Diaphragm, preparation method thereof and lithium ion battery

Abstract

The invention relates to the technical field of batteries, in particular to a diaphragm, a preparation method thereof and a lithium ion battery. The diaphragm comprises a base membrane and a coating located on the surface of at least one side of the base membrane, the base membrane comprises a polyvinyl alcohol membrane and first particles dispersed in the polyvinyl alcohol membrane, and the first particles comprise at least one of boehmite, aluminum oxide and zirconium oxide; the coating comprises second particles and a first binder, and the second particles comprise at least one of attapulgite, zeolite, sepiolite, montmorillonite and diatomite; the first binder comprises at least one of polyacrylonitrile, polyacrylic acid, an acrylic acid-acrylonitrile copolymer, an acrylic acid-acrylamide copolymer, an acrylonitrile-acrylamide copolymer, an acrylic acid-acrylonitrile-acrylate copolymer, an acrylic acid-acrylonitrile-acrylamide copolymer and an acrylic acid-acrylonitrile-acrylate copolymer. The diaphragm disclosed by the invention is good in thermal stability and liquid absorbency; comprising the diaphragm disclosed by the invention can improve lithium dendrites and cycle performance.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a diaphragm and a preparation method thereof, and a lithium ion battery. Background Art

[0002] The growing demand for advanced energy storage devices has stimulated researchers' interest in high energy density lithium-ion batteries. Lithium metal has an ideal theoretical specific capacity (3860 mAh / g), low potential (-3.04 V relative to standard hydrogen electrode) and low density (0.535 g / cm 3 ) is considered to be an ideal choice for anode materials, but in practical applications it is still hindered by unstable lithium ion stripping and deposition, the formation of lithium dendrites and "dead lithium" and potential safety issues. Therefore, solving the problems of lithium dendrites and "dead lithium" is of great significance to the use and development of lithium-ion batteries; on the other hand, the high-temperature safety of lithium-ion batteries is also very important for their application and promotion, and solving their thermal runaway problem is also of practical significance. Summary of the invention

[0003] The purpose of the present invention is to improve the problems of lithium dendrites and "dead lithium" in lithium-ion batteries (hereinafter referred to as batteries) and reduce the risk of thermal runaway of batteries. By modifying the diaphragm, on the one hand, the deposition and stripping behavior of lithium ions can be adjusted to inhibit the direct contact between lithium ions and the lithium anode to form lithium dendrites. On the other hand, a diaphragm with good thermal stability can be obtained to reduce the risk of thermal runaway of the battery.

[0004] Due to the microscopic roughness of the lithium anode surface, the electric field distribution on the electrode surface is uneven, resulting in uneven lithium ion flux during the electroplating process. Lithium ions are preferentially deposited at the tips of the protrusions, resulting in a self-reinforcement effect, and ultimately forming lithium dendrites and "dead lithium". The exposed lithium dendrites are further consumed by the electrolyte, resulting in an increase in charge transfer resistance and a decrease in coulombic efficiency. Therefore, how to promote the migration of lithium ions and reduce the overpotential is the key to achieving lithium dendrite-free.

[0005] In the prior art, strategies such as developing lithium metal alloys (Li-Au, Li-Mg, etc.), designing porous current collectors, and constructing artificial solid electrolyte interface (SEI) layers are often used to solve the lithium dendrite problem. In addition, optimizing electrolytes to produce stable SEI films and using solid electrolytes are also considered to be effective ways to reduce lithium dendrites, stabilize, and improve the electrochemical performance of lithium anodes. However, due to the high chemical activity of lithium, these methods involve cumbersome processes and dangerous manufacturing processes, which are not conducive to economic benefits and industrially feasible large-scale production. The inventors of the present invention have found that modifying the diaphragm to solve the problem of lithium dendrites not only has a good effect but also simplifies the process. The currently proposed diaphragm modification often suppresses the formation of lithium dendrites by adding a ceramic layer. The ceramic layer generally has a high density and a large interface resistance, which makes the ceramic layer too thick. On the other hand, the thickness of the diaphragm, mass load and other parameters can also have a great impact on the energy density of the lithium-ion battery. The inventors of the present invention propose a diaphragm and a preparation method thereof and a lithium-ion battery. The diaphragm is lightweight and can effectively regulate lithium deposition / stripping behavior. At the same time, it can also optimize the shortcomings of traditional polyethylene diaphragms, such as insufficient thermal stability and insufficient wetting to polar electrolytes.

[0006] Based on this, the inventor of the present invention proposes the following technical solution:

[0007] A first aspect of the present invention provides a diaphragm, comprising a base film and a coating located on at least one side of the base film, wherein the base film comprises a polyvinyl alcohol film and first particles dispersed in the polyvinyl alcohol film, wherein the first particles comprise at least one of boehmite, alumina and zirconium oxide; the coating comprises second particles and a first binder, wherein the second particles comprise at least one of attapulgite, zeolite, sepiolite, montmorillonite and diatomaceous earth, and the first binder comprises at least one of polyacrylonitrile, polyacrylic acid, acrylic acid-acrylonitrile copolymer, acrylic acid-acrylamide copolymer, acrylonitrile-acrylamide copolymer, acrylic acid-acrylonitrile-acrylate copolymer, acrylic acid-acrylonitrile-acrylamide copolymer and acrylic acid-acrylonitrile-acrylate salt copolymer.

[0008] A second aspect of the present invention provides a method for preparing a diaphragm, the method comprising at least the following steps:

[0009] Q1, taking polyvinyl alcohol and distilled water for a first mixing to obtain a polyvinyl alcohol solution;

[0010] Q2, performing a second mixing of the polyvinyl alcohol solution obtained in Q1 and the first particles to obtain a mixed solution;

[0011] Q3, vacuum-treating the mixed solution obtained in Q2, uniformly coating it on the substrate, drying it, and peeling it off to obtain a base film;

[0012] Q4, taking the second particles, the first binder and distilled water for a third mixing to obtain a slurry;

[0013] Q5, subjecting the base film and the slurry to a first treatment to obtain a diaphragm.

[0014] A third aspect of the present invention provides a lithium ion battery, wherein the lithium ion battery comprises the diaphragm provided by the first aspect of the present invention and / or the diaphragm prepared by the method provided by the second aspect of the present invention.

[0015] Through the above technical solution, the present invention has at least the following advantages compared with the prior art:

[0016] (1) The polyvinyl alcohol film of the present invention has high hydrophilicity and a porous structure, and has good liquid absorption performance. It is composed of a first particle and a polyvinyl alcohol film to form a base film, and a coating with a large number of regular and continuous pores is arranged on at least one side of the base film. The first particle and the second particle can not only improve lithium dendrites, but also improve thermal stability. The resulting diaphragm is conducive to the uniform deposition of metal ions, provides channels for lithium ion transmission, and also has good mechanical strength.

[0017] (2) The diaphragm prepared by the preparation method of the present invention has a strong bonding force between the base film and the coating, is not easy to fall off, has strong processability, and has good wettability to the electrolyte.

[0018] (3) The lithium-ion battery of the present invention has no lithium dendrite and lithium deposition problems and has good cycle performance.

[0019] The endpoints and any values ​​of the ranges disclosed in this article are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 FIG. 1 is a schematic diagram showing the structure of a diaphragm in one embodiment of the present invention.

[0021] Among them, 1 is the first particle, 2 is the base film, 3 is the coating, and 4 is the second particle. DETAILED DESCRIPTION

[0022] The specific embodiments of the present invention are described in detail below. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0023] A first aspect of the present invention provides a diaphragm, comprising a base film and a coating located on at least one side of the base film, wherein the base film comprises a polyvinyl alcohol film and first particles dispersed in the polyvinyl alcohol film, wherein the first particles comprise at least one of boehmite, alumina and zirconium oxide; the coating comprises second particles and a first binder, wherein the second particles comprise at least one of attapulgite, zeolite, sepiolite, montmorillonite and diatomaceous earth, and the first binder comprises at least one of polyacrylonitrile, polyacrylic acid, acrylic acid-acrylonitrile copolymer, acrylic acid-acrylamide copolymer, acrylonitrile-acrylamide copolymer, acrylic acid-acrylonitrile-acrylate copolymer, acrylic acid-acrylonitrile-acrylamide copolymer and acrylic acid-acrylonitrile-acrylate salt copolymer.

[0024] In one embodiment, the first particles include boehmite.

[0025] In one embodiment, the second particles include attapulgite.

[0026] In one embodiment, the first binder includes at least one of polyacrylonitrile, acrylonitrile-acrylic acid copolymer, acrylonitrile-acrylamide copolymer and acrylic acid-acrylonitrile-acrylate copolymer.

[0027] In the present invention, the polyvinyl alcohol film as a base film is the main component of the diaphragm. The polyvinyl alcohol film has higher hydrophilicity and porous structure, has good liquid absorption performance, and has more advantages for the cycle performance of the battery. In addition, the polyvinyl alcohol film also has good thermal stability; the first particle, as a halogen-free flame retardant, can achieve fire protection of lithium-ion batteries during thermal runaway. Among them, boehmite has a low hardness, and the base film formed by dispersing it in the polyvinyl alcohol film has good processing performance.

[0028] In the present invention, the second particles in the coating have a porous structure, which is conducive to the infiltration of the electrolyte, optimizes the lithium ion transmission path, reduces the blockage of the diaphragm pores, and reduces the internal resistance of the battery; wherein, attapulgite has a natural honeycomb structure, and the Si-O tetrahedron and Mg-O octahedron units are finely integrated into a 2:1 banded layer structure through covalent bonds. The attapulgite has abundant pores, a high aspect ratio, and a long-range continuous lithium ion transmission channel, which can make the deposition of lithium ions more uniform, the thermal stability of the diaphragm is good, and the ion migration number and ion conductivity are improved; on the other hand, attapulgite is also an inorganic metal hydroxide flame retardant, which decomposes into water vapor and an inorganic oxide isolation layer (MgO, Al 2 O 3), thereby inhibiting combustion. In addition, there are a large number of polar hydroxyl groups on the surface of attapulgite, which is suitable for its application in polar electrolyte systems; the coating can form a three-dimensional network structure in the electrolyte, has good mechanical strength, and combined with the base membrane, can further improve the mechanical strength and processability of the diaphragm.

[0029] The separator of the present invention can not only improve the transmission rate of lithium ions in the battery, but also avoid the formation of lithium dendrites and reduce the risk of thermal runaway of the battery.

[0030] In the present invention, the average particle size of the first particles is 100 nm-300 nm, for example, 100 nm, 110 nm, 120 nm, 130 nm, 140 nm, 160 nm, 180 nm, 200 nm, 220 nm, 240 nm, 260 nm or 300 nm.

[0031] In the present invention, the average particle size of the second particles is 500nm-800nm, for example, 500nm, 510nm, 520nm, 530nm, 540nm, 560nm, 580nm, 600nm, 620nm, 640nm, 660nm, 700nm, 750nm or 800nm.

[0032] In the present invention, the average particle size of the first particles and the second particles can be obtained by conventional testing methods in the art, such as using a laser particle size analyzer and measuring by laser diffraction.

[0033] In the present invention, the Mohs hardness of the first particles is 3-3.5, for example, 3, 3.1, 3.2, 3.3, 3.4 or 3.5.

[0034] In the present invention, the Mohs hardness of the first particles can be obtained by a conventional testing method in the art, specifically as follows:

[0035] (1) Take the first particle as a sample (make sure the sample surface is flat. If it is not flat, grind or cut it);

[0036] (2) Use a diamond pen with a grade of 1-10 to test the hardness of the sample;

[0037] (3) Fix the sample on a plane, and from the edge of the sample, move the selected diamond pen perpendicular to the sample surface and scratch the surface of the material with a slight force;

[0038] (4) Observe whether there are scratches on the sample surface, as well as the depth and length of the scratches. Then use a diamond pen with a higher hardness level to repeat the above test and observe the scratches. Repeat this process until you find the level of diamond pen that cannot produce scratches on the sample. This can be used to determine the Mohs hardness of the sample.

[0039] In the present invention, regulating the average particle size of the first particles and the second particles within a suitable range is conducive to improving the processing performance, consistency and electrochemical performance of the diaphragm. When the average particle size of the first particles is less than 100nm, it is difficult to disperse, and it is easy to agglomerate or unevenly distribute. When the average particle size of the first particles is greater than 300nm, the consistency of the resulting base film is poor; when the average particle size of the second particles is greater than 800nm, the transmission path of lithium ions is long and the electrochemical performance is reduced. When the average particle size of the second particles is less than 500nm, the pores of the diaphragm will be blocked, resulting in a decrease in porosity.

[0040] In the present invention, the ratio of the coating thickness to the base film thickness is 0.04-0.3, for example, 0.04, 0.06, 0.08, 0.1, 0.12, 0.14, 0.16, 0.18, 0.2, 0.22, 0.24, 0.26, 0.28 or 0.3.

[0041] In one embodiment, the ratio of the coating thickness to the base film thickness is 0.07-0.17.

[0042] In the present invention, the base film has a thickness of 5 μm-25 μm, for example, 5 μm, 6 μm, 8 μm, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, 20 μm, 22 μm, 23 μm, 24 μm or 25 μm.

[0043] In one embodiment, the base film has a thickness of 7 μm-12 μm.

[0044] In the present invention, the thickness of the coating is 0.3 μm-5 μm, for example, 0.3 μm, 0.5 μm, 0.6 μm, 0.8 μm, 1 μm, 1.2 μm, 1.4 μm, 1.6 μm, 1.8 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm or 5 μm.

[0045] In one embodiment, the thickness of the diaphragm is 0.5 μm-2 μm.

[0046] In the present invention, the thickness of the diaphragm is jointly determined by the thickness of the coating and the base film. When the diaphragm is too thick, the transmission distance of lithium ions increases and the transmission efficiency decreases. When the diaphragm is too thin, the puncture strength and mechanical strength are low, and the battery safety decreases. When the ratio of the thickness of the coating and the base film is adjusted within the above range, the coating thickness of the obtained diaphragm is adapted to the thickness of the base film. Otherwise, when the coating is too thin, the battery is subjected to external force or lithium dendrites grow, which will cause the diaphragm to be punctured, causing a short circuit, and the self-discharge of the battery increases; when the base film is too thick, the weight of the battery increases, the internal resistance increases, and the cycle stability decreases.

[0047] In the present invention, the structural schematic diagram of the diaphragm is as follows Figure 1 As shown, 1 is the first particle, 2 is the base film, 3 is the coating, and 4 is the second particle. It can be seen that the diaphragm has a layered structure, the first particles are dispersed in the base film, and the second particles are dispersed in the coating.

[0048] In the present invention, the thickness of the base film, the diaphragm and the coating layer can be obtained by conventional testing methods in the art, such as measuring with a micrometer.

[0049] In the present invention, the porosity of the diaphragm is 40%-60%, for example, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59% or 60%.

[0050] In one embodiment, the porosity of the membrane is 50%-60%.

[0051] In the present invention, the porosity of the coating and the diaphragm is regulated, the number and distribution of nanopores inside the coating can be regulated, the transmission of lithium ions in the diaphragm can be enhanced, the formation of lithium dendrites can be reduced, and the specific surface area of ​​the diaphragm material can be increased, thereby improving thermal stability and flame retardancy; when the porosity of the coating and the porosity of the diaphragm are within a suitable range, not only the liquid absorption of the diaphragm is better, more space can be filled with electrolyte, the transmission resistance of lithium ions in the diaphragm is reduced, the cycle performance of the battery is optimized, and the thermal runaway barrier effect is improved and the risk of thermal runaway is reduced. When the porosity of the diaphragm and the coating is too small, the optimization of the lithium ion transmission performance is insufficient, the temperature distribution in the battery is uneven, and it is not enough to alleviate the lithium dendrite problem and reduce the risk of thermal runaway; and when the porosity of the diaphragm and the coating is too large, the mechanical strength of the diaphragm is reduced, the diaphragm is punctured and ruptured, and the risk of battery short circuit increases.

[0052] In the present invention, the liquid absorption rate of the diaphragm is 150%-200%, for example, 150%, 152%, 154%, 156%, 158%, 160%, 165%, 170%, 175%, 180%, 185%, 190%, 195% or 200%.

[0053] In the present invention, the thermal shrinkage of the diaphragm at 200° C. is 0.8%-3%, for example, 0.8%, 1%, 1.3%, 1.5%, 1.7%, 2%, 2.3%, 2.5%, 2.7% or 3%.

[0054] In the present invention, by regulating the liquid absorption rate of the diaphragm, the ability of the diaphragm to adsorb electrolyte can be regulated, the contact degree and uniformity between the electrolyte and the diaphragm can be increased, the ionic conductivity can be improved, and electrode polarization and lithium precipitation can be inhibited; and because the diaphragm contains a large number of polar hydroxyl groups, the electrolyte affinity of the diaphragm is enhanced, the adsorption and infiltration of the electrolyte are increased, so that the diaphragm can adapt to the volume change during the lithium ion deposition / stripping process, effectively inhibit the growth of lithium dendrites, and improve the battery safety and cycle performance; the diaphragm has a small thermal shrinkage rate and good dimensional stability at 200°C, and the combination of the first particles and the second particles further reduces the risk of internal short circuit and thermal runaway caused by thermal shrinkage.

[0055] The second aspect of the present invention provides a method for preparing the diaphragm according to the first aspect of the present invention, the method comprising at least the following steps:

[0056] Q1, taking polyvinyl alcohol and distilled water for a first mixing to obtain a polyvinyl alcohol solution;

[0057] Q2, performing a second mixing of the polyvinyl alcohol solution obtained in Q1 and the first particles to obtain a mixed solution;

[0058] Q3, vacuum-treating the mixed solution obtained in Q2, uniformly coating it on the substrate, drying it, and peeling it off to obtain a base film;

[0059] Q4, taking the second particles, the first binder and distilled water for a third mixing to obtain a slurry;

[0060] Q5, subjecting the base film and the slurry to a first treatment to obtain a diaphragm.

[0061] In the present invention, the base film is prepared by a tape casting method, which can accurately control the temperature, and the obtained diaphragm has uniform thickness, good surface finish and flatness, can realize continuous production, simple process and high production efficiency; compared with the prior art, the sheet strength of the electrospinning diaphragm is low, the production efficiency is low, the dry diaphragm process has poor transverse mechanical strength and limited production efficiency, and the wet process has the problems of low melting temperature and poor heat resistance. The base film prepared by the method not only has good processability, but also can increase the porosity of the diaphragm and reduce thermal shrinkage; the improvement of its mechanical strength and thermal stability reduces the micro-deformation of the diaphragm when absorbing liquid and does not change the liquid absorption rate of polyvinyl alcohol. According to the structure-activity relationship between diaphragm swelling-pore structure-ion path-metal deposition, a diaphragm with a stable ion transmission path is prepared.

[0062] In the present invention, the first mixing condition is heating to 70°C-90°C (for example, 70°C, 71°C, 72°C, 73°C, 74°C, 75°C, 76°C, 78°C, 80°C, 82°C, 84°C, 86°C, 88°C or 90°C), and stirring for 12h-16h (for example, 12h, 12.5h, 13h, 13.5h, 14h, 14.5h, 15h, 15.5h or 16h).

[0063] In the present invention, the second mixing condition is heating to 85°C-90°C (e.g. 85°C,

[0064] 86°C, 87°C, 88°C, 89°C or 90°C), stirred for 3h-6h (e.g., 3h, 3.5h, 4h, 4.5h,

[0065] 5h, 5.5h or 6h).

[0066] In the present invention, the substrate includes a glass plate.

[0067] In the present invention, the first treatment may be to coat the slurry on at least one surface of the base film, to immerse one surface of the base film in the slurry, or to completely immerse the base film in the slurry.

[0068] In one embodiment, the first treatment includes immersing one side surface of the base film in the slurry.

[0069] In the present invention, the coating is attached to the surface of the base film through an impregnation process. The impregnation process is simple and suitable for products with high open porosity and the pores are interconnected. The addition of the second particles can form a coating with a large number of regular and continuous channels inside the base film, which is beneficial to the uniform deposition of metal ions and avoids the growth of lithium dendrites and lithium deposition. The coating has a strong bonding force with the diaphragm and is not easy to fall off, thereby improving the safety of the battery and reducing the risk of thermal runaway.

[0070] Furthermore, by forming a coating only on one surface of the base film through an immersion method, the internal resistance of the battery can be further reduced and the electrochemical performance of the battery can be improved.

[0071] In the present invention, the first treatment further includes a gradient temperature increase, which includes at least the following stages: 80°C-90°C (for example, 80°C, 82°C, 84°C, 86°C, 88°C or 90°C) for 1h-2h (for example, 1h, 1.2h, 1.5h or 2h), 120°C-135°C (for example, 120°C, 122°C, 124°C, 126°C, 128°C, 130°C, 131°C, 1 The temperature is preferably 32°C, 134°C or 135°C) for 1h-2h (for example, 1h, 1.2h, 1.5h or 2h), and then 160°C-180°C (for example, 160°C, 162°C, 164°C, 166°C, 168°C, 170°C, 172°C, 174°C, 176°C or 180°C) for 0.5h-1h (for example, 0.5h, 0.6h, 0.7h, 0.8h, 0.9h or 1h).

[0072] In the present invention, the gradient temperature increase method can gradually remove water from the diaphragm, cross-link the coating, and help form a more stable diaphragm.

[0073] In the present invention, the vacuum treatment time is 8 h-12 h, for example, 8 h, 8.5 h, 9 h, 9.5 h, 10 h, 10.5 h, 11 h, 11.5 h or 12 h.

[0074] In the present invention, the vacuum treatment can eliminate bubbles, improve the insulation of the battery, and prevent the diaphragm from cracking or rupturing, thereby avoiding internal short circuit of the battery caused by contact between the positive electrode and the negative electrode.

[0075] In the present invention, the step Q1 may further include adding a surfactant, wherein the surfactant includes at least one of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate and sodium laurate.

[0076] In the present invention, the content of the surfactant is 0.1%-2% (for example, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.8%, 1%, 1.2%, 1.4%, 1.6%, 1.8% or 2%).

[0077] In one embodiment, the content of the surfactant is 1%-1.5%.

[0078] In one embodiment, the surfactant comprises sodium lauryl sulfate.

[0079] In the present invention, a wetting agent and a dispersant may be further added in step Q4, wherein the wetting agent includes at least one of a polyether silane wetting agent and a nonionic siloxane wetting agent, and the dispersant includes at least one of sodium silicate, sodium carbonate and an acrylic polymer.

[0080] In the present invention, based on the total weight of the slurry, the content of the wetting agent is 1%-1.5% (for example, 1%, 1.1%, 1.2%, 1.3%, 1.4% or 1.5%), and the content of the dispersant is 0.3%-0.5% (for example, 0.3%, 0.35%, 0.4%, 0.45% or 0.5%).

[0081] In the present invention, based on the total weight of the mixed solution, the content of the polyvinyl alcohol powder is 10%-20% (for example, 10%, 11%, 12%, 14%, 15%, 16%, 17%, 18%, 19% or 20%), and the content of the first particles is 1%-5% (for example, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5% or 5%).

[0082] In the present invention, based on the total weight of the slurry, the content of the second particles is 25%-30% (for example, 25%, 26%, 27%, 28%, 29% or 30%), and the content of the first binder is 1%-1.5% (for example, 1%, 1.1%, 1.2%, 1.3%, 1.4% or 1.5%).

[0083] A third aspect of the present invention provides a lithium ion battery, comprising a negative electrode sheet and the separator according to the first aspect of the present invention.

[0084] In the present invention, the negative electrode sheet is arranged opposite to the side of the separator having the coating.

[0085] In the present invention, the lithium ion migration number is 0.3-0.75, for example, 0.3, 0.32, 0.34, 0.36, 0.38, 0.4, 0.42, 0.44, 0.46, 0.48, 0.5, 0.54, 0.58, 0.62, 0.66, 0.7, 0.72 or 0.75.

[0086] In the present invention, the negative electrode sheet is arranged opposite to the side of the separator having the coating, which can further shorten the transmission path of lithium ions, increase the migration rate of lithium ions, and further reduce the growth of lithium dendrites.

[0087] In the present invention, the lithium ion migration number of the battery is high, and the Li + The higher the ratio, the more efficient the electrolyte is in transferring charge between the positive and negative electrodes, the better the battery's charge and discharge efficiency, and the faster charging capability of the battery can be improved. On the other hand, a high lithium ion migration number helps reduce the overpotential of the battery during the charging process, thereby improving the battery's charging speed and efficiency, and having good electrochemical stability.

[0088] In the present invention, the negative electrode sheet also includes a negative electrode current collector, a negative electrode conductor and a negative electrode binder, and all of them are conventional choices of those skilled in the art. For example, the negative electrode current collector includes copper foil, the negative electrode binder includes at least one of polyacrylic acid (PAA), sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), sodium carboxymethyl cellulose, polyimide, styrene-butadiene rubber (SBR) and polyvinylidene fluoride, and the negative electrode conductor includes at least one of acetylene black, conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes and graphene.

[0089] In the present invention, the lithium-ion battery also includes a positive electrode sheet, and the positive electrode sheet also includes a positive electrode current collector, a positive electrode conductor and a positive electrode binder, and all of them are conventional choices of those skilled in the art. For example, the positive electrode current collector includes aluminum foil, the positive electrode binder includes at least one of polyvinylidene fluoride (PVDF), polyacrylate, polyimide and styrene-butadiene rubber, and the positive electrode conductor includes at least one of acetylene black, conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes and graphene.

[0090] It should be noted that the numerical expressions such as "first" and "second" in the present invention are only used to distinguish different substances or usages, and do not represent a difference in order.

[0091] The present invention will be described in detail below by way of examples. The examples described in the present invention are only a part of the examples of the present invention, rather than all of the examples. Based on the examples in the present invention, all other examples obtained by ordinary technicians in the field without creative work are within the scope of protection of the present invention.

[0092] In the following examples, unless otherwise specified, all materials used were commercially available of analytical grade.

[0093] The following examples are used to illustrate the lithium ion battery of the present invention.

[0094] Embodiment 1:

[0095] (1) Preparation of diaphragm:

[0096] Q1, based on the total weight of the mixed solution, 15% polyvinyl alcohol and distilled water are mixed for the first time, and the conditions for the first mixing are stirring at 90° C. on a hot plate stirrer for 16 hours until dissolved to obtain a polyvinyl alcohol solution;

[0097] Q2, based on the total weight of the mixed solution, 3% boehmite (average particle size of 160 nm) and the polyvinyl alcohol solution obtained in Q1 were mixed for the second time, and the second mixing condition was stirring at 90° C. for 6 h using a hot plate stirrer to obtain a mixed solution;

[0098] Q3, vacuum-treating the mixed solution obtained in Q2 for 8 hours, uniformly coating it on a glass plate, drying it at room temperature, and peeling it off to obtain a base film;

[0099] Q4, based on the total weight of the slurry, 28% attapulgite (average particle size of 630 nm), 1.3% LA133 binder (Sichuan Yindi Le), 1.5% sodium dodecyl sulfate, 1.5% non-ionic silicone wetting agent, 0.5% sodium carbonate and distilled water were mixed for the third time, and the condition of the third mixing was stirring at room temperature for 4 hours until the mixture was uniform, to obtain a slurry;

[0100] Q5, subjecting the base film and the slurry to a first treatment, the conditions of the first treatment are: immersing one side of the base film in the slurry for 60 seconds, taking it out and gradually heating it (80°C for 1 hour, 120°C for 2 hours, and 180°C for 0.5 hours) to obtain a diaphragm.

[0101] The ratio of the coating thickness to the base film thickness is 0.125, the base film thickness is 8 μm, and the coating thickness is 1 μm.

[0102] (2) Preparation of positive electrode sheet:

[0103] Lithium iron phosphate, acetylene black, and polyvinylidene fluoride were mixed in a ratio of 98:1:1, and N-methylpyrrolidone was used as a solvent to obtain a positive electrode slurry. The positive electrode slurry was coated on an aluminum foil current collector with a thickness of 12 μm, dried at 80°C, and heated and cured at 250°C for 3 hours. The electrode sheets were cut by roller pressing to obtain positive electrode sheets.

[0104] (3) Assembly of lithium-ion batteries:

[0105] Ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) are mixed in a volume ratio of 1:1:1 to obtain an organic solvent, 1M lithium hexafluorophosphate is added as a lithium salt, and the mixture is uniformly mixed to obtain an electrolyte.

[0106] Using metallic lithium as the negative electrode, a CR2032 button cell was assembled in a glove box filled with dry argon. The assembly order was positive electrode shell-gasket-lithium iron phosphate positive electrode sheet-electrolyte-diaphragm-electrolyte-lithium sheet-gasket-spring-negative electrode shell, and the sealing pressure was 50kg / cm 2 .

[0107] Embodiment 2:

[0108] Based on Example 1, the difference is that the diaphragm is different. The preparation of the diaphragm is as follows:

[0109] Q1, based on the total weight of the mixed solution, 20% polyvinyl alcohol and distilled water are mixed for the first time, and the conditions for the first mixing are stirring at 80° C. on a hot plate stirrer for 14 h until dissolved to obtain a polyvinyl alcohol solution;

[0110] Q2, based on the total weight of the mixed solution, 5% boehmite (average particle size of 175 nm) and the polyvinyl alcohol solution obtained in Q1 were mixed for the second time, and the second mixing condition was stirring at 87° C. for 5 h using a hot plate stirrer to obtain a mixed solution;

[0111] Q3, vacuum-treating the mixed solution obtained in Q2 for 10 h, uniformly coating it on a glass plate, drying it at room temperature, and peeling it off to obtain a base film;

[0112] Q4, based on the total weight of the slurry, 30% attapulgite (average particle size of 650 nm), 1.5% LA133 binder (Sichuan Yindi Le), 2% sodium dodecyl sulfate, 1.3% polyether silane wetting agent, 0.4% sodium silicate and distilled water were mixed for the third time, and the condition of the third mixing was stirring at room temperature for 4 hours until the mixture was uniform, to obtain a slurry;

[0113] Q5, subjecting the base film and the slurry to a first treatment, the conditions of the first treatment are: immersing one side of the base film in the slurry for 60 seconds, taking it out and gradually heating it (85°C for 1.5 hours, 125°C for 1.5 hours, and 170°C for 0.8 hours) to obtain a diaphragm.

[0114] The ratio of the coating thickness to the base film thickness is 0.167, the base film thickness is 12 μm, and the coating thickness is 2 μm.

[0115] Embodiment 3:

[0116] Based on Example 1, the difference is that the diaphragm is different. The preparation of the diaphragm is as follows:

[0117] Q1, based on the total weight of the mixed solution, 10% polyvinyl alcohol and distilled water are mixed for the first time, and the conditions for the first mixing are stirring at 70° C. on a hot plate stirrer for 12 h until dissolved to obtain a polyvinyl alcohol solution;

[0118] Q2, based on the total weight of the mixed solution, 1% boehmite (average particle size of 150 nm) and the polyvinyl alcohol solution obtained in Q1 were mixed for the second time, and the second mixing condition was stirring at 85° C. for 3 h using a hot plate stirrer to obtain a mixed solution;

[0119] Q3, vacuum-treating the mixed solution obtained in Q2 for 12 h, uniformly coating it on a glass plate, drying it at room temperature, and peeling it off to obtain a base film;

[0120] Q4, based on the total weight of the slurry, 25% attapulgite (average particle size of 625 nm), 1% LA133 type binder, 0.5% sodium dodecyl sulfate, 1% polyether silane wetting agent, 0.3% sodium silicate and distilled water were mixed for the third time, and the condition of the third mixing was stirring at room temperature for 4 hours until the mixture was uniform, to obtain a slurry;

[0121] Q5, subjecting the base film and the slurry to a first treatment, the conditions of the first treatment are: immersing one side of the base film in the slurry for 60 seconds, taking it out and gradually heating it (90°C for 2 hours, 135°C for 1 hour, and 160°C for 1 hour) to obtain a diaphragm.

[0122] The ratio of the coating thickness to the base film thickness is 0.07, the base film thickness is 7 μm, and the coating thickness is 0.5 μm.

[0123] Embodiment 4 group:

[0124] This group of examples is used to verify the impact of the change in the "average particle size of the first particles", as follows:

[0125] Example 4a, based on Example 1, except that the average particle size of the first particles is 100 nm;

[0126] Example 4b is based on Example 1, except that the average particle size of the first particles is 300 nm.

[0127] Embodiment 5 group:

[0128] This group of examples is used to verify the impact of the change in the "average particle size of the second particles", as follows:

[0129] Example 5a, based on Example 1, except that the average particle size of the second particles is 500 nm;

[0130] Example 5b is based on Example 1, except that the average particle size of the second particles is 800 nm.

[0131] Embodiment 6 group:

[0132] This group of examples is used to verify the impact of the change in the "ratio of coating thickness to diaphragm thickness", as follows:

[0133] Example 6a is based on Example 1, except that the ratio of the coating thickness to the base film thickness is 0.286, in which case the base film thickness is 7 μm and the coating thickness is 2 μm.

[0134] Example 6b is based on Example 1, except that the ratio of the coating thickness to the base film thickness is 0.042, in which case the base film thickness is 12 μm and the coating thickness is 0.5 μm.

[0135] Example 6c is based on Example 1, except that the ratio of the coating thickness to the base film thickness is 0.06. In this case, the base film thickness is 5 μm and the coating thickness is 0.3 μm.

[0136] Example 6d is based on Example 1, except that the ratio of the coating thickness to the base film thickness is 0.2. In this case, the base film thickness is 25 μm and the coating thickness is 5 μm.

[0137] Embodiment 7 group:

[0138] This group of examples is used to verify the effects of changes in "first particles, second particles and binder", as follows:

[0139] Example 7a is based on Example 1, except that the first particles are zirconia ceramics, the second particles are montmorillonite, and the binder is acrylic acid-acrylonitrile-acrylate copolymer.

[0140] Example 7b is based on Example 1, except that the first particle is alumina ceramic, the second particle is zeolite, and the binder is polyacrylic acid-polyacrylonitrile copolymer.

[0141] Embodiment 8 group:

[0142] This group of examples is used to verify the impact of "the formation method of the coating on the base film", as follows:

[0143] Example 8a is based on Example 1, and differs in that the diaphragm is obtained by coating the slurry on the surface of either side of the base film.

[0144] Example 8b is based on Example 1, except that the base membrane is completely immersed in the slurry to obtain a diaphragm.

[0145] Comparative Example 1:

[0146] Based on Example 1, the difference lies in that a polyvinyl alcohol film is used as the separator.

[0147] Comparative Example 2:

[0148] Based on Example 1, the difference lies in that the base film is used as the separator, that is, the coating layer is not prepared on the base film.

[0149] Comparative Example 3 Group:

[0150] This group of comparative examples is used to verify the impact of the change in "the ratio of the coating thickness to the diaphragm thickness", as follows:

[0151] Comparative Example 3a is based on Example 1, except that the ratio of the coating thickness to the base film thickness is 1, in which case the base film thickness is 5 μm and the coating thickness is 5 μm.

[0152] Comparative Example 3b is based on Example 1, except that the ratio of the coating thickness to the base film thickness is 0.012, in which case the base film thickness is 25 μm and the coating thickness is 0.3 μm.

[0153] Test example:

[0154] (1) Thermal shrinkage:

[0155] The diaphragms obtained in the embodiments of the present invention and the comparative examples were cut into samples of 10 cm×10 cm, 5 sheets each on the top and bottom were clamped and fixed with A4 paper, placed in a forced air drying oven and baked at 200°C for 1 hour, and the length and width of the diaphragm after shrinkage were measured and calculated to obtain the thermal shrinkage rate of the diaphragm at 200°C.

[0156] (2) Liquid absorption rate:

[0157] The membranes obtained in the embodiments of the present invention and the comparative examples were dried and weighed to obtain the dry membrane mass (referred to as m), and then the membranes were completely immersed in anhydrous ethanol for 2 hours, and then the membranes were quickly taken out, the solvent on the surface was gently wiped with filter paper, and the wet membrane mass (referred to as m') was weighed again. The calculation formula of the liquid absorption rate of the membranes is as follows:

[0158] Liquid absorption rate (%) = (m'-m) / m x 100%.

[0159] (3) Porosity:

[0160] The membranes obtained in the embodiments of the present invention and the comparative examples were dried and weighed as samples, recorded as m0, immersed in anhydrous ethanol solution for 2 hours, and then taken out to remove excess liquid on the surface with filter paper and weighed again, recorded as m1. The calculation formula of the membrane porosity is as follows:

[0161] Porosity (%) = [(m1-m0) / (ρ×V)]×100%;

[0162] Where m0 and m1 are the masses of the diaphragm before and after immersion in anhydrous ethanol, ρ is the density of anhydrous ethanol, and V is the volume of the sample.

[0163] The heat shrinkage, liquid absorption and porosity of the diaphragms prepared in the examples of the present invention and the comparative examples are recorded in Table 1.

[0164] (4) Battery performance test:

[0165] ① Cycle performance test:

[0166] The cyclic voltammetric characteristics test of lithium iron phosphate is carried out in the potential range of 2.0N-3.6V. The voltage scan rate can be 0.1mV / s-3.2mV / s, and the capacity retention rate of 50 cycles at a charge and discharge rate of 1C.

[0167] ②Lithium ion migration number:

[0168] The lithium ion migration number is calculated according to the following formula:

[0169] Lithium ion migration number = I s (ΔV-I 0 R 0 ) / I 0 (ΔV-I s R s );

[0170] Among them, I 0 is the initial current (mA), I s is the stable current (mA), R 0 is the AC impedance of the symmetrical battery before polarization (mΩ), R s is the AC impedance of the symmetrical battery after polarization (mΩ), and ΔV is the applied polarization voltage (mV).

[0171] ③Ionic conductivity test:

[0172] The test membrane was sandwiched between two stainless steel electrodes (SS) in an argon-filled glove box, and the SS|membrane|SS battery was assembled for testing. Electrochemical impedance spectroscopy (EIS) tests were performed in the frequency range of 8MHz-100mHz with an AC amplitude of 40mV. The ionic conductivity σ was calculated according to the following formula:

[0173] σ=L / RьS;

[0174] Where L is the thickness of the diaphragm in cm, R b is the volume resistance Ω, S is the area of ​​the stainless steel electrode cm 2 .

[0175] The performance tests of the batteries prepared in the examples of the present invention and the comparative examples are recorded in Table 2.

[0176] Table 1:

[0177]

[0178] Table 2:

[0179]

[0180] It can be seen from Table 1 that the diaphragm prepared by the present invention has good thermal stability, liquid absorption rate and porosity compared with the comparative example; it can be seen from Table 2 that the lithium ion battery prepared by the present invention improves lithium dendrite precipitation and has good cycle stability compared with the comparative example.

[0181] The preferred embodiments of the present invention are described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, the technical solution of the present invention can be subjected to a variety of simple modifications, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be regarded as the contents disclosed by the present invention and belong to the protection scope of the present invention.

Claims

1. A diaphragm, characterized in that: The diaphragm includes a base film and a coating located on at least one side of the base film, the base film includes a polyvinyl alcohol film and first particles dispersed in the polyvinyl alcohol film, the first particles include at least one of boehmite, alumina and zirconium oxide; the coating includes second particles and a first binder, the second particles include at least one of attapulgite, zeolite, sepiolite, montmorillonite and diatomaceous earth, the first binder includes at least one of polyacrylonitrile, polyacrylic acid, acrylic acid-acrylonitrile copolymer, acrylic acid-acrylamide copolymer, acrylonitrile-acrylamide copolymer, acrylic acid-acrylonitrile-acrylate copolymer, acrylic acid-acrylonitrile-acrylamide copolymer and acrylic acid-acrylonitrile-acrylate copolymer.

2. The diaphragm according to claim 1, wherein The first particles include boehmite; and / or, the second particles include attapulgite; And / or, the first binder includes at least one of polyacrylonitrile, acrylonitrile-acrylic acid copolymer, acrylonitrile-acrylamide copolymer and acrylic acid-acrylonitrile-acrylate copolymer.

3. The diaphragm according to claim 1 or 2, wherein: The average particle size of the first particles is 100nm-300nm; And / or, the average particle size of the second particles is 500nm-800nm; And / or, the Mohs hardness of the first particles is 3-3.

5.

4. The diaphragm according to claim 1, wherein The ratio of the coating thickness to the base film thickness is 0.04-0.3, preferably 0.07-0.17; and / or, the base film has a thickness of 5 μm-25 μm, preferably 7 μm-12 μm; And / or, the coating has a thickness of 0.3 μm-5 μm, preferably 0.5 μm-2 μm.

5. The diaphragm according to claim 1, wherein The porosity of the diaphragm is 40%-60%; Preferably, the porosity of the separator is 50%-60%.

6. The diaphragm according to claim 1, wherein The liquid absorption rate of the diaphragm at 200° C. is 150%-200%; And / or, the thermal shrinkage rate of the separator at 200° C. is 0.8%-3%.

7. A method for preparing the diaphragm according to any one of claims 1 to 6, characterized in that: The method comprises at least the following steps: Q1, taking polyvinyl alcohol and distilled water for a first mixing to obtain a polyvinyl alcohol solution; Q2, performing a second mixing of the polyvinyl alcohol solution obtained in Q1 and the first particles to obtain a mixed solution; Q3, vacuum-treating the mixed solution obtained in Q2, uniformly coating it on the substrate, drying it, and peeling it off to obtain a base film; Q4, mixing the second particles, the first binder and distilled water to obtain a slurry; Q5, subjecting the base film and the slurry to a first treatment to obtain a diaphragm; Among them, the first particles include at least one of boehmite, alumina, and zirconium oxide; the coating includes second particles and a first binder, the second particles include at least one of attapulgite, zeolite, sepiolite, montmorillonite, and diatomaceous earth, and the first binder includes at least one of acrylonitrile multipolymer, polyacrylic acid copolymer, and polyacrylic acid-polyacrylonitrile copolymer.

8. The method according to claim 7, wherein: The first mixing condition is heating to 70°C-90°C and stirring for 12h-16h; And / or, the second mixing condition is heating to 85°C-90°C and stirring for 3h-6h; And / or, the first treatment includes coating the slurry on at least one side surface of the base film and / or immersing at least one side surface of the base film in the slurry; And / or, step Q1 further comprises adding a surfactant, wherein the surfactant comprises at least one of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate and sodium laurate, and the content of the surfactant is 0.1%-2%, preferably 1%-1.5%.

9. The method according to claim 7, wherein: Based on the total weight of the mixed solution, the content of the polyvinyl alcohol is 10%-20%, and the content of the first particles is 1%-5%; And / or, the slurry also includes a dispersant and a wetting agent, and based on the total weight of the slurry, the content of the second particles is 25%-30%, the content of the first binder is 1%-1.5%, the content of the dispersant is 0.3%-0.5%, and the content of the wetting agent is 1%-1.5%.

10. A lithium ion battery, characterized in that: The lithium-ion battery comprises the diaphragm according to any one of claims 1 to 6 and / or the diaphragm prepared by the method according to any one of claims 7 to 9, and the lithium-ion battery further comprises a negative electrode sheet; And / or, the negative electrode sheet is arranged opposite to the side of the separator having the coating; And / or, the lithium ion transference number of the lithium ion battery is 0.3-0.75.

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