CN118231605A - Silicon-carbon composite material, preparation method thereof, negative plate, lithium ion battery and product - Google Patents

Abstract

The invention relates to the technical field of lithium ion batteries, and discloses a silicon-carbon composite material, a preparation method and application thereof. The silicon-carbon composite material comprises microspheres containing carbon shell layers and silicon cores, and the silicon element content ratio is gradually reduced from the centers of the microspheres to the carbon shell layers. The preparation method of the silicon-carbon composite material comprises the following steps: (1) Reacting the silicon nano particles, a carbon source, lauryl phosphate and/or citric acid and a solvent to obtain a product 1; (2) calcining the product 1 under anaerobic conditions. The silicon-carbon composite material has lower volume expansion rate and higher cycle stability. The silicon-carbon composite material provided by the invention is applied to a lithium ion battery, and has lower volume expansion rate and higher cycle stability.

Description

Silicon-carbon composite material, preparation method thereof, negative plate, lithium ion battery and product

Technical Field

The invention relates to the technical field of lithium ion batteries, in particular to a silicon-carbon composite material, a preparation method thereof, a negative plate, a lithium ion battery and a product.

Background

The lithium ion battery has the advantages of high energy density, long cycle life, small pollution, no memory effect and the like, and becomes the first choice of the power system of the new energy automobile industry. Currently, the main current cathode materials in the market are still graphite cathode materials, and the lower energy density of the graphite cathode materials is difficult to meet the increasing energy density requirements of people on power batteries. Therefore, finding a negative electrode material for a next generation lithium ion battery is becoming important. The silicon-based anode material is a novel anode material, has the advantages of ultrahigh theoretical specific capacity (4200 mAh/g), proper lithium intercalation potential, abundant and easily available resources, low cost and the like, and is expected to become an anode material of a next-generation lithium ion battery. However, during intercalation/deintercalation, silicon-based materials are subjected to a large volume expansion (300%), resulting in pulverization of their structure, and eventually irreversible capacity loss.

At present, the main methods for synthesizing the silicon-based negative electrode are solid-phase grinding sintering or vapor deposition, and the structure of a final product is difficult to regulate and control in the synthesis process, so that satisfactory physicochemical properties and electrochemical properties are difficult to obtain.

Disclosure of Invention

The invention aims to solve the problem that the physical and chemical properties and the electrochemical properties of a silicon-based negative electrode in the prior art are poor in application due to large volume expansion rate, and provides a silicon-carbon composite material, a preparation method thereof, a negative electrode plate, a lithium ion battery and a product.

In order to achieve the above object, a first aspect of the present invention provides a silicon-carbon composite material, wherein the composite material comprises microspheres comprising a carbon shell layer and a silicon core, and the silicon element content ratio gradually decreases from the center of the microspheres to the carbon shell layer.

In a second aspect, the present invention provides a method for preparing a silicon carbon composite material, wherein the method comprises:

(1) Reacting the silicon nano particles, a carbon source, lauryl phosphate and/or citric acid and a solvent to obtain a product 1;

(2) And roasting the product 1 under the anaerobic condition to obtain the silicon-carbon composite material.

In a third aspect, the present invention provides a silicon carbon composite material produced by the above method.

The fourth aspect of the invention provides a negative electrode sheet comprising the silicon carbon composite material of the invention.

A fifth aspect of the invention provides a lithium ion battery comprising the negative electrode sheet of the invention.

In a sixth aspect, the invention provides a product comprising a lithium ion battery of the invention;

the product is a battery pack, an automobile, an electronic product and electric equipment.

Through the technical scheme, the silicon-carbon composite material comprises the microsphere with the carbon shell layer and the silicon core, wherein the silicon element content ratio is gradually reduced from the center of the microsphere to the carbon shell layer, and the silicon-carbon composite material is beneficial to relieving the problem of battery capacity loss caused by the volume expansion rate of the silicon-based material when the silicon-carbon composite material is applied to a lithium ion battery. As can be seen from table 1 and fig. 2 of the present invention, when the silicon-carbon composite material of the present invention is applied to the negative electrode material of the lithium ion button cell, the expansion rate of the negative electrode sheet after 50 cycles of charge-discharge is lower than 35%, which is far lower than the expansion rate of the negative electrode sheet of the commercial silicon-carbon product and the silicon-based material without carbon shell layer in the application comparative example; and after 100 circles of charge-discharge cycles, the residual capacity of the lithium ion button cell using the silicon-carbon composite material is larger than 1000mAh/g, which is far higher than that of the lithium ion button cell using the commercial silicon-carbon product and the silicon-based material without the carbon shell layer in the comparative example. In addition, in the preparation method of the silicon-carbon composite material, citric acid is utilized to promote the generation of a core-shell structure, lauryl phosphate is utilized as a surfactant to further promote the generation of the core-shell structure of the composite material, and the content ratio of silicon element is gradually reduced from the center of the microsphere to the carbon shell layer; in addition, the step (1) of the preparation method adopts a vacuum stirring device, so that the obtained silicon-carbon composite material has lower volume expansion rate and higher cycle stability.

Drawings

FIG. 1 is a TEM photograph of a silicon carbon composite A prepared in example 1;

FIG. 2 is a graph comparing expansion rates after 50 cycles of a button cell using pole pieces comprising the silicon carbon composites prepared in examples 1-5 and the commercially available silicon carbon product of comparative example 1, respectively;

Detailed Description

The endpoints and any values of the ranges disclosed herein are not limited to the precise range or value, and are understood to encompass values approaching those ranges or values. For numerical ranges, one or more new numerical ranges may be found between the endpoints of each range, between the endpoint of each range and the individual point value, and between the individual point value, in combination with each other, and are to be considered as specifically disclosed herein.

The following describes specific embodiments of the present invention in detail with reference to the drawings. It should be understood that the detailed description and specific examples, while indicating and illustrating the invention, are not intended to limit the invention.

The first aspect of the invention provides a silicon-carbon composite material, wherein the composite material comprises microspheres containing carbon shell layers and silicon cores, and the silicon element content ratio gradually decreases from the centers of the microspheres to the carbon shell layers.

According to the present invention, the silicon-carbon composite material includes a microsphere having a core-shell structure, the microsphere including a plurality of nano-silicon particles as a core and a carbon layer as a shell. In the silicon-carbon composite material, the silicon core is nanocrystallized, the carbon material is coated, and the content ratio of silicon element is gradually reduced from inside to outside, so that the silicon-carbon composite material is favorable for volume expansion of the silicon core, and has higher first coulomb efficiency, first reversible capacity and cycle stability when being applied to a lithium ion battery anode material.

In the present invention, the smallest dimension unit of the composite is defined as a microsphere, i.e. a large number of microspheres constitute the composite of the present invention.

According to the invention, the silicon-carbon composite material is ensured to have higher first discharge specific capacity and first coulombic efficiency, and meanwhile, the cycle stability of the silicon-carbon composite material is effectively improved, preferably, the total weight of the composite material is taken as the reference, the carbon content is 25-50 wt%, and the silicon content is 50-75 wt%.

In some embodiments of the invention, the carbon content is 37 wt%, 39 wt%, or 24 wt%, respectively, and the silicon content is 62 wt%, 60 wt%, or 76 wt%, respectively, based on the total weight of the composite material.

According to the present invention, in order to alleviate the problem of lithium intercalation expansion, to improve the cycle performance of the silicon-based material, the size of the silicon-based material may be reduced, preferably, the average particle diameter (D50) of the silicon-carbon composite material is 0.1 to 15 μm; preferably, the average particle diameter (D50) of the silicon carbon composite material is 2 to 8 μm.

Preferably, the thickness of the carbon shell layer is 0.5-25nm; preferably, the thickness of the carbon shell layer is 5-15nm.

In some embodiments of the invention, the average particle size of the silicon-carbon composite material is 6 μm, 2 μm, 13 μm, or 8 μm, respectively; the thickness of the carbon shell layer is 8nm or 2nm respectively.

According to the invention, in order to provide the silicon carbon composite material with a lower expansion rate and a better electrical conductivity in applications, it is preferred that the silicon core consists of silicon nanoparticles.

Preferably, the particle size of the silicon core is 0.09-12 μm.

Preferably, the average particle size of the silicon nanoparticles is 20-40 nm.

According to the invention, the composite material is preferably also doped with P in order to further increase the conductivity of the composite material.

Further preferably, the content of P is 0.2-1 wt%, based on the total weight of the composite material.

In some embodiments of the invention, the content of P is1 wt%, respectively, based on the total weight of the composite material.

In the invention, the preparation method of the silicon-carbon composite material can adopt the method provided by the second aspect of the invention.

In the invention, the composition and structure of the silicon-carbon composite material can be determined by means of TGA, EDS, XRD, SEM, TEM, characterization means and the like, and also can be determined by the feeding amount of each material in the preparation process.

In a second aspect, the present invention provides a method for preparing a silicon carbon composite material, wherein the method comprises:

(1) Reacting the silicon nano particles, a carbon source, lauryl phosphate and/or citric acid and a solvent to obtain a product 1;

(2) And roasting the product 1 under the anaerobic condition to obtain the silicon-carbon composite material.

The invention synthesizes the silicon-carbon anode material for the lithium ion battery by adopting a liquid phase self-assembly method, and the liquid phase method has the characteristics of wide sources of raw materials, various solvent selections, easy control of product structure and the like. The silicon-carbon composite material is prepared in a solution, and has a core-shell structure, wherein the outside is a carbon coating, and the inside is a silicon core composed of silicon nano particles. In the preparation process of the invention, the carbon source, the solvent and the surfactant act on the surfaces of the nano silicon particles together, which is favorable for the dispersion and interfacial solvation of the silicon nano particles, thereby forming a stable silicon core structure and a carbon layer precursor. In the later carbonization process, based on the Kendall effect, the silicon nano particles serving as cores in the prepared composite material are gradually reduced from inside to outside, and the prepared carbon composite material has excellent conductivity, excellent electrochemical performance and extremely low expansion rate.

According to the preparation method, in order to promote the generation of the core-shell structure of the composite material in the preparation process, the content ratio of silicon element is gradually reduced from inside to outside, so that the cycle stability of the composite material is improved, and preferably, in the step (1), citric acid is contained in the preparation raw material.

According to the preparation method, in order to promote the generation of the core-shell structure of the composite material in the preparation process, the content ratio of silicon element is gradually reduced from inside to outside, so that the doping of element phosphorus is realized, the electrochemical performance and the cycle stability of the composite material are improved, and preferably, in the step (1), the preparation raw material contains lauryl alcohol phosphate serving as a surfactant.

According to the preparation method of the invention, the carbon source is at least one of melamine, polyvinylpyrrolidone, dopamine and glucose.

Preferably, the carbon source is melamine.

Further preferably, in the step (1), the mass ratio of the silicon nanoparticles, the citric acid, the carbon source and the lauryl phosphate is (95-105): (3-8): (12-17): (1-4).

Further preferably, in the step (1), the mass ratio of the silicon nanoparticles, citric acid and melamine is (95-105): (1-4): (12-17).

Further preferably, in the step (1), the mass ratio of the silicon nanoparticles, the melamine to the lauryl phosphate is (95-105): (12-17): (4-8).

According to the preparation method of the invention, in order to enable the prepared silicon-carbon composite material to have low expansion rate and good electric conductivity in application, the average particle size of the silicon nano particles in the step (1) is preferably 20-40 nm.

According to the production method of the present invention, in order to smoothly perform the liquid phase reaction, the solvent used is desirably a weak polar solvent which does not contain an oxygen element and is not easily reacted with other raw materials, preferably, in the step (1), the solvent is an alkane or an aromatic hydrocarbon.

Further preferably, the solvent is at least one selected from the group consisting of n-hexane, n-pentane, n-heptane, cyclohexane, cyclopentane, toluene. Through the synergistic effect between the solvent, the surfactant and the carbon source, the uniform liquid phase preparation of the composite material is realized.

In some embodiments of the invention, in step (1), the solvent is cyclopentane.

According to the production method of the present invention, in order to prevent oxidation of the silicon material and improve electrochemical performance and cycle stability of the silicon-carbon anode material, preferably, in step (1), the reaction is performed under vacuum stirring.

Further preferably, the vacuum degree is from-5 to-80 kPa, and the stirring speed is from 300 to 700 rpm;

Preferably, the temperature of the reaction is 25-50 ℃ and the time is 1-3h.

In some embodiments of the invention, in step (1), the vacuum level of the reaction is-15 kPa or-20 kPa, respectively, and the stirring speed is 400 rpm, respectively.

According to the production method of the present invention, a better quality of the carbon layer is obtained. The material itself and P are prevented from being oxidized, and the baking condition in step (2) is preferably an oxygen-free atmosphere.

Preferably, the roasting temperature is 550-900 ℃ and the time is 2-5h.

In some embodiments of the present invention, the firing conditions in step (2) are respectively: argon is used as shielding gas, the roasting temperature is 750 ℃, and the roasting time is 3 hours.

In a third aspect, the present invention provides a silicon carbon composite material produced by the above method.

The fourth aspect of the invention provides a negative electrode sheet comprising the silicon carbon composite material of the invention.

A fifth aspect of the invention provides a lithium ion battery comprising the negative electrode sheet of the invention.

In a sixth aspect, the invention provides a product comprising a lithium ion battery of the invention;

the product is a battery pack, an automobile, an electronic product and electric equipment.

The following examples are presented to illustrate the technical scheme of the present invention, wherein all raw materials used are commercially available.

Silicon powder with purity of 99.9 is purchased from Shanghai Ala Biochemical technology Co., ltd.

Citric acid, 99.5 purity, purchased from Shanghai Ala Biochemical technologies Co., ltd.

Melamine, 99.9 purity, purchased from Shanghai Ala Biochemical technologies Co., ltd.

Lauryl phosphate with purity of 99 is purchased from Jiangsu province sea-An petrochemical plant.

Cyclopentane, 99 purity, was purchased from Shanghai Ala Biochemical technologies Co., ltd.

Ethanol, 99.8 purity, was purchased from Shanghai Ala Biochemical technologies Co., ltd.

The composition and structure of the silicon-carbon composite material are determined by TGA, EDS, XRD, SEM, TEM, XRF, ICP and the like, or calculated by the feeding amount in the specific examples.

Example 1

(1) Adding 10g of fully ground silicon powder with the particle size in the range of 20-40nm into a reaction kettle, respectively adding 0.5g of citric acid, 1.5g of melamine and 0.3g of lauryl phosphate, pouring 30mL of solvent cyclopentane, placing the reaction kettle into a vacuum stirring device, vacuumizing to-15 kPa, stirring for 1.5 hours at the rotating speed of 400 rpm, transferring the reaction kettle into a 200 ℃ oven, reacting for 18 hours, washing for multiple times with deionized water and ethanol, and vacuum drying to obtain a product 1;

(2) And (3) putting the product 1 into a tube furnace, taking Ar gas as a protective gas (the oxygen content is lower than 0.5%), heating to 750 ℃ at a heating rate of 5 ℃/min, and preserving the heat for 3 hours to obtain the silicon-carbon composite material A.

TEM test is carried out on the silicon-carbon composite material A to obtain a TEM photo shown in figure 1, and as can be seen from the photo shown in figure 1, the composite material A has a core-shell structure, the average particle size is 6 mu m, the shell thickness of the composite material A is 8nm on average, and the average particle size of a silicon core is 5.5 mu m.

The method comprises the steps of performing DES-Maping test on the silicon-carbon composite material A, obtaining that the shell layer of the composite material A is composed of carbon, the inner core is composed of silicon, and sequentially selecting three points on a path from a silicon core to the surface of a carbon layer to test the content of silicon element respectively, wherein the mass ratio of silicon element gradually decreases from the microsphere center of the composite material A to the carbon shell layer as shown in figure 1.

XRF testing was performed on the silicon carbon composite a, and from the test results, elemental phosphorus was doped in the composite a, with the content of doped P being 1% based on the total weight of the composite a.

TGA test was performed on the silicon carbon composite a or calculated from the feed amount of example 1, to obtain: the silicon content was 62 wt% and the carbon content was 37 wt% based on the total weight of composite a.

Example 2

(1) Adding 10g of fully ground silicon powder with the particle size in the range of 20-40nm into a reaction kettle, respectively adding 1.5g of melamine and 0.3g of lauryl phosphate, pouring 30mL of solvent cyclopentane into the reaction kettle, placing the reaction kettle into a vacuum stirring device, vacuumizing to-15 kPa, stirring at the rotating speed of 400 rpm for 1.5 hours, transferring the reaction kettle into a 200 ℃ oven, reacting for 18 hours, washing for multiple times with deionized water and ethanol, and vacuum drying to obtain a product 1;

(2) And (3) putting the product 1 into a tube furnace, taking Ar gas as a protective gas (the oxygen content is lower than 0.5%), heating to 750 ℃ at a heating rate of 5 ℃/min, and preserving the heat for 3 hours to obtain the silicon-carbon composite material B.

TEM test is carried out on the silicon-carbon composite material B, and the test result shows that the composite material B has a core-shell structure, the average grain diameter is 2 mu m, the shell thickness of the composite material B is 2nm on average, and the average grain diameter of a silicon core is 1.8 mu m.

The DES-Maping test is carried out on the silicon-carbon composite material B, and the test result shows that the shell layer of the composite material B consists of carbon, the inner core consists of silicon, and in addition, three points are sequentially selected on the path from the silicon core to the surface of the carbon layer to test the content of silicon element respectively, and the test result shows that the mass ratio of the silicon element gradually decreases from the microsphere center of the composite material B to the carbon shell layer.

XRF testing was performed on the silicon carbon composite B, and from the test results, elemental phosphorus was doped in the composite B, with the content of doped P being 1% based on the total weight of the composite B.

TGA test was performed on the silicon carbon composite material B or calculated from the feed amount of example 2, to obtain: the carbon content was 39 wt% and the silicon content was 60 wt% based on the total weight of composite B.

Example 3

(1) Adding 10g of fully ground silicon powder with the particle size in the range of 20-40nm into a reaction kettle, respectively adding 0.5g of citric acid and 1.5g of melamine, pouring 30mL of solvent cyclopentane, stirring for 1.5 hours at the rotation speed of 400 rpm, transferring the reaction kettle into a 200 ℃ oven, reacting for 18 hours, washing for multiple times with deionized water and ethanol, and vacuum drying to obtain a product 1;

(2) And (3) putting the product 1 into a tube furnace, heating to 750 ℃ at a heating rate of 5 ℃/min by taking Ar gas as a protective gas, and preserving the heat for 3 hours to obtain the silicon-carbon composite material C.

TEM test is carried out on the silicon-carbon composite material C, and the test result shows that the composite material C has a core-shell structure, the average particle size is 13 mu m, the shell thickness of the composite material C is 2nm on average, and the average particle size of a silicon core is 12 mu m.

The method comprises the steps of performing DES-Maping test on a silicon-carbon composite material C, obtaining from test results that a shell layer of the composite material C comprises carbon, a core comprises silicon and carbon dioxide, and three points are sequentially selected on a path from a silicon core to the surface of the carbon layer to test the content of silicon element respectively.

XRF testing was performed on the silicon carbon composite C, and from the test results, elemental phosphorus was doped in the composite C, with the content of doped P being 1% based on the total weight of the composite C.

TGA test or calculation of the charge amount of example 3 was performed on the silicon carbon composite material C to obtain: the carbon content was 37 wt%, the silicon content was 41 wt%, the silica content was 21 wt% and the doped phosphorus content was 1wt%, based on the total weight of the composite material C.

Example 4

(1) Adding 10g of fully ground silicon powder with the particle size in the range of 20-40nm into a reaction kettle, respectively adding 1g of citric acid, 1g of melamine and 0.3g of sodium dodecyl benzene sulfonate, pouring 30mL of solvent cyclopentane, placing the reaction kettle into a vacuum stirring device, vacuumizing to-20 kpa, stirring at the rotating speed of 400 rpm for 1.5 hours, transferring the reaction kettle into a 200 ℃ oven, reacting for 18 hours, washing with deionized water and ethanol for multiple times, and vacuum drying to obtain a product 1;

(2) And (3) putting the product 1 into a tube furnace, taking Ar gas as a protective gas (the oxygen content is lower than 0.5%), heating to 750 ℃ at a heating rate of 5 ℃/min, and preserving the heat for 3 hours to obtain the silicon-carbon composite material D.

TEM test is carried out on the silicon-carbon composite material D, and the test result shows that the composite material D has a core-shell structure, the average particle size is 8 mu m, the shell thickness of the composite material D is 8nm, and the average particle size of a silicon core is 7.5 mu m.

And (3) performing a DES-Maping test on the silicon-carbon composite material D, wherein the shell layer of the composite material D is composed of carbon, the inner core is composed of silicon, and three points are sequentially selected on a path from a silicon core to the surface of a carbon layer to test the content of silicon element respectively.

XRF testing was performed on the silicon carbon composite D, and from the test results, elemental phosphorus was not doped in the composite D.

TGA test was performed on the silicon carbon composite material D or calculated from the feed amount of example 4, to obtain: the carbon content was 24 wt% and the silicon content was 76 wt% based on the total weight of the composite material D.

Comparative example 1

(1) Adding 10g of fully ground silicon powder with the particle size in the range of 20-40nm into a reaction kettle, adding 0.3g of lauryl phosphate, pouring a proper amount of cyclopentane, stirring at the rotation speed of 400 rpm for 1.5 hours, transferring the reaction kettle into a 200 ℃ oven, reacting for 18 hours, washing for multiple times with deionized water and ethanol, and vacuum drying to obtain a product 1;

(2) And (3) putting the product 1 into a tube furnace, heating to 750 ℃ at a heating rate of 5 ℃/min by taking Ar gas as a protective gas, and preserving the heat for 3 hours to obtain the silicon material E.

TEM and DES-Maping tests are carried out on the silicon-carbon composite material E, so that the composite material E has a core-shell structure, the average grain diameter is 7 mu m, the shell material of the composite material E is carbon, the thickness is 1nm, the coating is uneven, the core material is silicon and silicon dioxide, the average grain diameter is 6.9 mu m, and the density of the silicon-silicon dioxide core is close from the center of the microsphere of the composite material E to the carbon shell layer. In addition, XRF characterizes composite a as doped with elemental phosphorus and present in an amount of 1%.

TGA test was performed on silicon material E or calculated from the dose of comparative example 1 to give: the silicon content was 96.5% by weight and the carbon content was 2.5% by weight, based on the total weight of the composite E.

Comparative example 2

Commercially available silicon carbon products.

Test case

And (3) manufacturing a battery: the products A, B, C, D, E and the commercial silicon-carbon products prepared in the examples and the comparative examples are taken as anode active materials and are mixed with acetylene black and sodium carboxymethyl cellulose according to the mass ratio of 8:1:1, uniformly mixing, and coating the mixture on a copper foil to obtain a working negative plate; a metal lithium sheet is used as a counter electrode; the PE/PP composite membrane is used as an ion exchange membrane, and a button cell is manufactured by adopting a conventional method in the field.

And (3) testing charge and discharge performance: the button cell was discharged to 0.005V at normal temperature with a constant current of 0.5A, then charged to 1.5V with a constant current of 0.5mA, the discharge capacity and the charge capacity of the cell were recorded, and the charge-discharge efficiency (%) =charge capacity/discharge capacity×100% was calculated.

Expansion test:

The original thickness of each set of sample pole pieces was measured with a micrometer prior to assembly of the button cell. After 50 cycles, discharging to 0SOC, disassembling the button cell, taking out the negative plate, cleaning with DMC solution, air-drying, performing thickness test, and calculating the expansion ratio = (thickness after cycle-thickness before cycle)/thickness before cycle × 100%.

Battery testing instrument: LAND blue electric battery test system.

The test results are shown in table 1:

TABLE 1

Numbering device	First reversible capacity (mA h/g)	First coulombic efficiency (%)	Residual capacity after 100 circles (mA.h/g)
				Example 1	1850	89	1670
Example 2	1900	82	1400
				Example 3	1700	84	1100
Example 4	1800	85	1230
				Comparative example 1	2000	72	800
Comparative example 2	1900	75	500

As can be seen from Table 1, when the silicon-carbon composite materials prepared in examples 1 to 4 and the silicon material prepared in comparative example 1 were applied to lithium ion button cells, respectively, the initial coulombic efficiency of the silicon-carbon composite materials prepared in examples 1 to 4 was higher than that of the silicon-carbon composite materials prepared in comparative example 1 under the same other test conditions, and the residual capacity after 100 cycles was still higher, which means that the coating of the carbon shell layer in the silicon-carbon composite material was favorable for inhibiting the expansion rate of the silicon-based material and improving the cycle performance of the composite material.

Comparing the silicon-carbon composite material prepared in example 1 with the silicon-carbon composite material prepared in example 2, the silicon-carbon composite material prepared in example 1 has higher initial coulombic efficiency under the same other test conditions, and the residual capacity after 100 circles is higher than that of the composite material prepared in example 2, which indicates that the citric acid added in step (1) in example 1 can lead the silicon-carbon composite material to have lower expansion rate and better electrochemical performance, and the citric acid in the raw material not only serves as a carbon source, but also can promote the generation of a core-shell structure of the composite material. The effect of citric acid can be further verified from the pole piece expansion rate shown in figure 2.

Comparing the composite material prepared in example 1 with the composite material prepared in example 3, the silicon-carbon composite material prepared in example 1 has higher initial coulombic efficiency under the same other test conditions, the residual capacity after 100 cycles is higher than that of the composite material prepared in example 3, and the vacuum reaction conditions in step (1) of example 1 can prevent the silicon material from oxidizing into silicon dioxide, so that the core composed of silicon nanoparticles in the composite material has lower expansion rate (as shown in fig. 2).

Comparing the silicon-carbon composite material prepared in example 1 with the silicon-carbon composite material prepared in example 4, the silicon-carbon composite material prepared in example 1 has higher initial coulombic efficiency under the same other test conditions, the residual capacity after 100 circles is higher than that of the composite material prepared in example 4, and the puffing rate of the composite material can be reduced by doping of elemental phosphorus (as shown in fig. 2).

As can be seen from the above examples and comparative examples, the coating of the carbon shell layer is advantageous for reducing the expansion rate of the silicon-based material; gradually reducing the content of silicon element from the center of the composite microsphere to the carbon shell layer further relieves the volume expansion of the silicon-based material; in addition, the doping of the phosphorus element is beneficial to reducing the expansion rate of the silicon-based material and improving the cycle stability of the composite material. According to the preparation method provided by the invention, the reaction under the vacuum condition can effectively prevent the oxidation of the silicon-based material, and the prepared silicon-carbon composite material has good electrochemical performance and good cycling stability.

The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited thereto. Within the scope of the technical idea of the invention, a number of simple variants of the technical solution of the invention are possible, including combinations of the individual technical features in any other suitable way, which simple variants and combinations should likewise be regarded as being disclosed by the invention, all falling within the scope of protection of the invention.

Claims (14)

1. A silicon-carbon composite material, which is characterized by comprising a microsphere containing a carbon shell layer and a silicon core, wherein the silicon element content ratio gradually decreases from the center of the microsphere to the carbon shell layer.

2. The composite of claim 1, wherein the carbon is present in an amount of 24-50 wt% and the silicon is present in an amount of 50-76 wt%, based on the total weight of the composite.

3. The composite material according to claim 1 or 2, wherein the microspheres have an average particle size of 0.1-15 μm;

and/or the thickness of the carbon shell layer is 0.5-25nm;

And/or, the silicon core is composed of silicon nanoparticles;

Preferably, the average particle diameter of the silicon nanoparticles is 20-40nm.

4. A composite material according to any one of claims 1-3, wherein the carbon shell is doped with P;

preferably, the content of P is 0.2-1 wt%, based on the total weight of the composite material.

5. A method of making a silicon carbon composite, the method comprising:

(1) Reacting the silicon nano particles, a carbon source, lauryl phosphate and/or citric acid and a solvent to obtain a product 1;

(2) And roasting the product 1 under the anaerobic condition to obtain the silicon-carbon composite material.

6. The method according to claim 5, wherein in the step (1), the carbon source is at least one of melamine, polyvinylpyrrolidone, dopamine, and glucose.

7. The method of claim 5 or 6, wherein in step (1), the mass ratio of the silicon nanoparticles, citric acid, carbon source, and lauryl phosphate is (95-105): (3-8): (12-17): (1-4);

And/or the mass ratio of the silicon nano particles, the citric acid and the carbon source is (95-105): (1-4): (12-17);

and/or the mass ratio of the silicon nano particles, the carbon source and the lauryl phosphate is (95-105): (12-17): (4-8).

8. The method according to any one of claims 5 to 7, wherein in step (1), the silicon nanoparticles have an average particle diameter of 20 to 40nm;

and/or the solvent is alkane or aromatic hydrocarbon;

preferably, the solvent is at least one selected from n-hexane, n-pentane, n-heptane, cyclohexane, cyclopentane, toluene.

9. The process according to any one of claims 5-8, wherein in step (1), the reaction is carried out under vacuum agitation;

Preferably, the vacuum degree is-5 to-80 kPa, and the stirring speed is 300-700 rpm;

and/or the reaction temperature is 25-50 ℃ and the reaction time is 1-3h.

10. The method according to any one of claims 5 to 9, wherein in the step (2), the oxygen-free condition is an argon atmosphere, and the oxygen content is equal to or less than 0.5%;

Or/and, the roasting temperature is 550-900 ℃ and the time is 2-5h.

11. A silicon carbon composite material made by the method of any one of claims 5-10.

12. A negative electrode sheet, characterized in that the negative electrode sheet comprises a silicon-carbon composite material;

wherein the silicon carbon composite is the silicon carbon composite of any one of claims 1-4 and 11.

13. A lithium ion battery comprising the negative electrode sheet of claim 12.

14. A product comprising the lithium ion battery of claim 13;

the product is a battery pack, an automobile, an electronic product and electric equipment.