Composite material with core-shell structure as well as preparation and application of composite material

Abstract

The invention discloses a composite material with a core-shell structure as well as preparation and application of the composite material. The size of the composite material of the core-shell structure is nanoscale, the core is a nano material, the nano material is nano silicon, nano carbon, nano metal simple substance or nano metal oxide, the metal simple substance is tin or germanium, the metal oxide is titanium oxide or manganese oxide, and the shell layer comprises a transition metal silicate layer. The invention provides an application of the composite material with the core-shell structure as a lithium ion battery negative electrode active material, and also provides a lithium ion battery, the lithium ion battery comprises a negative electrode, and the negative electrode is obtained by coating a current collector with negative electrode slurry and then drying. The negative electrode slurry comprises the composite material with the core-shell structure, a conductive agent and a binder. The lithium ion battery taking the composite material as a negative electrode active material has good cycling stability and rate capability.

Description

Technical Field

[0001] The present invention belongs to the field of lithium-ion battery electrode materials and their preparation and applications, and particularly relates to a composite material with a core-shell structure, a preparation method thereof, and an application as a negative electrode active material for lithium-ion batteries. Background Art

[0002] Lithium-ion batteries have the advantages of long cycle life, high energy density, no memory effect, etc., and have been widely used in fields such as portable electronic products and electric vehicles. With the rapid development of electric vehicles, higher requirements for the energy density of lithium-ion batteries have been put forward. Currently, the theoretical specific capacity of commercial graphite negative electrode materials is relatively low, and it can no longer meet the needs of people for higher-capacity and high-rate lithium-ion batteries. Silicon (Si) has a theoretical specific capacity as high as 4200 mAh g -1 and is considered to be one of the most potential negative electrode materials for the next generation. Elements germanium and tin in the same main group as silicon are also negative electrode materials that are often studied at present. However, such alloying negative electrode materials have large volume changes during the cycling process, which will cause the pulverization of the electrode materials, resulting in the loss of electrical contact and a rapid decline in capacity. At the same time, under the repeated action of volume changes, the SEI film formed during the first charge and discharge process will also break and form a new SEI film, further consuming lithium ions and electrolyte. Among them, silicon materials have poor conductivity and slow lithium ion transport, which will lead to low cycling efficiency during the cycling process and poor charge and discharge performance at high current densities, and cannot meet the commercial requirements.

[0003] In order to effectively solve these problems of such alloying negative electrode materials when used as the negative electrode of lithium-ion batteries, researchers have made many attempts. For example, the silicon-based materials among them are designed into structures with flexibility and adaptability (such as porous structures, nanometerization treatment) to relieve volume expansion; silicon, germanium, and tin materials are compounded with conductive materials (such as carbon nanotubes, graphene, etc.); surface modification of such materials is carried out, such as carbon coating, oxide coating, etc. Surface modification and carbon coating of such materials can effectively relieve the volume expansion problem and improve their cycling performance. Although the above-mentioned various modification methods have improved the performance of silicon, germanium, and tin negative electrode materials, there are still some unsolved problems at present. For example, insufficient conductivity makes it difficult to ensure sufficient electron transport efficiency under high-rate conditions; the uniformity and thickness of carbon coating are difficult to effectively control; the interfacial stability with the electrolyte is poor; the uniformity and consistency of the product are poor during process scaling-up, which still limits its wide commercialization in practical applications.

[0004] Transition metal silicates have the potential to become excellent anode materials for lithium-ion batteries due to their wide sources, high specific capacity, environmental friendliness, etc. Coating such alloying materials with a shell containing transition metal silicate (MSiO) to prepare a core-shell structure with silicon, germanium, or tin particles as the core and a shell containing a transition metal silicate component can fully utilize the role of the silicate component and improve the performance of the battery. Summary of the Invention

[0005] To solve the problems of poor conductivity and short cycle life of such alloying anode materials, the present invention provides a composite material with a core-shell structure having a shell containing transition metal silicate (MSiO), and also provides its preparation method and application. The first object of the present invention is to provide a composite material with a shell containing transition metal silicate, aiming to improve the cycle life, conductivity, and rate performance of lithium-ion batteries.

[0006] The second object of the present invention is to provide a preparation method for the composite material with a shell containing transition metal silicate having the above structure.

[0007] The third object of the present invention is to provide an application of the composite material with a shell containing transition metal silicate having the above structure as an anode active material for lithium-ion batteries.

[0008] The fourth object of the present invention is to provide a lithium-ion battery containing the composite material with a shell containing transition metal silicate having the above structure.

[0009] To achieve the above-mentioned invention objects, the present invention adopts the following technical solutions:

[0010] In the first aspect, the present invention provides a composite material with a core-shell structure. The size of the composite material is nanoscale, where the core is a nanomaterial, and the nanomaterial is nanosilicon, nanocarbon, nanometallic element, or nano-metal oxide. The metallic element is tin or germanium, and the metal oxide is titanium oxide or manganese oxide. The shell layer includes a transition metal silicate layer.

[0011] For the composite material with a core-shell structure of the present invention, there may be obvious voids or no obvious voids between the core and the shell layer depending on the preparation method.

[0012] In the present invention, when the nanomaterial is nanocarbon, the nanocarbon can be nanoscale Super P carbon black, nanoscale graphite, carbon nanotubes, etc.

[0013] As an implementation manner, the shell layer is composed of a transition metal silicate layer.

[0014] As another implementation manner, the shell layer further includes a carbon layer, and the carbon layer is located inside or outside the transition metal silicate layer.

[0015] As another implementation manner, the shell layer further includes two carbon layers, which are respectively located inside and outside the transition metal silicate layer.

[0016] The transition metal in the transition metal silicate according to the present invention may be iron, cobalt, nickel, copper, manganese, etc., and cobalt is preferred.

[0017] For the composite material with a core-shell structure according to the present invention, the shell layer can provide a buffer space for the volume expansion of the core during cycling, improving the cycle life of the battery; and during cycling, the transition metal in the shell layer can increase the conductivity of the material, improving the rate performance of the battery; the carbon layer further improves the conductivity of the material and can also serve as a protective layer to avoid direct contact between the electrolyte and the inner core material, reducing the occurrence of side reactions.

[0018] In a second aspect, the present invention provides a preparation method of the composite material with a core-shell structure as described in the first aspect, and the preparation method includes the following steps:

[0019] Step 1: Obtain a silica-coated nanomaterial, where the nanomaterial is nanosilicon, nanocarbon, nanometallic element or nano-metal oxide, the metallic element is tin or germanium, and the metal oxide is titanium oxide or manganese oxide;

[0020] Step 2: In an aqueous solvent, uniformly mix the silica-coated nanomaterial obtained in Step 1 with a water-soluble transition metal salt, ammonium chloride and ammonia water, and generate a transition metal silicate layer on the surface of the silica-coated nanomaterial through a hydrothermal reaction. During the reaction process, silica is gradually etched and converted into transition metal silicate. After sufficient reaction, the nanomaterial@transition metal silicate, that is, a composite material with a core-shell structure, is obtained through separation and drying.

[0021] In Step 1 of the present invention, the silica-coated nanomaterial can be obtained in various ways, and the thickness of the silica coating layer determines the gap size between the core and the shell layer of the composite material with a core-shell structure.

[0022] As an implementation manner of Step 1, Step 1 is specifically implemented as follows: The nanomaterials and the surfactant are dispersed in a mixed solvent of absolute ethanol and water (preferably, the volume ratio of absolute ethanol to water is 3:1 - 5:1, more preferably 4:1). Subsequently, ammonia water and tetraethyl orthosilicate are added, and the mixture is stirred and reacted at room temperature. After sufficient reaction, it is separated and dried to obtain the nanomaterials coated with silica. The surfactant is used to regulate the surface activity of the nanomaterials. The surfactant is preferably cetyltrimethylammonium bromide (CTAB), and its addition amount is generally controlled at 30% - 60% of the mass of the nanomaterials. The purpose of adding ammonia water in this implementation manner is to provide the alkaline environment required for the hydrolysis of tetraethyl orthosilicate. Preferably, the feeding ratio of ammonia water to the mixed solvent is 1:250 - 1:125 in terms of the volume ratio of ammonia water with a mass fraction ≥ 28% to the mixed solvent; the volume ratio of tetraethyl orthosilicate (TEOS) to the mixed solvent is 1:375 - 1:100. This implementation manner can control the thickness of the silica coating layer, thereby controlling the void size between the core and the shell layer in the composite material with a core-shell structure. Preferably, the feeding mass-volume ratio of the nanomaterials to tetraethyl orthosilicate is 150 - 300 mg: 0.4 - 1.5 mL. Preferably, the stirring reaction time at room temperature is 5 - 8 h.

[0023] As another implementation manner of Step 1, Step 1 is specifically implemented as follows: The nanomaterial is nano-silicon. The nano-silicon is calcined at a high temperature in an air atmosphere to form a silica layer on its surface, thereby obtaining the nano-silicon material coated with silica. Preferably, the high-temperature calcination conditions are: calcination is carried out in an air atmosphere, the heating rate is 3 - 10 °C / min, the calcination temperature is 600 - 900 °C, and the heat preservation time is 1 - 6 h. The silica layer obtained by this implementation manner is relatively thin. Therefore, in the finally obtained composite material with a core-shell structure, the void between the core and the shell layer is not obvious.

[0024] In Step 2 of the present invention, the feeding ratio of the silica-coated nanomaterials, the water-soluble transition metal salt, and ammonium chloride is 100 mg: 0.8 - 1 mmol: 0.8 - 1 mmol. The ammonia water is used to provide an alkaline etching environment. Preferably, the feeding ratio of the ammonia water to the water solvent is 1:50 - 63 in terms of the volume ratio of ammonia water with a mass fraction ≥ 28% to the water solvent. The hydrothermal reaction conditions are preferably: carried out at 120 - 180 °C for 6 - 12 h. During the hydrothermal reaction process, the transition metal silicate layer is gradually formed, and the silica layer is gradually etched.

[0025] As a preferred implementation manner, the preparation method of the composite material with a core-shell structure further includes the following Steps 1 - 2:

[0026] Disperse the silica-coated nanomaterials and surfactants obtained in Step 1 in a mixed solvent of anhydrous ethanol and deionized water (the volume ratio is preferably 1:1 - 3, more preferably 1:2). Add resorcinol, ammonia water, and formaldehyde. Utilize the reaction of resorcinol and formaldehyde under alkaline conditions to form a polymer layer on the surface of the silica-coated nanomaterials, and then carbonize the polymer layer into a carbon layer by high-temperature calcination to obtain a carbon-coated product, namely nanomaterials@SiO2@C; Replace the silica-coated nanomaterials in Step 2 with the obtained carbon-coated product to perform the operation of Step 2, and obtain nanomaterials@C@transition metal silicate, namely a composite material with a core-shell structure containing a carbon layer.

[0027] Furthermore, in Steps 1 - 2, the surfactant is preferably cetyltrimethylammonium bromide (CTAB), and the mass ratio of the silica-coated nanomaterials to the surfactant is 1:8 - 10; the mass ratio of resorcinol to the silica-coated nanomaterials obtained in Step 1 is 1.5 - 2.5:1; the feeding ratio of ammonia water to the mixed solvent is calculated as the volume ratio of ammonia water with a mass fraction ≥ 28% to the mixed solvent, which is 1:300 - 800, and further preferably 1:300; the feeding ratio of formaldehyde to the mixed solvent is calculated as the volume ratio of a 37 wt.% formaldehyde solution to the mixed solvent, which is 1:200 - 1:100.

[0028] Furthermore, in Steps 1 - 2, the reaction conditions are as follows: First, stir and react at 35 - 50 °C for 6 - 8 h, and then stand at room temperature for 8 - 12 h.

[0029] Furthermore, in Steps 1 - 2, the high-temperature calcination temperature is 700 - 900 °C, and the holding calcination time is 2 - 4 h.

[0030] Furthermore, the preparation method of the composite material with the core-shell structure further includes the following Step 2 - 2:

[0031] Disperse the nanomaterials@C@transition metal silicate obtained in Step 2 in a mixed solvent of anhydrous ethanol and deionized water. Add resorcinol, ammonia water, and formaldehyde. Utilize the reaction of resorcinol and formaldehyde under alkaline conditions to form a polymer layer on the surface of the nanomaterials@C@transition metal silicate, and then carbonize the polymer layer into a carbon layer by high-temperature calcination to obtain nanomaterials@C@transition metal silicate@C, which is a composite material with a core-shell structure containing two carbon layers.

[0032] The raw material feeding ratios and reaction conditions in Step 2 - 2 can refer to the above Steps 1 - 2 and will not be elaborated here.

[0033] As another preferred embodiment, the preparation method of the composite material with the core-shell structure further includes the following Step 3:

[0034] Disperse the nanomaterial@transition metal silicate obtained in Step 2 in a mixed solvent of absolute ethanol and deionized water, add resorcinol, ammonia water and formaldehyde, and use the reaction of resorcinol and formaldehyde under alkaline conditions to form a polymer layer on the surface of the nanomaterial@transition metal silicate, and then carbonize the polymer layer into a carbon layer by high-temperature calcination to obtain nanomaterial@transition metal silicate@C, that is, a composite material with a core-shell structure containing a carbon layer.

[0035] The raw material feeding ratio and reaction conditions in Step 3 can refer to the above Steps 1-2 and will not be elaborated here.

[0036] In a third aspect, the present invention provides an application of the composite material with the core-shell structure described in the first aspect as a negative electrode active material for a lithium-ion battery.

[0037] In a fourth aspect, the present invention provides a lithium-ion battery, the lithium-ion battery includes a negative electrode, and the negative electrode is obtained by drying after coating a negative electrode slurry on a current collector, and the negative electrode slurry includes the composite material with the core-shell structure described in the first aspect, a conductive agent and a binder.

[0038] Preferably, the current collector is a copper foil.

[0039] Preferably, the conductive agent is conductive carbon black.

[0040] Preferably, the binder is carboxymethyl cellulose (CMC).

[0041] Preferably, the feeding mass ratio of the composite material containing transition metal silicate in the shell layer, the conductive agent and the binder is 60-80%: 6-30%: 6-30%.

[0042] The positive electrode and electrolyte of the lithium-ion battery described in the present invention can be selected according to the conventional method, and the lithium-ion battery can be assembled according to the conventional method.

[0043] Compared with the prior art, the present invention has the following advantages:

[0044] A composite material with a core-shell structure containing transition metal silicate in the shell layer is prepared. The prepared composite material has a nanoscale size and a large specific surface area, providing more lithium active sites. The shell layer containing transition metal silicate can be used as a protective layer to buffer the volume expansion during the cycle and enhance the cycle life of the material. At the same time, the carbon layer can further enhance the conductivity of the silicate electrode material and significantly enhance the rate performance of the material. After carbon coating, the conductivity of the material is further improved, and it can also be used as a protective layer to avoid direct contact between the electrolyte and the interior of the material, reducing the occurrence of side reactions. Under the combined action of these factors, the lithium-ion battery using this composite material as the negative electrode active material has good cycle stability and rate performance. Description of the Drawings

[0045] Figure 1 Scanning electron microscopy images ((a)-(c)), transmission electron microscopy images ((d)-(e)), high-resolution transmission electron microscopy image (f), and element distribution map (g) of the pure-phase cobalt silicate (Co2SiO4) prepared in Comparative Example 1.

[0046] Figure 2 : (a) XRD pattern of the pure-phase cobalt silicate prepared in Comparative Example 1, (b) XRD pattern of the Si@void@Co2SiO4 prepared in Example 1, and (c)-(e) scanning electron microscopy images of the pure-phase cobalt silicate obtained at hydrothermal temperatures of 120, 150, and 180 °C.

[0047] Figure 3 Cycling performance graph (a) and rate performance graph (b) of the pure-phase cobalt silicate (Co2SiO4) prepared in Comparative Example 1.

[0048] Figure 4 : (a)-(b) Scanning electron microscopy images of the composite material with a transition metal silicate-containing shell (Si@void@Co2SiO4) prepared in Example 1, (c)-(d) transmission electron microscopy images of the composite material with a transition metal silicate-containing shell (Si@void@Co2SiO4) prepared in Example 1, and (e)-(f) transmission electron microscopy images of the cobalt silicate, carbon composite material (C@Co2SiO4) prepared in Comparative Example 2.

[0049] Figure 5 : (a)-(b) Cycling performance and rate performance graphs of the composite material with a transition metal silicate-containing shell (Si@void@Co2SiO4) prepared in Example 1. (c)-(d) Cycling performance and rate performance graphs of the carbon, cobalt silicate composite material (C@Co2SiO4@C) prepared in Comparative Example 3.

[0050] Figure 6 Cycling performance and rate performance graphs of the composite material with a transition metal silicate-containing shell (Si@void@C@Co2SiO4) prepared in Example 2.

[0051] Figure 7 Cycling performance graphs of the composite material with a transition metal silicate-containing shell (G@void@C@Co2SiO4) prepared in Example 6 and the composite material (Si@void@C@Ni2SiO4) prepared in Example 9.

[0052] Figure 8 Scanning electron microscopy, transmission electron microscopy, and cycling performance graphs of the composite material with a transition metal silicate-containing shell (Si@Co2SiO4@C) prepared in Example 14. Detailed implementation mode

[0053] The present invention will be further described in detail below in conjunction with specific embodiments. The protection scope of the present invention includes but is not limited to the following embodiments.

[0054] The present application provides a composite material with a shell structure, which is composed of a core and a shell covering the core. The size of the composite material is nanoscale, wherein the core is a nanomaterial, and the nanomaterial is nanosilicon, nanocarbon, nanometallic element or nanometallic oxide. The metallic element is tin or germanium, and the metallic oxide is titanium oxide or manganese oxide. The shell layer includes a transition metal silicate layer. The nanocarbon can be nanoscale SuperP carbon black, nanoscale graphite, carbon nanotubes, etc. The transition metal in the transition metal silicate can be iron, cobalt, nickel, copper, manganese, etc., preferably cobalt.

[0055] In some embodiments, the shell layer is composed of a transition metal silicate layer (MSiO).

[0056] In some embodiments, the shell layer is composed of a carbon layer and a transition metal silicate layer (MSiO), and the carbon layer is located inside (C@MSiO) or outside (MSiO@C) the transition metal silicate layer.

[0057] In some embodiments, the shell layer is composed of two carbon layers and a transition metal silicate layer (MSiO), and the two carbon layers are respectively located inside and outside the transition metal silicate layer (C@MSiO@C).

[0058] In some embodiments, the structure of the electrode material can be one of Core-shell and Yolk-shell. The Core-shell structure is composed of a central core and a shell layer, and there is no obvious gap between the central core and the shell layer. The shell layer material can isolate the direct contact between the inner core part and the electrolyte, reducing the occurrence of side reactions. And during the cycling process, the transition metal in the shell layer can increase the conductivity of the material, improving the rate performance of the battery. The carbon layer further improves the conductivity of the material and can also act as a protective layer to avoid the direct contact between the electrolyte and the inner core material, reducing the occurrence of side reactions. The Yolk-shell structure is composed of a central core and a separated outer shell, and there are obvious pores between the core and the shell, providing a larger buffer space for the volume expansion during the cycling process of the inner core and prolonging the cycling life of the material.

[0059] In the embodiments of the present invention, those not specified in specific conditions are carried out according to conventional conditions or conditions recommended by the manufacturer. For the reagents or instruments not specified in the manufacturer, they are all conventional products that can be obtained by conventional technical means or purchased commercially.

[0060] In the embodiments of the present invention, ammonia water and formaldehyde used are both commercially available products, where the mass fraction of ammonia water is ≥28 wt%, and formaldehyde is 37 wt% in H₂O, containing 10 - 15% methanol stabilizer.

[0061] Comparative Example 1:

[0062] 1. Dissolve 150 mg of CTAB in 120 mL of absolute ethanol and 20 mL of deionized water by ultrasonic and stirring, then add 1 mL of ammonia water. After stirring for 30 minutes, add 1 mL of tetraethyl orthosilicate (TEOS) dropwise to the above mixed solution, and stir at room temperature for 6 h to obtain a silica precursor.

[0063] 2. Disperse 0.1 g of the obtained silica precursor in 20 mL of deionized water by ultrasonic, denoted as solution A. Dissolve 1 mmol of cobalt chloride hexahydrate, 1 mmol of ammonium chloride, and 1 mL of ammonia water in 30 mL of deionized water by stirring, denoted as solution B. Drop solution A into solution B, stir evenly, transfer it to a 100 mL polytetrafluoroethylene hydrothermal autoclave for hydrothermal reaction. The hydrothermal temperature is 150 °C, and the hydrothermal time is 12 h. After centrifugation and drying, pure-phase cobalt silicate (Co₂SiO₄) is obtained.

[0064] Performance test: The active material (pure-phase cobalt silicate prepared in Comparative Example 1), conductive agent (Super-P), and binder (CMC) are fully mixed to prepare a slurry, which is coated on a copper foil and dried to obtain a negative electrode sheet of a lithium-ion battery; the mass ratio of the active material, conductive agent, and binder is 8:1:1. The prepared negative electrode sheet of the battery is stamped into an electrode, and a metallic lithium foil is used as the counter electrode. The electrolyte is 1 M LiPF₆ dissolved in ethylene carbonate (EC) and dimethyl carbonate (DMC) (volume ratio 1:1), and 10 wt% fluoroethylene carbonate (FEC) is added as an additive, and a CR 2032 type button battery is assembled in a glove box.

[0065] The cycle performance of the battery is tested on a Land-CT3001A type battery test system. Test conditions: Put the battery after standing for 12 h into a 25 °C constant temperature oven, and the voltage range is 0.01 V - 3 V. The rate performance of the battery is tested on a Land-CT3001A type battery test system. Test conditions: Put the battery after standing for 12 h into a 25 °C constant temperature oven, the voltage range is 0.01 V - 3 V, and the current intensities are 0.1, 0.2, 0.5, 1, 2, 3 Ag -1 。

[0066] Figure 1(a)-(c) of Comparative Example 1 show the scanning electron microscope images of the pure-phase cobalt silicate (Co2SiO4) prepared. It can be seen that the prepared spherical nanoparticles have a size of approximately 300 nm and uniform particle size, and the particle surface presents a nanoflower structure; Figure 1 (d)-(e) of show the transmission electron microscope images of the pure-phase cobalt silicate. It can be seen from the pictures that the prepared nanoparticles present a hollow structure, and cobalt silicate nanoflowers composed of nanosheets are distributed on the particle surface; Figure 1 (f) of shows the HRTEM image of the pure-phase cobalt silicate. A lattice fringe spacing of about 0.205 nm can be seen, indicating the existence of cobalt silicate. Figure 1 (g) of shows the element distribution map of cobalt silicate. It can be seen that the three elements Si, Co, and O are uniformly distributed in the hollow sphere structure.

[0067] Figure 3 (a) of is the electrochemical cycling performance graph of the pure-phase Co2SiO4 prepared in Comparative Example 1. It was activated at a current density of 0.1 A / g for the first 10 cycles, and a current density of 0.3 A / g was used for the remaining cycling weeks. After 100 cycles, the specific capacity was only 383.79 mAh / g. Figure 3 (b) of is the rate performance graph of the pure-phase Co2SiO4 prepared in Comparative Example 1. It can provide a specific capacity of 130 mAh / g at a current density of 3 A / g. After the current density is restored to the initial current, the specific capacity can completely return to the initial level.

[0068] As a control, the hydrothermal temperature in Step 2 of Comparative Example 1 was changed to 120 °C and 180 °C, and other conditions remained unchanged to prepare cobalt silicate. Figure 2 (a) of shows the XRD patterns of the pure-phase cobalt silicate (Co2SiO4) prepared at three hydrothermal temperatures of 120 °C, 150 °C, and 180 °C. It can be seen from the figure that the characteristic peaks of the prepared material correspond to the Co2SiO4 standard card, indicating that the prepared material is pure-phase cobalt silicate. Through the morphology analysis shown in (c)-(e), it was found that the hydrothermal reaction was incomplete at 120 °C, and the hollow structure of the material obtained at 180 °C was damaged; therefore, the optimal hydrothermal temperature was determined to be 150 °C.

[0069] Example 1:

[0070] 1. Ultrasonically disperse 300 mg of silicon nanoparticles (particle size 50 - 100 nm) and 150 mg of CTAB (cetyltrimethylammonium bromide) into 120 mL of absolute ethanol and 30 mL of deionized water, then add 1 mL of ammonia water (commercial product, ≥28 wt% NH3 in H2O) and stir evenly. Slowly add 1 mL of TEOS (tetraethyl orthosilicate) dropwise to the above solution. After stirring at room temperature for 6 h, centrifuge and dry to obtain the Si@SiO2 precursor.

[0071] 2. 0.1 g of the Si@SiO2 precursor was ultrasonically dispersed in 20 mL of deionized water, denoted as solution A. 1 mmol of cobalt chloride hexahydrate, 1 mmol of ammonium chloride, and 1 mL of ammonia water were dissolved in 30 mL of deionized water by stirring, denoted as solution B. Solution A was added dropwise to solution B, stirred evenly, transferred to a 100 mL polytetrafluoroethylene hydrothermal autoclave for hydrothermal reaction. The hydrothermal temperature was 150 °C and the hydrothermal time was 12 h. After centrifugation and drying, a silicon-based composite material was obtained, denoted as Si@void@Co2SiO4.

[0072] Battery assembly: The active material (Si@void@Co2SiO4 prepared in Example 1), conductive agent (Super-P), and binder (CMC) were fully mixed to prepare a slurry, which was coated on a copper foil and dried to obtain a negative electrode sheet of a lithium-ion battery. The mass ratio of the active material, conductive agent, and binder was 8:1:1. The prepared negative electrode sheet of the battery was stamped into an electrode, and a metallic lithium foil was used as the counter electrode. The electrolyte was 1 M LiPF6 dissolved in ethylene carbonate (EC) and dimethyl carbonate (DMC) (volume ratio 1:1), and 10 wt% of fluoroethylene carbonate (FEC) was added as an additive. A CR 2032 type coin cell was assembled in a glove box. The test methods for the cycling performance and rate performance of the battery were the same as those in Comparative Example 1.

[0073] Figure 2 (b) is the XRD pattern of Si@void@Co2SiO4. It can be seen from the figure that the characteristic peaks of the prepared composite material with a shell containing transition metal silicate correspond to the characteristic peaks of nanosilicon and cobalt silicate, indicating the successful composite of nanosilicon and cobalt silicate.

[0074] Figure 4 (a)-(d) show the scanning electron microscope and transmission electron microscope images of Si@void@Co2SiO4. It can be seen from the scanning electron microscope image that the prepared composite material with a shell containing transition metal silicate is spherical in structure, the particle size is about 300 nm and the size is uniform, and there is a nanoscale flower-like structure on the surface. It can be seen from the transmission electron microscope image that it is a core-shell structure with nanosilicon as the core and cobalt silicate as the shell.

[0075] Figure 5 (a)-(b) The results show that: it was activated with a current density of 0.1 A / g in the first 10 cycles, and after 100 cycles with a current density of 0.3 A / g in the remaining cycles, the specific capacity was still 760 mAh / g, which is higher than the specific capacity of the pure-phase cobalt silicate prepared in Comparative Example 1. The rate performance graph shows that a specific capacity of 220 mAh / g can be provided at a current density of 3 A / g, and the specific capacity is still 835 mAh / g when the current density is restored to the initial 0.1 A / g.

[0076] Example 2:

[0077] 1. 300 mg of silicon nanoparticles (particle size 50 - 100 nm) and 150 mg of CTAB were ultrasonically dispersed in 120 mL of absolute ethanol and 30 mL of deionized water. Then, 1 mL of ammonia water was added and stirred evenly. 1 mL of TEOS was added dropwise to the above solution. After stirring at room temperature for 6 h, it was centrifuged and dried to obtain the Si@SiO2 precursor.

[0078] 2. 0.1 g of the Si@SiO2 precursor and 900 mg of CTAB were dispersed in 30 mL of deionized water and 15 mL of absolute ethanol by ultrasonic and stirring. Then, 160 mg of resorcinol and 0.15 mL of ammonia water were added to the above solution. After stirring in a 35 °C water bath for 30 min, 0.4 mL of formaldehyde was added, and stirring continued in the water bath for 6 h. Then, it was left standing at room temperature for 12 h, centrifuged and dried to obtain the carbon-coated precursor.

[0079] 3. The obtained carbon-coated precursor was calcined at high temperature in a tube furnace. First, argon was pre-passed for 30 min, then it was heated to 900 °C at a heating rate of 5 °C / min, held for 2 h, and after cooling to room temperature, the carbon-coated silica material, denoted as (Si@SiO2@C), was obtained.

[0080] 4. 0.1 g of the Si@SiO2@C precursor was ultrasonically dispersed in 20 mL of deionized water, denoted as solution A. 1 mmol of cobalt chloride hexahydrate, 1 mmol of ammonium chloride, and 1 mL of ammonia water were dissolved in 30 mL of deionized water by stirring, denoted as solution B. Solution A was added dropwise to solution B, stirred evenly, transferred to a 100 mL polytetrafluoroethylene hydrothermal autoclave for hydrothermal reaction. The hydrothermal temperature was 150 °C and the hydrothermal time was 12 h. After centrifugation and drying, a composite material with a shell containing transition metal silicate was obtained, denoted as Si@void@C@Co2SiO4.

[0081] The battery was assembled and the electrochemical cycling performance and rate performance of the battery were tested according to the method of Example 1. Figure 6 (a) in shows the electrochemical cycling performance diagram of the above-prepared CR 2032 type button battery. It was activated at a current density of 0.1 A / g for the first 10 weeks, and the current density of 0.3 A / g was used for the remaining cycling weeks. After 100 cycles, the specific capacity was still 871 mAh / g. Figure 6 (b) in shows that a specific capacity of 605 mAh / g could still be provided at a relatively large current density of 3 A / g, and when the current density was reduced to the original 0.1 A / g, the specific capacity basically recovered to the initial level.

[0082] Example 3:

[0083] Prepare the Si@void@C@Co2SiO4 composite material as a precursor according to the steps of Example 2, and then perform carbon coating treatment on the precursor according to Steps 2 and 3 of Example 2. The carbon content in the shell component of the material obtained in this example is higher, denoted as Si@void@C@Co2SiO4@C. Assemble the battery according to the method of Example 1 and test the electrochemical cycling performance and rate performance of the battery.

[0084] Example 4:

[0085] Prepare the material according to the steps of Example 2, replace the core material Si with germanium (Ge) powder (particle size 500 - 800 nm), and denote the obtained material as Ge@void@C@Co2SiO4. Assemble the battery according to the method of Example 1 and test the electrochemical cycling performance and rate performance of the battery.

[0086] Example 5:

[0087] Prepare the material according to the steps of Example 2, replace the core material Si with tin (Sn) powder (particle size 500 - 800 nm), and denote the obtained material as Sn@void@C@Co2SiO4. Assemble the battery according to the method of Example 1 and test the electrochemical cycling performance and rate performance of the battery.

[0088] Example 6:

[0089] Prepare the material according to the steps of Example 2, replace the core material Si with nano - graphite powder (Gr) (particle size 300 - 600 nm), and denote the obtained material as Gr@void@C@Co2SiO4. Assemble the battery according to the method of Example 1 and test the electrochemical cycling performance and rate performance of the battery.

[0090] Example 7:

[0091] Prepare the material according to the steps of Example 2, replace the core material Si with Super P carbon black (particle size 30 - 50 nm), and denote the obtained material as C@void@C@Co2SiO4. Assemble the battery according to the method of Example 1 and test the electrochemical cycling performance and rate performance of the battery.

[0092] Example 8:

[0093] Prepare the material according to the steps of Example 2, replace 1 mmol of cobalt chloride hexahydrate in Step 4 with 1 mmol of ferric chloride hexahydrate, so that the cobalt silicate component in the shell is transformed into iron silicate. Assemble the battery according to the method of Example 1 and test the electrochemical cycling performance and rate performance of the battery.

[0094] Example 9:

[0095] The materials were prepared according to the steps of Example 2. 1 mmol of cobalt chloride hexahydrate in Step 4 was replaced with 1 mmol of nickel chloride hexahydrate, so that the cobalt silicate component in the shell was transformed into nickel silicate. The battery was assembled according to the method of Example 1, and the electrochemical cycling performance and rate performance of the battery were tested.

[0096] Example 10:

[0097] The materials were prepared according to the steps of Example 2. 1 mmol of cobalt chloride hexahydrate in Step 4 was replaced with 1 mmol of manganese chloride tetrahydrate, so that the cobalt silicate component in the shell was transformed into manganese silicate. The battery was assembled according to the method of Example 1, and the electrochemical cycling performance and rate performance of the battery were tested.

[0098] Example 11:

[0099] The materials were prepared according to the steps of Example 2. 1 mmol of cobalt chloride hexahydrate in Step 4 was replaced with 1 mmol of copper chloride dihydrate, so that the cobalt silicate component in the shell was transformed into copper silicate. The battery was assembled according to the method of Example 1, and the electrochemical cycling performance and rate performance of the battery were tested.

[0100] Example 12:

[0101] 1. 300 mg of silicon nanoparticles (particle size 50 - 100 nm) were thoroughly ground in a mortar, then transferred to a porcelain boat and placed in a muffle furnace for calcination. The temperature was raised to 800 °C at a heating rate of 5 °C / min and held for 1 h, and then cooled to room temperature to obtain the Si@SiO2 precursor.

[0102] 2. 0.3 g of the Si@SiO2 precursor and 900 mg of CTAB were dispersed in 30 mL of deionized water and 15 mL of absolute ethanol by ultrasonic treatment and stirring. Then, 160 mg of resorcinol and 0.15 mL of ammonia water were added to the above solution. After stirring in a water bath at 35 °C for 30 min, 0.4 mL of formaldehyde was added, and stirring in the water bath was continued for 6 h. Then, it was left to stand at room temperature for 12 h, and the carbon-coated precursor was obtained by centrifugation and drying.

[0103] 3. The obtained carbon-coated precursor was subjected to high-temperature calcination in a tube furnace. Argon was pre-purged for 30 min first, then heated to 900 °C at a heating rate of 5 °C / min, held for 2 h, and after cooling to room temperature, the carbon-coated silica material, denoted as (Si@SiO2@C), was obtained.

[0104] 4. Disperse 0.1 g of the Si@SiO2@C precursor into 20 mL of deionized water by ultrasonic treatment, denoted as solution A. Dissolve 1 mmol of cobalt chloride hexahydrate, 1 mmol of ammonium chloride, and 1 mL of ammonia water in 30 mL of deionized water by stirring, denoted as solution B. Drop solution A into solution B, stir evenly, transfer it to a 100 mL Teflon hydrothermal reactor for hydrothermal reaction. The hydrothermal temperature is 150 °C and the hydrothermal time is 12 h. After centrifugation and drying, a composite material with a transition metal silicate-containing shell is obtained, denoted as Si@C@Co2SiO4. This composite material was tested by nitrogen adsorption and desorption, and has a large specific surface area of 155.94 m 2 / g.

[0105] Assemble the battery according to the method of Example 1 and test the electrochemical cycling performance and rate performance of the battery. The results show that: it was activated at a current density of 0.1 A / g for the first 10 cycles, and a current density of 0.3 A / g was used for the remaining cycles. After 100 cycles, the specific capacity was still 1368.8 mAh / g.

[0106] Example 13:

[0107] 1. Prepare the Si@SiO2 composite material according to step 1 of Example 12.

[0108] 2. Disperse 0.1 g of the Si@SiO2 precursor into 20 mL of deionized water by ultrasonic treatment, denoted as solution A. Dissolve 1 mmol of cobalt chloride hexahydrate, 1 mmol of ammonium chloride, and 1 mL of ammonia water in 30 mL of deionized water by stirring, denoted as solution B. Drop solution A into solution B, stir evenly, transfer it to a 100 mL Teflon hydrothermal reactor for hydrothermal reaction. The hydrothermal temperature is 150 °C and the hydrothermal time is 12 h. After centrifugation and drying, a composite material with a transition metal silicate-containing shell is obtained, denoted as Si@Co2SiO4. Assemble the battery and test the electrochemical cycling performance and rate performance of the battery according to the method of Example 1.

[0109] Example 14:

[0110] 1. Prepare the Si@Co2SiO4 composite material according to the steps of Example 13.

[0111] 2. Disperse 0.3 g of the Si@Co2SiO4 precursor and 900 mg of CTAB in 30 mL of deionized water and 15 mL of absolute ethanol by ultrasonic treatment and stirring. Then add 160 mg of resorcinol and 0.15 mL of ammonia water to the above solution. After stirring in a water bath at 35 °C for 30 min, add 0.4 mL of formaldehyde, continue stirring in the water bath for 6 h, then let it stand at room temperature for 12 h, and obtain a carbon-coated precursor after centrifugation and drying.

[0112] 3. The obtained carbon-coated precursor is subjected to high-temperature calcination in a tube furnace. First, argon is pre-purged for 30 min, then it is heated to 900 °C at a heating rate of 5 °C / min, held for 2 h, and after cooling to room temperature, the obtained composite material is denoted as (Si@Co2SiO4@C).

[0113] The battery was assembled and tested for its electrochemical cycling performance and rate performance according to the method of Example 1. The results showed that it was activated at a current density of 0.1 A / g for the first 10 cycles, and the current density of 0.3 A / g was used for the remaining cycles. After 100 cycles, the specific capacity was still 1598.5 mAh / g. Subsequently, long-cycle tests and rate performance tests were carried out. After 1000 cycles at a current density of 1 A / g, it could still provide a specific capacity of about 800 mAh / g; even at a large current density of 3 A / g, the specific capacity was still nearly 612.89 mAh / g.

[0114] Comparative Example 2:

[0115] 1. 150 mg of CTAB was dissolved in 120 mL of absolute ethanol and 20 mL of deionized water by ultrasonic treatment and stirring, then 1 mL of ammonia water was added, and after stirring for 30 minutes, 1 mL of tetraethyl orthosilicate (TEOS) was added dropwise to the above mixed solution, and it was stirred at room temperature for 6 h to obtain a silica precursor.

[0116] 2. 0.3 g of the silica precursor and 900 mg of CTAB were dispersed in 30 mL of deionized water and 15 mL of absolute ethanol by ultrasonic treatment and stirring, then 160 mg of resorcinol and 0.15 mL of ammonia water were added to the above solution. After stirring in a water bath at 35 °C for 30 min, 0.4 mL of formaldehyde was added, and stirring continued in the water bath for 6 h. Then it was left standing at room temperature for 12 h, and after centrifugation and drying, a carbon-coated silica precursor was obtained.

[0117] 3. The obtained carbon-coated silica precursor is subjected to high-temperature calcination in a tube furnace. First, argon is pre-purged for 30 min, then it is heated to 900 °C at a heating rate of 5 °C / min, held for 2 h, and after cooling to room temperature, a carbon-coated silica material is obtained, denoted as (SiO2@C).

[0118] 4. 0.1 g of the SiO2@C precursor was dispersed in 20 mL of deionized water by ultrasonic treatment, denoted as solution A. 1 mmol of cobalt chloride hexahydrate, 1 mmol of ammonium chloride, and 1 mL of ammonia water were dissolved in 30 mL of deionized water by stirring, denoted as solution B. Solution A was added dropwise to solution B, stirred evenly, transferred to a 100 mL polytetrafluoroethylene hydrothermal autoclave for hydrothermal reaction. The hydrothermal temperature was 150 °C and the hydrothermal time was 12 h. After centrifugation and drying, a carbon, cobalt silicate core-shell structure was obtained, denoted as C@Co2SiO4.

[0119] Figure 4 Panels (e)-(f) show the transmission electron microscope images of C@Co2SiO4. It can be seen from the figures that the prepared composite material has a large number of hollow sphere structures. The particle size is about 250 nm and the sizes are uniform. The spherical particles are composed of hollow carbon shells and cobalt silicate nanosheets loaded on the carbon shells.

[0120] The battery was assembled and the electrochemical cycling performance and rate performance of the battery were tested according to the method of Example 1. The results show that: it was activated at a current density of 0.1 A / g for the first 10 weeks, and after cycling 100 weeks at a current density of 0.3 A / g for the remaining cycles, the specific capacity was still 680 mAh / g, which is higher than that of pure phase cobalt silicate; the rate performance test shows that a specific capacity of 315 mAh / g can be provided at a current density of 3 A / g, and the specific capacity can also basically recover when the current density returns to the initial 0.1 A / g.

[0121] Comparative Example 3:

[0122] 1. 0.3 g of the product (C@Co2SiO4) obtained in Comparative Example 2 and 900 mg of CTAB were dispersed in 30 mL of deionized water and 15 mL of absolute ethanol by ultrasonic treatment and stirring. Then, 160 mg of resorcinol and 0.15 mL of ammonia water were added to the above solution. After stirring in a water bath at 35 °C for 30 min, 0.4 mL of formaldehyde was added, and stirring was continued in the water bath for 6 h. Then, it was left standing at room temperature for 12 h, and the carbon-coated product precursor was obtained by centrifugation and drying.

[0123] 2. The obtained precursor was calcined at high temperature in a tube furnace. Argon was pre-passed for 30 min first, and then it was heated to 900 °C at a heating rate of 5 °C / min, held for 2 h, and after cooling to room temperature, a carbon-coated composite material was obtained, denoted as (C@Co2SiO4@C).

[0124] The battery was assembled and the electrochemical cycling performance and rate performance of the battery were tested according to the method of Example 1. Figure 5 Panel (c) shows that: it was activated at a current density of 0.1 A / g for the first 10 weeks, and after cycling 100 weeks at a current density of 0.3 A / g for the remaining cycles, the specific capacity was still 770 mAh / g and the capacity had an upward trend; Figure 5 Panel (d) shows that: a specific capacity of about 460 mAh / g can still be provided at a large current density of 3 A / g, and the capacity can also basically recover to the initial level when the current density returns to 0.1 A / g.

[0125] Table 1

[0126]

[0127]

Claims

1. A composite material with a core-shell structure, characterized in that: The size of the composite material with a core-shell structure is nanoscale. The core is a nanomaterial, which can be nanosilicon, nanocarbon, nanometallic element or nano-metal oxide. The metallic element is tin or germanium, and the metal oxide is titanium oxide or manganese oxide. The shell layer includes a transition metal silicate layer.

2. The composite material with a core-shell structure according to claim 1, characterized in that: The shell layer is composed of a transition metal silicate layer; or the shell layer further includes a carbon layer, and the carbon layer is located inside or outside the transition metal silicate layer; or the shell layer further includes two carbon layers, and the two carbon layers are respectively located inside and outside the transition metal silicate layer.

3. The composite material with a core-shell structure according to claim 1, characterized in that: The transition metal in the transition metal silicate is iron, cobalt, nickel, copper or manganese; when the nanomaterial is nanocarbon, the nanocarbon is nanoscale SuperP carbon black, nanoscale graphite or carbon nanotube.

4. A method for preparing a composite material with a core-shell structure as described in claim 1, characterized in that: The preparation method includes the following steps: Step 1: Obtain a silica-coated nanomaterial, which can be nanosilicon, nanocarbon, nanometallic element or nano-metal oxide. The metallic element is tin or germanium, and the metal oxide is titanium oxide or manganese oxide. Step 2: In an aqueous solvent, uniformly mix the silica-coated nanomaterial obtained in Step 1 with a water-soluble transition metal salt, ammonium chloride and ammonia water. Through a hydrothermal reaction, a transition metal silicate layer is formed on the surface of the silica-coated nanomaterial. During the reaction process, silica is gradually etched and transformed into transition metal silicate. After sufficient reaction, it is separated and dried to obtain nanomaterial@transition metal silicate, which is a composite material with a core-shell structure.

5. The preparation method according to claim 4, characterized in that: Step 1 is specifically implemented in the following way: Disperse the nanomaterial and a surfactant in a mixed solvent of anhydrous ethanol and water, then add ammonia water and tetraethyl orthosilicate and stir at room temperature for reaction. After sufficient reaction, it is separated and dried to obtain a silica-coated nanomaterial. Or Step 1 is specifically implemented in the following way: The nanomaterial is nanosilicon. Nanosilicon is calcined at high temperature in an air atmosphere to form a silica layer on its surface, thereby obtaining a silica-coated nanosilicon material.

6. The preparation method according to claim 4, characterized in that: The preparation method of the composite material with a core-shell structure further includes the following Step 1-2: Disperse the silica-coated nanomaterial obtained in Step 1 and a surfactant in a mixed solvent of anhydrous ethanol and deionized water, add resorcinol, ammonia water and formaldehyde. Use the reaction of resorcinol and formaldehyde under alkaline conditions to form a polymer layer on the surface of the silica-coated nanomaterial, and then carbonize the polymer layer into a carbon layer through high-temperature calcination to obtain a carbon-coated product, that is, nanomaterial@SiO2@C; Replace the silica-coated nanomaterial in Step 2 with the obtained carbon-coated product and perform the operation of Step 2 to obtain nanomaterial@C@transition metal silicate, which is a composite material with a core-shell structure containing a carbon layer.

7. The preparation method according to claim 6, characterized in that: The preparation method of the composite material with a core-shell structure further includes the following Step 2-2: Disperse the nanomaterial@C@transition metal silicate obtained in step 2 in a mixed solvent of absolute ethanol and deionized water, add resorcinol, ammonia water and formaldehyde, and use the reaction of resorcinol and formaldehyde under alkaline conditions to form a polymer layer on the surface of the nanomaterial@C@transition metal silicate, and then carbonize the polymer layer into a carbon layer by high-temperature calcination to obtain nanomaterial@C@transition metal silicate@C, which is a composite material with a core-shell structure containing two carbon layers.

8. The preparation method according to claim 4, characterized in that: The preparation method of the composite material with the core-shell structure further includes the following step 3: Disperse the nanomaterial@transition metal silicate obtained in step 2 in a mixed solvent of absolute ethanol and deionized water, add resorcinol, ammonia water and formaldehyde, and use the reaction of resorcinol and formaldehyde under alkaline conditions to form a polymer layer on the surface of the nanomaterial@transition metal silicate, and then carbonize the polymer layer into a carbon layer by high-temperature calcination to obtain nanomaterial@transition metal silicate@C, that is, a composite material with a core-shell structure containing a carbon layer.

9. Application of the composite material with the core-shell structure according to claim 1 as a negative electrode active material for a lithium-ion battery.

10. A lithium-ion battery, the lithium-ion battery comprising a negative electrode, the negative electrode being obtained by drying after applying a negative electrode paste onto a current collector, characterized in that: The negative electrode slurry includes the composite material with the core-shell structure according to claim 1, a conductive agent and a binder.

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