Fluoride ion battery electrolyte based on pyrrolidinium ionic liquid and preparation method thereof

Abstract

This invention discloses a fluoride-ion battery electrolyte based on pyrrolidineonium ionic liquid and its preparation method, comprising the following steps: under a protective atmosphere, mixing and stirring a fluoride salt with the pyrrolidineonium ionic liquid to obtain the fluoride-ion battery electrolyte based on the pyrrolidineonium ionic liquid. This invention uses a pyrrolidineonium ionic liquid, whose cation is composed of a saturated five-membered nitrogen-containing heterocycle, achieving an excellent balance between electrochemical stability, fluoride ion solvation / desolvation kinetics, thermal stability, and structural tunability. By changing the alkyl chain length on the nitrogen atom of the five-membered nitrogen-containing heterocycle of the cation, introducing ether oxygen groups, or fluorinated side chains, the viscosity, conductivity, solvation ability, and electrode interface compatibility of the ionic liquid can be selectively adjusted. The molecular structure design of the ionic liquid can regulate the interaction between the ionic liquid and fluoride ions.

Description

Technical Field

[0001] This invention relates to the field of fluorine-ion battery technology, specifically to a fluorine-ion battery electrolyte based on pyrrolidone-onium ionic liquid and its preparation method. Background Technology

[0002] Lithium-ion battery technology has matured, but its theoretical energy density limit is gradually becoming a bottleneck restricting its expansion into heavy industry. Meanwhile, the cost fluctuations and supply chain risks caused by limited lithium resources are prompting the research community to re-examine novel electrochemical systems based on abundant elements. Against this backdrop, fluoride-ion batteries, due to their extremely high theoretical energy density and abundant raw material reserves, coupled with the absence of metal deposition, lithium dendrites that puncture the separator, and the risk of internal short circuits, are gradually moving from concept to the forefront of research.

[0003] Unlike the carrier migration mechanism of lithium-ion insertion and extraction in traditional lithium-ion batteries, fluoride-ion batteries rely on the migration of fluoride ions between the positive and negative electrodes to transfer charge. Fluoride ions maintain a -1 valence throughout the charging and discharging process, acting solely as charge carriers. This unique charge storage mechanism makes its reaction mechanism self-contained and also presents new challenges for the design of subsequent electrolyte materials.

[0004] Fluorine-ion batteries exhibit a natural advantage in ion migration dynamics due to the weak Coulomb effect of a single negative charge. Theoretical calculations show that their energy density can exceed 5000 Wh / L, far surpassing the theoretical limit of current commercial lithium-ion batteries.

[0005] However, looking back at the development of fluoride-ion batteries, the electrolyte has always been the core obstacle between theory and application. Early research mainly drew on high-temperature solid-state electrolyte systems, which verified the feasibility of fluoride ion conduction. However, the high-temperature operating conditions severely limited its practical application scenarios, and solid-state electrolytes suffered from poor contact at the interface and low ionic conductivity at room temperature. As research progressed, room-temperature liquid electrolyte systems were gradually introduced, but new problems quickly emerged: fluoride salts tend to exhibit significant solvation effects in traditional proton solvents, causing fluoride ions to be bound in the solvation sheath, making it difficult for them to effectively participate in the electrochemical reactions at the electrode interface; in addition, organic liquid electrolytes are volatile, flammable, and have a narrow electrochemical window. This bottleneck directly leads to the widespread problems of slow reaction kinetics, low coulombic efficiency, and poor cycle stability in reported fluoride-ion battery systems. Summary of the Invention

[0006] This invention addresses the shortcomings of existing technologies by providing a fluoride-ion battery electrolyte based on pyrrolidine-onium ionic liquid and its preparation method. The fluoride-ion battery electrolyte is prepared by mixing pyrrolidine-onium ionic liquid with fluoride salt. The operation is simple, safe, and stable, with high fluoride ion conductivity at room temperature and a wide electrochemical stability window. It effectively solves the technical problems of poor electrochemical reactivity and poor reversibility of fluoride-ion batteries, and has great application prospects.

[0007] To address the aforementioned technical problems, the first aspect of this invention provides a method for preparing a fluorine-ion battery electrolyte based on a pyrrolidone-onium ionic liquid, comprising the following steps:

[0008] Under a protective atmosphere, fluorine salts and pyrrolidine-onium ionic liquids are mixed and stirred to obtain the fluorine-ion battery electrolyte based on the pyrrolidine-onium ionic liquid.

[0009] The electrolyte for fluoride-ion batteries of this invention is prepared by mixing a fluoride salt with a pyrrolidine-onium ionic liquid. The pyrrolidine-onium ionic liquid cation consists of a saturated five-membered nitrogen-containing heterocycle without conjugated double bonds. Two alkyl groups (such as methyl, ethyl, butyl, etc.) are typically attached to the nitrogen atom, forming a quaternary ammonium salt structure. This saturated, rigid cyclic framework, along with the flexibly modifiable alkyl side chains, endows it with unique physicochemical properties. Thanks to its saturated five-membered ring structure, it achieves an excellent balance between electrochemical stability, fluoride ion solvation / desolvation kinetics, thermal stability, and structural tunability, providing a ready-to-use solution for constructing room-temperature fluoride-ion batteries.

[0010] Furthermore, the pyrrolidineonium ionic liquid is composed of cations and anions;

[0011] The cation is selected from one of the following: pyrrolidine-onium cations without β-H, pyrrolidine-onium cations containing hydroxyl groups, and pyrrolidine-onium cations containing fluorinated side chains;

[0012] The anion is selected from one of hexafluorophosphate, tetrafluoroborate, bis(trifluoromethanesulfonyl)imide, trifluoromethanesulfonate, and halide ions.

[0013] The cations of the pyrrolidineonium ionic liquid of this invention are selected from β-H-free pyrrolidineonium cations, hydroxyl-containing pyrrolidineonium cations, and fluorinated side-chain-containing pyrrolidineonium cations. By having the cations of the ionic liquid participate in the solvation structure of fluoride ions, interfacial side reactions are suppressed, the desolvation process of fluoride ions is promoted, the migration barrier is reduced, and the battery energy efficiency is improved.

[0014] Furthermore, the pyrrolidineonium ionic liquid is selected from one or more of the following: 1-methyl-1-propylpyrrolidine bis(trifluoromethanesulfonyl)imine salt, 1-allyl-3-methylpyrrolidineonium bis(trifluoromethanesulfonyl)imine salt, 1-ethyl-1-propylpyrrolidineonium bis(trifluoromethanesulfonyl)imine salt, 1-methyl-1-propylpyrrolidineonium bis(fluorosulfonyl)imine salt, and 1-methyl-1-propylpyrrolidineonium tetrafluoroborate.

[0015] Furthermore, the fluoride salt is selected from one or more of cesium fluoride, tetramethylammonium fluoride, tetraethylammonium fluoride, tetrapropylammonium fluoride, tetrabutylammonium fluoride, benzyltrimethylammonium fluoride, potassium fluoride, sodium fluoride, and ammonium fluoride.

[0016] Furthermore, the mass ratio of the fluoride salt to the pyrrolidineonium ionic liquid is 1:(5-60).

[0017] Furthermore, the concentration of the fluoride salt in the pyrrolidineonium ionic liquid is 0.1-2.0 mol / L.

[0018] The second aspect of the present invention provides a fluorine-ion battery electrolyte based on pyrrolidone ionic liquid prepared by the preparation method described in the first aspect.

[0019] A third aspect of the present invention provides a fluorine-ion battery, comprising a positive electrode, a negative electrode, and the fluorine-ion battery electrolyte described in the second aspect.

[0020] Furthermore, the active material of the positive electrode includes one or more of copper fluoride, bismuth fluoride, tin fluoride, a complex of copper fluoride and carbon, a complex of bismuth fluoride and carbon, a complex of tin fluoride and carbon, and a bismuth-carbon complex.

[0021] Furthermore, the active material of the negative electrode includes one or more of the following: metallic lead, metallic tin, metallic magnesium, metallic zinc, metallic aluminum, lead fluoride, lead-carbon complex, and complex of lead fluoride and carbon.

[0022] The beneficial effects of this invention are:

[0023] The electrolyte for the fluoride-ion battery of this invention uses a pyrrolidineonium ionic liquid, whose cation is composed of a saturated five-membered nitrogen-containing heterocycle. The ring lacks conjugated double bonds, and the nitrogen atom is typically connected to two alkyl groups (such as methyl, ethyl, butyl, etc.) to form a quaternary ammonium salt structure. This saturated, rigid cyclic framework, along with the flexibly modifiable alkyl side chains, endows it with unique physicochemical properties; achieving an excellent balance between electrochemical stability, fluoride ion solvation / desolvation kinetics, thermal stability, and structural tunability.

[0024] This invention selectively modulates the viscosity, conductivity, solvation ability, and electrode interface compatibility of ionic liquids by changing the alkyl chain length on the nitrogen atom of the five-membered nitrogen-containing heterocyclic cation, introducing etheroxy groups (-OR) or fluorinated side chains (-Rf); the molecular structure design of ionic liquids can regulate the interaction between ionic liquids and fluoride ions. Attached Figure Description

[0025] To more clearly illustrate the technical solution of the present invention, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 The electrochemical performance of the electrolyte-assembled battery of Example 1 of the present invention;

[0027] Figure 2 This describes the electrochemical performance of the electrolyte-assembled battery of Example 2 of the present invention;

[0028] Figure 3 This describes the electrochemical performance of the electrolyte-assembled battery of Comparative Example 1 of the present invention;

[0029] Figure 4 This describes the electrochemical performance of the electrolyte-assembled battery of Comparative Example 2 of the present invention. Detailed Implementation

[0030] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] This embodiment relates to a method for preparing a fluoride-ion battery electrolyte based on pyrrolidone ionic liquid, comprising the following steps: under a protective atmosphere, mixing and stirring a fluoride salt with a pyrrolidone ionic liquid to obtain the fluoride-ion battery electrolyte based on the pyrrolidone ionic liquid.

[0032] In this embodiment, the electrolyte for the fluoride-ion battery is prepared by mixing a fluoride salt with a pyrrolidine-onium ionic liquid. The pyrrolidine-onium ionic liquid cation consists of a saturated five-membered nitrogen-containing heterocycle without conjugated double bonds. Two alkyl groups (such as methyl, ethyl, butyl, etc.) are typically attached to the nitrogen atom, forming a quaternary ammonium salt structure. This saturated, rigid cyclic framework, along with the flexibly modifiable alkyl side chains, endows it with unique physicochemical properties. Thanks to its saturated five-membered ring structure, it achieves an excellent balance between electrochemical stability, fluoride ion solvation / desolvation kinetics, thermal stability, and structural tunability, providing a ready-to-use solution for constructing room-temperature fluoride-ion batteries.

[0033] In a preferred embodiment, the pyrrolidineonium ionic liquid is composed of cations and anions; the cation is selected from one of β-H-free pyrrolidineonium cations, hydroxyl-containing pyrrolidineonium cations, and fluorinated side-chain-containing pyrrolidineonium cations; the anion is selected from one of hexafluorophosphate, tetrafluoroborate, bis(trifluoromethanesulfonyl)imide, trifluoromethanesulfonate, and halide ions. Specifically, the pyrrolidineonium ionic liquid is selected from one or more of 1-methyl-1-propylpyrrolidineonium bis(trifluoromethanesulfonyl)imide, 1-allyl-3-methylpyrrolidineonium bis(trifluoromethanesulfonyl)imide, 1-ethyl-1-propylpyrrolidineonium bis(trifluoromethanesulfonyl)imide, 1-methyl-1-propylpyrrolidineonium bis(fluorosulfonyl)imide, and 1-methyl-1-propylpyrrolidineonium tetrafluoroborate. In this embodiment, the cation of the pyrrolidineonium ionic liquid is selected from pyrrolidineonium cations without β-H, pyrrolidineonium cations containing hydroxyl groups, and pyrrolidineonium cations containing fluorinated side chains. By having the cation of the ionic liquid participate in the solvation structure of fluoride ions, interfacial side reactions are suppressed, the desolvation process of fluoride ions is promoted, the migration barrier is reduced, and the battery energy efficiency is improved.

[0034] In a preferred embodiment, the fluoride salt is selected from one or more of cesium fluoride, tetramethylammonium fluoride, tetraethylammonium fluoride, tetrapropylammonium fluoride, tetrabutylammonium fluoride, benzyltrimethylammonium fluoride, potassium fluoride, sodium fluoride, and ammonium fluoride.

[0035] In a preferred embodiment, the mass ratio of the fluoride salt to the pyrrolidineonium ionic liquid is 1:(5-60); the concentration of the fluoride salt in the pyrrolidineonium ionic liquid is 0.1-2.0 mol / L.

[0036] Another embodiment provides a fluorine-ion battery electrolyte based on pyrrolidone-onium ionic liquid prepared by the preparation method described in the above embodiments.

[0037] Another embodiment provides a fluorine-ion battery, including a positive electrode, a negative electrode, and the fluorine-ion battery electrolyte described in the above embodiment. Preferably, the active material of the positive electrode includes one or more of copper fluoride, bismuth fluoride, tin fluoride, a complex of copper fluoride and carbon, a complex of bismuth fluoride and carbon, a complex of tin fluoride and carbon, and a bismuth-carbon complex. The active material of the negative electrode includes one or more of metallic lead, metallic tin, metallic magnesium, metallic zinc, metallic aluminum, lead fluoride, a lead-carbon complex, and a complex of lead fluoride and carbon.

[0038] Example 1

[0039] This embodiment relates to a method for preparing a fluorine-ion battery electrolyte based on a pyrrolidone-onium ionic liquid, comprising the following steps:

[0040] In an argon-filled glove box, tetramethylammonium fluoride (TMAF) was weighed and added to 2 mL of 1-methyl-1-propylpyrrolidine bis(trifluoromethanesulfonyl)imine salt (MppyTFSI) to achieve a TMAF concentration of 0.1 mol / L. The mixture was stirred vigorously at 30 °C for 12 hours to form a homogeneous and transparent solution, which is the electrolyte for fluorine-ion batteries based on pyrrolidine onium ionic liquid.

[0041] Example 2

[0042] This embodiment relates to a method for preparing a fluorine-ion battery electrolyte based on a pyrrolidone-onium ionic liquid, comprising the following steps:

[0043] In an argon-filled glove box, benzyltrimethylammonium fluoride (TBMAF) was weighed and added to 2 mL of 1-methyl-1-propylpyrrolidine bis(trifluoromethanesulfonyl)imine salt (MppyTFSI) to achieve a TBMAF concentration of 0.1 mol / L. The mixture was stirred vigorously at 30 °C for 12 hours to form a homogeneous and transparent solution, which is the electrolyte for fluorine-ion batteries based on pyrrolidine onium ionic liquid.

[0044] Comparative Example 1

[0045] The difference between this comparative example and Example 1 lies in the adjustment of the ionic liquid, specifically:

[0046] In an argon-filled glove phase, tetramethylammonium fluoride was weighed and placed in three glass bottles. A: 1-butyl-3-methylimidazolium hexafluorophosphate, B: 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, and C: 1-allyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt were added respectively to make the TMAF concentration reach 0.1 mol / L. The mixture was stirred vigorously at 30 ℃ for 12 hours to form a homogeneous and transparent solution, thus obtaining three fluoride ion battery electrolytes, denoted as A, B, and C.

[0047] Comparative Example 2

[0048] The difference between this comparative example and Example 1 is that the ionic liquid is replaced with a protic solvent, specifically:

[0049] In an argon-filled glove box, tetramethylammonium fluoride (TMAF) was weighed and added to 2 mL of ethylene glycol (EG) to achieve a TMAF concentration of 0.1 mol / L. The mixture was stirred vigorously at 30 °C for 12 hours to form a homogeneous and transparent solution, thus obtaining the electrolyte for fluorine-ion batteries.

[0050] Test case

[0051] The electrolytes obtained in the examples and comparative examples were used to assemble coin-type fluorine-ion batteries. The positive electrode active material (Bi / BiF3)@C (a composite of bismuth and bismuth fluoride coated with carbon, with both bismuth and bismuth fluoride treated with carbon coating, and the mass ratio of bismuth to bismuth fluoride is 1:1), the conductive agent super-p, the binder polyvinylidene fluoride (PVDF), and the solvent N-methylpyrrolidone (NMP) were mixed to form a positive electrode slurry, which was coated on a titanium foil to obtain the positive electrode, and the negative electrode was a metal lead foil.

[0052] Example 1: Electrochemical performance of the electrolyte-assembled battery (constant current charge-discharge test performed at 30 °C with a current density of 50 mA / g and a charge-discharge test voltage range of -0.8 to 1.2 V). Figure 1 As shown, the initial discharge specific capacity was 182 mAh / g, which decreased to 110 mAh / g after 100 cycles, with a capacity retention of approximately 60%. The results indicate that the electrode exhibits a reversible defluorination / fluorination reaction, proving that the electrolyte system can be used in fluoride-ion batteries.

[0053] Example 2: Electrochemical performance of the electrolyte-assembled battery (constant current charge-discharge test performed at 30 °C with a current density of 50 mA / g and a charge-discharge test voltage range of -0.8 to 1.2 V). Figure 2 As shown, the initial discharge specific capacity was 98 mAh / g, which decreased to 25 mAh / g after 200 cycles, with a capacity retention of approximately 25%. The results indicate that the electrode exhibits a reversible defluorination / fluorination reaction, proving that pyrrolidineonium ionic liquids are suitable for fluoride-ion batteries with different fluoride salts.

[0054] The electrochemical performance of the electrolyte-assembled battery in Comparative Example 1 (constant current charge-discharge test at 30 °C with a current density of 50 mA / g and a charge-discharge test voltage range of -0.8 to 1.2 V) is as follows: Figure 3As shown, compared with the pyrrolidone-onium ionic liquid ([Mppy][TFSI]) in Example 1, the initial discharge specific capacity of the battery with the imidazole ionic liquid is 150 mAh / g, which drops to 20 mAh / g after 100 cycles, with a capacity retention of about 13.3%. This proves that the battery performance of the pyrrolidone-onium ionic liquid system for fluoride ion batteries is significantly better than that of the imidazole ionic liquid.

[0055] Electrochemical performance of the electrolyte-assembled battery in Comparative Example 2 (constant current charge-discharge test at 30 °C with a current density of 50 mA / g and a charge-discharge test voltage range of -0.8–1.2 V) is as follows: Figure 4 As shown, the TMAF-EG coin cell exhibits a low initial discharge specific capacity (approximately 80 mAh / g) and rapid capacity decay during charging, with capacity retention of less than 20% after only a few cycles. This is attributed to the strong solvation effect of the protic solvent EG on fluoride ions, making it difficult for fluoride ions to desolvate and participate in the electrode reaction, resulting in sluggish reaction kinetics and poor electrochemical reversibility. This comparison illustrates that the protic solvent has an excessively strong solvation effect on fluoride ions, leading to an excessively high desolvation barrier at the electrode / electrolyte interface, severely limiting their reversible insertion / extraction behavior. In contrast, pyrrolidineonium ionic liquids, with their tunable weak hydrogen bonding interactions, can provide a moderate solvation environment, exhibiting superior rate performance and cycle stability in fluoride-ion batteries.

[0056] The present invention has been described in detail above with reference to specific embodiments and exemplary examples; however, these descriptions should not be construed as limiting the present invention. Those skilled in the art will understand that various equivalent substitutions, modifications, or improvements can be made to the technical solutions and embodiments of the present invention without departing from the spirit and scope of the invention, and all such modifications and improvements fall within the scope of the present invention. The scope of protection of the present invention is defined by the appended claims.

Claims

1. A method for preparing a fluoride-ion battery electrolyte based on a pyrrolidone-onium ionic liquid, characterized in that, Includes the following steps: Under a protective atmosphere, fluorine salts and pyrrolidine-onium ionic liquids are mixed and stirred to obtain the fluorine-ion battery electrolyte based on the pyrrolidine-onium ionic liquid.

2. The method for preparing the fluoride-ion battery electrolyte based on pyrrolidone-onium ionic liquid as described in claim 1, characterized in that, The pyrrolidone ionic liquid is composed of cations and anions; The cation is selected from one of the following: pyrrolidine-onium cations without β-H, pyrrolidine-onium cations containing hydroxyl groups, and pyrrolidine-onium cations containing fluorinated side chains; The anion is selected from one of hexafluorophosphate, tetrafluoroborate, bis(trifluoromethanesulfonyl)imide, trifluoromethanesulfonate, and halide ions.

3. The method for preparing the fluoride-ion battery electrolyte based on pyrrolidone-onium ionic liquid as described in claim 1, characterized in that, The pyrrolidineonium ionic liquid is selected from one or more of the following: 1-methyl-1-propylpyrrolidine bis(trifluoromethanesulfonyl)imine, 1-allyl-3-methylpyrrolidineonium bis(trifluoromethanesulfonyl)imine, 1-ethyl-1-propylpyrrolidineonium bis(trifluoromethanesulfonyl)imine, 1-methyl-1-propylpyrrolidineonium bis(fluorosulfonyl)imine, and 1-methyl-1-propylpyrrolidineonium tetrafluoroborate.

4. The method for preparing the fluoride-ion battery electrolyte based on pyrrolidone-onium ionic liquid as described in claim 1, characterized in that, The fluoride salt is selected from one or more of cesium fluoride, tetramethylammonium fluoride, tetraethylammonium fluoride, tetrapropylammonium fluoride, tetrabutylammonium fluoride, benzyltrimethylammonium fluoride, potassium fluoride, sodium fluoride, and ammonium fluoride.

5. The method for preparing the fluoride-ion battery electrolyte based on pyrrolidone-onium ionic liquid as described in claim 1, characterized in that, The mass ratio of the fluoride salt to the pyrrolidine onion ionic liquid is 1:(5-60).

6. The method for preparing the fluoride-ion battery electrolyte based on pyrrolidone-onium ionic liquid as described in claim 1, characterized in that, The concentration of the fluoride salt in the pyrrolidineonium ionic liquid is 0.1-2.0 mol / L.

7. The fluorine-ion battery electrolyte based on pyrrolidone ionic liquid prepared by the preparation method according to any one of claims 1-6.

8. A fluorine-ion battery, characterized in that, It includes a positive electrode, a negative electrode, and the fluorine-ion battery electrolyte as described in claim 7.

9. The fluorine-ion battery as described in claim 8, characterized in that, The active material of the positive electrode includes one or more of the following: copper fluoride, bismuth fluoride, tin fluoride, a complex of copper fluoride and carbon, a complex of bismuth fluoride and carbon, a complex of tin fluoride and carbon, and a bismuth-carbon complex.

10. The fluorine-ion battery as described in claim 8, characterized in that, The active material of the negative electrode includes one or more of the following: metallic lead, metallic tin, metallic magnesium, metallic zinc, metallic aluminum, lead fluoride, lead-carbon complex, and complex of lead fluoride and carbon.

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