Secondary battery, negative electrode sheet and electric device

Abstract

Disclosed in the present application are a secondary battery, a negative electrode sheet and an electric device. The secondary battery comprises a positive electrode, a negative electrode and a separator located between the positive electrode and the negative electrode, wherein the negative electrode comprises a negative electrode active layer. The negative electrode active layer comprises a linear conductive agent and a linear binder, wherein the linear binder comprises a linear polymer, and the linear conductive agent comprises linear conductive carbon, and the mass ratio of the linear conductive agent to the linear binder in the negative electrode active layer is (2:1)-(1:20).

Description

A secondary battery, a negative electrode plate, and an electrical device.

[0001] This application claims priority to Chinese application No. 202411515004.3, filed on October 28, 2024, entitled “A secondary battery, negative electrode sheet and electrical device”, the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to the field of batteries, specifically to a secondary battery, a negative electrode sheet, and an electrical device. Background Technology

[0003] With the market demand for high energy density and long lifespan, the problem of cell expansion in lithium-ion rechargeable batteries is becoming increasingly prominent. Cell expansion not only affects battery lifespan but also easily causes breakage of the module's end-side plate structure, leading to insulation failure and potentially safety accidents. For current high-energy-density, long-life, and fast-charging cells, the main expansion originates from the negative electrode. On one hand, when silicon-based or silicon-carbon negative electrodes are used, the expansion is significant due to the materials themselves. On the other hand, during the pressing process after coating, the residual stress within the graphite active material slowly releases over time, causing expansion of the active material. Furthermore, during charging, the interlayer spacing of lithium ions embedded in the graphite causes particle expansion, resulting in irreversible expansion.

[0004] Therefore, current secondary batteries, negative electrode plates, and electrical devices still need improvement. Summary of the Invention

[0005] In view of the above problems, this application provides a secondary battery, a negative electrode sheet, and an electrical device. By employing specific binders and conductive agents, the expansion problem of the negative electrode can be effectively alleviated. Specifically, by using linear polymer materials with lower swelling ratios in the electrolyte and linear conductive carbon capable of forming bridging support structures between the negative electrode active particles, the binding force on the negative electrode active particles can be better improved, thereby alleviating the expansion problem of the negative electrode.

[0006] In a first aspect, this application provides a secondary battery. The secondary battery includes a positive electrode, a negative electrode, and a separator located between the positive and negative electrodes. The negative electrode has a negative electrode active layer, which comprises a linear conductive agent and a linear binder. The linear binder comprises a linear polymer, and the linear conductive agent comprises linear conductive carbon. The mass ratio of the linear conductive agent to the linear binder in the negative electrode active layer is (2:1) to (1:20). This secondary battery exhibits a low negative electrode expansion rate and a low electrode rebound rate, thus possessing good lifespan and capacity retention.

[0007] In some embodiments, the aspect ratio of the linear conductive carbon is 50 to 5000. Linear conductive carbon with an aspect ratio within this range has better connectivity and confinement of the negative electrode active particles, which is beneficial for further reducing the rebound rate of the negative electrode sheet.

[0008] In some embodiments, the linear conductive carbon includes at least one of single-walled carbon nanotubes, multi-walled carbon nanotubes, and carbon fibers. This allows for further enhancement of the confinement effect on the negative electrode active particles while maintaining the conductivity of the conductive agent.

[0009] In some embodiments, the swelling rate of the linear polymer in the electrolyte is no higher than 5%. This further reduces the swelling rate of the negative electrode active layer.

[0010] In some embodiments, the linear polymer has hydrophilic functional groups, including at least one of amino, carboxyl, and hydroxyl groups. These hydrophilic functional groups facilitate interaction with structures such as negative electrode active materials and negative electrode current collectors through hydrogen bonding, thereby further enhancing the limiting effect of the linear binder on the expansion of the negative electrode sheet.

[0011] In some embodiments, the linear polymer includes at least one of polyacrylic acid, polyvinyl alcohol, polymethacrylate, acrylonitrile and its derivatives and copolymers. This is advantageous for further improving the limiting effect of the linear binder on the expansion of the negative electrode sheet.

[0012] In some embodiments, the sum of the mass of the linear conductive agent and the linear binder accounts for 0.5% to 6.0% of the total mass of the negative electrode active layer. This is beneficial for maintaining the conductivity of the negative electrode active layer and the adhesion between it and the current collector while reducing the negative electrode expansion rate.

[0013] In some embodiments, the negative electrode active layer further includes a second adhesive, wherein the mass percentage M of the second adhesive is 0 < M ≤ 1.6%, and the second adhesive includes styrene-butadiene rubber. This improves the overall toughness of the negative electrode active layer and mitigates the increased brittleness caused by the introduction of a linear structure.

[0014] In some embodiments, the mass ratio of the linear adhesive to the second adhesive is 0.1-5. This is beneficial for further improving the adhesion between the negative electrode active layer and the current collector.

[0015] In some embodiments, the negative electrode active layer further includes a second conductive agent, which includes at least one of graphite, conductive carbon black, and Ketjen black. This is beneficial for further improving the conductivity of the negative electrode active layer.

[0016] In some embodiments, the mass ratio of the linear conductive agent to the second conductive agent is (1:1) to (5:1). This is beneficial for further improving the conductivity of the negative electrode active layer.

[0017] In some embodiments, the total mass percentage of the binder in the negative electrode active layer is 1.0% to 5.0%. This is beneficial for further improving the performance of the secondary battery.

[0018] In some embodiments, the electrode rebound rate of the negative electrode is 16-22%. This is beneficial for further improving the performance of the secondary battery.

[0019] In some embodiments, the linear polymer comprises polyacrylic acid, the linear conductive agent comprises carbon nanotubes, and in the negative electrode active layer, the mass percentage of the polyacrylic acid is 0.3-1.5%, and the mass percentage of the carbon nanotubes is 0.01-1.5%. This is beneficial for further improving the performance of the secondary battery.

[0020] Secondly, this application proposes a negative electrode sheet. The negative electrode sheet includes a current collector and a negative electrode active layer located on the surface of the current collector. The negative electrode active layer has a linear conductive agent and a linear binder. The linear binder includes a linear polymer, and the linear conductive agent includes linear conductive carbon. The mass ratio of the linear conductive agent to the linear binder in the negative electrode active layer is (2:1) to (1:20). This negative electrode sheet has a low expansion rate.

[0021] In some embodiments, the negative electrode active layer comprises at least two sublayers, with at least one of the linear conductive agent and the linear binder located in the sublayer in contact with the current collector. This facilitates a further reduction in the expansion rate of the negative electrode sheet.

[0022] Thirdly, this application proposes an electrical device. This electrical device includes the aforementioned secondary battery, which provides electrical energy. Therefore, this electrical device possesses all the features and advantages of the aforementioned secondary battery, which will not be repeated here. In general, this electrical device has good cycle stability and a long service life. Attached Figure Description

[0023] Various other advantages and benefits will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiments below. The accompanying drawings are for illustrative purposes only and are not intended to limit the scope of this application. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings:

[0024] Figure 1 is a schematic diagram of a secondary battery according to an embodiment of this application.

[0025] Figure 2 is an exploded view of the secondary battery according to one embodiment of this application shown in Figure 1.

[0026] Figure 3 is a schematic diagram of an electrical device using a secondary battery as a power source according to an embodiment of this application.

[0027] Figure 4 is a scanning electron microscope image of the negative electrode sheet of Comparative Example 1 of this application.

[0028] Figure 5 is a scanning electron microscope image of the negative electrode sheet of Embodiment 1 of this application. Detailed Implementation

[0029] The embodiments of the technical solution of this application will now be described in detail with reference to the accompanying drawings. These embodiments are only used to more clearly illustrate the technical solution of this application and are therefore merely examples, and should not be used to limit the scope of protection of this application.

[0030] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0031] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0032] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0033] Unless otherwise stated, the terms used in this application have their common meanings as commonly understood by those skilled in the art. Unless otherwise stated, the values ​​of the parameters mentioned in this application can be measured using various measurement methods commonly used in the art (e.g., they can be tested according to the methods given in the embodiments of this application).

[0034] Currently, the application of power batteries is becoming increasingly widespread in the market. Power batteries are not only used in energy storage systems such as hydropower, thermal power, wind power, and solar power plants, but also extensively used in electric vehicles such as electric bicycles, electric motorcycles, and electric cars, as well as in military equipment and aerospace. With the continuous expansion of the application areas of power batteries, the market demand is also constantly increasing. Furthermore, with the continuous development of lithium-ion batteries in portable electronic devices, electric vehicles, and electric bicycles, the performance requirements for lithium-ion batteries are becoming increasingly stringent. Therefore, the compaction density and rate performance of the electrode sheets have been significantly improved. The increased compaction density leads to a higher probability and degree of electrode expansion. In addition, the properties of the negative electrode active material itself also make the negative electrode sheet more prone to expansion.

[0035] In existing lithium-ion batteries, besides silicon-oxygen and silicon-carbon materials which exhibit significant expansion, graphite materials, which are commonly used, are also prone to expansion. On one hand, during the compression of the negative electrode, residual stress within the graphite particles can cause the electrode to expand. On the other hand, the insertion and extraction of lithium ions during charging and discharging also leads to an increase in the interlayer spacing of the graphite. Especially under fast charging conditions, the rapid insertion and extraction of lithium ions between the positive and negative electrodes creates an extremely high lithium-ion concentration gradient within the battery, resulting in stress mismatch between active particles and exacerbating the expansion problem of the negative electrode.

[0036] While the aforementioned problems can be addressed by increasing the styrene-butadiene rubber (SBR) content in the negative electrode active layer to strengthen the electrode's structural strength, increase adhesion and cohesion, and restrain electrode expansion, this typically requires increasing the binder content to around 5% to achieve an effective result. However, this level of binder content sacrifices some energy density, and the improvement in electrode expansion is only minimal, with an initial full-charge rebound reduction of only about 1%, which cannot meet current demands for high energy density and long lifespan. Therefore, if a solution can be developed by adjusting the negative electrode slurry to enhance the binding force on the negative electrode active particles, alleviate rebound, and without significantly affecting the electrode's energy density, the aforementioned problems would be largely alleviated or even resolved.

[0037] Based on the above considerations, this application provides a secondary battery. The secondary battery includes a positive electrode, a negative electrode, and a separator located between the positive and negative electrodes. The negative electrode has a negative electrode active layer, which contains a linear conductive agent and a linear binder. The linear binder includes a linear polymer, and the linear conductive agent includes linear conductive carbon. The mass ratio of the linear conductive agent to the linear binder in the negative electrode active layer is (2:1) to (1:20). This secondary battery has a low negative electrode expansion rate and a low electrode rebound rate, thus exhibiting good lifespan and capacity retention.

[0038] This application employs a combination of linear conductive agents and linear binders, which work together to enhance the binding force of the negative electrode and alleviate electrode expansion. Compared to simply increasing the binder content, this application, by selecting the polymer type and microstructure of the conductive agent and binder and setting them in a specific ratio, significantly improves the binding force on the negative electrode active particles, especially the active particles, thus mitigating negative electrode expansion. Due to its superior expansion suppression effect, the secondary battery proposed in this application avoids controlling electrode expansion rate by significantly increasing the binder content, thereby achieving better overall performance.

[0039] Specifically, the linear conductive agent used in this application is linear conductive carbon. In addition to having a certain conductivity, which can play the role of traditional conductive agents in improving the conductivity of the negative electrode active layer, linear conductive carbon can also form "bridging" structures between negative electrode active particles by relying on its own microstructure characteristics, thereby confining the negative electrode active particles.

[0040] In some embodiments, the aspect ratio of the linear conductive carbon is 50 to 5000. For example, it can be 50, 100, 500, 800, 1000, 1500, 2000, 2500, 3000, 3500, 4000, 4500, or 5000. In some embodiments, the aspect ratio can be 2000-4000. Linear conductive carbon with an aspect ratio within the above range has a better effect on connecting and confining the negative electrode active particles, which is beneficial to further reducing the rebound rate of the negative electrode sheet. Specifically, linear conductive carbon with an appropriate aspect ratio has more suitable mechanical properties and dispersibility, can be better dispersed in the negative electrode active slurry, and is more likely to limit the expansion of the negative electrode active particles by overlapping or adhering to the surface of the negative electrode active particles.

[0041] In some embodiments, the linearly conductive carbon includes at least one of single-walled carbon nanotubes, multi-walled carbon nanotubes, and carbon fibers. This allows for further enhancement of the confinement effect on the negative electrode active particles while maintaining the conductivity of the conductive agent.

[0042] In some embodiments, the swelling ratio of the linear polymer in the electrolyte is no higher than 5%. For example, it can be 0.5%-4.9%, such as 1%, 2%, 3%, 4%, or 4.5%. Using a linear polymer with a swelling ratio within the above range as a linear binder is beneficial for further reducing the swelling ratio of the negative electrode active layer: selecting a linear polymer with a lower swelling ratio, due to its more ordered structure, can reduce the adsorption and confinement effect of long polymer chains on the electrolyte. Therefore, using a linear polymer with a smaller swelling ratio is beneficial for reducing the expansion of the binder itself in the electrolyte, thereby reducing the rebound of the negative electrode sheet. Furthermore, similar to linear conductive agents, linear polymers can also restrict and bind multiple adjacent negative electrode active particles in the negative electrode active layer through polymer chains, thereby further reducing the expansion of the negative electrode sheet.

[0043] Linear polymers

[0044] Linear polymers generally refer to unbranched polymers obtained by the condensation polymerization of monomers with a functionality of 2. In this application, the term "linear polymer" should be interpreted broadly. For example, some alkene monomers, such as styrene and methyl methacrylate, produce short side chains on the main chain after polymerization. These side chains are derived from the monomers, have relatively uniform lengths, and appear regularly within repeating units. In this case, the short side chains linked to the polymer main chain are not called branches, and the material still belongs to the category of linear polymers.

[0045] swelling rate

[0046] Substances that can swell in electrolyte can increase in volume and mass by adsorbing electrolyte after being soaked in electrolyte for a certain period of time.

[0047] The swelling ratio can be determined by a method including the following steps: The analyte, such as a linear polymer, is immersed in an electrolyte for 48 hours, then removed. The electrolyte can be a mixture of ethylene carbonate, diethyl carbonate, and dimethyl carbonate in a volume ratio of 1:1:1, with LiPF6 added at a concentration of 1 mol / L. The swelling ratio is the proportion of the difference between the mass of the polymer after immersion and the mass of the polymer before immersion, relative to the mass of the polymer before immersion.

[0048] In some embodiments, the linear polymer has hydrophilic functional groups, including at least one of amino, carboxyl, and hydroxyl groups. These hydrophilic functional groups facilitate interaction with structures such as the negative electrode active material and the negative electrode current collector via hydrogen bonds, further enhancing the limiting effect of the linear binder on the expansion of the negative electrode sheet. For example, the carboxyl groups of the linear polymer can interact with the hydroxyl groups on the surface of graphite and the current collector, enhancing the adhesion between the binder and the negative electrode active particles, and between the binder and the current collector, through hydrogen bonds. Furthermore, because the linear polymer has a certain length, it can indirectly confine the negative electrode active particles to the current collector surface through entanglement, chemical bonds, etc., while interacting with the current collector. This further enhances the limiting effect of the linear polymer on the negative electrode active particles and reduces electrode expansion.

[0049] In some embodiments, the linear polymer may include at least one of polyacrylic acid, polyvinyl alcohol, polymethacrylate, acrylonitrile, and their derivatives and copolymers. This is advantageous for further improving the limiting effect of the linear binder on the expansion of the negative electrode sheet. For example, the linear polymer may include polyacrylic acid (PAA), polyvinyl alcohol (PVA), polymethacrylate (PMAA), acrylonitrile, and may also include derivatives of the above polymers and copolymers. For example, it may include multi-component copolymers of acrylonitrile. The above compounds have a good "anchoring" effect on the active particles.

[0050] In general, as long as the selected linear polymer backbone has a high degree of linearity and a low swelling rate in the electrolyte, it can be dispersed on the surface of negative electrode active particles such as graphite through winding, bridging, etc., to confine the negative electrode active particles. For example, the linear polymer material selected in this application can act on the negative electrode active particles in a non-point-to-point bonding manner after the compaction of the electrode preparation and the injection of electrolyte into the secondary battery, thereby improving the anchoring degree and alleviating the expansion of the active layer.

[0051] derivative

[0052] Derivatives are substances containing one or more functional group segments with a parent core, with the parent core structure as the main body, through the substitution of functional groups or the replacement of segments. For example, polyacrylic acid derivatives are polymers with polyacrylic acid as the main structure, containing one or more substituted functional groups, or polymers with other monomer segments by replacing some of the acrylic acid monomers in the polymer.

[0053] copolymer

[0054] A copolymer is formed by the polymerization of two or more different monomers. The resulting polymer contains two or more monomer units. The arrangement of these monomers in the polymer molecular chain can vary, for example, it can be a block copolymer.

[0055] In some embodiments, the linear polymer can be added to the negative electrode active layer slurry via an aqueous dispersion. Adding the linear polymer via an aqueous dispersion facilitates its thorough dispersion.

[0056] The aforementioned linear conductive agent and binder act simultaneously on the negative electrode active layer, effectively inhibiting the expansion of the negative electrode active particles and alleviating electrode rebound without significantly increasing the binder and conductive agent content. Compared to simply adding a linear conductive agent or replacing it with a binder with stronger adhesion, the technical solution of this application can effectively limit the negative electrode active particles through their combined action while maintaining or even reducing the total content of conductive agent and binder. This alleviates the problem of insufficient energy density of the negative electrode caused by adding binders, conductive agents, and other additives.

[0057] In some embodiments, the ratio of linear conductive agent to linear binder can be adjusted to regulate the overall performance of the negative electrode active layer, mitigating electrode rebound while maintaining good conductivity and mechanical properties of the negative electrode active layer. Specifically, the mass ratio of the linear conductive agent to the linear binder in the negative electrode active layer is (2:1) to (1:20), for example, it can be 2:1, 1:1, 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, 1:11, 1:12, 1:13, 1:14, 1:15, 1:16, 1:17, 1:18, 1:19, or 1:20. When the mass ratio of linear conductive agent to linear binder is within the above range, the problem of electrode rebound can be effectively mitigated, while maintaining a low film resistance of the negative electrode.

[0058] For example, in some embodiments, the mass ratio of linear conductive agent to linear binder can be 0.2-1.5. When the mass ratio of linear conductive agent to linear binder is within the above range, the negative electrode can maintain a low film resistance while having a low electrode rebound rate. Furthermore, the strong bonding between the negative electrode active layer and the negative electrode current collector, combined with the low electrode rebound rate, allows the secondary battery to have good capacity retention.

[0059] In some embodiments, the sum of the mass of the linear conductive agent and the linear binder can be a low percentage of the total mass of the negative electrode active layer, for example, 0.5% to 6.0% of the total mass of the negative electrode active layer, and more specifically, 0.5% to 5%. This is beneficial for maintaining the conductivity of the negative electrode active layer and the adhesion between it and the current collector while reducing the negative electrode expansion rate. Specifically, the sum of the mass of the linear conductive agent and the linear binder can be 1.8%, 1.55%, 1.1%, 0.8%, 0.7%, 0.65%, 0.5%, etc., of the total mass of the negative electrode active layer.

[0060] In some embodiments, to further improve the performance and toughness of the negative electrode sheet, the negative electrode active layer may further include a binder other than a linear polymer. For example, the negative electrode active layer may further include a second binder, which may include binders commonly used in negative electrode active layers, such as styrene-butadiene rubber (SBR). Although SBR has certain adhesive properties, its swelling rate in the electrolyte is relatively large. Therefore, using only a second binder including SBR has limited effect on alleviating electrode rebound, but the addition of the second binder is beneficial to improving the toughness of the active layer. Since this application uses both a linear binder and a linear conductive agent, it can effectively alleviate electrode rebound even with a low total binder content in the electrode sheet. Therefore, the addition of the second binder can enhance the toughness of the active layer while providing an adhesive effect. Since the negative electrode sheet of this application has a linear polymer as a linear binder, the content of the second binder can be low. For example, the mass percentage M of the second binder in the negative electrode active layer is 0 < M ≤ 1.6%. This helps to improve the overall toughness of the negative electrode active layer and neutralize the increased brittleness of the negative electrode active layer caused by the introduction of a linear structure.

[0061] In some embodiments, the mass ratio of the linear binder to the second binder can be 0.1-5. Specifically, it can be 0.5, 1, 1.5, 2, 2.5, 3, 3.5, or 4. This improves the processability of the slurry forming the negative electrode active layer while maintaining a low electrode rebound rate and electrode performance.

[0062] In some embodiments, the negative electrode active layer may include a second conductive agent. Similar to the second binder, to further improve the conductivity of the negative electrode active layer and reduce the raw material cost of the negative electrode active layer, the negative electrode active layer may also include a second conductive agent composed of nonlinear conductive carbon. The second conductive agent may be selected from conductive agents commonly used in lithium-ion secondary batteries, for example, at least one of graphite, conductive carbon black, and Ketjen black.

[0063] In some embodiments, the mass ratio of the linear conductive agent to the second conductive agent can be (1:1) to (5:1). This allows for a good balance between the electrode's conductivity and production cost while maintaining a low electrode expansion rate.

[0064] In some embodiments, the total mass percentage of the conductive agent in the negative electrode active layer can be 0.5% to 2.0%. In some embodiments, the total mass percentage of the binder in the negative electrode active layer is 1.0% to 5.0%. When linear conductive agents and linear binders are used, and their mass ratio meets the aforementioned requirements, the total mass of the conductive agent and binder in the negative electrode active layer can be lower, thereby maintaining the content of negative electrode active particles in the negative electrode active layer, which is beneficial to further improving the performance of the secondary battery.

[0065] In some embodiments, the specific type of negative electrode active particles is not particularly limited. For example, they may include graphite particles, such as at least one of natural graphite and artificial graphite. The negative electrode active particles may also include carbon-based negative electrode active materials and silicon-based negative electrode active materials, such as natural graphite and / or artificial graphite, while containing at least one of silicon-carbon materials and silicon-oxygen materials.

[0066] For example, in some embodiments, the negative electrode active layer may include 93.0% to 96.5% by mass of a negative electrode active material and 0.5% to 2.0% by mass of a conductive agent, wherein the mass content of the linear conductive agent may be 0.05% to 1.0%. It may also include 1.0% to 5.0% by mass of a binder, wherein the mass content of the linear binder may be 0.5% to 4.0%.

[0067] In some embodiments, the electrode rebound rate of the negative electrode is 16-22%. This is beneficial for further improving the performance of the secondary battery. Furthermore, the reduced electrode rebound rate also helps improve the capacity retention of the secondary battery utilizing this negative electrode. In some embodiments, the capacity retention of the secondary battery after 500 cycles can be as high as 90% or more, for example, 93% or more.

[0068] In some embodiments, the linear polymer may include polyacrylic acid, and the linear conductive agent may include carbon nanotubes. In the negative electrode active layer, the mass percentage of polyacrylic acid is 0.3-1.5%, and the mass percentage of carbon nanotubes is 0.1-0.5%. This is beneficial for further improving the performance of the secondary battery: polyacrylic acid has a high degree of linearity in its main chain, resulting in low swelling in the electrolyte, and also exhibits high compatibility with the aqueous slurry of the negative electrode active layer. Carbon nanotubes are widely available and are relatively inexpensive linear conductive carbons. Furthermore, when the mass percentages of acrylic acid and carbon nanotubes are within the aforementioned ranges, it is beneficial to maintain the film resistance and mechanical properties of the electrode while effectively mitigating electrode rebound.

[0069] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 shows a square-structured secondary battery 1 as an example.

[0070] In some embodiments, referring to FIG2, the secondary battery may include an outer packaging and an electrode assembly 12. Specifically, the outer packaging may include a housing 11 and a cover plate 13. The housing 11 may include a bottom plate and side plates connected to the bottom plate, the bottom plate and side plates forming a receiving cavity. The housing 11 has an opening communicating with the receiving cavity, and the cover plate 13 can cover the opening to close the receiving cavity. A positive electrode, a negative electrode, and a separator can be formed into the electrode assembly 12 through a winding process or a stacking process. The electrode assembly 12 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 12. The secondary battery 1 may contain one or more electrode assemblies 12, which can be selected by those skilled in the art according to specific practical needs.

[0071] Secondly, this application proposes a negative electrode sheet. The negative electrode sheet includes a current collector and a negative electrode active layer located on the surface of the current collector. The negative electrode active layer has a linear conductive agent and a linear binder. The linear binder includes a linear polymer, and the linear conductive agent includes linear conductive carbon. This negative electrode sheet has a low expansion rate.

[0072] In some embodiments, the negative electrode may have the same technical features as the negative electrode in the secondary battery described above, and will not be repeated here.

[0073] In some embodiments, the negative electrode active layer of the negative electrode sheet may include at least two sublayers. Multiple sublayers may be stacked on the same side of the current collector of the negative electrode. For example, a negative electrode sheet with multiple sublayers can be formed by repeatedly coating the negative electrode active layer slurry. In this embodiment, at least one of the linear conductive agent and the linear binder may be located in the sublayer in contact with the current collector. For example, both the linear conductive agent and the linear binder may be located in the sublayer in contact with the current collector. This is beneficial for further reducing the expansion rate of the negative electrode sheet and alleviating the poor actual adhesion effect caused by binder floating, thereby improving the adhesion between the negative electrode active layer and the current collector.

[0074] In a third aspect, this application proposes an electrical device. This electrical device includes the aforementioned secondary battery, which is used to provide electrical energy. Therefore, this electrical device possesses all the features and advantages of the aforementioned secondary battery, which will not be repeated here. In general, this electrical device exhibits high fast-charging performance, good cycle stability, and a long service life.

[0075] In this application, the electrical device can be such as a mobile phone, tablet, laptop, electric toy, power tool, electric vehicle, electric car, ship, spacecraft, etc. Electric toys can include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc. Spacecraft can include airplanes, rockets, space shuttles, and spacecraft, etc.

[0076] As for the aforementioned electrical device, a secondary battery can be selected according to its usage requirements.

[0077] Figure 3 shows an example of an electrical device 2. This electrical device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of the secondary battery for this electrical device, a battery pack or battery module can be used.

[0078] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a rechargeable battery as their power source.

[0079] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0080] Example 1

[0081] Artificial graphite with a specific capacity of 350 mAh / g, conductive agent SuperP, linear carbon nanotubes (CNTs), binder SBR (styrene-butadiene rubber), linear binder PAA (polyacrylic acid), and thickener (CMC) were thoroughly stirred in an appropriate amount of deionized water at a mass ratio of 96.3:07:0.2:1.35:0.45:1 to form a negative electrode slurry. The negative electrode slurry was uniformly coated on the surface of a Cu current collector, the electrode was dried, and then cold-pressed and slit to obtain the negative electrode sheet.

[0082] Examples 2-7

[0083] The preparation process of the negative electrode sheets in Examples 2-7 is the same as that in Example 1. The differences in the negative electrode slurry compared to Example 1 are detailed in Table 1 below. The contents shown in Table 1 are the mass percentages of the corresponding components in the negative electrode slurry.

[0084] Comparative Example 1:

[0085] Without adding PAA and carbon nanotubes, artificial graphite with a specific capacity of 350 mAh / g, conductive agent (SuperP), binder (SBR), and thickener (CMC) are thoroughly stirred in an appropriate amount of deionized water at a mass ratio of 96.5:0.7:1.8:1 to form a negative electrode slurry. The negative electrode slurry is uniformly coated on the surface of a Cu current collector, the electrode is dried, and then cold-pressed and slit to obtain the negative electrode sheet.

[0086] Comparative Example 2

[0087] Without the addition of PAA, the mass ratio of graphite, conductive agent (SuperP), conductive agent (CNTs), binder SBR, and thickener CMC in the negative electrode slurry is 96.3:0.7:0.2:1.8:1, and all other conditions are the same as in Example 1.

[0088] Comparative Example 3

[0089] Without adding CNTs and binder SBR, only PAA binder is used. The mass ratio of graphite, conductive agent (SuperP), binder PAA and thickener CMC in the negative electrode slurry is 96.5:0.7:1.8:1. All other conditions are the same as in Example 1.

[0090] The negative electrode sheets prepared in Comparative Example 1 and Example 1 were tested by scanning electron microscopy. Referring to Figures 4 and 5, compared with Comparative Example 1 without the addition of PAA and CNTs, the negative electrode graphite particles of Example 1 with the addition of linear conductive agent and linear binder have carbon nanotube structure "bridging" between them, and carbon nanotubes and PAA can also be attached to the surface of graphite particles, systematically "limiting" the graphite particles.

[0091] Battery fabrication:

[0092] A secondary battery was prepared using the negative electrode sheet obtained in the above embodiments and comparative examples. The battery fabrication process is as follows:

[0093] [Positive electrode plate]

[0094] Lithium iron phosphate (LFP), conductive carbon black (SuperP), and binder (PVDF) were mixed in an N-methylpyrrolidone (NMP) solvent system at a weight ratio of 97.2:1.3:1.5 to obtain a positive electrode slurry. The positive electrode slurry was then coated on both sides of an Al foil substrate and subsequently dried, cold-pressed, slit, and cut to obtain a positive electrode sheet.

[0095] [Isolation membrane]

[0096] A polyethylene film with a thickness of 7 μm was used as the separator.

[0097] Electrolyte

[0098] Ethylene carbonate, diethyl carbonate, and dimethyl carbonate were mixed in a volume ratio of 1:1:1. LiPF6 was then dissolved in this solution to obtain an electrolyte. The concentration of LiPF6 in this electrolyte was 1 mol / L.

[0099] The positive electrode, separator, and negative electrode obtained above are assembled to form a stacked battery.

[0100] The performance of the negative electrode sheet obtained in the example and the assembled battery were tested:

[0101] Electrode film resistivity test

[0102] Cut small circular pieces with a diameter of 3mm from the left, center, and right of the electrode. Use a Yuaneng Technology electrode resistance meter for testing. Test two positions for each small circular piece, and calculate the average of six measurements to obtain the film resistance of that electrode.

[0103] Negative electrode sheet expansion rate test

[0104] The thickness of the cold-pressed negative electrode sheet is denoted as H0. The cold-pressed negative electrode sheet is then combined with the positive electrode sheet, separator, and electrolyte to form a secondary battery. The prepared battery is fully charged once after reaching its capacity, and the corresponding negative electrode sheet thickness H1 is determined. The expansion rate of the negative electrode sheet is then calculated as (H1 - H0 / H0) × 100%.

[0105] Cyclic capacity retention test

[0106] The assembled battery samples were placed in a 45°C incubator for 2 hours, and all batteries were subjected to charge-discharge tests.

[0107] The process is as follows:

[0108] Charge the battery with a constant current of 0.5C to 3.65V, then charge it with a constant voltage of 3.65V to 0.05C. Let it rest for 10 minutes, then discharge it with a constant current of 1C to 2.8V. Record the capacity as C1. This constitutes one charge-discharge cycle. Repeat this process 500 times (500cls), and record the battery capacity C500 at this point. The cycle capacity retention rate is then calculated as C500 / C1 × 100%.

[0109] The test results and the content of key components in the negative electrode sheets of the examples and comparative examples are shown in Table 1 below:

[0110] Table 1

[0111] As shown in Table 1 above, the electrode rebound rate and capacity retention rate of the embodiment containing both linear binder and linear conductive agent are higher than those of the comparative example, and the film resistance does not show a significant increase. This indicates that the technical solution of this application can effectively alleviate the expansion of the negative electrode while maintaining the conductivity of the negative electrode without significant reduction. Furthermore, compared to Comparative Examples 2 and 3, which only added linear conductive agent or only added linear binder, the embodiments of this application have a lower electrode rebound rate and a higher cycle retention rate, indicating a certain synergistic effect between the two.

[0112] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and not to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application, and they should all be covered within the scope of the claims and specification of this application. In particular, as long as there is no structural conflict, the various technical features mentioned in the embodiments can be combined in any way. This application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A secondary battery, characterized in that, include: Positive electrode, negative electrode, and a separator located between the positive and negative electrodes. The negative electrode has a negative electrode active layer, and the negative electrode active layer has a linear conductive agent and a linear binder. The linear binder comprises a linear polymer. The linear conductive agent includes linear conductive carbon. The mass ratio of the linear conductive agent to the linear binder in the negative electrode active layer is (2:1) to (1:20).

2. The secondary battery according to claim 1, characterized in that, The aspect ratio of the linear conductive carbon is 50 to 5000.

3. The secondary battery according to claim 1 or 2, characterized in that, The linear conductive carbon includes at least one of single-walled carbon nanotubes, multi-walled carbon nanotubes, and carbon fibers.

4. The secondary battery according to any one of claims 1-3, characterized in that, The swelling rate of the linear polymer in the electrolyte is no higher than 5%.

5. The secondary battery according to any one of claims 1-4, characterized in that, The linear polymer has hydrophilic functional groups, which include at least one of amino, carboxyl, and hydroxyl groups.

6. The secondary battery according to any one of claims 1-5, characterized in that, The linear polymer includes at least one of polyacrylic acid, polyvinyl alcohol, polymethacrylate, acrylonitrile and its derivatives and copolymers.

7. The secondary battery according to any one of claims 1-6, characterized in that, The sum of the mass of the linear conductive agent and the linear binder accounts for 0.5% to 6.0% of the total mass of the negative electrode active layer.

8. The secondary battery according to any one of claims 1-7, characterized in that, The negative electrode active layer further includes a second adhesive, wherein the mass percentage M of the second adhesive is 0 < M ≤ 1.6%. The second adhesive includes styrene-butadiene rubber.

9. The secondary battery according to claim 8, characterized in that, The mass ratio of the linear adhesive to the second adhesive is 0.1-5.

10. The secondary battery according to any one of claims 1-9, characterized in that, The negative electrode active layer further includes a second conductive agent, which includes at least one of graphite, conductive carbon black, and Ketjen black.

11. The secondary battery according to claim 10, characterized in that, The mass ratio of the linear conductive agent to the second conductive agent is (1:1) to (5:1).

12. The secondary battery according to any one of claims 1-11, characterized in that, The total mass percentage of the conductive agent in the negative electrode active layer is 0.5% to 2.0%.

13. The secondary battery according to any one of claims 1-12, characterized in that, The total mass percentage of the binder in the negative electrode active layer is 1.0% to 5.0%.

14. The secondary battery according to any one of claims 1-13, characterized in that, The negative electrode has a rebound rate of 16-22%.

15. The secondary battery according to any one of claims 1-14, characterized in that, The linear polymer includes polyacrylic acid, and the linear conductive agent includes carbon nanotubes. In the negative electrode active layer, the mass percentage of polyacrylic acid is 0.3-1.5%, and the mass percentage of carbon nanotubes is 0.01-1.5%.

16. The secondary battery according to any one of claims 1-15, characterized in that, The negative electrode active layer includes at least two sublayers, and at least one of the linear conductive agent and the linear binder is located in the sublayer that is in contact with the current collector of the negative electrode.

17. The secondary battery according to claim 16, characterized in that, Both the linear conductive agent and the linear binder are located in the sublayer that is in contact with the current collector.

18. A negative electrode sheet, characterized in that, It includes a current collector and a negative electrode active layer located on the surface of the current collector, the negative electrode active layer having a linear conductive agent and a linear binder. The linear binder comprises a linear polymer. The linear conductive agent includes linear conductive carbon. The mass ratio of the linear conductive agent to the linear binder in the negative electrode active layer is (2:1) to (1:20).

19. The negative electrode sheet according to claim 18, characterized in that, The negative electrode active layer includes at least two sublayers, and at least one of the linear conductive agent and the linear binder is located in the sublayer that is in contact with the current collector.

20. An electrical appliance, characterized in that, The electrical device includes a secondary battery as described in any one of claims 1-17, the secondary battery being used to provide electrical energy.

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