A high-capacity silicon-based lithium battery anode composite and a method of making the same

Abstract

The application belongs to the field of lithium ion battery negative electrode materials, and relates to a star-shaped structure polyacrylic acid and a preparation method thereof, and application of the star-shaped structure polyacrylic acid as a binder in a battery negative electrode material, and a corresponding high-capacity silicon-based lithium battery negative electrode composite material and a preparation method thereof, and a corresponding lithium battery negative electrode and lithium battery. First, a star-shaped structure polyacrylic acid binder is designed and synthesized, and a plurality of carboxyl groups are contained in linear side chains of the star-shaped structure polyacrylic acid binder, so that a self-repairing multiple hydrogen bond network structure can be formed in situ in a process of being used in combination with electrode active materials such as silicon powder or carbon powder. In addition, the silicon-based material and the carbon-based material are sequentially stacked after being dispersed by water, and the polymer binder is filled in the silicon-based material and the carbon-based material, the carbon-based material acts as a barrier between the silicon-based material and the electrolyte, and the volume expansion of the silicon-based material is inhibited together with the polymer binder, so that the volume expansion of the battery negative electrode in use is reduced, and the cycle stability of the lithium battery is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of lithium ion battery anode materials, in particular to a polyacrylic acid with a star structure and a corresponding high-capacity silicon-based lithium battery anode composite material. BACKGROUND

[0002] To meet the growing energy density requirements of future electrical energy storage and all-electric vehicles, various high-capacity electrode materials are currently being widely developed. Among them, silicon is considered to be the most promising candidate material for the next generation of lithium ion batteries (LIB) because of its high theoretical capacity, abundant natural resources, low electrochemical potential, and high potential as a lithium ion battery anode material. The main problem of silicon as a lithium ion battery anode material is the severe volume change that occurs during the working cycle, which leads to serious pulverization, loss of electrical contact, unstable solid electrolyte interface, and deterioration of electrochemical cycle reversibility.

[0003] To solve the stability problem of the above-mentioned silicon-based materials, for the application scenarios of lithium ion batteries, a large number of nano-structured silicon-based active materials, from zero-dimensional to three-dimensional silicon, especially complex layered structures (such as: porous or hollow structures, core-shell structures, yolk-shell structures, etc.) are proposed to solve the stability problem of silicon active materials. However, the preparation process of this part of the complex structure of silicon-based active materials is complicated and unfeasible, and it is difficult to scale up. In addition, these modified nano-structured or porous structured materials usually have a large specific surface area and inter-particle resistance, which in turn leads to low initial coulombic efficiency (ICE) and small volume capacity, and is not suitable for actual application scenarios. The silicon microparticles (SiMP) that are easily obtained commercially not only have a low price, but also have a higher ICE and energy density. Therefore, silicon microparticles are more promising anode materials in actual industrial applications.

[0004] To solve the problem of severe volume change of silicon microparticles during the working cycle, dispersing silicon microparticles P in a polymer binder can buffer the volume expansion of silicon microparticles during charging. The polymer binder acts as a flexible buffer during deep cycling, allowing the silicon microparticles to expand and break, while maintaining good electrical connections and SEI stability on the surface of the composite material.

[0005] Therefore, for silicon-based lithium batteries, the polymer binder as a non-active material of the battery plays a very important role in the formation of the electrode and the maintenance of the structure. Especially in maintaining the stability of the structure and adjusting the volume expansion. The carboxymethyl cellulose / styrene butadiene rubber (CMC / SBR) composite binder widely used at present has the advantages of high chemical stability, low cost, environmental friendliness, etc., but it is difficult to be used for silicon-based materials due to its weak bonding strength. Polyacrylic acid (PAA) polymer and its modified or derivative products have been widely studied due to their advantages of abundant carboxyl groups and water solubility. There are a large number of sites that can form hydrogen bonds between PAA molecules, but the topological structure of PAA molecules is linear, which limits the network structure formed in situ by PAA binder during the preparation of silicon-based electrodes, and the adaptability to the sharp volume change of high-activity silicon microparticles needs to be improved.

[0006] The present application starts from the topological structure of PAA molecules, designs and synthesizes star-shaped PAA binder, and the radial side chain contains multiple carboxyl groups, including hydrogen bond acceptors and donors. During the use of the electrode active material such as silicon powder or carbon powder, a self-repairing multiple hydrogen bond network structure can be formed in situ, so it can adapt to different types of binding matrix such as silicon-based and carbon-based materials. The existing hydrogen bond network structure has better inhibitory effect on the sharp volume change of silicon-based active materials. In addition, from the perspective of arrangement of silicon-based and carbon-based materials, the silicon-based material and the carbon-based material are sequentially stacked after being dispersed in water, and the polymer binder is filled in both of them. The carbon-based material acts as a barrier between the silicon-based material and the electrolyte, and together with the polymer binder layer, it plays a role in inhibiting the volume expansion and crushing of silicon active materials, reducing the volume expansion of the electrode during use, thereby improving the cycle stability of the lithium battery. SUMMARY

[0007] In view of the defects of the prior art, the present application provides a star-shaped polyacrylic acid and a preparation method thereof, and the use of the star-shaped polyacrylic acid as a binder in a battery negative material, as well as a corresponding high-capacity silicon-based lithium battery negative composite material and a preparation method thereof, and a corresponding lithium battery negative and lithium battery, which solves the technical problem that the volume expansion of the electrode during use of the existing silicon-based lithium battery and the cycle stability of the lithium battery are not ideal.

[0008] The present application is realized by the following technical solutions:

[0009] In a first aspect, the present application provides a star-shaped polyacrylic acid, which has a structure formula as shown in formula (I) or (II), and the structure includes a core A and a linear side chain, the linear side chain contains 2 to multiple carboxyl groups, and the linear side chain is sequentially connected to the core A and the number of linear side chains is greater than or equal to 3, and the number of repeating structural units n in the linear side chain is greater than or equal to 1.

[0010]

[0011]

[0012] Preferably, the number of linear side chains is 3-4;

[0013] Preferably, the chemical structure of core A is selected from:

[0014] In a second aspect, the present application provides a preparation method of the star-shaped structure polyacrylic acid, comprising: reacting 2-bromoisobutyryl bromide with a multi-side chain hydroxyl compound to obtain product I, a multi-arm intermediate, under catalysis of an organic base; polymerizing product I with tert-butyl acrylate to obtain product II, a star-shaped structure tert-butyl acrylate; and hydrolyzing tert-butyl ester groups in product II under catalysis of an organic acid to obtain final product III, a star-shaped structure polyacrylic acid.

[0015] Preferably, the polymerization reaction uses a transition metal catalyst, and more preferably, the transition metal catalyst is selected from CuBr.

[0016] Preferably, the polymerization reaction also uses a tertiary amine catalyst, and more preferably, the tertiary amine catalyst is selected from pentamethyldiethylenetriamine.

[0017] Preferably, the organic acid is selected from trifluoroacetic acid.

[0018] Preferably, the organic base is triethylamine.

[0019] Preferably, the multi-side chain hydroxyl compound comprises trihydroxyethyl isocyanurate or pentaerythritol.

[0020] Preferably, the process of reacting 2-bromoisobutyryl bromide with the multi-side chain hydroxyl compound comprises: reacting 2-bromoisobutyryl bromide with the multi-side chain hydroxyl compound under catalysis of the organic base at room temperature for at least 24 h; and more preferably, the process of reacting 2-bromoisobutyryl bromide with the multi-side chain hydroxyl compound further comprises: extracting, drying, and rotary-evaporating the reaction product to remove the solvent, and recrystallizing and purifying the crude product using methanol to obtain product I, the multi-arm intermediate.

[0021] Preferably, the process of polymerizing product I, the multi-arm intermediate, with tert-butyl acrylate comprises: polymerizing product I, the multi-arm intermediate, with tert-butyl acrylate, a transition metal catalyst, and a tertiary amine catalyst at 80-100°C for at least 24 h.

[0022] More preferably, the process of polymerizing product I, the multi-arm intermediate, with tert-butyl acrylate further comprises: removing metal ions from the reaction product by passing through an alumina column to obtain product II, the star-shaped structure tert-butyl acrylate.

[0023] Preferably, the process of hydrolyzing the t-butyl ester group in product II to obtain the final product III star-shaped polyacrylic acid comprises: precipitating product II in an alcohol / water mixed solution, drying the precipitate and then dissolving it in an organic solvent, adding an organic acid to the solution at a temperature of 0°C to room temperature, and reacting for at least 50 hours;

[0024] More preferably, the process of hydrolyzing the t-butyl ester group in product II to obtain the final product III star-shaped polyacrylic acid further comprises: purifying the precipitated product from the reaction system by dialysis, and then freeze-drying the precipitated product to obtain the final product III star-shaped polyacrylic acid.

[0025] In a third aspect, the present application provides a binder, which at least comprises: the star-shaped polyacrylic acid described above;

[0026] Further, the binder further comprises: a linear structure polymer. The linear structure polymer comprises a repeating structure of hydrogen bond donors or hydrogen bond acceptors.

[0027] Preferably, the repeating structure of hydrogen bond donors or hydrogen bond acceptors comprises one or more of: carboxyl, hydroxyl, amino, ester, ether, amide, urethane, urea, and cyano.

[0028] More preferably, the linear structure polymer comprises one or more of: linear structure polyacrylic acid, polyether, polyester, polyamide, polyurethane, polyurea, polyvinyl alcohol, and polyacrylonitrile.

[0029] In a fourth aspect, the present application provides a use, which comprises: the use of the binder described above as an electrode negative material in the preparation of a silicon / carbon nanotube lithium battery negative electrode.

[0030] In a fifth aspect, the present application provides a high-capacity silicon-based lithium battery negative electrode composite material, which comprises: a negative electrode current collector, a silicon-based material, a carbon-based material, and the binder composition described above.

[0031] The negative electrode current collector is selected from: electrolytic copper foil, rolled copper foil, double-sided bright copper foil, double-sided rough copper foil, single-sided rough copper foil, and porous copper foil, and the thickness of the negative electrode current collector is not more than 20 μm.

[0032] The silicon-based material is selected from silicon powder, and the particle size of the silicon powder is not more than 1 μm.

[0033] The carbon-based material is selected from carbon nanotubes (CNT) or multi-walled carbon nanotubes (MWCNT).

[0034] In a sixth aspect, the present application provides a method for preparing a silicon / carbon nanotube (Si / CNT) lithium battery anode by using the composite material described above, dissolving the binder composition in water, adding a silicon-based material to prepare a silicon-based dispersion, dissolving the binder composition in water, adding carbon nanotubes to prepare a carbon-based dispersion, using a spraying method to first spray the carbon-based dispersion onto the anode current collector, then alternately spraying the silicon-based dispersion and the carbon-based dispersion to obtain a stacked structure in which silicon layers and carbon layers are arranged alternately, the last layer sprayed is a carbon layer, and after the spraying is completed, the anode current collector on which the silicon-based dispersion layer and the carbon-based dispersion layer are sprayed is baked in a vacuum oven at 90-120°C for 1-3h, thereby preparing a silicon / carbon nanotube (Si / CNT) lithium battery anode with a stacked structure.

[0035] In the silicon-based dispersion, the binder composition used is selected from any one or a combination of the following: the star-shaped structure polyacrylic acid described above, the linear structure polymer with repeating structures containing hydrogen bond donors or hydrogen bond acceptors;

[0036] In the carbon-based dispersion, the binder composition used is selected from any one or a combination of the following: the star-shaped structure polyacrylic acid described above, the linear structure polymer with repeating structures containing hydrogen bond donors or hydrogen bond acceptors;

[0037] In the silicon-based dispersion or the carbon-based dispersion, at least one of the dispersions selects the star-shaped structure polyacrylic acid as a component of the binder composition;

[0038] Preferably, the repeating structure containing hydrogen bond donors or hydrogen bond acceptors includes one or more of the following: carboxyl, hydroxyl, amino, ester, ether, amide, urethane, urea, cyano;

[0039] Preferably, the linear structure polymer with repeating structures containing hydrogen bond donors or hydrogen bond acceptors includes one or more of the following: linear structure polyacrylic acid, polyether, polyester, polyamide, polyurethane, polyurea, polyvinyl alcohol, polyacrylonitrile;

[0040] More preferably, the linear structure polymer with repeating structures containing hydrogen bond donors or hydrogen bond acceptors includes any one or a combination of the following: linear structure polyacrylic acid, polyvinyl alcohol, or polyacrylonitrile.

[0041] Preferably, the silicon-based dispersion and the carbon-based dispersion are stirred for at least 12h before spraying;

[0042] Preferably, in the stacked structure, the number of silicon layers is ≥2, the number of carbon layers is ≥3, and the total number of layers in the stacked structure is not less than 5;

[0043] Preferably, the spraying uses the method of ultrasonic spraying, and the silicon-based dispersion or the carbon-based dispersion is sprayed on the negative current collector in sequence using compressed air with a pressure not less than 0.1 bar and an ultrasonic nozzle.

[0044] Preferably, the spraying rate of the ultrasonic nozzle is set to 0.05-0.5 mm / s.

[0045] Preferably, the spraying frequency of the ultrasonic nozzle is not less than 90 kHz.

[0046] Preferably, the spraying time is controlled to be 10-20 min.

[0047] Preferably, the mass ratio of the silicon powder in the silicon layer to the total carbon nanotubes in the carbon layer in the laminated structure is (8.0-9.0):(1.0-2.0).

[0048] In a seventh aspect, the application provides a lithium battery negative electrode prepared by the preparation method described above.

[0049] In an eighth aspect, the application provides a lithium battery, which comprises the lithium battery negative electrode described above.

[0050] The beneficial effects of the technical solutions proposed by the application are as follows:

[0051] The PAA binder with a star structure is designed and synthesized, and the radial side chain contains multiple carboxyl groups, including hydrogen bond acceptors and donors. During the use of the PAA binder and the electrode active material such as silicon powder or carbon powder, a multiple reversible hydrogen bond network can be formed in situ. On this basis, the silicon powder, the carbon nanotubes, the binder containing the star structure PAA, and water are mixed to prepare a corresponding dispersion, and the carbon-based dispersion and the silicon-based dispersion are sprayed on the battery-grade copper foil in sequence. The carbon layer of the carbon nanotube and the star structure PAA binder jointly act as a barrier between the silicon-based material and the electrolyte, and play the roles of isolation, buffering, and shielding. In particular, the star structure PAA binder can form a more dense hydrogen bond reversible crosslinking network through the side chain carboxyl group, reduce the volume expansion of the silicon-based dispersion layer, prevent direct contact between the silicon-based material and the electrolyte, thereby effectively reducing the side reaction between the electrolyte and the silicon-based material. In the charging and discharging process of the battery, the lower degree of volume change can avoid irreversible loss and rapid capacity decay of the electrode material, reduce the volume expansion of the electrode during use, and thus improve the cycle stability of the lithium battery. BRIEF DESCRIPTION OF DRAWINGS

[0052] Figure 1 NMR hydrogen spectrum of product I, product II, and product III in the preparation of the star structure polyacrylic acid in Example 1.

[0053] Figure 2A schematic diagram of a stack structure of a silicon / carbon nanotube (Si / CNT) lithium battery negative electrode, wherein 1 is a copper foil layer, 2 is a carbon layer, and 3 is a silicon layer. DETAILED DESCRIPTION

[0054] The application is further described in conjunction with the specific examples. It should be understood, however, that these examples are intended to illustrate the application and not to limit the scope of the application. Moreover, it should be understood that variations and modifications can be made to the application by those skilled in the art without departing from the spirit and scope of the application as hereinafter claimed.

[0055] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments in accordance with the present application. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and / or "comprising," when used in this specification, specify the presence of stated features, steps, operations, elements, components, and / or groups thereof, but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and / or groups thereof.

[0056] If the specific conditions of the experiment are not specified in the examples, they are generally in accordance with the conventional conditions in the art, or in accordance with the conditions recommended by the reagent company; the materials, reagents, etc. used in the examples can be purchased through commercial channels, unless otherwise specified.

[0057] The sources of reagents used in the examples: pentaerythritol (99%), triethylamine (TEA, 99.5%), 2-bromoisobutyryl bromide (98%), and cuprous bromide (CuBr, 99.9%) were purchased from Sigma-Aldrich. Trifluoroacetic acid (TFA, 99.5%), dichloromethane (DCM, 99.5%), tetrahydrofuran (THF, 99.5%), tert-butyl acrylate (99%), methanol (99.5%), pentamethyldiethylenetriamine (PMDETA, 99%), and N,N-dimethylformamide (DMF, 99.9%) were purchased from Aladdin. Deionized water with a resistivity of ≥18 MΩcm was used during the experiment. -1 (25°C).

[0058] Example 1

[0059] Star-shaped polyisobutyl acrylate was synthesized by atom transfer radical polymerization, and then the corresponding polyacrylic acid structure was obtained by ester hydrolysis.

[0060] S1, 2-bromoisobutyryl bromide (36.8 g) was added to a solution of triethylamine (168 g) and pentaerythritol (4.67 g) in tetrahydrofuran at 0 °C under a nitrogen atmosphere. The mixture was stirred for 24 h and then allowed to stand at room temperature. The mixture was poured into a separatory funnel with 250 mL of dichloromethane and washed sequentially with 10% HCl, 5% NaHCO3, and deionized water. The organic phase was dried over MgSO4and the solvent was removed by rotary evaporation. The crude product was recrystallized and purified using methanol and dried under vacuum to yield product I: multi-arm intermediate.

[0061] S2, The prepared multi-arm intermediate (25 mg), CuBr (40 mg), pentamethyldiethylenetriamine (48 mg), t-butyl acrylate (18.2 g), and solvent N,N-dimethylformamide (16 mL) were mixed and sealed in a vial in a glove box. The vial was then immersed in a constant temperature oil bath at 90 °C with stirring to allow polymerization of the t-butyl acrylate. After 24 h, the reaction was terminated by cooling to room temperature. The blue reaction product was passed through a basic alumina column to remove copper ions to yield product II: a colorless, transparent, viscous material.

[0062] S3, The transparent, viscous material was precipitated in a 1:1 volume ratio mixture of methanol / water and washed three times with solvent N,N-dimethylformamide. The precipitated white product was then dried under vacuum. Finally, 1 g of the precipitated product was dissolved in solvent dichloromethane (40 mL). Then, 5 mL of TFA was gradually added to the star polyisobutyl acrylate dissolved in dichloromethane at 0 °C ice bath. The mixture was kept at 0 °C and stirred for 2 h, and then stirred at room temperature for 48 h. The star polyisobutyl acrylate was purified by dialysis and the precipitated product was freeze-dried to yield the final product III: star polyacrylic acid.

[0063] Product characterization: The products I, II were dissolved in CDC13and product III was dissolved in D2O and characterized by1H-NMR (Bruker 400M) to confirm the molecular structure of the intermediate products I, II and final product III as shown in Figure 1

[0064] Example 2

[0065] ​Star-shaped polyisobutyl acrylate was synthesized by atom transfer radical polymerization, and then the corresponding polyacrylic acid was obtained by ester hydrolysis. Unlike Example 1, in Example 2, 9.16 g of trimethylolpropane was reacted with 2-bromoisobutyryl bromide to prepare a multi-arm intermediate in step S1, and 25 mg of the multi-arm intermediate was mixed with 12.9 g of tert-butyl acrylate to prepare star-shaped polyisobutyl acrylate in step S2.

[0066] Example 3

[0067] A silicon / carbon nanotube (Si / CNT) lithium battery anode was prepared by ultrasonic spraying. Silicon powder Brofos-Si-100 (0.48 g) with an average particle size of 100 nm and star-shaped polyacrylic acid (53.3 mg) prepared in Example 1 were dissolved in 42 mL of deionized water and stirred for 12 h to prepare a silicon-based dispersion. Subsequently, carbon nanotubes (A-CNT, DeFang Nanometer Co.) (0.06 g) and star-shaped polyacrylic acid binder prepared in Example 1 (6.7 mg) were dissolved in 34 mL of deionized water and then stirred for 12 h by using a homogenizer to prepare a carbon-based dispersion. Using compressed air (0.1 bar) and an ultrasonic nozzle, the carbon-based dispersion and the silicon-based dispersion were sprayed in sequence on a 6 μm double-sided photovoltaic-grade copper foil (Tongling Hui Li Fengke Electronic Materials), and a uniform carbon layer and a silicon layer were formed on a copper foil substrate area of 70 μm x 100 mm by spraying, with the spraying rate, frequency, and spraying time of the ultrasonic nozzle being set to 0.1 mm / s, 120 kHz, and 15 min, respectively. After spraying, the sample was baked in a vacuum oven at 105°C for 2 h, and finally a Si / CNT battery anode with a 5-layer structure was obtained, in which 3 layers were silicon layers and 4 layers were carbon layers. The mass ratio of the silicon layer and the carbon layer in each layer was 28.33% and 3.75%, respectively, and the total mass ratio of the silicon layer and the carbon layer in the final Si / CNT battery anode with a 5-layer structure was (8.5:1.5). The schematic diagram of the Si / CNT lithium battery anode with a 5-layer structure prepared in Example 3 is shown in FIG. 1. Figure 2

[0068] Example 4

[0069] The preparation method of the Si / CNT battery anode was the same as that in Example 3, except that the silicon powder Brofos-Si-100 (0.48 g) with an average particle size of 100 nm and the star-shaped polyacrylic acid (53.3 mg) prepared in Example 2 were dissolved in 42 mL of deionized water and stirred for 12 h to prepare a silicon-based dispersion. The carbon nanotubes (A-CNT, DeFang Nanometer Co.) (0.06 g) and the star-shaped polyacrylic acid binder prepared in Example 2 (6.7 mg) were dissolved in 34 mL of deionized water and stirred for 12 h by using a homogenizer to prepare a carbon-based dispersion.

[0070] ​Example 5

[0071] The Si / CNT battery anode was prepared as in Example 3, except that a silicon-based dispersion was prepared by dissolving 0.48 g of Brofos-Si-100 Si powder having an average particle size of 100 nm and 53.3 mg of the star-shaped polyacrylic acid binder prepared in Example 1 together with 10 mg of a polyvinyl alcohol binder in 42 mL of deionized water and stirring for 12 h.

[0072] Example 6

[0073] The Si / CNT battery anode was prepared as in Example 3, except that a silicon-based dispersion was prepared by dissolving 0.48 g of Brofos-Si-100 Si powder having an average particle size of 100 nm and 53.3 mg of the star-shaped polyacrylic acid binder prepared in Example 1 together with 8 mg of a polyacrylonitrile binder in 42 mL of deionized water and stirring for 12 h.

[0074] Example 7

[0075] The Si / CNT battery anode was prepared as in Example 3, except that a silicon-based dispersion was prepared by dissolving 0.48 g of Brofos-Si-100 Si powder having an average particle size of 100 nm and 53.3 mg of the star-shaped polyacrylic acid binder prepared in Example 1 together with 6 mg of a linear PAA binder in 42 mL of deionized water and stirring for 12 h.

[0076] Example 8

[0077] The Si / CNT battery anode was prepared as in Example 3, except that a silicon-based dispersion was prepared by dissolving 0.48 g of Brofos-Si-100 Si powder having an average particle size of 100 nm and 53.3 mg of the star-shaped polyacrylic acid binder prepared in Example 1 together with 6 mg of a linear PAA binder in 42 mL of deionized water and stirring for 12 h. A carbon-based dispersion was prepared by dissolving 0.06 g of carbon nanotubes (A-CNT, De-Fang Nanotech Co.) and 6.7 mg of the star-shaped polyacrylic acid binder prepared in Example 1 together with 3 mg of a linear PAA binder in 34 mL of deionized water using a homogenizer and then stirring for 12 h.

[0078] Example 9

[0079] The method is the same as Example 3, except that a silica-based dispersion is prepared by dissolving silica powder Brofos-Si-100 (0.48 g) with an average particle size of 100 nm and star-shaped polyacrylic acid prepared in Example 1 (53.3 mg) in 42 mL of deionized water and stirring for 12 h. A carbon-based dispersion is prepared by dissolving (0.06 g) carbon nanotubes (A-CNT, DeFOS Nanotech) and (6.7 mg) linear PAA binder together in 34 mL of deionized water using a homogenizer and then stirring for 12 h.

[0080] Example 10

[0081] The method is the same as Example 3, except that a silica-based dispersion is prepared by dissolving silica powder Brofos-Si-100 (0.48 g) with an average particle size of 100 nm and (53.3 mg) linear PAA binder in 42 mL of deionized water and stirring for 12 h. A carbon-based dispersion is prepared by dissolving (0.06 g) carbon nanotubes (A-CNT, DeFOS Nanotech) and star-shaped polyacrylic acid prepared in Example 1 (6.7 mg) together in 34 mL of deionized water using a homogenizer and then stirring for 12 h.

[0082] Comparative Example 1

[0083] The method is the same as Example 3, except that a silica-based dispersion is prepared by dissolving silica powder Brofos-Si-100 (0.48 g) with an average particle size of 100 nm and (53.3 mg) linear PAA binder in 42 mL of deionized water and stirring for 12 h. A carbon-based dispersion is prepared by dissolving (0.06 g) carbon nanotubes (A-CNT, DeFOS Nanotech) and (6.7 mg) linear PAA binder together in 34 mL of deionized water using a homogenizer and then stirring for 12 h.

[0084] Comparative Example 2

[0085] A silica powder-carbon nanotube composite dispersion is prepared by dissolving (0.06 g) carbon nanotubes (A-CNT, DeFOS Nanotech), silica powder Brofos-Si-100 (0.48 g) with an average particle size of 100 nm, and star-shaped polyacrylic acid binder prepared in Example 1 (60 mg) in 76 mL of deionized water using a homogenizer and then stirring for 12 h. The silica powder-carbon nanotube composite dispersion is sprayed on a 6 um double-sided photovoltaic cell grade copper foil (Tongling Hui Li Fengke Electronic Materials) using compressed air (0.1 bar) and an ultrasonic nozzle to form a uniform silica powder-carbon nanotube composite layer on a copper foil substrate area of 70 μm x 100 mm, with the spraying rate, frequency, and spraying time of the ultrasonic nozzle set to 0.1 mm / s, 120 kHz, and 15 min, respectively. After spraying is completed, the sample is baked in a vacuum oven at 105°C for 2 h, and a silica powder-carbon nanotube composite layer Si / CNT electrode is finally obtained.

[0086] Comparative Example 3

[0087] A silicon powder-carbon nanotube composite dispersion was prepared by dissolving (0.06 g) carbon nanotubes (A-CNT, DeFang Nanometer Co.), silicon powder Brofos-Si-100 with an average particle size of 100 nm (0.48 g), and linear PAA binder (60 mg) in 76 mL of deionized water by a homogenizer and stirring for 12 h. The silicon powder-carbon nanotube composite dispersion was sprayed on a 6 μm double-sided photocell grade copper foil (Huilifengke Electronic Materials, Tongling) using compressed air (0.1 bar) and an ultrasonic nozzle, and a uniform silicon powder-carbon nanotube composite layer was formed on a copper foil substrate area of 70 μm x 100 mm by spraying, with the spraying rate, frequency, and spraying time of the ultrasonic nozzle being set to 0.1 mm / s, 120 kHz, and 15 min, respectively. After spraying, the copper foil current collector was baked in a vacuum oven at 105°C for 2 h, and a silicon powder-carbon nanotube composite layer Si / CNT electrode was finally obtained by spraying.

[0088] The electrochemical performance test method of Example 3-10 and Comparative Examples 1-3 is as follows, and the test results are listed in Table 1.

[0089] The baked copper foil current collector was cut into a circular electrode with a diameter of 16 mm. A half battery was assembled in a glove box under high-purity argon, with a lithium metal sheet as the positive electrode, a porous polypropylene membrane Celgard 2400 as the separator, and 1.0 M LiPF6 (electrolyte solvent: a mixture of ethylene carbonate / dimethyl carbonate at a volume ratio of 1:1) as the electrolyte. The H2O and O2 contents in the air inside the glove box were less than 10 ppm.

[0090] The electrochemical impedance test first used an electrochemical workstation to test the open circuit potential, and after 300 s, when the open circuit potential was stable, the impedance test (EIS) was performed. The initial potential was the open circuit potential value recorded previously, and the frequency range of the impedance test was selected to be 10 -2 Hz-10 5 Hz, and the signal amplitude was 10 mV. The kinetics and resistance of each interface were checked. EIS was used to analyze the charge transfer mechanism that promotes Li ion diffusion, and thus the Warburg impedance (σ w ) corresponding to the resistance of lithium ion diffusion at the electrode was obtained. The lithium ion diffusion coefficient corresponding to the Si / CNT electrode assembled battery prepared in different examples and comparative examples was calculated according to the Warburg impedance (σ w ) obtained by the test and the formula (1) shown below.

[0091]

[0092] wherein, σ w is the Warburg impedance coefficient of the battery impedance obtained by EIS test, D, R, T, A, m, F and C correspond to the diffusion coefficient of lithium ion, gas constant, operating temperature, electrode area, number of electrons, Faraday constant and molar Li ion concentration, respectively. The lithium ion diffusion coefficient of the Si / CNT electrode assembled battery of Examples 3-10 and Comparative Examples 1-3 was calculated and the results are listed in Table 1.

[0093] Table 1

[0094]

[0095]

[0096] The performance of the batteries assembled with the Si / CNT electrodes of different examples and comparative examples was evaluated using a WBCS 3000-type battery cycler (Won A Tech) in a 0.1-1.5 V (vs. Li / Li + ) voltage range in a 25℃ constant temperature oven. Specifically, the initial energy density of the battery was tested, the energy density of the battery after 10 constant current charge-discharge cycles at a current density of 100 mA / g was tested, and then the energy density of the battery after 190 constant current charge-discharge cycles at a current density of 200 mA / g and 2000 mA / g was tested to evaluate the suitability of the Si / CNT electrodes prepared in Examples 3-10 and Comparative Examples 1-3 in lithium ion batteries, and the results are listed in Table 1.

[0097] The performance test data of Examples 3-10 and Comparative Examples 1-3 are shown in Table 1, which fully shows the excellent effect of the star-shaped polyacrylic acid binder introduced in the present application and the mutual laminated structure of the silicon-based material and the carbon-based material on improving the cycle performance of the silicon-based anode. In terms of initial energy density, the silicon anode can theoretically provide an energy density of 4200 mAh / g, which is more than 10 times that of graphite, a carbon-based material. Mixing the silicon-based material with the carbon-based material and the binder to obtain the Si / CNT electrode, the initial energy density of the battery prepared thereby is between 3000-4000 mAh / g, and no significant difference is observed between different examples and comparative examples.

[0098] The Si / CNT electrode prepared in Example 3-10 shows very good capacity retention rate after 10 cycles at 100 mA / g, and then 190 cycles at 200 mA / g or 190 cycles at 2000 mA / g, which is attributed to two aspects, on the one hand, the use of carbon nanotubes to form a carbon layer to physically isolate the silicon powder in the silicon layer from the electrolyte minimizes the reaction of the silicon-based material, thereby enhancing the cycle stability of the battery; on the other hand, the use of star-shaped polyacrylic acid binder in the Si / CNT electrode provides higher buffering capacity to alleviate the volume expansion compared to linear polyacrylic acid. The crosslinked network structure formed by the interaction between the side chain carboxyl groups -COOH is more dense than the linear polyacrylic acid, so as to reduce the side reaction of the silicon-based material with the electrolyte and improve the cycle life and stability. From the comparison of Comparative Example 1 and Example 3-10, it can be seen that the star-shaped polyacrylic acid binder used in the battery negative electrode Si-based material or carbon-based material has better cycle stability effect. From the comparison of Comparative Example 2 and Example 3-10, it can be seen that the Si / CNT electrode prepared by sequentially spraying the CNT layer and the Si layer on the copper foil has better cycle stability effect. The CNT layer designed on both sides of the silicon-based material plays a buffering role, inhibiting the overall volume expansion of the electrode, and can also have a synergistic effect with the star-shaped polyacrylic acid binder, greatly delaying the volume expansion of the silicon-based material. During the operation of the battery, lower degree of volume change can avoid irreversible loss of electrode material and rapid decay of capacity, thereby improving the cycle stability of the battery.

[0099] In addition, from the Warburg impedance coefficient obtained from the EIS test and the calculation of the lithium ion diffusion coefficient, the molecular topological shape of the negative electrode binder PAA has a great influence on the lithium ion diffusion coefficient. The star-shaped polyacrylic acid binder prepared in Example 1 or Example 2 is used in part or all of Example 3-10 and Comparative Example 2, and the corresponding Si / CNT electrode has a higher lithium ion diffusion coefficient. Even if the layer structure of Comparative Example 2 is changed, it also has a lithium ion diffusion coefficient of 9.4 x 10 -13 cm 2 / s, which is much higher than the conventional linear PAA binder. This is related to the star-shaped polyacrylic acid binder containing carboxyl groups COOH on the side chain and forming hydrogen bonds, thereby constructing a dense reversible hydrogen bond crosslinked network.

[0100] Finally, it should be noted that the above only for the preferred embodiments of the present application, and is not intended to limit the present application, although with reference to the foregoing embodiments of the present application has been described in detail, for those skilled in the art, it still can be modified, or part of the equivalent replacement of the technical solutions described in the foregoing embodiments. Any modification, equivalent replacement, improvement, etc. within the spirit and principles of the present application, should be included within the scope of the present application. Although the above describes the specific embodiments of the present application, but not the limitation of the scope of protection of the present application, the skilled in the art should be clear that, on the basis of the technical solutions of the present application, the skilled in the art without the need for creative labor can make various modifications or deformation still within the scope of the present application.

Claims

1. A method for preparing a silicon / carbon nanotube lithium battery anode using a high capacity silicon-based lithium battery anode composite material, characterized in that, The high-capacity silicon-based lithium battery negative electrode composite material comprises a negative electrode current collector, a silicon-based material, a carbon-based material, and a binder composition; The binder composition comprises star-shaped polyacrylic acid or a combination of star-shaped polyacrylic acid and linear structure polymer; The star-shaped polyacrylic acid is prepared by the following preparation method: 2-bromoisobutyryl bromide reacts with pentaerythritol under the catalysis of an organic base to obtain product I multi-arm intermediate, product I is subjected to polymerization reaction with tert-butyl acrylate to obtain product II star-shaped polytert-butyl acrylate, and finally the tert-butyl ester groups in product II are hydrolyzed under the catalysis of an organic acid to obtain the final product III star-shaped polyacrylic acid; The linear structure polymer is selected from linear structure polyacrylic acid; The negative electrode current collector is selected from electrolytic copper foil, rolled copper foil, double-sided bright copper foil, double-sided rough copper foil, single-sided rough copper foil, and porous copper foil, and the thickness of the negative electrode current collector is not more than 20 μm; The silicon-based material is selected from silicon powder, and the particle size of the silicon powder is not more than 1 μm; The carbon-based material is selected from carbon nanotubes; The silicon / carbon nanotube lithium battery negative electrode preparation method comprises the following steps: dissolving the binder composition in water, adding the silicon-based material to prepare a silicon-based dispersion, dissolving the binder composition in water, adding carbon nanotubes to prepare a carbon-based dispersion, using a spraying method to first spray the carbon-based dispersion onto the negative electrode current collector, then alternately spraying the silicon-based dispersion and the carbon-based dispersion, and obtaining a laminated structure in which silicon layers and carbon layers are alternately arranged, the last layer sprayed is a carbon layer, after the spraying is completed, the negative electrode current collector with the sprayed silicon-based dispersion layer and the carbon-based dispersion layer is baked in a vacuum oven at 90-120°C for 1-3 h, thereby preparing the silicon / carbon nanotube lithium battery negative electrode with the laminated structure; The mass ratio of the silicon powder in the silicon layer to the total amount of the carbon nanotubes in the carbon layer in the laminated structure is (8.0-9.0):(2.0-1.0); The binder composition used in the silicon-based dispersion is selected from star-shaped polyacrylic acid, linear structure polymer with repeating structure containing hydrogen bond donors or hydrogen bond acceptors, or a combination of the two; The binder composition used in the carbon-based dispersion is selected from star-shaped polyacrylic acid, linear structure polymer with repeating structure containing hydrogen bond donors or hydrogen bond acceptors, or a combination of the two; The linear structure polymer with repeating structure containing hydrogen bond donors or hydrogen bond acceptors comprises linear structure polyacrylic acid.

2. The method of claim 1, wherein the method of preparing a silicon / carbon nanotube lithium battery anode using a high-capacity silicon-based lithium battery anode composite is characterized by, The polymerization reaction uses a transition metal catalyst; And / or, the polymerization reaction also uses a tertiary amine catalyst; And / or, the organic acid is trifluoroacetic acid; And / or, the organic base is triethylamine.

3. The method of claim 1, wherein the method of preparing a silicon / carbon nanotube lithium battery anode using a high-capacity silicon-based lithium battery anode composite is characterized by, The silicon-based dispersion and the carbon-based dispersion are stirred for at least 12 h before spraying.

4. The method of claim 1, wherein the method of preparing a silicon / carbon nanotube lithium battery anode using a high-capacity silicon-based lithium battery anode composite is characterized by, In the laminated structure, the number of silicon layers is ≥2, the number of carbon layers is ≥3, and the total number of layers in the laminated structure is not less than 5.

5. The method of claim 1, wherein the method of preparing a silicon / carbon nanotube lithium battery anode using a high-capacity silicon-based lithium battery anode composite material is characterized by, The spraying uses an ultrasonic spraying method, and compressed air with a pressure of not less than 0.1 bar and an ultrasonic nozzle are used to sequentially spray the silicon-based dispersion or the carbon-based dispersion on the negative electrode current collector.

6. The method of claim 5, wherein the method of preparing a silicon / carbon nanotube lithium battery anode using a high-capacity silicon-based lithium battery anode composite is characterized by, The spraying rate of the ultrasonic nozzle is set to 0.05-0.5 mm / s; And / or, the spraying frequency of the ultrasonic nozzle is not less than 90 kHz; And / or, the spraying time is controlled between 10-20 min.

7. A lithium battery anode prepared using the method of any one of claims 1-6.

8. A lithium battery comprising: The lithium battery anode of claim 7.

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