A composite multi-level structure carbon material and its preparation method and application

Abstract

The present invention provides a composite multi-level structure carbon material and its preparation method and application. The composite multi-level structure carbon material is a three-layer composite structure, the outer layer is a dense carbon structure layer, the middle layer is a high microporous carbon structure layer, the inner layer is a micro-mesoporous composite carbon structure layer, and the electrode active material is loaded in the pore structure of the middle layer and the inner layer, and the electrode active material is a silicon-containing substance or a sulfur-containing substance. The present invention obtains a multi-level structure carbon material by controlling the steps of resin synthesis, carbonization, activation and coating, and the high-capacity electrode active material of the lithium-ion battery is loaded in the pore structure of the middle layer and the inner layer. The multi-level pore structure provides loading space, expansion space and conductive network for the active material, which can effectively alleviate the large volume expansion of the high-capacity electrode active material, and does not hinder the transmission of lithium ions. It shows the characteristics of high capacity, low expansion and good cycle stability, and can be applied to the development of high-energy density lithium-ion batteries.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium batteries, and in particular relates to a composite multi-level structure carbon material and a preparation method and application thereof. Background Art

[0002] Since its commercialization in the 1990s, lithium-ion batteries have made great progress and are widely used in various electronic equipment fields. In recent years, in order to reduce the consumption of non-renewable energy, achieve sustainable development of mankind, and achieve the "dual carbon" goal, clean energy storage systems based on lithium-ion batteries have developed rapidly, especially new energy vehicles that use lithium-ion batteries as the main power unit are becoming more and more popular. However, while developing, consumers' requirements for lithium-ion batteries are also getting higher and higher, hoping that batteries of the same size or weight have a longer driving range, which can alleviate the "range anxiety" problem of new energy vehicles to a certain extent.

[0003] Traditional lithium-ion batteries mainly use graphite as the negative electrode material. The theoretical specific capacity of graphite is 372mAh / g. The current mainstream graphite can actually exert a capacity of >350mAh / g, which is very close to its theoretical value. This also makes the energy density of lithium-ion batteries using graphite gradually approach its theoretical limit. The theoretical specific capacity of silicon negative electrode is 4200mAh / g, which is more than 10 times the theoretical specific capacity of graphite. In addition, it is abundant in the earth's crust and has low cost. It has been widely studied in recent years and is considered to be one of the most important negative electrode materials that can improve the energy density of lithium-ion batteries. However, the practical application of silicon negative electrode also faces huge challenges. Silicon itself is not conductive, and silicon will undergo huge volume expansion after lithium insertion. Repeated charging and discharging causes repeated expansion and contraction of volume, which can easily lead to the breakage of silicon particles, accelerate the consumption of electrolyte, and make the electrode unstable, resulting in poor cycle stability, which cannot meet practical applications. Therefore, it is very important to develop silicon-based negative electrodes with good conductivity, small volume expansion, and stable cycle.

[0004] Sulfur is abundant on earth, low-cost and environmentally friendly. Using sulfur as a positive electrode material for lithium batteries can achieve a theoretical specific capacity of 1675mAh / g, which is much higher than existing positive electrode materials such as lithium iron phosphate, lithium cobalt oxide, and ternary materials. It is a very promising positive electrode material and is expected to greatly improve the energy density of lithium batteries. However, at the same time, there are some thorny problems with sulfur positive electrodes that restrict their practical application. For example, sulfur itself is not conductive and is not conducive to the transmission of electrons or ions. During the charging and discharging process, sulfur will form lithium polysulfide compounds that can be dissolved in the electrolyte. Its shuttle effect causes the storage performance of lithium-sulfur batteries to be very poor. In addition, sulfur will also undergo a large volume change during the charging and discharging process, leading to electrode damage. The above factors have caused lithium-sulfur batteries to be unable to meet the requirements of practical applications.

[0005] Combining silicon or sulfur with carbon is a common solution, which can improve the conductivity of silicon or sulfur. At the same time, carbon materials can be used as carriers of silicon or sulfur, which can solve the problems of silicon negative electrode or sulfur positive electrode to a certain extent. However, due to the defects of silicon negative electrode or sulfur positive electrode itself, the usual silicon-carbon or sulfur-carbon composite still cannot meet the practical application, or the performance still has a lot of room for improvement. For example, the silicon-carbon composite material obtained by mechanically grinding nano-silicon and carbon materials such as graphite still has a large volume expansion, and the battery decays quickly. The amount added to the composite negative electrode with graphite negative electrode material cannot be too high (<5%). When silicon is deposited in a single or homogeneous microporous porous carbon substrate by chemical vapor deposition, silicon will still expand greatly when lithium is embedded, which easily leads to particle breakage; while when silicon is deposited in a mesoporous porous carbon substrate by chemical vapor deposition, the electrolyte can easily penetrate through the mesopores, accelerating the consumption of the electrolyte. Although the composite of elemental sulfur with carbon materials with low specific surface area or open pore structure can significantly improve the conductivity of sulfur, it cannot avoid the dissolution problem of lithium polysulfide, resulting in poor cycling performance and storage performance.

[0006] Therefore, it is necessary to further develop advanced composite structure carbon materials, accurately control the distribution state of high-capacity electrode active materials in carbon materials, and solve the problems existing in this type of materials from multiple dimensions, so as to significantly improve their electrochemical performance and make them meet practical applications.

[0007] CN118213508A discloses a silicon-carbon composite material, comprising a multi-level porous carbon material, silicon nanoparticles dispersed in the pores of the multi-level porous carbon material, and an amorphous carbon layer coated on the surface of the multi-level porous carbon material; the pore structure of the multi-level porous carbon material includes micropores, mesopores and macropores; the pore volume of the multi-level porous carbon material is 0.4cm 3 / g~1.5cm 3 / g; in the multi-level porous carbon material, the volume of micropores accounts for 60% to 92% of the total pore volume. CN117401667A discloses a multi-level porous carbon material, which is prepared by using a silicon sphere template and a carbon source solution. The synthesized multi-level porous carbon material has micropores, mesopores and macropores. CN115954481A discloses a silicon-carbon composite material, including nano-silicon, a primary buffer layer, a secondary buffer layer and a tertiary buffer layer; the primary buffer layer, the secondary buffer layer and the tertiary buffer layer are all carbon coating layers, and the carbon source of the secondary buffer layer is deformed graphite; the size of the deformed graphite is 4-8μm, and it is flat or ellipsoidal; micropores and mesopores are formed in the primary buffer layer and the secondary buffer layer. CN117790701A discloses a composite negative electrode material, wherein the core-shell structure comprises a core and a coating layer covering the core, wherein the core comprises a porous carbon material and Si particles, wherein Si particles are distributed on the surface and / or in the pores of the porous carbon material; the composite negative electrode material comprises micropores and mesopores, wherein the ratio of the pore volume of the micropores to the pore volume of the mesopores is (50-80):(20-50). The core-shell structure not only ensures the advantage of Si as a negative electrode with a large specific capacity, but also utilizes the porous characteristics of porous carbon to provide space for the volume expansion of Si, and further utilizes carbon with a certain strength as a skeleton to further ensure the overall stability of the composite negative electrode material, thereby improving the cycle performance of the composite negative electrode material. CN114287072A discloses a silicon-carbon composite material of a core-shell composite particle, wherein the preparation method comprises the following steps: (a) providing a plurality of precursor composite particles, wherein the precursor composite particles comprise: i. a porous carbon skeleton comprising micropores and / or mesopores, wherein the total pore volume of the micropores and mesopores measured by gas adsorption is at least 0.4 cm 3 / g, and wherein the PD50 pore size of the porous carbon skeleton is not more than 10nm; and ii. a plurality of nano-sized electroactive material domains arranged together with the porous carbon skeleton; (b) subjecting the plurality of precursor composite particles to a heat treatment in contact with a pyrolytic carbon precursor, so that a pyrolytic conductive carbon material shell is formed on the precursor composite particles, wherein the heat treatment is carried out at a temperature not exceeding 700°C. Summary of the invention

[0008] The purpose of the present invention is to solve the problems of poor electrochemical stability and rate performance caused by the existing high-capacity electrode materials of lithium batteries, such as serious volume expansion, easy dissolution, and easy side reactions with electrolytes. To this end, the present invention proposes a high-capacity electrode material loaded with a multi-level structure carbon material and a preparation method thereof, which can significantly improve the electrochemical stability and rate performance of the high-capacity electrode material. On the one hand, the multi-level structure carbon material provides a loading space for the high-capacity active material, and on the other hand, it buffers its volume expansion to the maximum extent, and avoids direct contact between the active material and the electrolyte and its dissolution problem, thereby improving the cycle stability and rate performance of the electrode material.

[0009] To achieve the above-mentioned effects, the present invention provides a composite multi-level structure carbon material, which is a three-layer composite structure, wherein the outer layer is a dense carbon structure layer, the middle layer is a high microporous carbon structure layer, and the inner layer is a micro-mesoporous composite carbon structure layer. The electrode active material is loaded in the pore structure of the middle layer and the inner layer, and the electrode active material is a silicon-containing substance or a sulfur-containing substance.

[0010] The composite multi-level structure carbon material provided by the present invention has a special structure, and the inner layer has both micropores and mesopores. The mesopores reserve a large space for the expansion of silicon. However, the specific surface area of ​​the material is large after simple micro-mesoporous silicon deposition, and it is still not easy to reduce the specific surface area after carbon coating, and the electrolyte can directly penetrate into the micro-mesoporous material, causing side reactions and consuming the electrolyte. Therefore, the present invention adds a high microporous layer as an intermediate layer on the basis of conventional materials with micro-mesoporous materials. After depositing silicon and carbon coating, the specific surface area is lower. At the same time, the intermediate layer dominated by micropores can hinder the infiltration of the electrolyte, but will not hinder the normal transmission of lithium ions. Especially after a long cycle, the surface carbon coating layer may be damaged, and the middle high microporous layer can still hinder the infiltration of the electrolyte, thereby alleviating the side reactions with the electrolyte and helping to extend the cycle life. The middle high microporous layer and the surface carbon coating layer play a dual protective role, especially the dual protective role of long-term circulation. The surface carbon coating layer can more effectively prevent the penetration of the electrolyte and avoid the occurrence of side reactions. Because the expansion space reserved for a single high micropore is limited, the material still expands severely, and the particles are easily broken due to expansion. The multi-level structure carbon material obtained by the present invention is loaded with high-capacity lithium-ion battery active materials, which helps to alleviate its volume expansion after lithium insertion. The outermost dense carbon structure layer prevents the active material from directly contacting the electrolyte to avoid electrochemical side reactions and avoids the dissolution of the active material, thereby helping to improve the cycle stability of the electrode material.

[0011] The particle size of the inner layer is 3-10 μm, the thickness of the middle layer is 1-3 μm, and the thickness of the outer layer is 3-10 nm.

[0012] The schematic diagram of the multi-level structure carbon material provided by the present invention is as follows Figure 1 shown.

[0013] Furthermore, the silicon-containing compound is selected from at least one of silicon and amorphous silicon-carbon compounds; the sulfur-containing compound is at least one of lithium sulfide, sulfur, lithium polysulfide, molybdenum sulfide, iron sulfide or titanium sulfide. Furthermore, the electrode active material content is 30-70wt%, preferably 40-60wt%.

[0014] Furthermore, the micro-mesoporous composite carbon structure layer contains both micropores (pore size < 2 nm) and mesopores (pore size 2-30 nm), and the micropores account for 40-70%, and the pore volume is 0.6-1.0 cm 3 / g, with a specific surface area of ​​1000-2500 m 2 / g. The micropore ratio or mesopore ratio in the present invention refers to the volume ratio, that is, the percentage of the micropore volume or mesopore volume in the total pore volume.

[0015] Furthermore, the micro-mesoporous composite carbon structure layer is obtained by reacting phenolic monomers and aldehyde monomers in a solvent to form a gel, and then crushing, drying and carbonizing. Activation treatment is performed when necessary to adjust the micro-mesoporous ratio. Activation can increase the proportion of micropores. The activation reagent is one or more of carbon dioxide, water vapor, potassium hydroxide, sodium hydroxide, and zinc chloride; the phenolic monomer is one or more of phenol, o-cresol, para-chloro-meta-cresol, catechol, resorcinol, 2,6-dichlorophenol, pyrogallol, nitrophenol, and biphenol; the aldehyde monomer is one or more of formaldehyde, paraformaldehyde, polyoxymethylene, acetaldehyde, valeraldehyde, benzaldehyde, glutaraldehyde, salicylaldehyde, and furanaldehyde. The molar ratio of phenolic monomers to aldehyde monomers is 1:1-3.

[0016] The high-microporous carbon structure layer has a micropore ratio of 90-96%; the high-microporous carbon structure layer is obtained by coating the surface of the inner micro-mesoporous composite structure layer, performing high-temperature treatment, and, if necessary, performing activation treatment; the coating is to coat at least one of phenolic resin, asphalt, polyacrylonitrile, and epoxy resin on the surface of the micro-mesoporous composite carbon structure layer particles.

[0017] The electrode active material is loaded in the pore structure of the multi-level structure carbon material to form a composite material of active material and porous carbon. When the electrode active material is a silicon-containing substance, a silicon-carbon composite material is obtained. Specifically, a silicon-containing compound gas, such as at least one of monosilane, disilane, trichlorosilane, and silicon tetrachloride, is contacted with a multi-level structure carbon material under high temperature conditions, and the silicon-containing compound is thermally decomposed under high temperature conditions to form elemental silicon, and the elemental silicon is filled in the pore structure of the above-mentioned porous carbon material, wherein the mass proportion of silicon is 30-60wt%, preferably 40-50wt%; when the electrode active material is a sulfur-containing compound, a sulfur-carbon composite material is obtained, specifically, the sulfur-containing compound is solid-phase mixed with the above-mentioned porous carbon material, or a solution of sulfur-containing or compound is mixed with the porous carbon material, and a sulfur-carbon composite material is obtained after melt infiltration and adsorption, wherein the mass proportion of sulfur compound is 30-60wt%, preferably 40-50wt%.

[0018] Furthermore, the outermost dense carbon structure layer is a surface dense carbon structure layer constructed on the composite material of the above-mentioned load battery active material to prevent the active material from contacting with the electrolyte or the active material from being dissolved; the construction of the dense carbon structure layer is completed by liquid phase coating or gas phase coating, and the raw materials for liquid phase coating are reagents such as asphalt, tar, and polycyclic aromatic hydrocarbons; gas phase coating is the contact of the carbon source gas with the composite material of the load battery active material under high temperature conditions, and the carbon source gas is selected from at least one of acetylene, ethylene, propylene, benzene, toluene, ethylbenzene, styrene, xylene, and trimethylbenzene.

[0019] Furthermore, the present invention provides a method for preparing the composite multi-level structure carbon material, comprising the following steps:

[0020] (S1) a phenolic monomer and an aldehyde monomer react in a solvent to form a gel, the gel is crushed and then dried to obtain a phenolic resin precursor; then pyrolysis carbonization is performed under a high temperature inert atmosphere, activation treatment is performed if necessary, and the micro-mesopore ratio is adjusted to obtain a carbon material having a micro-mesoporous composite pore structure, and after airflow crushing, a micro-mesoporous composite carbon structure is obtained as an inner layer;

[0021] (S2) coating the surface of the micro-mesoporous composite inner layer material as the inner layer with a polymer resin and performing a high temperature treatment, and performing an activation treatment if necessary, so that the coating layer forms a high microporous carbon structure layer as the middle layer, thereby obtaining a double-layer porous carbon structure;

[0022] (S3) loading the battery active material into the pore structure of the double-layer porous carbon structure obtained in step (S2) to obtain a loaded double-layer porous carbon;

[0023] (S4) Carbon coating is performed on the surface of the supported double-layer porous carbon to obtain a composite multi-level structure carbon material.

[0024] After air flow pulverization in step (S1), and / or high temperature treatment in step (S2), the carbon material is activated. Activation can increase the pore volume and micropore ratio. When the pore volume is insufficient or the micropore ratio needs to be increased, activation can be selected to load more battery active materials to improve the electrochemical performance of the material. The activation treatment is to use an activation reagent such as carbon dioxide, water vapor, or potassium hydroxide to activate the above-mentioned carbon material under an inert atmosphere at high temperature to mainly form a microporous structure with a pore size of less than 2nm. The high temperature treatment is 800-1000°C for 1-5h, such as 2-3.5h.

[0025] Furthermore, the phenolic monomer in step (S1) is selected from at least one of phenol, o-cresol, p-chloro-meta-cresol, catechol, resorcinol, 2,6-dichlorophenol, pyrogallol, nitrophenol, and biphenol; the aldehyde monomer is selected from at least one of formaldehyde, paraformaldehyde, polyoxymethylene, acetaldehyde, valeraldehyde, benzaldehyde, glutaraldehyde, salicylaldehyde, and furfural. The temperature for forming the gel is 30-125° C. The gel can be dried by freeze drying, heating drying, vacuum heating drying, etc. The specific surface area of ​​the resin obtained after drying is 100-300 m 2 / g; wherein the pyrolysis carbonization temperature is 500-900°C, and the median particle size of the crushed particles is 3-10μm.

[0026] Furthermore, in step (S2), the coating treatment of the polymer resin is to coat the polymer resin solution or liquid polymer resin on the surface of the particles of the inner layer micro-mesoporous composite carbon structure layer, and the polymer resin is selected from at least one of phenolic resin, asphalt, polyacrylonitrile, and epoxy resin; the coating amount is 10-30wt% of the mass of the micro-mesoporous composite carbon structure; the temperature range of the high temperature treatment is 600-750°C, and the high temperature treatment time is 2-5h.

[0027] Further, in step (S3): when the battery active material is a silicon-containing substance, a silicon-containing compound such as at least one of monosilane, disilane, trichlorosilane and silicon tetrachloride is contacted with the porous carbon material obtained in step (S2) under closed conditions at 500-800°C to perform a high-temperature thermal decomposition reaction, and the silicon element formed by the reaction is filled into the inner micropores and the middle micropores of the porous carbon material to obtain a silicon-carbon composite material, and the mass proportion of silicon in the formed silicon-carbon composite material is controlled by controlling the flow rate and introduction time of the silicon-containing compound;

[0028] When the battery active material is a sulfur-containing substance, the sulfur-containing compound is solid-phase mixed with the porous carbon material obtained in step (S2) at 150-200°C in an inert atmosphere or vacuum conditions, or a solution of the sulfur-containing compound is mixed with the porous carbon material obtained in step (S2), and then dried after adsorption and penetration to obtain a sulfur-carbon composite material.

[0029] Further, in step (S4), the carbon coating is carried out by liquid coating or gas coating, the raw material of the liquid coating is asphalt, tar, polycyclic aromatic hydrocarbons, etc., the raw material of the liquid coating is evenly coated on the surface of the loaded double-layer porous carbon and heat treated; the raw material of the gas coating is acetylene, ethylene, propylene, benzene, toluene, ethylbenzene, styrene, xylene, trimethylbenzene, one or a combination of two or more, and the temperature of the gas coating is 600-900°C. The dense carbon layer formed on the porous carbon surface after carbon coating can prevent the active material from directly contacting the electrolyte or avoid the dissolution of the active material, thereby helping to further improve the electrochemical performance of the material.

[0030] Another object of the present invention is to provide an application of the composite multi-level structure carbon material as an electrode material in a lithium battery, especially as a high-capacity silicon-based negative electrode material, which can improve the cycle stability and rate performance of existing silicon-based lithium-ion batteries; and as a high-capacity sulfur-based positive electrode material, which can significantly improve the cycle stability and self-discharge performance of lithium-sulfur batteries.

[0031] Compared with the prior art, the present invention constructs a multi-level structure carbon material and fills the high-capacity active material of the lithium-ion battery inside the multi-level structure carbon material. The multi-level structure carbon material provides a loading space for the high-capacity active material on the one hand, and buffers its volume expansion to the maximum extent on the other hand. In addition, the dense carbon structure layer on the surface can avoid direct contact between the active material and the electrolyte, reduce the occurrence of side reactions and avoid the dissolution of the active material, thereby effectively improving the cycle stability, rate performance and storage performance of the high-capacity electrode material, and can meet the demand for high-energy density lithium-ion battery materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 Schematic diagram of the composite multi-level structure carbon material of the present invention;

[0033] Figure 2 is a photograph of the phenolic resin gel block after drying in step (S1) of Example 1;

[0034] Figure 3 is the pore size distribution curve of the inner micro-mesoporous carbon structure layer of the hierarchical structure carbon material obtained in Example 1;

[0035] Figure 4 This is the pore size distribution curve of the high microporous carbon structure layer in the middle layer of the hierarchical structure carbon material obtained in Example 1. DETAILED DESCRIPTION

[0036] The present invention will be further described below in conjunction with specific embodiments, but the present invention is not limited to the following embodiments.

[0037] The experimental methods described in the following examples are conventional methods unless otherwise specified; the reagents and materials described are commercially available unless otherwise specified.

[0038] Example 1

[0039] (S1) 1500 g of phenol and 500 g of resorcinol are added to a 10 L polypropylene plastic reaction container, and then 2700 g of water is added, and stirred at room temperature for 1 hour until completely dissolved; then 2650 g of 37% formaldehyde aqueous solution is added to the above reaction container, and stirred at room temperature for 10 minutes to mix evenly; then 300 g of glacial acetic acid solution is dropped, and stirred at room temperature for 10 minutes to mix evenly; the above mixed system is placed in a 90° C. water bath for reaction, and heating is terminated after the reaction for 24 hours to obtain a solidified phenolic resin, which is coarsely broken and placed in a freeze dryer for freeze drying for 48 hours to obtain a dried phenolic resin gel as a precursor, and its appearance is as follows Figure 2 The dried phenolic resin gel block was added into a rotary kiln and heated to 800°C in a nitrogen atmosphere for 4 hours to obtain a micro-mesoporous carbon material, which was then crushed by a jet mill to obtain a micro-mesoporous carbon powder with a median particle size of 8 μm.

[0040] (S2) 800 g of the micro-mesoporous powder obtained in step (S1) is dispersed in 1 L of water, and then 200 g of liquid phenolic resin is added, stirred and mixed evenly at room temperature, and then dried at 60°C for 10 h, then dried at 90°C for 5 h, then dried at 120°C for 5 h, and dried at 150°C for 2 h to obtain a micro-mesoporous carbon material with a surface coated with resin after drying, and then high-temperature treated at 700°C in an inert atmosphere for 1 h, and then crushed and dispersed with an air flow disperser to obtain a double-layer porous carbon with a high microporous carbon layer coated on the surface; the median particle size is 11.5 μm.

[0041] (S3) 500 g of the core-shell porous carbon obtained in step (S2) is placed in a rotary kiln, and the temperature is raised to 800° C. under a nitrogen atmosphere, and then acetylene gas is introduced to carry out surface chemical vapor deposition coating of the carbon material, so that a dense carbon structure layer is formed on the surface, and finally a multi-level structure carbon material with a three-layer structure is obtained.

[0042] Example 2

[0043] Other conditions and operations are the same as those in Example 1, except that in step (S3), low-temperature asphalt is used to coat the core-shell porous carbon having a high microporous carbon layer on the surface obtained in step (S2). The specific operations are as follows:

[0044] (S3) Take 500 g of the core-shell structured porous carbon obtained in step (S2), then dissolve 100 g of low-temperature asphalt with a softening point of 60°C in 500 g of tetrahydrofuran solvent, mix and stir the above system for 2 hours, and then dry at 100°C to obtain porous carbon with surface coated with asphalt, and then place the above material in a rotary kiln, heat it to 900°C under a nitrogen atmosphere for high-temperature treatment for 1 hour to carbonize the surface-coated asphalt to form a dense carbon structure layer, thereby obtaining a multi-level structural carbon material with a three-layer structure.

[0045] Example 3

[0046] The other conditions and operations are the same as those in Example 1, except that in step (S2), an activation step is added, that is, after obtaining the core-shell structured porous carbon with a high microporous carbon layer coated on the surface, it is placed in an atmosphere rotary furnace, heated to 950°C under a nitrogen atmosphere, and then switched to introduce carbon dioxide gas for reaction for 3 hours, so that the surface coating layer is further activated to form pores.

[0047] Example 4

[0048] Other conditions and operations are the same as those in Example 1, except that in step (S1), the addition amounts of phenol and resorcinol are 1000 g and 333 g, respectively. The proportion of phenol in the phenolic ratio is reduced, and the overall solid content is reduced, resulting in the formation of more mesoporous structures, a reduced micropore ratio, and a reduced specific surface area.

[0049] Example 5

[0050] Other conditions and operations are the same as those in Example 1, except that in step (S1), the added amounts of phenol and resorcinol are 2500 g and 833 g, respectively. As the proportion of phenol in the phenolic ratio increases, the solid content increases, the proportion of micropores increases, and the specific surface area increases.

[0051] Example 6

[0052] Other conditions and operations are the same as those in Example 1, except that an activation step is added to step (S1), namely:

[0053] 1500 g of phenol and 500 g of resorcinol were added to a 10 L polypropylene plastic reaction container, and then 2700 g of water was added, and stirred at room temperature for 1 hour until completely dissolved; then 2650 g of 37% formaldehyde aqueous solution was added to the above reaction container, and stirred at room temperature for 10 minutes to mix evenly; then 300 g of glacial acetic acid solution was dropped, and stirred at room temperature for 10 minutes to mix evenly; the above mixed system was placed in a 90°C water bath for reaction, and the heating was terminated after the reaction for 24 hours to obtain a solidified phenolic resin, which was coarsely broken and placed in a freeze dryer for freeze drying for 48 hours to obtain a dried phenolic resin gel as a precursor, and its appearance is as follows: Figure 2 As shown. The dried phenolic resin gel block was added to a rotary kiln, heated to 800°C in a nitrogen atmosphere for pyrolysis and carbonization for 4 hours to obtain a micro-mesoporous carbon material, and the above material was crushed by a jet mill to obtain a micro-mesoporous carbon powder with a median particle size of 8 μm. The crushed micro-mesoporous carbon powder was mixed with potassium hydroxide at a mass ratio of 1:0.8, heated to 750°C in a nitrogen atmosphere for 3 hours, and after the reaction was completed and cooled, the product was washed with water and 2 mol / L hydrochloric acid aqueous solution, and finally washed with water three times, and dried under vacuum at 120°C to obtain further activated micro-mesoporous carbon powder.

[0054] Example 7

[0055] The other conditions and operations are the same as those in Example 6, except that an activation step is added to step (S2), that is, after obtaining the core-shell structured porous carbon with a high microporous carbon layer coated on the surface, it is placed in an atmosphere rotary furnace, heated to 950°C under a nitrogen atmosphere, and then switched to introduce carbon dioxide gas for reaction for 3 hours, so that the surface coating layer is further activated to form pores.

[0056] Comparative Example 1

[0057] The same as step (S1) of Example 1, that is, micro-mesoporous carbon powder with a median particle size of 8 μm is obtained, which is equivalent to the inner layer material of the multi-level structure carbon material with a three-layer structure.

[0058] Comparative Example 2

[0059] 1) Take 5 kg of liquid phenolic resin, let it stand for reaction at 90°C for 24 hours, crush the obtained phenolic resin solid and place it in a rotary kiln, heat it to 800°C in a nitrogen atmosphere and react for 4 hours to obtain carbonized phenolic resin;

[0060] 2) The carbonized phenolic resin powder was mixed with potassium hydroxide in a mass ratio of 1:2.85, and heated to 750°C for 3 hours under a nitrogen atmosphere. After cooling after the reaction, the product was washed with water and a 2 mol / L hydrochloric acid aqueous solution, respectively, and finally washed with water three times. The product was dried under vacuum conditions at 120°C to obtain an activated porous carbon material, which is a high microporous material, equivalent to the middle layer material of the three-layer multi-level structure carbon material.

[0061] The materials of the above-mentioned embodiments and comparative examples were subjected to BET and micropore ratio tests, and the results are shown in Table 1 below.

[0062] Table 1 Comparison of porous carbon pore parameters

[0063] .

[0064] Embodiment 8-14

[0065] Based on the above embodiments 1-7, after step (S2) and before the carbon coating in step (S3), a step of increasing the load of battery active materials into the pore structure of the porous carbon is performed:

[0066] 1 kg of double-layer porous carbon was placed in a vertical fluidized bed reactor, and silane gas was introduced at 500°C to control the mass proportion of silicon in the final silicon-carbon material to 40.5%, thereby obtaining a silicon-carbon composite material.

[0067] Comparative Example 3

[0068] The silicon-carbon composite material was prepared using the micro-mesoporous carbon material of Comparative Example 1:

[0069] 1 kg of the micro-mesoporous carbon material obtained in Comparative Example 1 was placed in a vertical fluidized bed reactor, and silane gas was introduced at 500° C. to control the mass proportion of silicon in the final silicon-carbon material to be 40.5%, thereby obtaining a silicon-carbon composite material.

[0070] The carbon coating step of the silicon-carbon composite material is the same as step (S3) of Example 1.

[0071] Comparative Example 4

[0072] The silicon-carbon composite material was prepared using the high microporous carbon material of Comparative Example 2:

[0073] 1 kg of the high microporous carbon material obtained in Comparative Example 2 was placed in a vertical fluidized bed reactor, and silane gas was introduced at 500° C. to control the mass proportion of silicon in the final silicon-carbon material to be 40.5%, thereby obtaining a silicon-carbon composite material.

[0074] The carbon coating step of the silicon-carbon composite material is the same as step (S3) of Example 1.

[0075] Embodiment 15

[0076] Other conditions and operations are the same as those in Example 7, except that after step (S2) and before the carbon coating in step (S3), a step of increasing the load of battery active materials into the pore structure of the porous carbon is performed:

[0077] 70 g of the double-layer porous carbon obtained in step (S2) and 30 g of sulfur powder were ball-milled and mixed for 3 h, and then the mixture was sealed and heated at 155° C. for 24 h to obtain a sulfur-carbon composite.

[0078] Example 16

[0079] Other conditions and operations are the same as those in Example 7, except that after step (S2) and before the carbon coating in step (S3), a step of increasing the load of battery active materials into the pore structure of the porous carbon is performed:

[0080] Take 20g of lithium sulfide and dissolve it in 100g of water, then add 50g of the double-layer porous carbon obtained in step (S2), stir at room temperature for 24h, filter and dry to obtain a sulfur-carbon composite material adsorbed with lithium sulfide.

[0081] Comparative Example 5

[0082] The sulfur-carbon composite material was prepared using the micro-mesoporous carbon material of Comparative Example 1:

[0083] Preparation of sulfur-carbon composite material: 20 g of lithium sulfide was dissolved in 100 g of water, and then 50 g of the porous carbon material obtained in Comparative Example 1 was added, stirred at room temperature for 24 h, and then filtered to obtain the sulfur-carbon composite material adsorbed with lithium sulfide.

[0084] The carbon coating step is the same as step (S3) of Example 1.

[0085] Comparative Example 6

[0086] The sulfur-carbon composite material was prepared using the high microporous carbon material of Comparative Example 2:

[0087] Step 1: Preparation of sulfur-carbon composite material: Take 20g of lithium sulfide and dissolve it in 100g of water, then add 50g of the porous carbon material obtained in Comparative Example 2, stir at room temperature for 24h, and then filter to obtain the sulfur-carbon composite material adsorbed with lithium sulfide.

[0088] The carbon coating step is the same as step (S3) of Example 1.

[0089] Test Case

[0090] The porous carbon material prepared by the present invention is tested for its specific surface area, pore size distribution and other pore parameters by nitrogen adsorption and desorption instrument. The electrochemical performance of the obtained silicon-carbon composite material is tested by the following method: the silicon-carbon composite material, conductive carbon and binder are mixed in a mass ratio of 8:1:1 to form a slurry, the slurry is evenly coated on the copper foil with a scraper, and after drying, it is placed in a vacuum oven at 80°C for 24 hours to obtain a silicon-carbon negative electrode plate, and cut into plates with a diameter of 1 cm for battery assembly; a half-cell test is performed using a lithium metal sheet as the counter electrode, a polyolefin diaphragm, 1 mol / L LiPF6 (the solvent is a mixture of ethylene carbonate and dimethyl carbonate in a volume ratio of 1:1) as the electrolyte, 2% VC and 5% FEC are added to the electrolyte by volume, and a 2032 button battery is assembled in an argon atmosphere; a LAND charge and discharge tester is used to perform charge and discharge tests on the above-assembled battery, the charge and discharge range is 50 mV ~ 1.5 V, and the charge and discharge range is 0.1 The reversible capacity test and the first coulombic efficiency were tested at 2C rate, and the capacity development (the specific capacity at 2C rate is the percentage of the specific capacity at 0.1C) and cycle stability test (capacity retention rate after 300 cycles at 2C rate) were performed at 2C rate. For the sulfur-carbon composite material, the pole piece coating and battery assembly were carried out in the same manner as above. The electrolyte used 1.0 mol / L LiTFSI (the solvent was DOL and DME in a volume ratio of 1:1, and 1% lithium nitrate was added) as the lithium-sulfur battery electrolyte. The self-discharge performance of the lithium-sulfur battery was obtained by standing for 7, 15, and 30 days after the sulfur positive electrode was inserted with lithium (discharge), and then the capacity was de-lithiated (charged) and the ratio of its capacity to the initial capacity was obtained. The results of the electrochemical performance test are shown in Table 2.

[0091] Table 2 Comparison of electrochemical performance of Examples 8-14 and Comparative Examples 3 and 4

[0092] .

[0093] Table 3 Comparison of electrochemical performance of Examples 15, 16 and Comparative Examples 5, 6

[0094] .

[0095] In summary, the present invention constructs a carbon material with a multi-level porous structure, and loads a high-capacity active material for a lithium-ion battery inside the multi-level porous structure carbon material. From the comparison of the pore parameters of Examples 1-7, it can be seen that a carbon material with a multi-level porous structure is successfully prepared, the inner layer is a micro-mesoporous structure layer, the middle layer is a high microporous structure layer, and the outermost layer is a dense carbon structure layer. From Examples 8-16, it can be seen that the silicon-carbon composite material or sulfur-carbon composite material loaded with the multi-level porous structure of the present invention is significantly better than the comparative examples 3-6 with a single structure in terms of first coulomb efficiency, rate performance, cycle stability and storage performance, indicating that the multi-level structure carbon material design of the present invention can effectively improve the electrochemical performance of high-capacity active materials for lithium-ion batteries, thereby meeting the material requirements of high-energy-density lithium-ion batteries.

Claims

1. A method for preparing a composite multi-level structure carbon material, characterized in that: The following steps are involved: (S1) a phenolic monomer and an aldehyde monomer react in a solvent to form a gel, which is crushed and dried to obtain a phenolic resin precursor; then, the phenolic resin is pyrolyzed and carbonized under a high temperature inert atmosphere, and after air flow crushing, a micro-mesoporous composite carbon structure is obtained as an inner layer; (S2) coating the surface of the micro-mesoporous composite inner layer material as the inner layer with a polymer resin and performing a high temperature treatment, wherein the coating layer forms a high microporous carbon structure layer as the middle layer, thereby obtaining a double-layer porous carbon structure; the polymer resin coating treatment is to coat the surface of the particles of the inner micro-mesoporous composite carbon structure layer with a liquid polymer resin, wherein the liquid polymer resin is a liquid phenolic resin; the coating amount is 10-30wt% of the mass of the micro-mesoporous composite carbon structure; and the temperature range of the high temperature treatment is 700-750°C; (S3) loading the battery active material into the pore structure of the double-layer porous carbon structure obtained in step (S2) to obtain a loaded double-layer porous carbon; (S4) performing carbon coating on the surface of the supported double-layer porous carbon to obtain a composite multi-level structure carbon material; The composite multi-level structure carbon material is a three-layer composite structure, the outer layer is a dense carbon structure layer, the middle layer is a high microporous carbon structure layer, and the inner layer is a micro-mesoporous composite carbon structure layer. The electrode active material is loaded in the pore structure of the middle layer and the inner layer. The electrode active material is a silicon-containing substance or a sulfur-containing substance; the micro-mesoporous composite carbon structure layer contains both micropores and mesopores, and the micropores account for 40-70%, and the pore volume is 0.6-1.0cm 3 / g, with a specific surface area of ​​1000-2500 m 2 / g; the high microporous carbon structure layer has a micropore ratio of 90-96%; the particle size of the inner layer is 3-10μm, the thickness of the middle layer is 1-3μm, and the thickness of the outer layer is 3-10nm.

2. The preparation method according to claim 1, characterized in that: The silicon-containing substance is selected from at least one of silicon and amorphous silicon-carbon compounds; the sulfur-containing substance is at least one of lithium sulfide, sulfur, lithium polysulfide, molybdenum sulfide, iron sulfide or titanium sulfide; the electrode active material content is 30-60wt%.

3. The preparation method according to claim 1, characterized in that: In step (S2), the high temperature treatment is followed by an activation treatment; the activation treatment is to perform thermal activation treatment on the carbon material under heating conditions using carbon dioxide, water vapor, or potassium hydroxide.

4. The preparation method according to claim 1, characterized in that: When the electrode active material is a silicon-containing substance, a silicon-carbon composite material is obtained; when the electrode active material is a sulfur-containing compound, the sulfur-containing compound is solid-phase mixed with the above-mentioned porous carbon material, or a solution containing sulfur or the compound is mixed with the porous carbon material, and a sulfur-carbon composite material is obtained after melt infiltration and adsorption, in which the mass proportion of the sulfur compound is 30-60%.

5. The preparation method according to claim 4, characterized in that: The silicon-carbon composite material is prepared by contacting a silicon-containing compound gas with a multi-level structured carbon material under high temperature conditions. The silicon-containing compound undergoes thermal decomposition under high temperature conditions to form elemental silicon, which is filled in the pore structure of the porous carbon material, wherein the mass proportion of silicon is 30-60%.

6. The preparation method according to claim 5, characterized in that: The silicon-containing compound gas is selected from at least one of monosilane, disilane, trichlorosilane and silicon tetrachloride.

7. The preparation method according to claim 1, characterized in that: After air flow pulverization in step (S1), the carbon material is activated to increase the pore volume and micropore ratio; the activation treatment is to perform thermal activation treatment on the carbon material under heating conditions using carbon dioxide, water vapor, or potassium hydroxide.

8. The preparation method according to claim 7, characterized in that: The phenolic monomer in step (S1) is selected from at least one of phenol, o-cresol, p-chloro-meta-cresol, catechol, resorcinol, 2,6-dichlorophenol, pyrogallol, nitrophenol, and biphenol; the aldehyde monomer is selected from at least one of formaldehyde, paraformaldehyde, polyoxymethylene, acetaldehyde, valeraldehyde, benzaldehyde, glutaraldehyde, salicylaldehyde, and furfural; the temperature for forming the gel is 30-125° C.; the pyrolysis carbonization temperature is 500-900° C., and the median particle size of the particles after crushing is 3-10 μm; and / or In step (S3): when the battery active material is a silicon-containing substance, the silicon-containing compound is brought into contact with the porous carbon material obtained in step (S2) under closed conditions at 500-800°C to perform a high-temperature thermal decomposition reaction to obtain a silicon-carbon composite material; when the battery active material is a sulfur-containing substance, the sulfur-containing compound and the porous carbon material obtained in step (S2) are solid-phase mixed at 150-200°C in an inert atmosphere or vacuum conditions, or a solution of the sulfur-containing compound is mixed with the porous carbon material obtained in step (S2), and the mixture is dried after adsorption and penetration to obtain a sulfur-carbon composite material; and / or In step (S4), carbon coating is carried out by liquid phase coating or gas phase coating. The raw material for liquid phase coating is asphalt, tar, polycyclic aromatic hydrocarbons. The liquid phase coated raw material is uniformly coated on the surface of the loaded double-layer porous carbon and heat treated. The raw material for gas phase coating is acetylene, ethylene, propylene, benzene, toluene, ethylbenzene, styrene, xylene, trimethylbenzene, one or a combination of two or more. The temperature of gas phase coating is 600-900°C.

9. Use of the composite multi-level structure carbon material obtained by the preparation method according to any one of claims 1 to 8 as an electrode material in a lithium battery.

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