Flame-retardant additive for electrolyte and application

Abstract

The invention provides a flame-retardant additive for an electrolyte and application, and relates to the technical field of lithium ion batteries. Specifically, the flame-retardant additive comprises a compound formed by connecting a nitrogen-containing six-membered ring and a phosphorus-containing five-membered ring through double bonds, and the compound further comprises hydroxyl, carboxyl and a plurality of fluorine atoms and at least comprises four fluorine atoms. When the electrolyte is prepared based on the flame-retardant additive, raw materials including a lithium salt, an organic solvent, a conductive additive and a film-forming additive are also adopted. The flame-retardant additive disclosed by the invention has an excellent flame-retardant effect, strong applicability and good stability, can improve the electrochemical performance of an electrolyte or a battery to a certain extent, and has a good application prospect.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to a flame retardant additive for an electrolyte and an application thereof. Background Art

[0002] As energy shortages and environmental pollution become increasingly serious, lithium-ion batteries, as an efficient and environmentally friendly energy storage device, have been widely used. However, lithium-ion batteries have safety hazards during use, such as overcharging, over-discharging, short circuits, etc., which may cause fires or even explosions. Therefore, improving the safety of lithium-ion batteries has become a hot topic in current research.

[0003] Electrolyte is an important component of lithium-ion batteries, and its performance directly affects the safety, cycle performance and rate performance of lithium-ion batteries. At present, commonly used electrolytes mainly consist of organic solvents, lithium salts and additives; among them, organic solvents are flammable and can easily cause fires under high temperature or short circuit conditions. In order to improve the safety of lithium-ion batteries and address the problem of thermal runaway of lithium-ion battery electrolytes, the prior art adds flame retardants to the electrolyte to reduce the flammability of the electrolyte, or develops new electrolyte solvents with flame retardant effects; however, this type of electrolyte with flame retardant effects has defects, such as unsatisfactory flame retardant effects and significant impact on the performance of lithium-ion batteries.

[0004] Specifically, electrolytes with flame retardant effects usually have the following technical defects: First, in terms of flame retardant effect, a single flame retardant has limitations and it is difficult to fully cope with complex thermal runaway situations; and there is a contradiction between the amount of flame retardant and the effect, and increasing the amount may have a negative impact on other battery properties. Second, for battery performance, flame retardant components will reduce ionic conductivity, reduce cycle performance, and accelerate capacity decay. Third, in terms of stability, the thermal stability of flame retardant components is insufficient, and flame retardants may decompose or lose their effect at high temperatures, resulting in poor chemical stability and easy reaction with electrolyte components to produce harmful by-products. The above technical defects limit the widespread application and further development of flame retardant electrolytes in lithium-ion batteries. Therefore, the development of flame retardant electrolytes that have good flame retardant effects while ensuring or even improving the electrochemical performance of lithium batteries remains a research hotspot in this field.

[0005] In view of this, the present invention is proposed. Summary of the invention

[0006] The first purpose of the present invention is to provide a flame retardant additive for electrolyte. In view of the technical problems that existing flame retardant additives often have a single or unsatisfactory flame retardant effect, have an adverse effect on the electrochemical performance of the battery, and have poor stability, the flame retardant additive of the present invention can simultaneously meet the three aspects of flame retardant performance, electrochemical performance and stability.

[0007] The second purpose of the present invention is to provide an electrolyte having good electrochemical properties and a strong flame retardant effect; it can improve the flame retardant properties of the electrolyte without reducing the conductivity of the electrolyte, and under its interface regulation, improve the high-temperature storage capacity retention rate and cycle capacity retention rate of the battery.

[0008] A third object of the present invention is to provide a lithium ion battery.

[0009] A fourth object of the present invention is to provide an electrical device.

[0010] In order to achieve the above-mentioned purpose of the present invention, the following technical solutions are particularly adopted:

[0011] A flame retardant additive for electrolyte, comprising a compound as shown in formula (I);

[0012]

[0013] Wherein, R1-R5 are independently selected from one of methylene, fluoromethylene, difluoromethylene, and C2-C3 fluoroalkyl; and R1-R5 contain at least 4 fluorine atoms.

[0014] An electrolyte comprises the flame retardant additive for the electrolyte.

[0015] A lithium ion battery comprises the electrolyte.

[0016] An electrical device comprises the lithium-ion battery.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] The present invention obtains a flame retardant additive with a specific structure through design, which has the advantages of good flame retardant effect, strong stability, and little effect on the performance of lithium-ion batteries. Specifically, the flame retardant additive of the present invention is a three-element cyclic compound containing phosphorus, nitrogen, and fluorine, which can effectively inhibit the occurrence and spread of combustion when the battery has dangerous situations such as thermal runaway. Among them, the phosphorus-containing part can form a heat-insulating and oxygen-isolating phosphoric acid substance, the nitrogen-containing part can release non-flammable gas to dilute the concentration of combustible gas, and the fluorine element can improve the oxidation stability of the electrolyte and reduce the decomposition of the electrolyte under high voltage, thereby reducing the risk of fire or even explosion of the battery. At the same time, the flame retardant additive also contains two functional groups, hydroxyl and carboxyl, which have better stability in the electrolyte while achieving a good flame retardant effect, and can participate in passivating the electrode interface to form a more solid electrode interface, thereby improving the cycle stability of the lithium-ion secondary battery. DETAILED DESCRIPTION

[0019] The technical scheme of the present invention will be clearly and completely described below in conjunction with specific embodiments, but those skilled in the art will appreciate that the following described embodiments are part of embodiments of the present invention, rather than all embodiments, and are only used to illustrate the present invention, and should not be considered as limiting the scope of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work are within the scope of protection of the present invention. If specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer is not specified in the reagents or instruments used, they are all conventional products that can be purchased commercially. In addition, the terms "first", "second" and "third" are only used for descriptive purposes and cannot be interpreted as indicating or implying relative importance.

[0020] A first aspect of the present invention is to provide a flame retardant additive for an electrolyte.

[0021] The flame retardant additive includes a compound as shown in formula (I);

[0022]

[0023] Wherein, R1-R5 are independently selected from one of methylene, fluoromethylene, difluoromethylene, and C2-C3 fluoroalkyl; and R1-R5 contain at least 4 fluorine atoms.

[0024] It is worth noting that R1 to R5 can be selected from the following ranges respectively, and the selection of R1 to R5 does not affect each other, as long as the total number of fluorine atoms in R1 to R5 is ≥4.

[0025] Specifically, the optional groups in R1 to R5 mainly include the following: methylene (-CH2-), fluoromethylene (-CHF-), difluoromethylene (-CF2-), C2 to C3 fluoroalkyl (including monofluoroalkyl and difluoroalkyl, including but not limited to: In some optional embodiments, the C2-C3 fluoroalkyl group may also be a polyfluoroalkyl group, including but not limited to: wait.

[0026] It is worth noting that although the present invention limits R1 to R5 to include one or more of monofluoroalkyl, difluoroalkyl and polyfluoroalkyl of C2 to C3, it is still necessary to ensure that the basic structure of the compound represented by formula (I) includes a phosphorus-containing five-membered ring and a nitrogen-containing six-membered ring.

[0027] As a preferred embodiment, R1 to R5 contain 4 to 10 fluorine atoms in total, more preferably 4 to 8 fluorine atoms.

[0028] The present invention provides a cyclic flame retardant additive containing three elements of phosphorus, nitrogen and fluorine, which exhibits excellent effects in the field of lithium-ion battery flame retardancy with its unique molecular structure; its cyclic structure is composed of a phosphorus-containing five-membered ring and a nitrogen-containing six-membered ring connected by a carbon-carbon double bond. This special cyclic design gives the molecule higher stability, allowing it to maintain structural integrity in a complex battery working environment.

[0029] Specifically, phosphorus plays a key flame retardant role in the five-membered ring; in dangerous situations such as thermal runaway of the battery, the phosphorus-containing part can form phosphoric acid substances, covering the surface of the electrode, playing a role of heat insulation and oxygen isolation, and effectively preventing the further spread of combustion. The nitrogen element is located in the six-membered ring and can release non-combustible gases, such as nitrogen, at high temperatures to dilute the concentration of combustible gases and reduce the possibility of combustion. The fluorine element introduced by means of substituents improves the oxidative stability of the electrolyte and reduces the decomposition of the electrolyte at high voltage, thereby reducing the risk of thermal runaway of the battery.

[0030] Furthermore, the hydroxyl and carboxyl functional groups introduced into the molecule as shown in formula (I) greatly enhance the interaction between the molecule and other components in the electrolyte. This not only improves the solubility and stability of the flame retardant additive in the electrolyte, but also enables the molecule as shown in formula (I) to better cooperate with the electrode material to form a more stable solid electrolyte interface film on the electrode surface, thereby improving the cycle performance and rate performance of the battery. It can be seen that the molecule as shown in formula (I) has excellent performance characteristics, which helps to improve the safety and performance of lithium-ion batteries.

[0031] As a preferred embodiment, in any battery electrolyte, the amount of the flame retardant additive added is 5% to 15% by mass; in some optional embodiments, the amount added includes but is not limited to any one of 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 14%, 15%, or a numerical range consisting of any two of them.

[0032] As a preferred embodiment, the compound shown in formula (I) can be obtained by the following preparation route: (1) synthesizing a phosphorus-containing five-membered ring structure and a nitrogen-containing six-membered ring structure separately; (2) polymerizing the phosphorus-containing five-membered ring structure and the nitrogen-containing six-membered ring structure to obtain two ring structures connected by a double bond; (3) performing a fluorine substitution reaction; (4) grafting carboxyl and hydroxyl functional groups; (5) purification and separation.

[0033] Specifically, the preparation route includes the following specific steps:

[0034] (1) Synthesis of phosphorus-containing five-membered ring monomers: dissolving a phosphorus-containing cyclic precursor in a solvent (such as anhydrous ethanol or dichloromethane), and then adding a catalyst (a specific metal catalyst or an organic catalyst) to promote the reaction; the reaction is carried out at a certain temperature and under stirring conditions to cause the phosphorus-containing cyclic precursor to undergo a cyclization reaction to form a stable phosphorus-containing five-membered ring structure.

[0035] Synthesis of nitrogen-containing six-membered ring monomers: dissolve the nitrogen-containing cyclic precursor in a suitable solvent, add a suitable initiator or catalyst to initiate the polymerization or cyclization reaction of the nitrogen-containing cyclic precursor; control the reaction conditions such as temperature, reaction time and stirring speed, so that the nitrogen-containing cyclic precursor forms a nitrogen-containing six-membered ring structure.

[0036] (2) Ring connection: The phosphorus-containing five-membered ring and the nitrogen-containing six-membered ring compound in step (1) are mixed in a reaction vessel, a reagent that can promote the formation of a carbon-carbon double bond is added, and a specific coupling agent or oxidant is used to react at an appropriate temperature and conditions to connect the two rings through the carbon-carbon double bond.

[0037] (3) Fluorine substitution: Select a suitable fluorine-containing substituent compound, which should have an active group that can react with the synthesized cyclic structure; add the fluorine-containing substituent compound to the reaction system of step (2), and introduce fluorine into the molecule through the substituent under certain temperature and reaction conditions.

[0038] (4) Grafting of hydroxyl and carboxyl functional groups: Select a reagent that can introduce hydroxyl groups, such as a mixed solution of sodium hydroxide and water, and perform a hydrolysis reaction to introduce hydroxyl groups into the molecule. Select a compound containing a carboxyl group for grafting reaction, or convert a specific group into a carboxyl group through an oxidation reaction, thereby introducing a carboxyl functional group.

[0039] (5) Purification and separation: Remove unreacted raw materials and by-products by filtration, distillation, extraction, etc. Use chromatography (column chromatography or high performance liquid chromatography) to purify and separate the product to obtain a pure phosphorus, nitrogen and fluorine cyclic flame retardant additive.

[0040] A second aspect of the present invention is to provide an electrolyte.

[0041] The electrolyte must include the flame retardant additive for electrolyte as described in the first aspect. It is understandable that, in addition to the flame retardant additive, the flame retardant electrolyte should also include other necessary ingredients to achieve the primary function of the ion transport carrier, at least including lithium salt and organic solvent. As shown below, the present invention will provide the preferred selection of lithium salt and organic solvent and other additive components to achieve better electrochemical performance and flame retardant effect.

[0042] As a preferred embodiment, the lithium salt includes at least one of lithium hexafluorophosphate, lithium tetrafluoroborate or lithium perchlorate.

[0043] As a preferred embodiment, in the electrolyte, the amount of the lithium salt added is 3% to 5% by mass; in some optional embodiments, the amount added includes but is not limited to any one of 3%, 3.5%, 4%, 4.5%, 5% or a numerical range consisting of any two of them.

[0044] As a preferred embodiment, the organic solvent includes a cyclic carbonate and a chain acid ester; wherein the cyclic carbonate includes at least one of ethylene carbonate, vinylene carbonate, propylene carbonate or butylene carbonate, and the chain acid ester includes at least one of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate or propyl acetate.

[0045] It is to be understood that the cyclic carbonate and the linear acid ester may be selected from one component or a combination of several components, such as some typical but non-limiting examples: a combination of ethylene carbonate and propylene carbonate, a combination of propylene carbonate and butylene carbonate, a combination of dimethyl carbonate and diethyl carbonate, a combination of ethyl methyl carbonate and methyl propyl carbonate, a combination of ethyl propyl carbonate and methyl formate, a combination of methyl formate and ethyl formate, a combination of propyl formate and methyl acetate, a combination of ethyl acetate and propyl acetate, etc.

[0046] As a more preferred embodiment, the volume ratio of the cyclic carbonate and the linear acid ester is (5-40):(60-95); in some optional embodiments, the volume ratio includes but is not limited to any one of 5:95, 10:90, 15:85, 20:80, 25:75, 30:70, 35:75, 40:60, 50:50:55:45, 60:40 or a ratio interval consisting of any two of them.

[0047] As a preferred embodiment, in the electrolyte, the amount of the organic solvent added is 70% to 80% by mass; in some optional embodiments, the amount added includes but is not limited to any one of 70%, 72%, 74%, 75%, 76%, 78%, 80% or a numerical range consisting of any two of them.

[0048] As a preferred embodiment, the electrolyte further includes a film-forming additive; the film-forming additive includes at least one of vinylene carbonate (VC), fluoroethylene carbonate (FEC), 1,3-propane sultone (PS), vinyl ethylene carbonate (VEC) or ethylene sulfate (DTD).

[0049] As a preferred embodiment, in the electrolyte, the amount of the film-forming additive added is 1% to 5% by mass; in some optional embodiments, the amount added includes but is not limited to 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, any one of them or a numerical range consisting of any two of them.

[0050] As a preferred embodiment, the electrolyte further includes a conductive additive; the conductive additive includes at least one of lithium difluorooxalatoborate (LiDFOB) and lithium trifluoromethanesulfonate (LiCF3SO3).

[0051] As a preferred embodiment, in the electrolyte, the amount of the conductive additive added is 1% to 5% by mass; in some optional embodiments, the amount added includes but is not limited to 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, any one of them or a numerical range consisting of any two of them.

[0052] A third object of the present invention is to provide a lithium ion battery.

[0053] The lithium-ion battery must include the electrolyte as described in the second aspect. It is understandable that, in addition to the electrolyte, the lithium-ion battery also includes at least a positive electrode, a negative electrode, a separator, and other necessary or non-essential functional elements or packaging components, etc., and those skilled in the art can make any selection and combination thereof. When an electrolyte containing a specific flame retardant additive is used in the lithium-ion battery, whether or not other types of electrolytes are used in the lithium-ion battery, it can be regarded as an embodiment of the present invention.

[0054] As a preferred embodiment, the active material of the positive electrode includes lithium transition metal oxide and / or lithium transition metal phosphate compound.

[0055] In some optional embodiments, the lithium transition metal oxide includes but is not limited to LiCoO2, LiNi x Co y Mn z O2、LiNi x Mn y At least one of O2, LiMn2O4, LiMnO2, Li2MnO4; 0≤x≤1, 0≤y≤1, 0≤z≤1.

[0056] In some optional embodiments, the lithium transition metal phosphate compound includes but is not limited to at least one of LiFePO4, LiMnPO4, and LiCoPO4.

[0057] As a preferred embodiment, the active material of the positive electrode includes at least one of natural graphite, artificial graphite, silicon-carbon composite material, soft carbon, hard carbon, lithium titanate or metallic lithium.

[0058] As a preferred embodiment, the diaphragm includes at least one of a polyethylene diaphragm, a polypropylene diaphragm, an ethylene-propylene copolymer film or a polyimide diaphragm.

[0059] A fourth aspect of the present invention is to provide an electrical device.

[0060] The electrical equipment includes the lithium-ion battery as described in the third aspect. It is understandable that the electrical equipment can be any equipment or device that relies on electric energy to work or operate, including but not limited to new energy vehicles, building electrical equipment, industrial electrical appliances, household and agricultural electrical appliances, etc.; when including the lithium-ion battery, any equipment equipped with the lithium-ion battery can belong to an embodiment of the present invention.

[0061] Example 1

[0062]

[0063] This embodiment uses the flame retardant additive shown in the above formula (a).

[0064] In this example, the compound of formula (a) was prepared according to the following preparation method.

[0065] S1: Synthesis of phosphorus-containing five-membered ring. Dissolve hexachlorocyclotriphosphazene (P3N3Cl6) in anhydrous ethanol, and then add triethylamine (Et3N) as a catalyst. The reaction is carried out at 60°C with stirring; the chlorine atoms in the hexachlorocyclotriphosphazene are gradually replaced by the ethoxy groups in ethanol under the action of the catalyst, and a cyclization reaction occurs to form a stable phosphorus-containing five-membered ring structure. During the reaction, continuous stirring is required to ensure that the reaction proceeds evenly. After 12 hours of reaction, the product is purified by cooling, filtering, washing and drying to obtain a phosphorus-containing five-membered ring compound.

[0066] S2: Synthesis of nitrogen-containing six-membered ring. Take the nitrogen-containing cyclic precursor caprolactam (C6H 11 NO) is dissolved in anhydrous ethanol, and concentrated sulfuric acid (H2SO4) is added as a catalyst. The reaction is carried out at 120°C with a stirring speed of 400 rpm for 8 hours, so that the nitrogen-containing cyclic precursor forms a nitrogen-containing six-membered ring structure.

[0067] S3: Connecting two ring structures. The phosphorus-containing five-membered ring and nitrogen-containing six-membered ring compounds prepared in steps S1 and S2 are mixed at a molar ratio of 1:1, and triethyl phosphite ((C2H5O)3P), a reagent that can promote the formation of carbon-carbon double bonds, and dibenzoyl peroxide (BPO), a coupling agent, are added, and the reaction is stirred at 100°C and nitrogen protection at a speed of 500 rpm to connect the two rings through the carbon-carbon double bond.

[0068] S4: Introducing fluorine atoms. Select a fluorine-containing substituent compound 2-fluoroethanol (CH2FCH2OH), add it to the reaction system of S3, and react at 120°C, nitrogen protection, and a stirring speed of 500 rpm to introduce fluorine elements into the molecule through substituents.

[0069] S5: Introducing hydroxyl and carboxyl groups (in no particular order). Prepare an aqueous solution of sodium hydroxide and slowly add it to the reaction system of S4. Perform a hydrolysis reaction at 50°C to introduce hydroxyl groups into the molecule. The reaction time can be controlled within 6 hours. Then add maleic anhydride (C4H2O3) to the above reaction system containing hydroxyl groups and react at 100°C. The hydroxyl groups react with maleic anhydride to form an esterification reaction, and then the ester undergoes hydrolysis and decarboxylation reactions to introduce carboxyl groups.

[0070] S6: Purification and separation. After the reaction is completed, dichloromethane (CH2Cl2) is used as an extractant for extraction, and then unreacted raw materials and by-products are removed by distillation. The product is purified and separated using high performance liquid chromatography to obtain a pure phosphorus, nitrogen, and fluorine ternary cyclic flame retardant additive as shown in formula (a).

[0071] Based on the flame retardant additive, this embodiment further provides a flame retardant electrolyte, including the following components; the preparation method is to fully stir and mix the following components.

[0072] Lithium salt: lithium hexafluorophosphate, 5wt.%;

[0073] Organic solvent: mixture of ethylene carbonate, diethyl carbonate and ethyl methyl carbonate (volume ratio of the three is 1:1:1), 80wt.%;

[0074] Flame retardant: as shown in formula (a), 10wt.%;

[0075] Film-forming additive: fluoroethylene carbonate, 5 wt.%.

[0076] Example 2

[0077]

[0078] It is basically the same as Example 1, except that the flame retardant additive is replaced by the compound shown in the above formula (b).

[0079] Example 3

[0080]

[0081] It is basically the same as Example 1, except that the flame retardant additive is replaced by the compound represented by the above formula (c).

[0082] Example 4

[0083]

[0084] It is basically the same as Example 1, except that the flame retardant additive is replaced by the compound represented by the above formula (d).

[0085] Example 5

[0086] It is basically the same as Example 1, except that: in the mass ratio of the electrolyte:

[0087] Flame retardant additive as shown in formula (a): 12 wt.%;

[0088] Organic solvent: 78wt.%.

[0089] Example 6

[0090] It is basically the same as Example 1, except that: in the mass ratio of the electrolyte:

[0091] Flame retardant additive as shown in formula (a): 15 wt.%;

[0092] Organic solvent: 75wt.%.

[0093] Example 7

[0094] It is basically the same as Example 1, except that: in the mass ratio of the electrolyte:

[0095] Flame retardant additive as shown in formula (a): 18 wt.%;

[0096] Organic solvent: 72wt.%.

[0097] Example 8

[0098] It is basically the same as Example 1, except that: in the mass ratio of the electrolyte:

[0099] Flame retardant additive as shown in formula (a): 20 wt.%;

[0100] Organic solvent: 70wt.%.

[0101] Example 9

[0102] It is basically the same as Example 1, except that: in the mass ratio of the electrolyte:

[0103] Flame retardant additive as shown in formula (a): 5 wt.%;

[0104] Organic solvent: 85wt.%.

[0105] Example 10

[0106] The method is basically the same as Example 1, except that: in the preparation of the electrolyte:

[0107] Lithium salt: lithium hexafluorophosphate, 4wt.%;

[0108] Organic solvent: mixture of ethylene carbonate, diethyl carbonate and ethyl methyl carbonate (volume ratio of the three is 1:1:1), 80wt.%;

[0109] Flame retardant: as shown in formula (a), 10wt.%;

[0110] Film-forming additive: fluoroethylene carbonate, 3wt.%;

[0111] Conductive additive: lithium difluorooxalatoborate, 3 wt.%.

[0112] Comparative Example

[0113] The method is basically the same as Example 1, except that: in the preparation of the electrolyte, the flame retardant additive as shown in formula (a) is not added, and the content of the organic solvent is adjusted to 90 wt.%.

[0114] Test example

[0115] The electrolytes prepared in the examples and comparative examples were assembled to obtain corresponding test batteries, and the specific steps were as follows:

[0116] (1) Preparation of positive electrode sheets: Lithium iron phosphate (LiFePO4), conductive carbon black (SP), carbon nanotubes (CNTs) and polyvinylidene fluoride (PVDF) are mixed in a solvent N-methylpyrrolidone at a weight ratio of 96:1:0.5:2.5, and stirred evenly to obtain a positive electrode slurry; aluminum foil is used as a positive electrode current collector, the positive electrode slurry is coated on the positive electrode current collector, and then baked at 120°C for 1h, followed by cold pressing, cutting and slitting to obtain a positive electrode sheet.

[0117] (2) Preparation of negative electrode sheets: The negative electrode active material artificial graphite, conductive agent (Super-P), thickener (CMC) and binder (SBR) are dissolved in deionized water at a mass ratio of 96.5:1.0:1.0:1.5, mixed evenly, and prepared into negative electrode slurry; the negative electrode slurry is evenly coated on the current collector copper foil, then dried at 90°C, cold pressed, trimmed, cut into pieces, and divided into strips, and then dried at 110°C under vacuum conditions for 4 hours, and the tabs are welded to prepare negative electrode sheets.

[0118] (3) Preparation of lithium-ion batteries: The positive electrode sheet, the separator, and the negative electrode sheet are arranged in order, and an electrode group is obtained by using a rapid lamination technology. The electrode group is welded into a shell (aluminum-plastic film) through positive and negative electrode ears, and the side of the top is sealed and dried. Then, the electrolyte (4Ah, injection coefficient 3.3g / Ah) in each embodiment and comparative example is injected respectively, and the corresponding lithium-ion secondary battery is obtained through processes such as standing, pre-charging, secondary injection, and formation.

[0119] The electrolytes of the above embodiments and comparative examples and the corresponding lithium-ion batteries were subjected to performance tests, and the test methods are as follows:

[0120] (1) Flame retardant performance test

[0121] (1.1) Thermal Stability Analysis: The thermal stability of the electrolytes of the embodiments and comparative examples at high temperatures was predicted by thermogravimetric analysis (TGA) test.

[0122] (1.2) Combustion performance test: The limiting oxygen index (LOI) test and vertical combustion test are used to evaluate the effect of flame retardant additives on the combustion performance of the electrolyte.

[0123] Table 1

[0124]

[0125]

[0126] The flame retardant test results in Table 1 show that each flame retardant additive has a high thermal decomposition temperature and can play a flame retardant role before the battery undergoes thermal runaway. At the same time, the flame retardant additive can significantly increase the LOI value of the electrolyte and reduce the combustion speed and flame propagation speed of the electrolyte, which shows that the nitrogen-fluorine ternary cyclic flame retardant compound of the present invention has a good effect of improving the flame retardancy of the battery electrolyte.

[0127] Specifically, based on the comparison between Example 1 and Example 2, it can be seen that when the number of fluorinated atoms in the nitrogen-fluorine three-element cyclic flame retardant compound molecules increases, its effect as a flame retardant tends to increase. In addition, from the concentration gradient comparison of Example 9, Example 1, and Examples 5 to 8, it can be seen that the flame retardant performance of the electrolyte increases with the increase in the concentration of the flame retardant additive in the electrolyte. When its addition concentration increases to 20% of the total electrolyte fraction, the thermal decomposition temperature of the electrolyte reaches 288°C, which is much higher than the comparative example.

[0128] (2) Electrochemical performance

[0129] (2.1) Ionic conductivity: The effect of flame retardant additives on the ionic conductivity of the electrolyte was studied by electrochemical impedance spectroscopy (EIS).

[0130] (2.2) High temperature storage performance: The storage capacity retention rate of each lithium-ion battery after storage at 55°C for 48 hours was tested.

[0131] (2.3) Cycle capacity retention rate: Charge each lithium-ion battery at room temperature at a constant current of 0.5C to 3.65V, then charge at a constant voltage until the current drops to 0.05C, and let it stand for 30 minutes. Discharge at a constant current of 0.5C to 2.0V, and let it stand for 30 minutes. In a constant temperature box at 55°C, charge at a constant current of 1C to 3.65V, then charge at a constant voltage until the current drops to 0.05C, and let it stand for 30 minutes. Discharge at a constant current of 1C to 2.0V, which is a complete charge and discharge cycle. After repeating the above charge and discharge cycles 300 times, calculate the cycle capacity retention rate.

[0132] Table 2

[0133]

[0134] The electrochemical performance test results in Table 2 show that the flame retardant additive of the present invention has a suitable molecular size and functional group, and will not have a significant negative impact on the ionic conductivity of the electrolyte; at the same time, because it can decompose at the electrode interface to promote the formation of a solid electrode interface, it can improve the storage capacity retention rate and cycle capacity retention rate of the lithium-ion battery at high temperature (55°C) to a certain extent.

[0135] Although the present invention has been illustrated and described with specific embodiments, it should be appreciated that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Those skilled in the art should understand that the technical solutions described in the above embodiments may be modified, or some or all of the technical features thereof may be replaced by equivalents without departing from the spirit and scope of the present invention. However, these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention. Therefore, this means that all such replacements and modifications within the scope of the present invention are included in the appended claims.

Claims

1. A flame retardant additive for electrolyte, characterized in that: The flame retardant additive for the electrolyte includes a compound as shown in formula (I); Wherein, R1-R5 are independently selected from one of methylene, fluoromethylene, difluoromethylene, and C2-C3 fluoroalkyl; and R1-R5 contain at least 4 fluorine atoms.

2. The flame retardant additive for electrolyte according to claim 1, characterized in that: R1 to R5 independently include the following groups: -CH2-, -CHF-, -CF2-, 3. The flame retardant additive for electrolyte according to claim 1, characterized in that: In the electrolyte, the flame retardant additive is added in an amount of 5wt.% to 15wt.%.

4. An electrolyte, characterized in that: The invention comprises the flame retardant additive for electrolyte as claimed in any one of claims 1 to 3.

5. The electrolyte according to claim 4, characterized in that The electrolyte includes a lithium salt; preferably, the lithium salt includes at least one of lithium hexafluorophosphate, lithium tetrafluoroborate or lithium perchlorate.

6. The electrolyte according to claim 4, characterized in that The electrolyte includes an organic solvent; the organic solvent includes a cyclic carbonate and a chain acid ester; Preferably, the volume ratio of the cyclic carbonate to the linear acid ester is (5-40):(60-95).

7. The electrolyte according to claim 4, characterized in that The electrolyte includes a film-forming additive; preferably, the film-forming additive includes at least one of vinylene carbonate, fluoroethylene carbonate, 1,3-propane sultone, vinyl vinyl carbonate or ethylene sulfate.

8. The electrolyte according to claim 4, characterized in that The electrolyte includes a conductive additive; preferably, the conductive additive includes at least one of lithium difluorooxalatoborate or lithium trifluoromethanesulfonate.

9. A lithium ion battery, characterized in that: Comprising the electrolyte as described in any one of claims 4 to 8.

10. An electrical device, characterized in that: Comprising the lithium ion battery as claimed in claim 9.

Images