Composite negative electrode active material, preparation method thereof, negative electrode sheet, secondary battery, and electric device

Abstract

The application discloses a composite negative electrode active material, a preparation method thereof, a negative electrode sheet, a secondary battery and an electric device. The composite negative electrode active material comprises silicon-based negative electrode active material particles and a coating layer coated on at least part of the surfaces of the silicon-based negative electrode active material particles, wherein the coating layer comprises a crosslinked polymer. The composite negative electrode active material is applied to the secondary battery, and the secondary battery has good cycle performance and high energy density.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of secondary batteries, and particularly relates to a composite negative electrode active material, a preparation method thereof, a negative electrode sheet, a secondary battery and an electric device. BACKGROUND

[0002] A secondary battery charges and discharges by virtue of active ions reciprocatingly deintercalating between a positive electrode and a negative electrode, and has the outstanding characteristics of high energy density, long cycle life, no pollution and no memory effect. Therefore, as a clean energy, the secondary battery has gradually been popularized from electronic products to large devices such as electric vehicles to adapt to the sustainable development strategy of the environment and energy. Therefore, higher requirements are put forward for the performance of the secondary battery.

[0003] Energy density is considered to be the biggest bottleneck restricting the development of the current secondary battery. Based on this, a large amount of research has been carried out around high-capacity negative electrode active materials. Among them, the gravimetric capacity of silicon-based materials is much higher than that of carbon materials, and the theoretical gravimetric capacity is several times that of graphite.

[0004] However, silicon-based materials have a serious volume effect and will produce a huge volume expansion during charging. Therefore, during the charging and discharging process, the silicon-based material is prone to crushing and pulverization, and it is difficult to form a stable solid electrolyte interface (SEI) film on the surface, thereby causing the capacity of the secondary battery to decay quickly and the cycle performance to be poor. SUMMARY

[0005] The purpose of the application is to provide a composite negative electrode active material, a preparation method thereof, a negative electrode sheet, a secondary battery and an electric device, aiming to make the secondary battery have good cycle performance on the premise of having a higher energy density.

[0006] In order to achieve the above-mentioned purpose of the application, the first aspect of the application provides a composite negative electrode active material, which comprises silicon-based negative electrode active material particles; and a coating layer coated on at least part of the surface of the silicon-based negative electrode active material particles, wherein the coating layer comprises a crosslinked polymer.

[0007] Not intended to be limited by any theory or explanation, when the coating layer contains a cross-linked polymer, the flexible molecular chains of the cross-linked polymer form a three-dimensional network through cross-linking. Such a three-dimensional cross-linked network can improve the polymer's ability to resist molecular chain slippage and deformation under stress, thus giving the coating layer both high mechanical strength and good toughness. Therefore, when the composite negative electrode active material of this application is applied to a secondary battery, during charge-discharge cycles, the coating layer can effectively suppress the volume expansion of silicon-based negative electrode active material particles and reduce the contact between silicon-based negative electrode active material and electrolyte, thereby reducing the risk of silicon-based negative electrode active material pulverization and deactivation, reducing the occurrence of side reactions, and thus effectively reducing the capacity loss of the secondary battery and improving the energy density and cycle performance of the secondary battery. In addition, on the one hand, the cross-linked polymer can interact with silicon-based negative electrode active material particles through cross-linking structures or specific functional groups in the molecules, thereby tightly adhering to the surface of silicon-based negative electrode active material; on the other hand, the cohesive energy density of the cross-linked polymer is increased, and the polymer molecules that form a three-dimensional cross-linked network through chemical bonding are not easily dissolved in water. Therefore, when the composite negative electrode active material of this application is applied to a secondary battery, the coating layer is not easily soluble in water during the preparation of the slurry and the processing, storage and use of the battery, and has a high adhesion to the silicon-based negative electrode active material particles. Thus, it can be stably coated on the surface of the silicon-based negative electrode active material particles for a long time, thereby effectively suppressing the volume expansion of the silicon-based negative electrode active material particles and the occurrence of side reactions, thereby effectively improving the long-term cycle performance of the secondary battery.

[0008] In any embodiment of this application, the volume average particle size Dv50 of the silicon-based anode active material particles is 10 nm to 30 μm, and can be selected as 1 μm to 10 μm. When the volume average particle size Dv50 of the silicon-based anode active material particles is within the above-mentioned suitable range, on the one hand, the silicon-based anode active material particles can have a low volume expansion rate, and on the other hand, the active lithium ions can have a suitable transport path. Therefore, the composite anode active material of this application, when applied to a secondary battery, can further improve the cycle performance of the secondary battery.

[0009] In any embodiment of this application, the thickness d of the coating layer is 20 nm to 1 μm, and can be selected as 20 nm to 200 nm. When the thickness of the coating layer is within the above-mentioned suitable range, it can not only effectively suppress the volume expansion of silicon-based anode active material particles by providing suitable mechanical strength and toughness, but also enable the composite anode active material to have a high theoretical specific capacity. Therefore, the composite anode active material of this application, when applied to a secondary battery, can not only improve the cycle performance of the secondary battery, but also allow the secondary battery to have a high energy density.

[0010] In any embodiment of this application, the thickness d of the coating layer and the volume average particle size Dv50 of the silicon active material satisfy the following: 1 / 150 ≤ d / Dv50 ≤ 1 / 9, optionally, 0.01 ≤ d / Dv50 ≤ 0.05. When the ratio of d / Dv50 is within the above-mentioned suitable range, not only can the coating layer have suitable mechanical strength and toughness, but the composite negative electrode active material can also have a high theoretical specific capacity. Therefore, when the composite negative electrode active material of this application is applied to a secondary battery, the secondary battery can have both good cycle performance and high energy density.

[0011] In any embodiment of this application, the uncrosslinked polymer contains one or more functional groups selected from carboxyl, carboxylic anhydride, hydroxyl, aldehyde, amide, ether, or epoxy groups. Optionally, the uncrosslinked polymer is selected from one or more of polyacrylic acid, sodium alginate, carboxymethyl cellulose, gum arabic, polyvinyl alcohol, polyethylene glycol, polyethylene oxide, guar gum, xanthan gum, chitosan, cyclodextrin, polyacrylamide, polyethyleneimine, epoxy resin, polymaleic anhydride, and polyvinyl alcohol formal. More preferably, the polymer is selected from one or more of polyvinyl alcohol, sodium alginate, gum arabic, polyethylene glycol, polyethylene oxide, guar gum, xanthan gum, chitosan, cyclodextrin, polyacrylamide, polyethyleneimine, epoxy resin, and polymaleic anhydride. The coating layer formed by crosslinking the polymer containing the above functional groups can possess high mechanical strength, good toughness, and high bonding force with silicon-based anode active material particles. Therefore, when the composite negative electrode active material of this application is applied to a secondary battery, the coating layer can effectively suppress the volume expansion of silicon-based negative electrode active material particles during charge-discharge cycles and reduce the contact between silicon-based negative electrode active material and electrolyte. This reduces the risk of silicon-based negative electrode active material pulverization and deactivation, reduces the occurrence of side reactions, and thus effectively improves the energy density and long-term cycle performance of the secondary battery.

[0012] In any embodiment of this application, the uncrosslinked polymer contains crosslinking functional groups selected from one or more of carboxyl, carboxylic anhydride, hydroxyl, aldehyde, amide, or epoxy groups. The crosslinked polymer is obtained by reacting the uncrosslinked polymer with the crosslinking functional groups and a crosslinking agent. The crosslinked polymer, through the reaction of the aforementioned crosslinking functional groups with the corresponding crosslinking agent, can rapidly crosslink on the surface of silicon-based anode active material particles under crosslinking reaction conditions to form the coating layer. This coating layer not only possesses high mechanical strength and good toughness but is also not easily soluble in water and exhibits high adhesion to the silicon-based anode active material particles. Therefore, this coating layer can stably coat the surface of silicon-based anode active material particles for a long period, thereby effectively suppressing the volume expansion and side reactions of the silicon-based anode active material particles, and thus effectively improving the long-term cycle performance of the secondary battery.

[0013] In any embodiment of this application, the weight-average molecular weight Mw of the uncrosslinked polymer is 5 × 10⁻⁶. 4 ~1×10 6 The equilibrium swelling ratio of the composite negative electrode active material with water as the solvent is 1% to 1000%. When the weight-average molecular weight of the uncrosslinked polymer is within the above-mentioned suitable range, and the equilibrium swelling ratio of the composite negative electrode active material in water is within the above-mentioned suitable range, the crosslinked polymer can have a high molecular weight and a suitable crosslinking density. Therefore, the coating layer is not easily detached from the surface of the silicon-based negative electrode active material particles during battery processing, storage, and use, thus enabling long-term and stable coating on the surface of the silicon-based negative electrode active material particles, effectively suppressing the volume expansion of the silicon-based negative electrode active material particles, and thereby improving the energy density and long-term cycle performance of secondary batteries using the composite negative electrode active material of this application.

[0014] In any embodiment of this application, the coating layer further includes a conductive agent. Optionally, the mass ratio of the conductive agent to the cross-linked polymer is 1:220 to 1:15. Including an appropriate amount of conductive agent in the coating layer can effectively improve the electron transport capability of the coating layer, thereby enhancing the electron transport capability of the composite negative electrode active material. Therefore, when the composite negative electrode active material of this application is applied to a secondary battery, it can reduce the interfacial charge transfer impedance on the surface of the negative electrode sheet, thereby improving the cycle performance of the secondary battery.

[0015] In any embodiment of this application, the conductive agent is selected from linear conductive agents, wherein the aspect ratio of the linear conductive agent is 30 to 10000, optionally 100 to 5000; the diameter of the conductive agent is 0.5 nm to 100 nm, optionally 1 nm to 20 nm; and the length of the conductive agent is 300 nm to 30 μm, optionally 1 μm to 5 μm. Linear conductive agents meeting the above conditions have high mechanical strength, and when applied to the coating layer, they can further improve the mechanical strength of the coating layer. Furthermore, the high aspect ratio of the linear conductive agent not only allows it to play a linear toughening role in the coating layer, expanding the toughening range, but also enables the formation of a conductive network, thereby improving the toughness and conductivity of the coating layer. Therefore, in the composite negative electrode active material of this application, the coating layer has high mechanical strength, high toughness, and good electron transport capability, thereby suppressing the volume expansion of silicon-based negative electrode active material particles during charge and discharge, and improving the electron transport capability of the composite negative electrode active material. Therefore, the composite negative electrode active material of this application, when applied to a secondary battery, can significantly improve the cycle performance of the secondary battery.

[0016] In any embodiment of this application, the conductive agent is selected from one or more of carbon nanotubes, vapor-grown carbon fiber reinforcements, and graphene. Conductive agents selected from the above categories possess good electrical conductivity and mechanical strength, which can improve the mechanical strength and electrical conductivity of the coating layer. This enhances the binding effect of the coating layer on the silicon-based anode active material particles while simultaneously improving the electron transport capability of the coating layer. Therefore, the composite anode active material of this application exhibits low volume expansion and good electron transport capability, enabling the secondary battery to possess excellent cycle performance when applied to it.

[0017] In any embodiment of this application, the powder resistivity of the composite negative electrode active material is 0.3 Ω·cm to 1.3 Ω·cm, optionally 0.4 Ω·cm to 0.8 Ω·cm. In the composite negative electrode active material of this application, the silicon-based negative electrode active material particles and the coating layer have suitable structures and are tightly bonded together, thereby giving the composite negative electrode active material a low resistivity. Therefore, when the composite negative electrode active material of this application is applied to a secondary battery, it can reduce the interfacial charge transfer impedance on the surface of the negative electrode sheet, thereby improving the cycle performance of the secondary battery.

[0018] The second aspect of this application provides a method for preparing the composite negative electrode active material of the first aspect of this application, comprising the following steps S1 to S2.

[0019] S1, the silicon-based anode active material particles, the uncrosslinked polymer, the crosslinking agent, and the optional conductive agent are mixed uniformly in a solvent to obtain a composite anode active material precursor.

[0020] S2, the composite negative electrode active material precursor is placed in an environment of 30℃~250℃, optionally 60℃~200℃, so that the uncrosslinked polymer is crosslinked on the surface of the silicon-based negative electrode active material particles under the action of the crosslinking agent, thereby obtaining the composite negative electrode active material.

[0021] Not intended to be limited by any theory or interpretation, the method of this application involves uniformly mixing the silicon-based anode active material particles, the uncrosslinked polymer, the crosslinking agent, and an optional conductive agent in a solvent, so that the uncrosslinked polymer, the crosslinking agent, and the optional conductive agent uniformly coat the surface of the silicon-based anode active material particles; then, at a certain temperature, the uncrosslinked polymer is crosslinked under the action of the crosslinking agent, thereby forming a coating layer on the surface of the silicon-based anode active material particles. In this coating layer, the flexible polymer molecular chains of the crosslinked polymer form a three-dimensional network through crosslinking; such a three-dimensional crosslinked network can improve the polymer's ability to resist molecular chain slippage and deformation under stress, thereby giving the coating layer both high mechanical strength and good toughness. Therefore, the composite negative electrode active material prepared according to the method of this application, when applied to a secondary battery, effectively suppresses the volume expansion of silicon-based negative electrode active material particles and reduces the contact between the silicon-based negative electrode active material and the electrolyte during charge-discharge cycles. This reduces the risk of silicon-based negative electrode active material pulverization and deactivation, reduces the occurrence of side reactions, and thus effectively reduces the capacity loss of the secondary battery and improves its energy density and cycle performance. Furthermore, on the one hand, the cross-linked polymer can interact with the silicon-based negative electrode active material particles through its cross-linked structure or specific functional groups in its molecules, thereby tightly adhering to the surface of the silicon-based negative electrode active material; on the other hand, the cohesive energy density of the cross-linked polymer is increased, and the polymer molecules that form a three-dimensional cross-linked network through chemical bonding are less likely to dissolve in water. Therefore, when the composite negative electrode active material prepared according to the method of this application is applied to a secondary battery, the coating layer is not easily soluble in water during the preparation of the slurry and the processing, storage and use of the battery, and has a high adhesion to the silicon-based negative electrode active material particles. Thus, it can be stably coated on the surface of the silicon-based negative electrode active material particles for a long time, thereby effectively suppressing the volume expansion of the silicon-based negative electrode active material particles and the occurrence of side reactions, thereby effectively improving the long-term cycle performance of the secondary battery.

[0022] Furthermore, the method described in this application operates under mild conditions, with the crosslinking reaction occurring at a lower temperature. This reduces the risk of disproportionation reactions in silicon-based anode active material particles, thereby enabling the composite anode active material to exhibit high initial coulombic efficiency. When applied to secondary batteries, the composite anode active material prepared according to the method described in this application can effectively improve the initial coulombic efficiency, energy density, and cycle performance of the secondary battery.

[0023] In any embodiment of this application, in step S1, the silicon-based anode active material particles are 100 parts by weight, the uncrosslinked polymer is 1 to 10 parts by weight, the crosslinking agent is 0.001 to 1 part by weight, and the conductive agent is 0.05 to 4 parts by weight. In step S1, the amount of silicon-based anode active material within the aforementioned suitable range enables the composite anode active material to have a high theoretical specific capacity. When the amounts of the uncrosslinked polymer, crosslinking agent, and conductive agent are within the aforementioned suitable range, the crosslinked polymer has a suitable crosslinking density, and the coating layer has a suitable thickness and good conductivity, enabling the composite anode active material to have a low volume expansion rate, high theoretical specific capacity, and good electron transport capability. Therefore, the composite anode active material prepared according to the method of this application, when applied to a secondary battery, enables the secondary battery to have good cycle performance and high energy density.

[0024] A third aspect of this application provides a negative electrode sheet, which includes a negative current collector and a negative electrode film layer located on at least one surface of the negative current collector. The negative electrode film layer includes a negative electrode active material, a binder, and a conductive agent. The negative electrode active material includes the composite negative electrode active material of the first aspect of this application, or a composite negative electrode active material prepared according to the method of the second aspect of this application.

[0025] In the negative electrode sheet of this application, the negative electrode film layer includes the composite negative electrode active material of the first aspect of this application, or the composite negative electrode active material prepared according to the method of the second aspect of this application. When applied to a secondary battery, it enables the secondary battery to have both good cycle performance and high energy density.

[0026] In any embodiment of this application, based on a total mass of 100%, the negative electrode film layer comprises 93.5% to 97% of the negative electrode active material, 2.0% to 5.0% of the binder, and 0.5% to 1.5% of the conductive agent. Optionally, the composite negative electrode active material has a mass percentage content of 1% to 50% in the negative electrode active material. When the content of each component is within the aforementioned suitable range, the negative electrode sheet can possess high energy density and good electron transport capability. In particular, the inclusion of a composite negative electrode active material in the negative electrode film layer enables it to have good electron transport capability, thereby reducing the amount of conductive agent used in the negative electrode film layer, thus allowing the negative electrode sheet to possess both low cost and good electrochemical performance.

[0027] The fourth aspect of this application provides a secondary battery, which includes the negative electrode sheet of the third aspect of this application.

[0028] The secondary battery of this application includes the negative electrode sheet of the third aspect of this application, thereby achieving both good cycle performance and high energy density.

[0029] The fifth aspect of this application provides an electrical device that includes the secondary battery of this application.

[0030] The electrical device of this application includes the secondary battery provided in this application, and therefore has at least the same advantages as the secondary battery. Attached Figure Description

[0031] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the embodiments of this application will be briefly introduced below. Obviously, the drawings described below are only some implementation methods of this application. For those skilled in the art, other drawings can be obtained based on the drawings without creative effort.

[0032] Figure 1 This is a schematic diagram of one embodiment of the secondary battery of this application.

[0033] Figure 2 This is an exploded view of one embodiment of the secondary battery of this application.

[0034] Figure 3 This is a schematic diagram of one embodiment of the device for using a secondary battery as a power source according to this application.

[0035] Figure 4 This is a scanning electron microscope (SEM) image of the composite negative electrode active material of Example 7 of this application. Detailed Implementation

[0036] To make the inventive objectives, technical solutions, and beneficial technical effects of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the embodiments described in this specification are merely illustrative and not intended to limit the scope of this application.

[0037] For simplicity, this paper only explicitly discloses some numerical ranges. However, any lower limit can be combined with any upper limit to form an undefined range; and any lower limit can be combined with other lower limits to form an undefined range, just as any upper limit can be combined with any other upper limit to form an undefined range. Furthermore, although not explicitly stated, every point or individual value between the endpoints of a range is included within that range. Therefore, each point or individual value can serve as its own lower or upper limit and be combined with any other point or individual value, or with other lower or upper limits, to form an undefined range.

[0038] In the description of this article, it should be noted that, unless otherwise stated, "above" and "below" include the number itself, and "several" in "one or several" means two or more.

[0039] In this description, unless otherwise stated, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0040] It should be understood that relational terms such as “first,” “second,” etc., are used only to distinguish one entity or operation from another, and do not necessarily require or imply any actual relationship or order between these entities or operations.

[0041] The foregoing description of this invention is not intended to describe every disclosed embodiment or implementation. Instead, the following description provides more specific examples of exemplary embodiments. Throughout the application, guidance is provided through a series of embodiments that can be used in various combinations. The examples listed are merely representative and should not be construed as exhaustive.

[0042] As described in the background section, silicon-based materials have a much higher specific capacity than carbon materials, making them highly promising anode active materials. However, the volume expansion of silicon-based materials during charging negatively impacts their capacity utilization and the cycle performance of secondary batteries. Therefore, reducing the volume expansion of silicon-based materials during charging is an urgent problem to be solved.

[0043] In related technologies, to reduce the volume expansion of silicon-based materials, silicon-based anode active material particles are often coated, and the coating layer suppresses the volume expansion of the silicon-based material. However, the inventors have found through research that the coating layers disclosed in related technologies have drawbacks such as low mechanical strength and low adhesion to the silicon-based anode active material, and their effect on suppressing the volume expansion of silicon-based materials is not ideal.

[0044] In view of this, the inventors, through in-depth research and extensive experiments, have provided a composite negative electrode active material and its preparation method, a negative electrode sheet, a secondary battery, and an electrical device.

[0045] Composite negative electrode active material

[0046] A first aspect of this application provides a composite negative electrode active material, comprising: silicon-based negative electrode active material particles; and a coating layer covering at least a portion of the surface of the silicon-based negative electrode active material particles, wherein the coating layer comprises a cross-linked polymer.

[0047] This application does not limit the silicon-based anode active material particles, which may include silicon-based anode active material particles known in the art. In some embodiments, the silicon-based anode active material particles may be selected from one or more of silicon particles, silicon-oxygen composite particles, silicon-carbon composite particles, silicon alloy particles, or modified versions of the above substances.

[0048] The aforementioned coating layer can cover a portion of the surface of the silicon-based anode active material particles. For example, the coating layer can cover more than 50%, 70%, or 90% of the surface area of ​​the silicon-based anode active material particles. Alternatively, the coating layer can substantially cover the entire surface of the silicon-based anode active material particles.

[0049] The cross-linked polymers described above may include polymers obtained by a cross-linking reaction of polymers. In some embodiments, the polymer may be selected from water-soluble polymers. Optionally, the polymer may include one or more polymer binders.

[0050] Although the mechanism is not yet clear, the inventors unexpectedly discovered that when the composite negative electrode active material has the composition described in this application, the coating layer can effectively suppress the volume expansion of silicon-based negative electrode active material particles, thereby reducing capacity loss during charge and discharge cycles, so that the secondary battery has both high energy density and good cycle performance.

[0051] Not intended to be limited by any theory or explanation, when the coating layer contains a cross-linked polymer, the flexible molecular chains of the cross-linked polymer form a three-dimensional network through cross-linking. Such a three-dimensional cross-linked network can improve the polymer's ability to resist molecular chain slippage and deformation under stress, thus giving the coating layer both high mechanical strength and good toughness. Therefore, when the composite negative electrode active material of this application is applied to a secondary battery, during charge-discharge cycles, the coating layer can effectively suppress the volume expansion of silicon-based negative electrode active material particles and reduce the contact between silicon-based negative electrode active material and electrolyte, thereby reducing the risk of silicon-based negative electrode active material pulverization and deactivation, reducing the occurrence of side reactions, and thus effectively reducing the capacity loss of the secondary battery and improving the energy density and cycle performance of the secondary battery. In addition, on the one hand, the cross-linked polymer can interact with silicon-based negative electrode active material particles through cross-linking structures or specific functional groups in the molecules, thereby tightly adhering to the surface of silicon-based negative electrode active material; on the other hand, the cohesive energy density of the cross-linked polymer is increased, and the polymer molecules that form a three-dimensional cross-linked network through chemical bonding are not easily dissolved in water. Therefore, when the composite negative electrode active material of this application is applied to a secondary battery, the coating layer is not easily soluble in water during the preparation of the slurry and the processing, storage and use of the battery, and has a high adhesion to the silicon-based negative electrode active material particles. Thus, it can be stably coated on the surface of the silicon-based negative electrode active material particles for a long time, thereby effectively suppressing the volume expansion of the silicon-based negative electrode active material particles and the occurrence of side reactions, thereby effectively improving the long-term cycle performance of the secondary battery.

[0052] In some embodiments, the volume average particle size Dv50 of the silicon-based anode active material particles can be 10 nm to 30 μm, for example, it can be 10 nm, 50 nm, 100 nm, 500 nm, 1 μm, 2 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, or within any range of the above values. Optionally, the volume average particle size Dv50 of the silicon-based anode active material particles can be 1 μm to 10 μm, for example, it can be 1 μm, 3 μm, 5 μm, 7 μm, 10 μm, or within any range of the above values.

[0053] Not intended to be limited by any theory or explanation, when the volume average particle size Dv50 of the silicon-based anode active material particles is within the aforementioned suitable range, it enables the silicon-based anode active material particles to have a low volume expansion rate and allows the active lithium ions to have a suitable transport path. Therefore, the composite anode active material of this application, when applied to secondary batteries, can further improve the cycle performance of secondary batteries.

[0054] In some embodiments, the thickness d of the coating layer can be from 20 nm to 1 μm, for example, it can be 20 nm, 50 nm, 100 nm, 500 nm, 800 nm, 1 μm or any range of the above values. Optionally, the thickness d of the coating layer can be from 20 nm to 200 nm, for example, it can be 20 nm, 40 nm, 60 nm, 80 nm, 100 nm, 120 nm, 140 nm, 180 nm, 200 nm or any range of the above values.

[0055] Not intended to be limited by any theory or explanation, when the thickness of the coating layer is within the aforementioned suitable range, it not only possesses appropriate mechanical strength and toughness to effectively suppress the volume expansion of silicon-based anode active material particles, but also enables the composite anode active material to possess a high theoretical specific capacity. Therefore, the composite anode active material of this application, when applied to secondary batteries, can not only improve the cycle performance of the secondary battery, but also allow the secondary battery to possess a high energy density.

[0056] In some embodiments, the thickness d of the coating layer and the volume average particle size Dv50 of the silicon active material can satisfy the following condition: 1 / 150 ≤ d / Dv50 ≤ 1 / 9. For example, d / Dv50 can be 1 / 150, 1 / 120, 1 / 100, 1 / 75, 1 / 50, 1 / 30, 1 / 9, or within any range of the above values. Optionally, 0.01 ≤ d / Dv50 ≤ 0.05. For example, d / Dv50 can be 0.01, 0.02, 0.03, 0.04, or 0.05.

[0057] Not intended to be limited by any theory or explanation, when the ratio of the coating thickness *d* to the volume average particle size *Dv50* of the silicon active material is within the aforementioned suitable range, the coating layer can possess appropriate mechanical strength and toughness, thereby effectively suppressing the volume expansion of the silicon-based anode active material particles. Furthermore, when *d* / *Dv50* is within the aforementioned suitable range, the coating layer can be considered to have a low mass proportion in the composite anode active material, thereby enabling the composite anode active material to possess a high theoretical specific capacity. Therefore, the composite anode active material of this application, when applied to secondary batteries, enables the secondary battery to possess both good cycle performance and high energy density.

[0058] In some embodiments, the uncrosslinked polymer comprises one or more functional groups selected from carboxyl, carboxylic anhydride, hydroxyl, aldehyde, amide, ether, or epoxy groups. Optionally, the uncrosslinked polymer is selected from one or more of polyacrylic acid, sodium alginate, carboxymethyl cellulose (CMC), gum arabic, polyvinyl alcohol, polyethylene glycol, polyethylene oxide, guar gum, xanthan gum, chitosan, cyclodextrin, polyacrylamide, polyethyleneimine, epoxy resin, polymaleic anhydride, and polyvinyl alcohol formal. It is readily understood that the polymer may also be selected from modified versions of the above substances. More preferably, the polymer is selected from one or more of polyvinyl alcohol, sodium alginate, gum arabic, polyethylene glycol, polyethylene oxide, guar gum, xanthan gum, chitosan, cyclodextrin, polyacrylamide, polyethyleneimine, epoxy resin, and polymaleic anhydride. It is readily understood that the polymer may also be selected from modified versions of the above substances.

[0059] Not intended to be limited by any theory or explanation, the coating layer formed by cross-linking polymers containing the aforementioned functional groups possesses both high mechanical strength and good toughness, as well as high bonding force with silicon-based anode active material particles. Therefore, when the composite anode active material of this application is applied to a secondary battery, during charge-discharge cycles, the coating layer can effectively and persistently suppress the volume expansion of silicon-based anode active material particles and reduce the contact between the silicon-based anode active material and the electrolyte. This reduces the risk of silicon-based anode active material pulverization and deactivation, minimizes side reactions, and effectively improves the energy density and long-term cycle performance of the secondary battery.

[0060] In some embodiments, the uncrosslinked polymer comprises a crosslinking functional group selected from one or more of carboxyl, carboxylic anhydride, hydroxyl, aldehyde, amide, or epoxy groups. The crosslinked polymer is obtained by reacting the uncrosslinked polymer with the crosslinking functional group and a crosslinking agent. In the uncrosslinked polymer, all or only some of the crosslinking functional groups may be crosslinked to obtain the crosslinked polymer. According to embodiments of this application, the crosslinking agent is a suitable compound having a plurality of crosslinking functional groups capable of reacting with the crosslinking functional groups to bond with the polymer molecular chain. Therefore, a suitable crosslinking agent can be selected based on the given crosslinking functional groups contained in the polymer, by selecting corresponding functional groups with appropriate reactivity to those crosslinking functional groups. Examples of crosslinking agents include compounds known in the art, especially those already used as crosslinking agents.

[0061] As an example, the uncrosslinked polymer can be selected from polymers containing carboxyl groups, such as polyacrylic acid and its modified forms or carboxymethyl cellulose (CMC) and its modified forms, or more thereof. Accordingly, the crosslinking agent can be selected from one or more of carbodiimide crosslinking agents, aziridine crosslinking agents, carbodiimide crosslinking agents, epoxy silane crosslinking agents, or blocked isocyanate crosslinking agents.

[0062] As an example, the uncrosslinked polymer can be selected from polymers containing carboxylic anhydride groups, for example, it can be selected from one or more of polymaleic anhydride and its modifiers, for example, it can be selected from one or more of methyl vinyl ether-maleic anhydride copolymers. Accordingly, the crosslinking agent can be selected from one or more of carbodiimide crosslinking agents, aziridine crosslinking agents, carbodiimide crosslinking agents, epoxy silane crosslinking agents, or blocked isocyanate crosslinking agents.

[0063] As an example, the uncrosslinked polymer can be selected from polymers containing hydroxyl groups, such as sodium alginate and its modified forms, carboxymethyl cellulose (CMC) and its modified forms, polyvinyl alcohol and its modified forms, polyethylene glycol and its modified forms, polyethylene oxide and its modified forms, guar gum and its modified forms, xanthan gum and its modified forms, chitosan and its modified forms, cyclodextrin and its modified forms, or gum arabic, or one or more of these. Correspondingly, the crosslinking agent can be selected from epoxy silane crosslinking agents, blocked isocyanate crosslinking agents, maleic anhydride, glutaraldehyde, glyoxal, glutaric anhydride, and succinic anhydride, or one or more of these.

[0064] As an example, the uncrosslinked polymer can be selected from polymers containing aldehyde groups, for example, it can be selected from polyvinyl formal. Accordingly, the crosslinking agent can be selected from amino-PEG4-amine.

[0065] As an example, the uncrosslinked polymer can be selected from polymers containing amide groups, such as polyacrylamide and / or polyethyleneimine. Accordingly, the crosslinking agent can be selected from one or more of aziridine crosslinking agents, epoxy silane crosslinking agents, or blocked isocyanate crosslinking agents.

[0066] As an example, the uncrosslinked polymer can be selected from polymers containing epoxy groups, for example, from hydrophilically modified epoxy resins (such as acrylic-modified epoxy resins). Accordingly, the crosslinking agent can be selected from amine curing agents, such as ethylenediamine, hexamethylenediamine, diethylenetriamine, triethylenetetramine, or diethylaminopropylamine; and / or anhydride curing agents, such as diacids and their anhydrides (such as maleic anhydride, phthalic anhydride, etc.).

[0067] Not intended to be limited by any theory or explanation, the uncrosslinked polymer reacts with the corresponding crosslinking agent through the aforementioned crosslinking functional groups, and under crosslinking reaction conditions, it can rapidly crosslink on the surface of silicon-based anode active material particles to form the coating layer. In this coating layer, the crosslinked polymer has a suitable crosslinking density and appropriate flexible segments, thereby giving the coating layer high mechanical strength and good toughness. Therefore, when the composite anode active material of this application is applied to a secondary battery, during charge-discharge cycles, the coating layer can effectively suppress the volume expansion of silicon-based anode active material particles and reduce the contact between the silicon-based anode active material and the electrolyte, thereby reducing the risk of silicon-based anode active material pulverization and deactivation, reducing the occurrence of side reactions, and thus effectively reducing the capacity loss of the secondary battery and improving the energy density and cycle performance of the secondary battery. Furthermore, on the one hand, the crosslinked polymer can interact with the silicon-based anode active material particles through the crosslinking structure or specific functional groups in the molecule, thereby tightly adhering to the surface of the silicon-based anode active material; on the other hand, the crosslinked polymer has a high cohesive energy density, a high molecular weight, and a crosslinking structure, thus exhibiting low solubility in water. Therefore, when the composite negative electrode active material of this application is applied to a secondary battery, the coating layer is not easily soluble in water during the preparation of the slurry and the processing, storage and use of the battery, and has a high adhesion to the silicon-based negative electrode active material particles. Thus, it can be stably coated on the surface of the silicon-based negative electrode active material particles for a long time, thereby effectively suppressing the volume expansion of the silicon-based negative electrode active material particles and the occurrence of side reactions, thereby effectively improving the long-term cycle performance of the secondary battery.

[0068] In some embodiments, the weight-average molecular weight Mw of the uncrosslinked polymer may be 5 × 10⁻⁶. 4 ~1×10 6 The equilibrium swelling ratio of the composite negative electrode active material using water as a solvent can be from 1% to 1000%.

[0069] Not intended to be limited by any theory or explanation, when the weight-average molecular weight of the uncrosslinked polymer is within the aforementioned suitable range, and the equilibrium swelling ratio of the composite negative electrode active material in water is within the aforementioned suitable range, the crosslinked polymer can have a higher molecular weight and a suitable crosslinking density. Therefore, the main chain functional groups in the crosslinked polymer have a higher uniformity of distribution, resulting in a coating layer exhibiting high strength, high toughness, and high anti-swelling properties. Consequently, the coating layer is less likely to detach from the surface of the silicon-based negative electrode active material particles during battery processing, storage, and use, thus enabling long-term and stable coating of the silicon-based negative electrode active material particles, effectively suppressing the volume expansion of the silicon-based negative electrode active material particles, and thereby improving the energy density and long-term cycle performance of secondary batteries using the composite negative electrode active material of this application.

[0070] In some embodiments, the coating layer further includes a conductive agent. Optionally, the mass ratio of the conductive agent to the crosslinked polymer is 1:220 to 1:15.

[0071] This application does not limit the type of conductive agent, which can be selected from conductive agents known in the art that can be used as the negative electrode of a secondary battery. As an example, the conductive agent can be selected from carbon-based materials, metal-based materials, conductive polymers, or any combination of the above. As an example, carbon-based materials can be selected from at least one of natural graphite, artificial graphite, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. Metal-based materials can be selected from metal powders and metal fibers. Conductive polymers may include polyphenylene derivatives.

[0072] Not intended to be limited by any theory or explanation, including an appropriate amount of conductive agent in the coating layer can effectively improve the electron transport capability of the coating layer, thereby enhancing the electron transport capability of the composite negative electrode active material. Therefore, when the composite negative electrode active material of this application is applied to a secondary battery, it can reduce the interfacial charge transfer impedance on the surface of the negative electrode sheet, thereby improving the cycle performance of the secondary battery.

[0073] In some embodiments, the conductive agent is selected from linear conductive agents, and the aspect ratio of the linear conductive agent can be 30 to 10000, for example, it can be 30, 50, 100, 200, 500, 1000, 3000, 5000, 8000, 10000 or within any range of the above values. Optionally, the aspect ratio of the linear conductive agent can be 100 to 5000. The diameter of the conductive agent can be 0.5 nm to 100 nm, for example, it can be 0.5 nm, 1 nm, 5 nm, 10 nm, 20 nm, 50 nm, 80 nm, 100 nm or within any range of the above values. Optionally, the diameter of the conductive agent can be 1 nm to 20 nm, for example, it can be 1 nm, 3 nm, 5 nm, 8 nm, 10 nm, 13 nm, 15 nm, 18 nm, 20 nm or within any range of the above values. The length of the conductive agent can be from 300 nm to 30 μm, for example, it can be 300 nm, 500 nm, 800 nm, 1 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm or within any range of the above values. Optionally, the length of the conductive agent can be from 1 μm to 5 μm, for example, it can be 1 μm, 2 μm, 3 μm, 4 μm, 5 μm or within any range of the above values.

[0074] In this application, a linear conductive agent may refer to a conductive agent with a high aspect ratio, for example, a conductive agent with an aspect ratio of 30 or higher.

[0075] Not intended to be limited to any particular theory or explanation, linear conductive agents that meet the above conditions possess high mechanical strength, and their application in the coating layer can further enhance the mechanical strength of the coating layer. Furthermore, linear conductive agents have a high aspect ratio, which not only allows them to exert a linear toughening effect in the coating layer, expanding the toughening range, but also enables the formation of a conductive network, thereby improving the toughness and conductivity of the coating layer. Therefore, in the composite negative electrode active material of this application, the coating layer possesses high mechanical strength, high toughness, and good electron transport capability, thus suppressing the volume expansion of silicon-based negative electrode active material particles during charge and discharge processes and improving the electron transport capability of the composite negative electrode active material. Therefore, the composite negative electrode active material of this application, when applied to secondary batteries, can significantly improve the cycle performance of secondary batteries.

[0076] In some embodiments, the conductive agent may be selected from one-dimensional conductive agents and / or two-dimensional conductive agents.

[0077] In some embodiments, the conductive agent may be selected from one or more of carbon nanotubes (CNTs), vapor-grown catbon fiber reinforcement (VGCF), and graphene.

[0078] Not intended to be limited to any theory or explanation, the conductive agents selected from the above categories possess good electrical conductivity and mechanical strength, which can enhance the mechanical strength and electrical conductivity of the coating layer. This, in turn, strengthens the binding effect of the coating layer on the silicon-based anode active material particles while simultaneously improving the electron transport capability of the coating layer. Therefore, the composite anode active material of this application exhibits low volume expansion and good electron transport capability, enabling the secondary battery to possess excellent cycle performance when applied.

[0079] In some embodiments, the powder resistivity of the composite negative electrode active material can be from 0.3 Ω·cm to 1.3 Ω·cm, for example, it can be 0.3 Ω·cm, 0.5 Ω·cm, 0.8 Ω·cm, 1.0 Ω·cm, 1.3 Ω·cm, or within any range of the above values. Optionally, the powder resistivity of the composite negative electrode active material can be from 0.4 Ω·cm to 0.8 Ω·cm.

[0080] Not intended to be limited by any theory or explanation, the composite negative electrode active material of this application comprises silicon-based negative electrode active material particles and a coating layer with suitable structures, which are tightly bonded together, thereby giving the composite negative electrode active material a low resistivity. Therefore, when applied to secondary batteries, the composite negative electrode active material of this application can reduce the interfacial charge transfer impedance on the surface of the negative electrode sheet, thereby improving the cycle performance of the secondary battery.

[0081] In this application, the volume average particle size Dv50 of the silicon-based anode active material particles has a meaning known in the art and can be determined by methods and instruments known in the art. Dv50 represents the particle size of 50% of the particles in a volume-based particle size distribution that is smaller than this value. The volume average particle size Dv50 of the silicon-based anode active material particles can be determined using a laser particle size analyzer (e.g., the Malvern Mastersizer 2000E) according to GB / T 19077-2016 Particle Size Distribution Laser Diffraction Method.

[0082] In this application, the thickness of the coating layer has a meaning known in the art and can be measured by methods and instruments known in the art. For example, it can be measured by transmission electron microscopy (TEM).

[0083] In this application, powder resistivity has a meaning known in the art and can be measured by methods and instruments known in the art. For example, it can be measured by a powder resistivity meter.

[0084] In this application, the equilibrium swelling ratio of the composite negative electrode active material using water as a solvent has a meaning known in the art. It can represent the ratio of the volume of the composite negative electrode active material to its volume before swelling when it reaches swelling equilibrium after absorbing a certain amount of water and swelling. As an example, the equilibrium swelling ratio of the composite negative electrode active material using water as a solvent can be determined by the following steps: Take an appropriate amount of composite negative electrode active material and measure its volume V1 using a hematometer; place the composite negative electrode active material in sufficient water and leave it at 25°C for a period of time (e.g., 10 to 12 hours) to allow it to reach swelling equilibrium; remove the composite negative electrode active material and gently blot the water adhering to its surface with filter paper; measure the volume V2 of the swollen composite negative electrode active material using a hematometer; calculate the equilibrium swelling ratio Q = V2 / V1 * 100%.

[0085] Method for preparing composite negative electrode active material

[0086] The second aspect of this application provides a method for preparing the negative composite negative electrode active material of the first aspect of this application, which includes the following steps S1 to S2.

[0087] S1, the silicon-based anode active material particles, the uncrosslinked polymer, the crosslinking agent, and the optional conductive agent are mixed uniformly in a solvent to obtain a composite anode active material precursor.

[0088] In step S1, the silicon-based anode active material particles, uncrosslinked polymer, crosslinking agent, and conductive agent can be selected from the silicon-based anode active material particles, uncrosslinked polymer, crosslinking agent, and conductive agent described in the first aspect of this application, and will not be repeated here. The solvent mentioned above can include organic or inorganic solvents that can be used to uniformly disperse the silicon-based anode active material particles, the uncrosslinked polymer, the crosslinking agent, and optionally the conductive agent. As an example, the solvent can be water. In step S1, the uncrosslinked polymer, crosslinking agent, and conductive agent can be uniformly dispersed in the solvent to form a slurry layer coating the surface of the silicon-based anode active material particles.

[0089] S2, the composite negative electrode active material precursor is placed in an environment of 30℃~250℃, optionally 60℃~200℃, so that the uncrosslinked polymer is crosslinked on the surface of the silicon-based negative electrode active material particles under the action of the crosslinking agent, thereby obtaining the composite negative electrode active material.

[0090] In step S2, the uncrosslinked polymer undergoes crosslinking on the surface of the silicon-based anode active material particles under the action of the crosslinking agent, thereby generating a crosslinked polymer with a suitable crosslinking density and appropriate flexible segments, which can tightly coat the surface of the silicon-based anode active material particles to form a coating layer.

[0091] Not intended to be limited by any theory or interpretation, the method of this application involves uniformly mixing the silicon-based anode active material particles, the uncrosslinked polymer, the crosslinking agent, and an optional conductive agent in a solvent, so that the uncrosslinked polymer, the crosslinking agent, and the optional conductive agent uniformly coat the surface of the silicon-based anode active material particles; then, at a certain temperature, the uncrosslinked polymer is crosslinked under the action of the crosslinking agent, thereby forming a coating layer on the surface of the silicon-based anode active material particles. In this coating layer, the flexible polymer molecular chains of the crosslinked polymer form a three-dimensional network through crosslinking; such a three-dimensional crosslinked network can improve the polymer's ability to resist molecular chain slippage and deformation under stress, thereby giving the coating layer both high mechanical strength and good toughness. Therefore, the composite negative electrode active material prepared according to the method of this application, when applied to a secondary battery, effectively suppresses the volume expansion of silicon-based negative electrode active material particles and reduces the contact between the silicon-based negative electrode active material and the electrolyte during charge-discharge cycles. This reduces the risk of silicon-based negative electrode active material pulverization and deactivation, reduces the occurrence of side reactions, and thus effectively reduces the capacity loss of the secondary battery and improves its energy density and cycle performance. Furthermore, on the one hand, the cross-linked polymer can interact with the silicon-based negative electrode active material particles through its cross-linked structure or specific functional groups in its molecules, thereby tightly adhering to the surface of the silicon-based negative electrode active material; on the other hand, the cohesive energy density of the cross-linked polymer is increased, and the polymer molecules that form a three-dimensional cross-linked network through chemical bonding are less likely to dissolve in water. Therefore, when the composite negative electrode active material prepared according to the method of this application is applied to a secondary battery, the coating layer is not easily soluble in water during the preparation of the slurry and the processing, storage and use of the battery, and has a high adhesion to the silicon-based negative electrode active material particles. Thus, it can be stably coated on the surface of the silicon-based negative electrode active material particles for a long time, thereby effectively suppressing the volume expansion of the silicon-based negative electrode active material particles and the occurrence of side reactions, thereby effectively improving the long-term cycle performance of the secondary battery.

[0092] Furthermore, the method described in this application operates under mild conditions, with the crosslinking reaction occurring at a lower temperature. This reduces the risk of disproportionation reactions in silicon-based anode active material particles, thereby enabling the composite anode active material to exhibit high initial coulombic efficiency. When applied to secondary batteries, the composite anode active material prepared according to the method described in this application can effectively improve the initial coulombic efficiency, energy density, and cycle performance of the secondary battery.

[0093] In some embodiments, in step S1, the silicon-based negative electrode active material particles may be 100 parts by weight, the uncrosslinked polymer may be 1 to 10 parts by weight, the crosslinking agent may be 0.001 to 1 part by weight, and the conductive agent may be 0.05 to 4 parts by weight.

[0094] Not intended to be limited by any theory or explanation, in step S1, the amount of silicon-based anode active material used within the aforementioned suitable range enables the composite anode active material to possess a high theoretical specific capacity. When the amounts of the uncrosslinked polymer, crosslinking agent, and conductive agent are within the aforementioned suitable range, the crosslinked polymer has a suitable crosslinking density, and the coating layer has a suitable thickness and good conductivity, enabling the composite anode active material to possess a low volume expansion rate, high theoretical specific capacity, and good electron transport capability. Therefore, the composite anode active material prepared according to the method of this application, when applied to a secondary battery, enables the secondary battery to possess good cycle performance and high energy density.

[0095] Negative electrode sheet

[0096] A third aspect of this application provides a negative electrode sheet, comprising a negative current collector and a negative electrode film layer located on at least one surface of the negative current collector. The negative electrode film layer comprises a negative electrode active material, a binder, and a conductive agent. The negative electrode active material comprises the composite negative electrode active material of the first aspect of this application, or a composite negative electrode active material prepared according to the method of the second aspect of this application.

[0097] The negative electrode active material may also include other negative electrode active materials known in the art for use in batteries. For example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, tin-based materials, and lithium titanate, etc. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0098] The binder may be selected from binders known in the art for use with negative electrodes. As an example, the binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0099] The conductive agent may be selected from conductive agents known in the art for use in negative electrodes. As an example, the conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0100] In the negative electrode sheet of this application, the negative electrode film layer includes the composite negative electrode active material of the first aspect of this application, or the composite negative electrode active material prepared according to the method of the second aspect of this application. When applied to a secondary battery, it enables the secondary battery to have both good cycle performance and high energy density.

[0101] In some embodiments, the negative electrode film layer may include 93.5% to 97% of negative electrode active material, 2.0% to 5.0% of binder, and 0.5% to 1.5% of conductive agent, based on a total mass of 100%.

[0102] Optionally, the composite negative electrode active material may have a mass percentage content of 1% to 50% in the negative electrode active material, for example, it may be 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or within any of the above values.

[0103] When the content of each component in the negative electrode film is within the aforementioned suitable range, the negative electrode sheet can possess high energy density and good electron transport capability. In particular, the inclusion of composite negative electrode active materials in the negative electrode film enables the negative electrode sheet to have good electron transport capability, thereby reducing the amount of conductive agent used in the negative electrode film, thus allowing the negative electrode sheet to combine low cost and good electrochemical performance.

[0104] The negative electrode film layer of this application may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0105] In the negative electrode of this application, the negative current collector can be a metal foil or a composite current collector (a composite current collector can be formed by depositing metal material on a polymer substrate). For example, copper foil can be used as the metal foil. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0106] In the negative electrode sheet of this application, the negative electrode film layer can be disposed on one side of the negative electrode current collector, or it can be disposed on both sides of the negative electrode current collector simultaneously. For example, the negative electrode current collector has two opposite sides in its own thickness direction, and the negative electrode film layer is disposed on any one or both of the opposite sides of the negative electrode current collector.

[0107] It should be noted that in the secondary battery of this application, the negative electrode sheet does not exclude other additional functional layers besides the negative electrode film layer. For example, in some embodiments, the negative electrode sheet of this application may also include a conductive undercoat layer (e.g., composed of a conductive agent and a binder) disposed between the negative electrode current collector and the negative electrode film layer. In other embodiments, the negative electrode sheet of this application also includes a protective layer covering the surface of the negative electrode film layer.

[0108] Secondary battery

[0109] Secondary batteries, also known as rechargeable batteries or storage batteries, are batteries that can be recharged after being discharged to activate the active materials and continue to be used.

[0110] Typically, a secondary battery consists of a positive electrode, a negative electrode, a separator, and an electrolyte. During charging and discharging, active ions (such as lithium ions) repeatedly insert and extract between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through. The electrolyte, also located between the positive and negative electrodes, mainly conducts the active ions.

[0111] [Negative electrode plate]

[0112] The negative electrode of the secondary battery of this application includes the negative electrode of the fourth aspect of this application. The embodiments of the negative electrode have been described and illustrated in detail above, and will not be repeated here. It is understood that the secondary battery of this application can achieve the beneficial effects of any of the above embodiments of the negative electrode of this application.

[0113] [Positive electrode plate]

[0114] In the secondary battery of this application, the positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector and comprising a positive electrode active material. For example, the positive current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0115] In the secondary battery of this application, the positive electrode active material may be any positive electrode active material known in the art for use in secondary batteries. For example, the positive electrode active material may include one or more of lithium transition metal oxides, lithium-containing phosphates with an olivine structure, and their respective modified compounds. 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, and their modified compounds. Examples of lithium-containing phosphates with an olivine structure 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 their respective modified compounds. This application is not limited to these materials, and other conventionally known materials that can be used as positive electrode active materials for secondary batteries may also be used.

[0116] In the secondary battery of this application, the positive electrode film layer typically comprises a positive electrode active material, an optional binder, and an optional conductive agent. It is usually formed by coating a positive electrode slurry and then drying and cold pressing it. The positive electrode slurry is typically formed by dispersing the positive electrode active material, optional conductive agent, and binder in a solvent and stirring until homogeneous. The solvent can be N-methylpyrrolidone (NMP).

[0117] As an example, the binder used for the positive electrode film may include one or more of polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE).

[0118] As an example, the conductive agent used for the positive electrode film may include one or more of superconducting carbon, carbon black (e.g., acetylene black, Ketjen black), carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0119] In the secondary battery of this application, the positive electrode current collector can be a metal foil or a composite current collector (a composite current collector can be formed by setting a metal material on a polymer substrate). As an example, the positive electrode current collector can be an aluminum foil.

[0120] [Electrolytes]

[0121] The secondary battery of this application does not have specific restrictions on the type of electrolyte, and can be selected according to needs. For example, the electrolyte can be selected from at least one of solid electrolytes and liquid electrolytes (i.e., electrolyte solutions).

[0122] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0123] In some embodiments, the electrolyte salt may be selected from one or more of LiPF6 (lithium hexafluorophosphate), LiBF4 (lithium tetrafluoroborate), LiClO4 (lithium perchlorate), LiAsF6 (lithium hexafluoroarsenate), LiFSI (lithium bis(fluorosulfonyl)imide), LiTFSI (lithium bis(trifluoromethanesulfonyl)imide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluorooxalate borate), LiBOB (lithium dioxalate borate), LiPO2F2 (lithium difluorophosphate), LiDFOP (lithium difluorodioxalate phosphate), and LiTFOP (lithium tetrafluorooxalate phosphate).

[0124] In some embodiments, the solvent may be selected from one or more of ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butyl 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), and diethyl sulfone (ESE).

[0125] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature performance, additives that improve battery low-temperature performance, etc.

[0126] [Isolation membrane]

[0127] Secondary batteries using electrolytes, and some secondary batteries using solid electrolytes, also include a separator. The separator is positioned between the positive and negative electrodes, serving a separating function. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected. In some embodiments, the separator material can be selected from one or more of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film. When the separator is a multi-layer composite film, the materials of each layer may be the same or different.

[0128] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0129] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0130] In some implementations, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, aluminum shell, or steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0131] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. Figure 1 This is an example of a square-structured secondary battery 5.

[0132] In some implementations, refer to Figure 2 The outer packaging may include a housing 51 and a cover plate 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 is used to cover the opening to close the receiving cavity. Positive electrode sheets, negative electrode sheets, and a separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated in the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The secondary battery 5 may contain one or more electrode assemblies 52, which can be adjusted according to requirements.

[0133] Electric device

[0134] This application also provides an electrical device, which includes the secondary battery of this application. The secondary battery can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may be, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0135] Figure 3 This is an example of an electrical device. The device can be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the device's requirements for high power and high energy density, a battery pack or battery module including the secondary battery described in this application can be used.

[0136] Another example of an electrical device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use rechargeable batteries as their power source.

[0137] Examples

[0138] The following embodiments describe the disclosure of this application 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 of this application. Unless otherwise stated, all parts, percentages, and ratios reported in the following embodiments are based on weight, 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.

[0139] Examples 1-31

[0140] Preparation of composite negative electrode active material

[0141] 100 parts by weight of silicon particles with a volume average particle size of Dv50, m1 parts by weight of uncrosslinked polymer, m2 parts by weight of crosslinking agent and m3 parts by weight of conductive agent are mixed evenly in deionized water to obtain a composite negative electrode active material precursor.

[0142] The composite negative electrode active material precursor is placed in an oven at a temperature of T℃ so that the uncrosslinked polymer is crosslinked on the surface of silicon particles under the action of the corresponding crosslinking agent, thereby obtaining the composite negative electrode active material.

[0143] In each embodiment, the preparation parameters, including the Dv50 of the silicon particles, the type of uncrosslinked polymer and its weight-average molecular weight Mw, the type of conductive agent, the diameter d1 of the conductive agent, the length 11 of the conductive agent, the aspect ratio 11 / d1, m1, m2, m3 of the conductive agent, and T, are shown in Table 1. Specifically, the crosslinking agent for polyvinyl alcohol is a blocked isocyanate crosslinking agent; the crosslinking agent for polyacrylamide is an epoxy silane crosslinking agent; the crosslinking agent for polyacrylic acid is a polyaziridinium crosslinking agent; the crosslinking agent for epoxy resin is diethylenetriamine; the crosslinking agent for polymaleic anhydride is a carbodiimide crosslinking agent; and the crosslinking agent for polyvinyl formal is amino-PEG4-amine.

[0144] Comparative Example 1

[0145] Uncoated silicon particles are used directly.

[0146] Comparative Example 2

[0147] Based on the preparation process of the composite negative electrode active material in Examples 1 to 31, the preparation parameters were adjusted according to Table 1 to prepare the composite negative electrode active material of Comparative Example 2.

[0148] The composite negative electrode active materials of Examples 1 to 31 and Comparative Example 2, and the silicon particles of Comparative Example 1 were tested as follows, and the test results are shown in Table 2.

[0149] Coating layer thickness test

[0150] The composite negative electrode active material or silicon particles were dispersed in ethanol solvent and dropped onto a microgrid support film. The coating thickness d was measured by high-resolution transmission electron microscopy.

[0151] The value of d / Dv50 can be calculated based on the measured coating thickness d and the Dv50 of the silicon particles.

[0152] Powder resistivity test

[0153] The powder resistivity was tested using a powder resistivity meter, referring to the four-probe method in GB / T 30835-2014 Carbon Composite Lithium Iron Phosphate Cathode Material for Lithium-ion Batteries.

[0154] Equilibrium swelling ratio test

[0155] Take an appropriate amount of composite negative electrode active material or silicon particles as a sample, and measure the sample volume V1 using a swelling meter; place the sample in a large test tube, add water, and fill the test tube to about one-third full; seal the large test tube containing the sample and solvent with a stopper and place it in a constant temperature bath to allow the sample to swell at about 25°C; after 12 hours, remove the sample and gently blot the water adhering to the sample surface with filter paper; measure the swollen sample volume V2 using a swelling meter; calculate the equilibrium swelling ratio Q = V2 / V1*100%.

[0156] Table 1

[0157]

[0158] Table 2

[0159] Serial number Coating layer thickness d d / Dv50 Powder resistivity Q Example 1 6.67 nm 1 / 150 0.92 Ω·cm 16.9％ Example 2 10 nm 1 / 100 0.98 Ω·cm 13.1％ Example 3 20 nm 1 / 50 1.04 Ω·cm 9.3％ Example 4 100 nm 1 / 10 1.30 Ω·cm 5.6％ Example 5 100 nm 1 / 10 0.79 Ω·cm 16.5％ Example 6 100 nm 1 / 10 0.71 Ω·cm 16.7％ Example 7 100 nm 1 / 10 0.64 Ω·cm 16.3％ Example 8 100 nm 1 / 10 0.56 Ω·cm 17.1％ Example 9 100 nm 1 / 10 0.49 Ω·cm 16.8％ Example 10 100 nm 1 / 10 0.42 Ω·cm 17.2％ Example 11 100 nm 1 / 10 0.38 Ω·cm 16.4％ Example 12 100 nm 1 / 10 0.35 Ω·cm 17.0％ Example 13 100 nm 1 / 10 0.33 Ω·cm 17.1％ Example 14 100 nm 1 / 10 0.30 Ω·cm 16.8％ Example 15 100 nm 1 / 10 0.31 Ω·cm 16.4％ Example 16 100 nm 1 / 10 0.41 Ω·cm 16.9％ Example 17 100 nm 1 / 10 0.36 Ω·cm 20.2％ Example 18 100 nm 1 / 10 0.38 Ω·cm 14.1％ Example 19 100 nm 1 / 10 0.37 Ω·cm 5.2％ Example 20 1 nm 1 / 10 0.35 Ω·cm 16.1％ Example 21 200 nm 1 / 50 0.37 Ω·cm 16.3％ Example 22 1 μm 1 / 100 0.37 Ω·cm 16.2％ Example 23 100 nm 1 / 10 0.35 Ω·cm 16.8％ Example 24 111 nm 1 / 9 0.36 Ω·cm 17.0％ Example 25 90 nm 1 / 110 0.37 Ω·cm 874％ Example 26 100 nm 1 / 10 0.35 Ω·cm 945％ Example 27 100 nm 1 / 10 0.34 Ω·cm 1％ Example 28 102 nm 1 / 10 0.35 Ω·cm 451％ Example 29 109 nm 1 / 9 0.34 Ω·cm 1.2％ Example 30 100 nm 1 / 10 0.56 Ω·cm 16.5％ Example 31 100 nm 1 / 10 0.63 Ω·cm 16.7％ Comparative Example 1 / / 0.86 Ω·cm / Comparative Example 2 100 nm 1 / 10 0.96 Ω·cm 1300％

[0160] The composite negative electrode active materials or silicon particles of Examples 1-31 and Comparative Examples 1-2 were used to prepare secondary batteries, and performance tests were conducted. The test results are shown in Table 3. The specific preparation and testing processes are as follows.

[0161] Preparation of negative electrode sheet

[0162] Add negative electrode active material, conductive carbon, thickener CMC-Na, and binder SBR to deionized water in a mass ratio of 96.6:0.8:1.1:1.5, mix evenly, and obtain negative electrode slurry;

[0163] The negative electrode slurry is uniformly coated onto Cu foil, and after drying in an oven and cold pressing, the negative electrode sheet is obtained.

[0164] In Examples 1-31 and Comparative Example 2, the negative electrode active material was a mixture of 15 wt% composite negative electrode active material and 85 wt% graphite. In Comparative Example 1, the negative electrode active material was a mixture of 15 wt% silicon particles and 85 wt% graphite. Except for the different negative electrode active materials, the preparation parameters of other negative electrode sheets in each example and comparative example were the same.

[0165] Preparation of positive electrode sheet

[0166] LiNi, the positive electrode active material 0.8 Co 0.1 Mn 0.1 O2 (NCM811), conductive agent Super P, binder polyvinylidene fluoride (PVDF), and dispersant are mixed thoroughly in an appropriate amount of solvent N-methylpyrrolidone (NMP) at a mass ratio of 96.94:1.7:0.3:1:0.06 to form a uniform positive electrode slurry. The positive electrode slurry is coated on the surface of the positive electrode current collector aluminum foil, and after drying and cold pressing, the positive electrode sheet is obtained.

[0167] Preparation of electrolyte

[0168] In a dry argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), organic solvents propylene carbonate (PC), ethylene carbonate (EC), and diethyl carbonate (DEC) were mixed evenly in a weight ratio of 1:1:1. Sufficiently dried lithium salt LiPF6 was then added and dissolved in the organic solvents. After thorough stirring and mixing, an electrolyte with a lithium salt concentration of 1.15 mol / L was obtained.

[0169] Separator film

[0170] A polypropylene separator membrane is used.

[0171] Preparation of secondary battery

[0172] The positive electrode, separator, and negative electrode are stacked and wound in sequence to obtain an electrode assembly. The electrode assembly is then placed in an outer package, and the electrolyte is added. After processes such as encapsulation, settling, formation, and aging, a secondary battery is obtained.

[0173] Test section

[0174] 1) First Coulomb efficiency test

[0175] At 25℃, the formed secondary battery was first discharged at a constant current rate of 1 / 3C (DC) to 2.8V and allowed to stand for 10 minutes; then it was charged at a constant current rate of 1 / 3C (CC) to 4.2V, and then charged at a constant voltage of 4.2V (CV) to a current of 0.05C and allowed to stand for 10 minutes. The charging capacity was recorded; then it was discharged at a constant current rate of 1 / 3C (DC) to 2.8V and the discharge capacity was recorded.

[0176] Initial coulombic efficiency = discharge capacity / charge capacity * 100%.

[0177] 2) Battery 25℃ Cycle Capacity Retention Test

[0178] At 25°C, the secondary battery is charged to 4.2V at a constant current of 1 / 3C, then charged to a current of 0.05C at a constant voltage of 4.2V. After resting for 5 minutes, it is discharged to 2.8V at 1 / 3C. The resulting capacity is recorded as the initial capacity C0. The above steps are repeated for the same battery, and the discharge capacity C of the battery after the nth cycle is recorded. n Then, the battery capacity retention rate P after the nth cycle n =C n / C0*100%.

[0179] Table 3

[0180] Serial number Initial coulombic efficiency P 100 ]]> Example 1 87.21％ 95.71％ Example 2 87.19％ 95.86％ Example 3 87.30％ 95.96％ Example 4 87.27％ 95.98％ Example 5 87.32％ 96.09％ Example 6 87.36％ 96.12％ Example 7 87.44％ 96.20％ Example 8 87.68％ 96.50％ Example 9 87.83％ 96.59％ Example 10 87.86％ 96.68％ Example 11 87.92％ 96.70％ Example 12 87.91％ 96.72％ Example 13 87.89％ 96.75％ Example 14 87.94％ 96.81％ Example 15 87.90％ 96.83％ Example 16 87.80％ 96.59％ Example 17 87.89％ 96.70％ Example 18 87.91％ 96.71％ Example 19 87.87％ 96.72％ Example 20 87.13％ 96.25％ Example 21 87.96％ 96.88％ Example 22 87.93％ 96.79％ Example 23 87.90％ 96.71％ Example 24 87.94％ 96.78％ Example 25 87.89％ 96.37％ Example 26 87.86％ 96v25％ Example 27 87.88％ 96.70％ Example 28 87.91％ 96.40％ Example 29 87.89％ 96.84％ Example 30 87.84％ 96.63％ Example 31 87.64％ 96.41％ Comparative Example 1 87.10％ 95.62％ Comparative Example 2 86.96％ 95.68％

[0181] As can be seen from Tables 1 to 3, the composite negative electrode active material of this application, when applied to secondary batteries, can effectively suppress the volume expansion of silicon-based negative electrode active material particles and the occurrence of side reactions in the long term, thereby effectively improving the long-term cycle performance of secondary batteries.

[0182] Specifically, as can be seen from Examples 1 to 4, under the same conditions, the thickness of the coating layer on the surface of silicon particles gradually increases with the increase of polyvinyl alcohol and crosslinking agent dosage. Furthermore, the equilibrium swelling ratio of the material decreases with the increase of crosslinking agent dosage, indicating a higher degree of crosslinking in the coating layer. The increased degree of crosslinking is beneficial for improving the strength of the coating layer and ensuring that the coating material does not detach during stirring. Therefore, the coating layer's inhibitory effect on the volume expansion of silicon particles gradually increases the capacity retention rate of the secondary battery after 100 cycles. However, in Examples 1-4, no CNTs were added to the coating layer, resulting in a relatively weak overall strength of the coating layer and a relatively high powder resistivity of the composite negative electrode active material, requiring further improvement.

[0183] As can be seen from Examples 4 to 10, when the coating layer thickness is the same, the composite negative electrode active material containing a conductive agent in the coating layer has a lower powder resistivity. With the increase of the conductive agent, the powder resistivity decreases, and the coating layer has better mechanical strength, which can better suppress the expansion of the silicon material. Therefore, the secondary battery not only has a higher initial coulombic efficiency but also a higher capacity retention rate after 100 cycles.

[0184] Based on Examples 9, 11 to 14, it can be seen that when the diameter of the conductive agent is equal, the powder resistivity of the composite negative electrode active material gradually decreases as the length of the conductive agent increases. Based on Examples 12, 15 to 16, it can be seen that when the length of the conductive agent is equal, the powder resistivity of the composite negative electrode active material gradually decreases as the diameter of the conductive agent decreases. Therefore, increasing the aspect ratio of the conductive agent in the coating layer can effectively reduce the powder resistivity of the composite negative electrode active material, thereby improving its electron transport capability.

[0185] As can be seen from Examples 12, 17 to 19, as the crosslinking temperature increases, the crosslinking reaction proceeds more smoothly. The increased degree of crosslinking of the coating layer can improve the strength of the coating layer, and the corresponding initial coulombic efficiency and capacity retention rate after 100 cycles of the secondary battery are both improved.

[0186] As can be seen from Examples 12, 20 to 22, with the increase of silicon particle Dv50, a thicker coating layer is more easily formed on the surface of the silicon particles. Therefore, the method of this application can simply and efficiently control the thickness of the coating layer within a suitable range. This effectively suppresses the volume expansion of silicon particles during cycling, thereby improving the long-term cycle performance of the secondary battery.

[0187] As can be seen from Examples 12, 23 to 24, when the weight-average molecular weight of the uncrosslinked polymer is within the range defined in this application, a coating layer with appropriate thickness and high strength can be formed, thereby suppressing the volume expansion of silicon particles and improving the long-term cycle performance of the secondary battery.

[0188] As can be seen from Examples 12, 25 to 29, the method of this application is applicable to a variety of uncrosslinked polymers and has the advantages of simple process and wide applicability.

[0189] As can be seen from Examples 12, 30 and 31, when the coating layer contains other one-dimensional conductive agents, such as VGCF and graphene, the coating layer can also have good electron transport capability and mechanical strength. Therefore, the composite negative electrode active material can have a low volume expansion rate and good electron transport capability, and when applied to secondary batteries, it can enable the secondary battery to have good long-term cycle performance.

[0190] In contrast, the silicon particles in Comparative Example 1 lack a coating layer, resulting in a significantly lower capacity retention rate after 100 cycles compared to Examples 1 to 31. While the silicon particles in Comparative Example 2 have a coating layer, the electrochemical activity of the silicon particles may have decreased due to excessively high crosslinking temperatures during the preparation of the composite negative electrode active material. Consequently, the secondary battery in Comparative Example 2 not only exhibits a lower initial coulombic efficiency but also a lower capacity retention rate after 100 cycles compared to Examples 1 to 31.

[0191] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in this application, and these modifications or substitutions should all be covered within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A composite negative electrode active material, comprising: Silicon-based anode active material particles; as well as A coating layer covering at least a portion of the surface of the silicon-based anode active material particles. The coating layer comprises a cross-linked polymer; The equilibrium swelling ratio of the composite negative electrode active material with water as the solvent is 1% to 1000%.

2. The composite negative electrode active material according to claim 1, wherein, The volume average particle size Dv50 of the silicon-based anode active material particles is 10 nm to 30 μm.

3. The composite negative electrode active material according to claim 2, wherein, The volume average particle size Dv50 of the silicon-based anode active material particles is 1μm~10μm.

4. The composite negative electrode active material according to any one of claims 1-3, wherein, The thickness d of the coating layer is 20nm~1μm.

5. The composite negative electrode active material according to claim 4, wherein, The thickness d of the coating layer is 20nm~200nm.

6. The composite negative electrode active material according to any one of claims 1-5, wherein, The thickness d of the coating layer and the volume average particle size Dv50 of the silicon-based negative electrode active material particles satisfy the following condition: 1 / 150≤d / Dv50≤1 / 9.

7. The composite negative electrode active material according to claim 6, wherein, The thickness d of the coating layer and the volume average particle size Dv50 of the silicon-based anode active material particles satisfy the following condition: 0.01≤d / Dv50≤0.

05.

8. The composite negative electrode active material according to any one of claims 1-7, wherein, The uncrosslinked polymer contains one or more functional groups selected from carboxyl, carboxylic anhydride, hydroxyl, aldehyde, amide, ether, or epoxy groups.

9. The composite negative electrode active material according to claim 8, wherein, The uncrosslinked polymer is selected from one or more of polyacrylic acid, sodium alginate, carboxymethyl cellulose, gum arabic, polyvinyl alcohol, polyethylene glycol, polyethylene oxide, guar gum, xanthan gum, chitosan, cyclodextrin, polyacrylamide, polyethyleneimine, epoxy resin, polymaleic anhydride, and polyvinyl formal.

10. The composite negative electrode active material according to claim 8, wherein, The polymer is selected from one or more of sodium alginate, gum arabic, polyethylene glycol, polyethylene oxide, guar gum, xanthan gum, chitosan, cyclodextrin, polyacrylamide, polyethyleneimine, epoxy resin, and polymaleic anhydride.

11. The composite negative electrode active material according to any one of claims 1-10, wherein, The uncrosslinked polymer contains a crosslinking functional group selected from one or more of carboxyl, carboxylic anhydride, hydroxyl, aldehyde, amide, or epoxy groups. The crosslinked polymer is obtained by reacting the uncrosslinked polymer with the crosslinking functional group and a crosslinking agent.

12. The composite negative electrode active material according to any one of claims 1-11, wherein, The weight-average molecular weight (Mw) of the uncrosslinked polymer is 5 × 10⁻⁶. 4 ~1×10 6 .

13. The composite negative electrode active material according to any one of claims 1-12, wherein, The coating layer also includes a conductive agent.

14. The composite negative electrode active material according to claim 13, wherein, The mass ratio of the conductive agent to the cross-linked polymer is 1:220 to 1:

15.

15. The composite negative electrode active material according to claim 13 or 14, wherein, The conductive agent is selected from linear conductive agents, and the aspect ratio of the linear conductive agent is 30~10000; The diameter of the conductive agent is 0.5 nm to 100 nm; The conductive agent has a length of 300 nm to 30 μm.

16. The composite negative electrode active material according to claim 15, wherein, The conductive agent is selected from linear conductive agents, and the aspect ratio of the linear conductive agent is 100~5000.

17. The composite negative electrode active material according to claim 15, wherein, The diameter of the conductive agent is 1 nm to 20 nm.

18. The composite negative electrode active material according to claim 15, wherein, The length of the conductive agent is 1μm to 5μm.

19. The composite negative electrode active material according to any one of claims 13-18, wherein, The conductive agent is selected from one or more of carbon nanotubes, vapor-grown carbon fiber reinforcements, and graphene.

20. The composite negative electrode active material according to any one of claims 1-19, wherein, The resistivity of the composite negative electrode active material is 0.3 Ω·cm ~ 1.3 Ω·cm.

21. The composite negative electrode active material according to claim 20, wherein, The powder resistivity of the composite negative electrode active material is 0.4 Ω·cm ~ 0.8 Ω·cm.

22. A method for preparing a composite negative electrode active material according to any one of claims 1-21, comprising the following steps: S1, the silicon-based anode active material particles, the uncrosslinked polymer, and the crosslinking agent are mixed evenly in a solvent to obtain a composite anode active material precursor; S2, the composite negative electrode active material precursor is placed in an environment of 30℃~250℃, so that the uncrosslinked polymer is crosslinked on the surface of the silicon-based negative electrode active material particles under the action of the crosslinking agent, thereby obtaining the composite negative electrode active material; the equilibrium swelling ratio of the composite negative electrode active material with water as solvent is 1%~1000%.

23. The method according to claim 22, wherein, Step S1 includes: mixing the silicon-based anode active material particles, the uncrosslinked polymer, the crosslinking agent, and the conductive agent uniformly in a solvent to obtain a composite anode active material precursor.

24. The method according to claim 22, wherein, Step S2 includes: placing the composite negative electrode active material precursor in an environment of 60°C to 200°C, so that the uncrosslinked polymer is crosslinked on the surface of the silicon-based negative electrode active material particles under the action of the crosslinking agent, thereby obtaining the composite negative electrode active material.

25. The method according to claim 23, wherein, In step S1, the silicon-based negative electrode active material particles are 100 parts by weight, the uncrosslinked polymer is 1 to 10 parts by weight, the crosslinking agent is 0.001 to 1 part by weight, and the conductive agent is 0.05 to 4 parts by weight.

26. A negative electrode sheet, comprising a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector, the negative electrode film layer comprising a negative electrode active material, a binder, and a conductive agent, wherein, The negative electrode active material includes the composite negative electrode active material according to any one of claims 1-21, or the composite negative electrode active material prepared by the method according to any one of claims 22-25.

27. The negative electrode sheet according to claim 26, wherein, Based on a total mass of 100%, the negative electrode film layer comprises 93.5% to 97% of the negative electrode active material, 2.0% to 5.0% of the binder, and 0.5% to 1.5% of the conductive agent.

28. The negative electrode sheet according to claim 26, wherein, The composite negative electrode active material has a mass percentage content of 1% to 50% in the negative electrode active material.

29. A secondary battery comprising a negative electrode sheet according to any one of claims 26-28.

30. An electrical device comprising a secondary battery according to claim 29.

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