Preparation method and application of silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly

Abstract

The invention discloses a preparation method and application of a silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly, and belongs to the technical field of electrode materials. The invention aims to solve the problems of fast capacity reduction and poor battery stability caused by the fact that silicon particles in a material fall off an electrode due to non-uniform dispersion between silicon and a graphite material, weak binding force and incapability of relieving volume expansion of silicon in a circulation process in the prior art. The method comprises the following steps: 1, preparing functionalized nano silicon particles; 2, preparing acidified graphite; 3, preparing a mixed dispersion liquid; 4, evaporating the mixed dispersion liquid; and 5, high-temperature calcination. The silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly is used as a negative electrode material of a lithium ion battery. Through two-step liquid-phase electrostatic self-assembly, chemical bonding force and physical bonding force are introduced to act on silicon and graphite at the same time, and the prepared silicon-carbon negative electrode material based on two-step electrostatic self-assembly has excellent rate capability and cycling stability and is suitable for large-scale industrial production.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrode materials, and in particular relates to a preparation method and application of a silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly. Background Art

[0002] As the performance of 3C digital products continues to improve, the requirements for batteries are also increasing. Traditional commercial lithium-ion batteries using only graphite as the negative electrode have reached a bottleneck. To meet emerging technological needs, combining graphite with silicon, which has a high theoretical specific capacity, has become a simple and effective approach. However, the two materials cannot work together due to their different charging and discharging mechanisms. Problems such as the inability to tightly bond between the materials and the severe volume expansion of silicon continue to restrict the further development of this strategy.

[0003] In order to solve the above problems, many researchers have dispersed silicon in graphite through methods such as ball milling, spray drying and liquid-phase electrostatic self-assembly. Although the problem of uniformity in mixing the two materials can be solved to a certain extent, the above dispersion methods cannot solve the problems of uneven dispersion and weak bonding between the two materials. This problem causes the volume expansion of silicon during the cycle to not only cause the silicon itself to detach from the electrode, but also interfere with the charge and discharge cycle of the graphite, ultimately leading to a decrease in the cycle stability of the battery.

[0004] Therefore, the present invention addresses the problem of weak binding forces due to the limited chemical bonding of conventional electrostatic self-assembly. By using a two-step liquid-phase electrostatic self-assembly, this approach introduces both chemical bonding and physical adhesion. This not only solves the dispersion and bonding issues between the two materials, but also, through the additional electrostatic self-assembly step, the carbon source introduced in a targeted manner increases physical adhesion while protecting the silicon particles, thereby unleashing the high theoretical specific capacity of silicon and the high cycling stability of graphite. Summary of the Invention

[0005] The purpose of the present invention is to solve the problems in the prior art of uneven dispersion, weak bonding between silicon and graphite materials, and inability to alleviate the volume expansion of silicon during the cycle, which causes silicon particles in the material to fall off the electrode, thereby leading to rapid capacity decline and poor battery stability. A preparation method and application of silicon-carbon negative electrode materials based on two-step liquid-phase electrostatic self-assembly are provided.

[0006] For the problems referred to above, the present invention is by two-step liquid phase electrostatic self-assembly, introduce chemical bonding force and physical cohesive force to act on silicon and graphite and solve both combination and dispersion problems simultaneously, by positively charging the nano-silicon surface, adopt mixed acid acidification graphite simultaneously so that graphite surface is rich in negatively charged groups such as carboxyl, by the effect of the electrostatic force produced by functionalized groups in solution, silicon is uniformly dispersed on the graphite surface by orientation, then again by hybrid material functionalization, make nano-silicon-graphite material surface rich in carboxylic acid particles, and be combined with positively charged chitosan in solution, finally complete chitosan carbonization at high temperature, further strengthen the bonding force between nano-silicon and graphite, glacial acetic acid provides space for the volume expansion of silicon as pore agent simultaneously.Complete the dispersion of nano-silicon particles and the anchoring on graphite by above two-step liquid phase electrostatic self-assembly.

[0007] The present invention provides a method for preparing a silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly, which is specifically completed by the following steps:

[0008] 1. Preparation of functionalized nano-silicon particles:

[0009] The nano-silicon is dispersed in deionized water, and then a polydiallyldimethylammonium chloride solution is added, ultrasonicated and stirred for a period of time, and then the excess polydiallyldimethylammonium chloride solution is removed by multiple centrifugal water washing and centrifugal alcohol washing, and finally vacuum dried to obtain functionalized silicon nanoparticles;

[0010] 2. Preparation of acidified graphite:

[0011] Graphite is placed in a container, and then a mixture of concentrated sulfuric acid and concentrated nitric acid is added, ultrasonicated and stirred at room temperature for a period of time, then stirred and refluxed at high temperature for a period of time, and finally subjected to multiple centrifugal water washing, centrifugal alcohol washing, and vacuum drying to obtain acidified graphite;

[0012] 3. Prepare mixed dispersion:

[0013] Dispersing the functionalized silicon nanoparticles and acidified graphite in a mixture of deionized water and N-methylpyrrolidone, stirring uniformly to obtain a dispersion of the functionalized silicon nanoparticles and acidified graphite, then adding glacial acetic acid, and adding chitosan after the solution becomes uniform, stirring uniformly to obtain a mixed dispersion;

[0014] Fourth, heating the mixed dispersion in a water bath and magnetically stirring until the solvent is completely evaporated to obtain a nano-silicon-graphite composite material precursor;

[0015] 5. High temperature calcination:

[0016] The nano-silicon-graphite composite material precursor is calcined at high temperature to obtain a silicon-carbon negative electrode material based on two-step liquid phase electrostatic self-assembly.

[0017] A silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly is used as a negative electrode material for lithium-ion batteries.

[0018] Beneficial effects of the present invention:

[0019] 1. The present invention functionalizes nano-silicon and graphite materials, then introduces chitosan as a carbon source and uses glacial acetic acid solution as a dispersion liquid to perform two electrostatic self-assembly treatments, thereby overcoming the problem of poor charge and discharge performance of the material due to the inability of nano-silicon and graphite to be tightly combined; the two functionalized materials complete the directional dispersion of nano-silicon on the surface of graphite through the first electrostatic self-assembly. The treatment with glacial acetic acid solution not only makes the surface of the nano-silicon-graphite composite material precursor rich in carboxyl groups, thereby further completing the second electrostatic self-assembly with chitosan, but also glacial acetic acid can act as a pore agent to provide space for the volume expansion of nano-silicon during high-temperature treatment; and chitosan as a carbon layer not only ensures the stable formation of the SEI film on the surface of nano-silicon, but also bonds nano-silicon and graphite; the silicon-carbon material that has undergone two electrostatic self-assemblies has high specific capacity and cycle stability;

[0020] 2. The silicon-carbon negative electrode material prepared by the present invention based on two-step liquid-phase electrostatic self-assembly has a simple process, low raw material cost, and is suitable for large-scale industrial production.

[0021] The present invention can obtain a lithium ion battery negative electrode material. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Scanning electron microscope images of the silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly prepared in Example 1 at different magnifications;

[0023] Figure 2 The figure shows the rate performance of the silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly prepared in Example 1, which was subjected to constant current charge and discharge tests at different current densities;

[0024] Figure 3 The silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly prepared in Example 1 was -1 Cyclic performance curve of constant current charge and discharge test under current density;

[0025] Figure 4 Scanning electron micrographs of the silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly prepared in Example 2 at different magnifications;

[0026] Figure 5 This is a rate performance diagram of the silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly prepared in Example 2 subjected to constant current charge and discharge tests at different current densities;

[0027] Figure 6The silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly prepared in Example 2 was -1 Cyclic performance curve of constant current charge and discharge test under current density;

[0028] Figure 7 Scanning electron micrographs of the silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly prepared in Example 3 at different magnifications;

[0029] Figure 8 This is a rate performance diagram of the silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly prepared in Example 3 subjected to constant current charge and discharge tests at different current densities;

[0030] Figure 9 The silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly prepared in Example 3 was -1 Cyclic performance curve of constant current charge and discharge test under current density;

[0031] Figure 10 The scanning electron microscope images of the silicon-carbon negative electrode material prepared in Comparative Example 1 at different magnifications;

[0032] Figure 11 The figure is a rate performance diagram of the silicon-carbon negative electrode material prepared in Comparative Example 1 under constant current charge and discharge tests at different current densities;

[0033] Figure 12 The silicon-carbon negative electrode material prepared in comparative example 1 is -1 Cyclic performance curve of constant current charge and discharge test under current density;

[0034] Figure 13 The figure is a rate performance diagram of the silicon-carbon negative electrode material prepared in comparative example 2 under constant current charge and discharge tests at different current densities;

[0035] Figure 14 The silicon-carbon negative electrode material prepared in comparative example 2 is 1A·g -1 Cycling performance curve of constant current charge and discharge test under current density. DETAILED DESCRIPTION

[0036] The following examples further illustrate the present invention, but should not be construed as limiting the present invention. Without departing from the essence of the present invention, modifications and substitutions made to the methods, steps or conditions of the present invention are within the scope of the present invention.

[0037] Specific embodiment 1: This embodiment is a method for preparing a silicon-carbon negative electrode material based on two-step liquid phase electrostatic self-assembly, which is specifically completed by the following steps:

[0038] 1. Preparation of functionalized nano-silicon particles:

[0039] The nano-silicon is dispersed in deionized water, and then a polydiallyldimethylammonium chloride solution is added, ultrasonicated and stirred for a period of time, and then the excess polydiallyldimethylammonium chloride solution is removed by multiple centrifugal water washing and centrifugal alcohol washing, and finally vacuum dried to obtain functionalized silicon nanoparticles;

[0040] 2. Preparation of acidified graphite:

[0041] Graphite is placed in a container, and then a mixture of concentrated sulfuric acid and concentrated nitric acid is added, ultrasonicated and stirred at room temperature for a period of time, then stirred and refluxed at high temperature for a period of time, and finally subjected to multiple centrifugal water washing, centrifugal alcohol washing, and vacuum drying to obtain acidified graphite;

[0042] 3. Prepare mixed dispersion:

[0043] Dispersing the functionalized silicon nanoparticles and acidified graphite in a mixture of deionized water and N-methylpyrrolidone, stirring uniformly to obtain a dispersion of the functionalized silicon nanoparticles and acidified graphite, then adding glacial acetic acid, and adding chitosan after the solution becomes uniform, stirring uniformly to obtain a mixed dispersion;

[0044] Fourth, heating the mixed dispersion in a water bath and magnetically stirring until the solvent is completely evaporated to obtain a nano-silicon-graphite composite material precursor;

[0045] 5. High temperature calcination:

[0046] The nano-silicon-graphite composite material precursor is calcined at high temperature to obtain a silicon-carbon negative electrode material based on two-step liquid phase electrostatic self-assembly.

[0047] Specific embodiment 2: This embodiment differs from specific embodiment 1 in that the particle size of the nano-silicon described in step 1 is 20 nm to 50 nm; the mass fraction of the polydiallyldimethylammonium chloride solution described in step 1 is 20% to 35%, and the molecular weight of the polydiallyldimethylammonium chloride is less than 100,000; and the mass volume ratio of the nano-silicon, polydiallyldimethylammonium chloride solution, and deionized water described in step 1 is (0.1 g to 0.2 g): 2 mL: 25 mL. Other steps are the same as specific embodiment 1.

[0048] Specific embodiment 3: This embodiment differs from specific embodiments 1 or 2 in that: the ultrasonic stirring time in step 1 is 3 to 6 hours; the number of centrifugal water washing and centrifugal alcohol washing in step 1 is not less than 3 times; the speed of centrifugal water washing and centrifugal alcohol washing in step 1 is 8000 rpm to 10000 rpm, and the duration of each centrifugation is 10 minutes; the vacuum drying temperature in step 1 is 60°C to 80°C, and the vacuum drying time is 12 to 14 hours. The other steps are the same as those in specific embodiments 1 or 2.

[0049] Specific embodiment 4: This embodiment differs from specific embodiments 1 to 3 in that: the mass fraction of the concentrated sulfuric acid in step 2 is 98%; the mass fraction of the concentrated nitric acid in step 2 is 68%; the volume ratio of the mass of the graphite in step 2 to the mixed solution of concentrated sulfuric acid and concentrated nitric acid is 1g:(10mL-15mL); the volume ratio of the concentrated sulfuric acid to the concentrated nitric acid in the mixed solution of concentrated sulfuric acid and concentrated nitric acid in step 2 is (3mL-10mL):(1mL-4mL); the ultrasonic stirring time in step 2 is 30min-60min. The other steps are the same as specific embodiments 1 to 3.

[0050] Specific embodiment 5: This embodiment differs from specific embodiments 1 to 4 in that the stirring and reflux under high temperature in step 2 is performed at a temperature of 80°C to 90°C for 4 to 6 hours; the number of centrifugal water washing and centrifugal alcohol washing in step 2 is no less than 3 times; the speed of the centrifugal water washing and centrifugal alcohol washing in step 2 is 8000 rpm to 10000 rpm, and the duration of each centrifugation is 10 minutes. The other steps are the same as specific embodiments 1 to 4.

[0051] Specific embodiment 6: This embodiment differs from specific embodiments 1 to 5 in that the volume ratio of the functionalized silicon nanoparticles, acidified graphite, and the mixture of deionized water and N-methylpyrrolidone in step 3 is (20 mg to 40 mg): (40 mg to 60 mg): (8 mL to 10 mL); the volume ratio of deionized water to N-methylpyrrolidone in the mixture of deionized water and N-methylpyrrolidone in step 3 is (5 mL to 7 mL): (3 mL to 5 mL); and the mass fraction of glacial acetic acid in step 3 is 97%. The other steps are the same as specific embodiments 1 to 5.

[0052] Specific embodiment 7: This embodiment differs from Specific embodiments 1 to 6 in that the volume ratio of glacial acetic acid to the dispersion of functionalized silicon nanoparticles and acidified graphite in step 3 is (100 μL to 125 μL):(10 mL to 12 mL); and the total mass ratio of chitosan to functionalized silicon nanoparticles and acidified graphite in step 3 is (15 mg to 40 mg):(60 mg to 100 mg). The other steps are the same as Specific embodiments 1 to 6.

[0053] Specific embodiment eight: This embodiment differs from specific embodiments one to seven in that the water bath heating temperature in step four is 80°C to 90°C; the high-temperature calcination process in step five is as follows: the nano-silicon-graphite composite material precursor is heated from room temperature to 800°C to 1000°C at a heating rate of 2°C / min to 5°C / min, kept at this temperature for 1 to 2 hours, then cooled to 100°C at a cooling rate of 2°C / min to 5°C / min, and finally cooled naturally to room temperature. The entire heat treatment process is carried out in argon or a hydrogen-argon mixed gas; the volume fraction of hydrogen in the hydrogen-argon mixed gas is 5% or 10%. The other steps are the same as those in specific embodiments one to seven.

[0054] Specific embodiment 9: This embodiment is a silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly used as a negative electrode material for lithium-ion batteries.

[0055] Specific embodiment 10: This embodiment differs from specific embodiments 1 to 9 in that the lithium-ion battery is prepared as follows: a slurry is prepared based on a two-step liquid-phase electrostatic self-assembly silicon-carbon negative electrode material, acetylene black, and a binder in a mass ratio of 7:1.5:1.5, and the slurry is evenly coated on a copper foil and dried to prepare a negative electrode, wherein the binder is battery-grade sodium carboxymethyl cellulose and styrene-butadiene rubber in a mass ratio of 2:3; the electrolyte is 1.0 mol·L -1 A mixed organic solution of lithium hexafluorophosphate is prepared, wherein the mixed organic solution is a mixture of ethylene carbonate and diethyl carbonate in a mass ratio of 1:1, and the additive is fluoroethylene carbonate at a mass fraction of 5%. The separator is a microporous polyethylene separator, and the positive electrode is a lithium sheet. The battery is assembled into a CR2025 button cell. The other steps are the same as those in Specific Embodiments 1 to 9.

[0056] The following examples are used to verify the beneficial effects of the present invention:

[0057] Example 1: A method for preparing a silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly is specifically completed by the following steps:

[0058] 1. Preparation of functionalized nano-silicon particles:

[0059] 0.1 g of nano-silicon was dispersed in 25 mL of deionized water, and then 2 mL of a 35% polydiallyldimethylammonium chloride solution was added. The mixture was ultrasonically stirred for 3 hours, and then the excess polydiallyldimethylammonium chloride solution was removed by multiple centrifugal water washing and centrifugal alcohol washing. Finally, the mixture was vacuum dried at 60°C for 12 hours to obtain functionalized silicon nanoparticles.

[0060] The particle size of the nano-silicon in step 1 is 20 nm to 50 nm;

[0061] The molecular weight of the polydiallyldimethylammonium chloride described in step 1 is less than 100,000;

[0062] The number of centrifugal water washing and centrifugal alcohol washing in step 1 is 3 times; the speed of centrifugal water washing and centrifugal alcohol washing in step 1 is 8000 rpm, and the time of each centrifugation is 10 minutes;

[0063] 2. Preparation of acidified graphite:

[0064] 1 g of graphite was placed in a container, followed by a 10 mL mixture of concentrated sulfuric acid and concentrated nitric acid. The mixture was ultrasonically stirred at room temperature for 30 minutes, then stirred and refluxed at 80°C for 6 hours. Finally, the mixture was centrifugally washed with water, centrifugally washed with alcohol, and vacuum dried multiple times to obtain acidified graphite.

[0065] The number of centrifugal water washing and centrifugal alcohol washing in step 2 is not less than 3 times; the speed of centrifugal water washing and centrifugal alcohol washing in step 2 is 8000 rpm, and the time of each centrifugation is 10 minutes;

[0066] The mass fraction of the concentrated sulfuric acid in step 2 is 98%; the mass fraction of the concentrated nitric acid in step 2 is 68%;

[0067] The volume ratio of concentrated sulfuric acid to concentrated nitric acid in the mixed solution of concentrated sulfuric acid and concentrated nitric acid described in step 2 is 3:1;

[0068] 3. Prepare mixed dispersion:

[0069] 30 mg of functionalized silicon nanoparticles and 50 mg of acidified graphite were dispersed in a mixture of 10 mL of deionized water and N-methylpyrrolidone and stirred to obtain a dispersion of functionalized silicon nanoparticles and acidified graphite. 125 μL of glacial acetic acid was then added. After the solution became homogeneous, 24 mg of chitosan was added and stirred to obtain a mixed dispersion.

[0070] The volume ratio of deionized water to N-methylpyrrolidone in the mixture of deionized water and N-methylpyrrolidone described in step 3 is 7 mL:3 mL;

[0071] The mass fraction of the glacial acetic acid in step 3 is 97%;

[0072] The chitosan deacetylation degree in step 3 is ≥ 95%;

[0073] 4. heating the mixed dispersion in a water bath at 90° C. and magnetically stirring until the solvent is completely evaporated to obtain a nano-silicon-graphite composite material precursor;

[0074] 5. High temperature calcination:

[0075] The nano-silicon-graphite composite material precursor is calcined at high temperature to obtain a silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly;

[0076] The high-temperature calcination process described in step five is as follows: heating the nano-silicon-graphite composite material precursor from room temperature to 1000°C at a heating rate of 5°C / min, keeping it warm for 2 hours, then cooling it to 100°C at a cooling rate of 5°C / min, and finally cooling it naturally to room temperature. The entire heat treatment process is carried out in argon.

[0077] Take a small amount of the silicon-carbon negative electrode material based on two-step liquid phase electrostatic self-assembly prepared in Example 1 and smear it on the surface of the conductive glue, place it on a metal copper sample holder, and observe the morphology using a field emission scanning electron microscope. The results are as follows: Figure 1 As shown;

[0078] Figure 1 Scanning electron microscope images of the silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly prepared in Example 1 at different magnifications;

[0079] from Figure 1 It can be seen that after the two-step electrostatic treatment, the material as a whole presents micron-sized secondary particles. The agglomeration phenomenon of nano-silicon particles after electrostatic treatment is alleviated. Graphite and nano-silicon are combined through electrostatic force and high-temperature carbonization of chitosan. Nano-silicon is irregularly distributed on the surface of graphite, and only a small amount of nano-silicon exists alone, indicating that the composite structure of nano-silicon and graphite is successfully constructed. At the same time, a small amount of carbonized chitosan can be observed coating the surface of nano-silicon, which not only consolidates the bonding force between nano-silicon and graphite, but also plays a role in protecting nano-silicon particles during the circulation process.

[0080] The silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly prepared in Example 1 was prepared into an electrode and assembled into a lithium-ion half-cell for constant current charge and discharge testing. The specific assembly method was as follows:

[0081] The silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly prepared in Example 1, acetylene black and a binder were prepared into a slurry in a mass ratio of 7:1.5:1.5, and the slurry was evenly coated on a copper foil and dried to prepare a negative electrode. The binder was battery-grade sodium carboxymethyl cellulose and styrene-butadiene rubber in a mass ratio of 2:3; the electrolyte was 1.0 mol·L -1 The mixed organic solution of lithium hexafluorophosphate is prepared by mixing ethylene carbonate and diethyl carbonate in a mass ratio of 1:1, and the additive is fluoroethylene carbonate with a mass fraction of 5%. The separator is a microporous polyethylene separator, and the positive electrode is a lithium sheet. The battery is assembled into a CR2025 button cell with a charge and discharge voltage range of 0.01 to 1.5 V. The test is carried out at a constant temperature of 25°C. Figures 2-3 As shown;

[0082] Figure 2The figure shows the rate performance of the silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly prepared in Example 1, which was subjected to constant current charge and discharge tests at different current densities;

[0083] Figure 3 The silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly prepared in Example 1 was -1 Cyclic performance curve of constant current charge and discharge test under current density;

[0084] from Figures 2 and 3 It can be seen that the material is at 0.1A·g -1 The reversible specific capacity at the current density is 1042 mA·h·g -1 The material also has a high first coulombic efficiency of 81.4%. When the current density increases to 1A·g -1 The reversible specific capacity is still 924 mA·h·g -1 , and when the current density is increased from 5A·g -1 Back to 0.5A·g -1 Its reversible specific capacity has basically no attenuation, showing good rate performance and recovery. -1 After 300 cycles, there is still 510 mA·h·g -1 , the capacity retention rate reaches 55%.

[0085] Example 2: This example differs from Example 1 in that, in step 3, 40 mg of functionalized silicon nanoparticles and 40 mg of acidified graphite were dispersed in a mixture of 10 mL of deionized water and N-methylpyrrolidone and stirred to obtain a dispersion of the functionalized silicon nanoparticles and acidified graphite. 125 μL of glacial acetic acid was then added, and after the solution became homogeneous, 24 mg of chitosan was added and stirred to obtain a mixed dispersion. All other steps and parameters were the same as in Example 1.

[0086] Take a small amount of the silicon-carbon negative electrode material based on two-step liquid phase electrostatic self-assembly prepared in Example 2 and smear it on the surface of the conductive glue, place it on a metal copper sample holder, and observe the morphology using a field emission scanning electron microscope. The results are as follows: Figure 4 As shown;

[0087] Figure 4 Scanning electron micrographs of the silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly prepared in Example 2 at different magnifications;

[0088] from Figure 4 It can be seen that in Example 2, the nano-silicon content is increased compared to Example 1, so that the nano-silicon distribution on the graphite surface is denser. However, too many nano-silicon particles also result in more nano-silicon particles failing to tightly combine with the graphite, resulting in a decrease in the structural stability of the material.

[0089] The silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly prepared in Example 2 was prepared into an electrode and assembled into a lithium-ion half-cell for constant current charge and discharge testing. The specific assembly method was as follows:

[0090] The silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly prepared in Example 2, acetylene black and a binder were prepared into a slurry in a mass ratio of 7:1.5:1.5, and evenly coated on a copper foil and dried to prepare a negative electrode. The binder was battery-grade sodium carboxymethyl cellulose and styrene-butadiene rubber in a mass ratio of 2:3; the electrolyte was 1.0 mol·L -1 The mixed organic solution of lithium hexafluorophosphate is prepared by mixing ethylene carbonate and diethyl carbonate in a mass ratio of 1:1, and the additive is fluoroethylene carbonate with a mass fraction of 5%. The separator is a microporous polyethylene separator, and the positive electrode is a lithium sheet. The battery is assembled into a CR2025 button cell with a charge and discharge voltage range of 0.01 to 1.5 V. The test is carried out at a constant temperature of 25°C. Figures 5-6 As shown;

[0091] Figure 5 This is a rate performance diagram of the silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly prepared in Example 2 subjected to constant current charge and discharge tests at different current densities;

[0092] Figure 6 The silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly prepared in Example 2 was -1 Cyclic performance curve of constant current charge and discharge test under current density;

[0093] from Figure 5 and Figure 6 It can be seen that although the silicon-carbon negative electrode material synthesized in Example 2 is -1 The reversible specific capacity is as high as 1328mAh·g at the current density -1 The coulombic efficiency reached 84.8% for the first time, but the rate performance and cycle stability of the material decreased. -1 The current density returns to 0.5A·g after charge and discharge -1 The current density and capacity decreased compared with the initial low current density cycle, indicating that the material stability decreased compared with Example 1. -1 The long-cycle capacity at this current density also decays rapidly, with a capacity retention rate of 31.9% after 300 cycles. Although increasing the nano-silicon content can increase the material's reversible specific capacity, it also leads to a significant decrease in the material's stability.

[0094] Example 3: This example differs from Example 1 in that, in step 3, 20 mg of functionalized silicon nanoparticles and 60 mg of acidified graphite were dispersed in a mixture of 10 mL of deionized water and N-methylpyrrolidone and stirred to obtain a dispersion of the functionalized silicon nanoparticles and acidified graphite. 125 μL of glacial acetic acid was then added, and after the solution became homogeneous, 24 mg of chitosan was added and stirred to obtain a mixed dispersion. All other steps and parameters were the same as in Example 1.

[0095] Take a small amount of the silicon-carbon negative electrode material based on two-step liquid phase electrostatic self-assembly prepared in Example 3 and smear it on the surface of the conductive glue, place it on a metal copper sample holder, and observe the morphology using a field emission scanning electron microscope. The results are as follows: Figure 7 As shown;

[0096] Figure 7 Scanning electron micrographs of the silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly prepared in Example 3 at different magnifications;

[0097] from Figure 7 It can be seen that in Example 3, compared with Example 1, due to the decrease in silicon content, nano-silicon is evenly distributed on the graphite surface, and the material has a stable secondary particle structure. However, due to the insufficient content of nano-silicon, the graphite surface is not completely covered with nano-silicon particles, and part of the graphite is exposed. The reduction in nano-silicon content limits the uniformity of the material.

[0098] The silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly prepared in Example 3 was prepared into an electrode and assembled into a lithium-ion half-cell for constant current charge and discharge testing. The specific assembly method was as follows:

[0099] The silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly prepared in Example 3, acetylene black and a binder were prepared into a slurry in a mass ratio of 7:1.5:1.5, and evenly coated on a copper foil and dried to prepare a negative electrode. The binder was battery-grade sodium carboxymethyl cellulose and styrene-butadiene rubber in a mass ratio of 2:3; the electrolyte was 1.0 mol·L -1 The mixed organic solution of lithium hexafluorophosphate is prepared by mixing ethylene carbonate and diethyl carbonate in a mass ratio of 1:1, and the additive is fluoroethylene carbonate with a mass fraction of 5%. The separator is a microporous polyethylene separator, and the positive electrode is a lithium sheet. The battery is assembled into a CR2025 button cell with a charge and discharge voltage range of 0.01 to 1.5 V. The test is carried out at a constant temperature of 25°C. Figures 8-9 As shown;

[0100] Figure 8 This is a rate performance diagram of the silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly prepared in Example 3 subjected to constant current charge and discharge tests at different current densities;

[0101] Figure 9 The silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly prepared in Example 3 was -1 Cyclic performance curve of constant current charge and discharge test under current density;

[0102] from Figure 8 and Figure 9 It can be seen that although Example 3 is at 0.1A·g -1 The reversible specific capacity at the current density reaches 841 mAh g -1 Due to the stability of the material structure, the material has good rate performance and cycle stability. -1 The current density returns to 0.5A·g after charge and discharge -1 The current density and capacity are basically unchanged compared to the initial low current density cycle, but the reversible specific capacity of the material is somewhat lower than that of Example 1. The insufficient nano-silicon content seriously affects the upper limit of the material's performance.

[0103] Comparative Example 1: A method for preparing a silicon-carbon negative electrode material is specifically prepared according to the following steps:

[0104] 1. Prepare mixed dispersion:

[0105] 30 mg of nano-silicon and 50 mg of graphite were dispersed in a mixture of 10 mL of deionized water and N-methylpyrrolidone, and stirred to obtain a dispersion of nano-silicon and graphite. 125 μL of glacial acetic acid was then added. After the solution was uniform, 24 mg of chitosan was added and stirred to obtain a mixed dispersion.

[0106] The volume ratio of deionized water to N-methylpyrrolidone in the mixture of deionized water and N-methylpyrrolidone described in step 3 is 7 mL:3 mL;

[0107] The mass fraction of the glacial acetic acid in step 3 is 97%;

[0108] The chitosan deacetylation degree in step 3 is ≥ 95%;

[0109] 2. heating the mixed dispersion in a water bath at 90° C. and magnetically stirring until the solvent is completely evaporated to obtain a nano-silicon-graphite composite material precursor;

[0110] 3. High temperature calcination:

[0111] calcining the nano-silicon-graphite composite material precursor at high temperature to obtain a silicon-carbon negative electrode material;

[0112] The high-temperature calcination process described in step three is as follows: heating the nano-silicon-graphite composite material precursor from room temperature to 1000°C at a heating rate of 5°C / min, keeping it warm for 2 hours, then cooling it to 100°C at a cooling rate of 5°C / min, and finally cooling it naturally to room temperature. The entire heat treatment process is carried out in argon.

[0113] Take a small amount of the silicon-carbon negative electrode material prepared in Control Example 1 and smear it on the surface of the conductive glue, place it on a metal copper sample holder, and observe the morphology using a field emission scanning electron microscope. The results are as follows: Figure 10 As shown;

[0114] Figure 10 The scanning electron microscope images of the silicon-carbon negative electrode material prepared in Comparative Example 1 at different magnifications;

[0115] from Figure 10 It can be seen that the composite material synthesized from non-functionalized nano-silicon and non-acidified graphite undergoes the same composite treatment steps as in Example 1. Only a small amount of nano-silicon particles remain on the graphite surface, while the majority of the nano-silicon particles form secondary particles that are not tightly bonded to the graphite material. This indirectly demonstrates that Example 1, based on the two-step electrostatic treatment, can effectively disperse the nano-silicon particles on the surface of the graphite material through electrostatic forces and effectively bond them.

[0116] The silicon-carbon negative electrode material prepared in Comparative Example 1 was prepared into an electrode and assembled into a lithium-ion half-cell for constant current charge and discharge testing. The specific assembly method was as follows:

[0117] The silicon-carbon negative electrode material prepared in Control Example 1, acetylene black, and a binder were prepared into a slurry in a mass ratio of 7:1.5:1.5, and evenly coated on a copper foil and dried to prepare a negative electrode. The binder was battery-grade sodium carboxymethyl cellulose and styrene-butadiene rubber in a mass ratio of 2:3; the electrolyte was 1.0 mol·L -1 The mixed organic solution of lithium hexafluorophosphate is prepared by mixing ethylene carbonate and diethyl carbonate in a mass ratio of 1:1, and the additive is fluoroethylene carbonate with a mass fraction of 5%. The separator is a microporous polyethylene separator, and the positive electrode is a lithium sheet. The battery is assembled into a CR2025 button cell with a charge and discharge voltage range of 0.01 to 1.5 V. The test is carried out at a constant temperature of 25°C. Figures 11-12 As shown;

[0118] Figure 11 The figure is a rate performance diagram of the silicon-carbon negative electrode material prepared in Comparative Example 1 under constant current charge and discharge tests at different current densities;

[0119] Figure 12 The silicon-carbon negative electrode material prepared in comparative example 1 is -1 Cyclic performance curve of constant current charge and discharge test under current density;

[0120] from Figure 11 and Figure 12 It can be seen that the composite material synthesized in Comparative Example 1 is -1 The reversible specific capacity at this current density is only 871 mAh g -1 Compared with the silicon-carbon negative electrode material of Example 1, the specific capacity is greatly reduced. Although at 5A·g -1 The current density is still 335mAh·g -1 The reversible specific capacity is 100%, but it is still significantly lower than that of Example 1, and the material cycle stability is poor, with multiple disturbance points. It is speculated that this is because the nano-silicon and the graphite material lack the first step of electrostatic self-assembly, resulting in the nano-silicon not being distributed in a directional manner on the graphite surface. Since the nano-silicon exists alone on the current collector, it falls off from the current collector due to volume expansion during the cycle, resulting in the destruction of the material structure. This also indirectly proves the importance of graphite acidification and nano-silicon functionalization. Although at 1A·g -1 After 300 cycles at the same current density, the capacity retention rate dropped to 45%, but the reversible specific capacity was still 317 mAh g -1 , which is higher than the performance of nano-silicon and graphite composite materials processed by traditional ball milling. This shows that although the first step of electrostatic self-assembly is missing, the second step of electrostatic self-assembly can also effectively improve the performance of nano-silicon and graphite composite materials.

[0121] Comparative Example 2: The preparation method of silicon-carbon negative electrode material is completed according to the following steps:

[0122] 1. Preparation of functionalized nano-silicon particles:

[0123] 0.1 g of nano-silicon was dispersed in 25 mL of deionized water, and then 2 mL of a 35% polydiallyldimethylammonium chloride solution was added. The mixture was ultrasonically stirred for 3 hours, and then the excess polydiallyldimethylammonium chloride solution was removed by multiple centrifugal water washing and centrifugal alcohol washing. Finally, the mixture was vacuum dried at 60°C for 12 hours to obtain functionalized silicon nanoparticles.

[0124] The particle size of the nano-silicon in step 1 is 20 nm to 50 nm;

[0125] The molecular weight of the polydiallyldimethylammonium chloride described in step 1 is less than 100,000;

[0126] The number of centrifugal water washing and centrifugal alcohol washing in step 1 is 3 times; the speed of centrifugal water washing and centrifugal alcohol washing in step 1 is 8000 rpm, and the time of each centrifugation is 10 minutes;

[0127] 2. Preparation of acidified graphite:

[0128] 1 g of graphite was placed in a container, followed by a 10 mL mixture of concentrated sulfuric acid and concentrated nitric acid. The mixture was ultrasonically stirred at room temperature for 30 minutes, then stirred and refluxed at 80°C for 6 hours. Finally, the mixture was centrifugally washed with water, centrifugally washed with alcohol, and vacuum dried multiple times to obtain acidified graphite.

[0129] The number of centrifugal water washing and centrifugal alcohol washing in step 2 is not less than 3 times; the speed of centrifugal water washing and centrifugal alcohol washing in step 2 is 8000 rpm, and the time of each centrifugation is 10 minutes;

[0130] The mass fraction of the concentrated sulfuric acid in step 2 is 98%; the mass fraction of the concentrated nitric acid in step 2 is 68%;

[0131] The volume ratio of concentrated sulfuric acid to concentrated nitric acid in the mixed solution of concentrated sulfuric acid and concentrated nitric acid described in step 2 is 3:1;

[0132] 3. Prepare mixed dispersion:

[0133] 20 mg of functionalized silicon nanoparticles and 80 mg of acidified graphite were dispersed in a mixture of 10 mL of deionized water and N-methylpyrrolidone and stirred to obtain a mixed dispersion.

[0134] The volume ratio of deionized water to N-methylpyrrolidone in the mixture of deionized water and N-methylpyrrolidone described in step 3 is 7 mL:3 mL;

[0135] 4. heating the mixed dispersion in a water bath at 90° C. and magnetically stirring until the solvent is completely evaporated to obtain a nano-silicon-graphite composite material precursor;

[0136] 5. High temperature calcination:

[0137] calcining the nano-silicon-graphite composite material precursor at high temperature to obtain a silicon-carbon negative electrode material;

[0138] The high-temperature calcination process described in step five is as follows: heating the nano-silicon-graphite composite material precursor from room temperature to 1000°C at a heating rate of 5°C / min, keeping it warm for 2 hours, then cooling it to 100°C at a cooling rate of 5°C / min, and finally cooling it naturally to room temperature. The entire heat treatment process is carried out in argon.

[0139] The silicon-carbon negative electrode material prepared in Comparative Example 2 was prepared into an electrode and assembled into a lithium-ion half-cell for constant current charge and discharge testing. The specific assembly method was as follows:

[0140] The silicon-carbon negative electrode material prepared in Control Example 2, acetylene black, and a binder were prepared into a slurry in a mass ratio of 7:1.5:1.5, and evenly coated on a copper foil and dried to prepare a negative electrode. The binder was battery-grade sodium carboxymethyl cellulose and styrene-butadiene rubber in a mass ratio of 2:3; the electrolyte was 1.0 mol·L -1 The mixed organic solution of lithium hexafluorophosphate is prepared by mixing ethylene carbonate and diethyl carbonate in a mass ratio of 1:1, and the additive is fluoroethylene carbonate with a mass fraction of 5%. The separator is a microporous polyethylene separator, and the positive electrode is a lithium sheet. The battery is assembled into a CR2025 button cell with a charge and discharge voltage range of 0.01 to 1.5 V. The test is carried out at a constant temperature of 25°C. Figures 13-14 As shown;

[0141] Figure 13 The figure is a rate performance diagram of the silicon-carbon negative electrode material prepared in comparative example 2 under constant current charge and discharge tests at different current densities;

[0142] Figure 14 The silicon-carbon negative electrode material prepared in comparative example 2 is 1A·g -1 Cyclic performance curve of constant current charge and discharge test under current density;

[0143] from Figure 13 and Figure 14 It can be seen that the silicon-carbon negative electrode material synthesized in Comparative Example 2 has a -1 The reversible specific capacity at this current density is only 742 mAh g -1 Compared with Example 3, the specific capacity of the silicon-carbon negative electrode material decreased. Although the first step of electrostatic self-assembly to provide chemical bonding between silicon and graphite was retained compared to Example 3, the lack of the second step of electrostatic self-assembly to introduce chitosan as a silicon coating and a binder between silicon and graphite resulted in poor cycling stability of the material. The specific capacity dropped several times during the cycle. It is speculated that due to the increase in the number of cycles, some nano-silicon fell off the graphite. This confirms the importance of the second step of electrostatic self-assembly to introduce a carbon source to provide physical bonding.

Claims

1. A method for preparing a silicon-carbon negative electrode material based on two-step liquid phase electrostatic self-assembly, characterized in that The preparation method is specifically completed according to the following steps:

1. Preparation of functionalized nano-silicon particles: The nano-silicon is dispersed in deionized water, and then a polydiallyldimethylammonium chloride solution is added, ultrasonicated and stirred for a period of time, and then the excess polydiallyldimethylammonium chloride solution is removed by multiple centrifugal water washing and centrifugal alcohol washing, and finally vacuum dried to obtain functionalized silicon nanoparticles; 2. Preparation of acidified graphite: Graphite is placed in a container, and then a mixture of concentrated sulfuric acid and concentrated nitric acid is added, ultrasonicated and stirred at room temperature for a period of time, then stirred and refluxed at high temperature for a period of time, and finally subjected to multiple centrifugal water washing, centrifugal alcohol washing, and vacuum drying to obtain acidified graphite; 3. Prepare mixed dispersion: Dispersing the functionalized silicon nanoparticles and acidified graphite in a mixture of deionized water and N-methylpyrrolidone, stirring uniformly to obtain a dispersion of the functionalized silicon nanoparticles and acidified graphite, then adding glacial acetic acid, and adding chitosan after the solution becomes uniform, stirring uniformly to obtain a mixed dispersion; Fourth, heating the mixed dispersion in a water bath and magnetically stirring until the solvent is completely evaporated to obtain a nano-silicon-graphite composite material precursor; 5. High temperature calcination: The nano-silicon-graphite composite material precursor is calcined at high temperature to obtain a silicon-carbon negative electrode material based on two-step liquid phase electrostatic self-assembly.

2. The method for preparing a silicon-carbon negative electrode material based on two-step liquid phase electrostatic self-assembly according to claim 1, characterized in that The particle size of the nano-silicon described in step 1 is 20 nm to 50 nm; the mass fraction of the polydiallyldimethylammonium chloride solution described in step 1 is 20% to 35%, and the molecular weight of the polydiallyldimethylammonium chloride is less than 100,000; the mass volume ratio of the nano-silicon described in step 1, the polydiallyldimethylammonium chloride solution and deionized water is (0.1 g to 0.2 g): 2 mL: 25 mL.

3. The method for preparing a silicon-carbon negative electrode material based on two-step liquid phase electrostatic self-assembly according to claim 1, characterized in that The ultrasonic and stirring time in step 1 is 3h to 6h; the number of centrifugal water washing and centrifugal alcohol washing in step 1 is not less than 3 times; the speed of centrifugal water washing and centrifugal alcohol washing in step 1 is 8000rpm to 10000rpm, and the time of each centrifugation is 10min; the temperature of the vacuum drying in step 1 is 60℃ to 80℃, and the vacuum drying time is 12h to 14h.

4. The method for preparing a silicon-carbon negative electrode material based on two-step liquid phase electrostatic self-assembly according to claim 1, characterized in that The mass fraction of the concentrated sulfuric acid described in step 2 is 98%; the mass fraction of the concentrated nitric acid described in step 2 is 68%; the volume ratio of the mass of the graphite described in step 2 to the mixed solution of concentrated sulfuric acid and concentrated nitric acid is 1g:(10mL~15mL); the volume ratio of concentrated sulfuric acid and concentrated nitric acid in the mixed solution of concentrated sulfuric acid and concentrated nitric acid described in step 2 is (3mL~10mL):(1mL~4mL); the time of ultrasonication and stirring described in step 2 is 30min~60min.

5. The method for preparing a silicon-carbon negative electrode material based on two-step liquid phase electrostatic self-assembly according to claim 1, characterized in that The temperature of the stirring reflux under high temperature described in step 2 is 80°C to 90°C, and the time is 4h to 6h; the number of centrifugal water washing and centrifugal alcohol washing in step 2 is not less than 3 times; the speed of the centrifugal water washing and centrifugal alcohol washing in step 2 is 8000rpm to 10000rpm, and the time of each centrifugation is 10min.

6. The method for preparing a silicon-carbon negative electrode material based on two-step liquid phase electrostatic self-assembly according to claim 1, characterized in that The volume ratio of the functionalized silicon nanoparticles, acidified graphite, and the mixture of deionized water and N-methylpyrrolidone described in step 3 is (20 mg to 40 mg): (40 mg to 60 mg): (8 mL to 10 mL); the volume ratio of deionized water to N-methylpyrrolidone in the mixture of deionized water and N-methylpyrrolidone described in step 3 is (5 mL to 7 mL): (3 mL to 5 mL); the mass fraction of the glacial acetic acid described in step 3 is 97%.

7. The method for preparing a silicon-carbon negative electrode material based on two-step liquid phase electrostatic self-assembly according to claim 1, characterized in that The volume ratio of the glacial acetic acid to the dispersion of functionalized silicon nanoparticles and acidified graphite described in step 3 is (100 μL~125 μL):(10 mL~12 mL); the total mass ratio of the chitosan to the functionalized silicon nanoparticles and acidified graphite described in step 3 is (15 mg~40 mg):(60 mg~100 mg).

8. The method for preparing a silicon-carbon negative electrode material based on two-step liquid phase electrostatic self-assembly according to claim 1, characterized in that The water bath heating temperature in step 4 is 80°C to 90°C; the high-temperature calcination process in step 5 is as follows: heating the nano-silicon-graphite composite material precursor from room temperature to 800°C to 1000°C at a heating rate of 2°C / min to 5°C / min, keeping the temperature for 1h to 2h, then cooling it to 100°C at a cooling rate of 2°C / min to 5°C / min, and finally cooling it naturally to room temperature. The entire heat treatment process is carried out in argon or a hydrogen-argon mixed gas; the volume fraction of hydrogen in the hydrogen-argon mixed gas is 5% or 10%.

9. Application of a silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly prepared by the preparation method according to claim 1, characterized in that A silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly is used as a negative electrode material for lithium-ion batteries.

10. The use of a silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly according to claim 9, characterized in that The preparation method of the lithium-ion battery is as follows: a silicon-carbon negative electrode material based on two-step liquid-phase electrostatic self-assembly, acetylene black and a binder are prepared into a slurry in a mass ratio of 7:1.5:1.5, and the slurry is evenly coated on a copper foil and dried to prepare a negative electrode, wherein the binder is battery-grade sodium carboxymethyl cellulose and styrene-butadiene rubber in a mass ratio of 2:3; the electrolyte is 1.0 mol·L -1 A mixed organic solution of lithium hexafluorophosphate is prepared, wherein the mixed organic solution is a mixture of ethylene carbonate and diethyl carbonate in a mass ratio of 1:1, and the additive is fluoroethylene carbonate with a mass fraction of 5%; the separator is a microporous polyethylene separator, the positive electrode is a lithium sheet, and they are assembled into a CR2025 button battery.

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