Electrolyte for sodium metal secondary battery and sodium metal secondary battery

Abstract

The application discloses an electrolyte for a sodium metal secondary battery and the sodium metal secondary battery, and belongs to the technical field of sodium metal secondary batteries. The electrolyte comprises an electrolyte additive and an ether mother liquor. The electrolyte additive is an ionic additive, the additive has a cation and a negatively charged H atom, the negatively charged H atom on the additive can form a dihydrogen bond with a positively charged alpha-H atom in the ether solvent, passivates the alpha-H atom in the ether solvent, and inhibits the dehydrogenation decomposition of the alpha-H atom. Meanwhile, the additive weakens the coordination effect of the ether solvent and sodium ions, the proportion of contact ion pairs and anion aggregates in the solvation structure of the sodium ions increases, therefore, the probability of the ether solvent moving to the surface of a sodium anode together with the sodium ions is low, the content of the ether solvent in an anode double electric layer is reduced, and the number of the ether solvent that can be reduced is less, and the side reaction of the chemical system is inhibited from the source.

Description

Technical Field

[0001] This application relates to an electrolyte for sodium metal secondary batteries and a sodium metal secondary battery, belonging to the technical field of sodium metal secondary battery. Background Technology

[0002] As the application of lithium-ion batteries in consumer electronics, electric vehicles, and energy storage systems continues to expand, the finite nature of global lithium resources and the associated supply risks are becoming increasingly prominent. Against this backdrop, sodium-based batteries, due to the abundance and widespread distribution of sodium on Earth, are considered a highly promising alternative technology.

[0003] However, sodium metal anodes exhibit extremely high intrinsic chemical activity. During charge-discharge cycles and long-term storage, continuous gas generation from side reactions has become a core technological bottleneck restricting the commercialization of sodium metal batteries. Gas generation in sodium metal batteries originates from irreversible side reactions at multiple interfaces and along multiple pathways: First, sodium metal undergoes strong reducing decomposition with the electrolyte, generating combustible gases such as H2, CO2, CO, and alkanes / olefins; second, the solid electrolyte interphase (SEI) film on the sodium anode surface has poor mechanical properties and an unstable structure, repeatedly breaking down and reconstructing during cycling, exposing fresh sodium surfaces and allowing them to continuously react with the electrolyte.

[0004] Therefore, there is an urgent need to develop an intrinsically stable electrolyte that does not continuously react with sodium metal to address issues such as gas generation, capacity decay, and safety risks. For example, CN202411621060.5 discloses a fluorine-rich composite additive and electrolyte, and its application in suppressing gas generation in sodium-ion batteries. This invention utilizes the preferential oxidation and graded reduction characteristics of different components in the composite additive to synergistically construct a cathode electrolyte membrane (CEI) and an anode solid electrolyte membrane (SEI), thereby suppressing gas generation in sodium-ion batteries. However, in sodium metal secondary batteries, due to the expansion and contraction of sodium metal on the anode side during cycling, the SEI formed by the electrolyte additive is difficult to reuse. After multiple cycles, it cannot suppress the reaction between the electrolyte and sodium metal. Therefore, the above-mentioned electrolyte is not suitable for sodium metal secondary batteries. Summary of the Invention

[0005] In view of the above problems, the first objective of this application is to provide an electrolyte for sodium metal secondary batteries to absorb hydrogen gas generated during the cycling process of sodium metal secondary batteries and alleviate the problem of cell bulging.

[0006] The technical solution adopted in this application is as follows: An electrolyte for sodium metal secondary batteries includes an electrolyte additive and an ether-based mother liquor. The electrolyte additive is an ionic additive, wherein the cation of the ionic additive is selected from one or more of sodium ions, lithium ions, potassium ions, magnesium ions, and zinc ions, and the anion of the ionic additive is selected from any one or more of the following structural formulas: Ⅰ-1; Ⅰ-2; Ⅰ-3; Ⅰ-4; Ⅰ-5.

[0007] Based on the above-mentioned cations and anions, the ionic additive is preferably any one of the following: Table 1 .

[0008] Preferably, the content of the ionic additive is 0.1%-5% of the mass percentage of the ether mother liquor, and more preferably 1%.

[0009] The ether mother liquor includes sodium salt and ether solvent.

[0010] The sodium salt is sodium hexafluorophosphate, with a concentration of 0.8-5 mol / L, preferably 1 mol / L. In some embodiments, some auxiliary salts may be added to optimize battery performance, including one or more of sodium tetrafluoroborate, sodium trifluoromethanesulfonate, sodium bis(oxalate)borate, sodium difluorooxalateborate, sodium difluorobis(oxalate)phosphate, sodium tetrafluorooxalate phosphate, sodium bis(trifluoromethanesulfonate)imide, sodium di(fluorosulfonyl)imide, and sodium perchlorate, with a concentration of 0.1-0.5 mol / L.

[0011] The solvent is one or more of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol dibutyl ether, tetrahydrofuran, 3-methyltetrahydrofuran, 1,3-dioxopentane, tetrahydropyran, 1,3-dioxane, and 1,4-dioxane, preferably ethylene glycol dimethyl ether.

[0012] This application also provides a sodium metal secondary battery containing the electrolyte for sodium metal secondary batteries described above.

[0013] The sodium metal secondary battery includes an anode electrode, a cathode electrode, a separator, and the electrolyte described above. The anode electrode, cathode electrode, and separator can be stacked or wound to form an electrode assembly.

[0014] The anode electrode is at least one of copper foil, aluminum foil, carbon-coated copper foil, carbon-coated aluminum foil, porous copper foil, and porous aluminum foil. In some embodiments, the anode electrode may also include a sodium-supplementing material, which includes one or more of sodium foil, sodium strip, sodium powder, and pre-sodiumization reagent.

[0015] The cathode electrode comprises an active material, conductive carbon, a binder, and a current collector.

[0016] The active substance is any one of sodium ferric pyrophosphate, sodium ferric sulfate, sodium vanadium phosphate, sodium vanadium fluorophosphate, layered oxides, Prussian blue, and Prussian white.

[0017] The conductive carbon is one or more of Superp, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, and graphene.

[0018] The adhesive is one or more of styrene-butadiene rubber, acrylonitrile, acrylate, polyvinylidene fluoride (PVDF), polytetrafluoroethylene, and carboxymethyl cellulose.

[0019] The current collector is any one of aluminum foil, carbon-coated aluminum foil, or porous aluminum foil.

[0020] The diaphragm is one or more of the following: polypropylene diaphragm, polyethylene diaphragm, inorganic ceramic coated diaphragm, or polypropylene or polyethylene composite ceramic diaphragm.

[0021] The beneficial effects of this application are as follows: (1) This application innovatively proposes an electrolyte for sodium metal secondary batteries, which is composed of an electrolyte additive and an ether mother liquor. By studying the electrolyte reaction pathway, it was found that the α-H atoms in the ether solvent are positively charged and easily detach to form hydrogen free radicals under the strong reducing effect of sodium metal, which further react to form hydrogen gas. This application designs a novel ionic additive with sodium ions and negatively charged H atoms. The negatively charged H atoms on the additive can form dihydrogen bonds with the positively charged α-H atoms in the ether solvent, passivating the α-H atoms in the ether solvent and inhibiting their dehydrogenation. At the same time, the additive weakens the coordination effect between the ether solvent and sodium ions, and the proportion of contact ion pairs and anion aggregates in the sodium ion solvation structure increases. Therefore, the probability of the ether solvent moving to the sodium anode surface with sodium ions is reduced, the content of ether solvent in the anode double layer is reduced, and the amount of ether solvent that can be reduced is less, thus inhibiting the side reactions of the chemical system from the root.

[0022] (2) When the cation of the ionic additive is sodium ion, it can replenish the active sodium consumed in the first cycle of battery film formation, offset the lithium loss caused by the formation of solid electrolyte interface film, improve the actual usable capacity of the battery, and improve the coulombic efficiency of the battery. When its cation is lithium ion, potassium ion, magnesium ion, or zinc ion, these cations can suppress the sodium metal tip effect, improve the sodium metal deposition morphology, and reduce the reactive specific surface area. Detailed Implementation

[0023] The present application will be further described below with reference to specific embodiments. Unless otherwise specified, the raw materials and reagents described in the embodiments are all existing technologies in the field or commercially available products.

[0024] Additives 1-8 in the examples are shown in the table below: .

[0025] Example 1 An electrolyte for sodium metal secondary batteries is composed of electrolyte additives and ether-based mother liquor.

[0026] Electrolyte additive: Select additive 1, with sodium ion as the cation and formula I-1 as the anion. The amount of electrolyte additive used is 1% of the mass of the ether mother liquor.

[0027] Ether mother liquor: sodium salt is sodium hexafluorophosphate, solvent is ethylene glycol dimethyl ether, and the concentration of sodium hexafluorophosphate is 1 mol / L.

[0028] Preparation method: In an argon-atmospheric glove box, where the water and oxygen contents are both less than 0.1 ppm, ethylene glycol dimethyl ether is used as the solvent. Sodium hexafluorophosphate is then dissolved in the solvent to obtain a homogeneous, colorless, and transparent ether mother liquor with a sodium hexafluorophosphate concentration of 1 mol / L. 1% (by weight) of additive 1 is weighed into the ether mother liquor and added, and the mixture is stirred until homogeneous to obtain the electrolyte for sodium metal secondary batteries.

[0029] To investigate the effects of different types of electrolyte additives on electrolyte performance, experiments as shown in Examples 2 to 8 were conducted.

[0030] Example 2

[0031] The preparation method is the same as in Example 1, except that the electrolyte additive is selected from Additive 2.

[0032] Example 3

[0033] The preparation method is the same as in Example 1, except that the electrolyte additive is selected from additive 3.

[0034] Example 4

[0035] The preparation method is the same as in Example 1, except that the electrolyte additive is selected from additive 4.

[0036] Example 5

[0037] The preparation method is the same as in Example 1, except that the electrolyte additive is selected from additive 5.

[0038] Example 6

[0039] The preparation method is the same as in Example 1, except that the electrolyte additive is selected from Additive 6.

[0040] Example 7

[0041] The preparation method is the same as in Example 1, except that the electrolyte additive is selected from additive 7.

[0042] Example 8

[0043] The preparation method is the same as in Example 1, except that the electrolyte additive is selected from Additive 8.

[0044] To investigate the effect of the amount of electrolyte additives on electrolyte performance, the experiments shown in Examples 9 and 10 were conducted.

[0045] Example 9

[0046] The preparation method is the same as in Example 1, except that the electrolyte additive is Additive 1, and the amount of electrolyte additive is 0.5% of the mass of the ether mother liquor.

[0047] Example 10

[0048] The preparation method is the same as in Example 1, except that the electrolyte additive is Additive 1, and the amount of electrolyte additive is 2% of the mass of the ether mother liquor.

[0049] To investigate the effect of the composition of the ether mother liquor on the performance of the electrolyte, the experiments shown in Examples 11 and 12 were conducted.

[0050] Example 11

[0051] The preparation method is the same as in Example 1, except that the composition of the ether mother liquor is: sodium hexafluorophosphate + sodium tetrafluoroborate, the solvent is ethylene glycol dimethyl ether, the concentration of sodium hexafluorophosphate is 1 mol / L, and the concentration of sodium tetrafluoroborate is 0.1 mol / L.

[0052] Example 12

[0053] The preparation method is the same as in Example 1, except that the composition of the ether mother liquor is: sodium hexafluorophosphate as sodium salt, diethylene glycol dimethyl ether as solvent, and sodium hexafluorophosphate concentration of 1 mol / L.

[0054] Comparative Example 1 The preparation method is the same as in Example 1, except that no electrolyte additives are added, and only ether mother liquor is used. The composition of the ether mother liquor is: sodium hexafluorophosphate as sodium salt, ethylene glycol dimethyl ether as solvent, and the concentration of sodium hexafluorophosphate is 1 mol / L.

[0055] Comparative Example 2 The preparation method is the same as in Example 1, except that no electrolyte additive is added, the sodium salt is sodium hexafluorophosphate + sodium tetrafluoroborate, the solvent is ethylene glycol dimethyl ether, the concentration of sodium hexafluorophosphate is 1 mol / L, and the concentration of sodium tetrafluoroborate is 0.1 mol / L.

[0056] Comparative Example 3 The preparation method is the same as in Example 1, except that no electrolyte additives are added, and the composition of the ether mother liquor is: sodium hexafluorophosphate as sodium salt, diethylene glycol dimethyl ether as solvent, and sodium hexafluorophosphate concentration of 1 mol / L.

[0057] Comparative Example 4 The preparation method is the same as in Example 1, except that the added electrolyte additive is Additive 9, the cation is sodium ion, and the anion is as shown in Formula II: Formula II.

[0058] Comparative Example 5 The preparation method is the same as in Example 1, except that the added electrolyte additive is Additive 10, the cation is sodium ion, and the anion is as shown in Formula III: Formula III.

[0059] Application Examples The electrolytes prepared in the aforementioned examples and comparative examples were applied to sodium metal secondary batteries to test their electrochemical performance.

[0060] Sodium metal secondary batteries consist of a cathode, an anode, a separator, and an electrolyte.

[0061] Cathode electrode: Active material sodium iron pyrophosphate, conductive agent Super P, and binder PVDF are mixed at a mass ratio of 98:1:1. N-methylpyrrolidone solvent is added and stirred until homogeneous, resulting in a cathode slurry with a viscosity of 40,000 mPas and a solid content of 65%. The cathode slurry is then coated on one side using a coating machine at a rate of 18 mg / cm². 2 The loading capacity is coated on both sides of the aluminum foil, dried thoroughly until the moisture content is below 150 ppm, and then cut into rectangles of 56 mm × 43 mm as cathode plates.

[0062] Anode electrode: Select carbon-coated aluminum foil with a thickness of 13 micrometers and cut it into rectangles with a size of 58 mm × 45 mm for later use.

[0063] Separator: Select a 12-micron thick polyethylene porous membrane and cut it into rectangles of 60 mm × 47 mm for later use.

[0064] Electrolyte: The electrolyte prepared using the aforementioned examples and comparative examples.

[0065] Battery assembly: The cut cathode plates, separator, and anode plates are stacked in sequence, with 10 layers of cathode plates and 11 layers of anode plates to obtain a bare cell. The bare cell is then welded with tabs and placed in an aluminum-plastic film outer packaging bag to obtain a dry cell. Electrolyte is injected into the dry cell and vacuum hot-pressed for sealing. After standing at room temperature for 10 hours, hot and cold pressing, formation, shaping, and capacity testing are carried out in sequence to obtain a sodium metal secondary battery with a rated capacity of 800mAh.

[0066] Performance testing: Cycle performance test: The ambient temperature was set to 25℃. The prepared battery was left to stand for 1 hour, then charged at a constant current of 0.5C to 3.35V, followed by constant voltage charging at 3.35V until the current dropped to 0.05C. After standing for 1 hour, it was discharged at a constant current of 1C to 2V to obtain the discharge capacity. The above steps were repeated for the same battery, and the number of cycles when the discharge capacity decayed to 80% was recorded. The number of cycles completed at this point is taken as the battery cycle life.

[0067] First-cycle coulombic efficiency test: The ambient temperature was set to 25℃. The prepared battery was left to stand for 1 hour, then charged at a constant current of 0.5C to 3.35V, followed by constant voltage charging at 3.35V until the current dropped to 0.05C, yielding the first-cycle charging capacity. After standing for 1 hour, it was discharged at a constant current of 1C to 2V, yielding the first-cycle discharging capacity. The first-cycle discharging capacity divided by the first-cycle charging capacity gives the first-cycle coulombic efficiency.

[0068] The test results are shown in Table 1: Table 1 Example 1 Additive 1 1% 1 mol / L sodium hexafluorophosphate Ethylene glycol dimethyl ether 3624 95.8 Example 2 Additive 2 1% 1 mol / L sodium hexafluorophosphate Ethylene glycol dimethyl ether 3567 95.5 Example 3 Additive 3 1% 1 mol / L sodium hexafluorophosphate Ethylene glycol dimethyl ether 3618 95.6 Example 4 Additive 4 1% 1 mol / L sodium hexafluorophosphate Ethylene glycol dimethyl ether 3617 95.4 Example 5 Additive 5 1% 1 mol / L sodium hexafluorophosphate Ethylene glycol dimethyl ether 3567 95.4 Example 6 Additive 6 1% 1 mol / L sodium hexafluorophosphate Ethylene glycol dimethyl ether 3631 95.6 Example 7 Additive 7 1% 1 mol / L sodium hexafluorophosphate Ethylene glycol dimethyl ether 3587 95.4 Example 8 Additive 8 1% 1 mol / L sodium hexafluorophosphate Ethylene glycol dimethyl ether 3595 95.6 Example 9 Additive 1 0.5% 1 mol / L sodium hexafluorophosphate Ethylene glycol dimethyl ether 3270 95.2 Example 10 Additive 1 2% 1 mol / L sodium hexafluorophosphate Ethylene glycol dimethyl ether 3317 95.2 Example 11 Additive 1 1% 1 mol / L sodium hexafluorophosphate + 0.1 mol / L sodium tetrafluoroborate Ethylene glycol dimethyl ether 3627 95.6 Example 12 Additive 1 1% 1 mol / L sodium hexafluorophosphate Diethylene glycol dimethyl ether 3628 95.7 Comparative Example 1 \ \ 1 mol / L sodium hexafluorophosphate Ethylene glycol dimethyl ether 2132 92.8 Comparative Example 2 \ \ 1 mol / L sodium hexafluorophosphate + 0.1 mol / L sodium tetrafluoroborate Ethylene glycol dimethyl ether 2415 93.1 Comparative Example 3 \ \ 1 mol / L sodium hexafluorophosphate Diethylene glycol dimethyl ether 2057 92.6 Comparative Example 4 Additive 9 1% 1 mol / L sodium hexafluorophosphate Ethylene glycol dimethyl ether 2567 93.8 Comparative Example 5 Additive 10 1% 1 mol / L sodium hexafluorophosphate Ethylene glycol dimethyl ether 2617 93.9 .

[0069] analyze: As can be seen from the comparison of Examples 1-8 and Comparative Example 1, the different types of electrolyte additives provided in this application can all achieve the construction of hydrogen bond networks in the electrolyte with anions, passivate α-H atoms in ether solvents, inhibit their dehydrogenation decomposition, reduce side reactions, and improve battery cycle life. At the same time, thanks to the lithium replenishment effect of cations or the suppression of sodium metal tip effect, the first-cycle coulombic efficiency is correspondingly improved.

[0070] As can be seen from the comparison between Examples 1 and Examples 9-10, the content of electrolyte additives provided in this application should not be too high or too low. If it is too high, there will be too many hydrogen bonds, the fluidity of the electrolyte will be poor, and the polarization of the battery will increase. If it is too low, the activity inhibition of solvent α-H atoms will not be sufficient, and a large number of active hydrogen atoms will still exist.

[0071] As can be seen from the comparison of Examples 1, 11-12 and Comparative Examples 1-3, the electrolyte additive provided in this application has excellent compatibility with ether mother liquor, which can enable sodium metal batteries to have a longer cycle life and a higher first-cycle coulombic efficiency.

[0072] As can be seen from the comparison of Example 1, Comparative Example 1, and Comparative Examples 4-5, the additive design provided by the present invention is unique. If the additive contains only one hydrogen atom, it cannot form a hydrogen bond network; if the additive contains only four hydrogen atoms, the additive itself is unstable and easily decomposes, leading to additive failure. However, the additive containing two hydrogen atoms is stable and can achieve an effective hydrogen bond network, thus making it suitable for sodium metal secondary battery electrolytes.

[0073] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. An electrolyte for sodium metal secondary batteries, characterized in that: The product includes an electrolyte additive and an ether mother liquor. The electrolyte additive is an ionic additive, wherein the cation of the ionic additive is selected from one or more of sodium ions, lithium ions, potassium ions, magnesium ions, and zinc ions, and the anion of the ionic additive is selected from any one or more of the following structural formulas: Ⅰ-1； Ⅰ-2； Ⅰ-3； Ⅰ-4； Ⅰ-5。 2. The electrolyte for a sodium metal secondary battery according to claim 1, characterized in that: The content of the ionic additive is 0.1%-5% of the mass percentage of the ether mother liquor.

3. The electrolyte for a sodium metal secondary battery according to claim 1, characterized in that: The ether mother liquor includes sodium salt and ether solvent.

4. The electrolyte for a sodium metal secondary battery according to claim 3, characterized in that: The sodium salt includes sodium hexafluorophosphate with a concentration of 0.8-5 mol / L.

5. The electrolyte for a sodium metal secondary battery according to claim 4, characterized in that: In addition to sodium hexafluorophosphate, the sodium salt also includes a secondary salt, which is selected from one or more of sodium tetrafluoroborate, sodium trifluoromethanesulfonate, sodium bis(oxalate)borate, sodium difluorooxalateborate, sodium di(oxalate)phosphate, sodium tetrafluorooxalate phosphate, sodium di(trifluoromethanesulfonate)imide, sodium di(fluorosulfonyl)imide, and sodium perchlorate, and the concentration of the secondary salt is 0.1-0.5 mol / L.

6. The electrolyte for a sodium metal secondary battery according to claim 3, characterized in that: The ether solvent is selected from one or more of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol dibutyl ether, tetrahydrofuran, 3-methyltetrahydrofuran, 1,3-dioxopentane, tetrahydropyran, 1,3-dioxane, and 1,4-dioxane.

7. A sodium metal secondary battery using the electrolyte for a sodium metal secondary battery as described in claim 1.

8. A sodium metal secondary battery according to claim 7, characterized in that: The sodium metal secondary battery includes an anode electrode, a cathode electrode, a separator, and an electrolyte. The anode electrode, cathode electrode, and separator are stacked or wound to form an electrode assembly.

9. A sodium metal secondary battery according to claim 8, characterized in that: The cathode electrode comprises an active material, conductive carbon, a binder, and a current collector; the active material is any one of sodium iron pyrophosphate, sodium iron sulfate, sodium vanadium phosphate, sodium vanadium fluorophosphate, layered oxides, Prussian blue, and Prussian white; the conductive carbon is one or more of Super P, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, and graphene; the binder is one or more of styrene-butadiene rubber, acrylonitrile, acrylate, polyvinylidene fluoride, polytetrafluoroethylene, and carboxymethyl cellulose; and the current collector is any one of aluminum foil, carbon-coated aluminum foil, and porous aluminum foil.

10. A sodium metal secondary battery according to claim 8, characterized in that: The anode electrode is at least one of copper foil, aluminum foil, carbon-coated copper foil, carbon-coated aluminum foil, porous copper foil, and porous aluminum foil; the diaphragm is one or more of polypropylene diaphragm, polyethylene diaphragm, inorganic ceramic coated diaphragm, and polypropylene or polyethylene composite ceramic diaphragm.

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