Diaphragm, battery, and electric device

Abstract

The application relates to a diaphragm, a battery and an electric device. The diaphragm comprises a base film and a foaming layer arranged on one side of the base film, wherein the foaming layer comprises composite particles, the composite particles are a melt blend of polymethyl methacrylate and polyvinylidene fluoride, and the foaming temperature T of the composite particles ranges from 110 DEG C to 170 DEG C.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a separator, a battery and an electric device. BACKGROUND

[0002] Lithium ion batteries can cause a series of side reactions and release a large amount of heat in a short time when used under electrical abuse (such as rapid charging and discharging) and thermal abuse (such as overheating), thereby continuously increasing the internal temperature of the battery and eventually causing a fire, which can cause serious safety problems. The common safety problems of lithium ion batteries mainly occur at the separator, and the function of the separator is lost under the abuse of the battery. Specifically, external extrusion, overcharge / discharge and uncontrollable lithium dendrite growth can cause the separator to be punctured and broken, and the blocking of the pores of the separator, the shrinkage and melting rupture of the separator due to the excessively high internal temperature can all cause safety problems such as combustion and explosion, thereby making the safety performance of the battery poor. SUMMARY

[0003] In view of this, the present application provides a separator, a battery and an electric device, which can effectively expand when the battery is in an overcharged state or an initial thermal runaway state to protect the battery, so that the battery has good safety performance in an overcharged state or a thermal runaway state.

[0004] The present application provides a separator, which comprises a base film and a foaming layer, the foaming layer is arranged on one side of the base film, the foaming layer comprises composite particles, the composite particles are a melt blend of polymethyl methacrylate and polyvinylidene fluoride, and the foaming temperature T of the composite particles ranges from 110°C to 170°C.

[0005] Further, in the composite particles, the mass fraction of the polymethyl methacrylate is greater than the mass fraction of the polyvinylidene fluoride.

[0006] Further, in the composite particles, the mass fraction a of the polyvinylidene fluoride ranges from 10% to less than 50%.

[0007] Further, the median particle size D of the composite particles ranges from 0.5 μm to 1.5 μm.

[0008] Further, after the composite particles are foamed, the expansion ratio a of the composite particles ranges from 1.5 to 15, wherein the expansion ratio of the composite particles is the ratio of the volume of the composite particles after volume expansion to the volume of the composite particles before volume expansion.

[0009] Further, the thickness h of the foaming layer ranges from 1 μm to 5 μm.

[0010] Furthermore, among the raw materials in the composite particles, the glass transition temperature T1 of the polymethyl methacrylate is in the range of 80°C ≤ T1 ≤ 120°C, and the melting temperature T2 of the polyvinylidene fluoride is in the range of 140°C ≤ T2 ≤ 160°C.

[0011] Furthermore, the composite particles are prepared by a melt-blending thermoforming process, and the thermoforming process includes at least one of extrusion molding, injection molding or calendering molding.

[0012] The present application provides a battery, which includes: a positive electrode sheet, a diaphragm provided in the present application, a negative electrode sheet and an electrolyte, wherein the diaphragm is arranged on one side of the positive electrode sheet; the negative electrode sheet is arranged on the side of the diaphragm away from the positive electrode sheet; the electrolyte is used to infiltrate at least a portion of the positive electrode sheet, the diaphragm and the negative electrode sheet.

[0013] The present application also provides an electrical device, which includes: a device body and a battery provided in the present application, and the battery supplies power to the device body.

[0014] In the present application, the diaphragm includes a base film and a foaming layer, and the foaming layer includes composite particles, which are a blend of the polymethyl methacrylate and the polyvinylidene fluoride. Among the composite particles, the polymethyl methacrylate is used as a foaming material. When the diaphragm is applied to a battery and the carbon dioxide content in the battery is high, PMMA can absorb a large amount of carbon dioxide to cause the volume of the foaming layer and the diaphragm to expand and touch the battery shell, thereby reducing the overcharge time of the battery and improving the safety performance of the battery in the case of overcharge. In addition, when the volume of the diaphragm expands and compresses the gas inside the battery, the air pressure inside the battery increases, which is conducive to opening the explosion-proof valve of the battery in advance, avoiding direct ignition due to excessive heat when the battery valve is opened, so that the battery also has better safety performance in the case of thermal runaway. In the composite particles, the role of the polyvinylidene fluoride is to assist PMMA in foaming. When the carbon dioxide in the environment of the composite particles reaches a certain concentration, the polyvinylidene fluoride crystallizes and the polyvinylidene fluoride produces a plasticizing effect during the crystallization process, thereby promoting the formation of bubble holes in the polymethyl methacrylate, thereby making the composite particles have a faster foaming rate and a higher foaming ratio. The polyvinylidene fluoride and the polymethyl methacrylate are melt-blended to form the composite particles. Compared with the agglomerates formed by the polyvinylidene fluoride particles and the polymethyl methacrylate particles, the degree of mixing of the polyvinylidene fluoride and the polymethyl methacrylate in the composite particles is higher, resulting in changes in the structure of the polyvinylidene fluoride and the structure of the polymethyl methacrylate. The polymethyl methacrylate can lower the crystallization temperature of the polyvinylidene fluoride and reduce the amount of carbon dioxide adsorbed to induce the crystallization of the polyvinylidene fluoride, making the polyvinylidene fluoride easier to crystallize, and ultimately allowing the composite particles to successfully foam at a lower temperature. By melt-blending the polymethyl methacrylate and polyvinylidene fluoride, the two interact, resulting in a foaming temperature (T) of the composite particles that satisfies the range of 110°C ≤ T ≤ 170°C. In other words, the addition of polyvinylidene fluoride to the composite particles effectively lowers the foaming temperature of the polymethyl methacrylate. This allows the composite particles to serve as a foaming material under low-pressure conditions when the battery is overcharged or in the early stages of thermal runaway. Timely foaming shortens the battery overcharge time and allows the explosion-proof valve to open earlier, reducing the concentration of combustible gases when the explosion-proof valve opens and preventing excessive battery heat from directly igniting the battery when the valve opens. This improves the safety of the battery in overcharge and thermal runaway conditions. Furthermore, the foamed composite particles exhibit excellent thermal insulation properties, blocking heat transfer when the battery experiences thermal runaway, reducing the probability of heat spread and further improving the battery's safety.When the foaming temperature T of the composite particles satisfies the range 110℃≤T≤170℃, the foaming temperature of the composite particles is within a reasonable range, compared with a scheme of only setting the polymethyl methacrylate as a foaming material, the composite particles provided by the present application further include the polyvinylidene fluoride, and the polyvinylidene fluoride and the polymethyl methacrylate have a high fusion degree, the plasticizing effect of the polyvinylidene fluoride when crystallizing can promote the polymethyl methacrylate to form bubble holes and foam, and the polymethyl methacrylate can reduce the crystallization temperature of the polyvinylidene fluoride and reduce the adsorption amount of carbon dioxide inducing the crystallization of the polyvinylidene fluoride, so that the composite particles can rapidly foam when the temperature of the battery is abnormal. When the battery is in an overcharged state or an overheated state, the composite particles can expand in time to achieve early valve opening of the battery, reduce the heat when the explosion-proof valve is opened, so that the battery has better safety performance in the overcharged state, and the probability of thermal runaway of the battery can be reduced. When the foaming temperature of the composite particles is too low, the composite particles may foam too early and cause the battery to open the valve, reducing the reliability of the battery. When the foaming temperature of the composite particles is too high, the composite particles absorb carbon dioxide and expand too late, which cannot expand in time to reduce the damage of overcharging and thermal runaway to the battery, and the safety performance of the battery is poor. BRIEF DESCRIPTION OF DRAWINGS

[0015] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed in the embodiments will be briefly introduced as follows. Obviously, the drawings in the following description are some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0016] Figure 1 A simple structure schematic diagram of a battery of an embodiment of the present application;

[0017] Figure 2 A structure schematic diagram of a separator of an embodiment of the present application;

[0018] Figure 3 A scanning electron microscope spectrum of an agglomerate formed by directly dispersing and blending the polymethyl methacrylate and the polyvinylidene fluoride in raw materials;

[0019] Figure 4 A scanning electron microscope spectrum of a composite particle of an embodiment of the present application;

[0020] Figure 5 A foaming process schematic diagram of a foaming layer in a battery of an embodiment of the present application;

[0021] Figure 6 A foaming principle schematic diagram of a composite particle of an embodiment of the present application;

[0022] Figure 7 This is a schematic structural diagram of a diaphragm according to another embodiment of the present application;

[0023] Figure 8 This is a schematic structural diagram of an electrical device according to an embodiment of the present application;

[0024] Figure 9 This is a circuit block diagram of an electrical device according to an embodiment of the present application.

[0025] Description of reference numerals:

[0026] 100-diaphragm, 110-base film, 120-foaming layer, 121-composite particles, 130-heat-resistant coating, 200-battery, 210-positive electrode sheet, 220-negative electrode sheet, 230-electrolyte, 240-shell, 250-top cover, 260-receiving chamber, 300-electrical equipment, 310-equipment body. DETAILED DESCRIPTION

[0027] The following will be combined with the drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0028] The terms "first," "second," and the like in the specification and claims of this application and the accompanying drawings are used to distinguish between different objects, not to describe a particular order. Furthermore, the terms "including," "having," and any variations thereof, are intended to cover non-exclusive inclusions. For example, a process, method, system, product, or apparatus comprising a series of steps or elements is not limited to the listed steps or elements but may optionally include steps or elements not listed, or may optionally include other steps or elements inherent to the process, method, product, or apparatus.

[0029] References herein to "embodiments" or "implementations" mean that a particular feature, structure, or characteristic described in connection with the embodiments or implementations may be included in at least one embodiment of the present application. The appearance of such phrases in various places in the specification does not necessarily refer to the same embodiment, nor do they constitute independent or alternative embodiments that are mutually exclusive of other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0030] When lithium-ion batteries are used under electrical abuse (such as rapid charge and discharge) and thermal abuse (such as overheating), they will trigger a series of side reactions and release a large amount of heat in a short period of time, causing the internal temperature of the battery to continue to rise, eventually causing a fire and causing serious safety problems. Common safety problems of lithium-ion batteries mainly occur in the electrolyte and diaphragm. The diaphragm loses its function under battery abuse. Specifically, external extrusion, overcharge / discharge and uncontrollable lithium dendrite growth can cause the diaphragm to be punctured and ruptured, while diaphragm pore blockage, diaphragm shrinkage due to excessive internal temperature and melting and rupture can all cause safety problems such as combustion and explosion, resulting in poor battery safety performance.

[0031] At present, the most widely used polyolefin microporous membranes in lithium-ion batteries, such as polyethylene (PE) and polypropylene (PP), have a low melting point, which makes the thermal stability of the diaphragm poor. When the battery temperature is too high, the polyolefin microporous membrane will shrink thermally, and the mechanical properties will drop sharply, eventually leading to a short circuit in the battery. The current patents mainly aim to improve the overcharge safety of lithium-ion batteries by adding special additives to the electrolyte to establish a self-protection mechanism inside the battery, or by coating a layer of low-melting-point polymer coating on the surface of the diaphragm to cause closed pores at high temperatures, thereby increasing polarization and shortening the overcharge time to achieve overcharge safety improvement. For thermal runaway, the heat resistance of the diaphragm is mainly enhanced by introducing a high-heat-resistant coating to avoid internal short circuits in the battery due to thermal shrinkage and melting of the diaphragm after thermal runaway, which may cause fire, explosion and other dangers. However, none of the above improvement ideas can simultaneously improve the safety performance of lithium-ion batteries during overcharge and thermal runaway.

[0032] The inventors have found through research that when the battery is in a condition of electrical abuse or thermal abuse, side reactions are likely to occur between the positive electrode and the electrolyte, and between the negative electrode and the electrolyte, thereby releasing a large amount of heat, a large amount of flammable and toxic gases. The gas released by the battery heat is composed of a variety of flammable components and is an important cause of fire and explosion in thermal safety tests. The composition of the gas is related to the positive electrode material and SOC value of the battery, but the main components are similar, including but not limited to: carbon dioxide CO2, hydrogen H2, carbon monoxide CO, and small molecule hydrocarbons. If the positive electrode material of the positive electrode is lithium iron phosphate (LFP), CO2 accounts for more than 50% of the gas produced. In the description of this application, the gas formed is carbon dioxide as an example. It should not be understood that the gas produced by the side reactions between the positive electrode and the electrolyte, and between the negative electrode and the electrolyte is only carbon dioxide.

[0033] The inventors discovered that when the electrolyte in the battery reacts with the positive and / or negative electrodes and generates gas, the pressure inside the battery gradually increases as the gas accumulates until the safety valve opens. Once the combustible gas is sprayed and mixes with the oxygen-containing air in the surrounding environment, its concentration, once reaching its flammable limit, is easily ignited by the hot battery surface and the ejected hot particles. The combustion of the gas can further ignite the electrolyte and cause thermal runaway in adjacent batteries, causing the fire to spread and expand.

[0034] See Figure 1 The present application provides a battery 200, which includes: a positive electrode sheet 210, a diaphragm 100 provided in the present application, a negative electrode sheet 220 and an electrolyte 230, wherein the diaphragm 100 is arranged on one side of the positive electrode sheet 210; the negative electrode sheet 220 is arranged on the side of the diaphragm 100 away from the positive electrode sheet 210; the electrolyte 230 is used to infiltrate at least part of the positive electrode sheet 210, the diaphragm 100 and the negative electrode sheet 220.

[0035] It can be understood that the positive electrode sheet 210 , the separator 100 and the negative electrode sheet 220 are stacked, and the positive electrode sheet 210 , the separator 100 and the negative electrode sheet 220 form an electrode assembly.

[0036] In this embodiment, the battery 200 includes the positive electrode sheet 210, the separator 100, the negative electrode sheet 220, and the electrolyte 230. The separator 100 includes a base film 110 and a foaming layer 120. The foaming layer 120 is disposed on one side of the base film 110 and includes composite particles 121. The composite particles 121 are a blend of polymethyl methacrylate and polyvinylidene fluoride. The foaming temperature T of the composite particles 121 is in the range of 110°C ≤ T ≤ 170°C. The polymethyl methacrylate and polyvinylidene fluoride in the composite particles 121 interact with each other. The polymethyl methacrylate can lower the crystallization temperature of the polyvinylidene fluoride and reduce the amount of carbon dioxide adsorption required to induce PVDF crystallization. The plasticizing effect of the polyvinylidene fluoride during the crystallization process is conducive to promoting the formation of bubble holes inside the polymethyl methacrylate, thereby making the composite particles 121 have a faster foaming rate and a higher foaming ratio. The polymethyl methacrylate and the polyvinylidene fluoride cooperate with each other, so that the foaming temperature of the composite particles 121 is low and the foaming performance is good. When the battery 200 is in an overcharged state or the internal temperature of the battery 200 rises abnormally, the composite particles 121 can foam and expand in time at the beginning of the thermal runaway of the battery 200, thereby shortening the overcharge time and reducing the excessive heat generated by the battery 200 during overcharging. It can also open the explosion-proof valve in advance, reducing the concentration of combustible gas ejected when the explosion-proof valve opens, thereby reducing the probability of combustible gas mixing with oxygen-containing air and then burning, thereby reducing the probability of thermal runaway of the battery 200. In addition, after foaming and expansion, the foaming layer 120 can also serve as a thermal insulation material to isolate heat transfer and reduce the probability of heat spread. The battery 200 has good safety performance in an overcharge state and a thermal runaway state.

[0037] Optionally, the battery 200 includes a housing 240 for housing the positive electrode sheet 210, the separator 100, the negative electrode sheet 220, and the electrolyte 230. When the battery 200 is in a normal charge and discharge cycle, a gap exists between the electrode assembly and the housing 240. When the battery 200 is overcharged, if the electrode assembly expands and contacts the housing 240, the voltage of the battery 200 can quickly reach the overcharge cutoff voltage, thereby shortening the overcharge time of the battery 200.

[0038] It can be understood that, in the terminology of the present application, “overcharging” refers to the process of continuing to charge the battery 200 when the battery 200 is fully charged.

[0039] Optionally, the battery 200 further comprises a top cover 250 and an explosion-proof valve (not shown in the figure), the top cover 250 is connected with the shell 240 and jointly encloses a receiving cavity 260 to accommodate the positive electrode plate 210, the separator 100, the negative electrode plate 220 and the electrolyte 230. The explosion-proof valve is arranged on the top cover 250, when the air pressure in the receiving cavity 260 reaches the safety pressure threshold of the explosion-proof valve, the explosion-proof valve bursts and releases the gas inside the battery 200, thereby avoiding explosion of the battery 200 and improving the safety performance of the battery 200.

[0040] Understandably, in the terms of the present application, the "safety pressure threshold of the explosion-proof valve" refers to the pressure threshold at which the explosion-proof valve suddenly opens and releases the internal gas after bearing a certain pressure.

[0041] Please see Figure 2 The present application provides a separator 100, which comprises a base film 110 and a foaming layer 120 arranged on one side of the base film 110, wherein the foaming layer 120 comprises composite particles 121, the composite particles 121 are a melt blend of polymethyl methacrylate (PMMA) and polyvinylidene fluoride (PVDF), and the foaming temperature T of the composite particles 121 ranges from 110℃ to 170℃.

[0042] Optionally, in some embodiments, the separator 100 comprises one foaming layer 120 arranged on one side of the base film 110; in other embodiments, the separator 100 comprises two foaming layers 120 arranged on opposite sides of the base film 110.

[0043] Understandably, the composite particles 121 are formed by melt blending of polymethyl methacrylate and polyvinylidene fluoride, and the polymethyl methacrylate and the polyvinylidene fluoride have high compatibility.

[0044] Understandably, in the terms of the present application, "foaming" refers to introducing bubble or pore structure into the material, and the foaming of the composite particles 121 refers to the formation of bubble pores inside the composite particles 121.

[0045] Specifically, the foaming temperature T of the composite particles 121 can be, but is not limited to, 110℃, 112℃, 115℃, 118℃, 120℃, 122℃, 125℃, 130℃, 135℃, 140℃, 145℃, 150℃, 155℃, 158℃, 160℃, 165℃ and 170℃, etc.

[0046] In this embodiment, the separator 100 includes a base film 110 and a foam layer 120. The foam layer 120 includes composite particles 121, which are a blend of polymethyl methacrylate and polyvinylidene fluoride. In the composite particles 121, the polymethyl methacrylate serves as the foaming material. When the separator 100 is applied to a battery 200 and the carbon dioxide content in the battery 200 is high, the PMMA can absorb a large amount of carbon dioxide, causing the foam layer 120 and the separator 100 to expand and contact the battery housing 240. This reduces the overcharge time of the battery 200 and improves the safety of the battery 200 during overcharge. Furthermore, when the separator 100 expands, it compresses the gas inside the battery 200, increasing the internal pressure. This facilitates the premature opening of the battery 200's explosion-proof valve, preventing direct ignition due to excessive heat when the valve opens. This ensures that the battery 200 also has better safety performance in the event of thermal runaway. In the composite particles 121, the polyvinylidene fluoride serves to assist in the foaming of PMMA. When the carbon dioxide in the environment surrounding the composite particles 121 reaches a certain concentration, the polyvinylidene fluoride crystallizes and plasticizes during the crystallization process, promoting the formation of pores in the polymethyl methacrylate, thereby enabling the composite particles 121 to have a faster foaming rate and a higher foaming ratio. The polyvinylidene fluoride and the polymethyl methacrylate are melt-blended to form the composite particles 121. Compared to the agglomerates formed by the polyvinylidene fluoride particles and the polymethyl methacrylate particles, the polyvinylidene fluoride and the polymethyl methacrylate in the composite particles 121 are more mixed, resulting in changes in the structures of the polyvinylidene fluoride and the polymethyl methacrylate. The polymethyl methacrylate can lower the crystallization temperature of the polyvinylidene fluoride and reduce the amount of carbon dioxide adsorbed to induce the crystallization of the polyvinylidene fluoride, making the polyvinylidene fluoride easier to crystallize and ultimately enabling the composite particles 121 to successfully foam at a lower temperature. By melt blending the polymethyl methacrylate and the polyvinylidene fluoride, the polymethyl methacrylate and the polyvinylidene fluoride influence each other, so that the foaming temperature T of the composite particles 121 satisfies the range of 110°C ≤ T ≤ 170°C. In other words, adding the polyvinylidene fluoride to the composite particles 121 can effectively lower the foaming temperature of the polymethyl methacrylate, so that when the battery 200 is in an overcharged state or in the early stage of thermal runaway, the composite particles 121 can be used as a foaming material under low pressure conditions, and timely foaming can shorten the overcharge time of the battery 200 and enable the explosion-proof valve to open in advance, thereby reducing the concentration of combustible gas when the explosion-proof valve opens, and avoiding excessive heat in the battery 200 when the valve is opened, which may directly ignite the battery 200, while improving the safety performance of the battery 200 in an overcharged state and a thermal runaway state.Furthermore, the composite particles 121 have excellent thermal insulation properties after foaming, which can block heat transfer when thermal runaway occurs in the battery 200, reduce the probability of heat spread, and further improve the safety performance of the battery 200. When the foaming temperature T of the composite particles 121 satisfies the range of 110°C ≤ T ≤ 170°C, the foaming temperature of the composite particles 121 is within a reasonable range. Compared with the solution of using only polymethyl methacrylate as the foaming material, the composite particles 121 provided in this embodiment also include polyvinylidene fluoride, and the polyvinylidene fluoride and polymethyl methacrylate have a high degree of fusion. The plasticization effect of the polyvinylidene fluoride during crystallization can promote the formation of bubble cells and foaming of the polymethyl methacrylate. The polymethyl methacrylate can lower the crystallization temperature of the polyvinylidene fluoride and reduce the amount of carbon dioxide adsorption that induces the crystallization of the polyvinylidene fluoride, thereby allowing the composite particles 121 to foam rapidly when the temperature of the battery 200 is abnormal. When the battery 200 is in an overcharged or overheated state, the composite particles 121 can expand in time to enable the battery 200 to open the valve in advance, reducing the heat generated when the explosion-proof valve opens, thereby improving the safety performance of the battery 200 in an overcharged state and reducing the probability of thermal runaway of the battery 200. When the foaming temperature of the composite particles 121 is too low, the composite particles 121 may foam prematurely and cause the battery 200 to open the valve, thereby reducing the reliability of the battery 200. When the foaming temperature of the composite particles 121 is too high, the composite particles 121 absorb carbon dioxide and expand too late, and cannot expand in time to reduce the damage to the battery 200 caused by overcharging and thermal runaway, resulting in poor safety performance of the battery 200.

[0047] Preferably, the foaming temperature T of the composite particles 121 is in the range of 115°C ≤ T ≤ 135°C.

[0048] Specifically, the polymethyl methacrylate and the polyvinylidene fluoride are formed by melt blending. PMMA is an amorphous polymer. The addition of PMMA can promote the rearrangement of PVDF molecular chains around the crystal nucleus, thereby promoting the crystallization of PVDF, thereby lowering the crystallization temperature of PVDF, increasing the crystallization rate, and widening the crystallization temperature range. This reduces the amount of carbon dioxide adsorption required to induce PVDF crystallization. The melt foaming process can eliminate the obstruction of crystals to foaming, significantly widening the foaming temperature range. When PVDF is at a low melt strength and in a crystalline state, it is conducive to inducing the opening of PMMA cells. Furthermore, the plasticizing effect of PVDF is enhanced during the crystallization process, and the microphases or microcrystals of PVDF can promote the generation of bubble cells in PMMA, thereby making the composite particles 121 have a faster foaming rate and a higher foaming ratio. Furthermore, the polyvinylidene fluoride is relatively soft, and PVDF can also reduce the curing temperature of the composite particles 121 to reduce the viscoelasticity of the composite particles 121, thereby promoting the polymethyl methacrylate to absorb carbon dioxide and increasing the solubility of the polymethyl methacrylate in carbon dioxide, so as to provide more sufficient gas for the polymethyl methacrylate to generate bubble pores, which is conducive to the formation of larger bubble pores.

[0049] Understandably, see Figure 3 , Figure 3 The morphology of the agglomerates formed by directly dispersing and blending the polymethyl methacrylate and the polyvinylidene fluoride in the raw materials is shown. Figure 3 It can be seen that the polymethyl methacrylate and the polyvinylidene fluoride are independent of each other. The polymethyl methacrylate cannot promote the reduction of the rearrangement of the polyvinylidene fluoride and cannot reduce the crystallization temperature of the polyvinylidene fluoride; the polyvinylidene fluoride cannot promote the formation of air bubbles in the polymethyl methacrylate through the plasticization effect during crystallization. Therefore, Figure 3 Although the polymethyl methacrylate in the agglomerate shown can also be foamed, its foaming temperature is high, its foaming efficiency is low, and its foaming time is long. When the battery 200 is in an overcharged state or in the early stage of thermal runaway, Figure 3 The agglomerates shown must reach a certain temperature before they begin to foam, and the foaming response speed is slow, and the foaming efficiency is low, and the battery 200 cannot be effectively protected. Figure 4 The morphology of the composite particles 121 provided in an embodiment of the present application is shown. The composite particles 121 are melt-blended by the polymethyl methacrylate and the polyvinylidene fluoride so that the polymethyl methacrylate and the polyvinylidene fluoride interact with each other. The composite particles 121 can start foaming at a lower temperature, and the foaming temperature satisfies the range of 110°C ≤ T ≤ 170°C. The composite particles 121 have a high foaming efficiency and can foam and expand in a shorter time to protect the battery 200.

[0050] Understandably, see Figure 5 , Figure 5 The foaming process of the foaming layer 120 is shown, specifically as follows: In the first stage S1, the battery 200 is in a normal charge-discharge cycle. In the second stage S2, a side reaction occurs within the battery 200, producing gas. The carbon dioxide concentration within the battery 200 increases. When the carbon dioxide concentration within the battery 200 reaches a certain level, the composite particles 121 begin to adsorb carbon dioxide. In the third stage S3, as the amount of carbon dioxide adsorbed by the composite particles 121 increases, the composite particles 121 expand, causing the volume of the separator 100 to expand. The separator 100 occupies a larger volume within the battery 200, thereby increasing the gas pressure within the battery 200. At this point, the composite particles 121 soften and enter a semi-solid state. In the fourth stage S4, as the air pressure within the battery 200 gradually rises to the safety pressure threshold of the explosion-proof valve, the explosion-proof valve opens. At this time, the concentration of the released carbon dioxide and other combustible gases is low, which can prevent the concentration of the combustible gas from reaching the flammable limit concentration range during pressure relief and directly combusting and causing thermal runaway. In addition, the air pressure within the battery 200 decreases, and the carbon dioxide can form bubble pores within the PMMA of the composite particles 121, realizing the foaming process of the composite particles 121. In the fifth stage S5, after the explosion-proof valve is depressurized, the temperature within the battery 200 gradually decreases. The composite particles 121 have good thermal insulation properties after foaming. At this time, the diaphragm 100 expands and takes shape, and acts as an insulator, thereby slowing the spread of heat.

[0051] It can be understood that the foaming temperature of the composite particles 121 satisfies the range of 110°C ≤ T ≤ 170°C. Compared to the solution using only polymethyl methacrylate as the foaming material, the foaming temperature of the composite particles 121 in this embodiment is lower, which can promote the progress of the third stage S3 and the fourth stage S4 of the foaming process of the foaming layer 120. Therefore, when the battery 200 is in an overcharged state or the temperature rises abnormally, the overcharge time can be reduced, allowing the explosion-proof valve to explode as soon as possible. After the composite particles 121 expand, the diaphragm 100 can serve as a better thermal insulation material to prevent the probability of heat spread. The composite particles 121 provide the battery 200 with better safety performance in the overcharge state and can reduce the probability of thermal runaway in the battery 200. When the composite particles 121 are used in the battery 200, they can simultaneously improve the safety performance of the battery 200 during overcharge and thermal runaway.

[0052] It is understood that during the foaming process of the composite particles 121, carbon dioxide is used as an initiator, see Figure 6 , Figure 6 The foaming principle of the composite particles 121 is shown as follows: In the second stage S2 of the foaming process of the foaming layer 120, i.e. Figure 6M1 stage, the composite particles 121 begin to adsorb carbon dioxide; in the third stage S3 of the foaming process of the foaming layer 120, i.e. Figure 6 M2 stage, the composite particles 121 form a homogeneous mixture with carbon dioxide as the temperature rises and the gas pressure increases; in the fourth stage S4 of the foaming process of the foaming layer 120, i.e. Figure 6 M3 stage, after the explosion-proof valve is opened to release pressure, the gas pressure inside the battery 200 decreases, and carbon dioxide begins to nucleate inside the composite particles 121; in the fifth stage S5 of the foaming process of the foaming layer 120, i.e. Figure 6 M4 stage, the temperature inside the battery 200 decreases, and the carbon dioxide adsorbed in the composite particles 121 begins to grow and eventually forms a bubble hole inside the composite particles 121.

[0053] Understandably, CO2 as the initiator of the composite particles 121 must reach a certain concentration in the environment where the composite particles 121 are located before the composite particles 121 can foam. The CO2 inside the battery 200 is derived from the side reaction of the electrolyte 230 with the positive electrode plate 210 and / or the negative electrode plate 220. Specifically, when the temperature inside the battery 200 is greater than 90°C, the side reaction intensifies, and the rate of temperature rise inside the battery 200 accelerates. At this time, the composite particles 121 begin to foam under the action of CO2, and the foaming time is within 40s-300s, which varies depending on the CO2 generation and speed.

[0054] Understandably, the polymethyl methacrylate and the polyvinylidene fluoride are commonly used as bonding materials, but in the embodiments of the present application, the polymethyl methacrylate and the polyvinylidene fluoride cooperate to make the polymethyl methacrylate have better foaming performance, and the composite particles 121 are ideal foaming materials. This is mainly reflected in the following aspects: first, compared with other common foaming materials such as polystyrene (PS) and polyurethane (PU), PMMA has a low carbon content, thus having a lower heat release rate, a smaller heat release amount and a smoke release amount, and will not worsen the safety of the battery 200. Second, PMMA has a high solubility for carbon dioxide, which can provide more gas for the nucleation and growth of bubble holes, achieve a higher foaming ratio, and make the expansion ratio of the foaming layer 120 within a reasonable range. Third, compared with traditional foaming materials such as PS and PU, PMMA has better rigidity and toughness and is not prone to aging. Fourth, PVDF and PMMA have good compatibility and can form a homogeneous blend. PVDF can reduce the viscoelasticity and curing temperature of the blend, and crystallization will occur during the adsorption stage. The plasticizing effect of PVDF can promote the growth of bubble holes, thereby reducing the optimal foaming temperature and obtaining a large-ratio foaming material under low pressure.

[0055] Optionally, in some embodiments, the preparation process of the composite particles 121 is: uniformly mixing the polymethyl methacrylate and the polyvinylidene fluoride according to the proportion, realizing melt blending at high temperature and extruding, and then drying to obtain the composite particles 121.

[0056] Optionally, the foaming layer 120 is arranged on one side of the base film 110 by roller coating. Specifically, the composite particles 121 and the solvent are mixed, wherein the mass percentage of the composite particles 121 is 20% to 40%, and the mass percentage of the solvent is 60% to 80%. The composite particles 121 and the solvent are fully stirred and dispersed for 90 minutes, and then roller coating is performed by selecting a 180 lines / in micro-concave roller. Extraction and drying are performed to obtain a foaming layer 120 with a thickness of 2 μm. The solvent includes but is not limited to organic solvents such as N-methyl pyrrolidone and dimethyl acetamide. Wherein, lines / in refers to the number of holes per inch (in) length.

[0057] In some embodiments, in the composite particles 121, the mass fraction of the polymethyl methacrylate is greater than the mass fraction of the polyvinylidene fluoride.

[0058] In the composite particles 121 provided in the present embodiment, the mass fraction of the polymethyl methacrylate is greater than the mass fraction of the polyvinylidene fluoride, so that the composite particles 121 have more foaming material. When the composite particles 121 are applied to the separator 100 and assembled in the battery 200, if a side reaction occurs in the battery 200 and the concentration of carbon dioxide in the battery 200 gradually increases, the polymethyl methacrylate can absorb carbon dioxide and form bubble holes, so that the composite particles 121 have a greater degree of expansion, and the expansion degree of the separator 100 is within a reasonable range, which facilitates the realization of the electrode assembly touching the shell 240, reduces the overcharge time, and makes the explosion-proof valve open early, so that the battery 200 has better safety performance in the overcharge state, and the probability of thermal runaway of the battery 200 can be reduced. Furthermore, after the explosion-proof valve is opened, the polyvinylidene fluoride can promote the carbon dioxide adsorbed in the polymethyl methacrylate to form larger bubble holes, so that the composite particles 121 have a faster foaming rate and a higher foaming ratio, so that the composite particles 121 have better heat insulation performance after foaming, so that the separator 100 can block heat transfer after the battery 200 is in thermal runaway, thereby reducing the probability of heat spread.

[0059] In some embodiments, in the composite particles 121, the mass fraction a of the polyvinylidene fluoride ranges from 10% to less than 50%.

[0060] It can be understood that, in the composite particles 121, the mass fraction of the polyvinylidene fluoride is the ratio of the mass of the polyvinylidene fluoride to the mass of the composite particles 121.

[0061] It can be understood that by regulating the blending ratio of the polyvinylidene fluoride and the polymethyl methacrylate, the foaming temperature of the composite particles 121 can be regulated.

[0062] Specifically, the mass fraction a of the polyvinylidene fluoride can be, but is not limited to, 10%, 12%, 15%, 18%, 20%, 22%, 25%, 28%, 30%, 32%, 35%, 38%, 40%, 42%, 45%, 48% and 49%, etc.

[0063] In the composite particles 121 provided in this embodiment, when the mass fraction a of the polyvinylidene fluoride satisfies the range of 10% ≤ a < 50%, the mass fractions of the polyvinylidene fluoride and the mass fractions of the polymethyl methacrylate are both within reasonable ranges. Polymethacrylic acid, as an amorphous polymer, can promote the rearrangement of the polyvinylidene fluoride molecular chains around the crystal nucleus, promoting the crystallization of PVDF, thereby lowering the crystallization temperature of PVDF, increasing the crystallization rate, and widening the crystallization temperature range, thereby reducing the amount of carbon dioxide adsorption required to induce PVDF crystallization. Furthermore, the microphases or crystallites of the polyvinylidene fluoride during the crystallization process can promote the formation of pores within the PMMA, thereby enabling the composite particles 121 to foam at a lower temperature and form larger pores, resulting in better thermal insulation properties. When the battery 200 is overcharged or in thermal runaway, the composite particles 121 can foam and expand in a timely manner, shortening the overcharge time and causing the explosion-proof valve to rupture prematurely, thereby reducing the probability of thermal runaway in the battery 200. When the mass fraction of the polyvinylidene fluoride is too high, the mass content of the polyvinylidene fluoride in the melt blend formed by the polyvinylidene fluoride and the polymethyl methacrylate is too high, and the mass content of the polymethyl methacrylate is too low. On the one hand, the polymethyl methacrylate, as the main foaming material, decreases in its content, which means that the foaming performance of the composite particles 121 is weakened. On the other hand, the function of the polymethyl methacrylate in lowering the crystallization temperature of the polyvinylidene fluoride is weakened, causing the polyvinylidene fluoride to crystallize only at higher temperatures and in an environment with a high carbon dioxide content. This in turn weakens the polyvinylidene fluoride's ability to form pores, and ultimately causes the foaming temperature of the composite particles 121 to be too high. When the foaming temperature of the composite particles 121 is too high, the composite particles 121 absorb carbon dioxide and expand too late, failing to expand in time to reduce damage to the battery 200 caused by overcharging and thermal runaway, resulting in poor safety performance of the battery 200. If the mass fraction of the polyvinylidene fluoride is too low, the polyvinylidene fluoride content in the melt blend formed by the polyvinylidene fluoride and the polymethyl methacrylate is too low, and the plasticizing effect of the polyvinylidene fluoride during the crystallization process is weak, making it difficult to enhance the nucleation of pores in the polymethyl methacrylate, resulting in a low rate of pore formation in the polymethyl methacrylate. When the battery 200 is overcharged or the temperature is too high, the composite particles 121 have difficulty rapidly foaming, resulting in poor safety performance of the battery 200 even in an overcharged state or thermal runaway state.Therefore, by regulating the mass fraction of the polyvinylidene fluoride and the mass fraction of the polymethyl methacrylate in the composite particles 121, and controlling the compatibility of the polyvinylidene fluoride and the polymethyl methacrylate, the foaming temperature of the composite particles 121 can be within a reasonable range. When the concentration of gas in the battery 200 exceeds the normal concentration, and / or the temperature inside the battery 200 exceeds the normal working temperature, the composite particles 121 can foam and expand in time, reduce the overcharge time, and achieve early valve opening of the battery 200, thereby improving the safety performance of the battery 200 in the overcharge state, and reducing the probability of thermal runaway of the battery 200.

[0064] Preferably, in the composite particles 121, the mass fraction a of the polyvinylidene fluoride ranges from 20% to 40%. Specifically, the value of the mass fraction a of the polyvinylidene fluoride can be, but is not limited to, 20%, 22%, 25%, 28%, 30%, 32%, 35%, 38%, 40%, and the like.

[0065] Alternatively, in the composite particles 121, the mass fraction b of the polymethyl methacrylate ranges from 50% to less than 90%. Specifically, the value of the mass fraction b of the polymethyl methacrylate can be, but is not limited to, 50%, 55%, 60%, 65%, 68%, 70%, 75%, 80%, 85%, 90%, and the like.

[0066] In some embodiments, the median particle size D of the composite particles 121 ranges from 0.5 μm to 1.5 μm.

[0067] Specifically, the value of the median particle size D of the composite particles 121 can be, but is not limited to, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, and the like.

[0068] In the present embodiment, when the median particle size D of the composite particles 121 satisfies the range 0.5 pm≤D≤1.5 pm, the median particle size of the composite particles 121 is within a reasonable range. On the one hand, when the battery 200 is in an overcharge state or the temperature inside the battery 200 abnormally rises, the composite particles 121 foam and expand, and the volume of the expanded composite particles 121 is within a reasonable range. This can not only make the electrode assembly touch the shell 240 to shorten the overcharge time and avoid generating too much heat, but also make the explosion-proof valve open in advance to release the combustible gas in the battery 200, thereby reducing the probability of thermal runaway of the battery 200. On the other hand, the median particle size of the composite particles 121 can be prevented from being too small to increase the processing difficulty, and the median particle size of the composite particles 121 can be prevented from being too large to make the thickness of the foaming layer 120 too large, thereby avoiding increasing the internal resistance of the battery 200 and reducing the performance of the battery 200. When the median particle size of the composite particles 121 is too large, when the foaming layer 120 is arranged on one side of the base film 110, the composite particles 121 are coated on one side of the base film 110 to form the foaming layer 120, and the difficulty of forming the foaming layer 120 is increased, and the thickness of the foaming layer 120 is too large, which affects the electrical performance of the electrode assembly. When the median particle size of the composite particles 121 is too small, on the one hand, the composite particles 121 can block the fine pores of the base film 110, thereby increasing the impedance of the active ion transmission in the battery 200. On the other hand, the processing difficulty of the composite particles 121 with a small median particle size is large, and the smaller the median particle size of the composite particles 121, the smaller the expansion degree of the composite particles 121 when foaming. Even if the composite particles 121 successfully foam, the volume of the composite particles 121 is still small, which results in a small volume of the expanded separator 100, and the separator 100 can be difficult to resist the shell 240 to shorten the overcharge time of the battery 200, and it is also difficult to achieve the early opening of the valve, thereby reducing the safety performance of the battery 200 when the composite particles 121 are applied to the separator 100 and assembled in the battery 200.

[0069] In some embodiments, after the composite particles 121 foam, the expansion ratio a of the composite particles 121 is in the range of 1.5≤a≤15, where the expansion ratio of the composite particles 121 is the ratio of the volume of the composite particles 121 after the volume expansion to the volume of the composite particles 121 before the volume expansion.

[0070] It can be understood that the expansion ratio of the composite particles 121 can represent the foaming ratio of the composite particles 121.

[0071] Specifically, the value of the expansion ratio a of the composite particles 121 can be, but is not limited to, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 6.5, 6.8, 7, 7.5, 8, 8.6, 9, 10, 11, 11.5, 12, 12.5, 13, 13.5, 14, 14.5, and 15, etc.

[0072] In this embodiment, when the expansion rate α of the composite particles 121 satisfies the range of 1.5≤α≤15, the expansion rate of the composite particles 121 is within a reasonable range. When the battery 200 is overcharged or the temperature within the battery 200 rises abnormally, the composite particles 121 foam and expand, and the volume of the expanded composite particles 121 is within a reasonable range, thereby ensuring that the volume of the expanded diaphragm 100 is also within a reasonable range. The electrode assembly contacts the housing 240 of the battery 200, thereby shortening the overcharge time of the battery 200 and improving the safety performance of the battery 200 in an overcharged state. At the same time, after the diaphragm 100 expands, it occupies more volume of the receiving cavity 260, increasing the pressure within the receiving cavity 260, causing the safety pressure threshold of the explosion-proof valve to be reached quickly, causing the explosion-proof valve to open, thereby preventing continued heat accumulation within the battery 200, reducing the risk of thermal runaway of the battery 200, and ultimately improving the safety performance of the battery 200 in a thermal runaway state. If the expansion rate of the composite particles 121 is too high, the explosion-proof valve may open prematurely, reducing the reliability of the battery 200. If the expansion rate of the composite particles 121 is too low, even if the composite particles 121 expand and foam when the battery 200 is overcharged or the temperature inside the battery 200 rises abnormally, the expanded volume of the composite particles 121 is small, resulting in a smaller volume of the diaphragm 100 after expansion. This makes it difficult for the electrode assembly to contact the battery housing 240, making it difficult to shorten the overcharge time of the battery 200 and improve the safety performance of the battery 200 in an overcharged state. Furthermore, the volume of the diaphragm 100 occupied by the expanded cavity 260 is small, making it difficult to increase the pressure in the cavity 260 to quickly reach the safety pressure threshold of the explosion-proof valve. In other words, it is difficult to open the explosion-proof valve prematurely, resulting in the continued accumulation of heat within the battery 200, increasing the risk of thermal runaway and reducing the safety performance of the battery 200 in a thermal runaway state.

[0073] Preferably, the expansion ratio α of the composite particles 121 is in the range of 2≤α≤10.

[0074] In some embodiments, the thickness h of the foaming layer 120 is in the range of 1 μm≤h≤5 μm.

[0075] Specifically, the thickness h of the foaming layer 120 can be, but is not limited to, 1 μm, 1.2 μm, 1.5 μm, 1.8 μm, 2 μm, 2.2 μm, 2.5 μm, 2.7 μm, 2.9 μm, 3 μm, 3.2 μm, 3.5 μm, 3.8 μm, 4 μm, 4.2 μm, 4.5 μm, 4.8 μm, and 5 μm, etc.

[0076] In this embodiment, when the thickness h of the foaming layer 120 satisfies the range of 1μm≤h≤5μm, the thickness of the foaming layer 120 is within a reasonable range. On the one hand, when the battery 200 is in an overcharged state or the temperature inside the battery 200 rises abnormally, the composite particles 121 foam and expand and the volume after expansion is within a reasonable range. Then, the volume of the foaming layer 120 after foaming and expansion is within a reasonable range, and the electrode assembly is able to touch the shell 240 to shorten the overcharge time and prevent the battery 200 from generating excessive heat in the overcharge state. The foaming layer 120 expands and squeezes the air in the receiving cavity 260, so that the air pressure in the receiving cavity 260 increases to the safety pressure threshold of the battery 200, so that the explosion-proof valve opens in advance to discharge the combustible gas in the battery 200, thereby reducing the probability of thermal runaway of the battery 200. Secondly, the volume of the foam layer 120 after expansion is within a reasonable range, making the diaphragm 100 an ideal thermal insulation material. Even if the battery 200 experiences thermal runaway, the diaphragm 100 can effectively block heat transfer and reduce the probability of heat spread. Therefore, when the thickness of the foam layer 120 is within a reasonable range, it can simultaneously improve the safety performance of the battery 200 in overcharge and thermal runaway states without affecting the safety performance of the battery 200, so that the battery 200 has better performance and a longer service life. When the thickness of the foam layer 120 is too large, during the normal charge and discharge cycle of the battery 200, the impedance of the active ions in the battery 200 passing through the diaphragm 100 is increased, the internal resistance of the battery 200 is increased, and the energy efficiency and performance of the battery 200 are reduced. When the thickness of the foaming layer 120 is too small, when the battery 200 is in an overcharged state or the temperature inside the battery 200 rises abnormally, even if the composite particles 121 foam and expand, the expansion degree of the foaming layer 120 is still too small, and the volume of the diaphragm 100 after expansion is still small. It may be difficult to resist the shell 240 and shorten the overcharge time of the battery 200, and it is also difficult to open the valve in advance, thereby reducing the safety performance of the battery 200 when the composite particles 121 are applied to the diaphragm 100 and assembled in the battery 200.

[0077] Optionally, the base film 110 may be, but is not limited to, a polyethylene base film 110 or a polypropylene base film 110 , and the thickness of the base film 110 ranges from 7 μm to 20 μm.

[0078] See Figure 7 Optionally, the diaphragm 100 further includes a heat-resistant coating 130, which is disposed between the base film 110 and the foaming layer 120 to isolate heat transfer between the base film 110 and the foaming layer 120. The heat-resistant coating 130 may be, but is not limited to, a ceramic layer, a polyimide layer, a cellulose layer, etc. The thickness of the heat-resistant coating 130 ranges from 2 μm to 5 μm.

[0079] In some embodiments, among the raw materials in the composite particles 121 , the glass transition temperature T1 of the polymethyl methacrylate is in the range of 80° C. ≤ T1 ≤ 120° C., and the melting temperature T2 of the polyvinylidene fluoride is in the range of 140° C. ≤ T2 ≤ 160° C.

[0080] Specifically, the glass transition temperature T1 of the polymethyl methacrylate can be, but is not limited to, 80°C, 82°C, 85°C, 86°C, 88°C, 90°C, 92°C, 95°C, 98°C, 100°C, 102°C, 105°C, 106°C, 108°C, 110°C, 112°C, 115°C, 118°C and 120°C, etc.

[0081] Specifically, the melting temperature T2 of the polyvinylidene fluoride can be, but is not limited to, 140°C, 142°C, 145°C, 146°C, 148°C, 150°C, 152°C, 153°C, 155°C, 157°C, 158°C, and 160°C.

[0082] In this embodiment, when the glass transition temperature T1 of the polymethyl methacrylate satisfies the range of 80°C ≤ T1 ≤ 120°C and the melting temperature T2 of the polyvinylidene fluoride satisfies the range of 140°C ≤ T2 ≤ 160°C, the melting temperature of the composite particles 121 is within a reasonable range. The composite particles 121 are in a molten state, so that the polymethyl methacrylate and the polyvinylidene fluoride have a high degree of intermixing. The polymethyl methacrylate and the polyvinylidene fluoride interact with each other, so that the foaming temperature of the composite particles 121 is within a reasonable range. When the battery 200 is in an overcharged state or the temperature inside the battery 200 rises abnormally, the composite particles 121 can foam and expand in time, shortening the overcharge time and causing the explosion-proof valve to open earlier, thereby improving the safety performance of the battery 200 in an overcharged state and a thermal runaway state.

[0083] In some embodiments, the composite particles 121 are prepared by a melt-blending thermoforming process, wherein the thermoforming process includes at least one of extrusion molding, injection molding, or calendering molding.

[0084] In this embodiment, the composite particles 121 are prepared by a melt-blending thermoforming process, which facilitates high compatibility between the polymethyl methacrylate and the polyvinylidene fluoride. The polymethyl methacrylate and polyvinylidene fluoride interact with each other, allowing the polymethyl methacrylate to lower the crystallization temperature of the polyvinylidene fluoride, reducing the amount of carbon dioxide adsorbed to induce crystallization. The plasticizing effect of the polyvinylidene fluoride during crystallization facilitates the formation of pores within the polymethyl methacrylate by the carbon dioxide, thereby increasing the foaming rate and expansion ratio of the composite particles 121. In other words, when the composite particles 121 are formed by at least one of extrusion molding, injection molding, or calendering molding, the composite particles 121 can rapidly foam at relatively low temperatures. When the battery 200 is overcharged or experiences an abnormally high temperature, the composite particles 121 can respond quickly, rapidly foaming and expanding, shortening the overcharge time and enabling the explosion-proof valve to open sooner, thereby improving the safety of the battery 200 in overcharged and thermal runaway conditions.

[0085] The following is a further introduction to the technical solution of this application in multiple embodiments:

[0086] Example 1 to Example 4, Comparative Example 1 and Comparative Example 2:

[0087] 1. Preparation of the diaphragm 100:

[0088] (1) Provide a base film 110.

[0089] (2) Coating a heat-resistant coating 130 on the surface of the base film 110: Deionized water, ammonium polyacrylate, sodium carboxymethyl cellulose (CMC1140), aluminum oxide powder (D50 is 600 nm) and lithium acrylate copolymer emulsion are mixed at a ratio of 55:0.08:0.8:40:

[0090] 1.2 Mix evenly and stir thoroughly to obtain a slurry for the heat-resistant coating 130. Use a 180 lines / in micro-concave roller for roller coating to apply the slurry for the heat-resistant coating 130 to the surface of the base film 110 to obtain a heat-resistant coating 130 with a thickness of 2 μm.

[0091] (3) Coating a foaming layer 120 on the surface of the heat-resistant coating 130: After uniformly mixing the polymethyl methacrylate and the polyvinylidene fluoride according to a ratio, add them to a twin-screw extruder for melt blending and extrusion to obtain the composite particles 121 of Examples 1 to 4, Comparative Example 1, and Comparative Example 2. The composite particles 121 and N-methyl pyrrolidone are mixed in a mass ratio of 30:70 and stirred for 90 minutes. A 180 lines / in micro-concave roller is selected for roller coating, extraction, and drying to obtain a foaming layer 120 with a thickness of 2 μm, thereby obtaining the diaphragms 100 of Examples 1 to 4, Comparative Example 1, and Comparative Example 2.

[0092] The composite particles 121 of Example 1 are applied to the diaphragm 100 of Example 1, the composite particles 121 of Example 2 are applied to the diaphragm 100 of Example 2, the composite particles 121 of Comparative Example 1 are applied to the diaphragm 100 of Comparative Example 1, and so on.

[0093] The mass fraction a of the polyvinylidene fluoride and the mass fraction b of the polymethyl methacrylate in the composite particles 121 of Examples 1 to 4 and Comparative Examples 1 and 2 are shown in Table 1.

[0094] 2. Preparation of positive electrode sheet 210:

[0095] The positive electrode active material lithium iron phosphate, the positive electrode conductive agent conductive carbon black (SP), and the positive electrode binder polyvinylidene fluoride (PVDF) are dispersed in a solvent N-methyl-2-pyrrolidone (NMP) according to a ratio and mixed evenly to obtain a positive electrode slurry with a solid content of 60 wt%. The positive electrode slurry is coated on the positive electrode current collector (aluminum foil). The coating weight of the positive electrode slurry is 300 mg / 1540.25 mm 2 After drying, cold pressing, slitting, and cutting, the positive electrode sheet 210 is obtained.

[0096] It can be understood that the separator 100 of the embodiment of the present application is applicable to the lithium iron phosphate system, and the material of the positive electrode plate 210 provided in the present application is only an example and should not be understood as being applicable only to the positive electrode plate 210 provided in the present application.

[0097] 3. Preparation of negative electrode sheet 220:

[0098] The negative electrode active material hard carbon, the negative electrode conductive agent conductive carbon black (SP), the negative electrode thickener carboxymethyl cellulose (CMC), the negative electrode binder styrene butadiene rubber (SBR) and asphalt are dispersed in deionized water according to a ratio and mixed evenly to obtain a negative electrode slurry with a solid content of 50wt%. The negative electrode slurry is coated on the negative electrode current collector (copper foil). The coating weight of the negative electrode slurry is 144mg / 1540.25mm 2 After drying, cold pressing, slitting and cutting, the negative electrode sheet 220 is obtained.

[0099] It can be understood that the separator 100 of the embodiment of the present application is applicable to the lithium iron phosphate system, and the material of the negative electrode plate 220 provided in the present application is only an example and should not be understood as being applicable only to the negative electrode plate 220 provided in the present application.

[0100] 4. Preparation of electrolyte 230:

[0101] In an argon atmosphere glove box with a moisture content of ≤1 ppm, ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate were mixed in a mass ratio of 1:1:1. Dry electrolyte lithium salt, lithium hexafluorophosphate, was then dissolved in the solvent and mixed thoroughly to obtain electrolyte 230. Electrolyte 230 was composed of the following: a solvent consisting of ethylene carbonate: dimethyl carbonate: ethyl methyl carbonate in a mass ratio of 1:1:1, and a lithium salt concentration of 1 mol / L.

[0102] 5. Preparation of Battery 200:

[0103] The prepared positive electrode sheet 210, separator 100, and negative electrode sheet 220 are stacked in order so that the separator 100 is located between the positive electrode sheet 210 and the negative electrode sheet 220. The positive electrode sheet 210, separator 100, and negative electrode sheet 220 are wound to obtain an electrode assembly. After welding the electrode ears, the electrode assembly is assembled into an outer package. After injecting the prepared electrolyte 230, the battery cell is packaged, allowed to stand, formed, shaped, and capacity tested, and finally, implementation batteries 1 to 6 are prepared.

[0104] The separator 100 of Example 1 is applied to the implementation battery 1, the separator 100 of Example 2 is applied to the implementation battery 2, the separator 100 of Comparative Example 1 is applied to the comparison battery 1, and so on.

[0105] Table 1 below shows the performance parameters of the electrolyte 230 of Examples 1 to 4, Comparative Example 1, and Comparative Example 2.

[0106] Table 1: Performance parameters of the electrolyte 230 of Examples 1 to 4, Comparative Example 1 and Comparative Example 2.

[0107]

[0108]

[0109] Battery 200 performance test:

[0110] The expansion temperature and expansion ratio of the separator 100 of Examples 1 to 4, Comparative Examples 1 and 2 were tested, and the overcharge cutoff temperature of the battery 1 to 4, the comparative battery 1 and the comparative battery 2 were tested:

[0111] Assemble the diaphragm 100 into a soft-pack battery and perform a single-cell overcharge test according to national standards. The temperature corresponding to the onset of expansion of the soft-pack battery 200 is used as the expansion temperature of the diaphragm 100. The temperature at which the voltage reaches 1.5 times the initial voltage is used as the overcharge cutoff temperature, and the time is recorded. The thickness of the electrode assembly after the test is measured and compared with the initial thickness of the electrode assembly to obtain the thickness of the diaphragm 100 before and after expansion, and the expansion ratio of the diaphragm 100 is calculated.

[0112] In addition, the battery 200 is observed to see whether it has passed the overcharge test. Overcharge test criteria: If the battery 200 does not catch fire or explode after standing for one hour after the overcharge is cut off, and no parts other than the explosion-proof valve are ruptured, the battery 200 is considered to have passed the test; otherwise, it is considered to have failed the test.

[0113] It can be understood that the expansion ratio of the diaphragm 100 can represent the expansion ratio of the composite particles 121 , and the expansion ratio of the diaphragm 100 is positively correlated with the expansion ratio of the composite particles 121 .

[0114] It is understood that the expansion temperature of the membrane 100 can represent the foaming temperature of the composite particles 121, and the expansion temperature of the membrane 100 corresponds to the foaming temperature of the composite particles 121. In other words, the foaming temperature of the composite particles 121 is equivalent to the expansion temperature of the membrane 100.

[0115] It can be understood that the overcharge cut-off temperature of the battery 200 represents the temperature of the battery 200 after the overcharge is cut off. The higher the overcharge cut-off temperature, the worse the safety performance of the battery 200 under overcharge.

[0116] The expansion temperature and expansion ratio of the diaphragm 100 of Examples 1 to 4, Comparative Examples 1 and 2 are shown in Table 2. The overcharge cutoff temperature and overcharge test results of Implementation Battery 1 to Implementation Battery 4, Comparative Battery 1 and Comparative Battery 2 are shown in Table 2.

[0117] Table 2 below shows the performance parameters of the separator 100 of Examples 1 to 4, Comparative Examples 1 and 2, and the performance parameters of Implementation Batteries 1 to 4, Comparative Batteries 1 and 2.

[0118] Table 2: Performance parameters of the separators 100 of Examples 1 to 4, Comparative Examples 1 and 2, and performance parameters of Implementation Batteries 1 to 4, Comparative Batteries 1 and 2.

[0119]

[0120] Please refer to Table 1 and Table 2. It can be seen from the data of Examples 1 to 4, Comparative Example 1 and Comparative Example 2 that the composite particles 121 of the diaphragms 100 of Examples 1 to 4, Comparative Example 1 and Comparative Example 2 are all melt blends of the polymethyl methacrylate and the polyvinylidene fluoride, so that the composite particles 121 have good foaming and expansion properties. The overcharge test results of the implementation battery 1 to the implementation battery 4, the comparative battery 1 and the comparative battery 2 are all passed, which shows that when the polymethyl methacrylate and the polyvinylidene fluoride are melt blended, the polymethyl methacrylate and the polyvinylidene fluoride affect each other, so that the polymethyl methacrylate can reduce the crystallization temperature of the polyvinylidene fluoride, and the plasticizing effect of the polyvinylidene fluoride during crystallization can promote the formation of bubble pores in the polymethyl methacrylate, thereby causing the diaphragm 100 to expand and resist the shell 240 of the battery 200, so as to shorten the overcharge time and improve the safety performance of the implementation battery 1 to the implementation battery 4, the comparative battery 1 and the comparative battery 2 under overcharge.

[0121] However, by adjusting the mass ratio of the polymethyl methacrylate and the polyvinylidene fluoride in the composite particles 121, the foaming performance of the composite particles 121 can be controlled. Specifically, in the composite particles 121 of the diaphragms 100 of Examples 1 to 4, the mass fraction a of the polyvinylidene fluoride satisfies the range of 10%≤a<50%, the mass fraction a of the polyvinylidene fluoride in the composite particles 121 of the diaphragm 100 of Comparative Example 1 is too small, and the mass fraction a of the polyvinylidene fluoride in the composite particles 121 of the diaphragm 100 of Comparative Example 2 is too small, so that the expansion temperature of the implementation battery 1 to the implementation battery 4 is lower than the expansion temperature of the comparison battery 1 and the comparison battery 2, the expansion ratio of the implementation battery 1 to the implementation battery 4 is lower than the expansion ratio of the comparison battery 2, the overcharge temperature of the implementation battery 1 to the implementation battery 4 is lower than the overcharge temperature of the comparison battery 1 and the comparison battery 2, and the implementation battery 1 to the implementation battery 4 is lower than the overcharge temperature of the comparison battery 1 and the comparison battery 2. The overcharge time of cell 4 is less than that of comparative cell 1 and comparative cell 2. This is because: when the mass fraction of the polyvinylidene fluoride is within a reasonable range, polymethacrylic acid, as an amorphous polymer, can promote the rearrangement of the polyvinylidene fluoride molecular chain around the crystal nucleus, promote the crystallization of PVDF, thereby lowering the crystallization temperature of PVDF, increasing the crystallization rate and widening the crystallization temperature range, and reducing the amount of carbon dioxide adsorption required to induce PVDF crystallization. The microphase or microcrystal of the polyvinylidene fluoride during the crystallization process can promote the generation of bubble pores in PMMA, thereby allowing the composite particles 121 to foam at a lower temperature and form larger bubble pores, so that the composite particles 121 have better thermal insulation properties. When the battery 200 is in an overcharged state or thermal runaway, the composite particles 121 can foam and expand in time and shorten the overcharge time, and cause the explosion-proof valve to explode in advance, thereby reducing the probability of thermal runaway of the battery 200. Therefore, battery 1 to battery 4 have lower expansion temperature and overcharge temperature, and have a larger expansion ratio, shorter overcharge time, and battery 1 to battery 4 have higher safety performance in overcharge state and thermal runaway state. When the mass fraction of the polyvinylidene fluoride is too small, the plasticizing effect of the polyvinylidene fluoride in the crystallization process of the composite particles 121 is weak, and it is difficult to enhance the nucleation of the bubble pores in the polymethyl methacrylate, resulting in a lower rate of bubble formation in the polymethyl methacrylate, and the polyvinylidene fluoride is also difficult to crystallize at a lower temperature, thereby reducing the foaming temperature of the composite particles 121. Therefore, in the comparative battery 1, although the comparative battery 1 has a higher expansion ratio and a lower overcharge time, its expansion temperature is still high and the overcharge time is still long. When the mass fraction of the polyvinylidene fluoride is too large, the polymethyl methacrylate is the main foaming material, and its reduced content means that the foaming performance of the composite particles 121 is weakened, resulting in an extremely low expansion ratio of the comparative battery 2.On the other hand, the poly(methyl methacrylate) weakens the function of reducing the crystallization temperature of the poly(vinylidene fluoride), so that the poly(vinylidene fluoride) crystallizes at a higher temperature and in an environment with a higher carbon dioxide content, which in turn weakens the function of the poly(vinylidene fluoride) to form bubble pores, and finally makes the foaming temperature of the composite particles 121 too high. Therefore, the swelling temperature and the overcharge temperature of the comparative battery 2 are both high, and the overcharge time is long.

[0122] Further, according to the data of Examples 1 to 4, as the mass fraction of the poly(vinylidene fluoride) gradually increases, the swelling temperature of the corresponding battery 200 first decreases and then increases, the swelling ratio first increases and then decreases, the overcharge temperature first decreases and then increases, and the overcharge time first decreases and then increases, which indicates that when the mass fraction of the poly(vinylidene fluoride) and the mass fraction of the poly(methyl methacrylate) are both within a reasonable range, the interaction between the poly(methyl methacrylate) and the poly(vinylidene fluoride) is better, so that the foaming temperature of the composite particles 121 satisfies the range 110℃≤T≤170℃, and when the battery 200 is in an overcharge state or an initial thermal runaway state, the separator 100 can protect the battery 200 to improve the safety performance of the battery 200 in the overcharge state and the thermal runaway state.

[0123] Please refer to Figure 8 and Figure 9 The application also provides a power consumption device 300, which comprises a device body 310 and the battery 200 provided by the application, and the battery 200 supplies power to the device body 310.

[0124] It can be understood that the battery 200 is electrically connected to the device body 310.

[0125] In this embodiment, the separator 100 comprises the composite particles 121, which have an optimal foaming temperature range, so that the composite particles 121 can effectively foam and swell when the battery 200 is in an overcharge state or an initial thermal runaway state, so as to shorten the overcharge time and make the explosion-proof valve open as soon as possible, thereby making the battery 200 have good safety performance in the overcharge state and the thermal runaway state, and the battery 200 has a long service life. When the battery 200 is applied to the power consumption device 300, the battery 200 can provide stable power for the device body 310, which is beneficial to improving the user experience.

[0126] Optionally, the power consuming device 300 of the embodiments of the present application can be, but is not limited to, a mobile phone, a tablet computer, a notebook computer, a desktop computer, a smart bracelet, a smart watch, an electronic reader, a game console, and the like portable electronic device. It can also be a vehicle such as a car, a truck, a sedan, a van, a motor car, a high-speed train, an electric automatic car, and the like. In addition, it can also be various household appliances, and the like. The present application Figure 8 The power consuming device 300 of the embodiments is a battery storage cabinet.

[0127] It can be understood that the power consuming device 300 described in the embodiments is only one form of the power consuming device 300 to which the battery 200 is applied, and should not be understood as a limitation on the power consuming device 300 provided by the present application, nor should it be understood as a limitation on the power consuming device 300 provided by each of the embodiments of the present application.

[0128] In the present application, the phrase "embodiment" or "embodiments" means that the specific features, structures or characteristics described in connection with the embodiment can be included in at least one embodiment of the present application. The appearance of the phrase in various places in the specification does not necessarily mean the same embodiment, nor is it an independent or alternative embodiment that is not mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described in the present application can be combined with other embodiments. In addition, it should also be understood that the features, structures or characteristics described in the embodiments of the present application can be combined with each other without contradiction, to form another embodiment of the present application without departing from the spirit and scope of the present application.

[0129] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application and not to limit it. Although the present application has been described in detail with reference to the above preferred embodiments, those skilled in the art should understand that the technical solutions of the present application can be modified or replaced by equivalents without departing from the spirit and scope of the present application.

Claims

1. A diaphragm, characterized in that: The diaphragm comprises: basement membrane; and A foaming layer is provided on one side of the base film, the foaming layer comprises composite particles, the composite particles are a melt blend of polymethyl methacrylate and polyvinylidene fluoride, and the foaming temperature T of the composite particles is in the range of 110°C≤T≤170°C.

2. The diaphragm according to claim 1, characterized in that In the composite particles, the mass fraction of the polymethyl methacrylate is greater than the mass fraction of the polyvinylidene fluoride.

3. The diaphragm according to claim 2, characterized in that In the composite particles, the mass fraction a of the polyvinylidene fluoride is in the range of 10%≤a<50%.

4. The diaphragm according to claim 1, characterized in that The range of the median particle size D of the composite particles is: 0.5 μm≤D≤1.5 μm.

5. The diaphragm according to any one of claims 1 to 4, characterized in that After the composite particles are foamed, the expansion rate α of the composite particles is in the range of 1.5≤α≤15, wherein the expansion rate of the composite particles is the ratio of the volume of the composite particles after volume expansion to the volume of the composite particles before volume expansion.

6. The diaphragm according to any one of claims 1 to 4, characterized in that The thickness h of the foaming layer is in the range of 1 μm≤h≤5 μm.

7. The diaphragm according to any one of claims 1 to 4, characterized in that Among the raw materials in the composite particles, the glass transition temperature T1 of the polymethyl methacrylate is in the range of 80° C. ≤ T1 ≤ 120° C., and the melting temperature T2 of the polyvinylidene fluoride is in the range of 140° C. ≤ T2 ≤ 160° C.

8. The diaphragm according to any one of claims 1 to 4, characterized in that The composite particles are prepared by a melt-blending thermoforming process, wherein the thermoforming process includes at least one of extrusion molding, injection molding or calendering molding.

9. A battery, characterized in that: The battery comprises: Positive electrode; The diaphragm according to any one of claims 1 to 8, wherein the diaphragm is provided on one side of the positive electrode plate; a negative electrode plate, the negative electrode plate being disposed on a side of the diaphragm away from the positive electrode plate; and An electrolyte is used to soak at least a portion of the positive electrode sheet, the separator, and the negative electrode sheet.

10. An electrical device, characterized in that: The electrical equipment includes: the device itself; and The battery according to claim 9, wherein the battery is used to power the device body.

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