Patent Publication Number: US-2021175497-A1

Title: Silicon-based composite battery anode material, preparation method thereof, and energy storage device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Patent Application No. PCT/CN2019/089010, filed on May 29, 2019, which claims priority to Chinese Patent Application No. 201811004238.6, filed on Aug. 30, 2018. The disclosures of the aforementioned applications are hereby incorporated by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to the field of secondary battery technologies, and in particular, to anode materials for a battery. 
     BACKGROUND 
     Cathode and anode materials of a lithium-ion battery are main parts in fulfilling an energy storage function, and directly reflect energy density, cycle performance, and security performance of an electrochemical cell. When lithium cobalt oxide, a current commercial cathode material, reaches a highest use limit (4.45 V, 4.2 g/cm 3 ), a capacity of the anode plays a crucial role in improving the energy density of the entire electrochemical cell. However, a current actual capacity of a commercial graphite anode is 360 mAh/g, which approaches a theoretical value (372 mAh/g). Therefore, it is necessary to develop a new high-capacity commercial anode material. 
     A silicon-based material is one of the most studied anode materials as an alternative to graphite. According to different degrees of reactions, silicon and lithium can generate different products, such as Li 12 Si 17 , Li 7 Si 3 , Li 13 Si 4 , and Li 22 Si 5 . A Li 4.4 Si alloy formed by lithium interacted with silicon has a theoretical ratio of 4200 mAh/g and is an anode material with a theoretically maximum capacity. However, the silicon-based material undergoes severe volume expansion (0-300%) and contraction during lithium intercalation and deintercalation reaction, causing damage and pulverization of a structure of an electrode material. In addition, a silicon surface and an electrolyte continuously generate a new SEI (solid electrolyte interface film) film, causing electrolyte exhaustion, and rapid battery capacity attenuation. 
     In order to resolve the foregoing problems, currently, nanocrystallization is commonly used in the industry to alleviate a silicon volume expansion effect. However, nanocrystallization causes a high surface area with a feature of congulation proneness and low probability of dispersion, a large contact area with the electrolyte, fast consumption of the electrolyte, and the like. In order to further resolve the foregoing problems caused by nanocrystallization, a coating layer (including soft coating or hard coating such as a carbon material layer) is disposed on a surface of a nano-silicon anode material. However, although the soft coating (such as carbon coating) is tough to some extent, pores of the soft coating cannot actually alleviate a side reaction between silicon and the electrolyte. In addition, although the hard coating is of relatively high hardness, the hard coating is brittle and is likely to break and fall off during expansion and contraction. 
     SUMMARY 
     In view of this, a first aspect of embodiments of the present invention provides a silicon-based composite anode material, and a coating layer of the silicon-based composite anode material can effectively alleviate a volume expansion effect of a silicon-based material core, and has high electrical conductivity and ionic conductivity performance, so as to resolve problems of pulverization, efficacy loss, and poor cycle performance that are caused by large expansion of an existing silicon-based material. 
     Specifically, a first aspect of the embodiments of the present invention provides a silicon-based composite anode material, including a silicon-based material core and a coating layer coated on a surface of the silicon-based material core, where the coating layer includes a first coating layer disposed on the surface of the silicon-based material core and a second coating layer disposed on a surface of the first coating layer, the first coating layer includes a two-dimensional quinone-aldehyde covalent organic framework material, and the second coating layer includes a material with high ionic conductivity. 
     The quinone-aldehyde covalent organic framework material includes a quinone substance and a trialdehyde substance, the quinone substance includes 2,6-diaminoanthraquinone (DAAQ) or 1,4-benzopuinone (DABQ), and the trialdehyde substance includes 2,4,6-triformylphloroglucinol (TFP). 
     A mass ratio of the quinone substance to the trialdehyde substance is 1:1 to 1:5. 
     The first coating layer is formed through in-situ growth of the two-dimensional quinone-aldehyde covalent organic framework material on the surface of the silicon-based material core and close layer-by-layer stacking, and the first coating layer completely coats the silicon-based material core. 
     A thickness of the first coating layer is 5 nm to 200 nm. 
     The material with high ionic conductivity includes at least one of lithium fluoride and an oxide solid-state electrolyte. Specifically, the oxide solid-state electrolyte includes one or more of a crystalline-state perovskite-type solid-state electrolyte, a crystalline-state NASICON-type solid-state electrolyte, a crystalline-state LISICON-type solid-state electrolyte, a garnet-type solid-state electrolyte, and a glass-state oxide solid-state electrolyte. 
     A thickness of the second coating layer is 10 nm to 200 nm, and the second coating layer completely coats the first coating layer. 
     The silicon-based material core includes one or more of monatomic silicon, a silicon-oxygen compound, a silicon-carbon compound, and a silicon alloy. Specifically, the silicon alloy includes one or more of a ferrosilicon alloy, an aluminum-silicon alloy, a copper-silicon alloy, or a silicon-tin alloy. 
     The silicon-based material core is in a shape of a sphere, a spheroid, or a plate, and a particle size of the silicon-based material core is 50 nm to 10 μm. 
     The silicon-based composite anode material provided in the first aspect of the embodiments of the present invention includes a silicon-based material core and a coating layer disposed on a surface of the core, and the coating layer includes a first coating layer and a second coating layer coating the first coating layer. With superb toughness and ordered pore structure, the two-dimensional quinone-aldehyde covalent organic framework material of the first coating layer can effectively absorb mechanical stress generated by expansion of the silicon-based material core, ensure integrity of the coating layer, improve structural stability of the silicon-based material, and have high electrical conductivity and ionic conductivity, thereby effectively improving electron conduction and ion conduction effects of the coating layer. With a relatively strong rigid structure, the material with high ionic conductivity of the second coating layer can maintain structural stability of an entire material during silicon expansion and contraction, to effectively alleviate volume expansion, and increases energy density of the silicon-based electrochemical cell. In addition, The fast-conducting ionic material layer can further effectively prevent the electrolyte from in contact with the silicon-based material core to cause side reactions, thereby ensuring cycle performance of the material. 
     A second aspect of the embodiments of the present invention provides a method for preparing a silicon-based composite anode material, including the following steps: 
     preparing a silicon-based material, and growing a two-dimensional quinone-aldehyde covalent organic framework material in situ on a surface of the silicon-based material, to form a first coating layer; and coating a surface of the first coating layer with a material with high ionic conductivity, to form a second coating layer, so that a silicon-based composite anode material is obtained, where the silicon-based composite anode material includes a silicon-based material core and a coating layer coated on a surface of the silicon-based material core, the coating layer includes the first coating layer disposed on the surface of the silicon-based material core and the second coating layer disposed on the surface of the first coating layer, the first coating layer includes the two-dimensional quinone-aldehyde covalent organic framework material, and the second coating layer includes the material with high ionic conductivity. 
     According to the foregoing preparation method in the present invention, a specific operation of growing a two-dimensional quinone-aldehyde covalent organic framework material in situ on a surface of the silicon-based material, to form a first coating layer is: adding the silicon-based material, a quinone substance, and a trialdehyde substance into an organic solvent, to obtain a mixed solution, leaving the mixed solution in reaction at 80° C. to 140° C. for 1 to 7 days in an anaerobic condition, and after the reaction is completed, obtaining a silicon-based material coated with the first coating layer through cooling and centrifugal separation, where the quinone substance includes 2,6-diaminoanthraquinone, and the trialdehyde substance includes 2,4,6-triformylphloroglucinol. 
     According to the foregoing preparation method in the present invention, a specific operation of growing a two-dimensional quinone-aldehyde covalent organic framework material in situ on a surface of the silicon-based material, to form a first coating layer is: adding the silicon-based material, a quinone substance precursor, and a trialdehyde substance into an organic solvent, to obtain a mixed solution, leaving the mixed solution in reaction at 80° C. to 140° C. for 1 to 7 days in an anaerobic condition, and after the reaction is completed, collecting solids through cooling and centrifugal separation, and adding the solids into the oxidant, to oxidize the quinone substance precursor into a quinone substance, so as to obtain a silicon-based material coated with the first coating layer, where the quinone substance precursor includes 2,5-diamino-1,4-dihydroxybenzo, the quinone substance includes 1,4-benzoquinone, and the trialdehyde substance includes 2,4,6-triformylphloroglucinol. 
     In the foregoing preparation method in the present invention, methods for coating the surface of the first coating layer with the material with high ionic conductivity, to form the second coating layer includes a hydrothermal method, a solvent-thermal method, a liquid phase precipitation method, a high energy ball milling method, or a high-temperature melting-casting method. 
     The method for preparing a silicon-based composite anode material provided in the second aspect of the embodiments of the present invention is simple in process and suitable for commercialized production. 
     According to a third aspect, an embodiment of the present invention further provides an energy storage device, including a cathode, an anode, and a separator located between the cathode and the anode, where the anode includes the silicon-based composite anode material according to the first aspect of the embodiments of present invention. 
     The energy storage device includes a lithium-ion battery, a sodium ion battery, a magnesium ion battery, an aluminum ion battery, or a supercapacitor. 
     The energy storage device provided in the embodiment of the present invention has high capacity and long cycle life by using the silicon-based composite anode material provided in the embodiments of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic structural diagram of a silicon-based composite anode material according to an embodiment of the present invention; 
         FIG. 2  is a flowchart of a method for preparing a silicon-based composite anode material according to an embodiment of the present invention; and 
         FIG. 3  is a comparison diagram of cycle performance between lithium ion batteries prepared in embodiments 1 to 2 of the present invention and a lithium ion battery in a comparison embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following describes the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. 
     To resolve problems of pulverization, efficacy loss, and poor cycle performance that are caused by large volume expansion of a silicon-based composite anode material, an embodiment of the present invention provides a silicon-based composite anode material. As shown in  FIG. 1 , the silicon-based composite anode material includes a silicon-based material core  10  and a coating layer coated on a surface of the silicon-based material core  10 . The coating layer is a double-layer structure, including a first coating layer  11  disposed on the surface of the silicon-based material core and a second coating layer  12  disposed on a surface of the first coating layer  11 , the first coating layer  11  includes a two-dimensional quinone-aldehyde covalent organic framework material, and the second coating layer  12  includes a material with high ionic conductivity. 
     The silicon-based composite anode material provided in this embodiment of the present invention is a particle of a core-shell structure, namely, an egg-like structure, coated by two layers. The silicon-based material core  10  is similar to yolk, the first coating layer  11  is similar to white, and the second coating layer  12  is similar to an eggshell. The two-dimensional quinone-aldehyde covalent organic framework material of the first coating layer  11  has high electrical conductivity and ionic conductivity, and an electrical conductivity and ionic conductivity network is not destroyed during a lithium intercalation and deintercalation process, thereby effectively improving electron conduction and ion conduction effects of the coating layer. In addition, with superb toughness and a regular and ordered porous pore structure, the two-dimensional quinone-aldehyde covalent organic framework material can effectively absorb mechanical stress generated by expansion of the silicon-based material core, and ensure integrity of the coating layer, playing a similar role as a sponge. The fast ion conduction material of the second coating layer  12  can maintain structural stability and a volume size of the entire silicon-based material with a double-coating structure during expansion and contraction of silicon, thereby effectively alleviating volume expansion. 
     In this implementation of the present invention, the covalent organic framework (COFs) material is a crystalline-state material, with a regular and ordered porous framework structure, formed by connecting organic building units such as light elements C, O, N, and B through covalent bonds. Strong covalent interaction exists between building units in the framework material, and has advantages such as low mass density, high thermal stability, a high surface area, and a uniform pore size. The two-dimensional quinone-aldehyde covalent organic framework material has high electrical conductivity and ionic conductivity performance, and can rapidly intercalate/deintercalate a lithium ion by utilizing redox reactions in the two-dimensional quinone-aldehyde covalent organic framework material. In addition, an ordered pore channel of the framework material facilitates transmission of the lithium ion, thereby improving electrochemical performance of the silicon-based composite material. Specifically, the quinone-aldehyde covalent organic framework material includes a quinone substance and a trialdehyde substance. Optionally, in this implementation of the present invention, the quinone substance includes 2,6-diaminoanthraquinone (DAAQ) or 1,4-benzopuinone (DABQ), and the trialdehyde substance includes 2,4,6-triformylphloroglucinol (TFP). In other words, the quinone-aldehyde organic framework material may be DAAQ-TFP or DABQ-TFP. Optionally, a ratio of the quinone substance to the trialdehyde substance is 1:1 to 1:5, for example, may be specifically 1:1, 1:2, 1:3, 1:4, or 1:5. 
     In this implementation of the present invention, the first coating layer  11  is formed through in-situ growth of the two-dimensional quinone-aldehyde covalent organic framework material on the surface of the silicon-based material core and close layer-by-layer stacking, and the first coating layer completely coats the silicon-based material core. Countless nucleation sites are provided on the surface of the silicon-based material core for growth and bonding of the two-dimensional quinone-aldehyde covalent organic framework material, and the two-dimensional quinone-aldehyde covalent organic framework material uniformly grows on the surface of the silicon-based material core by using the nucleation sites, to form a uniform-thickness first coating layer. Optionally, a thickness of the first coating layer  11  is 5 nm to 200 nm, and may be specifically 10 nm to 30 nm, 50 nm to 100 nm, 80 nm to 150 nm, or 120 nm to 180 nm. The thickness of the first coating layer  11  may be set based on a specific size of the silicon-based material core  10 . For example, when the core is a particle in a shape of a sphere or a spheroid, the thickness of the first coating layer  11  may be set to 5% to 30% of a radius of the silicon-based material core  10 . An appropriate thickness of the first coating layer can effectively strengthen a buffer effect of the first coating layer without affecting electrochemical performance of the silicon-based material. 
     In this implementation of the present invention, the second coating layer includes a material with high ionic conductivity. The material with high ionic conductivity includes at least one of lithium fluoride (LiF) and an oxide solid-state electrolyte. Specifically, the oxide solid-state electrolyte includes one or more of a crystalline-state perovskite-type solid-state electrolyte, a crystalline-state NASICON-type solid-state electrolyte, a crystalline-state LISICON-type solid-state electrolyte, a garnet-type solid-state electrolyte, and a glass-state oxide solid-state electrolyte. Specifically, the oxide solid-state electrolyte includes but is not limited to Li 3 PO 4 , Li 2 O, Li 6 BaLa 2 Ta 2 O 12 (LLZO), Li 7 La 3 Zr 2 O 12 , Li 5 La 3 Nb 2 O 12 , Li 5 La 3 M 2 O 12  (M=Nb,Ta), Li 7+x A x La 3-x Zr 2 O 12  (A=Zn), Li 3 Zr 2 Si 2 PO 12 , Li 5 ZrP 3 O 12 , Li 5 TiP 3 O 12 , Li 3 Fe 2 P 3 O 12 , Li 4 NbP 3 O 12 , Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP), and the like. 
     In this implementation of the present invention, the second coating layer completely coats the first coating layer, and a surface of the first coating layer provides countless nucleation sites for attaching and bonding of the materials with high ionic conductivity. The materials with high ionic conductivity are used to perform uniform attaching and bonding on the surface of the first coating layer by using the nucleation sites, to form a uniform-thickness second coating layer. 
     In this implementation of the present invention, a thickness of the second coating layer  12  is 10 nm to 200 nm, and may be specifically 20 nm to 30 nm, 50 nm to 100 nm, 80 nm to 150 nm, or 120 nm to 180 nm. An appropriate thickness of the second coating layer can effectively coat an core material and buffer volume expansion without causing degradation of electrochemical performance of the core material. In this implementation of the present invention, the silicon-based material core  10  includes but is not limited to monatomic silicon, a silicon-oxygen compound, a silicon-carbon compound, and a silicon alloy. The silicon alloy may be, for example, one or more of a ferrosilicon alloy, an aluminum-silicon alloy, a copper-silicon alloy, and a silicon-tin alloy. In this implementation of the present invention, the particle size of the silicon-based material core  10  is 50 nm to 10 μm. Optionally, the particle size of the silicon-based material core  10  is 100 nm to 500 nm, 300 nm to 800 nm, 1 μm to 5 μm, or 6 μm to 8 μm. A shape of the silicon-based material core  10  is not limited, and may be specifically in a shape of a sphere, a spheroid (for example, an ellipsoid) or a plate. The first coating layer  11  and the second coating layer  12  are a thin-layer structure coated on the surface of the core  10 , and specific shapes of the first coating layer  11  and the second coating layer  12  depend on a shape of the silicon-based material core  10 . To be specific, the silicon-based composite anode material is a core, of a core-shell structure, coated by two layers, and an overall outer shape of the particle mainly depends on the shape of the core  10 . 
     Correspondingly,  FIG. 2  shows a method for preparing the foregoing silicon-based composite anode material according to an embodiment of the present invention, and the method includes the following specific steps: 
     S 10 . Prepare a silicon-based material, and grow a two-dimensional quinone-aldehyde covalent organic framework material in situ on a surface of the silicon-based material, to form a first coating layer. 
     S 20 . Coat a surface of the first coating layer with a material with high ionic conductivity, to form a second coating layer, so that a silicon-based composite anode material is obtained, where the silicon-based composite anode material includes a silicon-based material core and a coating layer coated on a surface of the silicon-based material core, the coating layer includes the first coating layer disposed on the surface of the silicon-based material core and the second coating layer disposed on the surface of the first coating layer, the first coating layer includes the two-dimensional quinone-aldehyde covalent organic framework material, and the second coating layer includes the material with high ionic conductivity. 
     In an implementation of the present invention, in step S 10 , a specific operation of growing a two-dimensional quinone-aldehyde covalent organic framework material in situ on a surface of the silicon-based material, to form a first coating layer is: adding the silicon-based material, a quinone substance, and a trialdehyde substance into an organic solvent, to obtain a mixed solution; leaving the mixed solution in reaction at 80° C. to 140° C. for 1 to 7 days in an anaerobic condition; and after the reaction is completed, obtaining, through cooling and centrifugal separation, a silicon-based material coated with the first coating layer. The quinone substance includes 2,6-diaminoanthraquinone, and the trialdehyde substance includes 2,4,6-triformylphloroglucinol. Optionally, the organic solvent may be a mixed solvent including N,N-dimethylacetamide and mesitylene. Optionally, an operation of sequentially washing obtained solids by using N,N-dimethylformamide (DMF) and acetone is further performed after the centrifugal separation operation. 
     In another implementation of the present invention, in step S 10 , a specific operation of growing a two-dimensional quinone-aldehyde covalent organic framework material in situ on a surface of the silicon-based material, to form a first coating layer is: adding the silicon-based material, a quinone substance precursor, and a trialdehyde substance into an organic solvent, to obtain a mixed solution; leaving the mixed solution in reaction at 80° C. to 140° C. for 1 to 7 days in an anaerobic condition; and after the reaction is completed, collecting solids through cooling and centrifugal separation, and adding the solids into the oxidant, to oxidize the quinone substance precursor into a quinone substance, so as to obtain a silicon-based material coated with the first coating layer. The quinone substance precursor includes 2,5-diamino-1,4-dihydroxybenzo, the quinone substance includes 1,4-benzoquinone, and the trialdehyde substance includes 2,4,6-triformylphloroglucinol. Optionally, the organic solvent may be a mixed solvent including N,N-dimethylacetamide and mesitylene. Optionally, an operation of sequentially washing obtained solids by using N,N-dimethylformamide (DMF) and acetone is further performed after the centrifugal separation operation. Optionally, the oxidant is triethylamine, and the oxidization process is performed during 6 to 24 hours stirring under a room-temperature air atmosphere. 
     In this implementation of the present invention, in step S 10 , the silicon-based material core includes but is not limited to one or more of monatomic silicon, a silicon-oxygen compound, a silicon-carbon compound, and a silicon alloy. The silicon alloy may be, for example, one or more of a ferrosilicon alloy, an aluminum-silicon alloy, a copper-silicon alloy, and a silicon-tin alloy. In this implementation of the present invention, the particle size of the silicon-based material core is 50 nm to 10 μm. Optionally, the particle size of the silicon-based material core is 100 nm to 500 nm, 300 nm to 800 nm, 1 μm to 5 μm, or 6 μm to 8 μm. A shape of the silicon-based material core is not limited, and may be specifically in a shape of a sphere, a spheroid, a plate, or the like. 
     In this implementation of the present invention, the two-dimensional quinone-aldehyde covalent organic framework material includes a quinone substance and a trialdehyde substance. Optionally, the quinone substance includes 2,6-diaminoanthraquinone (DAAQ) or 1,4-benzopuinone (DABQ), and the trialdehyde substance includes 2,4,6-triformylphloroglucinol (TFP). In other words, the quinone-aldehyde organic framework material may be DAAQ-TFP or DABQ-TFP. Optionally, a ratio of the quinone substance to the trialdehyde substance is 1:1 to 1:5, for example, may be specifically 1:1, 1:2, 1:3, 1:4, or 1:5. 
     In this implementation of the present invention, the first coating layer is formed through in-situ growth of the two-dimensional quinone-aldehyde covalent organic framework material on the surface of the silicon-based material core and close layer-by-layer stacking, and the first coating layer completely coats the silicon-based material core. Countless nucleation sites are provided on the surface of the silicon-based material core for growth and bonding of the two-dimensional quinone-aldehyde covalent organic framework material, and the two-dimensional quinone-aldehyde covalent organic framework material uniformly grows on the surface of the silicon-based material core by using the nucleation sites, to form a uniform-thickness first coating layer. Optionally, a thickness of the first coating layer  11  is 5 nm to 200 nm, and may be specifically 10 nm to 30 nm, 50 nm to 100 nm, 80 nm to 150 nm, or 120 nm to 180 nm. The thickness may be adjusted by controlling a time in which the mixture reacts at 80° C. to 140° C. 
     In this implementation of the present invention, in step S 20 , methods for coating the surface of the first coating layer with the material with high ionic conductivity, to form the second coating layer includes a hydrothermal method, a solvent-thermal method, a liquid phase precipitation method, a high energy ball milling method, or a high-temperature melting-casting method. Specific operation parameters of the methods may be determined based on an actual condition. This is not particularly limited in the present invention. 
     In this implementation of the present invention, in step S 20 , the material with high ionic conductivity includes at least one of lithium fluoride (LiF) and an oxide solid-state electrolyte. Specifically, the oxide solid-state electrolyte includes one or more of a crystalline-state perovskite-type solid-state electrolyte, a crystalline-state NASICON-type solid-state electrolyte, a crystalline-state LISICON-type solid-state electrolyte, a garnet-type solid-state electrolyte, and a glass-state oxide solid-state electrolyte. Specifically, the oxide solid-state electrolyte includes but is not limited to Li 3 PO 4 , Li 2 O, Li 6 BaLa 2 Ta 2 O 12 , Li 7 La 3 Zr 2 O 12 , Li 5 La 3 Nb 2 O 12 , Li 5 La 3 M 2 O 12  (M=Nb,Ta), Li 7+x A x La 3-x Zr 2 O 12  (A=Zn), Li 3 Zr 2 Si 2 PO 12 , Li 5 ZrP 3 O 12 , Li 5 TiP 3 O 12 , Li 3 Fe 2 P 3 O 12 , Li 4 NbP 3 O 12 , Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP), and the like. 
     In this implementation of the present invention, the second coating layer completely coats the first coating layer, and the surface of the first coating layer provides countless nucleation sites for attaching and bonding of the materials with high ionic conductivity. The materials with high ionic conductivity are used to perform uniform attaching and bonding on the surface of the first coating layer by using the nucleation sites, to form a uniform-thickness second coating layer. 
     In this implementation of the present invention, a thickness of the second coating layer  12  is 10 nm to 200 nm, and may be specifically 20 nm to 30 nm, 50 nm to 100 nm, 80 nm to 150 nm, or 120 nm to 180 nm. 
     The method for preparing a silicon-based composite anode material provided in this embodiment of the present invention is easy to implement and facilitates large-scale production. 
     In addition, an embodiment of the present invention further provides an energy storage device, including a cathode, an anode, and a separator located between the cathode and the anode. The anode includes the silicon-based composite anode material in the foregoing embodiment of present invention. The energy storage device includes a lithium-ion battery, a sodium ion battery, a magnesium ion battery, an aluminum ion battery, or a supercapacitor. 
     The following further describe the embodiments of the present invention by using a plurality of embodiments. 
     Embodiment 1 
     This embodiment provides a method for preparing a silicon-based composite anode material (Si@DAAQ-TFP@LATP), and a method for assembling Si@DAAQ-TFP@LATP as a lithium ion battery anode into a lithium secondary battery: 
     S 10 . Prepare Si@DAAQ-TFP 
     Commercial nano-silicon of a median particle size of 100 nm and DAAQ and TFP with a stoichiometric ratio of 1:1 are dissolved in a mixed solvent of N,N-dimethylacetamide and mesitylene, to obtain a mixed solution, and the mixed solution is left in reaction at 80° C. to 140° C. for 1 to 7 days in a sealed anaerobic condition. After the solution cools to a room temperature, centrifugal separation is performed on obtained materials to obtain solids, and the solids are washed by sequentially using N,N-dimethylformamide (DMF) and acetone. Nano-silicon coated with DAAQ-TFP, namely, Si@DAAQ-TFP is obtained once the solids dry. 
     S 20 . Prepare Si@DAAQ-TFP@LATP 
     10 g Si@DAAQ-TFP is added into 100 mL deionized water, and after ultrasonic dispersion, lithium acetate dihydrate (Li(CH 3 COO).2H 2 O) of molar concentration of 0.26 mol/L, aluminum nitrate (Al(NO 3 ) 3 .9H 2 O) of molar concentration of 0.6 mol/L, and ammonium dihydrogen phosphate (NH 4 H 2 PO 4 ) of molar concentration of 0.6 mol/L are sequentially added into the water. Magnetical stirring is performed at a room temperature to implement complete dissolution, so that a mixed solution is obtained. 5 mL acetylacetone is added into the mixed solution and stirred for 15 minutes, and then titanium butoxide with a stoichiometric ratio of 0.34 mol/L is dropwise added and stirred for another 2 hours, to obtain Si@DAAQ-TFP@LATP sol. The sol maintains static for 24 hours for aging, and an obtained gel is dried in vacuum at 100° C. for 6 hours. Finally, the temperature is risen to 700° C. at 5° C./min, namely, for 2 hours, to obtain Si@DAAQ-TFP coated with Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP), namely, Si@DAAQ-TFP@LATP composite anode material. 
     Prepare a Lithium Secondary Battery 
     The Si@DAAQ-TFP@LATP composite anode material obtained from preparation in this embodiment and commercial graphite G49 are mixed into a 600 mAh/g anode material. The anode material and a conductive agent Super P, binder styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC) are dispersed in deionized water at a mass ratio of 95:0.3:3.2:1.5, and uniformly stirred to obtain an electrode slurry. The electrode slurry is coated on a surface of a copper foil, and the foil is dried at 85° C. to obtain an anode plate. A pouch cell battery of about 3.7 Ah is produced using the anode plate as an anode a commercial lithium cobalt oxide as an anode, a 1 mol/L LiPF6/EC+PC+DEC+EMC (a volume ratio 1:0.3:1:1) electrolyte as the electrolyte, and a PP/PE/PP three-layer separator of a thickness of 10 μm as a separator, so as to test full battery performance of the material. 
     Embodiment 2 
     This embodiment provides a method for preparing a silicon-based composite anode material (SiO@DABQ-TFP@LLZO), and a method for assembling SiO@DABQ-TFP@LLZO as a lithium ion battery anode material into a lithium secondary battery: 
     S 10 . Prepare SiO@DABQ-TFP 
     SiO of a particle size of 1 μm to 10 μm and 2,5-diamino-1,4-dihydroxybenzo (DABH) and TFP with a stoichiometric ratio of 7:2 are dissolved in a mixed solvent of N,N-dimethylacetamide and mesitylene, to obtain a mixed solution, and the mixed solution is left in reaction at 85° C. to 120° C. for 1 to 7 days in a sealed anaerobic condition. After the solution cools to a room temperature, centrifugal separation is performed on obtained materials to obtain solids, and the solids are washed by sequentially using N,N-dimethylformamide (DMF) and tetrahydrofuran. A SiO@DABH-TFP material is obtained once the solids dry. The SiO@DABH-TFP is gradually added into triethylamine to obtain a suspension. The suspension is stirred for 6 to 24 hours at a room-temperature open atmosphere to oxidize, and is leached. After leaching, a filter cake is washed by using tetrahydrofuran, acetone and methanol, and a SiO material coated with DABQ-TFP, namely, SiO@DABQ-TFP is obtained once the filter cake dries. 
     S 20 . Prepare SiO@DABQ-TFP@LLZO 
     Li 2 CO 3 , La 2 O 3  and ZrO(NO 3 ) 2 .6H 2 O are prepared as starting materials, and the materials are put into water at a molar ratio of 7.7:3:2 and dissolve in the water. pH is adjusted to 7, to obtain an LLZO precursor compound solution. The SiO@DABQ-TFP sample is dispersed in the LLZO precursor compound solution and thoroughly mixed, and the solution is filtered, to obtain solids. After the obtained solids are dried, the solids are sintered at 450° C. for 16 hours (at an argon atmosphere), to obtain a SiO@DABQ-TFP material coated with LLZO, namely, a SiO@DABQ-TFP@LLZO composite anode material. 
     Prepare a Lithium Secondary Battery 
     The SiO@DABQ-TFP@LLZO composite anode material obtained from preparation in this embodiment and commercial graphite G49 are mixed into a 600 mAh/g anode material. The anode material and a conductive agent carbon black, a binder (SBR), and CMC are dispersed in deionized water at a mass ratio of 95:0.3:3.2:1.5, and uniformly stirred to obtain an electrode slurry. The electrode slurry is coated on a surface of a copper foil, and the foil is dried at 85° C. to obtain an anode plate. A pouch cell battery of about 3.7 Ah is produced using the anode plate as an anode, a commercial lithium cobalt oxide as a cathode, a 1 mol/L LiPF6/EC+PC+DEC+EMC (a volume ratio 1:0.3:1:1) electrolyte as the electrolyte, a PP/PE/PP three-layer separator (of a thickness of 10 μm) as a separator, so as to test full battery performance of the material. 
     Comparative Embodiment 
     Commercial nano-silicon of a median particle size of 100 nm and commercial artificial graphite G49 are mixed into a 600 mAh/g anode material. The anode material and a conductive agent Super P, a binder SBR, and CMC are dispersed in deionized water at a mass ratio of 95:0.3:3.2:1.5, and uniformly stirred to obtain an electrode slurry. The electrode slurry is coated on a surface of a copper foil, and the foil is dried at 85° C. to obtain an anode plate. A pouch cell battery of about 3.7 Ah is produced, for performance testing, using the anode plate as an anode, a commercial lithium cobalt oxide as a cathode, a 1 mol/L LiPF6/EC+PC+DEC+EMC (a volume ratio is 1:0.3:1:1) electrolyte as the electrolyte, and a PP/PE/PP three-layer separator of a thickness of 10 μm as a separator. 
     Effect Embodiment 
     1. Table 1 shows a comparison among physicochemical parameters of the Si@DAAQ-TFP@LATP composite anode material in Embodiment 1 of the present invention, the SiO@DABQ-TFP@LLZO composite anode material in Embodiment 2 of the present invention, and the commercial nano-silicon: 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                 Semi-electrode 
               
               
                   
                 Surface area 
                 Tap density 
                 plate expansion 
               
               
                 Item 
                 m 2 /g 
                 g/cm 3   
                 rate % 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Si@DAAQ-TFP@LATP 
                 72 
                 0.6 
                 22% 
               
               
                 SiO@DABQ-TFP@LLZO 
                 68 
                 0.58 
                 23% 
               
               
                 Commercial nano-silicon 
                 80 
                 0.54 
                 30% 
               
               
                   
               
            
           
         
       
     
     The surface area is measured by using a gas adsorption BET principle; tap density is measured according to a GB5162 national standard; a semi-electrode plate expansion rate is a thickness increase rate, when an electrochemical cell is at a 50% battery power state (50% SOC), of a cathode (anode) plate compared with a cathode (anode) plate before formation. Usually, a thickness of the electrode plate before formation under 50% SOC is measured through a micrometer, and the expansion rate is calculated, where the expansion rate essentially reflects expansion of an active material. 
     It can be learned from data in Table 1 that the silicon-based composite anode materials with a double-layer coating layer structure in Embodiment 1 and Embodiment 2 of the present invention have an apparent advantage over the nano-silicon material. 
     (1) In Embodiment 1 and Embodiment 2 of the present invention, the expansion rates of the semi-electrode plate of the silicon-based composite anode material are 22% and 23% respectively, which are significantly improved compared with an expansion rate of 30% of the semi-electrode plate of the nano-silicon in the comparative embodiment. This is because the quinone-aldehyde covalent organic framework material of the first coating layer of the silicon-based composite anode material in this embodiment of the present invention has superior toughness and a regular ordered porous pore structure, can effectively absorb mechanical stress generated by expansion of the silicon-based material core, and can ensure integrity of the coating layer, functioning as a sponge. For the electrochemical cell, the silicon-based composite anode material can effectively alleviate impact caused by volume contraction of the silicon material on a volume of the electrochemical cell during an electrochemical lithium intercalation and deintercalation process of the silicon-based material. In addition, structural stability of the coating layer of the silicon-based material can be ensured, and interface performance of the anode and electrochemical cycle performance of the electrochemical cell can be improved. 
     (2) The silicon-based composite anode materials in Embodiment 1 and Embodiment 2 of the present invention have a lower surface area than the nano-silicon in the comparative embodiment because the coating layer directly coats on a surface of an original nano-silicon particle. To be specific, the particle size of the coated material increases, and the coating layer material effectively fills pores on the surface of the nano-silicon particle. As a result, a surface area is smaller as a whole. For the electrochemical cell, for an active material of a low surface area, a contact area of the surface of the particle and the electrolyte can be narrowed, thereby reducing a side reaction of the electrolyte in an electrochemical reaction (for example, an electrolyte solvent dissolves and generates gas (H 2 , O 2 ), and a SEI film forms), and improving cycle performance of the electrochemical cell as a whole. 
     (3) The silicon-based composite anode materials in Embodiment 1 and Embodiment 2 of the present invention have higher tap density than that of the nano-silicon in the comparative embodiment. Because the fast ion conductor layer of the outer second coating layer is relatively rigid, stability and hardness of an overall structure of the coated silicon-based material particle are ensured. In battery production craftsmanship, relatively high tap density of a material corresponds to better processing performance of the electrode, can improve packing density of the active material in an electrode, and can further improve energy density of the electrochemical cell. In addition, the rigid coating layer can further effectively alleviate impact caused by volume contraction of the silicon material on structural stability of the coated particle during the electrochemical lithium intercalation and deintercalation process of the silicon-based material, thereby preventing collapse, pulverization and shedding of the particle structure and improving electrochemical cycle performance of the electrochemical cell. 
     2. Cycle performance testing is separately performed on the pouch cell battery prepared in Embodiment 1 of the present invention, the pouch cell battery in Embodiment 2 of the present invention, and the pouch cell battery prepared in the comparative embodiment under the following conditions: a same electrochemical cell type (386174), a same capacity (about 3.7 Ah), same current density (0.7 C), and a test temperature (25° C.). Testing results are shown in  FIG. 3 , where curves 1, 2, 3 represent battery cycle curves of the pouch cell batteries prepared in Embodiment 1, Embodiment 2 and the comparative embodiment respectively. As shown in  FIG. 3 , capacity retention rates of the lithium ion batteries prepared in Embodiment 1, Embodiment 2, and the comparative embodiment after 50 cycles are respectively 97.5%, 96.2%, and 87.1%, and cycle performance of electrochemical cells, in Embodiment 1 and Embodiment 2, prepared using the silicon-based composite anode material of a double-coating layer structure in the present invention is significantly better than the electrochemical cell of the commercial nano-silicon in the comparative embodiment. This indicates that a silicon-based material coated with a two-dimensional quinone-aldehyde covalent organic framework material and a material with high ionic conductivity performs better in coating, has higher electrical conductivity and ionic conductivity, a lower expansion rate and better structural stability, and this is a root cause of improvement of cycle performance.