Functional separator for lithium-sulfur batteries and method of preparation and use thereof

Abstract

The application discloses a functional diaphragm for lithium-sulfur batteries and a preparation method and application thereof, and the preparation method comprises the following steps: coating a first slurry on the positive electrode side of a base film to obtain a first coating layer on the base film, coating a second slurry on the negative electrode side of the base film to form a second coating layer, and obtaining the functional diaphragm for lithium-sulfur batteries, wherein the preparation method of the first slurry is as follows: mixing a dispersing agent with a first solvent, uniformly stirring, then adding a carbon conductor and a first binder, uniformly stirring, and sanding to obtain the first slurry; and the preparation method of the second slurry is as follows: adding an unsaturated carboxylic acid and a second binder into a second solvent, uniformly stirring, and obtaining the second slurry, wherein the unsaturated carboxylic acid is an alkenoic acid. The preparation method can improve the ion conductivity of the diaphragm, is beneficial to reducing battery polarization, and thus improves the rate performance of the battery.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of battery separator, and particularly relates to a functional separator for a lithium-sulfur battery and a preparation method and application thereof. BACKGROUND

[0002] The lithium-sulfur battery has the advantages of high energy density and high theoretical capacity, and in addition, the rapid market growth is promoted by the characteristics of rich raw materials and low cost. However, the lithium-sulfur battery currently still has the problems of poor battery cycle performance, low rate performance and poor safety and stability performance, which are closely related to the characteristics of the battery separator.

[0003] The power lithium ion battery mainly consists of a positive electrode, a negative electrode, an electrolyte, a separator and a shell. The separator plays a role of blocking the electron conduction of the positive and negative electrodes in the battery, allowing the liquid ions to pass freely, so as to realize the ion conduction, and is an important determining factor of the battery cycle capacity and safety performance.

[0004] During the discharging process of the positive electrode sulfur, the cyclic S8 is converted into linear structure lithium polysulfide (Li2Sx, x = 8, 6, 4 and 2). Among the Li2S8, Li2S6, Li2S4 and Li2S2 lithium polysulfides, the lithium polysulfide with a high sulfur oxidation number (Li2Sx, usually x > 4) is particularly easy to dissolve in the hydrophilic electrolyte. Due to the concentration difference, the lithium polysulfide dissolved in the electrolyte diffuses from the positive electrode to a far place, that is, the “shuttle effect” is generated. The “shuttle effect” causes the loss of lithium polysulfide from the positive electrode, reduces the active material, reduces the utilization rate of sulfur, and causes the capacity to decrease. The pore structure of the traditional lithium battery separator cannot effectively block the shuttle of the lithium polysulfide, and in addition, the hydrophobicity, low polarity, low surface energy and other deficiencies of the traditional separator cause the low liquid absorption rate and poor liquid retention of the separator, and further cause the low ion conductivity, which affects the battery performance. SUMMARY

[0005] In view of the deficiencies of the prior art, the purpose of the present application is to provide a preparation method of a functional separator for a lithium-sulfur battery.

[0006] Another purpose of the present application is to provide a functional separator for a lithium-sulfur battery obtained by the above preparation method.

[0007] The purpose of the present application is achieved by the following technical solutions.

[0008] A preparation method of a functional separator for a lithium-sulfur battery, comprising the following steps:

[0009] The first slurry is coated on the positive electrode side of the base film to obtain a first coating layer on the base film, and the second slurry is coated on the negative electrode side of the base film to form a second coating layer, thereby obtaining the functional separator for the lithium-sulfur battery, wherein,

[0010] The preparation method of the first slurry comprises the following steps: mixing a dispersing agent with a first solvent, stirring uniformly, adding a carbon-based conductor and a first binder, stirring uniformly, and sanding to obtain the first slurry, wherein the dispersing agent is one or a mixture of two or more of ammonium polyacrylate, trimethylammonium hydrochloride and polyethylene glycol, the carbon-based conductor is graphene, Super-p or carbon black, and the first binder is PVDF (polyvinylidene fluoride) or polyacrylate.

[0011] The preparation method of the second slurry comprises the following steps: adding an unsaturated carboxylic acid and a second binder in a second solvent, and stirring uniformly to obtain the second slurry, wherein the unsaturated carboxylic acid is an alkenoic acid, and the second binder is polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA) or polyethylene oxide (PEO).

[0012] In the above technical solution, the first solvent is a mixture of water and alcohol, and the ratio of the water and alcohol is (1-20) : 1 by mass fraction, preferably (1-15) : 1.

[0013] In the above technical solution, the second solvent is a mixture of water and alcohol, and the ratio of the water and alcohol is (1-15) : (1-6) by mass fraction, preferably (1-10) : (1-5).

[0014] In the above technical solution, the alkenoic acid is acrylic acid, butenoic acid or octadecenoic acid.

[0015] In the above technical solution, the dispersing agent is 0.5-7wt% of the first solvent.

[0016] In the above technical solution, the ratio of the carbon-based conductor and the first binder is (10-90) : (0.5-5) by mass fraction.

[0017] In the above technical solution, the mass sum of the carbon-based conductor and the first binder is 1-25wt% of the mass sum of the dispersing agent and the first solvent.

[0018] In the above technical solution, in the preparation method of the first slurry, the stirring time is 10-40min.

[0019] In the above technical solution, the sanding time is 20-60min.

[0020] In the above technical solution, the coating method is roller coating.

[0021] In the above technical solution, the thickness of the first coating layer is 1-8 microns.

[0022] In the technical scheme, the ratio of the unsaturated carboxylic acid and the second binder is (25-60):(0.2-4) by mass fraction.

[0023] In the technical scheme, the total mass of the unsaturated carboxylic acid and the second binder is 3-20wt% of the second solvent.

[0024] In the technical scheme, in the preparation method of the second slurry, the stirring time is 10-50min.

[0025] In the technical scheme, the thickness of the second coating layer is 1-8 microns.

[0026] In the technical scheme, the base film is a polyolefin film, and the polyolefin film is a polyethylene film or a polypropylene film.

[0027] The functional separator for lithium-sulfur batteries obtained by the preparation method.

[0028] The functional separator for lithium-sulfur batteries is applied to improve the capacity retention rate and / or coulombic efficiency of the lithium-sulfur battery.

[0029] The beneficial effects of the present application are as follows:

[0030] The present application introduces a functional layer on the surface of the polyolefin film. On the one hand, the negative network formed by the unsaturated carboxylic acid can inhibit the diffusion of polysulfides with electronegativity through the separator to the negative electrode. At the same time, the presence of carbon conductors helps polysulfides to undergo redox reactions, reducing high-sulfur compounds to low-sulfur compounds, which are insoluble in electrolyte and will not produce shuttle effect. That is, it can prevent the generation of polysulfides and avoid the production of shuttle effect; on the other hand, it improves the lithium ion migration number of the separator, thereby improving the ionic conductivity of the separator, which is beneficial to reduce the battery polarization and improve the rate performance of the battery. DETAILED DESCRIPTION

[0031] The technical scheme of the present application will be further described below in combination with specific examples.

[0032] The sanding equipment is a full-ceramic nanometer grinding machine, model: PT-5L, and the manufacturer is Dongguan Pinno Machinery Equipment Co., Ltd.

[0033] The mixer is a double-planetary power mixer, model: HY-DLH43L, and the manufacturer is Guangzhou Hongshang Machinery Technology Co., Ltd.

[0034] Example 1

[0035] A preparation method of a functional separator for lithium-sulfur batteries, comprising the following steps:

[0036] The first slurry is coated on the positive electrode side of the base film to obtain a first coating layer on the base film, and the second slurry is coated on the negative electrode side of the base film to form a second coating layer, thereby obtaining the functional separator for lithium-sulfur batteries, wherein the coating method is roll coating, the thickness of the first coating layer is 2 microns, the thickness of the second coating layer is 2 microns, and the base film is a polyethylene film.

[0037] The preparation method of the first slurry is as follows: the dispersant is mixed with the first solvent, the dispersant accounts for 5% by weight of the first solvent, stirring is performed for 20 min until uniform, the carbon conductor and the first binder are then added, stirring is performed for 20 min until uniform, and sanding is performed for 20 min to obtain the first slurry, wherein the mass ratio of the carbon conductor to the first binder is 12:0.6, and the mass sum of the carbon conductor and the first binder accounts for 7% by weight of the mass sum of the dispersant and the first solvent. The first solvent is a mixture of water and alcohol, and the mass ratio of water to alcohol is 3:1; the dispersant is polyacrylate ammonium salt, the carbon conductor is carbon black, the carbon conductor can form reduction sites for sulfur ions to prevent the generation of polysulfides, and the first binder is PVDF (polyvinylidene fluoride).

[0038] The preparation method of the second slurry is as follows: the unsaturated carboxylic acid and the second binder are added to the second solvent, stirring is performed for 10 min until uniform to obtain the second slurry, wherein the mass ratio of the unsaturated carboxylic acid to the second binder is 10:1, and the mass sum of the unsaturated carboxylic acid and the second binder accounts for 4% by weight of the second solvent. The second solvent is a mixture of water and alcohol, and the mass ratio of water to alcohol is 1:1. The unsaturated carboxylic acid is acrylic acid, the negative nuclear network formed by the unsaturated carboxylic acid can inhibit the diffusion of the electronegative polysulfides through the separator to the negative electrode. The carboxyl contained in the unsaturated carboxylic acid can promote the dissociation of lithium salt, increase the number of “free” ions, and be beneficial to improving the ionic conductivity of the separator; in addition, the negative nuclear network also provides a fast channel for the migration of lithium ions, improves the lithium ion migration number of the separator, and is beneficial to reducing the battery polarization and thus improving the rate performance of the battery. The second binder is polyvinylpyrrolidone (PVP).

[0039] Example 2

[0040] A preparation method of a functional separator for lithium-sulfur batteries comprises the following steps:

[0041] The first slurry is coated on the positive electrode side of the base film to obtain a first coating layer on the base film, and the second slurry is coated on the negative electrode side of the base film to form a second coating layer, thereby obtaining the functional separator for lithium-sulfur batteries, wherein the coating method is roll coating, the thickness of the first coating layer is 2.5 microns, the thickness of the second coating layer is 2 microns, and the base film is a polyethylene film.

[0042] The preparation method of the first slurry is as follows: the dispersant is mixed with the first solvent, the dispersant is 5wt% of the first solvent, stirring for 30min to be uniform, then the carbon conductor and the first binder are added, stirring for 30min to be uniform, sanding for 36min to obtain the first slurry, the ratio of the carbon conductor and the first binder is 30:2.5 in mass fraction, and the mass sum of the carbon conductor and the first binder is 10wt% of the mass sum of the dispersant and the first solvent. The first solvent is a mixture of water and alcohol, the ratio of water and alcohol is 9:1 in mass fraction; the dispersant is trimethylammonium hydrochloride, the carbon conductor is graphene, and the first binder is polymethyl acrylate.

[0043] The preparation method of the second slurry is as follows: the unsaturated carboxylic acid and the second binder are added in the second solvent, stirring for 10min to be uniform to obtain the second slurry, the ratio of the unsaturated carboxylic acid and the second binder is 25:2 in mass fraction, and the mass sum of the unsaturated carboxylic acid and the second binder is 7wt% of the second solvent. The second solvent is a mixture of water and alcohol, the ratio of water and alcohol is 3:1 in mass fraction. The unsaturated carboxylic acid is butenoic acid. The second binder is polyvinyl alcohol (PVA).

[0044] Example 3

[0045] A preparation method of a functional separator for lithium-sulfur batteries, comprising the following steps:

[0046] The first slurry is coated on the positive side of the base film to obtain a first coating layer on the base film, and the second slurry is coated on the negative side of the base film to form a second coating layer, thereby obtaining the functional separator for lithium-sulfur batteries, wherein the coating method is roller coating, the thickness of the first coating layer is 3 microns, the thickness of the second coating layer is 2 microns, and the base film is a polypropylene film.

[0047] The preparation method of the first slurry is as follows: the dispersant is mixed with the first solvent, the dispersant is 5wt% of the first solvent, stirring for 40min to be uniform, then the carbon conductor and the first binder are added, stirring for 40min to be uniform, sanding for 40min to obtain the first slurry, the ratio of the carbon conductor and the first binder is 10:3 in mass fraction, and the mass sum of the carbon conductor and the first binder is 15wt% of the mass sum of the dispersant and the first solvent. The first solvent is a mixture of water and alcohol, the ratio of water and alcohol is 15:1 in mass fraction; the dispersant is polyethylene glycol, the carbon conductor is Super-p, and the first binder is polyethyl acrylate.

[0048] The second slurry is prepared by adding unsaturated carboxylic acid and second binder into the second solvent, stirring for 10 minutes until uniform, obtaining the second slurry, wherein the ratio of unsaturated carboxylic acid to second binder is 15:2, and the total mass of unsaturated carboxylic acid and second binder is 12% of the mass of the second solvent. The second solvent is a mixture of water and alcohol, and the ratio of water to alcohol is 9:4. The unsaturated carboxylic acid is octadecenoic acid. The second binder is polyethylene oxide (PEO).

[0049] The functional separator for lithium-sulfur batteries obtained in Examples 1-3 is tested as follows:

[0050] The functional separator for lithium-sulfur batteries obtained in Example 1 has a decomposition voltage of 4.7 V, a thermal shrinkage rate of 1.0% at 120°C for 1 hour, a tensile strength of 1450 Kg / cm 2 , and an ionic conductivity of 1.4 x 10 -3 s / cm-1. When a sulfur positive electrode and lithium negative electrode are assembled into a battery, the capacity retention rate is 98% after 100 cycles at a rate of 0.5C, and the average coulombic efficiency is 99.23% after 20 cycles.

[0051] The functional separator for lithium-sulfur batteries obtained in Example 2 has a decomposition voltage of 4.7 V, a thermal shrinkage rate of 1.2% at 120°C for 1 hour, a tensile strength of 1490 Kg / cm 2 , and an ionic conductivity of 1.5 x 10 -3 s / cm-1. When a sulfur positive electrode and lithium negative electrode are assembled into a battery, the capacity retention rate is 97% after 100 cycles at a rate of 0.5C, and the average coulombic efficiency is 98.48% after 20 cycles.

[0052] The functional separator for lithium-sulfur batteries obtained in Example 3 has a decomposition voltage of 4.7 V, a thermal shrinkage rate of 0.9% at 120°C for 1 hour, a tensile strength of 1550 Kg / cm 2 , and an ionic conductivity of 1.3 x 10 -3 s / cm -1 . When a sulfur positive electrode and lithium negative electrode are assembled into a battery, the capacity retention rate is 97% after 100 cycles at a rate of 0.5C, and the average coulombic efficiency is 97.57% after 20 cycles.

[0053] The above describes the present application by way of example, and it should be noted that any simple modification, change or equivalent replacement that does not depart from the core of the present application and can be made by those skilled in the art without creative effort falls within the scope of the present application.

Claims

1. A method for preparing a functional separator for a lithium-sulfur battery, characterized by, The method comprises the following steps: The first slurry is coated on the positive side of the base film to form a first coating layer, and the second slurry is coated on the negative side of the base film to form a second coating layer, thereby obtaining the functional separator for lithium-sulfur batteries, The preparation method of the first slurry comprises the following steps: mixing a dispersing agent with a first solvent, stirring until uniform, adding a carbon conductor and a first binder, stirring until uniform, and sanding to obtain the first slurry; the dispersing agent is polyacrylic acid ammonium salt, trimethyl ammonium chloride or polyethylene glycol; the carbon conductor is graphene, Super-p or carbon black; and the first binder is PVDF, polymethyl acrylate or polyethyl acrylate. The preparation method of the second slurry comprises the following steps: adding an unsaturated carboxylic acid and a second binder to a second solvent, and stirring until uniform, wherein the unsaturated carboxylic acid is an alkenoic acid, the alkenoic acid is acrylic acid, butenoic acid or octadecenoic acid, and the second binder is polyvinylpyrrolidone, polyvinyl alcohol or polyethylene oxide. The thickness of the first coating layer is 2-3 microns, and the thickness of the second coating layer is 2 microns.

2. The production method according to claim 1, characterized by, The dispersing agent accounts for 5wt% of the first solvent; the mass ratio of the carbon conductor to the first binder is (10-30):(0.6-3) by mass fraction; The mass sum of the carbon conductor and the first binder accounts for 7-15wt% of the mass sum of the dispersing agent and the first solvent. The mass ratio of the unsaturated carboxylic acid to the second binder is 10:1, 25:2 or 15:2 by mass fraction, and the mass sum of the unsaturated carboxylic acid and the second binder accounts for 4-12wt% of the second solvent.

3. The preparation method according to claim 2, characterized in that, The second solvent is a mixture of water and alcohol, and the mass ratio of the water to the alcohol is (1-15):1 by mass fraction; the first solvent is a mixture of water and alcohol, and the mass ratio of the water to the alcohol is (1-20):1 by mass fraction.

4. The production method according to claim 3, characterized by, The mass ratio of the water to the alcohol in the second solvent is (1-10):

1.

5. The preparation method according to claim 3, characterized in that, The mass ratio of the water to the alcohol in the first solvent is (1-15):

1.

6. The method of claim 1, wherein, In the preparation method of the first slurry, the stirring time is 10-40 min, and the sanding time is 20-60 min; in the preparation method of the second slurry, the stirring time is 10-50 min.

7. The preparation method according to claim 1, characterized in that, The coating method is roller coating.

8. The method of claim 1, wherein, The base film is a polyolefin film, and the polyolefin film is a polyethylene film or a polypropylene film.

9. The functional separator for lithium-sulfur batteries obtained by the preparation method of any one of claims 1-8.

10. Application of the functional separator for lithium-sulfur batteries of claim 9 to improving the capacity retention rate and / or the coulombic efficiency of lithium-sulfur batteries.