WO2024087081A1 - 复合负极活性材料及其制备方法、负极极片、二次电池及用电装置 - Google Patents

复合负极活性材料及其制备方法、负极极片、二次电池及用电装置 Download PDF

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WO2024087081A1
WO2024087081A1 PCT/CN2022/127837 CN2022127837W WO2024087081A1 WO 2024087081 A1 WO2024087081 A1 WO 2024087081A1 CN 2022127837 W CN2022127837 W CN 2022127837W WO 2024087081 A1 WO2024087081 A1 WO 2024087081A1
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negative electrode
active material
electrode active
silicon
composite negative
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PCT/CN2022/127837
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English (en)
French (fr)
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庄再裕
张明
杨继民
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宁德时代新能源科技股份有限公司
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Priority to CN202280088036.2A priority Critical patent/CN118511313A/zh
Priority to PCT/CN2022/127837 priority patent/WO2024087081A1/zh
Publication of WO2024087081A1 publication Critical patent/WO2024087081A1/zh

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers

Definitions

  • the present application belongs to the technical field of secondary batteries, and specifically relates to a composite negative electrode active material and a preparation method thereof, a negative electrode sheet, a secondary battery and an electrical device.
  • Secondary batteries rely on active ions to be reciprocated between the positive and negative electrodes for charging and discharging. They have outstanding features such as high energy density, long cycle life, no pollution, and no memory effect. Therefore, as a clean energy source, secondary batteries have gradually spread from electronic products to large-scale devices such as electric vehicles to adapt to the sustainable development strategy of the environment and energy. As a result, higher requirements are also placed on the performance of secondary batteries.
  • silicon-based materials have a serious volume effect, which will produce a huge volume expansion during the charging process.
  • silicon-based materials are very easy to break and pulverize, and it is difficult to form a stable solid electrolyte interface (SEI) film on the surface, resulting in rapid capacity decay and poor cycle performance of secondary batteries.
  • SEI solid electrolyte interface
  • the purpose of the present application is to provide a composite negative electrode active material and a preparation method thereof, a negative electrode plate, a secondary battery and an electrical device, so as to enable the secondary battery to have good cycle performance while having a high energy density.
  • the first aspect of the present application provides a composite negative electrode active material, which includes silicon-based negative electrode active material particles; and a coating layer coated on at least a portion of the surface of the silicon-based negative electrode active material particles, wherein the coating layer contains a cross-linked polymer.
  • the coating layer contains a cross-linked polymer
  • the polymer flexible molecular chain of the cross-linked polymer forms a three-dimensional network by cross-linking; such a three-dimensional cross-linked network can improve the ability of the polymer to resist molecular chain slippage and deformation under stress, so that the coating layer has both high mechanical strength and good toughness.
  • the composite negative electrode active material of the present application is applied to a secondary battery.
  • the coating layer can effectively inhibit the volume expansion of the silicon-based negative electrode active material particles and reduce the contact between the silicon-based negative electrode active material and the electrolyte, thereby reducing the risk of pulverization and inactivation of the silicon-based negative electrode active material and reducing the occurrence of side reactions, thereby effectively reducing the capacity loss of the secondary battery and improving the energy density and cycle performance of the secondary battery.
  • the cross-linked polymer can interact with the silicon-based negative electrode active material particles through the cross-linked structure or specific functional groups in the molecule, 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 by chemical bonding are not easily dissolved in water again. Therefore, when the composite negative electrode active material of the present application is applied to a secondary battery, during the preparation of the slurry and the processing, storage and use of the battery, the coating layer is not easily soluble in water and has a high adhesion to the silicon-based negative electrode active material particles.
  • the silicon-based negative electrode active material particles for a long time, thereby being able to effectively suppress the volume expansion and side reactions of the silicon-based negative electrode active material particles for a long time, thereby effectively improving the long-term cycle performance of the secondary battery.
  • the volume average particle size Dv50 of the silicon-based negative electrode active material particles is 10nm to 30 ⁇ m, and can be optionally 1 ⁇ m to 10 ⁇ m.
  • the volume average particle size Dv50 of the silicon-based negative electrode active material particles is within the above-mentioned suitable range, on the one hand, the silicon-based negative electrode active material particles can have a low volume expansion rate, and on the other hand, the active lithium ions can have a suitable transmission path. Therefore, the composite negative electrode active material of the present application is applied to secondary batteries, which can further improve the cycle performance of secondary batteries.
  • the thickness d of the coating layer is 20nm to 1 ⁇ m, and can be optionally 20nm to 200nm.
  • the thickness of the coating layer is within the above-mentioned suitable range, it can not only have suitable mechanical strength and toughness to effectively suppress the volume expansion of silicon-based negative electrode active material particles, but also enable the composite negative electrode active material to have a high theoretical gram capacity. Therefore, the composite negative electrode active material of the present application is applied to secondary batteries, which can not only improve the cycle performance of the secondary battery, but also allow the secondary battery to have a high energy density.
  • the thickness d of the coating layer and the volume average particle size Dv50 of the silicon active material satisfy: 1/150 ⁇ d/Dv50 ⁇ 1/9, optionally, 0.01 ⁇ d/Dv50 ⁇ 0.05.
  • the ratio of d/Dv50 is within the above-mentioned appropriate range, not only can the coating layer have appropriate mechanical strength and toughness, but also the composite negative electrode active material can have a high theoretical gram capacity. Therefore, the composite negative electrode active material of the present application is applied to secondary batteries, which can enable the secondary batteries to have both good cycle performance and high energy density.
  • the uncrosslinked polymer comprises one or more functional groups selected from carboxyl, carboxylic anhydride, hydroxyl, aldehyde, amide, ether or epoxy.
  • 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, polyethylene imine, epoxy resin, polymaleic anhydride, and polyvinyl formal.
  • 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, polyethylene imine, epoxy resin, and polymaleic anhydride.
  • the coating formed by the crosslinked polymer containing the above functional groups can have high mechanical strength, good toughness, and high binding force with silicon-based negative electrode active material particles. Therefore, the composite negative electrode active material of the present application is applied to secondary batteries.
  • the coating layer can effectively suppress the volume expansion of the silicon-based negative electrode active material particles for a long time and reduce the contact between the silicon-based negative electrode active material and the electrolyte, thereby reducing the risk of pulverization and deactivation of the silicon-based negative electrode active material and reducing the occurrence of side reactions, thereby effectively improving the energy density and long-term cycle performance of the secondary battery.
  • the uncrosslinked polymer contains a crosslinking functional group
  • the crosslinking functional group is selected from one or more of a carboxyl group, a carboxylic anhydride group, a hydroxyl group, an aldehyde group, an amide group or an epoxy group
  • the crosslinked polymer is obtained by reacting the uncrosslinked polymer with a crosslinking agent via the crosslinking functional group.
  • the crosslinked polymer reacts with the corresponding crosslinking agent through the above-mentioned crosslinking functional group, and can be rapidly crosslinked on the surface of the silicon-based negative electrode active material particles under the crosslinking reaction conditions to form the coating layer.
  • the coating layer not only has high mechanical strength and good toughness, but is also not easily soluble in water, and has high adhesion to the silicon-based negative electrode active material particles. Therefore, the coating layer can be stably coated on the surface of the silicon-based negative electrode active material particles for a long time, so that the volume expansion and side reactions of the silicon-based negative electrode active material particles can be effectively suppressed for a long time, thereby effectively improving the long-term cycle performance of the secondary battery.
  • the weight average molecular weight Mw of the uncrosslinked polymer is 5 ⁇ 10 4 to 1 ⁇ 10 6 ; the equilibrium swelling ratio of the composite negative electrode active material with water as solvent is 1% to 1000%.
  • the crosslinked polymer can have a higher molecular weight and a suitable crosslinking density.
  • the coating layer is not easy to fall off from the surface of the silicon-based negative electrode active material particles during battery processing, storage and use, so that it can be stably coated on the surface of the silicon-based negative electrode active material particles for a long time, effectively inhibiting the volume expansion of the silicon-based negative electrode active material particles, and thus can improve the energy density and long-term cycle performance of the secondary battery using the composite negative electrode active material of the present application.
  • the coating layer further includes a conductive agent, and 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 transmission capacity of the coating layer, thereby improving the electron transmission capacity of the composite negative electrode active material. Therefore, the composite negative electrode active material of the present application is applied to a secondary battery, which can reduce the interfacial charge transfer impedance on the surface of the negative electrode sheet, thereby improving the cycle performance of the secondary battery.
  • the conductive agent is selected from a linear conductive agent, and the aspect ratio of the linear conductive agent is 30 to 10000, and can be 100 to 5000; the diameter of the conductive agent is 0.5 nm to 100 nm, and can be 1 nm to 20 nm; the length of the conductive agent is 300 nm to 30 ⁇ m, and can be 1 ⁇ m to 5 ⁇ m.
  • the linear conductive agent that meets the above conditions has a high mechanical strength, and is applied to the coating layer, which can further improve the mechanical strength of the coating layer.
  • the linear conductive agent has a high aspect ratio, which can not only play a linear toughening role in the coating layer and expand the toughening range, but also form a conductive network, thereby improving the toughness and conductivity of the coating layer. Therefore, in the composite negative electrode active material of the present application, the coating layer has high mechanical strength, high toughness and good electron transmission ability, so that the volume expansion of the silicon-based negative electrode active material particles during the charging and discharging process can be suppressed, and the electron transmission ability of the composite negative electrode active material can be improved. Therefore, the composite negative electrode active material of the present application is applied to secondary batteries, which can significantly improve the cycle performance of secondary batteries.
  • the conductive agent is selected from one or more of carbon nanotubes, vapor-grown carbon fiber reinforcements, and graphene.
  • the conductive agent selected from the above types has good electrical conductivity and mechanical strength, and can improve the mechanical strength and electrical conductivity of the coating layer, thereby enhancing the binding effect of the coating layer on the silicon-based negative electrode active material particles while improving the electron transmission capacity of the coating layer.
  • the composite negative electrode active material of the present application can have a low volume expansion rate and good electron transmission capacity, and can be applied to secondary batteries, so that the secondary batteries can have good cycle performance.
  • the powder resistivity of the composite negative electrode active material is 0.3 ⁇ cm ⁇ 1.3 ⁇ cm, and can be optionally 0.4 ⁇ cm ⁇ 0.8 ⁇ cm.
  • the composite negative electrode active material of the present application the silicon-based negative electrode active material particles and the coating layer have a suitable structure, and the two are closely combined together, so that the composite negative electrode active material has a low resistivity. Therefore, the composite negative electrode active material of the present application is applied to secondary batteries, which can reduce the interfacial charge transfer impedance on the surface of the negative electrode plate, thereby improving the cycle performance of the secondary battery.
  • the second aspect of the present application provides a method for preparing the composite negative electrode active material of the first aspect of the present application, comprising the following steps S1 to S2.
  • the method of the present application is to mix the silicon-based negative electrode active material particles, the uncrosslinked polymer, the crosslinking agent and the optional conductive agent in a solvent so that the uncrosslinked polymer, the crosslinking agent and the optional conductive agent are uniformly coated on the surface of the silicon-based negative electrode active material particles; and then the uncrosslinked polymer is crosslinked under the action of the crosslinking agent at a certain temperature, so as to form a coating layer on the surface of the silicon-based negative electrode active material particles.
  • the polymer flexible molecular chains of the crosslinked polymer form a three-dimensional network through crosslinking; such a three-dimensional crosslinked network can improve the ability of the polymer to resist molecular chain slippage and deformation under stress, so that the coating layer has both high mechanical strength and good toughness. Therefore, the composite negative electrode active material prepared according to the method of the present application is applied to a secondary battery.
  • the coating layer can effectively inhibit the volume expansion of the silicon-based negative electrode active material particles and reduce the contact between the silicon-based negative electrode active material and the electrolyte, thereby reducing the risk of pulverization and inactivation of the silicon-based negative electrode active material and the occurrence of side reactions, thereby effectively reducing the capacity loss of the secondary battery and improving the energy density and cycle performance of the secondary battery.
  • the cross-linked polymer can interact with the silicon-based negative electrode active material particles through the cross-linking structure or specific functional groups in the molecule, 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 not easily dissolved in water again. Therefore, when the composite negative electrode active material prepared according to the method of the present application is applied to a secondary battery, during the preparation of the slurry and the processing, storage and use of the battery, the coating layer is not easily soluble in water and has a high adhesion to the silicon-based negative electrode active material particles.
  • the silicon-based negative electrode active material particles for a long time, thereby being able to effectively suppress the volume expansion and side reactions of the silicon-based negative electrode active material particles for a long time, thereby effectively improving the long-term cycle performance of the secondary battery.
  • the method of the present application has mild conditions, and the cross-linking reaction occurs at a lower temperature, which can reduce the risk of disproportionation reaction of silicon-based negative electrode active material particles, thereby making the composite negative electrode active material have a high first coulombic efficiency.
  • the composite negative electrode active material prepared according to the method of the present application is applied to secondary batteries, which can effectively improve the first coulombic efficiency, energy density and cycle performance of the secondary battery.
  • 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 parts by weight, and the conductive agent is 0.05 to 4 parts by weight.
  • the amount of the silicon-based negative electrode active material is within the above-mentioned appropriate range, which can make the composite negative electrode active material have a higher theoretical gram capacity.
  • the crosslinked polymer has a suitable crosslinking density
  • the coating layer has a suitable thickness and good conductivity, which can make the composite negative electrode active material have a low volume expansion rate, a high theoretical gram capacity and good electron transport ability. Therefore, the composite negative electrode active material prepared according to the method of the present application is applied to a secondary battery, which can make the secondary battery have good cycle performance and high energy density.
  • the third aspect of the present application provides a negative electrode plate, which includes 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 includes a negative electrode active material, a binder and a conductive agent, wherein the negative electrode active material includes the composite negative electrode active material of the first aspect of the present application, or the composite negative electrode active material prepared according to the method of the second aspect of the present application.
  • the negative electrode film layer includes the composite negative electrode active material of the first aspect of the present application, or the composite negative electrode active material prepared according to the method of the second aspect of the present application, and is applied to a secondary battery, which can enable the secondary battery to have both good cycle performance and high energy density.
  • the negative electrode film layer includes 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.
  • the mass percentage of the composite negative electrode active material in the negative electrode active material is 1% to 50%.
  • the content of each component is within the above-mentioned appropriate range, which can enable the negative electrode plate to have high energy density and good electron transmission ability.
  • the negative electrode film layer includes a composite negative electrode active material, so that it can have good electron transmission ability, thereby reducing the amount of conductive agent in the negative electrode film layer, so that the negative electrode plate has both low cost and good electrochemical performance.
  • a fourth aspect of the present application provides a secondary battery, which includes the negative electrode plate of the third aspect of the present application.
  • the secondary battery of the present application includes the negative electrode sheet of the third aspect of the present application, thereby being able to have both good cycle performance and high energy density.
  • a fifth aspect of the present application provides an electrical device, which includes the secondary battery of the present application.
  • the electric device of the present application includes the secondary battery provided by the present application, and thus has at least the same advantages as the secondary battery.
  • FIG. 1 is a schematic diagram of an embodiment of a secondary battery of the present application.
  • FIG. 2 is an exploded schematic diagram of an embodiment of a secondary battery of the present application.
  • FIG. 3 is a schematic diagram of an embodiment of a device using a secondary battery as a power source according to the present application.
  • FIG. 4 is a scanning electron microscope (SEM) image of the composite negative electrode active material of Example 7 of the present application.
  • any lower limit can be combined with any upper limit to form an unambiguous range; and any lower limit can be combined with other lower limits to form an unambiguous range, and any upper limit can be combined with any other upper limit to form an unambiguous range.
  • each point or single value between the range endpoints is included in the range.
  • each point or single value can be combined as its own lower limit or upper limit with any other point or single value or with other lower limits or upper limits to form an unambiguous range.
  • the term "or” is inclusive.
  • the phrase “A or B” means “A, B, or both A and B”. More specifically, any of the following conditions satisfies the condition "A or B”: 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).
  • the gram capacity of silicon-based materials is much higher than that of carbon materials, and they are negative electrode active materials with great development potential.
  • the volume expansion of silicon-based materials during charging will have a negative impact on their own capacity and the cycle performance of secondary batteries. Therefore, how to reduce the volume expansion of silicon-based materials during charging is an urgent problem to be solved.
  • the silicon-based negative electrode active material particles are usually coated to inhibit the volume expansion of the silicon-based materials through the coating layer.
  • the coating layer disclosed in the related art has the disadvantages of low mechanical strength and low bonding strength with the silicon-based negative electrode active material, and the effect of inhibiting the volume expansion of the silicon-based material is not ideal.
  • the inventors after in-depth research and extensive experiments, provide a composite negative electrode active material and a preparation method thereof, a negative electrode plate, a secondary battery and an electrical device.
  • a first aspect of the present application provides a composite negative electrode active material, comprising: silicon-based negative electrode active material particles; and a coating layer coated on 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.
  • the present application does not limit the silicon-based negative electrode active material particles, which may include silicon-based negative electrode active material particles known in the art.
  • the silicon-based negative electrode 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 products of the above substances.
  • the coating layer may cover part of the surface of the silicon-based negative electrode active material particles, for example, the coating layer may cover more than 50% of the surface area of the silicon-based negative electrode active material particles, more than 70% of the surface area of the surface area, or more than 90% of the surface area of the surface area.
  • the coating layer may also substantially cover the entire surface of the silicon-based negative electrode active material particles.
  • the cross-linked polymer may include a polymer obtained by a cross-linking reaction of a polymer.
  • the polymer may be selected from water-soluble polymers.
  • the polymer may include one or more polymer binders.
  • the coating layer can effectively inhibit the volume expansion of the silicon-based negative electrode active material particles, thereby reducing the capacity loss during the charge and discharge cycle, so that the secondary battery has both high energy density and good cycle performance.
  • the coating layer contains a cross-linked polymer
  • the polymer flexible molecular chain of the cross-linked polymer forms a three-dimensional network by cross-linking; such a three-dimensional cross-linked network can improve the ability of the polymer to resist molecular chain slippage and deformation under stress, so that the coating layer has both high mechanical strength and good toughness.
  • the composite negative electrode active material of the present application is applied to a secondary battery.
  • the coating layer can effectively inhibit the volume expansion of the silicon-based negative electrode active material particles and reduce the contact between the silicon-based negative electrode active material and the electrolyte, thereby reducing the risk of pulverization and inactivation of the silicon-based negative electrode active material and reducing the occurrence of side reactions, thereby effectively reducing the capacity loss of the secondary battery and improving the energy density and cycle performance of the secondary battery.
  • the cross-linked polymer can interact with the silicon-based negative electrode active material particles through the cross-linked structure or specific functional groups in the molecule, 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 by chemical bonding are not easily dissolved in water again. Therefore, when the composite negative electrode active material of the present application is applied to a secondary battery, during the preparation of the slurry and the processing, storage and use of the battery, the coating layer is not easily soluble in water and has a high bonding force with the silicon-based negative electrode active material particles.
  • the silicon-based negative electrode active material particles for a long time, thereby being able to effectively suppress the volume expansion and side reactions of the silicon-based negative electrode active material particles for a long time, thereby effectively improving the long-term cycle performance of the secondary battery.
  • the volume average particle size Dv50 of the silicon-based negative electrode active material particles may be 10nm to 30 ⁇ m, for example, 10nm, 50nm, 100nm, 500nm, 1 ⁇ m, 2 ⁇ m, 5 ⁇ m, 10 ⁇ m, 15 ⁇ m, 20 ⁇ m, 25 ⁇ m, 30 ⁇ m, or within the range of any of the above values.
  • the volume average particle size Dv50 of the silicon-based negative electrode active material particles may be 1 ⁇ m to 10 ⁇ m, for example, 1 ⁇ m, 3 ⁇ m, 5 ⁇ m, 7 ⁇ m, 10 ⁇ m, or within the range of any of the above values.
  • the volume average particle size Dv50 of the silicon-based negative electrode active material particles is within the above-mentioned suitable range, on the one hand, the silicon-based negative electrode active material particles can have a low volume expansion rate, and on the other hand, the active lithium ions can have a suitable transmission path. Therefore, the composite negative electrode active material of the present application is applied to secondary batteries, which can further improve the cycle performance of secondary batteries.
  • the thickness d of the coating layer can be 20nm to 1 ⁇ m, for example, 20nm, 50nm, 100nm, 500nm, 800nm, 1 ⁇ m or in the range of any of the above values.
  • the thickness d of the coating layer can be 20nm to 200nm, for example, 20nm, 40nm, 60nm, 80nm, 100nm, 120nm, 140nm, 180nm, 200nm or in the range of any of the above values.
  • the thickness of the coating layer when the thickness of the coating layer is within the above-mentioned suitable range, it can not only have suitable mechanical strength and toughness to effectively suppress the volume expansion of the silicon-based negative electrode active material particles, but also enable the composite negative electrode active material to have a high theoretical gram capacity. Therefore, the composite negative electrode active material of the present application is applied to a secondary battery, which can not only improve the cycle performance of the secondary battery, but also allow the secondary battery to have a high energy density.
  • the thickness d of the coating layer and the volume average particle size Dv50 of the silicon active material may satisfy: 1/150 ⁇ d/Dv50 ⁇ 1/9, for example, d/Dv50 may be 1/150, 1/120, 1/100, 1/75, 1/50, 1/30, 1/9 or within the range of any of the above values.
  • 0.01 ⁇ d/Dv50 ⁇ 0.05 for example, d/Dv50 may be 0.01, 0.02, 0.03, 0.04 or 0.05.
  • the coating layer when the ratio of the thickness d of the coating layer to the volume average particle size Dv50 of the silicon active material is within the above-mentioned suitable range, the coating layer can have suitable mechanical strength and toughness, thereby effectively suppressing the volume expansion of the silicon-based negative electrode active material particles.
  • d/Dv50 when d/Dv50 is within the above-mentioned suitable range, it can be considered that the coating layer has a low mass proportion in the composite negative electrode active material, thereby enabling the composite negative electrode active material to have a high theoretical gram capacity.
  • the composite negative electrode active material of the present application is applied to secondary batteries, which can enable the secondary batteries to have both good cycle performance and high energy density.
  • the uncrosslinked polymer comprises one or more functional groups selected from carboxyl, carboxylic anhydride, hydroxyl, aldehyde, amide, ether or epoxy.
  • 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, polyethylene imine, epoxy resin, polymaleic anhydride, polyvinyl formal, and it is easy to understand that the polymer can also be selected from the modified substance of the above-mentioned substances.
  • CMC carboxymethyl cellulose
  • 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, polyethylene imine, epoxy resin, polymaleic anhydride, and it is easy to understand that the polymer can also be selected from the modified substance of the above-mentioned substances.
  • the coating formed by the cross-linked polymer containing the above functional groups can have high mechanical strength, good toughness, and high binding force with the silicon-based negative electrode active material particles. Therefore, the composite negative electrode active material of the present application is applied to secondary batteries.
  • the coating layer can effectively suppress the volume expansion of the silicon-based negative electrode active material particles for a long time and reduce the contact between the silicon-based negative electrode active material and the electrolyte, thereby reducing the risk of pulverization and inactivation of the silicon-based negative electrode active material and reducing the occurrence of side reactions, thereby effectively improving the energy density and long-term cycle performance of the secondary battery.
  • the uncrosslinked polymer comprises a crosslinking functional group
  • the crosslinking functional group is selected from one or more of a carboxyl group, a carboxylic anhydride group, a hydroxyl group, an aldehyde group, an amide group or an epoxy group
  • the crosslinked polymer is obtained by reacting the uncrosslinked polymer with a crosslinking agent via the crosslinking functional group.
  • the crosslinking functional groups may all be crosslinked, or only part of the crosslinking functional groups may be crosslinked, thereby obtaining a crosslinked polymer.
  • a crosslinking agent is a suitable compound having a plurality of crosslinking functional groups that can react with the crosslinking functional groups and bond to the polymer molecular chain. Therefore, a corresponding functional group having appropriate reactivity with the crosslinking functional group can be selected according to the given crosslinking functional group contained in the polymer, so that an appropriate crosslinking agent can be selected.
  • crosslinking agents include compounds known in the art, especially those that have been used as crosslinking agents themselves.
  • the uncrosslinked polymer can be selected from polymers containing carboxyl groups, for example, can be selected from one or more of polyacrylic acid and its modified products or carboxymethyl cellulose (CMC) and its modified products.
  • the crosslinking agent can be selected from one or more of carbodiimide crosslinking agents, aziridine crosslinking agents, carbodiimide crosslinking agents, epoxysilane crosslinking agents or blocked isocyanate crosslinking agents.
  • the uncrosslinked polymer can be selected from polymers containing carboxylic anhydride groups, for example, can be selected from one or more of polymaleic anhydride and its modified products, for example, can be selected from one or more of methyl vinyl ether-maleic anhydride copolymers.
  • the crosslinking agent can be selected from one or more of carbodiimide crosslinking agents, aziridine crosslinking agents, carbodiimide crosslinking agents, epoxysilane crosslinking agents or blocked isocyanate crosslinking agents.
  • the uncrosslinked polymer can be selected from polymers containing hydroxyl groups, for example, it can be selected from one or more of sodium alginate and its modifications, carboxymethyl cellulose (CMC) and its modifications, polyvinyl alcohol and its modifications, polyethylene glycol and its modifications, polyethylene oxide and its modifications, guar gum and its modifications, xanthan gum and its modifications, chitosan and its modifications, cyclodextrin and its modifications or gum arabic.
  • CMC carboxymethyl cellulose
  • the crosslinking agent can be selected from one or more of epoxysilane crosslinking agents, blocked isocyanate crosslinking agents, maleic anhydride, glutaraldehyde, glyoxal, glutaric anhydride, and succinic anhydride.
  • the uncrosslinked polymer can be selected from polymers containing aldehyde groups, for example, can be selected from polyvinyl formal. Accordingly, the crosslinking agent can be selected from amino-PEG4-amine.
  • the uncrosslinked polymer can be selected from polymers containing amide groups, for example, can be selected from polyacrylamide and/or polyethyleneimine.
  • the crosslinking agent can be selected from one or more of aziridine crosslinking agents, epoxysilane crosslinking agents or blocked isocyanate crosslinking agents.
  • the uncrosslinked polymer can be selected from polymers containing epoxy groups, for example, can be selected from hydrophilically modified epoxy resins (such as acrylic acid modified epoxy resins).
  • 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 dibasic acids and their anhydrides (such as maleic anhydride, phthalic anhydride, etc.).
  • the uncrosslinked polymer reacts with the corresponding crosslinking agent through the above-mentioned crosslinking functional group, and can be rapidly crosslinked on the surface of the silicon-based negative electrode active material particles under the crosslinking reaction conditions to form the coating layer.
  • the crosslinked polymer has a suitable crosslinking density and an appropriate flexible segment, so that the coating layer has high mechanical strength and good toughness. Therefore, the composite negative electrode active material of the present application is applied to secondary batteries.
  • the coating layer can effectively inhibit the volume expansion of the silicon-based negative electrode active material particles and reduce the contact between the silicon-based negative electrode active material and the electrolyte, thereby reducing the risk of pulverization and inactivation of the silicon-based negative electrode active material and reducing the occurrence of side reactions, thereby effectively reducing the capacity loss of the secondary battery and improving the energy density and cycle performance of the secondary battery.
  • the crosslinked polymer can interact with the silicon-based negative electrode active material particles through the crosslinking structure or specific functional groups in the molecule, thereby tightly adhering to the surface of the silicon-based negative electrode active material; on the other hand, the crosslinked polymer has a higher cohesive energy density, a higher molecular weight and a crosslinking structure, so that it has low solubility in water. Therefore, when the composite negative electrode active material of the present application is applied to a secondary battery, during the preparation of the slurry and the processing, storage and use of the battery, the coating layer is not easily soluble in water and has a high adhesion to the silicon-based negative electrode active material particles.
  • the silicon-based negative electrode active material particles for a long time, thereby being able to effectively suppress the volume expansion and side reactions of the silicon-based negative electrode active material particles for a long time, thereby effectively improving the long-term cycle performance of the secondary battery.
  • the weight average molecular weight Mw of the uncrosslinked polymer may be 5 ⁇ 10 4 to 1 ⁇ 10 6 .
  • the equilibrium swelling ratio of the composite negative electrode active material with water as a solvent may be 1% to 1000%.
  • the crosslinked polymer can have a higher molecular weight and a suitable crosslinking density.
  • the main chain functional groups have a higher distribution uniformity, so that the coating layer exhibits high strength, high toughness and high swelling resistance.
  • the coating layer is not easy to fall off from the surface of the silicon-based negative electrode active material particles during battery processing, storage and use, so that it can be stably coated on the surface of the silicon-based negative electrode active material particles for a long time, effectively suppressing the volume expansion of the silicon-based negative electrode active material particles, and thus can improve the energy density and long-term cycle performance of the secondary battery using the composite negative electrode active material of the present application.
  • the coating layer further comprises a conductive agent.
  • the mass ratio of the conductive agent to the cross-linked polymer is 1:220 to 1:15.
  • the present application does not limit the type of conductive agent, which can be selected from conductive agents known in the art that can be used for the negative electrode of a secondary battery.
  • the conductive agent can be selected from carbon-based materials, metal-based materials, conductive polymers, or any combination of the above substances.
  • the carbon-based material 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.
  • the metal-based material can be selected from metal powders and metal fibers.
  • the conductive polymer may include polyphenylene derivatives.
  • the composite negative electrode active material of the present application is applied to a secondary battery, which can reduce the interfacial charge transfer impedance on the surface of the negative electrode sheet, thereby improving the cycle performance of the secondary battery.
  • the conductive agent is selected from a linear conductive agent, and the aspect ratio of the linear conductive agent may be 30 to 10000, for example, 30, 50, 100, 200, 500, 1000, 3000, 5000, 8000, 10000, or within the range of any of the above values.
  • the aspect ratio of the linear conductive agent may be 100 to 5000.
  • the diameter of the conductive agent may be 0.5 nm to 100 nm, for example, 0.5 nm, 1 nm, 5 nm, 10 nm, 20 nm, 50 nm, 80 nm, 100 nm, or within the range of any of the above values.
  • the diameter of the conductive agent may be 1 nm to 20 nm, for example, 1 nm, 3 nm, 5 nm, 8 nm, 10 nm, 13 nm, 15 nm, 18 nm, 20 nm, or within the range of any of the above values.
  • the length of the conductive agent may be 300 nm to 30 ⁇ m, for example, 300 nm, 500 nm, 800 nm, 1 ⁇ m, 5 ⁇ m, 10 ⁇ m, 15 ⁇ m, 20 ⁇ m, 25 ⁇ m, 30 ⁇ m, or any range thereof.
  • the length of the conductive agent may be 1 ⁇ m to 5 ⁇ m, for example, 1 ⁇ m, 2 ⁇ m, 3 ⁇ m, 4 ⁇ m, 5 ⁇ m, or any range thereof.
  • the linear conductive agent may refer to a conductive agent having a high aspect ratio, for example, a conductive agent having an aspect ratio of 30 or more.
  • the linear conductive agent that meets the above conditions has a high mechanical strength, and is applied to the coating layer to further improve the mechanical strength of the coating layer.
  • the linear conductive agent has a high aspect ratio, which can not only play a linear toughening role in the coating layer and expand the toughening range, but also form a conductive network, thereby improving the toughness and conductivity of the coating layer. Therefore, in the composite negative electrode active material of the present application, the coating layer has high mechanical strength, high toughness and good electron transmission ability, thereby being able to inhibit the volume expansion of the silicon-based negative electrode active material particles during the charging and discharging process and improve the electron transmission ability of the composite negative electrode active material. Therefore, the composite negative electrode active material of the present application is applied to secondary batteries, which can significantly improve the cycle performance of secondary batteries.
  • the conductive agent may be selected from a one-dimensional conductive agent and/or a two-dimensional conductive agent.
  • the conductive agent can be selected from one or more of carbon nanotubes (CNT), vapor growth carbon fiber reinforcement (VGCF), and graphene.
  • CNT carbon nanotubes
  • VGCF vapor growth carbon fiber reinforcement
  • graphene graphene
  • the conductive agent selected from the above types has good conductivity and mechanical strength, and can improve the mechanical strength and conductivity of the coating layer, thereby enhancing the binding effect of the coating layer on the silicon-based negative electrode active material particles while improving the electron transmission capacity of the coating layer. Therefore, the composite negative electrode active material of the present application can have a low volume expansion rate and good electron transmission capacity, and can be applied to secondary batteries to enable the secondary batteries to have good cycle performance.
  • the powder resistivity of the composite negative electrode active material may be 0.3 ⁇ cm to 1.3 ⁇ cm, for example, 0.3 ⁇ cm, 0.5 ⁇ cm, 0.8 ⁇ cm, 1.0 ⁇ cm, 1.3 ⁇ cm or within a range of any of the above values.
  • the powder resistivity of the composite negative electrode active material may be 0.4 ⁇ cm to 0.8 ⁇ cm.
  • the silicon-based negative electrode active material particles and the coating layer have a suitable structure, and the two are closely combined together, so that the composite negative electrode active material has a low resistivity. Therefore, the composite negative electrode active material of the present application is applied to a secondary battery, which can reduce the interfacial charge transfer impedance on the surface of the negative electrode plate, thereby improving the cycle performance of the secondary battery.
  • the volume average particle size Dv50 of the silicon-based negative electrode active material particles has a well-known meaning in the art and can be measured by methods and instruments known in the art. Wherein, Dv50 means that in the volume-based particle size distribution, 50% of the particle sizes are smaller than this value.
  • the volume average particle size Dv50 of the silicon-based negative electrode active material particles can be measured with reference to the particle size distribution laser diffraction method of GB/T 19077-2016, using a laser particle size analyzer (e.g., Malvern Mastersizer 2000E, UK).
  • the thickness of the coating layer has a well-known meaning in the art and can be measured by methods and instruments known in the art. For example, it can be measured by a transmission electron microscope (TEM).
  • TEM transmission electron microscope
  • powder resistivity has a meaning known in the art and can be measured by methods and instruments known in the art, for example, by a powder resistance measuring instrument.
  • the equilibrium swelling ratio of the composite negative electrode active material with water as solvent has a well-known meaning in the art, which can represent the ratio of the volume of the composite negative electrode active material when the swelling equilibrium is reached due to the absorption of a certain amount of water to the volume before swelling.
  • the equilibrium swelling ratio of the composite negative electrode active material with water as solvent can be determined by the following steps: take an appropriate amount of the composite negative electrode active material, and use a swelling meter to measure the volume of the composite negative electrode active material as V 1 ; place the composite negative electrode active material in sufficient water and place it at 25° C.
  • the second aspect of the present application provides a method for preparing the negative composite negative electrode active material of the first aspect of the present application, which comprises the following steps S1 to S2.
  • the silicon-based negative electrode active material particles, uncrosslinked polymers, crosslinking agents and conductive agents can be selected from the silicon-based negative electrode active material particles, uncrosslinked polymers, crosslinking agents and conductive agents described in the first aspect of the present application, which will not be repeated here.
  • the above-mentioned solvent may include an organic solvent or an inorganic solvent that can be used to uniformly disperse the silicon-based negative electrode active material particles, the uncrosslinked polymers, the crosslinking agents and the optional conductive agents.
  • the solvent can be water.
  • the uncrosslinked polymers, crosslinking agents and conductive agents can be uniformly dispersed in the solvent to form a slurry layer wrapped around the surface of the silicon-based negative electrode active material particles.
  • step S2 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 generating a crosslinked polymer having a suitable crosslinking density and appropriate flexible chain segments, which can be tightly coated on the surface of the silicon-based negative electrode active material particles to form a coating layer.
  • the method of the present application is to mix the silicon-based negative electrode active material particles, the uncrosslinked polymer, the crosslinking agent and the optional conductive agent in a solvent so that the uncrosslinked polymer, the crosslinking agent and the optional conductive agent are uniformly coated on the surface of the silicon-based negative electrode active material particles; then the uncrosslinked polymer is crosslinked under the action of the crosslinking agent at a certain temperature, thereby forming a coating layer on the surface of the silicon-based negative electrode active material particles.
  • the polymer flexible molecular chains of the crosslinked polymer form a three-dimensional network through crosslinking; such a three-dimensional crosslinked network can improve the ability of the polymer to resist molecular chain slippage and deformation under stress, so that the coating layer has both high mechanical strength and good toughness. Therefore, the composite negative electrode active material prepared according to the method of the present application is applied to a secondary battery.
  • the coating layer can effectively inhibit the volume expansion of the silicon-based negative electrode active material particles and reduce the contact between the silicon-based negative electrode active material and the electrolyte, thereby reducing the risk of pulverization and inactivation of the silicon-based negative electrode active material and the occurrence of side reactions, thereby effectively reducing the capacity loss of the secondary battery and improving the energy density and cycle performance of the secondary battery.
  • the cross-linked polymer can interact with the silicon-based negative electrode active material particles through the cross-linking structure or specific functional groups in the molecule, 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 not easily dissolved in water again. Therefore, when the composite negative electrode active material prepared according to the method of the present application is applied to a secondary battery, during the preparation of the slurry and the processing, storage and use of the battery, the coating layer is not easily soluble in water and has a high adhesion to the silicon-based negative electrode active material particles.
  • the silicon-based negative electrode active material particles for a long time, thereby being able to effectively suppress the volume expansion and side reactions of the silicon-based negative electrode active material particles for a long time, thereby effectively improving the long-term cycle performance of the secondary battery.
  • the method of the present application has mild conditions, and the cross-linking reaction occurs at a lower temperature, which can reduce the risk of disproportionation reaction of silicon-based negative electrode active material particles, thereby making the composite negative electrode active material have a high first coulombic efficiency.
  • the composite negative electrode active material prepared according to the method of the present application is applied to secondary batteries, which can effectively improve the first coulombic efficiency, energy density and cycle performance of the secondary battery.
  • 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 parts by weight
  • the conductive agent may be 0.05 to 4 parts by weight.
  • step S1 the amount of the silicon-based negative electrode active material is within the above-mentioned suitable range, which can make the composite negative electrode active material have a higher theoretical gram capacity.
  • the amount of the uncrosslinked polymer, crosslinking agent and conductive agent is within the above-mentioned suitable range, the crosslinked polymer has a suitable crosslinking density, and the coating layer has a suitable thickness and good conductivity, which can make the composite negative electrode active material have a low volume expansion rate, a high theoretical gram capacity and good electron transfer ability.
  • the composite negative electrode active material prepared according to the method of the present application is applied to a secondary battery, which can make the secondary battery have good cycle performance and high energy density.
  • the third aspect of the present application provides 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 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 the present application, or the composite negative electrode active material prepared according to the method of the second aspect of the present application.
  • the negative electrode active material may also include other negative electrode active materials for batteries known in the art.
  • 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.
  • artificial graphite natural graphite
  • soft carbon soft carbon
  • hard carbon hard carbon
  • tin-based materials tin-based materials
  • lithium titanate lithium titanate
  • present application is not limited to these materials, and other traditional 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.
  • the binder may be selected from binders for negative electrodes known in the art.
  • 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).
  • SBR styrene-butadiene rubber
  • PAA polyacrylic acid
  • PAAS sodium polyacrylate
  • PAM polyacrylamide
  • PVA polyvinyl alcohol
  • SA sodium alginate
  • PMAA polymethacrylic acid
  • CMCS carboxymethyl chitosan
  • the conductive agent may be selected from conductive agents for negative electrodes known in the art.
  • 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.
  • the negative electrode film layer includes the composite negative electrode active material of the first aspect of the present application, or the composite negative electrode active material prepared according to the method of the second aspect of the present application, and is applied to a secondary battery, which can enable the secondary battery to have both good cycle performance and high energy density.
  • the negative electrode film layer may include 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.
  • the mass percentage of the composite negative electrode active material in the negative electrode active material can be 1% to 50%, for example, 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50% or within the range of any of the above values.
  • the content of each component is within the above-mentioned suitable range, which can make the negative electrode plate have high energy density and good electron transmission ability.
  • the negative electrode film layer includes a composite negative electrode active material, which can make the negative electrode plate have good electron transmission ability, thereby reducing the amount of conductive agent in the negative electrode film layer, so that the negative electrode plate has both low cost and good electrochemical performance.
  • the negative electrode film layer of the present application may also optionally include other additives, such as a thickener (such as sodium carboxymethyl cellulose (CMC-Na)) and the like.
  • a thickener such as sodium carboxymethyl cellulose (CMC-Na)
  • CMC-Na sodium carboxymethyl cellulose
  • the negative electrode current collector may be a metal foil or a composite current collector (a metal material may be disposed on a polymer substrate to form a composite current collector).
  • a metal material may be disposed on a polymer substrate to form a composite current collector.
  • copper foil may 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 may 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.).
  • a metal material copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.
  • a polymer material substrate such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.
  • the negative electrode film layer can be arranged on one side of the negative electrode current collector, or can be arranged on both sides of the negative electrode current collector at the same time.
  • the negative electrode current collector has two opposite sides in its own thickness direction, and the negative electrode film layer is arranged on any one side or both sides of the two opposite sides of the negative electrode current collector.
  • the negative electrode plate does not exclude other additional functional layers in addition to the negative electrode film layer.
  • the negative electrode plate described in the present application may also include a conductive primer layer (e.g., composed of a conductive agent and a binder) disposed between the negative electrode current collector and the negative electrode film layer.
  • the negative electrode plate described in the present application also includes a protective layer covering the surface of the negative electrode film layer.
  • Secondary batteries also known as rechargeable batteries or storage batteries, refer to batteries that can continue to be used by activating the active materials by charging after the battery is discharged.
  • a secondary battery includes a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte.
  • active ions such as lithium ions
  • the separator is arranged between the positive electrode sheet and the negative electrode sheet, mainly to prevent the positive and negative electrodes from short-circuiting, while allowing ions to pass through.
  • the electrolyte is between the positive electrode sheet and the negative electrode sheet, mainly to conduct active ions.
  • the negative electrode sheet of the secondary battery of the present application includes the negative electrode sheet of the fourth aspect of the present application.
  • the embodiments of the negative electrode sheet have been described and illustrated in detail above, and will not be repeated here. It can be understood that the secondary battery of the present application can achieve the beneficial effects of any of the above embodiments of the negative electrode sheet of the present application.
  • the positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector and including a positive electrode active material.
  • the positive electrode 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 electrode current collector.
  • the positive electrode active material may adopt the positive electrode active material for secondary batteries known in the art.
  • the positive electrode active material may include one or more of lithium transition metal oxides, lithium phosphates containing olivine structures, and their respective modified compounds.
  • lithium transition metal oxides may include, but are not limited to, one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and their modified compounds.
  • lithium phosphates containing olivine structures may include, but are not limited to, one or more of lithium iron phosphate, a composite material of lithium iron phosphate and carbon, lithium manganese phosphate, a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, a composite material of lithium iron manganese phosphate and carbon, and their respective modified compounds.
  • the present 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.
  • the positive electrode film layer generally comprises a positive electrode active material and an optional binder and an optional conductive agent, and is generally formed by coating a positive electrode slurry, drying, and cold pressing.
  • the positive electrode slurry is generally formed by dispersing the positive electrode active material and the optional conductive agent and binder in a solvent and stirring them uniformly.
  • the solvent may be N-methylpyrrolidone (NMP).
  • the binder for the positive electrode film layer may include one or more of polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE).
  • PVDF polyvinylidene fluoride
  • PTFE polytetrafluoroethylene
  • the conductive agent used for the positive electrode film layer may include one or more of superconducting carbon, carbon black (eg, acetylene black, Ketjen black), carbon dots, carbon nanotubes, graphene, and carbon nanofibers.
  • carbon black eg, acetylene black, Ketjen black
  • carbon dots carbon nanotubes, graphene, and carbon nanofibers.
  • the positive electrode current collector may be a metal foil or a composite current collector (a metal material may be disposed on a polymer substrate to form a composite current collector).
  • the positive electrode current collector may be an aluminum foil.
  • the secondary battery of the present application has no specific restrictions on the type of electrolyte, which can be selected according to needs.
  • the electrolyte can be selected from at least one of a solid electrolyte and a liquid electrolyte (ie, an electrolyte solution).
  • the electrolyte is an electrolyte solution, which includes an electrolyte salt and a solvent.
  • the electrolyte salt may be selected from one or more of LiPF 6 (lithium hexafluorophosphate), LiBF 4 (lithium tetrafluoroborate), LiClO 4 (lithium perchlorate), LiAsF 6 (lithium hexafluoroarsenate), LiFSI (lithium bisfluorosulfonyl imide), LiTFSI (lithium bistrifluoromethanesulfonyl imide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluorooxalatoborate), LiBOB (lithium dioxalatoborate), LiPO 2 F 2 (lithium difluorophosphate), LiDFOP (lithium difluorobisoxalatophosphate) and LiTFOP (lithium tetrafluorooxalatophosphate).
  • LiPF 6 lithium hexafluorophosphate
  • LiBF 4 lithium tetrafluoroborate
  • the solvent can be selected from one or more of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), ethyl methyl sulfone (EMS) and diethyl sulfone (
  • the electrolyte may also optionally include additives.
  • the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, additives that improve battery high temperature performance, additives that improve battery low temperature performance, etc.
  • Secondary batteries using electrolytes and some secondary batteries using solid electrolytes also include a separator.
  • the separator is arranged between the positive electrode plate and the negative electrode plate to play an isolating role.
  • the present application has no particular restrictions on the type of separator, and any known porous structure separator with good chemical stability and mechanical stability can be selected.
  • the material of the separator 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 are the same or different.
  • the positive electrode sheet, the negative electrode sheet, and the separator may be formed into an electrode assembly by a winding process or a lamination process.
  • the secondary battery may include an outer package, which may be used to encapsulate the electrode assembly and the electrolyte.
  • the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc.
  • the outer packaging of the secondary battery can also be a soft package, such as a bag-type soft package.
  • the material of the soft package can be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), polybutylene succinate (PBS), etc.
  • FIG1 is a secondary battery 5 of a square structure as an example.
  • the outer package may include a shell 51 and a cover plate 53.
  • the shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity.
  • the shell 51 has an opening connected to the receiving cavity, and the cover plate 53 is used to cover the opening to close the receiving cavity.
  • the positive electrode sheet, the negative electrode sheet and the isolation film may form an electrode assembly 52 through a winding process or a lamination process.
  • the electrode assembly 52 is encapsulated in the receiving cavity.
  • the electrolyte is infiltrated in the electrode assembly 52.
  • the number of electrode assemblies 52 contained in the secondary battery 5 may be one or more, which can be adjusted according to demand.
  • the present application also provides an electrical device, which includes the secondary battery of the present application.
  • the secondary battery can be used as a power source for the electrical device, and can also be used as an energy storage unit for the electrical device.
  • the electrical device can be, but is not limited to, a mobile device (such as a mobile phone, a laptop computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship and a satellite, an energy storage system, etc.
  • FIG3 is an example of an electric device.
  • the electric device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle.
  • a battery pack or battery module including the secondary battery of the present application may be used.
  • the electric device may be a mobile phone, a tablet computer, a notebook computer, etc.
  • the electric device is usually required to be light and thin, and a secondary battery may be used as a power source.
  • the composite negative electrode active material precursor is placed in an oven at a temperature of T°C, so that the uncrosslinked polymer is crosslinked on the surface of the silicon particles under the action of the corresponding crosslinking agent, thereby obtaining the composite negative electrode active material.
  • the preparation parameters such as Dv50 of silicon particles, type of uncrosslinked polymer and its weight average molecular weight Mw, type of conductive agent, diameter d 1 of the conductive agent, length l 1 of the conductive agent, aspect ratio l 1 /d 1 , m 1 , m 2 , m 3 and T of the conductive agent are shown in Table 1.
  • the crosslinking agent corresponding to polyvinyl alcohol is a blocked isocyanate crosslinking agent;
  • the crosslinking agent corresponding to polyacrylamide is an epoxysilane crosslinking agent,
  • the crosslinking agent corresponding to polyacrylic acid is a polyaziridine crosslinking agent;
  • the crosslinking agent corresponding to epoxy resin is diethylenetriamine;
  • the crosslinking agent corresponding to polymaleic anhydride is a carbodiimide crosslinking agent;
  • the crosslinking agent corresponding to polyvinyl formal is amino-PEG4-amine.
  • Uncoated silicon particles are used directly.
  • the composite negative electrode active material or silicon particles are dispersed in an ethanol solvent and dropped onto the microgate support film, and the coating layer thickness d is tested using a high-resolution transmission electron microscope.
  • the value of d/Dv50 can be calculated.
  • Example 1 6.67nm 1/150 0.92 ⁇ cm 16.9%
  • Example 2 10nm 1/100 0.98 ⁇ cm 13.1%
  • Example 3 20nm 1/50 1.04 ⁇ cm 9.3%
  • Example 4 100nm 1/10 1.30 ⁇ cm 5.6%
  • Example 5 100nm 1/10 0.79 ⁇ cm 16.5%
  • Example 6 100nm 1/10 0.71 ⁇ cm 16.7%
  • Example 7 100nm 1/10 0.64 ⁇ cm 16.3%
  • Example 8 100nm 1/10 0.56 ⁇ cm 17.1%
  • Example 9 100nm 1/10 0.49 ⁇ cm 16.8%
  • Example 10 100nm 1/10 0.42 ⁇ cm 17.2%
  • Embodiment 11 100nm 1/10 0.38 ⁇ cm 16.4%
  • Example 12 100nm 1/10 0.35 ⁇ cm 17.0%
  • Example 13 100nm 1/10 0.33 ⁇ cm 17.1%
  • Embodiment 14 100nm 1/10 0.30 ⁇ cm 16.8% Embodiment 15 100nm 1/10 0.31 ⁇ cm 16.4%
  • Example 16 100
  • the negative electrode slurry is evenly coated on the Cu foil, and after oven drying and cold pressing, the negative electrode sheet is obtained.
  • the negative electrode active materials of Examples 1 to 31 and Comparative Example 2 are a mixture of 15wt% of a composite negative electrode active material and 85wt% of graphite
  • the negative electrode active material of Comparative Example 1 is a mixture of 15wt% of silicon particles and 85wt% of graphite.
  • the other negative electrode sheet preparation parameters of each embodiment and comparative example are the same.
  • the positive electrode active material LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811), the conductive agent Super P, the binder polyvinylidene fluoride (PVDF), and the dispersant are fully stirred and mixed in a proper amount of solvent N-methylpyrrolidone (NMP) in 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, a positive electrode sheet is obtained.
  • NMP N-methylpyrrolidone
  • Polypropylene separator is used.
  • the positive electrode sheet, the separator, and the negative electrode sheet are stacked and wound in order to obtain an electrode assembly; the electrode assembly is added to an outer package, the above-mentioned electrolyte is added, and a secondary battery is obtained after packaging, standing, forming, aging and other processes.
  • the formed secondary battery was first discharged at a constant current rate of 1/3C (DC) to 2.8V and allowed to stand for 10min; then charged at a constant current rate of 1/3C (CC) to 4.2V, then charged at a constant voltage rate of 4.2V (CV) to a current of 0.05C, allowed to stand for 10min, and the charging capacity was recorded; then discharged at a constant current rate of 1/3C (DC) to 2.8V, and the discharge capacity was recorded.
  • DC 1/3C
  • CV constant voltage rate of 4.2V
  • 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, left for 5 minutes, and then discharged to 2.8V at 1/3C.
  • the obtained capacity is recorded as the initial capacity C 0 .
  • the composite negative electrode active material of the present application is applied to secondary batteries, which can effectively and permanently inhibit the volume expansion and side reactions of silicon-based negative electrode active material particles, thereby effectively improving the long-term cycle performance of secondary batteries.
  • the composite negative electrode active material containing a conductive agent in the coating layer has a lower powder resistivity.
  • the powder resistivity decreases, and the coating layer has better mechanical strength, which can better inhibit the expansion of the silicon material.
  • the secondary battery not only has a higher first coulomb efficiency, but also has a higher 100-cycle cycle capacity retention rate.
  • the coating layer contains other one-dimensional conductive agents, such as VGCF and graphene
  • the coating layer can also have good electron transmission ability and mechanical strength. Therefore, the composite negative electrode active material can have a low volume expansion rate and good electron transmission ability, and can be applied to secondary batteries to enable the secondary batteries to have good long-term cycle performance.
  • the silicon particles in Comparative Example 1 do not have a coating layer on their surfaces, and the corresponding 100-cycle capacity retention rate of the secondary battery is much lower than that of Examples 1 to 31.
  • the silicon particles in Comparative Example 2 have a coating layer on their surfaces, the electrochemical activity of the silicon particles may be reduced due to excessively high crosslinking temperature during the preparation of the composite negative electrode active material.
  • the secondary battery in Comparative Example 2 not only has a reduced first coulombic efficiency, but also has a 100-cycle capacity retention rate lower than that of Examples 1 to 31.

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Abstract

一种复合负极活性材料及其制备方法、负极极片、二次电池及用电装置，复合负极活性材料包括硅基负极活性材料颗粒，以及包覆于硅基负极活性材料颗粒的至少部分表面的包覆层，其中，包覆层包含经交联的聚合物。复合负极活性材料应用于二次电池，能够使得二次电池具备良好的循环性能和高能量密度。

Description

复合负极活性材料及其制备方法、负极极片、二次电池及用电装置 技术领域
本申请属于二次电池技术领域，具体涉及一种复合负极活性材料及其制备方法、负极极片、二次电池及用电装置。
背景技术
二次电池依靠活性离子在正极和负极之间往复脱嵌来进行充电和放电，其具有能量密度高、循环寿命长，以及无污染、无记忆效应等突出特点。因此，二次电池作为清洁能源，已由电子产品逐渐普及到电动汽车等大型装置领域，以适应环境和能源的可持续发展战略。由此，也对二次电池的性能等提出了更高的要求。
能量密度被认为是制约当前二次电池发展的最大瓶颈。基于此，人们围绕高容量负极活性材料开展了大量研究。其中，硅基材料的克容量远高于碳材料，其理论克容量是石墨的数倍。
然而，硅基材料存在严重的体积效应，在充电过程中会产生巨大的体积膨胀。由此，在充放电过程中，硅基材料极易发生破碎粉化，表面难以形成稳定的固体电解质界面(solid electrolyte interface，SEI)膜，从而导致二次电池的容量衰减快，循环性能差。
发明内容
本申请的目的在于提供一种复合负极活性材料及其制备方法、负极极片、二次电池及用电装置，旨在使二次电池在具有较高能量密度的前提下，兼具良好的循环性能。
为了实现上述发明目的，本申请第一方面提供复合负极活性材料，其包括硅基负极活性材料颗粒；以及包覆于所述硅基负极活性材料颗粒的至少部分表面的包覆层，其中，所述包覆层包含经交联的聚合物。
并非意在受限于任何理论或解释，当包覆层包含经交联的聚合物时，该经交联的聚合物的聚合物柔性分子链通过交联形成三维网络；这样的三维交联网络能够提高聚合物抵抗在应力下分子链滑移、变形的能力，从而使得包覆层兼具高机械强度和良好的韧性。由此，本申请的复合负极活性材料应用于二次电池，在充放电循环过程中，包覆层能够有效抑制硅基负极活性材料颗粒的体积膨胀，并减少硅基负极活性材料与电解液的接触，从而能够降低硅基负极活性材料粉化失活的风险、减少副反应的发生，进而有效减少二次电池的容量损失、提升二次电池的能量密度和循环性能。此外，一方面，经交联的聚合物可通过交联结构或者分子中的特定官能团与硅基负极活性材料颗粒相互作用，从而紧密地附着于硅基负极活性材料表面；另一方面，经交联的聚合物的内聚能密度提高，且通过化学键合形成三维交联网络的聚合物分子不易再溶解到水中。由此，本申请的复合负极活性材料应用于二次电池时，在制备浆料及电池的加工、存储和使用过程中， 包覆层不易溶于水，且与硅基负极活性材料颗粒具有高粘结力，因此能够长期、稳定地包覆于硅基负极活性材料颗粒的表面，从而能够长期、有效地抑制硅基负极活性活性材料颗粒的体积膨胀和副反应的发生，进而有效提升二次电池的长期循环性能。
在本申请任意实施方式中，所述硅基负极活性材料颗粒的体积平均粒径Dv50为10nm～30μm，可选为1μm～10μm。当硅基负极活性材料颗粒的体积平均粒径Dv50在上述合适的范围内时，一方面能够使得硅基负极活性材料颗粒具有低体积膨胀率，另一方面能够使得活性锂离子具有合适的传输路径。由此，本申请的复合负极活性材料应用于二次电池，能够进一步提升二次电池的循环性能。
在本申请任意实施方式中，所述包覆层的厚度d为20nm～1μm，可选为20nm～200nm。当包覆层的厚度在上述合适的范围内时，不仅能够具有合适的机械强度和韧性有效地抑制硅基负极活性材料颗粒的体积膨胀，而且能够使得复合负极活性材料具备高理论克容量。由此，本申请的复合负极活性材料应用于二次电池，不仅能够提升二次电池的循环性能，而且能够允许二次电池具备高能量密度。
在本申请任意实施方式中，所述包覆层的厚度d与所述硅活性材料的粒径体积平均粒径Dv50满足：1/150≤d/Dv50≤1/9，可选地，0.01≤d/Dv50≤0.05。当d/Dv50之比在上述合适的范围内时，不仅能够使得包覆层具有合适的机械强度和韧性，而且能够使得复合负极活性材料具有高理论克容量。由此，本申请的复合负极活性材料应用于二次电池，能够使得二次电池兼具良好的循环性能和高能量密度
在本申请任意实施方式中，未交联的所述聚合物包含选自羧基、羧酸酐基、羟基、醛基、酰胺基、醚基或环氧基中的一种或几种的官能团。可选地，未交联的所述聚合物选自聚丙烯酸、海藻酸钠、羧甲基纤维素、阿拉伯胶、聚乙烯醇、聚乙二醇、聚氧化乙烯、瓜尔豆胶、黄原胶、壳聚糖、环糊精、聚丙烯酰胺、聚乙烯亚胺、环氧树脂、聚马来酸酐、聚乙烯醇缩甲醛中的一种或几种。更可选地，所述聚合物选自聚乙烯醇、海藻酸钠、阿拉伯胶、聚乙二醇、聚氧化乙烯、瓜尔豆胶、黄原胶、壳聚糖、环糊精、聚丙烯酰胺、聚乙烯亚胺、环氧树脂、聚马来酸酐中的一种或几种。包含上述官能团的聚合物经交联后形成的包覆层能够兼具高机械强度、良好的韧性，以及与硅基负极活性材料颗粒之间的高结合力。由此，本申请的复合负极活性材料应用于二次电池，在充放电循环过程中，包覆层能够长期且有效抑制硅基负极活性材料颗粒的体积膨胀，并减少硅基负极活性材料与电解液的接触，从而能够降低硅基负极活性材料粉化失活的风险、减少副反应的发生，进而有效提升二次电池的能量密度和长期循环性能。
在本申请任意实施方式中，未交联的所述聚合物包含交联官能团，所述交联官能团选自羧基、羧酸酐基、羟基、醛基、酰胺基或环氧基中的一种或几种，所述经交联的聚合物由未交联的所述聚合物经由所述交联官能团与交联剂反应得到。交联的聚合物通过上述交联官能团与对应的交联剂反应，可在交联反应条件下，于硅基负极活性材料颗粒表面迅速交联形成所述包覆层。该包覆层不仅具有高机械强度、良好的韧性，而且不易溶于水，并与硅基负极活性材料颗粒具有高粘结力。因此，该包覆层能够长期、稳定地包覆于硅基负极活性材料颗粒的表面，从而能够长期、有效地抑制硅基负极活性活性材料颗粒的体积膨胀和副反应的发生，进而有效提升二次电池的长期循环性能。
在本申请任意实施方式中，未交联的所述聚合物的重均分子量Mw为5×10 4～1×10 6；所述复合负极活性材料以水为溶剂的平衡溶胀比为1％～1000％。当未交联的所述聚合物的重均分子量在上述合适的范围内，且复合负极活性材料在水中的平衡溶胀比在上述合适的范围内时，经交联的聚合物能够具有较高的分子量和合适的交联密度。由此， 包覆层在电池加工、存储及使用过程中，均不易从硅基负极活性材料颗粒表面脱落，从而能够长期、稳定地包覆于硅基负极活性材料颗粒表面，有效地抑制硅基负极活性材料颗粒的体积膨胀，进而能够提升应用本申请复合负极活性材料的二次电池的能量密度和长期循环性能。
在本申请任意实施方式中，所述包覆层中还包括导电剂，可选地，所述导电剂与所述经交联的聚合物的质量之比为1:220～1:15。在包覆层中包括适量的导电剂，能够有效提升包覆层的电子传输能力，从而提升复合负极活性材料的电子传输能力。由此，本申请的复合负极活性材料应用于二次电池，能够降低负极极片表面的界面电荷转移阻抗，从而提升二次电池的循环性能。
在本申请任意实施方式中，所述导电剂选自线性导电剂，所述线性导电剂的长径比为30～10000，可选为100～5000；所述导电剂的直径为0.5nm～100nm，可选为1nm至20nm；所述导电剂的长度为300nm～30μm，可选为1μm～5μm。满足上述条件的线性导电剂具有较高的机械强度，应用于包覆层中，能够进一步提升包覆层的机械强度。此外，线性导电剂具有高长径比，不仅能够在包覆层中发挥线性增韧作用，扩大增韧范围，而且能够形成导电网络，从而提升包覆层的韧性以及导电性能。由此，本申请的复合负极活性材料中，包覆层具有高机械强度、高韧性以及良好的电子传输能力，从而能够抑制硅基负极活性材料颗粒在充放电过程中的体积膨胀，提升复合负极活性材料的电子传输能力。因此，本申请的复合负极活性材料应用于二次电池，能够显著提升二次电池的循环性能。
在本申请任意实施方式中，所述导电剂选自碳纳米管、气相生长碳纤维增强体、石墨烯中的一种或几种。选自上述种类的导电剂具有良好的导电性和机械强度，能够提升包覆层的机械强度和导电性能，从而能够在增强包覆层对硅基负极活性材料颗粒的束缚作用的同时，提升包覆层的电子传输能力。由此，本申请的复合负极活性材料能够具有低体积膨胀率和良好的电子传输能力，应用于二次电池，能够使得二次电池具备良好的循环性能。
在本申请任意实施方式中，所述复合负极活性材料的粉末电阻率为0.3Ω·cm～1.3Ω·cm，可选为0.4Ω·cm～0.8Ω·cm。本申请的复合负极活性材料中，硅基负极活性材料颗粒、包覆层具有合适的结构，且二者紧密地结合在一起，从而使得复合负极活性材料具备低电阻率。由此，本申请的复合负极活性材料应用于二次电池，能够降低负极极片表面的界面电荷转移阻抗，从而提升二次电池的循环性能。
本申请第二方面提供一种用于制备本申请第一方面的复合负极活性材料的方法，包括如下步骤S1～S2。
S1，将所述硅基负极活性材料颗粒、未交联的所述聚合物、交联剂以及可选的导电剂在溶剂中混合均匀，从而得到复合负极活性材料前驱体。
S2，将所述复合负极活性材料前驱体置于30℃～250℃、可选为60℃～200℃的环境中，以使未交联的所述聚合物在所述交联剂的作用下，于所述硅基负极活性材料颗粒的表面发生交联，从而得到所述复合负极活性材料。
并非意在受限于任何理论或解释，本申请的方法，通过将所述硅基负极活性材料颗粒、未交联的所述聚合物、交联剂以及可选的导电剂在溶剂中混合均，使得未交联的 所述聚合物、交联剂以及可选的导电剂均匀地包覆于硅基负极活性材料颗粒表面；再在一定温度下使未交联的所述聚合物在交联剂的作用下发生交联，从而在硅基负极活性材料颗粒表面形成包覆层。该包覆层中，经交联的聚合物的聚合物柔性分子链通过交联形成三维网络；这样的三维交联网络能够提高聚合物抵抗在应力下分子链滑移、变形的能力，从而使得包覆层兼具高机械强度和良好的韧性。由此，根据本申请的方法制备的复合负极活性材料应用于二次电池，在充放电循环过程中，包覆层能够有效抑制硅基负极活性材料颗粒的体积膨胀，并减少硅基负极活性材料与电解液的接触，从而能够降低硅基负极活性材料粉化失活的风险、减少副反应的发生，进而有效减少二次电池的容量损失、提升二次电池的能量密度和循环性能。此外，一方面，经交联的聚合物可通过交联结构或者分子中的特定官能团与硅基负极活性材料颗粒相互作用，从而紧密地附着于硅基负极活性材料表面；另一方面，经交联的聚合物的内聚能密度提高，且通过化学键合形成三维交联网络的聚合物分子不易再溶解到水中。由此，根据本申请方法制备的复合负极活性材料应用于二次电池时，在制备浆料及电池的加工、存储和使用过程中，包覆层不易溶于水，且与硅基负极活性材料颗粒具有高粘结力，因此能够长期、稳定地包覆于硅基负极活性材料颗粒的表面，从而能够长期、有效地抑制硅基负极活性活性材料颗粒的体积膨胀和副反应的发生，进而有效提升二次电池的长期循环性能。
此外，本申请的方法条件温和，交联反应在较低的温度下发生，能够降低硅基负极活性材料颗粒发生歧化反应的风险，从而使得复合负极活性材料具有高首次库伦效率。根据本申请的方法制备的复合负极活性材料应用于二次电池，能够有效提升二次电池的首次库伦效率、能量密度以及循环性能。
在本申请任意实施方式中，步骤S1中，所述硅基负极活性材料颗粒为100重量份，未交联的所述聚合物为1重量份～10重量份，所述交联剂为0.001重量份～1重量份，所述导电剂为0.05重量份～4重量份。步骤S1中，硅基负极活性材料的用量在上述合适的范围内，能够使得复合负极活性材料具有较高的理论克容量。当未交联的所述聚合物、交联剂和导电剂的用量在上述合适的范围内时，经交联的聚合物具有合适的交联密度，且包覆层具有合适的厚度和良好的导电性，能够使得复合负极活性材料具有低体积膨胀率、高理论克容量和良好的电子传输能力。由此，根据本申请的方法制备的复合负极活性材料应用于二次电池，能够使得二次电池具备良好的循环性能和高能量密度。
本申请第三方面提供一种负极极片，其包括负极集流体以及位于所述负极集流体至少一个表面上的负极膜层，所述负极膜层包括负极活性材料、粘结剂以及导电剂，其中，所述负极活性材料包括本申请第一方面的复合负极活性材料，或者根据本申请第二方面的方法制备的复合负极活性材料。
本申请的负极极片中，负极膜层包括本申请第一方面的复合负极活性材料，或者根据本申请第二方面的方法制备的复合负极活性材料，应用于二次电池，能够使得二次电池兼具良好的循环性能和高能量密度。
在本申请任意实施方式中，以总质量为100％计，所述负极膜层包括93.5％～97％的所述负极活性材料、2.0％～5.0％的所述粘结剂以及0.5％～1.5％的所述导电剂。可选地，所述复合负极活性材料在所述负极活性材料中的质量百分含量为1％～50％。各组份的含量在上述合适的范围内，能够使得负极极片具备高能量密度和良好的电子传输能力。特别 地，负极膜层中包括复合负极活性材料，从而能够具有良好的电子传输能力，由此，能够降低负极膜层中导电剂的用量，从而使得负极极片兼具低成本和良好的电化学性能。
本申请第四方面提供一种二次电池，其包括本申请第三方面的负极极片。
本申请的二次电池包括本申请第三方面的负极极片，由此能够兼具良好的循环性能和高能量密度。
本申请第五方面提供一种用电装置，其包括本申请的二次电池。
本申请的用电装置包括本申请提供的二次电池，因而至少具有与所述二次电池相同的优势。
附图说明
为了更清楚地说明本申请实施例的技术方案，下面将对本申请实施例中所需要使用的附图作简单地介绍，显而易见地，下面所描述的附图仅仅是本申请的一些实施方式，对于本领域普通技术人员来讲，在不付出创造性劳动的前提下，还可以根据附图获得其他的附图。
图1是本申请的二次电池的一实施方式的示意图。
图2是本申请的二次电池的一实施方式的分解示意图。
图3是本申请的二次电池用作电源的装置的一实施方式的示意图。
图4是本申请实施例7的复合负极活性材料的扫描电子显微镜(SEM)图。
具体实施方式
为了使本申请的发明目的、技术方案和有益技术效果更加清晰，以下结合实施例对本申请进行进一步详细说明。应当理解的是，本说明书中描述的实施例仅仅是为了解释本申请，并非为了限定本申请。
为了简便，本文仅明确地公开了一些数值范围。然而，任意下限可以与任何上限组合形成未明确记载的范围；以及任意下限可以与其它下限组合形成未明确记载的范围，同样任意上限可以与任意其它上限组合形成未明确记载的范围。此外，尽管未明确记载，但是范围端点间的每个点或单个数值都包含在该范围内。因而，每个点或单个数值可以作为自身的下限或上限与任意其它点或单个数值组合或与其它下限或上限组合形成未明确记载的范围。
在本文的描述中，需要说明的是，除非另有说明，“以上”、“以下”为包含本数，“一种或几种”中“几种”的含义是两种或两种以上。
在本文的描述中，除非另有说明，术语“或”是包括性的。举例来说，短语“A或B”表示“A，B，或A和B两者”。更具体地，以下任一条件均满足条件“A或B”：A为真(或存在)并且B为假(或不存在)；A为假(或不存在)而B为真(或存在)；或A和B都为真(或存在)。
应理解，术语“第一”、“第二”、等之类的关系术语仅仅用来将一个实体或者操作与另一个实体或操作区分开来，而不一定要求或暗示这些实体或操作之间存在任何实际的关系或顺序。
本申请的上述发明内容并不意欲描述本申请中的每个公开的实施方式或每种实现方式。如下描述更具体地举例说明示例性实施方式。在整篇申请中的多处，通过一系列实施例提供了指导，这些实施例可以以各种组合形式使用。在各个实例中，列举仅作为代表性组，不应解释为穷举。
如背景技术所述，硅基材料的克容量远高于碳材料，是极具发展潜力的负极活性材料，但是，硅基材料在充电过程中产生的体积膨胀会对其本身的容量发挥和二次电池的循环性能产生负面影响。因此，如何降低硅基材料在充电过程中的体积膨胀是当下亟待解决的问题。
相关技术中，为了降低硅基材料的体积膨胀，多是对硅基负极活性材料颗粒进行包覆，通过包覆层抑制硅基材料的体积膨胀。然而，发明人经研究发现，相关技术所披露的包覆层存在机械强度低、与硅基负极活性材料结合力低等缺点，对硅基材料体积膨胀的抑制效果并不理想。
鉴于此，发明人经深入研究与大量实验，提供了一种复合负极活性材料及其制备方法、负极极片、二次电池及用电装置。
复合负极活性材料
本申请的第一方面提供一种复合负极活性材料，其包括：硅基负极活性材料颗粒；以及包覆于所述硅基负极活性材料颗粒的至少部分表面的包覆层，其中，所述包覆层包含经交联的聚合物。
本申请对硅基负极活性材料颗粒不作限制，其可以包括本领域已知的硅基负极活性材料颗粒。在一些实施方式中，硅基负极活性材料颗粒可选自硅颗粒、硅氧复合物颗粒、硅碳复合物颗粒、硅合金颗粒或者上述物质的改性物中的一种或几种。
上述包覆层可以覆盖硅基负极活性材料颗粒的部分表面，例如，包覆层可以覆盖硅基负极活性材料颗粒的50％表面积以上的表面，70％表面积以上的表面或90％表面积以上的表面。上述包覆层也可以基本覆盖硅基负极活性材料颗粒的全部表面。
上述经交联的聚合物可以包括由聚合物经交联反应得到的聚合物。在一些实施例中，聚合物可选自水溶性聚合物。可选地，聚合物可包括一种或几种聚合物粘结剂。
虽然机理尚不明确，发明人意外地发现，当复合负极活性材料具有本申请所述的组成时，包覆层能够有效抑制硅基负极活性材料颗粒的体积膨胀，从而降低充放电循环过程中的容量损失，使得二次电池兼具高能量密度和良好的循环性能。
并非意在受限于任何理论或解释，当包覆层包含经交联的聚合物时，该经交联的聚合物的聚合物柔性分子链通过交联形成三维网络；这样的三维交联网络能够提高聚合物抵抗在应力下分子链滑移、变形的能力，从而使得包覆层兼具高机械强度和良好的韧性。由此，本申请的复合负极活性材料应用于二次电池，在充放电循环过程中，包覆层能够有效抑制硅基负极活性材料颗粒的体积膨胀，并减少硅基负极活性材料与电解液的接触，从而能够降低硅基负极活性材料粉化失活的风险、减少副反应的发生，进而有效减少二次电池的容量损失、提升二次电池的能量密度和循环性能。此外，一方面，经交联的聚合物可通过交联结构或者分子中的特定官能团与硅基负极活性材料颗粒相互作用，从而紧密地附着于硅基负极活性材料表面；另一方面，经交联的聚合物的内聚能密度提高，且通过化学键合形成三维交联网络的聚合物分子不易再溶解到水中。由此，本申请 的复合负极活性材料应用于二次电池时，在制备浆料及电池的加工、存储和使用过程中，包覆层不易溶于水，且与硅基负极活性材料颗粒具有高粘结力，因此能够长期、稳定地包覆于硅基负极活性材料颗粒的表面，从而能够长期、有效地抑制硅基负极活性活性材料颗粒的体积膨胀和副反应的发生，进而有效提升二次电池的长期循环性能。
在一些实施方式中，所述硅基负极活性材料颗粒的体积平均粒径Dv50可为10nm～30μm，例如可以为10nm，50nm，100nm，500nm，1μm，2μm，5μm，10μm，15μm，20μm，25μm，30μm或处于以上任何数值所组成的范围内。可选地，所述硅基负极活性材料颗粒的体积平均粒径Dv50可为1μm～10μm，例如可以为1μm，3μm，5μm，7μm，10μm或处于以上任何数值所组成的范围内。
并非意在受限于任何理论或解释，当硅基负极活性材料颗粒的体积平均粒径Dv50在上述合适的范围内时，一方面能够使得硅基负极活性材料颗粒具有低体积膨胀率，另一方面能够使得活性锂离子具有合适的传输路径。由此，本申请的复合负极活性材料应用于二次电池，能够进一步提升二次电池的循环性能。
在一些实施方式中，所述包覆层的厚度d可以为20nm～1μm，例如可以为20nm，50nm，100nm，500nm，800nm，1μm或处于以上任何数值所组成的范围内。可选地，所述包覆层的厚度d可以为20nm～200nm，例如，可以为20nm，40nm，60nm，80nm，100nm，120nm，140nm，180nm，200nm或处于以上任何数值所组成的范围内。
并非意在受限于任何理论或解释，当包覆层的厚度在上述合适的范围内时，不仅能够具有合适的机械强度和韧性有效地抑制硅基负极活性材料颗粒的体积膨胀，而且能够使得复合负极活性材料具备高理论克容量。由此，本申请的复合负极活性材料应用于二次电池，不仅能够提升二次电池的循环性能，而且能够允许二次电池具备高能量密度。
在一些实施方式中，所述包覆层的厚度d与所述硅活性材料的粒径体积平均粒径Dv50可满足：1/150≤d/Dv50≤1/9，例如，d/Dv50可为1/150，1/120，1/100，1/75，1/50，1/30，1/9或处于以上任何数值所组成的范围内。可选地，0.01≤d/Dv50≤0.05，例如，d/Dv50可为0.01，0.02，0.03，0.04或0.05。
并非意在受限于任何理论或解释，当所述包覆层的厚度d与所述硅活性材料的粒径体积平均粒径Dv50之比在上述合适的范围内时，包覆层能够具有合适的机械强度和韧性，从而能够有效抑制硅基负极活性材料颗粒的体积膨胀。此外，当d/Dv50在上述合适的范围内时，可认为包覆层在复合负极活性材料中具有低质量占比，从而能够使得复合负极活性材料具有高理论克容量。由此，本申请的复合负极活性材料应用于二次电池，能够使得二次电池兼具良好的循环性能和高能量密度。
在一些实施方式中，未交联的所述聚合物包含选自羧基、羧酸酐基、羟基、醛基、酰胺基、醚基或环氧基中的一种或几种的官能团。可选地，未交联的所述聚合物选自聚丙烯酸、海藻酸钠、羧甲基纤维素(CMC)、阿拉伯胶、聚乙烯醇、聚乙二醇、聚氧化乙烯、瓜尔豆胶、黄原胶、壳聚糖、环糊精、聚丙烯酰胺、聚乙烯亚胺、环氧树脂、聚马来酸酐、聚乙烯醇缩甲醛中的一种或几种，容易理解的，所述聚合物还可以选自上述物质的改性物。更可选地，所述聚合物选自聚乙烯醇、海藻酸钠、阿拉伯胶、聚乙二醇、聚氧化乙烯、瓜尔豆胶、黄原胶、壳聚糖、环糊精、聚丙烯酰胺、聚乙烯亚胺、环氧树脂、聚马来酸酐中的一种或几种，容易理解的，所述聚合物还可以选自上述物质的改性物。
并非意在受限于任何理论或解释，包含上述官能团的聚合物经交联后形成的包覆层能够兼具高机械强度、良好的韧性，以及与硅基负极活性材料颗粒之间的高结合力。由此，本申请的复合负极活性材料应用于二次电池，在充放电循环过程中，包覆层能够长期且有效抑制硅基负极活性材料颗粒的体积膨胀，并减少硅基负极活性材料与电解液的接触，从而能够降低硅基负极活性材料粉化失活的风险、减少副反应的发生，进而有效提升二次电池的能量密度和长期循环性能。
在一些实施方式中，未交联的所述聚合物包含交联官能团，所述交联官能团选自羧基、羧酸酐基、羟基、醛基、酰胺基或环氧基中的一种或几种，所述经交联的聚合物由未交联的所述聚合物经由所述交联官能团与交联剂反应得到。未交联的所述聚合物中，交联官能团可全部发生交联，也可以仅由部分交联官能团发生交联，从而得到经交联的聚合物。根据本申请的实施例，交联剂是具有多个能够与所述交联官能团反应而与聚合物分子链键合的合适化合物。因此，可以根据聚合物包含的给定的交联官能团，选定与该交联官能团具有适当反应性的对应官能团，从而可选定适当的交联剂。交联剂的示例包括本身是本领域已知的化合物，尤其是本身已经作为交联剂使用的那些。
作为示例，未交联的聚合物可选自包含羧基的聚合物，例如，可以选自聚丙烯酸及其改性物或羧甲基纤维素(CMC)及其改性物中的一种或几种。相应地，交联剂可选自碳化二亚胺类交联剂、氮丙啶类交联剂、碳化二亚胺类交联剂、环氧硅烷类交联剂或封闭型异氰酸酯交联剂中的一种或几种。
作为示例，未交联的聚合物可选自包含羧酸酐基的聚合物，例如，可以选自聚马来酸酐及其改性物中的一种或几种，例如，可以选自甲基乙烯基醚-马来酸酐共聚物中的一种或几种。相应地，交联剂可以选自碳化二亚胺类交联剂、氮丙啶类交联剂、碳化二亚胺类交联剂、环氧硅烷类交联剂或封闭型异氰酸酯交联剂中的一种或几种。
作为示例，未交联的聚合物可选自包含羟基的聚合物，例如，可以选自海藻酸钠及其改性物、羧甲基纤维素(CMC)及其改性物、聚乙烯醇及其改性物、聚乙二醇及其改性物、聚氧化乙烯及其改性物、瓜尔豆胶及其改性物、黄原胶及其改性物、壳聚糖及其改性物、环糊精及其改性或阿拉伯胶中的一种或几种。相应地，交联剂可选自环氧硅烷类交联剂、封闭型异氰酸酯交联剂、马来酸酐、戊二醛、乙二醛、戊二酸酐、丁二酸酐中的一种或几种。
作为示例，未交联的聚合物可选自包含醛基的聚合物，例如，可以选自聚乙烯醇缩甲醛。相应地，交联剂可以选自氨基-PEG4-胺(Amino-PEG4-amine)。
作为示例，未交联的聚合物可选自包含酰胺基的聚合物，例如，可以选自聚丙烯酰胺和/或聚乙烯亚胺。相应地，交联剂可以选自氮丙啶类交联剂、环氧硅烷类交联剂或封闭型异氰酸酯交联剂中的一种或几种。
作为示例，未交联的聚合物可选自包含环氧基的聚合物，例如，可以选自经亲水改性的环氧树脂(如丙烯酸改性的环氧树脂)。相应地，交联剂可以选自胺类固化剂，例如乙二胺、己二胺、二乙烯三胺、三乙烯四胺或二乙氨基丙胺；和/或酸酐类固化剂，例如二元酸及其酐(如顺丁烯二酸酐、邻苯二甲酸酐等)。
并非意在受限于任何理论或解释，未交联的聚合物通过上述交联官能团与对应的交联剂反应，可在交联反应条件下，于硅基负极活性材料颗粒表面迅速交联形成所述包 覆层。该包覆层中，经交联的聚合物具有合适的交联密度和适当的柔性链段，从而使得包覆层具有高机械强度和良好的韧性。由此，本申请的复合负极活性材料应用于二次电池，在充放电循环过程中，包覆层能够有效抑制硅基负极活性材料颗粒的体积膨胀，并减少硅基负极活性材料与电解液的接触，从而能够降低硅基负极活性材料粉化失活的风险、减少副反应的发生，进而有效减少二次电池的容量损失、提升二次电池的能量密度和循环性能。此外，一方面，经交联的聚合物可通过交联结构或者分子中的特定官能团与硅基负极活性材料颗粒相互作用，从而紧密地附着于硅基负极活性材料表面；另一方面，经交联的聚合物具有较高的内聚能密度、较高的分子量以及交联结构，从而在水中具有低溶解度。由此，本申请的复合负极活性材料应用于二次电池时，在制备浆料及电池的加工、存储和使用过程中，包覆层不易溶于水，且与硅基负极活性材料颗粒具有高粘结力，因此能够长期、稳定地包覆于硅基负极活性材料颗粒的表面，从而能够长期、有效地抑制硅基负极活性活性材料颗粒的体积膨胀和副反应的发生，进而有效提升二次电池的长期循环性能。
在一些实施方式中，未交联的所述聚合物的重均分子量Mw可为5×10 4～1×10 6。所述复合负极活性材料以水为溶剂的平衡溶胀比可以为1％～1000％。
并非意在受限于任何理论或解释，当未交联的所述聚合物的重均分子量在上述合适的范围内，且复合负极活性材料在水中的平衡溶胀比在上述合适的范围内时，经交联的聚合物能够具有较高的分子量和合适的交联密度。由此，经交联的聚合物中，主链官能团具有更高的分布均匀性，从而使得包覆层呈现高强度、高韧性及高抗溶胀性能。由此，包覆层在电池加工、存储及使用过程中，均不易从硅基负极活性材料颗粒表面脱落，从而能够长期、稳定地包覆于硅基负极活性材料颗粒表面，有效地抑制硅基负极活性材料颗粒的体积膨胀，进而能够提升应用本申请复合负极活性材料的二次电池的能量密度和长期循环性能。
在一些实施方式中，所述包覆层中还包括导电剂。可选地，所述导电剂与所述经交联的聚合物的质量之比为1:220～1:15。
本申请对导电剂的种类不作限制，其可以选自本领域公知的、可用于二次电池的负极的导电剂。作为示例，导电剂可选自基于碳的材料、基于金属的材料、导电聚合物或上述物质的任意组合。作为示例，基于碳的材料可选自天然石墨、人造石墨、超导碳、乙炔黑、炭黑、科琴黑、碳点、碳纳米管、石墨烯及碳纳米纤维中的至少一种。基于金属的材料可选自金属粉、金属纤维。导电聚合物可包括聚亚苯基衍生物。
并非意在受限于任何理论或解释，在包覆层中包括适量的导电剂，能够有效提升包覆层的电子传输能力，从而提升复合负极活性材料的电子传输能力。由此，本申请的复合负极活性材料应用于二次电池，能够降低负极极片表面的界面电荷转移阻抗，从而提升二次电池的循环性能。
在一些实施方式中，所述导电剂选自线性导电剂，所述线性导电剂的长径比可为30～10000，例如，可以为30，50，100，200，500，1000，3000，5000，8000，10000或处于以上任何数值所组成的范围内。可选地，线性导电剂的长径比可为100～5000。所述导电剂的直径可为0.5nm～100nm，例如，可以为0.5nm，1nm，5nm，10nm，20nm，50nm，80nm，100nm或处于以上任何数值所组成的范围内。可选地，导电剂的直径可以 为1nm至20nm，例如，可以为1nm，3nm，5nnm，8nm，10nm，13nm，15nm，18nm，20nm或处于以上任何数值所组成的范围内。所述导电剂的长度可为300nm～30μm，例如，可以为300nm，500nm，800nm，1μm，5μm，10μm，15μm，20μm，25μm，30μm或处于以上任何数值所组成的范围内。可选地，导电剂的长度可以为1μm～5μm，例如，可以为1μm，2μm，3μm，4μm，5μm或处于以上任何数值所组成的范围内。
本申请中，线性导电剂可表示具有高长径比的导电剂，例如，长径比在30以上的导电剂。
并非意在受限于任何理论或解释，满足上述条件的线性导电剂具有较高的机械强度，应用于包覆层中，能够进一步提升包覆层的机械强度。此外，线性导电剂具有高长径比，不仅能够在包覆层中发挥线性增韧作用，扩大增韧范围，而且能够形成导电网络，从而提升包覆层的韧性以及导电性能。由此，本申请的复合负极活性材料中，包覆层具有高机械强度、高韧性以及良好的电子传输能力，从而能够抑制硅基负极活性材料颗粒在充放电过程中的体积膨胀，提升复合负极活性材料的电子传输能力。因此，本申请的复合负极活性材料应用于二次电池，能够显著提升二次电池的循环性能。
在一些实施方式中，所述导电剂可选自一维导电剂和/或二维导电剂。
在一些实施方式中，所述导电剂可选自碳纳米管(carbon nanotubes，CNT)、气相生长碳纤维增强体(vapor disporsition Catbon fiber reinforcement，VGCF)、石墨烯中的一种或几种。
并非意在受限于任何理论或解释，选自上述种类的导电剂具有良好的导电性和机械强度，能够提升包覆层的机械强度和导电性能，从而能够在增强包覆层对硅基负极活性材料颗粒的束缚作用的同时，提升包覆层的电子传输能力。由此，本申请的复合负极活性材料能够具有低体积膨胀率和良好的电子传输能力，应用于二次电池，能够使得二次电池具备良好的循环性能。
在一些实施方式中，所述复合负极活性材料的粉末电阻率可以为0.3Ω·cm～1.3Ω·cm，例如可以为0.3Ω·cm，0.5Ω·cm，0.8Ω·cm，1.0Ω·cm，1.3Ω·cm或处于以上任何数值所组成的范围内。可选地，所述复合负极活性材料的粉末电阻率可以为0.4Ω·cm～0.8Ω·cm。
并非意在受限于任何理论或解释，本申请的复合负极活性材料中，硅基负极活性材料颗粒、包覆层具有合适的结构，且二者紧密地结合在一起，从而使得复合负极活性材料具备低电阻率。由此，本申请的复合负极活性材料应用于二次电池，能够降低负极极片表面的界面电荷转移阻抗，从而提升二次电池的循环性能。
本申请中，硅基负极活性材料颗粒的体积平均粒径Dv50具有本领域公知的含义，可以通过本领域已知的方法和仪器测定。其中，Dv50表示在体积基准的粒度分布中，50％的颗粒粒径小于该值，硅基负极活性材料颗粒的体积平均粒径Dv50可以参照GB/T 19077-2016粒度分布激光衍射法，采用激光粒度分析仪(例如英国马尔文Mastersizer 2000E)测定。
本申请中，包覆层的厚度具有本领域公知的含义，可以通过本领域已知的方法和仪器测定。例如，可以通过透射电子显微镜(Transmission Electron Microscope，TEM)测定。
本申请中，粉末电阻率具有本领域公知的含义，可以通过本领域已知的方法和仪器测定。例如，可以通过粉末电阻测定仪测定。
本申请中，复合负极活性材料以水为溶剂的平衡溶胀比具有本领域公知的含义，其可以表示复合负极活性材料因吸收一定量的水而发生溶胀，当达到溶胀平衡时，其体积与溶胀前的体积之比。作为示例，复合负极活性材料以水为溶剂的平衡溶胀比可以通过如下步骤测定：取适量的复合负极活性材料，用溶胀计测定复合负极活性材料的体积为V 1；将复合负极活性材料置于足量的水中，于25℃下放置一段时间(例如10h至12h)，以使复合负极活性材料达到溶胀平衡；取出复合负极活性材料，并用滤纸轻轻将复合负极活性材料表面附着的水吸干；用溶胀计测定溶胀后的复合负极活性材料的体积V 2；计算复合负极活性材料的平衡溶胀比Q＝V 2/V 1*100％。
用于制备复合负极活性材料的方法
本申请第二方面提供用于制备本申请第一方面负复合负极活性材料的方法，其包括如下步骤S1～S2。
S1，将所述硅基负极活性材料颗粒、未交联的所述聚合物、交联剂以及可选的导电剂在溶剂中混合均匀，从而得到复合负极活性材料前驱体。
在步骤S1中，硅基负极活性材料颗粒、未交联的聚合物、交联剂以及导电剂可以选自如本申请第一方面所述的硅基负极活性材料颗粒、未交联的聚合物、交联剂以及导电剂，在此不再赘述。上述溶剂可以包括可用于均匀分散所述硅基负极活性材料颗粒、未交联的所述聚合物、交联剂以及可选的导电剂的有机溶剂或无机溶剂。作为示例，溶剂可以为水。在步骤S1中，未交联的所述聚合物、交联剂及导电剂可均匀地分散于溶剂中，形成包裹于硅基负极活性材料颗粒表面的浆料层。
S2，将所述复合负极活性材料前驱体置于30℃～250℃、可选为60℃～200℃的环境中，以使未交联的所述聚合物在所述交联剂的作用下，于所述硅基负极活性材料颗粒的表面发生交联，从而得到所述复合负极活性材料。
在步骤S2中，未交联的所述聚合物在所述交联剂的作用下，于所述硅基负极活性材料颗粒的表面发生交联，从而能够生成经交联的聚合物，该聚合物具有合适的交联密度和适当的柔性链段，能够紧密地包覆于硅基负极活性材料颗粒表面，形成包覆层。
并非意在受限于任何理论或解释，本申请的方法，通过将所述硅基负极活性材料颗粒、未交联的所述聚合物、交联剂以及可选的导电剂在溶剂中混合均，使得未交联的所述聚合物、交联剂以及可选的导电剂均匀地包覆于硅基负极活性材料颗粒表面；再在一定温度下使未交联的所述聚合物在交联剂的作用下发生交联，从而在硅基负极活性材料颗粒表面形成包覆层。该包覆层中，经交联的聚合物的聚合物柔性分子链通过交联形成三维网络；这样的三维交联网络能够提高聚合物抵抗在应力下分子链滑移、变形的能力，从而使得包覆层兼具高机械强度和良好的韧性。由此，根据本申请的方法制备的复合负极活性材料应用于二次电池，在充放电循环过程中，包覆层能够有效抑制硅基负极活性材料颗粒的体积膨胀，并减少硅基负极活性材料与电解液的接触，从而能够降低硅基负极活性材料粉化失活的风险、减少副反应的发生，进而有效减少二次电池的容量损失、提升二次电池的能量密度和循环性能。此外，一方面，经交联的聚合物可通过交联结构或者分子中的特定官能团与硅基负极活性材料颗粒相互作用，从而紧密地附着于硅 基负极活性材料表面；另一方面，经交联的聚合物的内聚能密度提高，且通过化学键合形成三维交联网络的聚合物分子不易再溶解到水中。由此，根据本申请方法制备的复合负极活性材料应用于二次电池时，在制备浆料及电池的加工、存储和使用过程中，包覆层不易溶于水，且与硅基负极活性材料颗粒具有高粘结力，因此能够长期、稳定地包覆于硅基负极活性材料颗粒的表面，从而能够长期、有效地抑制硅基负极活性活性材料颗粒的体积膨胀和副反应的发生，进而有效提升二次电池的长期循环性能。
此外，本申请的方法条件温和，交联反应在较低的温度下发生，能够降低硅基负极活性材料颗粒发生歧化反应的风险，从而使得复合负极活性材料具有高首次库伦效率。根据本申请的方法制备的复合负极活性材料应用于二次电池，能够有效提升二次电池的首次库伦效率、能量密度以及循环性能。
在一些实施方式中，步骤S1中，所述硅基负极活性材料颗粒可为100重量份，未交联的所述聚合物可为1重量份～10重量份，所述交联剂可为0.001重量份～1重量份，所述导电剂可为0.05重量份～4重量份。
并非意在受限于任何理论或解释，步骤S1中，硅基负极活性材料的用量在上述合适的范围内，能够使得复合负极活性材料具有较高的理论克容量。当未交联的所述聚合物、交联剂和导电剂的用量在上述合适的范围内时，经交联的聚合物具有合适的交联密度，且包覆层具有合适的厚度和良好的导电性，能够使得复合负极活性材料具有低体积膨胀率、高理论克容量和良好的电子传输能力。由此，根据本申请的方法制备的复合负极活性材料应用于二次电池，能够使得二次电池具备良好的循环性能和高能量密度。
负极极片
本申请第三方面提供一种负极极片，包括负极集流体以及位于所述负极集流体至少一个表面上的负极膜层。所述负极膜层包括负极活性材料、粘结剂以及导电剂。其中，所述负极活性材料包括本申请第一方面的复合负极活性材料，或者根据本申请第二方面的方法制备的复合负极活性材料。
所述负极活性材料中还可以包括其他本领域公知的用于电池的负极活性材料，作为示例，负极活性材料可包括以下材料中的至少一种：人造石墨、天然石墨、软炭、硬炭、锡基材料和钛酸锂等。但本申请并不限定于这些材料，还可以使用其他可被用作电池负极活性材料的传统材料。这些负极活性材料可以仅单独使用一种，也可以将两种以上组合使用。
所述粘结剂可选自本领域公知的用于负极的粘结剂。作为示例，所述粘结剂可选自丁苯橡胶(SBR)、聚丙烯酸(PAA)、聚丙烯酸钠(PAAS)、聚丙烯酰胺(PAM)、聚乙烯醇(PVA)、海藻酸钠(SA)、聚甲基丙烯酸(PMAA)及羧甲基壳聚糖(CMCS)中的至少一种。
所述导电剂可选自本领域公知的用于负极的导电剂。作为示例，所述导电剂可选自超导碳、乙炔黑、炭黑、科琴黑、碳点、碳纳米管、石墨烯及碳纳米纤维中的至少一种。
本申请负极极片中，负极膜层包括本申请第一方面的复合负极活性材料，或者根据本申请第二方面的方法制备的复合负极活性材料，应用于二次电池，能够使得二次电池兼具良好的循环性能和高能量密度。
在一些实施方式中，以总质量为100％计，所述负极膜层可包括93.5％～97％的负极活性材料、2.0％～5.0％的粘结剂以及0.5％～1.5的％导电剂。
可选地，所述复合负极活性材料在所述负极活性材料中的质量百分含量可以为1％～50％，例如，可以为1％，5％，10％，15％，20％，25％，30％，35％，40％，45％，50％或处于以上任何数值所组成的范围内。
负极膜层中，各组份的含量在上述合适的范围内，能够使得负极极片具备高能量密度和良好的电子传输能力。特别地，负极膜层中包括复合负极活性材料，能够使得负极极片具有良好的电子传输能力，由此，能够降低负极膜层中导电剂的用量，从而使得负极极片兼具低成本和良好的电化学性能。
本申请的负极膜层中还可选地包括其他助剂，例如增稠剂(如羧甲基纤维素钠(CMC-Na))等。
本申请的负极极片中，所述负极集流体可采用金属箔片或复合集流体(可以将金属材料设置在高分子基材上形成复合集流体)。例如，作为金属箔片，可以采用铜箔。复合集流体可包括高分子材料基层和形成于高分子材料基材至少一个表面上的金属层。复合集流体可通过将金属材料(铜、铜合金、镍、镍合金、钛、钛合金、银及银合金等)形成在高分子材料基材(如聚丙烯(PP)、聚对苯二甲酸乙二醇酯(PET)、聚对苯二甲酸丁二醇酯(PBT)、聚苯乙烯(PS)、聚乙烯(PE)等的基材)上而形成。
本申请的负极极片中，负极膜层可以设置在负极集流体的一侧，也可以同时设置在负极集流体的两侧。例如，负极集流体具有在其自身厚度方向相对的两侧，负极膜层设置在负极集流体相对的两侧中的任意一侧或两侧上。
需要说明的是，本申请的二次电池中，负极极片并不排除除了负极膜层之外的其他附加功能层。例如在某些实施方式中，本申请所述的负极极片还可以包括设置在负极集流体和负极膜层之间的导电底涂层(例如由导电剂和粘结剂组成)。在另外一些实施方式中，本申请所述的负极极片还包括覆盖在负极膜层表面的保护层。
二次电池
二次电池又称为充电电池或蓄电池，是指在电池放电后可通过充电的方式使活性物质激活而继续使用的电池。
通常情况下，二次电池包括正极极片、负极极片、隔离膜及电解质。在电池充放电过程中，活性离子(例如锂离子)在正极极片和负极极片之间往返嵌入和脱出。隔离膜设置在正极极片和负极极片之间，主要起到防止正负极短路的作用，同时可以使离子通过。电解质在正极极片和负极极片之间，主要起到传导活性离子的作用。
[负极极片]
本申请的二次电池的负极极片包括本申请第四方面的负极极片。上文已对负极极片的实施例进行了详细描述和说明，在此不再重复。可以理解的是，本申请的二次电池可以实现本申请的负极极片的上述任一实施例的有益效果。
[正极极片]
本申请的二次电池中，正极极片包括正极集流体以及设置在正极集流体至少一个表面且包括正极活性材料的正极膜层。例如，正极集流体具有在自身厚度方向相对的两个表面，正极膜层设置于正极集流体的两个相对表面中的任意一者或两者上。
本申请的二次电池中，所述正极活性材料可采用本领域公知的用于二次电池的正极活性材料。例如，正极活性材料可包括锂过渡金属氧化物、橄榄石结构的含锂磷酸盐及其各自的改性化合物中的一种或几种。锂过渡金属氧化物的示例可包括但不限于锂钴氧化物、锂镍氧化物、锂锰氧化物、锂镍钴氧化物、锂锰钴氧化物、锂镍锰氧化物、锂镍钴锰氧化物、锂镍钴铝氧化物及其改性化合物中的一种或几种。橄榄石结构的含锂磷酸盐的示例可包括但不限于磷酸铁锂、磷酸铁锂与碳的复合材料、磷酸锰锂、磷酸锰锂与碳的复合材料、磷酸锰铁锂、磷酸锰铁锂与碳的复合材料及其各自的改性化合物中的一种或几种。本申请并不限定于这些材料，还可以使用其他可被用作二次电池正极活性材料的传统公知的材料。
本申请的二次电池中，所述正极膜层通常包含正极活性材料以及可选的粘结剂和可选的导电剂，通常是由正极浆料涂布，并经干燥、冷压而成的。正极浆料通常是将正极活性材料以及可选的导电剂和粘结剂等分散于溶剂中并搅拌均匀而形成的。溶剂可以是N-甲基吡咯烷酮(NMP)。
作为示例，用于正极膜层的粘结剂可以包括聚偏氟乙烯(PVDF)和聚四氟乙烯(PTFE)中的一种或几种。
作为示例，用于正极膜层的导电剂可以包括超导碳、炭黑(例如，乙炔黑、科琴黑)、碳点、碳纳米管、石墨烯及碳纳米纤维中的一种或几种。
本申请的二次电池中，所述正极集流体可采用金属箔片或复合集流体(可以将金属材料设置在高分子基材上形成复合集流体)。作为示例，正极集流体可采用铝箔。
[电解质]
本申请的二次电池对电解质的种类没有具体的限制，可根据需求进行选择。例如，电解质可以选自固态电解质及液态电解质(即电解液)中的至少一种。
在一些实施方式中，电解质采用电解液。电解液包括电解质盐和溶剂。
在一些实施方式中，电解质盐可选自LiPF 6(六氟磷酸锂)、LiBF 4(四氟硼酸锂)、LiClO 4(高氯酸锂)、LiAsF 6(六氟砷酸锂)、LiFSI(双氟磺酰亚胺锂)、LiTFSI(双三氟甲磺酰亚胺锂)、LiTFS(三氟甲磺酸锂)、LiDFOB(二氟草酸硼酸锂)、LiBOB(二草酸硼酸锂)、LiPO 2F 2(二氟磷酸锂)、LiDFOP(二氟二草酸磷酸锂)及LiTFOP(四氟草酸磷酸锂)中的一种或几种。
在一些实施方式中，溶剂可选自碳酸亚乙酯(EC)、碳酸亚丙酯(PC)、碳酸甲乙酯(EMC)、碳酸二乙酯(DEC)、碳酸二甲酯(DMC)、碳酸二丙酯(DPC)、碳酸甲丙酯(MPC)、碳酸乙丙酯(EPC)、碳酸亚丁酯(BC)、氟代碳酸亚乙酯(FEC)、甲酸甲酯(MF)、乙酸甲酯(MA)、乙酸乙酯(EA)、乙酸丙酯(PA)、丙酸甲酯(MP)、丙酸乙酯(EP)、丙酸丙酯(PP)、丁酸甲酯(MB)、丁酸乙酯(EB)、1,4-丁内酯(GBL)、环丁砜(SF)、二甲砜(MSM)、甲乙砜(EMS)及二乙砜(ESE)中的一种或几种。
在一些实施方式中，电解液中还可选地包括添加剂。例如添加剂可以包括负极成膜添加剂，也可以包括正极成膜添加剂，还可以包括能够改善电池某些性能的添加剂，例如改善电池过充性能的添加剂、改善电池高温性能的添加剂、改善电池低温性能的添加剂等。
[隔离膜]
采用电解液的二次电池、以及一些采用固态电解质的二次电池中，还包括隔离膜。隔离膜设置在正极极片和负极极片之间，起到隔离的作用。本申请对隔离膜的种类没有特别的限制，可以选用任意公知的具有良好的化学稳定性和机械稳定性的多孔结构隔离膜。在一些实施方式中，隔离膜的材质可以选自玻璃纤维、无纺布、聚乙烯、聚丙烯及聚偏二氟乙烯中的一种或几种。隔离膜可以是单层薄膜，也可以是多层复合薄膜。隔离膜为多层复合薄膜时，各层的材料相同或不同。
在一些实施方式中，正极极片、负极极片和隔离膜可通过卷绕工艺或叠片工艺制成电极组件。
在一些实施方式中，二次电池可包括外包装。该外包装可用于封装上述电极组件及电解质。
在一些实施方式中，二次电池的外包装可以是硬壳，例如硬塑料壳、铝壳、钢壳等。二次电池的外包装也可以是软包，例如袋式软包。软包的材质可以是塑料，如聚丙烯(PP)、聚对苯二甲酸丁二醇酯(PBT)、聚丁二酸丁二醇酯(PBS)等中的一种或几种。
本申请对二次电池的形状没有特别的限制，其可以是圆柱形、方形或其他任意的形状。如图1是作为一个示例的方形结构的二次电池5。
在一些实施方式中，参照图2，外包装可包括壳体51和盖板53。其中，壳体51可包括底板和连接于底板上的侧板，底板和侧板围合形成容纳腔。壳体51具有与容纳腔连通的开口，盖板53用于盖设所述开口，以封闭所述容纳腔。正极极片、负极极片和隔离膜可经卷绕工艺或叠片工艺形成电极组件52。电极组件52封装于所述容纳腔。电解液浸润于电极组件52中。二次电池5所含电极组件52的数量可以为一个或几个，可根据需求来调节。
用电装置
本申请还提供一种用电装置，所述用电装置包括本申请的二次电池。所述二次电池可以用作所述用电装置的电源，也可以用作所述用电装置的能量存储单元。所述用电装置可以但不限于是移动设备(例如手机、笔记本电脑等)、电动车辆(例如纯电动车、混合动力电动车、插电式混合动力电动车、电动自行车、电动踏板车、电动高尔夫球车、电动卡车等)、电气列车、船舶及卫星、储能系统等。
图3是作为一个示例的用电装置。该用电装置为纯电动车、混合动力电动车、或插电式混合动力电动车等。为了满足该装置对高功率和高能量密度的需求，可以采用包括本申请的二次电池的电池包或电池模块。
作为另一个示例的用电装置可以是手机、平板电脑、笔记本电脑等。该用电装置通常要求轻薄化，可以采用二次电池作为电源。
实施例
下述实施例更具体地描述了本申请公开的内容，这些实施例仅仅用于阐述性说明，因为在本申请公开内容的范围内进行各种修改和变化对本领域技术人员来说是明显的。除非另有声明，以下实施例中所报道的所有份、百分比、和比值都是基于重量计， 而且实施例中使用的所有试剂都可商购获得或是按照常规方法进行合成获得，并且可直接使用而无需进一步处理，以及实施例中使用的仪器均可商购获得。
实施例1～31
复合负极活性材料的制备
将100重量份的体积平均粒径为Dv50的硅颗粒、m 1重量份的未交联的聚合物、m 2重量份的交联剂以及m 3重量份的导电剂在去离子水中混合均匀，从而得到复合负极活性材料前驱体；
将复合负极活性材料前驱体置于温度为T℃的烘箱中，以使未交联的聚合物在对应的交联剂的作用下，于硅颗粒的表面发生交联，从而得到复合负极活性材料。
各实施例中，硅颗粒的Dv50、未交联的聚合物种类及其重均分子量Mw、导电剂种类、导电剂的直径d 1、导电剂的长度l 1、导电剂的长径比l 1/d 1、m 1、m 2、m 3以及T等制备参数如表1所示。其中，聚乙烯醇对应的交联剂为封闭型异氰酸酯交联剂；聚丙烯酰胺对应的交联剂为环氧硅烷交联剂，聚丙烯酸对应的交联剂为聚氮丙啶交联剂；环氧树脂对应的交联剂为二乙烯三胺；聚马来酸酐对应的交联剂为碳化二亚胺类交联剂；聚乙烯醇缩甲醛对应的交联剂为氨基-PEG4-胺。
对比例1
直接采用未经包覆的硅颗粒。
对比例2
基于实施例1～31的复合负极活性材料制备过程，根据表1中所示调整制备参数，制备对比例2的复合负极活性材料。
对实施例1～31及对比例2的复合负极活性材料、对比例1的硅颗粒进行如下测试，得到的测试结果如表2中所示。
包覆层厚度测试
将复合负极活性材料或硅颗粒分散于乙醇溶剂中，滴于微栅支撑膜，通过高分辨透射电子显微镜进行包覆层厚度d的测试。
根据测定的包覆层厚度d，以及硅颗粒的Dv50，可计算得到d/Dv50的值。
粉末电阻率测试
参照《GB/T 30835-2014锂离子电池用炭复合磷酸铁锂正极材料》中的四探针法，使用粉末电阻率测试仪进行测试。
平衡溶胀比测试
取适量的复合负极活性材料或硅颗粒作为试样，用溶胀计测定试样体积V 1；将试样置于大试管中，加入水，溶剂的量约至大试管三分之一处；将装有试样以及溶剂的大试管用塞子塞紧并置于恒温槽内，以使试样在25℃左右溶胀；12h后，取出试样，并用滤纸轻轻将试样表面附着的水吸干；用溶胀计测定溶胀后的试样体积V 2；计算平衡溶胀比Q＝V 2/V 1*100％。
表1
Figure PCTCN2022127837-appb-000001
表2
序号 包覆层厚度d d/Dv50 粉末电阻率 Q
实施例1 6.67nm 1/150 0.92Ω·cm 16.9％
实施例2 10nm 1/100 0.98Ω·cm 13.1％
实施例3 20nm 1/50 1.04Ω·cm 9.3％
实施例4 100nm 1/10 1.30Ω·cm 5.6％
实施例5 100nm 1/10 0.79Ω·cm 16.5％
实施例6 100nm 1/10 0.71Ω·cm 16.7％
实施例7 100nm 1/10 0.64Ω·cm 16.3％
实施例8 100nm 1/10 0.56Ω·cm 17.1％
实施例9 100nm 1/10 0.49Ω·cm 16.8％
实施例10 100nm 1/10 0.42Ω·cm 17.2％
实施例11 100nm 1/10 0.38Ω·cm 16.4％
实施例12 100nm 1/10 0.35Ω·cm 17.0％
实施例13 100nm 1/10 0.33Ω·cm 17.1％
实施例14 100nm 1/10 0.30Ω·cm 16.8％
实施例15 100nm 1/10 0.31Ω·cm 16.4％
实施例16 100nm 1/10 0.41Ω·cm 16.9％
实施例17 100nm 1/10 0.36Ω·cm 20.2％
实施例18 100nm 1/10 0.38Ω·cm 14.1％
实施例19 100nm 1/10 0.37Ω·cm 5.2％
实施例20 1nm 1/10 0.35Ω·cm 16.1％
实施例21 200nm 1/50 0.37Ω·cm 16.3％
实施例22 1μm 1/100 0.37Ω·cm 16.2％
实施例23 100nm 1/10 0.35Ω·cm 16.8％
实施例24 111nm 1/9 0.36Ω·cm 17.0％
实施例25 90nm 1/110 0.37Ω·cm 874％
实施例26 100nm 1/10 0.35Ω·cm 945％
实施例27 100nm 1/10 0.34Ω·cm 1％
实施例28 102nm 1/10 0.35Ω·cm 451％
实施例29 109nm 1/9 0.34Ω·cm 1.2％
实施例30 100nm 1/10 0.56Ω·cm 16.5％
实施例31 100nm 1/10 0.63Ω·cm 16.7％
对比例1 / / 0.86Ω·cm /
对比例2 100nm 1/10 0.96Ω·cm 1300％
将上述实施例1～31及对比例1～2的复合负极活性材料或硅颗粒用于制备二次电池后，进行性能测试，测试结果分别如表3中所示。具体制备过程及测试过程如下。
负极极片的制备
在去离子水中加入质量比为96.6:0.8:1.1:1.5的负极活性材料、导电碳、增稠剂CMC-Na、粘结剂SBR，混合均匀，得到负极浆料；
将负极浆料均匀涂覆于Cu箔上，经过烘箱烘干、冷压后，得到负极极片。
其中，实施例1～31、对比例2的负极活性材料为15wt％的复合负极活性材料与85wt％的石墨的混合物，对比例1的负极活性材料为15wt％的硅颗粒与85wt％的石墨的混合物，除负极活性材料不同外，各实施例及对比例的其他负极极片制备参数均相同。
正极极片的制备
将正极活性材料LiNi 0.8Co 0.1Mn 0.1O 2(NCM811)、导电剂Super P、粘结剂聚偏氟乙烯(PVDF)、分散剂按质量比96.94：1.7：0.3：1：0.06在适量的溶剂N-甲基吡咯烷酮(NMP)中充分搅拌混合，形成均匀的正极浆料；将正极浆料涂覆于正极集流体铝箔的表面，经干燥、冷压后，得到正极极片。
电解液的制备
在干燥氩气气氛手套箱中(H 2O<0.1ppm，O 2<0.1ppm)，将有机溶剂碳酸丙烯酯(PC)、碳酸乙烯酯(EC)、碳酸二乙酯(DEC)按照1：1：1的重量比混合均匀，加入充分干燥的锂盐LiPF 6溶解于上述有机溶剂中，充分搅拌混合均匀后，后得到锂盐浓度为1.15mol/L的电解液。
隔离膜
采用聚丙烯隔离膜。
二次电池的制备
将正极极片、隔离膜、负极极片按顺序堆叠并卷绕，得到电极组件；将电极组件加入外包装，加入上述电解液，经封装、静置、化成、老化等工序后得到二次电池。
测试部分
1)首次库伦效率测试
25℃下，对化成后的二次电池先以1/3C的倍率恒流放电(DC)至2.8V，静置10min；然后再以1/3C的倍率恒流充电(CC)至4.2V，再以4.2V恒电压充电(CV)至电流为0.05C，静置10min，记录充电容量；然后再以1/3C的倍率恒流放电(DC)至2.8V，记录放电容量。
首次库伦效率＝放电容量/充电容量*100％。
2)电池的25℃循环容量保持率测试
25℃下，将二次电池以1/3C恒流充电至4.2V，再以4.2V恒定电压充电至电流为0.05C，搁置5min，再以1/3C放电至2.8V，所得容量记为初始容量C 0。对上述同一个电池重复以上步骤，并同时记录循环第n次后电池的放电容量C n，则第n次循环后电池容量保持率P n＝C n/C 0*100％。
表3
序号 首次库伦效率 P 100
实施例1 87.21％ 95.71％
实施例2 87.19％ 95.86％
实施例3 87.30％ 95.96％
实施例4 87.27％ 95.98％
实施例5 87.32％ 96.09％
实施例6 87.36％ 96.12％
实施例7 87.44％ 96.20％
实施例8 87.68％ 96.50％
实施例9 87.83％ 96.59％
实施例10 87.86％ 96.68％
实施例11 87.92％ 96.70％
实施例12 87.91％ 96.72％
实施例13 87.89％ 96.75％
实施例14 87.94％ 96.81％
实施例15 87.90％ 96.83％
实施例16 87.80％ 96.59％
实施例17 87.89％ 96.70％
实施例18 87.91％ 96.71％
实施例19 87.87％ 96.72％
实施例20 87.13％ 96.25％
实施例21 87.96％ 96.88％
实施例22 87.93％ 96.79％
实施例23 87.90％ 96.71％
实施例24 87.94％ 96.78％
实施例25 87.89％ 96.37％
实施例26 87.86％ 96.25％
实施例27 87.88％ 96.70％
实施例28 87.91％ 96.40％
实施例29 87.89％ 96.84％
实施例30 87.84％ 96.63％
实施例31 87.64％ 96.41％
对比例1 87.10％ 95.62％
对比例2 86.96％ 95.68％
综合表1至表3可知，本申请的复合负极活性材料应用于二次电池中，能够长期、有效地抑制硅基负极活性活性材料颗粒的体积膨胀和副反应的发生，从而有效提升二次电池的长期循环性能。
具体地，综合实施例1至4可知，其他条件相同时，随着聚乙烯醇和交联剂用量的增加，硅颗粒表面的包覆层厚度逐渐增大，且随交联剂用量的增加，材料的平衡溶胀比降低，说明包覆层有更高的交联度。交联度的提升有利于提高包覆层的强度，并且保障包覆材料的包覆层在搅拌过程中不脱落。由此，包覆层对硅颗粒体积膨胀的抑制作用，二次电池的100圈循环容量保持率逐渐增高。但是，实施例1-4包覆层未添加CNT，所以包覆层整体强度较弱以及复合负极活性材料的粉末电阻率比较大，需要进一步提升。
综合实施例4至10可知，当包覆层厚度相同时，包覆层中包含导电剂的复合负极 活性材料具有更低的粉末电阻率。随着导电剂的增加粉末电阻率的降低，且包覆层的强度机械强度更好，能更好的抑制硅材料的膨胀。由此，二次电池不仅具有更高的首次库伦效率，而且具有更高的100圈循环容量保持率。
综合实施例9、11至14可知，当导电剂的直径相等时，随着导电剂长度的增大，复合负极活性材料的粉末电阻率逐渐降低。综合实施例12、15至16可知，当导电剂的长度相等时，随着导电剂直径的减小，复合负极活性材料的粉末电阻率逐渐降低。可见，增大包覆层中导电剂的长径比，能够有效降低复合负极活性材料的粉末电阻率，从而提升复合负极活性材料的电子传输能力。
综合实施例12、17至19可知，随着交联温度的提升，有利交联反应的进行，包覆层交联度提升可提高包覆层的强度，对应的二次电池的首次库伦效率和100圈循环容量保持率均有所提升。
综合实施例12、20至22可知，随着硅颗粒Dv50的增大，硅颗粒表面更容易形成厚度较大的包覆层。可见，根据本申请的方法，能够简单、高效地将包覆层的厚度控制在合适的范围内。由此，能够有效抑制硅颗粒在循环过程中的体积膨胀，从而提升二次电池的长期循环性能。
综合实施例12、23至24可知，当未交联的聚合物的重均分子量在本申请限定的范围内时，均能够形成厚度合适且具有较高强度的包覆层，从而抑制硅颗粒的体积膨胀，提升二次电池的长期循环性能。
综合实施例12、25至29可知，本申请的方法适用于多种未交联的聚合物，具有工艺简单、适用性广的优点。
综合实施例12、30至31可知，当包覆层包含其他一维导电剂，例如VGCF、石墨烯时，包覆层也能够具有良好的电子传输能力和机械强度。由此，复合负极活性材料能够具有低体积膨胀率和良好的电子传输能力，应用于二次电池，能够使得二次电池具备良好的长期循环性能。
而相对于此，对比例1中的硅颗粒表面不具有包覆层，其对应的二次电池的100圈循环容量保持率远低于实施例1至31。对比例2的硅颗粒表面虽然具有包覆层，但是在复合负极活性材料的制备过程中，可能由于交联温度过高，导致硅颗粒的电化学活性降低，由此，对比例2的二次电池不仅首次库伦效率有所降低，100圈循环容量保持率也低于实施例1至31。
以上所述，仅为本申请的具体实施方式，但本申请的保护范围并不局限于此，任何熟悉本技术领域的技术人员在本申请揭露的技术范围内，可轻易想到各种等效的修改或替换，这些修改或替换都应涵盖在本申请的保护范围之内。因此，本申请的保护范围应以权利要求的保护范围为准。

Claims (16)

  1. 一种复合负极活性材料，包括：
    硅基负极活性材料颗粒；以及
    包覆于所述硅基负极活性材料颗粒的至少部分表面的包覆层，
    其中，所述包覆层包含经交联的聚合物。
  2. 根据权利要求1所述的复合负极活性材料，其中，所述硅基负极活性材料颗粒的体积平均粒径Dv50为10nm～30μm，可选为1μm～10μm。
  3. 根据权利要求1或2所述的复合负极活性材料，其中，所述包覆层的厚度d为20nm～1μm，可选为20nm～200nm。
  4. 根据权利要求1-3中任一项所述的复合负极活性材料，其中，所述包覆层的厚度d与所述硅活性材料的粒径体积平均粒径Dv50满足：1/150≤d/Dv50≤1/9，可选地，
    0.01≤d/Dv50≤0.05。
  5. 根据权利要求1-4中任一项所述的复合负极活性材料，其中，未交联的所述聚合物包含选自羧基、羧酸酐基、羟基、醛基、酰胺基、醚基或环氧基中的一种或几种的官能团；
    可选地，未交联的所述聚合物选自聚丙烯酸、海藻酸钠、羧甲基纤维素、阿拉伯胶、聚乙烯醇、聚乙二醇、聚氧化乙烯、瓜尔豆胶、黄原胶、壳聚糖、环糊精、聚丙烯酰胺、聚乙烯亚胺、环氧树脂、聚马来酸酐、聚乙烯醇缩甲醛中的一种或几种；
    更可选地，所述聚合物选自海藻酸钠、阿拉伯胶、聚乙二醇、聚氧化乙烯、瓜尔豆胶、黄原胶、壳聚糖、环糊精、聚丙烯酰胺、聚乙烯亚胺、环氧树脂、聚马来酸酐中的一种或几种。
  6. 根据权利要求1-5中任一项所述的复合负极活性材料，其中，未交联的所述聚合物包含交联官能团，所述交联官能团选自羧基、羧酸酐基、羟基、醛基、酰胺基或环氧基中的一种或几种，所述经交联的聚合物由未交联的所述聚合物经由所述交联官能团与交联剂反应得到。
  7. 根据权利要求1-6中任一项所述的复合负极活性材料，其中，未交联的所述聚合物的重均分子量Mw为5×10 4～1×10 6；
    所述复合负极活性材料的以水为溶剂的平衡溶胀比为1％～1000％。
  8. 根据权利要求1-7中任一项所述的复合负极活性材料，其中，所述包覆层中还包括导电剂，可选地，所述导电剂与所述经交联的聚合物的质量之比为1:220～1:15。
  9. 根据权利要求7所述的复合负极活性材料，其中，所述导电剂选自线性导电剂，所述线性导电剂的长径比为30～10000，可选为100～5000；
    所述导电剂的直径为0.5nm～100nm，可选为1nm至20nm；
    所述导电剂的长度为300nm～30μm，可选为1μm～5μm。
  10. 根据权利要求8或9所述的复合负极活性材料，其中，所述导电剂选自碳纳米管、气相生长碳纤维增强体、石墨烯中的一种或几种。
  11. 根据权利要求1-9中任一项所述的复合负极活性材料，其中，所述复合负极活性材料的粉末电阻率为0.3Ω·cm～1.3Ω·cm，可选为0.4Ω·cm～0.8Ω·cm。
  12. 一种用于制备根据权利要求1-11中任一项所述的复合负极活性材料的方法，包括如下步骤：
    S1，将所述硅基负极活性材料颗粒、未交联的所述聚合物、交联剂以及可选的导电剂在溶剂中混合均匀，从而得到复合负极活性材料前驱体；
    S2，将所述复合负极活性材料前驱体置于30℃～250℃、可选为60℃～200℃的环境中，以使未交联的所述聚合物在所述交联剂的作用下，于所述硅基负极活性材料颗粒的表面发生交联，从而得到所述复合负极活性材料。
  13. 根据权利要求12所述的方法，其中，步骤S1中，所述硅基负极活性材料颗粒为100重量份，未交联的所述聚合物为1重量份～10重量份，所述交联剂为0.001重量份～1重量份，所述导电剂为0.05重量份～4重量份。
  14. 一种负极极片，包括负极集流体以及位于所述负极集流体至少一个表面上的负极膜层，所述负极膜层包括负极活性材料、粘结剂以及导电剂，其中，所述负极活性材料包括根据权利要求1-11中任一项所述的复合负极活性材料，或者根据权利要求12或13所述的方法制备的复合负极活性材料；
    可选地，以总质量为100％计，所述负极膜层包括93.5％～97％的所述负极活性材料、2.0％～5.0％的所述粘结剂以及0.5％～1.5％的所述导电剂；
    更可选地，所述复合负极活性材料在所述负极活性材料中的质量百分含量为1％～50％。
  15. 一种二次电池，包括根据权利要求14所述的负极极片。
  16. 一种用电装置，包括根据权利要求15所述的二次电池。
PCT/CN2022/127837 2022-10-27 2022-10-27 复合负极活性材料及其制备方法、负极极片、二次电池及用电装置 WO2024087081A1 (zh)

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