Composite diaphragm, lithium ion secondary battery and electric equipment

Abstract

The invention belongs to the technical field of secondary batteries, and particularly relates to a composite diaphragm, a lithium ion secondary battery and electric equipment. According to the composite diaphragm provided by the invention, thermal failure caused by crosstalk of a positive electrode and a negative electrode can be effectively avoided by limiting the composition of the coating, so that the thermal stability of the battery can be improved, and other performances, such as cycle performance, of the battery cannot be influenced. Specifically, keto groups in the ketone compounds react with hydrogen generated on a negative electrode interface, so that thermal runaway caused by hydrogen crosstalk to a positive electrode is avoided. And when the battery is in a hot box test or high-temperature state, the high temperature can accelerate the reaction of the ketone compound and hydrogen produced by the negative electrode, so that the overall thermal stability of the battery cell is improved. The idea and the method for isolating thermal runaway and improving the thermal stability of the battery provided by the invention are provided by the applicant for the first time, and have never occurred before.

Description

Technical Field

[0001] The present application belongs to the technical field of secondary batteries, and specifically relates to a composite diaphragm, a lithium-ion secondary battery and an electrical device. Background Art

[0002] With the development of science and technology, lithium batteries are widely used in various electronic devices, electric vehicles and other fields due to their high energy density, long cycle life and environmental protection. However, during the use of lithium batteries, due to the complex internal chemical reactions, thermal runaway and other problems may occur, which not only affects the safety of the battery, but also limits its application in some special fields. Therefore, how to improve the thermal stability of lithium batteries is an urgent problem to be solved in the current lithium battery manufacturing field.

[0003] The solutions in the existing technology mainly improve the thermal stability of the battery by improving the structural design of the battery, optimizing the material formula of the battery, etc. However, there are still some problems with the existing technology in improving the thermal stability of the battery. First, the existing solutions often require a significant change in the structure of the battery, which not only increases the manufacturing cost of the battery, but may also affect other performance of the battery, such as energy density, cycle life, etc.; secondly, some additives used in the existing solutions may affect the safety of the battery. For example, some additives may react with other substances in the battery to produce gas, causing the battery to swell or even explode.

[0004] In order to solve the above problems, some technologies have been proposed in the prior art to provide a coating on the surface of the diaphragm to improve the heat resistance of the battery. However, the coatings in the prior art are mostly ceramics or organic matter, which play the role of isolating the positive and negative electrodes or improving the heat resistance of the diaphragm itself, and cannot suppress the thermal failure caused by crosstalk between the positive and negative electrodes. Summary of the invention

[0005] Therefore, the technical problem to be solved by the present application is to overcome the defects of lithium-ion secondary batteries in the prior art such as thermal failure caused by crosstalk between the positive and negative electrodes, thereby providing a composite diaphragm, a lithium-ion secondary battery and an electrical device.

[0006] To this end, this application provides the following technical solutions:

[0007] According to one aspect of the present application, a composite diaphragm is provided, comprising:

[0008] The base film has two opposite surfaces in its thickness direction;

[0009] A coating layer is disposed on at least one side of the base film, wherein the coating layer comprises a ketone compound and a binder, and the mass percentage of the ketone compound is 0.5%-10% based on the mass of the coating layer;

[0010] The ketone compound is a solid ketone compound or a liquid ketone compound with a boiling point of ≥100°C.

[0011] As an example, based on the mass of the coating, the mass percentage of the ketone compound in the coating can be 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or within a range consisting of any of the above values.

[0012] In some optional embodiments, the carbon number of the ketone compound is C6 to C 25 .

[0013] In some optional embodiments, the ketone compound includes at least one of methyl undecafluoropentyl ketone, fluoropyranone, penanflurone, cyclohexanone, fluoropyridone, 5-fluoroindol-2-one, 3-fluorobenzophenone, 4-fluoro-1-indanone, 4,4'-difluorobenzophenone, 2-fluorobenzophenone, and 6-fluorochromone.

[0014] The ketone compound in the present application is preferably a fluorine-containing ketone compound. The ketone compound substituted with fluorine can further improve the thermal stability, thereby improving the electrical performance of the lithium-ion secondary battery.

[0015] In some optional embodiments, the coating also includes 70%-90% of a nickel-containing catalyst, which has no electrochemical activity; as an example, the mass percentage of the nickel-containing catalyst can be 70%, 73%, 75%, 78%, 80%, 82%, 85%, 88%, 90%, or within a range composed of any of the above values.

[0016] Those skilled in the art will understand that the term "electrochemically inactive" means that lithium insertion and de-lithiation will not occur, that is, it does not participate in the charging and discharging process of the lithium-ion battery, and lithium insertion and de-lithiation will not occur, thereby avoiding affecting the battery capacity.

[0017] In addition, it should be noted that when the coating does not include a nickel-containing catalyst, the coating may include a ceramic component in addition to a ketone compound and a binder; the ceramic component may include any prior art related to ceramic coatings known in the art. The present application does not specifically limit the composition of the ceramic component. As an example, it may be aluminum oxide.

[0018] In some optional embodiments, the nickel-containing catalyst includes at least one of crystalline nickel oxide, nickel-doped crystalline aluminum oxide, nickel-doped crystalline titanium dioxide, nickel-doped crystalline lanthanum oxide, nickel-doped crystalline zirconium oxide, and crystalline nickel aluminate.

[0019] In some optional embodiments, the mass percentage of nickel in the nickel-doped crystalline aluminum oxide, nickel-doped crystalline titanium dioxide, nickel-doped crystalline lanthanum oxide or nickel-doped crystalline zirconium oxide is 0.5%-10%. As an example, the mass percentage of nickel can be 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or within the range of any of the above values.

[0020] It should be noted that in the nickel-doped crystalline alumina, nickel-doped titanium dioxide, nickel-doped crystalline lanthanum oxide or nickel-doped crystalline zirconia in the present application, nickel plays a catalytic role and the corresponding crystalline oxide particles play a supporting role, which can avoid high-temperature film shrinkage. Among them, nickel can be doped with the corresponding oxide by physical or chemical methods, and the present application does not make specific limitations.

[0021] In some optional embodiments, the coating has a thickness of 2-20 μm; as an example, the coating has a thickness of 2 μm, 5 μm, 8 μm, 10 μm, 12 μm, 15 μm, 17 μm, 20 μm, or within a range consisting of any of the above values.

[0022] In some optional embodiments, the mass percentage of the binder is 3%-20% based on the mass of the coating; as an example, the mass percentage of the binder can be 3%, 5%, 7%, 9%, 10%, 12%, 14%, 15%, 17%, 18%, 20%, or within the range of any of the above values ​​based on the mass of the coating;

[0023] In the present application, the binder is a commonly used binder in the field. As an example, the binder includes but is not limited to at least one of polyacrylic acid adhesive, polyacrylonitrile adhesive, polyvinylidene fluoride adhesive, polyvinyl alcohol adhesive, sodium alginate, and carboxymethyl chitosan.

[0024] In some optional embodiments, the thickness of the base film is 6-10 μm; as an example, the thickness of the base film may be 6 μm, 6.5 μm, 7 μm, 7.5 μm, 8 μm, 8.5 μm, 9 μm, 9.5 μm, 10 μm, or within a range consisting of any of the above values.

[0025] In the present application, the base film is conventional in the field of lithium-ion batteries. As an example, the base film includes but is not limited to at least one of a polypropylene separator, a polyethylene separator or a cellulose separator.

[0026] It should be noted that the preparation method of the composite diaphragm described in the present application is conventional in the field, and the coating can be prepared by a coating method. Specifically, the preparation method of the composite diaphragm includes: uniformly mixing components such as a ketone compound, a nickel-containing catalyst (when the component is not present, it is replaced by a ceramic component), a binder, and a solvent (for example: NMP) to obtain a slurry, uniformly coating (spraying) the prepared slurry on a base film, and after drying, forming a composite diaphragm.

[0027] According to another aspect of the present application, a lithium-ion secondary battery is provided, comprising the above-mentioned composite separator.

[0028] According to another aspect of the present application, there is provided an electrical device comprising the above-mentioned lithium-ion secondary battery.

[0029] It is understood by those skilled in the art that the lithium-ion secondary battery provided by the present application includes not only the above-mentioned composite diaphragm, but also structural parts such as a positive electrode sheet, a negative electrode sheet, an electrolyte and a shell. During the battery charging and discharging process, lithium ions are embedded and extracted back and forth between the positive electrode sheet and the negative electrode sheet, and the electrolyte plays the role of conducting ions between the positive electrode sheet and the negative electrode sheet. The composite diaphragm is arranged between the positive electrode sheet and the negative electrode sheet, mainly to prevent the positive and negative electrodes from short-circuiting, and at the same time to allow lithium ions to pass through.

[0030] As an example, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer, the positive electrode current collector has two surfaces opposite to each other in its own thickness direction, and the positive electrode active material layer is arranged on any one or both of the two opposite surfaces of the positive electrode current collector. The material, composition and manufacturing method of the positive electrode sheet used in the lithium ion secondary battery of the present application may include any technology disclosed in the prior art.

[0031] As an example, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer, the negative electrode current collector has two surfaces opposite to each other in its own thickness direction, and the negative electrode active material layer is arranged on any one or both of the two opposite surfaces of the negative electrode current collector. The material, composition and manufacturing method of the negative electrode sheet used in the lithium ion secondary battery of the present application may include any technology disclosed in the prior art.

[0032] The shape used in the lithium ion secondary battery of the present application is not particularly limited, and may include any technology disclosed in the prior art.

[0033] The electrolyte used in the lithium ion secondary battery of the present application may include any technology disclosed in the prior art.

[0034] The present application does not specifically limit the preparation method of the lithium-ion secondary battery, and the lithium-ion secondary battery can be prepared by conventional preparation methods in the art. For example, the positive electrode sheet, the composite separator and the negative electrode sheet are stacked in sequence, so that the composite separator is located between the positive electrode sheet and the negative electrode sheet, and the battery core is obtained by stacking or winding, and then the lithium-ion secondary battery of the present application is obtained through baking, liquid injection, formation, packaging and other processes.

[0035] It is understood that in the electrical equipment provided in the present application, the lithium-ion secondary battery can be used as a power source for the electrical equipment, and can also be used as an energy storage unit for the electrical equipment. The electrical equipment can be, but is not limited to, mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0036] The technical solution of this application has the following advantages:

[0037] The composite diaphragm provided by the present application includes: a base film having two opposite surfaces in its own thickness direction; a coating disposed on at least one side surface of the base film, wherein the coating includes a ketone compound and a binder, and the mass percentage of the ketone compound is 0.5%-10% based on the mass of the coating; the ketone compound is a solid ketone compound or a liquid ketone compound with a boiling point of ≥100°C. The composite diaphragm provided by the present application can effectively avoid thermal failure caused by crosstalk between positive and negative electrodes by limiting the composition of the coating, which can not only improve the thermal stability of the battery, but also will not affect other performances of the battery, such as cycle performance. Specifically, in this application, the ketone group in the ketone compound can react with the hydrogen generated at the negative electrode interface (the side reaction between the negative electrode interface and the electrolyte will produce hydrogen, and the gas production is particularly obvious at high temperature), thereby avoiding hydrogen crosstalk to the positive electrode to produce thermal runaway; when the battery is in a hot box test or high temperature state, the high temperature will accelerate the reaction of the ketone compound with the hydrogen produced by the negative electrode, thereby improving the overall thermal stability of the battery cell; the use of a binder can ensure the formability of the coating, improve the bonding strength between the coating and the base film, and avoid stratification; and the ketone compound is limited to a solid ketone compound or a liquid ketone compound with a boiling point ≥100°C in order to ensure that it can be retained in the coating and avoid the loss of ketone compounds during the drying process after coating. The idea and method of isolating thermal runaway and improving the thermal stability of the battery provided in this application are proposed by the applicant for the first time and have never appeared before.

[0038] The composite diaphragm provided in the present application also includes 70%-90% of a nickel-containing catalyst in the coating, and the nickel-containing catalyst has no electrochemical activity. In the present application, the nickel-containing catalyst can catalyze the reaction between ketone compounds and hydrogen, improve the reaction efficiency, and thus further improve the thermal stability of the battery cell; the nickel-containing catalyst described in the present application has no electrochemical activity, and such a design can avoid the nickel-containing catalyst from taking away the capacity of the negative electrode active material when the battery is charged, affecting the cycle performance; in addition, if the nickel-containing catalyst participates in charging and discharging, it means volume expansion, which will cause the coating to break and fall off, affecting the thermal stability of the lithium-ion secondary battery.

[0039] Additional aspects and advantages of the embodiments of the present application will be described and shown in part in the subsequent description, or explained through the implementation of the embodiments of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the specific implementation methods of the present application or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0041] Figure 1 It is a schematic diagram of the structure of the composite diaphragm provided in this application;

[0042] Reference numerals:

[0043] 1. Base film; 2. Coating. DETAILED DESCRIPTION

[0044] The following examples are provided for a better understanding of the present application, but are not limited to the best implementation mode described, and do not limit the content and protection scope of the present application. Any product identical or similar to the present application obtained by anyone under the inspiration of the present application or by combining the features of the present application with other prior arts shall fall within the protection scope of the present application.

[0045] If no specific experimental steps or conditions are specified in the examples, the conventional experimental steps or conditions described in the literature in the field can be used. If no manufacturer is specified for the reagents or instruments used, they are all conventional reagent products that can be obtained commercially.

[0046] In all the examples and comparative examples of the present application, the unit % represents the mass percentage.

[0047] Example 1

[0048] This embodiment provides a composite diaphragm, and its structural schematic diagram is as follows Figure 1As shown, it includes a base film 1 and a coating 2 arranged on one side of the base film 1, wherein the base film 1 is a polypropylene diaphragm (PP film) with a thickness of 10 μm. Based on the total mass of the coating, the coating includes 5% of fluoropyrafuranone, 75% of crystalline nickel aluminate powder, and 20% of polyacrylic acid adhesive (manufacturer model LA136DL), and the thickness of the coating is 4 μm;

[0049] The preparation method of the composite diaphragm comprises the following steps:

[0050] Step 1: Prepare raw materials, weigh fluoropyrafuranone, crystalline nickel aluminate powder and polyacrylic acid adhesive according to the above composition, put fluoropyrafuranone, crystalline nickel aluminate powder and polyacrylic acid adhesive into a stirrer, add organic solvent N-methylpyrrolidone (NMP), stir at a speed of 500 rpm for 30 minutes to mix the raw materials evenly to form a slurry with a solid content of 20wt%, and control the viscosity of the slurry to be suitable for spraying;

[0051] Step 2: Spray the mixed slurry evenly on one side of the base film through a spraying device, the spraying pressure is 0.5 MPa, the spraying temperature is 25°C, and the spraying time is 10 minutes;

[0052] Step 3: Put the base film sprayed with the slurry into a dryer and dry it at 80° C. for 2 hours to form a coating on the base film to obtain the composite diaphragm.

[0053] Example 2

[0054] This embodiment provides a composite diaphragm, and its structural schematic diagram is as follows Figure 1 As shown, it includes a base film 1 and a coating 2 arranged on one side of the base film, wherein the base film 1 is a PP film with a thickness of 10 μm, and based on the total mass of the coating, the coating includes 0.5% of fluoropyrafuranone, 90% of crystalline nickel aluminate powder, and 9.5% of polyacrylic acid adhesive (manufacturer model LA136DL), and the thickness of the coating is 6 μm;

[0055] The preparation method of the composite diaphragm comprises the following steps:

[0056] Step 1: Prepare raw materials, weigh fluoropyrafuranone, crystalline nickel aluminate powder and polyacrylic acid adhesive according to the above composition, put fluoropyrafuranone, crystalline nickel aluminate powder and polyacrylic acid adhesive into a stirrer, add organic solvent N-methylpyrrolidone (NMP), stir at a speed of 500 rpm for 30 minutes to mix the raw materials evenly to form a slurry with a solid content of 30wt%;

[0057] Step 2: Spray the mixed slurry evenly on one side of the base film through a spraying device, the spraying pressure is 0.5 MPa, the spraying temperature is 25°C, and the spraying time is 10 minutes;

[0058] Step 3: Put the base film sprayed with the slurry into a dryer and dry it at 80° C. for 2 hours to form a coating on the base film to obtain the composite diaphragm.

[0059] Example 3

[0060] This embodiment provides a composite diaphragm, and its structural schematic diagram is as follows Figure 1 As shown, it includes a base film 1 and a coating 2 arranged on one side of the base film, the base film 1 is a PP film with a thickness of 10 μm, and based on the total mass of the coating, the coating includes 10% of fluoropyrafuranone, 75% of crystalline nickel aluminate powder, and 15% of polyacrylic acid adhesive (manufacturer model LA136DL), and the thickness of the coating is 2 μm;

[0061] The preparation method of the composite diaphragm comprises the following steps:

[0062] Step 1: Prepare raw materials, weigh fluoropyrafuranone, crystalline nickel aluminate powder and polyacrylic acid adhesive according to the above composition, put fluoropyrafuranone, crystalline nickel aluminate powder and polyacrylic acid adhesive into a stirrer, add organic solvent N-methylpyrrolidone (NMP), stir at a speed of 500 rpm for 30 minutes to mix the raw materials evenly to form a slurry with a solid content of 15wt%;

[0063] Step 2: Spray the mixed slurry evenly on one side of the base film through a spraying device, the spraying pressure is 0.5 MPa, the spraying temperature is 25°C, and the spraying time is 10 minutes;

[0064] Step 3: Put the base film sprayed with the slurry into a dryer and dry it at 80° C. for 2 hours to form a coating on the base film to obtain the composite diaphragm.

[0065] Example 4

[0066] This embodiment provides a composite diaphragm, and its structural schematic diagram is as follows Figure 1 As shown, it includes a base film 1 and a coating 2 arranged on one side of the base film, the base film 1 is a PE film with a thickness of 10 μm, and based on the total mass of the coating, the coating includes 8% of fluoropyrafuranone, 82% of crystalline nickel aluminate powder, and 10% of polyacrylic acid adhesive (manufacturer model LA136DL), and the thickness of the coating is 3 μm;

[0067] The preparation method of the composite diaphragm comprises the following steps:

[0068] Step 1: Prepare raw materials, weigh fluoropyrafuranone, crystalline nickel aluminate powder and polyacrylic acid adhesive according to the above composition, put fluoropyrafuranone, crystalline nickel aluminate powder and polyacrylic acid adhesive into a stirrer, add organic solvent N-methylpyrrolidone (NMP), stir at a speed of 500 rpm for 30 minutes to mix the raw materials evenly to form a slurry with a solid content of 17wt%;

[0069] Step 2: Spray the mixed slurry evenly on one side of the base film through a spraying device, the spraying pressure is 0.5 MPa, the spraying temperature is 25°C, and the spraying time is 10 minutes;

[0070] Step 3: Put the base film sprayed with the slurry into a dryer and dry it at 80° C. for 2 hours to form a coating on the base film to obtain the composite diaphragm.

[0071] Example 5

[0072] This embodiment provides a composite diaphragm, which is different from Embodiment 1 in that an equal mass of crystalline nickel oxide is used to replace crystalline nickel aluminate powder.

[0073] Example 6

[0074] This embodiment provides a composite diaphragm, which differs from Embodiment 1 in that an equal mass of nickel-doped alumina is used instead of crystalline nickel aluminate powder, wherein the nickel-doped alumina is prepared by physical mixing of nickel powder and crystalline alumina (mixing is performed by ball milling in this embodiment), and the nickel content is 5%.

[0075] Example 7

[0076] This embodiment provides a composite diaphragm, which is different from Embodiment 6 in that an equal mass of crystalline alumina is used to replace the crystalline nickel aluminate powder, that is, the coating does not include a nickel-containing catalyst.

[0077] Example 8

[0078] This embodiment provides a composite diaphragm, which is different from the embodiment 1 in that an equal mass of penicillin is used instead of flupyralidone.

[0079] Example 9

[0080] This embodiment provides a composite diaphragm, which is different from the embodiment 1 in that an equal mass of cyclohexanone is used instead of fluoropyrafuranone, that is, a ketone compound that does not contain fluorine is used in the coating.

[0081] Comparative Example 1

[0082] This comparative example provides a composite diaphragm, which is different from Example 1 in that an equal mass of crystalline nickel aluminate powder is used to replace fluoropyrafurone, that is, the coating does not contain fluoropyrafurone.

[0083] Comparative Example 2

[0084] This comparative example provides a composite diaphragm, which differs from Example 1 in that an electrochemically active nickel peroxide catalyst of equal mass is used instead of the crystalline nickel aluminate powder.

[0085] Comparative Example 3

[0086] The separator of this comparative example does not include a coating layer.

[0087] Comparative Example 4

[0088] This comparative example provides a composite diaphragm, which is different from Example 1 in that the coating includes 5% of fluopyrazone and 95% of crystalline nickel aluminate powder, that is, the coating does not include a binder.

[0089] Test Case

[0090] The composite diaphragms provided in the embodiments and comparative examples were prepared into lithium ion secondary batteries, and then the electrical performance was tested. The preparation method of the lithium ion secondary battery includes the following steps:

[0091] Positive electrode preparation: The positive electrode active material Li(Ni 0.9 Co 0.05 Mn 0.05 )O2, conductive agent (SP), single-walled carbon nanotube (SWNT) and binder polyvinylidene fluoride (PVDF) are added to an appropriate amount of N-methylpyrrolidone (NMP) solvent in a mass ratio of 96:2:1:1 and stirred thoroughly. The solid content is controlled to 68wt% to prepare a positive electrode mixture slurry. Thereafter, the formed positive electrode slurry is coated on aluminum foil, and the positive electrode sheet is formed by rolling and die-cutting after drying.

[0092] Negative electrode preparation: Graphite, conductive agent (SP), thickener CMC, and binder SBR were added to an appropriate amount of deionized water in a mass ratio of 95:2:1:2 and stirred thoroughly. The solid content was controlled to 55wt% to obtain a negative electrode mixture slurry. Thereafter, the formed negative electrode slurry was coated on copper foil, and the negative electrode sheet was formed by rolling and die-cutting after drying.

[0093] Battery assembly: positive electrode sheet, negative electrode sheet and composite separator are stacked in order of negative electrode, composite separator and positive electrode, wherein the coating of the composite separator faces the negative electrode sheet, and then the tabs are welded and encapsulated with aluminum-plastic film to obtain a soft-package dry battery cell. Finally, the electrolyte is injected into the battery cell. The composition of the electrolyte is: 1M LiPF6, the solvent is ethylene carbonate (EC) and dimethyl carbonate (DMC) in a mass ratio of 1:1, and 0.5M vinylene carbonate (VC) additive is added to prepare a lithium-ion secondary battery with a capacity of 5Ah.

[0094] 1. Thermal stability test

[0095] After the fully charged battery is stabilized at room temperature, place it in a constant temperature and humidity chamber with natural or circulating air convection. The test chamber is heated to 130℃±2℃ at a rate of 5℃ / min±2℃ / min. Maintain this temperature and stop the test after 30 minutes to check whether the battery catches fire or explodes. If it does not explode, continue to increase the temperature by 5℃ at a rate of 5℃ / min±2℃ / min and maintain it for 30 minutes. Repeat this cycle until the battery temperature is out of control.

[0096] In order to prove that the composite diaphragm provided in the present application can react with hydrogen, the composite diaphragm provided in each embodiment is placed in a vacuum crucible at room temperature, and the vacuum crucible is introduced into an Ar / H2 (H2 volume content 5%) mixed gas in a glove box, then sealed and placed in a tube furnace, and the test box is heated to 130℃±2℃ at a rate of 5℃ / min±2℃ / min. After 30 minutes, the temperature is increased by 5℃ at a rate of 5℃ / min±2℃ / min and maintained for 30 minutes, and this cycle is repeated until 180 degrees. After the test, the gas in the crucible is extracted, and the hydrogen content in the gas is tested with a gas composition analyzer (model Agilent 8860). The test results of each embodiment show that the H2 content is 0%.

[0097] 2. Normal temperature performance cycle test

[0098] At 25°C, the packaged lithium-ion secondary battery was subjected to the formation and capacity division steps and then began the cycle test. It was charged to 4.25V at 1C constant current, then charged at constant voltage until the current reached 0.05C. After being left for half an hour, it was discharged at 1C constant current to obtain the initial capacity of the first cycle of the battery and recorded as C1. After that, the first week of CCCV / CC charging and discharging cycles were repeated. The capacity after the 500th cycle of discharge was recorded as C 500 , then the capacity retention rate after 500 cycles at room temperature (%) = C 500 / C1×100%.

[0099] 3. High temperature performance cycle test

[0100] At 45°C, the packaged lithium-ion battery was subjected to a cycle test after the formation and capacity division steps. It was charged to 4.25V at a constant current of 1C, and then charged at a constant voltage until the current reached 0.05C. After being left for half an hour, it was discharged at a constant current of 1C to obtain the initial capacity of the first week of the battery cycle and recorded as C1. Thereafter, the charge and discharge cycle was repeated in the CCCV / CC manner of the first week. The capacity after the 500th week of discharge was recorded as C 500 , then the capacity retention rate after 500 cycles of high temperature cycling (%) = C 500 / C1×100%.

[0101] The specific test results are shown in the table below:

[0102] Table 1

[0103]

[0104] From the data in the above table, it can be seen that in Example 1, perfluorohexanone combined with high-temperature catalysis can remove H2 generated by the side reaction between the negative electrode and the electrolyte, improve thermal stability, prevent crosstalk, and improve high-temperature and room-temperature cycle stability; compared with Example 1, Example 2 reduces the proportion of ketone compounds, and the thermal stability will decrease to a certain extent; in Example 3 and Example 4, the ketone compound content is high, which will have a certain impact on the normal / high temperature cycle performance. Therefore, the mass proportion of ketone compounds is preferably 2%-7%. In Example 5 and Example 6, the catalyst is replaced to slightly reduce the thermal stability; Example 7 has no catalyst, and the ability to remove H2 is low The thermal stability is reduced, but the high temperature and room temperature cycle performance are good; in Comparative Example 4, no binder is added, the coating falls off, the thermal stability is not improved compared with Comparative Example 3, and there is no improvement in normal / high temperature cycles; in Example 8, the ketone compounds are replaced, and the thermal stability is reduced; in Example 9, fluorine-free ketone compounds are used. Compared with Example 1, it can be seen that fluorine-containing ketone compounds can better improve the thermal stability of the battery; Comparative Example 1 does not contain fluorine pyranone, and the thermal stability is not improved; the nickel-containing catalyst selected in Comparative Example 2 has electrochemical activity, and will participate in electrochemical reactions during the charge and discharge process, which greatly affects the cycle capacity retention rate, but the thermal stability is improved.

[0105] Obviously, the above embodiments are merely examples for the purpose of clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived from these are still within the scope of protection of the invention.

Claims

1. A composite diaphragm, characterized in that: include: The base film has two opposite surfaces in its thickness direction; A coating layer is disposed on at least one side of the base film, wherein the coating layer comprises a ketone compound and a binder, and the mass percentage of the ketone compound is 0.5%-10% based on the mass of the coating layer; The ketone compound is a solid ketone compound or a liquid ketone compound with a boiling point of ≥100°C.

2. The composite diaphragm according to claim 1, characterized in that: The carbon number of the ketone compound is C6~C 25 .

3. The composite diaphragm according to claim 2, characterized in that: The ketone compound includes at least one of methyl undecafluoropentyl ketone, fluoropyranone, penanflurone, cyclohexanone, fluoropyridone, 5-fluoroindol-2-one, 3-fluorobenzophenone, 4-fluoro-1-indanone, 4,4'-difluorobenzophenone, 2-fluorobenzophenone and 6-fluorochromone.

4. The composite diaphragm according to any one of claims 1 to 3, characterized in that: The coating also includes 70%-90% of a nickel-containing catalyst, which has no electrochemical activity.

5. The composite diaphragm according to claim 4, characterized in that: The nickel-containing catalyst comprises at least one of crystalline nickel oxide, nickel-doped crystalline aluminum oxide, nickel-doped crystalline titanium dioxide, nickel-doped crystalline lanthanum oxide, nickel-doped crystalline zirconium oxide, and crystalline nickel aluminate; The mass percentage of nickel in the nickel-doped crystalline aluminum oxide, nickel-doped crystalline titanium dioxide, nickel-doped crystalline lanthanum oxide or nickel-doped crystalline zirconium oxide is 0.5%-10%.

6. The composite diaphragm according to claim 1, characterized in that: The coating has a thickness of 2-20 μm.

7. The composite diaphragm according to claim 1, characterized in that: The mass percentage of the binder is 3%-20% based on the mass of the coating; And / or, the adhesive includes at least one of polyacrylic acid adhesive, polyacrylonitrile adhesive, polyvinylidene fluoride adhesive, polyvinyl alcohol adhesive, sodium alginate, and carboxymethyl chitosan.

8. The composite diaphragm according to any one of claims 1 to 3 or 5 to 7, characterized in that: The thickness of the base film is 6-10 μm; And / or, the base film includes at least one of a polypropylene membrane, a polyethylene membrane or a cellulose membrane.

9. A lithium ion secondary battery, characterized in that: The composite diaphragm comprises the composite diaphragm according to any one of claims 1 to 8.

10. An electrical device, characterized in that: A lithium ion secondary battery comprising the lithium ion secondary battery according to claim 9.

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