A co-crystal electrolyte, a preparation method and application in a secondary battery

Abstract

The present application relates to a kind of eutectic electrolyte, preparation method and application in secondary battery, eutectic electrolyte includes: lithium salt or sodium salt and sulfuric acid ester-containing compound;The mass ratio of lithium salt and sulfuric acid ester-containing compound is 1:1~1:10;The mass ratio of sodium salt and sulfuric acid ester-containing compound is 2:5~3:5;Eutectic electrolyte is the eutectic mixture formed by lithium salt or sodium salt and sulfuric acid ester-containing compound by hydrogen bond interaction;The ionic conductivity of eutectic mixture is greater than 10 ‑4 S / cm at room temperature;Since the eutectic electrolyte provided by the present application has high ionic conductivity, wide electrochemical window and other excellent electrochemical properties, the eutectic electrolyte of the present application is applied to secondary battery, has good cycle performance and rate performance at room temperature, and the interface is stable.In addition, eutectic electrolyte also has low volatility and flame retardancy, has excellent safety.

Description

Technical Field

[0001] This invention relates to the field of battery materials technology, and in particular to a eutectic electrolyte, its preparation method, and its application in secondary batteries. Background Technology

[0002] Secondary batteries (such as lithium-ion and sodium-ion batteries) are widely used in portable electronic devices, electric vehicles, and large-scale energy storage due to their high energy density and long cycle life. The electrolyte, as a key component of the battery, directly affects its safety, lifespan, and environmental adaptability.

[0003] Currently, commercially available rechargeable batteries mainly use organic carbonate liquid electrolytes (such as ethylene carbonate, dimethyl carbonate, etc.). However, these electrolytes have inherent defects such as flammability, volatility, and poor thermal stability, posing serious safety hazards and limiting the application of batteries in high energy density, high voltage, and wide temperature range scenarios.

[0004] For example, Chinese invention patent application CN107681198A (published on February 9, 2018) discloses a lithium-ion battery electrolyte and a lithium-ion battery thereof, which balances low-temperature and high-temperature performance by adding vinylene carbonate, sulfonyl lactone, fluoroethylene carbonate, and vinyl sulfate derivatives. However, this electrolyte still uses flammable carbonates and carboxylic acid esters as solvents, and essentially does not eliminate safety hazards. Its improvement on traditional organic solvent systems has not fundamentally solved the safety problem.

[0005] To overcome these problems, researchers have developed a variety of novel electrolyte systems, including all-solid-state electrolytes, ionic liquid electrolytes, and deep eutectic solvent (DES)-based electrolytes. Among them, DES-based electrolytes have attracted much attention due to their simple preparation, low cost, non-flammability, low volatility, and good ionic conductivity.

[0006] For example, Chinese invention patent application CN117766876A (publication date March 26, 2024) discloses a deep eutectic solvent (DES) based electrolyte for the cathode / solid electrolyte interface in solid-state batteries and its manufacturing method. The deep eutectic solvent based electrolyte contains lithium salts (such as LiTFSI, LiFSI) and sulfone compounds (such as sulfolane, dimethyl sulfone). This electrolyte, as an intermediate layer, can reduce the interfacial resistance between the cathode and the solid electrolyte. However, this technical solution still has the following shortcomings: (1) The DES system formed by the sulfone hydrogen bond donor and the lithium salt usually has a low ion transference number, which is not conducive to the rate performance of the battery; (2) This system is only designed for lithium-ion batteries and does not involve sodium-ion battery systems, so its scope of application is limited; (3) Its application scenario is limited to the interfacial wetting layer of solid-state batteries, rather than being used as an independent main electrolyte, thus failing to fully realize the potential of DES.

[0007] Chinese invention patent application CN120199890A (publication date: June 24, 2025) discloses a non-aqueous electrolyte and a lithium-ion battery. The non-aqueous electrolyte includes lithium salt, organic solvent, and additives, wherein the additives include 1, 2, 3, 4, and 5. 5(2) (cyanoethoxy)pentane and 4 Ethylene methyl sulfate is intended to improve the wettability and cycle performance of high-density negative electrodes. However, in this approach, 4-methylethylene sulfate is used only as a small amount (0.5%–2%) as an additive in a conventional carbonate solvent system. Its role is to assist in the formation of an SEI film on the negative electrode surface, rather than as a main component of the eutectic solvent to form a eutectic system with lithium salts. Therefore, the improvement in ionic conductivity and ion transference number is limited.

[0008] In summary, existing technologies in the field of eutectic electrolytes still have the following common shortcomings: (1) Insufficient safety: Most technologies still rely on flammable carbonate or carboxylic acid ester solvents, or although DES is used, it is limited to the application of the interface layer and fails to provide a highly safe and flame-retardant system that can be used as the main electrolyte.

[0009] (2) Low ion transport number: The known DES systems (such as sulfone / lithium salt) generally have an ion transport number of no more than 0.5, which affects the rate performance and power density of the battery.

[0010] (3) Single system: Most existing DES-based electrolytes are only suitable for lithium-ion batteries and lack effective adaptation to emerging systems such as sodium-ion batteries.

[0011] (4) Functional limitations: Sulfate compounds are only used as trace additives and have not been given full play to their potential as the main component of eutectic solvents.

[0012] Therefore, developing a eutectic electrolyte that combines high safety (inherent flame retardancy), high ion transference number, wide electrochemical window, and suitability for lithium / sodium dual systems is of significant practical importance and has broad application prospects. Summary of the Invention

[0013] The purpose of this invention is to address the shortcomings of existing technologies by providing a eutectic electrolyte, its preparation method, and its applications. This eutectic electrolyte aims to solve the problems of flammability, volatility, and poor safety associated with traditional organic electrolytes, while offering excellent electrochemical performance and interfacial stability.

[0014] To achieve the above objectives, in a first aspect, the present invention provides a eutectic electrolyte comprising: a lithium salt or a sodium salt, and a compound containing a sulfate ester.

[0015] The mass ratio of the lithium salt to the sulfate-containing compound is 1:1 to 1:10; the mass ratio of the sodium salt to the sulfate-containing compound is 2:5 to 3:5.

[0016] The eutectic electrolyte is a eutectic mixture formed by hydrogen bonding between the lithium salt or sodium salt and a sulfate-containing compound.

[0017] The eutectic mixture has an ionic conductivity greater than 10 at room temperature. -4 S / cm.

[0018] Preferably, the sulfate-containing compound includes: 1,3,2-dioxothiacyclopentane-2,2-dioxide, 1,3,2-dioxothiacycloheptane-2,2-dioxide, 4-methyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4,4-dimethyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-ethyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-fluoro-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-chloro-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-bromo-1,3,2-dioxothiacyclohexane-2,2-dioxide, and 4-trifluoromethyl 4-phenyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-benzyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-hydroxy-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-methoxy-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-allyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4,5-trans-dimethyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4,5-cis-dimethyl-1 3,2-Dioxothiacyclohexane-2,2-dioxide, 4,4,5,5-Tetramethyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-Carboxyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-Methyl carboxylate-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-Acetoxy-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-Amino-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-Acetamino-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-Cyano-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-Nitro- One or more of the following: 1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-vinyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-cyclohexyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-tert-butyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-isopropyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-methylthio-1,3,2-dioxothiacyclohexane-2,2-dioxide, sodium 4-sulfonate-1,3,2-dioxothiacyclohexane-2,2-dioxide, and diethyl phosphate-1,3,2-dioxothiacyclohexane-2,2-dioxide.

[0019] Preferably, the lithium salt comprises one or more of the following: lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium hexafluorophosphate (LiPF6), lithium difluorooxalate borate (LiDFOB), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium trifluoromethanesulfonate (LiOTf), and lithium difluorophosphate (LiPO2F2).

[0020] Preferably, the sodium salt comprises one or more of the following: sodium perchlorate (NaClO4), sodium hexafluorophosphate (NaPF6), sodium bis(fluorosulfonyl)imide (NaFSI), sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), sodium trifluoromethanesulfonate (NaOTf), sodium difluorophosphate (NaPO2F2), sodium tetrafluoroborate (NaBF4), and sodium difluorooxalate borate (NaDFOB).

[0021] Preferably, the eutectic electrolyte further includes additives, wherein the mass of the additives accounts for 0% to 10% of the mass of the eutectic electrolyte.

[0022] The additives include high molecular weight polymers and / or inorganic particles.

[0023] Preferably, the polymer specifically includes one or more of the following: polyvinylidene fluoride (PVDF) and its copolymers, polyethylene oxide (PEO) and its derivatives, polyacrylic acid (PAA), polymethyl methacrylate (PMMA) and its copolymers, polyurethane (PU), polyvinyl alcohol (PVA), cellulose and its nanofibers (CNF), silk fibroin, chitosan, polyacrylonitrile (PAN) and its derivatives, polyethoxylated trimethylolpropane triacrylate (PETPTA), and polyethylene glycol methyl ether methacrylate (PEGM).

[0024] The inorganic particles specifically include one or more of the following: LLZO solid electrolyte, LLZTO solid electrolyte, LATP solid electrolyte, silicon dioxide (SiO2), aluminum oxide (Al2O3), NASICON solid electrolyte, titanium dioxide (TiO2), zirconium oxide (ZrO2), zinc oxide (ZnO), magnesium oxide (MgO), yttrium oxide (Y2O3), barium titanate (BaTiO3), hexagonal boron nitride (h-BN), and sulfide electrolytes.

[0025] In a second aspect, the present invention provides a method for preparing the eutectic electrolyte described in the first aspect, the method comprising: in a protective gas environment, mixing a sulfate-containing compound with a lithium salt or sodium salt in a certain proportion, stirring at 30°C to 120°C for 10 min to 12 hours until the mixture is homogeneous, thereby obtaining the eutectic electrolyte.

[0026] Preferably, the mass ratio of the lithium salt to the sulfate-containing compound is 1:1 to 1:10; and the mass ratio of the sodium salt to the sulfate-containing compound is 2:5 to 3:5.

[0027] Preferably, the preparation method further includes: after stirring evenly, adding additives and continuing stirring to obtain a eutectic electrolyte.

[0028] Thirdly, the present invention provides a secondary battery, the secondary battery comprising the eutectic electrolyte described in the first aspect.

[0029] The present invention provides a eutectic electrolyte, its preparation method and application, which have the following technical effects.

[0030] (1) The eutectic electrolyte provided by this invention is a stable network structure formed by strong hydrogen bond interactions between sulfate ester compounds (hydrogen bond donors) and lithium / sodium salts (hydrogen bond acceptors). On the one hand, the intermolecular forces are significantly enhanced, solvent molecules are difficult to volatilize, and the vapor pressure of the eutectic electrolyte is much lower than that of traditional carbonate electrolytes, eliminating the prerequisite for combustion; on the other hand, the hydrogen bond network requires higher energy to be destroyed, and the thermal decomposition temperature is significantly increased. Therefore, the eutectic electrolyte provided by this invention has higher thermal stability and inherent flame retardant properties, fundamentally solving the safety problems of flammability and volatility of traditional electrolytes.

[0031] (2) The eutectic electrolyte of this invention exhibits excellent oxidative stability, mainly attributed to: ① the -SO2- group in the sulfate ester lowers the highest occupied orbital (HOMO) energy level of the molecule through electron-withdrawing effect, thereby improving antioxidant capacity; ② the bis(trifluoromethanesulfonyl)imide anion (TFSI) - ① It possesses a delocalized negative charge and a high oxidation potential; ③ The sulfate ester molecule does not contain easily oxidized α-hydrogen atoms. Regarding reduction stability, Li... + The strong coordination with sulfate reduces the activity of free solvent molecules, while the cyclic sulfate preferentially undergoes ring-opening polymerization on the negative electrode surface to form a stable solid electrolyte interphase (SEI) film, rather than continuously decomposing solvent molecules. Therefore, the oxidation potential of the eutectic electrolyte of this invention can reach above 4.7 V, which can be matched with high-voltage positive electrode materials and helps to improve the energy density of the battery.

[0032] (3) This invention can promote the cationic Li through its unique solvation structure. + Movement, limiting anion TFSI - Mobility is achieved through this solvation structure modulation to realize high ion transport numbers: the strong electron-donating ability of sulfate esters promotes the dissociation of lithium salts, forming compact Li... + -O coordination, coupled with the steric hindrance created by the cyclic structure of the sulfate ester, restricts the formation of large-volume anions (e.g., TFSI).- The movement of ions. Hydrogen bonding further anchors the anions in the eutectic network. In terms of transport mechanism, Li... + The ion transport occurs via hopping between coordination sites, rather than diffused along with the solvent molecules; and the viscosity of the eutectic system is lower than that of ionic liquids, which promotes ion migration. Therefore, the eutectic electrolyte provided by this invention has a high lithium-ion / sodium-ion transference number (>0.4), which is beneficial for improving the rate performance of the battery.

[0033] (4) In the eutectic electrolyte system of this invention, a stable interfacial film can be formed on the surfaces of the positive and negative electrodes. At the positive electrode interface, a LiF-rich positive electrode electrolyte interfacial film (CEI) can be formed, effectively inhibiting transition metal dissolution; at the negative electrode interface, cyclic sulfates are preferentially reduced, forming a dense, thin, and inorganic-rich SEI film with high mechanical strength and rapid LiF reduction. + Transmission capability; meanwhile, TFSI - The reduction and decomposition produce LiF and sulfur-containing compounds, further improving the rigidity and ionic conductivity of the SEI film. Therefore, the eutectic electrolyte provided by this invention ensures interfacial stability of the secondary battery during long-term cycling, effectively extending cycle life.

[0034] (5) The preparation method of the eutectic electrolyte provided by the present invention only requires mixing lithium salt / sodium salt with sulfate ester compound in proportion and stirring at a certain temperature until clear. No complicated purification steps are required. The operation is simple, the conditions are mild, and the raw materials are readily available, which is convenient for large-scale production and application. Attached Figure Description

[0035] Figure 1 The curve showing the change in ionic conductivity with temperature of the eutectic electrolyte prepared in Example 1 of this invention.

[0036] Figure 2 The ion transport number test curve is shown for the eutectic electrolyte prepared in Example 1 of this invention.

[0037] Figure 3 The linear sweep voltammetry curve is shown for the eutectic electrolyte prepared in Example 1 of this invention.

[0038] Figure 4 The thermogravimetric analysis curve is shown for the eutectic electrolyte prepared in Example 1 of this invention.

[0039] Figure 5 The image shows the long-cycle performance curve of a battery assembled using a lithium iron phosphate (LFP) cathode (2.5V-4V) with the eutectic electrolyte prepared in Example 1 of this invention at a 0.5C rate.

[0040] Figure 6The image shows the long-cycle performance curve of a battery assembled using a lithium cobalt oxide (LCO) cathode (3V-4.3V) with the eutectic electrolyte prepared in Example 1 of this invention at a rate of 0.5C.

[0041] Figure 7 The eutectic electrolyte prepared in Example 1 of this invention uses spinel lithium nickel manganese oxide (LiNi) 0.5 Mn 1.5 The long-cycle performance curve of a battery assembled with an O4 positive electrode (3V-4.9V) at a 0.5C rate.

[0042] Figure 8 The curve showing the change in ionic conductivity with temperature of the eutectic electrolyte prepared in Example 2 of this invention.

[0043] Figure 9 The ion transport number test curve is shown for the eutectic electrolyte prepared in Example 2 of this invention.

[0044] Figure 10 The linear sweep voltammetry curve is shown for the eutectic electrolyte prepared in Example 2 of this invention.

[0045] Figure 11 The thermogravimetric analysis curve of the eutectic electrolyte prepared in Example 2 of this invention.

[0046] Figure 12 The image shows the long-cycle performance curve of a battery assembled using sodium vanadium phosphate (NVP) cathode (2.5V-4V) with the eutectic electrolyte prepared in Example 2 of this invention at a rate of 0.5C. Detailed Implementation

[0047] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0048] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0049] This invention provides a eutectic electrolyte comprising a lithium salt or a sodium salt, and a sulfate-containing compound. The eutectic electrolyte is a eutectic mixture formed by hydrogen bonding between the lithium salt or sodium salt and the sulfate-containing compound.

[0050] Eutectic mixtures have an ionic conductivity greater than 10 at room temperature.-4 S / cm. Specifically, compounds containing sulfate esters include: 1,3,2-dioxothiacyclopentane-2,2-dioxide, 1,3,2-dioxothiacycloheptane-2,2-dioxide, 4-methyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4,4-dimethyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-ethyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-fluoro-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-chloro-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-bromo-1,3,2-dioxothiacyclohexane-2,2-dioxide, and 4-trifluoromethyl -1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-phenyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-benzyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-hydroxy-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-methoxy-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-allyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4,5-trans-dimethyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4,5-cis-dimethyl-1, 3,2-Dioxothiacyclohexane-2,2-dioxide, 4,4,5,5-Tetramethyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-Carboxyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-Methyl carboxylate-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-Acetoxy-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-Amino-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-Acetamino-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-Cyano-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-Nitro- One or more of the following: 1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-vinyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-cyclohexyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-tert-butyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-isopropyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-methylthio-1,3,2-dioxothiacyclohexane-2,2-dioxide, sodium 4-sulfonate-1,3,2-dioxothiacyclohexane-2,2-dioxide, and diethyl phosphate-1,3,2-dioxothiacyclohexane-2,2-dioxide.

[0051] Lithium salts include one or more of the following: lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium hexafluorophosphate (LiPF6), lithium difluorooxalate borate (LiDFOB), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium trifluoromethanesulfonate (LiOTf), and lithium difluorophosphate (LiPO2F2). The mass ratio of the lithium salt to the sulfate ester compound is 1:1 to 1:10, and can be any mass ratio within the above range, such as 1:1, 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, etc., but is not limited to the listed mass ratios; other unlisted mass ratios within this range are also applicable. The preferred mass ratio of the lithium salt to the sulfate ester compound is 1:5 to 4:5.

[0052] Sodium salts include one or more of the following: sodium perchlorate (NaClO4), sodium hexafluorophosphate (NaPF6), sodium bis(fluorosulfonyl)imide (NaFSI), sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), sodium trifluoromethanesulfonate (NaOTf), sodium difluorophosphate (NaPO2F2), sodium tetrafluoroborate (NaBF4), and sodium difluorooxalate borate (NaDFOB). The mass ratio of the sodium salt to the sulfate-containing compound is 2:5 to 3:5, and can be any mass ratio within the above range, such as 2:5, 1:2, 3:5, etc., but is not limited to the listed mass ratios; other unlisted mass ratios within this range are also applicable.

[0053] In an optional embodiment, the eutectic electrolyte further includes additives; the additives constitute 0% to 10% of the mass of the eutectic electrolyte, and can be any value within the above range, such as 0%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, etc., but are not limited to the listed values; other unlisted values ​​within this range are also applicable. Additives include polymers and / or inorganic particles. The eutectic mixture formed by lithium salts or sodium salts and sulfate-containing compounds through hydrogen bonding is generally liquid at room temperature; when polymers are added, the eutectic electrolyte of this invention can form a gel state, possessing both high ionic conductivity and good mechanical strength, effectively inhibiting dendrite growth; when inorganic nanoparticles are added, ion transport channels can be further broadened, improving rate performance.

[0054] The specific polymers include: polyvinylidene fluoride (PVDF) and its copolymers, polyethylene oxide (PEO) and its derivatives, polyacrylic acid (PAA), polymethyl methacrylate (PMMA) and its copolymers, polyurethane (PU), polyvinyl alcohol (PVA), cellulose and its nanofibers (CNF), silk fibroin, chitosan, polyacrylonitrile (PAN) and its derivatives, polyethoxylated trimethylolpropane triacrylate (PETPTA), and polyethylene glycol methyl ether methacrylate (PEGM) or one or more of these.

[0055] Inorganic particles specifically include one or more of the following: LLZO solid electrolyte, LLZTO solid electrolyte, LATP solid electrolyte, silicon dioxide (SiO2), aluminum oxide (Al2O3), NASICON solid electrolyte, titanium dioxide (TiO2), zirconium oxide (ZrO2), zinc oxide (ZnO), magnesium oxide (MgO), yttrium oxide (Y2O3), barium titanate (BaTiO3), hexagonal boron nitride (h-BN), and sulfide electrolytes. Among these, LLZO solid electrolyte includes Li7La3Zr2O. 12 LLZTO solid electrolyte includes Li 6.4 La3Zr 1.4 Ta 0.6 O 12 Li 6.5 La3Zr 1.5 Ta 0.5 O 12 Li 6.75 La3Zr 1.75 Ta 0.25 O 12 One or more of the following; LATP solid electrolytes include Li 1.3 Al 0.3 Ti 1.7 (PO4)3.

[0056] This invention provides a method for preparing the above-mentioned eutectic electrolyte, specifically comprising: stirring a sulfate-containing compound and a lithium salt or sodium salt in a certain proportion at 30°C to 120°C for 10 min to 12 hours under a protective gas environment until the mixture is homogeneous and melted into a clear liquid to obtain the eutectic electrolyte.

[0057] The protective gas environment includes one or more of nitrogen, argon, or helium; the water and oxygen content is preferably controlled below 0.2 ppm.

[0058] The stirring method is a conventional method, such as one or more of magnetic stirring, mechanical stirring, ultrasonic stirring or vacuum stirring, with a stirring speed of 100 rpm to 1000 rpm.

[0059] In an optional preparation method, additives are also added during the preparation process. Specifically, under a protective gas environment, a compound containing sulfate esters is stirred with lithium salt or sodium salt in a certain proportion at 30℃~120℃ for 10min~12h until homogeneous. Additives are then added, and stirring continues until a clear liquid is formed, yielding a eutectic electrolyte. The mass of the additives accounts for 0%~10% of the mass of the eutectic electrolyte. Additives include polymers and / or inorganic particles. Specifically, polymers include one or more of the following: polyvinylidene fluoride (PVDF) and its copolymers, polyethylene oxide (PEO) and its derivatives, polyacrylic acid (PAA), polymethyl methacrylate (PMMA) and its copolymers, polyurethane (PU), polyvinyl alcohol (PVA), cellulose and its nanofibers (CNF), silk fibroin, chitosan, polyacrylonitrile (PAN) and its derivatives, polyethoxylated trimethylolpropane triacrylate (PETPTA), and polyethylene glycol methyl ether methacrylate (PEGM). Inorganic particles specifically include one or more of the following: LLZO solid electrolyte, LLZTO solid electrolyte, LATP solid electrolyte, silicon dioxide (SiO2), aluminum oxide (Al2O3), NASICON solid electrolyte, titanium dioxide (TiO2), zirconium oxide (ZrO2), zinc oxide (ZnO), magnesium oxide (MgO), yttrium oxide (Y2O3), barium titanate (BaTiO3), hexagonal boron nitride (h-BN), and sulfide electrolytes.

[0060] This invention provides a secondary battery, which includes the aforementioned eutectic electrolyte, separator, positive electrode, and negative electrode. The secondary battery may be a lithium-ion battery or a sodium-ion battery.

[0061] The membrane includes one or more of the following: polyethylene (PE) membrane, polypropylene (PP) membrane, PP / PE / PP three-layer composite membrane, and glass fiber membrane.

[0062] The active materials in the positive electrode of a lithium-ion battery include one or more of the following: lithium iron phosphate, lithium manganese iron phosphate, lithium manganese oxide, lithium cobalt oxide, lithium nickel manganese oxide, nickel cobalt manganese ternary material (NCM), or nickel cobalt aluminum ternary material (NCA); the active materials in the negative electrode of a lithium-ion battery include one or more of the following: graphite, lithium metal, silicon, silicon-carbon negative electrode material, silicon-oxygen negative electrode material, and hard carbon.

[0063] One or more of the following: graphite anode, lithium metal anode, sodium metal anode, silicon anode, hard carbon anode, silicon-carbon anode, or silicon-oxygen anode.

[0064] The active materials in the positive electrode of a sodium-ion battery include one or more of sodium vanadium phosphate, sodium vanadium fluorophosphate, sodium manganate, sodium cobaltate, and sodium nickel manganate; the active materials in the negative electrode of a sodium-ion battery include one or more of metallic sodium and hard carbon.

[0065] Because the eutectic electrolyte provided in this embodiment of the invention possesses excellent electrochemical properties such as high ionic conductivity and a wide electrochemical window, its application in secondary batteries exhibits good cycle performance and rate capability at room temperature, along with interface stability. Furthermore, the eutectic electrolyte also exhibits low volatility and flame retardancy, demonstrating excellent safety.

[0066] To better understand the technical solution provided by the present invention, the preparation process and characteristics of the eutectic electrolyte of the present invention are illustrated below with several specific examples.

[0067] Example 1 This embodiment provides a process for preparing a eutectic electrolyte, specifically including: mixing lithium bis(trifluoromethanesulfonylimide) (LiTFSI) and 1,3,2-dioxothiacyclohexane-2,2-dioxide (PCS) at a mass ratio of 2:5, and stirring at 60°C until clear and transparent to prepare a eutectic electrolyte based on sulfate esters.

[0068] The ionic conductivity, cation transport number, oxidation potential, and thermogravimetric analysis of the eutectic electrolyte prepared in this embodiment were tested as follows.

[0069] The ionic conductivity of the eutectic electrolyte prepared in Example 1 was tested using the following method: Ionic conductivity was obtained by measuring AC impedance spectroscopy. A disc of the eutectic electrolyte, impregnated with a PE separator, was sandwiched between two stainless steel blocking electrodes. A spring clamp was then placed on top, and a coin cell was assembled. The cell was placed in a high-low temperature test chamber, and the temperature was slowly lowered from 70°C to 10°C. Tests were performed every 5°C. Before each temperature measurement, the cell was allowed to stand at that temperature for 2 hours to reach thermodynamic equilibrium. The tests were conducted using a Zennium X electrochemical workstation from Zahner Systems, USA, with a test frequency range of 4 MHz to 100 MHz and an amplitude of 5 mV. The formula for calculating ionic conductivity (σ) is: σ = L / (S R). Where L (cm) is the thickness of the PE membrane after the eutectic electrolyte is impregnated, and S (cm) is the thickness of the membrane. 2 R(Ω) represents the area of ​​the PE membrane after the eutectic electrolyte has been impregnated, and R(Ω) represents the bulk ohmic resistance measured by EIS.

[0070] Furthermore, using the same method, eutectic electrolytes were prepared by mixing LiTFSI and PCS at mass ratios of 1:2, 3:5, 7:10, and 4:5, respectively. The ionic conductivity of these eutectic electrolytes was then tested using the same method. The ionic conductivity curves of the eutectic electrolytes prepared at the five mass ratios of LiTFSI and PCS (2:5, 1:2, 3:5, 7:10, and 4:5) versus temperature are shown in the figure below. Figure 1 As shown in the figure, the five mass ratios are 40%, 50%, 60%, 70%, and 80%, respectively. It can be seen that the ionic conductivity of the eutectic electrolytes at room temperature is greater than 10 for all five mass ratios. -4 The S / cm value is high, and the eutectic electrolyte with a LiTFSI to PCS mass ratio of 40% provided in this embodiment exhibits an ionic conductivity as high as 2.18 × 10⁻⁶ at room temperature (25°C). -4 S / cm.

[0071] The cation transport number test employs a combined AC impedance and DC polarization technique. Specifically, a coin cell is assembled by sandwiching a disc of eutectic electrolyte-impregnated PE membrane between two lithium metal plates, covered with a stainless steel gasket and spring clamp. For lithium-ion transport number testing, the lithium metal symmetric cell is placed in a high-low temperature chamber and left to stand at 30°C for 12 hours to allow a stable interface to form between the eutectic electrolyte and the electrodes before testing. A Zennium X electrochemical workstation from Zahner (USA) is used. For DC polarization testing, the polarization voltage is set to 10mV, and the test duration is 20 hours. EIS tests are performed before and after the DC polarization test, with a test frequency range of 4 MHz-100 MHz and an amplitude of 5mV. The lithium-ion transport number (tLi) is calculated. + The formula for calculating tLi is: + =I s Rb s ( V-I0R0) / I0Rb0( VI s R s ), where ΔV is the applied polarization voltage of 10mV, I0(A) is the initial current, I s (A) represents the steady-state current, R0 (Ω) is the initial impedance of the electrolyte-electrode interface, Rbs (Ω) is the steady-state impedance of the electrolyte, and Rb0 (Ω) is the initial electrolyte impedance. s (Ω) represents the impedance of the electrolyte-electrode interface.

[0072] The ion transport number test curve of the eutectic electrolyte prepared in Example 1 is as follows: Figure 2 As shown, calculations show that the ion transference number of the eutectic electrolyte provided in this embodiment is approximately 0.96.

[0073] Electrochemical window testing was performed using linear sweep voltammetry (LSV). A disc (Φ16.2 mm) with eutectic electrolyte impregnated with a PE separator was placed on a lithium metal sheet (Φ12 mm), covered with a stainless steel gasket and spring clamp, and then assembled into a coin cell. The cell was placed in a high and low temperature test chamber and allowed to stand at 30°C for 12 h to allow a stable interface to form between the eutectic electrolyte and the electrode before testing. A CHI600E electrochemical workstation from Shanghai Chenhua Instruments Co., Ltd. was used, with a scan rate of 0.1 mV / s. The oxidation potential of the eutectic electrolyte was detected from the open circuit voltage (OCV) to 6 V, and the reduction potential was detected from the OCV to 0 V.

[0074] The linear sweep voltammetry curve of the eutectic electrolyte prepared in Example 1 is shown below. Figure 3 As shown, the oxidation potential of this eutectic electrolyte reaches above 5.45V, which can be matched with a high-voltage positive electrode.

[0075] Thermogravimetric analysis (TGA) is a characterization technique that measures the relationship between sample mass and temperature under programmed temperature control. A Thermogravimetric Analysis (TA) SDT Q600 thermal analyzer was used. Sample masses ranged from 5 mg to 8 mg, and the temperature was increased from room temperature to 500 °C at a rate of 10 °C / min in a nitrogen atmosphere at a rate of 10 mL / min.

[0076] The thermogravimetric analysis curve of the eutectic electrolyte prepared in Example 1 is as follows: Figure 4 As shown, the weight loss rate is less than 5% at 100℃, indicating that the eutectic electrolyte hardly decomposes or volatilizes below 100℃, and has excellent thermal stability.

[0077] The eutectic electrolyte prepared in this embodiment was used to assemble lithium-ion batteries with different positive electrodes and tested, as detailed below.

[0078] Battery assembly process: Using lithium metal as the negative electrode, lithium iron phosphate (LFP), lithium cobalt oxide (LCO), and LiNi are used respectively. 0.5 Mn 1.5The positive electrode was prepared using O4 (LNMO), and the eutectic electrolyte prepared in this embodiment was injected into the separator (PE). A lithium-ion coin cell was then assembled using conventional methods. The assembly process was as follows: a certain amount of eutectic electrolyte was added dropwise to the PE separator using a pipette, serving as the electrolyte for the battery. During battery assembly, the following order was used: negative electrode shell - lithium metal negative electrode - electrolyte - positive electrode - gasket - spring clip - positive electrode shell. During assembly, the center points of each part were aligned as much as possible. Finally, the assembled battery was packaged. With the coin cell negative electrode facing upwards, it was transferred using insulated tweezers to the pressure head mold of a pressure-controlled electric coin cell packaging machine. The pressure was set to 750 kg, the residence time to 4 s, and the pressure held for 3 s. The packaged coin cell was then removed using insulated tweezers and placed in a coin cell storage mold for testing.

[0079] Coin cell battery testing process: A full-cell test is performed. Before testing, the coin cells are left to stand at 30℃ for 10 hours, then activated twice at 0.1C rate at 30℃ to form a stable interface. Afterwards, a long-cycle test is conducted at 0.5C rate at 30℃. Different cathode materials correspond to different test voltages (discharge cut-off voltage - charge cut-off voltage): LFP corresponds to 2.5V-4V, LCO to 3V-4.3V, and LNMO to 3V-4.9V. The battery representations for different cathode materials are: LFP corresponds to LFP|PCS-Li|Li; LCO corresponds to LCO|PCS-Li|Li; and LNMO corresponds to LNMO|PCS-Li|Li.

[0080] The long-cycle performance test curves of the LFP|PCS-Li|Li battery assembled with the eutectic electrolyte and LFP cathode prepared in Example 1 are shown in the figure. Figure 5 As shown, at a 0.5C rate, the coulombic efficiency is 100% after 800 cycles, and the capacity retention is 71.02%. The long-cycle performance test curves of the LCO|PCS-Li|Li battery assembled with the eutectic electrolyte and LCO cathode prepared in Example 1 are shown in the figure. Figure 6 As shown, at a 0.5C rate, the coulombic efficiency of the battery after 800 cycles is 99.96%, and the capacity retention is 72.75%. The long-cycle performance test curves of the LNMO|PCS-Li|Li battery assembled with the eutectic electrolyte and LNMO cathode prepared in Example 1 are shown in the figure. Figure 7 As shown, at a 0.5C rate, the coulombic efficiency after 300 cycles is 99.19%, and the capacity retention is 76.99%.

[0081] Example 2 This embodiment provides a process for preparing a eutectic electrolyte, specifically including: mixing sodium bis(fluorosulfonyl)imide (NaFSI) and 1,3,2-dioxothiacyclohexane-2,2-dioxide (PCS) at a mass ratio of 2:5, and stirring at 60°C until clear and transparent to prepare a eutectic electrolyte based on sulfate esters.

[0082] The ionic conductivity, cation transport number, oxidation potential, and thermogravimetric analysis of the eutectic electrolyte prepared in this embodiment were tested as follows.

[0083] Using the same method, NaFSI and PCS were mixed at mass ratios of 2:5, 1:2, 3:5, 7:10, and 4:5 to prepare eutectic electrolytes. The ionic conductivity of these eutectic electrolytes was then tested using the same method as in Example 1. The ionic conductivity curves of the eutectic electrolytes prepared at the five NaFSI / PCS mass ratios of 2:5, 1:2, 3:5, 7:10, and 4:5 in this example, as a function of temperature, are shown below. Figure 8 As shown in the figure, these five mass ratios are represented by 40%, 50%, 60%, 70%, and 80%, respectively. Figure 8 It can be seen that among the five proportions of eutectic electrolytes, the ionic conductivity of the eutectic electrolytes containing NaFSI and PCS at proportions of 40%, 50%, and 60% is greater than 10 at room temperature. -4 The S / cm value is high, and the eutectic electrolyte with a NaFSI to PCS mass ratio of 40% provided in this embodiment has a room temperature ionic conductivity as high as 5.10 × 10⁻⁶. -4 S / cm, while the room temperature ionic conductivity at 70% and 80% ratios is less than 10. -4 The S / cm indicates that the ionic conductivity is better when the mass ratio of the sodium salt to the sulfate-containing compound is 2:5 to 3:5.

[0084] The cation transference number test of the eutectic electrolyte prepared in Example 2 was performed by replacing lithium metal with sodium metal when assembling the coin cell in Example 1. The rest of the battery assembly method, test method and parameter settings were the same as those in the lithium ion transference number test of Example 1.

[0085] The ion transport number test curve of the eutectic electrolyte prepared in Example 2 is as follows: Figure 9 As shown, the ion transference number of this eutectic electrolyte is approximately 0.55.

[0086] The redox potential test of the eutectic electrolyte prepared in Example 2 was conducted by replacing lithium metal with sodium metal when assembling the coin cell in Example 1. The rest of the battery assembly method, test method and parameter settings were the same as those of the lithium battery redox potential test in Example 1.

[0087] The linear sweep voltammetry curve of the eutectic electrolyte prepared in Example 2 is shown below. Figure 10 As shown, the oxidation potential of this eutectic electrolyte reaches above 5.22V, which can be matched with a high-voltage positive electrode. Figure 11 The thermogravimetric curve of the electrolyte prepared in Example 2 of the present invention is shown. The test method is the same as in Example 1, and the weight loss rate is less than 5% at 100°C.

[0088] The sodium-ion battery was assembled and tested using the eutectic electrolyte prepared in this embodiment and matched with sodium vanadium phosphate, as detailed below.

[0089] Battery assembly process: Using metallic sodium as the negative electrode and sodium vanadium phosphate (NVP) as the positive electrode, the eutectic electrolyte prepared in this embodiment is injected into the separator (PE), and assembled into a sodium-ion coin cell using conventional methods. Specifically, the assembly process involves using a pipette to add a certain amount of eutectic electrolyte to the PE separator to serve as the battery's electrolyte. During battery assembly, the following order is used: negative electrode shell - metallic sodium negative electrode - the electrolyte as described above - positive electrode - gasket - spring clip - positive electrode shell. During assembly, the center points of each part are aligned as much as possible. Finally, the assembled battery is packaged. With the coin cell negative electrode facing upwards, it is transferred using insulated tweezers to the pressure head mold of a pressure-controlled electric coin cell packaging machine. The pressure is set to 750 kg, the residence time is 4 seconds, and the pressure is maintained for 3 seconds. The packaged coin cell is then removed using insulated tweezers and placed in a coin cell storage mold for testing.

[0090] Coin cell battery (NVP|PCS-Na|Na) testing process: Full cell test is performed. Before the test, the coin cell battery is placed at 30℃ for 10 hours, and then activated at 30℃ for two cycles at a 0.1C rate to form a stable interface. Then, a long cycle test is performed at 30℃ using a 0.5C rate. The test voltage (discharge cut-off voltage - charge cut-off voltage) is 2.5V-4V.

[0091] The long-cycle performance test curves of the sodium-ion coin cell NVP|PCS-Na|Na assembled with the eutectic electrolyte prepared in Example 2 are shown in the figure. Figure 12 As shown, at a 0.5C rate, the coulombic efficiency of the battery after 800 cycles is approximately 99.9%, and the capacity retention is approximately 69%.

[0092] Example 3 This embodiment provides a process for preparing a eutectic electrolyte, specifically including: mixing lithium dioxaborate (LiDFOB) and 4-methyl-1,3,2-dioxothiacyclohexane 2,2-dioxide at a mass ratio of 1:2, and stirring at 80°C until clear and transparent to prepare a sulfate-based eutectic electrolyte.

[0093] The ionic conductivity, cation transference number, and oxidation potential of the eutectic electrolyte prepared in this embodiment were tested using the same method as in Example 1. The test results are detailed in Table 1.

[0094] The eutectic electrolyte prepared in this embodiment was used to assemble a coin cell with graphite as the negative electrode and lithium manganese iron phosphate as the positive electrode and was tested. The battery assembly and testing process was the same as in Example 1. The test results are detailed in Table 1.

[0095] Example 4 This embodiment provides a process for preparing a eutectic electrolyte, specifically including: mixing lithium perchlorate (LiClO4) and 4-cyano-1,3,2-dioxothiacyclohexane-2,2-dioxide at a mass ratio of 1:5, and stirring at 50°C until clear and transparent to prepare a sulfate-based eutectic electrolyte.

[0096] The ionic conductivity, cation transference number, and oxidation potential of the eutectic electrolyte prepared in this embodiment were tested using the same method as in Example 1. The test results are detailed in Table 1.

[0097] The eutectic electrolyte prepared in this embodiment was used to assemble a coin cell with lithium metal as the negative electrode and lithium cobalt oxide as the positive electrode and was tested. The battery assembly and testing process was the same as in Example 1. The test results are detailed in Table 1.

[0098] Example 5 This embodiment provides a process for preparing a eutectic electrolyte, specifically including: mixing sodium trifluoromethanesulfonate (NaOTf) and diethyl 4-phosphate-1,3,2-dioxothiacyclohexane-2,2-dioxide in a mass ratio of 3:5, and stirring at 90°C until clear and transparent to prepare a eutectic electrolyte based on sulfate esters.

[0099] The ionic conductivity, cation transference number, and oxidation potential of the eutectic electrolyte prepared in this embodiment were tested using the same method as in Example 1. The test results are detailed in Table 1.

[0100] The eutectic electrolyte prepared in this embodiment was used to assemble a coin cell with sodium metal as the negative electrode and sodium vanadium phosphate as the positive electrode and was tested. The battery assembly and testing process was the same as in Example 2. The test results are detailed in Table 1.

[0101] Example 6 This embodiment provides a process for preparing a eutectic electrolyte, specifically including: mixing sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) and diethyl 4-phosphate-1,3,2-dioxothiacyclohexane-2,2-dioxide in a mass ratio of 3:5, and stirring at 90°C until clear and transparent to prepare a sulfate-based eutectic electrolyte.

[0102] The ionic conductivity, cation transference number, and oxidation potential of the eutectic electrolyte prepared in this embodiment were tested using the same method as in Example 1. The test results are detailed in Table 1.

[0103] The eutectic electrolyte prepared in this embodiment was used to assemble a coin cell with sodium metal as the negative electrode and sodium vanadium phosphate as the positive electrode and was tested. The battery assembly and testing process was the same as in Example 2. The test results are detailed in Table 1.

[0104] Example 7 This embodiment provides a preparation process for a eutectic electrolyte, which differs from Example 1 in that an additive is added. The specific preparation process includes: mixing lithium bis(trifluoromethanesulfonyl)imide with 1,3,2-dioxothiacyclohexane-2,2-dioxide at a mass ratio of 2:5, stirring at 60°C until clear and transparent, adding polyvinylidene fluoride (PVDF), and continuing stirring to prepare a sulfate-based eutectic electrolyte. The amount of PVDF added accounts for 2% of the mass of the eutectic electrolyte.

[0105] The ionic conductivity, cation transference number, and oxidation potential of the eutectic electrolyte prepared in this embodiment were tested using the same method as in Example 1. The test results are detailed in Table 1.

[0106] The eutectic electrolyte prepared in this embodiment was used to assemble a coin cell with lithium metal as the negative electrode and lithium iron phosphate as the positive electrode and was tested. The battery assembly and testing process was the same as in Example 1. The test results are detailed in Table 1.

[0107] Example 8 This embodiment provides a preparation process for a eutectic electrolyte, which differs from Example 1 in that an additive is added. The specific preparation process includes: mixing lithium bis(trifluoromethanesulfonyl)imide with 1,3,2-dioxothiacyclohexane-2,2-dioxide at a mass ratio of 2:5, stirring at 60°C until clear and transparent, adding LATP solid electrolyte, and continuing stirring to prepare a sulfate-based eutectic electrolyte. The amount of LATP solid electrolyte added accounts for 3% of the mass of the eutectic electrolyte.

[0108] The ionic conductivity, cation transference number, and oxidation potential of the eutectic electrolyte prepared in this embodiment were tested using the same method as in Example 1. The test results are detailed in Table 1.

[0109] The eutectic electrolyte prepared in this embodiment was used to assemble a coin cell with lithium metal as the negative electrode and lithium iron phosphate as the positive electrode and was tested. The battery assembly and testing process was the same as in Example 1. The test results are detailed in Table 1.

[0110] Example 9 This embodiment provides a preparation process for a eutectic electrolyte, which differs from Example 2 in that an additive is added. The specific preparation process includes: mixing sodium bis(fluorosulfonyl)imide and 1,3,2-dioxothiacyclohexane-2,2-dioxide at a mass ratio of 2:5, stirring at 60°C until clear and transparent, adding polyethylene oxide (PEO), and continuing stirring to prepare a sulfate-based eutectic electrolyte. The amount of polyethylene oxide (PEO) added accounts for 3% of the mass of the eutectic electrolyte.

[0111] The ionic conductivity, cation transference number, and oxidation potential of the eutectic electrolyte prepared in this embodiment were tested using the same method as in Example 1. The test results are detailed in Table 1.

[0112] The eutectic electrolyte prepared in this embodiment was used to assemble a coin cell with sodium metal as the negative electrode and sodium vanadium phosphate as the positive electrode and was tested. The battery assembly and testing process was the same as in Example 2. The test results are detailed in Table 1.

[0113] Example 10 This embodiment provides a preparation process for a eutectic electrolyte, which differs from Example 2 in that an additive is added. The specific preparation process includes: mixing sodium bis(fluorosulfonyl)imide and 1,3,2-dioxothiacyclohexane-2,2-dioxide at a mass ratio of 2:5, stirring at 60°C until clear and transparent, adding zinc oxide, and continuing stirring to prepare a sulfate-based eutectic electrolyte. The amount of zinc oxide added accounts for 5% of the mass of the eutectic electrolyte.

[0114] The eutectic electrolyte prepared in this embodiment was used to assemble a coin cell with sodium metal as the negative electrode and sodium vanadium phosphate as the positive electrode and was tested. The battery assembly and testing process was the same as in Example 2. The test results are detailed in Table 1.

[0115] To better illustrate the effects of the embodiments of the present invention, the following comparative examples are compared with the embodiments described above.

[0116] Comparative Example 1 This comparative example provides a conventional carbonate-based solvent electrolyte for lithium-ion batteries, which is composed of LiPF6, ethylene carbonate (EC), and diethyl carbonate (DEC), wherein the molar concentration of LiPF6 in the electrolyte is 1 mol / L, and the volume ratio of EC to DEC is 1:1.

[0117] The ionic conductivity, cation transference number, and oxidation potential of the eutectic electrolyte prepared in this comparative example were tested using the same methods as in Example 1. The test results are detailed in Table 1.

[0118] This electrolyte system was used to assemble and test coin cells with lithium metal as the negative electrode and lithium iron phosphate as the positive electrode. Comparative Example 1 uses an LFP|EC / DEC-Li|Li battery. The battery assembly and testing process was the same as in Example 1. The long-cycle performance test curves of LFP|EC / DEC-Li|Li are shown in the figure. Figure 5 As shown, at a 0.5C rate, the coulombic efficiency after 350 cycles is 100%, and the capacity retention is 88.48%. Detailed test results are shown in Table 1. In this comparative LFP|EC / DEC-Li|Li battery, after 350 cycles, micro-short circuits occurred due to dendrite growth causing membrane puncture, resulting in poor cycle stability; therefore, further cycling was discontinued.

[0119] Furthermore, this comparative example uses LCO cathode material to prepare the battery, and the preparation method is the same as that of the LFP|EC / DEC-Li|Li battery in Comparative Example 1. The battery corresponding to LCO is represented as LCO|EC / DEC-Li|Li. The long-cycle performance test curve of the LCO|EC / DEC-Li|Li battery prepared in Comparative Example 1 is shown in the figure below. Figure 6 As shown, at 0.5C, the capacity retention of this battery drops to 72.00% after 200 cycles, which is much worse than that of the LCO|PCS-Li|Li battery in Example 1 (72.75% retention after 800 cycles). This indicates that the cycle performance of this battery is worse than that of the LCO|PCS-Li|Li battery in Example 1.

[0120] Comparative Example 2 This comparative example provides a conventional carbonate-based solvent electrolyte for sodium-ion batteries, which is composed of NaPF6, ethylene carbonate (EC), and diethyl carbonate (DEC), wherein the molar concentration of NaPF6 in the electrolyte is 1 mol / L, and the volume ratio of EC to DEC is 1:1.

[0121] The ionic conductivity, cation transference number, and oxidation potential of the eutectic electrolyte prepared in this comparative example were tested using the same methods as in Example 1. The test results are detailed in Table 1.

[0122] The electrolyte system was used to assemble a coin cell with sodium metal as the negative electrode and sodium vanadium phosphate as the positive electrode and the battery assembly and testing were performed. The battery assembly and testing process was the same as in Example 2. The test results are detailed in Table 1. Comparative Example 3 This comparative example provides a sulfone eutectic electrolyte for lithium-ion batteries. The electrolyte is composed of LiTFSI and ethyl methyl sulfone (EMS), which are mixed at a mass ratio of 2:5 and stirred at 40°C until clear and transparent to prepare the sulfone eutectic electrolyte.

[0123] The ionic conductivity, cation transference number, and oxidation potential of the eutectic electrolyte prepared in this comparative example were tested using the same methods as in Example 1. The test results are detailed in Table 1.

[0124] The eutectic electrolyte system was used to assemble and test a coin cell with lithium metal as the negative electrode and lithium iron phosphate as the positive electrode. The battery assembly and testing process was the same as in Example 1. The test results are detailed in Table 1.

[0125] Comparative Example 4 This comparative example provides a sulfone-based eutectic electrolyte for sodium-ion batteries. The electrolyte is composed of NaFSI and sulfolane (SL), which are mixed at a mass ratio of 2:5 and stirred at 30°C until clear and transparent to prepare a sulfone-based eutectic electrolyte.

[0126] The ionic conductivity, cation transference number, and oxidation potential of the eutectic electrolyte prepared in this comparative example were tested using the same methods as in Example 1. The test results are detailed in Table 1.

[0127] The electrolyte system was used to assemble a coin cell with sodium metal as the negative electrode and sodium vanadium phosphate as the positive electrode and the battery assembly and testing were performed. The battery assembly and testing process was the same as in Example 2. The test results are detailed in Table 1.

[0128] Table 1 summarizes the test data for Examples 1-10 and Comparative Examples 1-4.

[0129] Table 1 As can be seen from the test data in Table 1, in Examples 1-10 of this invention, the room temperature ionic conductivity of the eutectic electrolyte formed by the sulfate ester compound and the salt is all higher than 10. -4 The eutectic electrolyte exhibits a high oxidation potential and demonstrates excellent cycle stability using coin cells. The addition of polymer additives allows the eutectic electrolyte to form a gel state, improving interfacial contact and suppressing dendrite formation. Assembled batteries can cycle nearly 1000 times with a capacity retention rate exceeding 70%, effectively improving cycle life and stability. Furthermore, the addition of inorganic particulate additives enables the formation of multiple ion transport channels, resulting in room temperature ionic conductivity exceeding 10. -4 The S / cm effectively improves the electrolyte ionic conductivity and also provides excellent cycle stability.

[0130] Comparing Example 1 with the Comparative Example, although the ionic conductivity of Comparative Example 1 is higher than that of Example 1, its ion transference number is extremely low (0.32), resulting in severe concentration polarization and poor rate performance. The oxidation potential of Comparative Example 1 is only 4.40 V, which cannot match the high-voltage cathode. More importantly, Comparative Example 1 failed after 350 cycles due to lithium dendrites piercing the separator, while Example 1 remained stable after 800 cycles. The reason Example 1 performs better than Comparative Example 1 is that in Example 1, LiTFSI and PCS form a eutectic network through strong hydrogen bonding, anchoring solvent molecules and preventing volatility. Furthermore, the hydrogen bond network requires higher energy to break, resulting in excellent thermal stability (weight loss <5% at 100℃), fundamentally eliminating the risk of flammability and explosion. The -SO2- groups in the sulfate ester have an electron-withdrawing effect, improving antioxidant capacity. Li + The strong coordination with sulfate esters reduces the activity of free solvents, resulting in an oxidation potential as high as 5.45V; the unique solvation structure promotes Li + Movement, Restriction of TFSI - The ion transport number reaches 0.96, which is much higher than that of the carbonate system, which is beneficial to suppress concentration polarization and improve rate performance and cycle stability. The cyclic sulfate preferentially reduces on the negative electrode surface to form a dense, inorganic-rich SEI film, which effectively inhibits dendrite growth, thereby achieving a long cycle life of more than 800 cycles.

[0131] Comparing Example 2 with Comparative Example 2, although Comparative Example 2 has high conductivity, its coulombic efficiency is only 89.21%, indicating severe side reactions and continuous electrolyte decomposition. Comparative Example 2 also has a low oxidation potential (4.30V), making it difficult to match with a high-voltage cathode. Example 2, on the other hand, has a coulombic efficiency close to 100%, indicating a very stable interface. The reason Example 2 performs better than Comparative Example 2 is that carbonate-based solvent electrolytes are volatile, resulting in relatively poor safety performance and high-voltage stability. In the sulfate-based eutectic electrolyte prepared in Example 2, Na... + The strong coordination with PCS and the hydrogen bond network will bind the anion (FSI) - Anchoring effectively suppresses side reactions and significantly improves coulombic efficiency. The oxidation potential of Example 2 is 5.22V, which is nearly 1V higher than that of carbonate, allowing for safe matching with high-voltage sodium electrodes. The cyclic sulfate forms a stable SEI film on the sodium anode surface, avoiding the low coulombic efficiency problem caused by continuous reduction and decomposition of the carbonate system.

[0132] Comparing Example 1 with Comparative Example 3, the cycle life of Comparative Example 3 was only 400 cycles, less than half that of Example 1. Although the ionic conductivity of Comparative Example 3 (6.0 × 10⁻⁶) was higher... -4 Slightly higher than Example 1 (2.18 × 10) -4However, the ion transference number (0.52) was much lower than that of Example 1 (0.96). The reason why Example 1 performed better than Example 3 is that in the DES system formed by sulfone compounds and lithium salts in Comparative Example 3, the electron-donating ability of sulfones is weaker, and Li + Insufficient coordination strength with sulfones leads to a low ion transference number (0.52). Furthermore, sulfone-based eutectic electrolytes have a narrow electrochemical window, are unstable for lithium metal reduction, and exhibit rapid degradation when matched with a metal anode. In contrast, the strong electron-donating ability of the sulfate ester in Example 1 promotes lithium salt dissociation, forming a compact Li... + -O coordination, coupled with the steric hindrance of the ring structure restricting anion movement and hydrogen bonding further anchoring the anion, resulting in a migration number as high as 0.96; EMS in Comparative Example 3 exhibits poor stability to lithium metal reduction and continuously decomposes during cycling, leading to rapid capacity decay after only 400 cycles; while PCS in Example 1 preferentially undergoes ring-opening polymerization on the lithium anode surface to form a stable SEI film, effectively protecting the anode; the coulombic efficiency of Example 1 (100%) is higher than that of Comparative Example 3 (98.9%), indicating fewer side reactions and a more stable interface.

[0133] Comparing Example 2 with Comparative Example 4, Comparative Example 4 showed a decrease in capacity after 450 cycles, while Example 2 maintained 69% capacity after 800 cycles. The better performance of Example 2 is due to the fact that sulfolane (SL) in Comparative Example 4 is a five-membered ring sulfone, and the DES formed by it and NaFSI has poor interfacial stability with sodium metal, resulting in severe sodium dendrite growth during cycling. Furthermore, the electrochemical window of the sulfone eutectic electrolyte formed is narrow, making it unstable for sodium metal reduction and causing rapid capacity decay when matched with the metal anode. In contrast, PCS (six-membered ring sulfate) in Example 2 can form a denser and more stable SEI film on the sodium anode surface, effectively suppressing dendrite growth. The sodium ion transference number of Example 2 (0.55) is higher than that of Comparative Example 4 (0.50), which helps reduce concentration polarization and prolong cycle life. The coulombic efficiency of Example 2 (99.9%) is higher than that of Comparative Example 4 (99.21%), indicating fewer side reactions and slower electrolyte consumption.

[0134] In Examples 7-10, the addition of polymer or inorganic particles resulted in a slight decrease in ion transport number and conductivity, but a significant extension of cycle life to 960-1000 cycles, far exceeding Comparative Example 1. This is because the polymer PVDF forms a gel electrolyte, increasing mechanical strength and effectively inhibiting lithium dendrite penetration through the membrane; the LATP inorganic particles provide additional lithium-ion transport channels and act as a physical barrier to block dendrites, stabilizing the electrode / electrolyte interface. The additives and the sulfate-based eutectic system work synergistically to significantly improve cycle stability while maintaining high ionic conductivity.

[0135] The ion transference numbers (0.55-0.96) of Examples 1-6 without additives in this invention are significantly better than those of the sulfone systems in Comparative Examples 3 and 4. Although the ion transference numbers of Examples 7-10 with additives are slightly lower (0.42-0.47), the eutectic electrolytes of Examples 7-10 have higher mechanical strength due to the introduction of additives, which can effectively suppress lithium / sodium dendrite growth and achieve significant improvements in cycle life and mechanical strength. Their overall performance is still better than that of the comparative examples.

[0136] In summary, the sulfate-based eutectic electrolyte provided by this invention exhibits high ion transport number, wide electrochemical window, excellent long-cycle stability, and good thermal stability in both lithium and sodium systems. Adding polymers or inorganic particles can further extend cycle life or improve conductivity. Compared with existing technologies, this invention has significant advantages in ion transport number, oxidation stability, interfacial compatibility, and cycle life.

[0137] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A eutectic electrolyte, characterized in that, The eutectic electrolyte includes: lithium salt or sodium salt, and a compound containing sulfate ester; The mass ratio of the lithium salt to the sulfate-containing compound is 1:1 to 1:10; the mass ratio of the sodium salt to the sulfate-containing compound is 2:5 to 3:

5. The eutectic electrolyte is a eutectic mixture formed by hydrogen bonding between the lithium salt or sodium salt and a sulfate-containing compound. The eutectic mixture has an ionic conductivity greater than 10 at room temperature. -4 S / cm.

2. The eutectic electrolyte according to claim 1, characterized in that, The sulfate-containing compounds include: 1,3,2-dioxothiacyclopentane-2,2-dioxide, 1,3,2-dioxothiacycloheptane-2,2-dioxide, 4-methyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4,4-dimethyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-ethyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-fluoro-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-chloro-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-bromo-1,3,2-dioxothiacyclohexane-2,2-dioxide, and 4-trifluoromethyl-1 3,2-Dioxothiacyclohexane-2,2-dioxide, 4-phenyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-benzyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-hydroxy-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-methoxy-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-allyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4,5-trans-dimethyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4,5-cis-dimethyl-1,3 2-Dioxothiacyclohexane-2,2-dioxide, 4,4,5,5-Tetramethyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-Carboxyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-Methyl carboxylate-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-Acetoxy-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-Amino-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-Acetamino-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-Cyano-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-Nitro-1 One or more of the following: 3,2-dioxothiacyclohexane-2,2-dioxide, 4-vinyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-cyclohexyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-tert-butyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-isopropyl-1,3,2-dioxothiacyclohexane-2,2-dioxide, 4-methylthio-1,3,2-dioxothiacyclohexane-2,2-dioxide, sodium 4-sulfonate-1,3,2-dioxothiacyclohexane-2,2-dioxide, and diethyl phosphate-1,3,2-dioxothiacyclohexane-2,2-dioxide.

3. The eutectic electrolyte according to claim 1, characterized in that, The lithium salt includes one or more of the following: lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium hexafluorophosphate (LiPF6), lithium difluorooxalate borate (LiDFOB), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium trifluoromethanesulfonate (LiOTf), and lithium difluorophosphate (LiPO2F2).

4. The eutectic electrolyte according to claim 1, characterized in that, The sodium salt includes one or more of the following: sodium perchlorate (NaClO4), sodium hexafluorophosphate (NaPF6), sodium bis(fluorosulfonyl)imide (NaFSI), sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), sodium trifluoromethanesulfonate (NaOTf), sodium difluorophosphate (NaPO2F2), sodium tetrafluoroborate (NaBF4), and sodium difluorooxalate borate (NaDFOB).

5. The eutectic electrolyte according to claim 1, characterized in that, The eutectic electrolyte further includes additives, wherein the mass of the additives accounts for 0% to 10% of the mass of the eutectic electrolyte; The additives include high molecular weight polymers and / or inorganic particles.

6. The eutectic electrolyte according to claim 5, characterized in that, The specific polymers mentioned include: polyvinylidene fluoride (PVDF) and its copolymers, polyethylene oxide (PEO) and its derivatives, polyacrylic acid (PAA), polymethyl methacrylate (PMMA) and its copolymers, polyurethane (PU), polyvinyl alcohol (PVA), cellulose and its nanofibers (CNF), silk fibroin, chitosan, polyacrylonitrile (PAN) and its derivatives, polyethoxylated trimethylolpropane triacrylate (PETPTA), and polyethylene glycol methyl ether methacrylate (PEGM) or one or more of these. The inorganic particles specifically include one or more of the following: LLZO solid electrolyte, LLZTO solid electrolyte, LATP solid electrolyte, silicon dioxide (SiO2), aluminum oxide (Al2O3), NASICON solid electrolyte, titanium dioxide (TiO2), zirconium oxide (ZrO2), zinc oxide (ZnO), magnesium oxide (MgO), yttrium oxide (Y2O3), barium titanate (BaTiO3), hexagonal boron nitride (h-BN), and sulfide electrolytes.

7. A method for preparing a eutectic electrolyte according to any one of claims 1-6, characterized in that, The preparation method includes: under a protective gas environment, mixing a sulfuric acid ester-containing compound with a lithium salt or sodium salt in a certain proportion, stirring at 30℃~120℃ for 10min~12h until the mixture is homogeneous, to obtain a eutectic electrolyte.

8. The preparation method according to claim 7, characterized in that, The mass ratio of the lithium salt to the sulfate-containing compound is 1:1 to 1:10; the mass ratio of the sodium salt to the sulfate-containing compound is 2:5 to 3:

5.

9. The preparation method according to claim 7, characterized in that, The preparation method further includes: after stirring evenly, adding additives and continuing stirring to obtain a eutectic electrolyte.

10. A secondary battery, characterized in that, The secondary battery includes the eutectic electrolyte as described in any one of claims 1-6.

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