Lithium ion battery, C-Si material and preparation method and application of C-Si material

Abstract

The invention relates to the technical field of lithium ion batteries, and discloses a C-Si material, a preparation method and application thereof and a lithium ion battery, the material comprises graphite and a double-coating-layer nanometer silicon material distributed on the surface of the graphite, and the particle size of the nanometer silicon material is 50-300 nm; the thickness of the first coating layer in the double coating layers is 2-10nm, and the first coating layer comprises carbon, oxygen and silicon; the thickness of a second coating layer in the double coating layers is 5-20 nm, and the second coating layer mainly comprises carbon elements; and the double-coating-layer nano silicon material distributed on the surface of the graphite is adhered and connected with the graphite through the second coating layer. The nano silicon material provided by the invention is applied to the lithium ion battery, and has large specific capacity, high first coulombic efficiency and excellent cycling stability.

Description

Technical Field

[0001] The present invention relates to the field of C-Si materials, and in particular to a C-Si material, a preparation method and application thereof, and a lithium ion battery. Background Art

[0002] Silicon anode materials are the most promising anode materials for next-generation lithium-ion batteries because they have a theoretical specific energy density (4200mAh / g) that is nearly 10 times higher than the current commercial graphite anode (370mAh / g), low operating voltage, abundant reserves, good biocompatibility and environmental friendliness. The volume effect (300% to 400%) of silicon anode materials during battery charging and discharging will greatly reduce the cycle stability of the battery. Studies have found that when silicon powder is refined to the nanometer level, the stress tolerance of silicon materials on the phase interface increases, which can effectively avoid the breakage and crushing of silicon anode materials, and nano-silicon materials can effectively reduce the diffusion and transmission paths of particles and electrons, reduce the polarization of electrodes during charging and discharging, and increase the electrochemical performance of electrodes.

[0003] The mainstream commercial silicon negative electrode materials currently usually use nano-sized silicon particles composited with graphite negative electrode materials. Nano-silicon particles are coated on the surface of graphite materials through polymer carbon precursors, and after high-temperature pyrolysis, a silicon-carbon composite negative electrode material completely coated with carbon materials is formed. However, the presence of a large amount of coated carbon, on the one hand, limits the expansion space of silicon particles. When silicon particles expand too much, it is easy to cause the coating layer to rupture and the active material to fall off; on the other hand, the inactive lithium embedded in the coated carbon will greatly reduce the first coulomb efficiency of the material. Summary of the invention

[0004] The purpose of the present invention is to overcome the problems existing in the prior art and provide a C-Si material and a preparation method and application thereof. The C-Si material provided by the present invention is applied to lithium-ion batteries and has high first coulombic efficiency and excellent charge-discharge cycle stability.

[0005] To achieve the above-mentioned purpose, the present invention provides a C-Si material, which comprises: graphite and a double-coated layer of nano-silicon material distributed on the surface of the graphite, wherein the particle size of the nano-silicon material is 50 to 300 nm;

[0006] The thickness of the first coating layer in the double coating layer is 2 to 10 nm, preferably 3 to 6 nm; the first coating layer comprises carbon, oxygen and silicon;

[0007] The thickness of the second coating layer in the double coating layer is 5 to 20 nm, preferably 10 to 18 nm; the second coating layer mainly includes carbon elements; the double coating layer nano-silicon material distributed on the surface of graphite is adhered and connected to the graphite through the second coating layer. Figure 1 The structural schematic diagram of the C-Si material of the present invention is given as an example. Figure 2 The schematic diagram of the structure of the C-Si material of the prior art is shown; Figure 1 and Figure 2 By comparison, it can be seen that the carbon of the C-Si material of the present invention is mainly coated on the surface of nano-silicon, and there is almost no coating of asphalt pyrolysis carbon on the graphite surface; the silicon-carbon material ( Figure 2 )The graphite surface is also coated with carbon.

[0008] In the present invention, the C-Si material mainly contains graphite, and the graphite content is above 80 weight %.

[0009] In the present invention, the particle size is measured by a laser particle size analyzer, and the test conditions include: using ethanol as a solvent and a laser shading degree of 4-5%.

[0010] In the present invention, the coating thickness and composition are obtained by high-power transmission electron microscopy testing, and the testing conditions include: using a FEI Titan Cubed Themis G2 300 model spherical aberration corrected transmission electron microscope, an acceleration voltage of 200KV, a maximum magnification of 1.1 million times, a point resolution of 0.25nm, and an EDS detection line of 0.1wt%.

[0011] In the present invention, the scanning electron microscope test uses a JSM-7800F super-resolution field emission scanning electron microscope.

[0012] In the present invention, the optional range of the types of graphite is relatively wide, and commonly used types of graphite can be used in the present invention. This is an exemplary description, but it does not limit the scope of the present invention. For the present invention, preferably, the graphite is one or more of artificial graphite and natural graphite.

[0013] In the first coating layer, the content of each element can be selected in a wide range. According to a preferred embodiment of the present invention, in the first coating layer, the mass fraction of oxygen element is 15-35%, preferably 25-35%; the mass fraction of carbon element is 20-40%, preferably 21-29%.

[0014] The first coating layer may optionally include a surface modification element such as phosphorus. According to a preferred embodiment of the present invention, the first coating layer also includes phosphorus, and the mass fraction of phosphorus is 0.5-10%, preferably 1-3%.

[0015] According to a preferred embodiment of the present invention, the carbon in the second coating layer is asphalt pyrolysis carbon. The above preferred embodiment can achieve uniform coating of the surface of nano-silicon, alleviate the stress concentration and breakage caused by the volume expansion of silicon during the lithiation process, and the asphalt between silicon and graphite forms carbon material in situ during the pyrolysis process, increase the stability of silicon on the graphite surface, and effectively avoid electrochemical sintering of silicon during the lithiation process.

[0016] The C-Si materials having the above-mentioned characteristics of the present invention have obvious advantages, and there is no special requirement for their preparation methods. According to the present invention, a method for preparing the C-Si material is provided, and the method comprises the following steps:

[0017] 1) mixing nano silicon powder and a solvent and stirring to obtain a first suspension;

[0018] 2) Optionally, adding a carbon-containing surface modifier source and continuing to stir and mix evenly;

[0019] 3) After the stirring is completed, filtering and washing with a solvent, preferably 3 to 5 times, filtering, and drying in a vacuum or inert atmosphere to obtain a nano-silicon particle precursor coated with a first coating layer;

[0020] 4) mixing the nano silicon particle precursor coated by the first coating layer with a solvent, and stirring to obtain a second suspension;

[0021] 5) sand-milling the second suspension to disperse the nano silicon particle precursor coated by the first coating layer in the solvent;

[0022] 6) preparing an asphalt-solvent solution, adding it to the dispersed second suspension in step 5), and continuing sand grinding and dispersion;

[0023] 7) After mixing the graphite powder and the solvent, add the slurry obtained in step 6), mix well and then spray dry to obtain a C-Si material precursor;

[0024] 8) calcining the C-Si material precursor in an inert atmosphere in multiple stages;

[0025] The solvent in step 4) does not dissolve the asphalt;

[0026] The solvent in step 6) can dissolve asphalt and is miscible with the solvent in step 4).

[0027] In the present invention, in step 1), the optional range of the types of the solvent is relatively wide, and the following exemplary description is given, but the scope of the present invention is not limited thereby. According to a preferred embodiment of the present invention, the solvent is a mixed solvent of water and ethanol, wherein the ethanol concentration is 0-70%, and 0 means containing an amount infinitely close to 0. Preferably, in the mixed solvent, the ethanol concentration is 10-40wt%. The aforementioned ranges can achieve the purpose of the present invention. The embodiments are illustrative examples and cannot limit the scope of the present invention thereby.

[0028] In the present invention, in step 1), the formula of the first suspension can be selected in a wide range. Preferably, the mass fraction of nano-silicon powder in the first suspension is 2 to 30%, preferably 5 to 20%. The above ranges can achieve the purpose of the present invention. The embodiments are illustrative examples and cannot limit the scope of the present invention.

[0029] The types of the carbon-containing surface modifier source described in the present invention can be selected from a wide range, and commonly used carbon-containing surface modifiers can be used in the present invention. For the present invention, it is preferred that the carbon-containing surface modifier source also contains other surface modification elements such as P. The following exemplary description does not limit the scope of the present invention. According to a preferred embodiment of the present invention, preferably, in step 2), the phosphorus-containing surface modifier source is an organic matter containing a phosphate group, preferably an ester containing a phosphate group and / or an organic acid containing a phosphate group. Further, preferably, the organic matter containing a phosphate group is selected from at least one of phytic acid, phenylphosphonic acid, lecithin and n-dodecyl phosphoric acid. The aforementioned substances can achieve the purpose of the present invention. The embodiments are illustrative examples and cannot limit the scope of the present invention. In the embodiments of the present invention, phytic acid is provided in an aqueous solution with a concentration of 40-60wt% to illustrate the advantages of the present invention, but does not limit the scope of the present invention.

[0030] In the present invention, the optional range of the amount of the carbon-containing surface modifier source added is relatively wide, and is specifically determined based on the required coating thickness and modification composition.

[0031] In the present invention, any solvent that meets the above requirements can be used in the present invention. For the present invention, in step 4), the solvent is preferably a C2-C6 alcohol and / or water, preferably at least one selected from ethanol, isopropanol and butanol, more preferably ethanol.

[0032] In the present invention, in step 4), the formula of the second suspension can be selected in a wide range. Preferably, the mass fraction of the nano-silicon particle precursor coated by the first coating layer in the second suspension is 2-20%, preferably 5-15%, and more preferably 5-8%. The above ranges can achieve the purpose of the present invention. The embodiments are illustrative examples and cannot limit the scope of the present invention.

[0033] In the present invention, sand grinding is a prior art, and there is no special requirement for the equipment and conditions used therein. The following is an exemplary description, but the scope of the present invention is not limited thereby.

[0034] According to one embodiment of the present invention, the sand grinding conditions include: a rotation speed of 1000 to 3000 rpm, preferably 1500 to 2500 rpm; and a time of 0.25 to 3 hours, preferably 0.5 to 2 hours.

[0035] In the present invention, in step 6), the mass fraction of asphalt in the asphalt-solvent solution can be selected in a wide range, and is preferably 20-40% for the present invention. The above ranges can achieve the purpose of the present invention, and the embodiments are illustrative examples and cannot limit the scope of the present invention.

[0036] According to a preferred embodiment of the present invention, preferably, in step 6), the amount of asphalt added is 0.3 to 0.4 times the mass of nano-silicon. The aforementioned range can achieve the purpose of the present invention. The embodiments are illustrative examples and cannot limit the scope of the present invention. The aforementioned preferred embodiment can, on the one hand, be combined with the preferred asphalt mass fraction in the asphalt-solvent solution to achieve the precipitation of asphalt in the nano-silicon suspension and coating on the nano-silicon surface; on the other hand, it can achieve the control of the coating thickness of asphalt on the nano-silicon surface.

[0037] According to a preferred embodiment of the present invention, preferably, the solvent is selected from one or more of NMP and tetrahydrofuran, preferably a mixture of NMP and tetrahydrofuran, wherein the content of any one of them is not less than 10 wt %, preferably not less than 40 wt %.

[0038] In the present invention, the purpose of step 7) is to prepare a C-Si material precursor slurry. According to a preferred embodiment of the present invention, preferably, in step 7), the amount of graphite added is 10 to 40 times the mass of nano-silicon. The above range can achieve the purpose of the present invention. The embodiments are illustrative examples and cannot limit the scope of the present invention. The above preferred embodiment can achieve a specific capacity of the prepared C-Si material of 400 to 600 mAh / g, which is more conducive to achieving a good distribution interval of silicon on the graphite surface.

[0039] According to a preferred embodiment of the present invention, preferably, in step 7), a solvent is added to adjust the solid content of the slurry to 25-50%. The aforementioned range can achieve the purpose of the present invention. The embodiments are illustrative examples and cannot limit the scope of the present invention. The aforementioned preferred embodiment can adjust the slurry viscosity, which is beneficial to the spray drying.

[0040] According to a preferred embodiment of the present invention, preferably, in step 7), the solvent is selected from C2-C6 alcohols and / or water, preferably selected from at least one of ethanol, isopropanol and butanol; the aforementioned preferred embodiment can achieve good dispersion of graphite.

[0041] In the present invention, the optional range of the calcination conditions is relatively wide, and its purpose is to heat-treat the coating layer on the surface of nano-silicon. According to a preferred embodiment of the present invention, preferably, in step 8), the multi-stage calcination step includes: heating at a heating rate of 10-20℃ / min for 20-40min to 400-500℃, then heating at a heating rate of 1-10℃ / min for 50-100min to 550-650℃, keeping warm for 1-3h after heating, then heating at a heating rate of 4-8℃ / min for 20-50min to 700-1000℃, keeping warm for 15-120min after heating, and finally cooling naturally.

[0042] According to a preferred embodiment of the present invention, preferably, in step 8), the multi-stage roasting step includes: heating at a heating rate of 15°C / min for 20 to 40 minutes to 400-500°C, then heating at a heating rate of 2°C / min for 50 to 100 minutes to 550-650°C, keeping warm for 1 to 3 hours after heating, then heating at a heating rate of 5°C / min for 20 to 50 minutes to 700-1000°C, keeping warm for 15 to 120 minutes after heating, and finally cooling naturally.

[0043] The present invention provides application of the material of the present invention in lithium ion batteries.

[0044] The present invention provides a lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte, wherein the negative electrode sheet comprises the C-Si material of the present invention.

[0045] The C-Si material provided by the present invention is applied to lithium-ion batteries and has high initial coulombic efficiency and excellent charge-discharge cycle stability.

[0046] The C-Si material preparation method provided by the invention is simple and is conducive to industrial large-scale production.

[0047] The method of the present invention has the following advantages:

[0048] 1. By using the precipitation of asphalt in the solvent, the asphalt is in-situ coated with silicon without sintering. Graphite is added for sintering. The carbon layer formed after the in-situ sintering of graphite and asphalt is more tightly combined. After pyrolysis, the asphalt is carbonized and coated on the silicon surface, and the position connected with the graphite is fixed on the graphite surface, so that the nano-silicon particles are fixed on the graphite surface. The fusion of silicon during the lithiation process is avoided.

[0049] 2. The method of the present invention significantly reduces the amount of pyrolytic carbon used for coating and increases the initial efficiency and specific capacity of the material.

[0050] 3. The first coating layer of the present invention self-assembles into bonds on the silicon surface, which is conducive to the uniform coating of C on the silicon surface;

[0051] 4. The present invention preferably adjusts the amount of asphalt added so that the asphalt completely covers the silicon surface, increases the interaction between silicon and graphite, and makes the silicon evenly distributed on the graphite surface. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 It is a schematic structural diagram of the C-Si material of the present invention;

[0053] Figure 2 It is a schematic diagram of the structure of the C-Si material of the prior art;

[0054] Figure 3This is a TEM image of the material after pyrolysis of the first coating layer nano-silicon precursor prepared in Example 1 of the present invention;

[0055] Figure 4 This is a TEM image of the material after pyrolysis of the first coating layer nano-silicon precursor coated by the second coating layer prepared in Example 1 of the present invention;

[0056] Figure 5 is a SEM image of the C-Si material prepared in Example 1 of the present invention;

[0057] Figure 6 is a SEM image of the C-Si material prepared in Example 1 of the present invention;

[0058] Figure 7 This is the SEM image of the C-Si material prepared in comparative example. DETAILED DESCRIPTION

[0059] The endpoints and any values ​​of the ranges disclosed in this article are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

[0060] The present invention will be described in detail below through examples. In the following examples, the electrochemical performance and cycle stability of the battery are measured by a CT2001A model blue battery test system; the graphite raw material is a commercial product of Luoyang Yuexing Company with a brand number of G216; the silicon powder raw material is not limited, the purity is 99%, and the particle size is 100-200nm.

[0061] Example 1

[0062] 1) Add 1.5 kg of nano silicon powder (particle size: 100 nm to 200 nm) into 15 kg of a mixed solvent of ethanol and water in a mass ratio of 1:2 and stir evenly.

[0063] 2) Slowly add 0.75 kg of 50% phytic acid aqueous solution and stir for 4 hours.

[0064] 3) After stirring, the slurry is filtered, and the filter cake is washed to neutrality in a mixed solvent of ethanol and water in a mass ratio of 1:2, and placed in a vacuum drying oven for drying at 80° C. for more than 24 hours to obtain a nano-silicon precursor coated with the first coating layer.

[0065] 4) taking 80 g of the prepared nano-silicon precursor coated with the first coating layer, and stirring and mixing with 1520 g of ethanol solvent;

[0066] 5) After being dispersed in a nano sand mill for 30 minutes (rotation speed 2500 rpm), a nano silicon precursor slurry coated with the first coating layer with good dispersion was obtained.

[0067] 6) Prepare an asphalt-NMP solution with a solid content of 30%, take 237 g of the asphalt-NMP solution, add it dropwise into the nano-silicon precursor slurry coated with the first coating layer with good dispersibility, continue sand grinding and dispersion for 1 hour, and obtain the nano-silicon material precursor coated with the first coating layer by the second coating layer.

[0068] 7) 1604 g of graphite was added to 2000 g of ethanol solvent, stirred and mixed evenly, and then the slurry in step 6) was added, stirred and mixed evenly. Drying and granulation were performed by spray drying to obtain C-Si material precursor powder.

[0069] 8) The powder was placed in a nitrogen atmosphere and calcined by the following procedure: heating at a rate of 15°C / min for 30 minutes, then heating at a rate of 2°C / min for 80 minutes, then keeping the temperature for 2 hours, then heating at a rate of 5°C / min for 36 minutes, then keeping the temperature for 60 minutes, and finally cooling naturally. After the calcination, C-Si material was obtained.

[0070] Through high-magnification transmission electron microscopy, such as Figure 3 Shown is the nano-silicon material coated with the first coating layer (obtained by pyrolyzing the material obtained in step 3 under the pyrolysis conditions in step 8). The results show that the thickness of the first coating layer is about 5.5 nm, the phosphorus content is 2.7% by weight, the oxygen content is 25% by weight, and the carbon content is 29% by weight. Figure 4 The figure shows the nano-silicon material coated with the first coating layer (obtained by drying the material obtained in step 6 and then pyrolyzing it under the pyrolysis conditions in step 8) with the second coating layer coating the first coating layer. The thickness of the second coating layer is about 14 nm, and dense amorphous carbon material can be observed.

[0071] The C-Si material was characterized by scanning electron microscopy, such as Figure 5 and Figure 6 As shown, it can be observed that the spherical double-coated nano-silicon material is adhered to the surface of the graphite material through the second coating layer, and there is almost no pitch pyrolytic carbon coated graphite material.

[0072] test

[0073] The electrochemical performance of the prepared C-Si material was tested as follows:

[0074] The negative electrode was a mixture of the prepared C-Si material, acetylene black, sodium carboxymethyl cellulose, polyacrylic acid (J&K 25% aqueous solution, MW240000, Lot: LJ60T60), and styrene-butadiene rubber (Shanghai Sixin Industrial Co., Ltd., LB-422) (mass ratio 80:10:10 / 3:10 / 3:10 / 3); the positive electrode was a metal lithium sheet; the electrolyte was a mixed solution of 1 mol / L LiPF6 of ethyl carbonate, ethyl methyl carbonate, and dimethyl carbonate (volume ratio of 1:1:1); the separator was a lithium-ion battery PE isolation membrane; and the CR2025 button battery was assembled to obtain its electrochemical properties.

[0075] The electrochemical performance of CR2025 button cells at a rate of 0.1C (1C = 400 mAh / g) and the cycle stability at a rate of 0.5C were measured.

[0076] The results show that the lithium desorption capacity of the prepared C-Si material 1 is 413.6 mAh / g at a rate of 0.1C, the first coulombic efficiency reaches 90.1%, and the capacity retention rate is still greater than 90% after 300 cycles at a rate of 0.5C.

[0077] It can be seen that the C-Si material of the present invention has high first coulombic efficiency and excellent charge and discharge cycle stability when applied to lithium-ion batteries.

[0078] Example 2

[0079] Steps 1 to 3 are the same as in Example 1, except that 0.5 kg of phytic acid aqueous solution is added.

[0080] 4) taking 190 g of the prepared nano-silicon precursor coated with the first coating layer, and stirring and mixing 3610 g of ethanol solvent;

[0081] 5) After being dispersed in a nano sand mill for 30 minutes (rotation speed 2000 rpm), a nano silicon precursor slurry coated with the first coating layer with good dispersion was obtained.

[0082] 6) Prepare an asphalt-NMP solution with a solid content of 30%, take 316 g of the asphalt-NMP solution, add it dropwise into the nano-silicon precursor slurry coated with the first coating layer with good dispersibility, and continue sand grinding and dispersion for 1 hour.

[0083] 7) 5710 g of graphite was added to 10000 g of ethanol solvent, stirred and mixed evenly, and then the slurry in step 6) was added, stirred and mixed evenly. Drying and granulation were performed by spray drying to obtain C-Si material precursor powder.

[0084] 8) The powder was placed in a nitrogen atmosphere and calcined by the following procedure: heating at a rate of 15°C / min for 30 minutes, then heating at a rate of 2°C / min for 80 minutes, then keeping the temperature for 2 hours, then heating at a rate of 5°C / min for 36 minutes, then keeping the temperature for 60 minutes, and finally cooling naturally. After the calcination, C-Si material 2 was obtained.

[0085] The results show that the first coating layer has a thickness of about 3.4 nm, wherein the phosphorus content is 1.4% by weight, the oxygen content is 29% by weight, and the carbon content is 22% by weight. The second coating layer has a thickness of about 15 nm.

[0086] The test is the same as in Example 1.

[0087] The results show that the lithium desorption capacity of the prepared C-Si material 2 is 428.5 mAh / g at a rate of 0.1C, the first coulombic efficiency reaches 90.1%, and the capacity retention rate is still greater than 94% after 300 cycles at a rate of 0.5C.

[0088] Example 3

[0089] The method of Example 1 is followed, except that in step 4), 80 g of the prepared nano-silicon precursor coated with the first coating layer is taken and stirred with 760 g of ethanol solvent;

[0090] The test was the same as in Example 1, and the product parameters are shown in Table 1.

[0091] The results show that the lithium desorption capacity of the prepared C-Si material 3 is 404.3 mAh / g at a rate of 0.1C, the first coulombic efficiency reaches 88.2%, and the capacity retention rate is still greater than 84% after 300 cycles at a rate of 0.5C.

[0092] Example 4

[0093] The method of Example 1 is followed, except that the solvent used in step 4 and step 7 is water.

[0094] The test was the same as in Example 1, and the product parameters are shown in Table 1.

[0095] The results show that the lithium desorption capacity of the prepared C-Si material 1 is 387.2 mAh / g at a rate of 0.1C, the first coulombic efficiency reaches 84%, and the capacity retention rate is still greater than 87% after 300 cycles at a rate of 0.5C.

[0096] Example 5

[0097] Steps 1 to 7 are the same as in Example 1

[0098] 8) The obtained precursor powder was placed in a nitrogen atmosphere and calcined by the following procedure: heating at a heating rate of 15°C / min for 30 minutes, then keeping the temperature for 240 minutes, then heating at a heating rate of 5°C / min for 68 minutes, then keeping the temperature for 60 minutes, and finally cooling naturally. After the calcination, C-Si material 5 was obtained.

[0099] The test was the same as in Example 1, and the product parameters are shown in Table 1.

[0100] The results show that the lithium desorption capacity of the prepared C-Si material 5 is 403.1 mAh / g at a rate of 0.1C, the first coulombic efficiency is only 84.2%, and the capacity retention rate drops to 84% after 300 cycles at a rate of 0.5C.

[0101] Example 6

[0102] All are the same as Example 1, except that the asphalt-NMP solution is replaced by a mixture of asphalt-NMP-tetrahydrofuran, the amount of solvent remains unchanged, and the weight ratio of NMP:tetrahydrofuran is 1:1.

[0103] The results show that the lithium desorption capacity of the prepared C-Si material 5 is 449.8 mAh / g at a rate of 0.1C, the first coulombic efficiency is 90.5%, and the capacity retention rate is still higher than 95% after 300 cycles at a rate of 0.5C.

[0104] Table 1

[0105]

[0106] Comparative Example 1

[0107] Steps 1 to 5 are the same as those in Example 1, except that NMP solvent is used to disperse the nano-silicon.

[0108] 6) 5.71 kg of graphite was added to 10 kg of NMP solvent, and the mixture was stirred and mixed evenly. Then, the nano-silicon dispersion slurry prepared in step 5) was added, and the mixture was stirred and mixed evenly.

[0109] 7) Prepare a 30% solid content asphalt-NMP solution, take 632g of the asphalt-NMP solution, add it dropwise into the nano-silicon precursor slurry coated with the first coating layer with good dispersibility, continue stirring and dispersing for 10 hours, and dry and granulate by spray drying to obtain the comparative example material precursor powder.

[0110] 8) The powder was placed in a nitrogen atmosphere and calcined by the following procedure: heating at a rate of 15°C / min for 30 minutes, then heating at a rate of 2°C / min for 80 minutes, then keeping the temperature for 2 hours, then heating at a rate of 5°C / min for 36 minutes, then keeping the temperature for 60 minutes, and finally cooling naturally. After the calcination, a comparative example material was obtained.

[0111] The material of Comparative Example 1 was characterized by scanning electron microscopy. Figure 7 , it can be observed that there is a layer of asphalt pyrolytic carbon coating the silicon material and graphite together.

[0112] The test is the same as in Example 1.

[0113] The results showed that the lithium removal capacity of the prepared comparative example 1 material at a rate of 0.1C was 394.4 mAh / g, the first coulombic efficiency was only 75.2%, and the capacity retention rate dropped to 80% after 50 cycles at a rate of 0.5C.

[0114] It can be seen that the C-Si material of the present invention has a higher first coulombic efficiency and excellent charge-discharge cycle stability when applied to lithium-ion batteries than the comparative example.

[0115] The preferred embodiments of the present invention are described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, the technical solution of the present invention can be subjected to a variety of simple modifications, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be regarded as the contents disclosed by the present invention and belong to the protection scope of the present invention.

Claims

1. A C-Si material, characterized in that: The material comprises: graphite and a double-coated layer of nano silicon material distributed on the surface of the graphite, wherein the particle size of the nano silicon material is 50 to 300 nm; The thickness of the first coating layer in the double coating layer is 2 to 10 nm, preferably 3 to 6 nm; the first coating layer comprises carbon, oxygen and silicon; The thickness of the second coating layer in the double coating layer is 5 to 20 nm, preferably 10 to 18 nm; the second coating layer mainly comprises carbon element; The double-coated nano-silicon material distributed on the graphite surface is adhered and connected to the graphite through the second coating layer.

2. The C-Si material according to claim 1, wherein: The graphite is one or more of artificial graphite and natural graphite; and / or In the first coating layer, the mass fraction of oxygen is 15-35%, preferably 25-35%; the mass fraction of carbon is 20-40%, preferably 21-29%; and / or The first coating layer also includes phosphorus, and the mass fraction of phosphorus is 0.5-10%, preferably 1-3%; and / or The carbon in the second coating layer is asphalt pyrolysis carbon.

3. A method for preparing C-Si material, characterized in that: The method comprises the following steps: 1) mixing nano silicon powder and a solvent and stirring to obtain a first suspension; 2) Add the carbon-containing surface modifier source and continue to stir and mix evenly; 3) After the stirring is completed, filtering and washing with a solvent, filtering, and drying in a vacuum or inert atmosphere to obtain a nano-silicon particle precursor coated with a first coating layer; 4) mixing the nano silicon particle precursor coated by the first coating layer with a solvent, and stirring to obtain a second suspension; 5) dispersing the second suspension by sand milling; 6) preparing an asphalt-solvent solution, adding it to the dispersed second suspension in step 5), and continuing sand grinding and dispersion; 7) After mixing the graphite powder and the solvent, add the slurry obtained in step 6), mix well and then spray dry to obtain a C-Si material precursor; 8) calcining the C-Si material precursor in an inert atmosphere in multiple stages; The solvent in step 4) does not dissolve the asphalt; The solvent in step 6) can dissolve asphalt and is miscible with the solvent in step 4).

4. The method according to claim 3, wherein: In step 1), The solvent is a mixed solvent of water and ethanol, wherein the concentration of ethanol is 0-70wt%, preferably 10-40wt%; and / or The mass fraction of nano silicon powder in the first suspension is 2-30%, preferably 5-20%.

5. The method according to claim 3 or 4, wherein: In step 2), The carbon-containing surface modifier source is an organic substance containing a phosphate group, preferably an ester containing a phosphate group and / or an organic acid containing a phosphate group. Further, preferably, the organic substance containing a phosphate group is selected from at least one of phytic acid, phenylphosphonic acid, lecithin and n-dodecyl phosphoric acid; more preferably, phytic acid is provided in an aqueous solution with a concentration of 40-60wt%.

6. The method according to any one of claims 3 to 5, wherein: In step 4), The solvent is a C2-C6 alcohol and / or water, preferably at least one selected from ethanol, isopropanol and butanol, more preferably ethanol; and / or The mass fraction of the nano silicon particle precursor coated by the first coating layer in the second suspension is 2-20%, preferably 5-15%, and more preferably 5-8%.

7. The method according to any one of claims 3 to 6, wherein: In step 5), The sand grinding conditions include: a rotation speed of 1000 to 3000 rpm, preferably 1500 to 2500 rpm; and a time of 0.25 to 3 hours, preferably 0.5 to 2 hours.

8. The method according to any one of claims 3 to 7, wherein: In step 6), The mass fraction of asphalt in the asphalt-solvent solution is 20 to 40%; and / or The amount of asphalt added is 0.3 to 0.9 times the mass of nano silicon powder; and / or The solvent is selected from one or more of NMP and tetrahydrofuran, preferably a mixture of NMP and tetrahydrofuran, wherein the content of any one of them is not less than 10% by weight, preferably not less than 40% by weight; and / or In step 7), The amount of graphite added is 10 to 40 times the mass of nano silicon powder; and / or Adding solvent to adjust the solid content of the slurry to 25-50wt%; The solvent is selected from C2-C6 alcohol and / or water, preferably at least one selected from ethanol, isopropanol and butanol; and / or In step 8), the multi-stage roasting step comprises: The multi-stage roasting steps include: heating the temperature to 400-500°C at a heating rate of 10-20°C / min for 20-40min, heating the temperature to 550-650°C at a heating rate of 1-10°C / min for 50-100min, keeping the temperature for 1-3h after heating, heating the temperature to 700-1000°C at a heating rate of 4-8°C / min for 20-50min, keeping the temperature for 15-120min after heating, and finally cooling naturally.

9. Use of the material according to any one of claims 1 to 2 in a lithium ion battery.

10. A lithium ion battery, characterized in that: It comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the negative electrode sheet comprises the C-Si material as claimed in any one of claims 1 to 2.

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