Negative pole piece, preparation method thereof and electrochemical device

Abstract

The invention provides a negative pole piece, a preparation method thereof and an electrochemical device, and particularly relates to the technical field of electrode materials. The negative pole piece comprises a three-dimensional porous current collector material, a silicon material and conductive gel, the silicon material is coated on the three-dimensional porous current collector material, and the conductive gel is coated on the surfaces of the three-dimensional porous current collector material and the silicon material. The silicon material and the three-dimensional porous current collector are compounded, and the surface is coated with the conductive gel, so that the volume expansion of silicon material particles can be effectively inhibited, the electrode structure is stabilized, and meanwhile, the charge conduction efficiency is improved; the material has high reversible capacity and excellent charge-discharge dynamic performance.

Description

Technical Field

[0001] The present invention relates to the technical field of electrode materials, and particularly to a negative electrode sheet, a preparation method thereof, and an electrochemical device. Background Art

[0002] At present, the commercial negative electrode materials for lithium batteries are mainly graphite-based materials such as artificial graphite and natural graphite. The volume expansion during the lithium insertion and extraction process is basically below 9%, showing a relatively high Coulomb efficiency and excellent cycle stability. However, the relatively low theoretical lithium storage capacity (372 mAh / g) of the graphite electrode itself makes it difficult to meet the requirements of new lithium-ion batteries for high energy density.

[0003] Silicon-based negative electrode materials are high-capacity negative electrode materials, with a theoretical capacity of 4200 mAh / g, which is more than ten times higher than that of current commercial graphite negative electrodes. Therefore, the batch application of silicon-based negative electrode materials in power batteries can significantly improve the energy density of power batteries and greatly promote the process of energy transformation. However, silicon-based negative electrode materials have a relatively high volume expansion (>300%) during the charge and discharge process. With the increase in the number of cycles, the repeated volume expansion will lead to the pulverization and shedding of the electrode materials, resulting in a rapid decline in the performance of the silicon negative electrode. In addition, silicon is a semiconductor material with poor conductivity and poor kinetic performance. Summary of the Invention

[0004] In view of the above disadvantages of the prior art, the present invention provides a negative electrode sheet, a preparation method thereof, and an electrochemical device to improve the problem of performance attenuation caused by expansion during the cycling of silicon negative electrodes.

[0005] To achieve the above and other related objects, the present invention provides a negative electrode sheet, which includes: a three-dimensional porous current collector material, a silicon material, and a conductive gel. The silicon material is coated on the three-dimensional porous current collector material, and the conductive gel is coated on the surfaces of the three-dimensional porous current collector material and the silicon material.

[0006] In an example of the present invention, the three-dimensional porous current collector material is selected from nickel foam; the silicon content per unit area of the nickel foam is 0.4 g / cm 2 to 0.5 g / cm 2 .

[0007] In an example of the present invention, the mass ratio of the nickel foam to the silicon material is 1:(0.4 to 2); and / or, the coating thickness in 60% to 80% of the area of the conductive gel is 50 nm to 130 nm.

[0008] In an example of the present invention, the silicon material is selected from at least one of elemental silicon, silicon oxide compounds, or silicon-carbon composites, and the particle size of the silicon material is 20 nm to 200 nm.

[0009] In an example of the present invention, the three-dimensional porous current collector material is a current collector material surface-modified with a silane coupling agent, and the silane coupling agent is selected from at least one of 3-aminopropyltriethoxysilane, 3-2(2-aminoethylamine)propyltrimethoxysilane, and diallyldimethylammonium chloride.

[0010] In an example of the present invention, the specific surface area of the negative electrode sheet is 200 m 2 / g to 220 m 2 / g

[0011] On the other hand, the present invention provides a method for preparing a negative electrode sheet, and the preparation method at least includes the following steps:

[0012] Disperse the silicon material into a solvent to obtain a first dispersion;

[0013] Immerse the three-dimensional porous current collector material into a silane coupling agent for modification treatment to obtain a three-dimensional porous current collector material with polar groups on the surface;

[0014] Immerse the three-dimensional porous current collector material with polar groups on the surface into the first dispersion to obtain a current collector@coupling agent@silicon precursor;

[0015] Disperse the oxidant into a solvent to obtain a second dispersion;

[0016] Add an organic acid, a conductive monomer, and the second dispersion to the current collector@coupling agent@silicon precursor to polymerize and crosslink the conductive monomer to obtain a negative electrode sheet.

[0017] In an example of the present invention, before preparing the first dispersion, a pretreatment of the silicon material is further included, and the pretreatment includes: heating the silicon material in an air atmosphere to 500 °C to 1000 °C and holding for 60 min to 180 min.

[0018] In an example of the present invention, the solvents of the first dispersion and the second dispersion are each independently selected from at least one of water, methanol, ethanol, propanol, isopropanol, butanol, ethylene glycol, acetone, dimethylformamide, dimethyl sulfoxide, and ethyl acetate.

[0019] In an example of the present invention, the silane coupling agent is selected from at least one of 3-aminopropyltriethoxysilane, 3-2(2-aminoethylamine)propyltrimethoxysilane, and diallyldimethylammonium chloride.

[0020] In an example of the present invention, the oxidant is selected from at least one of ammonium persulfate, hydrogen peroxide, ferric chloride, benzoyl peroxide, ammonium cerium sulfate, sodium persulfate, and cerium sulfate.

[0021] In an example of the present invention, the organic acid is selected from at least one of phytic acid, citric acid, succinic acid, and tartaric acid.

[0022] In an example of the present invention, the conductive monomer is selected from at least one of aniline, aniline derivatives, thiophene, thiophene derivatives, pyrrole, and pyrrole derivatives.

[0023] In an example of the present invention, the mass of the organic acid accounts for 6% to 60% of the mass of the current collector @ coupling agent @ silicon precursor; the mass of the conductive monomer accounts for 10% to 50% of the mass of the current collector @ coupling agent @ silicon precursor.

[0024] In an example of the present invention, the mass of the organic acid accounts for 10% to 50% of the mass of the current collector @ coupling agent @ silicon precursor, and the mass of the conductive monomer accounts for 10% to 40% of the mass of the current collector @ coupling agent @ silicon precursor.

[0025] In an example of the present invention, the mass of the oxidant accounts for 0.15% to 10% of the mass of the solvent in the second dispersion.

[0026] The present invention also provides an electrochemical device, which includes the negative electrode sheet described above or the negative electrode sheet prepared by the above preparation method.

[0027] In the present invention, the silicon material is coupled with the current collector material having a three-dimensional porous structure by an electrostatic self-assembly method, and a conductive gel is in-situ coated on the surface by forming a molecular bond to prepare a silicon-based composite material with a three-dimensional porous conductive framework. The three-dimensional porous framework can provide a accommodation space for the volume expansion of the silicon material. At the same time, as a rigid current collector, the three-dimensional porous current collector material can effectively prevent the electrode from pulverizing during repeated volume expansion; the in-situ coating process is used to coat the conductive gel on the surfaces of the silicon material and the current collector material to form a three-dimensional conductive network, improving the stability and charge transport of the silicon negative electrode. The silicon-based composite material of the present invention has a high reversible capacity and excellent charge-discharge kinetic performance as a negative electrode sheet. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0029] Figure 1 It is a flowchart of an embodiment of the preparation method of the negative electrode sheet of the present invention;

[0030] Figure 2FT-IR spectra of the negative electrode sheets prepared in one embodiment of the present invention and the negative electrode sheets prepared in the comparative example;

[0031] Figure 3 N2 adsorption-desorption isotherms of the negative electrode sheet prepared in one embodiment of the present invention and the original silicon particles. Detailed implementation manners

[0032] The following specific examples illustrate the implementation manners of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific implementation manners. Various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that, without conflict, the following embodiments and the features in the embodiments can be combined with each other.

[0033] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. The terms used in the specification of the present invention herein are only for the purpose of describing specific embodiments, and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more of the related listed items.

[0034] Unless otherwise stated or there is a contradiction, the terms or phrases used herein have the following meanings:

[0035] As used herein, "a plurality of", "a variety of", "multiple times", etc., unless otherwise specified, mean greater than 2 or equal to 2 in number. For example, "one or more" means one or greater than or equal to two.

[0036] As used herein, "preferred", "better", "more preferable" are only used to describe embodiments or examples with better effects, and it should be understood that they do not constitute a limitation on the protection scope of the present invention. If there are multiple "preferred" in a technical solution, unless otherwise specified and there is no contradiction or mutual restriction relationship, each "preferred" is independent of each other.

[0037] As used herein, "further", "even further", "especially", etc. are used for descriptive purposes, indicating differences in content, but should not be construed as a limitation on the protection scope of the present invention.

[0038] As used herein, when referring to a numerical range, unless otherwise specified, the distribution of optional values within this numerical range is considered continuous, and includes the two numerical endpoints of this numerical range (i.e., the minimum value and the maximum value), as well as each numerical value between these two numerical endpoints. When providing multiple numerical ranges to describe features or characteristics, these numerical ranges can be combined.

[0039] Silicon has attracted much attention due to its high theoretical storage capacity (up to 4200 mAh / g), low reactivity with electrolytes, abundant natural reserves, and low price. However, during the highly reversible lithium insertion / extraction process of silicon-based materials, there is a very significant volume expansion (volume expansion rate > 300%), and the resulting mechanical stress causes the electrode material to gradually pulverize during cycling, the material structure is damaged, and the electrical contact between active substances is lost, leading to a decline in cycling performance. In addition, silicon is a semiconductor material with relatively poor electrical conductivity. The above reasons limit the commercial application of silicon-based materials. Therefore, while achieving high capacity, how to limit the volume expansion of silicon-based materials and improve their cycling stability has become the research focus and difficulty of this type of material.

[0040] Based on this, the present invention provides a negative electrode sheet, a preparation method thereof, and an electrochemical device. By compounding a silicon material with a current collector material having a three-dimensional porous structure to form a composite material having a three-dimensional porous structure, the three-dimensional porous framework is used to provide a accommodation space for the volume expansion of silicon particles; in addition, a conductive gel is coated on the silicon material and the three-dimensional porous current collector material by means of an in-situ coating process to improve the stability and electrical conductivity of the silicon-based negative electrode material.

[0041] In a first aspect of the present invention, a negative electrode sheet is provided. The negative electrode sheet includes a three-dimensional porous current collector material, a silicon material, and a conductive gel. Among them, the three-dimensional porous current collector material serves as a framework and has a rich pore structure; the silicon material is coated on the surface and / or inside the pores of the three-dimensional porous current collector material; the conductive gel is coated on the surfaces of the three-dimensional porous current collector material and the silicon material to form a silicon-based composite material having a three-dimensional porous structure. This material has a relatively high specific surface area (200 m 2 g -1 to 220 m 2 g -1 ), which is beneficial to the transport of liquid electrolytes, enables the electrolyte to fully contact with the active material, reduces the diffusion path of lithium ions; it can also provide reactive sites and improve the efficiency of electrochemical reactions; this material can be directly used as a negative electrode sheet in a battery without adding a binder and a conductive agent, effectively absorbing and buffering the volume expansion effect and improving the cycling performance of the material.

[0042] In one embodiment, the three-dimensional porous current collector material is an inert embedded metal material; it should be noted that inert embedded metal refers to a metal that cannot form an intermetallic compound or alloy with lithium. As an example, the three-dimensional porous current collector material can be any one of copper foil mesh, copper wire mesh, copper foam, and nickel foam; further, the three-dimensional porous current collector material is nickel foam. Furthermore, the average pore size of the three-dimensional porous current collector material is 100 to 500 μm, for example, 100 μm, 300 μm, or 500 μm, etc. The pore structure of the three-dimensional porous current collector material determines the final specific surface area of the negative electrode sheet.

[0043] Preferably, the three-dimensional porous current collector material is a current collector material modified by a silane coupling agent. After the three-dimensional porous current collector material is modified by the silane coupling agent, the surface carries positively charged molecular groups (such as -NH2, etc.), and -NH2 can generate electrostatic force and hydrogen bond interaction with the negatively charged -OH groups on the surface of the silicon material, thereby making the combination of the current collector and the silicon material more compact. In one example, the silane coupling agent is selected from at least one of 3-aminopropyltriethoxysilane, 3-2(2-aminoethylamine)propyltrimethoxysilane, and diallyldimethylammonium chloride. That is, the silane coupling agent can be any one of the materials listed above, or any two or three of them. For example, 3-aminopropyltriethoxysilane, or a mixture of 3-aminopropyltriethoxysilane and 3-2(2-aminoethylamine)propyltrimethoxysilane, or a mixture of 3-aminopropyltriethoxysilane, 3-2(2-aminoethylamine)propyltrimethoxysilane, and diallyldimethylammonium chloride, etc., and will not be listed one by one here.

[0044] In one embodiment, the silicon material is selected from any one of elemental silicon (Si), silicon oxide compound (SiO x , 0 < x < 2), or silicon carbide composite (SiC). Further, the particle size of the silicon material is 20 nm to 200 nm, for example, 20 nm, 50 nm, 150 nm, or 200 nm, etc. Furthermore, the particle size of the silicon material is 80 nm to 100 nm. Small particle size silicon materials have characteristics such as large specific surface area, short ion diffusion path, strong creepability, and high plasticity, which can alleviate the volume effect to a certain extent and improve their electrochemical performance. However, if the particles of the material are too small, severe agglomeration will occur during the cycling process. The agglomerated material will be difficult to exhibit the characteristics of nanoparticles and will also affect the silicon distribution uniformity, thus limiting the further improvement of its cycling performance. Therefore, selecting a silicon material with a particle size in the range of 20 nm to 200 nm can not only meet the coating requirements of this application but also prevent particle agglomeration.

[0045] In this application, the mass ratio of the three-dimensional porous current collector material to the silicon material is 1:(0.4 - 2). Optionally, the mass ratio of the two is 1:0.4, or 1:1, or 1:2, etc. Further, the silicon coating amount per unit area of the current collector is 0.4 g / cm 2 to 0.5 g / cm 2 , for example 0.45 g / cm 2 .

[0046] The coating of the conductive gel can improve the conductivity and stability of the silicon-based negative electrode. In this application, the conductive gel is formed by the polymerization and cross-linking of conductive monomers. The above-mentioned conductive monomers include at least one of aniline, aniline derivatives, thiophene, thiophene derivatives, pyrrole, and pyrrole derivatives; preferably, the conductive monomer is aniline and / or aniline derivatives. The coating amount of the conductive gel is based on completely coating the three-dimensional porous current collector material and the silicon material. Further, the coating thickness of the area with an area ratio of 60% to 80% in the conductive gel coating layer is 50 nm to 130 nm.

[0047] The negative electrode sheet of the present invention uses the three-dimensional porous current collector material as a framework, which can provide a space for accommodating the volume expansion of silicon particles. At the same time, as a rigid current collector, the three-dimensional porous material can effectively prevent the electrode from pulverizing during repeated volume expansion; a layer of conductive gel is coated on the surface of the three-dimensional porous current collector material and the silicon material, improving the stability and charge transfer of the silicon negative electrode, so that the negative electrode sheet has a high reversible capacity and excellent charge and discharge kinetic performance.

[0048] Please refer to Figure 1 , the second aspect of the present invention provides a method for preparing a negative electrode sheet, which at least includes steps S1 to S5:

[0049] S1. Disperse the silicon material into a solvent to obtain a first dispersion;

[0050] S2. Immerse the three-dimensional porous current collector material in a silane coupling agent for modification treatment to obtain a three-dimensional porous current collector material with polar groups on the surface;

[0051] S3. Immerse the three-dimensional porous current collector material with polar groups on the surface in the first dispersion to obtain a current collector@coupling agent@silicon precursor;

[0052] S4. Disperse the oxidant into a solvent to obtain a second dispersion;

[0053] S5. Add an organic acid, a conductive monomer, and the second dispersion to the current collector@coupling agent@silicon precursor, so that the conductive monomer polymerizes and cross-links on the surfaces of the current collector and the silicon material to form a conductive gel, and obtain the negative electrode sheet.

[0054] The specific process of step S1 is as follows: Weigh a certain amount of silicon material and place it in a container, add a solvent, and disperse it evenly to obtain a first dispersion. Among them, the silicon material is selected from any one of elemental silicon, silicon oxide compounds, or silicon-carbon composite materials, preferably elemental silicon. The particle size of the silicon material is 20 nm to 200 nm, such as 20 nm, 50 nm, 150 nm, or 200 nm, preferably 80 nm to 100 nm. The above-mentioned solvent is selected from at least one of water, methanol, ethanol, propanol, isopropanol, butanol, ethylene glycol, acetone, dimethylformamide, dimethyl sulfoxide, and ethyl acetate; further, the solvent is selected from at least one of deionized water, ethanol, isopropanol, and ethyl acetate, and still further, the solvent is deionized water and / or ethanol. The dispersion treatment can select conventional technical means in the art that can disperse solid particles evenly, such as ultrasonic treatment, high-speed stirring, and so on.

[0055] The purpose of preparing the first dispersion is to disperse the silicon material evenly so that the silicon material can be evenly attached to the skeleton of the three-dimensional porous current collector. In the first dispersion, the ratio of the solvent to the silicon material is not specifically limited, and the addition amount of the solvent increases adaptively with the increase in the mass of the silicon material, as long as the silicon material can be dispersed evenly. As an example, in the first dispersion, the mass of the silicon material is 0.2 g to 5 g, for example, 0.2 g, 2 g, 4 g, or 5 g, etc.; the addition amount of the solvent is 1 to 50 ml, for example, 1 ml, 10 ml, 30 ml, or 50 ml, etc.

[0056] Preferably, before performing step S1, the silicon material needs to be pretreated, and the process is as follows: Place the silicon material in an atmosphere furnace, heat it to 500 °C to 1000 °C, such as 800 °C, in an air atmosphere, and keep it at this temperature for 60 min to 180 min, such as 120 min. This process is the high-temperature treatment. On the one hand, this process can remove the dirt on the surface of the silicon material to achieve the purpose of cleaning. On the other hand, an oxide layer and hydroxyl groups (·OH) can be formed on the surface of the silicon material, which is convenient for the combination of the silicon material with the three-dimensional porous current collector material and the conductive gel.

[0057] The three-dimensional porous current collector material in step S2 is selected from inert intercalation metals that cannot form intermetallic compounds or alloys with lithium. For example, any one of copper foil mesh, copper wire mesh, copper foam, and nickel foam. In this embodiment, the three-dimensional porous current collector material is selected as nickel foam, and the average pore diameter of the nickel foam is 100 to 500 μm, such as 100 μm, 300 μm, or 500 μm, etc. The pore distribution of the three-dimensional porous current collector material will directly affect the specific surface area of the final material.

[0058] Step S2 is to perform surface modification on the three-dimensional porous current collector material so that the surface of the current collector carries polar groups, facilitating the combination with the silicon material. Specifically as follows: Cut the three-dimensional porous current collector material into small sizes according to actual needs, then immerse it in a beaker containing a silane coupling agent and let it stand for 5 - 20 minutes. After that, remove the excess silane coupling agent, and after drying treatment, a surface-modified current collector is obtained. In this step, conventional drying methods can be used for drying treatment. For example, dry it in a vacuum drying oven at 80°C. It should be noted that the dosage of the silane coupling agent is not limited here, as long as it covers the three-dimensional porous current collector material. For example, the three-dimensional porous current collector material is cut into small round pieces with a diameter of 12 mm and immersed in a beaker containing 10 ml - 50 ml of silane coupling agent and left to stand for 10 minutes.

[0059] Principle of surface modification of the three-dimensional porous current collector material using a silane coupling agent: The molecular formula of the silane coupling agent is Y-R-Si-X3, where Y represents an organic functional group, R represents an alkylene group, and X represents a group that can be hydrolyzed. Y mainly reacts with organic polymers, while the hydrolyzable group X mainly controls the hydrolysis rate. The three hydrolyzable groups connected to Si in the silane coupling agent react with water to form silanols; the silanols dehydrate and condense to form oligomeric siloxanes; after drying treatment, the oligomeric siloxanes adhere to the three-dimensional porous current collector to complete the modification of the current collector. In the subsequent synthesis process, the -Y functional group (i.e., -NH2) in the oligomeric siloxane forms intermolecular interactions such as electrostatic forces and hydrogen bonds with the -OH on the surface of the silicon particles, thereby achieving a strong bond with the silicon particles. The above silane coupling agent can be selected from at least one of 3-aminopropyltriethoxysilane, 3-2(2-aminoethylamine)propyltrimethoxysilane, and diallyldimethylammonium chloride, and can be any one of them, or any mixture of two or three of them.

[0060] Step S3 is to place the surface-modified current collector obtained in Step S2 into the first dispersion liquid obtained in Step S1. The nano-silicon particles in the dispersion liquid are in-situ coated on the framework of the three-dimensional porous current collector material through intermolecular interactions to obtain a current collector@coupling agent@Si precursor.

[0061] In step S4, the oxidant is evenly dispersed in the solvent to obtain an oxidant dispersion. The oxidant mainly plays a catalytic role in the polymerization process of the conductive gel. Therefore, the dosage of the oxidant in this step is related to the dosages of the conductive monomer and the organic acid in step S5. The role of the solvent in step S4 here is to disperse the oxidant, and the ratio of the solvent to the oxidant is not limited as long as the oxidant can be evenly dispersed. As an example, the mass of the oxidant in the second dispersion accounts for 0.15 to 10% of the mass of the solvent. For example, the mass of the oxidant is 0.15%, 5%, 8% or 10% of the mass of the solvent, etc. The oxidant includes at least one of ammonium persulfate, hydrogen peroxide, ferric chloride, benzoyl peroxide, ammonium cerium sulfate, sodium persulfate, cerium sulfate. The solvent is selected from at least one of water, methanol, ethanol, propanol, isopropanol, butanol, ethylene glycol, acetone, dimethylformamide, dimethyl sulfoxide, ethyl acetate. Further, the solvent is at least one of deionized water, ethanol, isopropanol, ethyl acetate. Still further, the solvent is deionized water and / or ethanol.

[0062] In step S5, the organic acid, the conductive monomer and the second dispersion prepared in step S4 are added to the current collector @ coupling agent @ Si precursor prepared in step S3. At room temperature, the conductive monomer undergoes polymerization crosslinking to form a layer of conductive polymer gel on the surfaces of the three-dimensional porous current collector material and the silicon material. Subsequently, the solvent is removed, and finally a three-dimensional porous silicon-based composite material (negative electrode sheet) with the three-dimensional porous current collector material as the skeleton is obtained.

[0063] Among them, the organic acid plays a crosslinking role for the gel. The organic acid includes at least one of phytic acid, citric acid, succinic acid, tartaric acid. The conductive monomer is the monomer for forming the conductive gel, and it can be selected from at least one of aniline, aniline derivatives, thiophene, thiophene derivatives, pyrrole, pyrrole derivatives. Preferably, the conductive monomer includes aniline and / or aniline derivatives.

[0064] In step S5, the added mass of the organic acid accounts for 6% to 60% of the mass of the current collector @ coupling agent @ Si precursor. Optionally, the added mass of the organic acid accounts for 10% to 50% of the mass of the current collector @ coupling agent @ Si precursor, such as 20%, 30% or 40%, etc. The added mass of the conductive monomer accounts for 10% to 50% of the mass of the current collector @ coupling agent @ Si precursor. Optionally, the mass of the conductive monomer accounts for 10% to 40% of the mass of the current collector @ coupling agent @ Si precursor, such as 10%, 20%, 30% or 40%, etc.

[0065] Preferably, when adding an organic acid, a conductive monomer, and a second dispersion containing an oxidant to the precursor, first add the organic acid and the conductive monomer, disperse them evenly by ultrasonic waves, and then add the second dispersion. Since the conductive monomer will rapidly polymerize and crosslink upon encountering the oxidant, dispersing the organic acid and the conductive monomer evenly first and then adding the oxidant will result in a more uniform conductive gel.

[0066] The method for removing the solvent in step S5 is one or a combination of methods such as vacuum drying, spray drying, freeze drying, and filtration drying. Preferably, it is a combination of filtration drying and vacuum drying.

[0067] After step S5 is completed, a silicon-based composite material with a three-dimensional porous current collector as the skeleton can be obtained. It can be directly used as a negative electrode sheet in electrochemical devices such as batteries without the need to additionally add a conductive agent and a binder, effectively suppressing the volume expansion of silicon particles, stabilizing the electrode structure, improving the charge conduction efficiency at the same time, and having a high reversible capacity and excellent charge-discharge kinetic performance.

[0068] The third aspect of the present invention provides an electrochemical device, which includes the above-mentioned negative electrode sheet or a negative electrode sheet prepared by the above-mentioned preparation method.

[0069] In some embodiments, the electrochemical device can be a primary lithium-ion battery, a secondary lithium-ion battery, a primary sodium-ion battery, a secondary sodium-ion battery, etc. Taking the secondary lithium-ion battery as an example, its structure will be described in detail below: The secondary lithium-ion battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator. The negative electrode sheet is the negative electrode sheet described above in the present invention or prepared by the above-mentioned preparation method, and will not be elaborated here.

[0070] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector. The positive electrode current collector is, for example, a foil formed by surface treatment of nickel, titanium, aluminum, silver, stainless steel, or carbon, etc. In addition to the foil, the positive electrode current collector can also be used in any one or a combination of various forms such as film, mesh, porous, and foam. Among them, the thickness of the positive electrode current collector is, for example, 8 μm to 15 μm. In this embodiment, the positive electrode current collector is, for example, an aluminum foil, and the thickness of the aluminum foil is, for example, 13 μm. The positive electrode current collector has two relatively arranged surfaces along its own thickness direction, and the positive electrode active material layer can be provided on any one of the surfaces or on both surfaces. The positive electrode active material layer includes a positive electrode active material, a conductive agent, and a binder. No specific limitations are imposed on the positive electrode active material, the conductive agent, and the binder here, and those skilled in the art can select according to actual needs.

[0071] As an example, the positive electrode active material is selected from one or more of lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium nickel cobalt manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt aluminum oxide, and lithium iron phosphate, but the present application is not limited to these materials. The binder is selected from any one or more of polyvinylidene fluoride (PVDF), poly (ethylene oxide) (PEO), polyamide (PA), polyacrylonitrile (PAN), polyacrylate, polyvinylether, polymethyl methacrylate (PMMA), ethylene-propylene-diene terpolymer (EPDM), polyhexafluoropropylene, or polymerized styrene butadiene rubber (SBR). The conductive agent is, for example, selected from one or at least two of super P, acetylene black, graphene, carbon nanotubes, carbon nanofibers, etc. The ratio of the positive electrode active material, the conductive agent, and the binder in the positive electrode active material layer can be set according to the conventional settings in the art.

[0072] The separator is disposed between the positive electrode plate and the negative electrode plate to play an isolating role. The separator can be, for example, a polyethylene film (PE), a polypropylene film (PP), a glass fiber film, a polyethylene film, or a composite film. The thickness of the separator is 9 to 18 μm, the air permeability is 180 s / 100 mL to 380 s / 100 mL; the porosity is 30% to 50%.

[0073] The electrolyte transports lithium ions between the positive and negative electrodes, and the electrolyte can be selected from conventional combination types in the art. The electrolyte includes an organic solvent and a lithium salt. Among them, the organic solvent can be selected from 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), etc., or a mixture of multiple kinds in any proportion. The lithium salt is selected from one or a mixture of multiple combinations of LiPF6 (lithium hexafluorophosphate), LiBF4 (lithium tetrafluoroborate), LiClO4 (lithium perchlorate), LiAsF6 (lithium hexafluoroarsenate), LiFSI (lithium bis(fluorosulfonyl)imide), LiTFSI (lithium bis(trifluoromethanesulfonyl)imide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluorooxalate borate), LiBOB (lithium bis(oxalate) borate), LiPO2F2 (lithium difluorophosphate), LiDFOP (lithium difluorodioxalate phosphate), LiTFOP (lithium tetrafluorooxalate phosphate). The molar concentration of the lithium salt in the electrolyte is 0.5 mol / L to 2 mol / L.

[0074] In other embodiments, some film-forming additives and functional additives that can improve battery performance can also be added to the electrolyte. For example, vinylene carbonate (VC), 1,3 - propane sultone (PS), ethylene sulfate (DTD), etc. Those skilled in the art can select according to actual needs.

[0075] The preparation process of the lithium-ion secondary battery is described below:

[0076] (1) Preparation of the positive electrode sheet

[0077] Mix the above-mentioned positive electrode active material, conductive agent, and binder in a certain mass ratio, and then add the solvent N-methylpyrrolidone (NMP) to a blender and stir evenly to obtain a positive electrode slurry; then evenly coat the positive electrode slurry on the positive electrode current collector, and after drying, rolling, and slitting, a positive electrode sheet is obtained.

[0078] (2) Preparation of the negative electrode sheet, see above.

[0079] (3) Preparation of the electrolyte

[0080] In an argon atmosphere glove box with a water content < 10 ppm, dissolve the fully dried lithium salt (LiPF6) in an organic solvent, and after mixing evenly, an electrolyte is obtained, where the concentration of LiPF6 is 1 mol / L.

[0081] (4) Preparation of the separator

[0082] Select a 12 μm thick polypropylene (PP) or polyethylene (PE) porous polymer film.

[0083] (5) Assembly of the battery:

[0084] The above-prepared positive electrode sheet, separator, and negative electrode sheet are stacked or wound in sequence to form a bare battery cell; then it is wrapped with an aluminum-plastic film, transferred to a vacuum drying oven for drying at 120 °C, filled with electrolyte and then sealed, and electrolytic liquefaction is carried out to finally prepare a soft-pack battery (i.e., a lithium-ion battery).

[0085] Those skilled in the art will understand that the preparation method of the lithium-ion secondary battery described above is only an example. Other commonly used methods in the art can be adopted without departing from the content disclosed in this application.

[0086] The present invention also provides an electronic device, which includes at least one of the above lithium-ion secondary batteries. The lithium-ion secondary battery can be used in the electronic device in the form of a single battery cell, a battery module, or a battery pack to supply power to it.

[0087] The electronic device can be a vehicle, a mobile phone, a portable device, a laptop computer, a ship, a spacecraft, an electric toy, and an electric tool, etc. The vehicle is, for example, a new energy vehicle, and the new energy vehicle can be a pure electric vehicle, a hybrid vehicle, or an extended-range vehicle, etc. The spacecraft includes an airplane, a rocket, a space shuttle, and a spaceship, etc. The electric toy includes a fixed or mobile electric toy, such as a game console, an electric vehicle toy, an electric ship toy, and an electric airplane toy, etc. The electric tool includes a metal cutting electric tool, a grinding electric tool, an assembly electric tool, and a railway electric tool, such as a drill, an electric grinding wheel, an electric wrench, an electric screwdriver, a hammer drill, an impact drill, a concrete vibrator, and a planer, etc. The electronic device includes the above lithium-ion secondary battery, so it has the advantages of the above lithium-ion secondary battery, which will not be elaborated here.

[0088] The technical solutions of the present invention will be described in detail below through several specific examples and comparative examples. Unless otherwise specified, the raw materials and reagents used in the following examples are all commercially available products, or can be prepared by conventional methods in the art, and the instruments used in the examples can be purchased commercially.

[0089] Example 1

[0090] This example provides a negative electrode sheet, which includes a three-dimensional porous current collector material, a silicon material, and a conductive gel. In this example, the three-dimensional porous current collector material is nickel foam, and the nickel foam is cut into a circular sheet with a diameter of 12 mm; the silicon material is elemental silicon, and the particle size of the elemental silicon is 200 nm. The conductive gel is formed by cross-linking and polymerization of the conductive monomer aniline under the action of phytic acid and ammonium persulfate.

[0091] The preparation process of the negative electrode sheet is as follows:

[0092] (1) Heat 1 g of silicon powder with a size of 200 nm at 800 °C for 60 min; take 0.5 g of the silicon powder after high-temperature treatment and add it to a sample bottle, then add 10 mL of deionized water and perform ultrasonic treatment to obtain a first dispersion.

[0093] (2) Immerse 0.5 g of nickel foam into a beaker containing 20 mL of silane coupling agent, let it stand for 10 min to make the surface of the nickel foam skeleton carry polar molecular groups, and then filter to remove the excess silane coupling agent to obtain surface-modified nickel foam.

[0094] (3) Put the surface-modified nickel foam obtained in step (2) into the sample bottle containing the first dispersion in step (1). The nano-silicon in the dispersion is in-situ coated on the surface-modified nickel foam skeleton through electrostatic force and hydrogen bond force to obtain nickel foam@coupling agent@Si precursor.

[0095] (4) Add 0.05 g of ammonium persulfate to 10 mL of deionized water and disperse it evenly to obtain a second dispersion.

[0096] (5) At room temperature, add 0.16 g of phytic acid and 0.1 g of aniline to the sample bottle in step (3), perform ultrasonic treatment, and then add the second dispersion containing the oxidant ammonium persulfate. Aniline rapidly polymerizes and crosslinks after adding the oxidant to form a layer of conductive polymer gel on the nickel foam skeleton and the surface of silicon particles; then remove the excess solvent to finally obtain a three-dimensional porous silicon-based composite material (negative electrode sheet) with nickel foam as the skeleton.

[0097] In the negative electrode sheet prepared in this example, the coating thickness in the area where the area ratio of the conductive gel coating layer is 60% - 80% is 50 nm to 130 nm. The thickness of the conductive gel is obtained by observing the cross-section of the negative electrode sheet under a high-resolution transmission electron microscope and measuring the thickness of the outermost coating layer.

[0098] Example 2

[0099] The difference between this example and Example 1 is that: in step (5), 0.2 g of aniline is added, and the other steps are the same.

[0100] Example 3

[0101] The difference between this example and Example 1 is that: in step (5), 0.38 g of aniline is added, and the other steps are the same.

[0102] Example 4

[0103] The difference between this example and Example 1 is that: in step (5), 0.32 g of phytic acid and 0.2 g of aniline are added, and the other steps are the same.

[0104] Example 5

[0105] The difference between this embodiment and Embodiment 1 is that: in step (5), 0.48 g of phytic acid and 0.2 g of aniline are added, and the remaining steps are the same.

[0106] Embodiment 6

[0107] The difference between this embodiment and Embodiment 1 is that: in step (5), 0.60 g of phytic acid and 0.2 g of aniline are added, and the remaining steps are the same.

[0108] Embodiment 7

[0109] The difference between this embodiment and Embodiment 1 is that: in step (5), 0.50 g of aniline is added, and the remaining steps are the same.

[0110] Comparative Example 1

[0111] The difference between this comparative example and Embodiment 4 is that: the step of preparing the second dispersion liquid in step (4) is omitted, and the second dispersion liquid containing an oxidant is not added in step (5).

[0112] Comparative Example 2

[0113] The difference between this comparative example and Embodiment 4 is that: organic acid phytic acid is not added in step (5).

[0114] Comparative Example 3

[0115] The difference between this comparative example and Embodiment 4 is that: conductive monomer aniline is not added in step (5).

[0116] Comparative Example 4

[0117] The difference between this comparative example and Embodiment 4 is that: the process of surface modification of nickel foam in step (2) is omitted, that is, the nickel foam used in this comparative example is nickel foam without surface modification.

[0118] The silicon-based composites prepared in Embodiments 1 to 7 and Comparative Examples 1 to 4 are directly used as the negative electrode sheets. Using a lithium sheet as the counter electrode, a CR2016 coin-type half cell is assembled in a glove box, and the batteries assembled in each embodiment and comparative example are tested. The test results are shown in Table 1, and the test process is as follows:

[0119] Place the battery on a NEWARE CT-4008T battery test system, set the voltage range to 0.01 - 1.5 V (vs. Li / Li+), the mass of the active material is the mass of the silicon powder added to the sample bottle in the synthesis step 1, set the gram capacity of the electrode to 1 Ah / g, and use A / g as the magnitude of the charge and discharge current.

[0120] Initial Coulombic efficiency: First, perform constant-current discharge at a current of 0.1 A / g until the cut-off voltage reaches 0.01 V, and record the discharge specific capacity. Then, perform constant-current charge at a current of 0.1 A / g until the cut-off voltage reaches 1.5 V, and record the charge specific capacity. The initial Coulombic efficiency = charge specific capacity / discharge specific capacity

[0121] Discharge specific capacity and cycle: Perform constant-current charge-discharge cycles at a current of 0.5 A / g within the voltage range of 0.01 - 1.5 V, and record the discharge specific capacity in each cycle until the test ends after 150 cycles. The capacity retention rate after 150 cycles = (discharge capacity retention rate after 150 cycles / initial discharge specific capacity) * 100%.

[0122] Table 1: Test results of the batteries assembled from Examples 1 - 7 and Comparative Examples 1 - 4

[0123]

[0124]

[0125] It can be concluded from Table 1 that for the negative electrode sheets prepared in Examples 1 - 7, compared with Comparative Examples 1 - 4, their initial Coulombic efficiency, initial discharge specific capacity, and cycle retention rate have been significantly improved, indicating that three-dimensional conductive gel networks have been formed in Examples 1 - 7, effectively improving the conductivity and stability of the electrode.

[0126] By comparing Examples 1, 2, 3, and 7, it can be concluded that appropriately increasing the addition amount of the conductive monomer can effectively improve the initial efficiency, discharge specific capacity, and cycle stability of the electrode. However, too high an addition amount of the conductive monomer will also affect its performance. This is because the conductive monomer and the organic acid undergo a cross-linking polymerization reaction under the catalysis of the oxidant to form a conductive gel. When the amount of the conductive monomer is too much or too little, it is not conducive to the formation of the conductive gel coating layer.

[0127] By comparing Examples 2, 4, 5, and 6, it can be seen that appropriately increasing the addition amount of the organic acid can effectively improve the initial efficiency, discharge specific capacity, and cycle stability of the electrode. However, too much organic acid will also affect its performance. The test results in Table 1 show that when the addition amount of the organic acid is about 0.3 g (30% of the mass of the precursor), the material performance is the best.

[0128] By comparing Example 4 with Comparative Example 1, it can be seen that without adding the oxidant, the organic acid and the conductive monomer cannot undergo a cross-linking reaction, so a three-dimensional conductive gel network cannot be formed on the surface of the silicon particles, resulting in a performance reduction. Figure 2 The infrared spectra of the materials prepared in Example 4 and Comparative Example 1 are shown. It can be seen from the figure that the material of Example 4 has absorption peaks at 1560 cm -1 and 1450 cm-1 Covalent bonds and hydrogen bonds are respectively formed at the [specific positions], confirming the formation of the conductive gel.

[0129] Comparing Example 4, Comparative Example 2, and Comparative Example 3, it can be seen that in any case where the organic acid or the conductive monomer is lacking, the oxidant cannot exert its due catalytic effect. Only when the organic acid and the conductive monomer coexist, the conductive gel will be formed through the catalysis of the oxidant, and the formed conductive gel will significantly improve the material properties.

[0130] Comparing Example 4 and Comparative Example 4, it can be seen that after the surface modification of the nickel foam, the silicon particles will be uniformly coated on the nickel foam skeleton. On the contrary, for the unmodified nickel foam, since no force can be generated between it and the silicon particles, the silicon particles will be disorderly distributed inside the nickel foam, resulting in serious agglomeration, thus seriously affecting the material properties.

[0131] The specific surface areas of the negative electrode sheets (Example 4) prepared in this application and the original silicon particles were respectively measured by the nitrogen adsorption-desorption (BET) method. The test results are shown in Figure 3 , and from Figure 3 it can be seen that the specific surface area of the negative electrode sheet prepared in the embodiment of this application is 210.122 m 2 g -1 , which is larger than that of the original Si particles (only 13.9220 m 2 g -1 ). This also results in the first Coulombic efficiency of the electrode being only about 75%, but its cycle stability has been significantly improved. This is because the three-dimensional porous structure effectively inhibits the volume expansion of the silicon particles, and at the same time, the surface-coated conductive gel improves the charge transfer ability inside the silicon negative electrode. The two work together to enhance the electrochemical stability and kinetic performance of the composite electrode.

[0132] The present invention uses a three-dimensional porous current collector material as the skeleton, couples the silicon material with the three-dimensional porous current collector material through the electrostatic self-assembly method, and in-situ coats a conductive gel on the surface to prepare a silicon-based composite material with a three-dimensional porous conductive skeleton. The three-dimensional porous skeleton can provide an accommodation space for the volume expansion of the silicon material. At the same time, as a rigid current collector, the three-dimensional porous current collector material can effectively prevent the electrode from pulverizing during repeated volume expansion; the in-situ polymerization process is used to coat a conductive gel on the surfaces of the silicon material and the current collector material to form a three-dimensional conductive network, improving the stability and charge transfer of the silicon negative electrode. The silicon-based composite material of the present invention has a high reversible capacity and excellent charge-discharge kinetic performance as a negative electrode sheet. Therefore, the present invention effectively overcomes some practical problems in the prior art and thus has high utilization value and practical significance.

[0133] The above embodiments are only illustrative of the principles and effects of the present invention and are not intended to limit the present invention. Any person familiar with this technology can modify or change the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or changes made by those with ordinary knowledge in the technical field without departing from the spirit and technical idea disclosed by the present invention should still be covered by the claims of the present invention.

Claims

1. A negative electrode plate, characterized in that, Comprising: A three-dimensional porous current collector material; A silicon material coated on the three-dimensional porous current collector material; And A conductive gel coated on the surfaces of the three-dimensional porous current collector material and the silicon material.

2. The negative electrode sheet according to claim 1, wherein, The three-dimensional porous current collector material is selected from nickel foam; the silicon content per unit area of the nickel foam is 0.4 g / cm 2 to 0.5 g / cm 2 .

3. The negative electrode sheet according to claim 2, characterized in that, The mass ratio of the nickel foam to the silicon material is 1:(0.4 to 2); and / or, the coating thickness in 60% to 80% of the area of the conductive gel is 50 nm to 130 nm.

4. The negative electrode sheet according to claim 1, characterized in that, The silicon material is selected from at least one of elemental silicon, silicon oxides, or silicon-carbon composites, and the particle size of the silicon material is 20 nm to 200 nm.

5. The negative electrode sheet according to claim 1, characterized in that, The three-dimensional porous current collector material is a current collector material surface-modified with a silane coupling agent, and the silane coupling agent is selected from at least one of 3-aminopropyltriethoxysilane, 3-2(2-aminoethylamine)propyltrimethoxysilane, and diallyldimethylammonium chloride.

6. A method for preparing the negative electrode sheet according to any one of claims 1 to 5, characterized in that, At least including the following steps: Disperse the silicon material into a solvent to obtain a first dispersion; Immerse the three-dimensional porous current collector material into the silane coupling agent for modification treatment to obtain a three-dimensional porous current collector material with polar groups on its surface; Immerse the three-dimensional porous current collector material with polar groups on its surface into the first dispersion to obtain a current collector@coupling agent@silicon precursor; Disperse the oxidant into a solvent to obtain a second dispersion; Add an organic acid, a conductive monomer, and the second dispersion to the current collector@coupling agent@silicon precursor to polymerize and crosslink the conductive monomer to obtain a negative electrode sheet.

7. The preparation method according to claim 6, wherein Before preparing the first dispersion, a pretreatment of the silicon material is further included, and the pretreatment includes: heating the silicon material in an air atmosphere to 500 °C to 1000 °C and holding for 60 min to 180 min.

8. The preparation method according to claim 6, characterized in that, Including one or more of the following: The solvents of the first dispersion and the second dispersion are each independently selected from at least one of water, methanol, ethanol, propanol, isopropanol, butanol, ethylene glycol, acetone, dimethylformamide, dimethyl sulfoxide, and ethyl acetate; The silane coupling agent is selected from at least one of 3-aminopropyltriethoxysilane, 3-2(2-aminoethylamine)propyltrimethoxysilane, and diallyldimethylammonium chloride; The oxidant is selected from at least one of ammonium persulfate, hydrogen peroxide, ferric chloride, benzoyl peroxide, ammonium cerium sulfate, sodium persulfate, and cerium sulfate; The organic acid is selected from at least one of phytic acid, citric acid, succinic acid, and tartaric acid; The conductive monomer is selected from at least one of aniline, aniline derivatives, thiophene, thiophene derivatives, pyrrole, and pyrrole derivatives.

9. The preparation method according to claim 6, characterized in that, Including one or more of the following: The mass of the organic acid accounts for 6% to 60% of the mass of the current collector@coupling agent@silicon precursor; The mass of the conductive monomer accounts for 10% to 50% of the mass of the current collector@coupling agent@silicon precursor; The mass of the oxidant accounts for 0.15% to 10% of the mass of the solvent in the second dispersion.

10. An electrochemical device, characterized in that, Including the negative electrode sheet according to any one of claims 1 to 5, or the negative electrode sheet prepared by the preparation method according to any one of claims 6 to 9.

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