Linear iron boride cathode catalyst for lithium-sulfur battery

Abstract

The invention 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, and has the characteristics of high length-diameter ratio and uniform size (the diameter is about 1 micron); according to the present invention, the excellent intrinsic conductivity provides rapid electron supply for the electrochemical reaction of sulfur, and the local limited chemical polarity between iron and boron provides rich adsorption and catalysis sites for the intermediate product lithium polysulfide, such that the shuttle effect is effectively inhibited, and the sulfur-electricity conversion reaction kinetics is improved so as to obtain the good lithium-sulfur battery performance;

Description

Technical Field

[0001] The invention relates to a lithium-sulfur battery functional material, in particular to a metal boride electrocatalyst for a lithium-sulfur battery cathode. Background Art

[0002] Among 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 sulfur and lithium sulfide reduces the sulfur reaction efficiency and the overall energy efficiency of the battery; (2) The volume change of active materials during the cycle is large, which can reach more than 80%, seriously damaging the electrode structure and affecting the stability of the battery; (3) The shuttle effect, that is, the intermediate product of the charge and discharge process, lithium polysulfide, can be dissolved in the electrolyte, shuttle back and forth between the positive and negative electrodes and produce side reactions at the two electrodes, causing the loss of active materials, 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 dead sulfur is easily formed, resulting in the loss of active materials and the decrease of battery capacity.

[0003] In view of these problems, the introduction of efficient electrocatalysts into the sulfur cathode is a key means to accelerate the polysulfide conversion reaction, inhibit the shuttle effect, and improve battery performance. Metal compounds have always been a hot topic in the research of cathode catalytic materials for lithium-sulfur batteries due to their intrinsic chemical polarity and good morphological and structural plasticity. Among them, metal borides are usually composed of boron and metal elements. Their filling structure inherits some metallic properties and exhibits conductivity much higher than other metal compounds. At the same time, there is an ionic polar structure between metal and boron, which can provide good adsorption sites for lithium polysulfide. However, the research on metal borides in lithium-sulfur batteries is still relatively limited. Compared with other metals, iron is abundant in the earth's crust and is also environmentally friendly. At the same time, the introduction of iron is expected to increase the d-band center of the catalyst and strengthen the interaction with lithium polysulfide, thereby obtaining a better adsorption effect than other borides. However, most common iron borides are block structures, which makes it difficult to fully expose their active sites; in addition, the preparation of metal borides usually requires harsh conditions such as high temperature and high pressure, which is not conducive to reducing production costs. For example, CN202310669168.0 discloses a method for preparing tungsten-boron materials, first pre-sintering the ball-milled mixed tungsten powder and boron powder at a temperature of 250°C to 650°C, and then sintering it at 1000°C to 1200°C for 2 to 5 hours to obtain the final product. This method has a high reaction temperature and includes a pre-sintering process, and the preparation steps are relatively cumbersome. Therefore, the development of an iron boride catalyst with excellent morphology and structure and a mild preparation method thereof are of great significance to the development of commercially viable high-performance lithium-sulfur batteries. Summary of the invention

[0004] In view of the above problems, the present invention provides an iron boride cathode catalyst with a unique linear structure, and uses it in lithium-sulfur batteries.

[0005] The iron boride cathode catalyst with a unique linear structure and the preparation method thereof are obtained based on a molten salt method of a double salt system, and specifically include the following steps:

[0006] (1) First, in a glove box filled with argon, weigh ferrous sulfate (FeSO 4 ) and sodium borohydride (NaBH 4 ), after grinding, weigh 10 times the total mass of potassium chloride (KCl) and sodium chloride (NaCl) of the reactants, and grind the reactants and the molten salt thoroughly and evenly;

[0007] (2) placing the powder obtained in step (1) in a crucible, calcining it at 1000° C. for 2 h in a tube furnace under an argon protective atmosphere, and obtaining a mixture of molten salt and iron boride after cooling;

[0008] (3) Soak the mixture obtained in step (2) in deionized water to separate the soluble salt and unreacted NaBH 4 Dissolve in water, centrifuge, and then wash repeatedly with deionized water and ethanol;

[0009] (4) Finally, the washed product is vacuum dried to obtain an iron boride 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 mixture is dried in a vacuum drying oven at 60° C. for more than 12 hours.

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

[0013] (1) The metal boride FeB material of the present invention effectively suppresses the self-propagating effect in the traditional solid phase reaction and the excessive growth of FeB particles by introducing NaCl / KCl as the dispersed phase. In addition, the NaCl / KCl molten salt system provides a liquid environment for the solid reactants at a medium temperature of 1000°C, which accelerates the diffusion of the 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 easy to control, the repeatability is good, it is more conducive to reducing production costs, and it is easy to scale up.

[0015] (3) Compared with the traditional bulk and particle structures, the FeB material of the present 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 nanowire structure interweaves to form a conductive network, providing a fast electron supply for electrochemical reactions. The synergistic effect of the two can greatly 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 of the present invention, the iron source is limited to FeSO 4 , the corresponding growth mechanism of FeB is the "template formation" mechanism, that is, when one of the two reactants is more soluble than the other, the fast-dissolving reactant will move to the surface of the slow-dissolving reactant, and further obtain the final product through localized reaction nucleation and growth. Compared with other iron sources, the orthorhombic FeSO 4 It is easier to induce the growth of products in one-dimensional direction, while other iron sources such as ferric chloride cannot obtain linear products. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0018] Figure 2 These are SEM images of FeB prepared in the present invention at different magnifications, where (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) curve of a lithium-sulfur battery using the FeB prepared in the present invention as a cathode catalyst.

[0020] Figure 4 The present invention shows the cycle performance of a lithium-sulfur battery using the FeB prepared in the present invention as a cathode catalyst at a rate of 0.2C. DETAILED DESCRIPTION

[0021] In order to make the contents of the present invention easier to understand, the present invention is further described in detail in conjunction with specific implementation examples and drawings.

[0022] The present invention is illustrated by the following examples, which are only used for illustration and cannot be regarded as limiting the scope of the invention or the application method of the invention. Unless otherwise specified, the raw materials of the present invention are commercially available.

[0023] Example 1

[0024] In a glove box, FeSO 4 and NaBH 4Weigh the mixture in a mortar at a molar ratio of 1:10. After a short grinding, weigh 10 times the total mass of KCl and NaCl (mass ratio of 1:1) and grind the reactants and salt thoroughly. Place the mixed powder in a corundum crucible and heat it to 1000℃ in a tube furnace with an argon atmosphere at a heating rate of 10℃ / min and keep it warm for 2 hours. After cooling, soak the obtained product in deionized water to allow the salt and unreacted NaBH 4 Finally, the product was centrifuged, washed and vacuum dried with deionized water and ethanol to obtain FeB powder.

[0025] Comparative Example 1

[0026] In a glove box, FeSO 4 and NaBH 4 Weigh the mixture in a mortar at a molar ratio of 1:5. After a short grinding, weigh 10 times the total mass of KCl and NaCl (mass ratio of 1:1) and grind the reactants and salt thoroughly. Place the mixed powder in a corundum crucible and heat it to 1000℃ in an argon atmosphere tube furnace at a heating rate of 10℃ / min and keep it warm for 2 hours. After cooling, soak the obtained product in deionized water to allow the salt and unreacted NaBH 4 Finally, the product was centrifuged, washed and vacuum dried with deionized water and ethanol to obtain FeB powder. 4 The content of is reduced, the reduction reaction is not sufficient, and the pure FeB phase cannot be synthesized. Figure 1 shown.

[0027] Comparative Example 2

[0028] In a glove box, FeSO 4 and NaBH 4 Weigh the mixture in a mortar at a molar ratio of 1:10. After a short grinding, weigh 20 times the total mass of KCl and NaCl (mass ratio of 1:1) and grind the reactants and salt thoroughly. Place the mixed powder in a corundum crucible and heat it to 1000℃ in a tube furnace with an argon atmosphere at a heating rate of 10℃ / min. Keep the mixture warm for 2 hours. After cooling, soak the obtained product in deionized water to allow the salt and unreacted NaBH 4 Finally, the product was centrifuged, washed and vacuum dried with deionized water and ethanol to obtain FeB powder. Due to the high salt dosage, the inhibitory effect on the reaction was too strong, resulting in FeSO 4 Not fully reacted, the yield is low and the crystallinity of the product is poor, such as Figure 1 shown.

[0029] Comparative Example 3

[0030] In a glove box, FeSO 4 and NaBH 4 Weigh the mixture in a mortar at a molar ratio of 1:10. After a short grinding, weigh 5 times the total mass of KCl and NaCl (mass ratio of 1:1) and grind the reactants and salt thoroughly. Place the mixed powder in a corundum crucible and heat it to 1000℃ in an argon atmosphere tube furnace at a heating rate of 10℃ / min and keep it warm for 2 hours. After cooling, soak the obtained product in deionized water to allow the salt and unreacted NaBH 4 Finally, the product was centrifuged, washed and vacuum dried with deionized water and ethanol to obtain FeB powder. Due to the small amount of salt, the reactants were unevenly dispersed and it was difficult to effectively inhibit the reaction rate. The product morphology was mostly micron particles (particle size of about 50μm), and it was impossible to obtain FeB with linear structure, such as Figure 2 shown.

[0031] Comparative Example 4

[0032] In a glove box, FeSO 4 and NaBH 4 Weigh the mixture in a mortar at a molar ratio of 1:10. After a short grinding, weigh 10 times the total mass of KCl and NaCl (mass ratio of 1:1) and grind the reactants and salt thoroughly. Place the mixed powder in a corundum crucible and heat it to 800°C in an argon atmosphere tube furnace at a heating rate of 10°C / min and keep it warm for 2 hours. After cooling, soak the obtained product in deionized water to allow the salt and unreacted NaBH 4 Finally, the product was centrifuged, washed and vacuum dried with deionized water and ethanol to obtain FeB powder. As the reaction temperature decreases, part of FeSO 4 Failed to fully react 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 XRD patterns of FeB prepared in Example 1 and Comparative Examples 1 and 2. Figure 1 In the figure, all diffraction peaks correspond well to the standard peaks of FeB, and there are no extra peaks, indicating that the sample prepared in Example 1 is pure phase orthorhombic FeB. 4 The amount is small and the FeB phase cannot be obtained. In Comparative Example 2, the salt amount is too high and the inhibition effect is too strong, resulting in insufficient reaction and its crystallinity is poorer than that of Example 1.

[0034] Figure 2 SEM images of FeB prepared in Example 1 and Comparative Examples 3 and 4 at different magnifications. Figure 2 (a, b) in the figure are FeB obtained in Example 1, which has a clear linear structure, a diameter of about 1 micron, uniform thickness, a smooth surface, and no obvious impurities. Figure 2 (c) is the sample prepared in Comparative Example 3. Since the amount of salt is small, the reaction rate cannot be fully inhibited, resulting in the microscopic morphology of particles with larger particle size. Figure 2 (d) is the sample prepared in Comparative Example 4. Due to its low reaction temperature, its reaction is incomplete and it cannot grow into a linear structure and needs to be further grown.

[0035] Figure 3 The cyclic voltammetry (CV) curve of a lithium-sulfur battery using FeB prepared in Example 1 as a cathode catalyst. Figure 3 It can be found that the lithium-sulfur battery using FeB catalyst has a stronger response current and a faster peak onset trend compared with the blank sample, 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 reactions.

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

Claims

1. A method for preparing a linear iron boride cathode catalyst, It is characterized in that The following steps are involved: (1) First, in a glove box filled with argon, weigh ferrous sulfate and sodium borohydride in a molar ratio of 1:10, grind them, weigh 10 times the total mass of potassium chloride and sodium chloride, and grind the reactants and molten salt thoroughly and evenly; (2) placing the powder obtained in step (1) in a crucible, calcining it at 1000° C. for 2 h in a tube furnace under an argon protective atmosphere, and obtaining a mixture of molten salt and iron boride after cooling; (3) soaking the mixture obtained in step (2) in deionized water to dissolve the soluble salt and unreacted sodium borohydride in water, centrifuging and then repeatedly washing with deionized water and ethanol for multiple times; (4) Finally, the washed product is vacuum dried to obtain an iron boride cathode catalyst with a unique linear structure.

2. The method according to claim 1, It is characterized in that The mass ratio of KCl and NaCl is 1:

1.

3. The method according to claim 1, It is characterized in that In step (4), the mixture is placed in a vacuum drying oven at 60° C. and dried for more than 12 hours.

4. A linear iron boride cathode catalyst prepared by the method according to any one of claims 1 to 3.

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

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