Composite coating material, composite coating and application thereof

Abstract

The invention discloses a composite coating material for a lithium ion battery positive electrode, a composite coating and application of the composite coating material, the inner core of the composite coating material is nano silicon dioxide particles, the shell of the composite coating material is of an organic-inorganic hybrid structure, and the organic-inorganic hybrid structure is formed by reaction of a quaternary ammonium salt surfactant and a silane coupling agent. The composite coating adopts a core-shell structure, the core is nano silicon dioxide particles, and the shell is an organic-inorganic hybrid layer formed by reaction of a quaternary ammonium salt surfactant and a silane coupling agent. The nano silicon dioxide of the inner core effectively prevents electron transfer through high insulativity, and the hybrid structure of the shell combines the flexibility of organic matters and the rigidity of inorganic matters, so that the composite material can adapt to the volume change in the charging and discharging process of the electrode and can resist the physical penetration of lithium dendrites. Due to the dual characteristics, the safety is remarkably improved while the high-efficiency operation of the battery is maintained by the coating.

Description

Technical Field

[0001] The present invention relates to the field of lithium batteries, and particularly to a positive electrode of a lithium battery, a preparation method thereof, and an application thereof. Background Art

[0002] A lithium-ion battery consists of a positive electrode, a negative electrode, and a thin polymer separator. The separator is usually prepared from porous PE (polyethylene) or PP (polypropylene). This separator is very thin, with a thickness of 7-20 μm. Once the separator is crushed or penetrated, the positive electrode and the negative electrode will come into direct contact to form an internal short circuit. For example, during the cycling of a lithium-ion battery, uneven lithium deposition will form "dead" lithium and lithium dendrites, which may lead to an internal short circuit of the battery. When the dendrites grow and finally penetrate the separator, an internal short circuit will occur in the battery, thus leading to a fire accident. In addition, during the assembly of the battery, damage to the separator under mechanical or thermal abuse conditions may result in an internal short circuit.

[0003] In order to reduce short circuits and improve the reliability of the battery, researchers have designed and manufactured many separators with high thermal stability and high mechanical strength. However, due to their high production costs, separators with high thermal stability and high mechanical strength cannot be industrialized. And now, the out-of-control of internal short circuit in the battery cell often occurs after the separator fails. Therefore, developing a method to improve the stability of internal short circuit in the battery cell after the separator fails is crucial for improving the safety performance of the battery cell. Summary of the Invention

[0004] To overcome the defects in the prior art, the first object of the present invention is to provide a composite coating material for the positive electrode of a lithium-ion battery, the second object is to provide a preparation method of a composite coating with the above composite coating material, the third object is to provide a positive electrode of a lithium-ion battery with the above composite coating, the fourth object is to provide a lithium-ion battery with the above positive electrode of a lithium-ion battery, and the fifth object is to provide an electrical device with the above lithium-ion battery.

[0005] To achieve the above object, the technical solution adopted by the present invention is:

[0006] In the first aspect, a composite coating material for the positive electrode of a lithium-ion battery, the core of the composite coating material is nano-silica particles, and the shell is an organic-inorganic hybrid structure formed by the reaction of a quaternary ammonium salt surfactant and a silane coupling agent.

[0007] When applied to the composite coating of the positive electrode of a lithium-ion battery, this composite coating material can act as a physical isolation barrier. The reasons are as follows: The core of the composite coating is composed of insulating nano-silica, and its dense structure forms a physical barrier on the surface of the positive electrode. When the separator fails due to lithium dendrite puncture or thermal runaway, the silica layer can effectively block the direct contact between the positive and negative electrodes, cut off the electron conduction path, and thus avoid the instantaneous surge of short-circuit current. The organic-inorganic hybrid structure of the outer shell (formed by crosslinking of quaternary ammonium salts and silane coupling agents) has both flexibility and rigidity, which can resist the mechanical stress caused by the volume change of the electrode or the growth of lithium dendrites during the cycle, and prevent the coating from cracking and causing secondary short circuit.

[0008] Preferably, the quaternary ammonium salt surfactant is selected from one of N-methylpyrrolidone, dimethylformamide, and dimethylacetamide. The silane coupling agent is tetraethyl orthosilicate.

[0009] The quaternary ammonium salt surfactant in the outer shell forms microporous channels through directional arrangement, allowing lithium ions to freely transport to maintain the normal operation of the battery, while blocking the direct leakage of electrons. The silane coupling agent reacts with the hydroxyl groups on the silica surface to form a stable Si-O-Si network, further strengthening the denseness of the coating and reducing the penetration of electrolyte decomposition products.

[0010] The microporous channels are mainly achieved through the following process: The hydrophilic head (quaternary ammonium group) of the quaternary ammonium salt is adsorbed on the negatively charged silica surface through electrostatic interaction, while the hydrophobic tail (long-chain alkyl group) spontaneously stretches outwards in the solvent to form an oriented molecular layer. The silanol groups (Si-OH) generated by the hydrolysis of the silane coupling agent condense with the hydroxyl groups on the silica surface to form Si-O-Si covalent bonds. At the same time, the hydrophobic segments of the silane interact with the tails of the quaternary ammonium salts through van der Waals forces, further stabilizing the oriented arrangement structure of the quaternary ammonium salts. This crosslinking process fixes the arrangement of the quaternary ammonium salts into an ordered microporous network.

[0011] Preferably, the molar ratio of the quaternary ammonium salt surfactant to the silane coupling agent is 1:(2.5 - 3.5). More preferably, the molar ratio of the quaternary ammonium salt surfactant to the silane coupling agent is 1:2.72.

[0012] When the molar ratio of the quaternary ammonium salt surfactant to the silane coupling agent is controlled at 1:(2.5 - 3.5), the hydrolysis and condensation reaction of the silane coupling agent can form a continuous and dense Si - O - Si cross - linked network. This network structure significantly improves the mechanical strength of the coating, effectively resisting physical damage caused by lithium dendrite penetration or external stress. At the same time, the hydrophobic chain segments of the quaternary ammonium salt surfactant and the polar groups of the silane act synergistically to form microporous channels with appropriate porosity, maintaining the efficient transport of lithium ions. In addition, the high thermal stability of the silane cross - linked network can delay the structural collapse at high temperatures, while the endothermic decomposition characteristics of the quaternary ammonium salt can further absorb heat. The synergistic effect of the two significantly inhibits the thermal runaway process.

[0013] It can be imagined that if the amount of the silane coupling agent is insufficient, resulting in an incomplete or uneven distribution of the Si - O - Si cross - linked network. The mechanical strength of the coating decreases, and it is easily broken due to the growth of lithium dendrites or the expansion of the electrode, losing its physical isolation function. The porosity may be too high to effectively block the penetration of the decomposition products of the electrolyte, accelerating side reactions and reducing the interfacial stability. In addition, the absence of the silane network weakens the heat resistance of the coating, and it is more likely to undergo structural failure at high temperatures, leading to a significant increase in the risk of thermal runaway. An excessive amount of the silane coupling agent will result in too high a Si - O - Si cross - linking density, a significant reduction in the porosity of the coating, hindering the normal transport of lithium ions, increasing the internal resistance of the battery and reducing the energy efficiency. The overly dense network will also reduce the flexibility of the coating, generating micro - cracks during the battery cycling process due to the volume change of the electrode, damaging the integrity of the coating. In addition, a high silane ratio may introduce excessive by - products (such as unreacted silane hydrolyzates), affecting the purity of the coating and even reacting with the electrolyte to accelerate the degradation of battery performance.

[0014] Preferably, the particle size range of the nano - silica particles is 10 - 100 nm. Further preferably, the particle size range of the nano - silica particles is 10 - 50 nm.

[0015] When the particle size of the nano - silica particles is less than 10 nm, the high activity on the particle surface is prone to cause agglomeration, resulting in a significant increase in the porosity of the coating and a decrease in the barrier performance. At the same time, the overly reactive surface hydroxyl groups may embrittle the material structure, reducing the mechanical stability; while when the particle size exceeds 50 nm, the coating formed by the particle accumulation has insufficient denseness, making it difficult to effectively block the physical penetration of lithium dendrites. The increased complexity of the ion transport path also leads to an increase in the internal resistance. At the same time, the limited specific surface area of the larger particles weakens the binding ability with the coating material, further affecting the thermal stability and electrochemical performance of the coating.

[0016] Preferably, the thickness of the outer shell coating layer is 2 - 10 nm.

[0017] Preferably, the specific surface area of the composite coating material is 200 - 400 m 2 / g.

[0018] Preferably, the contact angle of the organic-inorganic hybrid structure is 30 to 60°.

[0019] In a second aspect, a method for preparing a composite coating having the above composite coating material includes the following steps:

[0020] Prepare silica nanoparticles by the sol-gel method;

[0021] Compound the silica nanoparticles with a quaternary ammonium salt surfactant and a silane coupling agent to form a composite coating material having a core-shell structure, and the outer shell is an organic-inorganic hybrid structure formed by the reaction of the quaternary ammonium salt surfactant and the silane coupling agent;

[0022] Make the composite coating material into a coating slurry with a binder;

[0023] Coat the slurry on the surface of the positive electrode and dry it to obtain a composite coating.

[0024] The method for preparing the composite coating of the present invention mainly includes steps such as preparing silica nanoparticles, synthesizing a composite coating material with a core-shell structure, making a coating slurry, and coating and drying. The process route is relatively clear and concise, easy to operate and control, and is conducive to realizing industrial production.

[0025] In the above preparation method, the silica nanoparticles prepared by the sol-gel method have small and uniform particle sizes, which can make the coating have a delicate microstructure, help improve the denseness of the coating, and further enhance the barrier performance of the coating.

[0026] The prepared composite coating material with a core-shell structure can combine the advantages of different materials. The core material can provide basic properties such as mechanical strength, while the organic-inorganic hybrid structure of the outer shell (formed by the reaction of the quaternary ammonium salt surfactant and the silane coupling agent) can endow the coating with good flexibility, corrosion resistance, etc.

[0027] Preferably, the preparation of silica nanoparticles by the sol-gel method includes:

[0028] Mix tetraethyl orthosilicate and an alkaline catalyst, add a mixed solvent of ethanol and water, and stir and react to obtain a silica nanoparticle suspension;

[0029] Wash, ultrasonically treat and dry the silica nanoparticle suspension to obtain silica nanoparticles.

[0030] The above process involves the hydrolysis and condensation reaction of tetraethyl orthosilicate. The specific process is as follows:

[0031] Hydrolysis reaction: Tetraethyl orthosilicate undergoes hydrolysis in an alkaline catalyst and an ethanol / water mixed solvent to form silicic acid:

[0032]

[0033] The hydrolysis reaction generates active silicic acid monomers, providing raw materials for subsequent condensation.

[0034] Condensation reaction: Silicic acid monomers form Si-O-Si bonds through a condensation reaction and gradually grow into silicon dioxide nanoparticles:

[0035] Si(OH)4 → SiO2 + 2H2O.

[0036] By controlling the hydrolysis and condensation rates, the particle size (10 - 100 nm) and dispersion uniformity of the particles can be precisely regulated. A fast hydrolysis rate will cause the particles to grow rapidly, forming large-sized particles. A slow hydrolysis rate will generate small-sized, monodisperse nanoparticles. The surface of the silicic acid monomers generated by hydrolysis is rich in hydroxyl groups, endowing the particles with high chemical activity, facilitating subsequent reactions with quaternary ammonium salt surfactants and silane coupling agents, and forming a stable core-shell structure.

[0037] Preferably, the volume ratio of tetraethyl orthosilicate to the alkaline catalyst is 1:(0.5 - 2). More preferably, the volume ratio of tetraethyl orthosilicate to the alkaline catalyst is 1:1.

[0038] The dosage of the alkaline catalyst can balance the rates of hydrolysis and condensation reactions, ensuring that tetraethyl orthosilicate is fully hydrolyzed into silicic acid monomers, while avoiding particle aggregation caused by excessive condensation. Setting the volume ratio of tetraethyl orthosilicate to the alkaline catalyst to 1:(0.5 - 2) can ensure that the generated silicon dioxide nanoparticles have uniform particle sizes and surfaces rich in active hydroxyl groups (-Si-OH), providing sufficient active sites for subsequent reactions with silane coupling agents, thereby forming a dense and stable core-shell structure. Similarly, if the dosage of the alkaline catalyst is insufficient, the hydrolysis reaction of tetraethyl orthosilicate will be incomplete. The unhydrolyzed tetraethyl orthosilicate monomers will form large and uneven-sized particles, even dendritic structures, during the condensation stage, reducing the denseness and uniformity of the coating. Due to the insufficient number of hydroxyl groups (-Si-OH) on the particle surface, the bonding ability with silane coupling agents is weakened, and the hybrid outer shell is prone to peeling and failure. If the alkaline catalyst is excessive, it will accelerate the condensation reaction, causing the silicic acid monomers to rapidly polymerize into clusters. The overly dense particle arrangement results in too low porosity of the coating, hindering lithium ion transmission, increasing the internal resistance of the battery, and decreasing the rate performance. In addition, the residual excessive catalyst is difficult to completely remove, which may corrode the positive electrode active material or cause the decomposition of the electrolyte, leading to an aggravation of interfacial side reactions.

[0039] More preferably, the volume ratio of ethanol to water in the mixed solvent is (5-10): 1. More preferably, the volume ratio of ethanol to water in the mixed solvent is 7:1.

[0040] In the mixed solvent, ethanol is used to adjust the hydrolysis rate and inhibit agglomeration caused by excessive reaction, while water provides the environment required for the hydrolysis reaction. When the volume ratio of ethanol to water is controlled at (5-10):1, the high proportion of ethanol can effectively adjust the hydrolysis-condensation rate of tetraethyl orthosilicate. As a co-solvent, ethanol slows down the hydrolysis rate of tetraethyl orthosilicate by reducing the polarity of the reaction system, avoiding rapid growth and agglomeration of particles due to excessive reaction, thereby generating silica nanoparticles with uniform particle size. At the same time, an appropriate amount of water provides the necessary hydrolysis environment, ensuring that tetraethyl orthosilicate is fully hydrolyzed to generate active silica monomers, providing raw materials for subsequent condensation reactions.

[0041] More preferably, the stirring reaction temperature is 25 to 50° C., and the stirring time is 1 to 6 hours.

[0042] If the temperature is lower than 25°C, the reaction rate is too slow, resulting in uneven particle size, high coating porosity, decreased barrier properties, and the reaction time needs to be extended to more than 6 hours, significantly increasing energy consumption. If the temperature is higher than 50°C, the hydrolysis and condensation rate is out of control, the particles quickly agglomerate into a coarse structure or dendrite, the coating density deteriorates, and lithium dendrites easily penetrate to cause short circuits. At the same time, high temperature aggravates solvent volatilization, destroys the charge balance on the particle surface, and aggravates secondary agglomeration. If the time is less than 1 hour, the incomplete reaction causes residual unhydrolyzed T-ethyl orthosilicate monomer to contaminate the particle surface, weaken the bonding ability with the silane coupling agent, and the hybrid shell is discontinuous; if the time exceeds 6 hours, the particles grow excessively or densely agglomerate, the porosity decreases, the lithium ion transmission is blocked, the battery internal resistance increases, and extending the reaction time is not conducive to performance improvement, but increases production costs. This range ensures particle uniformity and activity by balancing reaction kinetics.

[0043] Further preferably, the alkaline catalyst is selected from one of ammonium hydroxide, sodium hydroxide or potassium hydroxide.

[0044] As a weak alkaline catalyst, ammonium hydroxide has a moderate hydrolysis reaction rate and can gently promote the hydrolysis and condensation of ethyl orthosilicate to generate silica nanoparticles with uniform particle size. Its decomposition products (NH3, H2O) are easy to volatilize or remove by washing, reducing the impact of residual impurities on coating performance. As strong alkaline catalysts, sodium hydroxide (NaOH) and potassium hydroxide (KOH) have a high OH- concentration that can significantly accelerate the hydrolysis reaction and are suitable for scenarios that require rapid synthesis. By adjusting the proportion of mixed solvents or the reaction temperature (25-50°C), particle agglomeration caused by excessive condensation can be suppressed to ensure product uniformity.

[0045] Preferably, for the washing, ultrasonic treatment, and drying of the silica nanoparticle suspension to obtain silica nanoparticles, the washing includes rinsing with ethanol at least 4 times and rinsing with distilled water at least 4 times;

[0046] The drying is carried out in a vacuum environment at 60 - 80 °C for 10 - 14 hours;

[0047] The ultrasonic treatment time is 10 - 30 minutes, and the ultrasonic power is 200 - 1200 W.

[0048] The reaction residues, including unreacted tetraethyl orthosilicate, alkaline catalyst, by-products (such as ethanol, ammonium salts), and free ions (such as Na + 、K + ), are removed through washing. If these impurities remain on the particle surface, they may cause electrolyte decomposition, corrode the electrode active material, or hinder the subsequent silane coupling reaction, resulting in deterioration of the coating performance. As an organic solvent, ethanol preferentially dissolves and removes non-polar or weakly polar impurities (such as unreacted tetraethyl orthosilicate, organic by-products), and at the same time, the low surface tension of ethanol can reduce particle agglomeration and maintain dispersion. Distilled water is used to remove water-soluble impurities (such as NH4 + 、OH - 、inorganic salts) to prevent the residual alkaline substances from corroding the positive electrode material (such as lithium iron phosphate) or reacting with the electrolyte (such as LiPF6) to generate harmful substances such as HF. Through multiple washings, thorough cleaning is ensured.

[0049] The purpose of drying is to completely remove the solvents (ethanol, water) adsorbed on the particle surface to prevent the residual solvents from affecting the subsequent composite reaction or coating uniformity. Residual solvents may introduce bubbles, reduce the binder compatibility, or trigger interfacial side reactions.

[0050] Ultrasonic treatment is used to break the van der Waals forces or electrostatic attractions between nanoparticles, eliminate the soft agglomerates formed during washing and drying, and restore the monodisperse state. Agglomerates will reduce the coating density and increase the risk of lithium dendrite penetration.

[0051] Preferably, the composite reaction includes the following steps:

[0052] Disperse the silica nanoparticles in a solvent to form a suspension;

[0053] Add a quaternary ammonium salt surfactant, an alkaline catalyst, deionized water, and ethanol to the suspension for premixing; the alkaline catalyst can be selected as concentrated ammonia solution;

[0054] After premixing, add a silane coupling agent to the suspension for coupling reaction;

[0055] After the coupling reaction, centrifugation, washing, and drying were carried out to obtain the nano-sized silica@quaternary ammonium salt composite coating material.

[0056] Through a step-by-step reaction, a quaternary ammonium salt-silane hybrid layer (outer shell) was coated on the surface of silica nanoparticles (core) to form an organic-inorganic composite structure, integrating the mechanical strength of inorganic substances and the flexibility of organic substances, and improving the puncture resistance and interfacial stability of the coating. The silane coupling agent (such as tetraethyl orthosilicate) forms a covalent bond (Si-O-Si) with the hydroxyl groups (-Si-OH) on the silica surface through hydrolysis and condensation reactions. At the same time, the quaternary ammonium salt surfactant binds to the hydrophobic segment of the silane through electrostatic adsorption to form a hybrid layer stabilized by dual forces, preventing the coating from peeling off. The introduction of the quaternary ammonium salt endows the coating with ion-selective transport ability, and the silane cross-linked network provides thermal stability, synergistically inhibiting thermal runaway.

[0057] Further preferably, the mass ratio of the silica nanoparticles to the quaternary ammonium salt surfactant is 1:(0.8 - 3). Further preferably, the mass ratio of the silica nanoparticles to the quaternary ammonium salt surfactant is 1:1.5.

[0058] This range ensures the formation of a continuous and appropriate organic coating layer on the silica surface by balancing the amount of the quaternary ammonium salt. If the amount of the quaternary ammonium salt is too small, the coating layer will be incomplete, unable to effectively block the penetration of harmful substances, and the protective performance of the coating will decrease significantly; if the amount of the quaternary ammonium salt is excessive, it may cover too many active sites on the particle surface, hinder the combination of the silane coupling agent and silica, resulting in a loose hybrid layer structure and even blocking the ion transport channels. By controlling the mass ratio within the range of 1:(0.8 - 3), the quaternary ammonium salt can not only fully cover the particle surface to form a stable coating, but also cooperate with the silane coupling agent to construct a hybrid structure with both flexibility and rigidity, thus achieving the optimal balance among the coating strength, ion conductivity, and thermal stability, and taking into account the battery safety and electrochemical performance.

[0059] Preferably, the molar ratio of the quaternary ammonium salt surfactant to the silane coupling agent is 1:(2.5 - 3.5). Further preferably, the molar ratio of the quaternary ammonium salt surfactant to the silane coupling agent is 1:2.72.

[0060] This range ensures the formation of a stable organic-inorganic hybrid structure on the silica surface by balancing the interaction between the silane coupling agent and the quaternary ammonium salt. If the amount of the silane coupling agent is insufficient, resulting in a low cross-linking density and an incomplete coating layer, the mechanical strength and thermal stability of the coating will decrease significantly. If the silane coupling agent is excessive, it may cover the active sites of the quaternary ammonium salt, weakening its directional adsorption ability. At the same time, excessive cross-linking will reduce the flexibility and ion transport efficiency of the coating. When the molar ratio of the quaternary ammonium salt surfactant to the silane coupling agent is in the range of 1:(2.5 - 3.5), the synergistic effect of the silane and the quaternary ammonium salt can not only achieve dense coating but also maintain the functional balance of the coating (such as puncture resistance and ion conductivity), thus optimizing the comprehensive performance of battery safety and electrochemical performance.

[0061] Preferably, preparing the composite coating material and the binder into a coating slurry includes: mixing the cathode active material, the conductive agent, and the first binder according to a mass ratio of (7 - 9):(0.5 - 1.5):(0.5 - 1.5), and adding a first solvent to form a cathode slurry; preferably, the mass ratio of the cathode active material, the conductive agent, and the first binder is 8:1:1;

[0062] Mixing the composite coating material and the second binder according to a mass ratio of (4 - 6):1, and adding a second solvent to form a coating slurry; preferably, the mass ratio of the composite coating material and the second binder is 5:1.

[0063] Preferably, the cathode active material is lithium iron phosphate.

[0064] The first solvent is selected from at least one of N-methylpyrrolidone, dimethylformamide, or dimethylacetamide.

[0065] The second solvent is N-methylpyrrolidone.

[0066] Preferably, coating the slurry on the cathode surface and drying to obtain a composite coating includes:

[0067] Using a doctor blade coating method or a spraying method to uniformly coat the slurry on the cathode surface, and the coating thickness is 50 - 300 μm;

[0068] Drying in a vacuum environment at 120 °C for 10 - 14 hours to obtain a composite coating with a thickness of 20 - 150 μm.

[0069] In a third aspect, a lithium-ion battery cathode includes a current collector and a cathode active material layer, and the surface of the cathode active material layer is covered with a composite coating, and the composite coating is prepared by the preparation method of the above composite coating.

[0070] In a fourth aspect, a lithium-ion battery includes the above lithium-ion battery cathode.

[0071] Fifth aspect, an electrical device, comprising the above-mentioned lithium-ion battery.

[0072] Due to the application of the above technical solution, the present invention has the following advantages compared with the prior art:

[0073] 1. The composite coating adopts a core-shell structure, with the core being nano-silica particles and the shell being an organic-inorganic hybrid layer formed by the reaction of a quaternary ammonium salt surfactant and a silane coupling agent. The nano-silica in the core effectively blocks electron transfer through its high insulation, while the hybrid structure of the shell combines the flexibility of the organic matter and the rigidity of the inorganic matter, which can not only adapt to the volume change during the charge and discharge process of the electrode, but also resist the physical penetration of lithium dendrites. This dual characteristic enables the coating to significantly improve safety while maintaining the efficient operation of the battery.

[0074] 2. The preparation method realizes precise control through the sol-gel method and step-by-step composite reaction. The sol-gel method generates nano-silica particles with uniform size by adjusting the solvent ratio and reaction conditions; the step-by-step composite reaction ensures the uniformity and stability of the coating layer through pre-mixing and silane cross-linking steps. The entire process does not require complex equipment or extreme conditions, has strong adaptability, is convenient for large-scale production, and the solvent can be recycled, reducing the environmental burden.

[0075] To make the above and other purposes, features and advantages of the present invention more obvious and understandable, the following specifically enumerates preferred embodiments and, in conjunction with the accompanying drawings, makes the following detailed description. Brief Description of the Drawings

[0076] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0077] Figure 1 It is the TEM spectrum of the composite coating material in Embodiment 1 of the present invention. Detailed Embodiments

[0078] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention.

[0079] Embodiment 1:

[0080] A composite coating material for the positive electrode of a lithium-ion battery. The core of the composite coating material is nano-silica particles, and the shell is an organic-inorganic hybrid structure formed by the reaction of a quaternary ammonium salt surfactant and a silane coupling agent. The quaternary ammonium salt surfactant is selected from one of N-methylpyrrolidone, dimethylformamide, and dimethylacetamide; the silane coupling agent is tetraethyl orthosilicate. The mass ratio of the silica nanoparticles to the quaternary ammonium salt surfactant is 1:(0.8 - 3). The molar ratio of the quaternary ammonium salt surfactant to the silane coupling agent is 1:(2.5 - 3.5).

[0081] This embodiment also discloses a preparation method of a composite coating with the above composite coating material and a positive electrode of a lithium-ion battery. The preparation method includes the following steps:

[0082] Step 1: Dissolve 10 mL of tetraethyl orthosilicate and 10 mL of ammonium hydroxide in a mixed solution of ethanol and water, where ethanol is 428 mL and water is 60 mL, and the volume ratio is 7:1. Stir and react gently at 30 °C for 2 hours to form a white product. Rinse with ethanol 4 times and distilled water 4 times in sequence, then perform ultrasonic treatment on the sample for 10 minutes with an ultrasonic power of 300 W, disperse it in distilled water, and dry it in a vacuum at 65 °C for 12 hours to remove moisture to obtain silica nanoparticles.

[0083] Step 2: Add 100 mg of silica nanoparticles to 20 mL of distilled water and ultrasonically disperse for 30 minutes. Add the dispersion to a mixed solution containing 150 mg of N-hexadecyltrimethylammonium bromide, 30 mL of deionized water, 30 mL of ethanol, and 0.55 mL of concentrated ammonia solution.

[0084] Step 3: After stirring at room temperature for 30 minutes, quickly add 0.25 mL of tetraethyl orthosilicate and continue to react for 6 hours. Centrifuge to collect the white product, rinse with ethanol and distilled water 4 times each, and then disperse it in 20 mL of distilled water to obtain nano-silica@N-hexadecyltrimethylammonium bromide composite coating material.

[0085] Step 4: Mix lithium iron phosphate powder, carbon black, and polyvinylidene fluoride in a mass ratio of 8:1:1, add N-methylpyrrolidone as a solvent, stir well until a uniform slurry is formed, coat it on the surface of aluminum foil, dry at room temperature, and then dry it in a vacuum oven at 60 °C for 12 hours.

[0086] Step 5: Mix the composite coating material nano-silica@N-hexadecyltrimethylammonium bromide prepared in Step 3 with polyvinylidene fluoride in a mass ratio of 5:1, disperse it in N-methylpyrrolidone, and stir until the solution is uniform to obtain a coating slurry.

[0087] Step 6: Use a doctor blade coating method to evenly coat the coating slurry on the surface of the lithium iron phosphate positive electrode with a coating thickness of 100 microns, and then dry it in a vacuum oven at 120°C overnight to prepare a composite coating disposed on the positive electrode surface. After completely removing the solvent, it is punched into a standard positive electrode sheet, that is, a lithium-ion battery positive electrode is obtained.

[0088] This embodiment also discloses a lithium-ion battery, including the lithium-ion battery positive electrode.

[0089] Comparative Example 1:

[0090] This comparative example also discloses a lithium ion battery positive electrode, the preparation method of which comprises the following steps:

[0091] Step 1: Mix lithium iron phosphate powder, carbon black and polyvinylidene fluoride in a mass ratio of 8:1:1, add N-methylpyrrolidone as a solvent, stir thoroughly to form a uniform slurry, apply it on the surface of aluminum foil, dry at room temperature, and then dry in a vacuum oven at 60°C for 12 hours.

[0092] Step 2: Mix nano-silicon dioxide and polyvinylidene fluoride in a mass ratio of 5:1, disperse in N-methylpyrrolidone, and stir until the solution is uniform to obtain a coating slurry. The nano-silicon dioxide can be purchased directly from the market, such as from MacLean Chemical Reagent Co., Ltd.

[0093] Step 3: Use a doctor blade coating method to evenly coat the coating slurry on the surface of the lithium iron phosphate positive electrode with a coating thickness of 100 microns, and then dry it in a vacuum oven at 120°C overnight to prepare a composite coating disposed on the positive electrode surface. After completely removing the solvent, it is punched into a standard positive electrode sheet, that is, a lithium-ion battery positive electrode is obtained.

[0094] This comparative example also discloses a lithium ion battery, comprising the lithium ion battery positive electrode.

[0095] Comparative Example 2:

[0096] This comparative example also discloses a lithium ion battery positive electrode, the preparation method of which comprises the following steps:

[0097] Lithium iron phosphate powder, carbon black and polyvinylidene fluoride are mixed in a mass ratio of 8:1:1, N-methylpyrrolidone is added as a solvent, and the mixture is fully stirred to form a uniform slurry, which is then coated on the surface of an aluminum foil, dried at room temperature and then dried in a vacuum oven at 60°C for 12 hours. After the solvent is completely removed, the mixture is punched into a standard positive electrode sheet to obtain a lithium-ion battery positive electrode.

[0098] This comparative example also discloses a lithium ion battery, comprising the lithium ion battery positive electrode.

[0099] The formula of the battery in the above-mentioned Example 1, Comparative Examples 1 and 2 is:

[0100] The positive electrode is the lithium-ion battery positive electrode prepared in Example 1, Comparative Example 1, and Comparative Example 2 respectively. The negative electrode is composed of graphite, LA136D, Super P, CNT, and CMC in a weight ratio of 96:2.3:0.9:0.4:0.4. The N / P value is close to 1.13. The electrolyte contains 1M LiPF6 / EC:EMC (volume ratio 3:7). The separator uses a ceramic-coated polyethylene film. All batteries use a copper foil with a thickness of 6.0 μm as the negative current collector. The charge-discharge range is 2.5 - 3.6V, and its cycling performance and energy efficiency are tested under 0.5C / 0.5C conditions.

[0101] The transmission electron microscope (TEM, JEM~2100F) was used to study the microstructure of the composite coating material nanosilica@N-hexadecyltrimethylammonium bromide, as shown in Figure 1 shown.

[0102] The characterization results by transmission electron microscope (TEM) confirmed that the composite coating material successfully achieved core-shell structure coating. Its microstructure presented clear bilayer characteristics:

[0103] Inner core layer: It is composed of silica nanoparticles with a uniform particle size distribution (10 - 100nm), which serves as a rigid support core to provide mechanical strength and thermal stability;

[0104] Outer shell layer: It is formed by the oriented arrangement of N-hexadecyltrimethylammonium bromide (CTAB), which uniformly coats the surface of silica. Through physical barrier action, it avoids direct contact between the electrolyte and silica and inhibits side reactions (such as the reaction between silica and HF generated by the hydrolysis of LiPF3).

[0105] This bilayer structure improves battery performance through the following mechanisms:

[0106] Interface stability: The hydrophobic chain segments of the CTAB shell form a dense barrier to block the penetration of the electrolyte and reduce the corrosion of active substances;

[0107] Ion conductivity: The hydrophilic quaternary ammonium groups of CTAB and the silane cross-linked network cooperate to construct microporous channels, allowing lithium ions to freely transport while blocking electron transfer.

[0108] It effectively solves the problem of the chemical inertness of silica in the electrolyte and provides a reliable solution for the positive electrode coating of high-safety lithium-ion batteries.

[0109] The short-circuit thermal runaway safety tests were conducted on the lithium-ion batteries of Example 1, Comparative Example 1, and Comparative Example 2. Test method: Under the condition that the battery is fully charged, an internal short-circuit fault is triggered to simulate extreme abuse conditions. The temperature evolution curve and the dynamic smoke release during the thermal runaway of the battery are monitored and recorded in real time. The test results are shown in the following table:

[0110] Test content Example 1 Comparative Example 1 Comparative Example 2 Short - circuit thermal runaway - Total thermal runaway time 269s 302s 320s Short - circuit thermal runaway - Maximum thermal runaway temperature 369.2℃ 403.7℃ 445.4℃ 25℃ 0.5C / 0.5C energy efficiency 94.50％ 90.50％ 93.50％ 25℃ 0.5C / 0.5C cycle performance 98.2% @500 cycles 92.7% @500 cycles 97.1% @500 cycles

[0111] It can be seen from the above test results that:

[0112] In Example 1, Comparative Example 1, and Comparative Example 2, the starting time of short-circuit thermal runaway is recorded as 0S, and the total thermal runaway times are 269S, 302S, and 320S respectively.

[0113] For the lithium-ion battery in Comparative Example 2, under the condition of simulating an internal short circuit, the total thermal runaway time is 320 seconds, the highest temperature climbs to 445.4 °C, and a large amount of smoke is released. This phenomenon is attributed to the direct contact between the positive and negative electrodes after the diaphragm melts, triggering a violent exothermic reaction, which advances the thermal runaway of the battery.

[0114] For the lithium-ion battery in Example 1, under the condition of simulating an internal short circuit, the total thermal runaway time is 269s, and the highest temperature climbs to 369.2 °C. Since the total thermal runaway time of the lithium iron phosphate battery protected by silica nanoparticles is 51 seconds shorter than that of the lithium-ion battery in Comparative Example 2, and the highest temperature during the thermal runaway process is also reduced by 76.2 °C. This is because even if the diaphragm melts, the physical barrier effect of the silica layer prevents the direct contact between the positive and negative electrodes, shortening the thermal runaway time. Compared with the lithium-ion battery in Comparative Example 2, the smoke generation and release of the battery with the positive electrode coated with the nano-silica@N-hexadecyltrimethylammonium bromide composite coating material are also greatly reduced. This is due to the adsorption characteristics of the silica layer, which greatly reduces the smoke release rate. Therefore, the positive electrode coated with the nano-silica@N-hexadecyltrimethylammonium bromide composite coating material can improve the safety performance of the battery.

[0115] Obviously, compared with the lithium-ion batteries in Comparative Example 1 and Comparative Example 2, the lithium-ion battery in Example 1 shows higher energy efficiency and cycling performance. This means that the coating with the nano-silica@N-hexadecyltrimethylammonium composite material coating can improve the interfacial stability between the positive electrode and the electrolyte, and the larger pores in the coating promote the ionic conductivity and specific capacity of the battery.

[0116] In the present invention, specific embodiments are used to illustrate the principle and implementation manner of the present invention. The description of the above embodiments is only used to help understand the method and its core idea of the present invention; at the same time, for those of ordinary skill in the art, according to the idea of the present invention, there will be changes in the specific implementation manner and application scope. In summary, the content of this specification should not be construed as a limitation to the present invention.

Claims

1. A composite coating material for a lithium-ion battery positive electrode, characterized in that: The core of the composite coating material is nano silicon dioxide particles, and the shell is an organic-inorganic hybrid structure, wherein the organic-inorganic hybrid structure is formed by the reaction of a quaternary ammonium salt surfactant and a silane coupling agent.

2. The composite coating material for lithium ion battery positive electrode according to claim 1, characterized in that: The quaternary ammonium salt surfactant is selected from one of N-methylpyrrolidone, dimethylformamide and dimethylacetamide; and the silane coupling agent is tetraethyl orthosilicate.

3. The composite coating material for lithium ion battery positive electrode according to claim 1, characterized in that: The molar ratio of the quaternary ammonium salt surfactant to the silane coupling agent is 1:(2.5-3.5).

4. A method for preparing a composite coating having the composite coating material according to any one of claims 1 to 3, characterized in that: The following steps are involved: Preparation of silica nanoparticles by sol-gel method; The silicon dioxide nanoparticles are subjected to a composite reaction with a quaternary ammonium salt surfactant and a silane coupling agent to form a composite coating material having a core-shell structure, wherein the shell is an organic-inorganic hybrid structure formed by the reaction of the quaternary ammonium salt surfactant and the silane coupling agent; preparing a coating slurry by combining the composite coating material and the binder; The slurry is coated on the surface of the positive electrode and dried to obtain a composite coating.

5. The method for preparing the composite coating according to claim 4, characterized in that: The method for preparing silicon dioxide nanoparticles by a sol-gel method comprises: Mixing tetraethyl orthosilicate and an alkaline catalyst, adding a mixed solvent of ethanol and water, stirring and reacting to obtain a suspension of silicon dioxide nanoparticles; The silica nanoparticle suspension is washed, ultrasonically treated and dried to obtain silica nanoparticles.

6. The method for preparing the composite coating according to claim 5, characterized in that: The volume ratio of the tetraethyl orthosilicate to the alkaline catalyst is 1:(0.5-2); The volume ratio of ethanol to water in the mixed solvent is (5-10):1; The stirring reaction temperature is 25 to 50°C and the stirring time is 1 to 6 hours; The alkaline catalyst is selected from one of ammonium hydroxide, sodium hydroxide or potassium hydroxide.

7. The method for preparing the composite coating according to claim 5, characterized in that: The silica nanoparticle suspension is washed, ultrasonically treated and dried to obtain silica nanoparticles, wherein the washing includes washing with ethanol at least 4 times and washing with distilled water at least 4 times; The drying is carried out in a vacuum environment at 60 to 80° C. for 10 to 14 hours; The ultrasonic treatment time is 10 to 30 minutes, and the ultrasonic power is 200 to 1200W.

8. The method for preparing the composite coating according to claim 4, characterized in that: The composite reaction comprises the following steps: dispersing the silicon dioxide nanoparticles in a solvent to form a suspension; Adding a quaternary ammonium surfactant, an alkaline catalyst, deionized water and ethanol to the suspension for premixing; adding a silane coupling agent to the suspension after premixing to carry out a coupling reaction; After the coupling reaction, the nano-silica@quaternary ammonium salt composite coating material is obtained by centrifugation, washing and drying.

9. The method for preparing the composite coating according to claim 8, characterized in that: The mass ratio of the silicon dioxide nanoparticles to the quaternary ammonium salt surfactant is 1:(0.8-3); The molar ratio of the quaternary ammonium salt surfactant to the silane coupling agent is 1:(2.5-3.5).

10. The method for preparing the composite coating according to claim 4, characterized in that: The composite coating material and the binder are made into a coating slurry, comprising: mixing the positive electrode active material, the conductive agent and the first binder in a mass ratio of (7-9):(0.5-1.5):(0.5-1.5), and adding a first solvent to form a positive electrode slurry; The composite coating material and the second binder are mixed in a mass ratio of (4-6):1, and a second solvent is added to form a coating slurry.

11. The method for preparing the composite coating according to claim 10, characterized in that: The positive electrode active material is lithium iron phosphate; The first solvent is selected from at least one of N-methylpyrrolidone, dimethylformamide or dimethylacetamide; The second solvent is N-methylpyrrolidone.

12. A lithium ion battery positive electrode, comprising a current collector and a positive electrode active material layer, characterized in that: The surface of the positive electrode active material layer is covered with a composite coating, and the composite coating is prepared by the preparation method of the composite coating according to any one of claims 4 to 11.

13. A lithium ion battery, characterized in that: The lithium-ion battery positive electrode comprises the positive electrode of claim 12.

14. An electrical device, characterized in that: Includes the lithium ion battery as claimed in claim 13.

Images