Silicon-carbon composite materials, their preparation methods, and lithium-ion batteries

Abstract

The embodiment of the present application provides a kind of silicon-carbon composite material, including core and carbon layer coated in the surface of core, wherein, core includes graphite framework, amorphous carbon filled in graphite framework structure, and silicon material uniformly distributed in amorphous carbon, only have pore structure with pore diameter less than or equal to 50nm in silicon-carbon composite material inside, there is no pore structure with pore diameter greater than 50nm.The pore size in this silicon-carbon composite material is small, which can effectively reduce the contact area of silicon material and electrolyte, reduce the occurrence of side reaction, prolong the service life of battery;While silicon material is uniformly dispersed around graphite framework, there is no agglomeration, so that graphite framework can effectively alleviate the volume expansion and shrinkage of silicon material, improve the structural stability and energy density of composite material.The embodiment of the present application also provides the preparation method of the silicon-carbon composite material and the lithium ion battery comprising the silicon-carbon composite material.

Description

[0001] This application is a divisional application. The original application has the application number 201910699789.7 and the original application date is July 31, 2019. The entire contents of the original application are incorporated herein by reference. Technical Field

[0002] This invention relates to the field of lithium-ion battery technology, and in particular to a silicon-carbon composite material, its preparation method, and a lithium-ion battery. Background Technology

[0003] Since the early 1990s, graphite has been the mainstream anode material for lithium-ion batteries. However, the current commercially available high-quality graphite has a specific capacity of 360 mAh / g, which is close to its theoretical value of 372 mAh / g. This represents the ceiling for energy density in full-cell batteries and cannot meet the energy density requirements of various consumer electronic devices, especially energy storage devices and electric vehicles. Therefore, there is an urgent need to find a high-energy-density anode material to replace graphite.

[0004] Silicon-based materials are among the most studied and promising alternatives to graphite as anode materials. Depending on the reaction depth, silicon and lithium can produce different products, such as Li7Si3 and Li... 13 Si4, Li 22 Si5, Li 12 Si 17 etc. Among them, Li formed by lithium intercalation into silicon 4.4 Si alloys have a theoretical specific capacity of 4200 mAh / g, making them the anode material with the highest theoretical capacity. However, silicon-based materials undergo drastic volume expansion (0-300%) and contraction during the lithium insertion / extraction reaction, leading to structural damage and pulverization of the electrode material. Furthermore, new SEI films (solid electrolyte interface) are continuously formed on the silicon surface with the electrolyte, resulting in the depletion of the electrolyte and a rapid decline in battery capacity.

[0005] To combine the properties of both graphite and silicon, silicon-carbon composite materials have been developed. Currently, the most widely accepted practical silicon-carbon composite materials are secondary particles formed by granulation of nano-silicon, graphite, and carbon. However, due to the two-order-of-magnitude difference in particle size between nano-silicon and graphite, and the high surface energy of nano-silicon leading to its tendency to agglomerate, it is difficult to uniformly disperse both nano-silicon and graphite. Nano-silicon often agglomerates on the graphite surface or concentrates in a certain location, resulting in a large local volume expansion and contraction rate. The graphite substrate cannot effectively absorb and mitigate the expansion of silicon, ultimately leading to structural damage and performance degradation in the composite material. Furthermore, existing silicon-carbon composite materials contain numerous large pores larger than 50 nm within the secondary particles, resulting in low structural stability, a large contact area between the silicon material and the electrolyte, severe side reactions, and ultimately, rapid performance degradation. Summary of the Invention

[0006] In view of this, the present invention provides a silicon-carbon composite material with graphite as the skeleton. Silicon material is uniformly dispersed in the gaps of the graphite skeleton by amorphous carbon, and the material is controlled to have no pores larger than 50 nm inside. This solves to a certain extent the problem of uneven dispersion of nano-silicon and large internal pores in existing silicon-carbon composite materials, which leads to low structural stability of the composite material and easy occurrence of side reactions.

[0007] Specifically, the first aspect of the present invention provides a silicon-carbon composite material for use as a battery negative electrode material. The silicon-carbon composite material includes a core and a carbon layer covering the surface of the core. The core includes a graphite skeleton, amorphous carbon filled in the graphite skeleton structure, and silicon material uniformly distributed in the amorphous carbon. The silicon-carbon composite material has only a pore structure with a pore size of less than or equal to 50 nm inside, and no pore structure with a pore size greater than 50 nm.

[0008] In this embodiment of the invention, the area ratio of silicon material in any cross-section of the silicon-carbon composite material is 20%-50% within a range of 5μm×5μm.

[0009] In this embodiment of the invention, the particle size of the silicon material is 50nm-150nm.

[0010] In this embodiment of the invention, the silicon material accounts for 10%-40% of the mass of the silicon-carbon composite material.

[0011] In this embodiment of the invention, the graphite skeleton accounts for 50%-70% of the mass of the silicon-carbon composite material.

[0012] In this embodiment of the invention, the amorphous carbon in the silicon-carbon composite material accounts for 10%-30% by mass.

[0013] In this embodiment of the invention, the particle size of the silicon-carbon composite material is D10 of 4μm-7μm, D50 of 8μm-18μm, D90 of 25μm-35μm, and D99 of 40μm-60μm.

[0014] In this embodiment of the invention, the graphite skeleton is composed of graphite material with a particle size of 5μm-15μm, and the graphite material includes at least one of artificial graphite and natural graphite.

[0015] In this embodiment of the invention, the graphite material is flake graphite with an aspect ratio of 1-3.

[0016] In this embodiment of the invention, the thickness of the carbon layer is 5nm-20nm.

[0017] In this embodiment of the invention, the tap density of the silicon-carbon composite material is 0.8-1.0 g / cm³. 3 Its specific surface area is 1.5-3.0 m². 2 / g.

[0018] The silicon-carbon composite material provided in the first aspect of this invention uses graphite connected by amorphous carbon, with silicon uniformly dispersed in the amorphous carbon. The small internal pore size reduces the contact area between the silicon and the electrolyte, decreasing side reactions and extending battery life. Simultaneously, the amorphous carbon ensures uniform silicon distribution around the graphite, effectively preventing excessive local expansion and contraction within the material. This allows the graphite to effectively mitigate the volume expansion and contraction of the silicon, improving the structural stability and energy density of the composite material.

[0019] Secondly, embodiments of the present invention also provide a method for preparing a silicon-carbon composite material, comprising:

[0020] Graphite material was added to an aqueous solution of sodium carboxymethyl cellulose and stirred to disperse evenly to obtain dispersion A;

[0021] Silicon material was wetted with C1-C4 alcohol and uniformly dispersed in water together with an amorphous carbon precursor to obtain dispersion B.

[0022] Add dispersion A and dispersion B to water and mix, stirring until uniformly dispersed to obtain a mixed dispersion.

[0023] The mixed dispersion was dried to obtain a primary precursor;

[0024] The primary precursor is subjected to primary heat treatment, molding treatment, cold isostatic pressing treatment and secondary heat treatment in sequence, and then crushed and classified to obtain the secondary precursor.

[0025] The secondary precursor is coated with carbon and then subjected to tertiary heat treatment to obtain a silicon-carbon composite material. The silicon-carbon composite material includes a core and a carbon layer covering the surface of the core. The core includes a graphite skeleton, amorphous carbon filling the graphite skeleton structure, and silicon material uniformly distributed in the amorphous carbon. The silicon-carbon composite material has only a pore structure with a pore size of less than or equal to 50 nm, and no pore structure with a pore size greater than 50 nm.

[0026] In this embodiment of the invention, the amorphous carbon precursor includes pitch or phenolic resin.

[0027] In this embodiment of the invention, the pressure during the molding process is 3 MPa-5 MPa, and the molding temperature is 300℃-400℃.

[0028] In this embodiment of the invention, the pressure of the cold isostatic pressing treatment is 90 MPa-120 MPa, and the time is 0.5-2 hours.

[0029] In this embodiment of the invention, the drying process includes spray drying or rotary evaporation drying.

[0030] In this embodiment of the invention, the temperature of the first-stage heat treatment is 180℃-200℃, and the treatment time is 2-3 hours.

[0031] In this embodiment of the invention, the specific operation of the secondary heat treatment is as follows: under nitrogen protection, first raise the temperature to 300℃-350℃ and hold for 1-2 hours, then raise the temperature to 450℃-550℃ and hold for 1-2 hours, and finally raise the temperature to 600℃-700℃ and hold for 2-4 hours, and then let it naturally cool to room temperature.

[0032] In this embodiment of the invention, the specific operation of carbon coating is as follows: the secondary precursor and the carbon layer precursor are mixed, the temperature is first raised to 200℃-250℃ and held for 60-90 minutes, then the temperature is slowly raised to 300℃-400℃ and held for 80-120 minutes; then the temperature is raised to 450℃-500℃ and held for 100-150 minutes; finally the temperature is raised to 650℃-700℃ and held for 100-150 minutes to obtain the carbon layer.

[0033] In this embodiment of the invention, the three-stage heat treatment is carried out in a tube furnace and maintained at 1000℃-1200℃ for 2-4 hours under nitrogen protection.

[0034] The method for preparing silicon-carbon composite materials provided in the second aspect of the present invention uses a three-step dispersion method to uniformly disperse the raw materials to obtain a uniform dispersion system, so that the silicon material in the composite material can be uniformly distributed with the graphite material and amorphous carbon, thereby improving the ability of the composite material to alleviate the volume expansion and contraction of silicon. Furthermore, by combining molding and isostatic pressing, the internal pore size of the composite material can be effectively reduced, the contact between the silicon material and the electrolyte can be reduced, and the service life of the battery cell can be extended.

[0035] The present invention also provides a lithium-ion battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the negative electrode comprises a negative electrode active material, and the negative electrode active material comprises the silicon-carbon composite material described in the first aspect of the present invention.

[0036] In addition, this embodiment of the invention also provides a terminal, which includes a housing, a circuit board and a battery located inside the housing, the battery including the lithium-ion battery provided in the above embodiment of the invention, the lithium-ion battery being used to power the terminal. This terminal can be a mobile phone, tablet computer, laptop computer, portable computer, smart wearable product, or other electronic product. Attached Figure Description

[0037] Figure 1 This is a schematic diagram of the structure of the silicon-carbon composite material provided in an embodiment of the present invention;

[0038] Figure 2 This is a schematic diagram of the terminal structure provided in an embodiment of the present invention;

[0039] Figure 3 This is a flowchart of the preparation method of the silicon-carbon composite material in Example 1 of the present invention;

[0040] Figure 4 The image is a scanning electron microscope (SEM) image of precursor A obtained after spray drying in Example 1 of the present invention.

[0041] Figure 5 This is a photograph of the precursor C obtained after molding and isostatic pressing in Embodiment 1 of the present invention;

[0042] Figure 6 This is a cross-sectional scanning electron microscope (SEM) image of the silicon-carbon composite material of Example 1 of the present invention;

[0043] Figure 7 This is a cross-sectional scanning electron microscope (SEM) image of the silicon-carbon composite material of the comparative example of the present invention;

[0044] Figure 8 The cycling curves are shown for batteries prepared using the silicon-carbon composite material of Example 1 of the present invention and the silicon-carbon material of the comparative example. Detailed Implementation

[0045] The embodiments of the present invention will now be described in conjunction with the accompanying drawings.

[0046] See Figure 1 This invention provides a silicon-carbon composite material 100 for use as a battery negative electrode. The silicon-carbon composite material 100 is a secondary particle, including a core 10 and a carbon layer 20 covering the surface of the core 10. The core 10 is composed of a graphite skeleton 101, amorphous carbon 102 filling the gaps in the graphite skeleton 101, and silicon material 103 uniformly distributed in the amorphous carbon 102. The silicon-carbon composite material 100 has only a pore structure with a pore size less than or equal to 50 nm inside, and no pore structure with a pore size greater than 50 nm.

[0047] In this embodiment of the invention, within the silicon-carbon composite material 100, amorphous carbon 102 serves as a connecting medium, effectively linking the graphite skeleton 101 together, ensuring that there are no pores larger than 50 nm between the graphite skeletons. Furthermore, uniformly dispersed silicon material 103 is distributed within the amorphous carbon 102, which also lacks pores larger than 50 nm. Because the secondary particles of the entire composite material lack large pores larger than 50 nm, when the electrolyte penetrates into the composite material, it effectively reduces the contact area between the silicon material and the electrolyte, minimizing side reactions and extending the cell's cycle life.

[0048] In this embodiment of the invention, silicon material 103 is uniformly dispersed in amorphous carbon 102, that is, silicon material 103 is uniformly dispersed in the gaps between graphite skeleton 101. Specifically, the high uniformity of silicon material 103 is manifested in that, within a 5μm × 5μm range of any cross-section of the silicon-carbon composite material 100, the area ratio of silicon material 103 is 20%-50%, and more preferably, the area ratio of silicon material 103 is 20%-30%. The uniform dispersion of silicon material 103 in the gaps between graphite skeleton 101, without agglomeration, effectively avoids the problem of excessive local expansion and contraction rates within the material, resulting in a more uniform distribution of expansion and contraction of silicon material within the material. This allows the graphite skeleton to more effectively mitigate the volume expansion and contraction of the absorbed silicon material, improving the structural stability and energy density of the composite material.

[0049] In this embodiment of the invention, the particle size of silicon material 103 can be 50nm-150nm, and further, the particle size of silicon material 103 can be 50nm-100nm or 60nm-80nm. Nanoscale silicon material can effectively reduce absolute expansion. Silicon material 103 can be spherical, near-spherical, or needle-like particles. Specifically, silicon material 103 is elemental silicon.

[0050] In this embodiment of the invention, the graphite skeleton 101 in the silicon-carbon composite material can account for 50%-70% by mass, and further, the graphite skeleton 101 can account for 55%-65% by mass. The graphite skeleton is the basic structure of the composite material, which can ensure the structural stability of the secondary particles. Moreover, the relatively soft texture of graphite can effectively alleviate the volume expansion and contraction of silicon materials, ultimately reducing the volume expansion of the secondary particles and effectively improving the energy density of the composite material.

[0051] In this embodiment of the invention, the mass percentage of silicon material 103 in the silicon-carbon composite material can be 10%-40%, and further, the mass percentage of silicon material 103 can be 15%-35% or 20%-30%. A suitable amount of silicon material can both increase the energy density of the composite material and ensure that it is uniformly dispersed throughout the structure, maintaining structural stability and preventing the composite material from being damaged by the expansion of the silicon material.

[0052] In this embodiment of the invention, the mass percentage of amorphous carbon 102 in the silicon-carbon composite material can be 10%-30%, and further, the mass percentage of amorphous carbon 102 can be 15%-25%. Filling the gaps in the graphite skeleton with amorphous carbon can, on the one hand, reduce the internal pore size and effectively reduce the contact area between the silicon material and the electrolyte; on the other hand, it can form an integral conductive network inside the material, which can effectively link the silicon material and graphite, improve the conductivity of secondary particles, compensate for the poor conductivity of silicon, and improve the structural stability of the material.

[0053] In this embodiment of the invention, the graphite skeleton 101 is composed of graphite material with a particle size of 5μm-15μm, and the graphite material can be at least one of artificial graphite and natural graphite. Optionally, the graphite material is flake graphite with an aspect ratio of 1-3. Selecting flake graphite with a suitable aspect ratio to construct the graphite skeleton structure is beneficial to enhancing the structural stability of the skeleton.

[0054] In this embodiment of the invention, the thickness of the carbon layer 20 can be 5nm-20nm, and further, the thickness of the carbon layer 20 can be 10nm-15nm. Specifically, the carbon layer 20 can be an amorphous carbon layer. Coating the outermost layer of the secondary particles with a carbon layer can further protect the core of the secondary particles, reduce the specific surface area, reduce the occurrence of side reactions, and improve the cycle performance of the composite material.

[0055] In this embodiment of the invention, the particle size of the silicon-carbon composite material 100 is D10 of 4μm-7μm, D50 of 8μm-18μm, D90 of 25μm-35μm, and D99 of 40μm-60μm.

[0056] In this embodiment of the invention, the silicon-carbon composite material 100 can be spherical or near-spherical particles with a tap density of 0.8-1.0 g / cm³.3 Its specific surface area is 1.5-3.0 m². 2 / g. The silicon-carbon composite material of this invention has a high tap density, which is beneficial to improving the processing performance of the material in subsequent cell fabrication; at the same time, it has a low specific surface area, which can effectively reduce side reactions between the material and the electrolyte and extend cycle life. The silicon-carbon composite material of this invention has a high initial coulombic efficiency, which can effectively improve the energy density of the full cell.

[0057] The silicon-carbon composite material provided in the embodiments of the present invention has graphite material connected by amorphous carbon, and silicon material uniformly dispersed in amorphous carbon. The internal pore size is small, which can reduce the contact area between silicon material and electrolyte, reduce the occurrence of side reactions, and extend battery life. At the same time, the amorphous carbon makes silicon material uniformly distributed around graphite material, which can effectively avoid excessive local expansion and contraction rate within the material. This allows graphite material to effectively alleviate the volume expansion and contraction of silicon material, improving the structural stability and energy density of the composite material.

[0058] Accordingly, embodiments of the present invention also provide a method for preparing a silicon-carbon composite material, comprising:

[0059] S101. Add the graphite material to the sodium carboxymethyl cellulose aqueous solution and stir to disperse evenly to obtain dispersion A;

[0060] S102. The silicon material is wetted with C1-C4 alcohol and uniformly dispersed in water together with the amorphous carbon precursor to obtain dispersion B.

[0061] S103. Add dispersion A and dispersion B to water and mix, stirring until uniform to obtain a mixed dispersion.

[0062] S104. The mixed dispersion is dried to obtain the primary precursor;

[0063] S105. The primary precursor is subjected to primary heat treatment, molding treatment, cold isostatic pressing treatment and secondary heat treatment in sequence, and then crushed and classified to obtain the secondary precursor.

[0064] S106. The secondary precursor is coated with carbon and then subjected to tertiary heat treatment to obtain a silicon-carbon composite material. The silicon-carbon composite material includes a core and a carbon layer covering the surface of the core. The core includes a graphite skeleton, amorphous carbon filled in the graphite skeleton structure, and silicon material uniformly distributed in the amorphous carbon. The silicon-carbon composite material has only a pore structure with a pore size of less than or equal to 50 nm, and no pore structure with a pore size greater than 50 nm.

[0065] In step S101, sodium carboxymethyl cellulose can act as a surfactant to modify the surface of the graphite material, adjusting the surface charge of the graphite and facilitating its affinity with silicon to form secondary particles. The graphite material can be dispersed in an aqueous solution of sodium carboxymethyl cellulose using a high-speed mixer at a speed of 2000-3000 rpm for 1-2 hours. Of course, in other embodiments of the present invention, other existing feasible methods can also be used to uniformly disperse the graphite material in the aqueous solution of sodium carboxymethyl cellulose.

[0066] The sodium carboxymethyl cellulose aqueous solution can be prepared by adding sodium carboxymethyl cellulose to water and stirring with a high-speed mixer until the sodium carboxymethyl cellulose is completely dissolved, thus obtaining the sodium carboxymethyl cellulose aqueous solution.

[0067] The graphite material can be at least one of artificial graphite and natural graphite. Optionally, the graphite material has a particle size of 5μm-15μm, specifically it can be flake graphite with an aspect ratio of 1-3.

[0068] In step S102, using C1-C4 alcohols to wet the silicon material can effectively improve the dispersibility of nano-silicon and prevent agglomeration. Specifically, the C1-C4 alcohols can be anhydrous ethanol, methanol, propanol, or butanol.

[0069] The specific procedure for wetting the silicon material with a C1-C4 alcohol and dispersing it together with the amorphous carbon precursor in water can be as follows: Add a C1-C4 alcohol to the silicon material, stir at low speed to fully wet the silicon material, add water after wetting, then add the amorphous carbon precursor. First, stir using a high-speed mixer, then add the precursor to a sand mill for circulating dispersion. The amorphous carbon precursor includes, but is not limited to, asphalt and phenolic resin. The stirring speed of the high-speed mixer can be 2000-3000 rpm.

[0070] In this embodiment of the invention, the particle size of the silicon material can be 50nm-150nm, and further, the particle size can be 50nm-100nm or 60nm-80nm. The silicon material can be spherical, near-spherical, or needle-shaped particles. Specifically, the silicon material is elemental silicon.

[0071] In this embodiment of the invention, the amorphous carbon precursor can be asphalt, phenolic resin, etc.

[0072] In step S103, the stirring speed can be 2000-3000 rpm, and the stirring and dispersion time can be 1-2 hours. Through the above-mentioned special three-step dispersion method, the graphite, silicon material, and amorphous carbon precursor are uniformly dispersed in the prepared mixed dispersion, thus ultimately obtaining a composite material with uniformly dispersed silicon material and graphite.

[0073] In step S104, the drying process can be spray drying or rotary evaporation drying.

[0074] In step S105, the primary heat treatment can be carried out in an electrically heated high-speed modified mixer. The heat treatment temperature can be 180℃-200℃, and the treatment time can be 2-3 hours. The stirring speed during the heat treatment is 1200-1500 rpm. During the heat treatment, when the temperature reaches above 200℃, nitrogen gas can be introduced (flow rate 200-300 mL / min) to control the volatile matter content to 1.5%-2.5%. The primary heat treatment can crosslink the amorphous carbon precursor, causing the light components in the system to volatilize.

[0075] To effectively reduce the internal pore size of the composite material, minimize the contact between the silicon material and the electrolyte during charging and discharging, and extend the cycle life of the battery cell, this invention employs molding and isostatic pressing methods after the primary heat treatment. Molding (also known as compression molding or compression molding) involves placing powdered, granular, or fibrous powder into a mold cavity at the molding temperature, then closing the mold and applying pressure to shape and solidify it. Isostatic pressing is an ultra-high pressure hydraulic technology that utilizes a sealed high-pressure container to mold the product under uniform ultra-high pressure in all directions.

[0076] In this embodiment of the invention, the pressure during the molding process is 3 MPa-5 MPa, and the molding temperature is 300℃-400℃. Specifically, the pressure during the molding process can be 3 MPa, 4 MPa, or 5 MPa, and the molding temperature can be 300℃, 350℃, or 400℃.

[0077] In this embodiment of the invention, the pressure of the cold isostatic pressing (COP) treatment is 90 MPa-120 MPa, the time is 0.5-2 hours, and the COP treatment is carried out at room temperature. Specifically, the pressure of the COP treatment can be 90 MPa, 100 MPa, 110 MPa, or 120 MPa, and the time can be 0.5 hours, 1 hour, 1.5 hours, or 2 hours.

[0078] By employing molding and cold isostatic pressing processes, and controlling appropriate operating parameters, the internal pore size of the composite material is significantly reduced.

[0079] In this embodiment of the invention, the secondary heat treatment can be carried out in a tube furnace. Under nitrogen protection, the temperature is first raised to 300℃-350℃ and held for 1-2 hours, then raised to 450℃-550℃ and held for 1-2 hours, and finally raised to over 600℃-700℃ and held for 2-4 hours, after which it is allowed to cool naturally to room temperature. During the secondary heat treatment, the amorphous carbon precursor undergoes pyrolysis and cross-linking, further releasing light components and fully mixing uniformly with graphite and silicon materials.

[0080] In this embodiment of the invention, the pulverization process can involve first crushing the blocky sample using a mortar and pestle, and then pulverizing it using a pulverizer. The grading process can employ methods such as jet grading or cyclone grading.

[0081] In step S106, specifically, a carbon layer can be obtained by coating with asphalt or phenolic resin, or by chemical vapor deposition. In one embodiment of the present invention, the specific operation of carbon coating is as follows: the secondary precursor and the carbon layer precursor are mixed and placed in an electrically heated high-speed modified mixer. The mixture is first heated to 200℃-250℃ and held for 60-90 minutes, then slowly heated to 300℃-400℃ and held for 80-120 minutes; then heated to 450℃-500℃ and held for 100-150 minutes; finally heated to 650℃-700℃ and held for 100-150 minutes to obtain the carbon layer.

[0082] In this embodiment of the invention, the third-stage heat treatment is a calcination treatment, specifically, it can be carried out in a tube furnace under a protective atmosphere at 1000℃-1200℃ for 2-4 hours. The protective atmosphere can be nitrogen, argon, etc. The nitrogen flow rate can be 1L / min.

[0083] The method for preparing silicon-carbon composite materials provided in this invention employs a three-step dispersion method to obtain a uniform dispersion system, enabling silicon materials, graphite materials, and amorphous carbon to be evenly distributed in the composite material. This effectively solves the problem of uneven dispersion of components in composite materials in the prior art, thereby improving the composite material's ability to mitigate silicon volume expansion and contraction. Furthermore, by combining molding and isostatic pressing treatments, the pore size in the secondary particles of the material is reduced to a predetermined level, forming a stable composite material resistant to expansion and contraction changes. This also reduces the overall specific surface area of ​​the material and improves its cycle performance.

[0084] This invention also provides a lithium-ion battery, including a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the negative electrode includes a negative electrode active material, and the negative electrode active material includes the silicon-carbon composite material described above in this invention.

[0085] like Figure 2 As shown, this embodiment of the invention also provides a terminal 200, which can be a mobile phone, tablet computer, laptop computer, portable computer, smart wearable product or other electronic product. The terminal 200 includes a shell 201 assembled on the outside of the terminal, and a circuit board and a battery (not shown in the figure) located inside the shell 201. The battery is the lithium-ion battery provided in the above embodiment of the invention. The shell 201 may include a display screen assembled on the front side of the terminal and a back cover assembled on the rear side. The battery can be fixed inside the back cover to supply power to the terminal 200.

[0086] The present invention will be further described below through specific embodiments.

[0087] Reference Figure 3 The preparation method of the silicon-carbon composite material in Example 1 of the present invention includes the following steps:

[0088] 1. Three-step dispersion of graphite / silicon materials / asphalt:

[0089] 1.1 Preparation of Graphite Dispersion A (One-Step Dispersion): Add 3000g of deionized water to a 10L container, turn on a high-speed mixer, and slowly add 20.5g of sodium carboxymethyl cellulose (CMC) while stirring at low speed. After adding, adjust the stirring speed to 900-1200 rpm to accelerate the dissolution of CMC, obtaining a completely dissolved CMC solution. Add 270g of graphite to the sodium carboxymethyl cellulose CMC solution, adjust the stirring speed to 2000-2400 rpm, and stir and disperse for 60-120 min to obtain a uniformly dispersed graphite slurry dispersion A.

[0090] 1.2 Preparation of Silicon Material / Asphalt Dispersion B: (Two-step dispersion): Take 180g of elemental silicon and add it to a 5L plastic bucket. Then add 270g of anhydrous ethanol, turn on the mixer, and stir at low speed to fully wet the elemental silicon. After wetting, add 1750g of deionized water, followed by 270g of asphalt powder. After stirring at high speed, add it to a sand mill for circulation dispersion to obtain a uniformly dispersed silicon material / asphalt mixture slurry, which is dispersion B.

[0091] 1.3 Preparation of Graphite / Silicon Material / Asphalt Mixed Dispersion (Three-Step Dispersion): The silicon material / asphalt mixed slurry prepared in step 1.2 is added to the graphite slurry prepared in step 1.1, followed by 2000g of deionized water. The stirring speed of the mixer is adjusted to 2000-3000 rpm, and the mixture is stirred for 60-120 minutes to obtain a uniformly dispersed graphite / silicon material / asphalt mixed slurry, thus obtaining the mixed dispersion. This three-step dispersion process ensures the uniform dispersion of silicon material, asphalt, and graphite.

[0092] 2. Spray drying and primary heat treatment:

[0093] 2.1 Turn on the sprayer and spray the mixed dispersion. Adjust the inlet temperature to 200-230℃. After the temperature is reached, turn on the atomizer and adjust the atomization speed to 30-45Hz. Turn on the peristaltic pump and adjust the feed pump feed speed to 20-30Hz to start spraying to obtain precursor A.

[0094] 2.2 The spray-dried material is put into an electrically heated high-speed modified mixer (thermal VC mixer), and the stirring is turned on (stirring speed 1200-1500 rpm) while the heating program is turned on (180-200℃) for 2-3 hours. The mixture is then stirred and cooled to below 100℃ before being discharged to obtain precursor B.

[0095] like Figure 4 As shown, after spray drying, the nano-silicon material, amorphous carbon precursor, and graphite granules become spherical secondary particles, with the nano-silicon material uniformly dispersed inside and on the surface of the secondary particles.

[0096] 3. Molding and isostatic pressing treatment

[0097] Precursor powders (graphite-based / nano-silicon / asphalt) after spray drying and primary heat treatment are loaded into a molding die (53mm diameter, 105mm length). The die is pressurized at 3-5 MPa for 10 minutes using a hydraulic gauge. Afterward, the powder is placed in a polyethylene bag, vacuum-sealed, and placed in an isostatic press. The pressure is increased to 90-120 MPa at 8-10 MPa / min, held for 0.5-2 hours, and then released at 8-10 MPa / min before discharge. After molding and isostatic pressing, the material forms a block, yielding precursor C, such as... Figure 5 As shown.

[0098] 4. Secondary heat treatment and grinding / grading

[0099] Precursor C is placed in a ceramic crucible and then placed in a tube furnace. The temperature is raised to 300°C and maintained for 1-2 hours; then raised to 450°C and maintained for 1-2 hours; finally, the temperature is raised to 600°C and maintained for more than 2 hours to end the process. The mixture is then allowed to cool naturally to room temperature under a protective atmosphere. Nitrogen protection is maintained throughout the process at a flow rate of 1 L / min. This process yields precursor D.

[0100] The above-mentioned blocks were crushed using a mortar and pestle, and then passed through an 80-mesh sieve before being added to a pulverizer. The pulverizer main unit frequency was 12-15Hz, the classifier frequency was 25-30Hz, and the feed rate was 5Hz. After this process, precursor E was obtained.

[0101] 5. Carbon coating treatment

[0102] Precursor E and asphalt are mixed evenly in a high-speed mixer at a mass ratio of 100:10-15. Then, the mixture is fed into an electrically heated high-speed modified mixer (thermal VC mixer) for coating treatment. The treatment conditions are as follows: first, the temperature is raised to 200℃ and held for 60-90 minutes; then, the temperature is slowly raised to 300℃ and held for 80-120 minutes; then, the temperature is raised to 450℃ and held for 100-150 minutes; finally, the temperature is raised to 650℃ and held for 100-150 minutes. The process is then completed, and precursor F is obtained.

[0103] 6. Three-stage heat treatment

[0104] The carbon-coated sample precursor F was placed in a crucible and then placed in a tube furnace. The temperature was rapidly increased to 1000℃-1100℃ and maintained for 2-3 hours. Nitrogen gas was used throughout the process at a flow rate of 1L / min to obtain the target material, silicon-carbon composite particles.

[0105] Comparative Example

[0106] The difference from Example 1 is that graphite, silicon material, and asphalt are dispersed in one step. Specifically, graphite, silicon material, and asphalt are stirred and dispersed together in water to obtain a mixed dispersion. The comparative example does not use molding and isostatic pressing treatment. Other steps are the same as in Example 1.

[0107] The cross-sections of the silicon-carbon composite materials prepared in Example 1 and the comparative example of the present invention were observed using a scanning electron microscope (SEM), and the results are as follows: Figure 6 and Figure 7 As shown, from Figure 6 As can be seen, in Embodiment 1 of the present invention, a silicon-carbon composite material prepared by a three-step dispersion method combined with molding and isostatic pressing is used. The silicon material (shown as white in the figure) and graphite (shown as black in the figure) are uniformly distributed inside the material, and the internal pore structure is small in size, with no large pores larger than 50 nm. Figure 7 The results show that the silicon and graphite are unevenly distributed inside the composite material prepared in the comparative example, and there are many large pore structures inside.

[0108] The comparison results of the physical and electrochemical properties of the silicon-carbon composite materials of Example 1 and the comparative example of the present invention are listed in Table 1.

[0109] Table 1

[0110] project Example 1: Silicon-Carbon Composite Material Comparative silicon-carbon composite materials D10(μm) 6.83 5.33 D50(μm) 17.67 12.55 D90(μm) 34.95 23.63 <![CDATA[Tap density (g / cm 3 )]]> 0.98 0.8 <![CDATA[Specific surface area (m 2 / g)]]> 1.85 2.17 Reversible capacity (mAh / g) 1083.4 1080.6 First-time coulomb efficiency (%) 86.7 84.8

[0111] Table 1 shows that the silicon-carbon composite material of Example 1 of the present invention has a high tap density, which is beneficial to improving the processing performance of the material in the later stage of cell manufacturing; secondly, the silicon-carbon composite material has a low specific surface area, which can effectively reduce the side reaction between the material and the electrolyte and extend the cycle life; the silicon-carbon composite material of Example 1 of the present invention has a high initial coulombic efficiency, which can effectively improve the energy density of the full cell.

[0112] The silicon-carbon composite material from Example 1 of this invention and the silicon-carbon composite material from the comparative example were used as negative electrodes. They were dispersed in deionized water with conductive agent SP, binder SBR, and CMC at a mass ratio of 95:0.3:3.2:1.5 and stirred until homogeneous to obtain an electrode slurry. This slurry was coated onto a copper foil surface and dried at 85°C to obtain a negative electrode sheet. Combined with a commercially available lithium cobalt oxide positive electrode sheet, an electrolyte of 1 mol / L LiPF6 / EC+PC+DEC+EMC (volume ratio 1:0.3:1:1), and a PP / PE / PP three-layer separator with a thickness of 10 μm, a soft-pack battery of approximately 3.7 Ah was fabricated for testing the full-cell performance of the material. Figure 8As shown, the battery using the silicon-carbon composite material of Embodiment 1 of the present invention exhibits excellent cycle performance, with a capacity retention rate of 80% after 500 cycles. In contrast, the battery using the comparative silicon-carbon composite material showed a capacity decay to 80% after 200 cycles. This is because the silicon-carbon composite material provided in the embodiments of the present invention has graphite material connected by amorphous carbon, and silicon material uniformly dispersed in the amorphous carbon, resulting in small internal pore sizes. This reduces the contact area between the silicon material and the electrolyte, decreases the occurrence of side reactions, and extends the battery life. At the same time, the amorphous carbon ensures that the silicon material is uniformly distributed around the graphite material, effectively preventing excessive local expansion and contraction rates within the material. This allows the graphite material to effectively mitigate the volume expansion and contraction of the silicon material, improving the structural stability of the composite material and extending the battery life.

Claims

1. A silicon-carbon composite material, characterized by, The silicon-carbon composite material comprises a core and a carbon layer coated on the surface of the core, the core comprises a graphite framework, amorphous carbon filled in the graphite framework structure, and silicon material uniformly distributed in the amorphous carbon, the graphite framework is composed of disordered arranged graphite material, the silicon material comprises elemental silicon, the silicon material is dispersed between the gaps of the graphite framework, the silicon-carbon composite material only has pore structure with pore diameter less than or equal to 50 nm inside, and there is no pore structure with pore diameter greater than 50 nm; the particle size D10 of the silicon-carbon composite material is 4-7 mu m, the D50 is 8-18 mu m, and the D90 is 25-35 mu m; the particle size of the silicon material is 50-150 nm, and the mass ratio of the silicon material in the silicon-carbon composite material is 10%-40%; the graphite material is flaky graphite with an aspect ratio of 1-3; the tap density of the silicon-carbon composite material is 0.8-1.0 g / cm 3 , and the specific surface area is 1.5-3.0 m 2 / g.

2. The silicon-carbon composite of claim 1, wherein, The silicon material accounts for 20-50% of the area in a 5-micron by 5-micron range of any cross section of the silicon-carbon composite material.

3. The silicon-carbon composite of claim 1, wherein, The silicon material is spherical, spherical-like or needle-like particles.

4. The silicon-carbon composite of claim 1, wherein, The graphite framework accounts for 50-70% of the mass in the silicon-carbon composite material.

5. The silicon-carbon composite of claim 1, wherein, The amorphous carbon accounts for 10-30% of the mass in the silicon-carbon composite material.

6. The silicon-carbon composite of claim 1, wherein, The particle size D99 of the silicon-carbon composite material is 40-60 microns.

7. The silicon-carbon composite of claim 1, wherein, The graphite framework is composed of graphite material with a particle size of 5-15 microns, and the graphite material includes at least one of artificial graphite and natural graphite.

8. The silicon-carbon composite of claim 1, wherein, The carbon layer has a thickness of 5-20 nanometers.

9. A method for producing a silicon-carbon composite material, characterized by, The method comprises the following steps: adding graphite material to an aqueous solution of surfactant, stirring and uniformly dispersing to obtain dispersion A, wherein the surfactant includes sodium carboxymethyl cellulose; wetting the silicon material with C1-C4 alcohol and uniformly dispersing the silicon material and amorphous carbon precursor in water to obtain dispersion B, wherein the amorphous carbon precursor includes pitch or phenolic resin; adding dispersion A and dispersion B to water and stirring and uniformly dispersing to obtain a mixed dispersion; subjecting the mixed dispersion to drying treatment to obtain a primary precursor, wherein the drying treatment includes spray drying or rotary evaporation drying; sequentially subjecting the primary precursor to primary heat treatment, molding treatment, cold isostatic pressing treatment and secondary heat treatment, and crushing and classifying to obtain a secondary precursor; subjecting the secondary precursor to carbon coating and subjecting to tertiary heat treatment to obtain a silicon-carbon composite material, wherein the silicon-carbon composite material includes a core and a carbon layer coated on the surface of the core, the core includes a graphite framework, amorphous carbon filled in the graphite framework structure, and silicon material uniformly distributed in the amorphous carbon, the silicon material is elemental silicon, the silicon material is uniformly dispersed in the interstices of the graphite framework, and the silicon-carbon composite material only has pore structures with a pore size less than or equal to 50 nanometers, and does not have pore structures with a pore size greater than 50 nanometers.

10. The method of claim 9, wherein the silicon-carbon composite material is prepared by the steps of: mixing a silicon source and a carbon source to form a mixture; and heating the mixture to form the silicon-carbon composite material. The silicon material accounts for 20-50% of the area in a 5-micron by 5-micron range of any cross section of the silicon-carbon composite material.

11. The method of claim 9, wherein the silicon-carbon composite material is prepared by the steps of: mixing a silicon source and a carbon source to form a mixture; and heating the mixture to form the silicon-carbon composite material. The pressure in the molding treatment process is 3-5 MPa, and the forming temperature is 300-400 DEG C.

12. The method of claim 9, wherein the silicon-carbon composite material is prepared by the steps of: mixing a silicon source and a carbon source to form a mixture; and heating the mixture to form the silicon-carbon composite material. The pressure of the cold isostatic pressing treatment is 90-120 MPa, and the time is 0.5-2 hours.

13. The method of claim 9, wherein the silicon-carbon composite material is prepared by the steps of: mixing a silicon source and a carbon source to form a mixture; and heating the mixture to form the silicon-carbon composite material. The temperature of the primary heat treatment is 180-200 DEG C, and the treatment time is 2-3 hours.

14. The method for preparing the silicon-carbon composite material as described in claim 9, characterized in that, The specific operation of the secondary heat treatment is: first, under nitrogen protection, the temperature is raised to 300-350 DEG C and maintained for 1-2 hours, then the temperature is raised to 450-550 DEG C and maintained for 1-2 hours, finally the temperature is raised to 600-700 DEG C and maintained for 2-4 hours, and then the temperature is naturally lowered to room temperature.

15. The method of claim 9, wherein the silicon-carbon composite material is prepared by the steps of: mixing a silicon source and a carbon source to form a mixture; and heating the mixture to form the silicon-carbon composite material. The specific operation of the carbon coating is mixing the secondary precursor with a carbon layer precursor, first heating to 200-250 DEG C and maintaining for 60-90 minutes, then slowly heating to 300-400 DEG C and maintaining for 80-120 minutes, then heating to 450-500 DEG C and maintaining for 100-150 minutes, and finally heating to 650-700 DEG C and maintaining for 100-150 minutes to obtain the carbon layer.

16. The method of making a silicon-carbon composite of claim 9, wherein, The third heat treatment is performed in a tube furnace under nitrogen protection at 1000-1200 DEG C for 2-4 hours.

17. A lithium-ion battery, characterized by, The lithium ion battery comprises a positive electrode, a negative electrode, a separator and an electrolyte, wherein the negative electrode comprises a negative electrode active material, and the negative electrode active material comprises the silicon-carbon composite material according to any one of claims 1-8 or is prepared by the preparation method according to any one of claims 9-16.

18. A terminal, characterized by The lithium ion battery comprises a shell, a circuit board and a battery in the shell, and the battery comprises the lithium ion battery according to claim 17.

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