CN114107925A

CN114107925A - Porous silicon-based film and preparation method thereof

Info

Publication number: CN114107925A
Application number: CN202111382139.3A
Authority: CN
Inventors: 王君; 葛旭辉; 孙中贵; 张致雅
Original assignee: Lanzhou University
Current assignee: Lanzhou University
Priority date: 2021-11-22
Filing date: 2021-11-22
Publication date: 2022-03-01

Abstract

The invention discloses a preparation method of a porous silicon-based film, which adopts a physical vapor deposition or chemical vapor deposition method to prepare a Si film and a selected other film into a multilayer film structure, wherein the selected other film material is used as an auxiliary material for Si diffusion and a composite material in a Si composite cathode; then heat treatment is carried out at a proper temperature, asymmetric diffusion occurs, and part of silicon particles are separated from TiO₂Then removing Si diffused to the surface to obtain the Si-based multi-layer film cathode with a void structure. Compared with other Si-based thin film electrodes, the invention has the advantages that: the Si-based film electrode has stable structure, high energy density and long service life; the Si film has convenient pore-forming, easy control and environmental protection; the film has good toughness, and can be used for preparing thicker film electrodes; the electrode slice prepared by the method not only can relieve the volume expansion of Si and improve the diffusion rate of lithium ions, but also can keep excellent cycle stability.

Description

Porous silicon-based film and preparation method thereof

Technical Field

The invention belongs to a preparation method of a silicon-based film negative plate of a lithium ion battery, which utilizes two-phase asymmetric diffusion, namely a Kirkendall effect, to realize that partial Si is diffused out of the film so as to form a porous Si film for meeting the volume change space requirement of a silicon negative electrode in the process of lithium release and intercalation.

Background

Thin film batteries do have some advantages over conventional solid state secondary batteries. The most obvious advantage is that it is much smaller in size and can be used to make smaller electronic devices. In addition, thin film batteries typically have higher average output voltages, lighter weight, greater flexibility, higher energy density, no electrolyte leakage, tighter packaging, and longer life cycles than bulky solid state batteries. Thin film batteries are used in many areas including renewable energy storage devices, smart cards, Radio Frequency Identification (RFID) tags, portable electronics, defibrillators, neurostimulators, pacemakers, and wireless sensors, to name a few.

A thin film battery is one of solid-state batteries, i.e., a battery using both a solid electrode and a solid electrolyte. However, unlike many other batteries, they are only a few hundred nanometers in size. As with all batteries, thin film batteries have both an anode and a cathode with an electrolyte and separator material in between. For many thin film batteries, the cathode is typically made of a lithium oxide composite, such as LiCoO₂、LiMn₂O₄And LiFePO₄. The anode material is typically made of a carbon-based material, such as graphite, but lithium and other metals may also be used.

Like the traditional lithium ion battery, the energy density of the thin film battery is improved mainly depending on the negative electrode material, and researches show that silicon is a negative electrode material with extremely high theoretical specific capacity, and the maximum value is 4200mAhg^-1In addition, silicon has a suitable lithiation potential and a rich reserve, and is therefore considered as the most likely negative electrode material to replace graphite.

To make the required thickness, the electrode material is typically deposited using Physical Vapor Deposition (PVD) techniques such as sputtering and thermal evaporation. Like Si particles, Si thin films crack during lithium deintercalation due to a large change in volume and may be detached from a current collector to cause battery failure.

The general strategy for solving the volume effect of the Si cathode is to reserve a gap on the design of the Si composite cathode structure to accommodate the silicon volume expansion and contraction, and the common method is to prepare a core-shell structure by utilizing a template method and an etching method. Specifically, a layer of template (such as SiO) is coated on the surface of silicon particles₂) The desired structure (e.g., Si @ viod @ C) is then obtained by coating a volume buffer material (e.g., carbon, etc.) and removing the template with a caustic substance (e.g., HF) after molding. However, for silicon thin film electrodes, the above method is not suitable, and how to reserve the volume expansion and contraction space in the Si thin film becomes a bottleneck restricting the use of Si in thin film batteries.

Disclosure of Invention

In view of the above, the present invention provides a method for generating defects by asymmetric diffusion at a two-phase interface to form holes in a silicon-based thin film for volume change of Si during li deintercalation. Specifically, a layer of material is deposited on the surface of Si film, and Si is deposited on the material (such as TiO)₂Or SiO₂) The diffusion coefficient of (a) is much larger than the diffusion of the substance in Si, so that Si will diffuse to the other side of the substance to form voids due to the loss of Si in the Si thin film.

The technical scheme of the invention is as follows: the Si film and the selected other film are prepared into a multilayer film structure by adopting a physical vapor deposition or chemical vapor deposition method, and the selected other film material is used as an auxiliary material for Si diffusion and also used as a composite material in a Si composite negative electrode. Then, heat treatment is carried out at a proper temperature, asymmetric diffusion occurs, part of silicon particles are diffused from another substance, and then Si diffused to the surface is removed to obtain the Si-based multi-layer film negative electrode with a void structure. (the structure is schematically shown in figure 1)

The other film is made of a substance with a diffusion coefficient different from that of Si, and the diffusion coefficient of Si in the substance is far larger than that of Si, so that Si diffuses to the other side of the substance to form a gap due to the loss of Si in the Si film.

Alternative materials for the film include, but are not limited to, TiO₂Or SiO₂。

The porous silicon-based film and the preparation method of the invention firstly utilize the magnetron sputtering method to prepare the multilayer film of silicon and another substance with the thickness of 400-2000nm, and then carry out heat treatment in the argon atmosphere of a tubular furnace at the heat treatment temperature of 300-500 ℃ for 2-6 hours; then sputtering a layer of thinner metal on the surface, repeating the process for several times to reach the ideal film thickness.

The thin metal includes but is not limited to copper, titanium.

The porous silicon-based film and the preparation method thereof generate the Kirkinjel effect under the proper heat treatment temperature and time, and the diffusion rate and the diffusion quantity of silicon are controlled by regulating and controlling the heat treatment temperature and time.

The purpose of the metal doping of the multilayer film is to improve the conductivity of the multilayer film and facilitate the rapid transmission of electrons on one hand, and to increase the adsorbability and facilitate the adsorptive contact between the metal and the next plated multilayer film on the other hand.

When another material is selected TiO₂The preparation method can be as follows: firstly, preparing Si/TiO by magnetron sputtering method₂Multilayer film (Si/TiO) of 10nm:3nm₂Can be adjusted) to a thickness of about 500nm, and then treated in an argon atmosphere at 350 ℃ for 2 hours, after which the surface is removedAfter the Si particles are sputtered, a layer of copper with the thickness of 8nm (film toughening) is formed on the surface, and the process is repeated for 4 times until the thickness is about 2 mu m. (as shown in the cross-sectional SEM of fig. 2).

The invention also provides the porous silicon-based film prepared by the method.

Compared with other Si-based thin film electrodes, the invention has the advantages that:

(1) the Si-based film electrode has stable structure, high energy density and long service life;

(2) the Si film has convenient pore-forming, easy control and environmental protection;

(3) the film has good toughness, and can be used for preparing thicker film electrodes;

(4) the electrode slice prepared by the method not only can relieve the volume expansion of Si and improve the diffusion rate of lithium ions, but also can keep excellent cycle stability.

Drawings

FIG. 1 is a schematic diagram of a process for preparing a porous silicon-based thin film material according to the present invention;

FIG. 2 is a sectional SEM image of a thin film electrode made of STO 350;

FIG. 3a is a SEM image of the surface topography of an STO electrode after 5 cycles, and FIG. 3b is a SEM image of the surface topography of an STO350 electrode after 5 cycles;

FIG. 4a is a surface topography SEM image after 200 cycles of the STO electrode; FIG. 4b is a SEM image of the surface topography after 200 cycles of the STO350 electrode;

FIG. 5 is a graph of the cycling performance of STO and STO 350;

FIG. 6 is a CV curve for STO 350.

Detailed Description

Example 1

Pre-polishing a rough Cu foil, washing an oxide layer on the surface of the Cu foil by using concentrated hydrochloric acid, scrubbing the Cu foil by using alcohol, pouring the Cu foil into a beaker with alcohol, carrying out ultrasonic treatment for 20min at the temperature of 30 ℃, and finally fixing the Cu foil on a substrate seat in a cavity of a sputtering table;

the vacuum degree of the cavity is pumped to 2.4 multiplied by 10^-4About Pa, the vacuum degree during film coating is adjusted to about 1.0Pa, the working gas is high-purity argon with the flow rate of 20sccm, the glow phenomenon can appear after the target material is electrified, and sputtering film coating is started。

Before preparing the multilayer film, firstly sputtering a Ti (C) transition layer (the thickness is about 25nm) on the surface of a Cu foil, wherein the current of C is increased from 0 to 0.3A, and the current rising speed is 0.05A/min; the current of Ti is gradually reduced from 0.6A to 0A, the current reduction speed is 0.1A/min, and Ti and C are sputtered simultaneously. (the purpose of depositing the transition layer is to enable the multilayer film to be better deposited and grown on the surface of the Cu foil without falling off, and in addition, C can also increase the conductivity)

After the transition layer sputtering is finished, the sputtering of Si and TiO is started₂Sputtered single layer of Si and TiO₂Is 10nm and 3nm, respectively, and the designed thickness of the thin film is 450nm (without the transition layer). Si and TiO₂35 times of total circulation, Si and TiO₂The deposition rates of (A) and (B) are respectively 7nm min^-1And 6nmmin^-1。

Then a further layer of copper, about 10nm, is sputtered and the process is repeated 4 more times to a film thickness of about 2 μm, the prepared electrode being designated STO.

And then cutting the processed electrode plate, and then filling a battery into a glove box, wherein the glove box is protected by argon. 2025 type electrode shell is selected, and electrolyte is LiPF₆And the diaphragm is polypropylene. The assembling method of the battery is the assembling method of the conventional button battery.

And standing the assembled battery for 24 hours to completely soak the electrolyte, and then carrying out electrochemical performance test. The battery is charged and discharged under the current density of 2.5A/g, the program selects reverse polarity, the battery is firstly discharged and recharged, and the cycle times are set to be 1000 circles.

Example 2

The other specific operations were the same as in example 1, except that Si/TiO was prepared each time₂After the film is formed, placing the film in a tubular furnace for calcination, setting the programmed temperature to be 350 ℃ for 2 hours, wherein the protective gas is argon, and the gas flow rate is 60 sccm; and sputtering a layer of copper. The above procedure was repeated 4 more times and the prepared electrode was named STO 350.

The thin film electrodes obtained in example 1 and example 2 were structurally characterized; the structural characterization is mainly SEM, the SEM dimensions are 10 micrometers and 50 micrometers, and the surface appearance of the electrode is obtained after 5-time and 200-time shooting circulation. As can be clearly seen in fig. 3a and b, the STO is compact and complete on the surface of the electrode at the initial stage of the cycle, and the pore left by the diffusion of the silicon particles can be seen on the surface of the electrode (STO350) after the heat treatment, which illustrates that the invention achieves the purpose of pore-forming;

in fig. 4a, b, it can be seen that the heat-treated electrode (STO350) does not have serious peeling after 200 cycles, and the surface electrode without heat treatment (STO) has serious peeling after 200 cycles; additionally some diffused silicon particles are visible on the electrode surface after the heat treatment. After the heat treatment, the silicon particles are diffused, the bonding force between the film and the substrate can be strengthened, the internal stress generated by deformation can be borne, and the film can be prevented from falling off, cracking and the like.

Example 2 electrochemical testing was carried out under the same conditions as in example 1, for 1000 cycles at a current of 2.5A/g. FIG. 5 is a graph of the cycle after no heat treatment and 350 ℃ heat treatment, in which it can be seen that after heat treatment, the specific capacity of STO350 at the initial cycle is not as high as STO due to capacity fade caused by diffusion of part of the silicon to the surface after heat treatment, but during 1000 cycles, the capacity of STO350 remains stable and shows a tendency to rise; the capacity of the inverse STO decays all the time in the long circulation process of 1000 times, and the specific capacity decays to 306.6mAh g after 1000 times of circulation^-1；

Due to the fact that copper with higher conductivity is doped, and the structural design is combined, the transmission speed of electrons and lithium ions is higher; in addition, the copper film also prevents silicon particles diffused to the surface layer from falling off;

therefore, the cycle stability after heat treatment is obviously improved compared with that without heat treatment, and the specific capacity is still 1000mAh g after 1000 times of charge-discharge cycles^-1The above.

FIG. 6 is a CV diagram of STO350, in which it can be seen that two delithiation peaks of amorphous Si appear near 0.37V and 0.53V during charging, and TiO appears at 1.52V₂At about 1.02V and 0.48V during discharge, both samples appearedTwo amorphous TiO₂Reduced peak of (2). Then, at the 0.18V position, a lithium insertion peak of silicon appears, which shows that Si and TiO are in 350 DEG C₂Still in an amorphous structure, Si and TiO not being altered at this temperature₂Structural properties of (a). In addition, the peak positions of the CV curves of the latter circles are basically overlapped, which shows that the stability of the battery is good.

From the above data, we can conclude that:

the invention makes up the defect that the large volume expansion of the silicon-based film material is difficult to solve by PVD, CVD or other methods, realizes the method for forming holes in the silicon-based film material by utilizing the Kerkdall effect, and relieves the defect of silicon material circulation; the porous structure not only reserves space for the volume expansion of silicon in the film electrode, but also provides a fast channel for the transmission of ions and electrons by large-area holes. In electrochemical performance, the prepared electrode has excellent performances of good cycle stability, high reversible capacity and the like, and the method can realize pore forming in the Si-based film.

Example 3

Firstly, preparing silicon and TiO by magnetron sputtering method₂Multi-layer film of (2), Si: TiO₂4nm under the condition of 10nm, and the thickness is 400nm, and then the mixture is subjected to heat treatment in a tubular furnace argon atmosphere at the temperature of 300 ℃ for 2 hours; then, titanium with a thickness of 10nm was sputtered on the surface, and this process was repeated 4 times.

Example 4

Firstly, preparing silicon and SiO by magnetron sputtering method₂Multi-layer film of (Si: SiO)₂3nm under the condition of 10nm and the thickness of 2000nm, and then performing heat treatment in a tubular furnace in an argon atmosphere at the temperature of 500 ℃ for 6 hours; then a layer of copper with the thickness of 8nm is sputtered on the surface, and the process is repeated for 2 times.

Example 5

Firstly, preparing silicon and TiO by magnetron sputtering method₂Multi-layer film of (2), Si: TiO₂The mixture is processed by heat treatment in a tubular furnace in argon atmosphere at the temperature of 400 ℃ for 4 hours, wherein the thickness of the mixture is 10nm to 5 nm; then a layer of titanium with a thickness of 12nm is sputtered on the surface, and the process is repeated for 3 times.

The above description is only for the preferred embodiment of the present invention, and is not intended to limit the present invention in any way. Any simple modifications, alterations and equivalent changes of the above embodiments according to the technical essence of the invention still fall within the protection scope of the technical solution of the invention. Although the present invention has been described in detail with reference to the foregoing embodiments, it will be apparent to those skilled in the art that changes may be made in the embodiments and/or equivalents thereof without departing from the spirit and scope of the invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A preparation method of a porous silicon-based film is characterized in that a multi-layer film structure is prepared by a Si film and a selected other film by adopting a physical vapor deposition or chemical vapor deposition method, and the selected other film material is used as an auxiliary material for Si diffusion and a composite material in a Si composite negative electrode; then, heat treatment is carried out at a proper temperature, asymmetric diffusion occurs, part of silicon particles are diffused from another substance, and then Si diffused to the surface is removed to obtain the Si-based multi-layer film negative electrode with a void structure.

2. The method of claim 1, wherein the other film is made of a material having a diffusion coefficient different from that of Si, and the diffusion coefficient of Si in the material is much greater than that of Si, so that Si diffuses to the other side of the material to form voids due to loss of Si in the Si film.

3. The method of claim 2, wherein the other material of the membrane includes but is not limited to TiO₂Or SiO₂。

4. The method for preparing a porous silicon-based film according to claim 2, wherein the multi-layer film of silicon and another substance is prepared by magnetron sputtering at a thickness of 400-2000nm, and then heat-treated in an argon atmosphere of a tubular furnace at a temperature of 300-500 ℃ for 2-6 hours; then sputtering a layer of thinner metal on the surface, repeating the process for several times to reach the ideal film thickness.

5. The method of claim 4, wherein the thin metal comprises but is not limited to copper and titanium.

6. The method of claim 1, wherein the kirkendall effect occurs at a suitable temperature and time for the thermal treatment, and the diffusion rate and amount of silicon are controlled by controlling the temperature and time for the thermal treatment.

7. The method of claim 5, wherein the metal is doped into the multilayer film to improve the conductivity of the multilayer film and facilitate the rapid transport of electrons, and to increase the adsorption property to facilitate the adsorption contact between the metal and the next plated multilayer film.

8. The method of claim 5, wherein the other material is TiO₂The Si and TiO are prepared by a magnetron sputtering method₂Multilayer film, Si/TiO₂The monolayer thickness ratio of (a) can be adjusted.

9. The method for preparing porous silicon-based film according to claim 8, wherein the prepared multilayer film is Si: TiO₂10nm to3 nm; the method specifically comprises the following steps:

the vacuum degree of the cavity is pumped to 2.4 multiplied by 10^-4Pa or so, the vacuum degree during film coating is adjusted to be about 1.0Pa, the working gas is high-purity argon with the flow rate of 20sccm, the glow phenomenon can occur after the target material is electrified, and sputtering film coating is started;

before preparing the multilayer film, firstly sputtering a Ti (C) transition layer (with the thickness of 25nm) on the surface of a Cu foil, wherein the current of C is increased from 0 to 0.3A, and the current increasing speed is 0.05A/min; the current of Ti is gradually reduced from 0.6A to 0A, the current reduction speed is 0.1A/min, and Ti and C are sputtered simultaneously;

after the transition layer sputtering is finished, the sputtering of Si and TiO is started₂Sputtered single layer of Si and TiO₂Has a thickness of 10nm and 3nm, Si and TiO respectively₂35 times of total circulation, Si and TiO₂The deposition rates of (A) and (B) are respectively 7nm min^-1And 6nm min^-1；

Then preparing Si/TiO₂Calcining the film in a tubular furnace, setting the programmed temperature to be 350 ℃ and the time to be 2 hours, wherein the protective gas is argon and the gas flow rate is 60 sccm;

sputtering a layer of metal with the thickness of about 10nm on the film, wherein copper is sputtered in the experiment;

then repeating the steps for 4 times to about 2 mu m.

10. A porous silicon-based film prepared by the method of any one of claims 1-9.