Electrolyte and secondary battery

Abstract

The application provides an electrolyte and a secondary battery, and particularly relates to the technical field of batteries. The electrolyte comprises an organic solvent, an electrolyte salt, a first additive and a second additive. The first additive has a chemical general formula as shown in formula I, and the second additive has a chemical general formula as shown in formula II. The second additive can promote the first additive to have a polymerization reaction, form a compact SEI film with high polymerization degree on the surface of an electrode, and reduce the loss of active lithium in the chemical conversion process. Meanwhile, the first additive and the second additive contain a large number of lone pair electrons, can construct a continuous lithium ion transmission channel in the compact SEI film, effectively eliminate the lithium ion kinetics conduction deterioration problem caused by the compact SEI film, reduce the capacity loss, and realize the significant improvement of the initial efficiency of the battery.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and more particularly to an electrolyte and a secondary battery. Background Technology

[0002] As the global energy structure accelerates its transition to renewable energy, the demand for portable, long-lasting products in consumer electronics, electric vehicles, and other fields is increasing daily. Lithium-ion batteries, with their outstanding advantages of high energy density, long cycle life, and excellent charge / discharge efficiency, are widely used in mobile phones, laptops, electric vehicles, and large-scale grid energy storage systems.

[0003] Currently, facing the global energy transition and carbon neutrality goals, the market has placed more stringent demands on the performance indicators of lithium-ion batteries. Among these, the first-time coulombic efficiency (CFE) of lithium-ion batteries is often limited by the excessive consumption of active lithium during the first charge-discharge (formation) process. The decrease in the first-time coulombic efficiency (CFE) of lithium-ion batteries has become one of the important bottlenecks to improving their energy density. Summary of the Invention

[0004] In view of the shortcomings of the prior art, the present invention provides an electrolyte and a secondary battery to improve the technical problem of low initial coulombic efficiency of the battery.

[0005] To achieve the above and other related objectives, the present invention provides an electrolyte comprising an organic solvent, an electrolyte salt, a first additive, and a second additive, wherein the chemical formula of the first additive is shown in Formula I:

[0006] Formula I; In Formula I, R1, R2, R3 and R4 are each independently selected from any one of hydrogen atoms, substituted or unsubstituted C1~C3 alkyl groups, substituted or unsubstituted C2~C3 alkenyl groups, and substituted or unsubstituted C3~C8 cycloalkyl groups, wherein the substituents in the substituted alkyl groups, substituted alkenyl groups and substituted cycloalkyl groups are each independently selected from alkyl groups or halogens. The general chemical formula of the second additive is shown in Formula II: Formula II; In Formula II, R5 and R6 are each independently selected from substituted or unsubstituted C1-C6 alkyl, substituted or unsubstituted C2-C6 alkenyl, or cyano; the substituents in the substituted alkyl and substituted alkenyl are each independently selected from alkyl, cyano, or halogen.

[0007] In one embodiment of the present invention, the mass of the first additive accounts for 0.1wt% to 3.0wt% of the total mass of the electrolyte; and the mass of the second additive accounts for 0.01wt% to 0.3wt% of the total mass of the electrolyte.

[0008] In one embodiment of the present invention, the mass of the first additive accounts for 0.3wt% to 1.0wt% of the total mass of the electrolyte; and the mass of the second additive accounts for 0.03wt% to 0.1wt% of the total mass of the electrolyte.

[0009] In one embodiment of the present invention, the organic solvent includes cyclic carbonates and chain carbonates, wherein the mass ratio of the chain carbonates to the cyclic carbonates is (1.5~4):1.

[0010] In one embodiment of the present invention, the cyclic carbonate includes one or more of ethylene carbonate and propylene carbonate.

[0011] In one embodiment of the present invention, the chain carbonate includes one or more of dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate.

[0012] In one embodiment of the present invention, the electrolyte further includes a third additive, the third additive being hexafluoroglutaric anhydride, wherein the mass of the hexafluoroglutaric anhydride accounts for 0.3wt% to 0.5wt% of the total mass of the electrolyte.

[0013] In one embodiment of the present invention, the electrolyte salt is a lithium salt, which includes at least one of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, and lithium hexafluoroarsenate; the mass of the lithium salt accounts for 8wt% to 18wt% of the total mass of the electrolyte.

[0014] In one embodiment of the present invention, the lithium salt includes lithium hexafluorophosphate and lithium difluorosulfonylimide, wherein the mass ratio of lithium hexafluorophosphate to lithium difluorosulfonylimide is (0.5~2):1.

[0015] The present invention also provides a secondary battery, wherein the secondary battery comprises any of the electrolytes described above.

[0016] The beneficial effects of this invention are as follows: The electrolyte of this application uses a combination of a first additive and a second additive. The azo bond (N=N) of the second additive preferentially reduces the nitrogen-centered free radicals generated during the first formation stage of the battery, efficiently initiating the polymerization reaction of the first additive and promoting the formation of a highly polymerized, cross-linked, dense SEI film. This effectively prevents electrolyte solvent molecules from embedding into the negative electrode, significantly reducing the irreversible loss of active lithium during the formation process. At the same time, both the first and second additives contain a large number of lone pairs of electrons, which can construct a continuous lithium-ion coordination-dissociation-recognition skipping conduction channel in the dense SEI film. Combined with a high sulfur-oxygen ratio, this effectively eliminates the problem of deteriorated lithium-ion kinetic conduction caused by the dense SEI film, effectively reducing capacity loss and achieving a significant improvement in the battery's first-stage efficiency.

[0017] Furthermore, by adding hexafluoroglutaric anhydride to the electrolyte, its high fluorine content and excellent decomposition characteristics allow the formation of a LiF-rich inorganic phase interface layer on the electrode surface. This interface layer exhibits high Young's modulus and good uniformity. This inorganic phase serves as the inner layer of the SEI film, working in conjunction with the organic cross-linked outer layer formed by the first and second additives to constitute a bilayer composite SEI film of inorganic inner layer and organic outer layer. This significantly suppresses side reactions of the electrolyte on the electrode surface. On one hand, the tightly bonded LiF-rich inorganic inner layer generated from the decomposition of hexafluoroglutaric anhydride and the organic cross-linked outer layer can greatly enhance the overall mechanical strength, interfacial flexibility, and structural stability of the SEI film, resisting electrode volume expansion and solvent erosion. On the other hand, this LiF-rich inorganic phase can synergistically interact with the lithium-ion conduction sites provided by lone pair electrons in the original system to construct a multi-channel, low-impedance lithium-ion conduction network, further improving lithium-ion conduction efficiency and enhancing battery rate performance and cycle stability. Detailed Implementation

[0018] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments. Various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. In the absence of conflict, the following embodiments and features in the embodiments can be combined with each other.

[0019] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0020] The term “and / or” as used herein includes any and all combinations of one or more of the related listed items.

[0021] In this document, when referring to numerical ranges, unless otherwise specified, the distribution of selectable values ​​within a numerical range is considered continuous, including the two endpoints of the range (i.e., the minimum and maximum values), and every value between these two endpoints. When multiple numerical ranges are provided to describe a feature or property, these numerical ranges can be combined.

[0022] In this document, terms such as "multiple," "various," and "repeatedly" are used unless otherwise specified, referring to a quantity greater than or equal to 2. For example, "one or more" indicates one or more types. Terms such as "further," "even further," and "particularly" are used to describe purposes and indicate differences in content, but should not be construed as limiting the scope of protection of this invention.

[0023] Unless otherwise specified, "wt%" and "%" in this article refer to the percentage content by mass.

[0024] Currently, to improve the first coulombic efficiency (first efficiency) of batteries, sacrificial additives are usually introduced into the electrolyte to reduce the consumption of active lithium by taking advantage of their preferential reduction characteristics, thereby improving the battery's first efficiency.

[0025] However, commonly used sacrificial additives have significant drawbacks: first, some additives exhibit poor reduction reaction kinetics, resulting in limited protection of active lithium and an inability to significantly improve initial efficiency; second, some additives contribute to the formation of the SEI film with high impedance, which can significantly increase the battery interface impedance. These drawbacks limit the large-scale application of sacrificial additives in high-energy-density lithium batteries and hinder further breakthroughs in lithium battery energy density.

[0026] Therefore, developing an additive that combines the advantages of significant first-efficiency improvement and low interfacial impedance is of great practical significance and application value for breaking through the technological bottleneck of high-energy-density lithium batteries and promoting their industrial upgrading.

[0027] Based on this, the present invention provides an electrolyte and a secondary battery containing the electrolyte. By simultaneously introducing a first additive and a second additive into the electrolyte, the second additive promotes the polymerization reaction of the first additive, thereby forming a dense SEI film with a high degree of polymerization on the electrode surface, reducing the loss of active lithium during the formation process. At the same time, a large number of lone pairs of electrons are introduced, effectively eliminating the problem of deteriorated kinetic conduction caused by the dense SEI film, reducing capacity loss, and achieving a significant improvement in the first efficiency of the battery.

[0028] The electrolyte provided by this invention comprises an organic solvent, an electrolyte salt, a first additive, and a second additive. The organic solvent, as the main component of the electrolyte, is used to dissolve the electrolyte salt and the additive. After dissolving in the organic solvent, the electrolyte salt releases a large number of active ions, giving the electrolyte good conductivity. The first and second additives, as functional additives, can form a dense, highly polymerized SEI film on the electrode surface during the battery formation stage, reducing the loss of active lithium during formation. The chemical formula of the first additive is shown in Formula I, and the chemical formula of the second additive is shown in Formula II. Formula I; In Formula I, R1, R2, R3, and R4 are each independently selected from any one of the following: hydrogen atom, substituted or unsubstituted C1-C3 (1-3 carbon atoms) alkyl group, substituted or unsubstituted C2-C3 (2-3 carbon atoms) alkenyl group, or substituted or unsubstituted C3-C8 (3-8 carbon atoms) cycloalkyl group. That is, R1, R2, R3, or R4 can each be a hydrogen atom, can be a C1-C3 alkyl group, such as C1 alkyl, C2 alkyl, or C3 alkyl, etc., and can be C2 alkyl, C3 alkyl, etc. The substituent can be a C3-C5 alkenyl group, such as a C2-C3 alkenyl group or a C3-C8 cycloalkyl group, such as a C3-C5-C8 cycloalkyl group or a C5-C8 cycloalkyl group. It can also be a substituted C1-C3 alkyl group, a substituted C2-C3 alkenyl group, or a substituted C3-C8 epoxy group. When R1, R2, R3, or R4 is a substituted alkyl group, a substituted alkenyl group, or a substituted epoxy group, the substituent can be an alkyl group or a halogen, and halogens include fluorine (F), chlorine (Cl), bromine (Br), etc. In other words, R1, R2, R3, and R4 are independent of each other and do not interfere with each other. They can be all the same, partially the same, or completely different. For example, R1, R2, R3, and R4 can all be hydrogen atoms, or R1 and R2 can be hydrogen atoms, R3 can be methyl, and R4 can be vinyl; or R1 can be a hydrogen atom, R2 can be methyl, R3 can be trifluoromethyl, and R4 can be cycloalkyl, etc.

[0029] Formula II; In Formula II, R5 and R6 are each independently selected from substituted or unsubstituted C1-C6 (1-6 carbon atoms) alkyl, substituted or unsubstituted C2-C6 (2-6 carbon atoms) alkenyl, or cyano. That is, R5 and R6 can each independently be C1-C6 alkyl, such as C1 alkyl, C3 alkyl, or C6 alkyl, etc.; can be C2-C6 alkenyl, such as C2 alkenyl, C4 alkenyl, or C6 alkenyl, etc.; or can be cyano; or can be substituted C1-C6 alkyl or substituted C2-C6 alkenyl. When R5 or R6 is a substituted alkyl or substituted alkenyl, its substituent can be alkyl, cyano, or halogen, and halogens include fluorine (F), chlorine (Cl), bromine (Br), etc. Those skilled in the art will understand that R5 and R6 are independent of each other and do not affect each other; they can be the same or different. For example, R5 and R6 can both be methyl, or R5 can be ethyl and R6 can be cyano, etc.

[0030] The first additive uses a five-membered ring carbonate containing carbon-carbon double bonds as its core framework. This active unsaturated structure is one of the key sites for subsequent polymerization reactions. A sulfate ester five-membered ring (DTD ring) is attached to each side of the core framework. This type of sulfur-containing cyclic structure possesses both high ring strain and electrochemical reactivity. Therefore, the first additive is a multifunctional monomer. Both the carbon-carbon double bond of its central carbonate ring and the DTD rings on both sides can undergo reduction reactions on the negative electrode surface, providing a structural basis for constructing a cross-linked polymeric SEI film. The azo bond (-N=N-) in the second additive molecule has high electrochemical activity and a reduction potential higher than that of the first additive and the electrolyte solvent. It can be preferentially reduced during the initial formation stage of the battery, generating a nitrogen-centered free radical active intermediate. This active intermediate can efficiently initiate multi-site polymerization reactions of the first additive, specifically including the addition polymerization of the carbon-carbon double bond of the central carbonate ring and the ring-opening polymerization of the DTD rings on both sides, ultimately forming a cross-linked polymeric SEI film with a significantly higher degree of polymerization than that formed by the first additive alone. This dense cross-linked structure effectively prevents electrolyte solvent molecules from embedding into the negative electrode layers, avoiding electrode expansion and a series of side reactions caused by solvent co-intercalation, thereby significantly reducing the irreversible consumption of active lithium during the formation process. The free radical initiation effect of the second additive can significantly reduce the polymerization reaction energy barrier, effectively suppressing the oligomerization reaction that is prone to occur when the first additive forms a film alone, ensuring the formation of an SEI film with a higher degree of polymerization and a denser structure, further reducing the loss of active lithium.

[0031] Furthermore, the first additive molecule contains a large number of oxygen atoms, such as the double-bonded oxygen (S=O) and bridging oxygen (OSO) of sulfate esters and the ester group oxygen (C=O and OCO) of carbonates; and the nitrogen atoms (amine and imine nitrogen) remaining after the reduction of the second additive. These atoms all contain lone pairs of electrons, which can serve as coordination sites for lithium ions, constructing a continuous lithium-ion "coordination-dissociation-coordination" skip-type conduction channel within the dense SEI film. This conduction mechanism can effectively avoid the lithium-ion diffusion obstruction problem easily caused by dense SEI films while maintaining the structural stability of the dense SEI film, eliminating the risk of kinetic deterioration caused by densification, and ensuring minimal capacity loss during the first charge and discharge cycle of the battery. The dual synergistic effect of the above-mentioned "highly dense cross-linked film formation" and "efficient lithium-ion conduction" balances the two core requirements of suppressing active lithium loss and maintaining lithium-ion conduction efficiency, achieving a significant improvement in the first-cycle efficiency of the battery cell. Meanwhile, the highly polymerized cross-linked SEI membrane possesses excellent structural stability, which can effectively suppress membrane damage and continuous electrolyte decomposition during subsequent cycling processes, further enhancing the cycle stability and storage reliability of the battery.

[0032] The applicant discovered in their research that the amount of the first and second additives affects the membrane structure and ion conduction of the SEI film. Appropriate amounts of the first and second additives can achieve a dual effect of a highly polymerized, dense film and efficient lithium-ion conduction. If the amount of the first additive is relatively high (and the amount of the second additive is relatively low), the total amount of nitrogen-centered free radicals provided by the second additive is insufficient to trigger the full polymerization of all the first additive molecules. This results in a large amount of unpolymerized first additive monomers and low-polymerization polymers remaining in the system, leading to a loose, porous, and poorly uniform SEI film on the electrode surface. Unreacted monomers may also trigger interfacial side reactions, causing localized loss of continuous and efficient lithium-ion conduction channels in the electrode. Impurity deposition further blocks these pathways, increasing transmission impedance. Simultaneously, the failure of the protective film layer highlights problems such as solvent co-intercalation and the consumption of active lithium by side reactions, thus affecting the cell's initial efficiency. If the amount of the second additive is relatively high (while the amount of the first additive is relatively low), the nitrogen-centered free radicals generated by the decomposition of the second additive will be excessive. The excessive nitrogen-centered free radicals will trigger overpolymerization and solvent side reactions. On the one hand, this will cause the first additive to quickly form a dense SEI film with excessive cross-linking and increased thickness. On the other hand, the excess free radicals will attack the electrolyte solvent and cause decomposition, generating inert deposits. The highly cross-linked film layer will compress the conduction channels, and the inert layer will hinder the transmission, resulting in an increase in lithium-ion transmission impedance, which will be mainly due to the film impedance. At the same time, both overpolymerization and solvent decomposition will irreversibly consume active lithium. Combined with the side reactions caused by the poor flexibility of the film layer and its susceptibility to cracking, this will ultimately lead to a decrease in the battery's initial efficiency.

[0033] In some optional embodiments, the first additive accounts for 0.1wt% to 3.0wt% of the total mass of the electrolyte, for example, it can be 0.1wt%, 0.5wt%, 1.0wt%, 2.0wt%, or 3.0wt%, etc.; the second additive accounts for 0.01wt% to 0.3wt% of the total mass of the electrolyte, for example, it can be 0.01wt%, 0.05wt%, 0.1wt%, 0.2wt%, or 0.3wt%, etc. Further, the first additive accounts for 0.3wt% to 1.0wt% of the total mass of the electrolyte, for example, it can be 0.3wt%, 0.5wt%, 0.8wt%, or 1.0wt%, etc.; the second additive accounts for 0.03wt% to 0.1wt% of the total mass of the electrolyte, for example, it can be 0.03wt%, 0.05wt%, 0.08wt%, or 0.1wt%, etc.

[0034] In some optional embodiments, the electrolyte further includes a third additive, which is hexafluoroglutaric anhydride. Hexafluoroglutaric anhydride has a high fluorine content in its molecular structure, exhibits a mild film-forming reaction, and has stable decomposition products. It can preferentially form a LiF-rich inorganic phase interface layer on the electrode surface, characterized by high Young's modulus and good uniformity. This inorganic phase can serve as the inner layer of the SEI film, working in conjunction with the organic cross-linked outer layer formed by the first and second additives to constitute a bilayer composite SEI film of inorganic inner layer and organic outer layer. This significantly suppresses side reactions of the electrolyte on the electrode surface. On one hand, the tightly bonded LiF-rich inorganic inner layer generated from the decomposition of hexafluoroglutaric anhydride and the organic cross-linked outer layer can greatly improve the overall mechanical strength, interfacial flexibility, and structural stability of the SEI film, resisting electrode volume expansion and solvent erosion. On the other hand, this LiF-rich inorganic phase can synergistically interact with the lithium-ion conduction sites provided by lone pair electrons in the original system to construct a multi-channel, low-impedance lithium-ion conduction network, further improving lithium-ion conduction efficiency and enhancing battery rate performance and cycle stability. Furthermore, the hexafluoroglutaric anhydride content is 0.3wt%~0.5wt% of the total electrolyte mass, specifically 0.3wt%, 0.4wt%, or 0.5wt%, etc. If the hexafluoroglutaric anhydride content in the electrolyte is too high, the excess hexafluoroglutaric anhydride releases excessive LiF, causing inorganic phase agglomeration and accumulation, leading to pore blockage of the composite SEI membrane. Simultaneously, the excessive inorganic phase covers the original lithium-ion conduction channels, resulting in increased battery impedance. Furthermore, the excess hexafluoroglutaric anhydride will undergo excessive electrochemical reactions during the first charge-discharge phase, not only consuming a large amount of lithium ions to generate excessive inorganic phase, but also causing a decrease in the battery's initial efficiency. In addition, excessive inorganic phase can also cause embrittlement and cracking of the SEI membrane, affecting cycle stability.

[0035] In some optional embodiments, the organic solvent comprises cyclic carbonate and chain carbonate, with a mass ratio of chain carbonate to cyclic carbonate of (1.5~4):1. Exemplarily, this can be 1.5:1, 2:1, 3:1, or 4:1, etc. Cyclic carbonate has a high dielectric constant, effectively disrupting lithium salt ion pairs and promoting lithium salt dissociation into lithium ions, thus providing sufficient charge carriers for ion conduction. Chain carbonate has a low viscosity, significantly reducing the overall viscosity of the electrolyte, decreasing resistance during lithium ion migration, and increasing the ion migration rate. Using a properly proportioned composition of cyclic and chain carbonate can control the electrolyte viscosity to a suitable range while ensuring sufficient lithium salt dissociation, thereby improving ionic conductivity and meeting the rapid lithium ion transport requirements during battery charging and discharging.

[0036] In some alternative embodiments, cyclic carbonates include one or more of ethylene carbonate (EC) and propylene carbonate (PC), that is, the cyclic carbonate can be ethylene carbonate, propylene carbonate, or a combination of both. Chain carbonates include one or more of dimethyl carbonate (DMC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC). Exemplarily, the chain carbonate can be dimethyl carbonate, or diethyl carbonate, or methyl ethyl carbonate, or a combination of dimethyl carbonate and diethyl carbonate, or a combination of dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate, etc.

[0037] The electrolyte salt can be selected according to the battery type. In one optional embodiment, the battery is a lithium-ion battery, and the electrolyte salt is a lithium salt. The lithium salt can be any material suitable for lithium-ion batteries. In some optional embodiments, the lithium salt includes at least one of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), and lithium hexafluoroarsenate (LiAsF6). That is, these lithium salts can be used alone or in combination. Exemplarily, the lithium salt is lithium hexafluorophosphate, or lithium tetrafluoroborate, or a combination of lithium hexafluorophosphate and other lithium salts. Preferably, the lithium salt is a composition of lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide, and the mass ratio of lithium hexafluorophosphate to lithium bis(fluorosulfonyl)imide is (0.5~2):1, for example, it can be 0.5:1, or 1:1, or 2:1, etc. The combination of lithium hexafluorophosphate and lithium difluorosulfonylimide as a lithium salt achieves synergistic performance enhancement and complementary advantages. It fully leverages the excellent ionic conductivity and good compatibility of lithium hexafluorophosphate in carbonate solvents to ensure the basic ion transport efficiency and system adaptability of the electrolyte. Simultaneously, lithium difluorosulfonylimide significantly improves the thermal stability and hydrolysis tolerance of the electrolyte, while optimizing the ion solvation structure and lithium-ion migration characteristics. The two components work synergistically to balance the basic performance and overall stability of the electrolyte, effectively avoiding the performance limitations of a single lithium salt and supporting the battery's wide temperature range, long cycle life, and high rate performance. In some embodiments, the mass fraction of the lithium salt in the electrolyte is 8wt%~18wt%, specifically 8wt%, 12wt%, 15wt%, or 18wt%. If the battery is a sodium-ion battery, any sodium salt suitable for sodium-ion batteries can be selected as the electrolyte salt, which will not be detailed here.

[0038] In other alternative embodiments, the electrolyte may also include other additives, such as vinylene carbonate (VC), ethylene ethylene carbonate (VEC), 1,3-propenesulfonate lactone (PST), tripropynyl phosphate (TPP), tetravinylsilane (TVSi), vinyl sulfate (DTD), lithium difluorophosphate (LiPO2F2), lithium tetrafluoroborate (LiBF4), etc. These additives can be used alone or in combination. Exemplarily, other additives may be VC, or PST, or a combination of VC and TVSi, or a combination of PST, TPP, and TVSi, etc. However, this is not a limitation; those skilled in the art can select the type and content of additives according to actual needs.

[0039] The electrolyte of this invention is prepared in a glove box filled with inert argon gas to ensure that the actual oxygen content in the glove box is less than 1 ppm and the moisture content is less than 10 ppm. An example of the preparation process is as follows: First, the organic solvent is mixed evenly according to a set ratio. Then, the lithium salt is added to the organic solvent, and stirring continues until the lithium salt is completely dissolved. Next, the first additive, the second additive, and other additives (if any) are added, and stirring continues until the mixture is evenly mixed to prepare the electrolyte.

[0040] The present invention also provides a secondary battery comprising the electrolyte described above. The type of secondary battery is not limited herein; it can be a lithium-ion battery or a sodium-ion battery.

[0041] The following section uses lithium-ion batteries as an example to describe the structure of secondary batteries in detail.

[0042] A lithium-ion battery also includes a positive electrode, a negative electrode, and a separator. The separator is located between the positive and negative electrodes to prevent short circuits between them and allows lithium ions to pass through. The electrolyte fills the space between the positive electrode, the separator, and the negative electrode to conduct ions.

[0043] Specifically, the positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one side of the positive current collector. The positive current collector is made of a material with good conductivity and mechanical strength, such as aluminum foil or carbon-coated aluminum foil. The positive current collector has two surfaces disposed opposite to each other along its thickness direction, and the positive active material layer can be disposed on one of the surfaces or on both surfaces. The positive active material layer includes a positive active material, a positive conductive agent, and a positive binder. The positive active material is selected from compounds capable of reversibly inserting and extracting lithium ions, including but not limited to one or more of lithium iron phosphate, lithium nickel manganese oxide, lithium-rich manganese-based compounds, lithium manganese iron phosphate, and ternary layered oxides. The ternary layered oxides include, but are not limited to, nickel-cobalt-manganese ternary materials (NCM) and nickel-cobalt-aluminum ternary materials (NCA). The materials listed above can be used alone or in combination. In some embodiments, the positive active material is lithium iron phosphate or a nickel-cobalt-manganese ternary material, such as LiNi. 0.6 Co 0.1 Mn 0.3 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.9 Co 0.05 Mn 0.05 O2, etc., the positive electrode active material can also be a combination of lithium manganese iron phosphate and nickel-cobalt-manganese ternary materials, such as lithium manganese iron phosphate and LiNi. 0.8 Co 0.1 Mn 0.1 Combinations of O2, etc. The positive electrode binder is selected from any one or more of the following: polyvinylidene fluoride (PVDF), polyethylene oxide (PEO), polyamide (PA), polyacrylonitrile (PAN), polyacrylate (polyacrylate), polyvinyl ether (polyvinyl ether), polymethyl methacrylate (PMMA), ethylene-propylene-diene terpolymer (EPDM), polyhexane-propylene, or styrene-butadiene rubber (SBR). The positive electrode conductive agent is selected from one or a combination of two or more of the following: conductive carbon black (Super P), Ketjen black, acetylene black, graphene, carbon nanotubes, carbon nanofibers, porous carbon, etc., in any proportion.

[0044] The negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one side of the negative current collector. The negative current collector can be made of a material with good conductivity and mechanical strength, such as copper foil, carbon-coated copper foil, or foamed copper. The negative current collector has two surfaces disposed opposite to each other along its thickness direction, and the negative active material layer can be disposed on one or both surfaces. The negative active material layer includes a negative active material, a negative binder, a thickener, and a negative conductive agent. The negative active material is selected from materials capable of inserting and extracting lithium ions, such as graphite and / or silicon-based materials. Graphite includes, but is not limited to, natural graphite, artificial graphite, soft carbon, and hard carbon; silicon-based materials include, but are not limited to, elemental silicon, silicon oxide compounds, and silicon-carbon composites. The negative active materials listed above can be used alone or in combination, without specific limitations. The negative electrode binder includes, but is not limited to, one or a combination of polyvinyl alcohol, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, styrene-butadiene rubber, or acrylated styrene-butadiene rubber in any proportion. The thickener is selected from sodium carboxymethyl cellulose (CMC-Na) or lithium carboxymethyl cellulose (CMC-Li). The negative electrode conductive agent is selected from one or a combination of two or more of the following: carbon black, Ketjen black, acetylene black, graphene, carbon nanotubes, carbon nanofibers, porous carbon, etc., in any proportion.

[0045] The separator can be made of conventional membranes, ceramic membranes, polymer membranes, non-woven fabrics, or inorganic-organic composite membranes, and its thickness is, for example, 9~15μm. Exemplarily, the separator can be a single-layer polypropylene (PP) membrane, a single-layer polyethylene (PE) membrane, a double-layer PP / PE membrane, a double-layer PP / PP membrane, or a triple-layer PP / PE / PP membrane. Alternatively, an 8μm~10μm polyethylene membrane can be selected as the base membrane, and a 2μm~4μm thick nano-alumina coating can be coated on the base membrane to improve its thermal stability, etc.

[0046] Those skilled in the art should know that, in addition to the structures described above, lithium-ion batteries also include some essential structures, such as the casing. The specific structure and material of the casing are not limited and can be selected according to the battery type. For example, for pouch batteries, the casing can be made of aluminum-plastic film; for cylindrical batteries, the casing can be made of steel, etc.

[0047] An example of the preparation method for the above-mentioned lithium-ion battery is as follows: (1) Preparation of positive electrode sheet The positive electrode active material, positive electrode conductive agent and positive electrode binder are dispersed in a solvent (such as N-methylpyrrolidone, abbreviated as NMP) to form a uniform positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode sheet is obtained.

[0048] (2) Preparation of negative electrode sheet The negative electrode active material, negative electrode binder, thickener and negative electrode conductive agent are dispersed in deionized water to form a uniform negative electrode slurry. The negative electrode slurry is coated on the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet is obtained.

[0049] (3) Electrolyte preparation: see above.

[0050] (4) Battery assembly The prepared positive electrode, separator, and negative electrode are placed sequentially, with the separator positioned between the positive and negative electrodes to provide insulation. The cells are then wound or stacked to obtain a bare cell. The bare cell is then installed in a casing, and electrolyte is injected into the casing in one or more stages to completely immerse the cell. Afterward, the cell undergoes processes such as settling, hot and cold pressing, formation, clamping, and capacity testing to obtain the finished lithium-ion battery.

[0051] The secondary battery of this invention can be used in the form of a single cell, battery module, or battery pack for powering electronic devices. Electronic devices include, but are not limited to, mobile phones, tablets, laptops, electric toys, electric vehicles, new energy vehicles, ships, spacecraft, etc. Electric toys can include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc. Spacecraft can include airplanes, rockets, space shuttles, and spacecraft, etc. New energy vehicles can be pure electric vehicles, hybrid electric vehicles, or range-extended electric vehicles, etc.

[0052] The technical solution of the present invention will be described in detail below through several specific embodiments and comparative examples. Unless otherwise stated, the raw materials and reagents used in the following embodiments are all commercially available products, or can be prepared by conventional methods in the art, and the instruments used in the embodiments are all commercially available.

[0053] Example 1 This embodiment provides an electrolyte comprising an organic solvent, a lithium salt, a first additive, a second additive, and hexafluoroglutaric anhydride. In the first additive, R1, R2, R3, and R4 are all hydrogen (H), and in the second additive, R5 and R6 are both isopropyl cyanide. The organic solvent comprises battery-grade ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC), with each solvent accounting for 30%, 50%, and 20% of the total organic solvent mass, respectively. The lithium salt is a composition of lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI).

[0054] The preparation process of the above electrolyte is as follows: In an argon-atmospheric glove box with a water content of <10 ppm, EC, EMC, and DEC are mixed in a mass ratio of 30:50:20 to form an organic solvent. Then, lithium salts LiPF6 and LiFSI are added and stirred until the lithium salts are completely dissolved. Next, the first additive, the second additive, and hexafluoroglutaric anhydride are added, and the mixture is stirred until homogeneous to obtain the electrolyte. The mass of LiPF6 accounts for 10 wt% of the total mass of the electrolyte, the mass of LiFSI accounts for 5 wt% of the total mass of the electrolyte, the mass of the first additive accounts for 0.1 wt% of the total mass of the electrolyte, the mass of the second additive accounts for 0.1 wt% of the total mass of the electrolyte, and the mass of hexafluoroglutaric anhydride accounts for 0.3 wt% of the total mass of the electrolyte.

[0055] This embodiment also provides a lithium-ion battery containing the above-mentioned electrolyte. The lithium-ion battery further includes a positive electrode, a separator, and a negative electrode, with the following specific composition: (1) Preparation of positive electrode sheet The positive electrode active material (LiFePO4), conductive agent acetylene black, and binder (polyvinylidene fluoride, PVDF) are mixed in a mass ratio of 95:3:2. N-methylpyrrolidone (NMP) is added, and the mixture is stirred until homogeneous under vacuum to obtain a positive electrode slurry. The positive electrode slurry is then uniformly coated onto an aluminum foil. After the aluminum foil is dried at room temperature, it is transferred to an oven for drying. Finally, it is rolled and cut to obtain the positive electrode sheet.

[0056] (2) Preparation of negative electrode sheet The negative electrode active material (silicon oxide compound SiOx, 0 < x < 2), conductive agent (acetylene black), and binder (sodium carboxymethyl cellulose, CMC-Na) are mixed at a mass ratio of 96:2:2. Deionized water is added, and the mixture is stirred evenly under vacuum to obtain a negative electrode slurry. The negative electrode slurry is then uniformly coated onto the negative electrode current collector copper foil. After the copper foil is dried at room temperature, it is transferred to an oven for drying. Finally, it is rolled and cut to obtain the negative electrode sheet.

[0057] (3) The electrolyte preparation is as shown above.

[0058] (4) Preparation of diaphragm A 12μm polypropylene membrane was used as the diaphragm.

[0059] (5) Battery assembly The prepared positive electrode, separator, and negative electrode are stacked sequentially and placed in an aluminum-plastic film to obtain a dry cell. The dry cell is then baked at 80°C to remove water. Electrolyte is injected into the cell at a ratio of 3.0 g / Ah, and the cell is sealed. After formation, a soft-pack battery (i.e., a lithium-ion battery) with a capacity of 1Ah is obtained.

[0060] Example 2 The difference between this embodiment and Embodiment 1 is that the mass of the first additive is 0.3 wt% of the total mass of the electrolyte.

[0061] Example 3 The difference between this embodiment and Embodiment 1 is that the mass of the first additive is 0.5 wt% of the total mass of the electrolyte.

[0062] Example 4 The difference between this embodiment and Embodiment 1 is that the mass of the first additive is 1 wt% of the total mass of the electrolyte.

[0063] Example 5 The difference between this embodiment and Embodiment 1 is that the mass of the first additive is 3 wt% of the total mass of the electrolyte.

[0064] Example 6 The difference between this embodiment and Embodiment 3 is that the mass of the second additive is 0.01 wt% of the total mass of the electrolyte.

[0065] Example 7 The difference between this embodiment and Embodiment 3 is that the mass of the second additive is 0.03 wt% of the total mass of the electrolyte.

[0066] Example 8 The difference between this embodiment and Embodiment 3 is that the mass of the second additive is 0.05 wt% of the total mass of the electrolyte.

[0067] Example 9 The difference between this embodiment and Embodiment 3 is that the mass of the second additive is 0.3 wt% of the total mass of the electrolyte.

[0068] Example 10 The difference between this embodiment and Example 8 is that hexafluoroglutaric anhydride was not added to the electrolyte.

[0069] Example 11 The difference between this embodiment and Example 8 is that R1, R2, and R3 in the first additive are H, and R4 is methyl; R5 in the second additive is cyano, and R6 is methyl.

[0070] Example 12 The difference between this embodiment and Example 8 is that R1 and R3 in the first additive are H, R2 is vinyl, and R4 is 1-fluorovinyl; R5 in the second additive is trifluoromethyl, and R6 is methyl.

[0071] Comparative Example 1 The difference between this comparative example and Example 1 is that no first additive and second additive were added to the electrolyte.

[0072] Comparative Example 2 The difference between this comparative example and Comparative Example 1 is that 0.3 wt% of hexafluoroglutaric anhydride was replaced with 0.5 wt% of the first additive, while the rest is the same as Comparative Example 1.

[0073] Comparative Example 3 The difference between this comparative example and Comparative Example 1 is that 0.3 wt% of hexafluoroglutaric anhydride was replaced with 0.1 wt% of the second additive, while the rest is the same as Comparative Example 1.

[0074] Table 1: Electrolyte components of Examples 1-12 and Comparative Examples 1-3

[0075] To verify the performance of the electrolytes in the embodiments and comparative examples, this application conducted performance tests on the lithium-ion batteries assembled in the embodiments and comparative examples. The test results are shown in Table 2, and the test methods are as follows: (1) First Coulomb efficiency (first-effect) test At 45℃, the lithium-ion battery is charged at a constant current of 0.01C to 3.2V, then charged at a constant current of 1 / 3C to 3.8V, and then charged at a constant voltage to a current of 0.05C. The charging capacity is recorded as C0. The battery is then discharged at a constant current of 1 / 3C to 2.0V. The capacity is recorded as C1. The initial efficiency is calculated as C1 / C0 × 100%.

[0076] (2) DCR (DC internal resistance) test At 25°C, an uncycled lithium-ion battery was charged at a constant current of 1 / 3C to 3.8V, then charged at a constant voltage to a current of 0.05C. The battery was then discharged at a constant current of 1 / 3C to 2.0V. This charging process was repeated, and the charging capacity was recorded as C1. The battery was then discharged at a constant current of 1 / 3C to (50% × C1), and the initial voltage was recorded as V2. The battery was then discharged at a constant current of 1C for 30 seconds, and the final voltage was recorded as V3. DCR = (V2 - V3) / (C1 × 1).

[0077] Table 2: Battery performance of Examples 1-12 and Comparative Examples 1-3

[0078] Referring to Tables 1 and 2, in Examples 1-5, compared to Comparative Example 1, the synergistic introduction of the first and second additives into the electrolyte significantly improved the initial coulombic efficiency of the battery and reduced the DC internal resistance (DCR). The reasons are as follows: The second additive effectively promotes the polymerization reaction of the first additive, forming a dense SEI film with a high degree of polymerization on the electrode surface, which greatly reduces the loss of active lithium during the battery formation stage. At the same time, the large number of lone pairs of electrons contained in the molecules of the first and second additives can build a continuous lithium-ion coordination-dissociation-recognition skipping conduction channel inside the dense SEI film, effectively eliminating the lithium-ion kinetic conduction degradation problem that is easily caused by the dense SEI film, reducing the capacity loss caused by ion transport obstruction, and ultimately achieving a significant improvement in the initial coulombic efficiency of the battery.

[0079] In addition, in Examples 1-5, the content of the first additive was adjusted while keeping the content of the second additive unchanged. The test results showed that when the content of the first additive was 0.3wt% to 1wt%, the improvement effect of the battery DCR value and first efficiency was better; when its content was lower than 0.3wt% or higher than 1wt%, the battery DCR value increased and the first efficiency decreased. The reasons are as follows: When the amount of the first additive is too low, the nitrogen-centered free radicals generated by the decomposition of the second additive are excessive. On the one hand, this causes the first additive to polymerize excessively, forming a dense SEI film with excessive cross-linking and excessive thickness. On the other hand, the excessive free radicals attack the electrolyte solvent, causing it to decompose and produce inert deposits. This not only significantly increases the membrane impedance of lithium-ion transmission, but also irreversibly consumes active lithium, ultimately leading to a decrease in the first-time efficiency of the battery cell. When the amount of the first additive is too high, the total amount of nitrogen-centered free radicals provided by the second additive is insufficient, which cannot promote the full polymerization of the first additive. A large number of unpolymerized monomers and low-polymerization polymers remain in the system, resulting in a loose, porous, and poorly uniform SEI film on the electrode surface. Furthermore, the unreacted monomers will cause interfacial side reactions, which not only significantly increase the lithium-ion transmission impedance, but also exacerbate solvent side reactions and active lithium consumption due to insufficient film protection, thus leading to a significant decrease in the first-time efficiency of the battery.

[0080] In Examples 3, 6-9, the content of the second additive was adjusted while keeping the content of the first additive constant. The test results showed that when the content of the second additive was 0.03wt% to 0.1wt%, the improvement effect on the battery DCR value and first efficiency was better; when its content was lower than 0.03wt% or higher than 0.1wt%, the battery DCR value increased and the first efficiency decreased. The reasons are as follows: When the amount of the second additive is too high, the nitrogen-centered free radicals generated by decomposition are excessive. On the one hand, this causes the first additive to polymerize excessively, forming a dense SEI film with excessive cross-linking and increased thickness. On the other hand, the excessive free radicals attack the electrolyte solvent, causing it to decompose and produce inert deposits, which significantly increases the membrane impedance of lithium-ion transport and irreversibly consumes active lithium, ultimately leading to a decrease in the battery's first efficiency. When the amount of the second additive is too low, the total amount of nitrogen-centered free radicals provided is insufficient to promote the full polymerization of the first additive. A large number of unpolymerized monomers and low-polymerization polymers remain in the system, causing a loose, porous, and poorly uniform SEI film to form on the electrode surface. Furthermore, the unreacted monomers will trigger interfacial side reactions, which not only significantly increase the lithium-ion transport impedance but also exacerbate solvent side reactions and active lithium consumption due to the failure of the film layer protection, thus leading to a decrease in the battery's first efficiency.

[0081] Compared to Example 8, Example 10 only added the first and second additives without introducing hexafluoroglutaric anhydride. Test results showed that the battery's DCR value increased while its initial efficiency decreased. This is because the electrolyte lacked hexafluoroglutaric anhydride, preventing the formation of a bilayer composite SEI film consisting of an inorganic inner layer and an organic outer layer. This resulted in insufficient structural stability and mechanical flexibility of the SEI film formed on the electrode surface, leading to reduced lithium-ion transport efficiency and ultimately an increase in the battery's DCR value and a decrease in its initial efficiency.

[0082] Comparative Example 2, compared to Example 10, only added the first additive, and Comparative Example 3, compared to Example 10, only added the second additive. The test results showed that while the DCR value of the battery in Comparative Example 2 decreased, its initial efficiency significantly decreased; while the DCR value of the battery in Comparative Example 3 increased, but its initial efficiency decreased. These results indicate that only by using the first and second additives in combination can a cross-linked, dense SEI film with a suitable degree of polymerization be formed on the electrode surface. This film can effectively block electrolyte solvent molecules from embedding into the negative electrode, significantly reducing the irreversible loss of active lithium during formation. Simultaneously, it can construct a continuous, skip-type conduction channel within the film, effectively solving the problem of deteriorated lithium-ion kinetics caused by dense SEI films, reducing capacity loss, and ultimately achieving a significant improvement in the initial efficiency of the cell and an effective reduction in the DCR value.

[0083] The test results of Examples 8, 11, and 12 show that various combinations of first and second additives can achieve similar improvements in battery initial efficiency. This is because the core reaction mechanisms of these first and second additives are consistent. Through their synergistic effect, both can form a dense, cross-linked SEI film with suitable polymerization on the electrode surface, effectively reducing the irreversible loss of active lithium during formation, thereby significantly improving the battery's initial efficiency.

[0084] This invention introduces a first additive and a second additive into the electrolyte. During the initial formation stage of the battery, the azo bonds of the second additive preferentially reduce the nitrogen-centered free radicals generated, efficiently initiating a polymerization reaction in the first additive. This promotes the formation of a highly polymerized, cross-linked, dense SEI film, effectively preventing electrolyte solvent molecules from embedding into the negative electrode and significantly reducing irreversible lithium loss during formation. Simultaneously, both the first and second additives contain a large number of lone pairs of electrons, which can construct continuous lithium-ion coordination-dissociation-recognition skipping conduction channels within the dense SEI film. This effectively eliminates the lithium-ion kinetic conduction degradation problem caused by the dense SEI film, reducing capacity loss and significantly improving the battery's initial efficiency. Therefore, this invention effectively overcomes some practical problems in the prior art, thus possessing high utilization value and practical significance.

[0085] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. An electrolyte, characterized in that, include: Organic solvent, electrolyte salt, first additive, and second additive, wherein the chemical formula of the first additive is shown in Formula I: Formula I; In Formula I, R1, R2, R3 and R4 are each independently selected from any one of hydrogen atoms, substituted or unsubstituted C1~C3 alkyl groups, substituted or unsubstituted C2~C3 alkenyl groups, and substituted or unsubstituted C3~C8 cycloalkyl groups, wherein the substituents in the substituted alkyl groups, substituted alkenyl groups and substituted cycloalkyl groups are each independently selected from alkyl groups or halogens. The general chemical formula of the second additive is shown in Formula II: Formula II; In Formula II, R5 and R6 are each independently selected from substituted or unsubstituted C1-C6 alkyl, substituted or unsubstituted C2-C6 alkenyl, or cyano; the substituents in the substituted alkyl and substituted alkenyl are each independently selected from alkyl, cyano, or halogen.

2. The electrolyte according to claim 1, characterized in that, The first additive accounts for 0.1wt% to 3.0wt% of the total mass of the electrolyte; the second additive accounts for 0.01wt% to 0.3wt% of the total mass of the electrolyte.

3. The electrolyte according to claim 2, characterized in that, The first additive accounts for 0.3wt% to 1.0wt% of the total mass of the electrolyte; the second additive accounts for 0.03wt% to 0.1wt% of the total mass of the electrolyte.

4. The electrolyte according to claim 1, characterized in that, The organic solvent includes cyclic carbonates and chain carbonates, wherein the mass ratio of the chain carbonate to the cyclic carbonate is (1.5~4):

1.

5. The electrolyte according to claim 4, characterized in that, The cyclic carbonates include one or more of ethylene carbonate and propylene carbonate.

6. The electrolyte according to claim 4, characterized in that, The chain carbonate includes one or more of dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate.

7. The electrolyte according to claim 1, characterized in that, The electrolyte also includes a third additive, which is hexafluoroglutaric anhydride, and the hexafluoroglutaric anhydride accounts for 0.3wt% to 0.5wt% of the total mass of the electrolyte.

8. The electrolyte according to claim 1, characterized in that, The electrolyte salt is a lithium salt, which includes at least one of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, and lithium hexafluoroarsenate; the lithium salt accounts for 8 wt% to 18 wt% of the total mass of the electrolyte.

9. The electrolyte according to claim 8, characterized in that, The lithium salt includes lithium hexafluorophosphate and lithium difluorosulfonylimide, wherein the mass ratio of lithium hexafluorophosphate to lithium difluorosulfonylimide is (0.5~2):

1.

10. A secondary battery, characterized in that, Includes the electrolyte as described in any one of claims 1 to 9.

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