A sulfur host material for lithium-sulfur batteries, its preparation method and application

Abstract

The present invention discloses a sulfur host material for lithium-sulfur batteries, its preparation method and application, belonging to the technical field of preparation of electrode materials. The preparation method includes the following steps: 1) Dissolve tetrabutylammonium tetrafluoroborate in dimethyl sulfoxide, and after electrochemical exfoliation, obtain thin-layer black phosphorus; 2) Add urea to the CNTs solution, stir evenly, remove the solvent, calcine in an Ar gas atmosphere, and after cooling, grind to obtain NC; 3) Mix the thin-layer black phosphorus and NC evenly and perform heat treatment in an Ar gas atmosphere to obtain FBP-NC. The present invention directly prepares a sulfur host material for lithium-sulfur batteries by using a low-temperature calcination method, which is convenient, simple and easy to operate, and has excellent electrochemical performance; compared with the simply physically mixed FBP-NC, it has a stronger anchoring ability and catalytic activity for LiPSs, proving the key role of the N-P covalent bond in lithium-sulfur batteries.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrode material preparation, and particularly relates to a sulfur host material for a lithium-sulfur battery, a preparation method thereof, and an application thereof. Background Art

[0002] The redox reaction of lithium polysulfides (LiPSs) in a lithium-sulfur battery is a very complex electrochemical process involving 16 electrons. So far, it is impossible to directly analyze the generation and subsequent reactions of its intermediate states experimentally. Due to the very slow conversion reaction kinetics of LiPSs, the ion-electron mobility of the sulfur cathode is very low. This slow kinetic process mainly stems from the liquid-solid conversion process from Li2S4 to Li2S. This leads to the accumulation of LiPSs in the electrolyte, causing a serious shuttle effect. Introducing a catalyst with high adsorption capacity for LiPSs and strong catalytic ability into the positive electrode of a lithium-sulfur battery can effectively anchor LiPSs and promote their conversion during charge and discharge. Thereby inhibiting the shuttle effect of LiPSs, improving the utilization rate of sulfur, and enhancing the electrochemical performance of the lithium-sulfur battery.

[0003] As is well known, compared with nanoparticles, two-dimensional (2D) nanostructures can significantly promote ion / electron transport, expose more active sites, and promote full contact between the electrode / electrolyte. Among them, due to the unique energy band structure and curly lattice of layered black phosphorus nanosheets, it is an extremely attractive electrocatalyst. In addition, black phosphorus nanosheets also exhibit a low density (2.69 g cm -3 ), a fast lithium ion diffusion constant, high sulfur affinity, and anisotropic thermal and electronic properties. At the same time, studies have shown that black phosphorus nanosheets can maintain structural stability within a potential window of 1.8 - 2.6 V. These characteristics indicate that black phosphorus nanosheets can effectively anchor LiPSs and catalyze their conversion without destroying the structure of the black phosphorus nanosheets themselves. Ideally, all phosphorus atoms in black phosphorus nanosheets can be exposed on the surface and serve as active catalytic sites. However, sulfur host materials based on black phosphorus nanosheets are still limited by electronic conductivity (the semiconductor nature of black phosphorus nanosheets) and limited active sites caused by severe layer stacking.

[0004] In the prior art, to solve these problems, surface modification of black phosphorus nanosheets is a commonly used method. By using FBP table for solution treatment and precisely controlling the adjusted properties. For example, by combining graphene, MXene, and metal compounds with black phosphorus nanosheets to construct black phosphorus-based heterostructures, the inherent characteristics of black phosphorus nanosheets can be retained and some new functions can be provided. However, this simple physical composite has a weak interfacial coupling and usually has a limited effect on adjusting the electronic properties of black phosphorus nanosheets. Summary of the Invention

[0005] In order to overcome the shortcomings of the above-mentioned prior art, the purpose of the present invention is to provide a sulfur host material for lithium-sulfur batteries, a preparation method thereof, and an application thereof, so as to solve the technical problem of weak physical composite interface coupling effect in the process of modifying the surface of thin-layer black phosphorus in the prior art.

[0006] In order to achieve the above purpose, the present invention adopts the following technical solutions to be realized:

[0007] A preparation method of a sulfur host material for lithium-sulfur batteries disclosed by the present invention includes: uniformly mixing thin-layer black phosphorus with nitrogen-doped carbon-coated carbon nanotubes (NC), and performing heat treatment in an inert gas atmosphere to obtain a thin-layer black phosphorus composite nitrogen-doped carbon nanotube sulfur host material (FBP-NC) with N-P covalent bonds.

[0008] Preferably, the mass ratio of the thin-layer black phosphorus to NC is 1:10.

[0009] Preferably, the temperature of the heat treatment is 200-300 °C, and the time of the heat treatment is 2 h.

[0010] Preferably, the preparation method of the thin-layer black phosphorus is: using a BP crystal and a platinum wire as the working electrode and the counter electrode respectively, dissolving tetrabutylammonium tetrafluoroborate in dimethyl sulfoxide as the electrolyte, and obtaining thin-layer black phosphorus through electrochemical exfoliation.

[0011] Preferably, the preparation method of the NC is: adding urea to the CNTs solution, stirring evenly, removing the solvent, calcining in an Ar gas atmosphere, and grinding after cooling to obtain NC.

[0012] Further preferably, the stirring time is 1-3 h.

[0013] Even more preferably, the stirring time is 2 h.

[0014] Further preferably, the temperature of the calcination is 800 °C, and the time of the calcination is 2 h.

[0015] The present invention also discloses a sulfur host material for lithium-sulfur batteries prepared by the above preparation method, and the thin-layer black phosphorus and the nitrogen-doped carbon-coated carbon nanotubes are combined through N-P covalent bonds.

[0016] Preferably, at a sulfur surface loading of 7.9 mg cm -2 and an electrolyte volume, after cycling 50 times at a current density of 0.1 C, the capacity can still be stabilized at 6.7 mAh cm -2 .

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] The present invention discloses a preparation method of a sulfur host material for a lithium-sulfur battery. Using thin-layer black phosphorus as a matrix, the thin-layer black phosphorus and NC are calcined at a low temperature to obtain a thin-layer black phosphorus composite nitrogen-doped carbon nanotube catalyst with N-P covalent bonds, that is, the sulfur host material for the lithium-sulfur battery. At a relatively low calcination temperature, black phosphorus and carbon nanotubes coated with nitrogen-doped carbon are combined through N-P covalent bonds. The formation of N-P covalent bonds has a strong promoting effect on the adsorption and catalysis of polysulfide lithium. The sulfur host material for the lithium-sulfur battery prepared by the preparation method disclosed in the present invention has a stronger anchoring ability and catalytic activity for LiPSs (polysulfide lithium) compared with the simply physically mixed FBP-NC (sulfur host material for the lithium-sulfur battery), and at the same time proves the key role of N-P covalent bonds in the lithium-sulfur battery.

[0019] The present invention also discloses the sulfur host material for the lithium-sulfur battery prepared by the above preparation method. Combining black phosphorus and carbon nanotubes coated with nitrogen-doped carbon through N-P covalent bonds is beneficial to improving the adsorption and catalytic performance of polysulfide lithium. When it is used as the sulfur cathode of the lithium-sulfur battery, it still has excellent electrochemical performance under high sulfur loading and lean electrolyte conditions.

[0020] The present invention also discloses the application of the above sulfur host material for the lithium-sulfur battery in the preparation of a lithium-sulfur battery cathode material. At a sulfur surface loading of 7.9 mg cm -2 and an electrolyte volume, after cycling 50 times at a current density of 0.1C, the capacity can still be stabilized at 6.7 mAh cm -2 . BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 are the XPS spectra of P 2p and N1s of the sulfur host material for the lithium-sulfur battery prepared in the present invention; among them, Figure (a) corresponds to the XPS spectrum of P 2p, and Figure (b) corresponds to the XPS spectrum of N1s;

[0022] Figure 2 is the cycling performance graph of the sulfur host material for the lithium-sulfur battery prepared in Example 1 of the present invention at 0.2C;

[0023] Figure 3 is the cycling performance graph of the sulfur host material for the lithium-sulfur battery prepared in Example 1 of the present invention at 0.1C under high sulfur surface loading and lean electrolyte conditions;

[0024] Figure 4 are the Raman spectrum and P 2p XPS spectrum of the sulfur host material for the lithium-sulfur battery prepared in Example 2 of the present invention, where Figure (a) corresponds to the Raman spectrum and Figure (b) corresponds to the XPS spectrum of P 2p.

[0025] Figure 5 is the Raman spectrum of the sulfur host material for the lithium-sulfur battery prepared in Comparative Example 1 of the present invention;

[0026] Figure 6 XPS spectra of P 2p and N 1s of the sulfur host material of the lithium-sulfur battery prepared in Comparative Example 2 of the present invention; wherein, Figure (a) corresponds to the XPS spectrum of P 2p, and Figure (b) corresponds to the XPS spectrum of N 1s;

[0027] Figure 7 Cycling performance graph of the sulfur host material of the lithium-sulfur battery prepared in Comparative Example 2 of the present invention at 0.2C. Detailed implementation manners

[0028] In order to enable those skilled in the art to better understand the solution of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0029] It should be noted that the terms "first", "second", etc. in the specification and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that such used data can be interchanged under appropriate circumstances so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "comprising" and "having" and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product or device that includes a series of steps or units does not necessarily have to be limited to those steps or units clearly listed, but may include other steps or units not clearly listed or inherent to these processes, methods, products or devices.

[0030] The present invention will be further described in detail below with reference to the accompanying drawings:

[0031] The present invention discloses a preparation method of a sulfur host material for a lithium-sulfur battery, comprising the following steps:

[0032] 1) BP crystals and platinum wires were used as the working electrode and the counter electrode respectively. 0.01 M tetrabutylammonium tetrafluoroborate (TBAB) salt dissolved in dimethyl sulfoxide (DMSO) was used as the electrolyte. The two electrodes were inserted into the electrolyte with a distance of 2 cm between them. Using the potentiostatic polarization function of the electrochemical workstation, electrochemical exfoliation was started at -5 V voltage. During the exfoliation process, the dissolved cations were embedded into the interlayer of the BP crystal under the action of the current. At the same time, the embedded cations interacted with DMSO under the action of the voltage to generate dimethyl sulfite and alkane gas, causing the BP crystal to expand into a sponge-like shape. Then, the obtained sponge-like BP was ultrasonically treated for 5 minutes to completely exfoliate it into EBP nanosheets and disperse them evenly. After that, it was washed repeatedly with DMSO and deionized water alternately in a high-speed centrifuge. The rotation speed was set to 10000 r min -1 , the centrifugation time was 10 min, and finally a small amount of incompletely exfoliated large pieces of BP was separated at a speed of 1500 r min -1 . The supernatant was taken and named FBP.

[0033] 2) 50 mg of CNTs was evenly dispersed in 100 mL of deionized water, then 1 g of urea was added and stirred for 2 h. After that, the solvent was removed by rotary evaporation. The collected solid mixture was calcined at 800 °C for 2 h in an Ar atmosphere with a heating rate of 2 °C min -1 . After cooling to room temperature, it was ground into powder with an agate mortar to obtain nitrogen-doped carbon-coated carbon nanotubes, namely NC.

[0034] 3) Then, the FBP nanosheets and NC were mixed evenly with a mass ratio of FBP to NC of 1:10. Then the obtained mixture was heat-treated at 200 - 400 °C for 2 h in an Ar atmosphere with a heating rate of 2 °C min -1 to obtain FBP-NC.

[0035] Note: The CNTs used in this invention were purchased from Jiangsu Xianfeng Nano Materials Technology Co., Ltd., with the product number 100776. The BP crystals used were purchased from Jiangsu Xianfeng Nano Materials Technology Co., Ltd., with the product number 100944.

[0036] The present invention discloses a thermoresponsive switch material prepared by the above preparation method. The thermoresponsive switch electrode material has a relatively low thermoresponsive temperature (90 °C). When the internal temperature of the battery abnormally rises to 90 °C, due to the expansion of the polymer matrix and the increase in the distance between the conductive fillers, the resistance of the switch material can increase by several orders of magnitude in a short time.

[0037] Example 1

[0038] A preparation method of a thermoresponsive switch material, comprising the following steps:

[0039] Step 1: Use a BP crystal and a platinum wire as the working electrode and the counter electrode respectively. 0.01 M tetrabutylammonium tetrafluoroborate (TBAB) salt dissolved in dimethyl sulfoxide (DMSO) is used as the electrolyte. Insert the two electrodes into the electrolyte with a distance of 2 cm between them. Use the constant potential polarization function of the electrochemical workstation to start electrochemical exfoliation at -5 V. During the exfoliation process, the dissolved cations are embedded between the layers of the BP crystal under the action of the current. At the same time, the embedded cations interact with DMSO under the action of the voltage to produce dimethyl sulfite and alkane gas, causing the BP crystal to expand into a sponge-like shape. Then, ultrasonicate the obtained sponge-like BP for 5 minutes to completely exfoliate it into EBP nanosheets and disperse them evenly. After that, wash it alternately with DMSO and deionized water multiple times in a high-speed centrifuge. Set the rotation speed to 10,000 r min -1 , centrifuge for 10 min, and finally separate a small amount of incompletely exfoliated large pieces of BP at a speed of 1,500 r min -1 . Take the supernatant and name it FBP;

[0040] Step 2: Uniformly disperse 50 mg of CNTs in 100 mL of deionized water, then add 1 g of urea and stir for 2 h. After that, remove the solvent by rotary evaporation. The collected solid mixture is calcined at 800 °C for 2 h in an Ar atmosphere with a heating rate of 2 °C min -1 . After cooling to room temperature, grind it into powder with an agate mortar to obtain NC;

[0041] Step 3: Then, uniformly mix the FBP nanosheets and NC. The ratio of FBP to NC is 1:10. Then heat-treat the mixture at 200 °C for 2 h in an Ar atmosphere with a heating rate of 2 °C min -1 to obtain FBP-NC.

[0042] Step 4: Mix FBP-NC and sublimed sulfur evenly in a carbon disulfide solution at a mass ratio of 3:7, and dry it. Load sulfur onto FBP-NC by capillary action to obtain a composite sulfur cathode (FBP-NC / S);

[0043] Step 5: Mix FBP-NC / S, acetylene black, and PVDF evenly in a 1-methyl-2-pyrrolidone (NMP) solvent at a mass ratio of 8:1:1. Then directly scrape the mixture onto an aluminum foil;

[0044] Step 6: Put the scraped electrode sheet into an oven at 60 °C to dry and remove NMP, and then it can be taken out for testing.

[0045] The prepared electrode sheets above were used as electrode materials to test their performance in coin cells. The dried film was cut into electrode sheets with a diameter of 10 mm using a manual cutter. Then, the electrode sheets were transferred to a glove box filled with argon (water and oxygen values ≤ 0.01 ppm) for subsequent battery assembly. In the glove box, CR2016 type batteries were assembled. The structure of the battery from bottom to top was: positive electrode case - gasket - film - gasket - negative electrode case. After the battery assembly was completed, it was placed in an electric packaging machine for battery packaging, and the packaging pressure was set to 0.85 T. The assembled device was tested for polar charge-discharge cycling using a Neware battery test system.

[0046] The test results are as follows: The XPS spectra and cycle stability of the black phosphorus-based composite sulfur host material in this example are as Figure 1 shown in Figures 2 and 3. In the XPS spectra, the presence of N-P covalent bonds can be clearly observed, indicating successful preparation. The material was made into electrode sheets and assembled into batteries for cycle testing. The battery was cycled 100 times at a current density of 0.2C, and the capacity retention rate was 82%. Even under high sulfur surface loading and lean electrolyte conditions, it had excellent electrochemical performance.

[0047] Example 2

[0048] The process in this example was the same as that in Example 1, except that some process parameters were changed: the calcination temperature was 300 °C. The prepared electrode sheets above were used as electrode materials to test their performance in coin cells. The dried film was cut into electrode sheets with a diameter of 10 mm using a manual cutter. Then, the electrode sheets were transferred to a glove box filled with argon (water and oxygen values ≤ 0.01 ppm) for subsequent battery assembly. In the glove box, CR2016 type batteries were assembled. The structure of the battery from bottom to top was: positive electrode case - gasket - film - gasket - negative electrode case. After the battery assembly was completed, it was placed in an electric packaging machine for battery packaging, and the packaging pressure was set to 0.85 T. The assembled device was tested for polar charge-discharge cycling using a Neware battery test system.

[0049] The test results are as follows: The Raman and XPS spectra of the black phosphorus-based composite sulfur host material prepared in this example are as Figure 4 shown. It can be seen from the figure that although black phosphorus has characteristic peaks at 300 °C, the intensity is very weak, indicating that black phosphorus is unstable at a calcination temperature of 300 °C and will undergo partial decomposition.

[0050] Comparative Example 1

[0051] The process of this embodiment is the same as that of Embodiment 1, except that some process parameters are changed: the calcination temperature is 400 °C. The prepared electrode sheet above is used as the electrode material to test its performance under a button battery. The dried film is cut into electrode sheets with a diameter of 10 mm using a manual cutting machine. Then, the electrode sheets are transferred to a glove box filled with argon (with water and oxygen values ≤ 0.01 ppm) for subsequent battery assembly. The CR2016 type battery is assembled in the glove box. The structure of this battery from bottom to top is: positive electrode case - gasket - film - gasket - negative electrode case. After the battery assembly is completed, it is placed in an electric packaging machine for battery packaging, and the packaging pressure is set to 0.85 T. The assembled device above is tested for polarization charge and discharge cycling using a Neware battery test system.

[0052] The test results are as follows: The Raman diagram of the black phosphorus-based composite sulfur host material prepared in this embodiment is as Figure 5 shown. It can be seen from the figure that the characteristic peak of black phosphorus disappears at 400 °C, indicating that black phosphorus is almost completely decomposed at this temperature.

[0053] Comparative Example 2

[0054] The process of this embodiment is the same as that of Embodiment 1, except that some process parameters are changed: direct mixing without calcination. The prepared electrode sheet above is used as the electrode material to test its performance under a button battery. The dried film is cut into electrode sheets with a diameter of 10 mm using a manual cutting machine. Then, the electrode sheets are transferred to a glove box filled with argon (with water and oxygen values ≤ 0.01 ppm) for subsequent battery assembly. The CR2016 type battery is assembled in the glove box. The structure of this battery from bottom to top is: positive electrode case - gasket - film - gasket - negative electrode case. After the battery assembly is completed, it is placed in an electric packaging machine for battery packaging, and the packaging pressure is set to 0.85 T. The assembled device above is tested for polarization charge and discharge cycling using a Neware battery test system.

[0055] The test results are as follows: The XPS spectrum and cycling performance diagram of the black phosphorus-based composite sulfur host material prepared in this embodiment are as Figure 6 shown in Figures 6 and 7. From the XPS diagram, it can be observed that there is no N-P covalent bond, and from the cycling performance diagram, it can be seen that the stability of the simple physical mixing composite material is poor and the capacity is also lower, indicating the importance of the N-P covalent bond for improving the performance of lithium-sulfur batteries.

[0056] In summary, the present invention covalently functionalizes thin-layer black phosphorus. By forming covalent bonds with foreign groups, the interfacial coupling can be enhanced, and the inherent properties of thin-layer black phosphorus nanosheets can be protected and regulated to a great extent. Especially under the actual working conditions of high sulfur loading and low electrolyte usage, a thin-layer black phosphorus-based sulfur host material with an optimized electronic structure is developed to promote the redox reaction of LiPSs and realize a high-energy-density lithium-sulfur battery.

[0057] The above content is only for explaining the technical idea of the present invention, and the protection scope of the present invention cannot be limited thereby. Any modification made on the basis of the technical solution according to the technical idea proposed by the present invention falls within the protection scope of the claims of the present invention.

Claims

1. A preparation method of a sulfur host material for a lithium-sulfur battery, characterized in that, Comprising: Mixing thin-layer black phosphorus and nitrogen-doped carbon-coated carbon nanotubes evenly, and performing heat treatment in an inert gas atmosphere to obtain a sulfur host material for a lithium-sulfur battery. The thin-layer black phosphorus and the nitrogen-doped carbon-coated carbon nanotubes in the sulfur host material for the lithium-sulfur battery are bonded by an N-P covalent bond; The temperature of the heat treatment is 200-300 °C, and the time of the heat treatment is 2 h; The preparation method of the thin-layer black phosphorus is as follows: Using a BP crystal and a platinum wire as the working electrode and the counter electrode respectively, dissolving tetrabutylammonium tetrafluoroborate in dimethyl sulfoxide as the electrolyte, and obtaining thin-layer black phosphorus through electrochemical exfoliation.

2. The preparation method of the sulfur host material for the lithium-sulfur battery according to claim 1, wherein The mass ratio of the thin-layer black phosphorus to the nitrogen-doped carbon-coated carbon nanotubes is 1:

10.

3. The preparation method of the sulfur host material for the lithium-sulfur battery according to claim 1, wherein, The preparation method of the nitrogen-doped carbon-coated carbon nanotubes is as follows: Adding urea to the carbon nanotube solution, stirring evenly, removing the solvent, and calcining in an inert gas atmosphere, followed by cooling and grinding to obtain nitrogen-doped carbon-coated carbon nanotubes.

4. The preparation method of the sulfur host material for the lithium-sulfur battery according to claim 3, wherein The stirring time is 1-2 h.

5. The preparation method of the sulfur host material for the lithium-sulfur battery according to claim 3, wherein The temperature of the calcination is 800 °C, and the time of the calcination is 2 h.

6. The sulfur host material of the lithium-sulfur battery prepared by the preparation method according to any one of claims 1 to 5, characterized in that, The thin-layer black phosphorus and the nitrogen-doped carbon-coated carbon nanotubes are bonded by an N-P covalent bond.

7. Use of the sulfur host material for the lithium-sulfur battery according to claim 6 in the preparation of the positive electrode material for the lithium-sulfur battery, characterized in that, At a sulfur surface loading of 7.9 mg cm -2 , the capacity can still remain stable at 6.7 mAh cm after 50 cycles at a current density of 0.1 C -2 .

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