Electrolyte, preparation method thereof, lithium battery and application

Abstract

The application provides an electrolyte, a preparation method thereof, a lithium battery and application. According to mass percentage, the electrolyte comprises 16.5-21% of lithium hexafluorophosphate, 0.5-2% of lithium difluoro(oxalato)borate, 0.1-2% of sodium hexafluorophosphate and 0.6-1% of a pyrrole ionic liquid, and the rest is a solvent. The compatibility of the electrolyte with lithium metal is improved. When the electrolyte is applied to the lithium battery, the phenomena of gas generation expansion and uneven deposition can be improved, and the cycle performance and safety performance of the lithium battery are improved.

Description

[0001] This application is a divisional application. The original application number is 202410548491.7, the application date is May 6, 2024, and the invention title is: Electrolyte and its preparation method, lithium battery and its application. Technical Field

[0002] This application relates to the field of electrolyte technology, specifically to an electrolyte and its preparation method, a lithium battery and its application. Background Technology

[0003] With the continuous development of portable electronic devices, electric vehicles, and the large-scale energy storage market, the demand for lithium batteries continues to grow. After years of development, lithium batteries have made significant progress in performance indicators and demonstrated enormous application potential; however, some shortcomings still exist in related technologies.

[0004] Taking traditional carbonate electrolyte systems containing lithium hexafluorophosphate as an example, traditional carbonate electrolyte systems suffer from poor interfacial stability. Furthermore, phenomena such as gas generation and uneven deposition caused by the reaction between the electrolyte and lithium metal can lead to a decrease in the cycle performance of lithium batteries and increase the risk of accidents such as gas expansion and explosion, thereby limiting the large-scale application of lithium batteries in the field of power batteries. Summary of the Invention

[0005] This application provides an electrolyte, a method for preparing the same, a lithium battery, and its applications.

[0006] In a first aspect, this application provides an electrolyte comprising, by mass percentage, 16.5% to 21% lithium hexafluorophosphate, 0.5% to 2% lithium difluorooxalate borate, 0.1% to 2% sodium hexafluorophosphate, and 1% to 2% pyrrole-based ionic liquid, with the remainder being solvent.

[0007] The pyrrole-based ionic liquid has the structure shown in the following general formula (Ⅰ):

[0008]

[0009] In general formula (Ⅰ), R11 and R2 are each independently selected from alkyl groups of C1 to C30.

[0010] Secondly, this application provides a method for preparing an electrolyte as described in the first aspect. The method for preparing the electrolyte includes the steps of: mixing lithium hexafluorophosphate, lithium difluorooxalate borate, sodium hexafluorophosphate, pyrrole ionic liquid and solvent according to a certain ratio to obtain the electrolyte.

[0011] Thirdly, this application provides an application of an electrolyte, wherein the electrolyte is the electrolyte described in the first aspect, or the electrolyte is an electrolyte prepared by the method described in the second aspect.

[0012] Fourthly, this application provides a lithium battery, including a positive electrode, a negative electrode, and a separator, wherein the lithium battery also includes an electrolyte as described in the first aspect.

[0013] Beneficial effects:

[0014] The embodiments of this application improve the compatibility between the electrolyte and lithium metal by compounding lithium hexafluorophosphate, lithium difluorooxalate borate, sodium hexafluorophosphate, pyrrole ionic liquid and solvent in a specific ratio. When the electrolyte is applied to lithium batteries, the phenomena of gas expansion and uneven deposition can be improved, which is beneficial to improving the cycle performance and safety performance of lithium batteries. Attached Figure Description

[0015] The technical solution and other beneficial effects of this application will become apparent from the following detailed description of specific embodiments in conjunction with the accompanying drawings.

[0016] Figure 1 This is a microscopic morphology diagram of the negative electrode of the lithium battery in Example 1 of this application after failure.

[0017] Figure 2 This is a microscopic morphology diagram of the negative electrode of the lithium battery in Comparative Example 1 of this application after failure. Detailed Implementation

[0018] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application. Furthermore, it should be understood that the specific embodiments described herein are only for illustration and explanation of this application and are not intended to limit this application.

[0019] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as are familiar to those skilled in the art. Furthermore, any methods and materials similar to or equivalent to those described herein may be used in this application. The preferred embodiments and materials described herein are for illustrative purposes only and do not limit the scope of this application.

[0020] It should be noted that the order of description of the following embodiments is not intended to limit the preferred order of embodiments. Various embodiments of this application may exist in the form of a range. It should be understood that the description in the form of a range is merely for convenience and simplicity and should not be construed as a rigid limitation on the scope of this application. Therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within that 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., and single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Furthermore, whenever a numerical range is indicated herein, it means including any referenced number (fraction or integer) within the indicated range.

[0021] In this application, the term "comprising" means "including but not limited to".

[0022] In this application, the term "and / or" is used to describe the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. A and B can be singular or plural.

[0023] In this application, the term "at least one" refers to one or more items, and "more than one" refers to two or more items. The terms "at least one," "at least one of the following," or similar expressions refer to any combination of these items, including any combination of a single or multiple items. For example, "at least one of a, b, or c" or "at least one of a, b, and c" can be expressed as: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can each be a single or multiple items.

[0024] This application provides an electrolyte comprising, by mass percentage, 16.5%–21% lithium hexafluorophosphate, 0.5%–2% lithium difluorooxalate borate, 0.1%–2% sodium hexafluorophosphate, and 1%–2% pyrrole-based ionic liquid, with the balance being solvent; wherein the pyrrole-based ionic liquid has the structure shown in the following general formula (Ⅰ):

[0025]

[0026] In general formula (I), R11 and R2 are each independently selected from C1 to C30 alkyl groups, for example, independently selected from C1 to C3 alkyl groups, C1 to C5 alkyl groups, C1 to C8 alkyl groups, C1 to C10 alkyl groups, C1 to C15 alkyl groups, or C1 to C20 alkyl groups. As an example, R1 and R2 are each independently selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, or tert-butyl.

[0027] In the electrolyte of this embodiment, lithium hexafluorophosphate, lithium difluorooxalate borate, sodium hexafluorophosphate, pyrrole ionic liquid and solvent are compounded in a specific ratio to improve the compatibility between the electrolyte and lithium metal. When the electrolyte is applied to lithium batteries, the phenomena of gas expansion and uneven deposition can be improved, which is beneficial to improving the cycle performance and safety performance of lithium batteries.

[0028] In the electrolyte, pyrrolidine ions in the pyrrole-based ionic liquid are preferentially reduced over lithium ions, thereby reducing lithium dendrite growth and improving the uniformity of lithium deposition. The mass percentage of the pyrrole-based ionic liquid can be, for example, 1%, 1.2%, 1.5%, 1.8%, 2%, or any two of the aforementioned values. This improves the uniformity of lithium deposition while avoiding a sharp increase in electrolyte viscosity, ensuring that the lithium battery using the electrolyte exhibits good rate performance, cycle performance, and low-temperature performance.

[0029] To further improve the cycle performance of lithium batteries using electrolytes, in some embodiments of this application, the pyrrole ionic liquid is selected from one or more of N-butyl-N-methylpyrrolidine hexafluorophosphate (CAS No. 330671-29-9), N-propyl-N-methylpyrrolidine hexafluorophosphate (CAS No. 327022-58-2), and N-ethyl-N-methylpyrrolidine hexafluorophosphate (CAS No. 121057-90-7).

[0030] It should be noted that the applicant found that replacing the pyrrole-based ionic liquid in the electrolyte with an imidazole-based ionic liquid while maintaining the same mass percentage would lead to a decrease in the cycle performance of the lithium battery. To obtain comparable cycle performance, the amount of imidazole-based ionic liquid needed to be increased, which increased production costs.

[0031] In the electrolyte, the mass percentage of lithium hexafluorophosphate can be, for example, 16.5%, 18%, 19%, 20%, 21%, or any two of the aforementioned values; the mass percentage of lithium difluorooxalate borate can be, for example, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, or any two of the aforementioned values; and the mass percentage of sodium hexafluorophosphate can be, for example, 0.1%, 0.3%, 0.5%, 0.8%, 1%, 1.5%, 2%, or any two of the aforementioned values.

[0032] In the electrolyte, the mass percentage of lithium hexafluorophosphate is 16.5% to 21%, which ensures a suitable electrolyte viscosity and good stability. Lithium batteries using this electrolyte exhibit good rate performance, cycle performance, and low-temperature performance, ensuring that the "inflection point" of the lithium battery's degradation trajectory occurs at an appropriate time. If the mass percentage of lithium hexafluorophosphate is too high, the electrolyte viscosity will increase, leading to a decrease in the rate performance, cycle performance, and low-temperature performance of the lithium battery using this electrolyte. Conversely, if the mass percentage of lithium hexafluorophosphate is too low, the electrolyte stability will decrease, causing the "inflection point" of the lithium battery's degradation trajectory to occur earlier.

[0033] In the electrolyte, sodium hexafluorophosphate is used to introduce sodium ions to stabilize lithium metal. Specifically, because the intrinsic potential of sodium ions is higher than that of lithium ions, sodium ions are more easily reduced to form precipitates during the formation of the solid electrolyte interface (SEI) in the lithium battery. Furthermore, the potential difference can guide sodium ions and lithium ions to spontaneously occupy different regions, thereby forming a more uniform and denser SEI layer. Too much or too little sodium hexafluorophosphate in the electrolyte will lead to a decrease in the cycle performance of the lithium battery.

[0034] In the electrolyte, both excessive and insufficient lithium difluorooxalate borate by mass percentage will lead to a decrease in the cycle performance of the lithium battery. Excessive addition of lithium difluorooxalate borate will result in lower battery capacity and thus poorer cycle performance; insufficient addition will reduce the electrolyte's cycle performance and accelerate electrolyte drying.

[0035] In some embodiments of this application, the molar ratio of lithium hexafluorophosphate to sodium hexafluorophosphate in the electrolyte is 1:(0.02-0.05), for example, it can be 1:0.02, 1:0.03, 1:0.04, 1:0.05, or any value between any two of the aforementioned values; and / or, the molar ratio of lithium hexafluorophosphate to lithium difluorooxalate borate is 1:(0.023-0.056), for example, it can be 1:0.023, 1:0.03, 1:0.04, 1:0.05, 1:0.056, or any value between any two of the aforementioned values. By rationally adjusting the ratio of lithium hexafluorophosphate, sodium hexafluorophosphate, and lithium difluorooxalate borate, the loss of lithium hexafluorophosphate during the operation of the lithium battery can be reduced, which is beneficial to improving the stability of the SEI layer, thereby further improving the cycle performance of the lithium battery.

[0036] In some embodiments of this application, the solvent is selected from one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, butene carbonate, methyl propyl carbonate, ethyl propyl carbonate, dipropyl carbonate, fluoroethylene carbonate, and diethyl carbonate.

[0037] In order to further improve the compatibility of the electrolyte with lithium metal, and thus further improve the cycle performance of lithium batteries using electrolyte, in some embodiments of this application, the solvent is composed of ethylene carbonate, diethyl carbonate, dimethyl carbonate and fluoroethylene carbonate.

[0038] It should be noted that the applicant found that if ethylene carbonate in the solvent is replaced with propylene carbonate, and the mass percentage of each component in the solvent remains the same, the improvement in the cycle performance of lithium batteries using electrolyte is limited.

[0039] Furthermore, in some embodiments of this application, the volume ratio of ethylene carbonate, diethyl carbonate, and dimethyl carbonate in the solvent is 1:(2-3):(1-2).

[0040] Furthermore, in some embodiments of this application, the mass ratio of fluoroethylene carbonate to the total mass of the electrolyte is 10% to 15%, for example, it can be 10%, 11%, 12%, 13%, 14%, 15%, or any two of the aforementioned values. Fluoroethylene carbonate can act not only as a solvent but also as a film-forming aid. It can participate in the construction of the SEI, reducing side reactions between the electrolyte and lithium metal, thereby improving the cycle performance of the lithium battery. Excessive addition of fluoroethylene carbonate will result in severe gas generation, leading to uneven lithium migration and deposition, thus reducing the cycle performance of the lithium battery; insufficient addition will have limited stabilizing effect on the SEI.

[0041] This application also provides a method for preparing an electrolyte, which can be used to prepare any of the electrolytes described above, including the steps of: mixing lithium hexafluorophosphate, lithium difluorooxalate borate, sodium hexafluorophosphate, pyrrole ionic liquid and solvent according to the specified ratio to obtain an electrolyte.

[0042] To further improve the stability of the electrolyte and thus the cycle performance of the lithium battery, in some embodiments of this application, the mixing of lithium hexafluorophosphate, lithium difluorooxalate borate, sodium hexafluorophosphate, pyrrole ionic liquid, and solvent according to the specified ratio includes the following steps: providing a solvent at room temperature, adding lithium hexafluorophosphate, lithium difluorooxalate borate, sodium hexafluorophosphate, and pyrrole ionic liquid to the solvent, and mixing at room temperature. The mixing method includes, but is not limited to, stirring, and the room temperature is, for example, 20°C to 25°C.

[0043] This application also provides an application of an electrolyte, wherein the electrolyte is any of the electrolytes described above, or the electrolyte is an electrolyte prepared by any of the electrolyte preparation methods described above.

[0044] This application also provides a lithium battery, including a positive electrode, a negative electrode, and a separator. The lithium battery also includes an electrolyte as described above, which effectively improves the problems of gas expansion and uneven deposition, and improves the cycle performance and safety performance of the lithium battery.

[0045] In this embodiment, the lithium battery can be a stacked structure formed by layering a positive electrode, a negative electrode, and a separator in a specific order, or it can be a wound body formed by winding the stacked structure. According to its shape, a lithium battery can be a laminated battery, a pouch battery, a prismatic battery, a cylindrical battery, a coin-shaped battery, or a button battery; according to its material, a lithium battery can be a lithium-ion battery, a lithium-sulfur battery, or a lithium-air battery.

[0046] In some embodiments of this application, the positive electrode includes a positive electrode current collector and a positive electrode active material layer disposed on the surface of the positive electrode current collector. The material of the positive electrode active material layer includes a positive electrode active substance, a positive electrode binder, and a positive electrode conductive agent. The material of the positive electrode current collector, the positive electrode active substance, the positive electrode binder, and the positive electrode conductive agent can be conventional materials in the art. For example, the material of the positive electrode current collector includes, but is not limited to, aluminum foil, platinum foil, or palladium foil; the positive electrode active substance includes, but is not limited to, one or more of lithium cobalt oxide, lithium manganese oxide, lithium permanganate, lithium iron phosphate, lithium nickel oxide, lithium manganese phosphate, lithium iron manganese phosphate, lithium nickel cobalt manganese oxide, and lithium nickel cobalt aluminum oxide. It is understood that for the compounds listed above as positive electrode active substances, their surface may have a coating layer, and the material of the coating layer includes, but is not limited to, carbon materials; the positive electrode binder includes, but is not limited to, one or more of vinylidene fluoride / hexafluoropropylene copolymer, polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, and polytetrafluoroethylene; the positive electrode conductive agent includes, but is not limited to, one or more of carbon black, graphite, and graphene.

[0047] The preparation method of the positive electrode may include the following steps: mixing a positive electrode active material, a conductive agent, a binder, and a first solvent to obtain a first mixture; then coating the first mixture onto a positive electrode current collector, followed by a drying process and a rolling process to obtain a positive electrode plate. It should be noted that the first mixture can also be cast onto a separate carrier to form a film layer, then the film layer is separated from the carrier, and then the film layer is stacked on one side of the positive electrode current collector. The first solvent includes, but is not limited to, N-methylpyrrolidone, acetone, and water.

[0048] In some embodiments of this application, the negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on the surface of the negative electrode current collector. The material of the negative electrode active material layer includes a negative electrode active substance, a negative electrode binder, and a negative electrode conductive agent. The materials for the negative electrode current collector, the negative electrode active material, the negative electrode binder, and the negative electrode conductive agent can be conventional materials in the art. For example, the materials for the negative electrode current collector include, but are not limited to, aluminum foil, platinum foil, or palladium foil; the negative electrode active material includes, but is not limited to, one or more of elemental lithium, lithium-alloyable metals, half-metals, transition metal oxides, non-transition metal oxides, and carbon materials. The lithium-alloyable metals or half-metals include, but are not limited to, Si, Sn, Al, Ge, Pb, Bi, Sb, Si-Y1 alloys (Y1 being an alkali metal, alkaline earth metal, group 13-16 elements, transition metals, rare earth elements, or combinations thereof, excluding Si), and Sn-Y2 alloys (Y2 being an alkali metal, alkaline earth metal, group 13-16 elements, transition metals, rare earth elements, or combinations thereof, excluding Sn); the transition metal oxides include, but are not limited to, one or more of lithium titanium oxide, vanadium oxide, lithium vanadium oxide, and titanium niobium oxide; and the non-transition metal oxides include, but are not limited to, SnO2 and SiO2. xOne or more of (0 < x < 2), the carbon material includes but is not limited to one or more of crystalline carbon (such as graphite) and amorphous carbon. The negative electrode binder can be the same as the positive electrode binder, and the negative electrode conductive agent can be the same as the positive electrode conductive agent. The preparation method of the negative electrode can be carried out by referring to the preparation method of the positive electrode.

[0049] The material of the separator includes but is not limited to one or more of glass fiber, polyester, Teflon, polyethylene, polypropylene, and polytetrafluoroethylene.

[0050] The preparation method of the lithium battery in the embodiment of the present application, for example, includes the steps of: first assembling the positive electrode, negative electrode, and separator to form a battery body, and then injecting an electrolyte into the battery body for infiltration to obtain a lithium battery.

[0051] Hereinafter, the technical solutions and technical effects of the present application will be described in detail through specific examples, comparative examples, and experimental examples. The following examples are only partial examples of the present application and do not specifically limit the present application.

[0052] Example 1

[0053] This example provides an electrolyte and a lithium battery containing the electrolyte. Calculated by mass percentage, the electrolyte includes 19.5% of lithium hexafluorophosphate, 1% of lithium difluorooxalate borate, 0.5% of sodium hexafluorophosphate, and 1.2% of N-propyl-N-methylpyrrolidinium hexafluorophosphate, and the balance is a solvent. Among them, the solvent is composed of ethylene carbonate, diethyl carbonate, dimethyl carbonate, and fluorinated ethylene carbonate. The volume ratio of ethylene carbonate, diethyl carbonate, and dimethyl carbonate is 2:5:3, and the mass of fluorinated ethylene carbonate accounts for 10% of the total mass of the electrolyte.

[0054] The preparation method of the electrolyte in this example includes the following steps:

[0055] S1.1. Take ethylene carbonate, diethyl carbonate, dimethyl carbonate, and fluorinated ethylene carbonate according to the formula ratio, mix them, and cool to room temperature to obtain a solvent;

[0056] S1.2. Add lithium hexafluorophosphate, N-propyl-N-methylpyrrolidinium hexafluorophosphate, lithium difluorooxalate borate, and sodium hexafluorophosphate in the formula ratio to the solvent prepared in step S1.1 in sequence, and stir at room temperature until uniform to obtain an electrolyte.

[0057] In a glove box filled with argon gas, NCM811 was used as the positive electrode material (positive electrode capacity of 3.2 mAh, positive electrode current collector material is aluminum foil), lithium metal was used as the negative electrode material (lithium metal layer thickness of 20 μm, negative electrode current collector material is copper foil), and a polyethylene film coated with Al2O3 on both sides (coating thickness of 2 μm on each side) was used as the separator. Under the condition of electrolyte injection volume of 1.6 g / Ah, a 6.2 Ah soft pack battery was fabricated. Then, electrolyte was injected into the battery for wetting to obtain the lithium battery of this embodiment.

[0058] Example 2

[0059] This embodiment provides an electrolyte and a lithium battery containing the electrolyte. Compared to the electrolyte in Example 1, the difference in this embodiment is that the mass percentage of sodium hexafluorophosphate in the electrolyte is replaced with "0.1%", and correspondingly, the mass percentage of the solvent is changed accordingly. It should be noted that the volume ratio of ethylene carbonate, diethyl carbonate, and dimethyl carbonate, as well as the mass percentage of fluoroethylene carbonate, are the same as in Example 1.

[0060] The preparation method of the electrolyte in this embodiment is the same as that in Example 1.

[0061] Compared to the lithium battery in Example 1, the difference of the lithium battery in this example is that the electrolyte is the electrolyte of this example.

[0062] Example 3

[0063] This embodiment provides an electrolyte and a lithium battery containing the electrolyte. Compared to the electrolyte in Example 1, the difference in this embodiment is that the mass percentage of sodium hexafluorophosphate in the electrolyte is replaced with "1%", and correspondingly, the mass percentage of the solvent is changed accordingly. It should be noted that the volume ratio of ethylene carbonate, diethyl carbonate, and dimethyl carbonate, as well as the mass percentage of fluoroethylene carbonate, are the same as in Example 1.

[0064] The preparation method of the electrolyte in this embodiment is the same as that in Example 1.

[0065] Compared to the lithium battery in Example 1, the difference of the lithium battery in this example is that the electrolyte is the electrolyte of this example.

[0066] Example 4

[0067] This embodiment provides an electrolyte and a lithium battery containing the electrolyte. Compared to the electrolyte in Example 1, the difference in this embodiment is that the mass percentage of lithium difluorooxalate borate in the electrolyte is replaced with "0.5%", and correspondingly, the mass percentage of the solvent is changed accordingly. It should be noted that the volume ratio of ethylene carbonate, diethyl carbonate, and dimethyl carbonate, as well as the mass percentage of fluoroethylene carbonate, are the same as in Example 1.

[0068] The preparation method of the electrolyte in this embodiment is the same as that in Example 1.

[0069] Compared to the lithium battery in Example 1, the difference of the lithium battery in this example is that the electrolyte is the electrolyte of this example.

[0070] Example 5

[0071] This embodiment provides an electrolyte and a lithium battery containing the electrolyte. Compared to the electrolyte in Example 1, the difference in this embodiment is that the mass percentage of lithium difluorooxalate borate in the electrolyte is replaced with "1.5%", and correspondingly, the mass percentage of the solvent is changed accordingly. It should be noted that the volume ratio of ethylene carbonate, diethyl carbonate, and dimethyl carbonate, as well as the mass percentage of fluoroethylene carbonate, are the same as in Example 1.

[0072] The preparation method of the electrolyte in this embodiment is the same as that in Example 1.

[0073] Compared to the lithium battery in Example 1, the difference of the lithium battery in this example is that the electrolyte is the electrolyte of this example.

[0074] Example 6

[0075] This embodiment provides an electrolyte and a lithium battery containing the electrolyte. Compared to the electrolyte in Example 1, the difference in this embodiment is that the mass percentage of lithium difluorooxalate borate in the electrolyte is replaced with "2%", and correspondingly, the mass percentage of the solvent is changed accordingly. It should be noted that the volume ratio of ethylene carbonate, diethyl carbonate, and dimethyl carbonate, as well as the mass percentage of fluoroethylene carbonate, are the same as in Example 1.

[0076] The preparation method of the electrolyte in this embodiment is the same as that in Example 1.

[0077] Compared to the lithium battery in Example 1, the difference of the lithium battery in this example is that the electrolyte is the electrolyte of this example.

[0078] Example 7

[0079] This embodiment provides an electrolyte and a lithium battery containing the electrolyte. Compared with the electrolyte in Embodiment 1, the difference in the electrolyte in this embodiment is that 1.2% N-propyl-N-methylpyrrolidine hexafluorophosphate is replaced with "1.2% N-butyl-N-methylpyrrolidine hexafluorophosphate".

[0080] The preparation method of the electrolyte in this embodiment is the same as that in Example 1.

[0081] Compared to the lithium battery in Example 1, the difference of the lithium battery in this example is that the electrolyte is the electrolyte of this example.

[0082] Example 8

[0083] This embodiment provides an electrolyte and a lithium battery containing the electrolyte. Compared with the electrolyte in Embodiment 1, the difference in the electrolyte in this embodiment is that 1.2% N-propyl-N-methylpyrrolidine hexafluorophosphate is replaced with "1.2% N-ethyl-N-methylpyrrolidine hexafluorophosphate".

[0084] The preparation method of the electrolyte in this embodiment is the same as that in Example 1.

[0085] Compared to the lithium battery in Example 1, the difference of the lithium battery in this example is that the electrolyte is the electrolyte of this example.

[0086] Example 9

[0087] This embodiment provides an electrolyte and a lithium battery containing the electrolyte. Compared with the electrolyte in Embodiment 1, the difference in the electrolyte in this embodiment is that 1.2% N-propyl-N-methylpyrrolidine hexafluorophosphate is replaced with "0.6% N-propyl-N-methylpyrrolidine hexafluorophosphate".

[0088] The preparation method of the electrolyte in this embodiment is the same as that in Example 1.

[0089] Compared to the lithium battery in Example 1, the difference of the lithium battery in this example is that the electrolyte is the electrolyte of this example.

[0090] Example 10

[0091] This embodiment provides an electrolyte and a lithium battery containing the electrolyte. Compared with the electrolyte in Embodiment 1, the difference in the electrolyte in this embodiment is that 1.2% N-propyl-N-methylpyrrolidine hexafluorophosphate is replaced with "2% N-propyl-N-methylpyrrolidine hexafluorophosphate".

[0092] The preparation method of the electrolyte in this embodiment is the same as that in Example 1.

[0093] Compared to the lithium battery in Example 1, the difference of the lithium battery in this example is that the electrolyte is the electrolyte of this example.

[0094] Comparative Example 1

[0095] This comparative example provides an electrolyte and a lithium battery containing the electrolyte. Compared to the electrolyte in Example 1, the electrolyte in this comparative example differs in that N-propyl-N-methylpyrrolidine hexafluorophosphate is omitted, and correspondingly, the mass percentage of the solvent is adapted accordingly. It should be noted that the volume ratio of ethylene carbonate, diethyl carbonate, and dimethyl carbonate, as well as the mass percentage of fluoroethylene carbonate, are the same as in Example 1.

[0096] The electrolyte in this comparative example was prepared according to Example 1.

[0097] Compared to the lithium battery in Example 1, the lithium battery in this comparative example differs in that the electrolyte is the electrolyte of this comparative example.

[0098] Comparative Example 2

[0099] This comparative example provides an electrolyte and a lithium battery containing the electrolyte. Compared with the electrolyte in Example 1, the electrolyte in this comparative example is different in that "1.2% N-propyl-N-methylpyrrolidine hexafluorophosphate" is replaced with "1.2% 1-ethyl-3-methylimidazolium hexafluorophosphate".

[0100] The electrolyte in this comparative example was prepared according to Example 1.

[0101] Compared to the lithium battery in Example 1, the lithium battery in this comparative example differs in that the electrolyte is the electrolyte of this comparative example.

[0102] Experimental Example

[0103] The performance of the lithium batteries in Examples 1 to 10, Comparative Examples 1 and 2 was tested. Specifically, each lithium battery after formation was subjected to a cycle test. Each lithium battery was charged at 25°C with a constant current at a rate of 0.33C to a voltage of 4.2V, and then discharged at a constant current at a rate of 0.5C until the voltage reached 3V. The above charge and discharge cycle was repeated 300 times, with a time interval of 10 minutes between two adjacent cycles. The capacity retention rate after 150 cycles at 25°C was obtained. Each lithium battery that completed the above cycle test was subjected to a national standard nail penetration test.

[0104] The test results are shown in Table 1 below:

[0105] Table 1

[0106]

[0107] As shown in Table 1, the lithium batteries in Examples 1 to 10 exhibit better overall performance compared to those in Comparative Examples 1 and 2. These batteries demonstrate good cycle performance and pass the nail penetration test. Taking the lithium battery in Example 1 as an example, its capacity retention rate after 150 cycles at 25°C is 11.8% higher than that of the lithium battery in Comparative Example 1, and 19.8% higher than that of the lithium battery in Comparative Example 2. While the lithium battery in Comparative Example 1 has a higher capacity retention rate after 150 cycles at 25°C than the lithium battery in Comparative Example 2, it failed the nail penetration test.

[0108] Furthermore, the lithium batteries in Example 1 and Comparative Example 1 were subjected to the aforementioned cycle tests after formation. The cycle tests were stopped when the capacity retention rate was below 80%. Then, the morphology of the negative electrode of the lithium battery was observed using a scanning electron microscope to obtain... Figure 1 and Figure 2 ,exist Figure 1 and Figure 2 In the diagram, both the A-side lithium layer and the B-side lithium layer are films deposited on the copper foil layer during long-cycle operation of lithium batteries. The A-side lithium layer and the B-side lithium layer each include an SEI layer.

[0109] Figure 1 The thickness of the lithium layer on side A is 49.6 μm to 51.4 μm, the thickness of the copper foil layer is 6.35 μm, and the thickness of the lithium layer on side B at one location is 55.4 μm. Figure 1 The combined thickness of the lithium layer on side A, the copper foil layer, and the lithium layer on side B at position one is 110 μm. Figure 2 The thickness of the lithium layer on surface A at position 1 is 72.2 μm. Figure 2 The sum of 50.6 μm and 21.6 μm (marked in the middle) indicates that the thickness of the copper foil layer is 7.94 μm, and the thickness of the lithium layer on the B-side at one location is 71.6 μm. Figure 2 (The sum of 33.9 μm and 37.7 μm marked in the middle), and Figure 2 The sum of the thicknesses of the lithium layer on side A, the copper foil layer, and the lithium layer on side B at a certain location is 147 μm. Therefore, it can be concluded that during the cycle test, the thickness of the lithium layer on side A of the lithium battery in Example 1 is lower than that of the lithium battery in Comparative Example 1, and the thickness of the lithium layer on side B of the lithium battery in Example 1 is lower than that of the lithium battery in Comparative Example 1. Furthermore, from... Figure 1 and Figure 2 It can be seen that the deposition uniformity of the lithium layer on side A of the lithium battery in Example 1 is significantly higher than that of the lithium layer on side A of the lithium battery in Comparative Example 1, and the deposition uniformity of the lithium layer on side B of the lithium battery in Example 1 is significantly higher than that of the lithium layer on side B of the lithium battery in Comparative Example 1.

[0110] This demonstrates that during the cycling test, the gas expansion phenomenon of the lithium battery in Example 1 was significantly improved. After long cycling, the film thickness deposited on the copper foil was low and the deposition uniformity was relatively high. Furthermore, the lithium battery in Example 1 exhibited good cycle performance, maintaining over 90% capacity retention even after 150 cycles at 25°C. In contrast, the lithium battery in Comparative Example 1 showed significant gas expansion during the cycling test. After long cycling, the film thickness deposited on the copper foil was high and the surface unevenness was significant, indicating uneven deposition. Therefore, the lithium battery in Comparative Example 1 failed the nail penetration test.

[0111] In summary, applying the electrolyte of this application embodiment to lithium batteries can improve the compatibility between the electrolyte and lithium metal, effectively improve the phenomena of gas expansion and uneven deposition, thereby improving the cycle performance and safety performance of lithium batteries.

[0112] The embodiments of this application have been described in detail above. Specific examples have been used to illustrate the principles and implementation methods of this application. The description of the above embodiments is only for the purpose of helping to understand the method and core ideas of this application. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of this application. Therefore, the content of this specification should not be construed as a limitation of this application.

Claims

1. An electrolyte, characterized in that, The electrolyte comprises, by mass percentage, 16.5%–21% lithium hexafluorophosphate, 0.5%–1.8% lithium difluorooxalate borate, 0.1%–2% sodium hexafluorophosphate, and 0.6% pyrrole ionic liquid, with the remainder being solvent. The pyrrole-based ionic liquid has the structure shown in the following general formula (Ⅰ): (Ⅰ) ； In general formula (Ⅰ), R1 and R2 are each independently selected from alkyl groups of C1 to C30.

2. The electrolyte according to claim 1, characterized in that, R1 and R2 are each independently selected from C1 to C10 alkyl groups.

3. The electrolyte according to claim 1 or 2, characterized in that, R1 and R2 are each independently selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, or tert-butyl. And / or, the pyrrole ionic liquid is selected from one or more of N-butyl-N-methylpyrrolidine hexafluorophosphate, N-propyl-N-methylpyrrolidine hexafluorophosphate, and N-ethyl-N-methylpyrrolidine hexafluorophosphate.

4. The electrolyte according to claim 1, characterized in that, The solvent is selected from one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, butene carbonate, methyl propyl carbonate, ethyl propyl carbonate, dipropyl carbonate, fluoroethylene carbonate, and diethyl carbonate.

5. The electrolyte according to claim 1, characterized in that, The solvent is composed of ethylene carbonate, diethyl carbonate, dimethyl carbonate, and fluoroethylene carbonate.

6. The electrolyte according to claim 4 or 5, characterized in that, In the solvent, the volume ratio of ethylene carbonate, diethyl carbonate, and dimethyl carbonate is 1:(2-3):(1-2); And / or, the mass of fluoroethylene carbonate accounts for 10% to 15% of the total mass of the electrolyte.

7. The electrolyte according to claim 1, characterized in that, In the electrolyte, the molar ratio of lithium hexafluorophosphate to sodium hexafluorophosphate is 1:(0.02~0.05), and / or the molar ratio of lithium hexafluorophosphate to lithium difluorooxalate borate is 1:(0.023~0.056).

8. A method for preparing an electrolyte, characterized in that, The method for preparing the electrolyte as described in any one of claims 1 to 7 includes the steps of: mixing lithium hexafluorophosphate, lithium difluorooxalate borate, sodium hexafluorophosphate, pyrrole ionic liquid and solvent according to a specified ratio to obtain the electrolyte.

9. The method for preparing the electrolyte according to claim 8, characterized in that, The step of mixing lithium hexafluorophosphate, lithium difluorooxalate borate, sodium hexafluorophosphate, pyrrole ionic liquid and solvent according to the specified ratio includes the following steps: providing a solvent at room temperature, adding lithium hexafluorophosphate, lithium difluorooxalate borate, sodium hexafluorophosphate and pyrrole ionic liquid to the solvent, and mixing at room temperature.

10. An application of an electrolyte, characterized in that, The electrolyte is the electrolyte as described in any one of claims 1 to 7, or the electrolyte is the electrolyte prepared by the method described in claim 8 or 9.

11. A lithium battery, comprising a positive electrode, a negative electrode, and a separator, characterized in that, The lithium battery further includes the electrolyte as described in any one of claims 1 to 7.

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