Sodium metal battery system

Abstract

The invention discloses a sodium metal battery system. The sodium metal battery system comprises a positive electrode, a negative electrode and an electrolyte, the electrolyte is an ionic liquid-based composite electrolyte, the negative electrode is a sodium metal negative electrode with a hydrophobic polymer interface protection layer on the surface, the ionic liquid-based composite electrolyte comprises a sodium salt, an ether compound and 1-butyl-3-methylimidazolium bis (trifluoromethanesulfonyl) imide salt, and the sodium salt is an ionic liquid-based composite electrolyte. The hydrophobic polymer interface protection layer comprises a fluorine-containing polymer and a sodium salt. Through the synergistic effect of the ionic liquid-based composite electrolyte and the hydrophobic polymer interface protection layer, the protection effect of the sodium metal negative electrode is remarkably improved, the influence of trace water in battery circulation is greatly reduced, direct contact between water molecules and sodium metal is effectively blocked, and the service life of the battery is prolonged. The severe side reaction in a high-humidity environment and the corrosion behavior of sodium salt to metal sodium are inhibited, so that the electrochemical performance and the stability of the sodium metal battery are effectively improved.

Description

Technical Field

[0001] The present invention relates to a metal battery system, in particular to a sodium metal battery system, and belongs to the technical field of sodium metal batteries. Background Art

[0002] Lithium-ion batteries, a core energy storage technology for portable electronic devices and new energy vehicles, are currently facing resource bottlenecks and cost constraints. The uneven geographical distribution of global lithium reserves and rising development costs are forcing the research community to accelerate the search for sustainable alternatives. Against this backdrop, sodium, an element with over a thousand times the reserves of lithium in the Earth's crust, has emerged as a cost-effective alternative, propelling sodium-ion battery systems into a strategically valuable energy storage technology.

[0003] Existing sodium ion battery technology presents two major development paths: although the system based on the intercalation reaction mechanism has been commercialized, its theoretical specific capacity is limited (usually less than 120 mAh g -1 ) makes it mainly suitable for grid-level energy storage scenarios, and it is difficult to meet the needs of consumer electronics and electric vehicles with high energy density requirements. The sodium metal anode using the deposition-dissolution mechanism has shown a revolutionary breakthrough, with a capacity of 1166 mAh g -1 The theoretical specific capacity is more than three times that of traditional graphite anodes. However, the journey of sodium metal batteries from the laboratory to industrialization is fraught with obstacles. The high chemical activity of metallic sodium is like a double-edged sword. While it provides high energy density, it also leads to environmental tolerance defects: the entire manufacturing and operation process must be carried out in a glove box protected by high-purity argon, which significantly increases operation and maintenance costs. At the same time, the sodium metal anode reacts with trace amounts of water that penetrates the electrolyte during operation to produce dangerous gases such as hydrogen, which significantly increases the safety risks of sodium metal batteries.

[0004] Patent publication number CN113764652A provides a method for protecting the metal anode of an aqueous battery with a hydrophobic organic layer. This method uses a hydrophobic membrane as a hydrophobic substrate and utilizes an impregnation technique to fully impregnate an organic electrolyte containing a metal salt, thereby obtaining a special hydrophobic membrane-organic electrolyte structure. This structure is then constructed on the surface of the metal anode. The hydrophobicity of the membrane is utilized to prevent direct contact between the aqueous electrolyte and the metal anode, thereby achieving efficient protection of the metal anode. However, this method, which relies solely on a single layer of hydrophobic membrane, has poor stability and the protective film easily falls off in ether electrolytes, making it difficult to effectively protect the sodium metal anode.

[0005] Therefore, how to improve the waterproof performance of the sodium metal negative electrode is crucial to improving the electrochemical performance and stability of sodium metal batteries. Summary of the Invention

[0006] To achieve the above technical objectives, the present invention provides a sodium metal battery system. This sodium metal battery can significantly reduce the impact of trace water during battery cycling, effectively block direct contact between water molecules and sodium metal, significantly inhibit violent side reactions in high humidity environments and the corrosion of sodium salts on metallic sodium, and effectively improve the electrochemical performance and stability of the sodium metal battery.

[0007] In order to achieve the above technical objectives, the present invention provides a sodium metal battery system, including a positive electrode, a negative electrode and an electrolyte, wherein the electrolyte is an ionic liquid-based composite electrolyte, the negative electrode is a sodium metal negative electrode having a hydrophobic polymer interface protective layer on its surface, the ionic liquid-based composite electrolyte comprises a sodium salt, an ether compound and 1-butyl-3-methylimidazolium bistrifluoromethanesulfonyl imide salt, and the hydrophobic polymer interface protective layer comprises a fluorine-containing polymer and a sodium salt.

[0008] The present invention enhances interfacial barrier protection for the sodium metal negative electrode through the synergistic effect of an ionic liquid-based composite electrolyte and a hydrophobic polymer interfacial protective layer. Compared to conventional ether electrolytes, the ionic liquid-based composite electrolyte employed in the present invention exhibits weaker interaction with the hydrophobic polymer interfacial layer, effectively preventing dissolution of the hydrophobic polymer interfacial layer, thereby improving the stability of the hydrophobic polymer interfacial layer and, consequently, enhancing the interfacial layer's protective properties against the sodium metal negative electrode. Furthermore, the hydrophobic polymer interfacial protective layer allows sodium ions to pass through, thereby ensuring the proper progress of the sodium deposition and dissolution reaction. The mixture of the hydrophobic ionic liquid (1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt (P13FSI)) and a diluent ether compound (such as TTE) improves the viscosity of the electrolyte and increases the migration rate of sodium ions.

[0009] As a preferred embodiment, the molar ratio of the ether compound to 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt is 1-10:1-10. Controlling the molar ratio of the ether compound (diluent) to the ionic liquid within an appropriate range is beneficial for improving the electrochemical performance of the system. However, if the amount of the ether compound (diluent) added is too high and the ionic liquid content is too low, the protective properties of the hydrophobic polymer interface protective layer will be reduced. Conversely, if the amount of diluent added is too low, the electrolyte viscosity will increase, which is not conducive to sodium ion migration.

[0010] As a preferred solution, the ether compound is a hydrofluoroether compound.

[0011] As a preferred solution, the hydrofluoroether compound is 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE). Using TTE as the electrolyte diluent can produce the best-performing composite electrolyte, which has the best compatibility with 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt and the best synergistic effect of the system.

[0012] As a preferred solution, the molar ratio of the sodium salt to the 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt in the ionic liquid-based composite electrolyte is 0.5-1.5:3-5.

[0013] As a preferred solution, the mass ratio of the fluorine-containing polymer to the sodium salt in the hydrophobic polymer interface protection layer is 1-10:1.

[0014] As a preferred solution, the fluorine-containing polymer includes at least one of vinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride, and polyvinyl fluoride.

[0015] As a preferred solution, the sodium salt includes at least one of sodium bis(fluoromethanesulfonyl)imide, sodium bis(trifluoromethanesulfonyl)imide, and sodium hexafluorophosphate.

[0016] As a preferred solution, the sodium salt in the ionic liquid-based composite electrolyte is sodium bis(trifluoromethanesulfonyl)imide (NaTFSI).

[0017] As a preferred solution, the sodium salt in the hydrophobic polymer interface protection layer is sodium bis(fluorosulfonyl)imide (NaFSI).

[0018] As a preferred solution, the thickness of the hydrophobic polymer interface protection layer is 4-10 μm.

[0019] As a preferred solution, the hydrophobic polymer interface protection layer is prepared by coating a solution containing a fluorine-containing polymer and a sodium salt on the surface of the sodium metal negative electrode and then volatilizing the solvent.

[0020] As a preferred solution, the total mass concentration of the fluorine-containing polymer and the sodium salt in the solution is 20-50%, more preferably 35-40%.

[0021] As a preferred solution, the molar concentration of the sodium salt in the solution is 0.1-1 mol / L, more preferably 0.5-1 mol / L.

[0022] As a preferred solution, the solvent in the solution containing the fluorine-containing polymer and the sodium salt is ethylene glycol dimethyl ether (DME).

[0023] As a preferred solution, the coating thickness is 45-55 μm.

[0024] As a preferred solution, the method of volatilizing the solvent is vacuum drying.

[0025] Compared with the prior art, the advantages of the present invention are:

[0026] (1) The present invention improves the protective performance of the sodium metal negative electrode through the synergistic effect of the ionic liquid-based composite electrolyte and the hydrophobic polymer interface protective layer, effectively and continuously and stably blocks the direct contact between water molecules and sodium metal, and significantly inhibits the corrosion behavior of sodium salt on metallic sodium, greatly improving the electrochemical performance and stability of the sodium metal battery. Therefore, the sodium metal negative electrode of the present invention can realize the assembly of Na||Na symmetric batteries in the air and can also operate stably in an electrolyte environment with a water content of up to 5000 ppm;

[0027] (2) The preparation method is simple, low-cost, and has the prospect of industrial-scale application. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are merely embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying any creative work.

[0029] Figure 1 1 is a comparison chart of battery performance between Example 1 of the present invention and Comparative Example 1 (bare sodium negative electrode).

[0030] Figure 2 1 is a comparison chart of battery performance between Example 1 of the present invention and Comparative Example 2 (conventional ether electrolyte).

[0031] Figures 3 to 5 1 is a comparison chart of the electrochemical performance of Examples 2 to 4 and a battery containing a bare sodium negative electrode using electrolytes with water contents of 1000 ppm, 3000 ppm, and 5000 ppm, respectively.

[0032] Figure 6 Schematic diagram of the preparation process of Example 6 of the present invention.

[0033] Figure 7 This is a performance diagram of the battery assembled in Example 6 of the present invention.

[0034] Figure 8 1 is a comparison chart of the water wetting test results of the sodium metal negative electrode in Example 1 of the present invention and Comparative Example 1.

[0035] Figure 9 This is a comparison chart of the air stability test results of the sodium metal negative electrode in Example 1 of the present invention and Comparative Example 1.

[0036] Figure 10 This is a microscopic morphology of the hydrophobic polymer protective layer in Example 1 of the present invention.

[0037] Figure 11This is a diagram showing the battery stability performance of Example 7 of the present invention.

[0038] Figure 12 This is a diagram of battery stability performance of Example 8 of the present invention.

[0039] Figure 13 This is a diagram of the battery stability performance of Comparative Example 3 of the present invention. DETAILED DESCRIPTION

[0040] The following will clearly and completely describe the technical solutions of the present invention in conjunction with the embodiments of the present invention and the accompanying drawings. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0041] Example 1

[0042] The following is a preparation process of a sodium metal negative electrode with a hydrophobic polymer protective layer and a sodium metal battery assembly process.

[0043] (1) In an argon atmosphere glove box (H2O < 0.01 ppm, O2 < 0.01 ppm), NaTFSI (sodium bis(trifluoromethanesulfonyl)imide), TTE (1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether) and P13FSI (1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide) were weighed in a molar ratio of 1:4:4, stirred and dissolved at room temperature, and evenly mixed to obtain an ionic liquid-based composite electrolyte.

[0044] (2) 0.5 g of PVDF-HFP (vinylidene fluoride-co-hexafluoropropylene) and 101.5 mg of NaFSI (sodium bis(fluorosulfonyl)imide) were dissolved in 1 mL of DME (ethylene glycol dimethyl ether). The solution was then cast and coated on a sodium metal negative electrode sheet supported on aluminum foil with a coating thickness of 50 μm. The solution was vacuum dried at room temperature for 6 h. After DME evaporated, a continuous, dense, hydrophobic polymer protective layer with a thickness of 4 μm was formed on the sodium metal negative electrode sheet.

[0045] (3) Using the sodium metal negative electrode sheet with a hydrophobic polymer protective layer prepared in step (2) as the positive electrode and the negative electrode, the cut PP film is used as the separator, and the ionic liquid-based composite electrolyte prepared in step (1) is used as the electrolyte, a Na|Na symmetric battery is assembled in a glove box.

[0046] Comparative Example 1

[0047] A sodium metal battery was assembled using the method of Example 1, except that two identical sodium metal sheets supported on aluminum foil served as the positive electrode and the negative electrode, respectively, that is, the surfaces of the sodium metal sheets did not contain a hydrophobic polymer protective layer.

[0048] Comparative Example 2

[0049] The sodium metal battery was assembled using the method of Example 1, except that the electrolyte was a mixture of NaPF6 and diethylene glycol dimethyl ether (G2).

[0050] Example 2

[0051] A sodium metal battery was assembled using the method of Example 1, except that 1 μl of water was added to the electrolyte to make the water content in the electrolyte 1000 ppm.

[0052] At the same time, as a comparative experiment, 1 microliter of water was also added to the electrolyte in Comparative Example 1, so that the water content of the electrolyte in Comparative Example 1 was 1000 ppm.

[0053] Example 3

[0054] A sodium metal battery was assembled using the method of Example 1, except that 3 μl of water was added to the electrolyte to make the water content in the electrolyte 3000 ppm.

[0055] At the same time, as a comparative experiment, 3 μl of water was also added to the electrolyte in Comparative Example 1, so that the water content of the electrolyte in Comparative Example 1 was 3000 ppm.

[0056] Example 4

[0057] A sodium metal battery was assembled using the method of Example 1, except that 5 μl of water was added to the electrolyte to make the water content in the electrolyte 5000 ppm.

[0058] At the same time, as a comparative experiment, 5 μl of water was also added to the electrolyte in Comparative Example 1, so that the water content of the electrolyte in Comparative Example 1 was 5000 ppm.

[0059] Example 6

[0060] The sodium metal battery was assembled using the method of Example 1, except that the battery was assembled in an open air environment.

[0061] The performance tests of the sodium metal batteries assembled in the embodiments and comparative examples were carried out respectively, as shown in the respective figures for details.

[0062] Figure 1This is a battery performance comparison between Example 1 and Comparative Example 1, comparing the electrochemical performance differences between a bare sodium symmetric cell and a hydrophobic polymer-modified sodium metal symmetric cell in an ionic liquid electrolyte. As can be seen from the figure, the potential curve of the dense hydrophobic polymer-protected sodium metal negative electrode constructed in Example 1 is stable, with a transition point around 0.1 V, while the potential curve of the bare sodium negative electrode in Comparative Example 1 exhibits large fluctuations in the electrolyte, with a transition point as high as 0.5 V, and is extremely unstable. This comparative experiment demonstrates that the hydrophobic polymer modification layer can form an effective interfacial barrier on the sodium metal surface, significantly inhibiting the corrosion behavior of NaTFSI salt on metallic sodium, demonstrating excellent negative electrode protection performance.

[0063] Figure 2 The battery performance of Example 1 and Comparative Example 2 is compared, which compares the hydrophobic polymer modified sodium metal symmetric battery in the ionic liquid based composite electrolyte of Example 1 ( Figure 2 a) and the conventional ether electrolyte of Comparative Example 2 ( Figure 2 b) Differences in electrochemical performance. As can be seen from the figure, the hydrophobic polymer-modified sodium metal anode can be stably cycled in the ionic liquid-based composite electrolyte of the present invention, and the hydrophobic polymer modification layer maintains its original appearance after cycling, effectively covering the surface of the sodium anode. In contrast, in conventional ether electrolytes, the hydrophobic polymer layer will fall off the surface of the sodium anode after cycling. This is because the dense hydrophobic polymer protective layer constructed does not dissolve in the ionic liquid electrolyte, forming an effective and stable interfacial barrier. In conventional ether electrolytes, the hydrophobic polymer protective layer slowly dissolves and is therefore unable to effectively block water corrosion on the sodium metal anode. This shows that only through the synergistic effect of the hydrophobic polymer protective layer and the ionic liquid-based composite electrolyte can the sodium metal anode be effectively protected.

[0064] Figures 3 to 5 The battery performance of Examples 2-4 is compared with that of each comparative experiment, respectively. The differences in electrochemical performance between the bare sodium symmetric battery and the hydrophobic polymer-modified sodium metal anode in electrolytes with water contents of 1000 ppm, 3000 ppm, and 5000 ppm, respectively, are shown. As can be seen from the figures, the potential curve of the sodium metal anode protected by the ionic liquid-based composite electrolyte and the hydrophobic polymer of the present invention is stable, while the potential curve of the bare sodium anode exhibits significant fluctuations. This shows that the synergistic effect of the nanoscale interface barrier constructed by the ionic liquid-based composite electrolyte and the hydrophobic polymer layer in the present invention can effectively block direct contact between water molecules and sodium metal, significantly suppressing violent side reactions in high humidity environments, and demonstrating superior interface protection for the sodium metal anode.

[0065] Figure 6 The schematic diagram of the direct assembly of the battery in open air in Example 6 of the present invention is shown. The performance of the assembled battery is shown in FIG. Figure 7 .from Figure 7 As can be seen in the figure, the potential curve of the assembled sodium|sodium symmetric battery is stable, indicating that the sodium metal negative electrode in the system of the present invention can operate highly reversibly. This is because the protective layer liquid casting method constructs a hydrophobic polymer protective layer, and under the synergistic effect of the ionic liquid-based composite electrolyte, it effectively blocks the corrosion of sodium metal by moisture and oxygen in the air, allowing the sodium metal negative electrode to maintain chemical stability under conventional environmental storage. It can be seen that the present invention can enable the assembly of sodium metal batteries to be carried out in a dry room, without the need for a high-purity argon glove box, significantly reducing the assembly cost of sodium metal batteries.

[0066] The sodium metal negative electrode in Example 1 of the present invention and Comparative Example 1 was subjected to water infiltration test. Figure 8 As shown in the figure, when a water droplet is dropped on the surface of the sodium metal negative electrode, the sodium metal surface (bare sodium) without the protection of the hydrophobic interface reacts violently upon contact with the water droplet, generating a large number of bubbles and accompanied by metal corrosion. In contrast, the water droplet on the hydrophobic polymer-modified sodium metal surface remains completely spherical, exhibiting superhydrophobic properties, and no significant reaction is observed within 3 minutes. This shows that the hydrophobic polymer interface layer of the present invention can form an effective hydrophobic polymer protective layer on the sodium metal surface, effectively isolating the sodium metal from direct contact reactions with water.

[0067] The air stability test of the sodium metal negative electrode in Example 1 of the present invention and Comparative Example 1 was carried out. Figure 9 As shown in the figure, the unprotected sodium metal anode (bare sodium) exhibits severe surface oxidation and produces white corrosion products after 24 hours of air exposure. In contrast, the hydrophobic polymer-modified sodium metal anode of the present invention maintains a silvery-white metallic luster after 24 hours, with no visible changes to the interface. This demonstrates that the present invention effectively inhibits permeation corrosion from oxygen and water molecules by creating a hydrophobic polymer protective layer, enabling the storage of sodium metal in conventional environments and significantly reducing reliance on inert gas protection facilities. This demonstrates significant cost advantages and practical potential for industrialization.

[0068] The microscopic morphology of the hydrophobic polymer protective layer in Example 1 of the present invention is analyzed. Figure 10 Figure 1 shows the microscopic morphology of a hydrophobic polymer protective layer (a) and the microscopic morphology of a hydrophobic polymer protective layer supported on a sodium metal surface (b). The figure shows a dense and uniform surface of the hydrophobic polymer film, exhibiting a nanoscale network structure. Its thickness was precisely measured to be 4 μm. This protective layer structure not only ensures efficient ion conduction but also effectively suppresses side reactions, demonstrating excellent interfacial engineering properties.

[0069] Example 7

[0070] The sodium metal battery was assembled using the method of Example 1, except that TTE was replaced by DME (dimethyl ether) and NaFSI in the preparation of the polymer protective layer was replaced by NaPF6. The corresponding battery performance is shown in FIG. Figure 11 As shown in the figure, it can be seen that the battery cycle stability is good.

[0071] Example 8

[0072] The sodium metal battery was assembled using the method of Example 1, except that during the preparation of the polymer protective layer, the amount of NaFSI added was controlled to be 203.1 mg, so that the concentration of sodium salt in the solution was 1 mol / L. The corresponding battery performance was as follows: Figure 12 As shown in the figure, it can be seen that the battery cycle stability is good.

[0073] Comparative Example 3

[0074] The sodium metal battery was assembled using the method of Example 1, except that P13FSI (1-butyl-3-methylimidazolium bistrifluoromethanesulfonyl imide) was replaced by 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIm]PF6), 1-hexyl-3-methylimidazolium hexafluorophosphate ([HMIM]PF6), and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm]BF4), respectively. The battery performance of the corresponding system is shown in FIG. Figure 13 , where (a) is [BMIm]PF6, (b) is [HMIM]PF6, and (c) is [BMIm]BF4. As can be seen from the figure, when the ionic liquid in the composite electrolyte is [BMIm]PF6, it is difficult to provide good protection for sodium metal. In the second cycle, the overpotential increases significantly and the cycle stability is poor. Similarly, when the ionic liquid is [HMIM]PF6 or [BMIm]BF4, the corresponding battery cycle stability is also poor.

[0075] Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention is not limited to the embodiments shown herein, but is to be construed in the widest manner consistent with the principles and novel features disclosed herein.

Claims

1. A sodium metal battery system comprising a positive electrode, a negative electrode, and an electrolyte, characterized in that: The electrolyte is an ionic liquid-based composite electrolyte, the negative electrode is a sodium metal negative electrode with a hydrophobic polymer interface protection layer on the surface, the ionic liquid-based composite electrolyte contains sodium salt, ether compound and 1-butyl-3-methylimidazole bistrifluoromethanesulfonyl imide salt, and the hydrophobic polymer interface protection layer contains fluorine-containing polymer and sodium salt.

2. A sodium metal battery system according to claim 1, characterized in that: The molar ratio of the ether compound to 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt is 1-10:1-10; The ether compound is a hydrofluoroether compound; The hydrofluoroether compound is 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

3. A sodium metal battery system according to claim 1 or 2, characterized in that: The molar ratio of the sodium salt to the 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt in the ionic liquid-based composite electrolyte is 0.5-1.5:3-5.

4. The sodium metal battery system according to claim 1, characterized in that: The mass ratio of the fluorine-containing polymer to the sodium salt in the hydrophobic polymer interface protection layer is 1 to 10:

1.

5. A sodium metal battery system according to claim 1 or 4, characterized in that: The fluorine-containing polymer includes at least one of vinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride, and polyvinyl fluoride.

6. The sodium metal battery system according to claim 1, characterized in that: The sodium salt includes at least one of sodium bis(trifluoromethanesulfonyl)imide, sodium bis(trifluoromethanesulfonyl)imide and sodium hexafluorophosphate.

7. The sodium metal battery system according to claim 1, characterized in that: The thickness of the hydrophobic polymer interface protection layer is 4-10 μm.

8. The sodium metal battery system according to claim 1, characterized in that: The preparation process of the hydrophobic polymer interface protection layer is as follows: coating a solution containing a fluorine-containing polymer and a sodium salt on the surface of the sodium metal negative electrode and then volatilizing the solvent to obtain the hydrophobic polymer interface protection layer.

9. The sodium metal battery system according to claim 8, characterized in that: The total mass concentration of the fluorine-containing polymer and the sodium salt in the solution is 20-50%.

10. A sodium metal battery system according to claim 8 or 9, characterized in that: The coating thickness is 45-55 μm.

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