Non-aqueous electrolyte solution and lithium ion secondary battery

Abstract

The invention discloses a non-aqueous electrolyte and a lithium ion secondary battery, and belongs to the technical field of secondary batteries. The non-aqueous electrolyte comprises a lithium salt, a non-aqueous solvent, lithium benzene tetraformate and a bicyclic sulfate compound with a structure as shown in a formula 1. The lithium ion secondary battery comprises a positive electrode, a negative electrode, a diaphragm and a non-aqueous electrolyte. The electrolyte provided by the invention can simultaneously improve the first efficiency, the high-low temperature cycle performance and the high-temperature storage performance of the high-voltage battery.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and in particular to a non-aqueous electrolyte and a lithium-ion secondary battery. Background Art

[0002] Lithium-ion batteries are widely used in portable devices such as mobile phones, camcorders, and laptops due to their high specific capacity, long cycle life, low self-discharge rate, no memory effect, and environmental friendliness. Currently, with the application of lithium-ion batteries in electric vehicles, higher requirements are being placed on the energy density of lithium-ion batteries.

[0003] Among them, increasing the operating voltage can improve the energy density of lithium-ion batteries. However, conventional lithium-ion electrolytes are prone to oxidation reactions on the surface of the positive electrode material, resulting in a rapid decline in the overall performance of high-voltage batteries, especially the first efficiency (initial charge and discharge efficiency) and high-temperature performance, thus restricting the further development of high-voltage lithium-ion batteries. Summary of the Invention

[0004] The object of the present invention is to provide a non-aqueous electrolyte and a lithium ion secondary battery.

[0005] To achieve the above objectives, the present invention proposes the following technical solutions: In a first aspect, the present invention provides a non-aqueous electrolyte, comprising: lithium salts, non-aqueous solvents, The first additive is lithium pyromellitic acid, and the second additive is lithium pyromellitic acid. A bicyclic sulfate compound having a structure shown in Formula 1; , Formula 1; Wherein, R1 is selected from an alkyl group, a cycloalkyl group, a phenyl group containing 2 to 8 carbon atoms, or a connecting group of a combination of the above groups.

[0006] Preferably, R1 is a linking group having a structure shown in Formula 2; , Formula 2; Wherein, R2 is selected from ; p is an integer from 0 to 6, n is an integer from 0 to 2, and m is an integer from 0 to 2; R3, R4, R5, and R6 are each independently selected from a single bond, a methylene group, or an ethylene group.

[0007] More preferably, the bicyclic sulfate compound is selected from at least one of the following compounds: , Preferably, the content of the bicyclic sulfate ester compound is 0.1 to 5%, based on the total mass of the non-aqueous electrolyte.

[0008] Preferably, the content of the lithium pyromellitic acid is 0.1-0.5%, based on the total mass of the non-aqueous electrolyte.

[0009] Preferably, the non-aqueous electrolyte further comprises a third additive, difluoromethylthiobenzenesulfonate.

[0010] More preferably, the content of the difluoromethylthiobenzenesulfonate is 0.1 to 1.0%, based on the total mass of the non-aqueous electrolyte.

[0011] Preferably, the non-aqueous solvent is selected from at least one of propylene carbonate, ethyl methyl carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, fluoroethylene carbonate, γ-butyrolactone, methyl formate, ethyl formate, propyl formate, butyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate, butyl butyrate, difluoroethyl acetate, and ethyl 2,2,2-trifluoroacetate; In addition to the lithium salt and additives in the electrolyte, the balance is the non-aqueous solvent content.

[0012] Preferably, the lithium salt is selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(oxalatoborate), lithium difluorooxalatophosphate, lithium difluorooxalatoborate, lithium tetrafluorooxalatophosphate, and lithium bis(fluorosulfonyl)imide. The content of the lithium salt is 5-20%, based on the total mass of the non-aqueous electrolyte.

[0013] In a second aspect, the present invention provides a lithium-ion secondary battery, comprising: A positive electrode; a negative electrode; a separator; and the non-aqueous electrolyte described in the first aspect.

[0014] The present invention has at least the following beneficial effects: The present invention uses lithium pyromellitic acid and a bicyclic sulfate compound as electrolyte additives. The bicyclic sulfate compound, because it contains high-valent sulfur, can form a stable film on the surfaces of the positive and negative electrodes. Furthermore, lithium pyromellitic acid can combine with and modify the film formed by the bicyclic sulfate compound, improving the film's stability and lithium ion conductivity, thereby enhancing the initial efficiency of the lithium-ion battery at high voltage, as well as its subsequent high- and low-temperature cycling performance and high-temperature storage performance. DETAILED DESCRIPTION

[0015] The following will be combined with the embodiments to clearly and completely describe the technical solutions of the present invention. Obviously, the embodiments described below are only some embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0016] It will be understood that when used in this specification and the appended claims, the terms “comprises” and “comprising” indicate the presence of described features, integers, steps, operations, elements and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or groups thereof.

[0017] It should also be understood that the terms used in this description of the embodiments of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the embodiments of the present invention. As used in the description of the embodiments of the present invention and the appended claims, the singular forms "a", "an", and "the" are intended to include the plural forms unless the context clearly indicates otherwise.

[0018] The non-aqueous electrolyte and the lithium-ion secondary battery of this embodiment are described in detail below.

[0019] First, the non-aqueous electrolyte solution according to the first aspect of this embodiment will be described.

[0020] non-aqueous electrolyte The non-aqueous electrolyte of this embodiment includes: a lithium salt, a non-aqueous solvent, lithium pyromellitic acid, and a bicyclic sulfate compound having a structure shown in the following formula 1: , Formula 1; Wherein, R1 is selected from an alkyl group, a cycloalkyl group, a phenyl group containing 2 to 8 carbon atoms, or a connecting group of a combination of the above groups.

[0021] It is understood that the lithium pyromellitic acid and bicyclic sulfate compounds are used as electrolyte additives. The bicyclic sulfate compound, due to its high-valent sulfur content, can form stable films on the positive and negative electrode surfaces. Furthermore, the lithium pyromellitic acid can combine with and modify the film formed by the bicyclic sulfate compound, further improving the film's stability and lithium ion conductivity, thereby enhancing the initial efficiency of lithium-ion batteries at high voltages, as well as their subsequent high- and low-temperature cycling performance and high-temperature storage performance.

[0022] Among them, due to the presence of lithium formate introduced on the phenyl group in lithium phenyltetracarboxylate, it overcomes the defect of poor cycle life of the battery caused by the instability of the phenyl additive; that is, the lithium phenyltetracarboxylate used in this embodiment has a more stable structure and a special lithium ion channel, which can significantly improve the stability of the film generated by the bicyclic sulfate compound and improve the lithium ion conductivity; therefore, it can better avoid the oxidation of the non-aqueous solvent at the positive electrode and the reduction at the negative electrode, and inhibit the collision of the positive electrode material at high voltage, so that the high voltage resistance of the lithium ion battery is significantly improved, and the first efficiency of the lithium ion battery at high voltage is improved.

[0023] Preferably, R1 is a linking group having a structure shown in Formula 2; , Formula 2; Wherein, R2 is selected from ; p is an integer from 0 to 6, n is an integer from 0 to 3, and m is an integer from 0 to 3; R3, R4, R5, and R6 are each independently selected from a single bond, a methylene group, or an ethylene group.

[0024] Illustratively, the bicyclic sulfate compound is selected from at least one of the following compounds:

[0025] As a preferred embodiment of this embodiment, the content of the bicyclic sulfate compound is 0.1-5%, more preferably 1-3%; the content of the lithium phenylene tetracarboxylate is 0.1-0.5%, more preferably 0.1-0.3%, based on the total mass of the non-aqueous electrolyte.

[0026] For example, the amount of the bicyclic sulfate compound in the non-aqueous electrolyte includes but is not limited to: 0.1%, 0.11%, 0.14%, 0.18%, 0.2%, 0.24%, 0.26%, 0.28%, 0.3%, 0.33%, 0.34%, 0.38%, 0.4%, 0.43%, 0.46%, 0.48%, 0.49%, 0.5%, 0.51%, 0.52%, 0.54%, 0.55%, 0.58%, 0.6%, 0.64%, 0.66%, 0.68%, 0.7%, 0.73%, 0.74%, 0.78%, 0.8% , 0.83%, 0.86%, 0.88%, 0.9%, 0.93%, 0.95%, 0.97%, 0.99%, 1.0%, 1.3%, 1.32%, 1.35%, 1.6%, 1.65%, 1.7%, 1.78%, 1.8%, 1.83%, 1.88%, 1.9%, 1.93%, 1.97%, 2.0%, 2.3%, 2.5%, 2.7%, 2.8%, 2.9%, 3.0%, 3.3%, 3.5%, 3.7%, 3.8%, 4%, 4.3%, 4.5%, 4.7%, 4.8%, 4.9%, 5.0%.

[0027] For example, the amount of the above-mentioned lithium phenylenetetracarboxylate in the non-aqueous electrolyte includes but is not limited to: 0.1%, 0.11%, 0.14%, 0.18%, 0.2%, 0.24%, 0.26%, 0.28%, 0.3%, 0.33%, 0.34%, 0.38%, 0.4%, 0.43%, 0.46%, 0.48%, 0.49%, and 0.5%.

[0028] The bicyclic sulfate compound of this embodiment can be prepared by the following preparation method.

[0029] The preparation method of compound 1 comprises: (1) At room temperature, 1 mol of thionyl chloride and 0.2 mol of pentaerythritol were added to a reaction flask (nitrogen circulation protection), stirred and mixed uniformly, and the reaction temperature was raised to 50°C. After stirring and reacting for 6 hours, the mixture was cooled to 25°C. After solid-liquid separation, the solid was a bicyclic sulfite (3,9-dioxide-2,4,8,10-tetraoxa-3,9-dithiaspiro[5.5]undecane) with a yield of 83.5% and a purity of 99.2%. (2) The above-mentioned bicyclic sulfite (0.185 mol), hydrated ruthenium trichloride (0.35% of the mass of the bicyclic sulfite) and acetonitrile (2.5 mol) were added to the reaction flask in sequence, cooled to -10°C and allowed to stand for 5 minutes; sodium hypochlorite (0.45 mol) solution was added dropwise, and the temperature was maintained at room temperature after the addition was completed. After the reaction was completed for 3 hours, the organic phase was collected, the acetonitrile was distilled off under reduced pressure, washed with water 3 times, recrystallized and dried to obtain pentaerythritol disulfate.

[0030] Compounds 2-6 listed in the technical solution of the present invention and other compounds listed in Formula 1 can all be prepared by existing technical means.

[0031] In the electrolyte of this embodiment, in addition to the first additive lithium pyromellitic acid and the second additive bicyclic sulfate compound, the additives may also include a third additive, wherein the third additive is difluoromethylthiobenzenesulfonate.

[0032] This embodiment uses difluoromethylthiobenzenesulfonate as an additive based on lithium pyromellitic acid and a bicyclic sulfate compound, which can further significantly improve the initial efficiency, high-low temperature cycling performance, and high-temperature storage performance of the high-voltage battery. The principle is as follows: the benzene structure in difluoromethylthiobenzenesulfonate contains a large amount of hydrogen, which can form hydrogen bonds with the film components formed by lithium pyromellitic acid and the bicyclic sulfate compound, thereby forming a more elastic interface, which can significantly improve the stability of the film and further suppress the collision of the positive electrode material under high voltage; at the same time, due to the presence of the strongly electronegative fluorine-containing group in difluoromethylthiobenzenesulfonate, the fluorine-containing group not only promotes the interface stability and lithium ion conduction ability formed by difluoromethylthiobenzenesulfonate, but the fluorine-containing group itself can also improve the interface oxidation resistance and ion conductivity.

[0033] Therefore, the electrolyte of this embodiment is applied to lithium-ion batteries, so that the high-voltage resistance of the lithium-ion batteries is further significantly improved, and the wide-temperature performance of the high-voltage batteries can be comprehensively improved.

[0034] In the present embodiment, the amount of difluoromethylthiobenzenesulfonate is not specifically limited within the scope of not impairing the effects of the present embodiment.

[0035] As a preferred embodiment of this embodiment, the amount of difluoromethylthiobenzenesulfonate used is 0.1-1.0%, based on the total mass of the non-aqueous electrolyte.

[0036] For example, the amount of the above-mentioned difluoromethylthiobenzenesulfonate in the electrolyte includes but is not limited to: 0.1%, 0.11%, 0.14%, 0.18%, 0.2%, 0.24%, 0.26%, 0.28%, 0.3%, 0.33%, 0.34%, 0.38%, 0.4%, 0.43%, 0.46%, 0.48%, 0.49%, 0.5%, 0.51%, 0.52%, 0.54%, 0.55%, 0.58%, 0.6%, 0.64%, 0.66%, 0.68%, 0.7%, 0.73%, 0.74%, 0.78%, 0.8%, 0.83%, 0.86%, 0.88%, 0.9%, 0.93%, 0.95%, 0.97%, 0.99%, and 1.0%.

[0037] lithium salts The lithium salt is an essential component of the non-aqueous electrolyte and can be selected from commonly used salts suitable for lithium-ion batteries. Specifically, the lithium salt is selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(oxalatoborate), lithium difluorooxalatophosphate, lithium difluorooxalatoborate, lithium tetrafluorooxalatophosphate, and lithium bis(fluorosulfonyl)imide.

[0038] When multiple lithium salts are used, their contents are not particularly limited as long as they do not significantly impair the effects of this embodiment.

[0039] For example, in the present invention, the content of the lithium salt is 5-25%, preferably 5-20%, more preferably 10-20%, and even more preferably 12-18%, based on the total mass of the electrolyte.

[0040] For example, the amount of the above lithium salt in the electrolyte includes but is not limited to: 5%, 5.1%, 5.2%, 5.5%, 5.8%, 6%, 6.4%, 7%, 7.3%, 7.4%, 7.8%, 8%, 8.3%, 8.6%, 9%, 9.3%, 9.5%, 10%, 13%, 13.5%, 14%, 14.5%, 15%, 15.5%, 16%, 16.5%, 17%, 17.2%, 17.6%, 18%, 18.6%, 18.8%, 19%, 19.2%, 19.5%, 19.8%, 20%, 21%, 22%, 23%, 24%, and 25%.

[0041] non-aqueous solvents The electrolyte of this embodiment is the same as a general non-aqueous electrolyte, and generally contains a non-aqueous solvent for dissolving the above-mentioned lithium salt as its main component. The non-aqueous solvent used herein is not particularly limited, and known organic solvents can be used. As the organic solvent, preferably, saturated cyclic carbonates, linear carbonates, linear carboxylates, cyclic carboxylates, etc. are selected, but are not particularly limited to the above types. The non-aqueous solvent can be used alone or in combination of two or more solvents.

[0042] Specifically, as a preferred embodiment of the present invention, the non-aqueous solvent is selected from at least one of propylene carbonate, ethyl methyl carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, fluoroethylene carbonate, γ-butyrolactone, methyl formate, ethyl formate, propyl formate, butyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate, butyl butyrate, difluoroethyl acetate, and ethyl 2,2,2-trifluoroacetate.

[0043] In addition to the lithium salt and additives in the electrolyte, the balance is the non-aqueous solvent content.

[0044] It should be understood that, in the electrolyte of the present invention, except for the lithium salt and additives, the remaining components are all non-aqueous solvents. Therefore, after determining the amounts of lithium salt and additives, the content of non-aqueous solvent can also be expressed as the remainder.

[0045] Other additives In addition to the various components listed above, the electrolyte of the present invention may reasonably use other functional additives within the scope that does not significantly impair the effects of the present invention, for example, at least one of 1,3-propane sultone, fluoroethylene carbonate, fluoromethylethylene carbonate, trimethyl phosphate, triethyl phosphate, triphenyl phosphate, triphenyl phosphite, tris(trimethylsilyl)borate, tris(trimethylsilyl)phosphate, and dimethyl sulfoxide.

[0046] In the present invention, the content of the other additives is 0.5-5%, preferably 0.8-4%, more preferably 1-3%, based on the total mass of the non-aqueous electrolyte.

[0047] For example, the amount of the above-mentioned other additives in the electrolyte includes but is not limited to: 0.5%, 0.54%, 0.55%, 0.58%, 0.6%, 0.64%, 0.68%, 0.7%, 0.74%, 0.78%, 0.8%, 0.86%, 0.88%, 0.9%, 0.93%, 0.95%, 1.0%, 1.3%, 1.6%, 1.65%, 1.7%, 1.8%, 1.88%, 1.9%, 1.97%, 2.0%, 2.3%, 2.5%, 2.7%, 2.8%, 2.9%, 3.0%, 3.5%, 3.8%, 4%, 4.5%, 4.8% or 5%.

[0048] Next, the method for preparing the non-aqueous electrolyte in this embodiment will be described.

[0049] Preparation method The electrolyte of the embodiment of the present invention can be prepared by a preparation method known in the art, for example: In an argon atmosphere glove box with a water content of <10 ppm, non-aqueous organic solvents are mixed in proportion, and then fully dried lithium salt is dissolved in the mixed non-aqueous organic solvents. Then, additives are added and mixed evenly to obtain an electrolyte.

[0050] The embodiment of the present invention does not impose any particular limitation on the preparation method of the electrolyte.

[0051] Next, the lithium ion secondary battery of this embodiment will be described.

[0052] lithium-ion secondary batteries The lithium-ion secondary battery comprises a positive electrode, a negative electrode, a separator and the non-aqueous electrolyte.

[0053] In some embodiments of the present invention, the positive electrode material may be selected from A 1+a (Ni x Co y M 1-x-y )O2、A(Ni p Mn q Co 2-p-q )O4 and A k M h (PO4) m One or more of the following; wherein 0≤a≤0.3, 0≤x≤1, 0≤y≤1, 0<x+y≤1; 0≤p≤2, 0≤q≤2, 0<p+q≤2; 0<h<5, 0<k<5, 0<m<5; A is Li, and M is Fe, Ni, Co, Mn, Al or V.

[0054] In some embodiments of the present invention, the negative electrode material may be selected from at least one of metallic lithium, lithium alloy, carbon, hard carbon, silicon-based negative electrode materials, and tin-based negative electrode materials.

[0055] It is understood that in order to prevent short circuits, a separator is usually provided between the positive electrode and the negative electrode. The present invention does not particularly limit the material or shape of the separator, and any known separator may be used as long as it does not significantly impair the effects of the present invention. Among them, resins, glass fibers, inorganic substances, etc. formed of materials that are stable to the non-aqueous electrolyte of this embodiment can be used, and preferably, materials in the form of porous sheets or non-woven fabrics with excellent liquid retention properties are used.

[0056] For example, polyolefins such as polyethylene and polypropylene, polytetrafluoroethylene, etc. can be used.

[0057] The present invention will be further described below with reference to the following examples. It should be understood that these examples are only intended to illustrate the present invention and are not intended to limit the scope of the present invention.

[0058] In the following examples, all materials, reagents and instruments used can be purchased from commercial sources unless otherwise specified. Examples 1-8 and Comparative Examples 1-10

[0059] The lithium ion secondary batteries of Examples 1-8 and Comparative Examples 1-10 were all prepared according to the following method.

[0060] (1) Preparation of positive electrode sheet The lithium nickel cobalt manganese oxide ternary material LiNi 0.6 Co 0.2 Mn 0.2 O2, conductive agent SuperP, adhesive PVDF and carbon nanotubes (CNT) are mixed in a mass ratio of 97.5:1.5:1:1, and N-methylpyrrolidone (NMP) is added. The mixture is stirred in a vacuum mixer to form a uniform lithium-ion battery positive electrode slurry. The positive electrode slurry is evenly coated on an aluminum foil with a thickness of about 12μm and dried in a blast oven at 120°C. It is then cold pressed and die-cut to form a positive electrode sheet.

[0061] (2) Preparation of negative electrode sheet Graphite was mixed with conductive agent SuperP, thickener CMC, and adhesive SBR (styrene-butadiene rubber latex) in a mass ratio of 95:1.5:1.0:2.5, deionized water was added, and the mixture was evenly mixed under the action of a vacuum mixer. The mixed slurry was evenly coated on a copper foil with a thickness of about 12 μm, and dried in a blast oven at 120°C. Then, the negative electrode sheet was obtained by cold pressing and slitting.

[0062] (3) Preparation of electrolyte In an argon atmosphere glove box with a water content of <10 ppm, ethylene carbonate (EC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC) are mixed in a mass ratio of EC:DEC:EMC of 1:1:1, and then the fully dried lithium salt LiPF6 is dissolved in the mixed organic solvent. Then, the first additive, the second additive (if any) and the third additive (if any) are added and mixed evenly to obtain an electrolyte.

[0063] The content of LiPF6 is 15% of the total weight of the electrolyte. The specific types and contents of the first additive, the second additive, and the third additive used in the electrolyte are shown in Table 1.

[0064] In the electrolyte of the present invention, except for the lithium salt LiPF6 and the additives, the remaining components are all non-aqueous solvents.

[0065] In Table 1, the contents of the first additive, the second additive (if any), and the third additive (if any) are expressed in mass percentage (%) calculated based on the total mass of the electrolyte.

[0066] (4) Preparation of isolation membrane A polyethylene film with a thickness of 15 μm (purchased from Celgard) was selected.

[0067] (5) Preparation of lithium-ion secondary batteries The positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator placed between the positive and negative electrode sheets to serve as an isolation. The bare battery cell is then wound and placed in an aluminum-plastic film shell. The electrolyte is injected and sealed. After standing, forming, final sealing, and capacity separation, a lithium-ion secondary battery is obtained.

[0068] In Table 1 below, the first additive is lithium pyromellitic acid, the second additive is a bicyclic sulfate compound, and the third additive is difluoromethylthiobenzenesulfonate.

[0069] Battery performance test

[0070] The lithium-ion batteries prepared in the above examples and comparative examples were subjected to the following performance tests, and the test results shown in Table 2 were obtained.

[0071] (1) Initial effectiveness test The battery was charged at 45°C at a constant current of 0.1C to 4.2V. The first charge capacity was recorded as Q1. After aging at 45°C and secondary sealing, the battery was charged at room temperature (25°C) at a constant current of 0.1C to 4.2V. Then, the battery was charged at a constant voltage of 4.2V to a cutoff current of 0.05C. The second charge capacity was recorded as Q2. The battery was then discharged at 0.2C to 2.75V. The first discharge capacity was recorded as Q3. The battery's first efficiency = Q3 / (Q1+Q2)*100%.

[0072] (2) High temperature cycle performance test The lithium-ion battery was charged at a constant current of 1C to 4.2V in a constant temperature box at 45°C, and then charged at a constant voltage of 4.2V to a cut-off current of 0.05C. The battery was then discharged at 1C to 3.0V. After 1000 charge / discharge cycles, the high-temperature capacity retention rate of the lithium-ion battery was measured. (3) Low temperature cycle performance test The lithium-ion battery was placed in a constant temperature box at -20°C, charged to 4.2V at a constant current and constant voltage of 0.2C, and then discharged to 3.0V at a constant current of 0.5C. After 200 charge / discharge cycles, the capacity retention rate of the lithium-ion battery was measured. The calculation formula for the capacity retention rate of the battery after the Nth cycle is: Capacity retention rate after the Nth cycle = discharge capacity after the Nth cycle / discharge capacity after the first cycle × 100%; In the high-temperature cycle and low-temperature cycle performance tests, three batteries were tested in parallel for each performance test and the average value was taken as the test result. The absolute deviation of the test was within ±0.5%.

[0073] (4) High temperature storage performance test The lithium-ion battery is charged at room temperature 25℃ with a constant current of 1C to 4.2V, and at a constant voltage of 4.2V to a cut-off current of 0.05C, and then the battery is discharged, and the discharge capacity is recorded as C1; at room temperature 25℃, it is charged at a constant current of 1C to 4.2V, and at a constant voltage of 4.42 to a cut-off current of 0.05C, and then the battery is transferred to a high temperature of 60℃ and left for 7 days, and then discharged at a constant current of 0.5C, and the discharge capacity is recorded as C2; 60℃ capacity retention rate = C2 / C1*100%.

[0074]

[0075] According to the test results in Table 2, Examples 1-5, using lithium pyromellitic acid and a bicyclic sulfate compound (Compound 1) as electrolyte additives, significantly improved the initial efficiency, subsequent high- and low-temperature cycling performance, and high-temperature storage performance of lithium-ion batteries. This indicates that lithium pyromellitic acid can combine with and modify the film formed by the bicyclic sulfate compound, thereby further improving the film's stability and lithium ion conductivity.

[0076] Specifically, Examples 6-8 further use difluoromethylthiobenzenesulfonate as an additive based on Example 3, which can further significantly improve the initial efficiency, high and low temperature cycle performance, and high temperature storage performance of the high-voltage battery. The principle is as follows: the benzene structure in difluoromethylthiobenzenesulfonate contains more hydrogen, which can form hydrogen bonds with the film components formed by lithium phenyltetracarboxylate and bicyclic sulfate compounds, thereby forming a more elastic interface, which can significantly improve the stability of the film, thereby further suppressing the collision of the positive electrode material under high voltage; at the same time, due to the presence of the strongly electronegative fluorine-containing group in difluoromethylthiobenzenesulfonate, the fluorine-containing group can not only promote the interface stability and lithium ion conduction ability formed by difluoromethylthiobenzenesulfonate, but the fluorine-containing group itself can also improve the interface oxidation resistance and ion conductivity.

[0077] Among them, Comparative Examples 1-3 used lithium pyromellitic acid alone, and Comparative Examples 4-6 used a bicyclic sulfate compound (Compound 1) alone. Compared with Comparative Example 10 (blank control group), the initial efficiency and subsequent high and low temperature cycle performance and high temperature storage performance of the lithium ion battery were improved, but compared with Examples 1-5, the relevant performance was significantly reduced, indicating that the present invention uses lithium pyromellitic acid, which can be combined with a film generated by a bicyclic sulfate compound, and lithium pyromellitic acid can modify the film generated by the bicyclic sulfate compound, thereby further improving the stability of the film and the lithium ion conductivity.

[0078] Further in conjunction with the test result of comparative example 7-9, comparative example 7-9 adopts difluoromethyl thiobenzene sulfonate as additive separately, and it can only improve the performance of lithium ion battery to a limited extent.When difluoromethyl thiobenzene sulfonate is used together with lithium pyromellitic acid and bicyclic sulfate compound (i.e. embodiment 6-8), because the benzene structure in difluoromethyl thiobenzene sulfonate contains more hydrogen, the film component formed with lithium pyromellitic acid and bicyclic sulfate compound forms hydrogen bond, thereby forming a more elastic interface, can significantly improve the stability of film, thereby further suppressing the collision of positive electrode material under high voltage; simultaneously, due to the presence of the fluorine-containing group of strong electronegativity in difluoromethyl thiobenzene sulfonate, the fluorine-containing group can not only promote the interface stability and the ability of conducting lithium ions formed by difluoromethyl thiobenzene sulfonate, but the fluorine-containing group itself can also improve interface oxidation resistance and ionic conductivity. Therefore, the lithium ion battery of embodiment 6-8 possesses more significant electrochemical performance. This indicates that the difluoromethylthiobenzenesulfonate additive in the present invention is not a conventional addition based on lithium pyromellitic acid and bicyclic sulfate compounds.

[0079] In summary, this embodiment uses lithium pyromellitic acid and bicyclic sulfate compounds as electrolyte additives. Among them, the bicyclic sulfate compound can form a stable film on the surface of the positive and negative electrodes due to the high-valent sulfur element; secondly, lithium pyromellitic acid can combine with the film generated by the bicyclic sulfate compound, and lithium pyromellitic acid can modify the film generated by the bicyclic sulfate compound, which can improve the stability of the film and the lithium ion conductivity, thereby improving the initial efficiency of the lithium-ion battery at high voltage and the subsequent high and low temperature cycle performance and high temperature storage performance. At the same time, on the basis of lithium pyromellitic acid and bicyclic sulfate compounds, the use of difluoromethylthiobenzenesulfonate can further improve the electrochemical properties of lithium ions.

[0080] In Examples 1-8 of the present invention, the lithium salt listed is LiPF6. The inventors have found that when the additives or additive combinations of Examples 1-8 are used, when the lithium salt is one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(oxalatoborate), lithium difluorooxalatophosphate, lithium difluorooxalatoborate, lithium tetrafluorooxalatophosphate, and lithium bis(fluorosulfonyl)imide, the battery's initial efficiency, high-temperature cycle performance, low-temperature cycle performance, and high-temperature storage performance are significantly improved compared to the comparative example without the additive, and the same or similar technical effects as adding LiPF6 as the lithium salt can be achieved.

[0081] In Examples 1-8 of the present invention, we listed compound 1 as the second additive. The inventors found that in some embodiments of the present invention, the second additive is replaced by compounds 2, 3, 4, 5, 6 and other compounds listed in Formula 1, and the addition amount is 1-5%. The first effect, high temperature cycle performance, low temperature cycle performance and high temperature storage performance of the battery are also significantly improved compared with the control ratio without additives, and the same or similar technical effects as adding compound 1 as the second additive can be achieved. And similarly, on the basis of lithium phenyltetracarboxylate and bicyclic sulfate compounds 2, 3, 4, 5, 6 and other bicyclic sulfate compounds listed in Formula 1, the use of difluoromethylthiobenzenesulfonate can further improve the electrochemical properties of lithium ions.

[0082] The technical solutions provided by the present invention are introduced in detail above. Specific examples are used herein to illustrate the principles and implementation methods of the embodiments of the present invention. The description of the above embodiments is only applicable to help understand the principles of the embodiments of the present invention. At the same time, for those skilled in the art, according to the embodiments of the present invention, there may be changes in the specific implementation methods and application scopes. In summary, the contents of this specification should not be understood as limiting the present invention.

Claims

1. A non-aqueous electrolyte, characterized in that The non-aqueous electrolyte comprises: lithium salt, non-aqueous solvent, a first additive of lithium phenyltetracarboxylate, and a second additive, The chemical formula of the bicyclic sulfate compound of the structure shown in Formula 1 is: ， Formula 1; Wherein, R1 is selected from an alkyl group, a cycloalkyl group, a phenyl group containing 2 to 8 carbon atoms, or a connecting group of a combination of the above groups.

2. The non-aqueous electrolyte according to claim 1, characterized in that R1 is a connecting group having a structure shown in Formula 2; ， Formula 2; Wherein, R2 is selected from ； p is an integer from 0 to 6, n is an integer from 0 to 2, and m is an integer from 0 to 2; R3, R4, R5, and R6 are each independently selected from a single bond, a methylene group, or an ethylene group.

3. The non-aqueous electrolyte according to claim 1 or 2, characterized in that The bicyclic sulfate compound is selected from at least one of the following compounds: 。 4. The non-aqueous electrolyte according to claim 1, wherein The content of the bicyclic sulfate ester compound is 0.1 to 5% based on the total mass of the non-aqueous electrolyte.

5. The non-aqueous electrolyte according to claim 1, characterized in that The content of the lithium pyromellitic acid salt is 0.1-0.5%, based on the total mass of the non-aqueous electrolyte.

6. The non-aqueous electrolyte according to claim 1, wherein The non-aqueous electrolyte further includes a third additive, difluoromethylthiobenzenesulfonate.

7. The non-aqueous electrolyte according to claim 6, characterized in that The content of the difluoromethylthiobenzenesulfonate is 0.1 to 1.0%, based on the total mass of the non-aqueous electrolyte.

8. The non-aqueous electrolyte according to claim 1, wherein The non-aqueous solvent is selected from at least one of propylene carbonate, ethyl methyl carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, fluoroethylene carbonate, γ-butyrolactone, methyl formate, ethyl formate, propyl formate, butyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate, butyl butyrate, difluoroethyl acetate, and ethyl 2,2,2-trifluoroacetate; except for the content of lithium salt and additives in the electrolyte, the balance is the content of the non-aqueous solvent.

9. The non-aqueous electrolyte according to claim 1, characterized in that The lithium salt is selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(oxalatoborate), lithium difluorooxalatophosphate, lithium difluorooxalatoborate, lithium tetrafluorooxalatophosphate, and lithium bis(fluorosulfonyl)imide; The content of the lithium salt is 5-20%, based on the total mass of the non-aqueous electrolyte.

10. A lithium ion secondary battery, characterized in that: The lithium-ion secondary battery comprises: positive electrode; negative electrode; A separator; and the non-aqueous electrolyte according to any one of claims 1 to 9.

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