A polyester film for an electric double layer capacitor and a lithium ion battery

Abstract

This invention discloses a polyester membrane comprising: multiple channels spaced apart within the polyester membrane, each channel including a closed chamber; a mixed flame-retardant liquid filled within the chamber, the mixed flame-retardant liquid comprising a free radical scavenging material and a thermal transition material, the thermal transition material comprising a thermosetting polymer and / or a thermo-induced phase change substance; when preset conditions are met, the polyester membrane ruptures, and the mixed flame-retardant liquid flows out from the chamber. The free radical scavenging material is used to block the combustion reaction, and the thermal transition material is used to form a physical barrier layer. The preset thermal runaway emergency condition corresponds to the condition when the battery cell has a risk of thermal runaway. The mixed flame-retardant liquid can be rapidly released based on the preset conditions, achieving combustion reaction blocking through free radical scavenging, combined with the physical barrier effect, effectively curbing the spread of thermal runaway and significantly improving the safety protection level of the battery cell. This invention also discloses a lithium-ion battery.

Description

Technical Field

[0001] This invention relates to the field of batteries, and particularly to a polyester film for battery cells and a lithium-ion battery. Background Technology

[0002] Due to their outstanding advantages such as high energy density and good reliability, lithium-ion batteries have been widely used in various fields such as mobile devices, electric vehicles, and smart grids, and the industry is currently iterating and developing towards even higher energy densities. Meanwhile, other rechargeable batteries such as sodium batteries, magnesium batteries, and zinc batteries have also received widespread attention within the industry due to their respective application advantages, becoming important development directions in the rechargeable battery field.

[0003] However, currently commercially available rechargeable batteries generally use volatile, flammable, and explosive organic electrolytes, an inherent characteristic that makes them inherently unsafe. When batteries are subjected to abuse conditions such as compression, overcharging, or puncture, they are highly susceptible to internal short circuits, rapid temperature increases, and other chain reactions, which can trigger thermal runaway. This manifests as smoke, fire, or even explosion, not only damaging the battery itself but also potentially causing serious safety accidents that threaten personal and property safety. The thermal runaway safety issue of lithium-ion batteries is particularly prominent and has long been a core challenge that industry professionals have been striving to overcome.

[0004] With the development of the new energy vehicle industry, the safety performance requirements for secondary batteries are constantly increasing. Taking thermal runaway safety as an example, the relevant mandatory national standard GB 38031-2025 "Safety Requirements for Power Batteries for Electric Vehicles" has raised the technical requirements from "providing an alarm signal 5 minutes before fire or explosion" to "no fire or explosion".

[0005] Currently, the industry has proposed a variety of protection solutions for battery thermal runaway, but these solutions all have obvious defects, making it difficult to balance safety and core cell performance, and they also have poor compatibility. For example, some existing flame-retardant protection solutions require modifications to the original structure of the battery cell, adding extra structural components. This not only increases the difficulty of battery cell design and assembly but also requires modifications to existing production lines, hindering large-scale industrial applications. Furthermore, such structures occupy valuable internal space within the battery cell, sacrificing internal structural integrity and directly impacting energy density. Some solutions directly add flame-retardant substances to the electrolyte, achieving a certain flame-retardant effect. However, these substances interfere with the ion transport process in the electrolyte, significantly increasing internal resistance and reducing charge / discharge efficiency. The flame-retardant substances dispersed in the electrolyte also react slightly with the electrolyte over a long period, affecting electrolyte stability. Uneven dispersion and delayed response of the flame-retardant substances are also issues. In addition, some flame-retardant solutions require additional control components for activation, further increasing the complexity and cost of the battery cell structure. The slow flame-retardant response also prevents rapid activation of the flame-retardant effect in the early stages of thermal runaway, hindering timely containment of the runaway and ultimately resulting in poor flame-retardant performance. Summary of the Invention

[0006] To address the aforementioned technical problems, embodiments of the present invention disclose a polyester film comprising:

[0007] Multiple channels are spaced apart within the polyester film, each channel comprising a closed chamber;

[0008] A mixed flame-retardant liquid is filled into the chamber. The mixed flame-retardant liquid includes a free radical scavenging material and a thermal transition material. The thermal transition material includes a thermosetting polymer and / or a thermo-induced phase change substance.

[0009] When the preset thermal runaway emergency conditions are met, the polyester film ruptures, the mixed flame-retardant liquid flows out of the chamber, the free radical capturing material is used to block the combustion reaction, and the thermal conversion material is used to form a physical barrier layer. The preset thermal runaway emergency conditions are the conditions corresponding to the risk of thermal runaway of the battery cell.

[0010] Using the above technical solution, flame retardant triggering requires no additional components and can achieve rapid response in situations such as thermal runaway, puncture, and deformation. The mixed flame retardant liquid can be rapidly released based on preset conditions, blocking the combustion reaction through free radical capture. Combined with physical barrier effects, it effectively curbs the spread of thermal runaway and significantly improves the safety protection level of the battery cell. Furthermore, it does not require significant modifications to the original battery cell structure, adapts to existing production processes, and does not occupy effective internal space, thus avoiding energy density loss and ensuring core battery cell performance. The mixed flame retardant liquid and electrolyte are independent of each other and do not affect electrolyte ion transport, preventing an increase in internal resistance and ensuring battery cell performance.

[0011] Optionally, in the mixed flame retardant liquid, the mass ratio of the free radical capturing material to the thermal conversion material is (6~9.5):(0.5~3).

[0012] Optionally, the mass ratio of the free radical capturing material to the thermal transformation material is (6~8):(2~3).

[0013] Optionally, the free radical scavenging material is selected from one or more of dimethyl methyl phosphate, tri(2,2,2-trifluoroethyl) phosphate, and triphenyl phosphate.

[0014] Optionally, the thermosetting polymer includes a polymerization monomer and / or an initiator, wherein the polymerization monomer is selected from one or more of phosphate ester monomers, styrene monomers, and cyclic monomers.

[0015] Optionally, the phosphate ester monomer is one or more of methacryloyloxyethyl phosphate and triallyl phosphate, the styrene monomer is one or more of bromostyrene, tribromostyrene and divinylbenzene, and the cyclic monomer is one or more of hexachlorocyclotriphosphazene and ethylene oxide.

[0016] Optionally, the thermally induced phase change material is selected from one or more of polyethylene glycol, poloxamer block copolymers, and poly(N-isopropylacrylamide).

[0017] Optionally, the polyester film includes a film body and a film flange folded from the film body, wherein the plurality of channels are uniformly distributed within the film body.

[0018] And / or, the preset thermal runaway emergency condition is that the pressure borne by the polyester film is greater than the rupture pressure threshold of the polyester film and / or the temperature of the polyester film is higher than the melting point of the polyester film.

[0019] And / or, the preset thermal runaway emergency condition is that the cell is at risk of thermal runaway due to at least one of the following:

[0020] The internal temperature of the battery cell rose abnormally.

[0021] The internal pressure of the battery cell increased abnormally.

[0022] The battery cell is subjected to abnormal mechanical stress.

[0023] Optionally, the pore size of the channel is 75–100 μm.

[0024] And / or, the spacing between adjacent channels is 25–45 μm.

[0025] And / or, the battery cell includes an explosion-proof valve, which is not opened under the preset thermal runaway emergency condition.

[0026] Another embodiment of the present invention discloses a lithium-ion battery comprising the polyester film as described above.

[0027] By adopting the above technical solutions, the safety of battery use can be significantly improved while ensuring high energy density and low internal resistance of the battery cells. Attached Figure Description

[0028] Figure 1 A schematic diagram of the structure of a polyester film according to an embodiment of the present invention is shown;

[0029] Figure 2 A cross-sectional schematic diagram of a polyester film according to an embodiment of the present invention is shown. Detailed Implementation

[0030] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Although the description of the present invention is presented in conjunction with preferred embodiments, this does not mean that the features of the invention are limited to these embodiments. On the contrary, the purpose of describing the invention in conjunction with embodiments is to cover other options or modifications that may be derived based on the claims of the present invention. To provide a deep understanding of the invention, many specific details will be included in the following description. The invention may also be implemented without using these details. Furthermore, to avoid confusion or obscuring the focus of the invention, some specific details will be omitted in the description. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0031] It should be noted that in this specification, similar reference numerals and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0032] The terms “first”, “second”, etc., are used only to distinguish descriptions and should not be interpreted as indicating or implying relative importance.

[0033] In the description of this embodiment, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set up," "connected," and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this embodiment based on the specific circumstances.

[0034] In embodiments of the present invention, the term "free radical scavenging material" refers to a flame retardant compound that can capture active free radicals such as ·H and ·OH in a combustion chain reaction, thereby interrupting the combustion chain reaction and inhibiting flame propagation and thermal runaway. Preferably, it is a phosphate ester or its derivative.

[0035] In embodiments of the present invention, the term "thermosetting polymer" refers to a substance that, under certain temperature or heating conditions, can undergo cross-linking polymerization of its corresponding monomers, transforming from a liquid / low viscosity state into an infusible and insoluble solid or gel state polymer.

[0036] In embodiments of the present invention, the term "thermally induced phase change material" refers to a substance that undergoes a reversible or irreversible phase transition (such as sol-gel transition, melt-solidification transition, or abrupt change in solubility) near a specific temperature threshold, resulting in a significant change in viscosity, morphology, or aggregation state.

[0037] In embodiments of the present invention, the term "phosphate ester monomer" refers to a monomeric compound whose molecular structure contains both phosphate ester groups and polymerizable unsaturated bonds, and which can undergo addition polymerization or crosslinking reactions under thermal initiation to form a polymer.

[0038] In embodiments of the present invention, the term "styrene monomer" refers to a monomer with a benzene ring as the parent core, containing polymerizable functional groups such as vinyl groups, which can undergo homopolymerization or copolymerization under thermal initiation to form a cross-linked polymer layer, and whose carbonizability and thermal stability can be improved through the benzene ring structure.

[0039] In embodiments of the present invention, the term "cyclic monomer" refers to a cyclic compound whose molecular structure contains a cyclic skeleton and can undergo ring-opening polymerization or cross-linking reaction under the action of heat or an initiator to form a stable polymer or network structure.

[0040] In embodiments of the present invention, the term "poloxam block copolymer" refers to an amphiphilic block copolymer composed of polyoxyethylene (PEO)-polypropylene (PPO)-polyoxyethylene (PEO) triblocks, which can undergo a sol-gel transition when the temperature changes, achieving a rapid increase in thermally responsive viscosity.

[0041] In embodiments of the present invention, the term "polyester film" is also known as Mylar film, which refers to a film made of polyethylene terephthalate (PET) as the main raw material, and is mainly used for insulation, support, fixation and protection inside the battery cell.

[0042] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0043] like Figure 1 and Figure 2 As shown, the first aspect of the present invention discloses a polyester film (Mylar film) 1, comprising:

[0044] Multiple channels 2 are spaced apart within the polyester film 1, each channel 2 comprising a closed chamber, for example, as shown in the reference. Figure 1 and Figure 2 Small elliptical channels are added inside the polyester film, and there is a certain distance between the elliptical channels, which can maintain the polyester film's ability to support and buffer forces.

[0045] A mixed flame retardant liquid is filled into a chamber. The mixed flame retardant liquid includes a free radical scavenging material and a thermal transition material. The thermal transition material includes a thermosetting polymer and / or a thermally induced phase change substance.

[0046] When the preset thermal runaway emergency conditions are met, the polyester film ruptures, and the mixed flame-retardant liquid flows out from the chamber. The free radical capture material is used to block the combustion reaction, and the thermal conversion material is used to form a physical barrier layer. The preset thermal runaway emergency conditions are the conditions corresponding to the risk of thermal runaway in the battery cell.

[0047] Through the above-described method, the channel adopts a spaced-out closed-chamber design, which can stably encapsulate the mixed flame-retardant liquid, preventing leakage during normal operation of the battery cell, while ensuring the insulation, support, and buffering performance of the polyester film itself. This does not affect the stability of the battery cell's internal structure, nor does it occupy the effective space of the battery cell, thus avoiding sacrifice of the battery cell's energy density and meeting the industry's demand for high-energy-density batteries. The polyester film does not rely on additional control components to trigger the protection function, matching the actual operating conditions of battery cell thermal runaway, with a fast response speed, effectively preventing thermal runaway from escalating into a serious safety accident. At the same time, the polyester film has a simple structure, requiring no significant modifications to the original battery cell structure, adapting to existing production lines, and enabling large-scale industrial application without additional line modifications. It balances safety, practicality, and economy. Furthermore, the mixed flame-retardant liquid and the battery cell electrolyte are independent of each other, not interfering with the electrolyte's ion transport process, effectively preventing an increase in the battery cell's internal resistance, and ensuring that the battery cell's core performance such as charge / discharge efficiency and cycle life are not affected.

[0048] In particular, the free radical scavenging material and the thermal transformation material in the mixed flame-retardant liquid of the present invention can work synergistically to achieve dual protection through the superposition of chemical blocking and physical barrier. When the battery cell is subjected to compression, puncture, or internal thermal runaway leading to a sudden increase in temperature and pressure, the polyester film will rupture in time. Preferably, its rupture pressure threshold is lower than the opening pressure of the battery cell's explosion-proof valve, ensuring that the mixed flame-retardant liquid flows out rapidly in the early stage of thermal runaway before the explosion-proof valve is opened. Among them, the free radical scavenging material decomposes upon heating to generate phosphorus-containing and other active free radicals, which can efficiently capture active free radicals such as ·H and ·OH in the combustion chain reaction, quickly interrupting the combustion reaction and inhibiting flame propagation and thermal runaway from the source. At the same time, the thermal transformation material undergoes rapid phase change, polymerization, or a sharp increase in viscosity under the action of thermal runaway temperature, forming a dense solid or gel-like physical barrier layer. This layer can not only block ion transport inside the battery cell and prevent the thermal runaway chain reaction from continuing, but also insulate against heat and oxygen, preventing the escape of internal combustible gases and the introduction of external heat, further curbing the spread of thermal runaway.

[0049] Meanwhile, since the polyester film directly wraps around or is adjacent to the electrode, separator, and electrolyte areas inside the battery cell, the mixed flame-retardant liquid, after flowing out from the ruptured channels, will rapidly diffuse, infiltrate, and fully mix with the electrolyte under the high temperature, high pressure, and capillary action inside the battery cell. This causes the electrolyte to change under the combined action of the mixed flame-retardant liquid and high temperature. On the one hand, the thermal transformation material rapidly undergoes thermally initiated polymerization or solution-gel phase transition at the thermal runaway temperature, becoming miscible with the electrolyte and causing the system viscosity to rise sharply. On the other hand, the free radical capturing material reacts rapidly with the active free radicals generated by the decomposition of the electrolyte, inhibiting the continuous decomposition and combustion of the electrolyte. This causes the electrolyte to rapidly transform from a volatile and flammable liquid state into a high-viscosity, flame-retardant, and low-reactivity gel state or semi-solid state, thereby further terminating the electrochemical reaction and the spread of thermal runaway.

[0050] Specifically, the preset thermal runaway emergency conditions are that the pressure on the polyester film is greater than the rupture pressure threshold of the polyester film and / or the temperature of the polyester film is higher than the melting point of the polyester film.

[0051] Specifically, the preset thermal runaway emergency condition is that the cell is at risk of thermal runaway due to at least one of the following:

[0052] The internal temperature of the battery cell rose abnormally.

[0053] The internal pressure of the battery cell increased abnormally.

[0054] The battery cell is subjected to abnormal mechanical stress.

[0055] Specifically, the battery cell includes an explosion-proof valve, which is preset to not open under thermal runaway emergency conditions.

[0056] More specifically, the preset conditions mainly correspond to various abuse conditions and abnormal states that may occur in the actual use of battery cells, including but not limited to:

[0057] When a battery is punctured, squeezed, collided, or bent, the internal structure of the cell deforms, and the polyester film is squeezed, sheared, or punctured by external forces, causing the local pressure it bears to quickly exceed its own rupture pressure threshold, thus causing it to rupture.

[0058] When the battery experiences abnormal conditions such as overcharging, internal short circuit, or thermal runaway, the internal temperature of the cell rises rapidly. When the temperature reaches or exceeds the melting point of the polyester film, the film melts, softens, and ruptures.

[0059] During thermal runaway, a large amount of gas is rapidly generated inside the battery cell, causing a sharp increase in internal pressure. The local pressure it experiences quickly exceeds its own rupture pressure threshold, resulting in rupture.

[0060] Under any of the above operating conditions, the polyester membrane can be automatically, quickly, and reliably triggered without relying on additional components such as sensors, control chips, and drive circuits. It can rupture in the early stage of thermal runaway and before the explosion-proof valve is opened, so that the mixed flame-retardant liquid in the chamber can be released in time and act on the inside of the cell, thereby achieving rapid suppression of combustion reaction and thermal runaway propagation.

[0061] Specifically, the channels can be formed by hot pressing, that is, by locally hot pressing the polyester film substrate, a closed cavity structure with intervals is formed inside the film. For example, two layers of polyester film are heated and pressurized in a preset area to form a composite. The non-hot-pressed area forms a hollow, closed channel because it is not compacted. The channels are separated by hot-pressed adhesive areas, thereby obtaining a stable and sealed cavity inside the polyester film for encapsulating mixed flame-retardant liquids.

[0062] In an embodiment of the present invention, the mass ratio of free radical scavenging material to thermal conversion material in the mixed flame-retardant liquid is (6~9.5):(0.5~3). This ratio ensures that the free radical scavenging material has a sufficient content to quickly and efficiently capture active free radicals in the combustion chain reaction, thus blocking combustion at a chemical level. It also allows the thermal conversion material to undergo phase change, polymerization, or viscosity increase at a reasonable dosage, forming a physical barrier layer and ensuring the flame-retardant effect. In a specific embodiment of the present invention, the mass percentage of the free radical scavenging material in the mixed flame-retardant liquid is 60%~95%, for example 60% or 80%, and the mass percentage of the thermal conversion material is 5%~30%, for example 20% or 30%.

[0063] Preferably, the mass ratio of free radical capturing material to thermal conversion material is (6~8):(2~3), which further enhances the synergistic effect of free radical capturing material and thermal conversion material. Chemical flame retardancy and physical barrier work together and reinforce each other, ensuring the ability to quickly suppress combustion reaction and forming a stable and dense physical barrier structure. When the battery cell experiences thermal runaway, crushing, puncture and other conditions, it can more quickly and effectively curb the spread of thermal runaway and further improve the safety protection performance of the battery cell.

[0064] In embodiments of the present invention, the free radical scavenging material is selected from one or more of dimethyl methyl phosphate (DMMP), tri(2,2,2-trifluoroethyl) phosphate (TFEP), and triphenyl phosphate (TPP). All three can efficiently capture highly reactive combustion free radicals such as ·H and ·OH generated during thermal runaway, thereby rapidly interrupting the combustion chain reaction and inhibiting flame propagation and thermal runaway spread.

[0065] Upon thermal decomposition, DMMP generates phosphorus-containing free radicals such as PO· and PO2·. These free radicals preferentially and efficiently capture ·H and ·OH free radicals in the combustion chain reaction, generating less reactive products (such as HPO·), thereby interrupting the combustion chain reaction. DMMP can significantly improve the flash point and fire extinguishing properties of the electrolyte with a very small amount added, exhibiting extremely high flame retardant efficiency.

[0066] TFEP not only generates phosphorus-containing free radicals such as PO· and PO2· to capture active combustion free radicals, but also contains the flame-retardant element fluorine (F), which enhances thermal stability from the source of the solute.

[0067] TPP contains phosphorus, and its free radical scavenging mechanism is similar to that of DMMP. However, it contains a unique benzene ring structure. When heated, the benzene ring structure tends to undergo cross-linking, aromatization, and carbonization reactions, forming a stable, continuous, and dense graphitized or aromatic coke layer. This carbonized layer acts like a "fireproof armor," insulating against heat and oxygen, preventing the escape of internal combustible gases and the influx of external heat, thus protecting the underlying material. More importantly, the benzene ring has a very high resonance stability energy (approximately 150 kJ / mol). Destroying its structure requires absorbing a large amount of heat, which effectively reduces the surface temperature of the material and delays thermal decomposition.

[0068] In embodiments of the present invention, the thermosetting polymer includes a polymeric monomer and / or an initiator. The polymeric monomer is selected from one or more of phosphate ester monomers, styrene monomers, and cyclic monomers. Under the influence of the thermal runaway temperature and the initiator, this type of polymeric monomer can rapidly undergo cross-linking polymerization or ring-opening polymerization reactions, forming an in-situ infusible and insoluble three-dimensional network polymer. This constructs a dense physical barrier layer inside the battery cell, blocking ion transport and isolating oxygen and heat transfer. Together with free radical scavenging materials, it achieves highly efficient suppression of thermal runaway.

[0069] In embodiments of the present invention, the initiator is a thermal initiator, specifically, the initiator is azobisisobutyronitrile (AIBN), or the initiator is a peroxide.

[0070] In embodiments of the present invention, the phosphate ester monomer is one or more of methacryloyloxyethyl phosphate (MEP) and triallyl phosphate (TAP).

[0071] In embodiments of the present invention, the styrene monomer is one or more of bromostyrene, tribromostyrene, and divinylbenzene.

[0072] In embodiments of the present invention, the cyclic monomer is one or more of hexachlorocyclotriphosphazene and ethylene oxide.

[0073] All of the aforementioned monomers possess a rapid thermal response, enabling rapid polymerization at the initial temperature of the battery cell's thermal runaway, achieving a response within seconds and promptly forming a physical barrier layer. This makes them particularly suitable for products like battery cells with high safety requirements. Furthermore, the polymerization of these monomers forms a three-dimensional cross-linked network structure with high strength and stability, effectively blocking ion transport and isolating oxygen and heat transfer. Their molecular structures contain flame-retardant functional groups such as phosphorus, bromine, benzene rings, and cyclic skeletons, which, while forming a physical barrier, can also help enhance the system's flame-retardant, flame-suppressing, and char-forming capabilities, forming a highly efficient synergy with free radical capture materials. Moreover, these monomers have good compatibility with the electrolyte, and after release, they can rapidly diffuse, wet, and act on the inside of the battery cell, causing the electrolyte to quickly transform into a gel / solid state, achieving thermal shutdown of the monomers. They also exhibit high stability and moderate viscosity at room temperature, making them easy to encapsulate within the pores of a polyester film without affecting the normal operation of the battery cell.

[0074] In embodiments of the present invention, the thermotropic phase change material is selected from one or more of polyethylene glycol (PEG), poloxamer-based block copolymers, and poly(N-isopropylacrylamide). PEG utilizes polymer phase change or monomer thermal polymerization to transform the liquid electrolyte into a gel / solid in the early stages of thermal runaway, drastically increasing internal resistance and achieving "thermal shutdown," physically blocking ion flow and thus extinguishing the thermal runaway chain reaction. Poloxamer-based block copolymers and poly(N-isopropylacrylamide), upon reaching a critical temperature, experience a sudden change in polymer solubility or self-assembly of gelling factors, leading to an exponential increase in electrolyte viscosity (sol-gel transition), slowing ion migration.

[0075] These thermo-induced phase change materials have advantages such as sensitive triggering, rapid response, good compatibility with electrolyte, and stable physical barrier effect, making them suitable for use in battery cells.

[0076] In embodiments of the present invention, such as Figure 1As shown, the polyester membrane 1 includes a membrane body 11 and a membrane flange 12 folded from the membrane body 11. Multiple channels 2 are evenly distributed within the membrane body 11, meaning that the channels 2 are only located on the large surface of the polyester membrane 1. On the one hand, this ensures the structural integrity and higher strength of the membrane flange area, facilitating wrapping, fixing, and sealing during the battery cell assembly process, thereby improving assembly stability and sealing performance. On the other hand, it allows the mixed flame-retardant liquid to be concentrated in the core area directly corresponding to the electrode and separator, enabling it to act more accurately and quickly on the key parts of the battery cell that are heating and reacting when thermal runaway is triggered, thus improving the effectiveness and timeliness of flame-retardant protection.

[0077] In embodiments of the present invention, the pore size of the channel is 75-100 μm, for example 75 μm, 77 μm, 80 μm, 82 μm, 85 μm, 88 μm, 92 μm, 95 μm, 98 μm, and 100 μm. The above pore size can ensure sufficient flame retardant liquid filling without significantly reducing the mechanical strength, insulation, and structural stability of the polyester film, thus balancing the liquid storage function and the mechanical properties of the membrane itself.

[0078] In embodiments of the present invention, the spacing between adjacent channels is 25 to 45 μm, for example, 25 μm, 28 μm, 30 μm, 32 μm, 35 μm, 37 μm, 39 μm, 41 μm, 43 μm, and 45 μm. The above spacing can ensure the overall support and buffering capacity of the membrane body, avoid the membrane body from being easily damaged due to excessively dense channels, and at the same time ensure that the flame retardant liquid can quickly and fully wet the internal area of ​​the battery cell when thermal runaway is triggered.

[0079] In embodiments of the present invention, the thickness of the polyester film is 75μm~100μm, which can effectively perform its own function and reliably support the pores and the mixed flame-retardant liquid therein.

[0080] A second aspect of the present invention discloses a lithium-ion battery comprising the polyester film described in the above embodiments. The lithium-ion battery further comprises a cell and a casing, wherein the cell is wrapped with the polyester film and placed within the casing, and the casing is provided with an explosion-proof valve.

[0081] Under abnormal operating conditions, this lithium-ion battery can quickly trigger dual flame-retardant protection through the polyester film, effectively suppressing the spread of thermal runaway without sacrificing energy density, increasing internal resistance, or requiring significant modifications to existing production lines, thus possessing excellent safety and practicality.

[0082] The following will describe the implementation methods in more detail.

[0083] Examples 1-4 and Comparative Examples 1-3 all consist of liquid substances filling the pores of the polyester membrane, and their specific compositions are shown in Table 1 below. Simultaneously, puncture tests were performed on each example and comparative example, and the test structures are also shown in Table 1.

[0084] Table 1:

[0085]

[0086] It can be seen that the use of polyethylene glycol and phosphorus-based substances significantly improves the number of needle penetration tests passed. When tris(2,2,2-trifluoroethyl) phosphate (TFEP) and triallyl phosphate monomers are used in combination, all tests pass. This is because TFEP not only contains phosphorus, whose free radicals preferentially and efficiently capture the active H and OH radicals in the combustion chain reaction, generating less reactive products (such as HPO₄·), but also contains fluorine, which, being non-flammable, further enhances its flame-retardant effect. Conversely, when only polyethylene glycol is used, its high viscosity and poor heat dissipation, while acting as insulation for the internal core, trigger premature diaphragm contraction and rupture, leading to short circuits between the positive and negative electrodes, premature thermal runaway, instantaneous heat release, and cell explosion, all of which hinder needle penetration testing. When phosphorus-based flame retardants are used alone, the release of flame-retardant free radicals is slow, and the efficiency of capturing ·H and ·OH active free radicals is low, which reduces the probability of thermal runaway. When these two substances are used in combination, the advantages of both can be combined to achieve a 1+1>2 effect.

[0087] Comparing Examples 2, 4, and Comparative Example 3, it can be seen that the optimal improvement effect can be achieved when TFEP is used in combination with monomers and initiators that can form solid substances. When using triallyl phosphate monomer, the internal temperature of the cell increases sharply at the beginning of needle penetration. Under the influence of temperature, azobisisobutyronitrile initiates the polymerization of triallyl phosphate monomer. This polymer penetrates into the various gaps inside the stacked core. When the positive and negative electrodes are about to short-circuit, the solid polymer formed fills the space between the positive and negative electrodes, acting as a physical firewall, thereby greatly reducing the phenomenon of gas and heat generation inside the cell. At the same time, TFEP absorbs heat in this process and generates PO· and PO2· free radicals, which capture the ·H and ·OH active free radicals generated in the early stage, thereby blocking the chain reaction of combustion and achieving a significant improvement effect.

[0088] While the present invention has been illustrated and described with reference to certain preferred embodiments, those skilled in the art should understand that the above description is a further detailed explanation of the invention in conjunction with specific embodiments, and should not be construed as limiting the specific implementation of the invention to these descriptions. Various changes in form and detail can be made by those skilled in the art, including several simple deductions or substitutions, without departing from the spirit and scope of the invention.

Claims

1. A polyester film for use in battery cells, characterized in that, include: Multiple channels are spaced apart within the polyester film, each channel comprising a closed chamber; A mixed flame-retardant liquid is filled into the chamber. The mixed flame-retardant liquid includes a free radical scavenging material and a thermal transition material. The thermal transition material includes a thermosetting polymer and / or a thermo-induced phase change substance. When the preset thermal runaway emergency conditions are met, the polyester film ruptures, the mixed flame-retardant liquid flows out of the chamber, the free radical capturing material is used to block the combustion reaction, and the thermal conversion material is used to form a physical barrier layer. The preset thermal runaway emergency conditions are the conditions corresponding to the risk of thermal runaway of the battery cell.

2. The polyester film for battery cells as described in claim 1, characterized in that, In the mixed flame-retardant liquid, the mass ratio of the free radical capturing material to the thermal conversion material is (6~9.5):(0.5~3).

3. The polyester film for battery cells as described in claim 2, characterized in that, The mass ratio of the free radical capturing material to the thermal conversion material is (6~8):(2~3).

4. The polyester film for battery cells as described in claim 1, characterized in that, The free radical scavenging material is selected from one or more of dimethyl methyl phosphate, tri(2,2,2-trifluoroethyl) phosphate, and triphenyl phosphate.

5. The polyester film for battery cells as described in claim 1, characterized in that, The thermosetting polymer includes a polymerization monomer and / or an initiator, wherein the polymerization monomer is selected from one or more of phosphate ester monomers, styrene monomers, and cyclic monomers.

6. The polyester film for battery cells as described in claim 5, characterized in that, The phosphate ester monomers are one or more of methacryloyloxyethyl phosphate and triallyl phosphate, the styrene monomers are one or more of bromostyrene, tribromostyrene and divinylbenzene, and the cyclic monomers are one or more of hexachlorocyclotriphosphazene and ethylene oxide.

7. The polyester film for battery cells as described in claim 1, characterized in that, The thermo-induced phase change material is selected from one or more of polyethylene glycol, poloxamer block copolymers, and poly(N-isopropylacrylamide).

8. The polyester film for battery cells as described in claim 1, characterized in that, The polyester film includes a film body and a film flange folded from the film body, wherein the plurality of channels are uniformly distributed within the film body. And / or, the preset thermal runaway emergency condition is that the pressure borne by the polyester film is greater than the rupture pressure threshold of the polyester film and / or the temperature of the polyester film is higher than the melting point of the polyester film. And / or, the preset thermal runaway emergency condition is that the cell is at risk of thermal runaway due to at least one of the following: The internal temperature of the battery cell rose abnormally. The internal pressure of the battery cell increased abnormally. The battery cell is subjected to abnormal mechanical stress.

9. The polyester film for battery cells as described in claim 1, characterized in that, The diameter of the channel is 75–100 μm. And / or, the spacing between adjacent channels is 25–45 μm. And / or, the battery cell includes an explosion-proof valve, which is not opened under the preset thermal runaway emergency condition.

10. A lithium-ion battery, characterized in that, Includes the polyester film for battery cells as described in any one of claims 1-9.

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