Electrolyte additive, electrolyte containing the additive and application thereof

Abstract

The application discloses an electrolyte additive, an electrolyte containing the additive and application of the electrolyte, relates to the technical field of electrolytes, and the electrolyte additive is at least one of 3-trifluoromethyl methyl benzoate and methyl benzoate; the electrolyte comprises a solvent, an electrolyte and the electrolyte additive. The electrolyte additive has low LUMO characteristics, can be preferentially reduced on the surface of a sulfur positive electrode, forms a dense and stable inorganic-organic composite interface film, and significantly improves the cycle stability, rate performance and capacity retention rate of a sodium-sulfur battery.

Description

Technical Field

[0001] This invention relates to the field of electrolyte technology, specifically to an electrolyte additive, an electrolyte containing the additive, and their applications. Background Technology

[0002] With the rapid development of large-scale energy storage and power battery industries, lithium-ion batteries, limited by the scarcity of lithium resources and high costs, struggle to meet the demands of long-term large-scale applications. Sodium-sulfur batteries, with their advantages of high energy density, abundant sodium and sulfur resources, and low cost, are considered a highly promising next-generation energy storage battery system. However, room-temperature sodium-sulfur batteries face several bottlenecks in practical applications: the sulfur cathode exhibits extremely poor conductivity, with volume expansion exceeding 160% during charge and discharge; carbonate electrolytes have poor compatibility with polysulfides, easily leading to irreversible side reactions and rapid capacity decay; and the sodium anode is prone to dendrite formation, damaging the solid electrolyte interface (SEI) and causing short circuits and safety risks.

[0003] Currently, strategies to improve the defects of sodium-sulfur batteries mainly include high-concentration electrolytes, locally high-concentration electrolytes, weakly solvated electrolytes, modification of the cathode carbon support, and functional membrane design. However, high-salt-concentration systems have high viscosity, reduced ion conduction efficiency, and high cost; although ether-based electrolytes can dissolve polysulfides, they exacerbate the shuttle effect; traditional additives are difficult to preferentially form films on the cathode surface and cannot fundamentally suppress the side reactions between carbonate solvents and sulfur cathodes.

[0004] Therefore, developing a low LUMO (lowest unoccupied orbital) functional additive that can preferentially reduce and decompose on the cathode surface, construct a stable CEI / SEI film, suppress side reactions and shuttle effects, and be compatible with carbonate electrolytes is crucial for promoting the commercialization of room temperature sodium-sulfur batteries. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide an electrolyte additive that can preferentially reduce sulfur cathode surface to form a dense and stable inorganic-organic composite interface film, which significantly improves the cycle stability, rate performance and capacity retention of battery.

[0006] The technical problem to be solved by this invention is achieved by the following technical solution:

[0007] The first objective of this invention is to provide an electrolyte additive, wherein the electrolyte additive is at least one of methyl 3-trifluoromethylbenzoate and methyl benzoate.

[0008] In some preferred embodiments, the electrolyte additive is methyl 3-trifluoromethylbenzoate.

[0009] A second object of the present invention is to provide an electrolyte comprising a solvent, an electrolyte, and an electrolyte additive.

[0010] Furthermore, the volume content of the electrolyte additive is 2-5% of the solvent.

[0011] Furthermore, the solvent is a carbonate solvent, including but not limited to at least one of ethylene carbonate (EC) and propylene carbonate (PC).

[0012] Furthermore, the electrolyte includes, but is not limited to, at least one of sodium bis(fluorosulfonyl)imide (NaFSI), sodium trifluoromethanesulfonate (NaOTf), sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), sodium perchlorate, sodium hexafluorophosphate, sodium tetrafluoroborate, and sodium bis(oxalatoborate) (NaBOB).

[0013] Furthermore, the concentration of the electrolyte is 1~3 mol / L.

[0014] A third objective of the present invention is to provide the application of the electrolyte in sodium-sulfur batteries.

[0015] The beneficial effects of this invention are:

[0016] 1) The highest occupied orbital (HOMO) energy level of the electrolyte additive methyl 3-trifluoromethylbenzoate provided by the present invention is -6.57 eV, which is much higher than the HOMO energy level of the solvent molecule, indicating that it is easily oxidized on the positive electrode surface to form SEI component, while its LUMO energy level is -2.58 eV, which is lower than the LUMO energy level of the solvent molecule, indicating that compared with carbonate solvents, methyl benzoate-based additives are more easily reduced on the sulfur electrode surface and participate in the formation of the interfacial film.

[0017] 2) Sodium-sulfur batteries using methyl 3-trifluoromethylbenzoate as an electrolyte additive exhibit superior cycle performance, with an initial charge capacity of 940 mAh·g. -1 The coulombic efficiency is 95%, and the capacity remains stable at 950 mAh·g after 100 cycles. -1 Its coulombic efficiency remains close to 100%, and its stability is better than that of electrolytes without additives. Attached Figure Description

[0018] Figure 1 This invention utilizes Materials Studio software to calculate and simulate the HUMO-LUMO energy level diagrams of electrolyte solvents and additives.

[0019] Figure 2The images shown are scanning electron microscope (SEM) images (a-c) of S, PAN and S@PAN in Embodiment 3 of the present invention, and X-ray energy dispersive spectroscopy (EDS) mapping image (d) of S@PAN.

[0020] Figure 3 To obtain the cyclic voltammetry (CV) curves of sodium-sulfur batteries, the electrolytes prepared in Examples 1, 2 and Comparative Example 1 of this invention were applied respectively.

[0021] Figure 4 To apply the electrolytes prepared in Examples 1, 2 and Comparative Example 1 of this invention to the electrochemical impedance spectroscopy (EIS) of sodium-sulfur batteries.

[0022] Figure 5 To obtain the cycle life curves of sodium-sulfur batteries by applying the electrolytes prepared in Examples 1, 2 and Comparative Example 1 of the present invention respectively;

[0023] Figure 6 The following are charge-discharge curves of the electrolytes prepared in Examples 1, 2 and Comparative Example 1 of this invention applied to sodium-sulfur batteries, respectively.

[0024] In this context, 1 M NaFSI + methyl benzoate refers to the electrolyte prepared in Example 1, 1 M NaFSI + methyl 3-trifluoromethylbenzoate refers to the electrolyte prepared in Example 2, and 1 M NaFSI refers to the electrolyte prepared in Comparative Example 1. Detailed Implementation

[0025] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below with reference to specific embodiments and illustrations.

[0026] Example 1

[0027] Using EC and PC in a volume ratio of 1:1 as solvents, NaFSI was stirred and dissolved in the solvent to prepare a NaFSI solution with a concentration of 1 mol / L. Methyl benzoate was added as an additive at a volume ratio of 5% of the solvent. The mixture was then thoroughly mixed to obtain the electrolyte.

[0028] Example 2

[0029] The electrolyte was prepared according to the method of Example 1, except that methyl benzoate was replaced with methyl 3-trifluoromethylbenzoate to obtain the electrolyte.

[0030] Comparative Example 1

[0031] An electrolyte was prepared according to the method of Example 1, except that no additives were added.

[0032] Example 3

[0033] The electrolytes prepared in Example 1, Example 2, and Comparative Example 1 were applied to sodium-sulfur batteries, respectively, and the specific steps are as follows:

[0034] (1) Cut the sodium tablets into round pieces with a diameter of 15.6 mm and set aside;

[0035] (2) Cut the glass fiber diaphragm into round pieces with a diameter of 19 mm and set aside.

[0036] (3) At room temperature, polyacrylonitrile (PAN) and sublimed sulfur (S) are ground and mixed evenly in a mortar and pestle at a mass ratio of 1:3. The resulting mixture is transferred to a crucible and then placed in a tube furnace. The temperature is increased to 350℃ at a heating rate of 5℃ / min and held for 2 h to obtain sulfur-containing cathode material (S@PAN).

[0037] (4) Using the above-mentioned sulfur-containing positive electrode material as the positive electrode and sodium sheet as the negative electrode. First, put the positive electrode into the positive electrode shell, then put in the glass fiber separator, then drop in 150 μL of electrolyte, then put in the negative electrode, gasket, and spring in sequence, and finally put the negative electrode shell on. The battery is then sealed using a battery packaging machine to obtain the CR2016 sodium-sulfur battery.

[0038] Figure 1 This invention utilizes the density functional theory (DFT) calculations in Materials Studio software to simulate the HUMO-LUMO energy level diagrams of the electrolyte solvent and additives. From... Figure 1 It can be seen that the HOMO energy levels of the additives methyl benzoate and methyl 3-trifluoromethylbenzoate are -6.2 eV and -6.57 eV, respectively, which are much higher than the HOMO energy levels of the solvent molecules, indicating that they are easily oxidized on the positive electrode surface to form CEI components; while their LUMO energy levels are -2.08 eV and -2.58 eV, respectively, which are lower than the LUMO energy levels of the solvent molecules, indicating that compared with EC / PC solvents, methyl benzoate-based additives are more easily reduced on the sulfur electrode surface and participate in the formation of SEI film.

[0039] Figure 2 The images show the SEM images (a-c) of S, PAN, and S@PAN in Example 3, and the EDS mapping image (d) of S@PAN. From... Figure 2It can be seen that the sulfur material exhibits an irregular, blocky structure with varying sizes; while PAN exhibits a near-spherical, relatively regular granular morphology. The synthesized S@PAN still has a near-spherical morphology with a diameter of 25-30 μm, a rough surface, and a secondary structure consisting of small particles with a diameter of 5-10 μm. Meanwhile, the EDS mapping shows that the material contains a small amount of N, and appropriate nitrogen doping is beneficial to improving the conductivity of PAN. Furthermore, the sulfur element is relatively evenly distributed, indicating that the S@PAN composite material was successfully synthesized.

[0040] The assembled zinc-sulfur battery was subjected to CV curve analysis (voltage range 0.01–3 V, scan rate 0.01 mV·s) using a Shanghai Chenhua Instrument CHI760E electrochemical workstation. -1 AC impedance spectroscopy and cycle life curve tests were performed, and the test results are shown in […]. Figures 3-5 .

[0041] Figure 3 To illustrate the CV curves of the electrolytes prepared in Examples 1, 2, and 1 (Comparative Example 1) applied to sodium-sulfur batteries, the following data was collected. Figure 3 It can be seen that the CV curve of the sodium-sulfur battery using the electrolyte prepared in Comparative Example 1 exhibits typical side reaction characteristics of the sodium battery system: a reduction peak appears at around 1.0 V, indicating significant irreversibility in the SEI film formation process, while the oxidation peak is located at 1.88 V. The reduction peak of the sodium-sulfur battery using the electrolyte prepared in Example 1 becomes significantly sharper, indicating that the additive preferentially participates in the positive electrode interface reaction, inducing the formation of a more stable SEI film, with better CV curve overlap and better reaction reversibility. The reduction peak of the sodium-sulfur battery using the electrolyte prepared in Example 2 becomes narrower and the current density is lower, which is attributed to the enhanced reduction activity of the fluorine-containing group (-CF3), which efficiently induces the formation of fluorine-containing SEI components.

[0042] Figure 4 AC impedance diagrams were generated to illustrate the application of the electrolytes prepared in Examples 1, 2, and 1 (Comparative Example 1) to sodium-sulfur batteries. From... Figure 4 It can be seen that all three EIS curves exhibit the characteristics of a high-frequency semicircle plus a low-frequency oblique line. First, the sodium-sulfur battery using the electrolyte prepared in Comparative Example 1 has the smallest EIS semicircle diameter, indicating that the interfacial charge transfer resistance is low. The sodium-sulfur battery using the electrolyte prepared in Example 1 has a significantly larger EIS semicircle diameter, indicating that insoluble products are generated inside the battery and deposited on the electrode surface, thus forming a stable SEI film. The sodium-sulfur battery using the electrolyte prepared in Example 1 has the largest EIS semicircle diameter.

[0043] Figure 5 To illustrate the cycle life curves of sodium-sulfur batteries prepared in Examples 1, 2, and 1, respectively, the electrolytes were applied. Figure 5It can be seen that the sodium-sulfur battery using the electrolyte prepared in Example 1 achieves a first-cycle charging capacity of nearly 1200 mAh·g. -1 The coulombic efficiency is around 85%. Although the initial capacity is lower than that of the comparative example 1, the cycle decay trend is significantly slowed down, and the capacity is maintained at 800 mAh·g after 100 cycles. -1 The coulombic efficiency remained stable at around 100%; the sodium-sulfur battery using the electrolyte prepared in Example 2 exhibited superior cycle performance, with an initial charge capacity of 940 mAh·g. -1 The coulombic efficiency is 95%, and the capacity remains stable at 950 mAh·g after 100 cycles. -1 The coulombic efficiency remained close to 100%; the sodium-sulfur battery using the electrolyte prepared in Comparative Example 1 had a higher first-cycle charging capacity, reaching 1350 mAh·g. -1 The coulomb efficiency was 95%, but then rapidly declined to 800 mAh·g. -1 And stabilized at 750 mAh·g -1 This may be because the dense SEI film induced by methyl benzoate effectively inhibits solvent decomposition, reduces irreversible capacity loss, and improves interfacial stability, demonstrating the optimizing effect of additives on cycling performance. Meanwhile, the SEI film induced by fluorinated additives exhibits superior chemical stability and ionic conductivity.

[0044] Figure 6 The image shows charge-discharge curves of the electrolytes prepared in Examples 1, 2, and 1 (Comparative Example 1) applied to sodium-sulfur batteries. Figure 6 It can be seen that the addition of additives to the electrolyte can significantly improve the first-cycle reaction kinetics of sodium-sulfur batteries, resulting in a higher initial voltage plateau, a more stable ending voltage, good reversibility, and a significantly slower capacity decay. The charge-discharge curves remain relatively stable with increasing cycle number, indicating that the addition of additives can effectively improve the cycle stability of the battery.

[0045] In summary, the addition of methyl benzoate and methyl 3-trifluoromethylbenzoate to the electrolyte increases the electrochemical window of the sodium-sulfur battery, making it easier for the SEI film of reduction decomposition products to form on the cathode surface. This hinders further reactions between carbonate molecules and the cathode active material, improving the battery's cycle stability and coulombic efficiency. Furthermore, with increasing cathode material synthesis temperature, carbon and sulfur are more deeply integrated, resulting in better electrochemical properties. Although this study has made some progress in regulating the electrochemical performance of sulfur-containing electrodes through electrolyte systems, clarifying that the combination of ester electrolytes and the benzene ring as a conjugated system has significant advantages in maintaining capacity, rate response, reducing LUMO, and improving interfacial stability in sodium-sulfur batteries.

[0046] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. An electrolyte additive, characterized in that: The electrolyte additive is at least one of methyl 3-trifluoromethylbenzoate and methyl benzoate.

2. The electrolyte additive according to claim 1, characterized in that: The electrolyte additive is methyl 3-trifluoromethylbenzoate.

3. An electrolyte, characterized in that: The electrolyte comprises a solvent, an electrolyte, and the electrolyte additive as described in claim 1.

4. The electrolyte according to claim 3, characterized in that: The volume content of the electrolyte additive is 2-5% of the solvent.

5. The electrolyte according to claim 3, characterized in that: The solvent is a carbonate solvent.

6. The electrolyte according to claim 5, characterized in that: The carbonate solvent is at least one of ethylene carbonate and propylene carbonate.

7. The electrolyte according to claim 3, characterized in that: The electrolyte is at least one of sodium bis(fluorosulfonyl)imide, sodium trifluoromethanesulfonate, sodium bis(fluoromethanesulfonyl)imide, sodium perchlorate, sodium hexafluorophosphate, sodium tetrafluoroborate, and sodium bis(oxalato)borate.

8. The electrolyte according to claim 3, characterized in that: The concentration of the electrolyte is 1~3 mol / L.

9. The application of the electrolyte according to any one of claims 3 to 8 in a sodium-sulfur battery.

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