Armored catalyst for lithium-sulfur battery and method for preparing the same

Abstract

The present application relates to a kind of " armors " catalysts for lithium-sulfur battery and its preparation method, using the method of hydrothermal synthesis and heat treatment reduction combination, first 3d transition metal catalyst precursor and urea or thiourea are uniformly distributed in graphite oxide dispersion liquid, after mixing, into hydrothermal kettle hydrothermal reaction, then centrifugal cleaning and freeze-drying are obtained Black powder, heat treatment is obtained " armors " catalyst for lithium-sulfur battery to black powder." Armors " catalyst includes armors layer and metal nanoparticles encapsulated in armors layer, armors layer is graphene sheet, and metal nanoparticles are iron or cobalt or nickel nanoparticles.The catalyst of the present application can catalyze Li2S x Conversion, with high catalytic activity and strong stability characteristics, can be directly applied to lithium-sulfur battery separator modification material, also can be directly used as sulfur carrier in lithium-sulfur battery positive electrode, it is expected to solve the technical problems of poor durability of existing lithium-sulfur battery catalyst, with broad application prospect.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium-sulfur catalytic material preparation, in particular to a "armor" catalyst for lithium-sulfur batteries and a preparation method thereof, and belongs to a technical application of a catalytic material prepared by a wet chemical method and a reduction method of heat treatment. BACKGROUND

[0002] Generally, a lithium-sulfur battery is composed of a sulfur positive electrode, a separator, a lithium negative electrode, an electrolyte, and a current collector, and its charge-discharge principle is the back-and-forth conversion of sulfur to polysulfide to lithium sulfide, with an energy density as high as 2600 Wh kg -1 , which is expected to become the next generation of high-energy-density energy storage devices. The discharge process of a lithium-sulfur battery is the reaction of S8 and Li to generate Li2S, and the charging process is the conversion of Li2S to S8, with a theoretical specific capacity of 1673 mA h g -1 . The overall reaction equation is as follows:

[0003]

[0004] The intermediate product in the reaction process is polysulfide Li2S x , and the equation of the reaction process is as follows:

[0005]

[0006] The separator of a lithium-sulfur battery is located between the positive and negative electrodes, and plays a role in blocking electron transmission and conducting ions, and needs to have a certain electrolyte wettability. The intermediate product of a lithium-sulfur battery, liquid polysulfide Li2S x (4≤x≤8), is easily dissolved in the electrolyte, and the kinetics of part of the conversion reaction is poor, so that the polysulfide is generated from the sulfur side and dissolved in the electrolyte, then passes through the separator to reach the lithium side, thereby causing a serious shuttle effect, and finally converted into Li2S and Li2S2 and deposited on the lithium negative electrode side, resulting in low sulfur utilization, poor cycle stability, and safety hazards in lithium-sulfur batteries, which hinders the commercial application of lithium-sulfur batteries. Therefore, it is necessary to introduce an electrocatalyst in a lithium-sulfur battery to reduce the activation energy of the conversion of Li2S x to Li2S, reduce the accumulation and shuttle of Li2S x , improve the conversion of Li2S x to Li2S, and improve the battery capacity, rate performance, and cycle stability.

[0007] Research has found that 3d transition metal nanoparticles have high catalytic activity due to a large number of free electrons and dangling bonds on their surfaces. Loading 3d transition metal nanoparticles on the battery separator or positive electrode material is an effective way.

[0008] Although the research on the application of 3d transition metal nanoparticles as catalysts in lithium-sulfur batteries has made great progress, its industrial application is still not optimistic. Through research of domestic and foreign literature, it is found that only a few catalytic materials can maintain good catalytic activity after more than 500 cycles in lithium-sulfur batteries. Therefore, in the actual application of energy storage devices, most companies prefer to use lithium batteries with lower energy density rather than lithium-sulfur batteries. The reason why most metal catalysts are deactivated during the cycle process is that the metal catalysts are easily deactivated in the corrosion environment, and the surface of the catalyst is easily covered with "dead sulfur" after multiple cycles, causing the catalyst to be deactivated. SUMMARY

[0009] To solve the above problems, the present application provides a "armor" catalyst for lithium-sulfur batteries and a preparation method thereof. The "armor" catalyst is prepared by a combination of hydrothermal synthesis and thermal reduction treatment. The prepared "armor" catalyst can catalyze the conversion of Li2S x (4≤x≤8) to Li2S, which has the characteristics of high catalytic activity and strong stability, can be directly applied to the modification of the separator of lithium-sulfur batteries, and can be directly used as a sulfur carrier for the positive electrode of lithium-sulfur batteries, which is expected to solve the technical problem that the catalytic activity and durability of the existing catalyst for lithium-sulfur batteries are difficult to balance, and has a wide application prospect.

[0010] The present application is realized by the following technical solutions. According to the "armor" catalyst for lithium-sulfur batteries provided by the present application, the "armor" catalyst comprises an armor layer and metal nanoparticles encapsulated in the armor layer. The armor layer is a graphene sheet, and the metal nanoparticles are nanoparticles of iron, cobalt or nickel, which are uniformly distributed between the graphene sheet layers. The catalytic activity of the "armor" catalyst is derived from the perturbation of the local electronic state of the carbon armor layer and the transfer of electrons from the metal nanoparticles to the carbon armor layer, and the catalytic durability is derived from the inherent stability of the carbon armor layer. The "armor" catalyst can place the catalytic reaction system outside the carbon armor layer to avoid direct contact between the encapsulated metal and the reaction system, thereby protecting the internal metal from damage, and the inherent stability of the carbon armor layer can improve the durability of the catalyst during long-term cycling. The encapsulated 3d transition metal nanoparticles can increase the surface electrons of the carbon armor layer and excite its activity, thereby achieving a synergistic improvement in catalytic activity and stability.

[0011] The present application also provides a preparation method of the "armor" catalyst for lithium-sulfur batteries, which specifically comprises the following steps:

[0012] (1) A certain amount of 3d transition metal catalyst precursor and urea or thiourea is weighed and dissolved in distilled water, and after being fully stirred, a solution A is obtained;

[0013] (2) A cell crusher is used to fully crush a certain concentration of graphite oxide dispersion liquid to obtain a solution B;

[0014] (3) While stirring, solution A is added dropwise into solution B and stirred thoroughly, and then a cell crusher is used to crush the mixed solution thoroughly, and the crushed mixed solution is transferred into a Teflon liner of a hydrothermal kettle, the hydrothermal kettle is sealed, and the hydrothermal reaction is carried out in an explosion-proof oven, and the kettle is cooled to room temperature after the reaction is completed;

[0015] (4) The reacted material in the Teflon liner is taken out and placed in a centrifuge tube, and distilled water is used for centrifugal washing 2-6 times, each time for 5-10 min, the supernatant is discarded, and the precipitate is placed in a freeze dryer for freeze drying, and a black powder is obtained after drying;

[0016] (5) The black powder obtained in step (4) is placed in a washed and dried porcelain boat and subjected to heat treatment reduction in a tube furnace, and the final product, a "armor" catalyst for lithium-sulfur batteries, is obtained after heat treatment.

[0017] Further, the 3d transition metal catalyst precursor in step (1) is at least one selected from iron nitrate, iron chloride, cobalt nitrate, cobalt chloride, cobalt acetate, cobalt oxalate, nickel nitrate, and nickel chloride.

[0018] Further, the concentration of the 3d transition metal catalyst precursor in solution A in step (1) is 6.7-33.3 mmol / L, and the concentration of urea or thiourea is 33.3-200 mmol / L. The amount and concentration of raw materials need to be strictly controlled. If the concentration and amount are too small, the catalytic effect is not obvious; on the contrary, if the concentration and amount are too large, the excess catalyst will be stacked and solidified on the outer surface of the stacked layered graphene, which is not conducive to the efficient release of the catalytic performance.

[0019] Further, the concentration of the graphite oxide dispersion liquid in step (2) is 0.5-2 mg / mL, and the crushing time is 0.5-4 h. The purpose of crushing the graphite oxide dispersion liquid here is to uniformly disperse the graphite oxide in distilled water and promote the uniform loading of the catalyst particles.

[0020] Further, the crushing time in step (3) is 5-45 min, the hydrothermal reaction temperature is 120-220 ℃, and the holding time is 8-18 h. The temperature and holding time need to be strictly controlled. If the temperature is too low, the graphene layers cannot be effectively crosslinked, and the catalyst cannot be protected. If the temperature is too high, the graphene layers are severely aggregated, the armor layer is too thick, and the catalyst cannot function.

[0021] Further, in step (5), the heat treatment temperature is 300~800 ℃, the heating rate is 3~10 ℃ / min, the heat treatment time is 2~6 h, and the heat treatment atmosphere is a hydrogen-argon mixture with a hydrogen content of 5~50%. The purpose of heat treatment is mainly to reduce the metal oxide particles obtained after freeze-drying into a single phase with higher catalytic activity. The temperature, heating rate, and holding time of heat treatment need to be strictly controlled. If the temperature is too low, it will be impossible to reduce all the metal oxides into a single metal with higher catalytic activity; if the temperature is too high, it will cause the catalyst particles to agglomerate into larger particles, which is not conducive to the effective performance of the catalyst. Similarly, the heating rate and holding time will also affect the properties and morphology of the product.

[0022] This invention also discloses an "armored" catalyst for lithium-sulfur batteries prepared according to the above method, and its application in lithium-sulfur batteries. The obtained "armored" catalyst for lithium-sulfur batteries comprises an armor layer and metal nanoparticles encapsulated in the armor layer. The armor layer is a graphene sheet, and the metal nanoparticles are iron, cobalt, or nickel nanoparticles, uniformly distributed between the graphene sheets. This "armored" catalyst for lithium-sulfur batteries can be used as a separator modifier for lithium-sulfur batteries, or directly as a sulfur carrier in the positive electrode of lithium-sulfur batteries.

[0023] Compared with existing technologies, this invention has significant advantages and beneficial effects. Through the above technical solution, this invention achieves considerable technological advancement and practicality, and has broad application value, possessing at least the following advantages:

[0024] (1) Current characterization and description of lithium-sulfur battery catalysts mainly focus on battery performance, lacking guidance on catalyst design. This invention introduces an "armored" catalyst into the lithium-sulfur battery catalytic system, utilizing electrocatalytic strategies to precisely control the rate of multi-step sulfur reduction reactions, achieving S8, Li2S x Matching the reaction rate between Li2S and Li2S improves Li2S x (4≤x≤8) The ability to convert to Li2S, reducing Li2S x The accumulation and movement of electrons are crucial for lithium-sulfur batteries. The "armored" catalyst consists of a carbon armor layer and metal nanoparticles encapsulated within it. The carbon armor layer is composed of graphene sheets, and the metal nanoparticles are iron, nickel, or cobalt nanoparticles, uniformly distributed between and covered by the graphene sheets. The catalytic reaction system is placed outside the carbon armor layer, preventing direct contact between the encapsulated metal and the reaction system, thus protecting the internal metal from damage. The inherent stability of the carbon armor layer enhances the catalyst's durability during long-cycle processes. The catalytic activity of the "armored" catalyst originates from the perturbation of local electronic states within the carbon armor layer and the transfer of electrons from the metal nanoparticles to the carbon armor layer; its catalytic durability stems from the inherent stability of the carbon armor layer.

[0025] (2) The present invention uses a combination of hydrothermal synthesis and thermal reduction to encapsulate catalytic phase 3d transition metal nanoparticles (iron, nickel or cobalt nanoparticles) between graphene sheets (armor layers). This not only preserves the stability of the armor layer, but also effectively regulates the electronic structure of the armor layer surface by utilizing the electron transfer between the metal nanoparticles and the armor layer, thereby stimulating its catalytic activity.

[0026] (3) Compared with existing catalysts that are directly exposed to the electrolyte environment, the "armor" catalyst for lithium-sulfur batteries of the present invention has the characteristics of high catalytic activity, stable catalytic performance, long service life and simple preparation process. Applying this highly catalytic catalyst protected by carbon armor as a membrane modification material to lithium-sulfur batteries is expected to solve the technical problem of the difficulty in balancing the catalytic activity and durability of existing lithium-sulfur battery catalysts, and has broad application prospects. Attached Figure Description

[0027] Figure 1 SEM image of the "armored" catalyst for lithium-sulfur batteries prepared in Example 1;

[0028] Figure 2 TEM image of the "armored" catalyst for lithium-sulfur batteries prepared in Example 1;

[0029] Figure 3 The XRD pattern of the "armored" catalyst for lithium-sulfur batteries prepared in Example 1;

[0030] Figure 4 This is an SEM image of the "armored" catalyst for lithium-sulfur batteries prepared in Example 2;

[0031] Figure 5 TEM image of the "armored" catalyst for lithium-sulfur batteries prepared in Example 3;

[0032] Figure 6 The Li2S6 symmetric cell I, assembled with the "armored" catalyst (Ni in RGO) prepared in Example 3, and the Li2S6 symmetric cell II, assembled with the catalyst (Ni out RGO) prepared in the comparative example, were tested at 3 mV s. -1 The following is a comparison chart of CV tests.

[0033] Figure 7 The image shows a comparison of the CV performance of the lithium-sulfur battery using the "armored" catalyst (Ni in RGO) prepared in Example 3 and the catalyst (Ni out RGO) prepared in the comparative example as membrane modification materials.

[0034] Figure 8The long cycle performance comparison chart of lithium-sulfur batteries using the "armor" catalyst (Ni in RGO) prepared for Example 3 and the catalyst (Ni out RGO) prepared for the comparative example as the separator modification material. DETAILED DESCRIPTION

[0035] To make the purposes, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions of the present application will be described clearly and completely below in combination with specific embodiments. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments of the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor fall within the scope of protection of the present application.

[0036] The "armor" catalyst for lithium-sulfur batteries provided by the present application is prepared according to the following method:

[0037] (1) A certain amount of 3d transition metal catalyst precursor and urea or thiourea is weighed and dissolved in distilled water, and solution A is obtained after sufficient stirring;

[0038] Here, the 3d transition metal catalyst precursor is at least one of ferric nitrate, ferric chloride, cobalt nitrate, cobalt chloride, cobalt acetate, cobalt oxalate, nickel nitrate and nickel chloride. Taking 30 mL of distilled water as an example, the amount of 3d transition metal catalyst precursor is 0.2-1 mmol, and the content of urea or thiourea is 1-6 mmol, that is, the concentration of 3d transition metal catalyst precursor in solution A is 6.7-33.3 mmol / L, and the concentration of urea or thiourea is 33.3-200 mmol / L. The amount and concentration of raw materials need to be strictly controlled. If the concentration and amount are too small, the catalytic effect will not be obvious; on the contrary, if the concentration and amount are too large, the excess catalyst will be stacked and solidified on the outer surface of the stacked layered graphene, which is not conducive to the efficient release of catalytic performance.

[0039] (2) A cell crusher is used to fully crush the oxidized graphite dispersion liquid of a certain concentration.

[0040] The purpose of crushing the oxidized graphite dispersion liquid here is to uniformly disperse the oxidized graphite in the distilled water and promote the uniform loading of the catalyst particles in the subsequent process. The concentration of the oxidized graphite dispersion liquid is 0.5-2 mg / mL, and the crushing time is 0.5-4 h.

[0041] (3) While stirring, the solution A obtained in step (1) is added dropwise into the pulverized graphite oxide dispersion liquid in step (2) and stirred for 0.5-3 h, and then the mixed solution is pulverized by a cell pulverizer, and then the pulverized solution is transferred into a Teflon liner of a stainless steel hydrothermal kettle, the hydrothermal kettle is tightly sealed, and the hydrothermal reaction is carried out in an explosion-proof oven, and the kettle is cooled to room temperature after the reaction is completed.

[0042] The purpose of adding solution A dropwise while stirring the graphite oxide dispersion liquid is to uniformly distribute the metal catalytic phase in the graphite oxide dispersion liquid.

[0043] The pulverization time is 5-45 min, and the purpose of pulverization is to uniformly distribute the metal catalytic phase between the graphite layers.

[0044] The hydrothermal temperature is 120-220 °C, and the hydrothermal holding time is 8-18 h. The temperature and holding time need to be strictly controlled. If the temperature is too low, the graphene layers cannot be effectively crosslinked, and the catalyst cannot be protected. If the temperature is too high, the graphene layers are severely aggregated, and the armor layer is too thick, which is not conducive to the function of the catalyst. Similarly, the holding time also affects the morphology and catalytic performance of the product.

[0045] (4) The reacted material is taken out from the Teflon liner and placed in a centrifuge tube, and washed with distilled water by centrifugation, the supernatant is discarded, and the precipitate is placed in a freeze dryer and freeze-dried for 20-60 h. A black powder is obtained after drying.

[0046] The centrifugation is performed 2-6 times, the centrifuge speed is 4000-8000 rpm, and the centrifugation time is 5-10 min each time. The purpose of centrifugal washing is to wash away the useless metal salt solution, and the purpose of freeze-drying is to remove the water in the sample. The number of centrifugal washing and the speed need to be strictly controlled. If the number of centrifugal washing is too much, the catalyst particles that are not anchored will be washed away during centrifugation. If the number of centrifugal washing is too small, the salt solution cannot be washed clean. Similarly, the speed and time of centrifugation also affect the morphology of the product catalyst.

[0047] (5) The black powder obtained in step (4) is placed in a clean and dry porcelain boat and placed in a tube furnace for heat treatment reduction, and the final product, a "armor" catalyst for lithium-sulfur batteries, is obtained after heat treatment.

[0048] The temperature of the heat treatment is 300-800 DEG C, the temperature rising rate is 3-10 DEG C / min, the heat treatment time is 2-6 h, and the heat treatment atmosphere is a hydrogen-argon mixed gas with a hydrogen content of 5-50%. The purpose of the heat treatment is mainly to reduce the metal oxide particles obtained after freeze-drying into elemental phases with higher catalytic activity. The temperature, temperature rising rate and holding time of the heat treatment need to be strictly controlled. If the temperature is too low, all the metal oxides cannot be reduced into elemental metals with higher catalytic activity. If the temperature is too high, the catalyst particles will be bonded and agglomerated into larger particles, which is not conducive to the effective performance of the catalyst. Similarly, the temperature rising rate and holding time will also affect the properties and morphology of the product.

[0049] The application further discloses a "armor" catalyst for lithium-sulfur batteries prepared by the method and application of the "armor" catalyst in lithium-sulfur batteries. The obtained "armor" catalyst for lithium-sulfur batteries comprises an armor layer and metal nanoparticles encapsulated in the armor layer. The armor layer is a graphene sheet layer, and the metal nanoparticles are iron or cobalt or nickel nanoparticles. The metal nanoparticles are uniformly distributed between the graphene sheet layers and covered by the graphene sheet layers. The encapsulated 3d transition metal nanoparticles can increase the surface electrons of the armor layer, stimulate the activity of the armor layer, and thus realize the synergistic improvement of catalytic activity and stability. The "armor" catalyst for lithium-sulfur batteries can be used as a lithium-sulfur battery separator modification material, and can also be directly used as a sulfur carrier in a lithium-sulfur battery positive electrode.

[0050] In the preparation process, the "armor" catalyst is prepared by controlling raw materials, hydrothermal conditions and heat treatment conditions, so that the obtained "armor" catalyst has a microstructure in which metal elemental catalytic phases are uniformly distributed and covered by thin-layer graphene sheet layers. The encapsulated metal and the reaction system can be prevented from directly contacting each other, so that the internal metal is protected from damage, the catalytic reaction can be fully carried out, and the service life of the catalyst is significantly improved.

[0051] The application will be described in detail below with reference to specific examples.

[0052] Example 1

[0053] (1) 0.6 mmol (0.175 g) of nickel nitrate hexahydrate and 3 mmol (0.18 g) of urea were weighed and dissolved in 30 mL of distilled water to obtain solution A.

[0054] (2) A cell crusher was used to fully crush 50 mL of a concentrated graphite oxide dispersion solution with a concentration of 2 g / mL.

[0055] (3) While stirring, the solution A obtained in step (1) is added dropwise into the graphite oxide dispersion solution of step (2) and stirred thoroughly for 1 h, and then the mixed solution is crushed using a cell crusher for 45 min, and the crushed solution is transferred into a polytetrafluoroethylene liner in a 100 mL stainless steel hydrothermal kettle, the hydrothermal kettle is tightly sealed, and the hydrothermal reaction is carried out in an explosion-proof oven, the temperature of the hydrothermal reaction is 120 ℃, and the time is 12 h, and the reaction kettle is cooled to room temperature after the reaction is completed.

[0056] (4) The reacted material is taken out from the polytetrafluoroethylene liner and placed in a centrifuge tube, and washed 4 times by centrifugation using distilled water, the centrifugal speed is 6000 rpm, and the centrifugal time is 10 min each time. After centrifugation, the supernatant is discarded, and the precipitate is placed in a freeze dryer for freeze drying, and a black powder is obtained after drying.

[0057] (5) The black powder obtained in step (4) is placed in a washed and dried porcelain boat and subjected to heat treatment reduction in a tube furnace, the heat treatment temperature is 500 ℃, the heating rate is 2 ℃ / min, the heat treatment time is 4 h, and the heat treatment atmosphere is a hydrogen-argon mixed gas with a hydrogen content of 5%, and the final product, a “armor” catalyst for lithium-sulfur batteries, is obtained after heat treatment.

[0058] The “armor” catalyst for lithium-sulfur batteries prepared in this example is micro-characterized, and the SEM and TEM images thereof are shown in Figure 1 and Figure 2 It can be seen from Figure 1 and Figure 2 that the “armor” catalyst for lithium-sulfur batteries prepared in this example is composed of uniformly distributed nickel nanoparticles and thin graphene sheet layers covering the nickel nanoparticles.

[0059] The “armor” catalyst for lithium-sulfur batteries prepared in this example is subjected to phase analysis, and the results are shown in Figure 3 It can be seen from Figure 3 that the XRD characteristic peaks of the obtained “armor” catalyst for lithium-sulfur batteries correspond to elemental Ni (PDF#04-0850) and graphene, which proves that the main component of the “armor” catalyst for lithium-sulfur batteries is elemental nickel, the elemental nickel contributes to the catalytic activity as a catalytic phase, and the graphene plays a protective role for the elemental nickel.

[0060] Example 2:

[0061] (1) 1 mmol (0.404 g) of iron nitrate nonahydrate and 3 mmol (0.18 g) of urea are dissolved in 30 mL of distilled water to obtain solution A after thorough stirring.

[0062] (2) The 50 mL of the oxidized graphite dispersion liquid with a concentration of 2 g / mL is fully pulverized by using a cell pulverizer.

[0063] (3) The solution A obtained in step (1) is added dropwise into the oxidized graphite dispersion liquid of step (2) under stirring, and fully stirred for 1.5 h, and then the mixed solution is pulverized for 15 min by using a cell pulverizer, and the pulverized solution is transferred into a polytetrafluoroethylene liner of a 100 mL stainless steel hydrothermal kettle, the hydrothermal kettle is tightly sealed, and is placed in an explosion-proof oven for hydrothermal reaction, the temperature of the hydrothermal reaction is 120 ℃, and the time is 12 h, and the reaction kettle is cooled to room temperature after the reaction.

[0064] (4) The material after the reaction is taken out from the polytetrafluoroethylene liner and is placed in a centrifugal tube, and is cleaned by using distilled water for centrifugation for 5 times, the centrifugal speed is 8000 rpm, and the centrifugation time is 10 min each time. After the centrifugation, the supernatant is discarded, and the precipitate is placed in a freeze dryer for freeze drying, and the black powder is obtained after the drying.

[0065] (5) The black powder obtained in step (4) is placed in a washed and dried porcelain boat and is placed in a tube furnace for heat treatment reduction, the heat treatment temperature is 700 ℃, the heating rate is 2 ℃ / min, the heat treatment time is 4 h, and the heat treatment atmosphere is a hydrogen argon mixed gas with a hydrogen content of 10%, and the final product, the “armor” catalyst for lithium-sulfur batteries, is obtained after the heat treatment.

[0066] The “armor” catalyst for lithium-sulfur batteries prepared in the embodiment is subjected to microscopic characterization, and the results are shown in Figure 4 It can be known from Figure 4 that the “armor” catalyst for lithium-sulfur batteries prepared in the embodiment is composed of uniformly distributed iron nanoparticles and thin graphene sheet layers covering the iron nanoparticles.

[0067] Example 3:

[0068] (1) 1 mmol (0.291 g) of nickel nitrate hexahydrate and 3 mmol (0.18 g) of urea are weighed and dissolved in 30 mL of distilled water to obtain solution A after fully stirring.

[0069] (2) The 50 mL of the oxidized graphite dispersion liquid with a concentration of 2 g / mL is fully pulverized by using a cell pulverizer.

[0070] (3) While stirring, the solution A obtained in step (1) was added dropwise into the graphite oxide dispersion solution of step (2) and stirred thoroughly for 1.5 h, and then the mixed solution was crushed using a cell crusher for 35 min. The crushed solution was transferred into a polytetrafluoroethylene liner of a 100 mL stainless steel hydrothermal kettle, the hydrothermal kettle was sealed, and the hydrothermal reaction was performed in an explosion-proof oven. The temperature of the hydrothermal reaction was 120 °C, and the time was 12 h. After the reaction, the reaction kettle was cooled to room temperature.

[0071] (4) The material after the reaction was taken out from the polytetrafluoroethylene liner and placed in a centrifuge tube. Distilled water was used for centrifugal washing 6 times at a centrifugal speed of 8000 rpm, and the centrifugal time was 10 min each time. After centrifugation, the supernatant was discarded, and the precipitate was placed in a freeze dryer for freeze drying. After drying, a black powder was obtained.

[0072] (5) The black powder obtained in step (4) was placed in a clean and dry porcelain boat and subjected to heat treatment reduction in a tube furnace. The heat treatment temperature was 500 °C, the heating rate was 2 °C / min, the heat treatment time was 4 h, and the heat treatment atmosphere was a hydrogen argon mixed gas with a hydrogen content of 5%. The final product, a "armor" catalyst for lithium-sulfur batteries (denoted as Ni in RGO catalyst), was obtained after heat treatment.

[0073] Comparative Example

[0074] (1) 3 mmol (0.18 g) of urea was dissolved in 30 mL of distilled water to obtain a urea solution.

[0075] (2) A cell crusher was used to crush 50 mL of a graphite oxide dispersion solution with a concentration of 2 g / mL.

[0076] (3) While stirring, the urea solution obtained in step (1) was added dropwise into the graphite oxide dispersion solution of step (2) and stirred thoroughly for 1.5 h. Then the mixed solution was crushed using a cell crusher for 35 min. The crushed solution was transferred into a polytetrafluoroethylene liner of a 100 mL stainless steel hydrothermal kettle, the hydrothermal kettle was sealed, and the hydrothermal reaction was performed in an explosion-proof oven. The temperature of the hydrothermal reaction was 120 °C, and the time was 12 h. After the hydrothermal reaction, 1 mmol (0.291 g) of nickel nitrate hexahydrate was dissolved in the product after the hydrothermal reaction and subjected to a second hydrothermal reaction. The temperature of the second hydrothermal reaction was 120 °C, and the holding time was 12 h.

[0077] The remaining steps were the same as steps (4) and (5) of Example 3, and finally a Ni out RGO catalyst was obtained, in which elemental nickel was loaded outside the graphene sheet.

[0078] The lithium-sulfur battery prepared in this embodiment was characterized by TEM electron microscopy using the "armored" catalyst, and the results are as follows: Figure 5 As shown. By Figure 5 It can be seen that the catalytically active nickel element is uniformly distributed in fine particles between the graphene sheets, laying the foundation for good catalytic activity and durability.

[0079] To test the catalytic performance of the "armored" catalyst prepared in this invention on liquid Li2S... x To demonstrate the conversion capability to solid Li₂S, the Ni in RGO catalyst prepared in this embodiment was ultrasonically dispersed in alcohol and then dropped onto carbon paper with a diameter of 12 mm. After drying at 60°C, it served as two electrodes, with Celgard 2500 as the separator. 25 µL of 0.5 mol / L Li₂S₆ electrolyte was added to each side to assemble a symmetrical cell I. The Li₂S₆ electrolyte preparation process was as follows: S and Li₂S were mixed and stirred at a molar ratio of 1:5, and then a lithium-sulfur electrolyte was added. The electrolyte formulation consisted of dissolving lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in a DME / DOL solvent with a volume ratio of 1:1. The concentration of LiTFSI in the DME / DOL solvent was 1 mol / L. The mixture was heated and stirred at 70°C for 12 h. To demonstrate the advantages of the "armor" protective layer in the Ni in RGO catalyst prepared in this invention, a symmetrical cell II was fabricated by loading the Ni outRGO catalyst prepared in the comparative example onto carbon paper using the same method described above. Cyclic voltammetry (CV) tests were performed on symmetrical cells I and II using an electrochemical workstation, i.e., a constant electrode potential rate (3 mV / s) was applied within a certain voltage range. -1 Perform one or more cyclic scans and record the curves of current versus voltage. The results are as follows: Figure 6 As shown, in the 5th cycle, the Ni in RGO electrode exhibits a narrower peak spacing and its peak current is slightly higher than that of the Ni out RGO electrode. With increasing test cycles, the peak current of the CV curve for the Ni out RGO electrode gradually decreases, indicating that the catalyst exposed outside the graphene sheets gradually degrades during the test. Conversely, the peak current of the CV curve for the Ni in RGO electrode does not decrease, suggesting that the graphene armor helps maintain the catalyst's activity.

[0080] In order to test the effect of the "armor" catalyst prepared by the present application on the redox reaction process in the lithium-sulfur battery, the lithium-sulfur battery prepared in the present example is loaded with the "armor" catalyst (denoted as Ni in RGO catalyst) as the separator modification material on the lithium-sulfur battery separator, and a blue light test system and an electrochemical workstation are used for demonstration. The lithium-sulfur electrolyte used is LiTFSI and LiNO3 as solutes, and the solvent is DME / DOL solvent in a volume ratio of 1:1. The concentration of LiTFSI in the electrolyte is 1 mol / L, and the mass percentage of LiNO3 in the electrolyte is 2%. The sulfur-carbon composite, Super P and PVDF are mixed in a mass ratio of 8:1:1, and after adding NMP solvent, stirring for 6 h. The stirring completed slurry is coated on a carbon-coated aluminum foil and dried, the drying temperature is 60°C, and the time is 12 h. The dried electrode sheet is punched, the diameter is 12 mm, and the electrode sheet mass is 0.6-1 mg / cm 2 . The electrode sheet is used as the positive electrode, lithium sheet as the negative electrode, Celgard 2500 modified with the Ni in RGO catalyst prepared in Example 3 of the present application as the separator to assemble lithium-sulfur battery I for testing.

[0081] In order to prove the advantage of the "armor" protection layer in the Ni in RGO catalyst prepared by the present application, the Ni out RGO catalyst prepared in the comparative example is used as the separator modification material to make lithium-sulfur battery II according to the same method as above. The separator modification materials of lithium-sulfur battery I and lithium-sulfur battery II are different. First, the CV test is performed on the two lithium-sulfur batteries to study the effect of the separator modified with different catalysts on the redox reaction process in the lithium-sulfur battery. The results are shown in Figure 7 . By comparing the CV curves of the two batteries, it can be found that compared with the battery using Ni out RGO modified separator, the lithium-sulfur battery using Ni in RGO modified separator has larger peak current value, narrower redox peak and smaller redox peak interval, which indicates that the "armor" catalyst prepared by the present application as the lithium-sulfur battery separator modification material can significantly accelerate the conversion of lithium polysulfide and improve the sulfur utilization rate. The charging and discharging test of the two lithium-sulfur button cells at 0.5C rate can obtain the long cycle performance of the battery, and the results are shown in Figure 8 . It can be seen that the attenuation rate of the battery using Ni in RGO modified separator after 1100 cycles is only 0.0418% per cycle, while the attenuation rate of the battery using Ni out RGO modified separator after 1100 cycles is as high as 0.0457% per cycle. It shows that the "armor" catalyst prepared by the present application as the lithium-sulfur battery separator catalyst can improve the utilization rate of active sulfur and is beneficial to the construction of long cycle life lithium-sulfur battery.

[0082] In other embodiments, the 3d transition metal catalyst precursor can also be selected from any one or more of ferric chloride, cobalt nitrate, cobalt chloride, cobalt acetate, cobalt oxalate, nickel chloride, and urea can be replaced by thiourea to finally prepare an iron nanoparticle or cobalt nanoparticle encapsulated "armor" catalyst for lithium-sulfur batteries, which will not be described here.

[0083] The above description is only individual embodiments of the present application, and does not limit the present application in any form. The present application can have other forms of embodiments according to the above features, which will not be listed one by one. Any skilled person in the art, without departing from the scope of the technical solutions of the present application, according to the technical essence of the present application, any simple modification, equivalent change and modification of the above embodiments, still belongs to the scope of the technical solutions of the present application.

Claims

1. A method for preparing an armoured catalyst for lithium-sulfur batteries, characterized in that The method comprises the following steps: (1) a certain amount of 3d transition metal catalyst precursor, urea or thiourea is weighed and dissolved in distilled water to obtain solution A after sufficient stirring; the concentration of the 3d transition metal catalyst precursor in solution A is 6.7-33.3 mmol / L, and the concentration of urea or thiourea is 33.3-200 mmol / L; the 3d transition metal catalyst precursor is at least one selected from the group consisting of ferric nitrate, ferric chloride, cobalt nitrate, cobalt chloride, cobalt acetate, cobalt oxalate, nickel nitrate and nickel chloride; (2) a cell crusher is used to sufficiently crush an oxidized graphite dispersion liquid with a concentration of 0.5-2 mg / mL to obtain solution B; (3) solution A is added dropwise into solution B while stirring and fully stirring, and then a cell crusher is used to sufficiently crush the mixed solution, and the crushed mixed solution is transferred to a polytetrafluoroethylene lining in a stainless steel hydrothermal kettle, the hydrothermal kettle is sealed, and the hydrothermal reaction is carried out in an explosion-proof oven, and the reaction kettle is cooled to room temperature after the reaction; (4) the reacted material in the polytetrafluoroethylene lining is taken out and placed in a centrifugal tube, and distilled water is used for centrifugal washing 2-6 times, each time for 5-10 min, the supernatant is discarded, and the precipitate is placed in a freeze dryer for freeze drying, and a black powder is obtained after drying; (5) the black powder obtained in step (4) is placed in a clean and dry porcelain boat and placed in a tube furnace, and is reduced by heat treatment under a hydrogen argon mixed gas atmosphere with a hydrogen content of 5-50% at 300-800 ℃, the heating rate is 3-10 ℃ / min, the heat treatment time is 2-6 h, and the final product, an armor catalyst for lithium-sulfur batteries, is obtained after heat treatment, the armor catalyst comprises an armor layer and metal nanoparticles encapsulated in the armor layer, the armor layer is a graphene sheet layer, and the metal nanoparticles are iron or cobalt or nickel nanoparticles, which are uniformly distributed between the graphene sheet layers and covered by the graphene sheet layers.

2. The method for preparing an armoring catalyst for lithium-sulfur batteries according to claim 1, characterized by The crushing time of the oxidized graphite dispersion liquid in step (2) is 0.5-4 h.

3. The method of claim 1, wherein the preparation of the armored catalyst for lithium-sulfur batteries is characterized by The crushing time in step (3) is 5-45 min, the hydrothermal reaction temperature is 120-220 ℃, and the holding time is 8-18 h.

4. The armor catalyst for lithium-sulfur batteries obtained by the preparation method of claim 1.

5. The application of the armor catalyst for lithium-sulfur batteries obtained by the preparation method of claim 1 in a lithium-sulfur battery separator modifier, or the direct application of the armor catalyst for lithium-sulfur batteries as a sulfur carrier in a lithium-sulfur battery positive electrode.

Images