Electrolyte and preparation method therefor, and secondary battery

Abstract

The present application relates to the technical field of secondary batteries. Disclosed are an electrolyte and a secondary battery. The electrolyte comprises a lithium salt, an ether solvent, a fluorinated solvent and an ether-based functionalized ionic liquid. When the electrolyte provided in the embodiments of the present application is applied to a lithium metal secondary battery, uniform deposition of lithium can be effectively regulated and controlled to generate a stable SEI film, interface charge transfer in a low-temperature environment is improved, and the service life of the secondary battery in a low-temperature environment is prolonged.

Description

Electrolyte and preparation method, secondary battery

[0001] This application claims priority to the Chinese patent application No. 202410982913.1, filed on July 22, 2024, with the Chinese Patent Office, the content of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0002] The present application relates to the field of batteries, in particular to an electrolyte and a preparation method thereof, and a secondary battery with the electrolyte. BACKGROUND

[0003] Lithium metal has the lowest electrochemical potential (-3.04 V vs. SHE), the highest theoretical specific capacity (3860 mAh·g -1 ), and the lowest density (0.534 g·cm -3 ), and is an ideal anode for the next generation of high-energy-density batteries.

[0004] However, it has encountered great obstacles in practical applications: on the one hand, lithium metal cannot form a stable SEI film in commercial electrolyte (such as traditional carbonate electrolyte 1 mol·L -1 LiPF6-EC / DEC), and non-uniform lithium deposition leads to continuous growth of dendrites and dead lithium, which continuously consumes electrolyte and active lithium, resulting in low coulombic efficiency and cycle life. In addition, the melting point of EC is 35-38℃, and the viscosity and conductivity of the electrolyte are high at low temperatures, which significantly reduces the performance of the battery and hinders the application of the battery in cold regions. TECHNICAL PROBLEM TECHNICAL SOLUTION

[0005] Therefore, the present application provides an electrolyte and a preparation method thereof, and a secondary battery, which can improve the service life of the secondary battery in a low-temperature environment.

[0006] In a first aspect, the present application provides an electrolyte for a secondary battery, which comprises a lithium salt, an ether solvent, a fluorinated solvent, and an ether-functionalized ionic liquid, the ether-functionalized ionic liquid comprising an anion and a cation, the cation comprising at least one of an ion represented by Formula I, an ion represented by Formula II, an ion represented by Formula III, or an ion represented by Formula IV:

[0007] wherein R1, R2, R3, R4, R5, R6, R7, R8, R9, R 10 , R 11 , R 12 , R 13 , R 14 are each independently selected from substituted or unsubstituted C1 to C 10 alkyl.

[0008] In some embodiments of the application, R1, R2, R4, R5, R7, R8, R 10 11 13 are each independently selected from any one of methyl, ethyl, propyl, butyl, pentyl, hexyl; and / or

[0009] R3, R6, R9, R 12 14 are each independently selected from any one of methyl, ethyl; and / or

[0010] the anion is selected from at least one of a bisfluorosulfonylimide ion, a bis(trifluoromethylsulfonyl)imide, a trifluoromethanesulfonate, a dicyanamide, a tetrafluoroborate, a hexafluorophosphate, a perchlorate, a dihydrogen bis(oxalato)borate, or a difluoro hydrogen bis(oxalato)borate.

[0011] In some embodiments of the application, the ether-functionalized ionic liquid is selected from at least one of N-methoxypropyl-N-methylpyrrolidinium hexafluorophosphate, (methoxybutyl)trimethylammonium bis(trifluoromethanesulfonyl)imide, N-methoxyethyl-N-ethylpiperidinium tetrafluoroborate, N-methoxyethyl-N-methylpyrrolidinium tetrafluoroborate, methyl(2-methoxyethyl)diethylammonium bis(fluorosulfonyl)imide, (methoxyethyl)trimethylammonium bis(fluorosulfonyl)imide, N-methoxybutyl-N-methylpiperidinium nitrate, 1-methoxyethyl-3-methylimidazolium tetrafluoroborate, 1-ethoxyethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide.

[0012] In some embodiments of the application, the mass ratio of the lithium salt to the electrolyte is 15% to 30%; and / or

[0013] the mass ratio of the ether solvent to the electrolyte is 5% to 30%; and / or

[0014] the mass ratio of the fluorinated solvent to the electrolyte is 30% to 75%; and / or

[0015] the mass ratio of the ether-functionalized ionic liquid to the electrolyte is 5% to 25%.

[0016] In some embodiments of the application, the mass ratio of the lithium salt to the electrolyte is 15% to 20%; and / or

[0017] the mass ratio of the ether solvent to the electrolyte is 10% to 20%; and / or

[0018] the mass ratio of the fluorinated solvent to the electrolyte is 50% to 65%; and / or

[0019] ​​​The mass ratio of the ether functionalized ionic liquid to the electrolyte is 5% to 15%.

[0020] In some embodiments of the present application, the mass ratio of the lithium salt to the electrolyte is 15% to 20%;

[0021] The mass ratio of the ether functionalized ionic liquid to the electrolyte is 5% to 15%;

[0022] The mass ratio of the ether solvent to the electrolyte is 10% to 20%;

[0023] The mass ratio of the fluorinated solvent to the electrolyte is 50% to 65%;

[0024] The sum of the mass of the ether solvent and the fluorinated solvent to the mass of the electrolyte is 70% to 75%.

[0025] In some embodiments of the present application, the lithium salt is selected from one or more of lithium difluoro(oxalato)borate (LiDFOB), lithium difluorophosphate (LiDFOP), lithium nitrate (LiNO3), lithium bis(oxalato)borate (LiBOB), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium difluorophosphate, lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium 2,2,2-(trifluoromethyl)sulfonyl-N-cyanamide (LiTFSAM); and / or

[0026] The ether solvent is selected from one or more of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, diethoxymethane, bis(2-methoxyethyl) ether, 1,4-dioxane, 1,3-dioxolane.

[0027] In some embodiments of the present application, the lithium salt is LiDFOB, the ether functionalized ionic liquid is N-methoxyethyl-N-ethylpiperidinium tetrafluoroborate, and the ether solvent is 1,4-dioxane.

[0028] Another aspect of the present application provides a secondary battery, which comprises a positive electrode sheet, a negative electrode sheet, and the electrolyte.

[0029] In some embodiments of the present application, the negative electrode sheet comprises lithium metal. Advantages

[0030] When the electrolyte provided by the embodiments of the present application is used in a lithium metal secondary battery, it can effectively regulate the uniform deposition of lithium to generate a stable SEI film, and improve the interface charge transfer in a low temperature environment, thereby improving the service life of the secondary battery in a low temperature environment. Embodiments of the present application

[0031] The technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments of the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of the present application. In addition, it should be understood that the specific implementations described herein are only for the purpose of illustration and explanation of the present application, and are not intended to limit the present application.

[0032] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description herein is for describing particular embodiments only and is not intended to be limiting of the application. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items.

[0033] In the present application, the orientation words such as "upper" and "lower" are generally used to refer to the upper and lower in the actual use or working state of the device, and "inner" and "outer" are used in relation to the outline of the device. In addition, in the description of the present application, the term "comprising" means "including but not limited to". The terms first, second, third, etc. are only used as labels and do not impose numerical requirements or establish an order.

[0034] In the present application, "and / or" describes the association relationship of the associated objects, which means that there can be three kinds of relationships, for example, A and / or B, which can represent the following cases: A exists alone, A and B exist together, and B exists alone. Wherein A and B can be singular or plural.

[0035] In the present application, "at least one" means one or more, and "multiple" means two or more. "One or more", "at least one of the following" or similar expressions mean any combination of these items, including any combination of single item or multiple items. For example, "at least one of a, b, or c", or "at least one of a, b, and c", can represent a, b, c, a-b (i.e. a and b), a-c, b-c, or a-b-c, where a, b, and c can be single or multiple.

[0036] Various embodiments of the present application can exist in a range of forms; it should be understood that the description in a range form is merely for the convenience and brevity, and should not be understood as a hard limitation on the scope of the present application; therefore, it should be considered that the range described has specifically disclosed all possible sub-ranges and single values within the range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. In addition, whenever a numerical range is indicated herein, it refers to any cited number (fraction or integer) within the indicated range.

[0037] At present, electrochemical devices (for example, lithium metal secondary batteries) have been widely applied in new energy electric vehicles, electronic products such as cameras, digital cameras, and 3C products, etc. due to their high energy density, high working voltage, long service life, green environmental protection, etc.

[0038] Lithium metal secondary batteries generally include a positive electrode, a negative electrode, a separator, and an electrolyte. As an important component, the electrolyte plays a role in transferring lithium ions between the positive and negative electrodes. At present, the electrolyte on the market cannot form a stable SEI film on the lithium metal negative electrode, causing low coulombic efficiency and cycle life. At the same time, the electrolyte has low conductivity in a low-temperature environment, resulting in significant performance degradation of the lithium metal secondary battery, hindering the application of the battery in cold regions.

[0039] In view of this, the embodiments of the present application provide an electrolyte for improving the performance of a lithium metal secondary battery in a low-temperature environment. The electrolyte and the lithium metal secondary battery in the present application are further described below in combination with embodiments.

[0040] The embodiments of the present application provide an electrolyte, which comprises: a lithium salt, an ether solvent, a fluorinated solvent, and an ether-functionalized ionic liquid, the ether-functionalized ionic liquid comprising an anion and a cation, the cation comprising at least one of an ion represented by Formula I, an ion represented by Formula II, an ion represented by Formula III, or an ion represented by Formula IV:

[0041] wherein R1, R2, R3, R4, R5, R6, R7, R8, R9, R 10 , R 11 , R 12 , R 13 , R 14 each independently is selected from substituted or unsubstituted C1 to C 10 alkyl.

[0042] It should be noted that C1 to C 10Alkyl refers to an alkyl group having 1 to 10 carbon atoms. Illustratively, C1to C10alkyl groups include methyl, ethyl, propyl, isopropyl, butyl, and the like. 10 Alkyl includes methyl, ethyl, propyl, isopropyl, butyl, and the like.

[0043] In some embodiments of the present application, R1, R2, R4, R5, R7, R8, R 10 , R 11 , R 13 each independently is selected from any one of methyl, ethyl, propyl, butyl, pentyl, hexyl.

[0044] In some embodiments of the present application, R3, R6, R9, R 12 , R 14 each independently is selected from any one of methyl, ethyl.

[0045] In some embodiments of the present application, the anion is selected from at least one of bisfluorosulfonylimide, bis(trifluoromethylsulfonyl)imide, trifluoromethanesulfonate, dicyanamide, tetrafluoroborate, hexafluorophosphate, perchlorate, dihydrogen bis(oxalato)borate, or difluoro hydrogen bis(oxalato)borate.

[0046] In some embodiments of the present application, the ether-functionalized ionic liquid is selected from at least one of N-methoxypropyl-N-methylpyrrolidinium hexafluorophosphate (Py1(30i)PF6), (methoxybutyl)trimethylammonium bis(trifluoromethanesulfonyl)imide (N(40i)niTFSI), N-methoxyethyl-N-ethylpiperidinium tetrafluoroborate (PP2(20i)BF4), N-methoxyethyl-N-methylpyrrolidinium tetrafluoroborate (Py1(20i)BF4), methyl(2-methoxyethyl)diethylammonium bis(fluorosulfonyl)imide (N(20i)33iFSI), (methoxyethyl)trimethylammonium bis(fluorosulfonyl)imide (N(20i)niFSI), N-methoxybutyl-N-methylpiperidinium nitrate (PP1(40i)NO3), 1-methoxyethyl-3-methylimidazolium tetrafluoroborate (EOMMImBF4), 1-ethoxyethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EOEMImTFSI). It is noted that the ether-functionalized ionic liquid in the present application can be purchased directly or obtained according to the prior art, which is not limited herein.

[0047] In some embodiments of the present application, the ether-functionalized ionic liquid has a mass fraction of 5% to 25% in the electrolyte. Illustratively, the ether-functionalized ionic liquid has a mass fraction of about 5%, about 10%, about 15%, about 20%, about 25%, and a value between any two values in the electrolyte. In the present application, the term "about" is used to describe and illustrate a small change. When used in conjunction with a number, it can refer to a change range of less than or equal to ±10% of the corresponding numerical value.

[0048] In some embodiments of the application, the lithium salt is selected from one or more of lithium difluoro(oxalato)borate (LiDFOB), lithium difluorophosphate (LiDFOP), lithium nitrate (LiNO3), lithium bis(oxalato)borate (LiBOB), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium difluorophosphate, lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium 2,2,2-(trifluoromethyl)sulfonyl-N-cyanamide (LiTFSAM).

[0049] In some embodiments of the application, the lithium salt is present in the electrolyte in a mass fraction of 15% to 30%. Illustratively, the lithium salt is present in the electrolyte in a mass fraction of about 15%, about 20%, about 25%, about 30%, and values between any two of these values.

[0050] In some embodiments of the application, the ether solvent (i.e., ether non-aqueous organic solvent) is selected from one or more of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, diethoxymethane, bis(2-methoxyethyl) ether, 1,4-dioxane, 1,3-dioxolane.

[0051] In some embodiments of the application, the ether solvent is present in the electrolyte in a mass fraction of 5% to 30%. Illustratively, the ether solvent is present in the electrolyte in a mass fraction of about 5%, about 10%, about 15%, about 20%, about 25%, about 30%, and values between any two of these values.

[0052] In some embodiments of the application, the fluorinated solvent is selected from one or more of fluorinated carbonates, fluorinated ethers, fluorobenzenes, fluorinated nitrile solvents.

[0053] Illustratively, the fluorinated solvent is selected from at least one of one or more of 1,1,2,2-tetrafluoroethyl ether, 1,1,2,2-tetrafluoroethyl ethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 1,1,2,3,3,3-hexafluoropropyl-2,2,2-trifluoroethyl ether, 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether, bis(2,2,3,3-tetrafluoropropyl) ether, tetrahydrofuran, fluorobenzene, 1,2-difluorobenzene, 1,3,5-trifluorobenzene, tris(2-fluoroethyl) borate, tris(2,2,2-trifluoroethyl) formate, bis(2,2,2-trifluoroethyl) carbonate.

[0054] In some embodiments of the present application, the fluorinated solvent has a mass fraction of 30% to 75% in the electrolyte. For example, the lithium salt has a mass fraction of about 30%, about 40%, about 50%, about 60%, about 70%, about 75%, and any value between any two values.

[0055] In some embodiments of the present application, the electrolyte comprises, by mass fraction:

[0056] lithium salt, 15% to 30%;

[0057] ether solvent, 5% to 30%;

[0058] fluorinated solvent, 30% to 75%;

[0059] ether functionalized ionic liquid, 5% to 25%.

[0060] In some embodiments of the present application, the lithium salt, the ether non-aqueous organic solvent, and the ether functionalized ionic liquid form a high-concentration system (e.g., lithium ion concentration > 5 mol / L). In the high-concentration solvation structure, the anions are more distributed in the inner layer of the solvation sheath than the solvent molecules, and the LUMO energy level is lower than that of the solvent molecules. The SEI film with high content of inorganic components LiF and Li3N is preferentially reduced to form, which has high stability, mechanical modulus, and lithium ion conductivity, effectively improving the cycle stability of the lithium negative electrode and promoting the transmission of charges in the SEI film at low temperatures.

[0061] In some embodiments of the present application, the ether solvent has the effect of dissociating the lithium salt to improve the conductivity of the electrolyte. The ether solvent has a low dielectric constant and a low melting point, which is beneficial to the desolvation process of Li + , reduces the interfacial transfer impedance, and improves the cycle performance of the lithium negative electrode and the charge transfer at the electrode / electrolyte interface at low temperatures.

[0062] In some embodiments of the present application, the ether functionalized ionic liquid has the effect of regulating uniform lithium deposition. The cation has a reduction potential lower than that of lithium ions, and competes with lithium ions for adsorption around the dendrites due to the large steric hindrance, thereby inhibiting dendrite growth. The anion participates in the formation of inorganic SEI film, and the synergistic effect of the anion and the cation improves the cycle stability of the lithium negative electrode. At the same time, the ether group has high flexibility, which can reduce the lattice energy of the ionic liquid, and ether functionalization can significantly reduce the melting point of the ionic liquid to improve the flowability of the ionic liquid in a low-temperature environment.

[0063] In the embodiments of the present application, the fluorinated solvent has low melting point, low viscosity and good wettability, and does not participate in the solvation of lithium ions, does not change the solvation structure of the high-concentration electrolyte, and plays a role in reducing the viscosity of the high-concentration electrolyte and the ionic liquid. The F atom has an electron-withdrawing effect, which can weaken the coordination between the ether-based solvent and the lithium ion, further accelerate the desolvation process of the lithium ion, reduce the interfacial impedance, and improve the cycle performance of the lithium anode and the low-temperature transmission kinetics of the lithium ion.

[0064] The electrolyte provided by the embodiments of the present application can effectively regulate the uniform deposition of lithium to form a stable SEI film, improve the interfacial charge transfer in a low-temperature environment, and improve the service life of the secondary battery in a low-temperature environment when used in a lithium metal secondary battery.

[0065] The embodiments of the present application also provide a preparation method of the electrolyte, which comprises the following steps,

[0066] (1) Under an inert atmosphere (H2O < 0.01 ppm, O2 < 0.01 ppm), a lithium salt, an ether-functionalized ionic liquid and an ether-based solvent with a predetermined ratio are mixed to obtain a pre-mixed liquid. For example, the lithium salt, the ether-functionalized ionic liquid and the ether-based solvent are stirred and mixed uniformly at 10-50°C.

[0067] (2) A predetermined amount of fluorinated solvent is added to the pre-mixed liquid to obtain the lithium metal secondary battery electrolyte. For example, the fluorinated solvent is added to the pre-mixed liquid at 10-50°C, and the electrolyte is obtained after stirring and mixing uniformly.

[0068] It can be understood that, since the fluorinated solvent has weak solubility for lithium salt and can be mutually soluble with the solvent, it is necessary to be added after the lithium salt and the ether-functionalized ionic liquid are dissolved in the ether-based solvent, which is conducive to improving the stability of the electrolyte.

[0069] The embodiments of the present application also provide a lithium metal secondary battery, which comprises a positive electrode sheet, a negative electrode sheet, a separator and the electrolyte described above. For example, the negative electrode sheet is lithium metal or lithium alloy.

[0070] In some embodiments of the present application, the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material arranged on the positive electrode current collector. For example, the positive electrode active material comprises lithium-containing phosphates, lithium transition metal oxides and their respective modified compounds. The lithium transition metal oxides include but are not limited to lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide. The lithium-containing phosphates include but are not limited to at least one of lithium iron phosphate, a composite material of lithium iron phosphate and carbon, lithium manganese phosphate, a composite material of lithium manganese phosphate and carbon, lithium manganese iron phosphate, a composite material of lithium manganese iron phosphate and carbon.

[0071] The application also provides an electronic device comprising the above lithium metal secondary battery.

[0072] Hereinafter, the embodiments of the present application will be described. In the embodiments, the specific techniques or conditions not mentioned are performed according to the techniques or conditions described in the literature in the art or according to the product manual. The reagents or instruments not mentioned by the manufacturer are all conventional products that can be commercially available.

[0073] Example 1

[0074] (1) Preparation of electrolyte

[0075] In an argon atmosphere glove box (H2O <0.01 ppm, O2 <0.01 ppm), LiPF6, N-methoxypropyl-N-methylpyrrolidinium hexafluorophosphate (Py1(30 1)PF6), and diethylene glycol dimethyl ether were mixed at 35°C to obtain a solution, and then the above prepared solution was mixed with 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether at 35°C to obtain the electrolyte of Example 1. According to the mass ratio, LiPF6:N-methoxypropyl-N-methylpyrrolidinium hexafluorophosphate:diethylene glycol dimethyl ether:1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether = 30:8:12:50.

[0076] (2) Preparation of positive electrode sheet

[0077] The ternary positive electrode material (for example, NCM811) is the positive electrode.

[0078] (3) Preparation of negative electrode sheet

[0079] The lithium metal is the negative electrode sheet.

[0080] (4) Separator

[0081] The polyethylene film is the separator.

[0082] (5) Preparation of battery

[0083] The positive electrode sheet, the separator, and the negative electrode sheet are stacked in order, and the separator is between the positive electrode and the negative electrode to play a role of isolation. The bare battery cell is placed in the outer package, the prepared electrolyte is injected, and the processes of packaging, liquid injection, formation, and degassing are performed to obtain the lithium ion battery.

[0084] Example 2

[0085] The difference between it and Example 1 is that:

[0086] (1) Preparation of electrolyte

[0087] In an argon atmosphere glove box (H2O < 0.01 ppm, O2 < 0.01 ppm), LiPF6, (methoxybutyl)trimethylammonium bis(trifluoromethanesulfonyl)imide salt (N(4o1)111TFSI), diethylene glycol dimethyl ether were mixed at 35°C to obtain a solution, and then the above prepared solution was mixed with fluorobenzene at 35°C to obtain the electrolyte of Example 2. According to the mass ratio, LiPF6:N(4o1)111TFSI:diethylene glycol dimethyl ether:fluorobenzene = 15:15:15:55.

[0088] Example 3

[0089] The difference from Example 1 is that:

[0090] (1) Preparation of electrolyte

[0091] In an argon atmosphere glove box (H2O < 0.01 ppm, O2 < 0.01 ppm), LiTFSI, N-methoxyethyl-N-ethylpiperidinium tetrafluoroborate (PP2(2o1)BF4), 1,3-dioxolane were mixed at 35°C to obtain a solution, and then the above prepared solution was mixed with bis(2,2,2-trifluoroethyl) carbonate at 35°C to obtain the electrolyte of Example 3. According to the mass ratio, LiTFSI:PP2(2o1)BF4:1,3-dioxolane: bis(2,2,2-trifluoroethyl) carbonate = 20:5:10:65.

[0092] Example 4

[0093] The difference from Example 1 is that:

[0094] (1) Preparation of electrolyte

[0095] In an argon atmosphere glove box (H2O < 0.01 ppm, O2 < 0.01 ppm), LiPF6, N-methoxypropyl-N-methylpyrrolidinium hexafluorophosphate (Py1(3o1)PF6), diethylene glycol dimethyl ether were mixed at 35°C to obtain a solution, and then the above prepared solution was mixed with 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether at 35°C to obtain the electrolyte of Example 4. According to the mass ratio, LiPF6:Py1(3o1)PF6:diethylene glycol dimethyl ether:1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether = 30:25:12:33.

[0096] Example 5

[0097] The difference from Example 1 is that:

[0098] (1) Preparation of electrolyte

[0099] In an argon atmosphere glove box (H2O < 0.01 ppm, O2 < 0.01 ppm), LiPF6, N-methoxypropyl-N-methylpyrrolidinium hexafluorophosphate (Py1(3o1)PF6), diethylene glycol dimethyl ether were mixed at 35°C to obtain a solution, and then the above prepared solution was mixed with 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether at 35°C to obtain the electrolyte of Example 5. LiPF6: Py1(3o1)PF6: diethylene glycol dimethyl ether: 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether = 30:25:15:30 by mass ratio.

[0100] Example 6

[0101] The difference from Example 1 is that:

[0102] (1) Preparation of electrolyte

[0103] In an argon atmosphere glove box (H2O < 0.01 ppm, O2 < 0.01 ppm), LiPF6, N-methoxypropyl-N-methylpyrrolidinium hexafluorophosphate (Py1(3o1)PF6), diethylene glycol dimethyl ether were mixed at 35°C to obtain a solution, and then the above prepared solution was mixed with 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether at 35°C to obtain the electrolyte of Example 6. LiPF6: Py1(3o1)PF6: diethylene glycol dimethyl ether: 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether = 15:5:5:75 by mass ratio.

[0104] Example 7

[0105] The difference from Example 1 is that:

[0106] (1) Preparation of electrolyte

[0107] In an argon atmosphere glove box (H2O < 0.01 ppm, O2 < 0.01 ppm), LiPF6, N-methoxypropyl-N-methylpyrrolidinium hexafluorophosphate (Py1(3o1)PF6), diethylene glycol dimethyl ether were mixed at 35°C to obtain a solution, and then the above prepared solution was mixed with 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether at 35°C to obtain the electrolyte of Example 7. LiPF6: Py1(3o1)PF6: diethylene glycol dimethyl ether: 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether = 30:8:5:75 by mass ratio.

[0108] Example 8

[0109] The difference from Example 1 is that:

[0110] (1) Preparation of electrolyte solution

[0111] In an argon atmosphere glove box (H2O < 0.01 ppm, O2 < 0.01 ppm), LiPF6, N-methoxypropyl-N-methylpyrrolidinium hexafluorophosphate (Py1(3o1)PF6), diethylene glycol dimethyl ether were mixed at 35°C to obtain a solution, and then the above prepared solution was mixed with 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether at 35°C to obtain the electrolyte solution of Example 2. According to the mass ratio, LiPF6: Py1(3o1)PF6: diethylene glycol dimethyl ether: 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether = 30:8:30:32.

[0112] Example 9

[0113] The difference from Example 1 is that:

[0114] (1) Preparation of electrolyte solution

[0115] In an argon atmosphere glove box (H2O < 0.01 ppm, O2 < 0.01 ppm), LiDFOB, N-methoxyethyl-N-methylpyrrolidinium tetrafluoroborate (Py1(2o1)BF4), 1,4-dioxane were mixed at 35°C to obtain a solution, and then the above prepared solution was mixed with (2,2,2-trifluoroethyl) formate at 35°C to obtain the electrolyte solution of Example 9. According to the mass ratio, LiDFOB: Py1(2o1)BF4: 1,4-dioxane: (2,2,2-trifluoroethyl) formate = 15:10:15:60.

[0116] Example 10

[0117] The difference from Example 1 is that:

[0118] (1) Preparation of electrolyte solution

[0119] In an argon atmosphere glove box (H2O < 0.01 ppm, O2 < 0.01 ppm), LiBOB, methyl(2-methoxyethyl)diethylammonium bis(fluorosulfonyl)imide (N(2o1)111FSI), ethylene glycol dimethyl ether were mixed at 35°C to obtain a solution, and then the above prepared solution was mixed with (2,2,2-trifluoroethyl) formate at 35°C to obtain the electrolyte solution of Example 10. According to the mass ratio, LiBOB: N(2o1)111FSI: ethylene glycol dimethyl ether: (2,2,2-trifluoroethyl) formate = 17:10:15:58.

[0120] Example 11

[0121] The difference from Example 1 is that:

[0122] (1) Preparation of electrolyte

[0123] In an argon atmosphere glove box (H2O < 0.01 ppm, O2 < 0.01 ppm), LiDFOB, N-methoxybutyl-N-methylpiperidinium nitrate (PP1(4o1)NO3), tetraethyleneglycol dimethyl ether were mixed at 35°C to obtain a solution, and then the above prepared solution was mixed with fluorobenzene at 35°C to obtain the electrolyte of Example 11. According to the mass ratio, LiDFOB:PP1(4o1)NO3: tetraethyleneglycol dimethyl ether: fluorobenzene = 20:5:20:55.

[0124] Example 12

[0125] The difference from Example 1 is that:

[0126] (1) Preparation of electrolyte

[0127] In an argon atmosphere glove box (H2O < 0.01 ppm, O2 < 0.01 ppm), LiFSI, 1-methoxyethyl-3-methylimidazolium tetrafluoroborate (EOMMImBF4), ethyleneglycol dimethyl ether were mixed at 35°C to obtain a solution, and then the above prepared solution was mixed with fluorobenzene at 35°C to obtain the electrolyte of Example 12. According to the mass ratio, LiFSI:EOMMImBF4: ethyleneglycol dimethyl ether: fluorobenzene = 18:15:13:54.

[0128] Example 13

[0129] The difference from Example 1 is that:

[0130] (1) Preparation of electrolyte

[0131] In an argon atmosphere glove box (H2O < 0.01 ppm, O2 < 0.01 ppm), LiFSI, 1-ethoxyethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EOEMImTFSI), ethyleneglycol dimethyl ether were mixed at 35°C to obtain a solution, and then the above prepared solution was mixed with 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethylether at 35°C to obtain the electrolyte of Example 13. According to the mass ratio, LiFSI:EOEMImTFSI: ethyleneglycol dimethyl ether: 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethylether = 20:10:20:50.

[0132] Comparative Example 1

[0133] The difference from Example 1 is that:

[0134] (1) Preparation of electrolyte

[0135] In an argon atmosphere glove box (H2O < 0.01 ppm, O2 < 0.01 ppm), LiPF6, N-methoxypropyl-N-methylpyrrolidinium hexafluorophosphate (Py1(30i)PF6), 1,1,2,2-tetrafluoroethyl ether-2,2,3,3-tetrafluoropropyl ether were mixed at 35°C to obtain the electrolyte of Comparative Example 1. According to the mass ratio, LiPF6: Py1(30i)PF6: 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether = 30:8:62.

[0136] Comparative Example 2

[0137] The difference from Example 1 is that:

[0138] (1) Preparation of electrolyte

[0139] In an argon atmosphere glove box (H2O < 0.01 ppm, O2 < 0.01 ppm), LiPF6, N-methoxypropyl-N-methylpyrrolidinium hexafluorophosphate (Py1(30i)PF6), diethylene glycol dimethyl ether were mixed at 35°C to obtain the electrolyte of Comparative Example 2. According to the mass ratio, LiPF6: Py1(30i)PF6: diethylene glycol dimethyl ether = 30:8:62.

[0140] Comparative Example 3

[0141] The difference from Example 1 is that:

[0142] (1) Preparation of electrolyte

[0143] In an argon atmosphere glove box (H2O < 0.01 ppm, O2 < 0.01 ppm), LiPF6, diethylene glycol dimethyl ether were mixed at 35°C to obtain a solution, and then the above prepared solution was mixed with 1,1,2,2-tetrafluoroethyl ether-2,2,3,3-tetrafluoropropyl ether at 35°C to obtain the electrolyte of Comparative Example 3. According to the mass ratio, LiPF6: diethylene glycol dimethyl ether: 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether = 30:20:50.

[0144] Comparative Example 4

[0145] The difference from Example 1 is that:

[0146] (1) Preparation of electrolyte

[0147] In an argon atmosphere glove box (H2O < 0.01 ppm, O2 < 0.01 ppm), LiPF6, methyl-N-propyl pyrrolidinium hexafluorophosphate (Py13PF6), diethylene glycol dimethyl ether were mixed at 35°C, and then the above prepared solution was mixed with 1,1,2,2-tetrafluoroethyl ether 2,2,3,3-tetrafluoropropyl ether at 35°C to obtain the electrolyte of Comparative Example 4. The mass ratio of LiPF6: Py13PF6: diethylene glycol dimethyl ether: 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether was 30:8:12:50.

[0148] The components and proportions of the electrolyte in Example 1 to Example 13 and Comparative Example 1 to Comparative Example 4 are shown in Table 1.

[0149] Table 1

[0150] Battery cycle life test:

[0151] The battery prepared in each example and comparative example was stored at 0°C for 6H before starting the test.

[0152] Step 1: constant current constant voltage charging to 4.3V at 0.33C, and then constant voltage charging to 0.1C at 4.3V;

[0153] Step 2: rest for 10 min.

[0154] Step 3: then constant current discharging to 2.75V at 0.5C.

[0155] Step 4: rest for 10 min.

[0156] Wherein, steps 1 to 4 are one cycle, cycle steps 1 to 4, the discharge capacity is recorded as Cn, the capacity retention rate is Rn = Cn / C1*100%, and the cycle number when Rn = 80% is recorded as the cycle life. The test results are shown in Table 1.

[0157] Battery cycle life test:

[0158] The battery to be tested was stored at 25°C for 2H. Constant current constant voltage charging to 4.3V at 0.33C, and then constant voltage charging to 0.1C at 4.3V, after resting for 10 min, then constant current discharging to 2.75V at 0.5C, and the discharge capacity was recorded as C0. Rest for another 10 min.

[0159] Then the battery was stored at 0°C for 6H, constant current constant voltage charging to 4.3V at 0.33C, and then constant voltage charging to 0.1C at 4.3V, after resting for 10 min, then constant current discharging to 2.75V at 0.5C, and the discharge capacity was recorded as C1. The capacity yield rate = C1 / C0*100%.

[0160] From Table 1, it can be seen that:

[0161] From Examples 1 to 13, it can be seen that each component (lithium salt, ether solvent, fluorinated solvent and ether functionalized ionic liquid) in the electrolyte and the proportion of each component in the electrolyte affect the cycle life and capacity release rate of the battery at low temperature. From Comparative Examples 3 and 4, it can be seen that the addition of ether functionalized ionic liquid in the electrolyte can improve the cycle life of the battery at low temperature.

[0162] From Examples 1, 4 and 5, it can be seen that when the content of ether functionalized ionic liquid in the electrolyte is relatively high (for example, about 25% by mass), the viscosity of the electrolyte is relatively large, the conductivity is relatively low, and the cycle life and capacity release rate of the battery at low temperature are not as good as those of Example 1.

[0163] From Examples 2 and 6, it can be seen that when the content of fluorinated solvent in the electrolyte is relatively high (for example, about 75% by mass), the viscosity of the electrolyte is relatively low, but the conductivity is also relatively low, and the cycle performance and capacity release performance of the battery decrease.

[0164] From Examples 1 and 7, it can be seen that when the concentration of lithium salt in the electrolyte is relatively high and the content of ether solvent is relatively low, the conductivity of the electrolyte is relatively low, and the cycle performance and capacity release performance of the battery decrease.

[0165] From Examples 1 and 8, it can be seen that when the content of ether in the electrolyte increases, the degree of dissociation of lithium salt increases, the conductivity increases, the capacity release is improved, but the electrochemical stability decreases, and the cycle performance decreases.

[0166] From Examples 1 and Comparative Example 1, it can be seen that the addition of ether solvent in the electrolyte to replace part of the fluorinated ether solvent is beneficial to the improvement of the low-temperature cycle performance.

[0167] From Examples 1 and Comparative Example 2, it can be seen that the addition of fluorinated ether solvent in the electrolyte to replace part of the ether solvent is beneficial to the improvement of the low-temperature cycle performance.

[0168] From Examples 1 and Comparative Example 3, it can be seen that the addition of ether functionalized ionic liquid in the electrolyte to replace part of the ether solvent is beneficial to the improvement of the low-temperature cycle performance.

[0169] The battery obtained in Example 6 has relatively outstanding low-temperature capacity release rate and cycle performance, especially the low-temperature cycle performance is significantly improved, and there is a synergistic effect of LiDFOB, Py1(2o1)BF4 and 1,4-dioxane in improving the low-temperature capacity release rate and cycle performance of the battery. The possible reason is that DFOB - (anion provided by lithium salt) and BF4 -The SEI film formed by coordination of the anion provided by the ionic liquid is better, and the solvating ability of 1,4-dioxane is weak, resulting in more DFOB in the solvated structure - and BF4 - participate in the formation of the SEI film, while Py1(2o1) + The cation provided by the ionic liquid is planar, has a short side chain and small volume, and has better kinetic performance, Py1(2o1) + can better regulate the uniformity of Li deposition, thereby making the obtained battery performance more excellent.

[0170] As can be seen from Example 1, Comparative Example 1 to Comparative Example 3, when the electrolyte contains an ether solvent, a fluoroether solvent and an ether-functionalized ionic liquid at the same time, the low-temperature performance of the electrolyte is the best, and has unexpected technical effects.

[0171] As can be seen from Example 2, Example 3, Example 9 to Example 12, when the mass ratio of lithium salt to electrolyte in the electrolyte is 15% to 20%, the mass ratio of ether-functionalized ionic liquid to electrolyte is 5% to 15%, the mass ratio of ether solvent to electrolyte is 10% to 20%, the mass ratio of fluorinated solvent to electrolyte is 50% to 65%, and the mass ratio of the sum of ether solvent and fluorinated solvent to electrolyte is 70% to 75%, the low-temperature capacity release rate and cycle performance of the obtained battery are both significantly improved.

[0172] The above describes the technical solutions provided by the embodiments of the present application in detail, and the principles and implementation modes of the present application are described by applying specific examples; the above description of the embodiments is only used to help understand the method and its core idea; meanwhile, for those skilled in the art, according to the idea of the present application, the specific implementation mode and application range will be changed, and the above description of the present application should not be understood as a limitation.

Claims

1. An electrolyte for a secondary battery, characterized by comprising: The electrolyte includes a lithium salt, an ether solvent, a fluorinated solvent, and an ether-functionalized ionic liquid including an anion and a cation, the cation including at least one of an ion of Formula I, an ion of Formula II, an ion of Formula III, or an ion of Formula IV: wherein R1, R2, R3, R4, R5, R6, R7, R8, R9, R 10 , R 11 , R 12 , R 13 , R 14 are each independently selected from substituted or unsubstituted C1 to C 10 alkyl.

2. The electrolyte of claim 1, wherein R1, R2, R4, R5, R7, R8, R 10 , R 11 , R 13 each independently is selected from any one of methyl, ethyl, propyl, butyl, pentyl, hexyl. 3.The electrolyte of claim 1, wherein the anion is at least one selected from a bisfluorosulfonylimide ion, a bis(trifluoromethylsulfonyl)imide, a trifluoromethanesulfonate, a dicyanamide, a tetrafluoroborate, a hexafluorophosphate, a perchlorate, a dihydrogenborate, or a difluorohydrogenborate. R3, R6, R9, R 12 , R 14 each independently is selected from any one of methyl, ethyl.

4. The electrolyte of claim 1, wherein The ether-functionalized ionic liquid is at least one selected from N-methoxypropyl-N-methylpyrrolidinium hexafluorophosphate, (methoxybutyl)trimethylammonium bis(trifluoromethanesulfonyl)imide, N-methoxyethyl-N-ethylpiperidinium tetrafluoroborate, N-methoxyethyl-N-methylpyrrolidinium tetrafluoroborate, methyl(2-methoxyethyl)diethylammonium bis(fluorosulfonyl)imide, (methoxyethyl)trimethylammonium bis(fluorosulfonyl)imide, N-methoxybutyl-N-methylpiperidinium nitrate, 1-methoxyethyl-3-methylimidazolium tetrafluoroborate, 1-ethoxyethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide.

5. The electrolyte according to any one of claims 1 to 4, wherein The mass ratio of the lithium salt to the electrolyte is 15% to 30%.

6. The electrolyte of claim 1, wherein The mass ratio of the ether solvent to the electrolyte is 5% to 30%.

7. The electrolyte of claim 1, wherein The mass ratio of the fluorinated solvent to the electrolyte is 30% to 75%.

8. The electrolyte of claim 1, wherein The mass ratio of the ether-functionalized ionic liquid to the electrolyte is 5% to 25%.

9. The electrolyte of claim 1, wherein The mass ratio of the lithium salt to the electrolyte is 15% to 20%.

10. The electrolyte of claim 6, wherein The mass ratio of the ether solvent to the electrolyte is 10% to 20%.

11. The electrolyte of claim 7, wherein The mass ratio of the fluorinated solvent to the electrolyte is 50% to 65%.

12. The electrolyte of claim 8, wherein The mass ratio of the ether-functionalized ionic liquid to the electrolyte is 5% to 15%.

13. The electrolyte of claim 9, wherein The mass ratio of the lithium salt to the electrolyte is 15% to 20%.

14. The electrolyte of claim 1, wherein The mass ratio of the ether-functionalized ionic liquid to the electrolyte is 5% to 15%. The mass ratio of the ether solvent to the electrolyte is 10% to 20%. The mass ratio of the fluorinated solvent to the electrolyte is 50% to 65%. The mass ratio of the sum of the mass of the ether solvent and the fluorinated solvent to the electrolyte is 70% to 75%. The lithium salt is one or more selected from lithium difluoro(oxalato)borate, lithium difluorophosphate, lithium nitrate, lithium bis(oxalato)borate, lithium tetrafluoroborate, lithium perchlorate, lithium difluorophosphate, lithium hexafluorophosphate, lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium 2,2,2-(trifluoromethyl)sulfonyl-N-cyanamide.

15. The electrolyte of claim 1, wherein The ether solvent is one or more selected from ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, diethoxymethane, bis(2-methoxyethyl)ether, 1,4-dioxane, 1,3-dioxolane.

16. The electrolyte of claim 1, wherein The lithium salt is LiDFOB, the ether-functionalized ionic liquid is N-methoxyethyl-N-ethylpiperidinium tetrafluoroborate, and the ether solvent is 1,4-dioxane.

17. The electrolyte of claim 1, wherein The method comprises the following steps:

18. A method for preparing an electrolyte for use in the electrolyte according to any one of claims 1 to 17, characterized by, mixing a lithium salt, an ether-functionalized ionic liquid, and an ether solvent in a preset ratio, obtaining a pre-mixed liquid, adding a fluorinated solvent to the pre-mixed liquid in a preset content to obtain the electrolyte. ​ 19. A secondary battery characterized by comprising: The battery includes a positive electrode sheet, a negative electrode sheet, and the electrolyte solution according to any one of claims 1 to 17.

20. The secondary battery of claim 19, wherein the cathode comprises a cathode active material, a cathode binder, and a cathode conductive agent. The negative electrode sheet includes lithium metal or a lithium alloy.

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