Silicon-carbon negative electrode material and preparation method and application thereof

Abstract

The invention belongs to the technical field of lithium ion battery materials, the silicon-carbon negative electrode material has a core-shell structure and comprises an inner core, a middle layer and a shell, and a gap is reserved between the inner core and the middle layer; the core is made of a silicon source, the middle layer is made of silicon carbide, and the shell is made of a carbon source; a binder layer is also arranged between the middle layer and the shell; and the binder layer is made of a modified binder. According to the invention, a silicon core-in-shell gap-silicon carbide intermediate layer-carbon shell double-shell stable structure is designed and constructed, and a flexible adhesive layer with excellent conductivity is additionally arranged between the silicon carbide intermediate layer and the carbon shell, so that a buffer space is reserved for volume expansion of the silicon core by the in-shell gap layer; and the outer-layer double-shell composite structure can compensate the outward stress during volume expansion of the silicon core, so that the structural stability is ensured, a lithium intercalation / deintercalation channel can be provided, the conductivity is further improved, and the cycle performance and the rate capability of the silicon-carbon material are improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion battery materials, and in particular to a silicon-carbon negative electrode material and a preparation method and application thereof. BACKGROUND

[0002] The negative electrode material of the traditional lithium ion battery is mainly graphite material, and the theoretical specific capacity is 372 mAh / g. At present, the actual specific energy density gradually approaches the theoretical limit, but it is still difficult to meet the market demand. Silicon is widely considered as the most promising negative electrode material of high-energy-density lithium ion batteries due to its high specific capacity (>3500 mAh / g) and abundant reserves.

[0003] However, silicon is prone to volume expansion (>300%) during lithiation / delithiation, which causes serious problems such as pulverization caused by repeated fracture of silicon, unstable solid electrolyte interface (SEI), and deformation of electrode structure, which can lead to rapid capacity decay and cycle performance degradation of the battery. In addition, electrode expansion can also squeeze the electrolyte filled in the pores of the separator, thereby hindering the transmission of lithium ions. Severe electrode expansion can cause the battery package to bulge, causing safety hazards.

[0004] Due to the structural stability of carbon materials, the volume change during charging and discharging is relatively small. Silicon materials and carbon materials are usually compounded to prepare silicon-carbon negative electrode materials, which utilize the high stability of carbon materials to coat silicon materials or embed silicon materials in carbon materials, so as to suppress the volume change of silicon materials and improve the stability of silicon-carbon negative electrode materials. The existing preparation methods of silicon-carbon negative electrode materials mainly include chemical vapor deposition and mechanical ball milling. However, the silicon-carbon negative electrode materials prepared by the above methods still have problems such as agglomeration, low compaction density, and poor structural stability, which leads to poor cycle stability and low coulombic efficiency in lithium ion batteries.

[0005] In view of the above related technologies, the applicant believes that it is very crucial to develop a silicon-carbon negative electrode material that can effectively reduce volume expansion, ensure the cycle stability of the battery, and at the same time has high initial charging efficiency and excellent cycle performance. SUMMARY

[0006] In order to effectively reduce the volume expansion problem of silicon-carbon negative electrode materials during charging and discharging, so as to ensure good cycle stability and high initial charging efficiency of the battery, the present application provides a silicon-carbon negative electrode material and a preparation method and application thereof.

[0007] In a first aspect, the present application provides a silicon-carbon negative electrode material, which adopts the following technical solution: A silicon-carbon negative electrode material has a core-shell structure, including an inner core, an intermediate layer, and an outer shell, the intermediate layer is coated on the inner core, the outer shell is coated on the intermediate layer, and a gap is left between the inner core and the intermediate layer; The constituting material of the inner core includes a silicon source, the constituting material of the intermediate layer includes silicon carbide, and the constituting material of the outer shell includes a carbon source; a binder layer is further arranged between the intermediate layer and the outer shell, and the constituting material of the binder layer includes a modified binder.

[0008] By adopting the above technical solution, a double-shell stable structure of silicon core-shell inner gap-silicon carbide intermediate layer-carbon shell is constructed, and a flexible binder layer is additionally arranged between the silicon carbide intermediate layer and the carbon shell. This structure design not only realizes that the inner gap layer has a buffer space for the volume expansion of the silicon core, but also compensates the stress outwardly when the silicon core expands, and solves the defects of low conductivity and low tap density and insufficient specific capacity of the existing yolk-shell structure silicon-carbon composite material due to the inner gap of the shell by additionally arranging the modified binder layer with excellent electrical conductivity.

[0009] By arranging a firm silicon carbide intermediate shell layer between the silicon core and the carbon shell, on the one hand, the silicon carbide intermediate shell layer has high Mohs hardness and excellent mechanical strength, and can effectively constrain the expansion direction of the silicon core. From the inside of the overall structure, the gap (buffer cavity) between the silicon carbide intermediate layer and the inner core has a synergistic effect, and can significantly reduce the risk of structure rupture by gradiently absorbing stress, thereby being beneficial to the formation of a more stable SEI film of the silicon-carbon material in the electrochemical reaction process, reducing the loss of active lithium in the battery, and improving the cycle performance and rate performance of the silicon-carbon material. On the other hand, the silicon carbide has semiconductor properties, and from the outside of the overall structure, the silicon carbide intermediate layer can have a synergistic effect with the modified binder layer to jointly form a continuous conductive network, construct a conductive gradient transition from the silicon core to the carbon shell, reduce the interface charge transfer impedance, thereby improving the conduction efficiency of electrons and further improving the first coulomb efficiency of the battery.

[0010] Preferably, the modified binder includes a conductive component and a binding base material, the conductive component is selected from any one or more of carbon fibers (VGCF), carbon nanotubes (CNT), and reduced graphene oxide (rGO); The binding base material is selected from any one or more of sodium alginate, carboxymethyl cellulose, polyvinylidene fluoride, and polyacrylic acid; And the mass ratio of the conductive component to the binding base material is (0.55-0.65):1.

[0011] By adopting the above technical scheme, different dimensional conductive materials are introduced into the bonding base material, and a modified binder layer with bonding performance and conductive efficiency is prepared by a simple and efficient blending method. Specifically, the conductive component as a connector can provide more connection areas (such as VGCF can be used as a conductive skeleton node to form a basic support frame in the binder layer; CNT can bridge isolated conductive islands, cross the gap between particles, and repair the conductive path broken due to volume expansion; rG0 can not only form strong adsorption with the bonding base material through π-π bond, but also can be chemically bonded to the surface of the silicon carbide interlayer through the oxygen-containing groups (-COOH / -OH) on the edge of the sheet layer to strengthen the interfacial bonding), and the good electrical and thermal properties, high surface area and flexibility of the material itself promote the conductive material to easily combine with the negative active material to form a stable three-dimensional conductive material under the condition of uniform distribution in the bonding base material, thereby improving the cycle stability of the silicon-carbon core-shell material as the battery negative electrode and making up for the low conductivity caused by the large amount of silicon component in the structure.

[0012] The selection of different bonding base materials and the use of different bonding base materials together will affect the electrochemical performance (and different lithium ion diffusion efficiency) of the composite negative electrode material. The above bonding base materials have good bonding strength and ductility, which can adapt to greater volume change to relieve the expansion effect caused by the internal structure. At the same time, the above bonding base material and the conductive component also have good compatibility, which is obviously better than other binder systems, such as sodium alginate or its complex system. The carboxyl group in it can esterify with the hydroxyl group on the surface of the silicon particles to form a hinge structure, enhance the bonding strength, reduce the shedding of active material particles, avoid the formation of a new electrode interface, form a new SEI, thereby reducing the loss of lithium and improving the cycle performance of the battery.

[0013] At the same time, by controlling the mass ratio of the conductive component and the bonding base material within a certain range (0.55-0.65):1, the functions of the two can be balanced and synergized. When the conductive component exceeds the appropriate threshold, the conductive components are more likely to agglomerate and lose part of the conductive activity, and the bonding effect and layer structure forming effect of the bonding base material are also adversely affected. When the proportion of the bonding base material exceeds the appropriate threshold, the electrical conductivity of the silicon-carbon composite material will be greatly reduced, thereby being difficult to meet the first coulombic efficiency requirement of the battery.

[0014] Preferably, the modified binder further comprises a dispersant selected from any one of polyvinylpyrrolidone and polyacrylamide; And the mass ratio of the conductive component, the bonding base material and the dispersant is (0.55-0.65):1:(0.18-0.24).

[0015] By adopting the above technical scheme, the dispersant is introduced into the modified binder system, which can effectively alleviate the agglomeration of the conductive component due to its high specific surface area and strong van der Waals force. Specifically, polyvinylpyrrolidone and polyacrylamide are selected as high molecular dispersants with water solubility, which can provide strong steric hindrance effect through multi-point adsorption of the high molecular chain on the surface of the conductive particle / sheet layer, so as to effectively disperse the conductive component and prevent its agglomeration and activity reduction. By controlling the mass ratio of the conductive component, the bonding base material and the dispersant within (0.55-0.65): 1:(0.18-0.24), the rheological properties of the binder basic slurry can be ensured, and the compatibility is more excellent.

[0016] Preferably, the conductivity of the binder layer is 10-120 S / cm, preferably 30-80 S / cm. The thickness of the binder layer is 30-60 nm, preferably 30-45 nm.

[0017] By adopting the above technical scheme, by adjusting the types and component proportions of the conductive component, the bonding base material and the dispersant in the binder layer, the conductivity of the binder layer is controlled within the range of 10-120 S / cm, so as to break through the electronic percolation threshold in the binder layer and establish a continuous and stable conductive network. The suitable conductivity range can promote rapid charge transfer, reduce lithium ion insertion barrier and reduce side reactions. At the same time, the conductive component (such as CNT, rGO) can also maintain the minimum conductive requirement by bridging or covering when the silicon core expands and generates micro-cracks in the outer carbon shell. By controlling the thickness of the binder layer within the range of 30-60 nm, the structure parameters and conductive properties of the binder layer are influenced each other, and reasonable layer thickness can effectively reduce the diffusion distance of lithium ions, so as to form a gradient conductive interface and eliminate the interface charge transition barrier in the multi-layer structure.

[0018] Preferably, the silicon source is selected from any one or more of nano-silicon particles, nano-porous silicon particles and nano-silicon-oxygen particles. The diameter of the inner core is 300-700 nm, preferably 500-600 nm.

[0019] By adopting the technical scheme, the silicon source is nano-sized, and any one or more of nano-silicon particles, nano-porous silicon particles and nano-silicon oxygen particles is selected, so as to reduce the material pulverization failure problem caused by the expansion of the silicon material in the electrochemical reaction, and a secondary coating treatment (i.e. a silicon carbide intermediate layer and an outer carbon shell) is performed on the outer layer of the silicon core, the coating of the silicon material can isolate the direct contact between the silicon material and the electrolyte and reduce the specific surface area of the whole material, the isolation of the contact between the silicon material and the electrolyte is beneficial to the formation of a more stable SEI film of the carbon-silicon composite material in the electrochemical reaction process, and the smaller specific surface area can reduce the secondary formation of the SEI film and reduce the loss of active lithium in the battery, thereby further improving the cycle stability of the silicon-based material. Meanwhile, the diameter of the silicon core is controlled in the range of 300-700 nm, which can effectively control the volume expansion stress of the inner core, so that the expansion direction is directionally absorbed by the intermediate layer gap (a reserved buffer cavity), and the diameter of the inner core can control the specific surface area in the range of 5-35 m 2 / g, which can not only avoid the lithium loss caused by the too large specific surface area, but also prevent the reaction activity from being reduced due to the too small specific surface area. The inner core diameter range is also matched with the thickness of the silicon carbide intermediate layer and the thickness of the outer carbon shell.

[0020] Preferably, the precursor of the silicon carbide is selected from dimethyldichlorosilane; and the thickness of the intermediate layer is 60-100 nm, preferably 60-80 nm. The precursor of the carbon source is selected from any one or more of sucrose, glucose, starch and phenolic resin; and the thickness of the outer shell is 100-200 nm, preferably 100-160 nm.

[0021] By adopting the technical scheme, on the one hand, the precursor of the silicon carbide is selected from dimethyldichlorosilane, which is pyrolyzed to generate high-purity β-SiC (the Mohs hardness and electrical conductivity are both improved), and the nanocrystalline structure is beneficial to stress dispersion. The thickness of the silicon carbide intermediate layer is controlled in the range of 60-100 nm, if the thickness is less than 60 nm, the intermediate layer is too thin to limit the impact of the volume expansion of the silicon core; if the thickness is greater than 100 nm, the intermediate layer is too thick to affect the lithium ion diffusion efficiency. The silicon carbide intermediate layer has good chemical compatibility with the inner core silicon and the outer shell carbon, and the thermal expansion coefficient of the silicon carbide intermediate layer is between the inner core silicon and the outer shell carbon, so as to be more consistent with the interlayer stress matching, thereby enhancing the interlayer interface stability. Meanwhile, the silicon carbide intermediate layer can also act as an ion sieve to regulate the diffusion rate of lithium ions to the inner core, avoid the stress concentration caused by the local too fast insertion into the silicon lattice, and reduce the risk of lithium metal precipitation on the surface of the silicon core.

[0022] In another aspect, the carbon source precursor is selected from any one or more of sucrose, glucose, starch and phenolic resin, all of which are low-cost carbon source materials. The outer carbon shell formed by the carbon source precursor can serve as a buffer to limit the volume expansion of the inner core, and can also serve as a conductive matrix to improve the electronic conductivity of the electrode, thereby enhancing the rate performance of the electrode. The thickness of the outer carbon shell is controlled in the range of 100-200 nm, which is based on the same balance between expansion limitation and conductivity efficiency as the thickness of the silicon carbide intermediate layer, and the diameter range of the inner core is matched with the thickness of the silicon carbide intermediate layer.

[0023] In a second aspect, the application provides a preparation method of a silicon-carbon negative electrode material, comprising the following steps: S1, silicon core preparation: a silicon source material, cetyltrimethylammonium bromide (CTAB) and polymethyl methacrylate (PMMA) are added to deionized water and mixed uniformly, ball-milled at a speed of 300-500 rpm for 3-4 h to obtain uniform dispersion, and then spray granulated to obtain silicon material microspheres, which are sintered at a temperature of 600-700℃ for 2-3 h to obtain a porous silicon core; S2, sacrificial layer coating: the porous silicon core in step S1 is added to a mixed liquid of anhydrous ethanol, ammonia water and deionized water, ultrasonically dispersed for 20-30 min, and then tetraethyl orthosilicate is added and mixed uniformly, and the mixture is stirred and mixed at a temperature of 40-45℃ for 4-5 h to obtain a silica-coated intermediate; S3, silicon carbide intermediate layer coating: the silica-coated intermediate in step S2 is placed in a CVD reactor, hydrogen gas and dimethyl dichlorosilane vapor are introduced at a flow rate of 200-300 sccm, and the temperature is raised to 900-1100℃ for 1.0-1.2 h to deposit a silicon carbide intermediate layer and coat the silica-coated intermediate, which is then immersed in a 10% HF solution and ultrasonically etched in an ice water bath for 1.5-2.0 h, and then centrifuged at a speed of 10,000-12,000 rpm, washed and dried to obtain a silicon-carbon core-shell intermediate; S4, preparation of modified binder layer: the conductive component is dissolved in NMP solvent, ultrasonically dispersed for 0.8-1.2 h, and then a binder base and a dispersant are added and the mixture is stirred and mixed at a temperature of 50-60℃ for 3-4 h to obtain a modified binder slurry, and the silicon-carbon core-shell intermediate in step S3 is mixed into the modified binder slurry, and the mixture is vacuum dried at a temperature of 100-120℃ to obtain a silicon-carbon core-shell preliminary sample; S5, carbon shell encapsulation: the silicon-carbon core-shell preliminary sample in step S4 is immersed in an ethanol solution of a carbon source precursor, and hexamethylenetetramine is added, vacuum impregnated for 30-40 min, and then subjected to stepwise temperature rise and solidification, and then carbonized at a temperature of 550-650℃ under argon protection for 1-2 h, and then graphitized at a temperature of 2600-2800℃ for 3-4 h to obtain a silicon-carbon negative electrode material.

[0024] By adopting the technical scheme, in the S1 silicon core preparation step, a small amount of CTAB is introduced into the nano-sized silicon source material as a dispersing agent and a small amount of PMMA is introduced as a sticky component, after blending, spray granulation and sintering treatment, a dense and uniform porous silicon core is prepared; in the S2 sacrificial layer coating step, tetraethyl orthosilicate is added dropwise into the porous silicon core mixed system to form a silicon dioxide layer to preliminarily wrap the silicon core, so as to serve as a basis for the intermediate layer gap and a substrate for the generation of the silicon carbide intermediate layer. At the same time, the thickness of the generated silicon dioxide layer can be controlled by the amount of tetraethyl orthosilicate added, thereby indirectly affecting the internal composite layer structure of the carbon-silicon core-shell material; in the S3 silicon carbide intermediate layer coating step, the silicon core intermediate wrapped with the sacrificial layer is subjected to CVD deposition under the condition of DMCS vapor and H2 mixed gas to preliminarily form a silicon carbide intermediate layer attached to the outer surface of the silicon dioxide layer, and then the silicon dioxide sacrificial layer is selectively etched by using an HF solution, thereby forming a buffer gap layer between the silicon core and the silicon carbide intermediate layer; in the S4 modified adhesive layer preparation step, a modified adhesive with adhesive and conductive properties is prepared by mixing in proportion, and then uniformly coated on the surface of the solid material and preliminarily solidified; in the S5 carbon shell packaging step, the initial sample is immersed in an ethanol solution of a carbon source precursor, and hexamethylenetetramine is added as a curing agent, and then sequentially subjected to stepwise solidification and high-temperature graphitization to firmly adhere the carbon shell to the surface of the initial sample, so as to form a stable carbon-silicon core-shell composite material.

[0025] Preferably, the volume ratio of hydrogen gas to dimethyldichlorosilane vapor in the step S3 is 1: (8-10). In the step S5, the stepwise temperature rising solidification is set as three-stage gradient temperature rising, the first stage is raised to 70-80℃ for preliminary solidification reaction for 1-2h, the second stage is raised to 110-120℃ for moderate solidification reaction for 1-2h, and the third stage is raised to 170-180℃ for severe solidification reaction for 40-60min.

[0026] By adopting the technical scheme, on the one hand, the volume ratio of hydrogen gas to dimethyldichlorosilane vapor in the step S3 is 1: (8-10), so that the cracking efficiency of the precursor is maximized, that is, hydrogen gas is sufficient to completely crack dimethyldichlorosilane, avoiding the generation of chlorine-containing by-products (such as SiCl4) and inhibiting the generation of free carbon. At the same time, the appropriate amount of hydrogen gas can also selectively etch amorphous silicon carbide, thereby promoting the preferential growth of the β-SiC crystal surface and further ensuring the coating quality of the silicon carbide intermediate layer; on the other hand, it can also cooperate with the subsequent HF etching process, so that the etching selectivity of HF is enhanced. Since HF solution is difficult to corrode high-purity β-SiC with a dense columnar crystal structure, it is ensured that the internal silicon dioxide sacrificial layer is completely removed while the loss of the silicon carbide intermediate layer is controlled within a suitable range. At the same time, it can also ensure the controllability of the pore structure of the silicon carbide intermediate layer, achieving the purpose of ensuring the lithium ion channel while avoiding excessive SEI generation.

[0027] A three-stage gradient temperature is set to control the gradient crosslinking of the carbon source precursor, so as to realize the ordered construction of the molecular structure in the outer carbon shell and the regulation of the pore structure on the surface of the carbon shell. For example, a first low-temperature stage is used to form pores with a large pore size to facilitate the expansion of the internal structure; a second medium-temperature stage is used to form mesopores with a moderate pore size to serve as the main lithium ion channel; and a third high-temperature stage is used to form closed micropores to reduce the penetration of electrolyte into the battery system.

[0028] Preferably, the method further comprises S6 post-processing, sieving the silicon-carbon negative electrode material particles, activating the particles at 750-850 DEG C for 40-60 min in a carbon dioxide atmosphere, and then modifying the surface of the silicon-carbon negative electrode material particles by acetylene cracking gas phase deposition to obtain the finished silicon-carbon negative electrode material.

[0029] By using the above technical solution, the silicon-carbon negative electrode material particles with incomplete structure are sieved to ensure the quality of the finished product, and the acetylene cracking gas phase deposition is used to modify the outer surface of the silicon-carbon negative electrode material particles with an ultrathin carbon layer again, so as to improve the interface stability of the silicon-carbon negative electrode material particles.

[0030] In a third aspect, the application provides the use of the above-mentioned silicon-carbon negative electrode material or the preparation method of the above-mentioned silicon-carbon negative electrode material in a lithium ion battery negative electrode material.

[0031] By using the above technical solution, a double-shell stable structure of silicon core-shell inner gap-silicon carbide intermediate layer-carbon shell is provided, and a flexible binder layer with excellent electrical conductivity is additionally arranged between the silicon carbide intermediate layer and the carbon shell. The inner gap layer of the double-shell composite structure can provide a buffer space for the volume expansion of the silicon core, and the outer double-shell composite structure can compensate for the outward stress when the silicon core expands, thereby ensuring the stability of the structure and providing a channel for lithium extraction / insertion, further improving the electrical conductivity, and improving the cycle performance and rate performance of the silicon-carbon core-shell composite material in the charge / discharge cycle of the lithium ion battery.

[0032] In summary, the application has the following beneficial effects: The present application coats a firm silicon carbide intermediate shell between the silicon core and the carbon shell. On the one hand, the silicon carbide intermediate shell has high Mohs hardness and excellent mechanical strength, which can effectively constrain the expansion direction of the silicon core. From the overall structure, the reserved gap (buffer cavity) between the silicon carbide intermediate layer and the inner core has a synergistic effect, which significantly reduces the risk of structure rupture by gradient stress absorption, thereby facilitating the formation of a more stable SEI film in the electrochemical reaction process of the silicon-carbon material, reducing the loss of active lithium in the battery, and improving the cycle performance and rate performance of the silicon-carbon material. On the other hand, silicon carbide has semiconductor properties. From the overall structure, the silicon carbide intermediate layer can have a synergistic effect with the modified binder layer to form a continuous conductive network, which builds a conductive gradient transition from the silicon core to the carbon shell, reduces the interface charge transfer impedance, thereby improving the conduction efficiency of electrons and further improving the first coulombic efficiency of the battery. 2. The present application selects different dimensional conductive materials to introduce into the binder base material, and prepares a modified binder layer with good adhesion and conductivity efficiency by a simple and efficient blending method. Specifically, the conductive component as a connector can provide more connection areas (such as VGCF which can be used as a conductive skeleton node to form a basic support frame in the binder layer; CNT which can bridge isolated conductive islands, cross the gap between particles, and repair the conductive path broken by volume expansion; rG0 which can not only form strong adsorption with the binder base material through π-π bond, but also can be chemically bonded to the surface of the silicon carbide intermediate layer through the oxygen-containing groups (-COOH / -OH) on the edge of the sheet, to strengthen the interfacial bonding between the layers), and the good electrical and thermal properties, high surface area and flexibility of the material itself, which facilitates the conductive material to easily combine with the negative active material to form a stable three-dimensional conductive material under the condition of uniform distribution in the binder base material, thereby improving the cycle stability of the silicon-carbon core-shell material as a battery negative electrode, and making up for the low conductivity caused by the high silicon content in this structure.

[0033] Different binder base materials and different combinations of different binder base materials will affect the electrochemical performance (and different lithium ion diffusion efficiency) of the composite negative electrode material. The above binder base materials have good adhesion and ductility, which can adapt to larger volume changes to relieve the expansion effect caused by the internal structure. At the same time, the above binder base materials also have good compatibility with the conductive component, which is significantly better than other binder systems, such as sodium alginate or its complex system. The carboxyl group in the above binder systems can react with the hydroxyl group on the surface of the silicon particles to form a hinge structure, enhance the adhesion strength, reduce the shedding of active material particles, avoid the formation of a new electrode interface, form a new SEI, thereby reducing the loss of lithium and improving the cycle performance of the battery. 3、In the preparation method, the volume ratio of hydrogen gas to dimethyldichlorosilane vapor in step S3 is 1:(8-10), so that the cracking efficiency of the precursor is maximized, that is, hydrogen gas is sufficient to completely crack dimethyldichlorosilane, avoid the generation of chlorine-containing by-products (such as SiCl4), and inhibit the generation of free carbon. At the same time, the appropriate amount of hydrogen gas can also selectively etch amorphous silicon carbide, thereby promoting the preferential growth of the beta-SiC crystal surface and further ensuring the coating quality of the silicon carbide intermediate layer; it can also cooperate with the subsequent HF etching process to enhance the etching selectivity of HF. Since HF solution is difficult to corrode high-purity beta-SiC with a dense columnar crystal structure, it can ensure the complete removal of the internal silicon dioxide sacrificial layer while controlling the loss of the silicon carbide intermediate layer within a suitable range. At the same time, it can also ensure the controllability of the pore structure of the silicon carbide intermediate layer, ensuring both lithium ion channels and avoiding excessive SEI generation.

[0034] A three-stage gradient temperature is set to control the gradient crosslinking of the carbon source precursor, so as to realize the ordered construction of the molecular structure in the outer carbon shell, and at the same time, the pore structure on the surface of the carbon shell is regulated. For example, a first low-temperature stage is used to form pores with a larger pore size to facilitate the expansion of the internal structure; a second medium-temperature stage is used to form mesopores with a moderate pore size to serve as the main lithium ion channel; and a third high-temperature stage is used to form closed micropores to reduce the penetration of electrolyte into the inside of the battery system. DETAILED DESCRIPTION

[0035] The present application can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present application more thorough and comprehensive.

[0036] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present application belongs. The terms used in the specification of the present application are only for the purpose of describing specific embodiments and are not intended to limit the present application. The term "and / or" used herein includes any and all combinations of one or more related listed items.

[0037] In the present application, "further", "further", "particularly", "for example", "as", "example", "for example" are used for description purposes, indicating that the preceding and subsequent technical solutions are related in terms of coverage, but should not be understood as limiting the previous technical solution, nor should it be understood as limiting the scope of protection of the present application. In the present application, unless otherwise stated, A (such as B) means that B is one non-limiting example of A. It can be understood that A is not limited to B.

[0038] In the present application, the technical features or technical solutions described in open language include closed technical features or technical solutions composed of listed contents, and also include open technical features or technical solutions containing listed contents.

[0039] In the present application, the exemplary description involving "in some embodiments (or examples)", "in an embodiment (or example)" and the like can cover but is not limited to the following meanings: these schemes can be combined with other schemes in a suitable manner to form new technical schemes.

[0040] In the present application, unless there is an explicit different description herein, the execution of the steps involved in the method flow has no strict order limitation, and can be executed in other orders than described. Moreover, any step can include multiple sub-steps or multiple stages, which do not necessarily be executed at the same time, but can be executed at different times, and the execution order is not necessarily sequential, but can be executed alternately or simultaneously with other steps or other steps or sub-steps or stages.

[0041] The raw materials used in the examples and comparative examples can be obtained by market purchase.

[0042] The application first discloses a core-shell silicon-carbon material with a bistable structure, which comprises, from inside to outside, an inner core, a void layer, an intermediate layer, a binder layer and an outer shell. The inner core is mainly composed of a silicon source material, the intermediate layer is mainly composed of a silicon carbide material, the modified binder layer is composed of a modified binder solidification, and the outer shell is mainly composed of a carbon source material. The silicon carbide intermediate layer and the outer carbon shell are both solid shell structures and are sequentially coated outside the silicon core, and the void layer is located between the silicon core and the silicon carbide intermediate layer. In terms of structural design, the diameter of the inner core is 300-700 nm, preferably 500-600 nm, and more preferably 500-560 nm; the thickness of the intermediate layer is 60-100 nm, preferably 60-80 nm, and more preferably 65-75 nm; the thickness of the binder layer is 30-60 nm, preferably 30-45 nm; and the thickness of the outer shell is 100-200 nm, preferably 100-160 nm, and more preferably 120-150 nm. It should be noted that the diameter-thickness matching relationship in the bistable core-shell structure is that the diameter (D) of the silicon core and the thickness (T1) of the silicon carbide intermediate layer and the thickness (T2) of the carbon shell satisfy 0.1 < (T1+T2) / D < 0.5, so as to ensure the integrity of the coating and avoid the hindering of lithium ion diffusion caused by the over-thickness of the shell. As for the design of the void layer, it mainly focuses on the construction of the silicon dioxide sacrificial layer, i.e. the initial thickness of the silicon dioxide sacrificial layer and the low-temperature etching degree of the HF solution. Usually, the reserved void of the void layer is 200-300 nm, and in view of the limited expansion effect of the outer double-shell structure, the thickness of the void layer can also be appropriately reduced in actual testing, so as to reduce the difficulty and cost of etching processing.

[0043] The modified binder includes a conductive component, a binding base material and a dispersant, and a small amount of other auxiliary functional agents can be added according to actual conditions to improve the basic rheological properties of the modified binder slurry. Specifically, the conductive component is selected from any one or more of carbon fibers (VGCF), carbon nanotubes (CNT) and reduced graphene oxide (rGO); the binding base material is selected from any one or more of sodium alginate, carboxymethyl cellulose, polyvinylidene fluoride and polyacrylic acid; and the dispersant is selected from any one of polyvinylpyrrolidone and polyacrylamide; and the mass ratio of the conductive component, the binding base material and the dispersant is controlled to be (0.55-0.65):1:(0.18-0.24), so that better adhesive properties (viscosity range of about 3000-5000 cP) are taken into account, and better electrical conductivity is also achieved to make up for the poor electrical conductivity that may be caused by too much silicon content in the system, and the electrical conductivity of the binder layer is controlled to be 10-120 S / cm, preferably 30-80 S / cm, which can be 30 S / cm, 35 S / cm, 40 S / cm, 50 S / cm, 58 S / cm, 65 S / cm, 75 S / cm or 80 S / cm, etc., and of course can also be other values within the above range, which are not limited here.

[0044] As for the selection of the material components of other main body structures, the silicon source is subjected to nanomodification treatment, and can be selected from any one or more of nanosilicon particles, nanoporous silicon particles and nanosilicon-oxygen particles; the precursor of the carbon source can be a high-molecular carbon material that is easy to carbonize and has low cost, and is selected from any one or more of sucrose, glucose, starch and phenolic resin. After multiple test tests, the applicant believes that the most preferred carbonized silicon precursor for the intermediate layer should be dimethyldichlorosilane, and a chemical vapor deposition (CVD) method is used to construct the carbonized silicon intermediate layer, and the implementability of subsequent low-temperature etching processing of the sacrificial layer also needs to be considered. In the experimental process, dimethyldichlorosilane (DMDCS) is used as the silicon source. DMDCS can produce two different pure silicon and silicon carbide products under different atmosphere conditions. Without considering complex organic reactions, as shown in the following reaction formula, the first decomposition of DMDCS at high temperature mainly produces CH3 and SiCl2.

[0045] (CH3)2SiCl2→CH3+SiCl2(1) Under normal conditions, the decomposition products will continue to react to produce SiC, HCl and H.

[0046] CH3+SiCl2→SiC+HCl+H(2) And when the hydrogen content is sufficient, this reaction process will be inhibited to produce Si and HCl.

[0047] SiCl2 + H → Si + HCl(3) Therefore, when the temperature and the C / Si ratio in the reaction are constant, the final product of methylsilane is silicon or silicon carbide by adjusting the content of hydrogen in the atmosphere.

[0048] The application further discloses a preparation method of the core-shell silicon-carbon material. (1) silicon core preparation: silicon source material, cetyltrimethylammonium bromide (CTAB) and polymethyl methacrylate (PMMA) are added into deionized water and uniformly mixed, ball milling is carried out at a rotating speed of 300-500 rpm for 3-4 h until uniform dispersion is achieved, a centrifugal spray dryer (inlet temperature 200 DEG C, outlet temperature 90 DEG C) is used to prepare silicon material microspheres by spray granulation, and the silicon material microspheres are sintered at 600-700 DEG C under nitrogen protection for 2-3 h to obtain a porous silicon core; (2) sacrificial layer coating: after the porous silicon core in step (1) is removed from the surface of the natural oxide layer, the porous silicon core is added into a mixed liquid of anhydrous ethanol, ammonia water and deionized water, ultrasonic dispersion is carried out for 20-30 min, tetraethyl orthosilicate is added and uniformly mixed, magnetic stirring is carried out at 40-45 DEG C for 4-5 h to obtain a silica-coated intermediate, and the reaction equation is Si(OC2H5)4 + 2H2O → SiO2 + 4C2H5OH; (3) silicon carbide intermediate layer coating: the silica-coated intermediate in step (2) is placed into a fluidized bed CVD reactor, hydrogen and dimethyldichlorosilane vapor are introduced at a flow rate of 200-300 sccm and a volume ratio of 1: (8-10), the temperature is raised to 900-1100 DEG C, and reaction is carried out for 1.0-1.2 h to deposit a silicon carbide intermediate layer and coat the silica-coated intermediate, then the silica-coated intermediate is immersed into a 10% HF solution (containing 0.1 M NH4F buffer) in an ice water bath and ultrasonic etching is carried out for 1.5-2.0 h, centrifugal separation is carried out at a rotating speed of 10000-12000 rpm, the residual HF solution is neutralized by washing with saturated boric acid solution, then deionized water is used for washing until the solution is neutral, and vacuum drying is carried out at 70-90 DEG C for 22-24 h to obtain a silicon-carbon core-shell intermediate; (4) modified binder layer preparation: the conductive component is dissolved in NMP solvent, ultrasonic dispersion is carried out for 0.8-1.2 h, the binder base material and the dispersant are added, the temperature is raised to 50-60 DEG C, and mixing and stirring are carried out for 3-4 h to obtain a modified binder slurry, the silicon-carbon core-shell intermediate in step (3) is mixed into the modified binder slurry, and vacuum drying is carried out at 100-120 DEG C to obtain a silicon-carbon core-shell preliminary sample; (5) Carbon shell encapsulation: the silicon-carbon core-shell precursor in step (4) is immersed into an ethanol solution of carbon source precursor, and hexamethylenetetramine is added, vacuum impregnation for 30-40 min, and then three-stage gradient heating is performed, the first stage is heated to 70-80℃ for preliminary curing reaction for 1-2 h, the second stage is heated to 110-120℃ for moderate curing reaction for 1-2 h, and the third stage is heated to 170-180℃ for severe curing reaction for 40-60 min, then carbonization reaction is carried out at 550-650℃ under argon protection for 1-2 h, and then graphitization reaction is carried out at 2600-2800℃ for 3-4 h, to obtain the silicon-carbon negative electrode material; (6) Post-treatment, screening the silicon-carbon negative electrode material particles in step (5), and activating reaction at 750-850℃ under carbon dioxide atmosphere for 40-60 min, then the surface of the silicon-carbon negative electrode material particles is modified by acetylene cracking gas phase deposition, to obtain the finished silicon-carbon negative electrode material.

[0049] The application discloses the application of the above-mentioned core-shell silicon-carbon material and its preparation method in lithium battery materials, especially as a negative electrode material of lithium ion battery. The rechargeable lithium ion battery is mainly composed of four parts: two electrodes (positive electrode and negative electrode), separator and electrolyte, wherein the positive and negative electrodes are separated by the separator and liquid electrolyte, allowing lithium ions to shuttle back and forth between the two electrodes. When the battery is charged, lithium ions released from the positive electrode will move to the negative electrode, and when discharged, lithium ions will move from the negative electrode to the positive electrode, and the electrons released by lithium atoms in the negative electrode will reach the positive electrode through the external circuit, thereby providing power through chemical energy. The core-shell silicon-carbon material disclosed in the application is used to construct the negative electrode of the lithium ion battery.

[0050] It is worth noting that although the silicon-carbon core-shell material provided by the application can significantly overcome the volume expansion of the core, making the SEI film on the surface less likely to crack, thereby improving the overall cycle stability and coulombic efficiency of the battery, in order to ensure the recyclable performance of the overall lithium ion battery, the lithium ion battery can also be subjected to conventional pre-lithiation treatment, and a lithium source is added in the full battery system, that is, a certain amount of active lithium can be stored in the negative electrode of the battery in advance, or a pre-lithiation additive is used in the positive electrode to achieve the purpose of ensuring that the lithium content in the battery is sufficient.

[0051] Preparation Example Preparation Example 1, a preparation method of a silicon-carbon negative electrode material, adopts the following steps: (1) Silicon core preparation: 100 g of nano-silicon particles, 10 g of CTAB and 30 g of PMMA are added to 800 ml of deionized water and mixed uniformly, ball milled at a speed of 400 rpm for 3.5 h to uniformly disperse, and then spray granulated to obtain silicon material microspheres, which are sintered at 650℃ under nitrogen protection for 2.5 h to obtain a porous silicon core; (2) Sacrificial layer coating: 100 g of the porous silicon core in step (1) was added to a mixed liquid of 400 ml of anhydrous ethanol, 15 ml of ammonia water (28 wt%), and 75 ml of deionized water, and ultrasonic dispersion was performed for 25 min. Then, 45 g of tetraethyl orthosilicate was added and mixed uniformly. The mixture was heated to 45°C and stirred for 4.5 h to prepare a silica-coated intermediate; (3) Silicon carbide intermediate layer coating: 100 g of the silica-coated intermediate in step (2) was placed in a CVD reactor. Hydrogen and dimethyldichlorosilane vapor were introduced at a flow rate of 300 sccm and a volume ratio of 1:9. The temperature was increased to 1000°C and reacted for 1.1 h. Then, the intermediate was immersed in 800 ml of a 10% mass fraction HF solution and ultrasonic etching was performed in an ice water bath for 1.8 h. Centrifugal separation was performed at a rotation speed of 11000 rpm. The intermediate was washed with saturated boric acid solution and deionized water until neutral. The temperature was increased to 80°C and dried for 23 h to prepare a silicon-carbon core-shell intermediate; (4) Preparation of a modified binder layer: 10 g of rGO was dissolved in 300 ml of NMP solvent and ultrasonic dispersion was performed for 1.0 h. Then, 16 g of sodium alginate and 3.4 g of polyvinylpyrrolidone were added, and the mixture was heated to 55°C and stirred for 3.5 h to prepare a modified binder slurry. The silicon-carbon core-shell intermediate in step (3) was mixed with the modified binder slurry, and vacuum drying was performed at a temperature of 110°C to prepare a silicon-carbon core-shell preliminary sample; (5) Carbon shell encapsulation: 100 g of the silicon-carbon core-shell preliminary sample in step (4) was immersed in an ethanol solution of 100 g of phenolic resin to which 10 g of hexamethylenetetramine was added. Vacuum impregnation was performed for 35 min. Then, three-stage temperature increasing was performed. In the first stage, the temperature was increased to 75°C, and preliminary curing reaction was performed for 1.5 h. In the second stage, the temperature was increased to 115°C, and moderate curing reaction was performed for 1.5 h. In the third stage, the temperature was increased to 175°C, and severe curing reaction was performed for 50 min. Then, carbonization reaction was performed at a temperature of 600°C for 1.5 h under argon protection. Graphitization reaction was performed at a temperature of 2700°C for 3.5 h to prepare a silicon-carbon negative electrode material; (6) Post-treatment: The silicon-carbon negative electrode material particles in step (5) were sieved. Activation reaction was performed at a temperature of 800°C for 50 min under a carbon dioxide atmosphere at a flow rate of 80 sccm. Surface modification of the silicon-carbon negative electrode material particles was performed by ethylene cracking gas phase deposition at a flow rate of 40 sccm, a deposition temperature of 750°C, and a deposition time of 20 min to prepare a silicon-carbon negative electrode material product.

[0052] Preparation Example 2 is a method for preparing a silicon-carbon negative electrode material, which comprises the following steps: (1) Silicon core preparation: 110 g of nano-silicon particles, 15 g of CTAB, and 40 g of PMMA were added to 1000 ml of deionized water and mixed uniformly. Ball milling was performed at a rotation speed of 500 rpm for 4 h until uniform dispersion was achieved. Then, spray granulation was performed to prepare silicon material microspheres. The silicon material microspheres were sintered at a temperature of 700°C for 3 h under nitrogen protection to prepare a porous silicon core; (2) Sacrificial layer coating: 110 g of the porous silicon core in step (1) was added to a mixed liquid of 500 ml of anhydrous ethanol, 20 ml of ammonia water (28 wt%), and 100 ml of deionized water, and ultrasonically dispersed for 30 min. Then, 60 g of tetraethyl orthosilicate was added and mixed uniformly. The mixture was heated to 45°C and stirred for 5 h to prepare a silica-coated intermediate; (3) Silicon carbide intermediate layer coating: 110 g of the silica-coated intermediate in step (2) was placed in a CVD reactor. Hydrogen and dimethyldichlorosilane vapor were introduced at a flow rate of 300 sccm and a volume ratio of 1:10. The temperature was raised to 1100°C and reacted for 1.2 h. Then, the intermediate was immersed in 1000 ml of 10% HF solution and ultrasonically etched in an ice water bath for 2 h. Centrifugal separation was performed at a rotation speed of 12000 rpm. The intermediate was washed with saturated boric acid solution and deionized water until neutral. The temperature was raised to 90°C and dried for 24 h to prepare a silicon-carbon core-shell intermediate; (4) Preparation of modified binder layer: 15 g of rGO was dissolved in 400 ml of NMP solvent and ultrasonically dispersed for 1.2 h. Then, 23 g of sodium alginate and 5.5 g of polyvinylpyrrolidone were added, and the mixture was heated to 60°C and stirred for 4 h to prepare a modified binder slurry. The silicon-carbon core-shell intermediate in step (3) was mixed into the modified binder slurry, and the temperature was raised to 120°C for vacuum drying to prepare a silicon-carbon core-shell preliminary sample; (5) Carbon shell encapsulation: 110 g of the silicon-carbon core-shell preliminary sample in step (4) was immersed in an ethanol solution containing 110 g of phenolic resin and 15 g of hexamethylenetetramine. Vacuum impregnation was performed for 40 min. Then, the temperature was raised in three stages: the first stage was to raise the temperature to 80°C for preliminary curing reaction for 2 h, the second stage was to raise the temperature to 120°C for moderate curing reaction for 2 h, and the third stage was to raise the temperature to 180°C for severe curing reaction for 60 min. Then, the temperature was raised to 650°C for carbonization reaction for 2 h, and then to 2800°C for graphitization reaction for 4 h to prepare a silicon-carbon negative electrode material; (6) Post-treatment: The silicon-carbon negative electrode material particles in step (5) were sieved. The temperature was raised to 850°C for activation reaction for 60 min in a carbon dioxide atmosphere at a flow rate of 100 sccm. Surface modification of the silicon-carbon negative electrode material particles was performed by ethylene cracking gas phase deposition at a flow rate of 50 sccm, a deposition temperature of 800°C, and a deposition time of 30 min to prepare a silicon-carbon negative electrode material product.

[0053] Preparation Example 3 is a method for preparing a silicon-carbon negative electrode material, which comprises the following steps: (1) Silicon core preparation: 90 g of nano-silicon particles, 5 g of CTAB, and 20 g of PMMA were added to 600 ml of deionized water and mixed uniformly. Ball milling was performed at a rotation speed of 300 rpm for 3 h until uniform dispersion was achieved. Then, spray granulation was performed to prepare silicon material microspheres. The temperature was raised to 600°C under nitrogen protection for sintering for 2 h to prepare a porous silicon core. (2) Sacrificial layer coating: 90 g of porous silicon core in step (1) is added to a mixed liquid of 300 ml of anhydrous ethanol, 10 ml of ammonia water (28 wt%), and 50 ml of deionized water, ultrasonic dispersion for 20 min, 30 g of tetraethyl orthosilicate is added and mixed uniformly, heated to 40℃ and stirred for 4.0 h, to prepare a silica-coated intermediate; (3) Silicon carbide intermediate layer coating: 90 g of the silica-coated intermediate in step (2) is placed in a CVD reactor, hydrogen and dimethyl dichlorosilane vapor are introduced at a flow rate of 200 sccm and a volume ratio of 1:8, heated to 900℃ and reacted for 1.0 h, then immersed in 600 ml of 10% mass fraction HF solution, ice water bath ultrasonic etching for 2 h, centrifugal separation at a rotation speed of 10000 rpm, washed with saturated boric acid solution and deionized water until neutral, heated to 70℃ and dried for 22 h, to prepare a silicon-carbon core-shell intermediate; (4) Preparation of modified binder layer: 5 g of rGO is dissolved in 200 ml of NMP solvent, ultrasonic dispersion for 0.8 h, 9 g of sodium alginate and 1.6 g of polyvinylpyrrolidone are added, heated to 50℃, mixed and stirred for 3 h, to prepare a modified binder slurry, the modified binder slurry is mixed with the silicon-carbon core-shell intermediate in step (3), and heated to 100℃ for vacuum drying, to prepare a silicon-carbon core-shell preliminary sample; (5) Carbon shell encapsulation: 90 g of the silicon-carbon core-shell preliminary sample in step (4) is immersed in an ethanol solution of 90 g of phenolic resin, 5 g of hexamethylenetetramine is added, vacuum impregnated for 30 min, and then heated in three stages, the first stage is heated to 70℃ for preliminary curing reaction for 1.0 h, the second stage is heated to 110℃ for moderate curing reaction for 1.0 h, and the third stage is heated to 170℃ for severe curing reaction for 40 min, then heated to 550℃ for carbonization reaction for 1.0 h, and then heated to 2600℃ for graphitization reaction for 3 h, to prepare a silicon-carbon negative electrode material; (6) Post-treatment, screen selection of the silicon-carbon negative electrode material particles in step (5), heated to 750℃ for activation reaction for 40 min under a carbon dioxide atmosphere at a flow rate of 60 sccm, and then surface modification of the silicon-carbon negative electrode material particles is performed by ethylene cracking gas phase deposition at a flow rate of 20 sccm, a deposition temperature of 700℃, and a deposition time of 10 min, to prepare a finished silicon-carbon negative electrode material.

[0054] Preparation Example 4, a method for preparing a silicon-carbon negative electrode material, which is different from Preparation Example 1 in that the carbon source precursor is selected from sucrose.

[0055] Preparation Example 5, a method for preparing a silicon-carbon negative electrode material, which is different from Preparation Example 1 in that the silicon source is selected from nano-porous silicon particles.

[0056] Preparation Example 6, a method for preparing a silicon-carbon negative electrode material, which is different from Preparation Example 1 in that the conductive component is selected from CNT.

[0057] Preparation Example 7, a method for preparing a silicon-carbon negative electrode material, differs from Preparation Example 1 in that the conductive component is selected from VGCF.

[0058] Preparation Example 8, a method for preparing a silicon-carbon negative electrode material, differs from Preparation Example 1 in that the conductive component is selected from a CNT and VGCF blended material.

[0059] Preparation Example 9, a method for preparing a silicon-carbon negative electrode material, differs from Preparation Example 1 in that the binder base material is selected from carboxymethyl cellulose.

[0060] Preparation Example 10, a method for preparing a silicon-carbon negative electrode material, differs from Preparation Example 1 in that the binder base material is selected from polyvinylidene fluoride.

[0061] Preparation Example 11, a method for preparing a silicon-carbon negative electrode material, differs from Preparation Example 1 in that the binder base material is selected from a sodium alginate and polyacrylic acid blended material.

[0062] Preparation Example 12, a method for preparing a silicon-carbon negative electrode material, differs from Preparation Example 1 in that the dispersant is selected from polyacrylamide.

[0063] Preparation Example 13, a method for preparing a silicon-carbon negative electrode material, differs from Preparation Example 1 in that the mass ratio of the conductive component to the binder base material is 0.3:1.

[0064] Preparation Example 14, a method for preparing a silicon-carbon negative electrode material, differs from Preparation Example 1 in that the mass ratio of the dispersant to the binder base material is 0.05:1.

[0065] Preparation Example 15, a method for preparing a silicon-carbon negative electrode material, differs from Preparation Example 1 in that the thickness of the binder layer is 120 nm.

[0066] Preparation Example 16, a method for preparing a silicon-carbon negative electrode material, differs from Preparation Example 1 in that the diameter of the inner core is 200 nm.

[0067] Preparation Example 17, a method for preparing a silicon-carbon negative electrode material, differs from Preparation Example 1 in that the diameter of the inner core is 1100 nm.

[0068] Preparation Example 18, a method for preparing a silicon-carbon negative electrode material, differs from Preparation Example 1 in that the thickness of the intermediate layer is 30 nm.

[0069] Preparation Example 19, a method for preparing a silicon-carbon negative electrode material, differs from Preparation Example 1 in that the thickness of the intermediate layer is 200 nm.

[0070] Preparation Example 20, a method for preparing a silicon-carbon negative electrode material, differs from Preparation Example 1 in that the thickness of the outer shell is 50 nm.

[0071] Preparation Example 21, a preparation method of a silicon-carbon negative electrode material, differs from Preparation Example 1 in that the volume ratio of hydrogen gas to dimethyldichlorosilane vapor in step (3) is 1:3.

[0072] Preparation Example 22, a preparation method of a silicon-carbon negative electrode material, differs from Preparation Example 1 in that the step (5) does not use the stepwise heating and curing method, and directly maintains the heating temperature at 180°C for 4h.

[0073] Preparation Example 23, a preparation method of a silicon-carbon negative electrode material, differs from Preparation Example 1 in that the post-treatment of step (6) is not included.

[0074] Example Example 1, a lithium ion battery, includes a positive electrode level sheet, a negative electrode level sheet, an electrolyte, and a separator disposed between the positive electrode level sheet and the negative electrode level sheet. (1) Preparation of the negative electrode level sheet: the finished silicon-carbon negative electrode material prepared by Preparation Example 1 is mixed in NMP solution, and mechanically stirred for 12h to obtain a negative electrode slurry. The negative electrode slurry is uniformly coated on a copper foil by a coating machine, vacuum dried, rolled, cut into 12mm diameter sheets and weighed to obtain the negative electrode level sheet.

[0075] (2) Preparation of the positive electrode level sheet: the positive electrode active material is selected as a ternary positive electrode material. The active material, PVDF and carbon black are ground and mixed in a ratio of 90:5:5, and mechanically stirred in NMP solution for 12h to obtain a positive electrode slurry. The positive electrode slurry is uniformly coated on an aluminum foil by a coating machine, vacuum dried, rolled, cut into 12mm diameter sheets and weighed to obtain the positive electrode level sheet.

[0076] (3) Assembly of CR2023 button cell: under argon protection, the negative electrode level sheet and the positive electrode level sheet prepared above are selected as the working electrode, 1mol / L LiPF6 is used as the electrolyte, EC / DMC(1:1)+10%FEC is used as the organic solvent, and Celgard2400 polypropylene porous film is used as the separator to separate the positive and negative electrodes. The assembly sequence is positive shell-positive electrode level sheet-separator-negative electrode level sheet-gasket-spring sheet-negative shell, and a few drops of electrolyte are added on the working electrode and the separator to obtain the lithium ion battery.

[0077] Example 2, a lithium ion battery, differs from Example 1 in that the negative electrode level sheet uses the finished silicon-carbon negative electrode material prepared by Preparation Example 2.

[0078] Example 3, a lithium ion battery, differs from Example 1 in that the negative electrode level sheet uses the finished silicon-carbon negative electrode material prepared by Preparation Example 3.

[0079] Example 4, a lithium-ion battery, which differs from Example 1 in that the negative electrode web employs the silicon-carbon negative electrode material finished product prepared in Preparation Example 4.

[0080] Example 5, a lithium-ion battery, which differs from Example 1 in that the negative electrode web employs the silicon-carbon negative electrode material finished product prepared in Preparation Example 5.

[0081] Example 6, a lithium-ion battery, which differs from Example 1 in that the negative electrode web employs the silicon-carbon negative electrode material finished product prepared in Preparation Example 6.

[0082] Example 7, a lithium-ion battery, which differs from Example 1 in that the negative electrode web employs the silicon-carbon negative electrode material finished product prepared in Preparation Example 7.

[0083] Example 8, a lithium-ion battery, which differs from Example 1 in that the negative electrode web employs the silicon-carbon negative electrode material finished product prepared in Preparation Example 8.

[0084] Example 9, a lithium-ion battery, which differs from Example 1 in that the negative electrode web employs the silicon-carbon negative electrode material finished product prepared in Preparation Example 9.

[0085] Example 10, a lithium-ion battery, which differs from Example 1 in that the negative electrode web employs the silicon-carbon negative electrode material finished product prepared in Preparation Example 10.

[0086] Example 11, a lithium-ion battery, which differs from Example 1 in that the negative electrode web employs the silicon-carbon negative electrode material finished product prepared in Preparation Example 11.

[0087] Example 12, a lithium-ion battery, which differs from Example 1 in that the negative electrode web employs the silicon-carbon negative electrode material finished product prepared in Preparation Example 12.

[0088] Comparative Example Comparative Example 1, a lithium-ion battery, which differs from Example 1 in that the negative electrode web employs the silicon-carbon negative electrode material finished product prepared in Preparation Example 13.

[0089] Comparative Example 2, a lithium-ion battery, which differs from Example 1 in that the negative electrode web employs the silicon-carbon negative electrode material finished product prepared in Preparation Example 14.

[0090] Comparative Example 3, a lithium-ion battery, which differs from Example 1 in that the negative electrode web employs the silicon-carbon negative electrode material finished product prepared in Preparation Example 15.

[0091] Comparative Example 4, a lithium-ion battery, which differs from Example 1 in that the negative electrode web employs the silicon-carbon negative electrode material finished product prepared in Preparation Example 16.

[0092] Comparative Example 5, a lithium ion battery, differs from Example 1 in that the negative electrode sheet uses the silicon-carbon negative electrode material finished product prepared from Preparation Example 17.

[0093] Comparative Example 6, a lithium ion battery, differs from Example 1 in that the negative electrode sheet uses the silicon-carbon negative electrode material finished product prepared from Preparation Example 18.

[0094] Comparative Example 7, a lithium ion battery, differs from Example 1 in that the negative electrode sheet uses the silicon-carbon negative electrode material finished product prepared from Preparation Example 19.

[0095] Comparative Example 8, a lithium ion battery, differs from Example 1 in that the negative electrode sheet uses the silicon-carbon negative electrode material finished product prepared from Preparation Example 20.

[0096] Comparative Example 9, a lithium ion battery, differs from Example 1 in that the negative electrode sheet uses the silicon-carbon negative electrode material finished product prepared from Preparation Example 21.

[0097] Comparative Example 10, a lithium ion battery, differs from Example 1 in that the negative electrode sheet uses the silicon-carbon negative electrode material finished product prepared from Preparation Example 22.

[0098] Comparative Example 11, a lithium ion battery, differs from Example 1 in that the negative electrode sheet uses the silicon-carbon negative electrode material finished product prepared from Preparation Example 23.

[0099] Performance detection test Lithium ion batteries were prepared according to the methods in each of the examples and comparative examples, and three lithium ion batteries were taken from each example or comparative example for first coulombic efficiency and capacity retention rate tests, the detection results were averaged and recorded in Table 1, and the detection methods were as follows: 1. First coulombic efficiency test: according to GB / T 44027.1-2024, each lithium ion battery was charged at 0.1C constant current to the cut-off voltage, converted to constant voltage until the current was less than or equal to 0.05C, and then rested for 5 minutes before discharging at 0.1C constant current to the cut-off voltage. The first coulombic efficiency (ICE) = (first discharge capacity / first charge capacity) x 100%.

[0100] 2. Capacity retention rate test: according to GB / T 44027.1-2024, each lithium ion battery was placed in a 25°C constant temperature box, first activated the SEI film by 0.1C charging and discharging for 1-3 weeks, then cycled at 1C for 100 weeks and recorded the discharge capacity every week, and calculated the capacity retention rate. Retention rate = Cn / C1 x 100% (C1 is the first cycle discharge capacity, Cn is the nth cycle discharge capacity).

[0101] Table 1 first coulomb efficiency (%) capacity retention rate (%) example 1 93.7 92.6 example 2 93.2 92.8 example 3 92.5 92.1 example 4 91.6 90.8 example 5 92.3 92.2 example 6 92.1 91.7 example 7 90.9 91.5 example 8 91.5 92.4 example 9 92.1 91.6 example 10 92.3 90.8 example 11 93.0 92.5 example 12 90.7 90.6 comparative example 1 78.5 82.0 comparative example 2 81.6 84.1 comparative example 3 82.3 83.5 comparative example 4 74.9 76.9 comparative example 5 78.2 69.2 comparative example 6 84.1 81.5 comparative example 7 79.7 80.6 comparative example 8 73.9 77.7 comparative example 9 75.7 73.8 comparative example 10 84.8 85.3 comparative example 11 89.5 88.2 The embodiments are only illustrative of the present application, and are not intended to limit the present application, and those skilled in the art can make modifications to the embodiments without creative contribution after reading the specification, but as long as the modifications are within the scope of the claims of the present application, they are protected by the patent law.

Claims

1. A silicon-carbon negative electrode material, characterized by, The silicon-carbon negative electrode material has a core-shell structure, including an inner core, an intermediate layer, and an outer shell, the intermediate layer is coated on the inner core, the outer shell is coated on the intermediate layer, and a gap is left between the inner core and the intermediate layer; The material constituting the inner core includes a silicon source, the material constituting the intermediate layer includes silicon carbide, and the material constituting the outer shell includes a carbon source; a binder layer is further arranged between the intermediate layer and the outer shell, and the material constituting the binder layer includes a modified binder.

2. The silicon-carbon negative electrode material of claim 1, wherein, The modified binder includes a conductive component and a bonding base material, the conductive component is selected from any one or more of carbon fibers (VGCF), carbon nanotubes (CNT), and reduced graphene oxide (rGO); The bonding base material is selected from any one or more of sodium alginate, carboxymethyl cellulose, polyvinylidene fluoride, and polyacrylic acid; And the mass ratio of the conductive component to the bonding base material is (0.55-0.65):

1.

3. The silicon-carbon negative electrode material of claim 2, wherein, The modified binder further includes a dispersant selected from any one of polyvinylpyrrolidone and polyacrylamide; And the mass ratio of the conductive component, the bonding base material, and the dispersant is (0.55-0.65):1:(0.18-0.24).

4. The silicon-carbon negative electrode material of claim 3, wherein, The conductivity of the binder layer is 10-120 S / cm, preferably 30-80 S / cm; The thickness of the binder layer is 30-60 nm, preferably 30-45 nm.

5. The silicon-carbon negative electrode material of claim 1, wherein, The silicon source is selected from any one or more of nano-silicon particles, nano-porous silicon particles, and nano-silicon-oxygen particles; And the diameter of the inner core is 300-700 nm, preferably 500-600 nm.

6. The silicon-carbon negative electrode material of claim 1, wherein, The precursor of the silicon carbide is selected from dimethyl dichlorosilane; and the thickness of the intermediate layer is 60-100 nm, preferably 60-80 nm; The precursor of the carbon source is selected from any one or more of sucrose, glucose, starch, and phenolic resin; and the thickness of the outer shell is 100-200 nm, preferably 100-160 nm.

7. A method for preparing the silicon-carbon negative electrode material according to any one of claims 1 to 6, characterized in that, The method includes the following steps: S1, silicon core preparation: silicon source material, cetyltrimethylammonium bromide (CTAB), and polymethyl methacrylate (PMMA) are added to deionized water and mixed uniformly, ball-milled at a speed of 300-500 rpm for 3-4 h until uniformly dispersed, then spray granulated to obtain silicon material microspheres, and sintered at a temperature of 600-700 ℃ for 2-3 h to obtain a porous silicon core; S2, sacrificial layer coating: the porous silicon core in step S1 is added to a mixed liquid of anhydrous ethanol, ammonia water, and deionized water, ultrasonically dispersed for 20-30 min, and then tetraethyl orthosilicate is added and mixed uniformly, heated to 40-45 ℃, and stirred and mixed for 4-5 h to obtain a silica-coated intermediate; S3, silicon carbide intermediate layer coating: the silicon dioxide coated intermediate in step S2 is placed in a CVD reactor, hydrogen and dimethyldichlorosilane vapor are passed in at a flow rate of 200-300 sccm, the temperature is raised to 900-1100°C for 1.0-1.2 h to deposit a silicon carbide intermediate layer and coat the silicon dioxide coated intermediate, then immersed in a 10% HF solution in an ice water bath for ultrasonic etching for 1.5-2.0 h, centrifuged at a speed of 10,000-12,000 rpm, washed and dried to obtain a silicon-carbon core-shell intermediate; S4, preparation of a modified binder layer: the conductive component is dissolved in NMP solvent, ultrasonically dispersed for 0.8-1.2 h, the binder base and dispersant are added and the temperature is raised to 50-60°C, mixed and stirred for 3-4 h to obtain a modified binder slurry, the silicon-carbon core-shell intermediate in step S3 is mixed into the modified binder slurry, and the temperature is raised to 100-120°C for vacuum drying to obtain a silicon-carbon core-shell preliminary sample; S5, carbon shell packaging: the silicon-carbon core-shell preliminary sample in step S4 is immersed in an ethanol solution of a carbon source precursor, hexamethylenetetramine is added, vacuum impregnated for 30-40 min, then stepwise temperature raised and cured, then heated to 550-650°C under argon protection for carbonization reaction for 1-2 h, then heated to 2600-2800°C for graphitization reaction for 3-4 h to obtain a silicon-carbon negative electrode material. 8.The method of claim 7, wherein the silicon-carbon negative electrode 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 negative electrode material. The volume ratio of hydrogen to dimethyldichlorosilane vapor in step S3 is 1:(8-10); The stepwise temperature raising and curing in step S5 is set to three stages, the first stage is raised to 70-80°C for preliminary curing reaction for 1-2 h, the second stage is raised to 110-120°C for moderate curing reaction for 1-2 h, and the third stage is raised to 170-180°C for severe curing reaction for 40-60 min.

9. The method of claim 7, wherein the silicon-carbon negative electrode 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 negative electrode material. S6, post-treatment, sieving to obtain silicon-carbon negative electrode material particles, heating to 750-850°C under a carbon dioxide atmosphere for activation reaction for 40-60 min, then surface modification of the silicon-carbon negative electrode material particles by acetylene cracking gas phase deposition to obtain a finished silicon-carbon negative electrode material.

10. Use of the silicon-carbon negative electrode material of any one of claims 1-6 or the preparation method of the silicon-carbon negative electrode material of any one of claims 7-9 in a lithium ion battery negative electrode material.