Low-overpotential uniform sodium deposition composite electrolyte and preparation method of sodium metal battery

Abstract

The invention relates to the technical field of sodium ion batteries, and discloses a low-overpotential uniform sodium deposition composite electrolyte and a preparation method of a sodium metal battery, the composite electrolyte comprises a sodium salt, a solvent, a film-forming additive and a sodium deposition regulating agent; the film forming additive is selected from at least one of NaPO2F2, NaDFOB and Na2SO3; the sodium deposition regulating agent is selected from at least one of CsTFSI, SnCl2 and Bi (OTf) 3. Through the synergistic effect of the film-forming additive and the sodium deposition regulating agent, a stable and firm SEI film can be formed on the interface of the negative electrode current collector, the overpotential of sodium deposition is obviously reduced to 20 mV or below, the growth of sodium dendrites is effectively inhibited, and dense and uniform deposition of sodium is realized. The negative-electrode-free sodium metal battery based on the composite electrolyte shows high first efficiency (greater than or equal to 90%), high coulombic efficiency (greater than 99.5%) and excellent cycling stability, and an effective scheme is provided for solving the core challenge of a sodium metal negative electrode.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, and more specifically to a composite electrolyte with low overpotential uniform sodium deposition and a method for preparing a sodium metal battery. Background Technology

[0002] With the global energy structure transitioning towards cleaner and lower-carbon energy, the development of efficient and safe energy storage technologies has become a research hotspot. Sodium metal batteries, due to their high theoretical capacity (1166 mAh / g) and low cost, have emerged as a strong candidate for next-generation energy storage systems. However, sodium metal anodes face significant challenges in practical applications: High deposition overpotential: leads to uneven local current density, triggering dendrite growth.

[0003] The solid electrolyte interface (SEI) is unstable: traditional liquid electrolytes are prone to react with sodium metal to form a loose and unstable SEI film, which continuously consumes active sodium and aggravates side reactions.

[0004] Uneven deposition: Dendrites can easily penetrate the separator, causing internal short circuits in the battery, which significantly reduces cycle life and safety performance.

[0005] Existing improvement methods have limitations: electrolyte additives (such as NaNO3, NaF) can optimize SEI to some extent, but cannot completely suppress dendrites; solid electrolytes (such as Na3Zr2Si2PO4) have limitations. 12 High mechanical strength, but poor interfacial contact and low room temperature ionic conductivity; three-dimensional current collectors (such as carbon-based porous materials) can reduce local current density, but the preparation process is complex and costly.

[0006] Based on this, the present invention proposes a composite electrolyte with low overpotential uniform sodium deposition and a method for preparing sodium metal batteries to solve the above problems. Summary of the Invention

[0007] In order to overcome the above-mentioned defects of the prior art, the present invention provides a composite electrolyte with low overpotential uniform sodium deposition and a method for preparing sodium metal batteries, so as to solve the problems existing in the background art.

[0008] This invention provides the following technical solution: a method for preparing a composite electrolyte with low overpotential uniform sodium deposition and a sodium metal battery, comprising the following components: Sodium salt; Solvent; Film-forming additive, wherein the film-forming additive is selected from at least one of NaPO2F2, sodium difluorooxalate borate (NaDFOB), and Na2SO3; Sodium deposition regulator, wherein the sodium deposition regulator is selected from at least one of bis(trifluoromethanesulfonyl)imide cesium (CsTFSI), SnCl2, and bismuth trifluoromethanesulfonate (Bi(OTf)3).

[0009] As a further aspect of the present invention: the concentration of the sodium salt is 1-3M, and the sodium salt is selected from at least one of NaPF6, sodium bis(fluorosulfonyl)imide (NaFSI), and sodium bis(trifluoromethanesulfonyl)imide (NaTFSI).

[0010] As a further aspect of the present invention: the sodium salt is 1.5M NaFSI.

[0011] As a further aspect of the present invention: the solvent is an ether solvent or a carbonate solvent; the ether solvent is a mixed solvent of 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL); the carbonate solvent includes ethylene carbonate (EC), dimethyl carbonate (DMC), and fluoroethylene carbonate (FEC).

[0012] As a further aspect of the present invention: the solvent is a mixture of DME and DOL in a volume ratio of 1:1, and contains 30 vol% of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) as a diluent.

[0013] As a further aspect of the present invention, the amount of the film-forming additive added is 0.5-2wt%, preferably 1wt% NaPO2F2.

[0014] As a further aspect of the present invention, the amount of sodium deposition regulator added is 0.05-0.3 wt%, preferably 0.2 wt% CsTFSI.

[0015] A sodium metal battery comprising a composite electrolyte as described in any one of the above.

[0016] As a further aspect of the present invention: the sodium metal battery is a sodium metal battery without a negative electrode, wherein the negative electrode side is a current collector that does not contain sodium metal.

[0017] As a further aspect of the present invention: a method for preparing a sodium metal battery, characterized in that, This includes the step of assembling a battery using a composite electrolyte as described in any of the above.

[0018] The technical effects and advantages of this invention are as follows: Ultra-low deposition overpotential: 0.5 mA / cm 2At the given current density, the sodium deposition overpotential can be stably below 20mV, which is more than 60% lower than that of unmodified conventional electrolytes (overpotential >50mV), indicating an extremely low nucleation and growth barrier.

[0019] Effective suppression of dendrites: Scanning electron microscopy (SEM) observation shows that a dense, non-porous bulk sodium layer is formed on the surface of the current collector using the electrolyte of this invention, which completely avoids the formation of porous, fibrous dendrites and greatly improves the safety of the battery.

[0020] High coulombic efficiency and long cycle life: In half-cells, the average coulombic efficiency reaches 99.7%. In full-cells without a negative electrode, the initial efficiency is ≥90%, the capacity retention rate exceeds 90% after 100 cycles, and the coulombic efficiency is stable at over 99.5%, demonstrating excellent cycle reversibility and stability.

[0021] Synergistic mechanism: Film-forming additives (such as NaPO2F2) preferentially decompose to form a robust SEI film rich in inorganic matter, while sodium deposition regulators (such as CsTFSI) guide sodium ions to achieve uniform and dense deposition through the "electrostatic shielding effect" or by inducing uniform nucleation. Attached Figure Description

[0022] The invention will now be further described with reference to the accompanying drawings.

[0023] Figure 1 This is a flowchart of the preparation method of a composite electrolyte with low overpotential uniform sodium deposition and a sodium metal battery according to the present invention. Figure 2 This is the SEM image from Embodiment 1 of the present invention; Figure 3 This is a SEM image from a comparative example of the present invention. Detailed Implementation

[0024] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. These embodiments are only used to explain the present invention and are not intended to limit the scope of protection of the present invention.

[0025] Example 1 This embodiment aims to illustrate a preferred formulation of the composite electrolyte described in this invention and its application effect in a sodium metal battery without a negative electrode.

[0026] Preparation of composite electrolytes: Sodium salt: Weigh out 1.5 M sodium difluorosulfonamide (NaFSI) as the conductive salt.

[0027] Solvent system: High-purity 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL) were mixed at a volume ratio of 1:1 to form the base solvent. Subsequently, 30 vol% of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) was added to this base solvent as a flame-retardant diluent to improve the safety of the electrolyte.

[0028] Film-forming additive: Sodium difluorophosphate (NaPO2F2) is added at 1 wt% of the total electrolyte mass.

[0029] Sodium deposition regulator: 0.2 wt% of bis(trifluoromethanesulfonyl)imide cesium (CsTFSI) was added.

[0030] All operations were performed in a glove box filled with argon gas, with water and oxygen content both below 0.1 ppm. The above components were mixed in a glass bottle and placed on a magnetic stirrer, and stirred continuously at room temperature for 12 hours until the sodium salt and additives were completely dissolved, yielding a clear, transparent homogeneous composite electrolyte solution.

[0031] Battery assembly: Negative electrode: A negative electrode-less configuration is adopted. Electrolytic copper foil with a thickness of 10 μm is used as the negative electrode current collector. Before use, it is ultrasonically cleaned sequentially with 1 M dilute hydrochloric acid, acetone and ethanol to thoroughly remove surface oxides and oil stains, and then vacuum dried for later use.

[0032] Positive electrode: Na3V2(PO4)3 (NVP) was used as the positive electrode active material, and was thoroughly mixed with acetylene black (conductive agent) and polyvinylidene fluoride (PVDF) (binder) in an N-methylpyrrolidone (NMP) solvent at a mass ratio of 8:1:1 to form a homogeneous slurry. This slurry was coated onto an aluminum foil current collector, vacuum dried at 120°C for 12 hours, and then cut into electrode sheets with a diameter of 12 mm. The areal loading of the positive electrode active material was approximately 3.5 mg / cm². 2 .

[0033] Separator: Whatman GF / D glass fiber diaphragm is used.

[0034] Battery configuration: In the glove box, the negative electrode shell, copper foil current collector (negative electrode), separator, prepared composite electrolyte (40 μL), NVP positive electrode sheet, gasket, spring sheet and positive electrode shell are assembled into a CR2032 coin cell in sequence, and sealed with a sealing machine at a pressure of 10 MPa.

[0035] Electrochemical performance testing: Testing environment: All tests were conducted in a constant temperature chamber at 25°C.

[0036] Sodium deposition / stripping test: The assembled Cu||Na half-cell was subjected to constant current cycling test on the Blue Battery test system. The current density was set to 0.5 mA / cm². 2 The deposition / stripping capacity per cycle is 0.5 mAh / cm³. 2 Record the voltage-time curve for each cycle.

[0037] Full cell testing: The assembled Cu||NVP electrodeless full cell was subjected to charge-discharge cycle testing within a voltage window of 2.0-3.8 V, with a charge-discharge current density of 0.2C (approximately 0.7 mA / cm²). 2 ).

[0038] Test Results and Analysis: Deposition overpotential: Measured from the voltage-time curve, at 0.5 mA / cm². 2 At the given current density, the overpotential of sodium deposition stabilizes at around 15 mV, exhibiting extremely low nucleation and growth barriers.

[0039] Deposition morphology analysis (SEM): After a certain number of cycles, the battery was disassembled in a glove box, the copper foil current collector was removed, and thoroughly rinsed with DME solvent to remove residual electrolyte. After drying, it was observed using a scanning electron microscope. SEM images showed that the sodium deposition layer on the copper current collector was dense and uniform, exhibiting a non-porous blocky morphology. No dendritic or moss-like sodium dendrites were observed (see attached instruction manual). Figure 2 ).

[0040] Coulombic efficiency: In the Cu||Na half-cell, the average coulombic efficiency is as high as 99.7% after multiple cycles, indicating that there are very few interfacial side reactions based on the electrolyte of this embodiment, and the sodium deposition / stripping process is highly reversible.

[0041] Full cell cycle performance: The initial discharge specific capacity of the Cu||NVP electrodeless full cell is 102 mAh / g. After 100 charge-discharge cycles, its discharge specific capacity retention rate is as high as 92%, and the coulombic efficiency remains stable at over 99.6%, demonstrating excellent cycle stability.

[0042] Example 2 This embodiment aims to illustrate the applicability and effectiveness of the composite electrolyte described in this invention in carbonate solvent systems.

[0043] Preparation of composite electrolytes: Sodium salt: Weigh out 2.0 M sodium hexafluorophosphate (NaPF6).

[0044] Solvent system: Ethylene carbonate (EC) and dimethyl carbonate (DMC) are mixed at a volume ratio of 1:1, and an additional 5 vol% of fluoroethylene carbonate (FEC) is added as a film-forming aid.

[0045] Film-forming additive: Sodium difluorooxalate borate (NaDFOB) is added at 2 wt% of the total electrolyte mass.

[0046] Sodium deposition regulator: 0.5 wt% of bismuth trifluoromethanesulfonate (Bi(OTf)3) was added.

[0047] The preparation process is the same as in Example 1.

[0048] Battery assembly: Negative electrode: Same as in Example 1, but using clean copper foil.

[0049] Positive electrode: using Prussian white analogue (Na) 1.5 Fe[Fe(CN)6] was used as the positive electrode active material to prepare the electrode sheet, with an active material loading of approximately 4.0 mg / cm³. 2 .

[0050] Separator: Celgard 2400 polypropylene separator is used.

[0051] Battery configuration and electrolyte filling: Same as in Example 1, with an electrolyte filling volume of 45 μL.

[0052] Electrochemical performance testing: The test conditions were the same as in Example 1.

[0053] Test Results and Analysis: Deposition overpotential: 0.5 mA / cm 2 At a current density of 18 mV, sodium deposition overpotential was achieved, thus realizing ultra-low overpotential deposition.

[0054] Deposition morphology analysis (SEM): The deposited sodium layer is flat and dense, consisting of tightly packed micron-sized sodium particles, and no sharp dendritic structures were observed.

[0055] Coulombic efficiency: The average coulombic efficiency of the Cu||Na half-cell is 99.5%.

[0056] Full cell cycle performance: The electrodeless full cell based on this electrolyte retained 90.5% of its capacity and maintained a stable coulombic efficiency of 99.4% after 80 cycles at a 0.5C rate. This result demonstrates that the composite electrolyte system of this invention is also effective in carbonate solvents and has good versatility.

[0057] Comparison Example This comparative example aims to highlight the technical advantages of the present invention by comparing it with conventional electrolytes.

[0058] Electrolyte preparation: Sodium salt: Weigh out 1.0 M sodium hexafluorophosphate (NaPF6).

[0059] Solvent system: Use a mixed solvent of EC and DMC (volume ratio 1:1).

[0060] Film-forming additives: None.

[0061] Sodium deposition regulator: None.

[0062] This formulation is a standard reference sodium-ion battery electrolyte.

[0063] Battery assembly and testing: Except for the electrolyte, the negative electrode, positive electrode (NVP), separator, assembly process and all test conditions of the battery are completely consistent with those in Example 1.

[0064] Test Results and Analysis: Deposition overpotential: 0.5 mA / cm 2 At a current density of 55 mV, the initial overpotential of sodium deposition is as high as 55 mV. As cycling progresses, the overpotential fluctuates continuously and increases significantly, indicating that the formed SEI film is unstable and the interfacial impedance is deteriorating.

[0065] Deposition morphology analysis (SEM): After cycling, numerous porous, fibrous sodium dendrites were observed on the surface of the copper current collector (see attached instruction manual). Figure 3 Its loose structure poses a very high risk of puncturing the diaphragm and causing a short circuit.

[0066] Coulombic efficiency: The average coulombic efficiency is only 85.3%, indicating that a large amount of metallic sodium is consumed in irreversible side reactions (such as electrolyte decomposition) and the formation of "dead sodium", and the irreversibility of the deposition / stripping process is very serious.

[0067] Full-cell cycle performance: The capacity of the electrodeless full cell exhibits a sharp decline. The capacity drops significantly after the first cycle, and after only 25 cycles, the capacity retention rate is already below 50%. At the same time, the coulombic efficiency fluctuates wildly between 85% and 93%, and the cell fails rapidly.

[0068] in conclusion A comparison of Examples 1 and 2 with the control examples clearly shows that the composite electrolyte system provided by the present invention, through the introduction of specific types of film-forming additives (such as NaPO2F2, NaDFOB) and sodium deposition modifiers (such as CsTFSI, Bi(OTf)3), produces a significant synergistic effect: The film-forming additive is preferentially reduced on the surface of the copper current collector, constructing a stable, robust solid electrolyte interface (SEI) film rich in inorganic matter, which effectively inhibits the continuous decomposition of the electrolyte and the irreversible consumption of sodium.

[0069] Sodium deposition modifiers, through their unique role at the interface (such as Cs) + The "electrostatic shielding effect" or the induced heterogeneous nucleation of Bi³⁺ effectively reduces the nucleation barrier of sodium and guides sodium ions to grow laterally in two dimensions, rather than forming dendrites in the vertical direction.

[0070] The synergistic effect of these two technologies has successfully achieved ultra-low sodium deposition overpotential and completely suppressed the growth of sodium dendrites, ultimately endowing sodium metal batteries, especially the highly challenging electrodeless batteries, with high coulombic efficiency, high initial efficiency, and excellent long cycle life.

[0071] The above are merely specific embodiments of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A composite electrolyte with low overpotential uniform sodium deposition, characterized in that, Includes the following components: Sodium salt; Solvent; Film-forming additive, wherein the film-forming additive is selected from at least one of NaPO2F2, sodium difluorooxalate borate (NaDFOB), and Na2SO3; Sodium deposition regulator, wherein the sodium deposition regulator is selected from at least one of bis(trifluoromethanesulfonyl)imide cesium (CsTFSI), SnCl2, and bismuth trifluoromethanesulfonate (Bi(OTf)3).

2. The composite electrolyte with low overpotential uniform sodium deposition according to claim 1, characterized in that: The concentration of the sodium salt is 1-3M, and the sodium salt is selected from at least one of NaPF6, sodium bis(fluorosulfonyl)imide (NaFSI), and sodium bis(trifluoromethanesulfonyl)imide (NaTFSI).

3. The composite electrolyte with low overpotential uniform sodium deposition according to claim 2, characterized in that: The sodium salt is 1.5M NaFSI.

4. The composite electrolyte with low overpotential uniform sodium deposition according to claim 1, characterized in that: The solvent is an ether solvent or a carbonate solvent; the ether solvent is a mixture of 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL); the carbonate solvent includes ethylene carbonate (EC), dimethyl carbonate (DMC), and fluoroethylene carbonate (FEC).

5. The composite electrolyte with low overpotential uniform sodium deposition according to claim 4, characterized in that: The solvent is a mixture of DME and DOL in a volume ratio of 1:1, and contains 30 vol% of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) as a diluent.

6. The composite electrolyte with low overpotential uniform sodium deposition according to claim 1, characterized in that: The amount of the film-forming additive added is 0.5-2 wt%, preferably 1 wt% NaPO2F2.

7. The composite electrolyte with low overpotential uniform sodium deposition according to claim 1, characterized in that: The amount of sodium deposition regulator added is 0.05-0.3 wt%, preferably 0.2 wt% CsTFSI.

8. A sodium metal battery, characterized in that, The sodium metal battery comprises a composite electrolyte as described in any one of claims 1-7.

9. The sodium metal battery according to claim 8, characterized in that, The sodium metal battery is a sodium metal battery without a negative electrode, and its negative electrode side is a current collector that does not contain sodium metal.

10. A method for preparing a sodium metal battery, characterized in that, The step includes assembling a battery using a composite electrolyte as described in any one of claims 1-7.

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