CN108448059A - A kind of silicon negative electrode for lithium ion battery and preparation method thereof - Google Patents

Abstract

The invention discloses a silicon negative electrode for a lithium ion battery and a preparation method thereof. The silicon negative electrode includes a silicon material arranged in an array on a substrate, and is characterized in that: the surface of the silicon material is coated with graphene Floor. The preparation method includes the following steps: forming a silicon film on the front and back sides of the substrate by chemical vapor deposition or magnetron sputtering coating method; carrying out isotropic etching through a mask plate to form an array of silicon materials; The vapor phase deposition method forms a graphene layer covering the surface of the silicon material. By coating graphene on the surface of the silicon array, the invention can restrain the expansion of silicon, improve the conductivity, avoid the direct contact between electrolyte and silicon, and utilize the good SEI film formed by graphene and electrolyte, thereby prolonging the charge-discharge cycle life of the material ; The invention improves the initial efficiency, improves the cycle performance, increases the rate performance and maximizes the energy density, and provides a very good method for the commercial application of silicon-based materials.

Description

A kind of silicon negative electrode for lithium ion battery and preparation method thereof

technical field

The invention relates to a lithium ion battery, in particular to a negative electrode plate for the lithium ion battery, especially a silicon negative electrode.

Background technique

With the increase in the mileage requirements of electric vehicles, the battery system puts forward higher requirements on the energy density of single lithium-ion batteries. For the inside of the battery, positive and negative electrode materials with higher gram capacity are required. Among them, the positive electrode material is transitioning from lithium iron phosphate to ternary, and high nickel in the ternary system has become a trend, while negative electrode materials are based on traditional graphite negative electrode materials, and silicon negative electrode materials have been developed. The development of silicon anodes has gradually become a hot spot in research and development applications.

The discharge capacity of graphite-based anode materials is 372mAh/g, while the discharge capacity of silicon is 4200mAh/g, more than 10 times that of graphite. If the traditional graphite-based anode material can be replaced by a silicon anode, the energy density of a single battery will be greatly improved. Graphite negative electrodes have been commercialized for nearly 30 years, but they have not been replaced by silicon as a conventional material. The main reason is that the volume expansion of silicon is 300% to 400% during charge and discharge, while graphite is only 10%. The drastic change in volume will lead to the shedding of active materials and repeated consumption of electrolyte to form SEI film, which is finally reflected in capacity fading. In addition, the direct contact between the silicon negative electrode and the electrolyte will be corroded by hydrofluoric acid to generate gas, which will pose a safety hazard to the battery.

Chinese invention patent CN102208632A discloses a silicon nanowire-fullerene composite negative electrode material for lithium-ion batteries, fullerene flexible conductive particles and silicon nanowires together form a binary composite composite; silicon nanowires are used as lithium storage In the main body, fullerene flexible conductive particles are loaded on the surface of silicon nanowires to form a topological network structure composed of silicon nanowire arrays and fullerenes. The invention utilizes the excellent elasticity of fullerenes to buffer the volume expansion of silicon, thereby hindering the fusion of adjacent silicon nanowires. However, it cannot solve the problem of repeated formation of the SEI film caused by the drastic change in the volume of the silicon nanowire column, nor can it prevent the direct contact between the silicon anode and the electrolyte.

Chinese invention patent application CN106784607A discloses a titanium dioxide nanotube array immobilized silicon negative electrode material for lithium batteries. First, the titanium dioxide nanotube array is prepared, and then magnetron sputtering is used to form silicon with a certain shape at the nanotube mouth. This scheme uses the special structure of nanotubes to suppress the breakage caused by the expansion and contraction of silicon in the two-dimensional direction, but the overall process is complicated, and, with the increase of charge and discharge times, the capacity of the battery decays faster. At the same time, silicon The material is likewise in direct contact with the electrolyte.

Contents of the invention

The object of the present invention is to provide a silicon negative electrode for lithium-ion batteries, through structural design, the volume change of silicon can be reduced, the initial efficiency can be improved, the cycle performance can be improved, the rate performance can be improved and the energy density can be maximized. Another object of the present invention is to provide a method for preparing such a silicon negative electrode.

In order to achieve the purpose of the above invention, the technical solution adopted in the present invention is: a silicon negative electrode for lithium-ion batteries, comprising silicon materials arranged in an array on a substrate, and a graphene layer is coated on the surface of the silicon materials.

In the above technical solution, by coating the silicon material with graphene, the expansion of silicon can be restrained, the conductivity can be improved, the direct contact between the electrolyte and silicon can be avoided, and the good SEI film formed by graphene and electrolyte can be used to prolong the charge of the material. Discharge cycle life.

In the above technical solution, the thickness of the graphene layer is 0.335-3.35 nm.

Due to the huge volume change of the silicon material during the charge and discharge process, even though it is coated with graphene, there is still volume expansion. On the other hand, the energy density of the battery requires a certain packing density. Therefore, the present invention provides a preferred solution for the array arrangement of silicon materials.

Preferred option 1, the array arrangement is to form a cylindrical silicon array on the surface of the substrate, the distance L between the central axes of adjacent cylinders, the radius R of the circular cylinder section, and the height h of the cylinders simultaneously satisfy the following conditions: ① ;② , where r is the D50 radius of graphite particles.

r is 10 to 20 μm. Generally, the D50 of energy type graphite is about 20 μm, and the D50 of power type graphite is about 10 μm.

Preferred option two, the array arrangement is to form a pyramid-shaped silicon array on the surface of the substrate, the bottom surface of the pyramid is a regular N-gon, the distance L between the centers of the inscribed circles of adjacent regular N-gons, the side lengths of the regular N-gons a, The pyramid height h satisfies the following conditions at the same time: ① ;② , where r is the D50 radius of graphite particles.

r is 10 to 20 μm.

In order to realize another object of the invention of the present invention, a kind of preparation method of silicon negative electrode for lithium ion battery is provided, comprising the following steps:

(1) Form a silicon film on the front and back of the substrate by chemical vapor deposition or magnetron sputtering coating method;

(2) Carry out isotropic etching through the mask to form an array of silicon materials;

(3) A graphene layer coated on the surface of the silicon material is formed by chemical vapor deposition.

Wherein, in step (2), the array arrangement is to form a cylindrical silicon array on the surface of the substrate, and the distance L between the central axes of adjacent cylinders, the radius R of the circular cylinder section, and the height h of the cylinders simultaneously satisfy the following conditions: ① ;② , where r is the D50 radius of graphite particles.

Alternatively, in step (2), the array arrangement is to form a pyramid-shaped silicon array on the surface of the substrate, the bottom surface of the pyramid is a regular N-gon, the distance L between the centers of the inscribed circles of adjacent regular N-gons, and the regular N-gons Side length a and pyramid height h satisfy the following conditions at the same time: ① ;② , where r is the D50 radius of graphite particles.

In the above technical solution, copper foil may be used as the substrate.

Due to the use of the above-mentioned technical solutions, the present invention has the following advantages compared with the prior art:

1. By coating graphene on the surface of the silicon array, the present invention can restrain the expansion of silicon, improve the conductivity, avoid direct contact between the electrolyte and silicon, and use the good SEI film formed by graphene and electrolyte to prolong the charge and discharge of the material. cycle life.

2. The present invention provides the rules for the arrangement of silicon under different array structures, which greatly reduces the volume change of silicon, improves the first-time efficiency, improves the cycle performance, improves the rate performance and maximizes the energy density, and provides silicon-based materials for commercial applications. very good method.

Description of drawings

Fig. 1 is a schematic diagram of the arrangement of silicon arrays in Embodiment 1;

FIG. 2 is a schematic diagram of the arrangement of silicon arrays in the second embodiment.

Detailed ways

The present invention will be further described below in conjunction with accompanying drawing and embodiment:

Embodiment 1: Copper foil of 6-8 μm is used as a current collector, and a silicon film is formed by chemical vapor deposition or magnetron sputtering coating and deposited on both sides of the copper foil. Through the mask, silicon is isotropically wet-etched with hydrofluoric acid, nitric acid, and acetic acid to form a cylindrical silicon array, and graphene is coated on the silicon surface by using chemical vapor deposition CVD.

Since the volume change of silicon during charging and discharging is 300%~400%, the maximum available space time between the cylinders after expanding 3~4 times is its tangency. Considering that the pole group is extruded in the central axis of the cylinder, the material with a high aspect ratio changes more in the radial direction than in the direction of the central axis. Therefore, it is assumed here that all silicon expands in the radial direction, eliminating the possibility of radial extrusion.

As shown in Figure 1, the circular area S ₂ of the cylindrical section after expansion and the area S ₁ before expansion satisfy S ₂ /S ₁ =3~4, that is =3~4, where r ₂ is the circular radius of the cylindrical section after expansion, and r ₁ is the circular radius of the cylindrical section before expansion. Center axis distance between cylinders . Generally, the D50 radius r of graphite particles is 10~20 μm, the D50 of energy type graphite is about 20 μm, and the D50 of power type graphite is about 10 μm. Changing the particle size will not only change the diffusion path, but also is very unfavorable to the process of homogenization and coating of ingredients, so the optimal solution is to make the volume of the silicon cylinder equal to the volume of the spherical carbon, so the height h of the cylinder satisfies .

According to the power density and energy density of the battery for electric vehicles, the surface density corresponding to the positive electrode material can be selected, and then the monolithic (single layer) capacity of the corresponding silicon negative electrode can be determined according to the gram capacity of the positive electrode material and the rational design of the negative electrode capacity excess ratio, and finally can be determined areal density of the negative electrode. Under the conditions of satisfying the above two formulas, the design scheme of the battery is finally formed.

Embodiment two:

6~8μm copper foil is used as the current collector, and the silicon film is deposited on the front and back sides of the copper foil by chemical vapor deposition or magnetron sputtering coating. Through the mask plate, silicon is anisotropically wet-etched by potassium hydroxide, sodium hydroxide, and ammonia water to form a pyramid-shaped silicon array, and graphene is coated on the silicon surface by using chemical vapor deposition CVD.

As shown in Figure 2, due to the regular polygon area , where N is the number of sides of the regular polygon, and r is the radius of the inscribed circle. The circular area S ₂ of the cylindrical section after expansion and the area S ₁ before expansion satisfy S ₂ /S ₁ =3~4. The distance between the centers of the inscribed circles between regular N-gons on the bottom surface, namely , where a ₁ and a ₂ are the side lengths of the regular N-gon before and after expansion, respectively, and r1 and r2 are the inscribed circle radii of the regular N-gon before and after expansion, respectively. Since the volume of the pyramid is , , so the height h of the pyramid on one side of the current collector satisfies , where r is the D50 radius of graphite particles, generally 10-20 μm, the D50 of energy type graphite is about 20 μm, and the D50 of power type graphite is about 10 μm.

According to the power density and energy density of the battery for electric vehicles, the surface density corresponding to the positive electrode material can be selected, and then the monolithic (single layer) capacity of the corresponding silicon negative electrode can be determined according to the gram capacity of the positive electrode material and the rational design of the negative electrode capacity excess ratio, and finally can be determined areal density of the negative electrode. Under the conditions of satisfying the above two formulas, the design scheme of the battery is finally formed.

Claims (9)

1. A silicon negative electrode for a lithium ion battery, comprising a silicon material arranged in an array on a substrate, characterized in that: the surface of the silicon material is covered with a graphene layer. 2. The silicon negative electrode for lithium ion battery according to claim 1, characterized in that: the thickness of the graphene layer is 0.335-3.35 nm. 3. The silicon negative electrode for lithium-ion batteries according to claim 1, characterized in that: the array arrangement is to form a cylindrical silicon array on the surface of the substrate, the central axis distance L between adjacent cylinders, the circular radius of the cylindrical section R and cylinder height h satisfy the following conditions at the same time: ① ;② , where r is the D50 radius of graphite particles. 4. The silicon negative electrode for lithium ion batteries according to claim 3, characterized in that: r is 10-20 μm. 5. The silicon negative electrode for lithium ion battery according to claim 1, characterized in that: the array arrangement is to form a pyramid-shaped silicon array on the surface of the substrate, the bottom surface of the pyramid is a regular N-gon, and the adjacent regular N-gon The center distance L of the inscribed circle, the side length a of the regular N-gon, and the height h of the pyramid satisfy the following conditions at the same time: ① ;② , where r is the D50 radius of graphite particles. 6 . The silicon negative electrode for lithium ion batteries according to claim 5 , wherein r is 10-20 μm. 7. A method for preparing a silicon negative electrode for a lithium ion battery, comprising the following steps: (1) Form a silicon film on the front and back of the substrate by chemical vapor deposition or magnetron sputtering coating method; (2) Carry out isotropic etching through the mask to form an array of silicon materials; (3) A graphene layer coated on the surface of the silicon material is formed by chemical vapor deposition. 8. The method for preparing a silicon negative electrode for a lithium-ion battery according to claim 7, characterized in that: in step (2), the array arrangement is to form a cylindrical silicon array on the surface of the substrate, and the center between adjacent columns is The axial distance L, the radius R of the cylindrical section circle, and the height h of the cylinder meet the following conditions at the same time: ① ;② , where r is the D50 radius of graphite particles. 9. The method for preparing a silicon negative electrode for a lithium-ion battery according to claim 7, characterized in that: in step (2), the array arrangement is to form a pyramid-shaped silicon array on the surface of the substrate, and the bottom surface of the pyramid is a positive N side The distance L between the centers of the inscribed circles of adjacent regular N-gons, the side length a of the regular N-gons, and the height h of the pyramids satisfy the following conditions at the same time: ① ;② , where r is the D50 radius of graphite particles.