Preparation method of nano sulfur / reduced graphene oxide composite self-supporting lithium sulfur battery positive electrode material

Abstract

The application discloses a preparation method of a nano sulfur / reduced graphene oxide composite self-supporting lithium sulfur battery positive electrode material, and comprises the following steps: dissolving elemental sulfur and graphene oxide in an organic solvent, adding an alkali source and a reducing agent, performing a graphene oxide reduction reaction and sulfur cooling crystallization deposition, and obtaining a nano sulfur particle loaded reduced graphene oxide composite material; dispersing the nano sulfur particle loaded reduced graphene oxide composite material, reduced graphene oxide, conductive carbon black and carbon fiber in an organic solvent, mixing, washing, and performing membrane extraction to obtain the nano sulfur / reduced graphene oxide composite self-supporting lithium sulfur battery positive electrode material. When the nano sulfur particle and the reduced graphene oxide composite self-supporting flexible electrode material are applied to a lithium sulfur battery, the lithium sulfur battery has high specific capacity, high surface capacity, high charging and discharging coulomb efficiency and a long cycle life.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, and particularly relates to a preparation method of a nano sulfur / reduced graphene oxide composite self-supporting lithium sulfur battery positive electrode material. BACKGROUND

[0002] The lithium sulfur battery uses elemental sulfur as a positive active material and metal lithium as a negative active material, and is a new type of electrochemical energy storage element; the theoretical specific capacity of elemental sulfur is 1675 mAh g -1 , and the theoretical specific energy is 2600 Wh kg -1 , which is superior to traditional lithium ion batteries. The lithium sulfur battery is environmentally friendly, has high energy density, and has low raw material prices, and is one of the research hot topics in the field of electrochemical energy storage.

[0003] Similar to traditional lithium batteries, the lithium sulfur battery is composed of a positive electrode, a negative electrode, an electrolyte and a separator, and the structure and performance of the positive electrode material have a decisive influence on the performance of the lithium sulfur battery. Because sulfur itself and the discharge product have low conductivity, the intermediate product of charging and discharging is soluble, and the volume changes dramatically during the charging and discharging process, so it cannot be directly used as an electrode material; the conventional method is to melt and composite elemental sulfur and a conductive host material (commonly used as a carbon-based material), but this method takes too long, and the obtained product is difficult to directly use as an electrode material. At the same time, the active material loading of the lithium sulfur battery needs to be considered when it is put into (market) application, and the electrode active material loading of the conventional coating method is below 5 mg cm -2 , but the addition of conductive agents and binders will reduce the active material content.

[0004] At present, most of the research on lithium sulfur batteries is limited to laboratory research level. Therefore, it is necessary to develop a lithium sulfur battery positive electrode material with simple preparation process, low cost and suitable for application. SUMMARY

[0005] In view of the technical problems existing in the prior art, the present application aims to provide a preparation method of a nano sulfur / reduced graphene oxide composite self-supporting lithium sulfur battery positive electrode material.

[0006] In order to achieve the above-mentioned purpose, the present application adopts the following technical scheme:

[0007] The first aspect of the present application provides a preparation method of a nano sulfur / reduced graphene oxide composite self-supporting lithium sulfur battery positive electrode material, comprising the following steps:

[0008] S1, dissolving elemental sulfur and graphene oxide in an organic solvent, adding an alkali source and a reducing agent, and performing a graphene oxide reduction reaction and sulfur cooling crystallization deposition to obtain a nano sulfur particle loaded reduced graphene oxide composite material;

[0009] S2, preparing reduced graphene oxide;

[0010] S3, dispersing the nano-sulfur particle loaded reduced graphene oxide composite material, reduced graphene oxide, conductive carbon black and carbon fiber in an organic solvent, mixing, then washing, suction filtering to form a film to obtain a nano-sulfur / reduced graphene oxide composite self-supporting lithium-sulfur battery positive electrode material.

[0011] Preferably, the amount of the elemental sulfur, the graphene oxide, the alkali source and the reducing agent is 10-1g:1g:4ml:2ml.

[0012] Preferably, the reduction reaction temperature is 80-120℃, and the reaction time is 5-10h.

[0013] Preferably, the preparation step of the reduced graphene oxide is: dissolving the graphene oxide in an organic solvent, adding an alkali source and a reducing agent, and performing a reduction reaction to obtain the reduced graphene oxide, which can also be obtained by conventional technical means in the art.

[0014] Preferably, the reduction reaction temperature is 90-100℃, and the reaction time is 2-3h.

[0015] Preferably, the organic solvent is selected from any one of N,N-dimethylformamide, N-methylpyrrolidone or dimethyl sulfoxide.

[0016] Preferably, the elemental sulfur is sublimed sulfur, the reducing agent is hydrazine hydrate, and the alkali source is ammonia.

[0017] Preferably, the mass ratio of sulfur, reduced graphene oxide, conductive carbon black and carbon fiber in the nano-sulfur particle loaded reduced graphene oxide composite material is (50-75):(30-13):(10-6):(10-6).

[0018] The second aspect of the present application provides a nano-sulfur / reduced graphene oxide composite self-supporting lithium-sulfur battery positive electrode material obtained by the above preparation method.

[0019] The third aspect of the present application provides a lithium-sulfur battery comprising the above nano-sulfur / reduced graphene oxide composite self-supporting lithium-sulfur battery positive electrode material.

[0020] Compared with the prior art, the present application has the following beneficial effects:

[0021] (1) The application utilizes organic solution assisted preparation of nano sulfur / reduced graphene oxide composite self-supporting lithium sulfur battery positive electrode material, increases reaction activity, reaction site and sulfur content; and obtains lithium sulfur battery electrode material with high specific capacity, high surface capacity, low resistance, high charge-discharge coulomb efficiency and the like, solves the problems of low loading, poor performance, difficult preparation, long process, low yield, high cost, complex preparation process and difficult large-scale production of existing lithium sulfur batteries, especially graphene-based lithium sulfur battery electrode materials;

[0022] (2) The application prepares nano sulfur particles with high sulfur content and high sulfur loading and reduced graphene oxide composite self-supporting electrode material by chemical reduction method, overcomes the shortcomings of low sulfur content and low sulfur surface loading of traditional coating process electrode material; the electrode material does not need to add adhesive, so the resistance can be effectively reduced, and the electrode material only contains three elements of carbon, oxygen and sulfur, which is environment-friendly, the lithium sulfur battery prepared by taking the electrode material as a positive electrode and lithium sheet as a negative electrode has good charge-discharge performance, high energy density, and meets the development trend of promoting the application of lithium sulfur battery. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 The scanning electron microscope test result graph of the nano sulfur / reduced graphene oxide composite self-supporting lithium sulfur battery positive electrode material prepared for Example 1;

[0024] Figure 2 The transmission electron microscope test result graph of the nano sulfur / reduced graphene oxide composite self-supporting lithium sulfur battery positive electrode material prepared for Example 1;

[0025] Figure 3 The element distribution test result graph of the nano sulfur / reduced graphene oxide composite self-supporting lithium sulfur battery positive electrode material prepared for Example 1;

[0026] Figure 4 The rate characteristic test result graph of the nano sulfur / reduced graphene oxide composite self-supporting lithium sulfur battery positive electrode material prepared for Example 1;

[0027] Figure 5 The cycle charge-discharge test result graph of the nano sulfur / reduced graphene oxide composite self-supporting lithium sulfur battery positive electrode material prepared for Example 1;

[0028] Figure 6 The transmission electron microscope test result graph of the nano sulfur / reduced graphene oxide composite self-supporting lithium sulfur battery positive electrode material prepared for Example 2;

[0029] Figure 7 The transmission electron microscope test result graph of the nano sulfur / reduced graphene oxide composite self-supporting lithium sulfur battery positive electrode material prepared for Example 3;

[0030] Figure 8 A transmission electron microscope test result graph of the nano-sulfur / reduced graphene oxide composite self-supporting lithium sulfur battery cathode material prepared for Example 4;

[0031] Figure 9 A rate characteristic test result graph and a cycle charge-discharge test result graph of the electrode material prepared for Comparative Example 1;

[0032] Figure 10 A rate characteristic test result graph and a cycle charge-discharge test result graph of the electrode material prepared for Comparative Example 2;

[0033] Figure 11 A rate characteristic test result graph and a cycle charge-discharge test result graph of the electrode material prepared for Comparative Example 3;

[0034] Figure 12 A scanning electron microscope test result graph of the electrode material prepared for Comparative Example 4;

[0035] Figure 13 A cycle charge-discharge test result graph of the electrode material prepared for Comparative Example 4;

[0036] Figure 14 A scanning electron microscope test result graph of the electrode material prepared for Comparative Example 5;

[0037] Figure 15 A cycle charge-discharge test result graph of the electrode material prepared for Comparative Example 5;

[0038] Figure 16 A size distribution result statistical graph of the nano-sulfur particles in the nano-sulfur / reduced graphene oxide composite self-supporting lithium sulfur battery cathode material prepared for Examples 1-4. DETAILED DESCRIPTION

[0039] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application.

[0040] Example 1

[0041] (1) 0.2 g of graphene oxide and 1.75 g of sublimed sulfur were weighed and added into 300 ml of N,N-dimethylformamide to be stirred until completely dissolved into a uniform solution. Then, 0.8 ml of ammonia water and 0.4 ml of hydrazine hydrate were added into the above solution to be stirred into a uniform solution. A reduction reaction was performed at 95℃ for 2 h, and then the solution was naturally cooled to obtain a reduced graphene oxide composite material loaded with nano-sulfur particles.

[0042] (2) Weigh 0.1 g of graphene oxide, add it to 100 ml of N,N-dimethylformamide and stir until it is completely dissolved to form a uniform solution. Then, add 0.4 ml of ammonia water (concentration is 30%) and 0.2 ml of hydrazine hydrate (concentration is 80%) to the above solution and stir to form a uniform solution. The reduction reaction is carried out at 95°C for 2 hours and the temperature is naturally lowered to obtain reduced graphene oxide.

[0043] (3) The reduced graphene oxide composite material loaded with nano-sulfur particles was filtered and washed three times with a cleaning solution, and then dispersed with 0.15 g of conductive carbon black, 0.15 g of carbon fiber, and 0.1 g of reduced graphene oxide in an organic solvent and mixed. The mixture was then washed, filtered and formed into a film according to the required loading amount, and freeze-dried to obtain a nano-sulfur / reduced graphene oxide composite self-supporting lithium-sulfur battery positive electrode material.

[0044] The nano-sulfur / reduced graphene oxide composite self-supporting lithium-sulfur battery positive electrode material prepared in Example 1 was subjected to scanning electron microscopy, transmission electron microscopy, and element distribution characterization tests. The results are as follows Figure 1 、 Figure 2 and Figure 3 .

[0045] Depend on Figure 1 The results show that in the nano-sulfur / reduced graphene oxide composite self-supporting lithium-sulfur battery positive electrode material prepared in Example 1, the morphology of the reduced graphene oxide material is well maintained without agglomeration, and the conductive carbon black and carbon fiber are evenly distributed therein.

[0046] Depend on Figure 2 The results show that in the nanosulfur / reduced graphene oxide composite self-supporting lithium-sulfur battery positive electrode material prepared in Example 1, the nanosulfur particles are evenly distributed on the reduced graphene oxide sheet, mainly showing a size distribution of 10-70 nanometers, the nanoparticles are mainly quasi-spherical, and the boundaries between the particles are clear.

[0047] Depend on Figure 3 The results show that, at the elemental level, the nano-sulfur / reduced graphene oxide composite self-supporting lithium-sulfur battery positive electrode material prepared in Example 1 can achieve a uniform degree as a whole, and there is no phenomenon of concentrated element distribution in some areas, indicating that the entire electrode material has good elemental uniformity, and has good uniformity both at the macroscopic and microscopic scales, proving that Example 1 has successfully prepared the target positive electrode material.

[0048] The obtained nanosulfur / reduced graphene oxide composite self-supporting lithium-sulfur battery cathode material was assembled into a lithium-sulfur battery 2032 button cell, using a lithium sheet as the counter electrode, 1M LiTFSI (solvent DME and DOL volume ratio 1:1, added 2% LiNO3) as the electrolyte, and a loading of 2 mg cm -2The magnification test results are as follows Figure 4 , the loading amount is 8 mg·cm -2 The cycle test results are as follows Figure 5 .

[0049] Example 2

[0050] (1) Weigh 0.2 g of graphene oxide and 1.75 g of sublimed sulfur, add them to 200 ml of N,N-dimethylformamide and stir until they are completely dissolved into a uniform solution. Then, add 0.8 ml of ammonia water and 0.4 ml of hydrazine hydrate to the above solution and stir until it becomes a uniform solution. Reduce the solution at 95 ° C for 2 h and cool it naturally to obtain a reduced graphene oxide composite material loaded with nano-sulfur particles;

[0051] (2) Weigh 0.1 g of graphene oxide and add it to 100 ml of N,N-dimethylformamide, stirring until it is completely dissolved into a uniform solution. Then, add 0.4 ml of ammonia water and 0.2 ml of hydrazine hydrate to the above solution and stir to form a uniform solution. Perform a reduction reaction at 95 ° C for 2 h, and cool naturally to obtain reduced graphene oxide.

[0052] (3) The reduced graphene oxide composite material loaded with nano-sulfur particles was filtered and washed three times with a cleaning solution; then dispersed with 0.15g of conductive carbon black, 0.15g of carbon fiber, and 0.1g of reduced graphene oxide in an organic solvent, mixed, washed, filtered according to the required loading amount to form a film, and freeze-dried to obtain a nano-sulfur / reduced graphene oxide composite self-supporting lithium-sulfur battery positive electrode material. The transmission electron microscope photo is shown as follows: Figure 6 As shown by Figure 6 The results show that the nanosulfur particles are evenly distributed on the reduced graphene oxide sheets, mainly showing a size distribution of 10-70 nanometers. The nanoparticles are mainly quasi-spherical, and the boundaries between the particles are clear, thus proving that the target positive electrode material has been successfully prepared in this example.

[0053] Example 3

[0054] (1) Weigh 0.2 g of graphene oxide and 1.75 g of sublimed sulfur, add them to 400 ml of N,N-dimethylformamide and stir until they are completely dissolved into a uniform solution. Then, add 0.8 ml of ammonia water and 0.4 ml of hydrazine hydrate to the above solution and stir until it becomes a uniform solution. The reduction reaction is carried out at 95 ° C for 2 h, and the temperature is naturally cooled to obtain a reduced graphene oxide composite material loaded with nanosulfur particles.

[0055] (2) Weigh 0.1 g of graphene oxide and add it to 100 ml of N,N-dimethylformamide, stirring until it is completely dissolved into a uniform solution. Then, add 0.4 ml of ammonia water and 0.2 ml of hydrazine hydrate to the above solution and stir to form a uniform solution. Perform a reduction reaction at 95 ° C for 2 h, and cool naturally to obtain reduced graphene oxide.

[0056] (3) The reduced graphene oxide composite material loaded with nano-sulfur particles was filtered and washed three times with a cleaning solution, and then dispersed with 0.15g of conductive carbon black, 0.15g of carbon fiber, and reduced graphene oxide in an organic solvent, mixed, washed, filtered according to the required loading amount to form a film, and freeze-dried to obtain a nano-sulfur / reduced graphene oxide composite self-supporting lithium-sulfur battery positive electrode material. Transmission electron microscopy photos are shown as follows. Figure 7 As shown by Figure 7 The results show that the nanosulfur particles are evenly distributed on the reduced graphene oxide sheets, mainly showing a size distribution of 10-70 nanometers. The nanoparticles are mainly quasi-spherical, and the boundaries between the particles are clear, thus proving that the target positive electrode material has been successfully prepared in this example.

[0057] Example 4

[0058] (1) Weigh 0.2 g of graphene oxide and 1.167 g of sublimed sulfur, add them to 200 ml of N,N-dimethylformamide and stir until they are completely dissolved into a uniform solution. Then, add 0.8 ml of ammonia water and 0.4 ml of hydrazine hydrate to the above solution and stir until it becomes a uniform solution. The solution is reduced at 95 ° C for 2 h and cooled naturally to obtain a reduced graphene oxide composite material loaded with nanosulfur particles.

[0059] (2) Weigh 0.1 g of graphene oxide and add it to 100 ml of N,N-dimethylformamide, stirring until it is completely dissolved into a uniform solution. Then, add 0.4 ml of ammonia water and 0.2 ml of hydrazine hydrate to the above solution and stir to form a uniform solution. Perform a reduction reaction at 95 ° C for 2 h, and cool naturally to obtain reduced graphene oxide.

[0060] (3) The reduced graphene oxide composite material loaded with nano-sulfur particles was filtered and washed three times with a cleaning solution, and then dispersed with 0.15g of conductive carbon black, 0.15g of carbon fiber, and reduced graphene oxide in an organic solvent, mixed, washed, filtered according to the required loading amount to form a film, and freeze-dried to obtain a nano-sulfur / reduced graphene oxide composite self-supporting lithium-sulfur battery positive electrode material. Transmission electron microscopy photos are shown as follows. Figure 8 As shown by Figure 8 The results show that the nanosulfur particles are evenly distributed on the reduced graphene oxide sheets, mainly showing a size distribution of 10-70 nanometers. The nanoparticles are mainly quasi-spherical, and the boundaries between the particles are clear, thus proving that the target positive electrode material has been successfully prepared in this example.

[0061] Comparative Example 1

[0062] (1) 0.23 g of graphene oxide and 1.75 g of sulfur were weighed out, added to 300 ml of N,N-dimethylformamide, and stirred until completely dissolved to form a uniform solution. Then, 1.2 ml of ammonia water and 0.6 ml of hydrazine hydrate were added to the above solution, and stirred to form a uniform solution. The reduction reaction was carried out at 95°C for 2 hours, and then naturally cooled to obtain a reduced graphene oxide composite material loaded with nano sulfur particles.

[0063] (2) The reduced graphene oxide composite material loaded with nano sulfur particles was washed three times by suction filtration using a washing solution, and then mixed with 0.15 g of conductive carbon black and 0.15 g of carbon fiber, which were dispersed in an organic solvent. The mixture was washed, suction filtered to form a film according to the desired loading amount, and freeze-dried to obtain a nano sulfur particle and reduced graphene oxide composite self-supporting positive electrode material.

[0064] The procedure was basically the same as in Example 1, except that the pure reduced graphene oxide prepared in step (2) of Example 1 was not added. The electrode material thus prepared had a similar morphology to that of Example 1. The lithium-sulfur battery positive electrode material thus obtained was assembled into a lithium-sulfur battery 2032 button cell, using a lithium sheet as the counter electrode, 1M LiTFSI (solvent DME and DOL in a volume ratio of 1:1, with 2% LiNO3 added) as the electrolyte, and a loading amount of 2 mg·cm -2 The results of the rate test and the cyclic charge-discharge test are shown in Figure 9 .

[0065] Comparative Example 2

[0066] The procedure was basically the same as in Example 1, except that conductive carbon black was not added. The lithium-sulfur battery positive electrode material thus obtained was assembled into a lithium-sulfur battery 2032 button cell, using a lithium sheet as the counter electrode, 1M LiTFSI (solvent DME and DOL in a volume ratio of 1:1, with 2% LiNO3 added) as the electrolyte, and a loading amount of 2 mg·cm -2 The results of the rate test and the cyclic charge-discharge test are shown in Figure 10 .

[0067] Comparative Example 3

[0068] The procedure was basically the same as in Example 1, except that carbon fiber was not added. The lithium-sulfur battery positive electrode material thus obtained was assembled into a lithium-sulfur battery 2032 button cell, using a lithium sheet as the counter electrode, 1M LiTFSI (solvent DME and DOL in a volume ratio of 1:1, with 2% LiNO3 added) as the electrolyte, and a loading amount of 2 mg·cm -2 The results of the rate test and the cyclic charge-discharge test are shown in Figure 11 .

[0069] Comparative Example 4

[0070] The steps are basically the same as those in Example 1, except that Na2S2O3 is used as the sulfur source. Specifically, 2g of Na2S2O3 and 0.3g of reduced graphene oxide are dissolved in 300ml of deionized water, followed by the addition of 1ml of concentrated hydrochloric acid. After continuous stirring for 6 hours, the active material is filtered, washed, and dried to obtain a scanning electron microscope image of the active material. Figure 12 .Depend on Figure 12 The results show that the sulfur particles of the active material prepared by Na2S2O3 have a wide distribution of micron-sized particles with regular geometric shapes. Subsequently, a positive electrode material slurry was prepared using NMP as a solvent with a ratio of 8:1:1 of active material: conductive carbon black: PVDF, and the slurry was coated on aluminum foil and dried to prepare the positive electrode. The obtained lithium-sulfur battery positive electrode was assembled into a lithium-sulfur battery 2032 button cell, using a lithium sheet as the counter electrode, 1M LiTFSI (solvent DME and DOL volume ratio 1:1, added 2% LiNO3) as the electrolyte, and a loading of 2mg cm -2 The test results of the cyclic charge and discharge test are as follows Figure 13 .

[0071] Comparative Example 5

[0072] Sublimed sulfur was used as the sulfur source and sulfur and graphene were composited by melting. Specifically, 0.7g of sublimed sulfur and 0.3g of reduced graphene oxide were mixed and ground for 2h, and then placed in a tube furnace at 155℃ under argon atmosphere for 12h. The scanning electron microscope photo is shown as follows: Figure 14 .Depend on Figure 14 The results show that sulfur is evenly distributed on the surface of reduced graphene oxide, with no obvious particles. At the same time, sulfur adheres to the reduced graphene oxide sheets, greatly reducing the overall specific surface area of ​​the material and causing serious hardening of the active material. Subsequently, a positive electrode material slurry was prepared using NMP as a solvent with a ratio of active material: conductive carbon black: PVDF of 8:1:1, and the slurry was coated on aluminum foil and dried to prepare a positive electrode. The obtained lithium-sulfur battery positive electrode was assembled into a lithium-sulfur battery 2032 button cell, using a lithium sheet as the counter electrode, 1M LiTFSI (solvent DME and DOL volume ratio 1:1, added 2% LiNO3) as the electrolyte, and a loading of 2mg cm -2 The cycle charge and discharge test results are as follows Figure 15 .

[0073] The electrode material prepared in Example 1 and Comparative Examples 1-5 was applied in lithium-sulfur button cell to prepare lithium-sulfur battery, including positive and negative electrodes, wherein the positive electrode was made of the prepared electrode material. The specific preparation method was as follows: in an argon atmosphere glove box, the electrode material was used as the positive electrode, the lithium sheet was used as the negative electrode, and the celgard 2500 separator was used as the separator; then the positive and negative electrodes were faced to each other, the two electrodes were separated by the separator, 1M LiTFSI (solvent DME and DOL in a volume ratio of 1:1, adding 2% LiNO3) electrolyte was added, and the conductive steel sheet and gasket were added, and then the lithium-sulfur battery was sealed in the 2032 battery shell. The rate performance test of the lithium-sulfur battery electrode material was carried out at a current density of 0.02C-1C, and the 0.5C cyclic charge-discharge test was carried out, and the specific results are shown in Table 1.

[0074] Table 1. Specific capacity test results of lithium-sulfur battery electrode material at a current density of 0.02C-1C

[0075]

[0076] From Figure 2 and Figure 6-8 The results show that, compared with the results of Example 1, the positive electrode materials prepared in Examples 2-4 have no obvious difference in morphology, and the difference in the preparation method mainly lies in the different concentrations of graphene oxide and sublimed sulfur, and the specific details lie in the different particle size distributions of the nano-sulfur particles of different sizes, which can be seen from Figure 16 , wherein Figure 16 a-d in the table correspond to Examples 1-4, respectively. The prepared positive electrode materials have little difference in electrochemical performance.

[0077] Example 1 and Comparative Example 1 are different in that the sample prepared in Comparative Example 1 does not add pure reduced graphene oxide, and the overall conductivity of the electrode material is greatly reduced compared with the sample prepared in Example 1, so that the overall electrochemical performance of the electrode is also reduced. In particular, at a high current rate, Comparative Example 1 only shows a specific capacity of 121 mAh·g-1, and only a small part of the sulfur participates in the reaction during the whole discharge process. It is shown that the addition of pure reduced graphene oxide can greatly improve the overall conductivity of the electrode and thus improve the electrochemical performance, mainly in the capacity improvement at a small rate and the large capacity improvement at a large rate of 1C.

[0078] Comparative Example 2 and Example 1 are different in that Comparative Example 2 does not add conductive carbon black, so that the overall conductivity of the electrode is reduced compared with the sample prepared in Example 1, and the overall electrochemical performance is also reduced, which is specifically shown in the capacity reduction at each rate. The specific values are shown in Table 1, and the rate and cycle diagrams are shown in Figure 10 . It is shown that the conductive carbon black can enhance the overall conductivity of the electrode to some extent, thereby improving the electrochemical performance of the electrode.

[0079] Comparative Example 3 is compared with Example 1, the difference is that Comparative Example 3 does not add carbon fibers, so that the gap between the remaining materials is completely compressed during the rolling operation in the electrode preparation process, resulting in that the electrolyte cannot penetrate into the electrode, so that the active material sulfur almost cannot participate in the reaction, so that the performance of the electrode material is almost completely degraded. Therefore, the addition of carbon fibers is very necessary, which can open the inside of the electrode material, so that it can still be fully infiltrated by the electrolyte after rolling, while connecting the whole electrode material, so that the nanosulfur particle composite reduced graphene active material in each part can fully react.

[0080] Comparative Example 4 is compared with Example 1, the difference is that the source of sulfur is different, the source of sulfur in Comparative Example 4 is the sulfur particles produced by the reaction of Na2S2O3 in an acidic environment, the size distribution of the sulfur particles is wide, and the size is micron level, with regular geometric shape. After the electrolyte is injected at the beginning, due to the large particle size, the amount of electrolyte adsorbed on the surface of the electrode material is limited, so that after a long time of charging and discharging process to generate a large number of new surfaces, the amount of electrolyte is not enough to fully infiltrate the whole electrode material, which is specifically shown in the obvious capacity attenuation after 100 cycles of battery recycling, and the capacity has been attenuated to less than one fifth of the initial value after 200 cycles, see Figure 13 .

[0081] Comparative Example 5 is compared with Example 1, the difference is that the source of sulfur is different, the source of sulfur in Comparative Example 5 is the sublimed sulfur melted at 155℃, which is redistributed on the surface of the reduced graphene oxide in liquid state. Since the sulfur is in liquid state during preparation, the sulfur is in continuous state in the final state, with hardening phenomenon, and the surface of the reduced graphene oxide is completely covered. Like Comparative Example 4, the amount of electrolyte adsorbed on the surface of the electrode material is limited, so that after a long time of charging and discharging process to generate a large number of new surfaces, the amount of electrolyte is not enough to fully infiltrate the whole electrode material, which is specifically shown in the obvious capacity attenuation after 100 cycles of battery recycling, and the capacity has been attenuated to less than one tenth of the initial value after 150 cycles, see Figure 15 .

[0082] As can be seen from Table 1, the lithium-sulfur battery prepared by the electrode material prepared by the present application has a very high loading capacity and a surface capacity, and the specific capacity reaches 1609.1 mAh·g -1 at a current of 0.02C, the loading capacity is 8 mg·cm -2 , and the surface capacity reaches 8.4 mAh·cm -2 .

[0083] From the above analysis results, the lithium-sulfur battery prepared by using the electrode material obtained in Example 1 has high electrochemical performance, mainly because: in the present application, by reducing the size of the sulfur particles to nanoscale, the utilization rate and specific capacity can be effectively improved; by compounding the nanosulfur particles on the surface of the reduced graphene oxide, the electrochemical activity of sulfur can be further improved by using the electrochemical activity of the reduced graphene oxide; and the addition of reduced graphene oxide, conductive carbon black and carbon fiber can significantly improve the conductivity of the electrode material, and the addition of pure reduced graphene oxide can increase the cycle life of the battery.

[0084] The electrode material provided by the present application contains sulfur nanoparticles and reduced graphene oxide, the electrochemical activity of nanoscale sulfur particles is higher than that of large particles, the reduced graphene oxide has high conductivity and stability, and the compounding of nanosulfur particles and reduced graphene oxide can effectively improve the conductivity and electrochemical activity of the electrode material.

[0085] In the above electrode material provided by the present application, the addition of conductive carbon black can further improve the conductivity of the material, thereby further improving the utilization rate of sulfur in the charging and discharging process and improving the electrochemical performance of the material. The addition of carbon fiber can significantly improve the mechanical and electrical properties of the material, reduce the internal resistance of the electrode material, improve the flexibility, open the inside of the electrode, and enable the nanosulfur particles to fully participate in the electrochemical reaction. The addition of pure reduced graphene oxide can improve the overall conductivity of the electrode material, reduce the overall internal resistance of the electrode material, provide adsorption sites for intermediate products of sulfur in the charging and discharging process, and slow down the capacity loss of the battery in the cyclic charging and discharging process.

[0086] In the present application, an electrode active material, specifically a reduced graphene oxide composite material loaded with nanosulfur particles, is prepared, which only contains reduced graphene oxide and nanosulfur particles, and the nanosulfur particles are uniformly distributed on the wrinkled reduced graphene oxide layers, which can well retain the layer morphology of the reduced graphene oxide and fully utilize the advantage of large specific surface area of the reduced graphene oxide; at the same time, the size of the sulfur particles is reduced to nanoscale, which can effectively overcome the defect that sulfur as an electrode active material is not conductive, can be fully combined with the reduced graphene oxide layers, can greatly enhance the reactivity and improve the utilization rate of sulfur, and thereby improve the specific capacity of the electrode material.

[0087] In the electrode material of the present application, (pure) reduced graphene oxide is selected, which has excellent conductivity and can be uniformly dispersed in the electrode to form an excellent conductive network, thereby greatly improving the overall conductivity of the electrode, and at the same time, the nanosulfur particles and the reduced graphene oxide layers in different parts of the electrode can be well connected to improve the utilization rate of the electrode active material sulfur; in addition, no adhesive is needed, and a self-supporting positive electrode material can be directly prepared, the content of sulfur is greatly improved, and the loading of sulfur is flexible and adjustable.

[0088] In the electrode material, conductive carbon black and carbon fiber are also selected, the addition of the two further increases the conductivity and integrity of the electrode material, so that the electrode material can still maintain a high utilization rate even in the case of a very high sulfur load, greatly improving the surface capacity of the electrode material, and the addition of carbon fiber can also increase the toughness of the electrode material, so that the electrode material has the characteristics of being bendable, and the electrode material has the potential to prepare a flexible battery.

[0089] In the present application, graphene oxide is used as a raw material instead of single-layer graphene, which has a lower cost; the chemical reduction method is simple, short in preparation time, low in energy consumption, and the preparation process can be accurately controlled; compared with the use of a melting method to prepare a sulfur-containing electrode material, the method of organic solvent assisted preparation can greatly reduce the time and energy consumption in the preparation process, the preparation result is stable, and is suitable for large-scale batch production, so as to realize the possibility of commercialization.

[0090] The nano sulfur / reduced graphene oxide composite self-supporting lithium sulfur battery positive electrode material prepared by the present application can improve the surface capacity of the lithium sulfur battery.

[0091] In summary, the lithium sulfur battery electrode material and the preparation method thereof provided by the present application have great advantages in terms of performance, cost and commercial production.

[0092] The above describes only the preferred specific embodiments of the present application, but the protection scope of the present application is not limited thereto, any person skilled in the art can make equivalent replacements or changes to the technical solutions and inventive concepts of the present application within the technical range disclosed by the present application, which should be covered within the protection scope of the present application.

Claims

1. A method for preparing a nano-sulfur / reduced graphene oxide composite self-supporting lithium-sulfur battery positive electrode material, characterized in that: The following steps are involved: S1, dissolving elemental sulfur and graphene oxide in an organic solvent, adding an alkali source and a reducing agent, performing a graphene oxide reduction reaction and sulfur cooling crystallization deposition, to obtain a nano-sulfur particle-supported reduced graphene oxide composite material; The ratio of elemental sulfur, graphene oxide, alkali source and reducing agent is 10~1g:1g:4ml:2ml; The organic solvent is selected from any one of N,N-dimethylformamide, N-methylpyrrolidone or dimethyl sulfoxide; the elemental sulfur is sublimed sulfur, the reducing agent is hydrazine hydrate, and the alkaline source is aqueous ammonia; The temperature of the graphene oxide reduction reaction is 80-120° C., and the reaction time is 5-10 hours; S2, preparing reduced graphene oxide; S3. Dispersing the reduced graphene oxide composite material loaded with nano-sulfur particles, reduced graphene oxide, conductive carbon black, and carbon fiber in an organic solvent, mixing the mixture, washing, and filtering to form a film, thereby obtaining a nano-sulfur / reduced graphene oxide composite self-supporting lithium-sulfur battery positive electrode material.

2. The method for preparing the nano-sulfur / reduced graphene oxide composite self-supporting lithium-sulfur battery positive electrode material according to claim 1, characterized in that: The preparation steps of the reduced graphene oxide are: dissolving graphene oxide in an organic solvent, adding an alkali source and a reducing agent, and performing a reduction reaction to obtain the reduced graphene oxide.

3. The method for preparing the nano-sulfur / reduced graphene oxide composite self-supporting lithium-sulfur battery positive electrode material according to claim 2, characterized in that: The temperature of the reduction reaction is 90-100° C., and the reaction time is 2-3 hours.

4. The method for preparing the nano-sulfur / reduced graphene oxide composite self-supporting lithium-sulfur battery positive electrode material according to claim 1, characterized in that: The mass ratio of sulfur, reduced graphene oxide, conductive carbon black and carbon fiber in the nano-sulfur particle-supported reduced graphene oxide composite material is (50-75): (30-13): (10-6): (10-6).

5. A nano-sulfur / reduced graphene oxide composite self-supporting lithium-sulfur battery positive electrode material prepared by the preparation method according to any one of claims 1 to 4.

6. A lithium-sulfur battery, characterized in that: The invention comprises a nano-sulfur / reduced graphene oxide composite self-supporting lithium-sulfur battery positive electrode material prepared by the preparation method according to any one of claims 1 to 5.

Images