CN112736232A

CN112736232A - Silicon-carbon composite material, preparation method thereof, negative plate and lithium ion secondary battery

Info

Publication number: CN112736232A
Application number: CN202110041752.2A
Authority: CN
Inventors: 陈云; 刘鹏; 褚春波; 张耀
Original assignee: Sunwoda Electric Vehicle Battery Co Ltd
Current assignee: Sunwoda Electric Vehicle Battery Co Ltd
Priority date: 2021-01-13
Filing date: 2021-01-13
Publication date: 2021-04-30

Abstract

The application discloses a silicon-carbon composite material, a preparation method thereof, a negative plate and a lithium ion secondary battery. The preparation method of the silicon-carbon composite material comprises the following steps: step one, grinding silicon powder in an oxygen atmosphere to obtain Si @ SiO_x(ii) a Step two, Si @ SiO_xAnd (5) carrying out carbon coating to obtain the silicon-carbon composite material. By adopting a grinding mode in an oxygen atmosphere, on one hand, the particle size of the silicon powder is integrally reduced, and the cycle performance of the material is improved; the other partyIn-situ generation of thin amorphous SiO on the surface of silicon powder_xA cladding layer that further efficiently buffers the volume expansion of silicon; subsequently on the formed Si @ SiO_xThe outer layer is uniformly coated with a layer of carbon material, so that the conductivity of the material can be enhanced, and the volume expansion of silicon in the reversible insertion and extraction processes of lithium ions can be further relieved; thereby leading the double-layer coated Si @ SiO_xthe/C silicon-carbon composite material has good electrochemical performance and energy density.

Description

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

Technical Field

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

Background

Lithium ion secondary batteries have rapidly monopolized the market for portable electronic products since their market introduction by Sony corporation of japan in 1991 due to their advantages of high specific energy, long charge-discharge life, no memory effect, low self-discharge rate, no pollution, safety and reliability, etc. In recent years, lithium ion secondary batteries have attracted much attention in new energy vehicles and energy storage fields, and have been developed explosively with an increased market concentration. However, with the rapid development of the 3C industry and the electric automobile field, the capacity of the graphite negative electrode used commercially at present has been developed to be very high (372mAh/g), and cannot meet the increasing demand, so that the development of a lithium ion secondary battery electrode material with high power, high energy density and high safety is the key point of the development of the lithium battery technology at present. The silicon-based material (silicon-containing oxide) is considered as an ideal negative electrode material of the next generation of high-performance lithium ion secondary battery due to the advantages of high specific capacity, moderate working potential, abundant storage capacity and environmental friendliness. However, silicon-based negative electrodes expand due to a large volume during charge and discharge (

/Li) causes structural destruction of the electrode and causes rapid deterioration of the battery capacity, which severely limits its wide application. The carbon material serving as the negative electrode material has small specific capacity, but not only has certain electrochemical activity, but also has a stable structure, and can be used as a 'buffer matrix' of a silicon-based material so as to solve the problems to a certain extent. Therefore, the preparation of the silicon-carbon composite anode material by combining the properties of both silicon and carbon is a very promising method.

Researchers take bamboo charcoal as a template, modify the surface of the bamboo charcoal and form pores by using metal chloride, further reduce the silicon dioxide in the bamboo charcoal into low-valent silicon oxide and simple substance silicon by calcining and a metal thermal reduction method, and simultaneously carbonize the silicon dioxide at high temperature to finally obtain the silicon-carbon composite material. Although the synthesis method is simpler, more impurity components may exist in the silicon-carbon negative electrode material system synthesized by the method, and the electrochemical performance of the material is influenced. In addition, the silicon-carbon composite material prepared by the method is of a porous structure, and has low compaction density and insufficient energy density.

In another method, inorganic acid is utilized to treat photovoltaic waste to obtain silicon powder, and then a magnesiothermic reduction method is adopted to prepare Si/SiO_xAnd finally, carrying out carbon coating by pyrolyzing acetylene gas at high temperature to obtain the silicon-carbon composite material. In the synthesis process of the method, on one hand, SiC in the photovoltaic material is difficult to dissolve in inorganic acid, so that more impurities may exist to influence the electrochemical performance of the material; on the other hand, the prepared porous silicon-carbon composite material also has the problems of low compaction density and insufficient energy density.

Therefore, there is a need for a silicon-carbon composite material having excellent electrochemical properties and high energy density.

Disclosure of Invention

The present application is directed to solving at least one of the problems in the prior art. Therefore, the application provides a silicon-carbon composite material with excellent electrochemical performance and high energy density, a preparation method thereof, a negative plate and a lithium ion secondary battery.

In a first aspect of the present application, a method for preparing a silicon-carbon composite material is provided, the method comprising the steps of:

step one, grinding silicon powder in an oxygen atmosphere to obtain silicon particles coated by silicon oxide, and marking the silicon particles as Si @ SiO_x；

Step two, Si @ SiO_xAnd (5) carrying out carbon coating to obtain the silicon-carbon composite material.

According to the preparation method of the embodiment of the application, at least the following beneficial effects are achieved:

by adopting the grinding mode in the oxygen atmosphere, on one hand, the particle size of the silicon powder is integrally reduced, the serious attenuation of the cycle performance of the prepared battery caused by obvious volume expansion of silicon with larger particles in the charging and discharging processes is avoided, and the cycle performance of the material is improved; on the other hand, the silicon powder is fully contacted with oxygen, and thin amorphous SiO is generated in situ on the surface of the silicon powder_xA cladding layer that further efficiently buffers the volume expansion of silicon; subsequently on the formed Si @ SiO_xThe outer layer is uniformly coated with a layer of carbon material, which not only can enhance the conductivity of the material and maintain the conductive network of the composite material, but also can further relieve the volume expansion of silicon in the reversible intercalation and deintercalation process of lithium ions; thereby leading the double-layer coated Si @ SiO_xthe/C silicon-carbon composite material has good electrochemical performance and energy density.

Wherein, Si @ SiO_xIs that the surface of Si particle is coated with SiO_xLayer, and for the embodiments of the present application, the SiO_xThe layer is generated in situ on the surface of the Si particles. SiO 2_xSiO in the layer_xRefers to amorphous silicon oxide, 0<x≤2。

According to some embodiments of the present application, the carbon coating comprises the following steps:

(1) reacting Si @ SiO_xMixing with a solution of an organic carbon source to obtain a silicon-carbon composite material precursor;

(2) and carbonizing the precursor of the silicon-carbon composite material to obtain the silicon-carbon composite material.

In solid phase coating, the carbon source is mixed with Si @ SiO_xThe mixing uniformity of (a) is poor and the effect of coating on performance improvement is limited. Although the problem of coating uniformity is solved to a certain extent by gas phase coating, the coating efficiency and safety need to be improved. Thus, the liquid-phase organic carbon source solution is used for the pair of Si @ SiO_xAnd (5) performing carbon coating.

According to some embodiments of the present application, the organic carbon source is at least one of tannic acid, dopamine, citric acid, polyvinylpyrrolidone (PVP), and cellulose.

According to the present applicationPlease note that in some embodiments, Si @ SiO_xThe mass ratio of the organic carbon source solution to the organic carbon source solution is 1: (15-100).

According to some embodiments of the present application, the concentration of the organic carbon source in the solution of the organic carbon source is 2 to 6.0 wt%.

According to some embodiments of the present application, Si @ SiO_xThe mass ratio of the carbon source to the organic carbon source is 1: (0.3-6).

According to some embodiments of the present application, in step (1), the mixing is performed by: reacting Si @ SiO_xDispersing in a buffer solution, adding a solution of an organic carbon source, adjusting the pH to 7.0-9.0, and stirring.

According to some embodiments of the present application, in the step (1), the stirring time is 4-24 hours.

According to some embodiments of the present application, in step (1), the Si @ SiO_xDispersing in a buffer solution, and then carrying out ultrasonic dispersion for 15-30 min.

According to some embodiments of the application, the buffer is at least one of Tris (hydroxymethyl) aminomethane (Tris) solution, barbiturate sodium-hydrochloric acid buffer, disodium hydrogen phosphate-citric acid buffer. The subsequent carbon coating is in a better environment by adding the buffer solution, so that the carbonization process of the carbon source is more complete, the carbon material coated on the outer layer is more uniform, and the electrochemical performance of the silicon-carbon composite material is further improved.

According to some embodiments of the present application, in the step (1), after the stirring, the product is washed, centrifuged, precipitated and dried to obtain the silicon-carbon composite material precursor.

According to some embodiments of the present application, in the step (1), the drying manner is at least one of forced air drying and vacuum drying.

According to some embodiments of the present application, in the step (1), the drying temperature is 80-90 ℃ and the drying time is 1-2 h.

According to some embodiments of the present application, in the step (2), the carbonization treatment is performed by: and carrying out heat treatment on the silicon-carbon composite material precursor in a protective atmosphere.

According to some embodiments of the present application, the protective atmosphere includes a nitrogen atmosphere and an inert gas atmosphere such as a helium atmosphere, a neon atmosphere, and the like.

According to some embodiments of the application, the heat treatment is performed by: heating to 400-600 ℃ at a heating rate of 1-5 ℃/min, then preserving heat for 2-5 h, then heating to 800-1200 ℃ at a heating rate of 5-10 ℃/min, and preserving heat for 3-24 h. The specific heating rate and the specific heat preservation time are strictly limited by a stage heating mode, so that the formed surface carbon layer is more uniformly coated, the thickness controllability is higher, the processing is easy, and the electrochemical performance of the material including the cycle performance is better.

According to some embodiments of the present application, Si @ SiO_xSieving with 400 mesh sieve before carbon coating. By keeping Si @ SiO in this way_xUniform particle size and regular morphology of the particles.

According to some embodiments of the present application, the sieving mode is a step sieving, i.e. Si @ SiO_xSequentially passing through two or more screens with continuously increasing meshes so as to remove Si @ SiO in a target particle size range_xScreening out as much as possible. The mesh number of the screens with different mesh numbers is more than 400 meshes.

According to some embodiments of the present application, the mesh number of the classifying screen is 400 mesh and 800 mesh, respectively.

According to some examples of the present application, the median particle size of the silicon powder is 0 < D50 ≦ 20 μm, and the purity of the silicon powder is 99.5% or more.

According to some embodiments of the present application, in the first step, the milling is ball milling. The particle size of the silicon powder is reduced as much as possible by ball milling for 6-24 h, so that the cycle performance of the silicon powder is improved efficiently.

According to some embodiments of the present application, in the first step, the ball milling is at least one selected from the group consisting of planetary ball milling, pendulum ball milling, and sand milling.

In a second aspect of the present application, there is provided a silicon carbon composite material prepared by the above-described preparation method.

In a third aspect of the present application, there is provided a negative electrode sheet comprising the above-described silicon-carbon composite.

According to some embodiments of the present application, the negative electrode sheet includes a current collector and an active material layer on a surface of the current collector, the active material layer including the above-described silicon carbon composite material or the silicon carbon composite material prepared by the above-described preparation method.

According to some embodiments of the present application, the active material layer further includes a binder and a conductive agent.

In a fourth aspect of the present application, there is provided a lithium ion secondary battery including the negative electrode sheet described above.

According to some embodiments of the present application, the lithium ion secondary battery further includes a positive electrode sheet, an electrolyte, and a separator.

Additional aspects and advantages of the present application will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the present application.

Drawings

The present application is further described with reference to the following figures and examples, in which:

FIG. 1 is Si @ SiO of example 1 of the present application_xX-ray diffraction pattern of the/C silicon-carbon composite material.

FIG. 2 is Si @ SiO of example 1 of the present application_xAnd (3) a transmission electron microscope image of the/C silicon-carbon composite material.

Fig. 3 is a particle size distribution diagram of the raw silicon powder of example 1 of the present application.

FIG. 4 is Si @ SiO of example 1 of the present application_xParticle size distribution profile of the composite.

FIG. 5 is Si @ SiO of example 1 of the present application_xAnd (3) a cycle performance diagram of the/C silicon-carbon composite material.

FIG. 6 is a graph of the cycle performance of the Si/C composite of comparative example 1 of the present application.

FIG. 7 shows Si @ SiO in a comparative experiment of the present application_xSilicon/carbon composite material and Si @ SiO_x-TiO₂And (3) a rate capability test result of the/C silicon-carbon composite material.

Detailed Description

The conception and the resulting technical effects of the present application will be clearly and completely described below in conjunction with the embodiments to fully understand the objects, features and effects of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, and not all embodiments, and other embodiments obtained by those skilled in the art without inventive efforts based on the embodiments of the present application belong to the protection scope of the present application.

Reference will now be made in detail to embodiments of the present application, examples of which are illustrated in the accompanying drawings. The embodiments described below with reference to the drawings are exemplary only for the purpose of explaining the present application and are not to be construed as limiting the present application.

In the description of the present application, the meaning of a plurality is one or more, the meaning of a plurality is two or more, and the above, below, exceeding, etc. are understood as excluding the present number, and the above, below, within, etc. are understood as including the present number. If the first and second are described for the purpose of distinguishing technical features, they are not to be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated or implicitly indicating the precedence of the technical features indicated.

In the description of the present application, unless otherwise expressly limited, terms such as set, mounted, connected and the like should be construed broadly, and those skilled in the art can reasonably determine the specific meaning of the terms in the present application by combining the detailed contents of the technical solutions.

In the description of the present application, reference to the description of the terms "one embodiment," "some embodiments," "an illustrative embodiment," "an example," "a specific example," or "some examples," etc., means that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the present application. In this specification, the schematic representations of the terms used above do not necessarily refer to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

Example 1

The embodiment provides a preparation method of a silicon-carbon composite material and the silicon-carbon composite material prepared by the preparation method, and the preparation method comprises the following steps:

the method comprises the following steps: si @ SiO_xPreparation of

7g of silicon powder (particle size D50: 19 μm) was placed in a ball mill pot, oxygen gas was charged into the ball mill pot to provide an oxygen atmosphere, and ball milling was carried out for 8 hours using a planetary ball mill. The ball-milled samples are sequentially sieved by 400 meshes and 800 meshes in a grading way, and the sieved powder is collected to obtain Si @ SiO_x。

Step two: carbon coating

Taking 2g of Si @ SiO obtained in the first step_xAnd (2) placing the mixture in 100ml of Tris buffer solution for ultrasonic treatment for 15min, then slowly adding 50g of tannic acid solution with the mass concentration of 3 wt%, adjusting the pH to be about 7.0, magnetically stirring the mixture at room temperature for 6h, centrifuging the mixture, collecting bottom sediment, and performing forced air drying at 80 ℃ for 1h to obtain the silicon-carbon composite material precursor.

Placing the silicon-carbon composite material precursor in a tube furnace, heating to 400 ℃ at a heating rate of 2 ℃/min under the argon atmosphere, preserving heat for 3h, then heating to 1000 ℃ at a heating rate of 5 ℃/min, preserving heat for 12h, and naturally cooling to obtain Si @ SiO_xa/C silicon-carbon composite material.

FIG. 1 is Si @ SiO_xX-ray diffraction pattern (XRD pattern) of/C silicon-carbon composite material, with Si @ SiO as upper layer_xXRD data of the/C silicon-carbon composite material, below that of the silicon powder used as a reference, it can be seen that Si @ SiO_xExcept the diffraction peak corresponding to silicon, amorphous SiO appears between 21 and 26 degrees in the/C silicon-carbon composite material_xAnd the diffraction peak package of carbon shows that the silicon-carbon composite material has SiO_xDouble-layer coating structure of-C, therefore, this example successfully synthesized Si @ SiO by the above method_xa/C silicon-carbon composite material.

FIG. 2 is Si @ SiO_xAnd (3) a Transmission Electron Microscope (TEM) image of the/C silicon-carbon composite material shows that the prepared silicon-carbon composite material has a core-shell structure, the overall size is about 400nm, and a peripheral shell layer in the core-shell structure of the composite material is a layer of uniformly coated amorphous carbon.

Respectively aligning the raw material silicon powder and the Si @ SiO prepared in the step one_xThe particle size was measured and the results are shown in FIG. 3 and FIG. 3, respectively4, as can be seen from fig. 3 and 4, the median particle diameter D50 of the raw material silicon powder before ball milling is about 19 μm, and Si @ SiO formed after ball milling_xThe median particle size of the particles has been on the nanometer scale, approximately 850 nm. Therefore, the particle size of the silicon material can be effectively reduced through ball milling, and the cycle performance of the material is improved.

Cycle performance test

The silicon-carbon composite material is used for preparing a negative plate, and further preparing a half cell, wherein the preparation process comprises the following steps:

taking the Si @ SiO obtained by the preparation_xThe preparation method comprises the following steps of uniformly mixing the/C silicon-carbon composite material, conductive carbon black and 12% lithium polyacrylate (LiPAA) as a binder in a mass ratio of 8:1:1, mixing slurry, coating the slurry on a copper foil current collector, and drying to prepare the electrode plate. The prepared pole piece is taken as a working electrode, a metal lithium piece is taken as a counter electrode, and the electrolyte is 1mol/L LiPF₆The mixed solution of EC/DEC/FEC (mixed solution of ethylene carbonate/diethyl carbonate/fluoroethylene carbonate, wherein the volume ratio of EC/DEC/FEC is 4: 4: 2) is assembled into a CR2032 type button cell.

The prepared CR2032 type button battery is tested for the first time and the charge-discharge specific capacity and the capacity retention rate after multiple cycles under the normal temperature condition that the charge-discharge voltage is 0.005-0.9V and the current density is 0.2C.

FIG. 5 shows Si @ SiO solid particles provided in this example_xAccording to a circulation performance diagram of the button battery prepared from the/C silicon-carbon composite material, the measured first discharge specific capacity is 864.3mAh/g, the first charge specific capacity is 730.3mAh/g, and the first coulombic efficiency can reach 84.5%; when the battery is cycled for 50 times under the multiplying power of 0.2C, the charging specific capacity of the battery is still 700.6mAh/g, and the capacity retention rate is as high as 96.0%.

Example 2

the method comprises the following steps: si @ SiO_xPreparation of

7g of silicon powder (particle size D50: 19 μm) was placed in a ball mill pot, and oxygen was charged into the ball mill pot to provide an oxygen atmosphereAnd ball-milling for 6 hours by using a swing ball mill. The ball-milled samples are sequentially sieved by 400 meshes and 800 meshes in a grading way, and the sieved powder is collected to obtain Si @ SiO_x。

Step two: carbon coating

Taking 2g of Si @ SiO obtained in the first step_xAnd (2) placing the precursor in 100ml of Tris buffer solution for ultrasonic treatment for 20min, then slowly adding 60g of dopamine solution with the mass concentration of 4 wt%, adjusting the pH value to be about 8.5, magnetically stirring the solution at room temperature for 12h, centrifuging the solution, collecting bottom precipitates, and carrying out forced air drying at 80 ℃ for 1h to obtain the silicon-carbon composite material precursor.

Placing the silicon-carbon composite material precursor in a tube furnace, heating to 500 ℃ at a heating rate of 2 ℃/min under the argon atmosphere, preserving heat for 4h, then heating to 1200 ℃ at a heating rate of 5 ℃/min, preserving heat for 6h, and naturally cooling to obtain Si @ SiO_xa/C silicon-carbon composite material.

The Si @ SiO prepared in this example was treated in the same manner as in example 1_xElectrochemical tests are carried out on the/C silicon-carbon composite material, the first charging specific capacity is 710.6mAh/g under the multiplying power of 0.2C, the first coulombic efficiency is 83.2%, after 50 cycles, the charging specific capacity still has 662.2mAh/g, and the capacity retention rate is 93.2%.

Example 3

the method comprises the following steps: si @ SiO_xPreparation of

7g of silicon powder (particle size D50: 19 μm) was placed in a ball mill pot, oxygen gas was charged into the ball mill pot to provide an oxygen atmosphere, and ball milling was carried out for 12 hours using a sand mill. The ball-milled samples are sequentially sieved by 400 meshes and 800 meshes in a grading way, and the sieved powder is collected to obtain Si @ SiO_x。

Step two: carbon coating

Taking 2g of Si @ SiO obtained in the first step_xPlacing in 100ml Tris buffer solution, performing ultrasonic treatment for 25min, slowly adding 90g polyvinylpyrrolidone (PVP) solution with mass concentration of 5 wt%, adjusting pH to about 8.0, magnetically stirring at room temperature for 18h, centrifuging, and collectingAnd collecting the bottom precipitate, and performing vacuum drying at 90 ℃ for 2h to obtain the silicon-carbon composite material precursor.

Placing the silicon-carbon composite material precursor in a tube furnace, heating to 600 ℃ at a heating rate of 3 ℃/min under an argon atmosphere, preserving heat for 4h, then heating to 800 ℃ at a heating rate of 5 ℃/min, preserving heat for 18h, and naturally cooling to obtain Si @ SiO_xa/C silicon-carbon composite material.

The Si @ SiO prepared in this example was treated in the same manner as in example 1_xElectrochemical tests are carried out on the/C silicon-carbon composite material, the first charging specific capacity is 653.7mAh/g under the multiplying power of 0.2C, the first coulombic efficiency is 81.8%, the charging specific capacity is still 605.3mAh/g after 50 cycles, and the capacity retention rate is 92.6%.

Example 4

the method comprises the following steps: si @ SiO_xPreparation of

7g of silicon powder (particle size D50: 19 μm) was placed in a ball mill pot, oxygen gas was charged into the ball mill pot to provide an oxygen atmosphere, and ball milling was carried out for 24 hours using a planetary ball mill. The ball-milled samples are sequentially sieved by 400 meshes and 800 meshes in a grading way, and the sieved powder is collected to obtain Si @ SiO_x。

Step two: carbon coating

Taking 2g of Si @ SiO obtained in the first step_xAnd (2) placing the mixture in 100ml of Tris buffer solution for ultrasonic treatment for 30min, then slowly adding 120g of citric acid solution with the mass concentration of 6 wt%, adjusting the pH to be about 7.5, magnetically stirring the mixture for 24h at room temperature, centrifuging the mixture, collecting bottom precipitates, and carrying out forced air drying for 2h at 80 ℃ to obtain the silicon-carbon composite material precursor.

Placing the silicon-carbon composite material precursor in a tube furnace, heating to 400 ℃ at a heating rate of 1 ℃/min under an argon atmosphere, preserving heat for 5h, then heating to 1000 ℃ at a heating rate of 5 ℃/min, preserving heat for 24h, and naturally cooling to obtain Si @ SiO_xa/C silicon-carbon composite material.

The Si @ SiO prepared in this example was treated in the same manner as in example 1_xSilicon carbon/CElectrochemical tests are carried out on the composite material, the first charging specific capacity is 688.2mAh/g under the multiplying power of 0.2C, the first coulombic efficiency is 82.5%, the charging specific capacity is 645.5mAh/g after 50 cycles, and the capacity retention rate is 93.8%.

Example 5

the method comprises the following steps: si @ SiO_xPreparation of

Step two: carbon coating

Taking 2g of Si @ SiO obtained in the first step_xAnd (2) placing the precursor in 100ml of Tris buffer solution for ultrasonic treatment for 15min, then slowly adding 30g of dopamine solution with the mass concentration of 2.0 wt%, adjusting the pH to be about 9.0, magnetically stirring the solution at room temperature for 6h, centrifuging the solution, collecting bottom precipitates, and drying the precipitates by air blowing at 80 ℃ for 1h to obtain the silicon-carbon composite material precursor.

The Si @ SiO prepared in this example was treated in the same manner as in example 1_xElectrochemical tests are carried out on the/C silicon-carbon composite material, the first charging specific capacity is 810.6mAh/g under the multiplying power of 0.2C, the first coulombic efficiency is 80.2%, after 50 cycles, the charging specific capacity still has 718.2mAh/g, and the capacity retention rate is 88.6%.

Example 6

the method comprises the following steps: si @ SiO_xPreparation of

Step two: carbon coating

Taking 2g of Si @ SiO obtained in the first step_xAnd (2) placing the mixture in 100ml of Tris buffer solution for ultrasonic treatment for 15min, then slowly adding 200g of tannic acid solution with the mass concentration of 3 wt%, adjusting the pH to be about 7.0, magnetically stirring the mixture at room temperature for 6h, centrifuging the mixture, collecting bottom sediment, and performing forced air drying at 80 ℃ for 1h to obtain the silicon-carbon composite material precursor.

The Si @ SiO prepared in this example was treated in the same manner as in example 1_xElectrochemical tests are carried out on the/C silicon-carbon composite material, the first charging specific capacity is 698.8mAh/g under the multiplying power of 0.2C, the first coulombic efficiency is 82.1%, the charging specific capacity is still 643.6mAh/g after 50 times of circulation, and the capacity retention rate is 92.1%.

It can be seen from the above embodiments that the organic carbon source and the micron-sized silicon powder are used as main materials to prepare the silicon-carbon composite negative electrode material in the embodiments of the present application, and the raw materials are wide in source, cheap and easy to obtain; meanwhile, the preparation method is simple and easy to implement, green and environment-friendly, low in production cost and suitable for mass production. The silicon-carbon composite material prepared by the method has uniform particle size and regular appearance. Amorphous SiO generated in situ on the surface of silicon particles_xThe volume expansion of silicon in the circulation process can be effectively relieved; and the carbon layer structure uniformly coated on the outermost layer can improve the conductivity of the material and can further relieve the volume expansion of silicon in the circulation process. The prepared silicon-carbon composite material has uniform surface carbon coating, controllable thickness and easy processingThe lithium ion secondary battery anode material is applied to the lithium ion secondary battery anode material, the first charging specific capacity can reach 653.7-810.6 mAh/g, and the capacity retention rate is more than 88% after 50 cycles of circulation at normal temperature, so that the lithium ion secondary battery anode material has excellent cycle stability and application prospect.

Comparative experiment

Comparative example 1

The comparative example provides a preparation method of a silicon-carbon composite material, which is different from the preparation method of example 1 only in that silicon powder is subjected to ball milling in an argon atmosphere, and the finally obtained silicon-carbon composite material is formed by coating a C layer on the surface of Si and does not contain SiO grown on the surface of Si in situ_xAnd (3) a layer.

By adopting the method in the embodiment 1, the Si/C silicon-carbon composite material prepared in the comparative example 1 is subjected to an electrochemical test, and fig. 6 is a cycle performance diagram of the button cell prepared from the Si/C silicon-carbon composite material provided in the comparative example 1, and the measured first charge specific capacity is 754.2mAh/g, and the first efficiency is 80.7%; the capacity retention rate is only 79.9 percent after 50 cycles at the rate of 0.2C. Comparing example 1 with comparative example 1, it can be seen that the Si @ SiO provided in the examples of the present application_xThe first effect and the cycle stability of the/C silicon-carbon composite material are greatly improved. Indicating in situ generation of SiO by oxygen atmosphere milling_xThe layer can effectively relieve the volume expansion of silicon and improve the circulation stability of the composite material.

Comparative example 2

This comparative example provides a method for preparing a silicon carbon composite, which is different from example 1 in that Si @ SiO is formed_x-TiO₂Rather than Si @ SiO_xThe preparation method comprises the following specific steps:

the method comprises the following steps: si @ SiO_x-TiO₂Preparation of

7g of silicon powder (particle size D50: 19 μm) was placed in a ball mill pot, oxygen gas was charged into the ball mill pot to provide an oxygen atmosphere, and ball milling was carried out for 8 hours using a planetary ball mill. The ball-milled samples are sequentially sieved by 400 meshes and 800 meshes in a grading way, and the sieved powder is collected to obtain Si @ SiO_x. The prepared Si @ SiO_xMixing with titanium dioxide powder, and performing high-energy ball milling for 6 hours in argon atmosphere to obtain Si @ SiO_x-TiO₂A composite material.

Step two: carbon coating

Taking 2g of Si @ SiO obtained in the first step_x-TiO₂And (2) placing the mixture in 100ml of Tris buffer solution for ultrasonic treatment for 15min, then slowly adding 50g of tannic acid solution with the mass concentration of 3 wt%, adjusting the pH to be about 7.0, magnetically stirring the mixture at room temperature for 6h, centrifuging the mixture, collecting bottom sediment, and performing forced air drying at 80 ℃ for 1h to obtain the silicon-carbon composite material precursor.

Placing the silicon-carbon composite material precursor in a tube furnace, heating to 400 ℃ at a heating rate of 2 ℃/min under the argon atmosphere, preserving heat for 3h, then heating to 1000 ℃ at a heating rate of 5 ℃/min, preserving heat for 12h, and naturally cooling to obtain Si @ SiO_x-TiO₂A silicon carbon composite material.

The Si @ SiO prepared in comparative example 2 was prepared as described in example 1_x-TiO₂Silicon/carbon composite and Si @ SiO from example 1_xThe button cell prepared from the/C silicon-carbon composite material is subjected to charge-discharge cycle test under the conditions of normal temperature, charge-discharge voltage range of 0.005-0.9V and different multiplying power currents, and the result is shown in FIG. 7. As can be seen from the figure, the Si @ SiO provided in comparative example 2_x-TiO₂After the/C material is subjected to three-layer coating, the rate performance of the material is obviously inferior to that of the double-layer coated Si @ SiO provided in the embodiment 1 of the application_xa/C composite material, in particular Si @ SiO at high rates of 1C, 1.5C and 2C_xThe charging specific capacity of the/C composite material is obviously superior; and upon return to low magnification, Si @ SiO_xthe/C composite also exhibits significant advantages. The reason for this analysis may be that the silicon-carbon composite material in comparative example 2 has a three-layer coating structure, which can buffer the volume expansion of silicon, but also increases the lithium ion migration path, so that the rate performance of the material is poor. The structure of the composite material provided by the embodiment of the application is double-layer coating, the lithium ion migration path is short, and the composite material has excellent rate performance while relieving volume expansion.

The embodiments of the present application have been described in detail with reference to the drawings, but the present application is not limited to the embodiments, and various changes can be made within the knowledge of those skilled in the art without departing from the gist of the present application. Furthermore, the embodiments and features of the embodiments of the present application may be combined with each other without conflict.

Claims

1. The preparation method of the silicon-carbon composite material is characterized by comprising the following steps:

Step two, subjecting the Si @ SiO_xAnd (5) carrying out carbon coating to obtain the silicon-carbon composite material.

2. The method of claim 1, wherein the carbon coating comprises the steps of:

(1) the Si @ SiO_xMixing with a solution of an organic carbon source to obtain a silicon-carbon composite material precursor;

3. The method of claim 2, wherein the Si @ SiO is_xThe mass ratio of the carbon source to the organic carbon source is 1: (0.3-6).

4. The method according to claim 2, wherein in the step (1), the mixing is performed by: the Si @ SiO_xDispersing in a buffer solution, adding the solution of the organic carbon source, adjusting the pH to 7.0-9.0, and stirring.

5. The production method according to claim 2, wherein in the step (2), the carbonization treatment is performed by: and carrying out heat treatment on the silicon-carbon composite material precursor under a protective atmosphere.

6. The method according to claim 5, wherein the heat treatment is performed by: heating to 400-600 ℃ at a heating rate of 1-5 ℃/min, then preserving heat for 2-5 h, then heating to 800-1200 ℃ at a heating rate of 5-10 ℃/min, and preserving heat for 3-24 h.

7. The method of claim 1, wherein the Si @ SiO is_xSieving the mixture before carbon coating, wherein the mesh number is more than 400.

8. Silicon-carbon composite material, characterized in that it is obtained by the method of preparation according to any one of claims 1 to 7.

9. A negative electrode sheet comprising the silicon-carbon composite material according to claim 8.

10. A lithium ion secondary battery comprising the negative electrode sheet according to claim 9.