Carbon nanofiber conductive agent-based micron silicon carbon negative electrode and lithium battery application thereof

Abstract

The invention discloses a carbon nanofiber conductive agent-based micron silicon carbon negative electrode and lithium battery application thereof, and belongs to the technical field of lithium battery negative electrodes. The positive electrode material is prepared from an active substance, a specific conductive agent and a binder, the active substance is obtained by mixing and ball-milling micron silicon, graphite and red phosphorus; the conductive agent is a network-shaped nano carbon material derived from a carbonaceous honeycomb structure, and is selected from one or more of a three-dimensional ordered macroporous (3DOM) carbon material formed by nano carbon, a three-dimensional carbon nanotube aggregate material and a homogeneous high-curvature porous nano carbon fiber material. The lithium storage specific capacity of the negative electrode of the lithium battery exceeds 1100mAh / g, and the negative electrode has excellent charge and discharge cycle stability.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium batteries, and particularly relates to a carbon nanofiber conductive agent-based micrometer silicon-carbon negative electrode and lithium battery application thereof. BACKGROUND

[0002] Under the background of energy transformation and new energy vehicle industry development, the market demand for lithium batteries as core energy storage equipment is rapidly growing. The negative active material plays a key role in the energy density and cycle life of the battery. However, for the currently dominant graphite negative electrode material, its specific capacity is close to the theoretical limit (372 mAh / g), and it is difficult to meet the market demand for high energy density batteries. Silicon element has become an ideal candidate material for the next generation of lithium battery negative electrode due to its advantages of ultra-high theoretical specific capacity (3579 mAh / g), abundant reserves and suitable working voltage. However, the silicon-based negative electrode material has problems such as poor electrical conductivity, serious volume expansion and crushing, which will lead to the collapse of the electrode structure and the failure of electrical connection, and thus seriously restrict the cycle stability and rate performance of the battery. Although the nanotechnology can alleviate the volume expansion problem of micrometer silicon, it will greatly increase the preparation cost of the material and the electrode, and at the same time, it will bring the disadvantages of increased specific surface area and reduced first coulomb efficiency, which will also limit the improvement of the energy density of the battery.

[0003] To solve the above problems, the micron silicon-carbon composite negative electrode formed by combining micron silicon and carbon materials has become a promising research direction due to its unique structure and performance advantages. The preparation techniques of the micron silicon-carbon composite negative electrode materials commonly used in business at present mainly include chemical vapor deposition method, high-temperature pyrolysis method, sol-gel method and mechanical ball milling method. Each of them has its own advantages, but also has certain limitations. Among them, the chemical vapor deposition method can give the material good electrochemical performance, but the process is complex and has certain danger. For example, patent CN111244417A uses chemical vapor deposition method, alkaline solution etching and hydrothermal reaction to prepare a three-dimensional graphene-silicon-carbon composite macroscopic material, which shows excellent electrochemical performance (can realize high specific capacity of 750 mAh / g and cycle life of 1000 cycles), but involves multiple operation steps, which is cumbersome and high preparation cost will not be conducive to its large-scale preparation. Patent CN116666583A simplifies the above process to some extent and prepares a multi-layer carbon-coated micron silicon-based negative electrode material (carbon content about 4wt%-10wt%) with higher specific capacity (1620-2605 mAh / g), which effectively alleviates the expansion and crushing problems of silicon-based materials, but the preparation process is still relatively complex and involves multiple dangerous chemicals, which is also not conducive to large-scale production. The high-temperature pyrolysis method has the advantages of simple preparation, but has the disadvantages of low specific capacity of the obtained negative electrode. For example, patent CN115621457A uses sand milling, hydrofluoric acid etching and high-temperature pyrolysis treatment to obtain a double-layer coated silicon-carbon composite material, which has a capacity retention rate of 99.5% after 100 cycles, but the specific capacity is only 520 mAh / g, which is difficult to adapt to the urgent demand for high specific capacity negative electrode in high-end application scenarios. The sol-gel method can prepare uniformly distributed silicon-carbon composite materials, thereby maintaining good specific capacity, but this method has the problems of complex preparation and poor cycle performance of the obtained samples. For example, patent CN115602809A uses sol-gel method to prepare Si / SiO x / C composite material, which has an initial specific capacity of only 566 mAh / g, and the specific capacity is less than 400 mAh / g after 300 cycles. The mechanical ball milling method generally grinds graphite and micron silicon as the matrix, which has the characteristics of simple process, low cost, high efficiency and is suitable for large-scale production, but the silicon particles are easy to agglomerate during the ball milling process. For example, patent CN119627069A uses ball milling combined with microwave digestion and high-temperature pyrolysis to prepare a nitrogen-doped graphene-coated silicon-based negative electrode material, which can achieve a high initial coulombic efficiency (about 83%), but the reversible specific capacity is only 543 mAh / g (capacity retention rate is about 88% after 10 weeks of cycle). Therefore, the ball milling method usually needs to be combined with other methods to break through the performance limitations.

[0004] In addition to the above preparation process, the selection of the conductive agent is also a key link to improve the battery performance. The conductive agents such as carbon black, multi-walled carbon nanotubes and single-walled carbon nanotubes widely used in the current commercial field each has its own characteristics, but there are still irreconcilable contradictions: the carbon black has a relatively low price, but its conductivity is poor and the addition amount is large, which is difficult to meet the demand of high-performance batteries; the carbon nanotube has excellent conductivity, but due to the complex preparation process and poor dispersibility (it needs to be made into a conductive slurry to be applied to the battery), the use cost is too high, and it is currently difficult to achieve comprehensive commercial application.

[0005] Recently, the inventor team developed a new type of carbon nanofiber conductive agent, which not only has outstanding cost advantage, but also has high electrical conductivity and excellent dispersibility, and can easily build a three-dimensional interconnected efficient electron conduction network inside the electrode, thereby effectively reducing the charge transfer impedance and significantly improving the rate energy storage performance of the battery. In addition, its special space network structure can also effectively buffer the volume expansion stress generated by the silicon particles during the charging and discharging process, and is expected to enhance the stability of the electrode structure through synergistic effect, thereby greatly improving the cycle stability performance of the battery.

[0006] In combination with the defects of the existing micron silicon-carbon composite negative electrode material technology in terms of complex preparation process, insufficient cycle stability and limited specific capacity (the commercial level is still lower than 650 mAh / g), the present application is based on the new type of carbon nanofiber network conductive agent developed by the team, uses inexpensive micron silicon material as raw material, adopts a simple and easy-to-scale production ball milling process, and further proposes a new type of micron silicon-carbon composite negative electrode with high cost performance and its preparation method, aiming to break through the bottleneck of traditional technology and inject new impetus for the technological innovation and industrial upgrading of lithium battery negative electrode materials. SUMMARY

[0007] The present application aims to provide a preparation scheme of a carbon nanofiber conductive agent-based micron silicon-carbon negative electrode and a lithium battery application method thereof, in order to solve the problems of insufficient conductivity, small specific capacity, large volume expansion and limited battery performance of the existing micron silicon-carbon negative electrode material.

[0008] To achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows:

[0009] The carbon nanofiber conductive agent-based microsilicon-carbon negative electrode is prepared from an active material, a specific conductive agent and a binder. Further, the active material is obtained by mixing and ball milling microsilicon, graphite and red phosphorus; the conductive agent is a network-like nanocarbon material derived from a carbonaceous honeycomb structure, and is selected from one or more of a three-dimensional ordered macroporous (3DOM) carbon material, a three-dimensional carbon nanotube aggregate material and a homogeneous high-curvature porous nanocarbon fiber material, and is preferably the homogeneous high-curvature porous nanocarbon fiber material, wherein the outer diameter of the carbon fiber conductive agent is uniformly distributed between 8-15 nm, the electrical conductivity is above 100 S / cm under a pressure of 200 MPa, and the carbon fiber conductive agent has high curvature and excellent water dispersion performance.

[0010] The preparation method of the carbon nanofiber conductive agent-based microsilicon-carbon negative electrode comprises the following steps:

[0011] (1) placing microsilicon, graphite and red phosphorus in a ball mill tank according to a specific ratio, and fully grinding and mixing them by a ball milling process to obtain a phosphorus-doped modified microsilicon-carbon composite material;

[0012] (2) adding the mixed mixture into deionized water by optimizing the ratio of the active material, the conductive agent and the binder, and then obtaining the microsilicon-carbon negative electrode with high carbon nanofiber conductive agent content after stirring, film coating and drying treatment.

[0013] Further, the particle size (R1) of the microsilicon raw material in step (1) is 1 μm≤R1≤20 μm.

[0014] Further, the mass ratio of the microsilicon, graphite and red phosphorus in step (1) is (27-40):(50-69):(1-10), preferably (27-40):(50-69):(2-4), and the sum of the microsilicon, graphite and red phosphorus is 100 (%); and the mixing method is mechanical ball milling.

[0015] Further, the ball milling in step (1) is as follows: the microsilicon is pretreated by ball milling at a speed of 350-600 rpm for 6-20 h in a ball mill tank, then mixed with graphite, and continuously ball milled at a speed of 350-600 rpm for 4-10 h to obtain a microsilicon-graphite composite material; then, the microsilicon-graphite composite material is mixed with red phosphorus, and ball milled at a speed of 350-600 rpm for 4-10 h to obtain a phosphorus-doped modified microsilicon-graphite composite material active material; and the ball milling atmosphere is an inert atmosphere such as argon or nitrogen.

[0016] Further, the conductive agent in step (2) is a carbon nanofiber network conductive agent derived from a carbonaceous honeycomb structure prepared by the inventors' team.

[0017] Further, the binder in step (2) is composed of a first binder and a second binder; the first binder comprises one or more of polyacrylic acid, sodium carboxymethyl cellulose, alginic acid and chitosan; the second binder comprises styrene-butadiene rubber or modified styrene-butadiene rubber; and the mass ratio of the first binder to the second binder is (1-2):1.

[0018] Further, the mass ratio of the active material, the conductive agent and the binder in step (2) is (60-80):(10-30):10.

[0019] The coating film is coated on a flat conductive substrate such as a copper foil.

[0020] The technical solution of the present application has the following advantages compared with the prior art:

[0021] The present application opens up a new path for improving the conductivity and dispersibility of micron silicon-carbon negative electrodes by a simple ball milling process combined with original carbon nanofiber conductive agents. The ingenious application of carbon nanofiber conductive agents, component ratio and process optimization effectively solve the problem of silicon volume expansion and significantly enhance the cycle stability of the battery. In addition, the preparation process is simple and efficient, easy to scale up, and the obtained negative electrode material can greatly improve the discharge specific capacity and cycle stability of lithium batteries, meeting the market demand for high-performance lithium battery negative electrode materials, and has great application potential. BRIEF DESCRIPTION OF DRAWINGS

[0022] In order to visually present the implementation method and technical solution of the present application, the following figures are attached. These figures are intended to provide visual references for those skilled in the art, so that they can more intuitively understand the present application, and skilled persons can deduce other related figures accordingly.

[0023] Figure 1 The scanning electron microscope (SEM) images of the micron silicon-carbon composite materials of Examples 1-3 and Comparative Example 1 of the present application show the microstructure characteristics of the materials under different conditions.

[0024] Figure 2 The SEM images of the micron silicon-carbon composite negative electrodes of Examples 1-3 and Comparative Example 1 of the present application show the microstructure characteristics of the electrode surfaces under different conditions.

[0025] Figure 3 The cycle performance of the micron silicon-carbon negative electrodes of Examples 1-3 and Comparative Example 1 of the present application under a current density of 0.1 A / g is compared, reflecting the rate performance and cycle stability differences of the samples of each example. DETAILED DESCRIPTION

[0026] The following examples are used to illustrate the technical solutions of the present application and their advantages, and are not intended to limit the protection scope of the present application. Those skilled in the art can obtain other examples from these examples, and these examples are all within the protection scope of the present application.

[0027] In the present application, the carbon nanofiber conductive agent used in the following examples is prepared based on the existing patent technology of the inventor team (see: ZL202110658668.5 and ZL202210230520.6), and the specific steps are as follows:

[0028] (1) Take nickel nitrate and citric acid with a molar ratio of 2:1, dissolve them in deionized water, and obtain a nickel salt precursor solution after sufficient stirring; (2) immerse the organic microspheres (such as PMMA, etc.) template in the precursor solution for 4 hours, then perform suction filtration and room temperature drying; (3) place the PMMA template impregnated with the nickel salt obtained in step (2) in the high-temperature zone downstream of the double-temperature-zone tube furnace, and place the MMA liquid in the low-temperature zone upstream of the tube furnace, and calcine under the conditions of normal pressure, inert gas carrier gas, wherein preferably: (i) the high-temperature zone is heated to 1000℃ at a heating rate of 10℃ / min and maintained for 11 minutes, (ii) the low-temperature zone is heated to 106℃ when the high-temperature zone reaches 390℃ and maintained for 69 minutes, and then heated to 200℃ when the high-temperature zone reaches 1000℃, and after the maintenance is completed, it is naturally cooled to room temperature; (4) place the sample obtained in step (3) in a nitric acid solution, for example, a HNO3 solution with a concentration of 2 mol / L and a temperature of 85℃, and soak for 12 hours to remove the nickel metal therein, then perform centrifugal separation and deionized water washing, and fully dry at 80℃ to obtain a new type of net-like conductive agent material powder formed by high-curvature nanocarbon fibers with uniform outer diameters (about 8-15nm), which has a powder electrical conductivity of more than 100S / cm under a pressure of 200MPa, and excellent water body dispersion stability.

[0029] Example 1

[0030] The micron silicon is ball milled in a ball mill tank at a speed of 480rpm for 20h, then mixed with graphite at a mass ratio of 35:65, and ball milled at a speed of 480rpm for 6h to obtain a micron silicon-graphite composite material. Then, it is mixed with red phosphorus at a mass ratio of 97:3, and ball milled at a speed of 480rpm for 6h to obtain a phosphorus-doped modified micron silicon-graphite composite material (active material). Finally, the active material, the above-mentioned new type of carbon nanofiber conductive agent, sodium carboxymethyl cellulose and butadiene rubber are mixed at a mass ratio of 60:30:5:5, and coated on a copper foil to obtain a negative electrode sheet.

[0031] Example 2

[0032] The micron silicon is ball milled at 480 rpm for 20 h, then mixed with graphite at a mass ratio of 35:65, and ball milled at 480 rpm for 6 h to obtain a micron silicon-graphite composite. Then, the active material is prepared by mixing the micron silicon-graphite composite with red phosphorus at a mass ratio of 97:3 and ball milling at 480 rpm for 6 h. Finally, the active material, the above-mentioned novel carbon nanofiber conductive agent, sodium carboxymethyl cellulose, and styrene-butadiene rubber are mixed at a mass ratio of 80:10:5:5, and coated on a copper foil to obtain a negative electrode sheet.

[0033] Example 3

[0034] The micron silicon is ball milled at 480 rpm for 20 h, then mixed with graphite at a mass ratio of 35:65, and ball milled at 480 rpm for 6 h to obtain a micron silicon-graphite composite. Then, the active material is prepared by mixing the micron silicon-graphite composite with red phosphorus at a mass ratio of 97:3 and ball milling at 480 rpm for 6 h. Finally, the active material, the above-mentioned novel carbon nanofiber conductive agent, sodium carboxymethyl cellulose, and styrene-butadiene rubber are mixed at a mass ratio of 80:10:5:5, and coated on a copper foil to obtain a negative electrode sheet.

[0035] Comparative Example 1

[0036] The micron silicon is ball milled at 480 rpm for 20 h, then mixed with graphite at a mass ratio of 35:65, and ball milled at 480 rpm for 6 h to obtain a micron silicon-graphite composite. Then, the active material, the above-mentioned novel carbon nanofiber conductive agent, sodium carboxymethyl cellulose, and styrene-butadiene rubber are mixed at a mass ratio of 80:10:5:5, and coated on a copper foil to obtain a negative electrode sheet.

[0037] Performance test

[0038] The electrode sheets obtained in Examples 1-3 and Comparative Example 1 are paired with a lithium metal foil to assemble CR2032 type button cells, with 1 mol / L LiPF6 dissolved in a mixed solution of EC:DEC:DMC (volume ratio of 1:1:1) as the electrolyte (10.0 wt% FEC and 1.0 wt% VC are added), and a polypropylene separator as the positive and negative electrode separator film. The assembled batteries are subjected to constant current charge-discharge test at 25°C and a current density of 0.1 A / g, with a voltage range of 1.0-0.01 V (vs. Li / Li + ). The charge-discharge specific capacity of the electrode is calculated based on the mass of the silicon-carbon active material, and the coulombic efficiency is obtained by dividing the charge specific capacity by the discharge specific capacity.

[0039] The results of the electrochemical performance test are shown in the following table (the conductive agent corresponds to the homogeneous high-curvature porous nanocarbon fiber material):

[0040]

[0041] By analyzing the electrochemical performance test results of Example 1, Example 2, Example 3 and Comparative Example 1, the following conclusions can be drawn: Example 1 exhibits the highest initial charge capacity (1114.6 mAh / g) and initial discharge capacity (1478.3 mAh / g), and has the best capacity retention rate in the first 30 cycles (94.9%), indicating that it can effectively maintain the capacity during charging and discharging, and has superior cycle stability. Compared with Comparative Example 1, Example 2 with appropriate phosphorus doping has significantly improved initial charge capacity, initial discharge capacity and capacity retention rate in the first 30 cycles, indicating that appropriate phosphorus doping helps to alleviate the volume expansion of silicon-based negative electrode and improve the electrochemical performance of the battery. However, Example 3 has a blocked lithium ion transport due to excessive phosphorus doping, and its initial charge capacity, first coulombic efficiency and capacity retention rate in the first 30 cycles are all lower than those of Example 2. In summary, Example 1 performs well in both charge and discharge capacity and cycle stability by optimizing the content of conductive agent and the proportion of phosphorus doping, which highlights the technical advantages of the present application.

[0042] Finally, it should be noted that the above examples are only used to illustrate the design scheme of the present application and do not constitute a limitation on the present application. Those skilled in the art can change, modify or replace some or all of the technical features on the basis of the above description, and these changes or modifications are still within the protection scope of the present application.

Claims

1. A carbon nanofiber conductive agent-based microsilicon carbon negative electrode, characterized by, The active material is prepared from micron silicon, graphite and red phosphorus modified by ball milling; the conductive agent is a network conductive agent derived from carbonaceous honeycomb structure, selected from one or more of three-dimensional ordered macroporous (3DOM) carbon material, three-dimensional carbon nanotube aggregate material and homogeneous high-curvature porous nanocarbon fiber material, preferably homogeneous high-curvature porous nanocarbon fiber material.

2. The carbon nanofiber conductive agent-based microsilicon carbon negative electrode according to claim 1, characterized in that, The conductive agent carbon fiber has a uniform size distribution of 8-15 nm and a high curvature, and has excellent water dispersion performance.

3. The carbon nanofiber conductive agent-based microsilicon carbon negative electrode according to claim 1, characterized in that, The method comprises the following steps: (1) micron silicon, graphite and red phosphorus are placed in a ball mill tank in a specific ratio, and are fully ground and mixed by a ball milling process to obtain a phosphorus-doped modified micron silicon-carbon composite active material; (2) the prepared mixture is added to deionized water by optimizing the ratio of the active material, the conductive agent and the binder, and after stirring, film coating and drying treatment, a micron silicon-carbon negative electrode with a certain content of carbon nanofiber conductive agent is obtained.

4. The method of claim 3, wherein, In step (1), the particle size (R1) of the micron silicon raw material is 1 μm≤R1≤20 μm.

5. The method of claim 3, wherein, In step (1), the mass ratio of the micron silicon, graphite and red phosphorus is (27-40):(50-69):(1-10), preferably (27-40):(50-69):(2-4), and the sum of the three is 100 (%). In step (1), the ball milling is as follows: the micron silicon is pretreated by ball milling at a speed of 350-600 rpm for 6-20 h in a ball mill tank, then mixed with graphite, and continuously ball milled at a speed of 350-600 rpm for 4-10 h to obtain a micron silicon-graphite composite material; then, the micron silicon-graphite composite material is mixed with red phosphorus and ball milled at a speed of 350-600 rpm for 4-10 h to obtain a phosphorus-doped modified micron silicon-graphite composite material (active material), and the ball milling atmosphere is an inert atmosphere such as argon or nitrogen.

6. The method of claim 3, wherein, In step (2), the binder is composed of a first binder and a second binder; the first binder comprises one or more of polyacrylic acid, sodium carboxymethyl cellulose, alginic acid and chitosan; the second binder comprises styrene-butadiene rubber or modified styrene-butadiene rubber; and the mass ratio of the first binder to the second binder is (1-2):

1.

7. The method of claim 3, wherein, In step (2), the mass ratio of the active material, the conductive agent and the binder is (60-80):(10-30):

10.

8. The method of claim 3, wherein, The film coating is coated on a flat conductive substrate.

9. Use of the carbon nanofiber conductive agent-based micron silicon-carbon negative electrode according to claim 1 or 2 in a lithium battery.

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