Battery cell, battery device, electric device, and method for preparing battery cell

Abstract

A battery cell, a battery device, an electric device, and a method for preparing a battery cell. The battery cell comprises a negative electrode sheet, wherein the negative electrode sheet comprises a current collector and a negative electrode active material layer which are stacked; the negative electrode active material layer comprises negative electrode active material particles and a first electrically conductive agent; the negative electrode active material particles comprise graphite particles; the first electrically conductive agent comprises first carbon nanotubes distributed among the graphite particles and second carbon nanotubes distributed on the surfaces of the graphite particles; the tortuosity δb of the first carbon nanotubes is 0-0.3; and the tortuosity δb is defined as δb=(L1-h1) / L1, where L1 is the length of the first carbon nanotube having two ends respectively connected to two graphite particles, and h1 is the straight-line distance between the two ends of the first carbon nanotube respectively connected to the two graphite particles. Thus, the rebound rate of the negative electrode plate and the expansion force of the battery cell can be reduced, so as to improve the cycle performance of the battery cell.

Description

Battery cells, battery devices and electrical devices, and methods for preparing battery cells.

[0001] Cross-references to related applications

[0002] This application claims priority to Chinese Patent Application No. 202510472312.0, filed on April 15, 2025, entitled “Battery cell, battery device and power-consuming device, method for preparing battery cell”, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This disclosure relates to the field of batteries, and in particular to a battery cell, a battery device, an electrical device, and a method for preparing the battery cell. Background Technology

[0004] Battery cells possess characteristics such as high capacity and long lifespan, and are therefore widely used in electronic devices, such as mobile phones, laptops, electric vehicles, electric cars, electric airplanes, electric ships, electric toy cars, electric toy ships, electric toy airplanes, and power tools. Due to the significant advancements in battery cell technology, higher performance requirements have been placed on them.

[0005] However, the cycle performance of individual battery cells is still relatively poor. Summary of the Invention

[0006] This disclosure provides a battery cell, a battery device, an electrical device, and a method for preparing a battery cell, which can improve the cycle performance of the battery cell.

[0007] In a first aspect, this disclosure provides a battery cell including a negative electrode sheet, the negative electrode sheet including a current collector and a negative electrode active material layer stacked together; the negative electrode active material layer including negative electrode active material particles and a first conductive agent, the negative electrode active material particles including graphite particles; the first conductive agent including a first carbon nanotube distributed between the graphite particles and a second carbon nanotube distributed on the surface of the graphite particles; the bending degree δb of the first carbon nanotube is 0 to 0.3;

[0008] The bending degree δb = (L1-h1) / L1, where L1 is the length of the first carbon nanotube connecting the two graphite particles at both ends, and h1 is the straight-line distance between the two ends of the first carbon nanotube particle connecting the two graphite particles.

[0009] The negative electrode includes first carbon nanotubes distributed between graphite particles and second carbon nanotubes distributed on the surface of graphite particles. On the one hand, the two ends of the first carbon nanotubes are connected to the graphite particles and their bending degree is 0-0.3, indicating that the L1 and h1 of the first carbon nanotubes are closer. This makes the first carbon nanotubes distributed between the graphite particles more taut, which can more tightly hold the adjacent graphite particles and further restrict the movement of the graphite particles. This limits the graphite particle flipping and orientation changes caused by repeated volume changes due to lithium ion insertion and extraction during charging and discharging, thus improving the stability of the graphite particle arrangement. On the other hand, the second carbon nanotubes distributed on the surface of the graphite particles can bind the graphite particles and conduct the stress originally concentrated inside and on the surface of the graphite particles to the surrounding area through the second carbon nanotubes. This improves the structural stability of the graphite particles, reduces the excessive stress concentration in the localized areas of the graphite particles, and prevents particle pulverization, which in turn leads to the increase in the thickness of the negative electrode due to the formation of an SEI film. Therefore, the first carbon nanotubes with a bending degree of 0-0.3 distributed between the graphite particles and the second carbon nanotubes distributed on the surface of the graphite particles work together to improve the stability of the graphite particle arrangement and the structural stability of the graphite particles, thereby reducing the thickness rebound rate of the negative electrode sheet and the expansion force of the battery cell, so as to improve the cycle performance of the battery cell.

[0010] In some embodiments, the bendability δb of the first carbon nanotube is 0 to 0.2.

[0011] Therefore, the bending degree of the first carbon nanotube is within the above range. The first carbon nanotube is relatively straighter, which can reduce the movement space of graphite particles, better suppress the tumbling and orientation changes of graphite particles during charging and discharging, and reduce the rebound rate of the negative electrode sheet, thereby further improving the cycle performance of the battery cell.

[0012] In some implementations, L1 is 0.1 μm to 5 μm.

[0013] Therefore, the first carbon nanotube in the above range can play a good mechanical buffering role while connecting graphite particles, reducing stress concentration in graphite particles, dispersing stress released by graphite particles during charging and discharging, and improving the structural stability of graphite particles.

[0014] In some implementations, h1 is 0.1 μm to 4 μm.

[0015] Thus, within the above range, h1 and L1 can obtain a first carbon nanotube with a bending degree in the range of 0 to 0.3, and can maintain a relatively stable distribution of graphite particles in three-dimensional space, reduce excessive aggregation or dispersion of particles, improve the structural integrity of the negative electrode sheet, and reduce the expansion force of the battery cell during long-term cycling.

[0016] In some embodiments, the length L2 of the second carbon nanotubes distributed on the surface of the graphite particles is 0.5 μm to 20 μm.

[0017] Therefore, the length of the second carbon nanotubes distributed on the surface of the graphite particles, within the aforementioned range, can constrain the expansion of the graphite particles through their high strength and toughness. This further enhances the ability to disperse the stress originally concentrated inside and on the surface of the graphite particles. The stress is then conducted to the surrounding area through the carbon nanotubes, reducing the excessive concentration of stress in localized areas that could lead to particle pulverization. The pulverized particles expose new interfaces, forming new SEI (Sediment-Insulated Plate), increasing the thickness of the negative electrode sheet. Thus, the stress in the graphite particles can be further dispersed, reducing pulverization and further decreasing the thickness expansion rate of the negative electrode sheet.

[0018] In some embodiments, the length L2 of the second carbon nanotube is 1.5 μm to 10 μm.

[0019] Therefore, as the length of carbon nanotubes increases, they are more prone to self-aggregation. Carbon nanotubes with a length in the range of 1.5μm to 10μm have good dispersibility and are not prone to self-aggregation. Under the same mass addition amount, the dispersibility of carbon nanotubes and the thickness expansion rate of the negative electrode sheet can be balanced.

[0020] In some embodiments, the diameters of the first carbon nanotube and the second carbon nanotube are each independently 1 nm to 50 nm.

[0021] Therefore, the first and second carbon nanotubes each have higher flexibility and strength within the aforementioned range, with their diameters being independent of each other.

[0022] In some embodiments, the total mass fraction of the first carbon nanotube and the second carbon nanotube is 0.01% to 2%, based on the total mass of the negative electrode active material layer being 100%.

[0023] Therefore, when the total mass fraction of the first and second carbon nanotubes is within the above range, the thickness rebound rate of the negative electrode sheet is low, and the negative electrode sheet has good conductivity.

[0024] In some embodiments, the negative electrode active material layer further includes a second conductive agent, which includes one or more of carbon black, acetylene black, superconducting carbon, carbon dots, Ketjen black, and graphene.

[0025] Therefore, in addition to the first conductive agent mentioned above, carbon nanotubes have good flexibility. The negative electrode active material layer also includes a second conductive agent such as carbon black. By using the dotted zero-dimensional conductive agent carbon black in combination with the one-dimensional conductive agent, a better conductive network and the structural stability of the conductive network are achieved.

[0026] In some embodiments, the mass ratio of the first conductive agent to the second conductive agent is 1:(0.01 to 30).

[0027] Therefore, by maintaining the mass ratio of the first conductive agent to the second conductive agent within the aforementioned range, the negative electrode sheet can achieve a low thickness rebound rate, thereby optimizing the formulation cost, reducing material costs, and enhancing the battery's market competitiveness.

[0028] In some embodiments, the particle size distribution Dv50 of the graphite particles is 10 μm to 20 μm.

[0029] In some embodiments, the compacted density of the graphite particles at 100 MPa to 300 MPa is 1.6 g / cm³. 3 ~1.8g / cm 3 .

[0030] Therefore, graphite particles with a particle size distribution Dv50 and / or powder compaction density within the above range can have high compaction density, thereby improving the energy density of the battery cell.

[0031] In some embodiments, the mass fraction of graphite particles is 96.5% to 97.5% based on the total mass of the negative electrode active material layer as 100%.

[0032] Therefore, within the above-mentioned range, the mass fraction of graphite particles can allow for the intercalation and deintercalation of more lithium ions, thereby obtaining high-energy-density battery cells.

[0033] In some embodiments, the negative electrode active material layer further includes a binder, wherein the mass fraction of the binder is 0.5% to 1.5% based on the total mass of the negative electrode active material layer (100%).

[0034] In some embodiments, the thickness of the negative electrode active material layer is 60 μm to 85 μm.

[0035] In some embodiments, the compaction density of the negative electrode active material layer is 1.55 g / cm³. 3 ~1.75g / cm 3 .

[0036] Therefore, the negative electrode active material layer has a compaction density within the above range, which can yield a battery cell with high energy density.

[0037] In some implementations, the thickness rebound rate of the negative electrode sheet under a 0.5C full charge condition in the range of 2.8 to 4.25V is 15% to 23%.

[0038] Therefore, the thickness rebound rate of the negative electrode sheet is low, and it has good cycle performance within the above range.

[0039] Secondly, embodiments of this disclosure provide a method for preparing a single battery cell, comprising:

[0040] Preparation of the first conductive agent slurry: Carbon nanotube powder is reacted with a first acrylic polymer to obtain polyacrylic acid grafted modified carbon nanotubes. The polyacrylic acid grafted modified carbon nanotubes are mixed with a second acrylic polymer at 60℃~80℃ for 2h~4h, wherein the weight average molecular weight of the second acrylic polymer is greater than that of the first acrylic polymer. The pH of the solution is adjusted to obtain a conductive adhesive.

[0041] A negative electrode slurry is prepared from materials including conductive adhesive and graphite particles. The negative electrode slurry is coated on the surface of at least one side of the current collector and dried to obtain a negative electrode sheet.

[0042] Electrode components, including negative electrode plates, are fabricated into battery cells;

[0043] The negative electrode active material layer includes graphite particles and a first conductive agent. The first conductive agent includes a first carbon nanotube distributed between the graphite particles and a second carbon nanotube distributed on the surface of the graphite particles. The bending degree δb of the first carbon nanotube is 0 to 0.3. The bending degree δb = (L1-h1) / L1, where L1 is the length of the first carbon nanotube that connects the two graphite particles at both ends, and h1 is the straight-line distance between the two ends of the first carbon nanotube particles that connect the two graphite particles.

[0044] First, carbon nanotube powder is grafted with a low weight-average molecular weight first acrylic polymer to reduce the surface polarity of the carbon nanotubes, provide steric hindrance, and improve the dispersibility of the carbon nanotubes, which facilitates subsequent grafting modification. Then, a high weight-average molecular weight second acrylic polymer is added and mixed at a high temperature of 60℃~80℃ for 2h~4h. The high weight-average molecular weight first acrylic polymer and the low weight-average molecular weight second acrylic polymer will undergo micro-crosslinking at 60℃~80℃, increasing steric hindrance, reducing the agglomeration and tortuosity of the carbon nanotubes, and helping to improve the dispersion uniformity of carbon nanotubes in the system, resulting in straighter carbon nanotubes. Thus, the above preparation method can obtain first carbon nanotubes distributed between graphite particles and second carbon nanotubes distributed on the surface of graphite particles, with the tortuosity of the first carbon nanotubes in the range of 0~0.3. At the same time, the conductive agent solution modified by high weight-average molecular weight grafting also has good adhesive properties and can play a bonding role.

[0045] Some embodiments of this disclosure also provide a method for preparing a single battery cell, comprising:

[0046] Preparation of the first conductive agent slurry: Carbon nanotube powder is mixed with a third acrylic polymer or with carboxymethyl cellulose, homogenized under high pressure, and the pH of the solution is adjusted to obtain a conductive solution; wherein the pressure of high pressure homogenization is 300 bar to 1500 bar, and the number of high pressure homogenization cycles is 10 to 20.

[0047] A negative electrode slurry is prepared by comprising a conductive solution and graphite particles. The negative electrode slurry is coated on the surface of at least one side of the current collector and dried to obtain a negative electrode sheet.

[0048] Electrode components, including the negative electrode, are fabricated into a single battery cell.

[0049] Therefore, by mixing carbon nanotube powder with a third acrylic polymer or with carboxymethyl cellulose and homogenizing it under high pressure for 10 to 20 times at a pressure of 300 bar to 1500 bar, the uniformity of carbon nanotube distribution can be improved, the degree of carbon nanotube agglomeration and bending can be reduced, and first carbon nanotubes distributed between graphite particles and second carbon nanotubes distributed on the surface of graphite particles can be obtained. The bending degree of the first carbon nanotube is also in the range of 0 to 0.3.

[0050] In some embodiments, the carbon nanotube powder has a diameter of 1 nm to 3 nm and a length of 1 μm to 50 μm. Using small-diameter single-walled carbon nanotubes as raw materials is advantageous because single-walled carbon nanotubes have high strength, high toughness, and high Young's modulus. Their distribution on the surface of graphite particles allows them to better bind the graphite particles and suppress their volume expansion.

[0051] In some embodiments, the optical density of the conductive adhesive is 1 to 2. When the optical density of the conductive adhesive is within this range, the carbon nanotubes exhibit good dispersibility in the solution.

[0052] In some embodiments, the weight-average molecular weight of the first acrylic polymer is 5,000 to 15,000.

[0053] In some embodiments, the weight-average molecular weight of the second acrylic polymer is 100,000 to 1,000,000.

[0054] In some embodiments, the solid content of the conductive adhesive is 2% to 6%.

[0055] In some embodiments, the viscosity of the conductive adhesive with a solid content of 4% is 10000 mPa·s to 30000 mPa·s.

[0056] In some implementations, the pH value of the conductive adhesive is 7 to 9.

[0057] Neutralize with an alkaline solution to adjust the pH of the conductive adhesive to 7-9, thereby improving the dispersibility of carbon nanotubes and the water solubility of the conductive adhesive.

[0058] In some embodiments, the weight-average molecular weight of the third acrylic polymer is 5,000 to 15,000.

[0059] In some embodiments, the weight-average molecular weight of carboxymethyl cellulose is 11,000 to 20,000.

[0060] In some embodiments, the optical density of the conductive solution is 1 to 2. When the optical density of the conductive solution is within this range, the carbon nanotubes exhibit good dispersibility in the solution.

[0061] In some embodiments, the solid content of the conductive solution is 0.2% to 1.6%.

[0062] In some embodiments, the viscosity of the conductive solution with a solid content of 0.5% is 100 mPa·s to 4000 mPa·s.

[0063] In some implementations, the pH of the conductive solution is 7 to 9.

[0064] Neutralize with an alkaline solution to improve the water solubility of the conductive solution.

[0065] In some embodiments, the diameter of the carbon nanotubes in the conductive solution is 1 nm to 50 nm. In the conductive solution, the carbon nanotubes can exist in clusters, and their diameter can be slightly larger than that of the raw carbon nanotubes.

[0066] In some embodiments, the carbon nanotubes in the conductive solution have a tube length of 1 μm to 25 μm.

[0067] In some embodiments, the step of preparing the negative electrode slurry from the material including conductive adhesive liquid or conductive solution and graphite particles also includes a stirring step, wherein the stirring speed is 800 rpm to 1800 rpm.

[0068] In some embodiments, the step of preparing a negative electrode slurry from materials including conductive adhesive liquid or conductive solution and graphite particles also includes a stirring step, with the stirring time being 60 min to 120 min.

[0069] Therefore, in the stirring process, when the stirring speed and / or time are within the above-mentioned range, it is possible to obtain first carbon nanotubes with a tortuosity in the range of 0 to 0.3 and second carbon nanotubes with a length L2 in the range of 0.5 μm to 20 μm, and the dispersion of single nanotubes or single clusters (diameter < 40 nm) is also better. Low stirring speed or short stirring time will increase the degree of carbon nanotube aggregation, while high stirring speed or long stirring time will disrupt the dispersion balance, leading to further aggregation of CNTs and deterioration of slurry gelation.

[0070] In some embodiments, the solid content of the negative electrode slurry is 40% to 60%.

[0071] In some embodiments, the viscosity of the negative electrode slurry is 5000 mPa·s to 10000 mPa·s.

[0072] In some embodiments, the first acrylic polymer, the second acrylic polymer, and the third acrylic polymer each independently include acrylic homopolymers and copolymers of acrylic acid with other monomers.

[0073] In some embodiments, the copolymer of acrylic acid with other monomers includes acrylic acid-acrylonitrile-acrylamide copolymer.

[0074] Thirdly, embodiments of this disclosure provide a battery device comprising a battery cell prepared by a method for preparing a plurality of battery cells according to any embodiment of the first aspect of this disclosure or a plurality of battery cells according to any embodiment of the second aspect of this disclosure.

[0075] Fourthly, embodiments of this disclosure provide an electrical device comprising a battery cell prepared by a method for preparing a plurality of battery cells according to any embodiment of the first aspect of this disclosure or a plurality of battery cells according to any embodiment of the second aspect of this disclosure, or a battery device according to any embodiment of the third aspect of this disclosure. Attached Figure Description

[0076] To more clearly illustrate the technical solutions of the embodiments of this disclosure, the accompanying drawings used in the embodiments of this disclosure will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this disclosure. For those skilled in the art, other drawings can be obtained based on the drawings without creative effort.

[0077] Figure 1 is a schematic diagram of a battery cell according to an embodiment of the present disclosure;

[0078] Figure 2 is a schematic diagram of an electrical device according to an embodiment of the present disclosure;

[0079] Figure 3 shows the distribution of carbon nanotubes in the negative electrode sheet of Example 1.

[0080] The following is an explanation of the reference numerals in the attached diagram: 5. Battery cell;

[0081] The accompanying drawings are not necessarily drawn to scale. Detailed Implementation

[0082] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the battery module, battery cell, battery device, and power-consuming device, as well as a method for manufacturing the battery cell. However, unnecessary details may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for a full understanding of this disclosure by those skilled in the art and are not intended to limit the subject matter of the claims.

[0083] The "range" disclosed in this disclosure is defined by a lower limit and an upper limit, whereby a given range is defined by selecting a lower limit and an upper limit, which define the boundaries of the particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also expected. Furthermore, if minimum range values ​​1 and 2 are listed, and if maximum range values ​​3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this disclosure, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0084] Unless otherwise specified, all embodiments and optional embodiments of this disclosure can be combined to form new technical solutions.

[0085] Unless otherwise specified, all technical features and optional technical features of this disclosure can be combined to form new technical solutions.

[0086] Unless otherwise specified, all steps of this disclosure may be performed sequentially or randomly, preferably sequentially. For example, if a method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if it is mentioned that the method may also include step (c), it means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0087] Unless otherwise specified, in this disclosure, the terms "first," "second," etc., are used to distinguish different objects, rather than to describe a specific order or primary / secondary relationship.

[0088] In this disclosure, the terms "multiple" or "a variety" refer to two or more kinds.

[0089] In the description of the embodiments of this disclosure, unless otherwise specified, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0090] Unless otherwise stated, the test temperature for all parameters mentioned in this disclosure is 25°C.

[0091] The battery cells mentioned in the embodiments of this disclosure are capable of charging and discharging independently. The battery cells may be cylindrical, cuboid, or other shapes, and this disclosure does not limit this. Figure 1 shows a cuboid battery cell 5 as an example.

[0092] The battery cells provided in the embodiments of this disclosure may include, but are not limited to, lithium battery cells and sodium battery cells, such as lithium-ion battery cells, sodium-ion battery cells, lithium metal battery cells, sodium metal battery cells, etc. The battery cells provided in the embodiments of this disclosure include electrode assemblies.

[0093] The electrode assembly can be a wound structure or a stacked structure, and this disclosure does not limit this. The battery cell also includes an outer packaging, which can be used to encapsulate the electrode assembly. The outer packaging can be a rigid shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging can also be a flexible package, such as a pouch-type flexible package. The material of the flexible package can be plastic, such as one or more of aluminum-plastic film, polypropylene, polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0094] The battery apparatus mentioned in the embodiments of this disclosure may include one or more battery cell assemblies for providing voltage and capacity. A battery cell assembly may include multiple battery cells connected in series, parallel, or mixed connections via a busbar. In some embodiments, the battery cell assembly is typically formed by arranging multiple battery cells.

[0095] As an example, a battery cell assembly can be a battery module, which is formed by arranging and fixing multiple battery cells together to form an independent module. As another example, a battery module can be formed by bundling multiple battery cells together with cable ties.

[0096] In some embodiments, the battery device may be a battery pack, which includes a housing and one or more individual battery cells housed within the housing.

[0097] As an example, the battery cell assembly can be a battery module, and the battery cell assembly can be housed in the housing by fixing the battery module in the housing.

[0098] As an example, battery cell assemblies can also be housed in a housing by directly fixing multiple battery cells to the housing.

[0099] As an example, the enclosure may include a first enclosure and a second enclosure. The first enclosure and the second enclosure are fastened together to form a closed space inside the enclosure to house the individual battery cells. Here, "closed" refers to covering or closing, and can be either sealed or unsealed. The first enclosure may be a top cover or a bottom plate.

[0100] As an example, the enclosure may include a top cover, a frame, and a bottom plate. The top cover and bottom plate are connected to the frame, creating an enclosed space inside the enclosure to house the individual battery cells.

[0101] In some embodiments, the housing may be part of the vehicle's chassis structure. For example, a portion of the housing may be at least a part of the vehicle's floor, or a portion of the housing may be at least a part of the vehicle's crossbeams and longitudinal beams.

[0102] The technical solutions described in this disclosure are applicable to various electrical devices that use battery cells or battery devices, such as, but not limited to, mobile devices (e.g., mobile phones, tablets, 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. Battery cells and battery devices are used to store or provide electrical energy.

[0103] Figure 2 is a schematic diagram of an example electrical device. This electrical device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc.

[0104] The continuous pursuit of high energy density in battery cells using graphite as the negative electrode active material has prompted the design of graphite negative electrode materials towards higher compaction density. However, the increase in compaction density of graphite negative electrode materials will increase the rebound and expansion force of the negative electrode sheet. At the same time, with the repeated insertion and extraction of lithium ions during charging and discharging (similar to the breathing effect), the repeated volume changes exacerbate the rebound of the electrode sheet and result in poor cycle performance.

[0105] In view of the above problems, this disclosure proposes a battery cell, including a negative electrode sheet, the negative electrode sheet including a current collector and a negative electrode active material layer stacked together; the negative electrode active material layer includes negative electrode active material particles and a first conductive agent, the negative electrode active material particles including graphite particles; the first conductive agent includes a first carbon nanotube distributed between the graphite particles and a second carbon nanotube distributed on the surface of the graphite particles; the bending degree δb of the first carbon nanotube is 0 to 0.3;

[0106] The bending degree δb = (L1-h1) / L1, where L1 is the length of the first carbon nanotube connecting the two graphite particles at both ends, and h1 is the straight-line distance between the two ends of the first carbon nanotube particle connecting the two graphite particles.

[0107] For example, the bendability of the first carbon nanotube can be 0, 0.05, 0.1, 0.15, 0.2, 0.22, 0.24, 0.26, 0.28, 0.3 or any range of two of the above values.

[0108] The negative electrode includes first carbon nanotubes distributed between graphite particles and second carbon nanotubes distributed on the surface of graphite particles. On the one hand, the two ends of the first carbon nanotubes are connected to the graphite particles and their bending degree is 0-0.3, indicating that the L1 and h1 of the first carbon nanotubes are closer. This makes the first carbon nanotubes distributed between the graphite particles more taut, which can more tightly hold the adjacent graphite particles and further restrict the movement of the graphite particles. This limits the graphite particle flipping and orientation changes caused by repeated volume changes due to lithium ion insertion and extraction during charging and discharging, thus improving the stability of the graphite particle arrangement. On the other hand, the second carbon nanotubes distributed on the surface of the graphite particles can bind the graphite particles and conduct the stress originally concentrated inside and on the surface of the graphite particles to the surrounding area through the second carbon nanotubes. This improves the structural stability of the graphite particles, reduces the excessive stress concentration in the localized areas of the graphite particles, and prevents particle pulverization, which in turn leads to the increase in the thickness of the negative electrode due to the formation of an SEI film. Therefore, the first carbon nanotubes with a bending degree of 0-0.3 distributed between the graphite particles and the second carbon nanotubes distributed on the surface of the graphite particles work together to improve the stability of the graphite particle arrangement and the structural stability of the graphite particles, thereby reducing the thickness rebound rate of the negative electrode sheet and the expansion force of the battery cell, so as to improve the cycle performance of the battery cell.

[0109] The technical solution disclosed herein will now be described in detail.

[0110] [Negative electrode plate]

[0111] In some embodiments, the negative electrode includes the first carbon nanotubes distributed between the graphite particles and the second carbon nanotubes distributed on the surface of the graphite particles; the bending degree δb of the first carbon nanotube is 0 to 0.3, optionally 0 to 0.25, and further optionally 0 to 0.2.

[0112] In some embodiments, the bendability δb of the first carbon nanotube is 0 to 0.2, optionally 0 to 0.15, and further optionally 0 to 0.13.

[0113] For example, the bendability of the first carbon nanotube can be 0, 0.02, 0.04, 0.06, 0.08, 0.1, 0.12, 0.14, 0.16, 0.18, 0.2 or any range of two of the above values.

[0114] The first carbon nanotube has a bendability within the above range. The first carbon nanotube is relatively straighter, which can reduce the movement space of graphite particles, better suppress graphite particle flipping and orientation changes, and result in a lower rebound rate of the negative electrode sheet, further improving the cycle performance of the battery cell.

[0115] In some implementations, L1 is 0.1μm to 5μm, optionally 0.1μm to 2μm, optionally 0.5μm to 4.5μm, optionally 0.8μm to 3.5μm, optionally 1.0μm to 3.0μm, optionally 1.5μm to 2.5μm, or optionally 1μm to 2μm.

[0116] For example, L1 can be 0.1μm, 0.2μm, 0.3μm, 0.4μm, 0.5μm, 1μm, 1.5μm, 2μm, 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm, 5μm or any range of two of the above values.

[0117] The first carbon nanotubes in the L1 range can play a good mechanical buffering role while connecting graphite particles, reducing stress concentration in graphite particles, dispersing the stress released by graphite particles during charging and discharging, and improving the structural stability of graphite particles.

[0118] In some implementations, h1 is 0.1μm to 4μm, optionally 0.1μm to 2μm, optionally 0.5μm to 3.5μm, optionally 0.6μm to 3.0μm, optionally 1.0μm to 2.5μm, optionally 1.5μm to 2.5μm, or optionally 1μm to 2μm.

[0119] For example, h1 can be 0.1μm, 0.2μm, 0.3μm, 0.4μm, 0.5μm, 1μm, 1.5μm, 2μm, 2.2μm, 2.4μm, 2.6μm, 2.8μm, 3μm, 3.5μm, 4μm or any range of two of the above values.

[0120] Within the aforementioned range, h1 and L1 can obtain first carbon nanotubes with a bendability in the range of 0 to 0.3, and can also maintain a relatively stable distribution of graphite particles in three-dimensional space, reduce excessive particle aggregation or dispersion, improve the structural integrity of the negative electrode sheet, and reduce the expansion force of the battery cell during long-term cycling.

[0121] In some embodiments, the length L2 of the second carbon nanotube is 0.5 μm to 20 μm, optionally 0.8 μm to 15 μm, optionally 1 μm to 15 μm, optionally 1 μm to 10 μm, optionally 1.5 μm to 10 μm, optionally 1 μm to 5 μm, or optionally 2 μm to 10 μm.

[0122] For example, the length L2 of the second carbon nanotube can be 0.5μm, 1μm, 2μm, 3μm, 4μm, 5μm, 8μm, 10μm, 12μm, 15μm, 20μm or any range of two of the above values.

[0123] The second carbon nanotubes distributed on the surface of the graphite particles, within the aforementioned length range, can constrain the expansion of the graphite particles through their high strength and toughness. This reduces the stress originally concentrated inside and on the surface of the graphite particles, which is then conducted to the surrounding area through the carbon nanotubes. This reduces the excessive stress concentration in localized areas that could lead to particle pulverization. The pulverized particles expose new interfaces, forming new SEI (Sediment-Insulated Plate), increasing the thickness of the negative electrode sheet. Therefore, the stress in the graphite particles can be further dispersed, reducing pulverization and further decreasing the thickness expansion rate of the negative electrode sheet.

[0124] In some embodiments, the length L2 of the second carbon nanotube is 1.5 μm to 10 μm, optionally 1.5 μm to 8 μm, optionally 2 μm to 10 μm, or optionally 2.5 μm to 10 μm.

[0125] For example, the length L2 of the second carbon nanotube can be 1.5μm, 2μm, 2.5μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm or any range of two of the above values.

[0126] Therefore, as the length of carbon nanotubes increases, they are more prone to self-aggregation. Carbon nanotubes with a length in the range of 1.5μm to 10μm have good dispersibility and are not prone to self-aggregation. Under the same mass addition amount, the dispersibility of carbon nanotubes and the thickness expansion rate of the negative electrode sheet can be balanced.

[0127] In some embodiments, the diameters of the first carbon nanotube and the second carbon nanotube are each independently 1 nm to 50 nm.

[0128] For example, the diameter of the first carbon nanotube and the second carbon nanotube can each independently be 1nm, 1.2nm, 1.4nm, 1.6nm, 1.8nm, 2nm, 2.2nm, 2.4nm, 2.6nm, 2.8nm, 3nm, 5nm, 10nm, 15nm, 20nm, 25nm, 30nm, 35nm, 40nm, 45nm, 50nm, or any range of two of the above values.

[0129] After selecting single-walled carbon nanotubes of 1 nm to 3 nm to prepare the negative electrode, the single-walled carbon nanotubes can exist in the negative electrode in a cluster or bundle manner, so that the diameter of the carbon nanotubes in the negative electrode is 1-50 nm.

[0130] Therefore, the first and second carbon nanotubes each have higher flexibility and strength within the aforementioned range, with their diameters being independent of each other.

[0131] In some embodiments, the total mass fraction of the first carbon nanotube and the second carbon nanotube is 0.01% to 2%, based on the total mass of the negative electrode active material layer being 100%.

[0132] For example, the total mass fraction of the first carbon nanotube and the second carbon nanotube can be 0.01%, 0.02%, 0.03%, 0.05%, 0.1%, 0.2%, 0.35%, 0.5%, 0.55%, 0.8%, 1.0%, 1.2%, 1.5%, 1.8%, 2.0%, or any range of two of the above values.

[0133] Therefore, when the total mass fraction of the first and second carbon nanotubes is within the above range, the thickness rebound rate of the negative electrode sheet is low, and the negative electrode sheet has good conductivity.

[0134] In some embodiments, the negative electrode active material layer further includes a second conductive agent, which includes one or more of carbon black, acetylene black, superconducting carbon, carbon dots, Ketjen black, and graphene.

[0135] In some embodiments, the mass ratio of the first conductive agent to the second conductive agent is 1:(0.01 to 30), optionally 1:(0.05 to 30), optionally 1:(10 to 30), optionally 1:(0.05 to 20), optionally 1:(0.05 to 10), optionally 1:(0.08 to 5), optionally 1:(0.05 to 1), optionally 1:(0.05 to 0.5).

[0136] For example, the mass ratio of the first conductive agent and the second conductive agent can be 1:0.01, 1:0.02, 1:0.03, 1:0.04, 1:0.05, 1:0.06, 1:0.07, 1:0.08, 1:0.09, 1:0.1, 1:1, 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, 1:15, 1:20, 1:25, 1:30, 1:35, 1:40, or any range of two of the above values.

[0137] Therefore, by maintaining the mass ratio of the first conductive agent to the second conductive agent within the aforementioned range, the negative electrode sheet can achieve a low thickness rebound rate, thereby optimizing the formulation cost, reducing material costs, and enhancing the battery's market competitiveness.

[0138] In some embodiments, the particle size distribution Dv50 of the graphite particles is 10 μm to 20 μm.

[0139] For example, the particle size distribution Dv50 of graphite particles can be 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm or a range of any two of the above values.

[0140] Therefore, graphite particles with a particle size distribution Dv50 and / or powder compaction density within the above range can have high compaction density, thereby improving the energy density of the battery cell.

[0141] The test method for the particle size distribution Dv50 of graphite particles is as follows: referring to GB / T 19077-2016 Particle size distribution laser diffraction method, the particle size distribution is conveniently determined using a laser particle size analyzer. The Mastersizer2000E laser particle size analyzer from Malvern Instruments Ltd. in the UK is used to determine the particle size distribution, where Dv50 is the particle size that reaches 50% of the particle size distribution.

[0142] In some embodiments, the compacted density of the graphite particles at 100 MPa to 300 MPa is 1.6 g / cm³. 3 ~1.8g / cm 3 Within the above-mentioned mass fraction of graphite particles, more lithium ions can be intercalated or deintercalated to obtain high-energy-density battery cells.

[0143] For example, the compaction density of graphite particles at 100 MPa to 300 MPa can be 1.6 g / cm³. 3 1.61 g / cm 3 1.63g / cm 3 1.65g / cm 3 1.67 g / cm3 1.69 g / cm 3 1.71 g / cm 3 1.73g / cm 3 1.75g / cm 3 1.77g / cm 3 1.79g / cm 3 1.80g / cm 3 Or, a range consisting of any two of the above values.

[0144] In some embodiments, the mass fraction of graphite particles is 96.5% to 97.5% based on the total mass of the negative electrode active material layer (100%). Within this range, a higher lithium-ion intercalation / deintercalation rate can be achieved to obtain a high-energy-density battery cell.

[0145] For example, the mass fraction of graphite particles can be 96.5%, 96.6%, 96.7%, 96.8%, 96.9%, 97%, 97.1%, 97.2%, 97.3%, 97.4%, 97.5%, or a range of any two of the above values.

[0146] In some embodiments, the thickness of the negative electrode active material layer is 60 μm to 85 μm.

[0147] For example, the thickness of the negative electrode active material layer can be 60μm, 61μm, 62μm, 63μm, 64μm, 65μm, 66μm, 67μm, 68μm, 69μm, 70μm, 71μm, 72μm, 73μm, 74μm, 75μm, 76μm, 77μm, 78μm, 79μm, 80μm, 81μm, 82μm, 83μm, 84μm, 85μm, or any range of two of the above values.

[0148] The thickness of the negative electrode active material layer was obtained through multiple measurements using a micrometer.

[0149] In some embodiments, the compaction density of the negative electrode active material layer is 1.55 g / cm³. 3 ~1.75g / cm 3 A negative electrode active material layer with a compaction density within the aforementioned range can yield battery cells with high energy density.

[0150] For example, the compaction density of the negative electrode active material layer can be 1.55 g / cm³. 3 1.56g / cm 3 1.57g / cm 3 1.58g / cm 3 1.59g / cm 31.60g / cm 3 1.61 g / cm 3 1.62g / cm 3 1.63g / cm 3 1.64 g / cm 3 1.65g / cm 3 1.66 g / cm 3 1.67 g / cm 3 1.68g / cm 3 1.69 g / cm 3 1.70g / cm 3 1.71 g / cm 3 1.72g / cm 3 1.73g / cm 3 1.74 g / cm 3 1.75g / cm 3 Or a range consisting of any two of the above values.

[0151] The test method for the compaction density of the negative electrode active material layer is as follows: Take a negative electrode sheet that has been coated on one side and cold-pressed, and cut it into a small circular piece with an area of ​​S1. Weigh it and record its weight as M1. Then wipe off the negative and positive electrode active material layers of the weighed negative electrode sheet, weigh the negative electrode current collector, and record it as M0. The areal density of the negative electrode active material layer = (weight of the negative electrode sheet M1 - weight of the positive electrode current collector M0) / S1. The compaction density of the negative electrode active material layer = areal density of the negative electrode active material layer / thickness of the negative electrode active material layer.

[0152] In some embodiments, the thickness rebound rate of the negative electrode sheet under a 0.5C full charge condition in the range of 2.8 to 4.25V is 15% to 23%. The low thickness rebound rate of the negative electrode sheet within the above range indicates good cycle performance.

[0153] For example, the thickness rebound rate can be 15%, 15.5%, 16%, 16.5%, 17%, 17.5%, 18%, 18.5%, 19%, 19.5%, 20%, 20.5%, 21%, 21.5%, 22%, 22.5%, 23%, or a range of any two of the above values.

[0154] In some embodiments, the negative electrode active material layer further includes a binder, wherein the mass fraction of the binder is 0.5% to 1.5% based on the total mass of the negative electrode active material layer (100%).

[0155] For example, the mass fraction of the adhesive can be 0.5%, 0.55%, 0.6%, 0.65%, 0.7%, 0.75%, 0.8%, 0.85%, 0.9%, 0.95%, 1.0%, 1.05%, 1.1%, 1.15%, 1.2%, 1.25%, 1.3%, 1.35%, 1.4%, 1.45%, 1.5%, or a range of any two of the above values.

[0156] In some embodiments, the adhesive may include, but is not limited to, one or more of the following: styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, polyacrylic acid, polymethacrylic acid, sodium polyacrylate, polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), carboxymethyl chitosan (CMCS), methyl vinyl silicone rubber, nitrile rubber (NBR), hydrogenated nitrile rubber (HNBR), thermoplastic styrene-butadiene rubber (SBS), isoprene rubber, cis-butadiene rubber (BR), ethyl cellulose, fluororubber, and acrylate rubber.

[0157] In some embodiments, the negative electrode active material layer may further include a negative electrode conductive agent, which may include, but is not limited to, one or more of conductive graphite (such as KS-6, SFG-6), superconducting carbon, acetylene black, carbon black (such as SP), Ketjen black, carbon dots, carbon nanotubes, graphene, carbon nanofibers, and vapor-grown carbon fibers (VGCF).

[0158] The embodiments of this disclosure also provide a method for preparing a single battery cell, comprising:

[0159] Preparation of the first conductive agent slurry: Carbon nanotube powder is reacted with a first acrylic polymer to obtain polyacrylic acid grafted modified carbon nanotubes. The polyacrylic acid grafted modified carbon nanotubes are mixed with a second acrylic polymer at 60℃~80℃ for 2h~4h, wherein the weight average molecular weight of the second acrylic polymer is greater than that of the first acrylic polymer. The pH of the solution is adjusted to obtain a conductive adhesive.

[0160] A negative electrode slurry is prepared from materials including conductive adhesive and graphite particles. The negative electrode slurry is coated on the surface of at least one side of the current collector and dried to obtain a negative electrode sheet.

[0161] Electrode components, including negative electrode plates, are fabricated into battery cells;

[0162] The negative electrode active material layer includes graphite particles and a first conductive agent. The first conductive agent includes a first carbon nanotube distributed between the graphite particles and a second carbon nanotube distributed on the surface of the graphite particles. The bending degree δb of the first carbon nanotube is 0 to 0.3. The bending degree δb = (L1-h1) / L1, where L1 is the length of the first carbon nanotube that connects the two graphite particles at both ends, and h1 is the straight-line distance between the two ends of the first carbon nanotube particles that connect the two graphite particles.

[0163] First, carbon nanotube powder is grafted with a low weight-average molecular weight first acrylic polymer to reduce the surface polarity of the carbon nanotubes, provide steric hindrance, and improve the dispersibility of the carbon nanotubes, which facilitates subsequent grafting modification. Then, a high weight-average molecular weight second acrylic polymer is added and mixed at a high temperature of 60℃~80℃ for 2h~4h. The high weight-average molecular weight first acrylic polymer and the low weight-average molecular weight second acrylic polymer will undergo micro-crosslinking at 60℃~80℃, increasing steric hindrance, reducing the agglomeration and tortuosity of the carbon nanotubes, and helping to improve the dispersion uniformity of carbon nanotubes in the system, resulting in straighter carbon nanotubes. Thus, the above preparation method can obtain first carbon nanotubes distributed between graphite particles and second carbon nanotubes distributed on the surface of graphite particles, with the tortuosity of the first carbon nanotubes in the range of 0~0.3. At the same time, the conductive agent solution modified by high weight-average molecular weight grafting also has good adhesive properties and can play a bonding role.

[0164] Some embodiments of this disclosure also provide a method for preparing a single battery cell, comprising:

[0165] Preparation of the first conductive agent slurry: Carbon nanotube powder is mixed with a third acrylic polymer or with carboxymethyl cellulose, homogenized under high pressure, and the pH of the solution is adjusted to obtain a conductive solution; wherein the pressure of high pressure homogenization is 300 bar to 1500 bar, and the number of high pressure homogenization cycles is 10 to 20.

[0166] A negative electrode slurry is prepared by comprising a conductive solution and graphite particles. The negative electrode slurry is coated on the surface of at least one side of the current collector and dried to obtain a negative electrode sheet.

[0167] Electrode components, including the negative electrode, are fabricated into a single battery cell.

[0168] By mixing carbon nanotubes with a third acrylic polymer or with carboxymethyl cellulose and homogenizing them under high pressure for 10 to 20 cycles at 300 bar to 1500 bar, the uniformity of carbon nanotube distribution can be improved, the degree of carbon nanotube aggregation and bending can be reduced, and first carbon nanotubes distributed between graphite particles and second carbon nanotubes distributed on the surface of graphite particles can be obtained. The bending degree of the first carbon nanotubes is also in the range of 0 to 0.3.

[0169] In some embodiments, the carbon nanotube powder has a diameter of 1 nm to 3 nm and a length of 1 μm to 50 μm. Using small-diameter single-walled carbon nanotubes as raw materials is advantageous because single-walled carbon nanotubes have high strength, high toughness, and high Young's modulus. Their distribution on the surface of graphite particles allows them to better bind the graphite particles and suppress their volume expansion.

[0170] For example, the diameter of the carbon nanotube powder can be 1 nm, 1.5 nm, 2.0 nm, 2.5 nm, 3.0 nm, or any combination of two of the above values.

[0171] For example, the length of carbon nanotube powder can be 1μm, 3μm, 5μm, 10μm, 19μm, 21μm, 25μm, 30μm, 35μm, 40μm, 50μm or any range of two of the above values.

[0172] The diameter and length of carbon nanotube powder can be measured as follows: Both diameter and length refer to the dimensional parameters of a single carbon nanotube material, meaning the data is obtained from measurements taken using the same carbon nanotube. The dimensions of the carbon nanotubes are measured using scanning electron microscopy (SEM). Specifically, an SEM image of the carbon nanotube surface can be obtained. The SEM voltage is set to 10.0 kV. The length of the carbon nanotubes is measured at the same magnification, such as 10000x (the specific magnification can be adjusted based on whether the carbon nanotubes can be clearly measured). The diameter of the carbon nanotubes is measured at 200000x. Fifty nanotubes are measured separately, and then the average value is calculated.

[0173] In some embodiments, the weight-average molecular weight of the first acrylic polymer is 5,000 to 15,000.

[0174] In some embodiments, the weight-average molecular weight of the second acrylic polymer is 100,000 to 1,000,000.

[0175] If the battery cell preparation method of this disclosure directly uses a second acrylic polymer with a high weight-average molecular weight for graft modification, the graft modification will be more difficult, and it will be difficult to control the dispersion of carbon nanotubes.

[0176] For example, the weight-average molecular weight of the first acrylic polymer can be 5000, 5500, 6000, 6500, 7000, 7500, 8000, 8500, 9000, 9500, 10000, 10500, 11000, 11500, 12000, 12500, 13000, 13500, 14000, 14500, 15000 or a range of any two of the above values.

[0177] For example, the weight-average molecular weight of the second acrylic polymer can be 100,000, 150,000, 200,000, 250,000, 300,000, 350,000, 400,000, 450,000, 500,000, 550,000, 600,000, 650,000, 700,000, 750,000, 800,000, 850,000, 900,000, 950,000, 1,000,000, or a range of any two of the above values.

[0178] In some embodiments, the weight-average molecular weight of the third acrylic polymer is 5,000 to 15,000.

[0179] In some embodiments, the weight-average molecular weight of carboxymethyl cellulose is 11,000 to 20,000, and optionally 12,000 to 20,000.

[0180] For example, the weight-average molecular weight of the third acrylic polymer can be 5000, 5500, 6000, 6500, 7000, 7500, 8000, 8500, 9000, 9500, 10000, 10500, 11000, 11500, 12000, 12500, 13000, 13500, 14000, 14500, 15000 or any range of two of the above values.

[0181] For example, the weight-average molecular weight of carboxymethyl cellulose can be 8000, 8500, 9000, 9500, 10000, 10500, 11000, 11500, 12000, 12500, 13000, 13500, 14000, 14500, 15000, 15500, 16000, 16500, 17000, 17500, 18000, 18500, 19000, 19500, 20000 or any range of two of the above values.

[0182] In some embodiments, the optical density of the conductive adhesive is 1 to 2, optionally 1.2 to 2, or optionally 1.2 to 1.5.

[0183] For example, the optical density of the conductive adhesive can be 1.00, 1.05, 1.10, 1.15, 1.20, 1.25, 1.30, 1.35, 1.40, 1.45, 1.50, 1.55, 1.60, 1.65, 1.70, 1.75, 1.80, 1.85, 1.90, 1.95, 2.00 or any range of two of the above values.

[0184] The optical density of the conductive adhesive is within the above range, and carbon nanotubes exhibit good dispersibility in the solution.

[0185] In some embodiments, the solid content of the conductive adhesive is 2% to 6%.

[0186] For example, the solid content of the conductive adhesive can be 2.0%, 2.2%, 2.4%, 2.6%, 2.8%, 3.0%, 3.2%, 3.4%, 3.6%, 3.8%, 4.0%, 4.2%, 4.4%, 4.6%, 4.8%, 5.0%, 5.2%, 5.4%, 5.6%, 5.8%, 6.0%, or a range of any two of the above values.

[0187] In some embodiments, the viscosity of the conductive adhesive with a solid content of 4% is 10000 mPa·s to 30000 mPa·s.

[0188] For example, the viscosity of a conductive adhesive with a solid content of 4% can be 10000 mPa·s, 12000 mPa·s, 14000 mPa·s, 16000 mPa·s, 18000 mPa·s, 20000 mPa·s, 22000 mPa·s, 24000 mPa·s, 26000 mPa·s, 28000 mPa·s, 30000 mPa·s, or a range of any two of the above values.

[0189] In some implementations, the pH value of the conductive adhesive is 7 to 9.

[0190] In some embodiments, the optical density (OD value, or absorbance) of the conductive solution is 1 to 2.

[0191] For example, the optical density of the conductive solution can be 1.00, 1.05, 1.10, 1.15, 1.20, 1.25, 1.30, 1.35, 1.40, 1.45, 1.50, 1.55, 1.60, 1.65, 1.70, 1.75, 1.80, 1.85, 1.90, 1.95, 2.00 or any range of two of the above values.

[0192] When the optical density of the conductive solution is within the above range, carbon nanotubes exhibit good dispersibility in the solution.

[0193] Optical density, also known as absorbance, is a physical quantity that measures the ability of a solution to absorb light. It represents the ratio of the amount of light absorbed to the amount of incident light when light of a specific wavelength passes through the solution.

[0194] In some embodiments, the solid content of the conductive solution is 0.2% to 1.6%.

[0195] For example, the solid content of the conductive solution can be 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, or a range of any two of the above values.

[0196] In some embodiments, the viscosity of the conductive solution with a solid content of 0.5% is 100 mPa·s to 4000 mPa·s.

[0197] For example, the viscosity of the conductive solution can be 100 mPa·s, 300 mPa·s, 500 mPa·s, 800 mPa·s, 1000 mPa·s, 1500 mPa·s, 2000 mPa·s, 2500 mPa·s, 3000 mPa·s, 3500 mPa·s, 4000 mPa·s, or a range of any two of the above values.

[0198] In some implementations, the pH of the conductive solution is 7 to 9.

[0199] In some embodiments, the diameter of the carbon nanotubes in the conductive solution is 1 nm to 50 nm. In the conductive solution, the carbon nanotubes can exist in clusters, and their diameter can be slightly larger than that of the raw carbon nanotubes.

[0200] For example, the diameter of carbon nanotubes in the conductive solution can be 1nm, 2nm, 3nm, 4nm, 5nm, 8nm, 10nm, 12nm, 15nm, 18nm, 20nm, 22nm, 25nm, 28nm, 30nm, 32nm, 35nm, 38nm, 40nm, 42nm, 45nm, 48nm, 50nm, or any range of two of the above values.

[0201] In some embodiments, the carbon nanotubes in the conductive solution have a tube length of 1 μm to 25 μm.

[0202] For example, the length of carbon nanotubes in the conductive solution can be 1 μm, 3 μm, 5 μm, 7 μm, 9 μm, 11 μm, 13 μm, 15 μm, 17 μm, 19 μm, 21 μm, 23 μm, 25 μm or any range of two of the above values.

[0203] In some embodiments, the step of preparing the negative electrode slurry from the material including conductive adhesive liquid or conductive solution and graphite particles also includes a stirring step, wherein the stirring speed is 800 rpm to 1800 rpm.

[0204] For example, the stirring speed can be 800 rpm, 900 rpm, 1000 rpm, 1100 rpm, 1200 rpm, 1300 rpm, 1400 rpm, 1500 rpm, 1600 rpm, 1700 rpm, 1800 rpm, or any range of two of the above values.

[0205] In some embodiments, the step of preparing a negative electrode slurry from materials including conductive adhesive liquid or conductive solution and graphite particles also includes a stirring step, with the stirring time being 60 min to 120 min.

[0206] For example, the stirring time can be 60 min, 65 min, 70 min, 75 min, 80 min, 85 min, 90 min, 95 min, 100 min, 105 min, 110 min, 115 min, 120 min, or any range of two of the above values.

[0207] Therefore, in the stirring process, when the stirring speed and / or time are within the above-mentioned range, it is possible to obtain first carbon nanotubes with a tortuosity in the range of 0 to 0.3 and second carbon nanotubes with a length L2 in the range of 0.5 μm to 20 μm, and the dispersion of single nanotubes or single clusters (diameter < 40 nm) is also better. Low stirring speed or short stirring time will increase the degree of carbon nanotube aggregation, while high stirring speed or long stirring time will disrupt the dispersion balance, leading to further aggregation of CNTs and deterioration of slurry gelation.

[0208] In some embodiments, the solid content of the negative electrode slurry is 40% to 60%, and optionally 40% to 50%.

[0209] For example, the solid content of the negative electrode slurry can be 40%, 41.4%, 42.9%, 44.3%, 45.7%, 47.1%, 48.6%, 50.0%, 51.4%, 52.9%, 54.3%, 55.7%, 57.1%, 58.6%, 60.0%, or a range of any two of the above values.

[0210] In some embodiments, the viscosity of the negative electrode slurry is 5000 mPa·s to 10000 mPa·s, and optionally 6000 mPa·s to 7000 mPa·s.

[0211] For example, the viscosity of the negative electrode slurry can be 5000 mPa·s, 5100 mPa·s, 5200 mPa·s, 5400 mPa·s, 5500 mPa·s, 5700 mPa·s, 5800 mPa·s, 6000 mPa·s, 6100 mPa·s, 6200 mPa·s, 6400 mPa·s, 6500 mPa·s, 6700 mPa·s, 6800 mPa·s, 7000 mPa·s, 8000 mPa·s, 9000 mPa·s, 10000 mPa·s, or a range of any two of the above values.

[0212] In some embodiments, the first acrylic polymer, the second acrylic polymer, and the third acrylic polymer each independently include acrylic homopolymers and copolymers of acrylic acid with other monomers.

[0213] In some embodiments, the copolymer of acrylic acid with other monomers includes acrylic acid-acrylonitrile-acrylamide copolymer.

[0214] [Positive electrode plate]

[0215] In some embodiments, the battery cell further includes a positive electrode sheet, which includes a current collector and a positive active material layer stacked together. The positive active material layer includes positive active material particles, which include one or more of lithium transition metal oxides, metal chalcogenides, lithium phosphates, and their respective modified compounds.

[0216] For example, the modified compound may include doping modification and / or coating modification, such as doping modification with metallic or non-metallic elements, carbon coating, etc.

[0217] Examples of lithium transition metal oxides may include, but are not limited to, one or more of lithium cobalt oxides, lithium nickel oxides, lithium manganese oxides, lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, lithium nickel cobalt manganese oxides, lithium nickel cobalt aluminum oxides, lithium titanium oxides, and their respective modified compounds. Lithium transition metal oxides may include, but are not limited to, layered structures and spinel structures.

[0218] Examples of lithium phosphates may include, but are not limited to, lithium iron phosphate, lithium iron phosphate and carbon composites, lithium manganese phosphate, lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, lithium manganese iron phosphate and carbon composites, and one or more of their respective modified compounds.

[0219] In some embodiments, the positive electrode active material particles may include those with the general formula Li a Ni b Co c M d O e Df One or more of lithium transition metal oxides and their modified compounds, wherein 0.8≤a≤1.2, 0.5≤b<1, 0<c<1, 0<d<1, 1≤e≤2, 0≤f≤1, M may include, but is not limited to, one or more of Ge, Mo, Sn, Mn, Al, Zr, Zn, Cu, Cr, Mg, Fe, V, Ti and B, and D may include, but is not limited to, one or more of N, F, S and Cl.

[0220] In some embodiments, the positive electrode active material particles may include, but are not limited to, LiCoO2, LiNiO2, LiMnO2, and LiNi 1 / 2 Mn 1 / 2 O2, LiMn2O4, Li 4 / 3 Ti 5 / 3 O4, LiNi 1 / 2 Mn 1 / 2 O2, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2NCM333), LiNi 0.5 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 0.80 Co 0.15 Al 0.05 O2, LiFePO4, LiMnPO4, Li 1.13 Ti 0.57 Fe 0.3 One or more of S2.

[0221] In some embodiments, the positive electrode active material particles may simultaneously comprise lithium transition metal oxides and lithium phosphates. This is advantageous for obtaining battery cells that balance high capacity and high reliability.

[0222] In some embodiments, the positive electrode active material layer further includes a positive electrode conductive agent, which may include, but is not limited to, one or more of conductive graphite (such as KS-6, SFG-6), superconducting carbon, acetylene black, carbon black (such as SP), Ketjen black (such as ECP), carbon dots, carbon nanotubes, graphene, carbon nanofibers, and vapor-grown carbon fibers (VGCF).

[0223] In some embodiments, the positive electrode active material layer further includes a positive electrode binder, which may include, but is not limited to, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-tetrafluoroethylene-propylene terpolymer, ethylene-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, fluorinated acrylate resin, water-soluble unsaturated resin SR-B, methyl vinyl silicone rubber, nitrile rubber (NBR), hydrogenated nitrile rubber (HNBR), styrene-butadiene rubber (SBR), thermoplastic styrene-butadiene rubber (SBS), isoprene rubber, cis-butadiene rubber (BR), ethyl cellulose, polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), carboxymethyl chitosan (CMCS), fluororubber, and acrylate rubber, or one or more of these.

[0224] Electrolyte

[0225] In some embodiments, the battery cell also includes an electrolyte, which comprises an electrolyte salt and an organic solvent.

[0226] In some embodiments, the concentration of the electrolyte salt can be 0.3 mol / L or more, optionally 0.7 mol / L or more, and further optionally 4 mol / L or less, optionally 2.5 mol / L or less, or 1.7 mol / L or less. When the concentration of the electrolyte salt is within the above range, the electrolyte can have a suitable ionic conductivity.

[0227] In some embodiments, the electrolyte salt comprises anion and cation, and the anion may include bis(fluorosulfonyl)imide anion (FSI). - ), bis(trifluoromethanesulfonyl)imide anion (TFSI) - ), dioxamate-borate anion

[0228] (BOB - ), difluorooxalate borate anion (DFOB) - ), difluorodioxanol phosphate anion (DFOP) - ), tetrafluorooxalate phosphate anion (TFOP) - ), difluorophosphate anion (PO2F2) - ), hexafluorophosphate anion (PF6) - ), tetrafluoroborate anion (BF4) - ), hexafluoroarsenate anion (AsF6) - ), trifluoromethanesulfonate anion (CF3SO3) - One or more of the following.

[0229] In some embodiments, the cation may include one or more of lithium ions and sodium ions.

[0230] In some embodiments, the mass percentage of the electrolyte salt in the electrolyte can be 8% or more, for example, 10% or more, optionally 10% to 20%, and further optionally 12% to 15%. When the concentration of the electrolyte salt is within the above range, the electrolyte can have a suitable ionic conductivity.

[0231] In some embodiments, the organic solvent includes ester-based organic solvents.

[0232] In some embodiments, the organic solvent may include, but is not limited to, one or more of esters, ethers, sulfones, nitriles, etc.

[0233] Esters may include, but are not limited to, one or more of carbonates, phosphate esters, carboxylic esters, sulfate esters, and sulfonates. Carbonates may include cyclic carbonates and / or chain carbonates; optionally, carbonates may include both cyclic and chain carbonates. Chain carbonates may include low-viscosity polar chain carbonates, aliphatic branched carbonates, etc.

[0234] For example, organic solvents may include, but are not limited to, ethylene carbonate (EC), propylene carbonate (PC), butene carbonate, ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), diethyl sulfone (ESE), dimethyl ether tetraethylene glycol (TEGDME), ethylene glycol dimethyl ether (... DME), 1,3-dioxolane (DOL), trimethyl phosphate, 3-methoxypropionitrile, 2-trifluoromethylhexafluoropropyl methyl ether, 2-trifluoromethylhexafluoropropyl ethyl ether, 2-trifluoromethylhexafluoropropyl propyl ether, 3-trifluoromethyloctafluorobutyl methyl ether, 3-trifluoromethyloctafluorobutyl ethyl ether, 3-trifluoromethyloctafluorobutyl propyl ether, 4-trifluoromethyldecafluoropentyl methyl ether, 4-trifluoromethyldecafluoropentyl ethyl ether, 4-trifluoromethyldecafluoropentyl propyl ether, 5-trifluoromethyldodecylfluorohexyl methyl ether, 5-trifluoromethyldodecylfluorohexyl ethyl ether, 5-trifluoromethyldodecylfluorohexyl propyl ether, 6-trifluoromethyltetradecylfluoroheptyl methyl ether, 6-trifluoromethyltetradecylfluoroheptyl ethyl ether, 6-trifluoromethyltetradecylfluoroheptyl propyl ether, 7-trifluoromethylhexadecylfluorooctyl methyl ether, 7-trifluoromethylhexadecylfluorooctyl ethyl ether, 7-trifluoromethylhexadecylfluorooctyl propyl ether.

[0235] In some embodiments, the organic solvent includes ethylene carbonate and ethyl methyl carbonate in a volume ratio of (2-5):(6-10).

[0236] For example, ethylene carbonate and methyl ethyl carbonate can be in a ratio of 2:8, 3:8, 4:8, 5:8, 2:8, 3:7, 4:6, or any range of two of the above values.

[0237] [Isolation Component]

[0238] In some implementations, the battery cell also includes a separator disposed between the positive and negative electrodes.

[0239] In some embodiments, the isolation chamber includes an isolation membrane. This application does not impose any particular limitation on the type of isolation membrane; any known porous membrane with good chemical and mechanical stability can be selected.

[0240] In some embodiments, the material of the separator may include at least one selected from glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator may be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer may be the same or different, without particular limitation. The separator may be a single component located between the positive and negative electrodes, or it may be attached to the surfaces of the positive and negative electrodes. An inorganic particle coating, an organic particle coating, or an organic / inorganic composite coating may also be applied to the surface of the separator.

[0241] In some embodiments, the separator is a solid electrolyte. The solid electrolyte is disposed between the positive and negative electrodes, serving both to transport ions and to isolate the positive and negative electrodes.

[0242] Example

[0243] The following embodiments describe the contents disclosed in this disclosure in more detail. These embodiments are merely illustrative, as various modifications and variations will be apparent to those skilled in the art within the scope of the disclosure. Unless otherwise stated, all parts, percentages, and ratios reported in the following embodiments are based on mass, and all reagents used in the embodiments are commercially available or synthesized by conventional methods and can be used directly without further processing, and the instruments used in the embodiments are commercially available.

[0244] Example 1

[0245] 1. Preparation of positive electrode sheet

[0246] LiNi, the positive electrode active material 0.8 Co 0.1 Mn 0.1 O2: conductive carbon black: carbon nanotubes (tube length: 0.5μm~10μm, tube diameter: 5nm~50nm, number of layers: 5-10 layers of oligowalled carbon nanotubes): PVDF are mixed in N-methylpyrrolidone at a mass ratio of 97.5:0.7:0.3:1.5 to prepare a positive electrode slurry. The positive electrode slurry is then uniformly coated on the positive electrode current collector, and then dried, cold-pressed and slit to obtain the positive electrode sheet.

[0247] 2. Preparation of negative electrode sheet

[0248] Preparation of conductive adhesive with a solid content of 4%: SWCNTs (single-walled carbon nanotubes) with a diameter of 1 nm to 3 nm and a length of 1 μm to 50 μm were reacted with an acrylic acid-acrylonitrile-acrylamide copolymer with a weight average molecular weight of 10,000 to obtain copolymer-grafted modified carbon nanotubes. The copolymer-grafted modified carbon nanotubes were mixed with an acrylic acid-acrylonitrile-acrylamide copolymer with a weight average molecular weight of 300,000 at 60 °C for 3 h. Lithium hydroxide was added to adjust the pH to 7 to 9 to obtain the conductive adhesive. The molar ratio of acrylic acid, acrylonitrile, and acrylamide in the acrylic acid-acrylonitrile-acrylamide copolymer was 50:35:15. Deionized water was added to adjust the solid content of the conductive adhesive to 4% and the viscosity to 15,000 mPa·s.

[0249] With a Dv50 of 15μm and a powder compaction density of 1.8g / cm³, the powder's density is determined. 3 The negative electrode active material, artificial graphite, conductive agent, conductive adhesive with a solid content of 4%, binder, styrene-butadiene rubber (SBR), and thickener, sodium carboxymethyl cellulose (CMC-Na), were dissolved in deionized water at a solid weight ratio of 97:0.5:1:1:0.5 and stirred until homogeneous at 1200 rpm for 90 minutes to prepare a negative electrode slurry. The negative electrode slurry was then uniformly coated onto a composite negative electrode current collector, and after drying, cold pressing, and slitting, negative electrode sheets were obtained. The coating weight of the negative electrode active material layer was 0.180 g / 1540.25 mm. 2 The thickness of the cold-pressed negative electrode sheet is 140 μm, and the compaction density of the negative electrode active material layer is 1.75 g / cm³. 3 .

[0250] As shown in Figure 3, the negative electrode active material layer of the negative electrode sheet includes graphite particles, first carbon nanotubes distributed between the graphite particles, and second carbon nanotubes distributed on the surface of the graphite particles. Examples of L1, h1, and L2 of carbon nanotubes in different distribution states can be found in Figure 3. The first carbon nanotube has a small bending degree.

[0251] 3. Separating membrane

[0252] Polypropylene (PP) (12μm) was selected.

[0253] 4. Preparation of electrolyte

[0254] In an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed uniformly at a volume ratio of 3:7. 12.5 wt% LiPF6 lithium salt was added and dissolved in the organic solvent and stirred until uniform to obtain the electrolyte.

[0255] 5. Battery manufacturing

[0256] The positive electrode, separator, and negative electrode obtained above are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. They are then wound to obtain a bare cell, tabs are welded onto the bare cell, and the cell is placed in an aluminum casing. It is then baked at 80°C to remove moisture, followed by the injection of electrolyte and sealing to obtain a non-charged battery. The non-charged battery then undergoes a series of processes including settling, hot and cold pressing, formation, shaping, and capacity testing.

[0257] Example 2

[0258] The difference between Example 2 and Example 1 is that the preparation of the negative electrode sheet is different.

[0259] Example 2: The preparation method of the negative electrode sheet is as follows:

[0260] Preparation of a conductive solution with a solid content of 0.5%: SWCNTs with a tube diameter of 1 nm to 3 nm and a tube length of 1 μm to 50 μm were mixed with an acrylic acid-acrylonitrile-acrylamide copolymer with a weight average molecular weight of 10,000, wherein the molar ratio of acrylic acid, acrylonitrile, and acrylamide in the acrylic acid-acrylonitrile-acrylamide copolymer was 50:35:15. The mixture was homogenized under high pressure at 1000 bar for 12 cycles. Lithium hydroxide was added to adjust the pH to 7 to 9 to obtain a conductive solution. Deionized water was added to adjust the solid content of the conductive solution to 0.5% and the viscosity to 2000 mPa·s.

[0261] The active material artificial graphite, conductive agent carbon black, conductive solution with a solid content of 0.5%, binder styrene-butadiene rubber (SBR), and thickener sodium carboxymethyl cellulose (CMC-Na) were dissolved in deionized water at a solid weight ratio of 96.5:0.4:0.1:2:1 and stirred until homogeneous at a speed of 1200 rpm for 90 minutes to prepare a negative electrode slurry. The negative electrode slurry was then uniformly coated onto a composite negative electrode current collector once or multiple times, and the negative electrode sheet was obtained after drying, cold pressing, and slitting.

[0262] Example 3

[0263] The difference between Example 3 and Example 2 lies in the preparation of the conductive solution with a solid content of 0.5%.

[0264] Example 3: Preparation of a conductive solution with a solid content of 0.5% was carried out as follows: SWCNTs with a tube diameter of 1 nm to 3 nm and a tube length of 1 μm to 50 μm were mixed with sodium carboxymethyl cellulose with a weight average molecular weight of 12000, and the mixture was dispersed by high-pressure homogenization to obtain a conductive solution. Lithium hydroxide was added to adjust the pH to 7 to 9, and the mixture was homogenized 15 times under high pressure at 1000 bar. Deionized water was added to adjust the solid content of the conductive solution to 0.5% and the viscosity to 1800 mPa·s.

[0265] Example 4

[0266] The difference between Example 4 and Example 2 lies in the stirring speed and time during the preparation of the negative electrode slurry. In Example 4, the stirring speed was 800 rpm and the stirring time was 90 min.

[0267] Comparative Example 1

[0268] The difference between Comparative Example 1 and Example 1 is that carbon nanotubes (CNTs) are not added to the negative electrode.

[0269] Comparative Example 1: Artificial graphite, conductive carbon black, PAA binder (Indira 136D), styrene-butadiene rubber (SBR) binder, and sodium carboxymethyl cellulose (CMC-Na) thickener were mixed in a weight ratio of 97:0.5:1:1:0.5 to prepare a negative electrode slurry, which was then coated to obtain a negative electrode sheet.

[0270] Comparative Example 2

[0271] The difference between Comparative Example 2 and Example 2 is that carbon nanotubes (CNTs) are not added to the negative electrode.

[0272] Comparative Example 2: Artificial graphite, conductive carbon black, binder styrene-butadiene rubber (SBR), and thickener sodium carboxymethyl cellulose (CMC-Na) were mixed in a weight ratio of 96.5:0.5:2:1 to prepare a negative electrode slurry, which was then coated to obtain a negative electrode sheet.

[0273] Comparative Example 3

[0274] The difference between Comparative Example 3 and Example 4 is that the preparation of the conductive solution with a solid content of 0.5% is different.

[0275] The preparation of the conductive solution with a solid content of 0.5% in Comparative Example 3 was as follows: SWCNTs with a tube diameter of 1 nm to 3 nm and a tube length of 1 μm to 50 μm were mixed with sodium carboxymethyl cellulose with a weight average molecular weight of 10,000. The mixture was then dispersed by milling with zirconium balls of 1 mm in diameter in a sand mill at a flow rate of 3 L / min for 4 h. Deionized water was added to adjust the solid content of the conductive solution to 0.5% and the viscosity to 2000 mPa·s.

[0276] Performance testing section

[0277] 1. OD value

[0278] The absorbance was measured using a visible spectrophotometer after taking a conductive solution or conductive adhesive sample and diluting the SWCNT concentration to 0.001%.

[0279] 2. Solid content, viscosity, and flowability of the negative electrode slurry

[0280] After the negative electrode slurry was prepared, a portion of the negative electrode slurry was taken to measure the solid content, viscosity, and filtration rate. After the slurry was left to stand for 48 hours, the sedimentation of the negative electrode slurry was observed visually. The test results are recorded in Table 1 below.

[0281] The solid content of the negative electrode slurry is tested as follows: The negative electrode slurry is placed in an aluminum-plastic box with a mass of M0, and the total weight is weighed as M1. The moisture is dried in an oven, and the remaining mass is weighed as M2. Solid content = (M2-M0) / (M1-M0).

[0282] The viscosity of the negative electrode slurry was tested in the following way: a 63r / 64r rotor was selected, the sample was tested at a constant temperature of 25±1℃, 12r / min, and the value was taken at the 6th minute; equipment model: DV-2TLV Bollefeld viscometer.

[0283] The gelation properties of the negative electrode slurry were tested in the following way: Take two cups (500 mL) of slurry from each example and comparative example, let them stand for 24 h and 48 h respectively, and use a steel ruler to scoop slurry to different depths to observe the fluidity of the slurry. If the slurry can flow continuously without lumpy gel, the fluidity is okay. If the slurry flows down in discontinuous pieces, the fluidity is not good.

[0284] 3. Bending test of carbon nanotubes

[0285] Take an uncharged electrode and observe the distribution of carbon nanotubes (CNTs) and graphite particles using SEM. Test the L1, h1, and L2 of the carbon nanotubes in the SEM image at a magnification of 20000x. Calculate the bending degree of a carbon nanotube using the formula (L1-h1) / L1*100%. Count 30 carbon nanotubes at the same magnification and calculate their bending degrees using the same method. Then, divide the sum of the total bending degrees by the number of carbon nanotubes (30) to obtain the average bending degree, which is recorded as the bending degree of the carbon nanotubes in the negative electrode.

[0286] Where L1 represents the length of the first carbon nanotube connecting two graphite particles at each end, h1 represents the straight-line distance between the two ends of the first carbon nanotube connecting the two graphite particles, and L2 represents the length of the carbon nanotubes distributed on the surface of the graphite particles. In Table 2, L1, h1, and L2 are the average values ​​obtained by statistically analyzing 30 carbon nanotubes at the same magnification. The test section and Table 1 show the average values. Figure 3 shows an example of L1 and h1 for a first carbon nanotube and an example of L2 for a second carbon nanotube.

[0287] 4. Thickness rebound rate test of negative electrode sheet

[0288] At 25°C, the secondary batteries corresponding to the examples and comparative examples were charged at a rate of 0.5C to 4.25V, then charged at a constant voltage until the current was below 0.05C, and then discharged at a rate of 1C to 2.8V. After cycling the secondary batteries 100 times, they were charged at a constant current of 1C to a voltage of 4.25V, and then charged at a constant voltage of 4.25V until the current was less than or equal to 0.05C. After that, they were left to stand for 5 minutes. At this time, the batteries were in a fully charged state. Then, the cycled batteries were disassembled in a dry room to obtain the negative electrode sheet after full charge cycling. The thickness of the negative electrode sheet was measured multiple times using a micrometer, and the average value was taken.

[0289] The thickness rebound rate is calculated as follows: the thickness of the fully charged negative electrode sheet is L2', the initial thickness of the negative electrode sheet after cold pressing is L1', and the thickness rebound rate is (L2'-L1') / L1'*100%.

[0290] 5. Cyclic performance test

[0291] Take a fresh battery cell, with three steel clamps, and install a KEPREI pressure sensor to monitor the expansion force. The preload force is 3000N.

[0292] In a constant temperature environment of 45℃, the prepared battery is charged at a constant current of 0.5C to the upper limit voltage of 4.25V, and then charged at a constant voltage at the upper limit voltage until the current is ≤0.05C. After standing for 5 minutes, it is discharged at a constant current of 1C to the lower limit voltage of 2.8V. The resulting capacity is recorded as the initial capacity C0. This is one cycle of charge and discharge. The above steps are repeated until the cell reaches 500 cycles. The discharge capacity C of the battery after the 500th cycle is recorded. Then, the SOH retention rate of the battery after each cycle is P=(C / C0)×100%. The SOH retention rate of the cell and the cell expansion force at this time are read when the cycle reaches 500 cycles.

[0293] The performance test results are shown in Table 2.

[0294] Table 1

[0295] Table 2

[0296] As shown in Tables 1-2, the carbon nanotubes in the conductive solution or conductive adhesive samples of this disclosure exhibit good dispersion and the negative electrode slurry has good fluidity. The negative electrode sheet includes first carbon nanotubes distributed between graphite particles and second carbon nanotubes distributed on the surface of graphite particles. The two ends of the first carbon nanotubes are connected to the graphite particles, and their bending degree is 0-0.3, which reduces the rebound rate of the negative electrode sheet and the expansion force of the battery cell, and also improves the cycle performance of the battery cell. In Comparative Examples 1-2, there are no carbon nanotubes, only carbon black particles, which cannot effectively connect and hold the graphite particles. The rebound rate and expansion force of the negative electrode sheet are higher than those of the examples. In Comparative Example 3, the bending degree of the first carbon nanotubes in the negative electrode sheet is greater than 0.3, the first carbon nanotubes are not straight enough, the graphite particle arrangement stability is poor, and the rebound rate and expansion force of the negative electrode sheet are also higher than those of the examples.

[0297] Although illustrative embodiments have been demonstrated and described, those skilled in the art should understand that the above embodiments should not be construed as limiting the present disclosure, and that changes, substitutions and modifications can be made to the embodiments without departing from the spirit, principles and scope of the present disclosure.

Claims

1. A battery cell, comprising a negative electrode sheet, the negative electrode sheet comprising a current collector and a negative electrode active material layer stacked thereon; the negative electrode active material layer comprising negative electrode active material particles and a first conductive agent, the negative electrode active material particles comprising graphite particles; the first conductive agent comprising a first carbon nanotube distributed between the graphite particles and a second carbon nanotube distributed on the surface of the graphite particles; The bendability δb of the first carbon nanotube is 0 to 0.

3. The bending degree δb = (L1-h1) / L1, where L1 is the length of the first carbon nanotube connecting the two graphite particles at both ends, and h1 is the straight-line distance between the two ends of the first carbon nanotube particle connecting the two graphite particles.

2. The battery cell according to claim 1, wherein, The bendability δb of the first carbon nanotube is 0 to 0.

2.

3. The battery cell according to claim 1 or 2, wherein, The first carbon nanotube satisfies at least one of the following characteristics: (1) The L1 is 0.1μm to 5μm; (2) The h1 is 0.1μm to 4μm.

4. The battery cell according to any one of claims 1 to 3, wherein, The length L2 of the second carbon nanotube is 0.5 μm to 20 μm.

5. The battery cell according to claim 4, wherein, The length L2 of the second carbon nanotube is 1.5 μm to 10 μm.

6. The battery cell according to any one of claims 1 to 5, wherein, The diameters of the first carbon nanotube and the second carbon nanotube are each independently between 1 nm and 50 nm.

7. The battery cell according to any one of claims 1 to 6, wherein, With the total mass of the negative electrode active material layer as 100%, the total mass fraction of the first carbon nanotube and the second carbon nanotube is 0.01% to 2%.

8. The battery cell according to any one of claims 1 to 7, wherein, The negative electrode active material layer further includes a second conductive agent, which includes one or more of carbon black, acetylene black, superconducting carbon, carbon dots, Ketjen black, and graphene.

9. The battery cell according to claim 8, wherein, The mass ratio of the first conductive agent to the second conductive agent is 1:(10-30).

10. The battery cell according to any one of claims 1 to 9, wherein, The graphite particles must satisfy at least one of the following characteristics: (1) The particle size distribution Dv50 of the graphite particles is 10μm to 20μm; (2) The compacted density of the graphite particles under a pressure of 100 MPa to 300 MPa is 1.6 g / cm³. 3 ~1.8g / cm 3 ; (3) The mass fraction of the graphite particles is 96.5% to 97.5%, based on the total mass of the negative electrode active material layer as 100%.

11. The battery cell according to any one of claims 1 to 10, wherein, The negative electrode active material layer must satisfy at least one of the following characteristics: (1) The thickness of the negative electrode active material layer is 60 μm to 85 μm; (2) The compaction density of the negative electrode active material layer is 1.55 g / cm³. 3 ~1.75 g / cm 3 .

12. The battery cell according to any one of claims 1 to 11, wherein, The thickness rebound rate of the negative electrode sheet under 0.5C full charge conditions in the range of 2.8 to 4.25V is 15% to 23%.

13. A method for preparing a single battery cell, comprising: Preparation of the first conductive agent slurry: Carbon nanotube powder is reacted with a first acrylic polymer to obtain polyacrylic acid grafted modified carbon nanotubes. The polyacrylic acid grafted modified carbon nanotubes are mixed with a second acrylic polymer at 60℃~80℃ for 2h~4h, wherein the weight average molecular weight of the second acrylic polymer is greater than that of the first acrylic polymer. The pH of the solution is adjusted to obtain a conductive adhesive. or, Carbon nanotube powder is mixed with a third acrylic polymer or with carboxymethyl cellulose, homogenized under high pressure, and the pH of the solution is adjusted to obtain a conductive solution. The high-pressure homogenization pressure is 300 bar to 1500 bar, and the number of high-pressure homogenization cycles is 10 to 20. A negative electrode slurry is prepared by comprising the conductive adhesive or the conductive solution and graphite particles. The negative electrode slurry is coated on the surface of at least one side of the current collector and dried to form a negative electrode active material layer, thereby obtaining a negative electrode sheet. The electrode assembly, including the negative electrode sheet, is fabricated into a single battery cell; The negative electrode active material layer comprises graphite particles and a first conductive agent, wherein the first conductive agent comprises a first carbon nanotube distributed between the graphite particles and a second carbon nanotube distributed on the surface of the graphite particles; the bending degree δb of the first carbon nanotube is 0 to 0.

3. The bending degree δb = (L1-h1) / L1, where L1 is the length of the first carbon nanotube connecting the two graphite particles at both ends, and h1 is the straight-line distance between the two ends of the first carbon nanotube particle connecting the two graphite particles.

14. The method for preparing a battery cell according to claim 13, wherein, The carbon nanotube powder has a diameter of 1 nm to 3 nm and a length of 1 μm to 50 μm.

15. The method for preparing a battery cell according to claim 13 or 14, wherein, The preparation process of the conductive adhesive liquid shall satisfy at least one of the following characteristics: (1) The weight-average molecular weight of the first acrylic polymer is 5,000 to 15,000; (2) The weight average molecular weight of the second acrylic polymer is 100,000 to 1,000,000.

16. The method for preparing a battery cell according to any one of claims 13 to 15, wherein, The optical density of the conductive adhesive is 1 to 2.

17. The method for preparing a battery cell according to any one of claims 13 to 16, wherein, The conductive adhesive liquid must satisfy at least one of the following characteristics: (1) The solid content of the conductive adhesive is 2% to 6%; (2) The viscosity of the conductive adhesive with a solid content of 4% is 10000mPa·s to 30000mPa·s; (3) The pH value of the conductive adhesive solution is 7 to 9.

18. The method for preparing a battery cell according to any one of claims 13 to 17, wherein, The preparation process of the conductive solution shall satisfy at least one of the following characteristics: (1) The weight-average molecular weight of the third acrylic polymer is 5,000 to 15,000; (2) The weight-average molecular weight of the carboxymethyl cellulose is 11,000 to 20,000.

19. The method for preparing a battery cell according to any one of claims 13 to 18, wherein, The optical density of the conductive solution is 1 to 2.

20. The method for preparing a battery cell according to any one of claims 13 to 19, wherein, The conductive solution must satisfy at least one of the following characteristics: (1) The solid content of the conductive solution is 0.2% to 1.6%; (2) The viscosity of the conductive solution with a solid content of 0.5% is 100 mPa·s to 4000 mPa·s; (3) The pH value of the conductive solution is 7 to 9.

21. The method for preparing a battery cell according to any one of claims 13 to 20, wherein, The step of preparing a negative electrode slurry from materials including the conductive adhesive or the conductive solution and graphite particles further includes a stirring process, wherein the stirring speed is 800 rpm to 1800 rpm and the stirring time is 60 min to 120 min.

22. The method for preparing a battery cell according to any one of claims 13 to 21, wherein, In the step of preparing a negative electrode slurry from materials including the conductive adhesive or the conductive solution and graphite particles, the negative electrode slurry shall at least satisfy one of the following characteristics: (1) The solid content of the negative electrode slurry is 40% to 60%; (2) The viscosity of the negative electrode slurry is 5000 mPa·s to 10000 mPa·s.

23. The method for preparing a battery cell according to any one of claims 13 to 22, wherein, The first acrylic polymer, the second acrylic polymer, and the third acrylic polymer each independently include one or more of acrylic homopolymers and acrylic-acrylonitrile-acrylamide copolymers.

24. A battery device comprising a plurality of battery cells as described in any one of claims 1 to 12 or battery cells prepared by a method according to any one of claims 13 to 23.

25. An electrical device comprising a plurality of battery cells as described in any one of claims 1 to 12, or battery cells prepared by a method for preparing a plurality of battery cells as described in any one of claims 13 to 23, or a battery device as described in claim 24.

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