A silicon-carbon negative electrode material and preparation method and system thereof

Abstract

The present invention relates to a silicon-carbon negative electrode material and a preparation method and system thereof. The silicon-carbon negative electrode material comprises a core and a shell, wherein the core is micron silicon particles, the shell is a porous carbon skeleton layer, a cavity is left between the core and the shell, a cross-linked structure composed of carbon nanotubes and nano-carbon particles is arranged in the cavity, and the carbon nanotubes in the cross-linked structure are embedded in the core and the shell; the length direction of the carbon nanotubes in the cross-linked structure is distributed along the radial direction of the micron silicon particles; and a dense carbon coating layer is arranged outside the carbon skeleton layer. The silicon-carbon negative electrode material in the present invention utilizes porous aluminum oxide-nanocarbon as a template on the surface of silicon particles, and adopts a temperature gradient method to directionally grow a layer of carbon nanotubes distributed along the radial direction. In the process of charging and discharging of the negative electrode material of the present invention, the volume change of silicon particles does not affect the shell structure, and a stable SEI film can be formed on the surface of the shell, which is conducive to ensuring the cycle performance of lithium-ion batteries.

Description

Technical Field

[0001] The invention relates to a silicon-carbon negative electrode material and a preparation method and system thereof, belonging to the technical field of preparation of negative electrode materials for lithium ion batteries. Background Art

[0002] Among the negative electrode materials of lithium-ion batteries, silicon has a very high specific capacity, but the volume of silicon expands greatly after lithium insertion and it is easy to pulverize; in addition, the conductivity of silicon itself is poor. At present, the direction of more research is to prepare silicon-carbon composite materials to overcome the defects of silicon as a negative electrode material for lithium-ion batteries. In the prior art, a silicon-carbon composite material with a core-shell structure is generally prepared, and a layer of carbon is coated on the surface of silicon particles. The carbon coating layer is used to increase the conductivity. In addition, the carbon coating layer also serves as a buffer layer for the volume expansion of silicon particles. However, the negative electrode material of this structure still has the problem of the carbon coating layer falling off after the silicon particles expand, and the battery performance decreases after multiple cycles. Another improved core-shell structure silicon-carbon negative electrode material is a composite material similar to the yolk-eggshell structure, leaving a gap space between the silicon particles and the carbon coating layer. The gap space serves as a space for the silicon particles to expand and prevents the composite structure from being destroyed due to the expansion of the silicon particles. However, the existence of this gap will reduce the conductivity between the silicon particles and the carbon coating layer, and the stacking density of the negative electrode material of this structure is low, and the specific capacity is insufficient. In addition, the existing methods for composite nano-silicon and carbon nanotubes are mostly achieved through grinding and mixing. Carbon nanotubes are a two-dimensional material with a large aspect ratio and are extremely easy to agglomerate. The grinding method is difficult to achieve the dispersion of carbon nanotubes, and it is also impossible to achieve the full expansion of carbon nanotubes along the length direction to form a cross-linked structure, and it is impossible to give full play to their excellent ionic and electronic conductivity. Summary of the invention

[0003] In view of the above problems, the present invention provides a silicon-carbon negative electrode material, method and system, and the specific scheme is as follows:

[0004] A silicon-carbon negative electrode material comprises an inner core and an outer shell, wherein the inner core is micron silicon particles, the outer shell is a porous carbon skeleton layer, a cavity is left between the inner core and the outer shell, a cross-linked structure consisting of carbon nanotubes and nano-carbon particles is contained in the cavity, the carbon nanotubes in the cross-linked structure are embedded in the inner core and the outer shell; the length direction of the carbon nanotubes in the cross-linked structure is distributed along the radial direction of the micron silicon particles; and a dense carbon coating layer is provided on the outside of the carbon skeleton layer.

[0005] Furthermore, the particle size of the micron silicon particles is 1-10 microns; the gap of the cavity is 100-500 nm; the particle size of the nano carbon particles is 10-50 nm; the thickness of the porous carbon skeleton layer is 1-10 microns; and the thickness of the carbon coating layer is 50-100 nm.

[0006] Furthermore, the carbon coating layer is a pyrolysis-deposited carbon film.

[0007] Furthermore, the nano carbon particles are graphene or carbon black.

[0008] A method for preparing a silicon-carbon negative electrode material comprises the following steps:

[0009] 1) Micron silicon particles, nano carbon particles and alumina sol are ultrasonically ground and dispersed, and then spray granulated. After calcination under protective atmosphere conditions, a precursor is obtained. The core of the precursor is micron silicon particles, and the outside is coated with a layer of nano alumina particles and nano carbon particles;

[0010] 2) The precursor is kneaded with asphalt, ground into powder, and carbonized under protective atmosphere to obtain formed particles;

[0011] 3) The formed particles are immersed in a catalyst salt solution, loaded with the catalyst salt solution, dried and placed in a reaction furnace, heated by introducing a reducing gas, and after the catalyst is reduced, a pyrolysis carbon source gas is introduced to deposit carbon nanotubes inside and on the surface of the formed particles to obtain skeleton particles;

[0012] 4) The skeleton particles are first dissolved in alkali to remove the alumina particles, and then acid washed and purified to remove the catalyst;

[0013] 5) The particles after acid washing and purification are heated in a pyrolysis furnace, and pyrolysis carbon source gas is introduced to deposit pyrolysis carbon film.

[0014] Furthermore, in the step 3), the molded particles are first heated to 700°C in the reaction furnace, and then kept warm at 650°C, while a carbon source gas at 650°C is introduced, so that a temperature gradient is generated from the inside to the outside of the molded particles. After a period of reaction, the molded particles are heated to 700°C again, the insulation temperature is increased to 670°C, and a carbon source gas at 670°C is introduced. After a period of reaction, the molded particles are heated to 700°C again, the insulation temperature is increased to 700°C, and a carbon source gas at 700°C is introduced until the reaction is completed.

[0015] A preparation system for silicon-carbon negative electrode materials comprises a reaction furnace, a gas-solid separator is arranged on the top of the reaction furnace, a movable fluidized bed is arranged inside the reaction furnace, and a lifting mechanism is arranged on the top of the fluidized bed; the reaction furnace is provided with an upper heating section and a lower heating section along the height direction, and the upper heating section and the lower heating section are provided with independent temperature control mechanisms; and an air inlet is arranged at the bottom of the reaction furnace.

[0016] The silicon-carbon negative electrode material in the present invention is to use porous aluminum oxide-nano carbon as a template on the surface of silicon particles, and adopt a temperature gradient method to directionally grow a layer of carbon nanotubes distributed in the radial direction, remove the aluminum oxide support after alkali dissolution and acid washing, and generate an intermediate layer with a cross-linked structure of carbon nanotubes-nano carbon particles. After removing the aluminum oxide support, most of the gap is retained between the shell and the kernel as a space for the expansion of silicon particles, and the carbon nanotubes distributed in the radial direction can improve the conductivity between the shell and the kernel, and at the same time serve as a channel for lithium ion migration. The nano-carbon particles dispersed in the cross-linked structure of the carbon nanotubes can also provide lithium insertion space, improve the capacity of the negative electrode material, and the nano-carbon particles are dispersed in the cross-linked structure of the carbon nanotubes, without forming a tight stack, and will not affect the expansion of the silicon particles. During the charge and discharge process, the volume change of the silicon particles does not affect the shell structure, and a stable SEI film can be formed on the shell surface, which is conducive to ensuring the cycle performance of the lithium ion battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 Schematic diagram of the structure of the silicon-carbon negative electrode material in the present invention;

[0018] Figure 2 It is a structural schematic diagram of the reaction furnace in the present invention. DETAILED DESCRIPTION

[0019] The scheme of the present invention is described in detail below with reference to specific examples.

[0020] Example 1

[0021] A silicon-carbon negative electrode material of this embodiment, such as Figure 1 As shown, the microstructure of the particles includes a core and a shell, wherein the core is a micron silicon particle 1, and the shell is a porous carbon skeleton layer 2. A cavity is left between the core and the shell, and a cross-linked structure composed of carbon nanotubes 3 and nano-carbon particles 4 is present in the cavity, wherein the carbon nanotubes in the cross-linked structure are embedded in the core and the shell; the length direction of the carbon nanotubes in the cross-linked structure is distributed along the radial direction of the micron silicon particle; and there is a dense carbon coating layer 5 outside the carbon skeleton layer.

[0022] The particle size of the micron silicon particles is 1-10 microns; the gap of the cavity is 100-500 nm; the particle size of the nano carbon particles is 10-50 nm; the thickness of the porous carbon skeleton layer is 1-10 microns; the thickness of the carbon coating layer is 50-100 nm. The carbon coating layer is a pyrolysis deposited carbon film. The nano carbon particles are graphene or carbon black.

[0023] Example 2

[0024] A method for preparing a silicon-carbon negative electrode material comprises the following steps:

[0025] 1) Micron silicon particles, nano carbon particles and alumina sol are ultrasonically ground and dispersed, and then spray granulated. After calcination under protective atmosphere conditions, a precursor is obtained. The core of the precursor is micron silicon particles, and the outside is coated with a layer of nano alumina particles and nano carbon particles; the mass fraction of the micron silicon particles is 15wt%, the mass fraction of the nano alumina particles is 50wt%, and the mass fraction of the nano carbon particles is 35wt%.

[0026] 2) The precursor is kneaded with asphalt, then ground into powder, and carbonized under protective atmosphere to obtain molded particles; the amount of asphalt added accounts for 12wt% of the precursor mass, and the asphalt is selected as medium-temperature asphalt with a softening point of 80~90℃, and the kneading temperature is above 160℃. After the asphalt is carbonized and coked, a porous carbon skeleton layer is formed.

[0027] 3) The formed particles are immersed in a catalyst salt solution, loaded with the catalyst salt solution, dried and placed in a reaction furnace, heated by introducing a reducing gas, and after the catalyst is reduced, a pyrolysis carbon source gas is introduced to deposit carbon nanotubes inside and on the surface of the formed particles to obtain skeleton particles; the catalyst salt solution is a mixed solution of ferric nitrate and nickel nitrate.

[0028] 4) The skeleton particles are first dissolved in alkali to remove the alumina particles, and then acid washed and purified to remove the catalyst;

[0029] 5) The particles after acid washing and purification are heated in a pyrolysis furnace, and pyrolysis carbon source gas is introduced to deposit pyrolysis carbon film.

[0030] In step 3), the formed particles are first heated to 700°C in the reaction furnace, and then kept at 650°C, while a carbon source gas at 650°C is introduced, so that a temperature gradient is generated from the inside to the outside of the formed particles. After a period of reaction, the formed particles are heated to 700°C, the holding temperature is increased to 670°C, and a carbon source gas at 670°C is introduced. After a period of reaction, the formed particles are heated to 700°C, the holding temperature is increased to 700°C, and a carbon source gas at 700°C is introduced until the reaction is completed. The carbon source gas is methane gas.

[0031] After the formed particles are heated to 700°C, they are kept at a relatively low temperature and a low-temperature carbon source gas is introduced. At this time, the surface temperature of the formed particles decreases, while the internal temperature remains at around 700°C, generating a temperature gradient from the inside to the outside. Since the deposition of carbon nanotubes is closely related to temperature, the optimal deposition temperature is 700°C, and the deposition rate is slow below 700°C. Under the condition of the temperature gradient, carbon atoms gradually deposit radially to form carbon nanotubes.

[0032] Example 3

[0033] like Figure 2A preparation system of silicon-carbon negative electrode materials includes a reactor 6, a gas-solid separator 7 is provided on the top of the reactor, a movable fluidized bed 8 is provided in the reactor, a lifting mechanism is provided on the top of the fluidized bed, and the lifting mechanism adopts a winch to drive a steel wire rope 9, and the winch drives the fluidized bed to rise and fall in the reactor when working; the reactor is provided with an upper heating section 10 and a lower heating section 11 along the height direction, and the upper heating section and the lower heating section are provided with independent temperature control mechanisms, and the temperatures of the upper and lower heating sections and the lower heating section can be independently controlled; an air inlet 12 is provided at the bottom of the reactor.

[0034] The structure of this embodiment is to realize the method of depositing carbon nanotubes from molded particles in Example 2. The molded particles are loaded into a moving fluidized bed. The bottom of the fluidized bed is provided with gas distribution holes. The carbon source gas enters from the bottom to keep the molded particles in the fluidized bed in a boiling state. When depositing carbon nanotubes, the temperature of the upper heating section is controlled at 700°C, and the temperature of the lower heating section is controlled at 650°C. The fluidized bed is first heated in the upper heating section. After the temperature stabilizes, it descends to the lower heating section. At the same time, the carbon source gas is introduced from the bottom to deposit carbon nanotubes; after a period of time, the fluidized bed is raised to the upper heating section, and the temperature of the lower heating section is raised to 670°C. After the temperature stabilizes, the fluidized bed is lowered to the lower heating section to continue the carbon nanotube deposition; until the temperature of the lower heating section is raised to 700°C, carbon deposition is carried out to the end. The carbon source gas can be preheated before entering the reactor, and the lower heating section in the reactor can further heat the preheated carbon source gas.

Claims

1. A method for preparing a silicon-carbon negative electrode material, characterized in that: The following steps are involved: 1) Ultrasonic grinding and dispersion of micron silicon particles, nano carbon particles and alumina sol, followed by spray granulation, and calcination under protective atmosphere to obtain a precursor, wherein the core of the precursor is micron silicon particles, and the outside is coated with a layer of nano alumina particles and nano carbon particles; 2) The precursor is kneaded with asphalt, then ground into powder, and carbonized under protective atmosphere to obtain formed particles; 3) The formed particles are immersed in a catalyst salt solution, loaded with the catalyst salt solution, dried and placed in a reaction furnace, heated by introducing a reducing gas, and after the catalyst is reduced, a pyrolysis carbon source gas is introduced to deposit carbon nanotubes inside and on the surface of the formed particles to obtain skeleton particles; 4) The skeleton particles are first dissolved in alkali to remove the alumina particles, and then acid washed and purified to remove the catalyst; 5) The particles after acid washing and purification are heated in a pyrolysis furnace, and pyrolysis carbon source gas is introduced to deposit pyrolysis carbon film; In step 3), a gas-solid separator is provided on the top of the reactor, a movable fluidized bed is provided in the reactor, and a lifting mechanism is provided on the top of the fluidized bed; the reactor is provided with an upper heating section and a lower heating section along the height direction, and the upper heating section and the lower heating section are provided with independent temperature control mechanisms; an air inlet is provided at the bottom of the reactor; the molded particles are loaded into the movable fluidized bed, and a gas distribution hole is provided at the bottom of the fluidized bed, and the carbon source gas enters from the bottom to keep the molded particles in the fluidized bed in a boiling state; when the carbon nanotubes are deposited, the upper heating section is provided with a gas distribution hole, and the carbon source gas enters from the bottom, so that the molded particles in the fluidized bed are kept in a boiling state; when the carbon nanotubes are deposited, the upper heating section is provided with a gas distribution hole, and the carbon source gas enters from the bottom, so that the carbon source gas enters from the bottom, and ... The temperature of the upper heating section is controlled at 700°C, and the temperature of the lower heating section is controlled at 650°C. The fluidized bed is first heated in the upper heating section, and after the temperature stabilizes, it descends to the lower heating section, and at the same time, carbon source gas is introduced from the bottom to deposit carbon nanotubes; after a period of time, the fluidized bed is raised to the upper heating section, and the temperature of the lower heating section is increased to 670°C. After the temperature stabilizes, the fluidized bed is descended to the lower heating section to continue depositing carbon nanotubes; after the temperature of the lower heating section is increased to 700°C, carbon deposition is continued to end.

Images