A safety coating for a battery electrode sheet and an electrode sheet

Abstract

This application provides a safety coating and electrode sheet for battery electrode sheets, belonging to the field of new energy battery technology. The safety coating is a composite layer structure, comprising: a barrier layer disposed on a current collector, the impedance of which increases stepwise with increasing potential difference at its location; and a thermally fused insulating layer disposed on the barrier layer, the impedance of which increases stepwise with increasing temperature. The barrier layer and the thermally fused insulating layer function synergistically, providing a bidirectional response mechanism for voltage and temperature. This safety coating for battery electrode sheets, through the synergistic effect of the barrier layer and the thermally fused insulating layer, can ensure the stability and performance of the battery during normal operation. In the event of internal short circuits or other faults, it responds rapidly and forms an effective insulating barrier, cutting off current and preventing safety accidents such as battery thermal runaway, thus providing reliable safety assurance for battery use.

Description

Technical Field

[0001] This application relates to the field of new energy battery technology, and in particular to a safety coating and electrode sheet for battery electrode sheets. Background Technology

[0002] New energy sources such as wind power and photovoltaics are characterized by high volatility and randomness, necessitating large-scale energy storage. Secondary battery energy storage systems, such as lithium-ion and sodium-ion batteries, offer advantages like high energy density, high conversion efficiency, short construction periods, and easy installation, making them suitable for large-scale energy storage and promising a bright future. However, during long-term operation, these secondary batteries are prone to thermal runaway due to internal or external faults, releasing large amounts of heat and explosive gases, potentially leading to fires and explosions. Recent years have seen numerous fires and explosions involving battery energy storage systems both domestically and internationally. The safety of lithium-ion and sodium-ion battery energy storage systems is a pressing technical challenge that needs to be addressed in the energy storage field.

[0003] Currently, during the manufacturing process of the positive electrode, a safety coating of a positive temperature coefficient (PTC) material is applied to the positive electrode current collector to enhance battery safety. When the battery temperature rises, the resistance of the PTC material increases, which in turn increases the resistance of the entire electrode active material layer, thus achieving the safety effect of cutting off power and preventing the electrochemical reaction from continuing.

[0004] Patent CN106229474A discloses a multifunctional coating for lithium-ion batteries, comprising the following components: 100 parts by weight of a conductive agent, 20-5000 parts by weight of ceramic material particles, and 10-500 parts by weight of a binder. Patent CN107437622A discloses an electrode and its preparation method, wherein an electrode coating is disposed between a current collector and an active material layer, comprising: a polymer matrix; and a conductive agent; the polymer matrix is ​​selected from one or more of low-density polyethylene, high-density polyethylene, epoxy resin, polyvinylidene fluoride, ethylene-ethyl acrylate copolymer, ethylene-vinyl acetate copolymer, polybutene, cellulose acetate, and polyamide; the conductive agent is selected from one or more of spiky Ni powder, spiky Cu powder, spiky Al powder, and spiky tungsten carbide.

[0005] In existing technologies, PTC materials exhibit high resistance even when not in use, significantly reducing battery energy conversion efficiency and other performance characteristics, increasing battery heat generation, and potentially inducing thermal runaway. Therefore, there is a need to develop coatings that exhibit low resistance during normal battery operation, do not affect battery safety, and can respond rapidly to internal battery faults, thereby enhancing battery safety. Summary of the Invention

[0006] This application is made in view of the above-mentioned problems, and its purpose is to provide a safety coating and electrode sheet for battery electrode sheets.

[0007] Specifically, the first aspect of this application provides a safety coating for battery electrode sheets, wherein the safety coating is a composite layer structure, comprising:

[0008] A barrier layer is disposed on the current collector, and the impedance of the barrier layer increases stepwise with the increase of the potential difference at the location.

[0009] A hot-melt insulating layer is disposed on the barrier layer, and the impedance of the hot-melt insulating layer increases stepwise with increasing temperature;

[0010] The barrier layer and the thermally fused insulating layer work synergistically to provide a bidirectional response mechanism for voltage and temperature.

[0011] Furthermore, the barrier layer includes a functional material with semiconductor properties, the Fermi level of which matches the Fermi level of the current collector to form an ohmic contact during normal battery operation.

[0012] Furthermore, the functional material with semiconductor properties is selected from at least one of polyaniline, polypyrrole, poly(3,4-ethylenedioxythiophene) and its derivatives, as well as composites thereof.

[0013] Further, the hot-melt insulating layer comprises:

[0014] Self-melting polymer matrix;

[0015] A conductive medium having a preset melting point dispersed in the self-melting polymer matrix;

[0016] Heat-resistant insulating particles dispersed in the self-melting polymer matrix;

[0017] The melting point of the low-melting-point conductive medium is higher than the glass transition temperature of the self-melting polymer matrix, and lower than or equal to the melting point of the self-melting polymer matrix.

[0018] Furthermore, the conductive medium having a preset melting point is a low-melting-point metal or alloy powder, selected from at least one of indium, tin, bismuth, gallium, and their alloys.

[0019] Furthermore, the hot-melt insulating layer comprises, by weight percentage: 20%-50% self-melting polymer matrix, 15%-25% low-melting-point conductive medium, 30%-50% highly stable heat-resistant insulating particles, 2%-5% binder, and 1%-5% processing aid.

[0020] Furthermore, the thickness T1 of the barrier layer is 50 nm-500 nm;

[0021] And / or, the thickness of the heat-fused insulation layer, T2, is greater than 10 × T1.

[0022] A second aspect of this application provides a method for preparing a safety coating, the method comprising the following steps:

[0023] A barrier layer slurry is prepared and coated onto the current collector, and a barrier layer is formed by a first drying process.

[0024] A hot-melt insulating layer slurry is prepared and coated onto the barrier layer, followed by a second drying process to form a hot-melt insulating layer.

[0025] The highest temperature of the second drying process is lower than that of the self-melting polymer matrix in the heat-melting insulating layer and lower than that of the conductive medium.

[0026] Furthermore, the second drying process is a programmed temperature drying process, including at least one heat preservation stage in the range of 80°C to 110°C.

[0027] A third aspect of this application provides an electrode sheet comprising a current collector and an active material layer, wherein the aforementioned safety coating is disposed between the current collector and the active material layer.

[0028] The present invention has the following beneficial effects:

[0029] (1) The barrier layer of the present invention is directly coated on the current collector, and its core function is voltage response. It is composed of functional materials with semiconductor properties, and its Fermi level matches the Fermi level of the current collector. Under normal battery operating voltage, this layer allows electrons to pass through (ohmic contact) with extremely low impedance. When the battery experiences a fault such as an internal short circuit, causing the local potential difference to increase sharply to a critical value, the layer will be suppressed due to the quantum tunneling effect or undergo electrochemical decomposition, and its impedance will increase by a step, thus effectively limiting the short-circuit current before the temperature rises significantly. The thermally molten insulating layer is coated on the barrier layer and contacts the electrode active material layer. Its core function is temperature response. This layer contains the improved self-melting polymer system and insulating support particles of the present invention. When the battery temperature rises due to a fault and exceeds the critical melting point, this layer melts, forming a dense insulating barrier that permanently cuts off the current.

[0030] (2) This coating combines voltage and temperature response mechanisms, enabling it to function under various battery fault scenarios. During normal battery operation, the barrier layer forms an ohmic contact with the current collector, resulting in minimal resistance. This does not significantly affect the battery's energy conversion efficiency or other performance characteristics, nor does it increase battery heat generation, thus avoiding the risk of inducing thermal runaway. When a fault occurs inside the battery, the coating responds rapidly, whether due to a sharp increase in local potential difference or a rise in temperature. When an internal short circuit occurs, causing a sharp increase in local potential difference, the barrier layer reacts quickly, with its impedance increasing dramatically, effectively limiting the short-circuit current and preventing further deterioration of the fault. Even if the temperature has not risen significantly under these conditions, the barrier layer still provides crucial protection. If the battery temperature rises due to various reasons, the thermally fused insulating layer will come into play. When the temperature exceeds the critical melting point, the self-melting polymer matrix melts, and the low-melting-point conductive medium originally dispersed within it also changes. Working together with the highly stable heat-resistant insulating particles, it forms a dense insulating barrier, permanently cutting off the current and preventing the electrochemical reaction from continuing, thereby greatly improving battery safety. Attached Figure Description

[0031] To more clearly illustrate the technical solutions in the embodiments of this drawing or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this drawing. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0032] Figure 1 This is a schematic diagram of the electrode sheet structure.

[0033] The purpose, features, and advantages of this accompanying drawing will be further explained in conjunction with the embodiments and with reference to the accompanying drawing. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of this application clearer, the following description and illustration are provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application. All other embodiments obtained by those skilled in the art based on the embodiments provided in this application without inventive effort are within the scope of protection of this application.

[0035] Obviously, the following description is merely some examples or embodiments of this application. Those skilled in the art can apply this application to other similar scenarios without any inventive effort. Furthermore, it is understood that although the effort involved in such development may be complex and lengthy, for those skilled in the art related to the content disclosed in this application, any changes to design, manufacturing, or production based on the technical content disclosed in this application are merely conventional technical means and should not be construed as insufficient disclosure of the content of this application.

[0036] An embodiment of the first aspect of this application provides a safety coating for a battery electrode sheet, the safety coating being a composite layer structure comprising:

[0037] A barrier layer is disposed on the current collector, and the impedance of the barrier layer increases stepwise with the increase of the potential difference at the location.

[0038] A hot-melt insulating layer is disposed on the barrier layer, and the impedance of the hot-melt insulating layer increases stepwise with increasing temperature;

[0039] The barrier layer and the thermally fused insulating layer work synergistically to provide a bidirectional response mechanism for voltage and temperature.

[0040] The barrier layer is directly coated on the current collector, and its core function is voltage response. It is composed of functional materials with semiconductor properties, and its Fermi level matches the Fermi level of the current collector. Under normal battery operating voltage, this layer allows electrons to pass through (ohmic contact) with extremely low impedance. When the battery experiences a fault such as an internal short circuit, causing the local potential difference to increase sharply to a critical value (e.g., >2V), the layer will be suppressed due to the quantum tunneling effect or undergo electrochemical decomposition, and its impedance will increase by a step, thus effectively limiting the short-circuit current before the temperature rises significantly.

[0041] A thermally molten insulating layer is coated on top of the barrier layer and contacts the electrode active material layer; its core function is temperature response. This layer contains the improved self-melting polymer system and insulating support particles of this invention. When the battery temperature rises due to a fault and exceeds the critical melting point, this layer melts, forming a dense insulating barrier that permanently cuts off the current.

[0042] In this embodiment, the barrier layer comprises a functional material with semiconductor properties, selected from at least one of polyaniline, polypyrrole, poly(3,4-ethylenedioxythiophene) and its derivatives, and composites thereof. These materials can have their conductivity and Fermi level tuned through chemical doping.

[0043] By precisely controlling the doping degree and material selection, a critical ohmic contact is formed with the current collector (such as Al foil). This is achieved within the normal battery operating voltage window (e.g., lithium iron phosphate system: 2.5-3.6V vs. Li / Li). + This layer exhibits good conductivity. Once an internal short circuit occurs, the local potential difference increases instantaneously, exceeding the stable voltage window of the conductive polymer, leading to rapid electrochemical dedoping and a decrease in conductivity by several orders of magnitude. These two mechanisms work together to achieve a voltage-triggered high-impedance response.

[0044] This layer is extremely thin, typically 50 nm to 500 nm, preferably 50 to 200 nm. This thickness is carefully designed to ensure both good electron tunneling effect (low resistance) at room temperature and rapid and complete transition to a high-resistivity state upon triggering.

[0045] Specifically, the barrier layer comprises the following raw materials by weight percentage:

[0046] Functional materials with semiconductor properties: 42-48%, selected from poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) dispersion (solid content 1.3wt%), whose work function is well matched with aluminum current collector.

[0047] Ionic liquid dopant: 2-8%, using 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMIM-TFSI) to precisely tune the conductivity and energy level structure of PEDOT.

[0048] Crosslinking agent: 1-5%, using (3-glycidyl ether oxypropyl)trimethoxysilane (GOPS) to improve the mechanical strength of the coating and its adhesion to the current collector.

[0049] Solvent: Deionized water / isopropanol mixed solvent (volume ratio 4:1), bring to 100%. This solvent system ensures that all components are fully dissolved and dispersed.

[0050] In this embodiment, the hot-melt insulating layer includes:

[0051] Self-melting polymer matrix;

[0052] A conductive medium having a preset melting point is dispersed in the self-melting polymer matrix;

[0053] Highly stable heat-resistant insulating particles dispersed in the self-melting polymer matrix;

[0054] The melting point of the low-melting-point conductive medium is higher than the glass transition temperature of the self-melting polymer matrix, and lower than or equal to the melting point of the self-melting polymer matrix.

[0055] Specifically, the self-fusing polymer matrix is selected from low-melting copolymers or blends of crystalline polyesters (such as PBT) and amorphous engineering plastics (such as polyaryletherketone). Through molecular design, there is a relatively wide softening range (such as 90-110 °C) between its melting point (Tm) and glass transition temperature (Tg). Within this temperature range, although the polymer is not completely melted, it has begun to soften and has a certain fluidity.

[0056] The conductive medium uses low-melting metal or alloy powders (such as bismuth-tin eutectic alloy, melting point 138 °C; or indium-gallium alloy, melting point 15.7 °C) as the conductive medium, and its average particle size is smaller than that of the insulating particles.

[0057] Insulating particles: Select alumina (Al2O3), etc.

[0058] Temperature response mechanism of the thermally fused insulating layer:

[0059] Primary stage (softening, Tg < T < Tm): When the temperature rises to the softening range, the polymer matrix becomes soft. At this time, under the mechanical stress (such as lithium dendrite piercing, extrusion) generated due to faults inside the battery, the low-melting metal powders dispersed in the polymer will be extruded and aggregated, gradually forming a continuous conductive path, resulting in the coating resistance not increasing but decreasing. This "abnormal" phenomenon causes the current at the short-circuit point to increase sharply, actively and quickly heating the local area, accelerating the local temperature rise until it reaches the complete melting temperature of the second layer.

[0060] Ultimate stage (melting, T ≥ Tm): After the temperature reaches the common melting point of the polymer and the low-melting metal, the polymer completely melts and levels off. At the same time, the liquid metal balls will automatically spheroidize and shrink due to the extremely high surface tension, disconnecting the temporary conductive path formed by them. The molten polymer and liquid metal are finally blocked and segmented by the underlying electron tunneling barrier layer and insulating particles, forming a completely insulating composite barrier.

[0061] This mechanism of "self-heating through forming a temporary conductive path in the initial stage (softening stage); forming a permanent insulating barrier in the later stage (melting stage)" solves the bottleneck problem that the thermal trigger response speed depends on external heat diffusion, realizes self-accelerated thermal triggering of the fault point, and greatly improves the response speed.

[0062] In this embodiment, the thermally fused insulating layer includes by mass percentage: 20%-50% of self-fusing polymer matrix, 15%-25% of conductive medium, 30%-50% of insulating particles, 2%-5% of binder, and 1%-5% of processing aids.

[0063] In this embodiment, the self-melting polymer matrix is ​​polybutylene adipate / terephthalate (PBAT), which has a melting point of 115-125℃, a glass transition temperature Tg≈-30℃, and significant softening properties in the 90-110℃ range.

[0064] Bi is selected as the low melting point conductive medium. 35 Sn 47 In 18 Eutectic alloy powder (melting point 138℃), particle size distribution D50 = 2.0μm, D90<5.0μm.

[0065] The highly stable heat-resistant insulating particles are made of spherical α-alumina (α-Al2O3), with a particle size distribution D50 = 4.5μm (>> metal powder particle size) and a purity ≥99.99%.

[0066] The binder is hydrogenated styrene-butadiene block copolymer (SEBS), which is compatible with PBAT and provides flexibility.

[0067] Processing aids include dispersants, thickeners, and solvents. The dispersant is polyether-modified polydimethylsiloxane. The thickener is hydrogenated castor oil, used to adjust the rheological properties of the slurry.

[0068] The solvent used was a mixture of terpineol and decanoic acid (volume ratio 9:1), which was brought to 100%. This high-boiling-point solvent system is suitable for subsequent programmed temperature drying.

[0069] In this embodiment, the thickness T1 of the barrier layer is 50 nm-500 nm; the thickness T2 of the hot-melt insulating layer is greater than 10 × T1.

[0070] An embodiment of the second aspect of this application provides a method for preparing a safety coating, the method comprising the following steps:

[0071] A barrier layer slurry is prepared and coated onto the current collector, and a barrier layer is formed by a first drying process.

[0072] A hot-melt insulating layer slurry is prepared and coated onto the barrier layer, and a second drying process is performed to form a hot-melt insulating layer; the second drying process is a programmed temperature rise drying, including at least one heat preservation stage in the range of 80°C-110°C.

[0073] The highest temperature of the second drying process is lower than the melting point of the self-melting polymer matrix and the low-melting-point conductive medium in the heat-melting insulating layer.

[0074] Specifically, the preparation and coating of the barrier layer (first layer):

[0075] Slurry preparation: In an argon-filled glove box (H2O, O2 < 0.1 ppm), 45 g PEDOT:PSS dispersion, 5 g EMIM-TFSI, and 3 g GOPS were sequentially added to 44.7 g of water / isopropanol mixed solvent. The mixture was transferred to a planetary mixer and stirred at 500 rpm for 30 min, followed by high-speed dispersion at 2000 rpm for 60 min to obtain the slurry.

[0076] Coating and Pre-curing: The slurry is continuously coated onto a clean 20μm aluminum foil current collector using a slit extrusion coating method.

[0077] After coating, the electrode sheet immediately enters a stepped drying oven: Zone 1, 60-70℃, 1-3 min; Zone 2, 80-90℃, 2-5 min, to form a barrier layer. Dry air with a dew point <-40℃ is circulated throughout the process for protection. After drying, the electrode sheet is wound up to obtain a semi-finished product coated with the first layer.

[0078] In this embodiment, the preparation and composite coating of the hot-melt insulating layer (second layer) are as follows:

[0079] Slurry preparation: First, add 18g BiSnIn alloy powder, 40g α-Al2O3 powder, and 35g mixed solvent (terpineol / decanoic acid) to the hopper of a planetary mixer. Add 2g dispersant and pre-disperse by stirring at 800 rpm for 20 minutes. Then switch to a high-speed dispersion mode with a revolution speed of 3000 rpm and a rotation speed of 1500 rpm for 90 minutes to ensure that the metal and ceramic powders are fully wetted and dispersed. Reduce the speed to 500 rpm and add 35g PBAT granules, 4g SEBS, and 1g hydrogenated castor oil in sequence. Stir for 120 minutes until the polymer is completely dissolved, forming a uniform paste slurry with a viscosity of approximately 4500 mPa·s (@25℃).

[0080] Composite Coating and Programmed Temperature Curing: A second set of slotted nozzles is used to directly apply the second layer of slurry online onto the surface of the electrode sheet that has already undergone the first coating layer but has not yet been wound up. The composite coating then enters a programmed temperature curing oven with the following parameters set:

[0081] Temperature zone 1: 80-85℃, 2-3 min, initially removes most of the solvent.

[0082] Temperature zone 2: 100-105℃, 4-5 min, to completely remove residual solvent and soften the PBAT matrix, allowing for molecular-level interpenetration with the first layer interface to achieve a strong bond.

[0083] Temperature zone 3: 60-65℃, 1-2 min, cooling and setting.

[0084] The highest temperature during the entire curing process must be far below the melting point of PBAT (115℃) and the melting point of the metal alloy (138℃), and any material must not undergo premature phase transformation.

[0085] Finally, the electrode sheet coated with the double-layer safety coating is coated with a positive electrode active material (such as lithium iron phosphate) slurry according to conventional processes, and then dried at 120°C for 8 hours to complete the preparation of the positive electrode sheet.

[0086] See Figure 1 A third aspect of this application provides an electrode sheet comprising a current collector and an active material layer, wherein the aforementioned safety coating is disposed between the current collector and the active material layer.

[0087] The safety coating and electrode sheet provided by this invention have significant advantages. This bidirectional response mechanism greatly improves battery safety. During normal battery operation, both the barrier layer and the thermally fused insulating layer of the safety coating ensure normal battery performance and stable operation. When a fault such as an internal short circuit occurs, the barrier layer responds quickly to abnormal voltage, limiting the short-circuit current, while the thermally fused insulating layer forms an insulating barrier in time when the temperature rises, cutting off the current. This effectively prevents safety accidents caused by short circuits, overheating, etc., such as fires and explosions, providing reliable safety assurance for battery use.

[0088] Example

[0089] The following examples describe the disclosure of this invention in more detail. These examples are merely illustrative, as various modifications and variations will be apparent to those skilled in the art within the scope of this disclosure. Unless otherwise stated, all parts, percentages, and ratios reported in the following examples are based on weight. Unless otherwise stated, all reagents used in the examples are available commercially or synthesized using conventional methods and are ready for use without further processing. Unless otherwise stated, all instruments used in the examples are available commercially.

[0090] Example 1

[0091] A safety coating for battery electrode sheets includes a barrier layer and a heat-melting insulating layer.

[0092] The barrier layer comprises the following raw materials by mass percentage: 45% PEDOT:PSS dispersion, 5% EMIM-TFSI, 3% GOPS, with the remainder made up to 100% by solvent;

[0093] The hot-melt insulating layer comprises the following raw materials by weight percentage: PBAT 33%, Bi 35 Sn 47 In18 Eutectic alloy powder 20%, α-Al2O3 40%, SEBS 3%, dispersant 1%, thickener 1%, terpineol / decanoic acid mixed solvent 2%.

[0094] The method for preparing the safety coating for battery electrode sheets includes the following steps:

[0095] A barrier layer slurry is prepared and coated onto the current collector. The first drying process is carried out in the first zone at 60-70℃ for 1-3 min and in the second zone at 80-90℃ for 2-5 min to form a barrier layer with a thickness of 150 nm.

[0096] A hot-melt insulating slurry is prepared and coated onto the barrier layer. The slurry is then subjected to a second drying process: temperature zone 1: 80-85℃, 2-3 min; temperature zone 2: 100-105℃, 4-5 min; temperature zone 3: 60-65℃, 1-2 min, to form a hot-melt insulating layer with a thickness of 8.0 μm.

[0097] Example 2

[0098] This embodiment is basically the same as Embodiment 1, except that the thickness of the barrier layer is 100 nm.

[0099] Example 3

[0100] This embodiment is basically the same as Embodiment 1, except that the low-melting-point conductive dielectric Bi is used. 35 Sn 47 In 18 The mass fraction of the eutectic alloy powder is 15%.

[0101] Example 4

[0102] This embodiment is basically the same as Embodiment 1, except that the high-stability heat-resistant insulating particles are boehmite (γ-AlOOH) with a particle size D50=3.0μm.

[0103] Example 5

[0104] This embodiment is basically the same as that of Embodiment 1, except that the conductive polymer is replaced with polyaniline (PANI).

[0105] Comparative Example 1

[0106] This comparative example is basically the same as Example 1, but the coating on the current collector is a single coating, which is a mixture of carbon black (conductive agent), polyethylene (PTC polymer matrix) and Al2O3, with a thickness of 10 μm.

[0107] Comparative Example 2

[0108] This comparative example is basically the same as Example 1, except that only the first layer of Example 1 is coated, with a thickness of 150nm.

[0109] Comparative Example 3

[0110] This comparative example is basically the same as Example 1, except that only the second layer of Example 1 is coated, with a thickness of 8.0 μm.

[0111] Comparative Example 4

[0112] This comparative example is basically the same as Example 1, except that the low melting point alloy in the second layer is replaced with an equal amount of carbon nanotubes (CNTs).

[0113] Comparative Example 5

[0114] This comparative example is basically the same as Example 1, except that the temperature of the drying program zone 2 is set to 130°C after the second coating is applied.

[0115] Comparative Example 6

[0116] This comparative example is basically the same as Example 1, except that only the first layer of Example 1 is coated, with a thickness of 450nm.

[0117] Comparative Example 7

[0118] This comparative example is basically the same as Example 1, except that no safety coating is applied to the current collector.

[0119] Experimental Case 1

[0120] The electrodes prepared in Examples 1-5 and Comparative Examples 1-5 were assembled into pouch lithium iron phosphate batteries with a rated capacity of 5 Ah. The AC internal resistance at room temperature (1 kHz) was measured using a battery internal resistance tester, and a needle penetration test was performed using an adiabatic accelerated calorimeter (ARC): a fully charged (100% SOC) battery was placed in the ARC, and a 3 mm diameter tungsten steel needle was used to pierce the battery at a speed of 80 mm / s, while simultaneously recording the battery voltage and temperature changes throughout the process. The trigger time was defined as the time from the moment of needle penetration to a voltage drop (more than 1 V) or current interruption, and the highest temperature was the peak temperature of the battery surface recorded during the needle penetration process. The test results are shown in Table 1.

[0121]

[0122] As shown in the table above, in Example 1, at the moment of needle penetration, the steel needle creates a direct short circuit between the positive and negative electrodes, and the local potential difference instantaneously exceeds 2V. The first layer of PEDOT rapidly undergoes electrochemical dedoping, resulting in a surge in impedance, which restricts most of the current before the heat generation can diffuse. Subsequently, the limited current and initial heat soften the second layer of PBAT. The low-melting-point alloy powder within it forms a temporary pathway under mechanical extrusion, generating a "self-heating" effect. The local temperature rapidly rises to above 138°C, and the alloy and polymer melt successively. The alloy spheroidizes and shrinks, while the polymer flows and levels, forming the final high-impedance insulating barrier. The voltage response initiates before the temperature response, and the temperature response is accelerated by self-heating. The two work together to achieve millisecond-level comprehensive protection. In Example 2, the first layer is thinner, resulting in a stronger electron tunneling effect and a slightly lower initial internal resistance. However, its mechanical strength and insulating barrier effect are slightly weaker, which may lead to partial damage to the barrier formed by the second layer during subsequent thermal processes. Therefore, the final temperature is slightly higher than in Example 1, but still far below the critical point. Example 3: The reduced alloy content weakens the "self-heating" effect of the second layer during the softening stage, slowing the temperature rise rate. This results in a slight delay in the time to complete melting and circuit breaking, leading to a higher peak temperature, but effective protection is still maintained. Example 4: Boehmite decomposes and absorbs heat at high temperatures, helping to lower the final temperature to some extent. However, its rigid support is slightly inferior to α-Al₂O₃, hence the temperature is higher than in Example 1. Example 5: The conductivity mechanism and response sensitivity of PANI differ slightly from PEDOT, resulting in a slight delay (2ms) in voltage response. However, the dual-layer synergistic mechanism remains effective, and the safety results are good.

[0123] Comparative Example 1: High initial internal resistance affected battery performance. After puncture, heat required time to conduct from the short-circuit point to the coating, resulting in a severely delayed response. During this delay, a chain reaction was triggered at the short-circuit point, leading to thermal runaway. Comparative Example 2: The voltage response layer quickly cut off most of the current, playing a crucial role. However, due to the lack of a second layer of ultimate thermal melting insulation barrier, a small residual current or arc may exist at the pinhole, continuously generating heat, leading to high local temperatures and electrolyte decomposition and gas ejection. Comparative Example 3: Extremely high initial internal resistance prevented the battery from charging and discharging normally. After puncture, heat was slowly conducted, causing the coating to melt. The response was too slow; before the response, a huge short-circuit current had already caused severe local overheating, leading to carbonization of some materials and electrolyte decomposition. Comparative Example 4: CNTs did not melt and break at high temperatures; instead, they remained conductive. Even after the second polymer layer melted, the CNT network remained connected and even became denser due to leveling, failing to form a high-resistivity state and completely losing its protective function. This demonstrates that a "low-melting-point conductive medium" is an irreplaceable key to achieving reversible "self-heating" and ultimately circuit breaking. In Comparative Example 5, an improper drying process caused the second polymer layer to melt prematurely. The low-melting-point alloy powder failed to form an effective conductive network without external force, instead being encapsulated and isolated by the molten polymer. This resulted in a coating with acceptable impedance under normal conditions, but completely losing its "self-heating" capability. Upon needle penetration, it functioned like an ordinary insulating layer, exhibiting an extremely slow response, leading to thermal runaway. This proves that a rigorous low-temperature curing process is crucial to ensuring that functional materials remain "latent" until activated when needed.

[0124] Experimental Case 2

[0125] Electrodes containing the coatings described in Examples 1-5 and Comparative Example 6, as well as an electrode without a safety coating, were used as Comparative Example 7 to assemble a battery, and the following performance tests were performed:

[0126] (1) Cyclic performance and energy efficiency test

[0127] Step 1: Let the battery stand at 25°C for 30 minutes, discharge it to 2.5V at 0.5P, and let it stand at 25°C for 30 minutes.

[0128] Step 2: Charge the battery to 3.65V at a constant power of 0.5P, let it stand at 25℃ for 30 minutes, discharge it to 2.5V at a constant power of 0.5P, let it stand at 25℃ for 30 minutes, repeat Step 2 for 1000 cycles, and record the battery's capacity retention rate and energy efficiency after 1000 cycles.

[0129] The capacity retention rate CR (%) after n battery cycles = discharge capacity of the nth cycle / discharge capacity of the first cycle × 100%.

[0130] (2) Safety performance test of battery under external short circuit

[0131] The test was conducted according to the test method of GB / T36276-2023. A fully charged battery was connected to the short-circuit test device. The connection between the short-circuit test device and the positive terminal of the battery cell was adjusted until the contact resistance of the positive terminal was less than or equal to 0.1 mΩ. The connection between the short-circuit test device and the negative terminal of the battery cell was adjusted until the contact resistance of the negative terminal was less than or equal to 0.1 mΩ. The short-circuit test device was activated to form a current loop between the positive and negative terminals of the battery cell. Observe whether the battery expands, leaks, smokes, or catches fire during the short circuit. The temperature at the center of the battery's surface was measured, and the highest temperature of the battery during the short circuit was recorded.

[0132] (3) Battery internal short-circuit performance test

[0133] The blunt needle test method was used, where a 1 / 4-inch diameter blunt needle was pressed against the center of the large surface area of ​​the cell until a 500 mV drop in the cell's open-circuit voltage was detected. Whether thermal runaway occurred was recorded. The temperature at the center of the large surface area of ​​the battery was also measured, and the highest temperature of the battery during an internal short circuit was recorded.

[0134] The test results are shown in Table 2.

[0135]

[0136] As can be seen from Examples 1-5, within the scope of patent protection, the battery exhibits good cycle performance and high energy efficiency during normal operation, indicating that the safety coating has little impact on the battery's internal resistance and long-term stability.

[0137] As can be seen from Comparative Example 6, although its 1000-cycle retention rate and energy efficiency are similar to those of the Example, it exhibited valve opening with smoke and thermal runaway during external and internal short-circuit tests. This is because while a relatively thick first coating layer can provide some protection for battery performance, it lacks the synergistic protection mechanism of a thermally fused insulating layer forming an insulating barrier when the temperature rises. When a short-circuit fault occurs, it cannot effectively cut off the current and prevent further heat accumulation, thus leading to thermal runaway.

[0138] In Comparative Example 7, the electrodes were not coated with a safety coating. While the battery's cycle performance and energy efficiency during normal operation were not significantly different from the examples and some comparative examples, the thermal runaway was much more severe in external and internal short-circuit tests, with the highest temperature far exceeding that of Comparative Example 6 and the examples. This fully demonstrates the importance of the safety coating for battery safety performance. Without the protection of a safety coating, when a battery encounters a short circuit or other fault, it cannot promptly limit or interrupt the short-circuit current. The large amount of heat generated by the short circuit will rapidly accumulate, leading to severe thermal runaway, which can not only damage the battery but also potentially cause serious safety accidents such as fires and explosions.

[0139] In summary, the safety coating for battery electrode sheets proposed in this invention, through the synergistic effect of the barrier layer and the thermally fused insulating layer, can ensure the stability and performance of the battery during normal operation. In the event of a fault such as an internal short circuit, it can quickly respond and form an effective insulating barrier to cut off the current and prevent safety accidents such as battery thermal runaway. This provides a reliable safety guarantee for the use of the battery and has significant application value and market prospects.

[0140] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A safety coating for battery electrode sheets, characterized in that, The safety coating has a composite layer structure, including a barrier layer and a hot-melt insulating layer: The barrier layer is disposed on the current collector, and the barrier layer includes a functional material with semiconductor properties. The Fermi level of the functional material matches the Fermi level of the current collector to form an ohmic contact when the battery is working normally. The impedance of the barrier layer increases stepwise with the increase of the potential difference at its location. The step increase is triggered by the electrochemical dedoping / doping characteristics of the functional material with semiconductor properties; when the potential difference at the location increases, causing the functional material to undergo rapid electrochemical dedoping, its conductivity decreases by several orders of magnitude. The functional material with semiconductor properties is selected from at least one of polyaniline, polypyrrole, poly(3,4-ethylenedioxythiophene) and its derivatives, as well as composites thereof; The hot-melt insulating layer is disposed on the barrier layer, and the impedance of the hot-melt insulating layer increases stepwise with increasing temperature; the hot-melt insulating layer includes: a self-melting polymer matrix; a conductive medium having a preset melting point dispersed in the self-melting polymer matrix; and heat-resistant insulating particles dispersed in the self-melting polymer matrix; wherein the melting point of the conductive medium is higher than the glass transition temperature of the self-melting polymer matrix and lower than or equal to the melting point of the self-melting polymer matrix; The hot-melt insulating layer is configured such that when the temperature rises to the softening range, the self-melting polymer matrix softens, and the conductive medium dispersed therein is squeezed and aggregated to form a temporary conductive path, resulting in a decrease in coating resistance to accelerate local heating; when the temperature reaches the melting point, the conductive medium spheroidizes and shrinks, and the polymer melts and flows level to form a permanent insulating barrier. The barrier layer and the thermally fused insulating layer work synergistically to provide a bidirectional response mechanism for voltage and temperature.

2. The safety coating for battery electrode sheets according to claim 1, characterized in that, The conductive medium having a preset melting point is a metal or alloy powder, selected from at least one of indium, tin, bismuth, gallium and their alloys.

3. The safety coating for battery electrode sheets according to claim 1, characterized in that, The hot-melt insulating layer comprises, by weight percentage: 20%-50% self-melting polymer matrix, 15%-25% conductive medium, 30%-50% insulating particles, 2%-5% binder, and 1%-5% processing aid.

4. The safety coating for battery electrode sheets according to claim 1, characterized in that, The thickness T1 of the barrier layer is 50 nm-500 nm; And / or, the thickness of the heat-fused insulation layer, T2, is greater than 10 × T1.

5. A method for preparing a safety coating, characterized in that, The method for preparing a safety coating for a battery electrode sheet according to any one of claims 1-4 includes the following steps: A barrier layer slurry is prepared and coated onto the current collector, and a barrier layer is formed by a first drying process. A hot-melt insulating layer slurry is prepared and coated onto the barrier layer, and then a second drying process is performed to form a hot-melt insulating layer. The highest temperature of the second drying process is lower than that of the self-melting polymer matrix in the heat-melting insulating layer and lower than that of the conductive medium.

6. The method for preparing the safety coating according to claim 5, characterized in that, The second drying process is a programmed temperature rise drying, including at least one heat preservation stage in the range of 80℃-110℃.

7. An electrode sheet comprising a current collector and an active material layer, characterized in that, A safety coating as described in any one of claims 1-4 is provided between the current collector and the active material layer.

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