A binder, a safety coating for electrodes, and a method for preparing and using the same

Abstract

This invention discloses an adhesive, a safety coating for electrodes, its preparation method, and its application, relating to the field of adhesive technology. The adhesive of this invention is formed by lithiation of dextran with an esterification reagent via a sulfate esterification reaction; the esterification reagent includes chlorosulfonic acid and / or concentrated sulfuric acid. The adhesive has hydroxyl and sulfate ester groups on its side chains, which can form strong hydrogen bonds and coordination interactions with the oxide layer on the current collector surface, significantly improving the adhesion of the coating to the substrate; it also has excellent film-forming properties, forming a continuous, dense, and flexible coating. Used in the preparation of safety coatings for electrode sheets, it provides better needle penetration protection, avoids thermal runaway, and improves the high-temperature cycle performance and overcharge performance of the battery. Furthermore, this adhesive possesses excellent electrolyte wettability and chemical stability, and can remain unswelled and undecomposed under long-term immersion, ensuring the coating's protective function remains effective and contributing to improved battery life.

Description

Technical Field

[0001] This invention relates to the field of adhesive technology, and in particular to an adhesive, a safety coating for electrodes, a method for preparing the same, and its application. Background Technology

[0002] As the energy density of lithium-ion batteries continues to increase, their safety hazards are becoming increasingly prominent, especially thermal runaway caused by internal short circuits under nail penetration conditions, which seriously threatens battery safety. Therefore, coating a safety layer between the positive electrode current collector and the active layer has become the mainstream protective measure. This coating needs to remain stable during normal charging and discharging of the battery, without interfering with lithium-ion transport and electrode reactions, ensuring that electrochemical performance is not affected; and in the event of extreme accidents such as nail penetration, it needs to function rapidly to effectively block the short circuit path and suppress the spread and escalation of thermal runaway.

[0003] In safety coating systems, binders are an indispensable core component, playing a crucial role throughout coating preparation and the entire battery lifecycle. On one hand, they must firmly bind the powder in the coating, forming a dense and resilient film to prevent powder from detaching and agglomerating during processing and cycling, or from being unable to withstand mechanical impacts such as needle penetration, thus ensuring the integrity of the coating. On the other hand, they must possess excellent substrate adhesion, ensuring a tight bond between the coating and the positive electrode current collector, preventing issues such as peeling and blistering. Furthermore, binders must also possess a certain degree of ionic conductivity to reduce the coating's obstruction of lithium-ion transport and decrease the battery's internal resistance.

[0004] However, traditional adhesives still have many defects. Fluorine-based adhesives, represented by polyvinylidene fluoride (PVDF) and its copolymers, have good chemical resistance, but have the following problems: (1) Insufficient adhesion: They only rely on van der Waals forces to bond with the aluminum foil substrate, resulting in weak interfacial adhesion and easy coating peeling; (2) Poor toughness: The molecular chains are rigid and have low elongation at break, making them prone to cracking under rolling, bending, or needle impact; (3) Single function: They cannot suppress the generation of metal burrs from the source, and the needle protection effect is limited; (4) Ionic insulation: PVDF is an electronic and ionic insulator, and increasing the coating thickness will significantly increase the battery internal resistance, affecting rate performance and cycle stability.

[0005] Therefore, developing a novel binder that combines high adhesion, excellent toughness, and good ionic conductivity is of great significance for improving the safety performance of lithium-ion batteries. Summary of the Invention

[0006] The present invention aims to at least solve one of the aforementioned technical problems existing in the prior art. Therefore, one objective of the present invention is to provide a lithium sulfate dextran salt binder, which is prepared by lithiation of dextran with chlorosulfonic acid or concentrated sulfuric acid via a sulfate esterification reaction. This binder incorporates a large number of lithium sulfate ester groups into its side chains, exhibiting high polarity, excellent film-forming properties, good flexibility, and ionic conductivity. It is used to prepare safety coatings for electrode sheets, providing better needle penetration protection.

[0007] The second objective of this invention is to provide a safety coating.

[0008] A third objective of this invention is to provide a method for preparing a safety coating.

[0009] The fourth objective of this invention is to provide an electrode sheet.

[0010] The fifth objective of this invention is to provide a battery.

[0011] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A first aspect of the present invention provides an adhesive formed by lithium lithiation of dextran with an esterification agent via a sulfate esterification reaction; the esterification agent includes chlorosulfonic acid and / or concentrated sulfuric acid.

[0012] This invention uses dextran as a raw material, and prepares an adhesive through sulfation modification followed by lithiation. Dextran is a polysaccharide composed of glucose units linked by glycosidic bonds, possessing advantages such as good biocompatibility, wide availability, and biodegradability. After sulfation modification, a large number of sulfate groups (-OSO3H) are introduced into the dextran molecular chain. Further lithiation transforms these sulfate groups into lithium sulfate groups (-OSO3Li), imparting the following properties to the material: (1) Strong interfacial adhesion: The sulfate ester groups and hydroxyl groups can form strong hydrogen bonds and coordination interactions with the oxide layer on the surface of the current collector (e.g., the alumina layer on the surface of aluminum foil, the copper oxide layer on the surface of copper foil). Taking the alumina on the surface of aluminum foil as an example, the oxygen atoms in the sulfate ester groups have lone pairs of electrons, which can form coordination bonds with the empty orbitals of aluminum ions; the hydroxyl groups form a hydrogen bond network with the hydroxyl groups on the surface of alumina. This multiple interaction significantly improves the adhesion of the coating to the substrate.

[0013] (2) Excellent film-forming properties: Dextran sulfate has good water solubility and film-forming properties, and can form a continuous, dense and uniform coating on the surface of the current collector, effectively coating the filler particles and preventing powder from falling off.

[0014] (3) Good flexibility: The main chain of dextran is a flexible glycosidic bond structure. The sulfation modification introduces polar groups while maintaining the flexibility of the molecular chain, so that the coating has a suitable elongation at break and can withstand the mechanical stress during electrode processing.

[0015] (4) Ionic conductivity: Lithium ions in the lithium sulfate ester group (-OSO3Li) can migrate under the action of an electric field, giving the binder a certain ionic conductivity, significantly reducing the coating's obstruction to lithium ion transport, reducing battery internal resistance, and improving electrochemical performance.

[0016] (5) Strong polarity and electrolyte wettability: The sulfate ester group has strong polarity, which can significantly improve the wettability of the coating to polar electrolyte, reduce interfacial impedance, and improve the electrochemical performance of the battery.

[0017] (6) Chemical stability: The sulfate ester bond has excellent stability in the electrochemical environment and is not easily decomposed, oxidized or corroded by the current collector, which can ensure that the coating maintains structural integrity and functional effectiveness during long-term battery cycling.

[0018] It should be understood that, when using the adhesive of the present invention, the adhesive can be mixed with other components to be bonded in a solvent to form a slurry, and after drying and curing, the components to be bonded can be bonded; alternatively, the adhesive can be dissolved in a solvent and then directly bonded to other materials.

[0019] In some embodiments, the mass ratio of the dextran to the esterification reagent is 1:(0.1~10). Preferably, the mass ratio of the dextran to the esterification reagent is 1:(2~10). More preferably, it is 1:(2~5); even more preferably, it is 1:(2~3). By optimizing the ratio range of the two, the degree of sulfation is moderate, which ensures that the molecular chain has sufficient polar groups to achieve strong adhesion, while avoiding excessive substitution that would lead to excessive rigidity of the polymer chain, increased intermolecular repulsion, and affect flexibility and adhesion.

[0020] In some embodiments, the esterification agent is preferably chlorosulfonic acid. Compared with concentrated sulfuric acid, chlorosulfonic acid (ClSO3H) provides milder esterification reaction conditions, effectively reducing the degradation of glycosidic bonds in the dextran backbone and maintaining the integrity of the polymer chain. Simultaneously, chlorosulfonic acid esterification exhibits better selectivity, forming uniformly distributed sulfate ester groups on the dextran molecular chain. This results in stronger hydrogen bonding and electrostatic adsorption between the binder, filler, and aluminum foil substrate, leading to superior film flexibility and adhesion.

[0021] It should be understood that sulfation refers to the reaction of hydroxyl groups in a material molecule with a reagent containing -SO3 (such as chlorosulfonic acid or concentrated sulfuric acid), resulting in the removal of smaller molecules and the introduction of sulfate ester groups (-OSO3H). Lithification refers to the acid-base neutralization or ion exchange reaction between the sulfate ester group and a lithium source, removing the acidic proton (H) from the sulfate ester group. + Replaced with lithium ions (Li) + The process of generating lithium sulfate groups (-OSO3Li).

[0022] In some embodiments, the method for preparing the adhesive includes the following steps: The dextran and esterification reagent are subjected to a sulfate esterification reaction, and the resulting product is then lithiated with a lithium salt solution to obtain the binder.

[0023] The sulfation reaction is carried out in an organic solvent, such as N,N-dimethylformamide. The lithiation involves a neutralization reaction between a lithium salt solution and the product of the sulfation reaction; the lithium salt can be, for example, lithium hydroxide.

[0024] A second aspect of the present invention provides a safety coating, the raw materials for preparing the safety coating including a binder, a filler and a conductive agent; the binder includes the binder described in the first aspect of the present invention.

[0025] In some embodiments, the mass ratio of the filler, the binder, and the conductive agent is (50~90):(4~20):(0.5~15). Further, the mass ratio of the filler, the binder, and the conductive agent is (60~80):(5~15):(5~15).

[0026] In some embodiments, the filler includes at least one selected from lithium iron phosphate, lithium vanadium phosphate, silica, silicate, alumina, bauxite, and aluminum titanate. These fillers possess high thermal stability, good mechanical strength, and chemical inertness, and can function to prevent short circuits, absorb heat, and suppress thermal runaway during needle penetration.

[0027] In some embodiments, the filler comprises aluminum hydroxya and silicate; the mass ratio of aluminum hydroxya to silicate is 1:(0.1~1), preferably, the mass ratio of aluminum hydroxya to silicate is 1:(0.1~0.5).

[0028] This invention preferably uses aluminosilicate hydroxyaluminate and silicate as fillers. The dense structure of the silicate layer can block the diffusion of acidic substances and electrolyte, reduce metal burrs, lower the risk of corrosion and short circuits, and improve battery safety. Aluminosilicate hydroxyaluminate can improve the hardness and modulus of the coating / separator, inhibit dendrite and burr penetration, and adsorb / neutralize acidic products. The two work synergistically to absorb / neutralize acidic substances such as HF, construct a barrier layer, reduce electrolyte corrosion of metals / active materials, and simultaneously suppress burrs at the source and wrap the tips, blocking short circuit paths within the needle punctures and ensuring safety in all scenarios. Among them, aluminosilicate hydroxyaluminate is preferably boehmite (γ-AlOOH), and silicate is preferably talc (hydrated magnesium silicate).

[0029] In some embodiments, the conductive agent includes at least one selected from conductive carbon black, acetylene black, carbon nanotubes, and graphene. The addition of the conductive agent can provide the necessary electronic conduction channels while ensuring the insulation of the coating, thus preventing a significant increase in interfacial impedance due to excessive coating thickness.

[0030] In some embodiments, the thickness of the safety coating is 2-8 μm. A coating that is too thin (<2 μm) is difficult to form a continuous and dense protective layer, resulting in limited needle penetration protection; a coating that is too thick (>8 μm) will increase the battery's internal resistance and reduce energy density. Preferably, the thickness is 3-7 μm; more preferably, it is 4-6 μm.

[0031] A third aspect of the present invention provides a method for preparing a safety coating, comprising the following steps: Fillers, binders, and conductive agents are mixed in a solvent to form a mixed slurry; the mixed slurry is coated onto a substrate and dried to obtain the safety coating.

[0032] In some embodiments, the mixture is first dispersed using a high-speed vacuum disperser, then dispersed using a sand mill, and finally dispersed using ultrasound. High-speed vacuum dispersion can quickly wet and initially disperse the powder; the sand mill achieves fine dispersion and particle size control of the filler through the shearing and impact action of the grinding media; and ultrasonic dispersion further breaks down agglomerates using the cavitation effect, ensuring a uniform and stable slurry. The synergistic effect of these three stages of dispersion yields a coating slurry with uniform dispersion and good stability.

[0033] A fourth aspect of the present invention provides an electrode sheet, the electrode sheet comprising a current collector, a safety coating, and an active material layer; the safety coating is disposed between the current collector and the active material layer; the safety coating is the safety coating described in the second aspect of the present invention.

[0034] In some embodiments, the electrode sheet is a positive electrode sheet or a negative electrode sheet; the current collector is an aluminum foil, a copper foil, or a composite foil.

[0035] A fifth aspect of the present invention provides a battery comprising the electrode plates described in the fourth aspect of the present invention.

[0036] In some embodiments, the battery is a lithium-ion battery or a sodium-ion battery.

[0037] Compared with the prior art, the beneficial effects of the present invention are: 1) This invention provides an adhesive formed by lithiation of dextran through a sulfation reaction. Through molecular structure design, lithium sulfate groups are introduced into the dextran molecular chain, resulting in an adhesive that combines high polarity, excellent film-forming properties, good flexibility, and ionic conductivity. This adhesive can form strong hydrogen bonds and coordination with aluminum foil substrates, exhibiting significantly better adhesion than traditional PVDF adhesives. It has high elongation at break, good toughness, and can withstand mechanical impact without breaking. Furthermore, the lithium sulfate groups provide lithium-ion transport channels, significantly reducing the coating's obstruction to ion transport and decreasing battery internal resistance.

[0038] 2) This invention provides a safety coating containing the binder of this invention, which exhibits good adhesion to the substrate and possesses certain toughness and elasticity, resulting in superior protective effects, particularly enhancing needle penetration protection. Further optimization of the filler composition in the safety coating can further improve puncture resistance. When used to prepare electrode sheets, the resulting battery can reduce damage after needle penetration, such as the number of foil leaks and temperature peaks, preventing burnout. Simultaneously, the safety coating suppresses burr formation, which helps prevent the expansion of short-circuit channels, avoiding thermal runaway caused by internal short circuits under needle penetration conditions, and improving the battery's high-temperature cycle stability and overcharge performance. Detailed Implementation

[0039] The present invention will be further described in detail below through specific embodiments. Unless otherwise specified, the raw materials, reagents, or apparatus used in the embodiments and comparative examples are all available from conventional commercial sources or can be obtained by existing technical methods. Unless otherwise specified, the test or experimental methods are conventional methods in the art.

[0040] In the following examples and comparative examples of this invention, the dextran (CAS No.: 9004-54-0), chlorosulfonic acid (CAS No.: 7790-94-5), concentrated sulfuric acid (CAS No.: 7664-93-9), N,N-dimethylformamide (DMF, CAS No.: 68-12-2), lithium hydroxide (battery grade), alumina hydroxide (boehmite, battery grade), silicate (talc, battery grade), lithium nickel cobalt manganese oxide (battery grade), N-methylpyrrolidone (NMP), conductive carbon black (Super P), graphite (battery grade), and carbon nanotubes (diameter 10~20nm, length 5~15μm) were all commercially available. The polyvinylidene fluoride (PVDF) was Arkema Kynar® HSV900, and the polyvinylidene fluoride-hexafluoropropylene polymer (PVDF-HFP) was Arkema Kynar Flex® LBG.

[0041] The following detailed description is provided in conjunction with specific embodiments and comparative examples.

[0042] Example 1 An adhesive, prepared by the following method: Dextran and chlorosulfonic acid were dissolved in anhydrous N,N-dimethylformamide (DMF) and stirred at 60°C for 3 hours. After cooling to room temperature, the solution was poured into water, and the pH was adjusted to 7.0-7.5 with 10wt% lithium hydroxide solution to complete lithiation. After vacuum drying (110°C, 24 hours), sulfated dextran lithium salt binder was obtained.

[0043] The mass ratio of dextran to chlorosulfonic acid is 1:2.5.

[0044] The safety coating is prepared using this binder. The raw materials for preparing the safety coating include binder, filler and conductive agent, with a mass ratio of filler, binder and conductive agent of 75:10:10. Among them, the filler is boehmite and talc powder with a mass ratio of 1:0.1; the conductive agent is conductive carbon black.

[0045] The method for preparing a safety coating using the adhesive of this embodiment is as follows: Fillers, binders, and conductive agents were added to water and dispersed for 60 minutes using a high-speed vacuum disperser, followed by 60 minutes using a sand mill. Then, they were dispersed again for 30 minutes using an ultrasonic treatment device with ultrasonic resonance to ensure that all components were uniformly dispersed, resulting in a slurry with a solid content of 25 wt%. The slurry was then coated onto the substrate surface using a comma-shaped scraper and dried at 110°C to form a 3 μm thick safety coating.

[0046] Example 2 An adhesive differs from Example 1 in that the mass ratio of dextran to chlorosulfonic acid is 1:0.1; otherwise, it is the same as Example 1.

[0047] A safety coating was prepared using this adhesive, and the raw materials and preparation method for the safety coating were the same as in Example 1.

[0048] Example 3 An adhesive that differs from Example 1 in that the mass ratio of dextran to chlorosulfonic acid is 1:5; otherwise, it is the same as Example 1.

[0049] A safety coating was prepared using this adhesive, and the raw materials and preparation method for the safety coating were the same as in Example 1.

[0050] Example 4 An adhesive that differs from Example 1 in that the mass ratio of dextran to chlorosulfonic acid is 1:10; otherwise, it is the same as Example 1.

[0051] A safety coating was prepared using this adhesive, and the raw materials and preparation method for the safety coating were the same as in Example 1.

[0052] Example 5 A safety coating differs from Example 1 in that the thickness of the safety coating is 1 μm, while the rest is the same as in Example 1.

[0053] Example 6 A safety coating differs from Example 1 in that the thickness of the safety coating is 5 μm, while the rest is the same as in Example 1.

[0054] Example 7 A safety coating differs from Example 1 in that the thickness of the safety coating is 10 μm, while the rest is the same as in Example 1.

[0055] Example 8 An adhesive, which differs from Example 1 in that concentrated sulfuric acid is used instead of chlorosulfonic acid; otherwise, it is the same as Example 1.

[0056] A safety coating was prepared using this adhesive, and the raw materials and preparation method for the safety coating were the same as in Example 1.

[0057] Example 9 A safety coating differs from Example 1 in that the mass ratio of boehmite to talc in the filler is 1:0.5, while the rest is the same as in Example 1.

[0058] Example 10 A safety coating differs from Example 1 in that the mass ratio of boehmite to talc in the filler is 1:1, while the rest is the same as in Example 1.

[0059] Example 11 A safety coating differs from Example 1 in that the filler is a single boehmite; otherwise, it is the same as Example 1.

[0060] Example 12 A safety coating differs from Example 1 in that the filler is a single talc powder; otherwise, it is the same as Example 1.

[0061] Comparative Example 1 An adhesive, which differs from Example 1 in that the adhesive is a polyvinylidene fluoride-hexafluoropropylene copolymer; a safety coating is prepared using this adhesive, and the raw materials and preparation method for the safety coating are the same as in Example 1.

[0062] Comparative Example 2 An adhesive, which differs from Example 1 in that the adhesive is polyvinylidene fluoride; a safety coating is prepared using this adhesive, and the raw materials and preparation method for the safety coating are the same as in Example 1.

[0063] Comparative Example 3 A safety coating differs from Comparative Example 1 in that the filler is a single boehmite; otherwise, it is the same as Comparative Example 1.

[0064] Comparative Example 4 A safety coating differs from Comparative Example 1 in that the filler is a single talc powder; otherwise, it is the same as Comparative Example 1.

[0065] Application Examples 1-12 Positive electrode sheets were prepared using the safety coatings described in Examples 1-12. The preparation methods are as follows: In Example 1, the substrate in the safety coating preparation process was replaced with a positive electrode current collector (smooth aluminum foil). A positive electrode active material was coated onto the surface of the safety coating, and after drying, an active material layer was formed, resulting in a positive electrode sheet. The positive electrode active material was obtained by mixing an active material, a conductive agent, and a binder in a solvent. The active material was lithium cobalt oxide, the binder was polyvinylidene fluoride, the conductive agent was conductive carbon black, and the solvent was N-methylpyrrolidone (NMP). The mass ratio of the active material, binder, and conductive agent was 95:3.5:1.5.

[0066] Among them, Application Examples 1 to 12 correspond to the safety coatings of Examples 1 to 12 respectively.

[0067] Compare and contrast examples 1-4 Positive electrode sheets were prepared using the safety coatings of Comparative Examples 1-4, and the preparation method was the same as in Application Example 1.

[0068] Among them, comparative application examples 1 to 4 correspond to the safety coatings of examples 1 to 4 respectively.

[0069] Comparative Application Example 5 A positive electrode sheet, without a safety coating, is prepared using the same method as in Application Example 1.

[0070] Result detection 1. Safety coating performance test: The safety coatings of the above embodiments and comparative examples were subjected to elongation at break and adhesion tests.

[0071] Adhesion Test: Take an electrode sheet 30mm wide and 200mm long. Simultaneously, take a clean stainless steel plate and apply a layer of double-sided tape. Smoothly attach the electrode sheet (current collector side down) onto the double-sided tape. Take a piece of standard test tape (3M 600, 19mm wide) and apply it to the electrode coating surface. Use a 2kg standard weight roller to roll back and forth on the tape three times at a speed of 300mm / min, ensuring full contact between the tape and the coating without air bubbles. Fix the steel plate in the lower clamp of the tensile testing machine. Fold the free end of the test tape 180° backwards and clamp it in the upper clamp. Set the tensile speed to 100mm / min (to be kept constant) and the peel angle to 180° for peeling. Record the test results. High adhesion means the tape cannot peel off any electrode material; medium adhesion means the tape can peel off a small amount of electrode material; low adhesion means the tape easily peels off a large amount of electrode material.

[0072] Elongation at break: Tested in accordance with GB / T 22638.11-2023 standard for elongation at break.

[0073] The test results are shown in Table 1 below.

[0074] Table 1 Comparison of safety coating performance between examples and comparative examples

[0075] Comparing the results in Table 1, it can be seen that Examples 1-4 only adjusted the mass ratio of dextran to chlorosulfonic acid (1:2.5, 1:0.1, 1:5, 1:10). Example 1 balanced high elongation at break and high adhesion. Example 2 had the highest elongation at break (4.3%), but adhesion decreased to a moderate level. This was because the amount of esterification reagent was too small, the degree of sulfate esterification of the adhesive was low, and the content of lithium sulfate groups was insufficient, weakening the coordination with the aluminum foil and reducing adhesion, although the molecular chain flexibility remained the best. Examples 3 and 4 maintained high adhesion, but with the increase of the chlorosulfonic acid ratio, the degree of substitution of the adhesive increased, the rigidity of the molecular chain increased, and the flexibility decreased, resulting in a slight decrease in elongation at break. Example 8 used concentrated sulfuric acid modification, resulting in an elongation at break of 3.5% and moderate adhesion. This is because the chlorosulfonation reaction is milder, the dextran backbone degrades less, and the sulfate groups are evenly distributed; while concentrated sulfuric acid is too acidic, easily causing glycosidic bond breakage and damaging the integrity of the polymer chain. Examples 9 and 10 adjusted the ratio of boehmite and talc (from 1:0.1 to 1:0.5 and 1:1), and the coating performance remained similar, maintaining high elongation at break and high adhesion. Examples 11 and 12 used a single filler, resulting in moderate adhesion and reduced elongation at break, demonstrating the synergistic optimization effect of the boehmite and talc blend. Boehmite particles themselves have no binding properties, limited interfacial bonding with the binder, and single particles are prone to agglomeration, leading to stress concentration and cracking. The SiO2 network structure formed after pure silicate curing is extremely rigid and lacks deformation capacity. The blend of alumina hydroxyacid and silicate can increase density through particle size distribution, enhance the interface through surface chemical complementarity, and buffer stress concentration through morphological complementarity and the introduction of a flexible phase. Therefore, the adhesion and elongation are superior to those of a single filler. Comparative Examples 1 and 2 used PVDF-HFP and traditional PVDF binders, resulting in coating elongation at break of only 3.1%~3.3%, low adhesion, and performance far below that of Example 1. Comparative Examples 3 and 4, using traditional adhesives and single fillers, exhibited poor adhesion and elongation at break of only 2.3% to 2.5%. This fully demonstrates the significant advantages of lithium dextran sulfate adhesive in terms of adhesion and toughness compared to traditional fluorinated adhesives, as well as the synergistic effect of the combination of alumina hydroxyl and silicate.

[0076] 2. Battery performance test: The positive electrode sheet containing the safety coating prepared in the above application example and comparative application example was used to prepare a ternary NCM622 system lithium-ion battery, resulting in a 5Ah lithium-ion pouch battery.

[0077] The internal resistance (ACR) of the lithium-ion pouch battery was tested according to GB / T 18287 / 36276, and the cell internal resistance was recorded.

[0078] A nail penetration test was performed on the lithium-ion pouch battery. Referring to GB / T 31485-2015, under room temperature conditions (25℃, 101.3KPa), the lithium-ion battery to be tested is fully charged (100% SOC) using a standard charge and left to stand for 24 hours. The battery is then fixed in a needle penetration testing machine, with the center of the largest surface of the battery directly below the probe. The explosion-proof box is closed and locked. A 5mm diameter probe (made of high-hardness alloy steel) is used to completely pierce the battery in one pass at a speed of 25mm / s. The probe is then removed after 60 seconds. The standard for passing the test is that the battery cell does not smoke, catch fire, or explode during the entire process from the moment the needle touches the battery until the battery surface temperature naturally drops below 50℃.

[0079] Temperature changes were recorded during the needle puncture experiment, and the number of foils exposed in the needle puncture notch area was recorded after the experiment, which is the number of aluminum foil areas exposed after the active material layer in the needle puncture area was torn and detached.

[0080] High-temperature cycle performance test of lithium-ion pouch battery: 45℃ 1C1C cycle test was carried out in accordance with GB / T 31484-2015, and the capacity retention rate and thickness expansion rate were recorded after 1000 cycles.

[0081] Overcharge performance testing of lithium-ion pouch batteries: A 2C, 6.0V overcharge test is performed according to IEC 62133-2-2017. The batteries are charged at a constant current of 2C to 6.0V or for 1.5 hours (whichever comes first), and the highest temperature during the charging process is recorded. The judgment standard is: no smoke, no fire, and no explosion constitutes a pass.

[0082] The test results are shown in Tables 2 and 3 below.

[0083] Table 2. Test results of nail penetration and overcharge tests on lithium-ion batteries

[0084] Table 3 Electrochemical performance of lithium-ion batteries

[0085] Examples 1-4 and 6-10 all passed the needle penetration and overcharge tests. Example 5 failed the needle penetration test, with ≥5 missing foils and a peak temperature of 162℃, indicating that the coating was too thin to form an effective protective layer. Comparative Examples 1 and 2 used traditional PVDF-based binders; despite having a safety coating, they still failed the needle penetration and overcharge tests, demonstrating the effective improvement of the battery cell's needle penetration resistance by the lithium dextran sulfate binder. Examples 11 and 12 and Comparative Examples 3 and 4 used single fillers and all failed the needle penetration and overcharge tests, demonstrating the synergistic effect of compound fillers.

[0086] Comparing the cell resistance data, it can be seen that the internal resistance of cells in Application Examples 1-12 is 8.5-11.0 mΩ, significantly lower than that in Comparative Application Examples 1 and 2. This fully demonstrates the ionic conductivity advantage of lithium dextran sulfate binder; the lithium sulfate groups provide a transport channel for lithium ions, significantly reducing the coating's hindrance to ion migration. Application Example 2 has a higher resistance because the content of lithium sulfate groups is insufficient, resulting in limited ion transport channels. Application Examples 3 and 4 have slightly higher resistances than Application Example 1, but are still significantly lower than those of the traditional binder, indicating that good ionic conductivity is maintained over a wide range. Application Example 5 has the lowest resistance because the thin coating minimizes ion transport hindrance. Application Example 6 has the highest resistance, but is still lower than that of the traditional binder, indicating that even under thicker coating conditions, the ionic conductivity of lithium dextran sulfate can partially compensate for the increase in resistance due to increased thickness.

[0087] Application Examples 1-10 exhibited a capacity retention of ≥82.5% over 1000 cycles and a thickness expansion rate of ≤11.5%, significantly outperforming Comparative Application Examples 1-4. Application Example 2 showed slightly lower performance due to insufficient polar groups in the binder, resulting in slightly poorer interfacial stability. Application Example 7 showed a slight decrease in performance because the thicker coating increased the lithium-ion transport path, but it still significantly outperformed conventional binders.

[0088] In summary, the sulfated dextran lithium salt binder provided by this invention is prepared by lithiation after sulfation reaction of dextran with chlorosulfonic acid or concentrated sulfuric acid. The lithium sulfate groups introduced into the molecular chain and the retained hydroxyl groups can form strong hydrogen bonds and coordination interactions with the aluminum foil surface, significantly improving adhesion; at the same time, the maintained flexible glycosidic bond structure endows the coating with excellent toughness. When used as a safety coating for lithium-ion batteries, it can significantly improve the needle penetration protection effect, avoid thermal runaway, and improve the high-temperature cycle stability and overcharge safety of the battery.

[0089] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. An adhesive, characterized in that, The binder is formed by lithium lithiation of dextran with an esterification agent via a sulfate esterification reaction; the esterification agent includes chlorosulfonic acid and / or concentrated sulfuric acid.

2. The adhesive according to claim 1, characterized in that, The mass ratio of the dextran to the esterification reagent is 1:(0.1~10).

3. A safety coating, characterized in that, The raw materials for preparing the safety coating include binders, fillers, and conductive agents; the binder includes the binder described in claim 1 or 2.

4. The safety coating according to claim 3, characterized in that, The mass ratio of the filler, the binder and the conductive agent is (50~90):(4~20):(0.5~15).

5. The safety coating according to claim 3 or 4, characterized in that, The filler includes at least one of lithium iron phosphate, lithium vanadium phosphate, silicon dioxide, silicate, alumina, aluminum hydroxyalumina, bauxite, and aluminum titanate.

6. The safety coating according to claim 5, characterized in that, The filler comprises aluminum hydroxya and silicate; the mass ratio of aluminum hydroxya to silicate is 1:(0.1~1).

7. The safety coating according to claim 3, characterized in that, The thickness of the safety coating is 2~8μm.

8. A method for preparing the safety coating according to any one of claims 3 to 7, characterized in that, Includes the following steps: Fillers, binders, and conductive agents are mixed in a solvent to form a mixed slurry; the mixed slurry is coated onto a substrate and dried to obtain the safety coating.

9. An electrode sheet, characterized in that, The electrode sheet includes a current collector, a safety coating, and an active material layer; the safety coating is disposed between the current collector and the active material layer; the safety coating is the safety coating according to any one of claims 3 to 7.

10. A battery, characterized in that, The battery includes the electrode plates as described in claim 9.

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