A composite material of MoO 2 nanosheets in-situ grown on the surface of mesoporous hollow carbon spheres

Abstract

The present invention discloses a composite material of mesoporous hollow carbon spheres with in-situ grown MoO2 nanosheets on the surface. A molybdenum-dopamine chelate nanosheet is loaded on the core-shell structure of SiO2@SiO2 / RF. After carbonization and etching away the SiO2 template, the obtained composite material of mesoporous hollow carbon spheres with in-situ grown MoO2 nanosheets on the surface has a high specific surface area and a porous structure, which is beneficial to the penetration of the electrolyte and the infiltration of sulfur, improving the conductivity of the material. At the same time, the mesoporous shell and the large internal voids can provide sufficient sulfur storage space and buffer the volume expansion of sulfur species during charge and discharge to ensure structural stability. In addition, the ultrathin MoO2 nanosheets can expose more active sites, greatly improving the chemical adsorption and catalytic conversion efficiency for Li2S x and preventing the dissolution of Li2S x in the electrolyte, effectively inhibiting the shuttle effect. Therefore, when the prepared HMC@MoO2-S cathode is under the condition of a high sulfur loading (5.0 mg cm −2 ), the lithium-sulfur battery exhibits excellent cycle stability.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-sulfur batteries, and particularly relates to a composite material of mesoporous hollow carbon spheres with in-situ grown MoO 2 nano-sheets on the surface. Background Art

[0002] With the popularization of portable electronic devices, electric vehicles, and the development of energy storage battery systems, the demand for high-energy and long-life batteries is increasing day by day. Lithium-ion batteries can no longer meet the needs of the development of new commercial batteries. The theoretical energy density of lithium-sulfur batteries is as high as 2600 Wh·kg -1 , and sulfur has outstanding advantages such as rich reserves, low cost, and environmental friendliness. Therefore, lithium-sulfur batteries are considered to be the most competitive next-generation secondary batteries. However, the practical application of lithium-sulfur batteries is still faced with many technical challenges. Among them, the problems existing in the sulfur cathode are: (1) The conductivity of sulfur and the discharge end product lithium sulfide (Li 2 S) is poor, which hinders the transport of electrons and ions, resulting in slow reaction kinetics; (2) The densities of sulfur (2.03 g cm -3 ) and the reduction product Li 2 S (1.66 g cm -3 ) are different, and the volume fluctuates by up to 80% during charge and discharge, resulting in damage or even pulverization of the electrode material structure; (3) The reaction intermediate product (Li 2 S x , 4≤x≤8) is easily soluble in the electrolyte. During the charge and discharge process, Li 2 S x repeatedly moves between the positive and negative electrodes (the so-called "shuttle effect"). Among them, part of Li 2 S x forms insoluble Li 2 S 2 / Li 2 S on the surface of the lithium negative electrode, resulting in irreversible loss of sulfur, attenuation of battery capacity, and passivation of the surface of the lithium negative electrode; (4) The charge and discharge process of lithium-sulfur batteries is a redox process that proceeds step by step in solid (S)-liquid (Li 2 S x )-solid (Li 2 S), involving complex disproportionation reactions and normalization reactions, with slow reaction kinetics, which seriously affects the overall performance of the battery.

[0003] To solve the above problems, researchers have focused on preparing sulfur cathodes with unique structures and compositions to improve conductivity and suppress the shuttle effect. One of the most effective methods is to combine sulfur with carbon materials having high conductivity and high specific surface area, such as microporous / mesoporous carbon, hollow carbon spheres, carbon nanocages, carbon nanofibers, carbon nanotubes, graphene, and their hybrids. However, carbon materials with weak polarity have limited ability to physically confine Li 2 S x dissolution, resulting in severe shuttle effect during charge / discharge processes, especially at high sulfur loadings. In recent years, with the deepening understanding of the electrochemical processes of lithium-sulfur batteries, researchers have proposed an adsorption-catalysis synergistic strategy, and polar metal compounds have been widely used as sulfur host materials or additives for host materials in lithium-sulfur batteries. For example, metal oxides (Bi 2 O 3 , Mn 3 O 4 ), metal sulfides (Sb 2 S 3 , MoS 2 ), metal carbides (Ti 3 C 2 , Mo 2 C), metal nitrides (TiN, VN), metal phosphides (CoP, MoP), and heterostructures (TiO 2 / TiN, CoS 2 / Fe 7 S8), etc. Polar metal compounds can strongly chemisorb Li 2 S x by establishing chemical bonds, while reducing the energy barrier of the sulfur species conversion reaction, accelerating the redox reaction kinetics, thereby effectively suppressing the shuttle effect, improving the utilization rate of sulfur, and achieving high capacity and long cycle life of lithium-sulfur batteries. However, most polar metal compounds have poor conductivity, limited catalytic activity, or insufficient specific surface area, resulting in a limited number of exposed catalytic active sites and cannot provide enough active sites to adsorb Li 2 S x and catalyze its conversion. Therefore, combining high-catalytic-activity metal compounds with rich active sites and high-conductivity carbon materials with reasonable structures to develop multifunctional sulfur host materials is an effective strategy to comprehensively improve the electrochemical performance of lithium-sulfur batteries. Summary of the Invention

[0004] To solve the problems existing in the cathode materials in the above lithium-sulfur batteries, the present invention provides a composite material of ultrathin MoO 2 nanosheets grown on the surface of mesoporous hollow carbon spheres (HMC) (HMC@MoO 2 ).

[0005] The present invention provides a composite material of ultrathin MoO nanosheets in-situ grown on the surface of mesoporous hollow carbon spheres and a method thereof, comprising the following steps: 2

[0006] (1) Prepare a mesoporous hollow carbon sphere precursor SiO 2 @SiO 2 / RF;

[0007] (2) Add hydrochloric acid dopamine to an ammonium molybdate solution, stir evenly to obtain solution A; under vigorous stirring, add the SiO 2 @SiO 2 / RF obtained in step (1) to ethanol to obtain solution B; slowly inject solution A into solution B, centrifuge and dry to obtain an HMC@MoO 2 precursor;

[0008] (3) In an argon atmosphere, calcine the precursor obtained in step (2) at a certain temperature for a period of time, etch away the SiO 2 template of the obtained product, wash and vacuum dry to obtain an HMC@MoO 2 composite material.

[0009] Preferably, in step (1), SiO 2 @SiO 2 / RF is prepared by the following steps: Add NH 2 •H 3 O to a mixed solvent of ethanol and H 2 O, add tetrapropyl orthosilicate under stirring, stir for 15 minutes to obtain a milky colloidal solution, add resorcinol and formaldehyde, stir at room temperature for more than 24 h, centrifuge, wash and dry to obtain SiO 2 @SiO 2 / RF.

[0010] Preferably, in step (2), the mass ratio of hydrochloric acid dopamine to ammonium molybdate tetrahydrate is 1:1.

[0011] Preferably, in step (2), the mass ratio of ammonium molybdate tetrahydrate to SiO 2 @SiO 2 / RF is 11~33:500.

[0012] Preferably, in step (3), calcine at 600~800 °C for 3~5 h.

[0013] Preferably, in step (4), use a 4 M NaOH solution to etch away the SiO 2 template.

[0014] ​The present invention also provides a cathode material based on the above composite material. The above composite material and sublimed sulfur are mixed and ground evenly at a mass ratio of x:100 - x, where x = 15 - 25. Then the mixed powder is placed in a high-pressure reactor filled with Ar protection and reacted at 155 °C for 12 h to obtain the cathode material.

[0015] The present invention also provides an application of the cathode material based on the above composite material in a lithium-sulfur battery.

[0016] Compared with the prior art, the present invention has the following advantages: First, HMC@MoO 2 has a relatively high specific surface area and a porous structure, which is beneficial to the penetration of the electrolyte and the infiltration of sulfur, and improves the conductivity of the material. Second, the mesoporous shell and the relatively large internal voids can provide sufficient sulfur storage space and buffer the volume expansion of sulfur species during charge and discharge to ensure the structural stability. Third, the ultrathin MoO 2 nanosheets can expose more active sites, greatly improving the chemical adsorption and catalytic conversion efficiency of Li 2 S x and preventing the dissolution of Li 2 S x in the electrolyte, effectively inhibiting the shuttle effect. Due to the combined action of the above advantages, the prepared HMC@MoO 2 -S cathode exhibits excellent cycle stability in a lithium-sulfur battery under the condition of a high sulfur loading (5.0 mg cm −2 ). Description of the Drawings

[0017] Figure 1 is a schematic diagram of the synthesis of a composite material with ultrathin MoO 2 nanosheets grown on the surface of mesoporous hollow carbon spheres.

[0018] Figure 2 is the SEM image (a, b), TEM image (c), HRTEM image (d) (the inset is the magnified IFFT image), and HAADF image and EDS elemental mapping image (e) of the HMC@MoO 2 composite material prepared in Example 1.

[0019] Figure 3 is the XRD spectrum (a), N 2 adsorption-desorption isotherm (b) and the corresponding pore size distribution (c) of the HMC@MoO 2 prepared in Example 1.

[0020] Figure 4 is the SEM image of the composite material of mesoporous hollow carbon spheres doped with MoO 2 nanoparticles prepared in Comparative Example 1.

[0021] Figure 5 The XRD pattern (a) and TGA curve (b) of HMC@MoO 2 -S prepared in Example 2.

[0022] Figure 6 The cycling performance at 0.1 C of the HMC-S, HMC / p-MoO 2 -S, HMC@MoO 2 -S, HMC@MoO 2 -S-1, HMC@MoO 2 -S-2 cathodes prepared in this invention.

[0023] Figure 7 The cycling performance at 0.1 C of the HMC@MoO 2 -S cathode prepared in Example 5 at a high areal sulfur loading of 5.0 mg cm −2 ².

[0024] Figure 8 The schematic diagram of the principle of the multifunctional HMC@MoO 2 -S prepared in this invention. Detailed implementation manners

[0025] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0026] (1) Combining Figure 1 , the preparation process and principle of the HMC@MoO 2 in this invention are as follows:

[0027] In the first step, using three small molecules, resorcinol, formaldehyde, and tetrapropyl orthosilicate (TPOS), as reactants, and adopting a one-pot surfactant-free synthesis method, taking advantage of the difference in reaction kinetics between the polymerization of resorcinol-formaldehyde (RF) and the hydrolysis of TPOS in an alkaline solution of ethanol / water: (1) In the initial stage of the reaction, TPOS hydrolyzes to form SiO 2 primary particles, and the primary particles grow and polymerize to form a SiO 2 core. (2) In the later stage of the reaction, the SiO 2 primary particles and the resorcinol-formaldehyde (RF) polymer co-condense on the SiO 2 core to form a SiO 2 @SiO 2 / RF, that is, the HMC precursor. In this step, the difference between the first step of this invention and the prior art is that in the prior art, first, SiO 2 spheres are prepared by using tetraethyl orthosilicate (TEOS) with the assistance of a surfactant. Then, on the SiO 2The surface of the sphere is coated with polydopamine or phenolic resin. Finally, after carbonization and etching of SiO 2 template, hollow carbon spheres are obtained. The hollow carbon spheres obtained by such methods have a thin and dense carbon shell and a low specific surface area. When used as a sulfur-carrying material, sulfur often adheres to its surface, and high sulfur loading cannot be achieved. The thick-carbon-shell hollow carbon spheres with a high specific surface area and abundant mesopores obtained by carbonization and etching of the HMC precursor of the present invention provide a guarantee for the molten sulfur to diffuse into the mesoporous carbon shell and achieve high sulfur loading.

[0028] In the second step, molybdate reacts with dopamine to form molybdenum-dopamine chelate nanosheets. The hydrophilic groups (-NH 2 of the molybdenum-dopamine chelate) and the abundant hydroxyl groups (-OH) on the surface of SiO 2 @SiO 2 / RF undergo hydrogen bonding, so that the molybdenum-dopamine chelate is uniformly distributed on the SiO 2 @SiO 2 / RF spheres to obtain the HMC@MoO 2 precursor.

[0029] In the third step, the HMC@MoO 2 precursor is carbonized, and the molybdenum-dopamine chelate nanosheets are transformed into ultrathin MoO 2 nanosheets. The SiO 2 @SiO 2 / RF composite is transformed into SiO 2 @SiO 2 / C; after removing the SiO 2 template, a composite material (HMC@MoO 2 nanosheets grown on the surface of mesoporous hollow carbon spheres is obtained. 2 )

[0030] (2) Combining Figure 8 , the principle of the multiple functions of the HMC@MoO 2 -S of the present invention is as follows:

[0031] Firstly, the mesoporous hollow carbon provides sufficient space for the storage of sulfur and the volume expansion of sulfur species during the charge and discharge process; secondly, the mesoporous carbon shell serves as the first line of defense to prevent the dissolution of Li 2 S x through physical confinement. The MoO 2 nanosheets on the surface of the carbon shell serve as the second line of defense to capture Li 2 S x through chemical adsorption, further preventing the dissolution of Li 2 S x . Thirdly, the ultrathin MoO 2The nanosheets expose more active sites, improving the capture and catalytic conversion efficiency of Li 2 S x and effectively inhibiting the shuttle effect of Li 2 S x Based on these advantages, the HMC@MoO 2 -S cathode exhibits excellent long-term cycling stability at high sulfur loadings.

[0032] Example 1: Preparation of mesoporous hollow carbon spheres with ultrathin MoO 2 nanosheets grown on their surface (HMC@MoO 2 ) composite

[0033] First step: Add 3 ml of NH 2 •H 3 O (25 wt%) to 80 ml of ethanol / H 2 O solvent (v / v = 7:1), and add 3.46 ml of tetrapropyl orthosilicate (TPOS) under magnetic stirring. After 15 minutes, a milky colloidal solution is obtained, then add 0.4 g of resorcinol and 0.56 ml of formaldehyde (37 wt%), and stir at room temperature for 24 h. Centrifuge to collect the precipitate, wash it with water and ethanol, and dry it overnight at 50°C to obtain the mesoporous hollow carbon sphere HMC precursor (SiO 2 @SiO 2 / RF).

[0034] Second step: First, dissolve 88 mg of ammonium molybdate tetrahydrate ((NH 4 ) 6 Mo 7 O 24 •4H 2 O) in 15 mL of deionized water and stir for 5 min, then add 88 mg of dopamine hydrochloride and stir for 30 min. The color of the solution turns reddish-brown (denoted as solution A). Secondly, under vigorous stirring, add 2 g of the HMC precursor to 50 ml of ethanol (denoted as solution B), slowly inject solution A into solution B, and after stirring for 1 h, the suspension shows a deep orange-red color. Centrifuge to collect the precipitate and dry it at 50°C to obtain the precursor of the HMC@MoO 2 composite.

[0035] Third step: Place the HMC@MoO 2 precursor in a tubular furnace, carbonize it at 700°C for 3 h in an argon atmosphere, etch away the SiO 2 template with 4 M NaOH solution, wash it with water and ethanol, and dry it under vacuum to obtain the mesoporous hollow carbon spheres with ultrathin MoO 2 nanosheets grown on their surface composite (HMC@MoO 2 ).

[0036] The obtained HMC@MoO 2 composite material was characterized, and its morphology is as Figure 2 shown. The XRD pattern is as shown in (a) of Figure 3 , and the N 2 adsorption-desorption isotherm is as shown in (b) of Figure 3 , and the corresponding pore size distribution is as shown in (c) of Figure 3 .

[0037] Comparative Example 1: Preparation of mesoporous hollow carbon sphere doped with MoO 2 nanoparticles (HMC / p-MoO 2 ) composite material

[0038] First step: The same as in Example 1, the mesoporous hollow carbon sphere HMC precursor (SiO 2 @SiO 2 / RF) was prepared.

[0039] Second step: First, 88 mg of ammonium molybdate tetrahydrate ((NH 4 ) 6 Mo 7 O 24 •4H 2 O) was dissolved in 15 mL of deionized water and stirred for 5 min (denoted as solution A). Secondly, under vigorous stirring, 2 g of the HMC precursor was added to 50 ml of ethanol (denoted as solution B), and solution A was slowly injected into solution B, and stirred at 50 °C until the solvent evaporated to dryness, obtaining the precursor of the mesoporous hollow carbon sphere doped with MoO 2 nanoparticle composite material.

[0040] Third step: The precursor of the mesoporous hollow carbon sphere doped with MoO 2 nanoparticles was placed in a tube furnace, carbonized at 700 °C for 3 h in an argon atmosphere, etched with 4 M NaOH solution to remove the SiO 2 template, washed with water and ethanol, and dried under vacuum to obtain the mesoporous hollow carbon sphere doped with MoO 2 nanoparticle composite material (HMC / p-MoO 2 ), and its morphology is as Figure 4 shown.

[0041] Comparative Example 2: Preparation of mesoporous hollow carbon sphere-sulfur (HMC-S) composite material and its application in lithium-sulfur batteries

[0042] First step: The same as in Example 1, the mesoporous hollow carbon sphere HMC precursor ((SiO 2 @SiO 2 / RF) was prepared.

[0043] Step 2: Place the HMC precursor in a tube furnace, carbonize it at 700 °C for 3 h in an argon atmosphere, etch away the SiO 2 template with 4 M NaOH solution, wash it with water and ethanol, and dry it under vacuum to obtain mesoporous hollow carbon spheres (HMC).

[0044] Step 3: Mix the HMC material and sublimed sulfur in a mass ratio of 25:75, and grind them evenly. Then place the mixed powder in a high-pressure autoclave filled with Ar protection and react at 155 °C for 12 h. After cooling to room temperature, obtain the HMC-S composite material.

[0045] Step 4: Mix HMC-S, Ketjen black, and polyvinylidene fluoride (PVDF) in a mass ratio of 8:1:1 in an appropriate amount of N-methylpyrrolidone (NMP) to form a uniform slurry. Coat the slurry evenly on carbon-coated aluminum foil, then dry it overnight in a vacuum oven at 60 °C, and further cut it into discs (d = 1.2 cm) to make the positive electrode, with an average sulfur loading of 1.0 mg cm −2 . Use Celgard 2400 membrane as the separator and lithium foil as the negative electrode. The electrolyte consists of 1 M LiTFSI, 1 wt% LiNO 3 and DOL / DME (v / v = 1:1) solvent. The ratio of electrolyte to sulfur (E / S μL mg −1 ) is 20. Assemble it into a CR2032 coin cell in a glove box filled with argon. In the voltage range of 1.7 - 2.8 V, perform constant current charge-discharge tests on a Neware battery test system at a rate of 0.1 C (1 C = 1675 mAh g −1 ), and the results are as Figure 6 shown.

[0046] Comparative Example 3: Preparation of mesoporous hollow carbon sphere doped with MoO 2 nanoparticle-sulfur (HMC / p-MoO 2 ) composite material and its application in lithium-sulfur batteries

[0047] Step 1: Mix the HMC / p-MoO 2 composite material prepared in Comparative Example 1 and sublimed sulfur in a mass ratio of 25:75, and grind them evenly. Then place the mixed powder in a high-pressure autoclave filled with Ar protection and react at 155 °C for 12 h. After cooling to room temperature, obtain the HMC / p-MoO 2 -S composite material.

[0048] Step 2: HMC / p-MoO 2, Ketjen black and polyvinylidene fluoride (PVDF) were mixed thoroughly in a mass ratio of 8:1:1 in an appropriate amount of N-methylpyrrolidone (NMP) to form a homogeneous slurry. The slurry was evenly coated on carbon-coated aluminum foil and then dried overnight at 60 °C in a vacuum oven, and further cut into discs (d = 1.2 cm) to make the positive electrode, with an average sulfur loading of 1.0 mg cm −2 . Celgard 2400 membrane was used as the separator, and lithium foil was used as the negative electrode. The electrolyte was composed of 1 M LiTFSI, 1 wt% LiNO 3 and DOL / DME (v / v = 1:1) solvent. The ratio of electrolyte to sulfur (E / S μL mg −1 ) was 20. The CR2032 coin cells were assembled in an argon-filled glove box. In the voltage range of 1.7 - 2.8 V, a constant current charge-discharge test was carried out on a Neware battery test system at a rate of 0.1 C (1 C = 1675 mAh g −1 ), and the results are as Figure 6 shown.

[0049] Example 2: Preparation of HMC@MoO 2 -S composite and its application in lithium-sulfur batteries

[0050] The first step: The HMC@MoO 2 composite prepared in Example 1 and sublimed sulfur were mixed and ground evenly in a mass ratio of 25:75. Then the mixed powder was placed in a high-pressure reactor filled with Ar protection and reacted at 155 °C for 12 h. After cooling to room temperature, the HMC@MoO 2 -S composite was obtained. Its XRD pattern is as shown in (a) of Figure 5 , and the TGA curve is as shown in (b) of Figure 5 .

[0051] The second step: HMC@MoO 2 -S, Ketjen black and polyvinylidene fluoride (PVDF) were mixed thoroughly in a mass ratio of 8:1:1 in an appropriate amount of N-methylpyrrolidone (NMP) to form a homogeneous slurry. The slurry was evenly coated on carbon-coated aluminum foil and then dried overnight at 60 °C in a vacuum oven, and further cut into discs (d = 1.2 cm) to make the positive electrode, with an average sulfur loading of 1.0 mg cm −2 . Celgard 2400 membrane was used as the separator, and lithium foil was used as the negative electrode. The electrolyte was composed of 1 M LiTFSI, 1 wt% LiNO 3 and DOL / DME (v / v = 1:1) solvent. The ratio of electrolyte to sulfur (E / S μL mg −1) was 20. The CR2032 coin cells were assembled in an argon-filled glove box. In the voltage range of 1.7 - 2.8 V, the constant current charge-discharge tests were carried out on a Neware battery test system at a rate of 0.1 C (1 C = 1675 mAh g −1 ) and the results are as Figure 6 shown.

[0052] Example 3: Preparation of HMC@MoO 2 -S-1 composite and its application in lithium-sulfur batteries

[0053] First step: Similar to Example 1, the mesoporous hollow carbon sphere HMC precursor (SiO 2 @SiO 2 / RF) was prepared. Second step: First, 44 mg of ammonium molybdate tetrahydrate ((NH 4 ) 6 Mo 7 O 24 •4H 2 O) was dissolved in 15 mL of deionized water and stirred for 5 min, then 44 mg of dopamine hydrochloride was added and stirred for 30 min, and the solution color changed to reddish-brown (denoted as solution A). Secondly, under vigorous stirring, 2 g of the HMC precursor was added to 50 ml of ethanol (denoted as solution B), and solution A was slowly injected into solution B. After stirring for 1 h, the suspension showed a deep orange-red color. The precipitate was collected by centrifugation and dried at 50 °C to obtain the precursor of the HMC@MoO 2 composite.

[0054] Third step: The HMC@MoO 2 precursor was placed in a tube furnace and carbonized at 700 °C for 3 h in an argon atmosphere. The SiO 2 template was etched off with 4 M NaOH solution, washed with water and ethanol, and dried under vacuum to obtain the composite of mesoporous hollow carbon spheres with ultrathin MoO 2 nanosheets grown on the surface (HMC@MoO 2 ).

[0055] Fourth step: The HMC@MoO 2 composite and sublimed sulfur were mixed and ground evenly at a mass ratio of 25:75. Then the mixed powder was placed in a high-pressure reactor filled with Ar protection and reacted at 155 °C for 12 h. After cooling to room temperature, the HMC@MoO 2 -S composite was obtained.

[0056] Fifth step: HMC@MoO 2-S, Ketjen black, and polyvinylidene fluoride (PVDF) were thoroughly mixed in a mass ratio of 8:1:1 in an appropriate amount of N-methylpyrrolidone (NMP) to form a homogeneous slurry. The slurry was evenly coated on carbon-coated aluminum foil and then dried overnight at 60 °C in a vacuum oven. It was further cut into circular pieces (d = 1.2 cm) to make the positive electrode, with an average sulfur loading of 1.0 mg cm −2 . Celgard 2400 membrane was used as the separator, and lithium foil was used as the negative electrode. The electrolyte was composed of 1 M LiTFSI, 1 wt% LiNO 3 and DOL / DME (v / v = 1:1) solvent. The ratio of electrolyte to sulfur (E / S μL mg −1 ) was 20. A CR2032 coin cell was assembled in a glove box filled with argon. In the voltage range of 1.7 - 2.8 V, a constant current charge-discharge test was carried out on a Neware battery test system at a rate of 0.1 C (1 C = 1675 mAh g −1 ), and the results are as Figure 6 shown.

[0057] Example 4: Preparation of HMC@MoO 2 -S-2 composite and its application in lithium-sulfur batteries

[0058] First step: Similar to Example 1, a mesoporous hollow carbon sphere HMC precursor (SiO 2 @SiO 2 / RF) was prepared. Second step: First, 132 mg of ammonium molybdate tetrahydrate ((NH 4 ) 6 Mo 7 O 24 •4H 2 O) was dissolved in 15 mL of deionized water and stirred for 5 min. Then, 132 mg of dopamine hydrochloride was added and stirred for 30 min. The color of the solution turned reddish-brown (denoted as solution A). Secondly, under vigorous stirring, 2 g of the HMC precursor was added to 50 ml of ethanol (denoted as solution B). Solution A was slowly injected into solution B. After stirring for 1 h, the suspension showed a deep orange-red color. The precipitate was collected by centrifugation and dried at 50 °C to obtain the precursor of the HMC@MoO 2 composite.

[0059] Third step: The HMC@MoO 2 precursor was placed in a tubular furnace and carbonized at 700 °C for 3 h in an argon atmosphere. The SiO 2 template was etched away with 4 M NaOH solution, washed with water and ethanol, and dried in vacuum to obtain a composite of mesoporous hollow carbon spheres with ultrathin MoO 2 nanosheets grown on the surface (HMC@MoO 2).

[0060] Step 4: Mix the HMC@MoO 2 composite material and sublimed sulfur in a mass ratio of 25:75, mix and grind evenly. Then place the mixed powder in a high-pressure reactor filled with Ar protection, and react at 155 °C for 12 h. After cooling to room temperature, obtain HMC@MoO 2 -S composite material.

[0061] Step 5: HMC@MoO 2 -S, Ketjen black and polyvinylidene fluoride (PVDF) are fully mixed in a mass ratio of 8:1:1 in an appropriate amount of N-methylpyrrolidone (NMP) to form a uniform slurry. Coat the slurry evenly on carbon-coated aluminum foil, and then dry it overnight in a vacuum oven at 60 °C, and further cut it into circular pieces (d = 1.2 cm) to make a positive electrode, with an average sulfur loading of 1.0 mg cm −2 . Use Celgard 2400 membrane as the separator, lithium foil as the negative electrode, and the electrolyte is composed of 1 M LiTFSI, 1 wt% LiNO 3 and DOL / DME (v / v = 1:1) solvent. The ratio of electrolyte to sulfur (E / S μL mg −1 ) is 20. Assemble into a CR2032 coin cell in a glove box filled with argon. In the voltage range of 1.7 ~ 2.8 V, perform constant current charge and discharge tests on a Neware battery test system at a rate of 0.1 C (1 C = 1675 mAh g −1 ), and the results are as Figure 6 shown.

[0062] Example 5: Preparation of HMC@MoO −2 under the condition of high sulfur loading (5.0 mg cm 2 -S-2 composite material and its application in lithium-sulfur batteries

[0063] Step 1: Mix the HMC@MoO 2 composite material prepared in Example 1 and sublimed sulfur in a mass ratio of 15:85, mix and grind evenly. Then place the mixed powder in a high-pressure reactor filled with Ar protection, and react at 155 °C for 12 h. After cooling to room temperature, obtain HMC@MoO 2 -S composite material.

[0064] Step 2: HMC@MoO 2-S, Ketjen black and polyvinylidene fluoride (PVDF) were mixed in an appropriate amount of N-methylpyrrolidone (NMP) at a mass ratio of 8:1:1 to form a uniform slurry. The slurry was evenly coated on the carbon-coated aluminum foil by coating in batches, then dried in a vacuum oven at 60 °C overnight, and further cut into discs (d = 1.2 cm) to make the positive electrode with an average sulfur loading of 5.0 mg cm −2 Celgard 2400 membrane was used as the separator, lithium foil was used as the negative electrode, and the electrolyte consisted of 1 MLiTFSI, 1 wt % LiNO 3 With DOL / DME (v / v = 1:1) solvent composition, the ratio of electrolyte to sulfur (E / S μL mg −1 ) was 20. CR2032 button cells were assembled in an argon-filled glove box. The voltage range was 1.7 to 2.8 V at 0.1 C (1 C = 1675 mAh g −1 ) rate to conduct constant current charge and discharge test on the Xinwei battery test system. The results are as follows Figure 7 shown.

[0065] From the test results of the above embodiments and comparative examples, it can be seen that when HMC is used as a sulfur-carrying material, since HMC has a high specific surface area and abundant mesoporous voids, sulfur is uniformly compounded with it, which greatly enhances the conductivity of the carbon-sulfur positive electrode. Therefore, in Comparative Example 2, at a current density of 0.1 C, the first discharge specific capacity of the HMC-S positive electrode can reach 1182.9 mA h / g. However, after 100 cycles, the discharge capacity rapidly dropped to 631.7 mA h / g, and the average attenuation rate per cycle was 0.44% (based on the second discharge capacity). This is because: (1) The weakly polar HMC carbon material is not very effective for Li 2 S x The adsorption effect of mesoporous carbon shell on Li 2 S x The physical confinement effect of Li 2 S x Dissolved in the electrolyte, the active sulfur is irreversibly lost, and the capacity decays rapidly. 2 After nanomaterials, due to MoO 2 Strong adsorption of Li 2 S x and catalyze their mutual conversion. Therefore, in Comparative Example 3, at a current density of 0.1 C, HMC / p-MoO 2The initial discharge specific capacity of the -S cathode can reach 1169.7 mA h / g. After 100 cycles, the discharge capacity decays to 746.4 mA h / g, and the average decay rate per cycle is 0.33% (based on the second discharge capacity). To expose as many active sites as possible and improve the chemisorption and catalytic conversion efficiency of Li 2 and effectively slow down the shuttle effect of Li 2 S x . Therefore, a composite material with ultrathin MoO 2 nanosheets grown on the surface of mesoporous hollow carbon spheres (HMC) is prepared and used as the sulfur host material, which exhibits more excellent cycle stability in the examples. However, a small amount of MoO x nanosheets have limited active sites. For the HMC@MoO 2 -S-1 cathode in Example 3, the average decay rate per cycle is 0.32% (based on the second discharge capacity). Excessive MoO 2 nanosheets are prone to stacking. For the HMC@MoO 2 -S-2 cathode in Example 4, the average decay rate per cycle is 0.25% (based on the second discharge capacity). Further, for the optimized HMC@MoO 2 -S cathode in Example 2, the initial discharge specific capacity is as high as 1316.3 mA h / g. After 100 cycles, the discharge capacity is still 946.4 mA h / g, and the average decay rate per cycle is only 0.22% (based on the second discharge capacity). In particular, for Example 5, the HMC@MoO 2 -S cathode has an initial areal capacity of 4.48 mAh cm 2 at 0.1 C under the condition of a high areal sulfur loading of 5.0 mg cm 2 . After 500 cycles, its capacity remains at 3.32 mAh cm −2 . -2 . After 500 cycles, its capacity remains at 3.32 mAh cm -2 .

[0066] In addition, the raw materials used for the HMC@MoO 2 composite material only involve 5 small molecule compounds, which are cheap and easy to obtain, and the synthesis steps are simple and easy to operate. No expensive equipment is required, which is suitable for large-scale production and is expected to be applied to the actual production of lithium-sulfur batteries.

Claims

1. A method for preparing a composite material of ultrathin MoO 2 nanosheets in-situ grown on the surface of mesoporous hollow carbon spheres, It is characterized in that It includes the following steps: (1) Preparation of mesoporous hollow carbon sphere precursor SiO 2 @SiO 2 / RF; (2) Dissolve ammonium molybdate tetrahydrate in deionized water, and then add dopamine hydrochloride. The mass ratio of dopamine hydrochloride to ammonium molybdate tetrahydrate is 1:

1. Stir evenly to obtain solution A; Under vigorous stirring, add the mesoporous hollow carbon sphere precursor SiO 2 @SiO 2 / RF obtained in step (1) into ethanol to obtain solution B; Slowly inject solution A into solution B, centrifuge and dry to obtain the HMC@MoO 2 precursor. Among them, the mass ratio of ammonium molybdate tetrahydrate to the mesoporous hollow carbon sphere precursor SiO 2 @SiO 2 / RF is 11~33:500; (3) In an argon atmosphere, the precursor obtained in step (2) is calcined at 600 - 800 °C for 3 - 5 h, and the resulting product is etched to remove the SiO 2 template, and after washing and vacuum drying, the composite material of ultrathin MoO 2 nanosheets in-situ grown on the surface of the mesoporous hollow carbon spheres is obtained; Among them, the mesoporous hollow carbon sphere precursor SiO 2 @SiO 2 / RF is prepared by the following steps: Add ammonia water to the mixed solvent of ethanol and H 2 O, add tetrapropyl orthosilicate under stirring, add resorcinol and formaldehyde after stirring for a period of time, stir at room temperature for more than 24 h, and obtain the mesoporous hollow carbon sphere precursor SiO 2 @SiO 2 / RF after centrifugation, washing and drying.

2. The method according to claim 1, It is characterized in that In step (1), after stirring for 15 minutes, a milky colloidal solution is obtained, and resorcinol and formaldehyde are added.

3. The method according to claim 1, It is characterized in that In step (4), the SiO template is etched away using a 4M NaOH solution. 2 template.

4. A composite material prepared by the method according to any one of claims 1-3.

5. A cathode material for a lithium-sulfur battery, It is characterized in that The composite material prepared by the method according to any one of claims 1-3 is mixed with sublimed sulfur in a mass ratio of x:100-x and ground evenly, where x = 15-25, and then the mixed powder is placed in a high-pressure reaction kettle filled with Ar protection and reacted at 155 °C for 12 h to obtain the cathode material.

6. A cathode material for a high sulfur-loading lithium-sulfur battery, It is characterized in that The composite material prepared by the method according to any one of claims 1-3 is mixed with sublimed sulfur in a mass ratio of 15:85 and ground evenly, and then the mixed powder is placed in a high-pressure reaction kettle filled with Ar protection and reacted at 155 °C for 12 h to obtain a cathode material for a high sulfur-loading lithium-sulfur battery.

Images