mass of the negative electrode film.

4. The secondary battery according to claim 3, characterized in that: The carbon nanotubes account for 0.3% to 0.6% of the mass of the negative electrode film.

5. The secondary battery according to claim 1, characterized in that: The adhesive includes polyacrylate; And / or, the binder content in the negative electrode film is 3%-9%.

6. The secondary battery according to claim 5, characterized in that: The adhesive includes sodium polyacrylate; And / or, the binder content in the negative electrode film is 4%-6%.

7. The secondary battery according to claim 1, characterized in that: SiO in the negative electrode active material x The mass percentage content W is 15% ≤ W ≤ 40%.

8. The secondary battery according to claim 7, characterized in that: SiO in the negative electrode active material x The mass percentage content W is 20% ≤ W ≤ 40%.

9. The secondary battery according to claim 1, characterized in that: The graphite is selected from one or both of artificial graphite and natural graphite.

10. The secondary battery according to claim 1, characterized in that: The thickness of the negative electrode current collector is 4μm-10μm; And / or, the surface roughness Ra of the negative electrode current collector is in the range of 1.6 μm ≤ Ra ≤ 3.2 μm; And / or, the bonding force F between the negative electrode membrane and the negative electrode current collector is in the range of 10 N / m ≤ F ≤ 90 N / m.

11. The secondary battery according to claim 1, characterized in that: The thickness of the negative electrode current collector is 4μm-8μm; And / or, the bonding force F between the negative electrode membrane and the negative electrode current collector is in the range of 30 N / m ≤ F ≤ 80 N / m.

12. The secondary battery according to claim 1, characterized in that: The compaction density (PD) of the negative electrode film is 1.6 g / cm³. 3 ≤PD≤2.0 g / cm 3 ; and / or, The coating weight (CW) of the negative electrode membrane is 0.045 mg / mm². 2 ≤CW≤0.09 mg / mm 2 .

13. The secondary battery according to claim 12, characterized in that: The compaction density (PD) of the negative electrode film is 1.65 g / cm³. 3 ≤PD≤1.8 g / cm 3 ; and / or, The coating weight (CW) of the negative electrode membrane is 0.06 mg / mm². 2 ≤CW≤0.08 mg / mm 2 .

14. The secondary battery according to claim 1, characterized in that: The secondary battery further includes a positive electrode plate, which includes a positive electrode current collector and a positive electrode film disposed on at least one surface of the positive electrode current collector and containing a positive electrode active material. The positive electrode active material includes Li 1+y Ni a Co b M c O 2-z A z , where -0.2 ≤ y ≤ 0.2, 0.5 ≤ a < 1, 0 < b ≤ 0.3, 0 < c ≤ 0.2, 0 ≤ z < 0.2, M is selected from one or two of Mn and Al, and A is selected from one or more of S, N, F, Cl, Br, and I.

15. An electrical appliance, characterized in that, Includes the secondary battery according to any one of claims 1 to 14.

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