A local high-concentration ionic liquid modified electrolyte, a preparation method and application thereof

Abstract

The application belongs to the technical field of lithium batteries, and provides a locally high-concentration ionic liquid modified electrolyte, a preparation method and application thereof.The ionic liquid modified electrolyte provided by the application takes a stachydrine ionic liquid and a diluent hydrofluoroether and / or fluorophosphate as a main body, adds specific cosolvents and lithium salts, well dissolves the stachydrine ionic liquid and the diluent, and forms a homogeneous phase.Compared with a traditional carbonate electrolyte, the ionic liquid modified electrolyte has high conductivity, low viscosity, a wide electrochemical window, non-flammability and non-volatility, and high lithium ion transference number;further used for assembling lithium batteries, still can maintain good charge and discharge capacity after running at different rates, has good cycle stability and service life, lithium battery cycle number is greater than or equal to 80 cycles, and even can reach 220 cycles, average coulombic efficiency is greater than or equal to 96.5%, and even can reach 98.6%.

Description

Technical Field

[0001] This invention relates to the field of lithium battery technology, and more specifically, to a locally highly concentrated ionic liquid modified electrolyte, its preparation method, and its application. Background Technology

[0002] Lithium-ion batteries, with their high energy density, cycle stability, and designability, have attracted widespread attention in energy storage fields such as portable devices (mobile phones, cameras, computers, etc.), electric vehicles (pure electric vehicles, hybrid electric vehicles, etc.), and smart grids. Furthermore, lithium-ion batteries are considered the most suitable storage medium for realizing high-performance off-site energy storage units. Lithium-ion batteries have the potential to become the most promising energy storage option for next-generation electric vehicles and smart grid technologies.

[0003] Commercial lithium-ion battery electrolytes typically consist of organic solvents (carbonates) and lithium salts (LiPF6). However, moisture and heat can cause LiPF6 to thermally dissociate, generating Lewis acids (PF5) that attack solvent molecules. The resulting combustion of numerous active free radicals leads to thermal runaway. Lithium-ion battery fires are usually caused by heat generated from short circuits within one or more battery cells. The heat generated in a lithium-ion battery ignites the chemical substances inside, leading to thermal runaway. Thermal runaway temperatures can reach as high as 500°C, creating immense pressure inside the battery and causing the flammable electrolyte to burn and explode, posing a significant safety hazard. Furthermore, when carbonate-based electrolytes are used in lithium-ion batteries, the extremely low reduction potential of lithium metal (relative to the standard hydrogen electrode of -3.040V) makes it prone to chemical reactions with carbonate solvents, resulting in continuous electrolyte depletion. Moreover, during charge-discharge cycles, the solid electrolyte film formed gradually thickens, hindering lithium-ion migration and increasing the battery's internal resistance. In addition, batteries assembled with carbonate-based electrolytes are prone to lithium dendrite formation after multiple cycles. These dendrites penetrate the solid electrolyte membrane, triggering numerous side reactions, which in turn lead to continuous electrolyte consumption and ultimately battery failure.

[0004] Next-generation batteries require higher energy density (higher voltage or capacity) and a wider operating temperature range, and must meet higher safety standards for use in smartphones, electric vehicles, smart grids, and other applications. These shortcomings of traditional electrolytes hinder their development. Methods to improve battery performance and enhance the cycle stability of lithium batteries include increasing lithium salt concentration, constructing high-concentration electrolytes, introducing additives, and developing novel electrolytes. However, high-concentration electrolytes have high viscosity, which significantly affects their conductivity. Furthermore, the excessive amount of lithium salt used increases costs, thus limiting their widespread application. Diluents are substances that do not participate in the dissolution and solvation processes of lithium salts. Introducing diluents does not change the original solvation structure and composition of high-concentration electrolytes; it only reduces the electrolyte viscosity, thereby improving conductivity to some extent and reducing its cost. Therefore, introducing diluents to construct locally high-concentration electrolytes based on existing high-concentration electrolytes can solve the aforementioned problems associated with high-concentration electrolytes and improve battery performance. Room temperature ionic liquids are molten salts with melting points below 100°C. They typically consist of a large, asymmetric organic cation and a weakly coordinated inorganic / organic anion. They are characterized by non-flammability, extremely low vapor pressure, wide electrochemical window, high chemical and thermal stability, and high ionic conductivity. Therefore, ionic liquids are one of the promising electrolyte materials for lithium-ion batteries and supercapacitors.

[0005] However, the diluents currently used are mainly fluorine-containing compounds, which have very weak polarity. When they are used together with strongly polar ionic liquids to prepare electrolytes, the two cannot dissolve in each other, resulting in poor performance of the prepared electrolytes.

[0006] Therefore, there is an urgent need to provide a locally highly concentrated ionic liquid electrolyte that can maintain high cycle stability while ensuring the normal charge and discharge capacity of the battery. Summary of the Invention

[0007] The present invention aims to at least solve one of the technical problems existing in the prior art. To this end, the present invention proposes a locally highly concentrated ionic liquid modified electrolyte, its preparation method, and its application. The ionic liquid modified electrolyte provided by the present invention can be used to further prepare lithium batteries, maintaining good charge and discharge capacity even after operation at different rates, while possessing high cycle stability, with a lithium battery cycle count ≥80 cycles, and even reaching 220 cycles, and an average coulombic efficiency ≥96.5%, and even reaching 98.6%.

[0008] A first aspect of the present invention provides a locally highly concentrated ionic liquid modified electrolyte.

[0009] Specifically, a locally highly concentrated ionic liquid modified electrolyte comprises the following components:

[0010] Ionic liquids, diluents, cosolvents, lithium salts;

[0011] The ionic liquid is a stachyose base-type ionic liquid;

[0012] The diluent is hydrofluoroether and / or fluorophosphate;

[0013] The co-solvent is at least one of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0014] This invention utilizes stachydrine-based ionic liquids as the main solvent. By introducing diluents, the viscosity of the electrolyte is reduced, forming a locally highly concentrated electrolyte and improving its conductivity. More importantly, it alters the solvation structure of lithium ions in the electrolyte, forming a structure dominated by ionic liquid anions. This can be used to fabricate solid electrolyte membranes with high mechanical strength and high conductivity, primarily composed of inorganic materials, thereby preventing the formation of lithium dendrites and significantly improving battery performance while enhancing battery safety and stability. However, because ionic liquids are composed of cations and anions, they are highly polar, especially stachydrine-based ionic liquids, which have numerous CO bonds and are even more polar than typical ionic liquids. The diluents used in this invention, hydrofluoroethers and / or fluorophosphates, are very weakly polar; therefore, the ionic liquid and diluent are immiscible. Therefore, this invention adds specific co-solvents that effectively make the stachydrine-based ionic liquid and diluent miscible, forming a homogeneous phase. Furthermore, using the ionic liquid modified electrolyte of the present invention to prepare lithium batteries can significantly improve the charge and discharge capacity of lithium batteries, and enable them to maintain good charge and discharge capacity and high cycle stability after operating at different rates.

[0015] Preferably, the mass percentage of ionic liquid in the ionic liquid modified electrolyte is 30-60%.

[0016] Preferably, the mass percentage of the diluent in the ionic liquid modified electrolyte is 25-45%.

[0017] Preferably, the mass percentage of the co-solvent in the ionic liquid modified electrolyte is 3-10%.

[0018] Preferably, the lithium salt in the ionic liquid modified electrolyte has a mass percentage of 5-20%.

[0019] More preferably, the product comprises the following components by mass percentage:

[0020]

[0021] More preferably, it comprises the following components by mass percentage:

[0022]

[0023] Preferably, the structural formula of the cation of the stachydrine-type ionic liquid is shown in formula (Ⅰ):

[0024]

[0025] R1, R2, and R3 each independently represent C1-C6 alkyl groups.

[0026] Preferably, R1, R2, and R3 are each independently selected from one of -CH3, -CH2CH3, -CH2CH2CH3, -CH2CH2CH2CH3, and -CH2CH2CH2CH2CH3.

[0027] Preferably, the anion of the stachydrine-based ionic liquid is at least one of hexafluorophosphate (PF6-), bis(fluorosulfonyl)imide (FSI-), and bis(trifluoromethyl)sulfonyl)imide (TFSI-).

[0028] Preferably, the diluent is a fluorophosphate.

[0029] Preferably, the co-solvent is ethylene glycol dimethyl ether.

[0030] Preferably, the lithium salt is at least one selected from lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethylsulfonyl)imide. The anion in the lithium salt may be the same as or different from the anion in the ionic liquid.

[0031] A second aspect of the present invention provides a method for preparing an ionic liquid modified electrolyte.

[0032] A method for preparing an ionic liquid modified electrolyte includes the following steps:

[0033] The ionic liquid modified electrolyte is prepared by mixing an ionic liquid, a diluent, a co-solvent, and a lithium salt.

[0034] Preferably, the preparation method of the ionic liquid modified electrolyte includes the following steps:

[0035] First, the lithium salt is dissolved in the ionic liquid, then a diluent is added, followed by a co-solvent, to prepare the ionic liquid modified electrolyte.

[0036] A third aspect of the present invention provides an application of an ionic liquid modified electrolyte.

[0037] Application of an ionic liquid modified electrolyte in the preparation of lithium batteries.

[0038] A lithium battery includes the ionic liquid modified electrolyte, a negative electrode, a positive electrode, and a separator.

[0039] Preferably, the negative electrode active material of the negative electrode sheet is at least one of lithium metal, artificial graphite, natural graphite, mesophase carbon microspheres, soft carbon, hard carbon, mesophase carbon fiber, lithium titanate, and lithium metal alloy.

[0040] Preferably, the positive electrode is a copper foil or the positive electrode active material is lithium iron phosphate.

[0041] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0042] (1) The ionic liquid modified electrolyte provided by the present invention is based on stachydrine ionic liquid and diluent hydrofluoroether and / or fluorophosphate, with the addition of specific cosolvents and lithium salts, thereby regulating the solvation structure of lithium ions, improving the chemical environment of lithium ions, forming a solvation structure dominated by anions, and then forming a solid electrolyte membrane mainly composed of inorganic matter and supplemented by organic matter during the first charge and discharge process. This membrane has high strength and stability, and has good ionic conductivity and electronic insulation, thus ensuring that the lithium battery has good acclimatization stability and service life. Moreover, the locally high concentration of ionic liquid modified electrolyte of the present invention, due to the use of specific cosolvents, can make stachydrine ionic liquid and diluents miscible and form a homogeneous phase. Compared with traditional carbonate electrolytes, the ionic liquid modified electrolyte of the present invention has high conductivity, low viscosity, wide electrochemical window, non-flammability and non-volatility, and high lithium ion transport number.

[0043] (2) The ionic liquid modified electrolyte of the present invention can be further used to assemble lithium batteries, which can reduce the occurrence of side reactions between electrodes and electrolytes, form a more stable and good conductivity solid electrolyte membrane, and maintain good charge and discharge capacity after operation at different rates. It has better cycle stability and service life, with lithium battery cycle count ≥80 cycles, or even up to 220 cycles, and average coulombic efficiency ≥96.5%, or even up to 98.6%. Attached Figure Description

[0044] Figure 1 The diagram shows the battery cycle performance of Application Examples 1-3 and Comparative Application Examples 1-2 of the present invention. Detailed Implementation

[0045] To enable those skilled in the art to more clearly understand the technical solutions described in this invention, the following embodiments are provided for illustration. It should be noted that the following embodiments do not constitute a limitation on the scope of protection claimed by this invention.

[0046] Unless otherwise specified, the raw materials, reagents or devices used in the following examples are available from conventional commercial sources or can be obtained by existing known methods.

[0047] Example 1

[0048] An ionic liquid modified electrolyte comprises the components shown in Table 1.

[0049] The preparation method of the above-mentioned ionic liquid modified electrolyte includes the following steps:

[0050] At room temperature, lithium salt is first dissolved in ionic liquid to form a solution, then a diluent and a co-solvent are added and mixed evenly to obtain a locally highly concentrated ionic liquid modified electrolyte.

[0051] Table 1. Components and amounts of the ionic liquid modified electrolytes in each example and comparative example (unit: mass percentage).

[0052]

[0053]

[0054] Example 2-12

[0055] The difference between Examples 2-12 and Example 1 is that the components and their contents are different, as shown in Table 1, but the preparation methods are the same.

[0056] Comparative Example 1

[0057] Comparative Example 1 used a conventional electrolyte, with the solvent consisting of a 1:1 volume ratio of 1,3-dioxolane (DOL) and dimethyl ethylene glycol (DME), and 1 wt% lithium nitrate (LiNO3) as an additive. The lithium salt was lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) at a concentration of 1.0 mol / L. Comparative Example 1 served as a control group.

[0058] Comparative Examples 2-3

[0059] The difference between Comparative Examples 2-3 and Example 1 is that the components and their contents are different, as shown in Table 1, but the preparation methods are the same.

[0060] Application Example 1-12

[0061] Application Examples 1-12: Using the electrolyte samples prepared in Examples 1-12 (named A1-A12 electrolytes respectively), half-cells were assembled with lithium foil as the negative electrode and copper foil as the positive electrode, and the cycle stability of the batteries was tested.

[0062] Comparative Application Examples 1-3

[0063] Comparative Application Examples 1-3: Using the electrolyte samples prepared in Comparative Examples 1-3, lithium foil was used as the negative electrode and copper foil as the positive electrode to assemble half-cells, and the cycle stability of the batteries was tested.

[0064] Product effectiveness test

[0065] The lithium batteries prepared in Application Examples 1-12 and Comparative Application Examples 1-3 were respectively tested at 0.1 mA / cm. 2 0.5mA / cm 2 1.0 mA / cm 2 2.0mA / cm 2 The circuit was cycled at a current density to test its cycle count and coulomb efficiency.

[0066] Table 2. Test results of various lithium batteries at different rate.

[0067] Number of lithium battery cycles (revolutions) Average coulombic efficiency (%) Application Example 1 220 98.6 Application Example 2 200 98.3 Application Example 3 130 98.1 Application Example 4 120 96.5 Application Example 5 150 97.5 Application Example 6 200 98.1 Application Example 7 130 97.4 Application Example 8 160 98.2 Application Example 9 120 97.3 Application Example 10 100 98.1 Application Example 11 120 97.4 Application Example 12 110 96.9 Comparative Application Example 1 (Control Group) 35 95.3 Comparative Application Example 2 80 96.2 Comparative Application Example 3 85 95.7

[0068] Figure 1 The battery cycle performance diagrams for Application Examples 1-3 and Comparative Application Examples 1-2 of the present invention are shown in conjunction with... Figure 1 As shown in Table 2, compared with Comparative Application Example 1 (control group), under the same assembly and testing conditions, the lithium battery cycle count increased and the average coulombic efficiency improved after using the locally high-concentration ionic liquid electrolyte provided by the present invention in Application Examples 1-12. The lithium battery cycle count was ≥80 cycles, and even reached 220 cycles, and the average coulombic efficiency was ≥96.5%, and even reached 98.6%.

[0069] Comparative Application Example 2 uses a pyrrolidine-based ionic liquid, while Comparative Application Example 3 uses an imidazole-based ionic liquid. Because the pyrrolidine-based and imidazole-based ionic liquids have high electroreduction potentials after multiple cycles, they decompose earlier. After multiple cycles, they decompose completely and adhere to the electrode surface, making lithium-ion transport difficult. As a result, the number of lithium battery cycles and the average coulombic efficiency of Comparative Application Example 2 and Comparative Application Example 3 both decrease.

Claims

1. An ionic liquid modified electrolyte, characterized in that, The ionic liquid modified electrolyte comprises the following components: an ionic liquid, a diluent, a cosolvent, a lithium salt; the ionic liquid is a stachysin ionic liquid; the diluent is tris(trimethylsilyl) phosphate; the cosolvent is at least one of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether; the mass percentage of the ionic liquid in the ionic liquid modified electrolyte is 30-60%; the mass percentage of the diluent in the ionic liquid modified electrolyte is 25-45%; the mass percentage of the cosolvent in the ionic liquid modified electrolyte is 3-5%; the mass percentage of the lithium salt in the ionic liquid modified electrolyte is 5-20%; the structure of the cation of the stachysin ionic liquid is shown in the following formula (I): formula (I); wherein R1, R2 and R3 independently represent C1-C6 alkyl.

2. The method for preparing the ionic liquid-modified electrolyte according to claim 1, characterized in that, The method comprises the following steps: mixing the ionic liquid, the diluent, the cosolvent and the lithium salt to prepare the ionic liquid modified electrolyte.

3. The production method according to claim 2, characterized by, The method comprises the following steps: first dissolving the lithium salt in the ionic liquid, then adding the diluent, and then adding the cosolvent to prepare the ionic liquid modified electrolyte.

4. Use of the ionic liquid modified electrolyte of claim 1 in the preparation of a lithium battery.

5. A lithium battery, characterized by The lithium battery comprises the ionic liquid modified electrolyte of claim 1, a negative electrode sheet, a positive electrode sheet and a separator.

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