A lithium ion electrolyte and a preparation method thereof

Abstract

The invention discloses a lithium ion electrolyte and a preparation method thereof, belonging to the technical field of electrolyte preparation; the electrolyte comprises a non-aqueous organic solvent, a lithium salt, an additive and a modified phosphorus-containing flame-retardant vesicle; the additive comprises 2,3-pyridinedicarboxylic anhydride, a modified layered inorganic material, a modified aramid fiber and diethylene glycol diethyl ether; the modified layered inorganic material is obtained by first modifying the layered inorganic material with a lithium material and then with a high molecular polymer; the modified aramid fiber is obtained by first treating the aramid fiber with plasma and then grafting polypropylene ethyl ester; the modified phosphorus-containing flame-retardant vesicle is obtained by first wrapping a flame retardant with poly-N-isopropylacrylamide and then modifying it with an acrylate monomer; the components are mixed to obtain the electrolyte, and a battery prepared from the electrolyte has relatively excellent electrical cycle performance at high temperatures of 45°C and 60°C; and the electrolyte also has relatively strong flame retardancy, and can even reach a level of non-combustion, thereby achieving true flame retardancy.

Description

Technical Field

[0001] The invention belongs to the technical field of electrolyte preparation, and specifically relates to a lithium ion electrolyte and a preparation method thereof. Background Art

[0002] Lithium-ion batteries have the advantages of high specific energy, long cycle stability, high operating voltage, and low environmental pollution. They are widely used in new energy vehicles, energy storage power stations, portable devices and other fields.

[0003] The electrolyte of lithium-ion batteries is generally composed of lithium hexafluorophosphate and carbonates. Lithium hexafluorophosphate itself has low heat resistance and is easily decomposed at high temperatures. Carbonates include ethylene carbonate (EC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC). Their flash points are relatively low, especially linear carbonates, which have a flash point below 25°C and are generally flammable.

[0004] Therefore, when lithium-ion batteries containing LiPF6 electrolyte are subjected to improper operation such as overcharging, internal short circuit (metal lithium negative electrode and electrolyte react with each other to form lithium dendrites), extrusion, high temperature, etc., there will be serious safety problems. When the heating rate of the battery is greater than the heat dissipation rate, thermal runaway may occur, leading to fire or even explosion.

[0005] The invention patent with application number 201910732764.2 discloses a lithium-ion battery electrolyte and a preparation method thereof. On the basis of commercial electrolyte, lithium-modified inorganic layered materials and high molecular polymers with ion conductivity are introduced to achieve a strong lithium negative electrode and inhibit the growth of lithium metal dendrites, thereby achieving the purpose of improving thermal stability and flame retardancy.

[0006] The above-mentioned electrolyte can play a more significant role at room temperature, including inhibiting the growth of lithium metal dendrites and having excellent electrical cycle performance. However, once the temperature is higher than 50°C, the above-mentioned effects will be significantly reduced. At the same time, once thermal runaway occurs, even if it has a certain flame retardancy, it is still difficult to achieve the true flame retardant purpose due to its poor flame retardant effect. Summary of the invention

[0007] The purpose of the present invention is to provide a lithium ion electrolyte and a preparation method thereof for the first time; the lithium ion electrolyte can maintain relatively stable high temperature and long cycle performance at high temperature, and at the same time has an extremely excellent flame retardant effect (self-extinguishing time ≤ 1s), and once the battery has thermal runaway, it can be flame retardant or fire extinguished in time.

[0008] The purpose of the present invention is achieved through the following technical solutions:

[0009] A lithium ion electrolyte comprising a non-aqueous organic solvent, a lithium salt, an additive and a modified phosphorus-containing flame-retardant vesicle;

[0010] The additives include 2,3-pyridinedicarboxylic anhydride, modified layered inorganic material, modified aramid fiber and diethylene glycol diethyl ether;

[0011] The modified layered inorganic material is obtained by first modifying the layered inorganic material with a lithium material and then with a high molecular polymer;

[0012] The modified aramid fiber is obtained by first subjecting the aramid fiber to plasma treatment and then grafting polypropylene ethyl ester;

[0013] The modified phosphorus-containing flame-retardant vesicles are obtained by firstly encapsulating the phosphorus-containing flame retardant with poly-N-isopropylacrylamide and then modifying it with an acrylate monomer.

[0014] As one possible embodiment of the present application, the non-aqueous organic solvent consists of vinylene carbonate and trifluoropropylene carbonate; and the lithium salt consists of lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide.

[0015] As one of the embodiments of the present application, the lithium ion electrolyte includes the following components in parts by weight:

[0016] 45-65 parts of vinylene carbonate, 10-20 parts of trifluoropropylene carbonate, 10-25 parts of lithium hexafluorophosphate, 1-5 parts of lithium bis(fluorosulfonyl)imide, 3-10 parts of modified phosphorus-containing flame retardant vesicles, 0.1-5 parts of 2,3-pyridinedicarboxylic anhydride, 0.1-5 parts of modified layered inorganic material, 0.1-5 parts of modified aramid fiber and 0.1-5 parts of diethylene glycol diethyl ether.

[0017] As one of the embodiments of the present application, the lithium ion electrolyte includes the following components in parts by weight:

[0018] 48-60 parts of vinylene carbonate, 12-18 parts of trifluoropropylene carbonate, 15-24 parts of lithium hexafluorophosphate, 1.2-3.5 parts of lithium bis(fluorosulfonyl)imide, 3.5-9 parts of modified phosphorus-containing flame retardant vesicles, 0.5-2 parts of 2,3-pyridinedicarboxylic anhydride, 0.5-4 parts of modified layered inorganic material, 1-3 parts of modified aramid fiber and 0.5-2 parts of diethylene glycol diethyl ether.

[0019] As one of the embodiments of the present application, the lithium ion electrolyte includes the following components in parts by weight:

[0020] 57 parts of vinylene carbonate, 16 parts of trifluoropropylene carbonate, 23 parts of lithium hexafluorophosphate, 1.8 parts of lithium bis(fluorosulfonyl)imide, 7 parts of modified phosphorus-containing flame retardant vesicles, 1.5 parts of 2,3-pyridinedicarboxylic anhydride, 2 parts of modified layered inorganic material, 1.8 parts of modified aramid fiber and 1.2 parts of diethylene glycol diethyl ether.

[0021] As one of the embodiments of the present application, 0.2-1.5 parts of a fluorine-containing surfactant is also included.

[0022] As one possible embodiment of the present application, the melting temperature of the modified phosphorus-containing flame-retardant vesicles is 100-120°C.

[0023] As one of the possible embodiments of the present application, the acrylate monomers include methyl acrylate (MA), ethyl acrylate (EA), butyl acrylate (BA) and dodecyl acrylate (DA).

[0024] As one of the possible embodiments of the present application, the flame retardant is selected from trimethyl phosphate (TMP), triethyl phosphate (TEP), tributyl phosphate (TBP), triphenyl phosphate (TPP), and toluene diphenyl phosphate (CDP).

[0025] In addition, to achieve the above-mentioned purpose, the present application also provides a method for preparing a lithium ion electrolyte, comprising the following steps:

[0026] S1: uniformly mixing vinylene carbonate and trifluoropropylene carbonate to obtain a non-aqueous organic solvent;

[0027] S2 adds lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide and fluorine-containing surfactant to a non-aqueous organic solvent under an inert gas environment at room temperature and stirs until completely dissolved; then adds 2,3-pyridinedicarboxylic anhydride, modified layered inorganic material, modified aramid fiber, diethylene glycol diethyl ether and modified flame retardant capsules, stirs thoroughly and allows to stand to obtain a lithium ion electrolyte.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] 1. The electrolyte provided in the present application can effectively improve the high-temperature and long-cycle performance of the battery. The battery prepared by the electrolyte of the present application has relatively excellent electrical cycle performance at high temperatures of 45°C and 60°C, and the battery capacity retention rate after 600 cycles at 45°C is not less than 91.5%; the battery capacity retention rate after 100 cycles at 60°C is not less than 96.1%, the battery capacity retention rate after 200 cycles at 60°C is not less than 90.7%, and the battery capacity retention rate after 300 cycles at 60°C is not less than 80.5%.

[0030] 2. The electrolyte provided in this application has good flame retardant properties, and its self-extinguishing time is ≤1s, which can reach the non-combustible level, or even the level of almost non-combustible, thereby achieving the purpose of true flame retardancy and fire extinguishing. DETAILED DESCRIPTION

[0031] The present application provides a lithium ion electrolyte, comprising the following components:

[0032] A lithium ion electrolyte comprises a non-aqueous organic solvent, a lithium salt, an additive and a modified phosphorus-containing flame retardant vesicle; the flame retardant is selected from trimethyl phosphate (TMP), triethyl phosphate (TEP), tributyl phosphate (TBP) and triphenyl phosphate (TPP);

[0033] The additives include 2,3-pyridinedicarboxylic anhydride, modified layered inorganic material, modified aramid fiber and diethylene glycol diethyl ether;

[0034] The modified layered inorganic material is obtained by first modifying the layered inorganic material with a lithium material and then with a high molecular polymer (such as polyethylene oxide or polyvinyl alcohol);

[0035] The modified aramid fiber is obtained by first subjecting the aramid fiber to plasma treatment and then grafting polypropylene ethyl ester;

[0036] The modified phosphorus-containing flame-retardant vesicles are obtained by first encapsulating the phosphorus-containing flame retardant with poly-N-isopropylacrylamide and then modifying it with an acrylate monomer.

[0037] Wherein, the non-aqueous organic solvent is composed of vinylene carbonate and trifluoropropylene carbonate; and the lithium salt is composed of lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide.

[0038] Among them, the preparation method of the modified layered inorganic material refers to the preparation method of Example 1 in the patent application number 201910732764.2;

[0039] The preparation method of the modified phosphorus-containing flame retardant vesicles is as follows:

[0040] S1: A dihedral core-shell structure microcapsule flame retardant (i.e., phosphorus-containing flame retardant vesicles) is prepared according to the preparation method of Example 2 in the patent application number 202010329878.5;

[0041] S2: React phosphorus-containing flame-retardant vesicles, 2% acrylate monomer by weight of vesicles, 0.5% sodium dodecyl sulfate by weight of vesicles, 1% cross-linking agent (bisacrylamide) by weight of acrylate monomers, and 1% persulfate initiator by weight of vesicles at a temperature of 45-50°C for 24-28h, dry, extract with anhydrous ethanol to remove small molecules such as sodium dodecyl sulfate, and then dry under reduced pressure at 50°C to obtain modified phosphorus-containing flame-retardant vesicles, the melting temperature of which is 100-120°C.

[0042] The preparation method of modified aramid fiber is as follows: nano-aramid fiber is placed between two electrodes of a plasma generator, with an air pressure of 18Pa and an output power of 150W, and treated at room temperature for 20 minutes; then the aramid fiber pretreated by plasma is quickly taken out of the generator, immersed in ethyl acrylate (the mass wall ratio of the pretreated fiber and ethyl acrylate is 1:2.2), maintained at 52°C for 1 hour under N2 protection, and then washed and dried to obtain the modified aramid fiber.

[0043] In the above-mentioned non-aqueous organic solvent, trifluoropropylene carbonate is added on the basis of conventional vinylene carbonate, which can improve the high temperature resistance of vinylene carbonate, participate in the formation of positive and negative electrode passivation films, and effectively improve the cycle performance of the battery.

[0044] In lithium salt, lithium bis(fluorosulfonyl)imide is added to conventional lithium hexafluorophosphate, which has high thermal stability and strong electrical conductivity, and can effectively improve the thermal stability and electrical conductivity of the electrolyte, thereby effectively improving the high-temperature long-cycle performance of the battery. At the same time, lithium bis(fluorosulfonyl)imide can also reduce the impedance of the electrolyte / electrode interface, and form a stable SEI film on the negative electrode surface, while helping to form a stable CEI film on the positive electrode surface, further improving the long-cycle performance of the battery.

[0045] Among the additives, the added modified layered inorganic material has a strong lithium-philic property and can restrain the spread of lithium dendrites. The polymer, due to its molecular structure and spatial structure, can provide a sufficiently high electron-donating group density and has a flexible polyether segment, so that the modified layered inorganic material can be evenly suspended in the electrolyte, preventing it from stacking and agglomerating, which is conducive to the uniform formation of the SEI film on the surface of the lithium negative electrode.

[0046] However, the electrolyte prepared by the above components can play a more significant role at room temperature or below 40°C, including inhibiting the growth of lithium metal dendrites and having excellent electrical cycle performance. However, once the temperature is higher than 45°C, the above effects will be significantly reduced; at the same time, once thermal runaway occurs, even if it has a certain flame retardancy, it is still difficult to achieve the true flame retardant purpose due to poor flame retardant effect.

[0047] Based on this, the present invention adds modified phosphorus-containing flame-retardant vesicles and diethylene glycol diethyl ether on the basis of the above components.

[0048] Phosphorus-containing flame-retardant vesicles can protect the phosphorus-containing flame retardant in the inner core, avoiding the decrease of electrolyte conductivity caused by directly adding the phosphorus-containing flame retardant into the electrolyte system, thereby affecting the electrochemical performance of the battery; however, after the phosphorus-containing flame-retardant vesicles are added to the electrolyte system, the compatibility with the electrolyte is not good, resulting in uneven dispersion of the electrolyte system and a significant increase in the internal resistance of the lithium-ion battery, which seriously affects the electrochemical cycle performance of the battery. Based on this, the present invention obtains phosphorus-containing flame-retardant vesicles modified with acrylate monomers, and the acrylate monomers include methyl acrylate (MA), ethyl acrylate (EA) and butyl acrylate (BA). After modification, the compatibility of the modified phosphorus-containing flame-retardant vesicles with other components in the electrolyte can be effectively improved. At the same time, after modification, the melting temperature of the modified phosphorus-containing flame-retardant vesicles can be made to be 100-120°C, and the phosphorus-containing flame-retardant vesicles can be melted after the battery thermal runaway occurs and reaches 100-120°C, and the internal phosphorus-containing flame retardant can be released, thereby achieving the flame retardant purpose. However, in actual implementation, it was found that in the electrolyte system of the present invention, if complete fire extinguishing is to be achieved, the electrolyte must be made to reach a non-combustible level (i.e., self-extinguishing time ≤ 5s). At this time, the amount of phosphorus-containing flame retardant added is at least greater than 16.8wt%. This amount added will result in a larger mass of modified phosphorus-containing flame retardant vesicles added to the electrolyte, thereby seriously affecting the conductivity and electrical properties of the electrolyte. Based on this, the present invention adds diethylene glycol diethyl ether to the electrolyte system, which has a flash point of up to 74°C and good solubility in lithium salts. After being added to the electrolyte system, the problem of low organic solvent flash point can be effectively improved. At the same time, diethylene glycol diethyl ether can synergize with the phosphorus-containing flame retardant to achieve the purpose of complete fire extinguishing on the basis of reducing the dosage of the phosphorus-containing flame retardant.

[0049] In addition, although the added modified phosphorus-containing flame-retardant vesicles are evenly dispersed in the electrolyte, the conductivity of the electrolyte is reduced due to the large particle size of the modified phosphorus-containing flame-retardant vesicles and the large amount added. At the same time, the modified phosphorus-containing flame-retardant vesicles themselves are more sensitive to temperature, which has a certain impact on the high-temperature stability of the electrolyte and affects the high-temperature electrical cycle performance.

[0050] Based on this, the present invention adds an appropriate amount of nano-modified aramid fiber, which can not only significantly improve the high-temperature stability of the electrolyte, but also improve the ionic conductivity to improve the problem of reduced conductivity caused by the addition of modified phosphorus-containing flame-retardant vesicles.

[0051] Specifically, because the aramid fiber has a smooth surface and few active groups, it is not ideally compatible with the electrolyte components. Therefore, the present invention selects acrylate monomers as grafting monomers to modify it. Acrylate monomers can be effectively compatible with organic solvents, thereby allowing good physical interactions between the fiber surface and the electrolyte components, and the molecular chains can penetrate each other to a certain extent, thereby improving the interface bonding force. In addition, compared with aramid fibers, the modified aramid fiber can effectively reduce the battery impedance. Furthermore, the carbonyl group in the acrylate polymer in the modified aramid fiber can absorb and retain the electrolyte, making the lithium ion transmission channel unobstructed, reducing the battery impedance, inhibiting the battery gas production in a high temperature environment, and effectively improving the high temperature cycle performance of the battery.

[0052] Furthermore, since the electrolyte requires a high degree of uniform dispersion of components, it is one of the important factors that determine the electrochemical cycle performance of the battery. Although the synergistic effect of the above-mentioned components can improve the compatibility of the electrolyte system to a certain extent, the long-term circulation of the electrolyte and the long-term exposure to high temperature (greater than 45°C) will cause the electrolyte structure, SEI membrane and CEI membrane to be damaged, thereby seriously affecting the stability of the electrolyte components and the high-temperature cycle performance.

[0053] Based on this, on the basis of the above, the present invention also adds fluorinated surfactants and 2,3-pyridine dicarboxylic anhydride, and the fluorinated surfactant can effectively improve the dispersion uniformity and uniform stability of the electrolyte. 2,3-pyridine dicarboxylic anhydride can broaden the redox window of the electrolyte, and form a passivation film on the surface of the positive and negative electrodes before the solvent, effectively improve the high temperature stability of the SEI film and the CEI film, effectively inhibit the decomposition of the electrode liquid and the interface side reactions, etc., thereby improving the cycle performance of the battery under high temperature and high pressure, etc., and effectively inhibiting the high temperature thickness expansion rate.

[0054] Through the synergistic effect of the above components, the electric cycle performance of the battery can be significantly improved, and at the same time, the electrolyte can achieve the purpose of complete fire extinguishing.

[0055] In order to effectively improve the cycle performance and flame retardant performance of the battery at high temperature, as one of the embodiments of the present application, the weight percentage of each component in the lithium ion electrolyte is further limited, that is, the lithium ion electrolyte includes the following components in weight percentage:

[0056] 45-65 parts of vinylene carbonate, 10-20 parts of trifluoropropylene carbonate, 10-25 parts of lithium hexafluorophosphate, 1-5 parts of lithium bis(fluorosulfonyl)imide, 3-10 parts of modified phosphorus-containing flame retardant vesicles, 0.1-5 parts of 2,3-pyridinedicarboxylic anhydride, 0.1-5 parts of modified layered inorganic material, 0.1-5 parts of modified aramid fiber and 0.1-5 parts of diethylene glycol diethyl ether.

[0057] In order to effectively improve the cycle performance and flame retardant performance of the battery at high temperature, as one of the embodiments of the present application, the weight percentage of each component in the lithium ion electrolyte is further limited, that is, the lithium ion electrolyte includes the following components in weight percentage:

[0058] 48-60 parts of vinylene carbonate, 12-18 parts of trifluoropropylene carbonate, 15-24 parts of lithium hexafluorophosphate, 1.2-3.5 parts of lithium bis(fluorosulfonyl)imide, 3.5-9 parts of modified phosphorus-containing flame retardant vesicles, 0.5-2 parts of 2,3-pyridinedicarboxylic anhydride, 0.5-4 parts of modified layered inorganic material, 1-3 parts of modified aramid fiber and 0.5-2 parts of diethylene glycol diethyl ether.

[0059] In order to effectively improve the cycle performance and flame retardant performance of the battery at high temperature, as one of the embodiments of the present application, the weight percentage of each component in the lithium ion electrolyte is further limited, that is, the lithium ion electrolyte includes the following components in weight percentage:

[0060] 57 parts of vinylene carbonate, 16 parts of trifluoropropylene carbonate, 23 parts of lithium hexafluorophosphate, 1.8 parts of lithium bis(fluorosulfonyl)imide, 7 parts of modified phosphorus-containing flame retardant vesicles, 1.5 parts of 2,3-pyridinedicarboxylic anhydride, 2 parts of modified layered inorganic material, 1.8 parts of modified aramid fiber and 1.2 parts of diethylene glycol diethyl ether.

[0061] In order to effectively improve the cycle performance of the battery at high temperature, as one of the possible embodiments of the present application, the weight percentage of each component in the lithium ion electrolyte is further limited, that is, the electrolyte also includes 0.2-1.5 parts of fluorine-containing surfactant.

[0062] In addition, to achieve the above-mentioned purpose, the present application also provides a method for preparing a lithium ion electrolyte, comprising the following steps:

[0063] S1: uniformly mixing vinylene carbonate and trifluoropropylene carbonate to obtain a non-aqueous organic solvent;

[0064] S2 adds lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide and fluorine-containing surfactant to a non-aqueous organic solvent under an inert gas environment at room temperature and stirs until the composite lithium salt is completely dissolved; then adds 2,3-pyridinedicarboxylic anhydride, modified layered inorganic material, modified aramid fiber, diethylene glycol diethyl ether and modified flame retardant capsules, stirs thoroughly and allows to stand to obtain a lithium ion electrolyte.

[0065] Next, the preparation method of the lithium ion electrolyte in the present application is described in detail through examples.

[0066] Example 1

[0067] S1: 65 parts of vinylene carbonate and 13 parts of trifluoropropylene carbonate are mixed uniformly to obtain a non-aqueous organic solvent;

[0068] S2: In an inert gas environment at room temperature, 17 parts of lithium hexafluorophosphate, 1.2 parts of lithium bis(fluorosulfonyl)imide and 1 part of fluorinated surfactant are added to a non-aqueous organic solvent and stirred until the composite lithium salt is completely dissolved; then 1.5 parts of 2,3-pyridinedicarboxylic anhydride, 1.8 parts of modified layered inorganic material, 2 parts of modified aramid fiber, 1 part of diethylene glycol diethyl ether and 8 parts of modified flame retardant capsules are added, stirred sufficiently, and allowed to stand to obtain a lithium ion electrolyte. The water content and acidity of the electrolyte meet the GB / T 19282-2014 standard.

[0069] Example 2

[0070] S1: 60 parts of vinylene carbonate and 15 parts of trifluoropropylene carbonate are mixed uniformly to obtain a non-aqueous organic solvent;

[0071] S2: In an inert gas environment at room temperature, 16 parts of lithium hexafluorophosphate, 1.5 parts of lithium bis(fluorosulfonyl)imide and 1 part of fluorinated surfactant are added to a non-aqueous organic solvent and stirred until the composite lithium salt is completely dissolved; then 1.5 parts of 2,3-pyridinedicarboxylic anhydride, 2.2 parts of modified layered inorganic materials, 1.5 parts of modified aramid fibers, 1.2 parts of diethylene glycol diethyl ether and 7 parts of modified flame retardant capsules are added, stirred sufficiently, and allowed to stand to obtain a lithium ion electrolyte. The water content and acidity of the electrolyte meet the GB / T19282-2014 standard.

[0072] Example 3

[0073] S1: 57 parts of vinylene carbonate and 16 parts of trifluoropropylene carbonate are mixed uniformly to obtain a non-aqueous organic solvent;

[0074] S2: Under an inert gas environment at room temperature, 23 parts of lithium hexafluorophosphate, 1.8 parts of lithium bis(fluorosulfonyl)imide and 0.8 parts of fluorine-containing surfactant are added to a non-aqueous organic solvent and stirred until the composite lithium salt is completely dissolved; then 1.5 parts of 2,3-pyridinedicarboxylic anhydride, 2 parts of modified layered inorganic material, 1.8 parts of modified aramid fiber, 1.2 parts of diethylene glycol diethyl ether and 7 parts of modified flame retardant capsules are added, stirred sufficiently, and allowed to stand to obtain a lithium ion electrolyte. The water content and acidity of the electrolyte meet the GB / T19282-2014 standard. Comparative Example 1

[0075] Compared with Example 1, diethylene glycol diethyl ether was not added, and the other components and parameters were the same as those in Example 1.

[0076] Comparative Example 2

[0077] Compared with Example 1, pyridinedicarboxylic anhydride was not added, and the other components and parameters were the same as Example 1.

[0078] Comparative Example 3

[0079] Compared with Example 1, no modified aramid fiber is added, and the other components and parameters are the same as Example 1.

[0080] Experimental example

[0081] 1. Battery cycle performance test (as shown in Table 1).

[0082] Preparation of positive electrode sheet: Lithium cobalt oxide, conductive agent CNT and binder polyvinylidene fluoride in a mass ratio of 95:2.5:2.5 were fully stirred and mixed in N-methylpyrrolidone solvent to form a slurry with a viscosity of 6000mPa·s. The slurry was then coated on an 8μm thick aluminum foil current collector, dried, and cold pressed to obtain a positive electrode sheet.

[0083] Preparation of negative electrode sheet: Graphite, conductive agent acetylene black, binder styrene butadiene rubber and thickener sodium carboxymethyl cellulose in a mass ratio of 95:2.5:1.5:1 were fully stirred and mixed in an appropriate amount of deionized water solvent to form a negative electrode slurry with a viscosity of 3000mPa·s. The slurry was then coated on a 6μm thick Cu foil current collector, dried, and cold pressed to obtain a negative electrode sheet.

[0084] Preparation of lithium-ion batteries: stack the positive electrode sheet, the isolation film and the negative electrode sheet in order, so that the isolation film is located between the positive and negative electrodes to play an isolation role, and then wind them into a bare battery cell. Thereafter, the bare battery cell is placed in an outer packaging bag, and the electrolytes in the above-mentioned embodiments 1-3 and comparative examples 1-3 are respectively injected into the battery cells, and the corresponding lithium-ion batteries are prepared through vacuum packaging, standing, formation, shaping and other processes.

[0085] Among them, the lithium-ion battery cycle performance test conditions are: voltage of 3.0~4.5V, cycle test temperature of 45℃ and 60℃, and charge and discharge rate of 1.0C.

[0086] Table 1

[0087]

[0088] It can be seen from Table 1 that the batteries prepared using the electrolytes in Examples 1-3 have relatively excellent electrical cycle performance at high temperatures of 45°C and 60°C, and the battery capacity retention rate after 600 cycles at 45°C is not less than 91.5%; the battery capacity retention rate after 100 cycles at 60°C is not less than 96.1%, the battery capacity retention rate after 200 cycles at 60°C is not less than 90.7%, and the battery capacity retention rate after 300 cycles at 60°C is not less than 80.5%; while the electrolytes in Comparative Examples 1-3 are not strictly in accordance with the components or dosages in this application, and their high temperature cycle performance is worse than that of Examples 1-3.

[0089] 2. Flame retardant performance test.

[0090] Ignition-self-extinguishing test: A spherical glass wool core (diameter 0.5-1 cm) was adsorbed with a small amount of electrolyte prepared in Examples 1-3 and Comparative Examples 1-3 and ignited, and the burning time was measured with a stopwatch. Generally, a self-extinguishing time of more than 20 seconds is considered flammable; between 5-20 seconds is flame retardant, and less than 5 seconds is considered non-combustible.

[0091] Puncture test: Take three batteries, charge them to 4.4V at a constant current of 1C, then charge them at a constant voltage of 4.4V, with the cut-off current being 0.05C, then pierce the batteries with a 2.5mm nail to observe whether the batteries explode or catch fire.

[0092] The test results are shown in Table 2.

[0093] Table 2

[0094] Self-extinguishing time(s) Acupuncture experiment Example 1 1 No fire, no explosion Example 2 1 No fire, no explosion Example 3 0, almost no burning No fire, no explosion Comparative Example 1 9 No fire, no explosion Comparative Example 2 10 No fire, no explosion Comparative Example 3 15 No fire, no explosion

[0095] It can be seen from Table 2 that the self-extinguishing time of the electrolyte in Comparative Examples 1-3 is ≤1s, and almost no combustion occurs, while the self-extinguishing time in Example 3 is 0s, and almost no combustion occurs, thereby achieving the purpose of real fire extinguishing. In Comparative Example 1, no diethylene glycol diethyl ether that cooperates with the flame retardant is added, resulting in a relatively low flame retardant effect of the electrolyte; in Comparative Example 2, no pyridine dicarboxylic anhydride is added, and the overall effect on the flame retardant effect is not significant; in Comparative Example 3, no high temperature resistant material modified aramid fiber is added, resulting in a reduced flame retardant effect.

[0096] Because modified phosphorus-containing flame-retardant vesicles are added to Comparative Examples 1-3 and Comparative Examples 1-3, when the temperature of the needle puncture test reaches 100-120° C., the vesicles can release the phosphorus-containing flame retardant, thereby allowing the electrolyte to exert flame retardant properties.

[0097] In summary, the battery prepared with the electrolyte provided by the present invention has excellent electrical cycle performance at high temperatures of 45°C and 60°C; it also has strong flame retardant properties and can even reach a level of non-combustion.

Claims

1. A lithium ion electrolyte, characterized in that: It includes non-aqueous organic solvent, lithium salt, additive and modified phosphorus-containing flame retardant vesicle; The additives include 2,3-pyridinedicarboxylic anhydride, modified layered inorganic material, modified aramid fiber and diethylene glycol diethyl ether; The modified layered inorganic material is obtained by first modifying the layered inorganic material with a lithium material and then with a high molecular polymer; The modified aramid fiber is obtained by first subjecting the aramid fiber to plasma treatment and then grafting polypropylene ethyl ester; The modified phosphorus-containing flame-retardant vesicles are obtained by first encapsulating the phosphorus-containing flame retardant with poly-N-isopropylacrylamide and then modifying it with an acrylate monomer.

2. A lithium ion electrolyte according to claim 1, characterized in that: The non-aqueous organic solvent consists of vinylene carbonate and trifluoropropylene carbonate; the lithium salt consists of lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide.

3. A lithium ion electrolyte according to claim 1, characterized in that: It includes the following components by weight: 45-65 parts of vinylene carbonate, 10-20 parts of trifluoropropylene carbonate, 10-25 parts of lithium hexafluorophosphate, 1-5 parts of lithium bis(fluorosulfonyl)imide, 3-10 parts of modified phosphorus-containing flame retardant vesicles, 0.1-5 parts of 2,3-pyridinedicarboxylic anhydride, 0.1-5 parts of modified layered inorganic material, 0.1-5 parts of modified aramid fiber and 0.1-5 parts of diethylene glycol diethyl ether.

4. A lithium ion electrolyte according to claim 1, characterized in that: It includes the following components by weight: 48-60 parts of vinylene carbonate, 12-18 parts of trifluoropropylene carbonate, 15-24 parts of lithium hexafluorophosphate, 1.2-3.5 parts of lithium bis(fluorosulfonyl)imide, 3.5-9 parts of modified phosphorus-containing flame retardant vesicles, 0.5-2 parts of 2,3-pyridinedicarboxylic anhydride, 0.5-4 parts of modified layered inorganic material, 1-3 parts of modified aramid fiber and 0.5-2 parts of diethylene glycol diethyl ether.

5. A lithium ion electrolyte according to claim 1, characterized in that: It includes the following components by weight: 57 parts of vinylene carbonate, 16 parts of trifluoropropylene carbonate, 23 parts of lithium hexafluorophosphate, 1.8 parts of lithium bis(fluorosulfonyl)imide, 7 parts of modified phosphorus-containing flame retardant vesicles, 1.5 parts of 2,3-pyridinedicarboxylic anhydride, 2 parts of modified layered inorganic material, 1.8 parts of modified aramid fiber and 1.2 parts of diethylene glycol diethyl ether.

6. A lithium ion electrolyte according to claim 3, characterized in that: Also included is 0.2-1.5 parts of a fluorinated surfactant.

7. A lithium ion electrolyte according to claim 1, characterized in that: The melting temperature of the modified phosphorus-containing flame-retardant vesicle is 100-120°C.

8. A lithium ion electrolyte according to claim 1, characterized in that: The acrylic acid ester monomers include methyl acrylate, ethyl acrylate and butyl acrylate.

9. A lithium ion electrolyte according to claim 1, characterized in that: The flame retardant is selected from trimethyl phosphate, triethyl phosphate, tributyl phosphate, triphenyl phosphate and toluene diphenyl phosphate.

10. The method for preparing a lithium ion electrolyte according to claim 6, characterized in that: The steps include: S1: uniformly mixing vinylene carbonate and trifluoropropylene carbonate to obtain a non-aqueous organic solvent; S2 adds lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide and fluorine-containing surfactant to a non-aqueous organic solvent under an inert gas environment at room temperature and stirs until completely dissolved; then adds 2,3-pyridinedicarboxylic anhydride, modified layered inorganic material, modified aramid fiber, diethylene glycol diethyl ether and modified flame retardant capsules, stirs thoroughly and allows to stand to obtain a lithium ion electrolyte.

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