A cathode material for lithium-sulfur batteries based on a tungsten-carbon substrate dual-metal single-atom catalyst and a preparation method thereof

Abstract

The present invention relates to the technical field of lithium-sulfur batteries, and particularly relates to a cathode material for a lithium-sulfur battery based on a tungsten-carbon substrate dual-metal single-atom catalyst and a preparation method thereof. The preparation method is to first use a porous tungsten-carbon material to stabilize metal atoms Fe and Ni to obtain a dual-metal single-atom catalyst WCx-FeNi; then mix sulfur (S) and WCx-FeNi and perform heat treatment to form an S / WCx-FeNi composite material; finally, mix it with a conductive agent and a binder to obtain an S / WCx-FeNi cathode material. The present invention uses the tungsten-carbon substrate dual-metal single-atom catalyst WCx-FeNi material as the carrier of sulfur at the cathode. This material can effectively adsorb and fix polysulfide and has a strong accelerating effect on the redox conversion kinetics of sulfur, thereby effectively suppressing the "shuttle effect" of lithium-sulfur batteries. The lithium-sulfur battery based on the S / WCx-FeNi cathode material exhibits excellent cycling performance.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-sulfur batteries, and particularly relates to a cathode material for a lithium-sulfur battery based on a tungsten-carbon substrate bimetallic single-atom catalyst and a preparation method thereof. Background Art

[0002] With the increasing development of electric vehicles and portable electronic products, traditional lithium-ion batteries have been difficult to meet the requirements due to their limited specific capacity. Lithium-sulfur batteries have a theoretical specific capacity of 1675 mAh / g and are considered to be one of the most promising next-generation energy storage batteries.

[0003] At present, there are three technical difficulties in lithium-sulfur batteries that need to be overcome: (1) The insulating property of sulfur leads to low utilization rate of active substances; (2) The volume change during the lithiation and delithiation processes of the sulfur cathode reduces the mechanical integrity of the electrode; (3) The "shuttle effect" brings a series of negative problems. During the discharge process of the battery, the reaction intermediate long-chain polysulfide lithium (Li2S x , 4 ≤ x ≤ 8) dissolved in the electrolyte will cause the loss of active substance sulfur during the discharge / charge cycle; polysulfide lithium will move to the negative electrode and deposit on the lithium surface, react with lithium metal, and form insoluble and insulating lithium sulfide (Li2S and Li2S2) on the lithium surface, resulting in poor contact between lithium and the electrolyte and low Coulomb efficiency; the dissolution of polysulfide will cause the redistribution of sulfur in the electrode material, resulting in poor cycle performance of the battery.

[0004] The cathode of a lithium-sulfur battery usually uses carbon materials such as porous carbon, carbon nanotubes (CNT) / fibers, and graphene as a conductive matrix to encapsulate sulfur. While improving the conductivity of the sulfur cathode, it can also buffer the volume change during the lithiation and delithiation processes, and to a certain extent, inhibit the dissolution and reaction problems of polysulfide using the pore structure. However, pure carbon materials are non-polar, and the intermolecular force between them and polar polysulfide lithium is very weak. Therefore, the effect of inhibiting the "shuttle effect" of polysulfide is not ideal. Modifying carbon materials, such as nitrogen and sulfur doping, can further effectively alleviate the "shuttle effect" of lithium-sulfur batteries. However, the doping concentration of heteroatoms is limited. Introducing heteroatoms at a high concentration requires high requirements for the design and preparation of materials and will reduce the conductivity of the materials. In addition, forming strong chemical bonds between metal compounds and polysulfide lithium can also effectively limit the migration of polysulfide. However, due to the poor conductivity and low specific surface area of metal oxides, the loading of active materials is limited, resulting in poor rate performance and specific capacity as a sulfur cathode material. Summary of the Invention

[0005] To overcome the deficiencies of the above-mentioned prior art, the object of the present invention is to provide a cathode material for a lithium-sulfur battery based on a tungsten-carbon substrate dual-metal single-atom catalyst. This material uses a tungsten-carbon substrate Fe-Ni dual-metal single-atom catalyst (WCx-FeNi) as the carrier material for sulfur at the cathode. The WCx-FeNi material has many advantages in suppressing the "shuttle effect": (1) Metal atoms have a strong affinity for polysulfides and a strong catalytic acceleration effect on the redox conversion kinetics of sulfur; (2) The nanostructure and polar surface of the tungsten-carbon carrier can effectively adsorb and fix polysulfides and achieve high sulfur regulation and rapid ion migration; (3) The high conductivity of the tungsten-carbon material is conducive to electron transport. The lithium-sulfur battery based on this cathode material exhibits excellent cycling performance.

[0006] The present invention provides a cathode material for a lithium-sulfur battery based on a tungsten-carbon substrate dual-metal single-atom catalyst. The cathode material includes WCx-FeNi, S, a conductive agent, and a binder. The WCx-FeNi is made by loading Fe and Ni dual metals on a tungsten-carbon substrate.

[0007] The present invention also provides a preparation method for a cathode material for a lithium-sulfur battery based on a tungsten-carbon substrate dual-metal single-atom catalyst, which is characterized by including the following steps:

[0008] S1. Preparation of WCx-FeNi: Add iron salt and nickel salt to a dopamine solution with a pH of 1 to 3, then add a tungstate solution to the resulting mixed solution to form a DA-W-FeNi precursor solid product, and finally perform high-temperature carbonization on the solid product in an inert atmosphere to obtain WCx-FeNi;

[0009] S2. Preparation of the sulfur cathode: First, heat-treat S powder and WCx-FeNi powder in an inert atmosphere to form an S / WCx-FeNi composite material, then use NMP as a solvent to make an electrode slurry from the S / WCx-FeNi composite material, a conductive agent, and a binder, and finally coat the electrode slurry on an aluminum foil substrate to obtain a cathode material for a lithium-sulfur battery based on a tungsten-carbon substrate dual-metal single-atom catalyst.

[0010] Preferably, in step 1, the concentration of the dopamine solution is 0.1 mol / L, and the solvent of the dopamine solution is water.

[0011] More preferably, the method for preparing the dopamine solution with a pH of 1 to 3 is to dissolve dopamine hydrochloride in water and then adjust the pH to 1 to 3 with hydrochloric acid.

[0012] Preferably, in step 1, the iron salt is ferric chloride hexahydrate, the nickel salt is nickel chloride hexahydrate, and the molar ratio of the iron salt, nickel salt, and dopamine is (1 to 2):(1 to 2):20.

[0013] Preferably, in step 1, the volume ratio of the mixed solution to the tungstate solution is 1:(1 - 1.2), and the concentration of the tungstate solution is 0.1 - 0.15 mol / L.

[0014] Preferably, in step 1, the sodium tungstate solution is added in a slow dropwise manner.

[0015] More preferably, when adding the sodium tungstate solution, the color of the mixed solution changes from dark brown to bright yellow, and then a green-yellow precipitate is formed.

[0016] Preferably, in step 1, the carbonization conditions are to heat up to 900 - 1000 °C at a heating rate of 2 - 3 °C / min in an argon atmosphere and carry out the carbonization reaction for 2 - 3 h.

[0017] Preferably, in step 2, the S powder and the WCx-FeNi powder are mixed at a mass ratio of (7 - 8):(2 - 3), and the heat treatment conditions are to heat up to 155 - 170 °C at a heating rate of 2 - 3 °C / min in an argon atmosphere and heat treat for 12 - 13 h.

[0018] Preferably, in step 2, the S / WCx-FeNi composite material, the conductive agent, and the binder are mixed at a mass ratio of (6 - 7):(1 - 2):(1 - 2), the conductive agent is graphene, and the binder is polyvinylidene fluoride (PVDF).

[0019] More preferably, the mass ratio of the S / WCx-FeNi composite material, the conductive agent, and PVDF is 7:2:1.

[0020] Preferably, in step 2, the coating is to coat the electrode slurry onto the aluminum foil substrate at a surface loading of 0.8 - 1.2 mg / cm 2 ².

[0021] Compared with the prior art, the beneficial effects of the present invention are:

[0022] The present invention discloses a lithium-sulfur battery cathode material based on a tungsten-carbon substrate dual-metal single-atom catalyst. This cathode material uses a tungsten-carbon substrate dual-metal single-atom catalyst (WCx-FeNi) as the carrier material for the cathode sulfur. The WCx-FeNi material has many advantages in inhibiting the "shuttle effect": (1) The metal atoms have a strong affinity for polysulfide lithium and have a strong catalytic acceleration effect on the redox conversion kinetics of sulfur; (2) The nanostructure and polar surface of the tungsten-carbon carrier can effectively adsorb and fix polysulfide lithium and obtain high sulfur regulation and fast ion migration; (3) The high conductivity of the tungsten-carbon material is beneficial to electron transport. The lithium-sulfur battery based on this cathode material (S / WCx-FeNi) exhibits excellent cycle performance. Brief Description of the Drawings

[0023] Figure 1 Schematic diagram of the preparation process of WCx-FeNi;

[0024] Figure 2 X-ray diffraction pattern of WCx-FeNi;

[0025] Figure 3 N2 adsorption-desorption isotherm diagram of WCx-FeNi;

[0026] Figure 4 Pore size distribution diagram of WCx-FeNi;

[0027] Figure 5 Cycling performance diagrams of S / WCx-FeNi battery, S / WCx battery, and S / CNT battery at a current density of 1C;

[0028] Figure 6 Cycling performance diagrams of S / WCx-FeNi battery, S / WCx battery, and S / CNT battery at a current density of 0.5C;

[0029] Figure 7 Charge-discharge curve diagram of S / WCx-FeNi battery. Detailed implementation manners

[0030] The following further describes the detailed implementation manners of the present invention. It should be noted here that the description of these implementation manners is for helping to understand the present invention, but does not constitute a limitation to the present invention. In addition, the technical features involved in the following various implementation manners of the present invention can be combined with each other as long as they do not conflict with each other.

[0031] The experimental methods in the following examples are all conventional methods unless otherwise specified, and the test materials used in the following examples are all commercially available through conventional channels unless otherwise specified.

[0032] Example 1 Preparation of S / WCx-FeNi cathode

[0033] 1. The preparation process of WCx-FeNi is as Figure 1 shown, and the specific steps are as follows:

[0034] (1) Dissolve dopamine (DA; 0.948 g, 5 mmol) in 50 mL of water and stir for 0.5 h to prepare a 0.1 mol / L dopamine aqueous solution. Then add 2 mL of hydrochloric acid (1 mol / L) to the dopamine aqueous solution to make the pH value of the dopamine aqueous solution 2, denoted as DA solution;

[0035] (2) adding ferric chloride hexahydrate (0.25 mmol) and nickel chloride hexahydrate (0.25 mmol) to the DA solution and stirring to obtain a mixed solution, which is recorded as DA-FeNi;

[0036] (3) 50 mL of sodium tungstate solution (5 mmol, Na2WO4·2H2O) was slowly dripped into the DA-FeNi solution (drip time was about 10 min). During the dripping of sodium tungstate, the color of the suspension changed from dark brown to bright yellow, and finally a green-yellow precipitate was formed. The reaction was further stirred for 1 h, and the solid product was collected by centrifugation. The product was washed with deionized water and anhydrous ethanol for 3 times respectively; the product was dried in an oven at 60 ° C overnight to obtain a DA-W-FeNi precursor;

[0037] (4) Under an argon atmosphere, the DA-W-FeNi precursor was carbonized at a heating rate of 2°C / min to 900°C for 2 h, and finally a tungsten-carbon-based bimetallic single-atom catalyst was obtained in the form of a black powder, denoted as WCx-FeNi.

[0038] 2. Preparation of S / WCx-FeNi positive electrode:

[0039] 0.8 g S and 0.2 g WCx-FeNi were mixed, and then heated to 155 °C at a rate of 2 °C / min in an argon atmosphere for 12 hours to allow sulfur to fully enter the WCx-FeNi pores to obtain a S / WCx-FeNi composite material; 0.7 g S / WCx-FeNi material, 0.2 g graphene and 0.1 g PVDF were fully mixed, and then an appropriate amount of NMP was added as a solvent to obtain an adhesive slurry, which was coated on a clean aluminum foil (the surface loading of active substance S was 1.0 mg / cm 2 ); the aluminum foil coated with the slurry was dried in an oven at 60°C for 12 hours and then taken out and cut into discs with a diameter of 14 mm using a tablet press as the positive electrode of the button battery for later use.

[0040] Comparative Example 1 Preparation of S / WCx positive electrode

[0041] The difference between this comparative example and Example 1 is that during the preparation of the carrier, ferric chloride hexahydrate and nickel chloride hexahydrate are not added to the DA solution, but the sodium tungstate solution is directly and slowly dripped into the DA solution to prepare the WCx material. Subsequently, the WCx material is used to prepare the S / WCx positive electrode by the same preparation method.

[0042] Comparative Example 2 Preparation of S / CNT positive electrode

[0043] The difference between this comparative example and Example 1 is that CNT (purity 95%, length 10 - 30 μm, diameter 5 - 15 nm) is directly used as the carrier material of S, and then the S / CNT material is prepared into the S / CNT positive electrode by the same positive electrode preparation method.

[0044] Material Structure and Battery Performance Characterization of Example 1

[0045] 1. X-ray Diffraction Spectrum of WCx-FeNi

[0046] X-ray diffraction analysis was performed on WCx-FeNi prepared in Example 1 using an X-ray diffractometer (JCPDS no.01-070-1849), and the analysis spectrum is as Figure 2 shown. This material has good crystallinity. WC crystallization peaks appear at 31° and 35°, W2C crystallization peaks appear at 34°, 37°, 39° and 52°, and WC 1-x crystallization peaks appear in the range of 36° and 41 - 45°, indicating that this material is a WCx crystal material. After loading metals Fe and Ni on WCx, no crystallization peaks of metals Fe and Ni were characterized, indicating that there are no Fe, Ni, and FeNi alloy particles in this material, proving that Fe and Ni are highly dispersed on the WCx carrier.

[0047] 2. Nitrogen Adsorption-Desorption Isotherm of WCx-FeNi

[0048] Nitrogen adsorption-desorption performance testing was performed on WCx-FeNi prepared in Example 1 using BELSORP-mini, and the analysis results are as Figure 3 shown. The nitrogen adsorption-desorption isotherm of the sample indicates that WCx-FeNi has a hierarchical micro-mesoporous structure. Based on the nitrogen adsorption-desorption experimental results, the specific surface area of this material is calculated to be 225 m 2 / g.

[0049] 3. Pore Size Distribution Curve of WCx-FeNi

[0050] The pore size distribution curve of WCx-FeNi is as Figure 4 shown. It indicates that this material has a hierarchical micro-mesoporous structure.

[0051] 4. Element Distribution of WCx-FeNi

[0052] Inductively coupled plasma spectrometry (ICP) was used to analyze the element distribution in the WCx-FeNi material prepared in Example 1. The loadings of Fe and Ni in WCx-FeNi are 0.68 wt% and 0.58 wt% respectively.

[0053] 5. Battery Cycling Performance at 1C and 0.5C Current Densities

[0054] Using S / WCx-FeNi, S / WCx, and S / CNT as the positive electrodes respectively, a lithium chip (purity ≥ 99.5%) as the negative electrode, a battery case of model CR2032, and a PP separator of model Celgard2400, a coin-type lithium-sulfur battery was assembled with 10 - 20 μL of electrolyte (1M LiTFSI, DOL / DME (1:1, v / v), 1wt% LiNO3), denoted as S / WCx-FeNi battery, S / WCx battery, and S / CNT battery; and each battery was placed in a glove box (argon, (O2 < 0.1 ppm, H2O < 0.1 ppm)) for testing.

[0055] The cycling performance of each battery at a current density of 1C is as Figure 5 shown. The lithium-sulfur battery cathode material prepared based on WCx-FeNi endows the battery with excellent cycling stability. The battery has an initial discharge specific capacity of 820 mAh / g and still maintains a discharge specific capacity of 790 mAh / g after 200 cycles; the battery based on the S / WCx cathode in Comparative Example 1 has an initial discharge specific capacity of 820 mAh / g, but the discharge specific capacity only remains 490 mAh / g after cycling; the performance of the battery based on the S / CNT cathode in Comparative Example 2 is even worse. The above proves that the WCx-FeNi material can effectively inhibit the "shuttle effect" and improve the performance of lithium-sulfur batteries.

[0056] The cycling performance of each battery at a current density of 0.5C is as Figure 6 shown. The lithium-sulfur battery cathode material prepared based on WCx-FeNi endows the battery with excellent cycling stability. At 0.5C, the battery has a high initial discharge specific capacity of 1398 mAh / g and still maintains a discharge specific capacity of 610 mAh / g after 500 cycles; the battery based on the S / WCx cathode in Comparative Example 1 has an initial discharge specific capacity of 1050 mAh / g, but the discharge specific capacity only remains 300 mAh / g after cycling; the performance of the battery based on the S / CNT cathode in Comparative Example 2 is even worse. The above proves that the WCx-FeNi material can effectively inhibit the "shuttle effect" and improve the performance of lithium-sulfur batteries.

[0057] 6. Charge-discharge performance of the battery based on the S / WCx-FeNi cathode

[0058] The charge-discharge curve of the battery based on the S / WCx-FeNi cathode is as Figure 7 shown. The curve shows the typical voltage plateau of a Li-S battery, that is, the discharge curve presents two typical plateaus, corresponding to the two-step reaction from sulfur to Li2S2 / Li2S respectively, and the charge curve corresponds to its reverse reaction.

[0059] In summary, the WCx-FeNi material in the present invention has many advantages in suppressing the "shuttle effect" of lithium-sulfur batteries: (1) Metal atoms have a strong affinity for polysulfide lithium and a strong catalytic acceleration effect on the redox conversion kinetics of sulfur; (2) The nanostructure and polar surface of the tungsten-carbon carrier can effectively adsorb and fix polysulfide lithium, and achieve high sulfur regulation and rapid ion migration; (3) The high conductivity of the tungsten-carbon material is conducive to electron transport. Therefore, the present invention uses WCx-FeNi as the positive sulfur carrier to prepare the S / WCx-FeNi positive electrode material. The lithium-sulfur battery based on this positive electrode material exhibits excellent cycling performance.

[0060] The above has described the embodiments of the present invention in detail, but the present invention is not limited to the described embodiments. For those skilled in the art, without departing from the principle and spirit of the present invention, various changes, modifications, substitutions, and variations made to these embodiments still fall within the protection scope of the present invention.

Claims

1. A preparation method of a cathode material for a lithium-sulfur battery based on a tungsten-carbon substrate dual-metal single-atom catalyst, characterized in that, The positive electrode material includes WCx-FeNi, S, a conductive agent, and a binder. The WCx-FeNi is made by loading a tungsten-carbon substrate with Fe and Ni bimetals; The preparation method of the lithium-sulfur battery positive electrode material based on the tungsten-carbon substrate bimetallic single-atom catalyst includes the following steps: S1. Preparation of WCx-FeNi: Add iron salt and nickel salt to a dopamine solution with a pH of 1-3, then add a tungstate solution to the resulting mixed solution to form a DA-W-FeNi precursor solid product. Finally, perform high-temperature carbonization on the solid product in an inert atmosphere to obtain WCx-FeNi; S2. Preparation of the sulfur positive electrode: First, heat-treat S powder and WCx-FeNi powder in an inert atmosphere to form an S / WCx-FeNi composite material. Then, use N-methylpyrrolidone as a solvent to make an electrode slurry from the S / WCx-FeNi composite material, a conductive agent, and a binder. Finally, coat the electrode slurry on an aluminum foil to obtain the lithium-sulfur battery positive electrode material based on the tungsten-carbon substrate bimetallic single-atom catalyst; In step 1, the addition method of the tungstate solution is slow dropping.

2. The preparation method of the cathode material for the lithium-sulfur battery based on the tungsten-carbon substrate dual-metal single-atom catalyst according to claim 1, characterized in that, In step 1, the concentration of the dopamine solution is 0.1-0.15 mol / L, and the solvent of the dopamine solution is water.

3. The preparation method of the cathode material for the lithium-sulfur battery based on the tungsten-carbon substrate dual-metal single-atom catalyst according to claim 1, characterized in that, In step 1, the iron salt is ferric chloride hexahydrate, the nickel salt is nickel chloride hexahydrate, and the molar ratio of the iron salt, nickel salt, and dopamine is (1-2):(1-2):

20.

4. The preparation method of the cathode material for the lithium-sulfur battery based on the tungsten-carbon substrate dual-metal single-atom catalyst according to claim 1, characterized in that, In step 1, the volume ratio of the mixed solution to the tungstate solution is 1:(1-1.2), and the concentration of the tungstate solution is 0.1-0.15 mol / L.

5. The preparation method of the cathode material for the lithium-sulfur battery based on the tungsten-carbon substrate dual-metal single-atom catalyst according to claim 1, characterized in that, In step 1, the high-temperature carbonization conditions are to heat up to 900-1000 °C at a heating rate of 2-3 °C / min in an argon atmosphere and carry out the carbonization reaction for 2-3 h.

6. The preparation method of the cathode material for the lithium-sulfur battery based on the tungsten-carbon substrate dual-metal single-atom catalyst according to claim 1, characterized in that, In step 2, the S powder and WCx-FeNi powder are mixed in a mass ratio of (7-8):(2-3), and the heat-treatment conditions are to heat up to 155-170 °C at a heating rate of 2-3 °C / min in an argon atmosphere and heat-treat for 12-13 h.

7. The preparation method of the cathode material for the lithium-sulfur battery based on the tungsten-carbon substrate dual-metal single-atom catalyst according to claim 1, characterized in that, In step 2, the S / WCx-FeNi composite material, the conductive agent, and the binder are mixed in a mass ratio of (6-7):(1-2):(1-2). The conductive agent is graphene, and the binder is polyvinylidene fluoride (PVDF).

8. The preparation method of the cathode material for the lithium-sulfur battery based on the tungsten-carbon substrate dual-metal single-atom catalyst according to claim 1, characterized in that, In Step 2, the coating is to coat the electrode paste onto the aluminum foil substrate at a surface loading of 0.8 to 1.2 mg / cm 2 .

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