Electrolyte, preparation method thereof and secondary battery

Abstract

The invention belongs to the technical field of batteries, and particularly relates to an electrolyte and a preparation method thereof, and a secondary battery. The electrolyte comprises a main lithium salt, an auxiliary lithium salt, a main solvent, a film-forming additive, an ionic liquid and a reducing additive, wherein the reducing additive contains at least one functional group of cyano group, aldehyde group, thioether group and thioketone group; the ionic liquid comprises at least one of pyrroles, imidazoles, piperidines, quaternary ammonium salts, morpholines and phosphorus onium salts. After the ionic liquid and the reducing additive are added into the electrolyte, a stable interface layer can be formed on the surface of an electrode, so that direct contact between the electrolyte and an electrode material is prevented, side reactions are reduced, the high-voltage and high-temperature working stability of the battery is improved, and the service life of the battery is prolonged. The high-temperature cycle life and the safety of the battery can be obviously improved. The electrolyte can be applied to stable circulation at a high temperature of 60 DEG C or above.

Description

Technical Field

[0001] The present application belongs to the field of battery technology, and in particular relates to an electrolyte and a preparation method thereof, and a secondary battery. Background Art

[0002] In recent years, lithium metal batteries have attracted widespread attention due to their high specific energy. However, their poor electrochemical performance at high and low temperatures has seriously hindered their practical application. The main problem faced by the design of wide-temperature electrolytes is that under low temperature (below 0°C) or high temperature (above 40°C) conditions, battery performance drops sharply, and even causes safety accidents, making it difficult for lithium batteries to adapt to the diversity and variability of the global climate. Therefore, the development of wide-temperature lithium batteries has become a hot research direction that researchers are currently paying close attention to. Traditional lithium battery electrolytes are easily decomposed at high temperatures, resulting in accelerated lithium dendrite growth, reduced interface stability, and rapid degradation of electrochemical performance.

[0003] To develop high-temperature, high-voltage electrolytes, researchers have explored various approaches. While these technologies have demonstrated significant progress in high-temperature lithium metal electrolytes, several challenges remain. For example, the high cost of some highly stable solvents and lithium salts hinders large-scale application, and interfacial ion transport requires further optimization. Furthermore, the interfacial reaction mechanisms between electrolyte and electrode under high-temperature conditions are not fully understood, limiting precise guidance for electrolyte design. Summary of the Invention

[0004] The purpose of this application is to provide an electrolyte and a preparation method thereof, as well as a secondary battery, which aims to solve the problem of poor high temperature resistance of existing electrolytes for lithium batteries to a certain extent.

[0005] To achieve the above application objectives, the technical solutions adopted in this application are as follows:

[0006] In a first aspect, the present application provides an electrolyte comprising a main lithium salt, an auxiliary lithium salt, a main solvent, a film-forming additive, an ionic liquid, and a reducing additive; wherein the reducing additive has at least one functional group selected from the group consisting of a cyano group, an aldehyde group, a thioether group, and a thioketone group; and the ionic liquid comprises at least one selected from the group consisting of pyrroles, imidazoles, piperidines, quaternary ammonium salts, morpholines, and phosphonium salts.

[0007] In some possible implementations, the cation in the ionic liquid includes at least one of a side chain-containing pyrrole, a side chain-containing imidazole, a side chain-containing piperidine, a side chain-containing quaternary ammonium salt, a side chain-containing morpholine, and a side chain-containing phosphonium.

[0008] In some possible implementations, the anions in the ionic liquid include at least one of bistrifluoromethanesulfonyl imide ions, bisfluorosulfonyl imide ions, and tetrafluoroborate ions.

[0009] In some possible implementations, the side chain of the ionic liquid contains at least one functional group selected from the group consisting of an ether group, a fluorinated alkyl group, and a nitrile group.

[0010] In some possible implementations, in the ionic liquid, the cation includes a pyrrole containing a side chain, and the anion includes a bistrifluoromethanesulfonyl imide ion.

[0011] In some possible implementations, the ionic liquid includes At least one of the following, wherein TFSI - It is a bis(trifluoromethanesulfonyl)imide ion.

[0012] In some possible implementations, the reducing additive includes a nitrile additive.

[0013] In some possible implementations, the main solvent includes at least one of an ether solvent, a fluorinated ether solvent, and a siloxane solvent.

[0014] In some possible implementations, the film-forming additive includes at least one of fluoroethylene carbonate, vinylene carbonate, LiPO2F2, and fluorinated silicone.

[0015] In some possible implementations, the main lithium salt includes at least one of LiTFSI, LiPF6, LiDFOB, and LiBF4.

[0016] In some possible implementations, the auxiliary lithium salt includes at least one of LiNO 3 , LiBOB, and LiDFOB.

[0017] In some possible implementations, the nitrile additive includes at least one of acetonitrile, adiponitrile, succinonitrile, glutaronitrile, and suberonitrile.

[0018] In some possible implementations, the main solvent includes at least one of ethylene glycol dimethyl ether, fluoro-1,4-dimethoxybutane, dimethylmethoxy (3,3,3-trifluoropropyl) silane, trimethoxy (3,3,3-trifluoropropyl) silane, ethylene glycol monomethyl ether, and propylene glycol methyl ether.

[0019] In some possible implementations, the volume percentage of the ionic liquid is 20% to 50%.

[0020] In some possible implementations, the volume percentage of the reducing additive is 5% to 15%.

[0021] In some possible implementations, the concentration of the main lithium salt is 0.8 mol / L to 1.5 mol / L.

[0022] In some possible implementations, the concentration of the auxiliary lithium salt is 0.1 mol / L to 0.3 mol / L.

[0023] In some possible implementations, the volume percentage of the main solvent is 50% to 70%.

[0024] In some possible implementations, the volume percentage of the film-forming additive is 2% to 15%.

[0025] In some possible implementations, the electrolyte is suitable for a circulation condition with a temperature of 0°C to 70°C.

[0026] In a second aspect, the present application provides a method for preparing an electrolyte, comprising the following steps:

[0027] In a dry inert atmosphere, a main lithium salt, an auxiliary lithium salt, a main solvent, a film-forming additive, an ionic liquid and a reducing additive are mixed to prepare an electrolyte; wherein the reducing additive has at least one functional group selected from the group consisting of a cyano group, an aldehyde group, a sulfide group and a thioketone group; and the ionic liquid includes at least one selected from the group consisting of pyrroles, imidazoles, piperidines, quaternary ammonium salts, morpholines and phosphonium salts.

[0028] In a third aspect, the present application provides a secondary battery, comprising a positive electrode, a negative electrode, an electrolyte and a separator, wherein the electrolyte comprises the above-mentioned electrolyte or the electrolyte prepared by the above-mentioned method.

[0029] The electrolyte provided in the first aspect of the present application comprises a main lithium salt, an auxiliary lithium salt, a main solvent, a film-forming additive, an ionic liquid and a reducing additive. On the one hand, the reducing additive has at least one reducing functional group of a cyano group, an aldehyde group, a thioether group, or a thioketone group, and these reducing functional groups in the electrolyte have a strong electron-attracting effect, making it difficult to be further reduced, thereby making the electrolyte have better stability on the battery electrode side. And these reducing additives can form a SEI (solid electrolyte interface film) or CEI (positive electrode electrolyte interface film) rich in inorganic components on the electrode surface, improve interface stability, improve high-voltage stability and reduce interface resistance, thereby improving the overall antioxidant capacity of the electrolyte, improving the kinetic performance of the electrolyte under high temperature conditions in the battery, so as to promote excellent cycle stability. On the other hand, the ionic liquid includes at least one of pyrroles, imidazoles, piperidines, quaternary ammonium salts, morpholines, and phosphonium salts, so that the ionic liquid has a lower HOMO energy level and a higher LUMO energy level. The lower HOMO (highest occupied molecular orbital) energy level of the ionic liquid means that the electrons in its highest occupied orbital are more difficult to be oxidized, thereby improving the antioxidant capacity of the electrolyte, making the electrolyte more stable at high potentials, and not easily decomposed on the positive electrode side during battery operation. The higher LUMO (lowest unoccupied molecular orbital) energy level means that the ionic liquid is more difficult to accept electrons, thereby improving the electrolyte's anti-reduction ability, making the electrolyte not easily decomposed on the negative electrode side during battery operation. Therefore, after adding ionic liquids and reducing additives to the electrolyte, the high-temperature cycle life and safety of the battery can be significantly improved in this application. The electrolyte can be used for stable circulation at high temperatures above 60°C.

[0030] The second aspect of the present application is to prepare a secondary battery electrolyte with excellent cycle performance and the ability to operate under high temperature conditions. Among them, the ionic liquid affects the charge distribution and the interaction between cations and anions through functional groups, has a lower HOMO and a higher LUMO energy level, and has good electrochemical stability, thereby preventing the electrolyte from producing oxidation reactions with the surface of the electrode material during the continuous circulation process. In addition, by adding a reducing additive, the electrolyte can achieve high voltage and high temperature operation stability, significantly improving the high temperature cycle life of the battery. Moreover, by mixing ionic liquids, reducing additives with main solvents, film-forming additives and other components, it is beneficial to reduce the cost of the electrolyte.

[0031] The secondary battery provided in the third aspect of the present application has excellent high-temperature cycle life and safety performance because the electrolyte contains the electrolyte with good high-temperature cycle stability and high safety. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the embodiments or descriptions of the prior art. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0033] Figure 1 This is a schematic diagram comparing the lithium negative electrode interface after using the electrolyte provided in the embodiment of the present application;

[0034] Figure 2 The room temperature cycling performance of the Li||LFP lithium metal battery of the present application in the electrolyte of Example 1;

[0035] Figure 3 The room temperature cycling performance of the Li||LFP lithium metal battery of the present application in the electrolyte of Example 2;

[0036] Figure 4 The room temperature cycling performance of the Li||LFP lithium metal battery of the present application in the electrolyte of Example 3;

[0037] Figure 5 The room temperature cycling performance of the Li||LFP lithium metal battery of the present application in the electrolyte of Example 4;

[0038] Figure 6 The room temperature cycling performance of the Li||LFP lithium metal battery of the present application in the electrolyte of Example 5;

[0039] Figure 7 The room temperature cycling performance of the Li||LFP lithium metal battery of the present application in the electrolyte of Example 6;

[0040] Figure 8 The room temperature cycling performance of the Li||LFP lithium metal battery of the present application in the electrolyte of Example 7;

[0041] Figure 9 This is the room temperature cycling performance of the Li||LFP lithium metal battery of the present application in the commercial electrolyte of Comparative Example 1 (1M LiPF6 inEC:DMC (volume ratio 1:1));

[0042] Figure 10 This is the room temperature cycling performance of the Li||LFP lithium metal battery of the present application in the electrolyte of Comparative Example 2;

[0043] Figure 11 This is the room temperature cycling performance of the Li||LFP lithium metal battery of the present application in the electrolyte of Comparative Example 3;

[0044] Figure 12This is the cycling performance of the Li||LFP lithium metal battery of the present application under high rate 4C conditions in the electrolyte of Example 3;

[0045] Figure 13 This is a comparison of the high temperature (60°C) cycling performance of the Li||LFP lithium metal battery of the present application in the electrolyte of Example 3 and the commercial electrolyte of Comparative Example 1 (1MLiPF6 in EC:DMC (volume ratio 1:1));

[0046] Figure 14 This is the high temperature (60°C) cycling performance of the Li||LFP lithium metal battery of the present application in the electrolyte of Example 1;

[0047] Figure 15 This is the high temperature (60°C) cycling performance of the Li||LFP lithium metal battery of the present application in the electrolyte of Example 2;

[0048] Figure 16 This is the high temperature (60°C) cycling performance of the Li||LFP lithium metal battery of the present application in the electrolyte of Example 4;

[0049] Figure 17 This is the high temperature (60°C) cycling performance of the Li||LFP lithium metal battery of the present application in the electrolyte of Example 5;

[0050] Figure 18 This is the high temperature (60°C) cycling performance of the Li||LFP lithium metal battery of the present application in the electrolyte of Example 6;

[0051] Figure 19 This is the high temperature (60°C) cycling performance of the Li||LFP lithium metal battery of the present application in the electrolyte of Example 7;

[0052] Figure 20 This is the high temperature (60°C) cycling performance of the Li||LFP lithium metal battery of the present application in the electrolyte of Comparative Example 2;

[0053] Figure 21 This is the high temperature (60°C) cycling performance of the Li||LFP lithium metal battery of the present application in the electrolyte of Comparative Example 3. DETAILED DESCRIPTION

[0054] In order to make the technical problems, technical solutions and beneficial effects to be solved by this application more clearly understood, the present application is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit this application.

[0055] In this application, the term "and / or" describes the relationship between associated objects, indicating that three possible relationships exist. For example, A and / or B can represent: A exists alone, A and B exist simultaneously, and B exists alone. A and B can be singular or plural. The character " / " generally indicates that the associated objects are in an "or" relationship.

[0056] In this application, "at least one" means one or more, and "plurality" means two or more. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single items or plural items. For example, "at least one of a, b or c", or "at least one of a, b and c" can all mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, c can be single or multiple.

[0057] It should be understood that in the various embodiments of the present application, the size of the serial numbers of the above-mentioned processes does not mean the order of execution. Some or all of the steps can be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0058] The terms used in the embodiments of the present application are only for the purpose of describing specific embodiments and are not intended to limit the present application. The singular forms "a", "an" and "the" used in the embodiments of the present application and the appended claims are also intended to include plural forms unless the context clearly indicates otherwise.

[0059] The weights of the relevant components mentioned in the examples of this specification may not only refer to the specific content of each component, but also represent the weight ratio between the components. Therefore, as long as the content of the relevant components in the examples of this specification is proportionally enlarged or reduced according to the examples of this specification, it is within the scope disclosed in the examples of this specification. Specifically, the mass described in the examples of this specification may be μg, mg, g, kg, etc., which are mass units commonly known in the chemical industry.

[0060] The terms "first" and "second" are used solely for descriptive purposes to distinguish objects, such as substances, from one another and should not be understood to indicate or imply relative importance or to implicitly specify the quantity of the technical features being referred to. For example, without departing from the scope of the embodiments of this application, a first XX may also be referred to as a second XX, and similarly, a second XX may also be referred to as a first XX. Thus, features defined as "first" or "second" may explicitly or implicitly include one or more of such features.

[0061] Conventional lithium battery electrolytes are prone to decomposition at high temperatures, leading to accelerated lithium dendrite growth, reduced interfacial stability, and rapid degradation of electrochemical performance. Ideal high-temperature electrolytes typically need to simultaneously meet the following conditions: (1) ensuring high ionic conductivity of the SEI / CEI by regulating interfacial properties; (2) improved thermal stability; (3) excellent electrochemical stability; and (4) a stable SEI / CEI membrane at high temperatures. Currently, electrolyte systems commonly used to broaden the operating temperature of lithium metal batteries can be divided into locally concentrated electrolytes, weakly solvated electrolytes, liquefied gas electrolytes, and polymer electrolytes. Furthermore, electrolyte additives can also broaden the battery's temperature range to a certain extent.

[0062] To address these issues, researchers have explored various approaches: 1. developing high-temperature stable solvents, 2. developing highly stable lithium salts, 3. developing functional additives, 4. developing solid electrolytes as novel electrolyte systems, and 5. developing composite electrolyte systems. While these technologies have demonstrated significant progress in high-temperature lithium metal electrolytes, several challenges remain. For example, the high cost of some highly stable solvents and lithium salts makes large-scale application difficult, and interfacial ion transport requires further optimization. Furthermore, the interfacial reaction mechanisms between electrolytes and electrodes at high temperatures are not fully understood, limiting precise guidance for electrolyte design.

[0063] Based on the above considerations, in order to solve the problem of poor high temperature resistance of existing electrolytes for lithium batteries, the research and development of high temperature lithium metal electrolytes in the embodiments of this application will focus on low cost of materials, in-depth research on interfacial reaction mechanisms, and comprehensive optimization of electrolyte performance through the coordinated design of multifunctional additives and composite systems.

[0064] In a first aspect, an embodiment of the present application provides an electrolyte, comprising a main lithium salt, an auxiliary lithium salt, a main solvent, a film-forming additive, an ionic liquid, and a reducing additive; wherein the reducing additive has at least one functional group selected from the group consisting of a cyano group, an aldehyde group, a thioether group, and a thioketone group; and the ionic liquid comprises at least one selected from the group consisting of pyrroles, imidazoles, piperidines, quaternary ammonium salts, morpholines, and phosphonium salts.

[0065] The electrolyte provided in the first aspect of the embodiment of the present application includes a main lithium salt, an auxiliary lithium salt, a main solvent, a film-forming additive, an ionic liquid and a reducing additive. On the one hand, the reducing additive has at least one reducing functional group of a cyano group, an aldehyde group, a thioether group, and a thioketone group. These reducing functional groups in the electrolyte have a strong electron-attracting effect, making it difficult to be further reduced, so that the electrolyte has better stability on the battery electrode side. And these reducing additives can form a SEI (solid electrolyte interface film) or CEI (positive electrode electrolyte interface film) rich in inorganic components on the electrode surface, improve interface stability, improve high-voltage stability and reduce interface resistance, thereby improving the overall antioxidant capacity of the electrolyte, improving the kinetic performance of the electrolyte under high temperature conditions in the battery, so as to promote excellent cycle stability. On the other hand, the ionic liquid includes at least one of pyrroles, imidazoles, piperidines, quaternary ammonium salts, morpholines, and phosphonium salts, so that the ionic liquid has a lower HOMO energy level and a higher LUMO energy level. The lower HOMO (highest occupied molecular orbital) energy level of the ionic liquid means that the electrons in its highest occupied orbital are more difficult to be oxidized, thereby improving the antioxidant capacity of the electrolyte, making the electrolyte more stable at high potentials, and not easily decomposed on the positive electrode side during battery operation. The higher LUMO (lowest unoccupied molecular orbital) energy level means that the ionic liquid is more difficult to accept electrons, thereby improving the electrolyte's anti-reduction ability, making the electrolyte not easily decomposed on the negative electrode side during battery operation. Therefore, after adding ionic liquids and reducing additives to the electrolyte in the embodiment of the present application, the high temperature cycle life and safety of the battery can be significantly improved. This allows the electrolyte to be used for stable circulation at high temperatures above 60°C.

[0066] In some possible implementations, the cations in the ionic liquid include at least one of a side chain-containing pyrrole, a side chain-containing imidazole, a side chain-containing piperidine, a side chain-containing quaternary ammonium salt, a side chain-containing morpholine, and a side chain-containing phosphonium. In the electrolyte of the present application, imidazole ionic liquids have high conductivity, low viscosity, and strong antioxidant stability; piperidine ionic liquids have good chemical stability and a wide electrochemical window; quaternary ammonium salt ionic liquids have high electrochemical stability and low volatility; morpholine ionic liquids have good structural rigidity and moderate viscosity; phosphonium salt ionic liquids have good thermal stability, are suitable for high temperature systems, and have high density. Active functional groups can be further introduced through side chain modification to better improve the high temperature cycle stability and safety of the electrolyte.

[0067] In some possible implementations, the anion in the ionic liquid includes a bis(trifluoromethanesulfonyl)imide ion (TFSI) - ), bis(fluorosulfonyl imide) ion (FSI - ), tetrafluoroborate ion (BF4 -) at least one of these anions. These anions can improve the stability of ionic liquids, among which the bis(trifluoromethanesulfonyl)imide ion TFSI - With high stability, low viscosity and excellent ionic conductivity; bis(fluorosulfonyl)imide FSI - Able to optimize low temperature performance and ionic conductivity; tetrafluoroborate BF4 - Has higher oxidation stability.

[0068] In some possible implementations, the ionic liquid contains at least one functional group selected from the group consisting of an ether group, a fluorinated alkyl group, and a nitrile group in its side chain. In this case, the presence of functional groups such as ether, fluorinated alkyl, and nitrile groups in the ionic liquid's side chain influences charge distribution. Furthermore, through interactions between cations and anions, ionic liquids containing functional groups in their side chains have lower HOMO and higher LUMO energy levels, resulting in better electrochemical stability.

[0069] In some possible implementations, the side chain of the ionic liquid contains an ether group, and the ether functional group (-O-) has a low coordination ability and can react with metal cations (such as Li + ) form a weaker coordination interaction, which is conducive to the desolvation of lithium ions and improves the interfacial dynamics. The presence of this functional group makes ionic liquids more stable in electrochemical processes, especially under high temperature conditions.

[0070] In some possible implementations, the cation in the ionic liquid includes a pyrrole containing a side chain, and the anion includes a bistrifluoromethanesulfonyl imide ion. In this case, the ionic liquid can better improve the high-temperature cycling performance of the electrolyte.

[0071] In some possible implementations, the ionic liquid comprises P14, At least one of P14o, wherein TFSI - In this case, the pyrrole cation has a lower reduction potential and higher cathode stability among many ionic liquids; the anion of the ionic liquid is preferably TFSI. - (The chemical formula is [(CF3SO2)2N] - , the structural formula is ). In addition, the performance of ionic liquids can be further improved by using side chain structures with different chain lengths and functional groups in the cations. The side chains of these ionic liquids are three carbon atoms (P13), four carbon atoms (P14), and oxygen substituted for the carbon in the four carbon atoms (P14O). The ether functional group (-O-) in the side chain of P14O can reduce the interaction between cations and anions, reduce the viscosity of the ionic liquid, and improve the conductivity. +The solvation structure of P13 and P14 facilitates desolvation and improves lithium ion transport. Oxygen's lone pair electrons facilitate the formation of a more stable solvation structure for cations, improving interfacial stability and inhibiting lithium dendrites. The carbon chain structure in P13 and P14 provides a certain steric hindrance effect, which can also reduce cation-anion interactions, improve the fluidity of the ionic liquid, and provide better reduction stability. However, the effect may not be as good as P14O in terms of high temperature and ion migration.

[0072] In some possible implementations, the ionic liquid of the embodiment of the present application can also be used to better optimize the electrochemical stability by adjusting other components of the electrolyte, such as adding appropriate co-solvents or additives.

[0073] In some possible implementations, the volume percentage of the ionic liquid is 20% to 50%. For example, the volume percentage of the ionic liquid can be any typical but non-limiting value such as 20%, 30%, 40%, or 50%, or an interval between any two values. In this case, the ionic liquid content is beneficial for comprehensively improving the interfacial stability and conductivity of the electrolyte, as well as the viscosity and wettability of the electrolyte.

[0074] In some possible implementations, the reducing additive includes a nitrile additive. In this case, the introduction of nitrile additives is conducive to improving the kinetic performance of secondary batteries such as lithium metal batteries under high temperature conditions, wherein the reducing cyano functional group plays a key role in the process of reducing transition metals. The cyano group can form an SEI (solid electrolyte interface film) or CEI (positive electrode electrolyte interface film) rich in inorganic components on the electrode surface, thereby improving the interface stability and reducing the interface resistance. The cyano group can also change the solvation shell structure of the Li ion, improve the desolvation ability of the lithium ion at low temperature, and thus improve the low temperature performance. In addition, the cyano group has a strong electron attraction effect, making it more difficult to be further reduced, thereby having better stability on the negative electrode side, improving the overall antioxidant capacity of the electrolyte, and allowing the electrolyte to be used for stable circulation at high temperatures above 60°C.

[0075] In some possible implementations, the nitrile additive includes at least one of acetonitrile, adiponitrile, succinonitrile, glutaronitrile, and suberonitrile. These nitrile additives, when introduced into the electrolyte, can improve the kinetic performance of secondary batteries, such as lithium metal batteries, under high-temperature conditions. The reducing cyano functional group plays a key role in the reduction of transition metals. They can form a stable SEI / CEI interface on the electrode surface, promoting excellent cycling stability and enabling stable cycling of the electrolyte at temperatures above 60°C.

[0076] In some possible implementations, the volume percentage of the reducing additive is 5% to 15%. For example, the volume percentage of the reducing additive can be any typical but non-limiting value such as 5%, 10%, or 15%, or an interval between any two values. In this case, the content of the reducing additive is beneficial to the uniformity of lithium metal deposition, reduces side reactions, and provides a stable protective effect on the formation of the SEI film.

[0077] In some possible implementations, the main solvent includes at least one of an ether solvent, a fluorinated ether solvent, and a siloxane solvent. These main solvents can all dissolve lithium salt components, which is beneficial to ion transport. Among them, ether solvents have low viscosity and high ionic conductivity, can effectively dissolve lithium salts and promote lithium ion transport. They have good compatibility with lithium metal negative electrodes and help form a stable solid electrolyte interface (SEI). Fluorinated ether solvents improve oxidative stability by introducing -F groups while maintaining the ability to dissolve lithium salts. Siloxane solvents expand the voltage window through a multi-salt system and reduce the side reactions of free solvent molecules on the negative electrode.

[0078] In some possible implementations, the primary solvent includes at least one of ethylene glycol dimethyl ether, 1,4-dimethoxybutane fluoride, dimethylmethoxy(3,3,3-trifluoropropyl)silane, trimethoxy(3,3,3-trifluoropropyl)silane, ethylene glycol monomethyl ether, and propylene glycol methyl ether. These primary solvents are all capable of effectively dissolving lithium salts and promoting lithium ion transport.

[0079] In some possible implementations, the volume percentage of the main solvent is 50% to 70%. For example, the volume percentage of the main solvent can be any typical but non-limiting value such as 50%, 60%, or 70%, or an interval between any two values. In this case, the main solvent can fully dissolve the lithium salt and promote lithium ion transport.

[0080] In some possible implementations, the film-forming additive includes at least one of fluoroethylene carbonate, vinylene carbonate, LiPO2F2, and fluorinated silicone. These film-forming additives are all conducive to the formation of a LiF-rich SEI layer. Among them, fluoroethylene carbonate (FEC), as a film-forming additive, preferentially decomposes on the surface of the negative electrode to form a lithium fluoride (LiF)-rich SEI layer, inhibiting the growth of lithium dendrites. Improve the high-voltage stability of the electrolyte and reduce the oxidative decomposition of the solvent on the positive electrode side. Vinylene carbonate (VC) can form a stable SEI layer through polymerization reaction. LiPO2F2 strengthens the mechanical strength of SEI through phosphate groups. Fluorinated silicone has both film-forming and solvent functions and can reduce interfacial impedance.

[0081] In some possible implementations, the volume percentage of the film-forming additive is 2% to 15%. For example, the volume percentage of the film-forming additive can be any typical but non-limiting value, such as 2%, 5%, 10%, or 15%, or an interval between any two values. In this case, it is beneficial to ensure the formation of a LiF-rich SEI layer of appropriate thickness, and avoid the formation of an overly dense SEI layer that affects lithium ion transport, interface stability, and Coulombic efficiency.

[0082] In some possible implementations, the primary lithium salt includes at least one of LiTFSI (lithium bis(trifluoromethanesulfonyl)imide), LiPF6 (lithium hexafluorophosphate), LiDFOB (lithium difluorooxalatoborate), and LiBF4 (lithium tetrafluoroborate). LiTFSI has high conductivity and thermal stability; LiPF6 is low-cost; the combination of LiDFOB and LiTFSI can improve SEI stability; and LiBF4 is suitable for low-temperature applications.

[0083] In some possible implementations, the concentration of the main lithium salt is 0.8 mol / L to 1.5 mol / L. Exemplarily, the concentration of the main lithium salt can be 0.8 mol / L, 1.0 mol / L, 1.2 mol / L, 1.4 mol / L, 1.5 mol / L, and other typical but non-limiting arbitrary point values or interval values between any two point values. In this case, sufficient lithium ions can be provided to ensure ion transport. In some embodiments, when the main lithium salt is LiTFSI, the concentration is preferably 1 mol / L to 1.5 mol / L; when the main lithium salt is LiPF6, the concentration is preferably 0.8 mol / L to 1.2 mol / L; when the main lithium salt is LiDFOB, the concentration is preferably 0.2 mol / L to 0.5 mol / L; when the main lithium salt is LiBF4, the concentration is preferably 0.5 mol / L to 1 mol / L.

[0084] In some possible implementations, the auxiliary lithium salt includes at least one of LiNO3 (lithium nitrate), LiBOB (lithium dioxalatoborate or lithium bisoxalatoborate), and LiDFOB (lithium difluorooxalatoborate). The main differences between these auxiliary lithium salts and the main lithium salts are as follows: the main lithium salt provides the main lithium ion source with a high concentration and dominates the electrolyte ion transport. The auxiliary lithium salt is only used for interface regulation and does not contribute to the main ion conduction. As a sacrificial additive, it takes precedence over solvent decomposition and reduces the activity of side reactions. Among them, LiNO3 promotes the formation of SEI containing Li3N at the negative electrode interface, improving the interface ion conductivity and lithium deposition uniformity. LiBOB stabilizes SEI through borate groups. LiDFOB has both film-forming and antioxidant properties and is suitable for high voltage systems.

[0085] In some possible implementations, the auxiliary lithium salt concentration is 0.1 mol / L to 0.3 mol / L. For example, the auxiliary lithium salt concentration can be any typical but non-limiting value, such as 0.1 mol / L, 0.2 mol / L, or 0.3 mol / L, or an interval between any two values. In this case, the interfacial impedance can be effectively reduced, the positive electrode stability can be improved, lithium dendrites can be effectively suppressed, and Coulombic efficiency can be ensured.

[0086] In some possible implementations, the electrolyte is suitable for circulation conditions with a temperature of 0°C to 70°C. After adding ionic liquids and reducing additives to the electrolyte in the embodiment of the present application, these additives will form a stable interface layer on the surface of the electrode. This not only prevents direct contact between the electrolyte and the electrode material, but also reduces the occurrence of side reactions, thereby improving the cycle stability and safety of the battery under high temperature conditions. The electrolyte is suitable for application conditions such as low temperature, room temperature, and high temperature at the same time, and in particular, it can be used to stably operate in secondary batteries such as lithium metal batteries at high temperatures above 60°C. Exemplarily, the circulation temperature of the electrolyte can be 0°C, 10°C, 20°C, 30°C, 40°C, 50°C, 60°C, 70°C, etc., typical but non-restrictive arbitrary point values or interval values between any two point values.

[0087] The electrolytes of the above embodiments of the present application can be prepared by the following methods.

[0088] In a second aspect, an embodiment of the present application provides a method for preparing an electrolyte, comprising the following steps:

[0089] S10. In a dry inert atmosphere, a main lithium salt, an auxiliary lithium salt, a main solvent, a film-forming additive, an ionic liquid and a reducing additive are mixed to prepare an electrolyte; wherein the reducing additive has at least one functional group selected from the group consisting of a cyano group, an aldehyde group, a sulfide group and a thioketone group; and the ionic liquid includes at least one selected from the group consisting of pyrrole, imidazole, piperidine, quaternary ammonium salt, morpholine and phosphonium salt.

[0090] The embodiment of the present application prepares an electrolyte for secondary batteries such as lithium metal batteries that have excellent cycle performance and can operate under high temperature conditions. Among them, the ionic liquid affects the charge distribution and the interaction between cations and anions through functional groups, has a lower HOMO and a higher LUMO energy level, and has good electrochemical stability, thereby preventing the electrolyte from producing oxidation reactions with the surface of the electrode material during the continuous circulation process. In addition, by adding a reducing additive, the electrolyte can achieve high voltage and high temperature operation stability, significantly improving the high temperature cycle life of the battery. Moreover, by mixing ionic liquids, reducing additives with main solvents, film-forming additives and other components, it is beneficial to reduce the cost of the electrolyte.

[0091] The electrolyte of the embodiment of the present application is prepared in a dry inert atmosphere to avoid interference of moisture and oxygen on the electrolyte components. In some embodiments, in a glove box filled with nitrogen / argon / helium (H2O<1ppm, O2<1ppm), the main solvent, ionic liquid, film-forming aid and reducing additive are mixed in a volume ratio of (50-70): (20-50): (2-15): (5-15), and the main lithium salt and auxiliary lithium salt are added for dissolution. The concentration of the main lithium salt in the prepared electrolyte is 0.8mol / L~1.5mol / L, and the concentration of the auxiliary lithium salt is 0.1mol / L~0.3mol / L.

[0092] In a third aspect, an embodiment of the present application provides a secondary battery, the secondary battery comprising a positive electrode, a negative electrode, an electrolyte and a separator, wherein the electrolyte comprises the above-mentioned electrolyte.

[0093] Since the electrolyte of the secondary battery of the embodiment of the present application contains the electrolyte with good high-temperature cycle stability and high safety, the secondary battery also has excellent high-temperature cycle life and safety performance.

[0094] In some embodiments, in a secondary battery, the positive electrode material can be lithium cobalt oxide, lithium iron phosphate, ternary material, lithium manganese oxide, etc., the negative electrode material can be graphite, silicon-based material, lithium titanate, metallic lithium, etc., and the diaphragm can be polyolefin, ceramic coating diaphragm, nanofiber diaphragm, etc.

[0095] In some embodiments, the secondary battery includes a lithium metal battery, a lithium ion battery, and the like.

[0096] In order to make the above implementation details and operations of this application clearly understood by those skilled in the art, as well as to demonstrate the significant improvement in performance of the electrolyte and its preparation method, and the secondary battery of the embodiment of this application, the above technical solution is illustrated by multiple embodiments below.

[0097] Example 1

[0098] An electrolyte, the preparation of which comprises the steps of:

[0099] In an argon-filled glove box (H2O<1ppm, O2<1ppm), ethylene glycol dimethyl ether (DME), ionic liquid P 13 TFSI, fluoroethylene carbonate (FEC), and adiponitrile were mixed in a volume ratio of 5:3:1:1 to prepare an electrolyte of 1 mol / L LiFSI+0.3 mol / L LiNO3, and magnetic stirring was performed for 24 h to obtain the electrolyte of Example 1.

[0100] Example 2

[0101] An electrolyte solution, the preparation of which comprises the steps of:

[0102] In an argon-filled glove box (H2O<1ppm, O2<1ppm), ethylene glycol dimethyl ether (DME), ionic liquid P 14 TFSI, fluoroethylene carbonate (FEC), and adiponitrile were mixed in a volume ratio of 5:3:1:1 to prepare an electrolyte of 1 mol / L LiFSI+0.3 mol / L LiNO3, and magnetic stirring was performed for 24 h to obtain the electrolyte of Example 2.

[0103] Example 3

[0104] An electrolyte solution, the preparation of which comprises the steps of:

[0105] In an argon-filled glove box (H2O<1ppm, O2<1ppm), ethylene glycol dimethyl ether (DME), ionic liquid P 14O TFSI, fluoroethylene carbonate (FEC), and adiponitrile were mixed in a volume ratio of 5:3:1:1 to prepare an electrolyte of 1 mol / L LiFSI+0.3 mol / L LiNO3, and magnetic stirring was performed for 24 h to obtain the electrolyte of Example 3.

[0106] Example 4

[0107] An electrolyte solution, the preparation of which comprises the steps of:

[0108] In an argon-filled glove box (H2O<1ppm, O2<1ppm), ethylene glycol dimethyl ether (DME), ionic liquid P 14O TFSI, fluoroethylene carbonate (FEC), and adiponitrile (HN) were mixed in a volume ratio of 5:3:1:1 to prepare an electrolyte of 1 mol / L LiFSI+0.1 mol / L LiNO3, and magnetic stirring was performed for 24 h to obtain the electrolyte of Example 4.

[0109] Example 5

[0110] An electrolyte solution, the preparation of which comprises the steps of:

[0111] In an argon-filled glove box (H2O<1ppm, O2<1ppm), ethylene glycol dimethyl ether (DME), ionic liquid P 14O TFSI, fluoroethylene carbonate (FEC), and acetonitrile (AN) were mixed in a volume ratio of 5:3:1:1 to prepare an electrolyte of 1 mol / L LiFSI+0.3 mol / L LiNO3, and magnetic stirring was performed for 24 h to obtain the electrolyte of Example 5.

[0112] Example 6

[0113] An electrolyte solution, the preparation of which comprises the steps of:

[0114] In an argon-filled glove box (H2O<1ppm, O2<1ppm), ethylene glycol dimethyl ether (DME), ionic liquid P 14O TFSI, fluoroethylene carbonate (FEC), and succinonitrile (BAN) were mixed in a volume ratio of 5:3:1:1 to prepare an electrolyte of 1 mol / L LiFSI+0.3 mol / L LiNO3, and magnetic stirring was performed for 24 h to obtain the electrolyte of Example 6.

[0115] Example 7

[0116] An electrolyte solution, the preparation of which comprises the steps of:

[0117] In an argon-filled glove box (H2O<1ppm, O2<1ppm), ethylene glycol dimethyl ether (DME), ionic liquid P 14O TFSI, fluoroethylene carbonate (FEC), and glutaronitrile (GAN) were mixed in a volume ratio of 5:3:1:1 to prepare an electrolyte of 1 mol / L LiFSI + 0.3 mol / L LiNO3, and magnetic stirring was performed for 24 h to obtain the electrolyte of Example 7.

[0118] Comparative Example 1

[0119] A commercial electrolyte 1M LiPF6 in EC:DMC (volume ratio 1:1) was used as comparative example 1.

[0120] Comparative Example 2

[0121] An electrolyte solution, the preparation of which comprises the steps of:

[0122] In a glove box filled with argon (H2O < 1ppm, O2 < 1ppm), DME:P 14O TFSI:FEC were mixed in a volume ratio of 5:3:2 (without nitrile solvent) to prepare an electrolyte of 1 mol / L LiFSI+0.3 mol / L LiNO3, and magnetic stirring was performed for 24 h to obtain the electrolyte of Comparative Example 2.

[0123] Comparative Example 3

[0124] An electrolyte solution, the preparation of which comprises the steps of:

[0125] In an argon-filled glove box (H2O <1ppm, O2 <1ppm), DME:FEC:HN was mixed in a volume ratio of 8:1:1 (without ionic liquid) to prepare an electrolyte of 1 mol / L LiFSI + 0.3 mol / L LiNO3, and magnetic stirring was performed for 24 hours to obtain the electrolyte of Comparative Example 3.

[0126] In order to verify the progress of the examples of the present application, the electrolytes prepared in the above examples and comparative examples were applied to lithium metal batteries to conduct electrochemical performance tests:

[0127] After the active material LiFePO4 (LFP) and conductive carbon black (Super P) are thoroughly mixed, the binder polyvinylidene fluoride (PVDF) and N-methylpyrrolidone (NMP) solvent are added to form a uniform slurry. The mass ratio of active material: conductive agent: binder is controlled to be 80:10:10. The electrode slurry is then scraped onto aluminum foil, and the electrode loading is controlled at 12 mg / cm 2 Finally, the dried electrode is punched into The circular electrodes were then dried in a 120°C vacuum oven for 12 hours. The dried electrodes were weighed and set aside. In an argon-filled glove box with a water and oxygen content of ≤0.1 ppm, the electrolytes prepared in the above examples and comparative examples were added, in the order from bottom to top: negative electrode shell, negative electrode (lithium metal), separator, positive electrode, flat gasket, spring sheet, and positive electrode shell. After assembly and packaging, a button-type half-cell model R2302 was obtained. The half-cell was allowed to stand at room temperature for 24 hours before testing its electrochemical performance.

[0128] 1. Room temperature cycle test: measured by the cycle life curve at a rate of 1C, a voltage range of 2.5-4.2V, and room temperature conditions;

[0129] 2. 60℃ high temperature cycle test: The cycle life curve is measured at a rate of 1C, a voltage range of 2.5-4.2V, and high temperature (60℃) conditions;

[0130] 3. Voltage test: measured by linear sweep voltammetry;

[0131] 4. Discharge capacity retention rate test: Cycle at a rate of 1C, with a voltage range of 2.5-4.2V. The discharge capacity of the last cycle is divided by the discharge capacity of the first cycle and multiplied by 100%.

[0132] The above test results are shown in Table 1 and the accompanying drawings:

[0133] Table 1

[0134]

[0135]

[0136] According to the test results of the above test table 1, combined with the attached Figures 1 to 13 It can be seen that:

[0137] Attachment Figure 1This is a schematic diagram comparing the lithium negative electrode interface after using the comparative commercial electrolyte and the electrolyte of the embodiment of the present application. It can be seen that the electrolyte of the embodiment of the present application can promote lithium nucleation and form a uniform solid electrolyte SEI film layer on the electrode surface; while the comparative commercial electrolyte will cause the growth of lithium dendrites and form a non-uniform solid electrolyte SEI film layer on the electrode surface.

[0138] Attachment Figure 2 The comparison of the electrolyte designed in Example 1 at the negative electrode interface of the lithium metal battery is revealed. The ionic liquid-based electrolyte helps to inhibit the formation of lithium dendrites and form a stable SEI film, which will be beneficial to the long-term stable operation of the battery at high temperature and high voltage.

[0139] Attachment Figure 3 -Attached Figure 11 Cycle life curves for lithium metal batteries (Li||LFP) were obtained using the electrolytes of Examples 1-7 and Comparative Example 1 (commercial electrolyte), Comparative Example 2 (without reducing additives), and Comparative Example 3 (without ionic liquid) at a rate of 1C, a voltage range of 2.5-4.2V, and room temperature. Examples 1-8 investigated the effects of different ionic liquid cation structures, formulation ratios, and additive content on their cycling performance. In particular, Example 3 maintained a capacity retention of 97% after 2000 cycles, outperforming all other examples and commercial electrolytes. This is due to the fact that the ether functional groups of the ionic liquid influence charge distribution and the interaction between cations and anions. Ionic liquids with ether functional groups in their side chains have lower HOMO and higher LUMO energy levels, resulting in better electrochemical stability. This electrolyte can form a stable SEI film on the lithium anode surface, enabling stable operation under high temperature and high pressure for long periods of time. Furthermore, Example 3 maintained a capacity retention of 95% after 1000 cycles at a high rate of 4C, demonstrating its high fast-charge cycling stability.

[0140] Attachment Figure 12 This is a cycle life curve of a lithium metal battery Li||LFP full cell using the electrolyte of Example 3 under high rate 4C conditions. It can be seen that the electrolyte of Example 3 of the present application exhibits excellent high rate cycle performance.

[0141] Attachment Figure 13 The cycle life curves of lithium metal battery Li||LFP full cell using the electrolyte of Example 3 and the commercial electrolyte of Comparative Example 1 1MLiPF6 in EC:DMC at a rate of 1C, a voltage range of 2.5-4.2V, and a high temperature (60°C) are shown. Figure 14-21The cycle life of a full lithium metal Li||LFP cell using the electrolytes of Examples 1-2, Examples 4-7, and Comparative Examples 2-3 under the same conditions is shown. It can be seen that after 400 cycles, the capacity retention rate of the electrolyte of Example 3 reaches 82%, while the commercial electrolyte can only work for a few dozen cycles. This is mainly due to the high temperature resistance of the ionic liquid and nitrile solvent in the electrolyte of the present application, and the formation of a strong and stable SEI interface.

[0142] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present application should be included in the scope of protection of the present application.

Claims

1. An electrolyte, characterized in that: The invention comprises a main lithium salt, an auxiliary lithium salt, a main solvent, a film-forming additive, an ionic liquid and a reducing additive; wherein the reducing additive has at least one functional group selected from the group consisting of a cyano group, an aldehyde group, a thioether group and a thioketone group; and the ionic liquid comprises at least one selected from the group consisting of pyrroles, imidazoles, piperidines, quaternary ammonium salts, morpholines and phosphonium salts.

2. The electrolyte according to claim 1, wherein The cation in the ionic liquid includes at least one of pyrrole containing a side chain, imidazole containing a side chain, piperidine containing a side chain, quaternary ammonium salt containing a side chain, morpholine containing a side chain, and phosphonium containing a side chain; And / or, the anions in the ionic liquid include at least one of bis(trifluoromethanesulfonyl)imide ions, bis(fluorosulfonyl)imide ions, and tetrafluoroborate ions.

3. The electrolyte according to claim 2, wherein The side chain of the ionic liquid contains at least one functional group selected from the group consisting of an ether group, a fluorinated alkyl group, and a nitrile group; And / or, in the ionic liquid, the cation includes a pyrrole containing a side chain, and the anion includes a bistrifluoromethanesulfonyl imide ion.

4. The electrolyte according to claim 2, wherein The ionic liquid includes At least one of the following, wherein TFSI - It is a bis(trifluoromethanesulfonyl)imide ion.

5. The electrolyte according to any one of claims 1 to 4, characterized in that The reducing additive includes a nitrile additive; and / or, the main solvent comprises at least one of an ether solvent, a fluorinated ether solvent, and a siloxane solvent; and / or, the film-forming additive comprises at least one of fluoroethylene carbonate, vinylene carbonate, LiPO2F2, and fluorinated siloxane; And / or, the main lithium salt includes at least one of LiTFSI, LiPF6, LiDFOB, and LiBF4; And / or, the auxiliary lithium salt includes at least one of LiNO 3 , LiBOB, and LiDFOB.

6. The electrolyte according to claim 5, wherein The nitrile additive includes at least one of acetonitrile, adiponitrile, succinonitrile, glutaronitrile, and suberonitrile; And / or, the main solvent includes at least one of ethylene glycol dimethyl ether, fluoro-1,4-dimethoxybutane, dimethylmethoxy (3,3,3-trifluoropropyl) silane, trimethoxy (3,3,3-trifluoropropyl) silane, ethylene glycol monomethyl ether, and propylene glycol methyl ether.

7. The electrolyte according to any one of claims 1 to 4 or 6, characterized in that The volume percentage of the ionic liquid is 20% to 50%; And / or, the volume percentage of the reducing additive is 5% to 15%; and / or, the concentration of the main lithium salt is 0.8 mol / L to 1.5 mol / L; and / or, the concentration of the auxiliary lithium salt is 0.1 mol / L to 0.3 mol / L; And / or, the volume percentage of the main solvent is 50% to 70%; And / or, the volume percentage of the film-forming additive is 2% to 15%.

8. The electrolyte according to claim 7, wherein In the electrolyte, the electrolyte is suitable for a circulation condition with a temperature of 0°C to 70°C.

9. A method for preparing an electrolyte, characterized in that: The following steps are involved: In a dry inert atmosphere, a main lithium salt, an auxiliary lithium salt, a main solvent, a film-forming additive, an ionic liquid and a reducing additive are mixed to prepare an electrolyte; wherein the reducing additive has at least one functional group selected from the group consisting of a cyano group, an aldehyde group, a sulfide group and a thioketone group; and the ionic liquid includes at least one selected from the group consisting of pyrroles, imidazoles, piperidines, quaternary ammonium salts, morpholines and phosphonium salts.

10. A secondary battery, characterized in that: The secondary battery comprises a positive electrode, a negative electrode, an electrolyte and a separator, wherein the electrolyte comprises the electrolyte according to any one of claims 1 to 8 or the electrolyte prepared by the method according to claim 9.

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