WO2021082074A1

WO2021082074A1 - Polymer electrolyte film and preparation method therefor, and application in all-solid-state lithium battery

Info

Publication number: WO2021082074A1
Application number: PCT/CN2019/117835
Authority: WO
Inventors: 李峰; 徐胜军; 孙振华; 成会明
Original assignee: 中国科学院金属研究所
Priority date: 2019-11-01
Filing date: 2019-11-13
Publication date: 2021-05-06
Also published as: CN110690497A; CN110690497B

Abstract

The present invention relates to the technical field of all-solid-state batteries. Disclosed are a polymer electrolyte film and a preparation method therefor, and an application in an all-solid-state lithium battery. The polymer film is composed of polyethylene oxide and a lithium salt, and has the characteristics of high ionic conductivity, excellent mechanical performance, and a wide electrochemical stability window. The preparation method of the present invention is environment-friendly, low in cost, simple in process and compatible with existing processes, can effectively simplify a production matching process of solid-state lithium batteries, improve battery performance, and thus have great application prospects.

Description

Polymer electrolyte film, preparation method thereof and application in all-solid-state lithium battery

Technical field

The invention relates to the technical field of all-solid-state batteries, in particular to a polymer electrolyte film, a preparation method thereof, and application in all-solid-state lithium batteries.

Background technique

Polyethylene oxide (PEO) is a common solid electrolyte matrix. It has high lithium salt solubility, and has the advantages of stable contact with lithium metal, high flexibility, and easy processing. It is considered to be an ideal preparation for high One of the high-performance solid electrolyte materials has been extensively studied in recent years.

At present, the preparation method of PEO-based solid electrolyte is mainly the solution casting method, that is, a certain quality of PEO is dissolved in an organic solvent (acetonitrile, acetone, chloroform, etc.) to prepare a uniform solution, and then the solvent is evaporated under a certain temperature and vacuum conditions, thereby A polymer film is obtained. However, the PEO-based solid electrolyte prepared by the solution casting method has problems such as low ionic conductivity, poor mechanical properties, and narrow electrochemical stability window. In addition, the solution pouring method requires the use of a large amount of organic solvents, which has problems such as high cost, high toxicity and environmental pollution.

Summary of the invention

In order to solve the above-mentioned shortcomings in the prior art, the purpose of the present invention is to provide a polymer electrolyte film, a preparation method thereof, and an application in an all-solid-state lithium battery. The polymer electrolyte membrane is obtained by using deionized water as the solvent to dissolve the polymer and the lithium salt, and adopting a freeze-drying method. The polymer film all-solid electrolyte of the present invention has the characteristics of high ionic conductivity, wide electrochemical stability window and excellent mechanical properties, can be applied to all-solid lithium batteries, and can have a higher capacity when working at room temperature and high temperature. .

In order to achieve the above objectives, the technical solutions adopted by the present invention are as follows:

A method for preparing a polymer electrolyte membrane includes the following steps:

(1) Dissolve polyethylene oxide and lithium salt in a solvent in proportions, and stir to obtain a mixed solution;

(2) Transfer the mixed solution obtained in step (1) to the substrate and place it in a low-temperature environment for freezing. After the solvent is completely solidified, move it to a freeze dryer to freeze-dry the sample to obtain the polymer electrolyte film

The polyethylene oxide has a molecular weight of 300,000 to 7 million.

The lithium salt is one or more of LiTFSI, LiClO4, LiPF6, LiBF4 and LiAsF6.

In the above step (1), the weight of the lithium salt is 5% to 50% by weight of the total weight of the polyethylene oxide and the lithium salt.

The concentration of polyethylene oxide in the mixed solution in the above step (1) is 5wt% to 15wt%.

In the mixed solution in the above step (1), the solvent used is deionized water, or the solvent used is a mixed solvent formed by mixing deionized water and an organic solvent (ethanol, etc.) in any ratio.

In the above step (1), the stirring time is 12-48 hours; in the step (2), the low-temperature environment refers to a temperature of -30°C to -10°C, and the freezing time in a low-temperature environment is 12-48 hours. 36 hours; in step (2), the temperature of the cold trap of the freeze dryer is -90°C to -50°C, and the freeze-drying time in the freeze dryer is 24-48 hours.

The polymer electrolyte membrane prepared by the invention is applied to all-solid-state lithium batteries under room temperature and high temperature working conditions.

The design principle of the present invention is as follows:

Crystalline PEO has potential high ionic conductivity, and PEO film with high crystallinity is an excellent solid electrolyte. Based on this, the present invention uses deionized water as a solvent to dissolve PEO and lithium salt, and freeze-drying is used to remove the solvent, thereby obtaining a high crystallinity PEO film with internal interconnection structure, improving the mechanical properties and ionic conductivity of the solid electrolyte. Realize the all-solid-state lithium battery that works at room temperature and high temperature.

By adopting the freeze-drying method to prepare the interconnected high crystallinity PEO film, a rapid transmission channel of lithium ions is formed inside the solid electrolyte, thereby effectively improving the mechanical properties and ionic conductivity of the solid electrolyte, and preparing a complete film that works at room temperature and high temperature. Solid-state lithium battery.

The advantages and beneficial effects of the present invention are as follows:

1. The method of the present invention has the characteristics of low cost and environmental protection.

2. As a solid electrolyte, the polymer film of the present invention has the characteristics of high ionic conductivity, wide electrochemical stability window and excellent mechanical properties.

3. The all-solid-state lithium battery assembled by the method proposed in the present invention exhibits a higher capacity at room temperature and high temperature.

4. The polymer film designed in the present invention has a simple preparation process, good repeatability, and is easy to scale up and produce on a large scale.

Description of the drawings

Figure 1 is a schematic diagram of the preparation of a polymer electrolyte membrane.

FIG. 2 is a physical view of a polymer film prepared according to Example 1. FIG.

FIG. 3 shows the mechanical performance test of the polymer film prepared according to Example 1. FIG.

Figure 4 shows the mechanical performance test of the polymer film prepared according to Comparative Example 1.

Figure 5 shows the electrochemical impedance of a polymer film prepared according to Example 1 at 50°C.

Figure 6 shows the electrochemical impedance of a polymer film prepared according to Example 1 at 25°C.

Figure 7 shows the electrochemical impedance of the polymer film prepared according to Example 3 at 50°C.

Figure 8 shows the electrochemical impedance of a polymer film prepared according to Comparative Example 1 at 25°C.

FIG. 9 shows the electrochemical stability window test of the polymer film prepared according to Example 1. FIG.

Fig. 10 is a charging and discharging curve of an all-solid-state battery prepared according to Example 3 at 50°C.

FIG. 11 is a charging and discharging curve of an all-solid-state battery prepared according to Example 3 at 25°C.

Fig. 12 is a charging and discharging curve of an all-solid-state battery prepared according to Example 5 at 50°C.

Fig. 13 is a charging and discharging curve of an all-solid-state battery prepared according to Comparative Example 2 at 25°C.

Detailed ways

The present invention is described below in conjunction with embodiments.

The present invention proposes a polymer electrolyte membrane. According to an embodiment of the present invention, the polymer film includes: polyethylene oxide and lithium salt. Therefore, the polymer film has the characteristics of high ionic conductivity, wide electrochemical stability window and high mechanical properties, and the preparation process is simple.

Example 1

This embodiment is the preparation of the polymer electrolyte membrane, and the process is as follows:

Dissolve polyethylene oxide and LiTFSI with a molecular weight of 600,000 in deionized water and stir for 24 hours to obtain a uniform solution. Among them, the concentration of polyethylene oxide in the solution is 5 wt.%, and the weight of LiTFSI accounts for 35% of the total mass of LiTFSI and polyethylene oxide. Subsequently, the homogeneous solution was transferred to a polytetrafluorovinyl sheet, and frozen at minus 18°C for 24 hours, and then the substrate was transferred to a freeze dryer for 36 hours and then taken out to obtain a polymer electrolyte film.

Figure 1 shows a schematic diagram of the polymer electrolyte membrane preparation process. Figure 2 shows the physical picture of the polymer film prepared in this embodiment. The elastic modulus of the polymer film is as high as 55MPa, has high mechanical properties (as shown in Figure 3), and the preparation process is simple.

Example 2

Dissolve polyethylene oxide and LiClO4 with a molecular weight of 600,000 in deionized water and stir for 24 hours to obtain a uniform solution. Among them, the concentration of polyethylene oxide in the solution is 5 wt.%, and the weight of LiClO 4 accounts for 10% of the total mass of LiTFSI and polyethylene oxide. Subsequently, the uniform solution was transferred to a polytetrafluorovinyl sheet, and frozen at minus 18°C for 24 hours, and then transferred to a freeze dryer for 36 hours and then taken out to obtain a polymer electrolyte film.

Example 3

This embodiment is to prepare a high-performance all-solid-state lithium battery, and the process is as follows:

The polyethylene oxide, succinonitrile, and LiTFSI with a molecular weight of 600,000 are mixed in acetonitrile to obtain a uniform mixed electrolyte slurry with a certain viscosity. Among them, succinonitrile accounts for 30% of the total mass of succinonitrile and polyethylene oxide, and LiTFSI accounts for 15% of the total mass of LiTFSI and polyethylene oxide. The obtained mixed electrolyte slurry, lithium iron phosphate, polyvinylidene fluoride, and conductive carbon black are uniformly mixed in NMP at a mass ratio of 10:60:15:15 to obtain a composite positive electrode slurry, and the positive electrode slurry is coated On the side of carbon-coated aluminum foil. After vacuum drying at 80° C., acetonitrile and NMP are removed to obtain a composite positive electrode. The composite positive electrode formed is composed of lithium iron phosphate, conductive carbon black, polyvinylidene fluoride, and a polymer film all-solid electrolyte.

The obtained composite positive electrode is cut into a positive electrode sheet, and a lithium sheet is used for the negative electrode. The polymer film in Example 1 was sandwiched between the positive electrode sheet and the negative electrode sheet, and packed into a 2025 battery case, and assembled into a button battery for testing.

Example 4

This embodiment is a high-performance all-solid-state lithium battery prepared, and the process is as follows:

The obtained composite positive electrode is cut into a positive electrode sheet, and a lithium sheet is used for the negative electrode. The polymer film in Example 1 was sandwiched between the positive electrode sheet and the negative electrode sheet for aluminum-plastic packaging to obtain a soft pack battery.

Example 5

Mix polyethylene oxide and LiTFSI with a molecular weight of 600,000 in acetonitrile to obtain a uniform mixed electrolyte slurry with a certain viscosity. Among them, LiTFSI accounts for 15% of the total mass of LiTFSI and polyethylene oxide. The obtained mixed electrolyte slurry, S@CMK-3, polyvinylidene fluoride, and conductive carbon black are mixed uniformly in NMP at a mass ratio of 10:70:10:10 to obtain a composite positive electrode slurry, and the positive electrode slurry Coated on the side of carbon-coated aluminum foil. After vacuum drying at 60°C, acetonitrile and NMP are removed, a composite positive electrode is obtained. The composite positive electrode formed is composed of sulfur, CMK-3, conductive carbon black, polyvinylidene fluoride, and polymer film all-solid electrolyte.

The obtained composite positive electrode was cut into positive electrode sheets. The negative electrode adopts lithium sheet. The polymer film in Example 1 was sandwiched between the positive electrode sheet and the negative electrode sheet, and packed into a 2025 battery case, and assembled into a button battery for testing.

Comparative example 1

This example is the preparation of polymer electrolyte membrane:

Dissolve polyethylene oxide and LiTFSI with a molecular weight of 600,000 in acetonitrile solvent and stir for 24 hours to obtain a uniform solution. Among them, the concentration of polyethylene oxide in the solution is 6 wt.%, and the weight of LiTFSI accounts for 35% of the total mass of LiTFSI and polyethylene oxide. Subsequently, the uniform solution was transferred to a polytetrafluorovinyl sheet, and vacuum-dried at 40°C for 48 hours, and then taken out to obtain a polymer film. The elastic modulus of the polymer film is only 0.2 MPa, and the mechanical properties are poor (Figure 4).

Comparative example 2

This example is to prepare an all-solid-state lithium battery, the process is as follows:

The obtained composite positive electrode is cut into a positive electrode sheet, and a lithium sheet is used for the negative electrode. The polymer film in Comparative Example 1 was sandwiched between the positive electrode sheet and the negative electrode sheet, and packed into a 2025 battery case, and assembled into a button battery for testing.

The following is the performance test of the samples prepared in each embodiment and comparative example:

1. Mechanical performance test:

The polymer film prepared in Example 1 was cut into a sample to be tested with a length of 40 mm, a width of 3 mm, and a thickness of 400 μm. A dynamic thermomechanical analyzer was used to test the mechanical properties of the sample in Example 1. The specific test method is: at room temperature, the sample to be tested is placed in the sample holder of the instrument, preloaded with a load of 0.1N, and the stress loading speed is 1.5N/min , The load-strain curve of the sample is shown in Figure 3. As a result, the elastic modulus of the polymer film is as high as 55 MPa and has high mechanical strength.

The polymer film prepared in Comparative Example 1 was cut into a sample to be tested with a length of 40 mm, a width of 3 mm, and a thickness of 400 μm. A dynamic thermomechanical analyzer was used to test the mechanical properties of the sample of Comparative Example 1. The specific test method is: place the sample to be tested in the sample holder of the instrument at room temperature, preload a load of 0.1N, and the stress loading speed is 1.5N/min , The load-strain curve of the sample is shown in Figure 4. Therefore, the elastic modulus of the polymer film is only 0.2 MPa, and the mechanical strength is low.

2. Conductivity test:

The polymer film prepared in Example 1 was punched with a punching machine to obtain a polymer film disc. The test showed that the sample had a thickness of 400 microns and a film diameter of 20 mm. The electrical conductivity of the sample of Example 1 was tested. The specific test method is: adding stainless steel sheets at both ends of the sample to form a battery test, the diameter of the stainless steel sheet is 12 mm, and the test frequency range is 0.1Hz-3MHz (electrochemical workstation). The impedance diagram at 50°C is shown in Figure 5, and the impedance diagram at 25°C is shown in Figure 6. Finally, according to the electrochemical impedance, the thickness of the sample and the area of the electrode, the ionic conductivity of the sample is calculated. The ionic conductivity of the sample of Example 1 measured at 50°C is 8.3×10-4 S/cm; the ionic conductivity measured at 25°C is 6.4×10-5 S/cm. Therefore, the polymer film has high ionic conductivity at high temperature and room temperature.

The polymer film prepared in Example 2 was punched with a punching machine to obtain a polymer film disc. The thickness of the sample was 450 micrometers and the diameter of the film was 20 mm. The electrical conductivity of the sample in Example 2 was tested. The specific test method is: adding stainless steel sheets at both ends of the sample to form a battery test. The diameter of the stainless steel sheet is 12 mm. The test frequency range is 0.1Hz-3MHz (electrochemical workstation). The impedance diagram at 50°C is shown in Figure 7. Finally, according to the electrochemical impedance, the thickness of the sample and the area of the electrode, the ionic conductivity of the sample is calculated. The ionic conductivity of the sample of Example 2 measured at 50°C is 2.0×10-4 S/cm. As a result, the polymer film has higher ionic conductivity.

The polymer film prepared in Comparative Example 1 was punched with a punching machine to obtain a polymer film disc. The thickness of the sample was 140 micrometers and the diameter of the film was 20 mm. The electrical conductivity of the sample is tested. The specific test method is: add stainless steel sheets at both ends of the sample to form a battery test, the diameter of the stainless steel sheet is 12 mm, and the test frequency range is 0.1Hz-3MHz (electrochemical workstation), which is at 25℃ The impedance diagram is shown in Figure 8. Finally, according to the electrochemical impedance, the thickness of the sample and the area of the electrode, the ionic conductivity of the sample is calculated. The ionic conductivity of the sample of Comparative Example 1 measured at 25°C is only 8.0×10-6S/cm.

3. Electrochemical stable working window test:

The polymer film prepared in Example 1 was punched with a punching machine to obtain a polymer film disc. The test showed that the sample had a thickness of 400 microns and a film diameter of 20 mm. The electrochemical stability window of the sample of Example 1 was tested. The specific test method was as follows: a stainless steel sheet and a metal lithium sheet were added to the two ends of the sample to form a battery for testing, and an electrochemical workstation was used to perform an electrochemical working window test to obtain a linear scan voltage. Ampere curve (Figure 9), as a result, the oxidation voltage of the polymer film is as high as 5.1V, showing a wide electrochemical stability window.

4. Charge and discharge test:

The all-solid-state battery prepared in Example 3 was tested at 50°C. The charge cut-off voltage is 4.2V, and the discharge cut-off voltage is 2.5V. The charge and discharge current is set to 0.2C. As shown in FIG. 10, it is a charging and discharging curve diagram of the all-solid-state battery prepared according to Example 3 at 50°C. It can be seen from the figure that the specific discharge capacity of the all-solid-state battery containing polymer film is as high as 158.2mAh/g at 50°C. As a result, such all-solid-state batteries also have higher charge and discharge capabilities at high temperatures.

The all-solid-state battery prepared in Example 3 was tested at room temperature (25°C). The charge cut-off voltage is 4.2V, and the discharge cut-off voltage is 2.5V. The charge and discharge current is set to 0.1C. As shown in FIG. 11, it is a charging and discharging curve diagram of the all-solid-state battery prepared according to Example 3 at room temperature. It can be seen from the figure that the specific discharge capacity of the all-solid-state battery containing polymer film is as high as 135.8mAh/g at 25°C. As a result, this kind of all-solid-state battery also has a higher charge and discharge capacity at room temperature. Therefore, the polymer film all-solid electrolyte is suitable for lithium ion battery systems.

The all-solid-state battery prepared in Example 5 was tested at 50°C. The charge cut-off voltage is 2.7V, and the discharge cut-off voltage is 1.8V. The charge and discharge current is set to 0.01C. As shown in FIG. 12, it is a charging and discharging curve diagram of the all-solid-state battery prepared according to Example 5 at room temperature. It can be seen from the figure that the specific discharge capacity of the all-solid-state battery containing polymer film is as high as 1066.3mAh/g. Therefore, the polymer film all-solid electrolyte is also suitable for lithium-sulfur battery systems.

The all-solid-state battery prepared in Comparative Example 2 was tested at room temperature (25°C). The charge cut-off voltage is 4.2V, and the discharge cut-off voltage is 2.5V. The charge and discharge current is set to 0.05C. As shown in FIG. 13, it is a charge and discharge curve diagram of an all-solid-state battery prepared according to Comparative Example 2 at room temperature (25° C.). It can be seen from the figure that the specific discharge capacity of the all-solid-state battery containing the polymer film prepared in Comparative Example 1 is only 11.9 mAh/g.

Therefore, based on the above expression, the present invention provides a polymer thin film electrolyte and a preparation method thereof, which can effectively improve the mechanical properties, electrochemical stability window and ionic conductivity of the solid electrolyte. The all-solid-state lithium battery assembled by the polymer film has a higher capacity at room temperature and high temperature. It is conducive to the extensive production and application of all-solid-state batteries and has great practical application prospects.

The above are only the preferred embodiments of the present invention. It should be pointed out that the above examples are exemplary in nature and should not be understood as limiting the present invention. Several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

A method for preparing a polymer electrolyte membrane, which is characterized in that the method includes the following steps:

(1) Dissolve polyethylene oxide and lithium salt in a solvent in proportions, and stir to obtain a mixed solution;

(2) The mixed solution obtained in step (1) is transferred to the substrate and placed in a low-temperature environment for freezing. After the solvent is completely solidified, the sample is transferred to a freeze dryer to freeze-dry the sample to obtain the polymer electrolyte film.
The method for preparing a polymer electrolyte membrane according to claim 1, wherein the molecular weight of the polyethylene oxide is 300,000-7 million.
The method for preparing a polymer electrolyte membrane according to claim 1, wherein the lithium salt is one or more of LiTFSI, LiClO4, LiPF6, LiBF4 and LiAsF6.
The method for preparing a polymer electrolyte membrane according to claim 1, wherein in step (1), the weight of the lithium salt is 5wt% of the total mass of the polyethylene oxide and the lithium salt. 50wt%.
The method for preparing a polymer electrolyte membrane according to claim 1, wherein the concentration of polyethylene oxide in the mixed solution in step (1) is 5wt%-15wt%.
The method for preparing a polymer electrolyte membrane according to claim 1, wherein the solvent in step (1) is deionized water, or the solvent is formed by mixing deionized water and organic solvent in any ratio. Mixed solvents.
The method for preparing a polymer electrolyte membrane according to claim 1, characterized in that: in step (1), the stirring time is 12-48 hours; in step (2), the low temperature environment means that the temperature is -30℃～-10℃, the freezing time in a low temperature environment is 12-36 hours; in step (2), the cold trap temperature of the freeze dryer is -90℃～-50℃, and the freezing time in the freeze dryer Dry time is 24-48 hours.
A polymer electrolyte membrane prepared by the method of any one of claims 1-7.
The application of the polymer electrolyte film in an all-solid-state lithium battery according to claim 7, characterized in that: the polymer electrolyte film is applied to an all-solid-state lithium battery operating in a room temperature and high temperature environment.