High ionic conductivity concave porous silicon-carbon particles and method for preparing the same

Abstract

The present application relates to the technical field of carbon composite materials, in particular to a concave porous silicon-carbon particle with high ion conductivity and a preparation method thereof. The concave porous silicon-carbon particle is provided with a part of surface concave, forming a crosslinked body of silicon debris and carbon nanomaterials with a continuous boundary and an arc shape. Compared with the prior art, the nanocarbon material in the particle is in an ordered structure, improving the internal conductive and ion transmission capacity of the silicon-carbon particle, and also serving as a buffer structure to absorb the expansion stress of silicon, avoiding the cracking of the silicon-carbon particle and the cracking and falling of the negative electrode.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of carbon composite materials, in particular to a concave porous silicon-carbon particle with high ion conductivity and a preparation method thereof. BACKGROUND

[0002] The specific capacity of a lithium ion battery is jointly determined by the specific capacity of the positive electrode and the specific capacity of the negative electrode, and the greater the specific capacity of the negative electrode, the better. The specific capacity of a conventional graphite negative electrode product is about 350-370 mAh / g, which has basically reached the theoretical limit, while the theoretical specific capacity of silicon can be as high as 4200 mAh / g, which has a significant advantage.

[0003] Silicon will expand by 300% in volume during charging and discharging, and a composite of silicon and porous amorphous carbon is usually used to buffer the stress generated by expansion, but the strength and toughness of the porous amorphous carbon are weak, and after long-term cyclic charging and discharging, the frequent expansion and contraction will cause the bonding and connecting interfaces between the silicon-carbon composite particles to break, resulting in particle collapse, negative electrode film cracking and other phenomena, which will significantly reduce the capacity of the negative electrode. In the research of silicon-carbon, there are many technologies for forming secondary particles by mixing nano- and sub-micron silicon particles with carbon, or forming a carbon coating layer on the surface of the silicon particles and then forming secondary particles. In long-term cycling, the alloying of silicon causes expansion and extrusion between the secondary particles, and the collapse during dealloying causes the surface of the secondary particles to be concave and the gap between the secondary particles in the negative electrode to increase, which will reduce the conductivity of the negative electrode. In addition, the expansion and extrusion will also cause the secondary particles to crack, forming new solid-liquid interfaces, and a large amount of irreversible amorphous lithium / silicon / oxide will be produced on the surface of the secondary particles in the negative electrode. The gap and the above-mentioned products not only hinder the ion transport in the negative electrode, but also cause the negative electrode layer to powder and fall off, thus the overall capacity of the silicon-carbon negative electrode is prone to rapid decay and poor working stability. SUMMARY

[0004] The present application provides a concave porous silicon-carbon particle with high ion conductivity and a preparation method thereof to overcome the deficiencies of the prior art.

[0005] To achieve the above-mentioned purpose, a concave porous silicon-carbon particle with high ion conductivity is designed, and the surface of the concave porous silicon-carbon particle is provided with a part of the surface concave, forming a continuous boundary and an arc-shaped cross-linking body of silicon debris and carbon nanomaterials.

[0006] To achieve the above-mentioned purpose, a preparation method of a concave porous silicon-carbon particle with high ion conductivity is designed, which comprises the following steps:

[0007] S1, mixing, stirring and drying a modifier, a surface modification solvent and silicon powder to obtain surface-modified silicon powder;

[0008] S2, the surface modified silicon powder, water-soluble carbon source, soluble inorganic salt and carboxylated carbon nanomaterial are added into an acidic aqueous solution and mixed uniformly;

[0009] S3, the mixed solution is prepared into micron-sized silicon-carbon secondary particles by a spray dryer, and the silicon-carbon secondary particles are sintered at high temperature in a vacuum furnace under the protection of inert gas and cooled to room temperature.

[0010] S4: the product after high-temperature sintering is washed to remove soluble inorganic salt, and low-temperature drying is performed under vacuum to obtain the final product.

[0011] In the step S1, the modifier is selected from one of an organosiloxane, a polyether modified hydrophilic water-soluble silicone oil, and a PVP hydrophilic solvent. The modifier mainly changes the hydrophilic property of the silicon surface to improve the hydrophilicity.

[0012] The surface modification solvent is selected from one of a silane coupling agent, a titanate, and stearic acid; the silane coupling agent includes but is not limited to one of an amino group, an epoxy group, and a vinyl group.

[0013] In the step S1, the silicon powder is selected from a crystal silicon particle with an arbitrary shape, and the size is 100-500 nm, and the size dimension of the particle in a certain direction is below 200 nm; the mass ratio of the modifier to the silicon powder is 2.5-8:16, so that the thickness of the molecular layer covering the surface of the silicon particle is 3-20 nm.

[0014] In the step S2, the water-soluble carbon source is selected from a polar polymer aqueous solution, including but not limited to one of a water-based polyurethane resin solution, a water-soluble polyethylene glycol solution, and a methyl cellulose solution; the carboxylated carbon nanomaterial is selected from one or more of a carboxylated carbon nanotube, carboxylated graphene, and carboxylated carbon nanofiber; and the radial size of the carboxylated carbon nanomaterial in different directions is less than 300 nm.

[0015] In the step S2, the soluble inorganic salt is selected from one or more of an inorganic salt that can be crystallized at 80-100℃ and dissolved in water, including but not limited to one or more of a potassium salt, a sodium salt, an ammonium salt, a nitrate salt, an acetate salt, and other inorganic salts that can be crystallized at 100℃ and dissolved in water.

[0016] In the step S2, the acidic aqueous solution includes at least one of an acetic acid solution with weak acidity, a citric acid solution, an acetic acid solution, and a carbonic acid solution, and the PH value is 3-5.

[0017] In the step S2, the mass ratio of silicon to carbon in the water-soluble carbon source is 1:1-3, the mass ratio of the soluble inorganic salt to silicon is 1:1-1.5, and the mass ratio of silicon to the carboxylated carbon nanomaterial is 1:0.1-0.5.

[0018] In step S3, the inert gas is either nitrogen or argon, the sintering temperature is 800-900℃, and the holding time is 0.5-1.5 hours.

[0019] Compared with the prior art, the present invention has an ordered structure of nano-carbon material in the particles, which improves the internal conductivity and ion transport capacity of silicon-carbon particles. At the same time, it can also act as a buffer structure to absorb silicon expansion stress, avoid cracking of silicon-carbon particles, and also avoid cracking and shedding of the negative electrode. Attached Figure Description

[0020] Figure 1 SEM image of concave porous silicon-carbon particles (SCT) with added carbon nanotubes, as shown in this invention.

[0021] Figure 2 A comparison chart showing the pore size test results for particles without carbon nanotubes (SC) and particles with carbon nanotubes (SCT).

[0022] Figure 3 SEM image of concave porous silicon carbide particles (SCG) with added carboxyl-based graphene.

[0023] Figure 4 SEM images of an electrode without carbon nanotubes (a) and an electrode with carbon nanotubes (b) after charge-discharge cycles.

[0024] Figure 5 Simulation diagram of the failure mechanism during cycling for particles without carbon nanotubes (SC) and particles with carbon nanotubes (SCT). Detailed Implementation

[0025] The present invention will now be further described with reference to the accompanying drawings.

[0026] like Figure 1 As shown, the surface of the concave porous silicon-carbon particles has partial surface depressions, forming a cross-linked body of silicon debris and carbon nanomaterials with continuous and arc-shaped boundaries.

[0027] The method for preparing the above-mentioned concave porous silicon-carbon particles with high ionic conductivity includes the following steps:

[0028] S1, the modifier, surface modification solvent and silicon powder are mixed, stirred and dried to obtain surface-modified silicon powder;

[0029] S2, surface-modified silicon powder, water-soluble carbon source, soluble inorganic salt and carboxylated carbon nanomaterials are added to an acidic aqueous solution and mixed evenly;

[0030] S3, the mixed solution is spray-dried to prepare micron-sized secondary silicon-carbon particles. Under the protection of inert gas, the secondary silicon-carbon particles are sintered at high temperature in a vacuum furnace and then cooled to room temperature.

[0031] S4: The product after high-temperature sintering is washed to remove soluble inorganic salts, and low-temperature drying is performed under vacuum to obtain a final product.

[0032] In step S1, the modifier is selected from one of an organosiloxane, a polyether-modified hydrophilic water-soluble silicone oil, and a PVP hydrophilic solvent. The modifier mainly changes the hydrophilic properties of the silicon surface to improve the hydrophilicity.

[0033] The surface modification solvent is selected from one of a silane coupling agent, a titanate, and stearic acid; the silane coupling agent includes but is not limited to one of an amino group, an epoxy group, and a vinyl group.

[0034] In step S1, the silicon powder is selected from a crystal silicon particle with an arbitrary shape, and the size is 100-500 nm, and the size dimension of the particle in a certain direction is below 200 nm; the mass ratio of the modifier to the silicon powder is 2.5-8:16, so that the thickness of the molecular layer covering the silicon particle surface is 3-20 nm.

[0035] In step S2, the water-soluble carbon source is selected from a polar polymer aqueous solution, including but not limited to one of a water-based polyurethane resin solution, a water-soluble polyethylene glycol solution, and a methyl cellulose solution; the carboxylated carbon nanomaterial is selected from one or more of a carboxylated carbon nanotube, carboxylated graphene, and a carboxylated carbon nanofiber; and the radial size of the carboxylated carbon nanomaterial in different directions is less than 300 nm.

[0036] In step S2, the soluble inorganic salt is selected from one or more of inorganic salts that can crystallize at 80-100℃ and are soluble in water, including but not limited to one or more of potassium salts, sodium salts, ammonium salts, nitrate salts, and acetate salts that can crystallize at 100℃ and are soluble in water. In specific use, potassium chloride, calcium chloride, sodium sulfate, etc. can be selected. During high-temperature sintering, the crystals and the silicon particles are mixed to form secondary particles wrapped by carbon, and finally the inorganic salt is dissolved and removed.

[0037] In step S2, the acidic aqueous solution includes at least one of acetic acid solution, citric acid solution, vinegar solution, and carbonic acid solution with weak acidity, and the PH value is 3-5.

[0038] In step S2, the mass ratio of silicon to carbon elements in the water-soluble carbon source is 1:1-3, the mass ratio of the soluble inorganic salt to silicon is 1:1-1.5, and the mass ratio of silicon to the carboxylated carbon nanomaterial is 1:0.1-0.5.

[0039] In step S3, the inert gas is selected from one of nitrogen and argon, the sintering temperature is 800-900℃, and the holding time is 0.5-1.5 hours.

[0040] In the S3 process, the suspension containing silicon powder, water-soluble carbon source, soluble inorganic salt and carboxylated carbon nanomaterial is pumped into the nozzle, transmitted into the high-temperature drying cavity by air pressure, and the droplets enter the drying chamber. As the water on the surface of the microspheres evaporates, the inorganic salt precipitates on the surface of the microspheres, together with the water-soluble carbon source, to hinder the overflow of the internal steam. Then, the internal steam pressure of the microspheres gradually increases, and the heat of water evaporation changes the surface temperature gradient of the droplets. Under the action of inertia, the surface of the droplets deforms and forms a part of the surface concave 1-1. The particles are formed from the collapse of the spherical particles to form a boundary continuous and arc-shaped silicon debris 1-2 and carbon nanomaterial 1-3 crosslinker, and the water-soluble carbon source is coated on the surface of the silicon and carbon nanomaterial when the particles are formed.

[0041] When the concave porous silicon-carbon particles of the present application are prepared into a negative electrode of a lithium ion battery, the above-mentioned silicon-carbon composite material is mixed with carbon black (SP) microspheres as an active material for storing lithium and coated into a film layer. SP is an electrically conductive medium in the film layer.

[0042] When the silicon-carbon particles are in a surface concave shape, the tap density of the negative electrode is large, the gap is small, the filling of SP is compact, and the gap between the silicon-carbon particles is small. Therefore, the contact area between the particles in the prepared negative electrode is large, and the conductive path is more. When Li ions enter the silicon-carbon material, the lithium-silicon alloyed particles expand, and the concave surface of the particles provides space for the expansion of the outside.

[0043] The nanocarbon material in the composite particles is an ordered structure, which improves the internal conductivity and ion transport capacity of the silicon-carbon particles, and also can act as a buffer structure to absorb the expansion stress of silicon, avoid the cracking of silicon-carbon particles, and also avoid the cracking and falling off of the negative electrode. Example 1

[0044] This embodiment is a preparation method of concave porous silicon-carbon particles with carbon nanotubes, which comprises the following steps:

[0045] S1: The modifier, amino silane coupling agent and silicon powder are mixed and stirred, and the mass ratio of the modifier to the Si powder is 1:4, so that the surface coverage of the silicon particles is about 10 nm. After drying, the surface-modified silicon powder is obtained.

[0046] S2: The modified silicon powder, water-based polyurethane solution, soluble potassium chloride and carboxylated carbon nanotubes are added to the aqueous acetic acid solution and mixed uniformly. The raw materials are stirred for 48 hours to form a uniform gel solution.

[0047] S3: The gel solution is prepared into micron-sized silicon-carbon particles by a spray dryer. The parameters such as air pressure, air flow and speed of peristaltic pump of the spray drying are controlled, and the matching degree of the viscosity and flowability of the gel solution with the above-mentioned parameters is utilized to prepare silicon-carbon composite particles SCT with a particle size of 0.5-8 μm. The sample without adding carbon nanotubes is SC.

[0048] The embodiment utilizes the change of potassium chloride concentration to change the surface tension and viscous force of the droplet, and utilizes the solid content and polymerization amount of polyurethane to change the cross-linking force of the carboxylated carbon nanotube on the surface of the modified silicon particles. Specifically, the spray dryer pumps the suspension liquid containing silicon powder, water-soluble carbon source, soluble inorganic salt, and carboxylated carbon nanotube into the nozzle, and then is transmitted into the high-temperature drying cavity through air pressure. The droplet enters the drying chamber, and as the water on the surface of the microsphere evaporates, the inorganic salt is precipitated on the surface of the microsphere, and together with the water-soluble carbon source, hinders the overflow of the internal steam. Then, the internal steam pressure of the microsphere gradually increases, the heat of water evaporation changes the surface temperature gradient of the droplet, and under the action of inertia, the surface of the droplet deforms and forms a part of the surface depression 1-1. The particles are collapsed from the spherical shape to form a boundary continuous and arc-shaped silicon debris 1-2 and carbon nanotube 1-3 cross-linking body, and the water-soluble carbon source is coated on the surface of the silicon and carbon nanotube when the particles are formed.

[0049] Under the action of the vortex airflow, the micron-sized silicon-carbon composite particles are screened out, and the silicon-carbon particles are sintered at high temperature to further carbonize the water-soluble carbon. In the sintering process, inert gas such as nitrogen is used as a protective gas to remove H and O components while avoiding oxidation of the carbon wrapping layer and the surface of the silicon particles. The carbon layer on the surface of the silicon particles undergoes carbonization reaction during high-temperature sintering, forming a short-range ordered and long-range disordered carbon composite layer.

[0050] S4: The product after high-temperature sintering is washed with a solvent that can dissolve the inorganic salt and does not react with silicon and carbon, the soluble inorganic salt is removed, and then dried to obtain the product, which is a recessed porous silicon-carbon particle added with carbon nanotubes. The SEM morphology of the particle is shown in FIG. 1-4. Figure 1

[0051] Specifically, the solvent can be acetic acid, ethanol, water, or other reagents that can dissolve potassium chloride and do not react with silicon and carbon. After washing, the recessed structure of the particle does not collapse, and more pores are formed inside. The pore condition of the particle is shown in FIG. 1-5. Compared with the particle without added carbon nanotubes, the particle with added carbon nanotubes has a larger pore volume. Figure 2

[0052] The acid aqueous solution in step S1 of the embodiment includes at least one of acetic acid solution and citric acid solution, and the pH value is 3-5. Specifically, the acid aqueous solution facilitates the combination of the water-soluble carbon source with the surface of the modified silicon particles and the surface of the carbon nanotubes. Further, the mass ratio of silicon to carbon element in the water-soluble carbon source in step S1 is 1:2, the mass ratio of soluble inorganic salt to silicon is 1:1, and the mass ratio of silicon to carbon nanotube is 1:0.3.

[0053] Specifically, in the case that the mass of carbon element is greater than that of silicon element, the carbon element forms a wrapping effect on the silicon particles in the form of amorphous carbon and disordered carbon structure layer. The amount of soluble KCl and carbon nanotubes can be controlled to achieve the control of the surface depression. ​​

[0054] Further, the airflow outlet temperature in the spray dryer granulation process in step S3 of the embodiment is 90 °C, and the gas pressure is 22.4 kg / cm 2 . Nitrogen is added during high-temperature sintering in step S3, and the temperature is raised to 850 °C, and then the temperature is maintained for 1.5 h before naturally cooling to room temperature. The carbon nanotube-added recessed porous silicon-carbon particles obtained by the above method have a size of about 3-8 μm.

[0055] After the carbon nanotube-added silicon-carbon particles are formed into a film to prepare a lithium battery negative electrode and are cycled, the particles are different from those without the addition of silicon-carbon particles, and the particle structure is complete after the cycle of the particles added with carbon nanotube, and the change in the particles during the cycle is as shown in Figure 4 . The carbon nanotube-added silicon-carbon particles have a complete particle structure after the negative electrode is cycled and charged and discharged, which is beneficial to the cycle life of the battery. Example Two

[0056] In the embodiment, 2.0 g of modified silicon powder, 2.0 g of polyethylene glycol PEG6000, 1.35 g of CaCl2, and 0.15 g of carboxylated graphene are added to deionized water. After the above mixture is stirred for 24 h, micron-sized silicon-carbon particles are prepared by a spray dryer. During the granulation process, the airflow inlet temperature is 140 °C, the airflow outlet temperature is 95 °C, and the gas pressure is 33 Kg / cm 2 .

[0057] The silicon-carbon particles are heated to 900 °C under nitrogen protection, and then naturally cooled to room temperature after maintaining the temperature for 1 h. The final product of sintering is first washed with acetic acid three times to remove CaCl2, and then washed with ethanol and dried.

[0058] The prepared carboxylated graphene-added recessed porous silicon-carbon particles SCG have a surface morphology as shown in Figure 3 .

[0059] The mechanism of the formation of the non-spherical particles prepared by the present application is as shown in Figure 5 , in which the carbon nanotubes CNTs can be replaced by graphite or nanocarbon fibers. The addition of the silicon-carbon particles prepared by the above method to the negative electrode material of a lithium battery can effectively improve the specific capacity of the negative electrode, and at the same time, can improve the stability of the negative electrode and the cycle life of the negative electrode material.

Claims

1. A high ionic conductivity concave porous silicon-carbon particle for use as a negative electrode material for a lithium ion battery, characterized by: The concave porous silicon-carbon particle surface is provided with partial surface depressions (1-1), forming a continuous boundary and arc-shaped crosslinked body of silicon debris (1-2) and carbon nanomaterial (1-3); A preparation method thereof comprises the following steps: S1, mixing, stirring and drying a modifier, a surface modification solvent and silicon powder to obtain surface-modified silicon powder; S2, adding the surface-modified silicon powder, a water-soluble carbon source, a soluble inorganic salt and a carboxylated carbon nanomaterial into an acidic aqueous solution and mixing uniformly; S3, preparing micron-sized silicon-carbon secondary particles by a spray dryer, and sintering the silicon-carbon secondary particles at high temperature in a vacuum furnace under the protection of an inert gas, and cooling to room temperature; S4: removing the soluble inorganic salt by washing the product after high-temperature sintering, and drying at low temperature under vacuum to obtain the final product; In the step S1, the modifier is selected from one of an organosiloxane, a polyether-modified hydrophilic water-soluble silicone oil, and a PVP hydrophilic solvent; The surface modification solvent is selected from one of a silane coupling agent, a titanate, and stearic acid; the silane coupling agent includes one of an amino group, an epoxy group, and a vinyl group; In the step S2, the water-soluble carbon source is selected from one of a water-based polyurethane resin solution, a water-soluble polyethylene glycol solution, and a methyl cellulose solution; and the carboxylated carbon nanomaterial is selected from one or more of a carboxylated carbon nanotube, carboxylated graphene, and carboxylated carbon nanofiber; The soluble inorganic salt includes one or more of potassium salt, sodium salt, and ammonium salt that can crystallize at 100℃ and dissolve in water; In the step S3, the inert gas is selected from one of nitrogen and argon, the sintering temperature is 800-900℃, and the holding time is 0.5-1.5 hours.

2. The high ionic conductivity concave porous silicon-carbon particles for use as an anode material for lithium ion batteries according to claim 1, characterized in that: In the step S1, the silicon powder is selected from a crystal silicon particle with an arbitrary shape, the size is 100-500nm, and the dimension of the particle in a certain direction is below 200nm; the mass ratio of the modifier to the silicon powder is 2.5-8:16, so that the thickness of the molecular layer covering the surface of the silicon particle is 3-20nm.

3. The high ionic conductivity concave porous silicon-carbon particles for use as an anode material for lithium ion batteries according to claim 1, characterized in that: The radial dimension of the carboxylated carbon nanomaterial in different directions is less than 300nm.

4. The high ionic conductivity concave porous silicon-carbon particles for use as an anode material for lithium ion batteries according to claim 1, characterized in that: In the step S2, the acidic aqueous solution includes at least one of acetic acid solution, citric acid solution, vinegar solution, and carbonic acid solution with weak acidity, and the PH value is 3-5.

5. The high ionic conductivity concave porous silicon-carbon particles for use as an anode material for lithium ion batteries according to claim 1, characterized in that: In the step S2, the mass ratio of silicon to carbon in the water-soluble carbon source is 1:1-3, the mass ratio of the soluble inorganic salt to silicon is 1:1-1.5, and the mass ratio of silicon to the carboxylated carbon nanomaterial is 1:0.1-0.5.

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