Lithium ion battery electrolyte, preparation method and application thereof

Abstract

The invention discloses a lithium ion battery electrolyte, a preparation method and application thereof, and the electrolyte comprises: a lithium salt with a concentration of 0.5-3 mol / L; the organic solvent comprises ethylene carbonate and dimethyl carbonate, and the volume ratio of the ethylene carbonate to the dimethyl carbonate is 1: (2-9); and the flame retardant additive is urea resin coated phosphonitrilic chloride trimer and accounts for 1-10% of the total mass of the electrolyte. A microencapsulated flame-retardant additive (urea-formaldehyde resin-coated phosphonitrilic chloride trimer) is introduced into the electrolyte, so that the electrolyte can quickly release phosphorus / nitrogen free radicals during thermal runaway, the combustion chain reaction is interrupted, and the second-level fire extinguishing capability is realized. The urea-formaldehyde resin coating layer isolates the flame-retardant additive and the electrolyte in a normal state, side reaction is avoided, a physical barrier is formed only through carbonization at high temperature, and oxygen diffusion and heat transfer are inhibited.

Description

Technical Field

[0001] The present invention relates to the field of lithium ion batteries, and in particular to a lithium ion battery electrolyte, a preparation method and application thereof. Background Art

[0002] Lithium-ion batteries have become the core power source for energy storage devices, electric vehicles and consumer electronics due to their advantages such as high energy density and long cycle life. However, with the continuous improvement of battery energy density, the risk of combustion and explosion caused by thermal runaway has become a key bottleneck restricting its large-scale application. Under extreme conditions such as overcharging, short circuit, and mechanical abuse, the violent exothermic reaction between the electrolyte and the electrode material will trigger a chain thermal runaway, releasing a large amount of flammable gases (such as H2, CH4), causing the battery to catch fire or even explode. Therefore, the development of an electrolyte system with both high safety and stable electrochemical performance has become an important direction for the technological innovation of lithium-ion batteries.

[0003] In order to improve the flame retardant properties of electrolytes, traditional solutions mostly use the addition of flame retardant additives, such as phosphates (such as TPP), fluorinated solvents (such as FEC) or phosphorus / nitrogen compounds (such as hexachlorocyclotriphosphazene). However, these technologies have the following defects: Chemical incompatibility between flame retardant additives and electrolytes: For example, although hexachlorocyclotriphosphazene (HCP) has excellent free radical capture ability, its active Cl - It will react with LiPF6 in the electrolyte to generate acidic substances (such as HCl, POF3), accelerate the thickening of the SEI layer and block the lithium ion transmission channel. Significant attenuation of electrochemical performance: The introduction of flame retardant additives often destroys the solvation structure of the electrolyte, resulting in a decrease in ionic conductivity and triggering interlayer peeling of the negative electrode graphite. The contradiction between flame retardant efficiency and added amount: In order to achieve effective fire extinguishing, flame retardant additives need to be added at 10%-30%, but high content will aggravate the increase in viscosity and interfacial side reactions, resulting in a sharp drop in battery cycle life. Summary of the invention

[0004] In order to overcome the defects in the prior art, the first purpose of the present invention is to provide a lithium ion battery electrolyte, the second purpose of the present invention is to provide a method for preparing a lithium ion battery electrolyte, the third purpose of the present invention is to provide a lithium ion battery, and the fourth purpose of the present invention is to provide an electrical device.

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

[0006] In a first aspect, a lithium-ion battery electrolyte comprises:

[0007] Lithium salt, concentration is 0.5~3mol / L;

[0008] An organic solvent comprises ethylene carbonate and dimethyl carbonate, wherein the volume ratio of ethylene carbonate to dimethyl carbonate is 1:(2-9);

[0009] The flame retardant additive is hexachlorocyclotriphosphazene coated with urea-formaldehyde resin, accounting for 1% to 10% of the total mass of the electrolyte.

[0010] The lithium-ion battery electrolyte of the present invention uses urea-formaldehyde resin-coated hexachlorocyclotriphosphazene as a flame retardant additive, and synergizes with a specific ratio of ethylene carbonate / dimethyl carbonate solvent system and lithium salt, while significantly improving the flame retardant properties of the electrolyte, it effectively inhibits the side reactions between the flame retardant additive and the electrode material and electrolyte components. The urea-formaldehyde resin coating isolates the flame retardant additive from direct contact with the electrolyte at room temperature, avoiding the attenuation of electrochemical performance; it quickly releases the flame retardant active ingredients in the event of thermal runaway, and efficiently inhibits the combustion chain reaction through the dual mechanisms of gas phase free radical quenching and condensed phase carbonization. This system breaks through the technical bottleneck of traditional flame retardant electrolytes that are difficult to balance high safety and long cycle life, and achieves a good balance between flame retardant efficiency, ionic conductivity and interface stability, without the need to modify existing production processes, and has excellent prospects for industrial application.

[0011] The gas-phase free radical quenching mechanism means that when the electrolyte is heated or in the early stage of combustion, the urea-formaldehyde resin coating breaks due to high temperature, releasing the hexachlorocyclotriphosphazene inside. After the substance decomposes, it produces chlorine, phosphorus and nitrogen active free radicals, which capture hydrogen free radicals and hydroxyl free radicals in the combustion chain reaction, blocking the continuous oxidation reaction of combustible gases (such as hydrogen and methane), thereby inhibiting the spread of flames. This mechanism directly interrupts the chain transfer process of combustion at the gas phase level, achieving rapid fire extinguishing.

[0012] The condensed phase carbonization mechanism refers to the following: under high temperature conditions, the urea-formaldehyde resin coating undergoes pyrolysis and carbonization, forming a carbon layer on the surface of the electrode and electrolyte. This carbon layer acts as a physical barrier, effectively isolating the external oxygen from the internal combustibles, and blocks the heat from transferring into the battery, delaying the thermal runaway process. At the same time, the carbon layer further inhibits the continued spread of combustion by adsorbing and fixing combustible volatiles.

[0013] The gas phase mechanism extinguishes the open flame through chemical means, and the condensed phase mechanism isolates the combustion conditions through physical means. The two complement each other at different stages of combustion. The urea-formaldehyde resin coating not only protects the flame retardant additives from side reactions at room temperature, but also accurately triggers the dual flame retardant pathways at high temperatures, ensuring the safety of the electrolyte under extreme working conditions while maintaining the electrochemical stability required for normal charging and discharging of the battery.

[0014] Preferably, the particle size of the flame retardant additive is 0.1-10 μm.

[0015] Particle size control is the key to the efficient function of flame retardant additives in liquid electrolytes. The range of 0.1 to 10 μm solves the performance attenuation problem caused by improper size of traditional flame retardant additives by optimizing the balance between dispersion stability, flame retardant efficiency and battery performance, ensuring the synergistic improvement of electrolyte safety and electrochemical performance. First, the particles in this range have moderate Brownian motion ability in liquid electrolytes, and can be evenly dispersed with the help of stirring and ultrasound to avoid agglomeration or sedimentation. If the particle size is too small (<0.1 μm), it is easy to spontaneously agglomerate due to excessive surface energy; if the particle size is too large (>10 μm), it will quickly settle due to gravity, resulting in uneven distribution of flame retardant additives. Secondly, when the microcapsules of 0.1 to 10 μm are thermally runaway, the urea-formaldehyde shell can quickly break and release flame retardant additives (such as HCP), and the particle specific surface area is moderate, ensuring that the active ingredients are fully exposed to combustible gases. If the particle size is too large, it will delay heat conduction and reduce the flame retardant response speed; if it is too small, it will leak prematurely due to the thin coating layer, causing side reactions. Furthermore, this size range avoids blocking the pores of the separator or covering the electrode surface, ensuring the free migration of lithium ions. Ultra-small particles may penetrate the separator and cause a short circuit, while ultra-large particles will hinder ion transmission and increase interface impedance.

[0016] Preferably, the coating layer formed by urea-formaldehyde resin in the flame retardant additive has a thickness of 50 to 500 nm.

[0017] When the thickness of the urea-formaldehyde resin coating is ≥50nm, it can effectively block the direct contact between hexachlorocyclotriphosphazene and the lithium salt in the electrolyte, and avoid the leakage of chloride ions at room temperature to cause side reactions (such as the generation of HCl to corrode the SEI layer). At the same time, the coating in this thickness range has sufficient mechanical strength to prevent the coating from breaking due to volume changes during battery charging and discharging. When the thickness is ≤500nm, the urea-formaldehyde resin will decompose rapidly at the thermal runaway temperature to ensure that hexachlorocyclotriphosphazene is released in time and exerts a gas-phase flame retardant effect. An overly thick coating (>500nm) will delay heat conduction, resulting in a delayed release of flame retardant additives, and will not be able to effectively suppress initial combustion. At the same time, the oxygen-containing functional groups (such as C=O, -OH) on the surface of the urea-formaldehyde resin provide active sites for the transmission of lithium ions, and excessive thickness will hinder the ion migration path. Therefore, the coating thickness range (50-500nm) balances the barrier performance and ion transmission efficiency, ensuring that the urea-formaldehyde resin can completely isolate the side reactions of hexachlorocyclotriphosphazene with the electrolyte / electrode and provide active sites for lithium ion transmission through surface functional groups; at the same time, its thickness optimizes the high-temperature carbonization response speed, allowing the flame retardant additive to quickly release and quench free radicals in the early stage of thermal runaway, while the carbonized residue forms a physical barrier to inhibit the spread of combustion.

[0018] Preferably, the lithium salt is selected from at least one of lithium hexafluorophosphate, lithium bis(trifluoromethanesulfonyl imide) and lithium bis(fluorosulfonyl imide), and the concentration is 1 to 2 mol / L.

[0019] The decomposition products of the lithium hexafluorophosphate (such as phosphorus pentafluoride and phosphorus oxytrifluoride) can react with the phosphorus / nitrogen radicals released by hexachlorocyclotriphosphazene to generate phosphate flame retardant intermediates (such as lithium phosphate), and enhance the flame retardant efficiency through the condensed phase carbonization mechanism. Lithium hexafluorophosphate has excellent solubility and ionization ability in ethylene carbonate / dimethyl carbonate organic mixed solvents, which can maintain the high ionic conductivity of the electrolyte and match the dispersion stability of the microencapsulated flame retardant additive. The decomposition temperature range of lithium hexafluorophosphate (about 60-80°C) cooperates with the carbonization temperature of the urea-formaldehyde resin coating layer (the initial stage of thermal runaway) to ensure that the release of the flame retardant additive is consistent with the timing of the decomposition of the lithium salt, avoiding premature or delayed reactions.

[0020] When the lithium salt concentration is lower than 0.5 mol / L, insufficient free lithium ions lead to a decrease in conductivity and intensified battery polarization; when the lithium salt concentration is higher than 3 mol / L, the excessive viscosity increases the resistance to ion migration, and the self-decomposition of lithium salts accelerates the generation of acidic by-products that corrode the electrode interface. At the same time, the high viscosity environment causes the flame retardant additives to agglomerate, causing local thermal runaway protection failure.

[0021] Preferably, the volume ratio of ethylene carbonate to dimethyl carbonate in the organic solvent is 1:(2.3-3). The organic solvent ratio (ethylene carbonate: dimethyl carbonate = 1:2.3-3) optimizes the lithium salt dissolution efficiency and ion migration rate by balancing the high dielectric constant and low viscosity characteristics, and adapts to the dispersion requirements of microencapsulated flame retardant additives. The high polarity of ethylene carbonate (EC) ensures that lithium hexafluorophosphate is fully dissociated, while the low viscosity of dimethyl carbonate (DMC) reduces the flow resistance of the electrolyte, and the two work together to maintain high ionic conductivity; this ratio also inhibits the oxidation and decomposition of the solvent at high temperature to produce gas, and enhances the dispersion stability of the microcapsules through the hydrogen bonding of the solvent molecules and the surface polar groups of the flame retardant additives (such as -OH, C=O). When the ratio is too low, the excess EC leads to increased viscosity and hinders ion transmission; if the ratio is too high, the proportion of DMC is too large, which weakens the dissolution ability of the lithium salt and aggravates the agglomeration of the flame retardant additive. This range accurately coordinates the contradiction between electrochemical performance and thermal runaway protection performance.

[0022] Preferably, the flame retardant additive accounts for 3% to 8% of the total mass of the electrolyte.

[0023] When the flame retardant additive accounts for 1% to 10%, urea-formaldehyde resin-coated hexachlorocyclotriphosphazene (HCP@UR) can release sufficient active ingredients to achieve rapid self-extinguishing by interrupting the combustion chain reaction. If it is less than 1%, the concentration of the flame retardant additive is insufficient and the fire extinguishing efficiency is significantly reduced; when it exceeds 10%, the flame retardant effect tends to be saturated, and further increasing the dosage cannot further improve the performance. On the contrary, the electrolyte viscosity increases and the risk of side reactions surges due to the high proportion of particles. At the same time, the addition of 1% to 10% of the flame retardant additive can maintain the ionic conductivity of the electrolyte, ensure the efficient migration of lithium ions, and avoid excessive microcapsules from obstructing the ion transmission path or inducing interfacial side reactions. When it is less than 1%, the flame retardant additive is not dispersed enough, resulting in local protection failure; when it exceeds 10%, the accumulation of particles exacerbates the interfacial corrosion and capacity decay, which damages the battery cycle life. This addition range is suitable for conventional stirring-ultrasonic processes to ensure that the microcapsules are evenly dispersed without the need for additional energy consumption or additives. When it is less than 1%, insufficient raw material utilization leads to low cost-effectiveness; when it exceeds 10%, higher power equipment and more raw materials are required, significantly increasing production costs.

[0024] In a second aspect, a method for preparing a lithium ion battery electrolyte comprises the following steps:

[0025] The urea-formaldehyde-coated hexachlorocyclotriphosphazene is mixed with an organic solvent, stirred under an inert atmosphere, and subjected to ultrasonic treatment to obtain a premixed solution;

[0026] The premixed solution is filtered, and then a lithium salt is added to the premixed solution to prepare a lithium ion battery electrolyte.

[0027] The urea-formaldehyde-coated hexachlorocyclotriphosphazene is first mixed with an organic solvent under an inert atmosphere to avoid side reactions caused by oxygen or moisture. The microcapsules are then broken up by ultrasonic treatment to ensure that they are evenly dispersed in the solvent, thus avoiding flame retardant failure or interface blockage caused by excessive local concentration.

[0028] Before adding lithium salt, filter the premixed solution to remove undispersed large particles or impurities to prevent them from reacting with lithium salt to generate acidic substances (such as HCl, HF) or clogging the pores of the diaphragm. This step avoids the difficulty of filtering caused by the increased viscosity of the solution after the lithium salt is dissolved, while reducing the risk of impurities introduced and improving the purity of the electrolyte.

[0029] Adding lithium salt after ultrasonic dispersion is completed can prevent lithium salt from decomposing due to mechanical stress or local high temperature during ultrasonic or stirring. At the same time, lithium salt is dissolved in the pre-dispersed organic solvent-flame retardant additive system to form a uniform solvation structure, reducing the interface side reactions caused by concentration gradient (such as uneven thickening of SEI film).

[0030] Preferably, the urea-formaldehyde coated hexachlorocyclotriphosphazene accounts for 3% to 8% of the total mass of the electrolyte;

[0031] The organic solvent is a mixed solution of ethylene carbonate and dimethyl carbonate, the volume ratio of ethylene carbonate to dimethyl carbonate is 1:(2-9), preferably, the volume ratio of ethylene carbonate to dimethyl carbonate is 1:(2.3-3);

[0032] The lithium salt is selected from at least one of lithium hexafluorophosphate, lithium bis(trifluoromethanesulfonyl)imide and lithium bis(fluorosulfonyl)imide, and has a concentration of 1 to 2 mol / L.

[0033] Preferably, the inert atmosphere is argon or nitrogen, the stirring rate is 500-1000 rpm, and the stirring time is 2-4 hours.

[0034] Preferably, the power of the ultrasonic treatment is 200-500 W, the frequency is 20-40 kHz, and the treatment time is 30-90 minutes.

[0035] Preferably, the filtration uses a 0.22 μm polytetrafluoroethylene filter membrane.

[0036] In a third aspect, a lithium-ion battery comprises a positive electrode, a negative electrode, a separator and the above-mentioned electrolyte.

[0037] In a fourth aspect, an electrical device comprises the above-mentioned lithium-ion battery.

[0038] Due to the application of the above technical solution, the present invention has the following advantages compared with the prior art:

[0039] 1. By introducing microencapsulated flame retardant additives (urea-formaldehyde resin coated hexachlorocyclotriphosphazene) into the electrolyte, the electrolyte quickly releases phosphorus / nitrogen free radicals when thermal runaway occurs, interrupting the combustion chain reaction and achieving fire extinguishing capability in seconds. The urea-formaldehyde resin coating isolates the flame retardant additives from the electrolyte under normal conditions to avoid side reactions, and only carbonizes at high temperatures to form a physical barrier, inhibiting oxygen diffusion and heat transfer.

[0040] 2. The high dielectric constant and low viscosity characteristics are balanced by the ratio of organic solvents ethylene carbonate and dimethyl carbonate, and the efficient dissociation ability of lithium salt is coordinated to ensure ion conductivity and migration rate. The content of flame retardant additives accurately matches the solvation structure to avoid viscosity surge or interface deterioration caused by excessive addition, so that the battery capacity retention rate is greater than 97% after 700 cycles, breaking through the restrictions of traditional flame retardant additives on electrochemical performance.

[0041] 3. The step-by-step preparation method (first mix the flame retardant additive and the organic solvent → then add the lithium salt) simplifies the multi-component synergistic reaction process. Ultrasonic treatment is used to break up the agglomeration while protecting the integrity of the coating layer. The filtration step removes the undispersed particles to avoid the failure of thermal runaway protection caused by local concentration gradients.

[0042] 4. Microencapsulated flame retardant additives form hydrogen bonds with solvent molecules through surface polar groups (-OH, C=O), reduce the adsorption of electrode active substances, and inhibit abnormal thickening of SEI film. Lithium salt decomposition products (such as Li3PO4) are embedded in the negative electrode interface film, improving its mechanical strength and ion transmission efficiency, and reducing the risk of interface degradation during the cycle.

[0043] In order to make the above and other purposes, features and advantages of the present invention more obvious and easy to understand, preferred embodiments are specifically cited below and described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0045] Figure 1 It is the SEM morphology and particle size distribution diagram of urea-formaldehyde resin-coated hexachlorocyclotriphosphazene in the embodiment of the present invention.

[0046] Figure 2 It is a TEM core-shell structure diagram of hexachlorocyclotriphosphazene coated with urea-formaldehyde resin in an embodiment of the present invention.

[0047] Figure 3 It is a Fourier transform infrared spectrum diagram in the embodiment of the present invention. DETAILED DESCRIPTION

[0048] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0049] Embodiment 1:

[0050] This embodiment discloses a lithium-ion battery electrolyte, comprising:

[0051] Lithium salt, concentration is 0.5~3mol / L;

[0052] An organic solvent comprises ethylene carbonate and dimethyl carbonate, wherein the volume ratio of ethylene carbonate to dimethyl carbonate is 1:(2-9);

[0053] The flame retardant additive is hexachlorocyclotriphosphazene coated with urea-formaldehyde resin, accounting for 1% to 10% of the total mass of the electrolyte.

[0054] This embodiment also discloses a method for preparing the above-mentioned lithium ion battery electrolyte, comprising the following steps:

[0055] 1. Synthesis of hexachlorocyclotriphosphazene coated with urea-formaldehyde resin

[0056] (1) 3 g of urea, 10.4 mL of formaldehyde aqueous solution (8-14 wt%) and 20 mL of deionized water were added to a 500 mL round-bottom flask and stirred until completely dissolved to obtain a polymer precursor solution.

[0057] (2) 0.5 mol / L sodium hydroxide solution was added dropwise to adjust the pH of the polymer precursor solution to 8-9, and the solution was heated in a 75° C. oil bath for 1 hour, followed by the addition of 60 mL of deionized water and continued mixing.

[0058] (3) After the system is cooled to 25° C., 2.5 g of hexachlorocyclotriphosphazene is added and stirred to disperse. After 10 minutes, a mixed emulsifier of octylphenol polyoxyethylene ether-10 and fatty alcohol polyoxyethylene ether in a mass ratio of 1:1 (150 μL each) is added.

[0059] (4) The temperature was raised to 50°C for 10 minutes, 10 mL of 0.5 g / mL ammonium chloride aqueous solution was added, and heating was continued for 1 hour.

[0060] (5) Adjust the pH to 3-4 with citric acid, heat to 70°C for reaction for 2 hours, let stand at room temperature for 24 hours, filter, wash three times with deionized water and ethanol in sequence, and dry at 60°C for 2 hours to obtain the product urea-formaldehyde resin-coated hexachlorocyclotriphosphazene.

[0061] 2. Preparation of electrolyte

[0062] (1) The synthesized urea-formaldehyde resin-coated hexachlorocyclotriphosphazene was added into a mixed solvent of ethylene carbonate / dimethyl carbonate (volume ratio 3:7) at 5 wt %, and stirred at 600 rpm for 3 hours under argon protection to ensure uniform dispersion.

[0063] (2) Use a 300W, 30kHz ultrasonic probe to treat for 50 minutes to break up the particle agglomeration, let it stand for 12 hours, and continue ultrasonic treatment if there is precipitation. After the precipitation is eliminated, filter with a 0.22μm polytetrafluoroethylene filter to remove the undispersed particles.

[0064] (3) Add lithium hexafluorophosphate to the filtrate to a final concentration of 1 mol / L and stir until completely dissolved to prepare a lithium ion battery electrolyte.

[0065] See also Figure 1The figure shows the SEM morphology and particle size distribution of urea-formaldehyde resin-coated hexachlorocyclotriphosphazene (SEM equipment: QUANTA250FEG). From the figure, it can be seen that the microscopic morphology of urea-formaldehyde resin-coated hexachlorocyclotriphosphazene is spherical or quasi-spherical particles, which are uniform microspheres. The functional groups on the resin serve as active sites, which can improve the kinetic performance of lithium ions, and the polar groups on it can enhance the wetting efficiency.

[0066] See also Figure 2 The TEM core-shell structure of hexachlorocyclotriphosphazene coated with urea-formaldehyde resin is shown in FIG. 1 (TEM equipment: Tecnai F30). The core-shell structure of hexachlorocyclotriphosphazene coated with urea-formaldehyde resin can be seen from the figure, and the coating layer formed by the urea-formaldehyde resin is clearly shown.

[0067] Comparative Example 1:

[0068] (1) Hexachlorocyclotriphosphazene was added into a mixed solvent of ethylene carbonate / dimethyl carbonate (volume ratio 3:7) at 5 wt %, and stirred at 600 rpm for 3 hours under argon protection to ensure uniform dispersion.

[0069] (2) Use a 300W, 30kHz ultrasonic probe to treat for 50 minutes to break up the particle agglomeration, let it stand for 12 hours, and continue ultrasonic treatment if there is precipitation. After the precipitation is eliminated, filter with a 0.22μm polytetrafluoroethylene filter to remove the undispersed particles.

[0070] (3) Add lithium hexafluorophosphate to the filtrate to a final concentration of 1 mol / L and stir until completely dissolved to prepare a lithium ion battery electrolyte.

[0071] The electrolytes in Example 1 and Comparative Example 1 were used to prepare lithium-ion batteries according to conventional processes. The battery compositions are as follows:

[0072] 1. Electrode composition and coating parameters

[0073] Positive electrode (double-sided coating):

[0074] Material ratio (mass ratio): lithium iron phosphate (LFP): conductive carbon black (SuperP): polyvinylidene fluoride (PVDF5130): carbon nanotube (CNT) = 96:1.8:1.7:0.5. Coating amount: single-sided mass loading 16.0mg / cm 2 (Total double-sided loading 32.0 mg / cm 2 ).

[0075] Negative electrode (double-sided coating):

[0076] Material ratio (mass ratio): graphite: binder (LA136D): conductive carbon black (SuperP): carbon nanotube (CNT): sodium carboxymethyl cellulose (CMC) = 96:2.3:0.9:0.4:0.4. Coating amount: single-sided mass loading 10.1 mg / cm 2 (Total double-sided loading 20.2 mg / cm 2 ).

[0077] N / P Ratio:

[0078] The capacity ratio of negative electrode to positive electrode is ≈1.13.

[0079] 2. Electrolyte system (using the electrolytes prepared in Example 1 and Comparative Example 1 respectively)

[0080] 3. Separator and current collector

[0081] Diaphragm: Polyethylene (PE) diaphragm, total thickness 9+3μm.

[0082] Current collector: The negative electrode uses 6.0μm thick copper foil.

[0083] The lithium ion batteries prepared in Example 1 and Comparative Example 1 were tested as follows:

[0084] 1. Fourier transform infrared spectroscopy (FTIR) analysis

[0085] The NIKOLIS 50 infrared spectrometer (test range 4000-600cm -1 ) The chemical structures of Example 1 (urea-formaldehyde resin-coated hexachlorocyclotriphosphazene) and Comparative Example 1 (uncoated hexachlorocyclotriphosphazene) were characterized.

[0086] from Figure 3 It can be seen that the characteristic peak of comparative example 1 (uncoated) is 1218 cm -1 There is a stretching vibration peak of PN bond at 870cm -1 There is a bending vibration peak of PN bond at 522cm -1 and 610cm -1 The above peak position completely matches the molecular structure of hexachlorocyclotriphosphazene, indicating that the flame retardant additive in Comparative Example 1 has not been chemically modified.

[0087] Example 1 (after coating) characteristic peak: 1632cm -1 There is a stretching vibration peak of C=O bond in urea-formaldehyde resin at 1018cm -1 There is a vibration peak of CO bond at 522 / 610cm -1) is significantly weakened, indicating that the urea-formaldehyde resin completely covers the surface of the flame retardant additive, blocking it from contact with the outside world. Therefore, the urea-formaldehyde coating completely covers the surface of hexachlorocyclotriphosphazene, preventing it from directly reacting with the electrolyte (such as LiPF6) and the graphite negative electrode, avoiding the generation of by-products (such as HCl) and abnormal thickening of the SEI film. The oxygen-containing functional groups (C=O, CO) in the urea-formaldehyde resin provide active sites for lithium ion transport, optimize the electrode reaction kinetics, and thus improve the battery cycle stability.

[0088] 2. Limiting oxygen index (LOI) test

[0089] Using FTT0077 limiting oxygen index tester and referring to ASTMD2863 standard, the diaphragm was immersed in the electrolyte of Example 1 (containing hexachlorocyclotriphosphazene coated with urea-formaldehyde resin) and Comparative Example 1 (uncoated) for 2 hours, the samples were taken out, drained and cut into standard size samples, and fixed vertically in the test cavity. By adjusting the concentration of oxygen-nitrogen mixed gas and igniting it, the minimum oxygen volume percentage (LOI value) required to maintain combustion was measured to evaluate the difference in flame retardant efficiency.

[0090] 3. Combustion test

[0091] The actual fire extinguishing ability of the flame retardant additive was verified by simulating a thermal runaway scenario. The diaphragm was immersed in an electrolyte containing Example 1 (urea-formaldehyde coated hexachlorocyclotriphosphazene) or Comparative Example 1 (uncoated) for 2 hours, drained and fixed vertically, and ignited from the top using a propane igniter (flame height 10-20 mm) and the combustion process was recorded.

[0092] The results showed that Example 1 caught fire briefly 2 seconds after ignition and the open flame was completely extinguished within 4 seconds, while Comparative Example 1 continued to burn; its mechanism is that the urea-formaldehyde coating layer quickly releases Cl·, P·, and N· free radicals at high temperature, efficiently quenching the H· active intermediates in the combustion chain reaction, thereby inhibiting the spread of flame and achieving self-extinguishing, verifying the rapid response and efficient fire extinguishing characteristics of the coated flame retardant additive.

[0093] 4. Normal temperature cycle performance test

[0094] The LAND battery test system was used to test the batteries equipped with the electrolytes of Example 1 (containing urea-formaldehyde coated hexachlorocyclotriphosphazene) and Comparative Example 1 (uncoated). Under normal temperature (25±2°C), the voltage range was set to 2.0–4.5V, and 700 charge and discharge cycles were performed continuously at a constant current charge and discharge rate of 0.5C. The charge and discharge capacity and voltage curve changes of each cycle were monitored and recorded in real time to evaluate the long-term stability of the electrolyte system.

[0095] The test results are shown in the following table:

[0096] Test items Example 1 Comparative Example 1 Limiting oxygen index 33％ 25％ Burn 0s No open flame No open flame Burn 2s Fire Fire Burn 4s No open flame Fire Normal temperature cycle test 97% @ 700 laps 86% @ 700 laps

[0097] From the limiting oxygen index (LOI) test results, it can be seen that the limiting oxygen index of Example 1 (33%) is higher than that of Comparative Example 1 (25%), which shows that the material of Example 1 is relatively less flammable and the coating of urea-formaldehyde resin improves its flame retardant properties.

[0098] From the combustion test results, it can be seen that when burning for 0s, no open flame was seen in both Example 1 and Comparative Example 1, indicating that neither of them caught fire immediately in the initial state. When burning for 2s, both of them caught fire, indicating that the combustion characteristics of the two were similar at this time point. When burning for 4s, no open flame was seen in Example 1 while Comparative Example 1 caught fire. Example 1 has good flame retardancy. This is because the flame temperature is high, the shell of hexachlorocyclotriphosphazene @ urea formaldehyde will unfold, and hexachlorocyclotriphosphazene will be released immediately, generating Cl·, P·, N· free radicals, which combine with H· to prevent combustion. Since the amount of H is reduced or even completely eliminated, the combustion intensity is suppressed until it is extinguished.

[0099] From the results of the normal temperature cycle test, it can be seen that the retention rate of Example 1 after 700 normal temperature cycles (97%) is higher than that of Comparative Example 1 (86%). This shows that after multiple normal temperature cycles, the material of Example 1 has better performance stability and can better maintain its original characteristics. The protection of urea-formaldehyde resin avoids direct contact with the electrolyte and reduces side reactions. Hexachlorocyclotriphosphazene@urea-formaldehyde has excellent cycle performance.

[0100] The present invention uses specific embodiments to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only used to help understand the method of the present invention and its core idea. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as a limitation on the present invention.

Claims

1. A lithium ion battery electrolyte, characterized in that: include: Lithium salt, concentration is 0.5~3mol / L; An organic solvent comprises ethylene carbonate and dimethyl carbonate, wherein the volume ratio of ethylene carbonate to dimethyl carbonate is 1:(2-9); The flame retardant additive is hexachlorocyclotriphosphazene coated with urea-formaldehyde resin, accounting for 1% to 10% of the total mass of the electrolyte.

2. The electrolyte according to claim 1, characterized in that: The particle size of the flame retardant additive is 0.1-10 μm.

3. The electrolyte according to claim 1, characterized in that: The thickness of the coating layer formed by urea-formaldehyde resin in the flame retardant additive is 50-500 nm.

4. The electrolyte according to claim 1, characterized in that: The lithium salt is selected from at least one of lithium hexafluorophosphate, lithium bis(trifluoromethanesulfonyl imide) and lithium bis(fluorosulfonyl imide), and the concentration is 1-2 mol / L.

5. The electrolyte according to claim 1, characterized in that: The volume ratio of ethylene carbonate to dimethyl carbonate in the organic solvent is 1:(2.3-3).

6. The electrolyte according to claim 1, characterized in that: The flame retardant additive accounts for 3% to 8% of the total mass of the electrolyte.

7. A method for preparing a lithium ion battery electrolyte, characterized in that: The following steps are involved: The urea-formaldehyde-coated hexachlorocyclotriphosphazene is mixed with an organic solvent, stirred under an inert atmosphere, and subjected to ultrasonic treatment to obtain a premixed solution; The premixed solution is filtered, and then a lithium salt is added to the premixed solution to prepare a lithium ion battery electrolyte.

8. The method for preparing a lithium ion battery electrolyte according to claim 7, characterized in that: The urea-formaldehyde coated hexachlorocyclotriphosphazene accounts for 3% to 8% of the total mass of the electrolyte; The organic solvent is a mixed solution of ethylene carbonate and dimethyl carbonate, the volume ratio of ethylene carbonate to dimethyl carbonate is 1:(2-9), preferably, the volume ratio of ethylene carbonate to dimethyl carbonate is 1:(2.3-3); The lithium salt is selected from at least one of lithium hexafluorophosphate, lithium bis(trifluoromethanesulfonyl imide) and lithium bis(fluorosulfonyl imide), and the concentration is 1-2 mol / L.

9. The method for preparing a lithium ion battery electrolyte according to claim 7, characterized in that: The inert atmosphere is argon or nitrogen, the stirring rate is 500-1000 rpm, and the stirring time is 2-4 hours; The power of the ultrasonic treatment is 200-500W, the frequency is 20-40kHz, and the treatment time is 30-90 minutes.

10. The method for preparing a lithium ion battery electrolyte according to claim 7, characterized in that: The filtration adopts a 0.22 μm polytetrafluoroethylene filter membrane.

11. A lithium ion battery, characterized in that: The invention comprises a positive electrode, a negative electrode, a separator and an electrolyte as claimed in any one of claims 1 to 6.

12. An electrical device, characterized in that: Comprising the lithium ion battery as claimed in claim 12.

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