A silicon-carbon anode material with a regular morphology for lithium-ion batteries and a preparation method thereof

Abstract

The present invention discloses a silicon-carbon negative electrode material with a regular morphology for lithium-ion batteries and a preparation method thereof. The present invention describes the preparation method of spherical materials, the ratio range of the longest diameter to the shortest diameter, and the control of the dispersion of spherical particles. The present invention also elaborates on the influence of spherical particles with different characteristic parameters on the electrochemical performance after subsequent chemical vapor deposition of silane. The present invention provides a practical solution and idea for solving the problems that greatly affect the industrialization progress of silicon-carbon negative electrode materials, such as huge volume expansion and easy loss of electrical contact during the process of lithium deintercalation and intercalation.

Description

Technical Field

[0001] The present invention belongs to the field of lithium-ion battery materials, and particularly relates to a silicon-carbon anode material for lithium-ion batteries with a regular morphology and a preparation method thereof. Background Art

[0002] At present, in order to cope with the phenomena of global warming and energy shortage, the development and rational utilization of new energy are extremely urgent. Currently, in the global new energy market, lithium-ion batteries are still the mainstay. With the progress of the global living and technological levels, the development of the next-generation lithium-ion battery materials with higher energy density has become the top priority.

[0003] In the existing lithium-ion battery system, if the anode and cathode materials are considered separately, without major technological breakthroughs in the current cathode materials, they have all approached the limit of their specific capacities (lithium iron phosphate 150 / 175 mAh / g, nickel cobalt manganese ternary cathode 190 / 205 mAh / g, lithium manganate 142 / 148 mAh / g); compared with the cathode materials, the graphite anode still occupies the main market, but the graphite anode has infinitely approached the limit of its theoretical capacity (355 / 372 mAh / g). Therefore, compared with the development of cathode materials with higher energy density, the development of anode materials with high energy density and long cycle stability is the decisive force to promote the next generation of lithium battery technology innovation. After years of exploration, the silicon-based anode is considered to be the most likely breakthrough due to its extremely high energy density (4200 mAh / g). However, its own huge volume expansion during the lithium insertion and extraction process, as well as the poor conductivity of silicon itself, also hinder the use of the silicon-based anode.

[0004] The exploration of silicon-based anodes mainly includes: 1. The silicon-oxygen route: The initial efficiency of the material is too low (~76%), which will cause a large loss of active lithium ions in the cathode - the improvement of ED is limited; 2. To make up for the low initial efficiency problem of the silicon-oxygen route, pre-lithiated silicon-oxygen materials have been developed: The lithium source used is expensive, and the improvement of the initial efficiency has also approached the limit (82%) - due to the instability of the material-electrolyte interface caused by pre-lithiation, a large amount of SEI film is formed, resulting in excessive swelling of the battery cell; 3. Conventional grinding method silicon-carbon materials: The limit of the silicon particle grinding method has been reached (flake with a thickness of 20nm and a length of 100nm), but its large-scale use still cannot solve the structural impact on the battery cell caused by the lithium insertion expansion. Because its mixing method is physical mixing, there is no stable interface and material structure between silicon and carbon, and the silicon distribution is relatively uneven, so the high-capacity characteristics of silicon cannot be effectively exerted, and the overall conductivity of the composite material is not excellent. And the uneven silicon distribution cannot effectively relieve the volume expansion of silicon during lithium deintercalation and insertion. In summary, the key to realizing the use of a large amount of silicon components to improve the ED of the battery cell lies in improving the dispersion degree of silicon components in the silicon anode structure, and the current industrial introduction verification has proved that the capacity design limits of the 1, 2, and 3 solution routes have been reached: 390, 420, 400 mAh / g. Summary of the Invention

[0005] The present invention aims to solve at least one of the technical problems existing in the above-mentioned prior art. For this reason, the present invention proposes a novel silicon-carbon spherical material, which can effectively solve the application problems of existing silicon-based anodes.

[0006] According to a first aspect of the present invention, a silicon-carbon anode material is proposed. The silicon-carbon anode material is spherical-like, with a carbon layer coated on its surface. The ratio of the longest diameter to the shortest diameter of the silicon-carbon anode material is 1.0 to 1.2, and the ratio of the overlapping area between the particles of the silicon-carbon anode material to the total surface area of the overlapping particles is not more than 10%.

[0007] The longest diameter is the longest straight-line distance between any two points of the same material particle, and the shortest diameter is the shortest straight-line distance between any two points of the material particle.

[0008] Preferably, the ratio of the overlapping area between the particles of the silicon-carbon anode material to the total surface area of the overlapping particles is not more than 5%.

[0009] Preferably, the particle size D V90 / D V50 is 1.5 to 5.5.

[0010] Preferably, the particle size of the silicon-carbon anode material is 1.8μm < D V50 < 6.7μm, 6.7μm < D V90 < 12.3μm.

[0011] Preferably, the mass ratio of the carbon layer is 2% - 3%.

[0012] Preferably, the first efficiency range of the silicon-carbon anode material in the coin cell test satisfies: 86.5% < FCE 2.0 V < 94.5%.

[0013] Preferably, the lithium intercalation specific capacity range of the silicon-carbon anode material is 1850 - 2300 mAh / g.

[0014] Preferably, the mass ratio W of Si element in the silicon-carbon anode material Si is 35% - 50%.

[0015] According to the second aspect of the present invention, a preparation method of the silicon-carbon anode material as described in the first aspect of the present invention is provided, including the following steps:

[0016] D1: Heat the solution of the carbon skeleton precursor and then carry out polymerization, and then continue to heat for pyrolysis to obtain spherical carbon; D2: Activate and create pores in the spherical carbon, and then crush it to obtain a carbon skeleton;

[0017] D3: Carry out silicon deposition and carbon deposition on the carbon skeleton in sequence to obtain a silicon-carbon anode material.

[0018] Preferably, in step D1, the carbon skeleton precursor includes aldehyde compounds and phenolic compounds. By using specific organic substances and appropriate processes, spherical polymer can be prepared, and then spherical porous carbon skeleton can be obtained by heating and pyrolysis.

[0019] More preferably, the carbon skeleton precursor is bisphenol A and formaldehyde.

[0020] More preferably, in the carbon skeleton precursor, the molar ratio of the phenolic compound to the aldehyde compound is 0.5 - 1.3:1; and / or, the concentration of the phenolic compound is 0.6 - 1.2 mol / L, and the concentration of the aldehyde compound is 0.6 - 1.2 mol / L.

[0021] More preferably, the solid content after polymerization of the carbon skeleton precursor is 4% - 12%.

[0022] Preferably, in step D1, the temperature of the polymerization is 60 - 120 °C, and the time of the polymerization is 2 - 3 h.

[0023] Preferably, in step D2, the method of activating and creating pores includes but is not limited to physical method, chemical method or a method combining physical and chemical methods. By carrying out pore modification, it is beneficial to realize the reasonable accommodation of the carbon skeleton for the subsequent introduced silicon component.

[0024] Preferably, in step D3, the methods of silicon deposition and carbon deposition are independently selected from chemical vapor deposition.

[0025] More preferably, the process of silicon deposition includes but is not limited to: the pyrolysis of gaseous silicon source followed by diffusion into the pores of the carbon skeleton; or the diffusion of gaseous silicon source into the pores of the carbon skeleton followed by pyrolysis; or the simultaneous occurrence of the above two processes.

[0026] Preferably, in step D3, the silicon source used for silicon deposition is: SiH 4 、Si 2 H 6 、Si 2 H 4 、Si 3 H 8 、Si 3 H 6 、Si 3 H 4 、Si 4 H 10 、Si 4 H 8 、Si 4 H 6 、Si 5 H 12 、Si 5 H 10 、Si 5 H 8 、Si 5 H 6 、Si 6 H 14 、Si 6 H 12 、Si 6 H 10 、Si 6 H 8 、Si 6 H 6 、Si 7 H 16 、Si 7 H 14 、Si 7 H 12 、Si 7 H 10 、Si 7 H 8 、Si 7 H 6 、Si 8 H 18 、Si 8 H 16 、Si 8 H 14 、Si8 H 12 、 Si 8 H 10 、 Si 8 H 8 、 Si 9 H 20 、 Si 9 H 18 、 Si 9 H 16 、 Si 9 H 14 、 Si 9 H 12 、 Si 9 H 10 、 Si 9 H 8 、 Si 10 H 22 、 Si 10 H 20 、 Si 10 H 18 、 Si 10 H 16 、 Si 10 H 14 or Si 10 at least one of H.

[0027] Preferably, in step D3, during the silicon deposition, a mixed gas of a gaseous silicon source and a carrier gas is introduced, and the volume ratio of the gaseous silicon source in the mixed gas is about 35%; the carrier gas is N 2 .

[0028] More preferably, the flow rate of the mixed gas is 300 - 800 sccm.

[0029] Preferably, in step D3, the temperature of the silicon deposition is 450 - 800 °C; and / or, the time of the silicon deposition is 3 - 6 h.

[0030] Preferably, in step D3, the carbon source used for the carbon deposition is a hydrocarbon.

[0031] Preferably, in step D3, during the carbon deposition, the gas flow rate of the carbon source is 1.5 - 7.5 L / min.

[0032] Preferably, the temperature of the carbon deposition is 350 - 750 °C; and / or, the time of the carbon deposition is 0.5 - 6 h.

[0033] The process of carbon deposition can prevent the outer silicon component of the silicon-carbon negative electrode material from being eroded and oxidized during the stirring and pulping processes in the aqueous binder during the preparation process of the battery cell, thus avoiding the generation of other gases such as hydrogen that are unfavorable to the production process of the battery cell. At the same time, it can also reduce the excessive formation of the SEI film during processes such as formation in the battery cell manufacturing process and the cycling process during actual use, thereby reducing phenomena such as gas generation in the battery cell.

[0034] The preparation method of the silicon-carbon negative electrode material proposed by the present invention prepares a monodisperse spherical silicon-carbon material, which solves the problems of uneven distribution of silicon on the carbon carrier and incomplete penetration into the material interior after chemical vapor deposition of traditional silicon-carbon negative electrode materials. At the same time, the spherical silicon-carbon material minimizes the contact area with the electrolyte to the greatest extent, reduces the formation of unnecessary SEI film and the occurrence of other side reactions, and further reduces the influence of the SEI film on the expansion of the battery cell.

[0035] Compared with the ordinary silicon oxide material, pre-lithiated silicon oxide material and silicon-carbon material prepared by the traditional grinding method described above, the silicon raw material used in the preparation method of the silicon-carbon negative electrode material proposed by the present invention is a product relatively upstream in the silicon industrial chain, and has an obvious cost advantage when calculated by unit capacity.

[0036] According to the third aspect of the present invention, there is provided an application of the silicon-carbon negative electrode material as described in the first aspect of the present invention in the preparation of lithium-ion batteries.

[0037] According to a preferred embodiment of the present invention, it has at least the following beneficial effects:

[0038] (1) The present invention proposes a novel monodisperse silicon-carbon spherical material. On the one hand, the monodisperse spherical silicon-carbon particles can effectively alleviate the influence of the volume expansion of silicon during charge and discharge on the material, and compared with traditional silicon-carbon negative electrode materials, can release the volume stress generated during the lithium insertion and extraction process of the negative electrode to a greater extent in the topological structure. On the other hand, since the surface area of the sphere is the smallest among all shapes of the same volume, the consumption of the electrolyte can be minimized to the greatest extent, thereby reducing the formation of the SEI film. Moreover, the spherical particles can be well compounded with graphite to play the role of "filling holes and inserting seams" and better adapt to the existing negative electrode. The monodisperse silicon-carbon spherical composite material proposed by the present invention combines high energy density, long cycle stability, and excellent ionic and electronic conductivity, and provides a practical solution for the new generation of silicon-based negative electrodes.

[0039] (2) Compared with the silicon-carbon material prepared by the traditional grinding method, the silicon-carbon negative electrode material with a spherical morphology of the present invention has a more than 50% reduction in full-insertion expansion; compared with other silicon-carbon negative electrode materials with irregular morphologies of the same type, the full-insertion expansion is reduced by more than 5%. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The following further describes the present invention in conjunction with the drawings and embodiments, wherein:

[0041] Figure 1 SEM image of the silicon-carbon negative electrode material prepared in Example 1 of the present invention magnified 3000 times;

[0042] Figure 2 SEM image of the silicon-carbon negative electrode material prepared in Example 1 of the present invention magnified 5000 times;

[0043] Figure 3 Schematic diagram of the silicon deposition process and the lithium insertion / extraction process of the obtained particles in Example 1 of the present invention;

[0044] Figure 4 SEM image of the silicon-carbon negative electrode material prepared in Comparative Example 2 of the present invention magnified 5000 times;

[0045] Figure 5 SEM image of the silicon-carbon negative electrode material prepared in Comparative Example 10 of the present invention magnified 10000 times;

[0046] Figure 6 SEM image of the silicon-carbon negative electrode material prepared in Comparative Example 11 of the present invention magnified 2000 times. Detailed implementation manners

[0047] The following will clearly and completely describe the concept of the present invention and the technical effects generated in combination with the embodiments, so as to fully understand the purpose, features and effects of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, other embodiments obtained by those skilled in the art without creative efforts shall fall within the scope of protection of the present invention. The raw materials used in the following examples and comparative examples, unless otherwise specified, are all conventional products that can be obtained by commercial purchase.

[0048] Example 1

[0049] This example provides a silicon-carbon negative electrode material, and its SEM images are as shown in Figure 1 、 2 . The ratio of the longest diameter to the shortest diameter of the silicon-carbon negative electrode material is 1.05, and the ratio of the overlapping area between the material particles to the total surface area of the overlapping particles is 3.2%.

[0050] This example also proposes a preparation method for the above silicon-carbon negative electrode material, and the specific steps are as follows:

[0051] D1. Place an aqueous formaldehyde solution with a concentration of 0.8 mol / L in a three-necked flask. Take an aqueous bisphenol A solution with a concentration of 0.8 mol / L, and add it dropwise to the above three-necked flask at a rate of 30 mL / min according to a molar ratio of 1:1. Stir at 100 rpm, and carry out mixing and polymerization at 120 °C for 3 h. The solid content of the resulting resin is about 10%. After drying, place the resin in an inert atmosphere and heat and pyrolyze it at 900 °C for 2 h to obtain spherical carbon.

[0052] D2. Place the spherical carbon obtained in step D1 in a tube furnace and use water vapor to activate and create pores in the particles.

[0053] D3. Carry out air flow pulverization on the carbon particles after activation and pore creation obtained in step D2 for 3 h, and classify them to the particle size distribution shown in Table 1 below to obtain a carbon skeleton as a silicon deposition carrier.

[0054] D4. Place the carbon skeleton obtained in step D3 in a chemical vapor deposition chamber. Use SiH 4 as the silicon source and N 2 as the carrier gas. The volume ratio of SiH 4 in the mixed gas is 35%, and the flow rate is 800 sccm. Carry out silicon deposition at 600 °C for 6 h. The process and the lithium deintercalation process of the resulting particles are as Figure 3 shown.

[0055] D5. Continue to place the product obtained in step D4 in a chemical vapor deposition chamber, introduce acetylene, and carry out CVD coating (carbon deposition). By controlling the acetylene gas flow rate at 4 L / min, the temperature at 550 °C, and the deposition time at 2 h, control the amount of carbon coating to obtain a silicon-carbon negative electrode material coated with a carbon shell.

[0056] Example 2

[0057] This example presents a preparation method of a silicon-carbon negative electrode material. The difference from Example 1 is only that:

[0058] In step D1, the molar ratio of bisphenol A to formaldehyde is 1.3:1, the concentrations of bisphenol A and formaldehyde are both 1 mol / L, the dropping rate is 40 mL / min, the stirring rate is 200 rpm, and the temperature of mixing and polymerization is 60 °C;

[0059] In step D3, classify the carbon particles after activation and pore creation obtained in step D2 to the particle size distribution shown in Table 1 below.

[0060] Example 3

[0061] This example presents a preparation method of a silicon-carbon negative electrode material. The difference from Example 1 is only that:

[0062] In step D4, the time for silicon deposition is 5 h.

[0063] Comparative Example 1

[0064] This example presents a method for preparing a silicon-carbon anode material, which is only different from Example 1 in that:

[0065] In step D1, the carbon skeleton precursor is mixed and polymerized at a temperature of 300 °C for 2 h; then it is heated and pyrolyzed at 850 °C for 5 h to obtain massive carbon;

[0066] In step D3, the activated and pore-formed carbon particles obtained in step D2 are subjected to air jet milling for 0.5 h and classified to the particle size distribution shown in Table 1 below.

[0067] Comparative Example 2

[0068] This example presents a method for preparing a silicon-carbon anode material, which is only different from Example 1 in that:

[0069] In step D1, the carbon skeleton precursor is mixed and polymerized at a temperature of 300 °C for 2 h; then it is heated and pyrolyzed at 900 °C for 2 h to obtain massive carbon;

[0070] In step D3, the activated and pore-formed carbon particles obtained in step D2 are subjected to air jet milling for 1.5 h and classified to the particle size distribution shown in Table 1 below.

[0071] The SEM image of the prepared silicon-carbon anode material is as Figure 4 shown.

[0072] Comparative Example 3

[0073] This example presents a method for preparing a silicon-carbon anode material, which is only different from Example 1 in that:

[0074] In step D1, the carbon skeleton precursor is mixed and polymerized at a temperature of 300 °C for 2 h; then it is heated and pyrolyzed at 850 °C for 5 h to obtain massive carbon;

[0075] In step D3, the activated and pore-formed carbon particles obtained in step D2 are subjected to air jet milling for 4 h and classified to the particle size distribution shown in Table 1 below.

[0076] Comparative Example 4

[0077] This example presents a method for preparing a silicon-carbon anode material, which is only different from Example 1 in that:

[0078] In step D1, the molar ratio of bisphenol A to formaldehyde is 0.5:1, the concentrations of both bisphenol A and formaldehyde are 0.6 mol / L, the dropping rate is 5 mL / min, the stirring rate is 80 rpm, and the temperature for mixing and polymerization is 100 °C.

[0079] In step D3, the activated and pore - formed carbon particles obtained in step D2 are classified to the particle size distribution shown in Table 1 below.

[0080] Comparative Example 5

[0081] This example presents a preparation method of a silicon - carbon anode material, and the difference from Example 1 is only that:

[0082] In step D1, the molar ratio of bisphenol A to formaldehyde is 1.8:1, the concentrations of both bisphenol A and formaldehyde are 0.7 mol / L, the dropping rate is 50 mL / min, the stirring rate is 250 rpm, and the temperature for mixing and polymerization is 140 °C.

[0083] In step D3, the activated and pore - formed carbon particles obtained in step D2 are classified to the particle size distribution shown in Table 1 below.

[0084] Comparative Example 6

[0085] This example presents a preparation method of a silicon - carbon anode material, and the difference from Example 1 is only that the silicon deposition process in step D4 is not carried out.

[0086] Comparative Example 7

[0087] This example presents a preparation method of a silicon - carbon anode material, and the difference from Example 1 is only that:

[0088] In step D4, the time for silicon deposition is 8 h.

[0089] Comparative Example 8

[0090] This example presents a preparation method of a silicon - carbon anode material, and the difference from Example 1 is only that:

[0091] In step D4, the time for silicon deposition is 9 h.

[0092] Comparative Example 9

[0093] This example presents a preparation method of a silicon - carbon anode material, and the difference from Example 1 is only that the carbon deposition process in step D5 is not carried out.

[0094] Comparative Example 10

[0095] This example presents a preparation method of a silicon - carbon anode material, and the difference from Example 1 is only that:

[0096] In step D1, the concentration of the aqueous formaldehyde solution is 1.6 mol / L, and the concentration of the aqueous bisphenol A solution is 1.6 mol / L; after the carbon skeleton precursor polymerizes, the solid content of the system is about 20%.

[0097] The SEM of the prepared silicon-carbon anode material is as Figure 5 shown.

[0098] Comparative Example 11

[0099] This example proposes a preparation method of a silicon-carbon anode material, which is only different from Example 1 in that:

[0100] In step D1, the concentration of the aqueous formaldehyde solution is 2.4 mol / L, and the concentration of the aqueous bisphenol A solution is 2.4 mol / L; after the carbon skeleton precursor polymerizes, the solid content of the system is about 30%.

[0101] The SEM image of the prepared silicon-carbon anode material is as Figure 6 shown.

[0102] Test Example

[0103] This test example tested the physical and chemical properties of the anode materials prepared in the examples and comparative examples, and the results are shown in Table 1 and Table 2 below. Among them, the test methods are as follows:

[0104] 1. The electrochemical performance of the anode material in a coin cell, and the specific method is as follows:

[0105] Electrode formula: Si 84% + CNTs slurry 1% (solid content 1%, where CNTs account for 40% and CMC accounts for 60%) + 10% PAA-Li (lithiated PAA binder) + 5% Super p conductive carbon black;

[0106] Battery case: 2430 type coin cell case, and a shrapnel, nickel foam or gasket is set between the test electrode or the anode and the battery case to avoid open circuit and accurately reflect the test results.

[0107] Electrolyte: 1M lithium hexafluorophosphate carbonate solution containing 15% FEC + 1 - 2% VC.

[0108] Coating weight of the test electrode: 7 - 8 mg / cm 2 ;

[0109] Test voltage range: 2V - 0.005V.

[0110] Test process:

[0111] Stand still for 8h - 12h; Stand still for 5min with 0.05C lithium insertion to 5mV; Stand still for 5min with 50uA lithium insertion to 5mV; Stand still for 5min with 10uA lithium insertion to 5mV; Stand still for 5min with 0.05C lithium extraction to 2V.

[0112] The specific capacity per gram, the first cycle efficiency and the percentage increase in thickness at full lithium insertion thickness of the obtained silicon-carbon anode material were tested.

[0113] Meanwhile, at a charge-discharge rate of 0.33C (1C current is 1700 mAh / g), the number of cycles when the specific capacity decays to 90% (compared with the first week at 0.33C rate) was tested.

[0114] 2. Test scheme for the mass ratio of silicon element and the mass ratio of carbon coating in the anode material:

[0115] The difference method was adopted, and the corresponding deposited silicon mass and coated carbon mass were obtained by recording the mass of the material particles before and after silicon deposition and carbon deposition processes respectively.

[0116] 3. Test scheme for the longest diameter / shortest diameter of the anode material particles:

[0117] Under the electron scanning microscope, 100 particles were randomly selected. The two points with the longest distance and the two points with the shortest distance were found on the particle contour in the image. The connection distance between them was the longest diameter and the shortest diameter. The average value of the longest diameter / shortest diameter ratio of 100 particle samples was the longest diameter / shortest diameter ratio of this sample.

[0118] 4. Test scheme for the percentage of the overlapping area between the particles of the anode material:

[0119] Select several particles in the SEM image of the particles at random. Determine the unit particle area through the values of the longest diameter and the shortest diameter, and then measure the overlapping area S through the scanning electron microscope processing software (DM) 重叠 , the sum of all single particle areas was obtained as S 总 , S 重叠 / S 总 was the percentage of the overlapping area between the particles.

[0120] Table 1 Data table of the morphology of the anode material

[0121]

[0122] Table 2 Data table of the electrochemical performance of the anode material

[0123]

[0124]

[0125] It can be analyzed from the above table that:

[0126] For Example 1 and Comparative Examples 1-3: Since the spheres are isotropic, the impact of the expansion of the silicon negative electrode on the material itself can be alleviated to the greatest extent. Example 1 with a more regular particle morphology has lower lithium intercalation expansion of the material, and thus also has better cycle life, because the loss of ionic conductivity and electronic conductivity in the electrode caused by particle expansion is reduced. In Comparative Examples 1-3, due to the relatively high polymerization temperature, the particle morphology is irregular, and the ratio of the longest diameter to the shortest diameter further increases, which will lead to a more significant effect of expanding the porosity of the electrode when the particles expand, reducing the cycle life of the material, and the capacity rapidly decays during long-term cycling. Therefore, spherical carbon with a longest diameter / shortest diameter exceeding a certain range has poor ability to alleviate volume expansion, resulting in serious pulverization and fragmentation during the cycling process.

[0127] For Example 1, 2 and Comparative Examples 4, 5: The comparison of these four examples illustrates the influence of the particle size distribution of the material on the cycle life. In Comparative Example 4, it can be considered that most of the material has a uniform particle size, but it is difficult to achieve a good match between large and small particles, and thus fully reduce the porosity of the electrode caused by material stacking. Therefore, a reasonable particle size match can ensure that the material has a longer cycle life, while too large a D V90 / D V50 also cannot guarantee a further increase in the cycle life.

[0128] For Example 1, 3 and Comparative Examples 6-8: In the silicon-carbon negative electrode material designed in the present invention, the silicon content can reach acceptable performance indicators within a certain range. The lower silicon content in Example 3 will inevitably result in lower lithium intercalation expansion, but at the same time will lead to a lower specific capacity per gram of the material, while in Comparative Examples 7 and 8, the situation is opposite; in Comparative Example 6, no silicon deposition is carried out, and at this time, because the initial Coulomb efficiency of the carbon skeleton is lower, its cycle performance is poor and it is difficult to be used as a material to improve the energy density of the battery cell in lithium-ion batteries. However, too high a silicon content (Comparative Examples 7 and 8) will impact the stability of the carbon skeleton, easily cause particle fragmentation during lithium intercalation, and thus affect the cycle life of the battery.

[0129] For Example 1 and Comparative Example 9: The silicon-carbon negative electrode material in Comparative Example 9 is not carbon-coated. From the start of homogenization in the aqueous slurry, gas generation easily occurs, causing surface oxidation of the material; moreover, during the cycling process of the battery cell, the "core" of the material directly contacts the electrolyte, which will lead to the formation of a large amount of SEI film, increasing the impedance of the material and the electrode, increasing the porosity and nuclear conductivity of the electrode, and thus reducing the cycle life of the material.

[0130] For Example 1, Comparative Example 10, and Comparative Example 11: During the long cycle of the electrode sheet, the overlap and tightness between spherical silicon-carbon materials will affect the release of expansion stress. The smaller the overlapping area between the silicon-carbon anode material particles, the higher the monodispersity, which is more conducive to giving full play to the advantages of the stable cycling performance of the smallest unit spherical material. Moreover, during the process of chemical vapor deposition of silane, the smaller the overlapping part between the particles, the more uniform and stable the deposition effect. Compared with Example 1, Comparative Examples 10 and 11 have more overlapping parts between the particles and poorer cycling performance.

[0131] The present invention describes the preparation method of spherical materials, the ratio range of the longest diameter to the shortest diameter, and the control of the dispersibility of spherical particles. The present invention also elaborates on the influence of spherical particles with different characteristic parameters on the electrochemical performance after subsequent chemical vapor deposition of silane. The present invention provides a practical solution and idea for solving the problems that affect the industrialization progress of silicon-carbon anode materials, such as huge volume expansion and easy loss of electrical contact during the process of lithium deintercalation and intercalation of silicon-carbon anode materials.

[0132] The embodiments of the present invention have been described in detail above in conjunction with the accompanying drawings. However, the present invention is not limited to the above embodiments, and various changes can be made without departing from the gist of the present invention within the knowledge scope of those of ordinary skill in the art. In addition, the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

Claims

1. A method for preparing a silicon-carbon negative electrode material, characterized in that: The following steps are involved: D1: The solution of the carbon skeleton precursor is heated to polymerize, and then the temperature is further increased to crack to obtain spherical carbon; D2: activating the spherical carbon to form pores, and then crushing it to obtain a carbon skeleton; D3: sequentially depositing silicon and carbon on the carbon skeleton to obtain a silicon-carbon negative electrode material; In step D1, the carbon skeleton precursor includes formaldehyde and bisphenol A, and the solution of the carbon skeleton precursor is prepared by adding an aqueous solution of bisphenol A dropwise to an aqueous solution of formaldehyde and mixing them; The mixing and polymerization temperature is 60-120°C, and the polymerization time is 2-3h; In the carbon skeleton precursor, the molar ratio of bisphenol A to formaldehyde is 0.5-1.3:1; The solid content of the carbon skeleton precursor after polymerization is 4%-12%; The silicon-carbon negative electrode material is spherical, with a carbon layer coated on the surface, the ratio of the longest diameter to the shortest diameter of the silicon-carbon negative electrode material is 1.0-1.05, and the ratio of the overlapping area between the particles of the silicon-carbon negative electrode material to the total surface area of ​​the overlapping particles is not more than 10% and not less than 3.2%; The mass proportion of the carbon layer is 2% to 3%; The particle size D of the silicon-carbon negative electrode material V90 / D V50 1.5~5.5; The test scheme for the ratio of the area of ​​overlap between particles of the silicon-carbon negative electrode material to the total surface area of ​​the overlapped particles is as follows: Select a number of particles in the SEM image, determine the unit particle area by the values ​​of the longest diameter and the shortest diameter, and then measure the overlapping area S using the SEM processing software. 重叠 , S is the sum of the areas of all single particles 总 , S 重叠 / S 总 It is the percentage of the area that overlaps between particles.

2. The preparation method according to claim 1, characterized in that: The ratio of the overlapping area between the particles of the silicon-carbon negative electrode material to the total surface area of ​​the overlapping particles is no more than 5% and no less than 3.2%.

3. The preparation method according to claim 1, characterized in that: The mass proportion of Si element in the silicon-carbon negative electrode material is W Si It is 35%~50%.

4. The preparation method according to claim 1, characterized in that: The concentration of the bisphenol A is 0.6-1.2 mol / L, and the concentration of the formaldehyde is 0.6-1.2 mol / L.

5. Application of the silicon-carbon negative electrode material prepared by the preparation method according to any one of claims 1 to 4 in the preparation of lithium-ion batteries.

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