Diaphragm, preparation method thereof and lithium ion battery

Abstract

The diaphragm provided by the embodiment of the invention comprises a base film layer and a lithium supplementing layer arranged on the base film layer, the lithium supplementing layer comprises a lithium supplementing agent, a conductive agent and polyimide, and the lithium supplementing agent and the conductive agent are coated with the polyimide. The diaphragm provided by the embodiment of the invention can be used for supplementing active lithium ions due to the lithium supplementing layer, and in addition, the lithium supplementing agent is coated with the polyimide, so that direct contact between the lithium supplementing agent and the outside can be isolated, and the side reaction of the lithium supplementing agent is reduced. According to other embodiments of the invention, the invention also provides a method for preparing the diaphragm and a lithium ion battery.

Description

Technical Field

[0001] The embodiments of the present application relate to the field of batteries, and more specifically, to a separator and a preparation method thereof, and a lithium-ion battery. Background Art

[0002] During the formation process of lithium-ion batteries, the formation of a solid electrolyte interface (SEI) film at the negative electrode consumes a large amount of active lithium. This irreversible lithium consumption is particularly severe in batteries with high-capacity silicon materials added to the negative electrode. Furthermore, during the subsequent continuous charge and discharge process, the number of active lithium ions continues to decrease due to various side reactions, resulting in battery capacity decay and shortened cycle life.

[0003] Therefore, there is an urgent need for a technical solution that can solve the technical problem of the decrease in the number of active lithium ions in lithium-ion batteries.

[0004] The separator is a crucial component of lithium-ion batteries. It primarily separates the positive and negative electrodes, preventing direct contact and short circuits. It also conducts lithium ions. Therefore, the performance of the separator significantly impacts the overall performance of lithium-ion batteries. Currently, improvements in separators are needed to enhance the performance of lithium-ion batteries. Summary of the Invention

[0005] In view of the problems existing in the prior art, the purpose of the present application is to provide a separator having a lithium replenishing layer, a method for preparing the separator having a lithium replenishing layer, and a lithium-ion battery using the separator.

[0006] To achieve the above objectives, the present application provides a diaphragm, comprising: a base film layer and a lithium replenishing layer arranged on the base film layer, the lithium replenishing layer comprising: a lithium replenishing agent, a conductive agent, and polyimide, wherein the polyimide coats the lithium replenishing agent and the conductive agent.

[0007] In some embodiments, the separator further includes a first polyimide layer, and the first polyimide layer and the lithium replenishing layer are located on opposite sides of the base film layer.

[0008] In some embodiments, the separator further includes a second polyimide layer located between the lithium replenishing layer and the base film layer.

[0009] In some embodiments, the diaphragm further includes a first polyimide layer and a second polyimide layer located on opposite sides of the base film layer, and the lithium replenishing layer is disposed on the second polyimide layer.

[0010] In some embodiments, the thickness of the lithium-replenishing layer is 10%-30% of the total thickness of the lithium-replenishing layer and the second polyimide layer.

[0011] In some embodiments, the base film layer is one or more of a polypropylene film and a polyethylene film.

[0012] In some embodiments, the lithium supplement is one or more of Li2O, Li2O2, Li6CoO4, Li2NiO2, and Li5FeO4, and the conductive agent is one or more of ordered mesoporous carbon, conductive carbon black, conductive graphite, graphite carbon, activated carbon, carbon nanotubes, carbon fibers, and conductive polymers.

[0013] On the other hand, the present application provides a method for preparing a diaphragm, comprising the following steps: using polyamic acid as a precursor solution, uniformly mixing the polyamic acid, a conductive agent, and a lithium replenisher to prepare a slurry; forming the slurry on a base film layer, and then performing a heat treatment to form a lithium replenisher layer on the base film layer, wherein the heat treatment dehydrates and condenses the polyamic acid into polyimide, and the polyimide coats the lithium replenisher and the conductive agent.

[0014] In some embodiments, the method for preparing the diaphragm also includes forming a first polyimide layer, and the lithium-replenishing layer and the first polyimide layer are formed on opposite sides of the base film layer, wherein the first polyimide layer is formed by coating a polyamic acid solution on the base film layer and then performing a heat treatment, and the first polyimide layer is formed before or after the lithium-replenishing layer.

[0015] In some embodiments, before forming the slurry on the base film layer, a polyamic acid solution is coated on one side of the base film layer, heat treatment is performed to form a second polyimide layer, and then the lithium replenishing layer is formed on the base film layer by coating the slurry on the second polyimide layer.

[0016] In some embodiments, before forming a slurry on a base film layer, a polyamic acid solution is coated on opposite sides of the base film layer, a heat treatment is performed to form a first polyimide layer and a second polyimide layer, and then a lithium replenishing layer is formed on the base film layer by coating the slurry on the second polyimide layer.

[0017] In some embodiments, the polyamic acid, the conductive agent, and the lithium supplement agent are mixed in a mass ratio of 3%-10%: 3%-30%: 60%-94%.

[0018] In another aspect, the present application provides a lithium-ion battery, comprising: a positive electrode sheet, a negative electrode sheet, an electrolyte, and the separator of any one of the above embodiments, wherein the lithium supplement layer of the separator faces the positive electrode sheet.

[0019] The beneficial technical effects of this application are:

[0020] The separator has a lithium-replenishing layer that can be used to replenish active lithium ions. Furthermore, the polyimide-coated lithium-replenishing agent isolates it from direct contact with the outside world, thereby reducing side effects of the lithium-replenishing agent.

[0021] Due to the application of a separator with a lithium replenishment layer, the active lithium ions in the lithium-ion battery can be replenished, thereby improving the initial efficiency of the lithium-ion battery, reducing the irreversible capacity and extending the cycle life. In addition, in lithium-ion batteries, polyimide-coated lithium replenishers can isolate the lithium replenishers from direct contact with the electrolyte, thereby reducing the side reactions of the lithium replenishers, improving battery gas production (the side reactions of the lithium replenishers under high-temperature storage and overcharge and over-discharge conditions will produce a large amount of gas) and reducing the internal resistance of the cycle, thereby improving the stability of the lithium replenisher system and improving the safety of the lithium-ion battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1A is a schematic side view of a diaphragm according to an embodiment of the present application.

[0023] Figure 1B is a schematic side view of a diaphragm according to another embodiment of the present application.

[0024] Figure 2 Schematic top view of a lithium replenishing layer according to one embodiment of the present application.

[0025] Figure 3 is a schematic side view of a diaphragm according to another embodiment of the present application.

[0026] Figure 4 is a schematic side view of a diaphragm according to another embodiment of the present application.

[0027] Figure 5 is a schematic side view of a diaphragm according to another embodiment of the present application.

[0028] Figure 6 Schematic diagram of a diaphragm preparation device according to one embodiment of the present application.

[0029] Figure 7 It is a three-dimensional schematic diagram of various steps of forming a diaphragm according to an embodiment of the present application. DETAILED DESCRIPTION

[0030] The following fully describes exemplary embodiments in detail, however, these exemplary embodiments may be implemented in different ways and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided to make this disclosure thorough and complete and to fully convey the scope of this application to those skilled in the art.

[0031] Figure 1A FIG is a schematic side view of a diaphragm 10 according to an embodiment of the present application. Figure 1AAs shown, the present application provides a diaphragm 10, characterized in that the diaphragm 10 includes: a base film layer 11 and a lithium replenishing layer 13 provided on the base film layer 11, the lithium replenishing layer 13 having: a lithium replenishing agent 131, a conductive agent 132, and a polyimide (PI) 133, wherein the polyimide 133 coats the lithium replenishing agent 131 and the conductive agent 132. The diaphragm 10 of the present application can be used to replenish active lithium ions because it has the lithium replenishing layer 13. In addition, the polyimide 133 in the lithium replenishing layer 13 of the present application coats the lithium replenishing agent 131, which can prevent the lithium replenishing agent 131 from directly contacting the outside world, thereby reducing the side reactions of the lithium replenishing agent 131.

[0032] Coating in the present invention refers to any situation where the surface of the lithium supplement agent and the conductive agent is completely or partially covered by polyimide at a microscopic level (e.g., under an electron microscope at 1K-5K magnification), and the polyimide contains through-holes that allow lithium ions to pass through, thereby achieving the purpose of lithium supplementation. Compared to lithium supplement agents not coated with polyimide, the lithium supplement agent and the conductive agent of the present invention are completely or partially covered with polyimide, which can, to a certain extent, prevent the lithium supplement agent from being completely exposed to the external environment (e.g., the electrolyte), thereby achieving protection for the lithium supplement agent. The larger the area of ​​the lithium supplement agent covered by polyimide, the more it can prevent the lithium supplement agent from contacting the external environment, the lower the probability of side reactions in the lithium supplement agent, and the more stable the lithium supplement layer.

[0033] Figure 1A The schematic diagram shows that polyimide 133 completely encapsulates lithium replenisher 131 and conductive agent 132. In other words, polyimide 133 completely encapsulates lithium replenisher 131 and conductive agent 132, embedding them within polyimide 133. The layer formed by polyimide 133 is a continuous structure, and conductive agent 132 within polyimide 133 forms a conductive network connecting lithium replenisher 131.

[0034] Figure 1B It is schematically shown that the polyimide 133 partially covers the lithium replenisher 131 and the conductive agent 132 , that is, the surfaces of the lithium replenisher 131 and the conductive agent 132 are partially covered by the polyimide 133 .

[0035] It should be understood that the polyimide 133 coated lithium replenisher 131 and conductive agent 132 in the present application may be that the polyimide 133 completely covers the lithium replenisher 131 and conductive agent 132, or that the polyimide 133 partially covers the lithium replenisher 131 and conductive agent 132, or that the polyimide 133 completely covers the lithium replenisher 131 and conductive agent 132 in some areas, and partially covers the lithium replenisher 131 and conductive agent 132 in the remaining areas.

[0036] Figure 2FIG2 is a top view of a lithium replenishing layer 13 according to an embodiment of the present application. Referring to FIG2 , the polyimide 133 covers the lithium replenishing agent 131 and the conductive agent 132 .

[0037] Figure 3 FIG is a side view of a diaphragm 10 according to another embodiment of the present application. Figure 3 As shown, the diaphragm 10 may further include a first polyimide layer 15 , and the first polyimide layer 15 and the lithium replenishing layer 13 are located on opposite sides of the base film layer 11 . In a preferred embodiment, the first polyimide layer 15 and the lithium replenishing layer 13 are both in direct contact with the base film layer 11 . Figure 4 FIG is a side view of a diaphragm 10 according to another embodiment of the present application. Figure 4 As shown, the separator 10 may further include a second polyimide layer 17 located between the lithium replenishing layer 13 and the base film layer 11 . In a preferred embodiment, the second polyimide layer 17 is in direct contact with both the lithium replenishing layer 13 and the base film layer 11 . Figure 5 FIG is a side view of a diaphragm 10 according to another embodiment of the present application. Figure 5 As shown, the diaphragm 10 can also be located on the first polyimide layer 15 and the second polyimide layer 17 on opposite sides of the base film layer 11, and the lithium supplement layer 13 is arranged on the second polyimide layer 17. In a preferred embodiment, the first polyimide layer 15 and the second polyimide layer 17 are both in direct contact with the base film layer 11. The diaphragm formed of pure polypropylene and polyethylene is prone to shrinkage and melting at higher temperatures. When used in lithium-ion batteries, the diaphragm is prone to failure, resulting in battery short circuit and thermal runaway. The diaphragm 10 of the present application has the first polyimide layer 15 and / or the second polyimide layer 17, so that the diaphragm 10 can withstand high temperatures and improve the safety of the diaphragm 10. In addition, the diaphragm 10 has the advantages of excellent mechanical stability, electrochemical stability, high temperature stability, and small shrinkage, which reduces the risk of the diaphragm 10 shrinking and melting in a high temperature environment.

[0038] refer to Figure 4 As shown, in some embodiments, the thickness h of the lithium replenishing layer 13 is 10%-30% of the total thickness H of the lithium replenishing layer 13 and the second polyimide layer 17. Figure 5 As shown, in some embodiments, the thickness h of the lithium-replenishing layer 13 is 10%-30% of the total thickness H of the lithium-replenishing layer 13 and the second polyimide layer 17. When the thickness of the lithium-replenishing layer 13 is greater than 30% of the total thickness H of the lithium-replenishing layer 13 and the second polyimide layer 17, compared to the separator 10 in which the thickness of the lithium-replenishing layer 13 is 10%-30% of the total thickness of the lithium-replenishing layer 13 and the second polyimide layer 17, the lithium-replenishing agent 131 in the lithium-replenishing layer 13 is dispersed in an area closer to the base film layer 11, resulting in poor stability of the separator 10.

[0039] In some embodiments, the base film layer 11 may be one or more of a polypropylene film and a polyethylene film. In some embodiments, the thickness of the base film layer 11 may be 5-10 μm. In some embodiments, the thickness of the lithium replenishing layer 13 may be 2-6 μm. In some embodiments, the thickness of the first polyimide layer 15 may be 2-6 μm. In some embodiments, the thickness of the second polyimide layer 17 may be 2-6 μm. In some embodiments, the lithium replenishing agent 131 may be one or more of Li2O, Li2O2, Li6CoO4, Li2NiO2, and Li5FeO4, and the conductive agent 132 may be one or more of ordered mesoporous carbon, conductive carbon black, conductive graphite, graphite carbon, activated carbon, carbon nanotubes, carbon fibers, and conductive polymers. The conductive agent 132 in the lithium replenishing layer 13 of the present application, in particular the ordered mesoporous carbon, may be CMK-3, which can enhance the conductivity of the lithium replenishing agent 131. When the separator 10 is used in a lithium-ion battery, it can also enhance the conductivity of the active material of the positive electrode material.

[0040] An embodiment of the present application provides a method for preparing a diaphragm, comprising the following steps: using polyamic acid (PAA) as a precursor solution, uniformly mixing the polyamic acid, a conductive agent, and a lithium replenisher to prepare a slurry a; forming the slurry a on a base film layer, and then performing a heat treatment to form a lithium replenisher layer on the base film layer, wherein the heat treatment dehydrates and condenses the polyamic acid into polyimide, the layer formed by the polyimide is a continuous structure, and the polyimide coats the lithium replenisher and the conductive agent. In the embodiments of the present application, polyamic acid is used as the precursor solution. The numerous carboxyl groups (-COOH) and amide groups (-CONH) in the polyamic acid can form hydrogen bonds and van der Waals forces with the lithium supplement, acting as a binder to secure the lithium supplement, ensuring a safe distance between the lithium supplement and the substrate film. Furthermore, after heat treatment, the polyamic acid (thermal amidation) undergoes dehydration and condensation to form polyimide, which coats the lithium supplement and the conductive agent. The polyimide acts as a protective layer, preventing the lithium supplement from penetrating the substrate film and isolating the lithium supplement from direct contact with the outside world, thereby reducing side reactions of the lithium supplement. For example, in lithium-ion batteries, this can prevent excessive reactions of the lithium supplement from causing battery bloating and safety risks. In some embodiments, the polyamic acid, conductive agent 132, and lithium supplement 131 are mixed in a mass ratio of 3%-10%: 3%-30%: 60%-94%. The lithium supplement content can be adjusted by adjusting the slurry formula.

[0041] Figure 6Schematic diagram of a diaphragm preparation device 20 according to an embodiment of the present application, comprising an unwinding device 1, a base film layer detection device 2, a coating device 3, a heat treatment device 4, a transmission device 5, a coating detection device 6, and a winding device 7. The diaphragm preparation method comprises: transferring the base film layer to the base film layer detection device 2 via the unwinding device 1, testing the thickness and surface density of the base film layer to ensure the quality of the base film layer; applying slurry a at the coating device 3, and then transferring the diaphragm to the heat treatment device 4 for heat treatment to form a lithium replenishing layer on the base film layer, wherein the heat treatment temperature range is 60-150°C; transferring the diaphragm to the coating detection device 6 via the transmission device 5, testing the thickness and surface density; and finally transferring the diaphragm to the winding device 7 for completion.

[0042] In some embodiments, the above method may further include forming a first polyimide layer, wherein the lithium-replenishing layer and the first polyimide layer are formed on opposite sides of the base film layer, wherein the first polyimide layer is formed by coating a polyamic acid solution on the base film layer and then performing a heat treatment, and the first polyimide layer is formed before or after the lithium-replenishing layer.

[0043] In some embodiments, the above method may further include coating a polyamic acid solution on one side of the base film layer before forming the slurry on the base film layer, performing heat treatment to form a second polyimide layer, and then forming a lithium replenishing layer on the base film layer by coating the slurry on the second polyimide layer.

[0044] In some embodiments, the above method may further include coating a polyamic acid solution on opposite sides of the base film layer before forming the slurry on the base film layer, performing heat treatment to form a first polyimide layer and a second polyimide layer, and then forming a lithium replenishing layer on the base film layer by coating the slurry on the second polyimide layer.

[0045] In the embodiment of forming the first polyimide layer and / or the second polyimide layer, Figure 6 The diaphragm preparation device 20 shown may further include other coating devices, heat treatment devices, and coating detection devices, which are arranged before the coating device 3 or after the coating detection device 6. The heat treatment temperature range of the heat treatment device for forming the first polyimide layer and / or the second polyimide layer is 100-350°C.

[0046] The heat treatment in this application is gradual heating to dry and solidify the polyamic acid solution into polyimide. The curing effect and adhesion of the lithium replenishing layer, the first polyimide layer, and the second polyimide layer can be controlled by regulating the temperature and time of the heat treatment.

[0047] Figure 7 1 is a perspective schematic diagram of various steps of forming the diaphragm 10 according to an embodiment of the present application. Figure 7 As shown, first a base film layer 11 is provided, then a first polyimide layer 15 and a second polyimide layer 17 are formed on opposite sides of the base film layer 11 , and finally a lithium replenishing layer 13 is formed on the second polyimide layer 17 to complete the preparation of the separator 10 .

[0048] The present application also provides a lithium-ion battery, comprising: a positive electrode sheet, a negative electrode sheet, an electrolyte, and a diaphragm of the above embodiment, wherein the lithium replenishing layer of the diaphragm faces the positive electrode sheet. In the lithium-ion battery, due to the presence of the lithium replenishing layer, it can be used to replenish active lithium ions, thereby improving the initial efficiency of the lithium-ion battery, reducing the irreversible capacity and extending the cycle life. Moreover, in the lithium-ion battery, the polyimide-coated lithium replenisher can isolate the lithium replenisher from direct contact with the electrolyte, so as to reduce the side reactions of the lithium replenisher, improve the gas production of the battery (the side reactions of the lithium replenisher under high-temperature storage and overcharge and over-discharge conditions will produce a large amount of gas) and reduce the internal resistance of the cycle, reduce the influence of the residual alkali on the surface of the lithium replenisher on the battery system, so as to improve the stability of the lithium replenisher system and improve the safety of the lithium-ion battery. In addition, in lithium-ion batteries, the addition of a polyimide layer (a first polyimide layer and / or a second polyimide layer) to the diaphragm can solve the following technical problems: During the charge and discharge process, as the lithium replenisher in the diaphragm is consumed by the electrochemical reaction, the pores on the diaphragm gradually increase. At the same time, due to the low tensile strength of the base film layer (commonly used polypropylene and polyethylene), especially in high temperature and high pressure environments, the base film layer will shrink and melt, causing battery short circuits and thermal runaway, posing a threat to the safety performance of the battery. The polyimide layer has a tensile strength of more than 10MPa, is electrochemically stable and resistant to high temperatures (500°C), and has small shrinkage, which can make the diaphragm have excellent mechanical stability, electrochemical stability, high temperature stability, and small diaphragm shrinkage, thereby avoiding battery short circuits and thermal runaway, which pose a threat to the safety performance of the battery. The diaphragm with both a lithium replenishment layer and a polyimide layer can effectively replenish active lithium ions to the battery while being resistant to high temperatures, safe and stable.

[0049] Preparation of positive electrode

[0050] The positive electrode active material lithium nickel cobalt manganese oxide (LiNi 0.1 Co 0.1 Mn 0.1 O2), binder polyvinylidene fluoride, and conductive agent acetylene black are mixed in a mass ratio of 88-97:1-10:2, N-methylpyrrolidone is added, and the mixture is stirred until stable and uniform under the action of a vacuum stirrer to obtain a positive electrode slurry; the positive electrode slurry is coated on an aluminum foil with a thickness of 12 μm; the aluminum foil is dried at room temperature and then transferred to a 120°C forced air oven for drying for 1 hour, and then cold pressed and cut to obtain a positive electrode sheet.

[0051] Preparation of negative electrode sheet

[0052] The negative electrode active material artificial graphite, Si, conductive agent acetylene black, thickener sodium carboxymethyl cellulose (CMC), and binder styrene-butadiene rubber latex were mixed in a mass ratio of 89:8:1:1, deionized water was added, and the mixture was stirred until stable and uniform under the action of a vacuum mixer to obtain a negative electrode slurry; the negative electrode slurry was coated on a copper foil with a thickness of 8 μm; the copper foil was dried at room temperature and transferred to a 120°C forced air oven for drying for 1 hour, and then cold pressed and slit to obtain a negative electrode sheet.

[0053] Preparation of diaphragm

[0054] It may be any of the separators described above.

[0055] Preparation of electrolyte

[0056] The organic solvent is a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC), wherein the weight ratio of EC, EMC, and DEC is 1:1:1. The electrolyte is LiPF6, and the total content of LiPF6 accounts for 12.5% ​​of the total weight of the electrolyte.

[0057] Preparation of lithium-ion batteries

[0058] The positive electrode sheet, negative electrode sheet and separator are wound to obtain a battery cell. After the battery cell is placed in a packaging shell, the electrolyte is injected and then sealed in sequence. After standing, hot and cold pressing, formation, exhaust, capacity testing and other processes, a lithium-ion battery is obtained.

[0059] The present application will be described in more detail below through examples.

[0060] Example 1

[0061] (1) Preparation of positive electrode sheet

[0062] The positive electrode active material lithium nickel cobalt manganese oxide (LiNi 0.1 Co 0.1 Mn 0.1 O2), binder polyvinylidene fluoride, and conductive agent acetylene black are mixed in a mass ratio of 94:4:2, N-methylpyrrolidone is added, and the mixture is stirred until stable and uniform under the action of a vacuum stirrer to obtain a positive electrode slurry; the positive electrode slurry is coated on an aluminum foil with a thickness of 12 μm; the aluminum foil is dried at room temperature and then transferred to a 120°C forced air oven for drying for 1 hour, and then cold pressed and cut to obtain a positive electrode sheet.

[0063] (2) Preparation of negative electrode sheet

[0064] The negative electrode active material artificial graphite, Si, conductive agent acetylene black, thickener sodium carboxymethyl cellulose (CMC), and binder styrene-butadiene rubber latex were mixed in a mass ratio of 89:8:1:1, deionized water was added, and the mixture was stirred until stable and uniform under the action of a vacuum mixer to obtain a negative electrode slurry; the negative electrode slurry was coated on a copper foil with a thickness of 8 μm; the copper foil was dried at room temperature and transferred to a 120°C forced air oven for drying for 1 hour, and then cold pressed and slit to obtain a negative electrode sheet.

[0065] (3) Preparation of diaphragm

[0066] Using polyamic acid as a precursor solution, polyamic acid, a conductive agent (CMK-3), and a lithium replenisher (Li2NiO2) are evenly mixed in a mass ratio of 12.5%:37.5%:50% to prepare slurry a; the slurry a is coated on a polypropylene film, and then heat treated (150°C) to form a lithium replenishing layer on the polypropylene film, wherein the heat treatment causes the polyamic acid to dehydrate and condense into polyimide, and the polyimide coats Li2NiO2 and CMK-3.

[0067] (4) Preparation of electrolyte

[0068] The organic solvent is a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC), wherein the weight ratio of EC, EMC, and DEC is 1:1:1. The electrolyte is LiPF6, and the total content of LiPF6 accounts for 12.5% ​​of the total weight of the electrolyte.

[0069] (5) Preparation of lithium-ion batteries

[0070] The positive electrode sheet, negative electrode sheet and separator are wound to obtain a battery cell, in which the lithium supplement layer of the separator faces the positive electrode sheet. After the battery cell is placed in a packaging shell, the electrolyte is injected and then sealed in sequence. After standing, hot and cold pressing, formation, exhaust, capacity testing and other processes, a lithium-ion battery is obtained.

[0071] Example 2

[0072] The preparation process of the lithium ion battery is the same as that of Example 1, except that:

[0073] (3) Preparation of diaphragm

[0074] The preparation method is similar to that of the separator in Example 1, except that the polyamic acid, the conductive agent (CMK-3), and the lithium supplement agent (Li2NiO2) are uniformly mixed in a mass ratio of 10%:30%:60%.

[0075] Example 3

[0076] The preparation process of the lithium ion battery is the same as that of Example 1, except that:

[0077] (3) Preparation of diaphragm

[0078] The preparation method is similar to that of the separator in Example 1, except that the polyamic acid, the conductive agent (CMK-3), and the lithium supplement agent (Li2NiO2) are uniformly mixed in a mass ratio of 5%:15%:80%.

[0079] Example 4

[0080] The preparation process of the lithium ion battery is the same as that of Example 1, except that:

[0081] (3) Preparation of diaphragm

[0082] The preparation of the separator is similar to that of Example 1, except that the polyamic acid, the conductive agent (CMK-3), and the lithium supplement (Li2NiO2) are uniformly mixed in a mass ratio of 2.5%:7.5%:90%.

[0083] Example 5

[0084] The preparation process of the lithium ion battery is the same as that of Example 1, except that:

[0085] (3) Preparation of diaphragm

[0086] Using polyamic acid as a precursor solution, polyamic acid, a conductive agent (CMK-3), and a lithium replenisher (Li2NiO2) are evenly mixed in a mass ratio of 10%:30%:60% to prepare slurry a; the slurry a is coated on a polypropylene film, and then heat treated (150°C) to form a lithium replenishing layer on the polypropylene film, wherein the heat treatment dehydrates and condenses the polyamic acid into polyimide (negative electrode side PI layer), and the polyimide coats Li2NiO2 and CMK-3; the polyamic acid solution is coated on the side of the polypropylene film opposite to the lithium replenishing layer, and then heat treated (150°C) to form a first polyimide layer.

[0087] Example 6

[0088] The preparation process of the lithium ion battery is the same as that of Example 1, except that:

[0089] (3) Preparation of diaphragm

[0090] A polyamic acid solution is coated on one side of a polypropylene film, and then heat treated (150°C) to form a second polyimide layer (positive electrode side PI layer); polyamic acid, a conductive agent (CMK-3), and a lithium replenisher (Li2NiO2) are mixed uniformly in a mass ratio of 10%:30%:60% to prepare slurry a; slurry a is then coated on the second polyimide layer; heat treated (150°C) to form a lithium replenishing layer, and the heat treatment causes the polyamic acid to dehydrate and condense into polyimide, and the polyimide coats Li2NiO2 and CMK-3, wherein the thickness of the lithium replenishing layer is 10% of the total thickness of the lithium replenishing layer and the second polyimide layer.

[0091] Example 7

[0092] The preparation process of the lithium ion battery is the same as that of Example 1, except that:

[0093] A polyamic acid solution is coated on opposite sides of a polypropylene film, and then a heat treatment (150°C) is performed to form a first polyimide layer (negative electrode side PI layer) and a second polyimide layer (positive electrode side PI layer) on opposite sides of the polypropylene film; polyamic acid, a conductive agent (CMK-3), and a lithium replenisher (Li2NiO2) are mixed uniformly in a mass ratio of 12.5%:37.5%:50% to prepare a slurry a; and then the slurry a is coated on the second polyimide layer; a heat treatment (150°C) is performed to form a lithium replenishing layer, and the heat treatment causes the polyamic acid to dehydrate and condense into polyimide, and the polyimide coats Li2NiO2 and CMK-3, wherein the thickness of the lithium replenishing layer is 10% of the total thickness of the lithium replenishing layer and the second polyimide layer.

[0094] Example 8

[0095] The preparation process of the lithium-ion battery is the same as that of Example 7, except that:

[0096] (3) Preparation of diaphragm

[0097] The preparation method is similar to that of the separator in Example 7, except that the polyamic acid, the conductive agent (CMK-3), and the lithium supplement agent (Li2NiO2) are uniformly mixed in a mass ratio of 10%:30%:60%.

[0098] Example 9

[0099] The preparation process of the lithium-ion battery is the same as that of Example 7, except that:

[0100] (3) Preparation of diaphragm

[0101] The preparation method is similar to that of the separator in Example 7, except that the polyamic acid, the conductive agent (CMK-3), and the lithium supplement agent (Li2NiO2) are uniformly mixed in a mass ratio of 5%:15%:80%.

[0102] Example 10

[0103] The preparation process of the lithium-ion battery is the same as that of Example 7, except that:

[0104] (3) Preparation of diaphragm

[0105] The preparation method is similar to that of the separator in Example 7, except that the polyamic acid, the conductive agent (CMK-3), and the lithium supplement agent (Li2NiO2) are uniformly mixed in a mass ratio of 2.5%:7.5%:90%.

[0106] Example 11

[0107] The preparation process of the lithium-ion battery is the same as that of Example 7, except that:

[0108] (3) Preparation of diaphragm

[0109] The preparation method is similar to that of the separator in Example 7, except that the polyamic acid, the conductive agent (CMK-3), and the lithium supplement agent (Li2NiO2) are uniformly mixed in a mass ratio of 1.5%:4.5%:94%.

[0110] Example 12

[0111] The preparation process of the lithium-ion battery is the same as that of Example 6, except that:

[0112] (3) Preparation of diaphragm

[0113] The preparation process is similar to that of the separator of Example 6, except that the thickness of the lithium replenishing layer is 5% of the total thickness of the lithium replenishing layer and the second polyimide layer.

[0114] Example 13

[0115] The preparation process of the lithium ion battery is the same as that of Example 6.

[0116] Example 14

[0117] The preparation process of the lithium-ion battery is the same as that of Example 6, except that:

[0118] (3) Preparation of diaphragm

[0119] The preparation process is similar to that of the separator of Example 6, except that the thickness of the lithium replenishing layer is 30% of the total thickness of the lithium replenishing layer and the second polyimide layer.

[0120] Example 15

[0121] The preparation process of the lithium-ion battery is the same as that of Example 6, except that:

[0122] (3) Preparation of diaphragm

[0123] The preparation process is similar to that of the separator of Example 6, except that the thickness of the lithium replenishing layer is 50% of the total thickness of the lithium replenishing layer and the second polyimide layer.

[0124] Example 16

[0125] The preparation process of the lithium-ion battery is the same as that of Example 6, except that:

[0126] (3) Preparation of diaphragm

[0127] The preparation process is similar to that of the separator of Example 6, except that the thickness of the lithium replenishing layer is 70% of the total thickness of the lithium replenishing layer and the second polyimide layer.

[0128] Those skilled in the art will understand that the above-described method for preparing a lithium-ion battery is merely an example, and other methods commonly used in the art may be employed without departing from the disclosure of this application.

[0129] Some specific comparative examples are listed below to better illustrate the present application.

[0130] Comparative Example 1

[0131] The preparation process of the lithium ion battery is the same as that of Example 1, except that:

[0132] (3) Preparation of diaphragm

[0133] Polypropylene film is used as the diaphragm.

[0134] Comparative Example 2

[0135] The preparation process of the lithium ion battery is the same as that of Example 1, except that:

[0136] (3) Preparation of diaphragm

[0137] A polyamic acid solution was coated on one side of a polypropylene film, and then heat-treated (150° C.) to form a polyimide layer. The composite film of the polypropylene film and the polyimide layer served as a separator, wherein the polyimide layer faced the negative electrode sheet.

[0138] Comparative Example 3

[0139] The preparation process of the lithium ion battery is the same as that of Example 1, except that:

[0140] (3) Preparation of diaphragm

[0141] A polyamic acid solution was coated on opposite sides of a polypropylene film, and then heat-treated (150° C.) to form polyimide layers on opposite sides of the polypropylene film. The composite film of the polypropylene film and the polyimide layers on opposite sides served as a separator.

[0142] Comparative Example 4

[0143] The preparation process of the lithium ion battery is the same as that of Example 1, except that:

[0144] (3) Preparation of diaphragm

[0145] Using polyimide as a binder, polyimide, a conductive agent (CMK-3), and a lithium replenisher (Li2NiO2) are evenly mixed in a mass ratio of 12.5%:37.5%:50% to prepare a lithium replenishing slurry, which is then coated on a polypropylene film to form a lithium replenishing layer.

[0146] Comparative Example 5

[0147] The preparation process of the lithium-ion battery is the same as that of Comparative Example 4, except that:

[0148] (3) Preparation of diaphragm

[0149] The preparation of the separator is similar to that of Comparative Example 4, except that the polyimide, the conductive agent (CMK-3), and the lithium supplement (Li2NiO2) are uniformly mixed in a mass ratio of 10%:30%:60%.

[0150] Comparative Example 6

[0151] The preparation process of the lithium-ion battery is the same as that of Comparative Example 4, except that:

[0152] (3) Preparation of diaphragm

[0153] The preparation of the separator is similar to that of Comparative Example 4, except that the polyimide, the conductive agent (CMK-3), and the lithium supplement (Li2NiO2) are uniformly mixed in a mass ratio of 5%:15%:80%.

[0154] Comparative Example 7

[0155] The preparation process of the lithium-ion battery is the same as that of Comparative Example 4, except that:

[0156] (3) Preparation of diaphragm

[0157] The preparation of the separator is similar to that of Comparative Example 4, except that the polyimide, the conductive agent (CMK-3), and the lithium supplement (Li2NiO2) are uniformly mixed in a mass ratio of 2.5%:7.5%:90%.

[0158] Comparative Example 8

[0159] The preparation process of the lithium-ion battery is the same as that of Comparative Example 7, except that:

[0160] (3) Preparation of diaphragm

[0161] The preparation of the separator is similar to that of Comparative Example 7, except that a polyamic acid solution is first coated on opposite sides of the polypropylene film, and then a heat treatment (150°C) is performed to form a polyimide layer on opposite sides of the polypropylene film; and then a lithium replenishing slurry is coated on the polyimide layer on one side.

[0162] Performance testing:

[0163] The gram capacity, 45-degree cycle number, and volume increase rate (%) of the lithium-ion batteries obtained in the test examples and comparative examples after 45-day storage at 60°C and 100% SOC

[0164] (1) Capacity test of lithium-ion batteries

[0165] First, adjust the constant temperature of the thermostat to 25°C, place the prepared lithium-ion battery in the thermostat for 30 minutes, discharge it at a constant current of 1 / 3C to 2.5V, rest for 30 minutes, then charge it at a constant current of 1 / 3C to 4.25V, rest for 30 minutes, and repeat this cycle three times to calculate the average discharge capacity of the lithium-ion battery. The higher the discharge capacity, the better the lithium replenishment effect. The lithium replenishment effect is defined as: relative to the capacity increase of the non-lithium replenishment group, A represents a capacity increase of more than 3%; B represents a capacity increase of 1-3%; C represents an increase of less than 1%.

[0166] (2) Lithium-ion battery cycle performance test

[0167] Place the lithium-ion battery in a 45°C constant temperature box and let it stand for 30 minutes to allow the lithium-ion battery to reach a constant temperature. The lithium-ion battery that has reached a constant temperature is charged at a constant current of 0.5C to a voltage of 4.25V, then charged at a constant voltage of 4.25V to a current of 0.05C, and then discharged at a constant current of 1C to a voltage of 2.5V. This is considered a charge and discharge cycle. With the initial discharge capacity as 100%, the charge and discharge cycle is repeated until the discharge capacity decays to 80%. The test is stopped and the number of cycles is recorded as an indicator for evaluating the cycle performance of the lithium-ion battery.

[0168] (3) High temperature storage performance test of lithium-ion batteries

[0169] Place the lithium-ion battery in a 25°C constant temperature box and let it stand for 30 minutes to allow the lithium-ion battery to reach a constant temperature. Charge the lithium-ion battery that has reached a constant temperature at a constant current of 0.5C to a voltage of 4.25V, and then charge it at a constant voltage of 4.25V to a current of 0.05C. Let it stand for 30 minutes, and test the volume of the lithium-ion battery, which is recorded as the initial volume. Then store the lithium-ion battery in a 60°C constant temperature box, and take it out on the 3rd, 6th, 9th, 12th, 15th, 20th, 25th, 30th, 45th, and 60th days, let the lithium-ion battery stand for 2 hours, and test the battery volume after reaching room temperature. At the end of each test, charge it at a constant current of 0.5C to a voltage of 4.25V, and then charge it at a constant voltage of 4.25V to a current of 0.05C for replenishment. Record the volume increase rate of each test as an indicator for evaluating the gas production performance of the lithium-ion battery.

[0170] Table 1 shows the experimental results of Examples 1-11.

[0171] Table 1

[0172]

[0173]

[0174] (Note: A represents a capacity increase of more than 3%; B represents a capacity increase of 1-3%; C represents an increase of <1%). Table 2 shows the experimental results of Examples 12-15.

[0175] Table 2

[0176]

[0177] (Note: A represents a capacity increase of more than 3%; B represents a capacity increase of 1-3%; C represents an increase of <1%) Table 3 shows the experimental results of Comparative Examples 1-8.

[0178] Table 3

[0179]

[0180]

[0181] (Note: A represents a capacity increase of more than 3%; B represents a capacity increase of 1-3%; C represents an increase of less than 1%)

[0182] Comparing Comparative Example 1 with Examples 1-4 demonstrates that the separators in the lithium-ion batteries provided by the examples of this application, because they include a lithium-supplementing layer, contain a lithium-supplementing agent capable of replenishing active lithium ions in the lithium-ion battery. As the content of the lithium-supplementing agent in the lithium-supplementing layer increases, the lithium-supplementing effect gradually increases, increasing the capacity of the lithium-ion battery and the number of 45-degree cycles (extending the cycle life). Furthermore, when the mass ratio of the lithium-supplementing agent exceeds 60%, the lithium-supplementing effect is significantly enhanced.

[0183] By comparing Comparative Examples 4-7 with Examples 1-4, and Comparative Example 8 with Example 10, it can be seen that the 45-degree cycle number of the embodiments of the present application is significantly increased compared to the 45-degree cycle number of the comparative examples, and the volume increase rate / % of the embodiments of the present application after 45 days of storage at 60 degrees and 100% SOC is significantly reduced compared to the volume increase rate / % of the comparative examples after 45 days of storage at 60 degrees and 100% SOC. In other words, the lithium-ion batteries of the embodiments of the present application can increase the cycle life of the batteries and improve the gas production of the batteries. Because the lithium replenishing layer in the comparative example directly uses polyimide as a binder, polyimide is blended with a conductive agent (CMK-3) and a lithium replenishing agent (Li2NiO2) to form a lithium replenishing layer, the lithium replenishing agent is not coated by polyimide, and the lithium replenishing agent contacts the electrolyte to cause side reactions; while the lithium replenishing layer in the embodiment of the present application uses polyamic acid as a precursor solution, and a slurry prepared by uniformly mixing polyamic acid, a conductive agent and a lithium replenishing agent is heat-treated to form a lithium replenishing layer, wherein the polyamic acid solution is condensed and polymerized after heat treatment to form a polyimide protective layer, and the polyimide protective layer has a continuous structure. The polyimide protective layer covers the conductive agent and the lithium replenishing agent, which can prevent the lithium replenishing agent from piercing the base film layer (polypropylene film) and at the same time prevent the lithium replenishing agent from contacting the electrolyte, thereby reducing the side reactions of the lithium replenishing agent, improving battery gas production, reducing cycle internal resistance, and improving safety. In addition, in the comparative example, polyimide is directly selected as the binder, and the lithium replenisher cannot be evenly dispersed in the polyimide, resulting in a significant decrease in the bonding force between the base film layer and the lithium replenisher layer in the separator.

[0184] As can be seen from Comparative Example 1, a simple base film layer (polypropylene film) is prone to shrinkage and melting at higher temperatures, resulting in diaphragm failure, so the 45-degree cycle number is low (<500). Comparative Examples 2 and 3 are coated with a polyimide layer on both sides or one side of the base film layer, which can effectively increase the 45-degree cycle number, but it is still low (<700). For Examples 5 and 6, a polyimide layer is formed on one side of the base film layer (positive side or negative side), and a lithium supplement layer is provided. Although the 45-degree cycle number is increased compared to Comparative Example 1 (due to the increase in the number of active lithium ions by the lithium supplement layer), the 45-degree cycle number is reduced compared to Comparative Example 3. This is because the lithium supplement agent is consumed during the charge and discharge process as the electrochemical reaction produces gas, so the pores on the diaphragm gradually increase, the strength of the diaphragm decreases, and the cycle deteriorates. Referring to Example 8, a polyimide layer is formed on both sides of the base film layer, and a lithium supplement layer is provided, which can effectively increase the 45-degree cycle number by up to 850. By comparing Examples 2, 5-6 and 8, it can be seen that since the polyimide layer is added to the diaphragm, the strength of the diaphragm is increased, and the number of 45-degree cycles can be increased. Among them, the embodiment of adding polyimide layers on both sides of the base film layer has a greater effect on improving the number of 45-degree cycles than the embodiment of adding polyimide layers on the positive side of the base film layer, which is greater than the embodiment of adding polyimide layers on the negative side of the base film layer.

[0185] By comparing Examples 7-11, it can be seen that adding polyimide layers on both sides of the base film layer and forming a lithium replenishment layer on the positive electrode side can effectively increase the 45-degree cycle number (>700 cycles). However, due to the low proportion of lithium replenisher (50%), the lithium replenishment effect is general. Adjusting the lithium replenisher ratio to 60%-94% significantly improves the lithium replenishment effect (45-degree cycle number >850 cycles). As the lithium replenisher content increases, the 45-degree cycle number increases. Continuing to increase the lithium replenisher ratio, although the lithium replenishment effect will be improved, the 45-degree cycle number will decrease because too much lithium replenisher brings greater reaction activity and increases side reactions in the system.

[0186] By comparing Examples 12-16, it can be seen that when the thickness of the lithium replenishing layer is 10% and 30% of the total thickness of the lithium replenishing layer and the second polyimide layer (positive electrode side PI layer), the 45-degree cycle number of the lithium-ion battery is greater than 600 cycles, and the volume increase rate / % of the lithium-ion battery stored at 60°C 100% SOC for 45 days is 25%. That is to say, the cycle performance and high-temperature storage performance of the lithium-ion battery are relatively excellent; when the thickness of the lithium replenishing layer is less than 10% (for example, 5%) of the total thickness of the lithium replenishing layer and the second polyimide layer (positive electrode side PI layer), the lithium replenishing capacity is improved less, the 45-degree cycle number is 500 cycles, and the volume increase rate of 60-degree 100% SOC storage for 45 days reaches 27%. That is to say, when the thickness of the lithium replenishing layer is 10% of the total thickness of the lithium replenishing layer and the second polyimide layer (positive electrode side PI layer), the volume increase rate / % of the lithium replenishing layer is 27%. When the thickness of the lithium replenishing layer is less than 10% of the total thickness, the lithium replenishing effect is poor. The reason for the poor lithium replenishing effect is that it is difficult to ensure uniform distribution when the thickness is too low, and the thinner the thickness, the more difficult it is to control. When the thickness of the lithium replenishing layer is more than 30% of the total thickness of the lithium replenishing layer and the second polyimide layer (the positive electrode side PI layer), the number of cycles of the lithium-ion battery at 45 degrees drops to less than 600 cycles, and the volume increase rate / % after 45 days of storage at 60°C and 100% SOC also increases to 27%. In other words, when the thickness of the lithium replenishing layer is more than 30% of the total thickness of the lithium replenishing layer and the second polyimide layer (the positive electrode side PI layer), the lithium replenishing effect is weakened. The principle is that the lithium replenishing agent is too dispersed, the distance between the lithium replenishing agent and the positive electrode active material increases, and the lithium replenishing effect is impaired. At the same time, because the lithium replenishing agent is dispersed in an area close to the base film layer, the stability of the base film layer is poor. Therefore, controlling the thickness of the lithium replenishing layer to 10%-30% of the total thickness of the lithium replenishing layer and the second polyimide layer (the positive electrode side PI layer) can achieve relatively good cycle stability and lithium replenishing effect.

[0187] In summary, the present application relates to a diaphragm with a lithium replenishing layer, a preparation method thereof, and a lithium-ion battery using the diaphragm. In the embodiment of the present application, a lithium replenishing agent, a conductive agent, and a polyamic acid solution are first mixed evenly to form a slurry, and then coated. The content of the lithium replenishing agent can be precisely controlled by adjusting the slurry formula and other methods; ordered mesoporous carbon acts as a conductive network to enhance the conductivity between the lithium replenishing agent and the active material; the polyamic acid solution is heat-treated to dehydrate and condense into polyimide, which acts as both a binder and a coating agent for the lithium replenishing agent, isolating the lithium replenishing agent from direct contact with the electrolyte, thereby reducing the side reactions of the lithium replenishing agent, improving battery gas production, and reducing cycle internal resistance. The embodiments of the present application have the following advantages, including: being able to supplement active lithium ions, thereby improving the initial efficiency of lithium-ion batteries, reducing irreversible capacity and extending cycle life; the surface of the lithium supplement is coated with polyimide to isolate direct contact with the electrolyte, reduce side reactions of the lithium supplement, improve battery gas production, reduce cycle internal resistance, and reduce the impact of residual alkali on the surface of the lithium supplement on the battery system; because the contact between the surface of the lithium supplement material and the electrolyte during high-temperature storage is isolated, high-temperature storage gas production is significantly improved; the diaphragm also includes a first polyimide layer and / or a second polyimide layer, so that the diaphragm has excellent mechanical stability, high-temperature stability, and small diaphragm shrinkage, which makes it an effective solution to improve the safety of the diaphragm. In addition, the preparation equipment of the diaphragm can be continuously and efficiently produced to meet mass production needs, and the components on both sides of the base film layer can be adjusted to achieve different improvement effects; at the same time, the surface density and thickness of the diaphragm are controllable, and it has good application prospects.

[0188] It should be understood by those skilled in the art that the above embodiments are merely exemplary embodiments and that various changes, substitutions, and alterations may be made without departing from the spirit and scope of the present application.

Claims

1. A diaphragm, characterized in that: include: A base film layer and a lithium replenishing layer provided on the base film layer, wherein the lithium replenishing layer comprises a lithium replenishing agent, a conductive agent, and polyimide, wherein the polyimide covers the lithium replenishing agent and the conductive agent.

2. The diaphragm according to claim 1, characterized in that The invention also includes a first polyimide layer, wherein the first polyimide layer and the lithium replenishing layer are located on two opposite sides of the base film layer.

3. The diaphragm according to claim 1 or claim 2, characterized in that: The invention also includes a second polyimide layer located between the lithium supplement layer and the base film layer.

4. The diaphragm according to claim 3, characterized in that The thickness of the lithium replenishing layer is 10%-30% of the total thickness of the lithium replenishing layer and the second polyimide layer.

5. The diaphragm according to claim 1, characterized in that The base film layer is one or more of a polypropylene film and a polyethylene film, the lithium supplement agent is one or more of Li2O, Li2O2, Li6CoO4, Li2NiO2, and Li5FeO4, and the conductive agent is one or more of ordered mesoporous carbon, conductive carbon black, conductive graphite, graphite carbon, activated carbon, carbon nanotubes, carbon fibers, and conductive polymers.

6. A method for preparing a diaphragm, characterized in that: The steps include: Using polyamic acid as a precursor solution, the polyamic acid, a conductive agent, and a lithium supplement agent are uniformly mixed to prepare a slurry; The slurry is formed on a base film layer, and then a heat treatment is performed to form a lithium replenishing layer on the base film layer, wherein the heat treatment dehydrates and condenses the polyamic acid into polyimide, and the polyimide covers the lithium replenishing agent and the conductive agent.

7. The method for preparing a diaphragm according to claim 6, wherein: It also includes forming a first polyimide layer, wherein the lithium-replenishing layer and the first polyimide layer are formed on opposite sides of the base film layer, wherein the first polyimide layer is formed by coating a polyamic acid solution on the base film layer and then performing a heat treatment, and the first polyimide layer is formed before or after the lithium-replenishing layer.

8. The method for preparing a diaphragm according to claim 6, wherein: Before forming the slurry on the base film layer, A polyamic acid solution is coated on one side of the base film layer, a heat treatment is performed to form a second polyimide layer, and then the lithium replenishing layer is formed on the base film layer by coating the slurry on the second polyimide layer.

9. The method for preparing a diaphragm according to claim 7, wherein: The polyamic acid, the conductive agent, and the lithium supplement agent are mixed according to the mass ratio of 3%-10%: 3%-30%: 60%-94%.

10. A lithium ion battery, characterized in that: include: A positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator according to any one of claims 1 to 5, wherein the lithium supplement layer of the separator faces the positive electrode sheet.

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