A linear iron boride cathode catalyst for lithium-sulfur batteries

Abstract

The application discloses a linear iron boride cathode catalyst for a lithium-sulfur battery. The catalyst is prepared based on a molten salt method of a double salt system, has a high length-diameter ratio and a uniform size (about 1 microns in diameter), and has excellent intrinsic conductivity, which provides fast electron supply for electrochemical reaction of sulfur, and meanwhile, local limited chemical polarity between iron and boron provides abundant adsorption and catalytic sites for intermediate product lithium polysulfide, effectively inhibits a shuttle effect, improves sulfur electro-conversion reaction kinetics, and thus good lithium-sulfur battery performance is obtained.

Description

Technical Field

[0001] This invention relates to functional materials for lithium-sulfur batteries, and more particularly to a metal boride electrocatalyst for lithium-sulfur battery cathodes. Background Technology

[0002] Among the many new energy storage technologies, lithium-sulfur batteries have attracted much attention due to their advantages such as high energy density, abundant raw materials, low price, and environmental friendliness. However, lithium-sulfur batteries still face some key problems and challenges, mainly manifested as follows: (1) The intrinsic insulation of elemental sulfur and lithium sulfide reduces the sulfur reaction efficiency and the overall energy efficiency of the battery; (2) The volume change of active material during cycling is very large, reaching more than 80%, which seriously damages the electrode structure and affects the stability of the battery; (3) The shuttle effect, that is, the intermediate product of charging and discharging, lithium polysulfide, is soluble in the electrolyte, shuttles back and forth between the positive and negative electrodes and causes side reactions on the two electrodes, causing loss of active material, passivation of the lithium negative electrode surface, and further leading to a series of problems such as low coulombic efficiency and rapid performance decay; (4) The deposition and dissolution kinetics of lithium sulfide are poor, the reaction energy barrier is high, and it is easy to form dead sulfur, resulting in loss of active material and decrease in battery capacity.

[0003] To address these issues, introducing highly efficient electrocatalysts into sulfur cathodes is a key approach to accelerate polysulfide conversion reactions, suppress shuttle effects, and improve battery performance. Metal compounds, due to their intrinsic chemical polarity and excellent morphological flexibility, have always been a research hotspot for lithium-sulfur battery cathode catalysts. Among them, metal borides, typically composed of boron and metal elements, inherit some metallic properties in their filled structure, exhibiting conductivity far exceeding that of other metal compounds. Furthermore, the ionic polarity between the metal and boron provides excellent adsorption sites for lithium polysulfides. However, research on metal borides in lithium-sulfur batteries remains relatively limited. Compared to other metals, iron is abundant in the Earth's crust and is environmentally friendly. The introduction of iron is expected to enhance the d-band center of the catalyst, strengthening its interaction with lithium polysulfides, thereby achieving a superior adsorption effect compared to other borides. However, common iron borides are mostly bulk structures, making it difficult to fully expose their active sites; moreover, the preparation of metal borides typically requires harsh conditions such as high temperature and high pressure, which is detrimental to reducing production costs. For example, CN202310669168.0 discloses a method for preparing tungsten-boron materials. First, ball-milled tungsten and boron powders are pre-sintered at 250℃–650℃, and then sintered at 1000℃–1200℃ for 2–5 hours to obtain the final product. This method involves high reaction temperatures and a pre-sintering process, making the preparation steps rather cumbersome. Therefore, developing iron boride catalysts with excellent morphology and structure, along with their mild preparation methods, is of great significance for the development of commercially viable high-performance lithium-sulfur batteries. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides an iron boride cathode catalyst with a unique linear structure, and applies it to lithium-sulfur batteries.

[0005] The iron boride cathode catalyst with a unique linear structure and its preparation method described in this invention are obtained based on a molten salt method using a dual-salt system, specifically including the following steps:

[0006] (1) First, weigh ferrous sulfate (FeSO4) and sodium borohydride (NaBH4) in a 1:10 molar ratio in a glove box filled with argon. After grinding, weigh potassium chloride (KCl) and sodium chloride (NaCl) at 10 times the total mass of the reactants. Grind the reactants and molten salt thoroughly and evenly.

[0007] (2) Place the powder obtained in step (1) in a crucible and calcine it at 1000°C for 2 hours in a tube furnace under an argon protective atmosphere. After cooling, a mixture of molten salt and iron boride is obtained.

[0008] (3) Soak the mixture obtained in step (2) in deionized water to dissolve the soluble salt and unreacted NaBH4 in the water. After centrifugation, wash it repeatedly with deionized water and ethanol.

[0009] (4) Finally, the washed product is vacuum dried to obtain a boron iron cathode catalyst with a unique linear structure.

[0010] Preferably, the mass ratio of KCl to NaCl is 1:1.

[0011] Preferably, in step (4), the product is placed in a vacuum drying oven at 60°C and dried for more than 12 hours.

[0012] The technical solution of the present invention has the following advantages compared with the prior art:

[0013] (1) The FeB metal boride material of the present invention effectively suppresses the self-propagation effect in the traditional solid-phase reaction by introducing NaCl / KCl as the dispersed phase, thus inhibiting the excessive growth of FeB particles. In addition, the NaCl / KCl molten salt system provides a liquid environment for the solid reactants at a medium temperature of 1000℃, which accelerates the diffusion of reactants and effectively reduces the synthesis temperature compared with the traditional solid-phase method.

[0014] (2) The synthesis method of the present invention is simpler, the experimental parameters are easier to control, the repeatability is better, and it is more conducive to reducing production costs and easy to scale up.

[0015] (3) Compared with traditional bulk and particulate structures, the FeB material of this invention has a micron-wire structure that can more fully expose active sites. At the same time, FeB has a high intrinsic conductivity, and the interwoven nanowire structure forms a conductive network, providing a rapid electron supply for electrochemical reactions. The synergistic effect of the two can significantly improve the adsorption and catalytic activity of the material, significantly accelerate the sulfur reaction kinetics, and effectively improve the actual capacity and cycle performance of lithium-sulfur batteries.

[0016] (4) In the preparation process described in this invention, the iron source is limited to FeSO4. The corresponding growth mechanism of FeB is the "template formation" mechanism, that is, when one of the two reactants has greater solubility than the other, the fast-dissolving reactant will move to the surface of the slow-dissolving reactant and further obtain the final product through local reaction nucleation growth. Compared with other iron sources, orthorhombic FeSO4 is more likely to induce the growth of the product in one dimension, while other iron sources such as ferric chloride cannot obtain linear products. Attached Figure Description

[0017] Figure 1 The XRD patterns of FeB prepared in Example 1 and Comparative Examples 1-2 of this invention are shown.

[0018] Figure 2 These are SEM images of FeB prepared according to the present invention at different magnifications, wherein (a) and (b) are samples of Example 1, (c) is comparative example 3, and (d) is comparative example 4.

[0019] Figure 3 The cyclic voltammetry (CV) curves of a lithium-sulfur battery using FeB prepared in this invention as the cathode catalyst are shown.

[0020] Figure 4 The cycling performance of a lithium-sulfur battery using FeB prepared in this invention as the cathode catalyst at a rate of 0.2C is shown. Detailed Implementation

[0021] To make the content of this invention easier to understand, the invention will be further described in detail with reference to specific embodiments and accompanying drawings.

[0022] This invention is illustrated by the following examples, but these examples are for illustrative purposes only and should not be construed as limiting the scope or application of the invention. Unless otherwise specified, all materials used in this invention are commercially available.

[0023] Example 1

[0024] In a glove box, FeSO4 and NaBH4 were weighed into a mortar at a molar ratio of 1:10 and briefly ground. Then, 10 times the total mass of the reactants, KCl and NaCl (mass ratio 1:1) were weighed and thoroughly ground together. The powdered mixture was placed in a corundum crucible and heated to 1000°C in a tube furnace under an argon atmosphere at a heating rate of 10°C / min, and held at this temperature for 2 hours. After cooling, the resulting product was soaked in deionized water to fully dissolve the salt and unreacted NaBH4. Finally, the product was centrifuged with deionized water and ethanol, washed, and vacuum dried to obtain FeB powder.

[0025] Comparative Example 1

[0026] In a glove box, FeSO4 and NaBH4 were weighed into a mortar at a molar ratio of 1:5 and briefly ground. Then, KCl and NaCl (mass ratio 1:1) were weighed out at a mass ratio of 1:1, ten times the total mass of the reactants, and the reactants and salts were thoroughly ground. The powder mixture was placed in a corundum crucible and heated to 1000°C in a tube furnace under an argon atmosphere at a heating rate of 10°C / min, and held at that temperature for 2 hours. After cooling, the resulting product was soaked in deionized water to fully dissolve the salts and unreacted NaBH4. Finally, the product was centrifuged, washed, and vacuum dried with deionized water and ethanol to obtain FeB powder. Due to the reduced NaBH4 content, the reduction reaction was insufficient, and pure FeB phase could not be synthesized. Figure 1 As shown.

[0027] Comparative Example 2

[0028] In a glove box, FeSO4 and NaBH4 were weighed into a mortar at a molar ratio of 1:10 and briefly ground. Then, 20 times the total mass of the reactants, KCl and NaCl (mass ratio 1:1) were weighed and thoroughly ground together. The powder mixture was placed in a corundum crucible and heated to 1000°C in a tube furnace under an argon atmosphere at a heating rate of 10°C / min, and held at that temperature for 2 hours. After cooling, the resulting product was soaked in deionized water to fully dissolve the salt and unreacted NaBH4. Finally, the product was centrifuged, washed, and vacuum dried with deionized water and ethanol to obtain FeB powder. Due to the excessively high salt dosage, the reaction was inhibited too strongly, resulting in insufficient reaction of FeSO4, low yield, and poor crystallinity of the product. Figure 1 As shown.

[0029] Comparative Example 3

[0030] In a glove box, FeSO4 and NaBH4 were weighed into a mortar at a molar ratio of 1:10 and briefly ground. Then, KCl and NaCl (mass ratio 1:1), five times the total mass of the reactants, were weighed and thoroughly ground together. The powder mixture was placed in a corundum crucible and heated to 1000°C in a tube furnace under an argon atmosphere at a heating rate of 10°C / min, and held at that temperature for 2 hours. After cooling, the resulting product was soaked in deionized water to fully dissolve the salt and unreacted NaBH4. Finally, the product was centrifuged, washed, and vacuum dried with deionized water and ethanol to obtain FeB powder. Due to insufficient salt content, the reactants were unevenly dispersed, making it difficult to effectively suppress the reaction rate. The product morphology consisted mostly of micron-sized particles (approximately 50 μm in diameter), and a linear FeB structure could not be obtained. Figure 2 As shown.

[0031] Comparative Example 4

[0032] In a glove box, FeSO4 and NaBH4 were weighed into a mortar at a molar ratio of 1:10 and briefly ground. Then, 10 times the total mass of the reactants, KCl and NaCl (mass ratio 1:1) were weighed and thoroughly ground together. The powder mixture was placed in a corundum crucible and heated to 800°C in a tube furnace under an argon atmosphere at a heating rate of 10°C / min, and held at this temperature for 2 hours. After cooling, the resulting product was soaked in deionized water to fully dissolve the salt and unreacted NaBH4. Finally, the product was centrifuged, washed, and vacuum dried with deionized water and ethanol to obtain FeB powder. Due to the lower reaction temperature, some FeSO4 failed to react completely and could not grow uniformly into a linear structure, such as... Figure 2 As shown, the crystallinity is also weaker than that of the sample in Example 1.

[0033] Figure 1 The XRD patterns of FeB prepared in Example 1, Comparative Examples 1 and 2 are shown. Figure 1 In the analysis, all diffraction peaks corresponded well to the FeB standard peaks, with no extraneous peaks, indicating that the sample prepared in Example 1 was a pure-phase orthorhombic FeB. However, Comparative Example 1, due to its low NaBH4 content, could not obtain the FeB phase, while Comparative Example 2, due to its excessively high salt content and strong inhibitory effect, resulted in incomplete reaction and poorer crystallinity compared to Example 1.

[0034] Figure 2 SEM images of FeB prepared in Example 1, Comparative Examples 3 and 4 at different magnifications. Figure 2 In the example, (a, b) represents FeB obtained in Example 1, which has a distinct linear structure with a diameter of about 1 micrometer. It is uniform in thickness, has a smooth surface, and has no obvious impurities. Figure 2(c) in the figure is the sample prepared in Comparative Example 3. Due to its low salt content, it is not enough to fully suppress the reaction rate, resulting in a microstructure of large-sized particles. Figure 2 (d) in the figure represents the sample prepared in Comparative Example 4. Due to its low reaction temperature, the reaction was incomplete, and it could not grow into a linear structure, requiring further growth.

[0035] Figure 3 The cyclic voltammetry (CV) curves for a lithium-sulfur battery using FeB prepared in Example 1 as the cathode catalyst are shown. Figure 3 It can be observed that, compared with the blank sample, the lithium-sulfur battery using FeB catalyst has a stronger response current and a faster peak trend, and the polarization potential difference between the oxidation peak and the reduction peak is also significantly reduced, indicating that the sulfur cathode has better redox kinetics and the obtained FeB has good catalytic activity for sulfur reaction.

[0036] Figure 4 The cycling performance of a lithium-sulfur battery using FeB prepared in Example 1 as the cathode catalyst at a rate of 0.2C is shown. Figure 4 In this study, FeB-based lithium-sulfur batteries exhibited higher initial discharge capacity and slower capacity decay rate, indicating that FeB can effectively improve the utilization rate of active materials, suppress the shuttle effect, and enhance the reversibility of battery reactions, showing great potential for improving the performance of lithium-sulfur batteries.

Claims

1. A method for producing a linear iron boride cathode catalyst, characterized by, Comprising the following steps: (1) First, in an argon-filled glove box, ferrous sulfate and sodium borohydride were weighed according to a molar ratio of 1:10, and then potassium chloride and sodium chloride were weighed in an amount of 10 times the total mass of the reactants, and the reactants were thoroughly ground with the molten salt; (2) The powder obtained in step (1) was placed in a crucible and calcined at 1000℃ for 2h in a tube furnace under an argon protective atmosphere, and after cooling, a mixture of molten salt and iron boride was obtained; (3) The mixture obtained in step (2) was soaked in deionized water, and the soluble salt and unreacted sodium borohydride were dissolved in water, and after centrifugation, the product was repeatedly washed with deionized water and ethanol for several times; (4) Finally, the washed product was vacuum dried to obtain an iron boride cathode catalyst with a unique linear structure.

2. The method of claim 1, wherein, The mass ratio of KCl and NaCl is 1:

1.

3. The method of claim 1, wherein, In step (4), the drying was carried out in a vacuum drying oven at 60℃ for more than 12 hours.

4. The linear iron boride cathode catalyst prepared by the method of any one of claims 1-3.

5. The application of the linear iron boride cathode catalyst prepared by the method of any one of claims 1-3 as a lithium-sulfur battery cathode material.

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