Method for prolonging cycle life of sodium metal battery

Abstract

The invention relates to the field of battery preparation metal, in particular to a method for prolonging the cycle life of a sodium metal battery, which comprises the following steps: (S1) electrolyte preparation: NaPF6 and NaBF4 are respectively weighed and placed in a mixed solvent of DEM, FEC and TFDA, DEM is ethylene glycol dimethyl ether, FEC is fluoroethylene carbonate, and TFDA is trimethylsilyl 2-(fluorosulfonyl)-difluoroacetate; placing the mixture in an ultrasonic cleaning machine; (S2) pretreating the sodium metal foil in an FSTC solvent, and forming a soft package battery with the positive plate after the solvent on the surface of the sodium metal foil is volatilized; the FSTC is 3-(fluorosulfonyl)-2-thiophene carboxylic acid methyl ester; (S3) injecting an electrolyte into the soft package battery; and (S4) carrying out process treatment on the soft package battery injected with the electrolyte to obtain a product. According to the method for prolonging the cycle life of the sodium metal battery, the problem that the cycle life of a sodium metal negative electrode is limited can be solved.

Description

Technical Field

[0001] The present invention relates to the field of battery preparation metals, and particularly to a method for improving the cycle life of sodium metal batteries. Background Art

[0002] With the rapid development of the new energy industry and the increasing global attention to environmental protection and sustainable development, the research and application of high-efficiency, low-cost, and environmentally friendly energy storage devices have become a hot topic in the current scientific and technological field. As an important supplement and alternative to lithium-ion batteries, sodium metal batteries are regarded as an important development direction in the future energy storage field due to their advantages such as rich resources, low cost, environmental friendliness, and stable electrochemical performance.

[0003] However, despite the many advantages of sodium metal batteries, their cycle life and performance stability are still the key factors restricting their large-scale commercial applications. In particular, sodium metal anodes are prone to form sodium dendrites during charge and discharge processes, which not only cause internal short circuits in the battery but also accelerate the decomposition of the electrolyte and the decay of battery performance, thus seriously affecting the cycle life and safety of sodium metal batteries.

[0004] Improving the electrolyte formulation is one of the common methods to improve the cycle life and performance stability of sodium metal batteries. Common methods include: adjusting the chemical composition of the electrolyte and optimizing additives.

[0005] The chemical composition adjustment method refers to a method of optimizing the performance of the electrolyte by precisely controlling the solvent, electrolyte type, and its concentration of the electrolyte. The limitation of this method is the high implementation difficulty, and the types and ratios of the solvent and electrolyte need to be strictly screened and precisely matched. Inappropriate ratios may lead to an increase in the viscosity of the electrolyte and a decrease in the ion mobility, affecting the charge and discharge efficiency and cycle life of the battery. In addition, some solvent or electrolyte components may react with other materials in the battery to generate harmful impurities, damaging the battery performance and safety.

[0006] The additive optimization method refers to a method of improving certain key properties of the electrolyte by introducing a small amount of functional compounds. The limitation of this method is that the selection, concentration, and compatibility of the additives with the electrolyte all face challenges. There are a wide variety of additives, and their effects are affected by multiple factors such as the electrolyte composition, the type of electrode material, and the battery operating conditions, making it difficult to precisely control. Summary of the Invention

[0007] In order to overcome the deficiencies in the prior art, the present invention provides a method for improving the cycle life of sodium metal batteries.

[0008] To achieve the above object, a method for improving the cycle life of sodium metal batteries disclosed by the present invention includes the following steps:

[0009] (S1) Prepare the electrolyte: Weigh NaPF6 and NaBF4 separately and dissolve them in a mixed solvent of DEM, FEC, and TFDA, where DEM is dimethoxyethane, FEC is fluoroethylene carbonate, and TFDA is trimethylsilyl 2-(fluorosulfonyl)-difluoroacetate; place the above mixture in an ultrasonic cleaner;

[0010] (S2) Pretreat the sodium metal foil in the FSTC solvent. After the solvent on the surface of the sodium metal foil has evaporated, form a soft-pack battery with the positive electrode sheet; FSTC is methyl 3-(fluorosulfonyl)-2-thiophenecarboxylate;

[0011] (S3) Inject the electrolyte into the soft-pack battery;

[0012] (S4) Perform process treatment on the soft-pack battery injected with the electrolyte to obtain the product.

[0013] Preferably, the process treatment in step (S4) includes high-temperature infiltration, formation, secondary final sealing, and aging.

[0014] Preferably, high-temperature infiltration means placing the soft-pack battery after injection under the condition of 40-50°C and standing still to ensure that the electrolyte fully infiltrates the battery core.

[0015] Preferably, formation means charging the battery to 4.3V with a constant current of 0.05C.

[0016] Preferably, secondary final sealing means releasing the gas in the soft-pack battery after formation and performing secondary sealing on the side of the soft-pack battery.

[0017] Preferably, aging means performing high-temperature aging on the soft-pack battery under the condition of 40-50°C.

[0018] Preferably, in step (S1), the concentration of NaPF 6 is between 0.5 and 3.0 mol·L -1 , the concentration of NaBF 4 is between 0.05 and 0.5 mol·L -1 , the volume fraction of FEC is between 2 and 5 vol%, the volume fraction of TFDA is between 5 and 10 vol%, define the volume fraction of DEM as X vol%, and define the volume fraction of TFDA as Y vol%, where X + Y = 100.

[0019] Furthermore, the mass ratio of NaPF6, NaBF4, DEM, FEC, and TFDA is 18.8:1.24:69:3.96:7.1.

[0020] The present invention has the following technical effects:

[0021] 1. Solve the problem of limited cycle life of the sodium metal negative electrode;

[0022] As a type of potential energy storage device, the cycle life of sodium metal batteries is closely related to the growth of sodium dendrites on the sodium metal anode. Traditional sodium metal battery designs often suffer from limited cycle life due to the lack of an effective sodium dendrite inhibition mechanism. This solution pre-treats sodium metal foil with methyl 3-(fluorosulfonyl)-2-thiophenecarboxylate (FSTC) to form an initial interfacial film; combined with the use of a SEI film enhancer (TFDA) added to the electrolyte, it can further evolve into an enhanced solid electrolyte interface film during the first charging process, effectively inhibiting the formation of sodium dendrites, thus significantly improving the cycle life of sodium metal batteries.

[0023] 2. Introduce an enhancer to enhance the comprehensive interfacial performance;

[0024] This patent introduces trimethylsilyl 2-(fluorosulfonyl)difluoroacetate (TFDA) as a SEI film enhancer into the electrolyte, which not only optimizes the interfacial chemical stability and the conductivity of interfacial sodium ions, but also enhances the performance of the SEI film through the further reaction of TFDA with the initial interfacial film, providing higher energy density, more stable cycle performance, and better safety for sodium metal batteries.

[0025] 3. Reduce the preparation cost and improve the technical economy;

[0026] While pursuing high performance, this solution also fully considers the control of preparation cost. By introducing a SEI film enhancer, a "building block type" connected interface strategy is derived, which reduces the dependence on expensive materials or complex processes while ensuring a significant improvement in battery performance, thus enhancing the technical economy and commercial potential. Description of the Drawings

[0027] Figure 1 It is a schematic diagram of the FSTC molecular structure;

[0028] Figure 2 It is a schematic diagram of the TFDA molecular structure;

[0029] Figure 3 It is the XPS·F1s spectrum of the comparative example;

[0030] Figure 4 It is the XPS·F1s spectrum of the example;

[0031] Figure 5 It is the rate performance graph of the soft-pack batteries of the comparative example and the example;

[0032] Figure 6 It is the cycle performance graph of the soft-pack batteries of the comparative example and the example. Detailed Description of the Invention

[0033] The principles and features of the present invention will be described below in conjunction with embodiments; the examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0034] A method for improving the cycle life of a sodium metal battery, comprising the following steps:

[0035] (S1) Prepare the electrolyte: Weigh NaPF 6 and NaBF 4 respectively in a mixed solvent of DEM, FEC and TFDA, where DEM is ethylene glycol dimethyl ether, FEC is fluoroethylene carbonate, and TFDA is trimethylsilyl 2-(fluorosulfonyl)-difluoroacetate (its molecular structure is as shown in the appendix Figure 2 ); Place the above mixture in an ultrasonic cleaner and ultrasonicate for 10 min. The concentration of NaPF 6 ranges from 0.5 to 3.0 mol·L -1 , preferably, the concentration of NaPF 6 is 1.0 mol·L -1 ; The concentration of NaBF 4 ranges from 0.05 to 0.5 mol·L -1 , preferably, the concentration of NaF 4 is 0.1 mol·L -1 ; The volume fraction of FEC ranges from 2 to 5 vol%, preferably, the volume fraction of FEC is 2.5 vol%; The volume fraction of TFDA ranges from 5 to 10 vol%, preferably, the volume fraction of TFDA is 5 vol%. According to the above components and content ranges, the following Examples 1 to 20 are made, as shown in Table 1, where NaPF 6 is 167 g, NaBF 4 is 11 g, DME is 926.25 ml, FEC is 23.75 ml, and TFDA is 50 ml.

[0036] (S2) Place the cut sodium metal foil in the FSTC solvent for 30 s and then take it out. After the solvent on the surface of the sodium metal foil evaporates, it is combined with the positive electrode sheet to form a soft-pack battery. The material of the positive electrode sheet is selected as NVOPF, and FSTC is methyl 3-(fluorosulfonyl)-2-thiophenecarboxylate, and its molecular structure is as shown in the appendix Figure 1 .

[0037] (S3) Inject the electrolyte into the soft-pack battery, and the injection coefficient is 3.0 g / Ah.

[0038] (S4) After subjecting the soft-pack battery injected with the electrolyte to process treatment, a battery product is obtained. The process treatment includes high-temperature infiltration, formation, secondary final sealing and aging.

[0039] High-temperature infiltration means placing the pouch cell after injection under the condition of 40-50 °C and standing for 24 h to ensure that the electrolyte fully infiltrates the cell core.

[0040] Formation means placing the battery at 25 °C and charging the battery to 4.3 V with a constant current of 0.05C.

[0041] Secondary final sealing means releasing the gas in the pouch cell after formation and performing secondary encapsulation on the side of the pouch cell.

[0042] Aging means placing the pouch cell at 45 °C and standing for 48 h for high-temperature aging.

[0043] Table 1 Electrolyte formulations of each example

[0044]

[0045]

[0046] To verify the superiority of this solution, the following comparative examples are made. Specifically, it includes the following steps:

[0047] (S1) Electrolyte preparation: Weigh 168 g of NaPF 6 and 11 g of NaBF 4 and dissolve them in a mixed solvent of 975 mL of DME and 25 mL of FEC, and place them in an ultrasonic cleaner for ultrasonic treatment for 10 min.

[0048] (S2) Assemble the original Na foil and the NVOPF positive electrode sheet into a pouch cell.

[0049] (S3) Add electrolyte to the pouch cell, and the injection coefficient is 3.0 g / Ah.

[0050] (S4) Place the pouch cell after injection at 45 °C and stand for 24 h to ensure that the electrolyte fully infiltrates the cell core.

[0051] (S5) Place the battery at 25 °C and charge the battery to 4.3 V with a constant current of 0.05C.

[0052] (S6) Release the gas in the pouch cell after formation and perform secondary encapsulation on the side of the pouch cell.

[0053] (S7) Place the pouch cell at 45 °C and stand for 48 h to perform high-temperature aging on the pouch cell.

[0054] Perform a cycle test on Example 1 and the comparative example. Place Example 1 and the comparative example at 25 °C for long-cycle testing. The test voltage range is 2.5-4.3 V, and the cycle rate is 0.5C.

[0055] Figure 3 and Figure 4 shows the XPS F1s spectra of the negative electrode materials in the examples and comparative examples. The solid electrolyte interface (SEI) film formed by the original metallic Na in the base electrolyte has a relatively low content of inorganic components (especially LiF), which is roughly equivalent to the content of organic components (such as C-F bond compounds), each accounting for about 50%. This composition ratio indicates that such SEI films have limited effectiveness in suppressing sodium dendrite growth because a high content of inorganic components usually means that the SEI film has a higher hardness and can better regulate the transport process of Na+ ions. In contrast, the metallic Na pretreated by FSTC in the electrolyte containing the SEI film enhancer (TFDA) in-situ generates an enhanced SEI film (ESEI). The remarkable feature of this ESEI film is its extremely high content of inorganic components, which enables the ESEI to effectively inhibit the precipitation of sodium dendrites and ensure the uniform distribution of sodium ions during the transport process.

[0056] Figure 5 shows the rate performance graphs of the pouch cells made in Example 1 and the comparative example. Among them, NVOPF||Na@SEI represents the pouch cell of the comparative example, and NVOPF||Na@ESEI represents the pouch cell of Example 1. Due to the high LiF content in the ESEI, the impedance of the NVOPF||Na@ESEI cell is significantly reduced; while the LiF content in the SEI is low, resulting in a large impedance. This phenomenon further confirms the positive effect of the ESEI on regulating Na+ transport.

[0057] Figure 6 shows the cycle performance graphs of the pouch cells of the comparative example and Example 1. Due to the high LiF content in the ESEI, the interface is stable and plays a positive regulatory role in Na+ transport, resulting in a significant increase in the NVOPF||Na@ESEI cell; while the LiF content in the SEI is low, which cannot inhibit the precipitation of Na dendrites, leading to serious side reactions in the NVOPF||Na@SEI cell and a rapid decay of the cycle life.

[0058] In this solution:

[0059] (1) An innovative "building block type" connection interface design is proposed to provide protection for the sodium metal negative electrode. The Na@ASEI film is formed by pretreating the sodium metal foil through FSTC and then combined with the NVOPF positive electrode to construct a sodium metal battery with excellent performance. This design simplifies the battery assembly process and improves the cycle stability and safety.

[0060] (2) Based on the "building block type" connection interface, the present invention modifies the electrolyte and introduces an enhanced SEI film. This SEI film has a dense interface and a high Young's modulus, which can effectively prevent the growth of sodium dendrites and improve the cycle life. At the same time, its dense interface structure reduces side reactions, and the high Young's modulus enhances the mechanical strength, ensuring the safety performance of the battery.

[0061] (3) The enhanced SEI film also has excellent ion conduction performance and chemical stability. The inorganic components act as ion channels, reducing the internal resistance of the battery and improving the charge and discharge performance. The stable chemical properties avoid performance degradation caused by the rupture of the SEI film, making the sodium metal battery perform excellently in terms of cycle life, safety performance, and charge and discharge performance.

[0062] The above are only the preferred embodiments of the present invention; they are not intended to limit the present invention; any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for improving the cycle life of a sodium metal battery, characterized in that: The following steps are involved: (S1) preparing an electrolyte: weighing NaPF6 and NaBF4 respectively into a mixed solvent of DEM, FEC and TFDA, wherein DEM is ethylene glycol dimethyl ether, FEC is fluoroethylene carbonate, and TFDA is trimethylsilyl 2-(fluorosulfonyl)-difluoroacetate; placing the mixture in an ultrasonic cleaning machine; (S2) placing the sodium metal foil in a FSTC solvent for pretreatment, and after the solvent on the surface of the sodium metal foil evaporates, forming a soft-pack battery with the positive electrode sheet, wherein FSTC is 3-(fluorosulfonyl)-2-thiophenecarboxylic acid methyl ester; (S3) injecting electrolyte into the soft pack battery; (S4) Processing the soft-pack battery injected with the electrolyte to obtain a product.

2. The method for improving the cycle life of a sodium metal battery according to claim 1, characterized in that: The process treatment in step (S4) includes high temperature soaking, chemical formation, secondary sealing and aging.

3. The method for improving the cycle life of a sodium metal battery according to claim 2, characterized in that: High temperature soaking means placing the soft-pack battery after filling at 40-50°C to ensure that the electrolyte fully soaks the battery cell.

4. The method for improving the cycle life of a sodium metal battery according to claim 2, characterized in that: Formation refers to charging the battery to 4.3V using a constant current of 0.05C.

5. The method for improving the cycle life of a sodium metal battery according to claim 2, characterized in that: Secondary final sealing refers to releasing the gas in the soft-pack battery after formation and performing secondary sealing on the side of the soft-pack battery.

6. The method for improving the cycle life of a sodium metal battery according to claim 2, characterized in that: Aging refers to placing the soft-pack battery under high-temperature aging conditions of 40 to 50°C.

7. The method for improving the cycle life of a sodium metal battery according to claim 1, characterized in that: In step (S1), the concentration of NaPF6 is between 0.5 and 3.0 mol·L -1 , NaBF4 concentration is between 0.05 and 0.5 mol·L -1 , the volume fraction of FEC is between 2 and 5 vol%, the volume fraction of TFDA is between 5 and 10 vol%, the volume fraction of DEM is defined as X vol%, and the volume fraction of TFDA is defined as Y vol%, where X+Y=100.

8. The method for improving the cycle life of a sodium metal battery according to claim 1, characterized in that: The mass ratio of NaPF6, NaBF4, DEM, FEC and TFDA is: 18.8:1.24:69:3.96:7.1.

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