Electrolyte and preparation method thereof, lithium battery and application thereof

Abstract

The present application provides an electrolyte, a preparation method thereof, a lithium battery and applications thereof. Calculated by mass percentage, the electrolyte comprises 16.5% to 21% of lithium hexafluorophosphate, 0.5% to 2% of lithium difluorooxalatoborate, 0.1% to 2% of sodium hexafluorophosphate and 1% to 2% of a pyrrole ionic liquid, with the remainder being a solvent. This improves the compatibility of the electrolyte with lithium metal. When the electrolyte is applied to a lithium battery, the phenomena of gas expansion and uneven deposition can be improved, which is beneficial to improving the cycle performance and safety performance of the lithium battery.

Description

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

[0002] The present application relates to the technical field of electrolytes, and in particular to an electrolyte and a preparation method thereof, a lithium battery and applications thereof. Background Art

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

[0004] Taking the traditional carbonate system electrolyte containing lithium hexafluorophosphate as an example, the traditional carbonate system electrolyte has the problem of poor interface stability, and the phenomena such as gas production and uneven deposition caused by the reaction between the electrolyte and lithium metal will lead to the decline of the cycle performance of the lithium battery, increase the risk of accidents such as flatulence and explosion, thereby limiting the large-scale application of lithium batteries in the field of power batteries. Summary of the Invention

[0005] The present application provides an electrolyte, a preparation method thereof, a lithium battery and applications thereof.

[0006] In a first aspect, the present application provides an electrolyte, which comprises, by mass percentage, 16.5% to 21% of lithium hexafluorophosphate, 0.5% to 2% of lithium difluorooxalatoborate, 0.1% to 2% of sodium hexafluorophosphate, and 1% to 2% of a pyrrole ionic liquid, with the remainder being a solvent;

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

[0008]

[0009] In the general formula (I), R1 and R2 are each independently selected from a C1 to C30 alkyl group.

[0010] In a second aspect, the present application provides a method for preparing an electrolyte, which is used to prepare the electrolyte as described in the first aspect, and the method for preparing the electrolyte comprises the steps of: mixing lithium hexafluorophosphate, lithium difluorooxalatoborate, sodium hexafluorophosphate, pyrrole ionic liquid and solvent according to a ratio to obtain the electrolyte.

[0011] In a third aspect, the present application provides an application of an electrolyte, wherein the electrolyte is the electrolyte described in the first aspect, or the electrolyte is the electrolyte prepared by the preparation method of the electrolyte described in the second aspect.

[0012] In a fourth aspect, the present application provides a lithium battery comprising a positive electrode, a negative electrode, and a separator, wherein the lithium battery further comprises the electrolyte as described in the first aspect.

[0013] Beneficial effects:

[0014] In the embodiments of the present application, lithium hexafluorophosphate, lithium difluorooxalatoborate, sodium hexafluorophosphate, pyrrole ionic liquid and solvent are compounded in specific proportions to improve the compatibility of the electrolyte with lithium metal. The electrolyte is applied to a lithium battery, which can improve the phenomena of gas expansion and uneven deposition, and is beneficial to improving the cycle performance and safety performance of the lithium battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The following detailed description of the specific embodiments of the present application in conjunction with the accompanying drawings will make the technical solutions and other beneficial effects of the present application apparent.

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

[0017] Figure 2 This is a microscopic morphology of the negative electrode of the lithium battery in Comparative Example 1 of the present application after failure. DETAILED DESCRIPTION

[0018] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in 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. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without making creative work are within the scope of protection of this application. In addition, it should be understood that the specific embodiments described herein are only used to illustrate and explain the present application and are not used to limit the present application.

[0019] Unless otherwise defined, all technical and scientific terms used herein have the same meanings as those familiar to those skilled in the art. Furthermore, any methods and materials similar or equivalent to those described herein may be applied to this application. The preferred embodiments and materials described herein are for illustrative purposes only and are not intended to 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 the embodiments. The various embodiments of the present application may be presented in the form of a range. It should be understood that the description in the form of a range is merely for convenience and brevity and should not be understood as a rigid limitation on the scope of the present application. Therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within the range. For example, the range description from 1 to 6 should be considered to have 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, which apply regardless of the range. In addition, whenever a numerical range is indicated herein, it is intended to include any cited number (fractional or integer) within the indicated range.

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

[0022] In this application, the term "and / or" is used to describe the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone. A and B can be singular or plural.

[0023] In this application, the term "at least one" refers to one or more, and "multiple" refers to two or more. The term "at least one", "the following at least one" or similar expressions refer to any combination of these items, including any combination of a single or multiple. 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 be single or multiple.

[0024] The present embodiment provides an electrolyte, which comprises, by weight percentage, 16.5% to 21% of lithium hexafluorophosphate, 0.5% to 2% of lithium difluorooxalatoborate, 0.1% to 2% of sodium hexafluorophosphate, and 1% to 2% of a pyrrole ionic liquid, with the remainder being a solvent; wherein the pyrrole ionic liquid has a structure represented by the following general formula (I):

[0025]

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

[0027] In the electrolyte of this embodiment, lithium hexafluorophosphate, lithium difluorooxalatoborate, sodium hexafluorophosphate, pyrrole ionic liquid and solvent are compounded in a specific proportion to improve the compatibility of the electrolyte with lithium metal. The application of the electrolyte in a lithium battery can improve the phenomena of gas expansion and uneven deposition, which is beneficial to improving the cycle performance and safety performance of the lithium battery.

[0028] In the electrolyte, pyrrolidine ions in the pyrrole ionic liquid are reduced prior to lithium ions, thereby reducing the growth of lithium dendrites and improving the uniformity of lithium deposition. The mass percentage of the pyrrole ionic liquid can be, for example, 1%, 1.2%, 1.5%, 1.8%, 2%, or any value between the foregoing two values. This improves lithium deposition uniformity while preventing a sharp increase in electrolyte viscosity, ensuring that lithium batteries using the electrolyte have good rate performance, cycling performance, and low-temperature performance.

[0029] In order to further improve the cycle performance of lithium batteries using electrolytes, in some embodiments of the present 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 has found that if the pyrrole ionic liquid in the electrolyte is replaced with an imidazole ionic liquid and the mass percentage remains the same, the cycle performance of the lithium battery will decrease. If equivalent cycle performance is to be obtained, at least the amount of imidazole ionic liquid needs to be increased, which increases the production cost.

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

[0032] In the electrolyte, the mass percentage of lithium hexafluorophosphate is 16.5% to 21%, which can ensure that the electrolyte has an appropriate viscosity and good stability. The lithium battery using this electrolyte has good rate performance, cycle performance, and low-temperature performance, ensuring that the "inflection point" of the lithium battery degradation trajectory is at an appropriate time point. If the mass percentage of lithium hexafluorophosphate is too high, the viscosity of the electrolyte will increase, and the rate performance, cycle performance, and low-temperature performance of the lithium battery using this electrolyte will decrease. If the mass percentage of lithium hexafluorophosphate is too low, the stability of the electrolyte will decrease, and the "inflection point" of the lithium battery degradation trajectory will be premature.

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

[0034] In the electrolyte, either too much or too little lithium difluorooxalatoborate by mass can lead to a decrease in the cycling performance of the lithium battery. Adding too much lithium difluorooxalatoborate reduces the capacity of the lithium battery, thereby impairing the cycling performance. Adding too little lithium difluorooxalatoborate reduces the cycling performance of the electrolyte and accelerates the drying out of the electrolyte.

[0035] In some embodiments of the present application, the molar ratio of lithium hexafluorophosphate to sodium hexafluorophosphate in the electrolyte is 1:(0.02-0.05), for example, 1:0.02, 1:0.03, 1:0.04, 1:0.05, or a value between any two of the foregoing values; and / or the molar ratio of lithium hexafluorophosphate to lithium difluorooxalatoborate is 1:(0.023-0.056), for example, 1:0.023, 1:0.03, 1:0.04, 1:0.05, 1:0.056, or a value between any two of the foregoing values. By rationally adjusting the ratios among lithium hexafluorophosphate, sodium hexafluorophosphate, and lithium difluorooxalatoborate, 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 the present application, the solvent is selected from one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, butylene 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, thereby further improving the cycle performance of the lithium battery using the electrolyte, in some embodiments of the present application, the solvent is composed of ethylene carbonate, diethyl carbonate, dimethyl carbonate and fluoroethylene carbonate.

[0038] It should be noted that the applicant has found that if ethylene carbonate in the solvent is replaced with propylene carbonate and the mass percentages of the various components in the solvent remain consistent, the improvement in the cycle performance of the lithium battery using the electrolyte is limited.

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

[0040] Furthermore, in some embodiments of the present application, the mass of fluoroethylene carbonate accounts for 10% to 15% of the total mass of the electrolyte, for example, it can be 10%, 11%, 12%, 13%, 14%, 15% or a value between any two of the aforementioned values. Fluoroethylene carbonate can not only serve as a solvent, but also as a film-forming aid. Fluoroethylene carbonate can participate in the construction of SEI, reduce the side reactions between the electrolyte and lithium metal, and thus improve the cycle performance of the lithium battery. If the amount of fluoroethylene carbonate added is too much, serious gas production will occur, resulting in uneven lithium migration and deposition, thereby reducing the cycle performance of the lithium battery; if the amount of fluoroethylene carbonate added is too little, the stabilizing effect on the SEI is limited.

[0041] An embodiment of the present application also provides a method for preparing an electrolyte, which can be used to prepare any of the electrolytes described above, comprising the steps of: mixing lithium hexafluorophosphate, lithium difluorooxalatoborate, sodium hexafluorophosphate, a pyrrole ionic liquid, and a solvent in a proportion to obtain an electrolyte.

[0042] To further enhance the stability of the electrolyte and thereby improve the cycling performance of the lithium battery, in some embodiments of the present application, mixing lithium hexafluorophosphate, lithium difluorooxalatoborate, sodium hexafluorophosphate, a pyrrole-based ionic liquid, and a solvent according to a ratio includes the steps of providing a solvent at room temperature, adding the lithium hexafluorophosphate, lithium difluorooxalatoborate, sodium hexafluorophosphate, and the pyrrole-based 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] An embodiment of the present application also provides an application of an electrolyte, wherein the electrolyte is any one of the electrolytes described above, or the electrolyte is an electrolyte prepared by the preparation method of any one of the electrolytes described above.

[0044] An embodiment of the present 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 in any one of the 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 the embodiments of the present application, the lithium battery can be a laminated structure formed by stacking a positive electrode, a negative electrode, and a separator in a specific order, or a wound body formed by winding the laminated structure. According to the form, the lithium battery can be a laminated battery, a soft-pack battery, a prismatic battery, a cylindrical battery, a coin battery, or a button battery; according to the material, the lithium battery can be a lithium battery, a lithium-ion battery, a lithium-sulfur battery, or a lithium-air battery.

[0046] In some embodiments of the present 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, and the materials of the positive electrode active material layer include a positive electrode active substance, a positive electrode binder, and a positive electrode conductive agent. The materials 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 materials of the positive electrode current collector include but are 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 the positive electrode active material, their surfaces may have a coating layer, and the materials of the coating layer include but are 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 positive electrode preparation method may include the steps of: 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 on a positive electrode current collector, followed by a drying process and a roll pressing process to obtain a positive electrode plate. It should be noted that the first mixture may also be cast onto a separate carrier to form a film layer, which is then separated from the carrier and laminated onto 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 the present 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 material, a negative electrode binder and a negative electrode conductor. The material of 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 material of the negative electrode current collector is, but is not limited to, aluminum foil, platinum foil or palladium foil. The negative electrode active material is, but is not limited to, lithium, metals that can be alloyed with lithium, semi-metals, transition metal oxides, non-transition metal oxides and carbon materials. The metals or semi-metals that can be alloyed with lithium include, but are not limited to, Si, Sn, Al, Ge, Pb, Bi, Sb, Si-Y1 alloy (Y1 is an alkali metal, an alkaline earth metal, an element of Group 13 to 16, a transition metal, a rare earth element, or a combination thereof, excluding Si), and Sn-Y2 alloy (Y2 is an alkali metal, an alkaline earth metal, an element of Group 13 to 16, a transition metal, a rare earth element, or a combination thereof, excluding Sn). The transition metal oxide is, but is not limited to, one or more of lithium titanium oxide, vanadium oxide, lithium vanadium oxide and titanium niobium oxide. The non-transition metal oxide is, but is not limited to, SnO2 and SiO 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] In the embodiments of the present application, the preparation method of the lithium battery, for example, includes the steps of: first assembling the positive electrode, negative electrode, and separator to form a battery body, and then injecting the electrolyte into the battery body for infiltration to obtain the 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 the solvent. Among them, the solvent is composed of ethylene carbonate, diethyl carbonate, dimethyl carbonate, and fluoroethylene carbonate. The volume ratio of ethylene carbonate, diethyl carbonate, and dimethyl carbonate is 2:5:3, and the mass of fluoroethylene 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 fluoroethylene carbonate according to the formula ratio, mix them, and cool to room temperature to obtain the 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 the electrolyte.

[0057] In a glove box filled with argon, NCM811 was used as the positive electrode material (the positive electrode loading was 3.2 mAh, and the material of the positive electrode current collector was aluminum foil), lithium metal was used as the negative electrode material (the thickness of the lithium metal layer was 20 μm, and the material of the negative electrode current collector was copper foil), and a polyethylene film coated with Al2O3 on both sides (the coating thickness of each side was 2 μm) was used as a diaphragm. Under the condition of an injection volume of 1.6 g / Ah, a 6.2 Ah soft-pack battery was made, and then the electrolyte was injected into the interior of the battery for infiltration 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 electrolyte in this embodiment differs in that the mass percentage of sodium hexafluorophosphate in the electrolyte is replaced with "0.1%," and the mass percentage of the solvent is correspondingly modified. It should be noted that the volume ratios of ethylene carbonate, diethyl carbonate, and dimethyl carbonate, as well as the mass percentage of fluoroethylene carbonate, are the same as those in Example 1.

[0060] The preparation method of the electrolyte in this embodiment is carried out with reference to Example 1.

[0061] Compared with the lithium battery in Example 1, the lithium battery in this embodiment is different in that the electrolyte is the electrolyte of this embodiment.

[0062] Example 3

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

[0064] The preparation method of the electrolyte in this embodiment is carried out with reference to Example 1.

[0065] Compared with the lithium battery in Example 1, the lithium battery in this embodiment is different in that the electrolyte is the electrolyte of this embodiment.

[0066] Example 4

[0067] This embodiment provides an electrolyte and a lithium battery containing the electrolyte. Compared to the electrolyte in Example 1, the electrolyte in this embodiment differs in that the mass percentage of lithium difluorooxalatoborate in the electrolyte is replaced with "0.5%," and the mass percentage of the solvent is correspondingly modified. It should be noted that the volume ratios 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 carried out with reference to Example 1.

[0069] Compared with the lithium battery in Example 1, the lithium battery in this embodiment is different in that the electrolyte is the electrolyte of this embodiment.

[0070] Example 5

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

[0072] The preparation method of the electrolyte in this embodiment is carried out with reference to Example 1.

[0073] Compared with the lithium battery in Example 1, the lithium battery in this embodiment is different in that the electrolyte is the electrolyte of this embodiment.

[0074] Example 6

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

[0076] The preparation method of the electrolyte in this embodiment is carried out with reference to Example 1.

[0077] Compared with the lithium battery in Example 1, the lithium battery in this embodiment is different in that the electrolyte is the electrolyte of this embodiment.

[0078] Example 7

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

[0080] The preparation method of the electrolyte in this embodiment is carried out with reference to Example 1.

[0081] Compared with the lithium battery in Example 1, the lithium battery in this embodiment is different in that the electrolyte is the electrolyte of this embodiment.

[0082] Example 8

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

[0084] The preparation method of the electrolyte in this embodiment is carried out with reference to Example 1.

[0085] Compared with the lithium battery in Example 1, the lithium battery in this embodiment is different in that the electrolyte is the electrolyte of this embodiment.

[0086] Example 9

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

[0088] The preparation method of the electrolyte in this embodiment is carried out with reference to Example 1.

[0089] Compared with the lithium battery in Example 1, the lithium battery in this embodiment is different in that the electrolyte is the electrolyte of this embodiment.

[0090] Example 10

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

[0092] The preparation method of the electrolyte in this embodiment is carried out with reference to Example 1.

[0093] Compared with the lithium battery in Example 1, the lithium battery in this embodiment is different in that the electrolyte is the electrolyte of this embodiment.

[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-methylpyrrolidino hexafluorophosphate is omitted, and the mass percentage of the solvent is correspondingly modified. 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 those in Example 1.

[0096] The preparation method of the electrolyte in this comparative example is carried out with reference to Example 1.

[0097] Compared with the lithium battery in Example 1, the lithium battery in this comparative example is different 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 difference of the electrolyte in this comparative example is that "1.2% N-propyl-N-methylpyrrolidine hexafluorophosphate" is replaced by "1.2% 1-ethyl-3-methylimidazole hexafluorophosphate".

[0100] The preparation method of the electrolyte in this comparative example is carried out with reference to Example 1.

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

[0102] Experimental example

[0103] Performance testing was performed on the lithium batteries of Examples 1 to 10, Comparative Examples 1, and 2. Specifically, each formed lithium battery was subjected to a cycle test. Each lithium battery was charged at 25°C at a constant current rate of 0.33C to a voltage of 4.2V, then discharged at a constant current rate of 0.5C until the voltage reached 3V. This charge and discharge cycle was repeated 300 times, with a 10-minute interval between each cycle. The capacity retention rate after 150 cycles at 25°C was determined, and each lithium battery that completed the cycle test was subjected to a national standard needle penetration test.

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

[0105] Table 1

[0106]

[0107] As can be seen from Table 1, compared to the lithium batteries in Comparative Examples 1 and 2, the lithium batteries in Examples 1 to 10 have better overall performance. The lithium batteries in Examples 1 to 10 have good cycle performance and can pass the needle penetration test. Taking the lithium battery in Example 1 as an example, the capacity retention rate of the lithium battery in Example 1 after 150 cycles at 25°C is 11.8% higher than that of the lithium battery in Comparative Example 1, and the capacity retention rate of the lithium battery in Example 1 after 150 cycles at 25°C is 19.8% higher than that of the lithium battery in Comparative Example 2. The capacity retention rate of the lithium battery in Comparative Example 1 after 150 cycles at 25°C is higher than that of the lithium battery in Comparative Example 2, but the lithium battery in Comparative Example 1 fails the needle penetration test.

[0108] In addition, the lithium batteries of Example 1 and Comparative Example 1 were subjected to the aforementioned cycle test after formation. When the capacity retention rate was lower than 80%, the aforementioned cycle test was stopped. Then, the negative electrode morphology 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 figure, the A-side lithium layer and the B-side lithium layer are both film layers deposited on the copper foil layer during the long cycle of the lithium battery, and the A-side lithium layer and the B-side lithium layer respectively include SEI layers.

[0109] Figure 1 The thickness of the lithium layer on the A side 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 the B side is 55.4 μm at one position. Figure 1 The total thickness of the lithium layer on the A side, the copper foil layer, and the lithium layer on the B side at a position in the middle is 110 μm. Figure 2 The thickness of the lithium layer on the A side at the middle position is 72.2 μm ( Figure 2 The thickness of the copper foil layer is 7.94 μm, and the thickness of the lithium layer at one position on the B side is 71.6 μm ( Figure 2 The sum of 33.9 μm and 37.7 μm marked in ), and Figure 2 The sum of the thicknesses of the A-side lithium layer, the copper foil layer, and the B-side lithium layer at a position in the middle is 147 μm. It can be seen that during the cycle test, the thickness of the A-side lithium layer of the lithium battery in Example 1 is lower than the thickness of the A-side lithium layer of the lithium battery in Comparative Example 1, and the thickness of the B-side lithium layer of the lithium battery in Example 1 is lower than the thickness of the B-side lithium layer of the lithium battery in Comparative Example 1. In addition, 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 the deposition uniformity 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 the deposition uniformity of the lithium layer on side B of the lithium battery in Comparative Example 1.

[0110] This demonstrates that during the cycling test, the lithium battery in Example 1 exhibited significant improvement in gas expansion. After extended cycling, the film layer deposited on the copper foil exhibited a low thickness and relatively high deposition uniformity. Furthermore, the lithium battery in Example 1 exhibited excellent cycling performance, maintaining a capacity retention rate exceeding 90% after 150 cycles at 25°C. However, the lithium battery in Comparative Example 1 exhibited significant gas expansion during the cycling test. After extended cycling, the film layer deposited on the copper foil exhibited a high thickness and high surface roughness, indicating uneven deposition. Consequently, the lithium battery in Comparative Example 1 failed the needle penetration test.

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

[0112] The above is a detailed introduction to the embodiments of the present application. Specific examples are used herein to illustrate the principles and implementation methods of the present application. The description of the above embodiments is only used to help understand the method and core idea of ​​the present application. At the same time, for those skilled in the art, based on the ideas of the present application, there may be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as a limitation on the present application.

Claims

1. An electrolyte, characterized in that: Calculated by mass percentage, the electrolyte includes 16.5% to 21% of lithium hexafluorophosphate, 0.5% to 1.8% of lithium difluorooxalatoborate, 0.1% to 2% of sodium hexafluorophosphate, and 1% to 2% of pyrrole ionic liquid, with the remainder being solvent; The pyrrole ionic liquid has a structure shown in the following general formula (I): In the general formula (I), R1 and R2 are independently selected from C1 to C30 alkyl groups; In the solvent, the volume ratio of ethylene carbonate, diethyl carbonate and dimethyl carbonate is 1:(2-3):(1-2).

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

3. The electrolyte according to claim 1 or 2, characterized in that The R1 and R2 are 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, butylene carbonate, methyl propyl carbonate, ethyl propyl carbonate, dipropyl carbonate, fluoroethylene carbonate and diethyl carbonate.

5. The electrolyte according to claim 4, characterized in that The solvent consists of ethylene carbonate, diethyl carbonate, dimethyl carbonate and fluoroethylene carbonate.

6. The electrolyte according to claim 4, characterized in that The mass of the 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 difluorooxalatoborate is 1:(0.023-0.056).

8. A method for preparing an electrolyte, characterized in that: Used to prepare the electrolyte as described in any one of claims 1 to 7, the preparation method of the electrolyte comprises the steps of: mixing lithium hexafluorophosphate, lithium difluorooxalatoborate, sodium hexafluorophosphate, pyrrole ionic liquid and solvent according to a ratio to obtain the electrolyte.

9. The method for preparing the electrolyte according to claim 8, characterized in that: The mixing of lithium hexafluorophosphate, lithium difluorooxalatoborate, sodium hexafluorophosphate, pyrrole ionic liquid and solvent according to a ratio comprises the steps of providing a solvent at room temperature, adding lithium hexafluorophosphate, lithium difluorooxalatoborate, 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 claimed in any one of claims 1 to 7, or the electrolyte is the electrolyte prepared by the method for preparing the electrolyte as claimed in claim 9 or 8.

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

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