Silicon / hard carbon composite negative electrode material with porous structure and preparation method thereof

Abstract

The invention discloses a silicon / hard carbon composite negative electrode material with a porous structure and a preparation method of the silicon / hard carbon composite negative electrode material. The preparation method of the negative electrode material comprises the following steps: S1, dispersing a cationic or anionic surfactant and a silicon source in water, and separating out a solid after the dispersion is completed, so as to obtain the silicon source of which the surface is positively or negatively charged; s2, dispersing a silicon source with positive electricity or negative electricity on the surface in water, adding a soluble carbon source and soluble salt, stirring, and freezing to obtain a precursor material; and S3, carbonizing the precursor material at 600-900 DEG C under the protection of inert gas, washing with water to remove soluble salt, and drying to obtain the negative electrode material. According to the invention, agglomeration of a nano-scale silicon material in an aqueous solution is relieved by utilizing the principle that same charges repel, meanwhile, a salt template is introduced to enable the material to generate holes for accommodating silicon expansion, expansion of the material in a circulation process is relieved, and the obtained negative electrode material has good circulation stability, is simple in process, is safe and environment-friendly and is low in cost.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and in particular relates to a silicon / hard carbon composite negative electrode material with a porous structure and a preparation method thereof. Background Art

[0002] The specific capacity of the positive electrode materials of current lithium-ion batteries has reached its theoretical upper limit, making further improvement difficult. Therefore, the advancement of negative electrode materials has a critical impact on the safety, energy density and life of batteries. Currently, the theoretical specific capacity of silicon-based materials in negative electrode materials is extremely large, and the maximum lithium storage capacity at high temperature can reach 4200mAh g -1 (Li 22 Si5), and has good safety (lithium insertion potential is less than 0.5V, and it is not easy to have the problem of solvent molecules co-insertion). In addition, silicon reserves are abundant, occupying more than a quarter of the mass of the earth's crust, and it is low in cost. It is considered to be one of the most promising lithium-ion battery negative electrode materials. However, during the lithium insertion process, silicon will undergo a huge volume change (the capacity is the highest at high temperature, Li 22 The volume change rate of Si5 phase is 312%, and the capacity is the highest at room temperature. 15 The volume change rate of the Si4 phase is 281%), which causes the material to pulverize, fall off, lose electrical contact, and rapidly decay in capacity, leading to bulging of the battery cell. Therefore, research on silicon negative electrodes has basically focused on alleviating the volume change of silicon materials and improving cycle stability. In order to buffer the huge stress caused by the rapid expansion of silicon, the material maintains relative structural stability during the cycle and ensures good electrical contact between the material, electrolyte, and current collector. People have tried a variety of methods, including silicon nanoparticles, silicon nanowires, porous silicon, silicon oxides, silicon alloys, and silicon-carbon composites.

[0003] Patent CN117996047A discloses a method for preparing a biomass hard carbon composite silicon-carbon anode material. The preparation process includes the following steps: Step 1: pre-carbonizing, pulverizing, and calcining the biomass raw material to obtain a biomass carbon material; Step 2: preparing a nano-silicon slurry; Step 3: adding the biomass carbon material, nano-silicon slurry, and asphalt in a certain proportion into a dispersion tank, stirring with ethanol as a solvent, and then spray granulating to obtain a reaction product III; Step 4: adding asphalt to reaction product III, performing solid-phase mixing, and performing a primary granulation to obtain a reaction product V; Step 5: adding asphalt to reaction product V, performing solid-phase mixing, and performing a secondary granulation to obtain a reaction product VII, which is then sieved to obtain the final product. While the resulting composite material alleviates the poor cycling performance of silicon materials to some extent, the silicon dispersion and hard carbon coating uniformity are insufficient, and the preparation process is relatively complex.

[0004] Patent CN110571409A first creates a high-capacity active material precursor with a high-molecular-weight compound and polyacrylonitrile solution by coagulating it in a coagulation bath. This precursor is then subjected to pre-oxidation and carbonization to produce the negative electrode material. Polyacrylonitrile is used in the material preparation process. While non-toxic at low temperatures, it can release harmful gases at high temperatures, posing a safety risk.

[0005] Therefore, new green and environmentally friendly, simple process preparation methods for silicon-carbon negative electrode materials that can alleviate the expansion of materials during the cycle still need to be further developed. Summary of the Invention

[0006] The purpose of the present invention is to provide a silicon / hard carbon composite negative electrode material with a porous structure and a preparation method thereof. The preparation method is simple in process, safe, environmentally friendly, and low in cost, and the prepared negative electrode material has good cycle stability.

[0007] To achieve the above object, the present invention adopts the following technical solutions: In a first aspect, the present invention provides a method for preparing a silicon / hard carbon composite negative electrode material having a porous structure, comprising the following steps: S1. Dispersing a cationic or anionic surfactant and a silicon source in water, and separating the solid after dispersion to obtain a silicon source with a positive or negative surface charge; The silicon source is nano-silicon or silicon monoxide; S2. dispersing the silicon source with positive or negative surface charge in water, adding a soluble carbon source and a soluble salt, stirring, and freezing to obtain a precursor material; The soluble carbon source is one or two of carrageenan, xanthan gum and gelatin; The soluble salt is sodium chloride or potassium chloride; S3. Carbonizing the precursor material at 600-900° C. under inert gas protection, washing with water to remove the soluble salt, and drying to obtain the silicon / hard carbon composite negative electrode material with a porous structure.

[0008] Based on the above technical solution, the present invention first uses a cationic or anionic surfactant to modify the surface of silicon, so that the surface of the silicon source has a positive charge or a negative charge. The interaction between the same charges makes it have a greater repulsive force in the aqueous solution, and then mixes it with a soluble biomass solution and adds a soluble salt to stir. After stirring evenly, liquid nitrogen is quickly added to freeze it, and then it is placed in a freeze dryer to freeze the water. It is then carbonized in an inert atmosphere, and after carbonization, the soluble salt in the material is washed with water to remove the soluble salt to obtain a silicon-carbon composite negative electrode material with a porous structure. The preparation method of the present invention uses the principle of like charge repulsion to alleviate the problem that nano-scale silicon materials are easy to agglomerate in aqueous solution. At the same time, the introduction of the salt template causes the material to produce pores that accommodate silicon expansion, which is conducive to alleviating the expansion of the material during the cycle. The prepared negative electrode material has good cycle stability, simple process, safety, environmental protection, and low cost.

[0009] In the above preparation method, further, the cationic surfactant is polydimethyldiallylammonium chloride, hexadecyltrimethylammonium chloride or octadecyldimethylbenzylammonium chloride; The anionic surfactant is dodecylbenzenesulfonic acid or sodium fatty alcohol ether sulfate; The ratio of the surfactant to the silicon source is (1-5) mL:1 g, such as 4 mL:1 g. The amount of surfactant added is based on the amount that can introduce sufficient positive and negative charges on the surface of the silicon source to generate repulsive forces between them, thereby enhancing its dispersibility in the aqueous solution. Excess surfactant can be removed by washing with water in a subsequent step. The particle size of the nano-silicon is 50 to 250 nm, preferably 150 nm; The particle size of the silicon monoxide is 300-600 nm, such as 500 nm.

[0010] In the above preparation method, further, in step S1, the volume ratio of the surfactant to water is (2-6):100, such as 4:100; The dispersion comprises sequentially performing stirring dispersion at 200-1000 rpm (such as 600 rpm) for 20-60 min (such as 30 min) and ultrasonic dispersion at 100-600 W (such as 300 W) for 20-60 min (such as 30 min).

[0011] As an example, the solid is separated by centrifugation, and the steps of washing with water and centrifuging are repeated three times before drying.

[0012] In the above-mentioned preparation method, the mass ratio of the soluble carbon source to the silicon source with a positive or negative surface charge is (2-8):1, such as 2:1, 4:1, or 8:1. In the present invention, soluble carbon sources such as gelatin have good solubility and can completely dissolve at 40°C. Their excellent film-forming properties allow them to evenly coat the silicon and NaCl during freezing. During high-temperature carbonization, the derived carbon is in situ coated on the silicon and NaCl surfaces. The NaCl solubility is then utilized to wash the material away with deionized water, resulting in a silicon / hard carbon porous composite anode material. The gelatin-derived carbon coating the silicon effectively isolates the electrolyte, preventing its decomposition and facilitating the stable formation of the SEI film. Furthermore, direct contact between the silicon particles and the carbon network facilitates stable ion and electron flow during cycling, significantly improving the material's conductivity and accelerating lithium ion transport. Furthermore, the porous structure of the carbon material helps disperse stress during silicon volume changes, making the electrode more stable. The mass ratio of the soluble carbon source to the silicon source is controlled within this range, which can ensure that the carbon source such as gelatin forms a hard carbon skeleton to disperse silicon after carbonization, and can also form a carbon coating on the outer layer of silicon.

[0013] The mass ratio of the soluble salt to the positively or negatively charged silicon source is (2.5-25):1, preferably (5-20):1, such as 10:1, 5:1, or 20:1, preferably 10:1. After carbonization, both the soluble salt and the silicon are coated with a carbon layer. After the salt is washed away, the carbon coated on the salt surface forms a porous skeleton. The carbon on the silicon surface acts as a coating. If the salt ratio is too low, the porous structure cannot be effectively formed, which is not conducive to alleviating material expansion during cycling. If the salt ratio is too high, the skeleton is excessively formed, and the coating layer on the silicon surface is very thin, which is not conducive to improving cycling performance.

[0014] In the above preparation method, further, in step S2, the mass ratio of the silicon source with positive or negative surface charge to water is (0.2-1.2):100, such as 0.5:100; In step S2, the dispersion is performed by ultrasonic dispersion at 100-600 W (e.g., 300 W) for 0.5 h; The stirring temperature is 20-80°C, such as 60°C. This temperature setting can promote the dissolution of the carbon source on the one hand, and on the other hand, it is conducive to the dispersion of silicon in the solution. Taking gelatin as an example, this temperature range is conducive to the dissolution of gelatin. In addition, the gelatin solution has a certain viscosity within a certain temperature range, which is conducive to the dispersion of silicon in the solution. After cooling, the gelatin has film-forming properties and can prevent the dispersed silicon from settling and reaggregating due to gravity. The stirring process is performed at a rotation speed of 200 to 1000 rpm (eg, 600 rpm) and for a time of 20 to 60 min (eg, 60 min).

[0015] In the above-mentioned preparation method, the temperature of the carbonization treatment can specifically be 750°C. Furthermore, in the carbonization treatment step, the heating rate is 2-10°C / min, and the holding time is 2-4 hours, such as heating to 750°C at 5°C / min and holding for 3 hours. In the above-mentioned preparation method, further, the water washing is performed by placing the product in a funnel and performing suction filtration and water washing, such as multiple suction filtration and water washing.

[0016] As an example, the drying is performed in a vacuum oven at 80°C.

[0017] In a second aspect, the present invention provides a silicon / hard carbon composite negative electrode material having a porous structure prepared by the method described in any one of the above.

[0018] In the aforementioned silicon / hard carbon composite negative electrode material having a porous structure, the mass content of silicon in the silicon / hard carbon composite negative electrode material having a porous structure is 25% to 75%.

[0019] In a third aspect, the present invention provides a lithium-ion battery comprising a negative electrode plate, wherein the negative electrode plate comprises a current collector and the silicon / hard carbon composite negative electrode material having a porous structure as described above disposed on the current collector.

[0020] Compared with other existing technologies, the present invention has at least the following beneficial effects: (1) The porous silicon / hard carbon composite negative electrode material of the present invention is prepared by first modifying the surface of nano-silicon or silicon monoxide to increase the absolute value of its zeta potential and enhance its dispersibility in aqueous solution by utilizing the principle that like charges repel each other.

[0021] (2) The carbon source used in the preparation of the porous silicon / hard carbon composite negative electrode material of the present invention forms a colloid after dissolving in water, which can prevent the dispersed silicon from settling and reaggregating due to gravity.

[0022] (3) In the preparation of the porous silicon / hard carbon composite negative electrode material described in the present invention, the silicon, soluble salt, and carbon source suspension are stirred evenly and then rapidly frozen with liquid nitrogen. The moisture is removed by freeze drying. During the freeze drying process, the several carbon sources all have film-forming properties and can be evenly coated on the surface of silicon and soluble salt.

[0023] (4) The carbon sources in the preparation of the porous silicon / hard carbon composite negative electrode material of the present invention all contain heteroatom N, which forms an in-situ doping of the coating and supporting carbon layers after carbonization, which is beneficial to improving the conductivity of the material.

[0024] (5) In the preparation of the porous silicon / hard carbon composite negative electrode material of the present invention, the soluble salt plays a space-occupying role during the freezing and carbonization process. After being washed away, the space occupied by the original salt becomes a hole to buffer the volume expansion of silicon.

[0025] (6) The porous silicon / hard carbon composite negative electrode material of the present invention is prepared using soluble salts. The raw materials can be reused after simple collection without the use of acid or alkali. It has the advantages of simple process, easy operation, safety, environmental protection, and industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a flow chart for preparing the porous silicon / hard carbon composite negative electrode material provided by the present invention.

[0027] Figure 2 2 is the zeta potential diagram of silicon 2 oxide before and after modification by surfactant in Example 1 of the present invention.

[0028] Figure 3 These are the SEM and EDS images of the composite negative electrode material prepared in Example 1 of the present invention.

[0029] Figure 4 This is a TEM image of the composite negative electrode material prepared in Example 1 of the present invention.

[0030] Figure 5 The composite negative electrode material prepared in Example 1 of the present invention has a capacitance of 0.2 mV s -1 The first five cycles of cyclic voltammetry curves at the scanning rate. DETAILED DESCRIPTION

[0031] The present invention will be further described in detail below in conjunction with specific embodiments. The examples provided are only for illustrating the present invention and are not intended to limit the scope of the present invention. The examples provided below can serve as a guide for further improvements by those skilled in the art and are not intended to limit the present invention in any way.

[0032] Unless otherwise specified, the methods used in the following examples are all conventional methods and are performed according to the techniques or conditions described in the literature in the field or according to the product instructions. The materials, reagents, etc. used in the following examples are all commercially available unless otherwise specified.

[0033] Example 1 according to Figure 1 The flowchart shown is a method for preparing a silicon / hard carbon composite negative electrode material having a porous structure, and the specific steps are as follows: First, 4 mL of the surfactant polydimethyldiallyl ammonium chloride (molecular weight: 491.06) was weighed and dispersed in 100 mL of distilled water. After thorough stirring for 5 minutes, 1 g of silicon oxide with a diameter of about 500 nm was added. The mixture was stirred and dispersed at 600 rpm for 30 minutes, ultrasonically dispersed at 300 W for 30 minutes, and centrifuged. The water washing and centrifugation steps were repeated three times, and then dried to obtain a silicon oxide material with a positively charged surface.

[0034] Take 0.5 g of the silicon dioxide material with a positively charged surface prepared above, ultrasonically disperse it in 100 mL of deionized water at 300 W for 30 min, then add 1 g of gelatin and 5 g of sodium chloride, place it in a water bath at 60 °C and stir at 600 rpm for 60 min. After stirring, quickly freeze it in liquid nitrogen and then freeze-dry it in a freeze dryer for 48 h to obtain the precursor material.

[0035] The precursor material was placed in a crucible and transferred to a high-temperature tube furnace under an argon atmosphere for high-temperature carbonization. The temperature was raised to 750°C at a rate of 5°C / min and held for 3 hours. After carbonization, the sample was cooled to room temperature and placed in a sand-core funnel. It was washed several times with deionized water to remove the template NaCl. The product was then collected on filter paper and dried in a vacuum oven at 80°C. After drying, the sample was removed and ground to obtain a porous silicon oxide / hard carbon composite anode material.

[0036] Figure 2 The zeta potential of silicon oxide and the silicon oxide material with a positive surface charge after surfactant modification is shown. The zeta potential value before modification is -22.4, and the zeta potential after modification is +33.6. The absolute value of the zeta potential increases, which confirms that the silicon oxide surface has a strong positive charge.

[0037] Depend on Figure 3 and Figure 4 As can be seen from (a) and (c), the silicon monoxide / hard carbon composite negative electrode material prepared in this embodiment has a porous structure, and the C layer in the negative electrode material prepared using the carbon source of the present invention contains the impurity element N. The in-situ doping of the N element is beneficial to the conductivity of the material.

[0038] Depend on Figure 4 (b) It can be seen that the surface of the silicon 2 Oxide / hard carbon composite negative electrode material prepared in this example has a carbon coating layer (the portion between the dotted lines is the carbon layer).

[0039] Example 2 according to Figure 1 The flow chart shows a method for preparing a silicon / hard carbon composite negative electrode material having a porous structure. The preparation steps are the same as those in Example 1, except that the amount of sodium chloride added is adjusted. The specific steps are as follows: First, 4 mL of the surfactant polydimethyldiallyl ammonium chloride (molecular weight: 491.06) was weighed and dispersed in 100 mL of distilled water. After thorough stirring for 5 minutes, 1 g of silicon oxide with a diameter of about 500 nm was added. The mixture was stirred and dispersed at 600 rpm for 0.5 h, ultrasonically dispersed at 300 W for 0.5 h, and centrifuged. The water washing and centrifugation steps were repeated three times, and then dried to obtain a silicon oxide material with a positively charged surface.

[0040] Take 0.5 g of the silicon dioxide material with a positive surface charge prepared above, and ultrasonically disperse it in 100 mL of deionized water at 300 W for 30 min. Then add 1 g of gelatin and 2.5 g of sodium chloride, place it in a water bath at 60 °C and stir at 600 rpm for 1 h. After stirring, quickly freeze it in liquid nitrogen, and then freeze-dry it in a freeze dryer for 48 h to obtain the precursor material.

[0041] The precursor material was placed in a crucible and transferred to a high-temperature tube furnace under an argon atmosphere for high-temperature carbonization. The temperature was raised to 750°C at a rate of 5°C / min and held for 3 hours. After carbonization, the sample was cooled to room temperature and placed in a sand-core funnel. It was washed several times with deionized water to remove the template NaCl. The product was then collected on filter paper and dried in a vacuum oven at 80°C. After drying, the sample was removed and ground to obtain a porous silicon oxide / hard carbon composite anode material.

[0042] Example 3 according to Figure 1 The flow chart shows a method for preparing a silicon / hard carbon composite negative electrode material having a porous structure. The preparation steps are the same as those in Example 1, except that the amount of sodium chloride added is adjusted. The specific steps are as follows: First, 4 mL of the surfactant polydimethyldiallyl ammonium chloride (molecular weight: 491.06) was weighed and dispersed in 100 mL of distilled water. After thorough stirring for 5 minutes, 1 g of silicon oxide with a diameter of about 500 nm was added. The mixture was stirred and dispersed at 600 rpm for 0.5 h, ultrasonically dispersed at 300 W for 0.5 h, and centrifuged. The water washing and centrifugation steps were repeated three times, and then dried to obtain a silicon oxide material with a positively charged surface.

[0043] Take 0.5 g of the silicon dioxide material with a positive surface charge prepared above, and ultrasonically disperse it in 100 mL of deionized water at 300 W for 30 min. Then, add 1 g of gelatin and 10 g of sodium chloride, place it in a water bath at 60 °C and stir at 600 rpm for 1 h. After stirring, quickly freeze it in liquid nitrogen, and then freeze-dry it in a freeze dryer for 48 h to obtain the precursor material.

[0044] The precursor material was placed in a crucible and transferred to a high-temperature tube furnace under an argon atmosphere for high-temperature carbonization. The temperature was raised to 750°C at a rate of 5°C / min and held for 3 hours. After carbonization, the sample was cooled to room temperature and placed in a sand-core funnel. It was washed several times with deionized water to remove the template NaCl. The product was then collected on filter paper and dried in a vacuum oven at 80°C. After drying, the sample was removed and ground to obtain a porous silicon oxide / hard carbon composite anode material.

[0045] Example 4 according to Figure 1 The flow chart shows a method for preparing a silicon / hard carbon composite negative electrode material having a porous structure. The preparation steps are the same as those in Example 1, except that the silicon source is adjusted. The specific steps are as follows: First, 4 mL of the surfactant polydimethyldiallyl ammonium chloride (molecular weight: 491.06) was weighed and dispersed in 100 mL of distilled water. After thorough stirring for 5 minutes, 1 g of spherical nanosilicon with a diameter of about 150 nm was added. The mixture was stirred and dispersed at 600 rpm for 0.5 h, ultrasonically dispersed at 300 W for 0.5 h, and centrifuged. The washing and centrifugation steps were repeated three times, and then dried to obtain nanosilicon material with a positive surface charge.

[0046] Take 0.5 g of the above-prepared nano-silicon material with a positive surface charge, dispers it ultrasonically in 100 mL of deionized water at 300 W for 30 min, then add 1 g of gelatin and 5 g of sodium chloride, place it in a 60 °C water bath environment and stir it at 600 rpm for 1 h. After stirring, quickly freeze it in liquid nitrogen and then freeze-dry it in a freeze dryer for 48 h to obtain the precursor material.

[0047] The precursor material was placed in a crucible and transferred to a high-temperature tube furnace under an argon atmosphere for high-temperature carbonization. The temperature was raised at 5°C / min to 750°C and held for 3 hours. After carbonization, the sample was cooled to room temperature and placed in a sand-core funnel. It was washed several times with deionized water to remove the template NaCl. The product on the filter paper was collected and dried in a vacuum oven at 80°C. After drying, the sample was removed and ground to obtain a porous nano-silicon / hard carbon composite anode material.

[0048] Example 5 according to Figure 1 The flow chart shows a method for preparing a silicon / hard carbon composite negative electrode material having a porous structure. The preparation steps are the same as those in Example 1, except that the cationic surfactant is adjusted. The specific steps are as follows: First, 4 mL of the surfactant hexadecyltrimethylammonium chloride (molecular weight: 320) was weighed and dispersed in 100 mL of distilled water. After thorough stirring for 5 minutes, 1 g of silicon dioxide with a diameter of approximately 500 nm was added. The mixture was stirred and dispersed at 600 rpm for 0.5 h, ultrasonically dispersed at 300 W for 0.5 h, and centrifuged. The washing and centrifugation steps were repeated three times, and then dried to obtain a silicon dioxide material with a positively charged surface.

[0049] Take 0.5 g of the silicon dioxide material with a positive surface charge prepared above, dispers it ultrasonically in 100 mL of deionized water at 300 W for 30 min, then add 1 g of gelatin and 5 g of sodium chloride, place it in a water bath at 60 °C and stir it at 600 rpm for 1 h. After stirring, quickly freeze it in liquid nitrogen and then freeze-dry it in a freeze dryer for 48 h to obtain the precursor material.

[0050] The precursor material was placed in a crucible and transferred to a high-temperature tube furnace under an argon atmosphere for high-temperature carbonization. The temperature was raised to 750°C at a rate of 5°C / min and held for 3 hours. After carbonization, the sample was cooled to room temperature and placed in a sand-core funnel. It was washed several times with deionized water to remove the template NaCl. The product was then collected on filter paper and dried in a vacuum oven at 80°C. After drying, the sample was removed and ground to obtain a porous silicon oxide / hard carbon composite anode material.

[0051] Example 6 according to Figure 1 The flow chart shows a method for preparing a silicon / hard carbon composite negative electrode material having a porous structure. The preparation steps are the same as those in Example 1, except that the cationic surfactant is changed to an anionic surfactant. The specific steps are as follows: First, 4 mL of the surfactant dodecylbenzenesulfonic acid (molecular weight: 326.49) was weighed and dispersed in 100 mL of distilled water. After thorough stirring for 5 minutes, 1 g of silicon dioxide with a diameter of about 500 nm was added. The mixture was stirred and dispersed at 600 rpm for 30 minutes, ultrasonically dispersed at 300W for 30 minutes, and centrifuged. The washing and centrifugation steps were repeated three times, and then dried to obtain a silicon dioxide material with a stronger negative charge on the surface.

[0052] Take 0.5 g of the silicon dioxide material with a stronger negative charge on the surface prepared above, and ultrasonically disperse it in 100 mL of deionized water at 300 W for 30 min. Then add 1 g of gelatin and 5 g of sodium chloride and place it in a water bath at 60 °C and stir at 600 rpm for 60 min. After stirring, quickly freeze it in liquid nitrogen and then freeze-dry it in a freeze dryer for 48 h to obtain the precursor material.

[0053] The precursor material was placed in a crucible and transferred to a high-temperature tube furnace under an argon atmosphere for high-temperature carbonization. The temperature was raised to 750°C at a rate of 5°C / min and held for 3 hours. After carbonization, the sample was cooled to room temperature and placed in a sand-core funnel. It was washed several times with deionized water to remove the template NaCl. The product was then collected on filter paper and dried in a vacuum oven at 80°C. After drying, the sample was removed and ground to obtain a porous silicon oxide / hard carbon composite anode material.

[0054] Example 7 according to Figure 1 The flow chart shows a method for preparing a silicon / hard carbon composite negative electrode material having a porous structure. The preparation steps are the same as those in Example 1, except that the amount of gelatin added is adjusted. The specific steps are as follows: First, 4 mL of the surfactant polydimethyldiallyl ammonium chloride (molecular weight: 491.06) was weighed and dispersed in 100 mL of distilled water. After thorough stirring for 5 minutes, 1 g of silicon oxide with a diameter of about 500 nm was added. The mixture was stirred and dispersed at 600 rpm for 30 minutes, ultrasonically dispersed at 300 W for 30 minutes, and centrifuged. The water washing and centrifugation steps were repeated three times, and then dried to obtain a silicon oxide material with a positively charged surface.

[0055] Take 0.5 g of the silicon dioxide material with a positively charged surface prepared above, ultrasonically disperse it in 100 mL of deionized water at 300 W for 30 min, then add 2 g of gelatin and 5 g of sodium chloride, place it in a water bath at 60 °C and stir it at 600 rpm for 60 min. After stirring, quickly freeze it in liquid nitrogen and then freeze-dry it in a freeze dryer for 48 h to obtain the precursor material.

[0056] The precursor material was placed in a crucible and transferred to a high-temperature tube furnace under an argon atmosphere for high-temperature carbonization. The temperature was raised to 750°C at a rate of 5°C / min and held for 3 hours. After carbonization, the sample was cooled to room temperature and placed in a sand-core funnel. It was washed several times with deionized water to remove the template NaCl. The product was then collected on filter paper and dried in a vacuum oven at 80°C. After drying, the sample was removed and ground to obtain a porous silicon oxide / hard carbon composite anode material.

[0057] Example 8 according to Figure 1 The flow chart shows a method for preparing a silicon / hard carbon composite negative electrode material having a porous structure. The preparation steps are the same as those in Example 1, except that the amount of gelatin added is adjusted. The specific steps are as follows: First, 4 mL of the surfactant polydimethyldiallyl ammonium chloride (molecular weight: 491.06) was weighed and dispersed in 100 mL of distilled water. After thorough stirring for 5 minutes, 1 g of silicon oxide with a diameter of about 500 nm was added. The mixture was stirred and dispersed at 600 rpm for 30 minutes, ultrasonically dispersed at 300 W for 30 minutes, and centrifuged. The water washing and centrifugation steps were repeated three times, and then dried to obtain a silicon oxide material with a positively charged surface.

[0058] Take 0.5 g of the silicon dioxide material with a positively charged surface prepared above, ultrasonically disperse it in 100 mL of deionized water at 300 W for 30 min, then add 4 g of gelatin and 5 g of sodium chloride, place it in a water bath at 60 °C and stir it at 600 rpm for 60 min. After stirring, quickly freeze it in liquid nitrogen and then freeze-dry it in a freeze dryer for 48 h to obtain the precursor material.

[0059] The precursor material was placed in a crucible and transferred to a high-temperature tube furnace under an argon atmosphere for high-temperature carbonization. The temperature was raised to 750°C at a rate of 5°C / min and held for 3 hours. After carbonization, the sample was cooled to room temperature and placed in a sand-core funnel. It was washed several times with deionized water to remove the template NaCl. The product was then collected on filter paper and dried in a vacuum oven at 80°C. After drying, the sample was removed and ground to obtain a porous silicon oxide / hard carbon composite anode material.

[0060] Comparative Example 1 The operation is the same as in Example 1, except that sodium chloride is not added and the step of removing sodium chloride is omitted, as follows: First, 4 mL of the surfactant polydimethyldiallyl ammonium chloride (molecular weight: 491.06) was weighed and dispersed in 100 mL of distilled water. After thorough stirring for 5 minutes, 1 g of silicon oxide with a diameter of about 500 nm was added. The mixture was stirred and dispersed at 600 rpm for 0.5 h, ultrasonically dispersed at 300 W for 0.5 h, and centrifuged. The water washing and centrifugation steps were repeated three times, and then dried to obtain a silicon oxide material with a positively charged surface.

[0061] Take 0.5 g of the silicon dioxide material with a positive surface charge prepared above, ultrasonically disperse it in 100 mL of deionized water at 300 W for 30 min, then add 1 g of gelatin and place it in a water bath at 60 °C and stir at 600 rpm for 1 h. After stirring, quickly freeze it in liquid nitrogen and then freeze-dry it in a freeze dryer for 48 h to obtain the precursor material.

[0062] The precursor material was placed in a crucible and transferred to a high-temperature tube furnace under an argon atmosphere for high-temperature carbonization. The temperature was increased at 5°C / min to 750°C and maintained at this temperature for 3 hours. After carbonization, the sample was cooled to room temperature and ground to obtain a porous silicon oxide / hard carbon composite anode material.

[0063] Comparative Example 2 The operation is the same as in Example 1, except that the surfactant is not used to modify the silicon source. The details are as follows: Take 0.5 g of untreated silicon dioxide material with a diameter of about 500 nm, ultrasonically disperse it in 100 mL of deionized water at 300 W for 30 min, then add 1 g of gelatin and 5 g of sodium chloride, place it in a water bath at 60°C and stir it at 600 rpm for 60 min. After stirring, quickly freeze it in liquid nitrogen and then freeze-dry it in a freeze dryer for 48 h to obtain the precursor material.

[0064] The precursor material was placed in a crucible and transferred to a high-temperature tube furnace under an argon atmosphere for high-temperature carbonization. The temperature was raised to 750°C at a rate of 5°C / min and held for 3 hours. After carbonization, the sample was cooled to room temperature and placed in a sand-core funnel. It was washed several times with deionized water to remove the template NaCl. The product was then collected on filter paper and dried in a vacuum oven at 80°C. After drying, the sample was removed and ground to obtain a porous silicon oxide / hard carbon composite anode material.

[0065] Performance test case The negative electrode materials prepared in the above examples and comparative examples were mixed with a conductive agent (SP), a binder (CMC), and a binder (SBR) in a mass ratio of 9:0.5:0.25:0.25 to prepare an electrode slurry. The slurry was evenly coated on a copper current collector, dried under vacuum at 105°C for 12 hours, and punched out using a sheet puncher to obtain negative electrode sheets. CR2032 batteries were assembled using lithium sheets as counter electrodes in a glove box maintained at water and oxygen levels less than 0.1 ppm. The button cells were tested for charge and discharge performance using a Blue Electric battery test system, with a charge and discharge cutoff voltage range of 0.01 V to 1.5 V and a test temperature of 25°C. The same testing method was used in the following examples and comparative examples.

[0066] The first discharge capacity, first efficiency and capacity retention after 400 cycles of Comparative Examples 1-2 and Examples 1-8 at 1 A / g are shown in Table 1: Table 1. Power-off test results

[0067] As can be seen from the data in Table 1, Example 4 has the highest first discharge capacity and first coulombic efficiency, but its performance in long-term cycling is poor, with a capacity retention rate of only 41.1% after 400 cycles. Among the other samples using silicon monoxide as the silicon source, the button cell prepared with the silicon-carbon composite material of Example 1 as the active material has the highest first discharge capacity compared to the button cells prepared with other samples, and the capacity retention rate after 400 cycles is also the highest, and the first efficiency is also at an intermediate level among all samples. This shows that the preparation method of Example 1 of the present invention can significantly improve the cycle performance of the silicon-carbon composite material ( Figure 5 ).

[0068] Compared with Example 2 and Example 3, the amount of salt added in Example 1 is more appropriate. Too little salt will not cause the silicon coating to be thicker and too few pores to accommodate silicon expansion, nor will too much salt cause the silicon coating to be too thin and the porous carbon skeleton to be too fragile and the structure to be destroyed during the cycle.

[0069] Compared with Example 1, Example 6 adjusts the cationic surfactant to an anionic surfactant. The initial capacity, first-effect performance and capacity retention rate of the negative electrode material prepared from the silicon source modified with the anionic surfactant are lower than those in Example 1. Cationic surfactants are preferred in the method of the present invention.

[0070] Compared with Example 1, Example 7 and Example 8 adjusted the amount of carbon source added, and the initial performance was similar, with a slightly higher capacity retention rate. However, the initial capacity was low, which was insufficient to reflect the high capacity advantage of silicon-based materials.

[0071] Compared with Comparative Example 1, Example 1 added sodium chloride and removed the sodium chloride in the sample after carbonization. Compared with Comparative Example 1, the first discharge capacity and capacity retention rate of Example 1 were significantly improved. This is because the introduction of the salt template enables the material to produce pores that accommodate silicon expansion, which is more conducive to alleviating the expansion of the material during the cycle, and the prepared negative electrode material has better electrochemical properties.

[0072] Compared with Comparative Example 2, Example 1 performs surface modification on the silicon source. Compared with Comparative Example 2, Example 1 has improved initial discharge capacity, initial coulombic efficiency and capacity retention rate. This is because the surface of the silicon source is modified by using a surfactant to make it positively charged, which can improve its dispersibility in aqueous solution. The principle of like charges repel each other is used to alleviate the problem of easy agglomeration of nano-scale silicon materials in aqueous solution, thereby improving the cycle performance of the battery.

[0073] In summary, the purpose of the present invention is to provide a method for preparing a silicon-carbon composite negative electrode material for lithium-ion batteries. The prepared silicon-carbon composite negative electrode material has the advantage of high cycle stability. Compared with the traditional liquid-phase coating method for preparing silicon-carbon composite negative electrode materials, the silicon surface is first modified to improve its dispersibility in aqueous solution. Secondly, the confinement effect of the template is utilized to enable gelatin to uniformly encapsulate the template and silicon. After washing away the template, a large number of holes are left. Controlling the amount of template added can achieve controllable preparation of silicon / hard carbon porous materials. The derived amorphous carbon serves as the skeleton of the material and coats the silicon. It not only effectively disperses the silicon and improves the utilization efficiency of the active material, but the carbon in situ coated on the silicon surface can also effectively block the electrolyte to prevent it from continuously decomposing on the silicon surface. In addition, this reaction uses a soluble salt as a template, which is cheap, safe, environmentally friendly, and reusable.

[0074] The present invention has been described in detail above. For those skilled in the art, without departing from the purpose and scope of the present invention, can implement the present invention in a wider range under equivalent parameters, concentration and conditions. Although the present invention provides specific embodiments, it should be understood that the present invention can be further improved. In a word, according to the principle of the present invention, the application is intended to include any variation, purposes or improvements of the present invention, including departing from the disclosed scope in the application and the changes made with conventional techniques known in the art.

Claims

1. A method for preparing a silicon / hard carbon composite negative electrode material having a porous structure, characterized in that: The steps include: S1. Dispersing a cationic or anionic surfactant and a silicon source in water, and separating the solid after dispersion to obtain a silicon source with a positive or negative surface charge; The silicon source is nano-silicon or silicon monoxide; S2. dispersing the silicon source with positive or negative surface charge in water, adding a soluble carbon source and a soluble salt, stirring, and freezing to obtain a precursor material; The soluble carbon source is one or two of carrageenan, xanthan gum and gelatin; The soluble salt is sodium chloride or potassium chloride; S3. Carbonizing the precursor material at 600-900° C. under inert gas protection, washing with water to remove the soluble salt, and drying to obtain the silicon / hard carbon composite negative electrode material with a porous structure.

2. The method for preparing a porous silicon / hard carbon composite negative electrode material according to claim 1, wherein: The cationic surfactant is polydimethyldiallylammonium chloride, hexadecyltrimethylammonium chloride or octadecyldimethylbenzylammonium chloride; The anionic surfactant is dodecylbenzenesulfonic acid or sodium fatty alcohol ether sulfate; The ratio of the surfactant to the silicon source is (1-5) mL:1 g; The particle size of the nano-silicon is 50 to 250 nm; The particle size of the silicon monoxide is 300-600 nm.

3. The method for preparing a porous silicon / hard carbon composite negative electrode material according to claim 2, wherein: In step S1, the volume ratio of the surfactant to water is (2-6):100; The dispersion comprises sequentially performing stirring dispersion at 200 to 1000 rpm for 20 to 60 minutes and ultrasonic dispersion at 100 to 600 W for 20 to 60 minutes.

4. The method for preparing a porous silicon / hard carbon composite negative electrode material according to claim 1, wherein: The mass ratio of the soluble carbon source to the silicon source with a positive or negative surface charge is (2-8):1; The mass ratio of the soluble salt to the silicon source with positive or negative charge on the surface is (2.5-25):

1.

5. The method for preparing a silicon / hard carbon composite negative electrode material having a porous structure according to claim 4, wherein: In step S2, the mass ratio of the silicon source with positive or negative surface charge to water is (0.2-1.2):100; In step S2, the dispersion is performed by ultrasonic dispersion at 100-600W for 0.5 h; The stirring temperature is 20 to 80°C; The stirring process is performed at a rotation speed of 200 to 1000 rpm and for a time of 20 to 60 min.

6. The method for preparing a silicon / hard carbon composite negative electrode material having a porous structure according to claim 1, wherein: In the carbonization treatment step, the heating rate is 2 to 10° C. / min and the holding time is 2 to 4 hours.

7. The method for preparing a porous silicon / hard carbon composite negative electrode material according to claim 1, wherein: The water washing is performed by placing the mixture in a funnel and performing suction filtration and water washing.

8. A silicon / hard carbon composite negative electrode material with a porous structure prepared by the method according to any one of claims 1 to 7.

9. The silicon / hard carbon composite negative electrode material with a porous structure according to claim 8, characterized in that: In the silicon / hard carbon composite negative electrode material with a porous structure, the mass content of silicon is 25% to 75%.

10. A lithium-ion battery comprising a negative electrode plate, characterized in that: The negative electrode plate includes a current collector and the silicon / hard carbon composite negative electrode material with a porous structure according to claim 8 or 9 arranged on the current collector.

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