A soft package lithium battery flame-retardant film and a preparation method and application thereof

Abstract

The present application relates to a kind of soft package lithium battery flame-retardant film and its preparation method and application, belong to lithium ion battery technical field.Solve the existing technology in battery cell directly adding flame retardant usually can sharply deteriorate the comprehensive performance of battery, and in module and battery package carry out flame-retardant design only act on the technology problem after cell fire.The soft package lithium battery flame-retardant film of the present application is composed of first polypropylene film, intumescent flame retardant coating and second polypropylene film arranged in sequence;Intumescent flame retardant coating is composed of ammonium polyphosphate, calcium gluconate and sodium carboxymethyl cellulose.The soft package lithium battery flame-retardant film is placed to the interior of cell with flame retardant, can play flame-retardant efficiency at the first time of cell fire, and the contact of flame retardant and electrolyte is isolated, the deterioration of electrical performance is avoided, with the advantages of efficient, good flame-retardant, green environmental protection, no toxic gas release.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, specifically relating to a flame-retardant film for soft-pack lithium batteries, its preparation method and application, and particularly to the application of the flame-retardant film for soft-pack lithium batteries in the preparation of flame-retardant cells. Background Technology

[0002] The lithium battery industry has ushered in unprecedented development opportunities. As the largest application of lithium batteries in the power sector, new energy vehicles have benefited from multiple factors such as policy support, rapid demand, cost reduction, and ample supply, showing a rapid growth trend. China is the world's largest new energy vehicle market. According to statistics from the Ministry of Industry and Information Technology, China's production and sales of new energy vehicles have ranked first in the world for six consecutive years. In 2021, China's sales of new energy passenger vehicles reached 3.334 million units, accounting for nearly half of global sales. In 2022, China's new energy vehicle production was approximately 6.584 million units, a year-on-year increase of 88.8%; the installed capacity of power batteries in the Chinese market was approximately 302.3 GWh (gigawatt-hours), a year-on-year increase of 89.7%. However, while lithium batteries are developing rapidly, a series of safety issues have also attracted widespread attention. The electrolyte, solution, and electrode materials in lithium batteries are all flammable. Therefore, battery safety has always been a top concern for researchers, manufacturers, and consumers. According to statistics, in 2022 alone, the economic and personal losses caused by battery safety accidents in China showed a doubling trend. Meanwhile, in the nearly 10 years from 2012 to 2022, a total of 32 fire and explosion accidents occurred at energy storage power stations worldwide. In the current context of the rapid development of the lithium battery industry, the safety behind its success is even more crucial: safety is the cornerstone of the battery industry's sustainable development and the very foundation upon which batteries exist.

[0003] To address the serious risks of combustion and explosion associated with lithium batteries, academia and industry have implemented various measures to suppress combustion. Adding flame retardants directly to the battery cell is a relatively straightforward method; however, the chemical properties of flame retardants often drastically degrade the overall performance of the battery. Currently, industry often focuses on innovations in battery or module structure to delay or reduce fire risks. For example, adding flame-retardant materials to the gaps between modules and battery packs can suppress the rapid spread and explosion of fire after a cell catches fire, buying time for evacuation. However, this effect only occurs after the cell has already caught fire. Summary of the Invention

[0004] One of the objectives of this invention is to provide a flame-retardant film for soft-pack lithium batteries, which has the advantages of high efficiency, good flame retardancy, green environmental protection, and no release of toxic gases.

[0005] The second objective of this invention is to provide a method for preparing a flame-retardant film for soft-pack lithium batteries. The method and equipment for preparing this flame-retardant film are compatible with existing soft-pack battery production lines, and production can be carried out without the need to purchase new equipment or modify existing production lines, thus being economical.

[0006] The third objective of this invention is to provide an application of a flame-retardant film for soft-pack lithium batteries in the preparation of flame-retardant cells and the prepared flame-retardant cells. The flame-retardant cells have a "sandwich" structure of film-intumescent flame retardant coating-film, which not only places the flame retardant inside the cell so that it can exert its flame-retardant effect at the first moment when the cell catches fire, but also isolates the flame retardant from the electrolyte, thus avoiding the deterioration of electrical performance.

[0007] The technical solution adopted by the present invention to achieve the above objectives is as follows.

[0008] The flame-retardant film for soft-pack lithium batteries of the present invention is composed of a first polypropylene film, an intumescent flame retardant coating, and a second polypropylene film arranged sequentially.

[0009] The intumescent flame retardant coating is composed of ammonium polyphosphate, calcium gluconate, and sodium carboxymethyl cellulose in a mass ratio of 100:40-10:0.5-1.

[0010] Preferably, the thickness of the flame-retardant film for the soft-pack lithium battery is 40-60 μm.

[0011] Preferably, the median particle size (D50) of the ammonium polyphosphate is not greater than 6 μm.

[0012] The method for preparing the flame-retardant film for soft-pack lithium batteries of the present invention includes the following steps:

[0013] Step 1: Weigh out ammonium polyphosphate, calcium gluconate and sodium carboxymethyl cellulose according to the ratio, stir evenly, add purified water, stir evenly, and obtain a flame retardant slurry with a viscosity of 2700-9900 mPa·s.

[0014] Step 2: Apply flame-retardant paste to the upper surface of the first polypropylene film, dry it to form a flame-retardant coating, roll it up, and then cover the upper surface of the flame-retardant coating with a second polypropylene film. After rolling, a flame-retardant film for soft-pack lithium batteries is obtained.

[0015] Preferably, in step one, after weighing ammonium polyphosphate, calcium gluconate and sodium carboxymethyl cellulose, they are stirred evenly at a stirring speed of 10-60 rpm.

[0016] Preferably, in step one, the amount of purified water added is 60-80% of the total mass of ammonium polyphosphate, calcium gluconate, and sodium carboxymethyl cellulose.

[0017] Preferably, in step one, after adding purified water, the stirring time is 4-8 hours.

[0018] Preferably, in step two, before rolling, the thickness of the first polypropylene film is 8-15 μm, the thickness of the second polypropylene film is 8-15 μm, and the thickness of the flame-retardant coating is 40-60 μm.

[0019] Preferably, in step two, when the flame-retardant slurry is coated on the upper surface of the first polypropylene film, a 4-6mm margin is left on both the left and right sides of the first polypropylene film.

[0020] Preferably, in step two, the second polypropylene film and the first polypropylene film are aligned.

[0021] Preferably, in step two, the rolling process is hot pressing, and the temperature of the main roller is 150±5℃.

[0022] The present invention also provides the application of the above-mentioned soft-pack lithium battery flame retardant film in the preparation of flame retardant cells, the steps of which are: drying the soft-pack lithium battery flame retardant film to a moisture content of less than 150 ppm, and winding the dried soft-pack lithium battery flame retardant film after the cell is stacked.

[0023] Preferably, the drying is vacuum drying, with a vacuum degree of 50-200 Pa and a drying temperature of 115-125℃.

[0024] Preferably, the winding thickness is the maximum thickness allowed for winding to the flame-retardant battery cell.

[0025] Preferably, the number of wrapping turns is 5-8.

[0026] The present invention also provides a flame-retardant cell prepared from the above-mentioned flame-retardant film for soft-pack lithium batteries.

[0027] The principle of this invention is as follows: The flame-retardant film of this invention adopts a "sandwich" structure of polypropylene film-intumescent flame retardant coating-polypropylene film. The intumescent flame retardant is composed of a carbon source, an acid source, and a gas source, and its system has a synergistic effect. It is halogen-free and does not use antimony oxide as a synergist, making it environmentally friendly. This invention adds a flame-retardant film containing an intumescent flame retardant inside the flame-retardant battery cell. During combustion, a charcoal insulating layer is generated on the surface of the flame-retardant battery cell, providing heat insulation, oxygen isolation, smoke suppression, and anti-drip effects. It has excellent flame-retardant performance, thereby directly and quickly preventing or delaying the spread of fire, reducing the risk of battery fire, and producing low smoke, low toxicity, and no corrosive gases. The two layers of polypropylene film provide good insulation to prevent contact between the flame retardant and the electrolyte, thus not affecting the original electrical performance of the flame-retardant battery cell.

[0028] Compared with the prior art, the present invention has the following advantages:

[0029] 1. The flame-retardant film of the present invention uses ammonium polyphosphate, calcium gluconate, and sodium carboxymethyl cellulose as an intumescent flame retardant. Among them, calcium gluconate has high expansion properties after heating and can work synergistically with ammonium polyphosphate. At the same time, sodium carboxymethyl cellulose acts as both a char source to participate in synergistic flame retardancy and a binder to promote the uniform coating of flame-retardant slurry on polypropylene film.

[0030] 2. The flame-retardant slurry used in the flame-retardant membrane of this invention is a halogen-free system, which has the advantages of high efficiency, good flame retardancy, green environmental protection, and no release of toxic gases. All three raw materials are mature industrial products, which are convenient for industrial production. Among them, calcium gluconate and sodium carboxymethyl cellulose are both biorenewable and biodegradable materials, which have significant environmental friendliness.

[0031] 3. The flame-retardant film of this invention utilizes a structure of polypropylene film-intumescent flame retardant coating-polypropylene film. This structure not only integrates the intumescent flame retardant into the flame-retardant battery cell, allowing it to exert its flame-retardant effect immediately upon ignition of the battery cell, but also isolates the intumescent flame retardant from the electrolyte, preventing deterioration of electrical performance. Vertical burning tests show that the flame-retardant film of this invention cannot be ignited, exhibiting excellent flame-retardant properties. Flame-retardant battery cells prepared with this film are also difficult to ignite, and their electrical performance is not significantly reduced.

[0032] 4. The method and equipment for preparing the flame-retardant film of the present invention are compatible with existing soft-pack battery production lines. Production can be carried out without the need to purchase new equipment or modify the production line, which is economical. Attached Figure Description

[0033] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0034] Figure 1 The charge-discharge cycle curves of the batteries assembled in Example 1 and Comparative Example 2 of this invention at 25°C and 1C rate are shown. Detailed Implementation

[0035] To further understand the present invention, preferred embodiments of the present invention are described below. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, and not for limiting the scope of the claims of the present invention.

[0036] The flame-retardant film for soft-pack lithium batteries of the present invention is composed of a first polypropylene film, an intumescent flame retardant coating, and a second polypropylene film arranged sequentially, usually arranged from bottom to top, but not limited thereto; the intumescent flame retardant coating is composed of ammonium polyphosphate, calcium gluconate, and sodium carboxymethyl cellulose in a mass ratio of 100:40-10:0.5-1.

[0037] In the above technical solution, the first polypropylene film and the second polypropylene film can block the contact between the intumescent flame retardant and the electrolyte, thus avoiding their impact on the battery's electrical performance.

[0038] In the above technical solution, the intumescent flame retardant coating has the characteristics of high efficiency, good flame retardancy, green source, and strong adhesion. Among them, the median particle size (D50) of ammonium polyphosphate is preferably no greater than 6μm.

[0039] In the above technical solution, the thickness of the flame-retardant film for soft-pack lithium batteries is preferably 40-60μm.

[0040] The flame-retardant film for soft-pack lithium batteries of the present invention is prepared by roll forming.

[0041] The method for preparing the flame-retardant film for soft-pack lithium batteries of the present invention includes the following steps:

[0042] Step 1: Weigh out ammonium polyphosphate, calcium gluconate and sodium carboxymethyl cellulose according to the ratio, stir evenly, add purified water, and stir to form a uniform viscous slurry. The resulting flame retardant slurry has a viscosity of 2700-9900 mPa·s (the viscosity is adjusted by adding purified water, which can be added all at once or in batches. There are no special restrictions as long as the final viscosity control requirements are met).

[0043] Step 2: Apply flame-retardant paste to the upper surface of the first polypropylene film, dry it to form a flame-retardant coating, roll it up, and then cover the upper surface of the flame-retardant coating with the second separator film. After rolling, a flame-retardant film for soft-pack lithium batteries is obtained.

[0044] In the above technical solution, in step one, after weighing ammonium polyphosphate, calcium gluconate, and sodium carboxymethyl cellulose, the mixture is preferably stirred at a speed of 10-60 rpm, gradually increasing the speed until it is homogeneous. The stirring equipment is usually a mixing tank.

[0045] In the above technical solution, after adding purified water in step one, the stirring time is preferably 4-8 hours.

[0046] In the above technical solution, in step two, the thickness of both the first and second polypropylene films is preferably controlled at 8-15 μm, and the width is cut to match the width of the flame-retardant battery cell, with a more preferred thickness of 12 μm. When coating the upper surface of the first polypropylene film with flame-retardant paste, a 4-6 mm margin is preferably left on both sides of the first polypropylene film. After coating the upper surface of the first polypropylene film with flame-retardant paste and winding it up, the thickness of the flame-retardant coating is 40-60 μm, and the total thickness of the first polypropylene film and the flame-retardant coating is preferably 48-75 μm. The second polypropylene film is preferably aligned with the first polypropylene film. A coating machine is typically used for coating. A dryer is typically used for drying.

[0047] In the above technical solution, in step two, the roll forming is preferably carried out by hot pressing, and the temperature of the main roll is 150℃.

[0048] The flame-retardant film for soft-pack lithium batteries of the present invention can be used in the preparation of flame-retardant cells. The application steps are as follows: the flame-retardant film for soft-pack lithium batteries is dried to a moisture content of less than 150 ppm. After the cells are stacked, the dried flame-retardant film for soft-pack lithium batteries is wrapped around the outside of the dry cell stack.

[0049] The above technical solution involves vacuum drying, preferably using a vacuum chamber with a vacuum level of 50-200 Pa and a temperature of 115-125°C. The process involves repeated nitrogen filling and vacuuming cycles until the moisture content of the flame-retardant film drops below 150 ppm.

[0050] The above technical solution involves a winding thickness that is the maximum allowable thickness for winding flame-retardant battery cells, typically 5-8 turns.

[0051] The method for applying the flame-retardant cell prepared from the flame-retardant film of the present invention to a soft-pack lithium battery can be carried out in accordance with the procedures known to those skilled in the art and with reference to the prior art, without any special limitations.

[0052] The terminology used in this invention generally has the meanings commonly understood by those skilled in the art, unless otherwise stated. To enable those skilled in the art to better understand the technical solutions of this invention, the invention will be further described in detail below with reference to embodiments.

[0053] In the following embodiments, various processes and methods not described in detail are conventional methods known in the art. Unless otherwise specified, the materials, reagents, apparatus, instruments, equipment, etc., used in the following embodiments are commercially available.

[0054] In the following examples and comparative examples, conventional battery cells were assembled using a nickel-cobalt-manganese ternary cathode (Ni65 product) and graphite anode system. The cathode was prepared by mixing the main material (ternary nickel-cobalt-manganese material), conductive agent (super li, CNT), binder (polyvinylidene fluoride PVDF), and N-methylpyrrolidone into a slurry, which was then coated onto aluminum foil and die-cut to obtain a cathode sheet. The anode was prepared by mixing the main material (graphite), conductive agent (CNT), binder (sodium carboxymethyl cellulose CMC), and water into a slurry, which was then coated onto copper foil and die-cut to obtain a anode sheet. The cathode sheet, anode sheet, and separator were then stacked in a "Z" shape using a stacking machine to form a dry battery cell. In the examples, a flame-retardant separator was then wound around the dry battery cell. Finally, the finished battery cell was obtained after conventional processes such as welding tabs, encapsulating with an aluminum-plastic film, electrolyte injection, and formation.

[0055] Additionally, it is worth noting that all parts by weight are used in the following examples and comparative examples, and the test data were obtained according to GB / T 31486-2015 "Electrical Performance Requirements and Test Methods for Power Batteries for Electric Vehicles" and GB 38031-2020 "Safety Requirements for Power Batteries for Electric Vehicles". The flame retardancy evaluation of the film was performed in accordance with ASTM D4804 standard.

[0056] Example 1

[0057] Weigh 100 parts of ammonium polyphosphate, 40 parts of calcium gluconate, and 0.5 parts of sodium carboxymethyl cellulose into a mixing tank. Start stirring, gradually increasing the stirring speed from 10 rpm to 60 rpm. After the powders are evenly mixed, add 60% (by weight) of purified water and stir until a homogeneous slurry is formed. Measure the viscosity of the slurry and control it to 3000±300 mPa·s by adding a small amount of purified water. Coat the prepared slurry onto a polypropylene film using a coating machine. Adjust the process to control the following: the polypropylene film thickness is 15 μm, and the clearance width on both sides of the polypropylene film is controlled to 5±1 mm. After drying in a dryer, the coating thickness after winding is 50±10 μm. During roller pressing, a layer of polypropylene film is hot-pressed onto the coated polypropylene film, with the main roller temperature at 150±5℃. Adjust the process to control the total thickness of the flame-retardant film after compaction to 50±10 μm, thus obtaining the flame-retardant film.

[0058] The flame-retardant film obtained in Example 1 was subjected to a vertical burning test. The flame-retardant film could not be ignited, indicating that it has good flame retardancy.

[0059] The flame-retardant film obtained in Example 1 was vacuum dried (moisture content below 150 ppm) and wound around the outside of the stacked dry cell to form a flame-retardant cell. Subsequent processes were completed according to the conventional battery manufacturing procedure, and the cells were assembled into a battery for electrical performance testing. The cycle performance results are as follows: Figure 1As shown, after 1000 cycles at room temperature, the capacity retention rate is 90.3%, the cell capacity is 60.5Ah, and the energy density is 262.1Wh / kg.

[0060] Example 2

[0061] Weigh 100 parts of ammonium polyphosphate, 35 parts of calcium gluconate, and 0.7 parts of sodium carboxymethyl cellulose into a mixing tank. Start stirring, gradually increasing the stirring speed from 10 rpm to 60 rpm. After the powders are evenly mixed, add 63% (by weight) of purified water and stir until a homogeneous slurry is formed. Measure the viscosity of the slurry and control it to 4500±450 mPa·s by adding a small amount of purified water. Coat the prepared slurry onto a polypropylene film using a coating machine. Adjust the process to control the following: the polypropylene film thickness is 12 μm, and the clearance width on both sides of the polypropylene film is controlled to 5±1 mm. After drying in a dryer, the coating thickness after winding is 50±10 μm. During roller pressing, a layer of polypropylene film is hot-pressed onto the coated polypropylene film, with the main roller temperature at 150±5℃. Adjust the process to control the total thickness of the flame-retardant film after compaction to 50±10 μm, thus obtaining the flame-retardant film.

[0062] The flame-retardant film obtained in Example 2 was subjected to a vertical burning test. The flame-retardant film could not be ignited, indicating that it has good flame retardancy.

[0063] The flame-retardant film obtained in Example 2 was vacuum dried (moisture content below 150 ppm) and wound around the outside of the stacked dry cell to form a flame-retardant cell. Subsequent processes were completed according to the conventional battery manufacturing procedure, and the cells were assembled into batteries for electrical performance testing. The test results were as follows: after 1000 cycles at room temperature, the capacity retention rate was 89.8%, the cell capacity was 60.9 Ah, and the energy density was 261.2 Wh / kg.

[0064] Example 3

[0065] Weigh 100 parts of ammonium polyphosphate, 30 parts of calcium gluconate, and 0.8 parts of sodium carboxymethyl cellulose into a mixing tank. Start stirring, gradually increasing the stirring speed from 10 rpm to 60 rpm. After the powders are evenly mixed, add 66% (by weight) of purified water and stir until a homogeneous slurry is formed. Measure the viscosity of the slurry and control it to 5000±500 mPa·s by adding a small amount of purified water. Coat the prepared slurry onto a polypropylene film using a coating machine. Adjust the process to control the following: the polypropylene film thickness is 12 μm, and the clearance width on both sides of the polypropylene film is controlled to 5±1 mm. After drying in a dryer, the coating thickness after winding is 50±10 μm. During roller pressing, a layer of polypropylene film is hot-pressed onto the coated polypropylene film, with the main roller temperature at 150±5℃. Adjust the process to control the total thickness of the flame-retardant film after compaction to 50±10 μm, thus obtaining the flame-retardant film.

[0066] The flame-retardant film obtained in Example 3 was subjected to a vertical burning test. The flame-retardant film could not be ignited, indicating that it has good flame retardancy.

[0067] The flame-retardant film obtained in Example 3 was vacuum dried (moisture content below 150 ppm) and wound around the outside of the stacked dry cell to form a flame-retardant cell. Subsequent processes were completed according to conventional battery manufacturing procedures, and the cells were assembled into batteries for electrical performance testing. The test results were as follows: after 1000 cycles at room temperature, the capacity retention rate was 91.1%, the cell capacity was 60.9 Ah, and the energy density was 262.6 Wh / kg.

[0068] Example 4

[0069] Weigh 100 parts of ammonium polyphosphate, 25 parts of calcium gluconate, and 0.7 parts of sodium carboxymethyl cellulose into a mixing tank. Start stirring, gradually increasing the stirring speed from 10 rpm to 60 rpm. After the powders are evenly mixed, add 68% (by weight) of purified water and stir until a homogeneous slurry is formed. Measure the viscosity of the slurry and control it to 5000±500 mPa·s by adding a small amount of purified water. Coat the prepared slurry onto a polypropylene film using a coating machine. Adjust the process to control the following: the polypropylene film thickness is 12 μm, and the clearance width on both sides of the polypropylene film is controlled to 5±1 mm. After drying in a dryer, the coating thickness after winding is 50±10 μm. During roller pressing, a layer of polypropylene film is hot-pressed onto the coated polypropylene film, with the main roller temperature at 150±5℃. Adjust the process to control the total thickness of the flame-retardant film after compaction to 50±10 μm, thus obtaining the flame-retardant film.

[0070] The flame-retardant film obtained in Example 4 was subjected to a vertical burning test. The flame-retardant film could not be ignited, indicating that it has good flame retardancy.

[0071] The flame-retardant film obtained in Example 4 was vacuum dried (moisture content below 150 ppm) and wound around the outside of the stacked dry cell to form a flame-retardant cell. Subsequent processes were completed according to the conventional battery manufacturing procedure, and the cells were assembled into batteries for electrical performance testing. The test results were as follows: after 1000 cycles at room temperature, the capacity retention rate was 90.3%, the cell capacity was 61.0 Ah, and the energy density was 261.1 Wh / kg.

[0072] Example 5

[0073] Weigh 100 parts of ammonium polyphosphate, 20 parts of calcium gluconate, and 0.7 parts of sodium carboxymethyl cellulose into a mixing tank. Start stirring, gradually increasing the stirring speed from 10 rpm to 60 rpm. After the powders are evenly mixed, add 70% (by weight) of purified water and stir until a homogeneous slurry is formed. Measure the viscosity of the slurry and control it to 6500±650 mPa·s by adding a small amount of purified water. Coat the prepared slurry onto a polypropylene film using a coating machine. Adjust the process to control the following: the polypropylene film thickness is 12 μm, and the clearance width on both sides of the polypropylene film is controlled to 5±1 mm. After drying in a dryer, the coating thickness after winding is 50±10 μm. During roller pressing, a layer of polypropylene film is hot-pressed onto the coated polypropylene film, with the main roller temperature at 150±5℃. Adjust the process to control the total thickness of the flame-retardant film after compaction to 50±10 μm, thus obtaining the flame-retardant film.

[0074] The flame-retardant film obtained in Example 5 was subjected to a vertical burning test. The flame-retardant film could not be ignited, indicating that it has good flame retardancy.

[0075] The flame-retardant film obtained in Example 5 was vacuum dried (moisture content below 150 ppm) and wound around the outside of the stacked dry cell to form a flame-retardant cell. Subsequent processes were completed according to the conventional battery manufacturing procedure, and the cells were assembled into batteries for electrical performance testing. The test results were as follows: after 1000 cycles at room temperature, the capacity retention rate was 90.9%, the cell capacity was 60.2 Ah, and the energy density was 261.7 Wh / kg.

[0076] Example 6

[0077] Weigh 100 parts of ammonium polyphosphate, 15 parts of calcium gluconate, and 0.9 parts of sodium carboxymethyl cellulose into a mixing tank. Start stirring, gradually increasing the stirring speed from 10 rpm to 60 rpm. After the powders are evenly mixed, add 75% (by weight) of purified water and stir until a homogeneous slurry is formed. Measure the viscosity of the slurry and control it to 7300±730 mPa·s by adding a small amount of purified water. Coat the prepared slurry onto a polypropylene film using a coating machine. Adjust the process to control the following: the polypropylene film thickness is 15 μm, and the clearance width on both sides of the polypropylene film is controlled to 5±1 mm. After drying in a dryer, the coating thickness after winding is 50±10 μm. During roller pressing, a layer of polypropylene film is hot-pressed onto the coated polypropylene film, with the main roller temperature at 150±5℃. Adjust the process to control the total thickness of the flame-retardant film after compaction to 50±10 μm, thus obtaining the flame-retardant film.

[0078] The flame-retardant film obtained in Example 6 was subjected to a vertical burning test. The flame-retardant film could not be ignited, indicating that it has good flame retardancy.

[0079] The flame-retardant film obtained in Example 6 was vacuum dried (moisture content below 150 ppm) and wound around the outside of the stacked dry cell to form a flame-retardant cell. Subsequent processes were completed according to conventional battery manufacturing procedures, and the cells were assembled into batteries for electrical performance testing. The test results were as follows: after 1000 cycles at room temperature, the capacity retention rate was 91.6%, the cell capacity was 60.4 Ah, and the energy density was 262.3 Wh / kg.

[0080] Example 7

[0081] Weigh 100 parts of ammonium polyphosphate, 10 parts of calcium gluconate, and 1 part of sodium carboxymethyl cellulose into a mixing tank. Start stirring, gradually increasing the stirring speed from 10 rpm to 60 rpm. After the powders are evenly mixed, add 80% (by weight) of purified water and stir until a homogeneous slurry is formed. Measure the viscosity of the slurry and control it to 9000±900 mPa·s by adding a small amount of purified water. Coat the prepared slurry onto a polypropylene film using a coating machine. Adjust the process to control the following: the polypropylene film thickness is 8 μm, and the clearance width on both sides of the polypropylene film is controlled to 5±1 mm. After drying in a dryer, the coating thickness after winding is 50±10 μm. During roller pressing, a layer of polypropylene film is hot-pressed onto the coated polypropylene film, with the main roller temperature at 150±5℃. Adjust the process to control the total thickness of the flame-retardant film after compaction to 50±10 μm, thus obtaining the flame-retardant film.

[0082] The flame-retardant film obtained in Example 7 was subjected to a vertical burning test. The flame-retardant film could not be ignited, indicating that it has good flame retardancy.

[0083] The flame-retardant film obtained in Example 7 was vacuum dried (moisture content below 150 ppm) and wound around the outside of the stacked dry cell to form a flame-retardant cell. Subsequent processes were completed according to conventional battery manufacturing procedures, and the cells were assembled into batteries for electrical performance testing. The test results were as follows: after 1000 cycles at room temperature, the capacity retention rate was 90.9%, the cell capacity was 61.4 Ah, and the energy density was 263.2 Wh / kg.

[0084] Comparative Example 1

[0085] Two 15μm thick polypropylene films are hot-pressed together with the main roller at a temperature of 150±5℃ to obtain the film to be tested.

[0086] The test film obtained in Comparative Example 1 was subjected to a vertical burning test. The test film was directly ignited, could not self-extinguish, and burned completely.

[0087] The test film obtained from Comparative Example 1 was dried (moisture content below 150 ppm) and wound around the outside of the stacked dry cell to form a flame-retardant cell. Subsequent processes were completed according to the standard battery manufacturing procedure, and the cells were assembled into a battery for electrical performance testing. The test results were as follows: after 1000 cycles at room temperature, the capacity retention rate was 91.1%, the cell capacity was 60.8 Ah, and the energy density was 262.5 Wh / kg.

[0088] Comparative Example 2

[0089] The battery was assembled according to the method in Example 1, with no additional thin film wrapped around the stacked electrodes. The assembled battery was then subjected to electrical performance testing, and the cycle test results are as follows: Figure 1 As shown, after 1000 cycles at room temperature, the capacity retention rate is 91.8%, the cell capacity is 61.9Ah, and the energy density is 264.1Wh / kg.

[0090] from Figure 1 It can be seen that after adding the flame-retardant film (Example 1), the battery capacity retention rate did not decrease significantly compared with the blank group (Comparative Example 2).

[0091] Comparative Example 3

[0092] 100 parts of ammonium polyphosphate, 40 parts of calcium gluconate, and 0.4 parts of sodium carboxymethyl cellulose were weighed into a mixing tank. The mixing speed was gradually increased from 10 rpm to 60 rpm. After the powders were evenly mixed, 60% (by weight) of purified water was added, and the mixture was stirred until a homogeneous slurry was formed. The viscosity of the slurry was measured to be only 2000 ± 100 mPa·s. The prepared slurry was then coated onto a polypropylene film using a coating machine. Through process adjustments, the following parameters were controlled: the polypropylene film thickness was 15 μm, and the allowable width on both sides of the polypropylene film was controlled to be 5 ± 1 mm. Due to the excessively thin slurry, a uniform and stable coating could not be formed despite trying various process conditions. Subsequent preparation was therefore impossible.

[0093] Comparative Example 4

[0094] 100 parts of ammonium polyphosphate, 10 parts of calcium gluconate, and 1.1 parts of sodium carboxymethyl cellulose were weighed into a mixing tank. The mixing speed was gradually increased from 10 rpm to 60 rpm. After the powders were evenly mixed, 80% of the total powder mass of purified water was added. Due to the high content of sodium carboxymethyl cellulose, the mixing time was long and the mixing torque was high, resulting in additional energy costs. The mixture was stirred for an extended period until a homogeneous slurry was formed. The viscosity of the slurry was measured, and it was only through repeated small additions of purified water that the viscosity was controlled to 9000±900 mPa·s. However, the stability of the slurry decreased; during the transfer from the slurry to the coating process, some slurry settled and agglomerated. The prepared slurry was coated onto a polypropylene film using a coating machine. Through process adjustments, the following parameters were controlled: the polypropylene film thickness was 8 μm, and the clearance width on both sides of the polypropylene film was controlled to 5±1 mm. However, after multiple attempts, it was impossible to guarantee the formation of a uniform and stable coating in the later stages of coating. Subsequent preparation could not proceed.

[0095] Comparative Example 5

[0096] Weigh 100 parts of ammonium polyphosphate, 45 parts of calcium gluconate, and 0.5 parts of sodium carboxymethyl cellulose into a mixing tank. Start stirring, gradually increasing the stirring speed from 10 rpm to 60 rpm. After the powders are evenly mixed, add 60% (by weight) of purified water and stir until a homogeneous slurry is formed. Measure the viscosity of the slurry and control it to 3000±300 mPa·s by adding a small amount of purified water. Coat the prepared slurry onto a polypropylene film using a coating machine. Adjust the process to control the following: the polypropylene film thickness is 15 μm, and the clearance width on both sides of the polypropylene film is controlled to 5±1 mm. After drying in a dryer, the coating thickness after winding is 50±10 μm. During roller pressing, a layer of polypropylene film is hot-pressed onto the coated polypropylene film, with the main roller temperature at 150±5℃. Adjust the process to control the total thickness of the flame-retardant film after compaction to 50±10 μm, thus obtaining the flame-retardant film.

[0097] Vertical burning tests were conducted on the flame-retardant films obtained in Comparative Example 5. Some of the flame-retardant film samples were ignited, indicating that the flame retardancy was unstable and failed the test. Analysis suggests that the excessively high calcium gluconate content caused its expansion rate to be too fast upon heating; the catalytic char formation rate of ammonium polyphosphate did not match the expansion rate of calcium gluconate; resulting in an insufficiently dense carbon layer formed by the intumescent flame retardant, which was then broken through by the flame and unable to effectively exert its flame-retardant effect.

[0098] Comparative Example 6

[0099] Weigh 100 parts of ammonium polyphosphate, 5 parts of calcium gluconate, and 1 part of sodium carboxymethyl cellulose into a mixing tank. Start stirring, gradually increasing the stirring speed from 10 rpm to 60 rpm. After the powders are evenly mixed, add 80% (by weight) of purified water and stir until a homogeneous slurry is formed. Measure the viscosity of the slurry and control it to 9000±900 mPa·s by adding a small amount of purified water. Coat the prepared slurry onto a polypropylene film using a coating machine. Adjust the process to control the following: the polypropylene film thickness is 8 μm, and the clearance width on both sides of the polypropylene film is controlled to 5±1 mm. After drying in a dryer, the coating thickness after winding is 50±10 μm. During roller pressing, a layer of polypropylene film is hot-pressed onto the coated polypropylene film, with the main roller temperature at 150±5℃. Adjust the process to control the total thickness of the flame-retardant film after compaction to 50±10 μm, thus obtaining the flame-retardant film.

[0100] Vertical burning tests were conducted on the flame-retardant films obtained in Comparative Example 6. Most of the films were ignited, indicating poor flame retardancy. Analysis suggests that when the calcium gluconate content is low, its expanded volume is too small to effectively coat the sample, thus reducing its flame-retardant efficacy and failing to meet requirements.

[0101] Obviously, the above embodiments are merely examples for clear illustration and are not intended to limit the embodiments. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all embodiments here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A flame-retardant film for soft-pack lithium batteries, characterized in that, It consists of a first polypropylene film, an intumescent flame retardant coating, and a second polypropylene film arranged sequentially. The intumescent flame retardant coating is composed of ammonium polyphosphate, calcium gluconate, and sodium carboxymethyl cellulose in a mass ratio of 100:40-10:0.5-1. The preparation method of the flame-retardant film for the soft-pack lithium battery includes the following steps: Step 1: Weigh out ammonium polyphosphate, calcium gluconate and sodium carboxymethyl cellulose according to the ratio, stir evenly, add purified water, stir evenly, and obtain a flame retardant slurry with a viscosity of 2700-9900 mPa·s. Step 2: Apply flame retardant paste to the upper surface of the first polypropylene film, dry it to form a flame retardant coating, roll it up, and then cover the upper surface of the flame retardant coating with the second polypropylene film. After rolling, a flame retardant film for soft-pack lithium batteries is obtained. In step two, before rolling, the thickness of the first polypropylene film is 8-15 μm, the thickness of the second polypropylene film is 8-15 μm, and the thickness of the flame-retardant coating is 40-60 μm. When coating the upper surface of the first polypropylene film with flame retardant paste, leave 4-6mm on both sides of the first polypropylene film. The second polypropylene film is aligned with the first polypropylene film.

2. The flame-retardant film for soft-pack lithium batteries according to claim 1, characterized in that, The thickness of the flame-retardant film for the soft-pack lithium battery is 40-60 μm.

3. The flame-retardant film for soft-pack lithium batteries according to claim 1, characterized in that, The median particle size of the ammonium polyphosphate is no greater than 6 μm.

4. The method for preparing the flame-retardant film for soft-pack lithium batteries according to any one of claims 1-3, characterized in that, Includes the following steps: Step 1: Weigh out ammonium polyphosphate, calcium gluconate and sodium carboxymethyl cellulose according to the ratio, stir evenly, add purified water, stir evenly, and obtain a flame retardant slurry with a viscosity of 2700-9900 mPa·s. Step 2: Apply flame retardant paste to the upper surface of the first polypropylene film, dry it to form a flame retardant coating, roll it up, and then cover the upper surface of the flame retardant coating with a second polypropylene film. After rolling, a flame retardant film for soft-pack lithium batteries is obtained.

5. The method for preparing the flame-retardant film for soft-pack lithium batteries according to claim 4, characterized in that, In step one, Weigh out ammonium polyphosphate, calcium gluconate and sodium carboxymethyl cellulose, and stir at a speed of 10-60 rpm until homogeneous. The amount of purified water added is 60-80% of the total mass of ammonium polyphosphate, calcium gluconate, and sodium carboxymethyl cellulose. After adding purified water, stir for 4-8 hours.

6. The method for preparing the flame-retardant film for soft-pack lithium batteries according to claim 4, characterized in that, In step two, before rolling, the thickness of the first polypropylene film is 8-15 μm, the thickness of the second polypropylene film is 8-15 μm, and the thickness of the flame-retardant coating is 40-60 μm. When coating the upper surface of the first polypropylene film with flame retardant paste, leave 4-6mm on both sides of the first polypropylene film. The second polypropylene film is aligned with the first polypropylene film.

7. The method for preparing the flame-retardant film for soft-pack lithium batteries according to claim 4, characterized in that, In step two, the rolling process is performed using hot pressing, with the main roller temperature at 150±5℃.

8. The application of the flame-retardant film for soft-pack lithium batteries prepared by the method of any one of claims 1-3 or any one of claims 4-7 in the preparation of flame-retardant battery cells, characterized in that, The flame-retardant film for soft-pack lithium batteries is dried to a moisture content of less than 150 ppm, and then the dried flame-retardant film is wound around the cells after they are stacked.

9. The application of the flame-retardant film for soft-pack lithium batteries according to claim 8 in the preparation of flame-retardant cells, characterized in that, The drying process is vacuum drying, with a vacuum degree of 50-200Pa and a drying temperature of 115-125℃; the winding thickness is the maximum allowable thickness for winding to the flame-retardant battery cell.

10. A flame-retardant battery cell prepared from the flame-retardant film of a soft-pack lithium battery prepared by any of the methods described in claims 1-3 or 4-7.

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