Silicon-carbon negative electrode material and preparation method thereof, lithium ion battery negative electrode and lithium ion battery

Abstract

The invention provides a silicon-carbon negative electrode material and a preparation method thereof, a lithium ion battery negative electrode and a lithium ion battery. The preparation method of the silicon-carbon negative electrode material comprises the following steps: performing reduction treatment on a mixture of silicon dioxide aerogel, molten salt and a reducing agent in a protective atmosphere to obtain a porous nano silicon skeleton; wherein the mass ratio of the silicon dioxide aerogel to the molten salt is 0.05 to 0.5; and mixing the porous nano silicon skeleton with a carbon source, and carrying out heat treatment in a protective atmosphere to obtain the silicon-carbon negative electrode material. The silicon-carbon negative electrode material obtained by the invention can effectively improve the conductivity and the stability of the lithium ion battery.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion battery material preparation, and in particular to a silicon-carbon negative electrode material and a preparation method thereof, a lithium ion battery negative electrode, and a lithium ion battery. Background Art

[0002] As a new type of clean rechargeable power source, lithium-ion batteries have the advantages of high energy density, long service life, small volume and mass, and low environmental pollution. They have shown broad application prospects in the fields of transportation, energy storage, mobile communications, etc.

[0003] Negative electrode materials are an important component of lithium-ion batteries. At present, the negative electrode of commercial lithium-ion batteries generally uses graphite materials or carbon as negative electrode materials, and its theoretical specific capacity is only 372mAh / g, which cannot meet people's demand for high-capacity electrodes. Silicon-based negative electrode materials have a theoretical specific capacity of 4200mAh / g, which is a very promising negative electrode material. However, lithium has a large volume change during charging and discharging, resulting in severe pulverization and unstable interface. During the charging process, lithium is unevenly deposited to form dendrites, which easily penetrate the diaphragm and cause internal short circuits in the battery, release a large amount of heat to decompose the electrolyte, cause battery expansion, and rapidly reduce performance. In severe cases, the battery shell explodes and the battery cell catches fire, causing unpredictable disasters. Researchers have proposed solutions such as nano-sizing, alloying and carbon coating. These studies can alleviate volume expansion, improve the cycle performance of silicon-based negative electrode materials, reduce irreversible capacity, and improve initial efficiency. However, these improvements still fail to meet the requirements for cycle performance and volume expansion restrictions in industrial applications, and the achievements made in research are still far from industrialization. Therefore, more and more researchers are turning their attention to silicon-carbon negative electrode materials, which have great potential because they can improve the conductivity and initial efficiency of negative electrode materials while slowly releasing silicon volume expansion and ensuring stability.

[0004] Patents involving silicon-carbon negative electrode materials have been reported. CN 112751007 A discloses a method for preparing a porous silicon / carbon composite lithium-ion battery negative electrode material, wherein a porous silicon dioxide block is prepared from methyl orthosilicate, and then porous silicon is obtained by magnesium thermal reduction, and then carbon is coated with dopamine hydrochloride, but the pore structure of the obtained product is macroporous and the specific surface area is low. CN 110071277 A discloses a silicon-carbon composite material and a preparation method thereof, wherein the silicon-carbon composite material includes a porous nano-silicon-carbon core layer and a coating layer coating the porous nano-silicon-carbon core layer, but the charge-discharge specific capacity is low. CN 114792781A discloses a method for preparing a porous silicon negative electrode material, wherein the porous silicon is carbon coated by a chemical vapor deposition method, and the process is complex and the cost is high. CN 114975946 A discloses a method for preparing a silicon-carbon negative electrode composite material, wherein lithium salt is deposited on the surface of a silicon-carbon precursor by an electrochemical deposition method, and the process is complex and not environmentally friendly. CN 112897514 A discloses a porous nano silicon carbon negative electrode material and a preparation method thereof, wherein transition metal and graphene oxide are coated on the surface of silicon material, graphene oxide is reduced and the transition metal is etched to remove the transition metal, and then carbon coating is performed to prepare a porous nano silicon carbon negative electrode, wherein the process is relatively complex and the pore structure is difficult to control. CN 113851627A discloses a porous nano silicon carbon negative electrode material and a preparation method thereof, wherein several carbon sources with different properties are compounded in stages, and the temperature during heat treatment is controlled to form pores of a certain size inside the material, wherein the process is quite complex and the carbon coating process is difficult to control. CN 114105145 A discloses a carbon-coated three-dimensional porous silicon negative electrode material and a preparation method and application thereof, wherein the process utilizes metallurgical silicon powder and industrial magnesium powder to react to generate magnesium silicide, and then nitridation and carbonization are performed, and chemical vapor deposition is performed to prepare a "core-shell type" carbon-coated porous silicon negative electrode material, wherein the process is complex and the preparation process and pore structure are also difficult to control. CN 114695887 A discloses a lithium ion battery porous silicon / carbon composite negative electrode material and a preparation method thereof, and a lithium ion battery. The silicon-carbon composite material is obtained by heat treatment of a porous silicon skeleton, a graphite / carbon nanotube composite material and an organic carbon source, and has a single pore structure and uneven carbon coating. CN 109273680 B discloses a porous nano silicon-carbon negative electrode material and a preparation method thereof, and a lithium ion battery. The porous nano silicon-carbon material is a core-shell three-layer composite structure, and the core is an amorphous porous silicon-oxygen material SiO x The middle layer is a mesh conductive agent coating layer, and the outermost layer is an amorphous carbon coating layer, which has a single pore structure and a low reversible specific capacity. CN 109830673 A discloses a porous nano silicon carbon negative electrode material and its preparation method and application. First, silicon / silicon dioxide / conductive carbon black composite particles are prepared, and silicon dioxide is etched away using hydrofluoric acid to obtain a coated silicon@carbon negative electrode material. The process is simple, but the specific capacity is low and there is environmental pollution.

[0005] There are many patents related to porous nano silicon carbon negative electrode materials. Most of them use silicon powder, silicon microspheres or directly porous silicon as silicon source, conductive carbon black, graphene, carbon nanotubes, etc. as carbon source, and mostly prepare porous nano silicon carbon negative electrode by direct coating. These preparation methods face many problems, such as too low reversible specific capacity, complex process, high cost and environmental pollution, single or difficult to control pore structure of prepared materials, difficult to control and uneven carbon coating layer, etc. The characteristics of silicon and carbon as negative electrode materials for lithium-ion batteries cannot be fully utilized or coordinated, which restricts the development of negative electrode materials for lithium-ion batteries.

[0006] In view of this, the present invention is proposed. Summary of the invention

[0007] In order to solve the above problems in the prior art, the purpose of the present invention is to provide a silicon-carbon negative electrode material and a preparation method thereof, a lithium-ion battery negative electrode, and a lithium-ion battery. The silicon-carbon negative electrode material can effectively improve the conductivity and stability of the lithium-ion battery.

[0008] In order to achieve the above object, the present invention provides a method for preparing a silicon-carbon negative electrode material, wherein the preparation method comprises:

[0009] The mixture of silica aerogel, molten salt and reducing agent is subjected to reduction treatment under a protective atmosphere to obtain a porous nano-silicon skeleton; wherein the mass ratio of the silica aerogel to the molten salt is 0.05 to 0.5;

[0010] The porous nano-silicon skeleton is mixed with a carbon source and subjected to heat treatment under a protective atmosphere to obtain the silicon-carbon negative electrode material.

[0011] According to a specific embodiment of the present invention, preferably, the mass ratio of the silica aerogel to the molten salt is 0.05 to 0.5, preferably 0.1.

[0012] According to a specific embodiment of the present invention, preferably, the molten salt includes one or a combination of two or more of magnesium chloride, sodium chloride and aluminum chloride; preferably, the molten salt is sodium chloride.

[0013] In the present invention, sodium chloride is preferably used as the molten salt because it has a high melting point (801°C), which means that it can better absorb heat and thus better protect the pore structure. In addition, the melting point of sodium chloride is not too high, which is also conducive to better protecting the pore structure. In addition, sodium chloride is low in cost and easy to obtain.

[0014] According to a specific embodiment of the present invention, preferably, the weight ratio of the silica aerogel to the reducing agent is 0.5 to 4, preferably 1; such an amount of reducing agent is more conducive to fully realizing the reduction effect. When the amount of reducing agent is too much, the excess reducing agent will react with the molten salt to generate salt compounds that are difficult to remove, such as magnesium chloride, which will cause the introduction of impurities, thereby affecting the capacity and stability. When the amount of reducing agent is insufficient, the reduction reaction is not thorough, which will result in excessive silica residue, which will also affect the capacity performance.

[0015] Preferably, the reducing agent includes one or a combination of two or more of carbon powder, magnesium powder, and aluminum powder; preferably, the reducing agent is magnesium powder. The reasons why magnesium powder is preferred include: its properties are relatively active, magnesium thermal reduction reaction is fast and efficient, and the cost of magnesium is low. In addition, the by-product of the magnesium reduction method is magnesium oxide, which is a non-toxic compound and will not cause environmental pollution problems. In addition, the reaction temperature of magnesium reduction is relatively low, which is conducive to saving production energy consumption.

[0016] According to a specific embodiment of the present invention, preferably, the weight ratio of the porous nano-silicon skeleton to the carbon source is 0.5 to 4, preferably 1; preferably, the carbon source includes one or a combination of two or more of asphalt, acetylene, and sucrose.

[0017] According to a specific embodiment of the present invention, preferably, the heat treatment comprises a first temperature rising stage, a first temperature keeping stage, a second temperature rising stage and a second temperature keeping stage which are performed sequentially;

[0018] Preferably, the heating rate in the first heating stage is 2-7°C / min;

[0019] Preferably, the temperature of the first insulation stage is 200-400°C and the time is 20-40 minutes;

[0020] Preferably, the first heat preservation stage is carried out under vacuum conditions, more preferably, under a vacuum degree of 1 to 10 torr; through vacuum treatment, the carbon source can be quickly and evenly filled into the pores of the porous nano-silicon skeleton, so as to better realize the silicon-carbon negative electrode material of the present invention;

[0021] Preferably, the heating rate of the second heating stage is 5-15°C / min; preferably, the temperature of the second heat preservation stage is 600-800°C, and the time is 50-70min.

[0022] In the present invention, the different heating rates have a great influence on the coating effect. First, if the heating rate is too slow, the overall heating time will be prolonged, thus causing certain damage to the pore structure; second, if the heating rate is too fast, the molten asphalt cannot completely and evenly fill the pores, and the heat treatment process ends, so the asphalt may only cover the silicon surface and cannot enter the pores.

[0023] Different heat treatment temperatures also affect the coating effect. If the temperature of the first insulation stage is too low, the asphalt is not completely melted, and the filling effect may be poor; conversely, it may damage the pores that are not filled with asphalt. The temperature of the second insulation stage is the temperature at which the asphalt is cracked into carbon. Whether it is high or low will affect the effect.

[0024] According to a specific embodiment of the present invention, preferably, the heating rate of the first heating stage is 5°C / min; preferably, the temperature of the first insulation stage is 300°C and the time is 30min; preferably, the heating rate of the second heating stage is 10°C / min; preferably, the temperature of the second insulation stage is 700°C and the time is 60min. By controlling the heat treatment procedure and heating rate in such a preferred manner, the carbon source can be better controlled to melt into the pores of the silicon-carbon negative electrode material, thereby more fully achieving uniform coating of the silicon skeleton with carbon.

[0025] According to a specific embodiment of the present invention, preferably, the temperature of the reduction treatment is 200-900°C, preferably 300-800°C, and more preferably 650°C; preferably, the time of the reduction treatment is 6-24h, and more preferably 12h. The temperature of the reduction treatment of the present invention is relatively low, which can further and better avoid the pore structure of the silicon-carbon negative electrode material from being destroyed.

[0026] According to a specific embodiment of the present invention, preferably, before the reduction treatment, the method further comprises preheating the mixture; preferably, the heating rate of the preheating treatment is 1 to 5°C / min, more preferably 2°C / min.

[0027] According to a specific embodiment of the present invention, preferably, the method for preparing the silica aerogel comprises: converting a silicon precursor into the silica aerogel by supercritical drying.

[0028] According to a specific embodiment of the present invention, preferably, the silicon precursor includes one or more of water glass, tetraethyl orthosilicate, and methyltrimethoxysilane; preferably, the silicon precursor is water glass and / or tetraethyl orthosilicate; water glass and / or tetraethyl orthosilicate are preferred because they contain less carbon;

[0029] Preferably, the supercritical drying temperature is 50-70° C., the pressure is 13-21 MPa, the time is 3-7 hours, and the pH is 5-9; the supercritical drying method can better retain the mesoporous structure of the silica aerogel;

[0030] More preferably, the supercritical drying temperature is 60° C., the pressure is 17 MPa, the time is 5 h, and the pH is 7.

[0031] In some embodiments of the present invention, the means for controlling pH includes: regulating the pH value of the reaction system by regulating the concentration of the hydrochloric acid solution. The concentration of the hydrochloric acid solution is selected to be 2 to 4 M, preferably 3 M. When the concentration of the hydrochloric acid solution is too high, the adjusted Ph value is also relatively small, which will affect the morphology of the porous nano-silicon skeleton and may lead to a decrease in the final electrochemical performance.

[0032] The gel at supercritical temperature has been solvent-displaced, so the pH has been adjusted to about 5. The pH is adjusted before the sol-gel process when the solution system is configured.

[0033] Preferably, after obtaining the silica aerogel, the method for preparing the silica aerogel further comprises drying at normal pressure.

[0034] In addition, the method for preparing the silica aerogel of the present invention has the advantage that no other gel accelerators need to be added during the preparation process.

[0035] The present invention also provides a silicon-carbon negative electrode material obtained by the above preparation method. The negative electrode material comprises a porous nano-silicon skeleton and carbon; at least part of the carbon is filled in the pore structure of the porous nano-silicon skeleton, and at least part of the carbon is coated on the surface of the porous nano-silicon skeleton.

[0036] According to a specific embodiment of the present invention, preferably, the silicon-carbon negative electrode material includes macropores, mesopores and micropores, wherein the pore diameter of the macropores is greater than 50nm, the pore diameter of the mesopores is 2-50nm, and the pore diameter of the micropores is less than 2nm; wherein the total pore volume of the mesopores accounts for 68-78% of the volume percentage of the porous nano-silicon skeleton, and the total pore volume of the micropores accounts for 0.8-1.2% of the volume percentage of the porous nano-silicon skeleton.

[0037] According to a specific embodiment of the present invention, preferably, before being coated with carbon source, the specific surface area of ​​the porous nano-silicon skeleton is 200 to 300 m 2 / g, the empty volume is 0.5~1cm 3 / g, and the average pore size is 10-30nm.

[0038] According to a specific embodiment of the present invention, preferably, after being coated with carbon using a carbon source, the specific surface area of ​​the silicon-carbon negative electrode material is 10 to 20 m 2 / g, the empty volume is 0.01~0.1cm 3 / g, and the average pore size is 10-20nm.

[0039] The silicon-carbon negative electrode material obtained by the preparation method of the present invention better retains the mesoporous structure, and such a mesoporous structure can better provide a large specific surface area, increase the contact area with the electrolyte, increase the active sites, and enhance the lithium ion conductivity.

[0040] The present invention also provides a lithium ion battery negative electrode, which is made of the above silicon-carbon negative electrode material.

[0041] The present invention also provides a lithium ion battery, which comprises the above-mentioned lithium ion battery negative electrode.

[0042] The silicon-carbon negative electrode material provided by the present invention has a rich porous structure. Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0043] (1) The reduction of silica is assisted by molten salt, which fully retains the porous structure of silica aerogel. Therefore, the silicon-carbon negative electrode material of the present invention has a high specific surface area. When used in lithium batteries, it can fully contact with the electrolyte, has many active sites and high capacity.

[0044] (2) The silicon-carbon negative electrode material prepared by the present invention has a rich porous structure, so it can alleviate volume expansion, overcome the technical difficulty that silicon-based negative electrode materials may be damaged due to volume expansion during charging and discharging, thereby improving the stability of lithium-ion batteries.

[0045] (3) In the present invention, the porous nano-silicon skeleton is coated with carbon, which can improve the conductivity of the negative electrode material, solve the problem of low intrinsic conductivity of silicon material, and also improve its first coulombic efficiency.

[0046] (4) The preparation method of the present invention controls the amount of molten salt added. Within this amount range, it can effectively protect silicon without introducing too many impurities.

[0047] (5) The preparation method of the present invention controls the heat preservation stage during the heat treatment to be carried out under vacuum conditions, which enables the molten carbon to enter the pores of the porous nano-silicon skeleton, thereby achieving uniform coating of the silicon skeleton with carbon. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1a and Figure 1b The SEM image of the silicon-carbon negative electrode material prepared in Example 1 is shown.

[0049] Figure 1c A BET image of a silicon skeleton is shown.

[0050] Figure 2 The SEM image of the silicon-carbon negative electrode material prepared in Example 1 is shown.

[0051] Figure 3 The TEM image of the silicon-carbon negative electrode material prepared in Example 1 is shown.

[0052] Figure 4 The cycle capacity diagram of the lithium-ion battery made from the silicon-carbon negative electrode material prepared in Example 1 is shown.

[0053] Figure 5 The efficiency curve of the lithium-ion battery made from the silicon-carbon negative electrode material prepared in Example 1 is shown.

[0054] Figure 6 The SEM image of the silicon-carbon negative electrode material prepared in Example 5 is shown.

[0055] Figure 7 The SEM image of the silicon-carbon negative electrode material prepared in Comparative Example 1 is shown.

[0056] Figure 8 The SEM image of the silicon-carbon negative electrode material prepared in Comparative Example 1 is shown. DETAILED DESCRIPTION

[0057] In order to have a clearer understanding of the technical features, purposes and beneficial effects of the present invention, the technical solution of the present invention is now described in detail below, but it should not be construed as limiting the applicable scope of the present invention.

[0058] Example 1

[0059] This embodiment provides a method for preparing a porous nano-silicon carbon negative electrode material, which comprises the following steps:

[0060] 1. Preparation of silica aerogel

[0061] 1) Preparation of sol: Mix water glass and water at a volume ratio of 0.75, stir for 30 minutes, and then quickly add to hydrochloric acid solution, wherein the volume ratio of water glass to hydrochloric acid solution is 0.6, and the concentration of hydrochloric acid solution is 3M. ​​Stir the obtained solution at room temperature for 30 minutes to obtain a transparent viscous liquid;

[0062] 2) Gel aging: The obtained transparent viscous liquid was aged at 65°C for 24h;

[0063] 3) Solvent replacement: Soak in ethanol at 50 °C for 24 h, for a total of 5 times;

[0064] 4) Supercritical drying to obtain silica aerogel; the supercritical drying temperature is 60° C., the pressure is 17.5 MPa, and the time is 8 h.

[0065] 2. Silica aerogel reduction

[0066] 1) Weigh 0.8 g of silica aerogel and 8 g of sodium chloride, mix and grind to remove large particles, dissolve in an appropriate amount of deionized water, heat and stir the solution in a 50° C. water bath for 1 h, and then air dry at 105° C. for 2 h to remove moisture;

[0067] 2) Take out the dried sample, add 0.8g magnesium powder, mix evenly, grind thoroughly and spread on a porcelain boat;

[0068] 3) Place the porcelain boat in a tube furnace, heat the temperature to 650°C at a rate of 2°C / min under argon atmosphere, keep the temperature for 12 hours, and then cool it down to room temperature at a rate of 10°C / min;

[0069] 4) The reduced product was acid-washed with 2M dilute hydrochloric acid for 12 h to remove by-products, and then vacuum-dried at 80° C. for 6 h to obtain a porous nano-silicon skeleton.

[0070] 3. Porous nano-silicon skeleton coated with carbon

[0071] 1) Weigh equal masses of porous nano-silicon skeleton and asphalt, grind and mix thoroughly, and spread them evenly on a porcelain boat;

[0072] 2) The porcelain boat was placed in a tube furnace, and the temperature was raised to 300°C at a rate of 5°C / min under an argon atmosphere, and kept warm for 30 minutes. Vacuum was evacuated while keeping warm, and then the temperature was raised to 700°C at a rate of 10°C / min, and kept warm for 1 hour. The temperature was then cooled to room temperature at a rate of 10°C / min to complete the carbon coating treatment of the porous nano-silicon skeleton to obtain a carbon-silicon negative electrode material.

[0073] Specific surface area of ​​porous nano-silicon skeleton before carbon coating: 245.37m 2 / g, empty volume: 0.74cm 3 / g, average pore size 13.36nm;

[0074] Data of the negative electrode material prepared after carbon coating: Specific surface area: 13.43m 2 / g, empty volume: 0.04cm 3 / g, average pore size 19.45nm.

[0075] Preparation of lithium-ion batteries

[0076] The electrochemical tests were performed by assembling standard 2025-type button cells in an argon-filled glove box. The separator was Canrd 2500, and the electrolyte formula was 1.0M LiPF6, which was a 1:1:1 volume ratio of ethylene carbonate (EC), dimethyl carbonate (DMC) and diethyl carbonate (DEC), plus 5% mass fraction of fluoroethylene carbonate (FEC). The electrode slurry was prepared by mixing 96% silicon-carbon negative electrode material, 1% conductive agent (carbon nanotubes) and 3% binder (lithiated polyacrylic acid) with an appropriate amount of deionized water as solvent. The electrode slurry was evenly coated on copper foil and vacuum dried at 105°C for 6h. All coating thicknesses of the slurry were set to 100μm to maintain uniform mass loading. The test process was carried out by the Land battery test system, and the charge and discharge tests were carried out in the potential range of 0.01-0.8V. Cyclic voltammetry (CV) tests were performed in an electrochemical workstation (PARSTAT2273) at 0.05 mV s -1 The scan rate was carried out in the range of 0.01 to 0.3 V.

[0077] The relevant properties of the prepared lithium-ion battery were tested:

[0078] At a current density of 500mA / g, the first charge specific capacity is 2009mAh / g, the first discharge specific capacity is 2239mAh / g, and the first coulombic efficiency is 89.72%; after 20 cycles, the discharge specific capacity and charge specific capacity can be maintained at 1866mAh / g and 1838mAh / g, respectively.

[0079] The SEM image of the silicon-carbon negative electrode material prepared in Example 1 is as follows: Figure 1a and Figure 1b As shown, it can be seen that when the magnesium thermal reduction time is 12h, the pore structure of the original silica aerogel is still retained without obvious collapse, and its pore size also meets the standard of mesopores.

[0080] Figure 1c The BET image of the silicon skeleton is shown, which conforms to the typical type IV hysteresis loop, thus confirming the mesoporous structure.

[0081] Figure 2 The SEM image of the silicon-carbon negative electrode material obtained in this embodiment is further shown. Figure 2 It can be seen more clearly that in the silicon-carbon negative electrode material obtained by the present invention, carbon is effectively wrapped in the silicon pores.

[0082] Figure 3 The TEM image of the silicon-carbon negative electrode material obtained in this embodiment is further shown. Figure 3It can be seen that in the silicon-carbon negative electrode material obtained by the present invention, carbon is effectively wrapped in the silicon pores.

[0083] The cycle capacity of the lithium-ion battery made of the silicon-carbon negative electrode material of this embodiment is as follows: Figure 4 shown. Figure 4 In the figure, the hollow circles represent the charge capacity and the solid circles represent the discharge capacity.

[0084] The efficiency curve of the lithium-ion battery made of the silicon-carbon negative electrode material of this embodiment is as follows: Figure 5 shown.

[0085] Example 2

[0086] This embodiment provides a method for preparing a porous nano-silicon carbon negative electrode material, which comprises the following steps:

[0087] 1. Preparation of silica aerogel

[0088] The only difference from Example 1 is that the precursor is replaced with ethyl orthosilicate.

[0089] 2. Silica aerogel reduction

[0090] 1) Weigh 0.8 g of silica aerogel and 16 g of sodium chloride, mix and grind to remove large particles, dissolve in an appropriate amount of deionized water, heat and stir the solution in a 50° C. water bath for 1 h, and then air dry at 105° C. for 2 h to remove moisture;

[0091] 2) Take out the dried sample, add 1.6g of carbon powder, mix evenly, grind thoroughly and spread on a porcelain boat;

[0092] 3) Place the porcelain boat in a tube furnace, heat the temperature to 300°C at a rate of 1°C / min under a hydrogen atmosphere, keep the temperature for 6 hours, and then cool it down to room temperature at a rate of 10°C / min;

[0093] 4) The reduced product was acid-washed with 2M dilute hydrochloric acid for 12 h to remove by-products, and then vacuum-dried at 80° C. for 6 h to obtain a porous nano-silicon skeleton.

[0094] 3. Porous nano-silicon skeleton coated with carbon

[0095] 1) Weigh 0.8 g of porous nano-silicon skeleton and 1.6 g of asphalt, grind and mix thoroughly, and spread them evenly on a porcelain boat;

[0096] 2) The porcelain boat was placed in a tube furnace, and the temperature was raised to 200°C at a rate of 2°C / min under an argon atmosphere, and kept at this temperature for 20 minutes. During the heating, the temperature was evacuated, and then the temperature was raised to 600°C at a rate of 5°C / min, and kept at this temperature for 50 minutes. The temperature was then lowered to room temperature at a rate of 10°C / min to complete the carbon coating treatment of the porous nano-silicon skeleton.

[0097] The preparation method of the lithium ion battery is the same as that of Example 1.

[0098] The relevant properties of the prepared lithium-ion battery were tested:

[0099] At a current density of 500mA / g, the first charge specific capacity is 857mAh / g, the first discharge specific capacity is 1083mAh / g, and the first coulombic efficiency is 79.13%; after 20 cycles, the discharge specific capacity and charge specific capacity can be maintained at 618mAh / g and 614mAh / g.

[0100] Example 3

[0101] This embodiment provides a method for preparing a porous nano-silicon carbon negative electrode material, which comprises the following steps:

[0102] 1. The preparation method of silica aerogel is the same as that in Example 2.

[0103] 2. Silica aerogel reduction

[0104] 1) Weigh 0.8 g of silica aerogel with tetraethyl orthosilicate as a precursor and 1.6 g of sodium chloride, mix and grind to remove large particles, dissolve in an appropriate amount of deionized water, heat and stir the solution in a 50° C. water bath for 1 h, and then air dry at 105° C. for 2 h to remove moisture;

[0105] 2) Take out the dried sample, add 0.2g aluminum powder, mix evenly, grind thoroughly and spread on a porcelain boat;

[0106] 3) Place the porcelain boat in a tube furnace, heat the temperature to 800°C at a rate of 5°C / min under nitrogen atmosphere, keep the temperature for 24 hours, and then cool it down to room temperature at a rate of 10°C / min;

[0107] 4) The reduced product was acid-washed with 2M dilute hydrochloric acid for 12 h to remove by-products, and then vacuum-dried at 80° C. for 6 h to obtain a porous nano-silicon skeleton.

[0108] 3. Porous nano-silicon skeleton coated with carbon

[0109] 1) Weigh 0.8 g of porous nano-silicon skeleton and 0.2 g of asphalt, grind and mix thoroughly, and spread them evenly on a porcelain boat;

[0110] 2) The porcelain boat was placed in a tube furnace, and the temperature was raised to 400°C at a rate of 7°C / min under an argon atmosphere, and kept at that temperature for 40 minutes. Vacuum was then evacuated while the temperature was raised to 800°C at a rate of 15°C / min, and kept at that temperature for 70 minutes. The temperature was then lowered to room temperature at a rate of 10°C / min to complete the carbon coating treatment of the porous nano-silicon skeleton.

[0111] The preparation method of the lithium ion battery is the same as that of Example 1.

[0112] The relevant properties of the prepared lithium-ion battery were tested:

[0113] At a current density of 500mA / g, the first charge specific capacity is 1584mAh / g, the first discharge specific capacity is 2107mAh / g, and the first coulombic efficiency is 75.18%; after 20 cycles, the discharge specific capacity and charge specific capacity are 872mAh / g and 906mAh / g.

[0114] Example 4

[0115] The only difference from Example 1 is that the molten salt is changed to magnesium chloride.

[0116] The relevant properties of the prepared lithium-ion battery were tested:

[0117] At a current density of 500mA / g, the first charge specific capacity is 1466mAh / g, the first discharge specific capacity is 1790mAh / g, and the first coulombic efficiency is 81.89%; after 20 cycles, the discharge specific capacity and charge specific capacity can be maintained at 1341mAh / g and 1322mAh / g.

[0118] Example 5

[0119] The only difference from Example 1 is that the molten salt is changed to aluminum chloride.

[0120] The relevant properties of the prepared lithium-ion battery were tested:

[0121] At a current density of 500mA / g, the first charge specific capacity is 2005mAh / g, the first discharge specific capacity is 2274mAh / g, and the first coulombic efficiency is 88.17%; after 20 cycles, the discharge specific capacity and charge specific capacity can be maintained at 1843mAh / g and 1819mAh / g.

[0122] The SEM image of the silicon-carbon negative electrode material prepared in Example 5 is as follows: Figure 6 As shown, it can be seen that in this embodiment, aluminum chloride is used to assist reduction, and the retention of its pore structure is poorer than that of Example 1 using sodium chloride.

[0123] Comparative Example 1

[0124] The only difference from Example 1 is that the mass ratio of silica aerogel to sodium chloride is 0.01.

[0125] As a result, the excess molten salt is difficult to wash, and the remaining impurities clog the pore structure and affect the capacity of the final electrode material.

[0126] The SEM image of the silicon-carbon negative electrode material prepared in Comparative Example 1 is as follows: Figure 7 As shown, the small particles in the figure are residual impurities.

[0127] Comparative Example 2

[0128] The only difference from Example 1 is that the mass ratio of silica aerogel to sodium chloride is 1.

[0129] The mass of the molten salt is too small to protect the pore structure, causing the pore structure to be destroyed during the reduction process, ultimately affecting the stability of the electrode material.

[0130] The SEM image of the silicon-carbon negative electrode material prepared in Comparative Example 2 is as follows: Figure 8 As shown, it can be seen that the pore structure is destroyed and the pores are blocked.

[0131] It can be seen from the above examples that the preparation method of the present invention better retains the porous structure of the silica aerogel, and the prepared silicon-carbon negative electrode material has macroporous, mesoporous and microporous structures at the same time, and the carbon coating is not only on the surface of the porous nano-silicon skeleton, but also in its pores, achieving full and uniform coating. Therefore, the silicon-carbon negative electrode material of the present invention can fully enable silicon and carbon to play a synergistic role.

[0132] It can be seen from the measured electrochemical performance data that when the silicon-carbon negative electrode material of the present invention is applied to a lithium-ion battery, its charge-discharge specific capacity remains good after 20 cycles. The silicon-carbon negative electrode material of the present invention can effectively alleviate volume expansion, overcome the technical problem that silicon-based negative electrode materials may be damaged due to volume expansion during the charge and discharge process, thereby improving the stability of lithium-ion batteries.

[0133] The lithium-ion battery using the silicon-carbon negative electrode material provided by the present invention also has the advantage of high electrical conductivity, high initial coulombic efficiency, low cost, and broad market prospects.

Claims

1. A method for preparing a silicon-carbon negative electrode material, characterized in that: include: The mixture of silica aerogel, molten salt and reducing agent is subjected to reduction treatment under a protective atmosphere to obtain a porous nano-silicon skeleton; wherein the mass ratio of the silica aerogel to the molten salt is 0.05 to 0.5; The porous nano-silicon skeleton is mixed with a carbon source and subjected to heat treatment under a protective atmosphere to obtain the silicon-carbon negative electrode material.

2. The preparation method according to claim 1, characterized in that: The molten salt includes one or a combination of two or more of magnesium chloride, sodium chloride and aluminum chloride.

3. The preparation method according to claim 1, characterized in that: The weight ratio of the silica aerogel to the reducing agent is 0.5-4; preferably, the reducing agent includes one or a combination of two or more of carbon powder, magnesium powder and aluminum powder.

4. The preparation method according to claim 1, characterized in that: The weight ratio of the porous nano-silicon skeleton to the carbon source is 0.5-4; preferably, the carbon source includes asphalt.

5. The preparation method according to claim 1, characterized in that: The heat treatment comprises a first temperature rising stage, a first temperature keeping stage, a second temperature rising stage and a second temperature keeping stage which are performed in sequence; The heating rate of the first heating stage is 2-7°C / min; Preferably, the temperature of the first insulation stage is 200-400°C and the time is 20-40 minutes; Preferably, the first insulation stage is carried out under vacuum conditions, more preferably, under a vacuum degree of 1 to 10 torr; Preferably, the heating rate of the second heating stage is 5 to 15°C / min; Preferably, the temperature of the second insulation stage is 600-800° C., and the time is 50-70 minutes.

6. The preparation method according to claim 1, characterized in that: The temperature of the reduction treatment is 200-900°C, preferably 300-800°C; preferably, the time of the reduction treatment is 6-24h; Preferably, before the reduction treatment, the method further comprises preheating the mixture; preferably, the heating rate of the preheating treatment is 1 to 5°C / min.

7. The preparation method according to claim 1, characterized in that: The preparation method of the silica aerogel comprises: The silicon precursor is converted into the silicon dioxide aerogel by supercritical drying; Preferably, the silicon precursor comprises one or a combination of two or more of water glass, ethyl orthosilicate, and methyltrimethoxysilane; Preferably, the supercritical drying temperature is 50-70° C., the pressure is 13-21 MPa, the time is 3-7 h, and the pH is 5-9.

8. A silicon-carbon negative electrode material obtained by the preparation method according to any one of claims 1 to 7, characterized in that: It comprises a porous nano-silicon skeleton and carbon; at least part of the carbon is filled in the pore structure of the porous nano-silicon skeleton, and at least part of the carbon is coated on the surface of the porous nano-silicon skeleton.

9. The silicon-carbon negative electrode material according to claim 8, characterized in that: The silicon-carbon negative electrode material includes macropores, mesopores and micropores, wherein the pore diameter of the macropores is greater than 50nm, the pore diameter of the mesopores is 2-50nm, and the pore diameter of the micropores is less than 2nm; wherein the total pore volume of the mesopores accounts for 68-78% of the volume percentage of the porous nano-silicon skeleton, and the total pore volume of the micropores accounts for 0.8-1.2% of the volume percentage of the porous nano-silicon skeleton.

10. A negative electrode for a lithium ion battery, characterized in that: The lithium-ion battery negative electrode is made of a silicon-carbon negative electrode material obtained by the preparation method described in any one of claims 1 to 7 or a silicon-carbon negative electrode material described in claim 8 or 9.

11. A lithium ion battery, characterized in that: The lithium-ion battery negative electrode comprises the negative electrode of claim 10.

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