CN113659142A - Nitrogen-doped graphene aerogel and preparation method thereof, lithium-sulfur battery positive electrode material and preparation method thereof, and lithium-sulfur battery - Google Patents
Nitrogen-doped graphene aerogel and preparation method thereof, lithium-sulfur battery positive electrode material and preparation method thereof, and lithium-sulfur battery Download PDFInfo
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Abstract
The invention provides a nitrogen-doped graphene aerogel and a preparation method thereof, a lithium-sulfur battery positive electrode material and a preparation method thereof, and a lithium-sulfur battery. The preparation method of the nitrogen-doped graphene aerogel comprises the following steps: step S1, mixing and stirring the soft template solvent, the graphene oxide suspension and the emulsifier to form a graphene oxide emulsion; the soft template solvent is selected from one or more of cyclohexane, normal hexane, normal butanol, dichloromethane and tetrahydrofuran; step S2, adding an auxiliary reducing agent into the oxidized graphene emulsion, and carrying out hydrothermal reduction reaction to obtain graphene hydrogel; the auxiliary reducing agent comprises ethylenediamine and/or ammonia water; step S3, freezing and drying the graphene hydrogel to obtain graphene aerogel; step S4, calcining the graphene aerogel in an inert atmosphere to obtain the nitrogen-doped graphene aerogel. The nitrogen-doped graphene aerogel prepared by the invention has a better pore structure, and can effectively improve the battery performance when being applied to a lithium-sulfur battery cathode material.
Description
Technical Field
The invention relates to the technical field of new energy, and particularly relates to a nitrogen-doped graphene aerogel and a preparation method thereof, a lithium-sulfur battery positive electrode material and a preparation method thereof, and a lithium-sulfur battery.
Background
Lithium-sulfur batteries are one of the new batteries that are currently relatively close to commercial use. Although lithium sulfur batteries have great advantages in energy density and low cost compared to lithium ion batteries, there are still many problems that are difficult to solve, hindering their large-scale commercial road. Firstly, elemental sulfur and reaction product Li of lithium sulfur battery2S has very low conductivity, which leads to the reduction of the utilization rate of active substances and indirectly reduces the capacity; the second is the shuttling effect of polysulfides. The intermediate reaction product polysulfide dissolves in the electrolyte and reacts with the lithium metal of the negative electrode through the separator. Shuttling can lead to deposition of insulating products that further lead to increased resistivity at the solid electrolyte interface, loss of lithium metal anode active material, and decreased coulombic efficiency.
In order to solve the shuttle effect and improve the electrochemical performance of lithium sulfur batteries, various strategies have been proposed in which the addition of a high porosity conductive matrix is a very effective method. Graphene Aerogels (GA) have received much attention due to their high electrical conductivity and rich porous structure, and the internal pores of GA can serve as Li2The excellent template of the S/functional material not only can increase the content of active substance sulfur and provide abundant polysulfide loading sites, but also can construct a conductive network and inhibit the shuttling effect of polysulfide in a circulating process. For example, in chinese patent CN 106450209 a, by controlling the sulfur source to be uniformly attached to the surface of graphene oxide in the form of polythiophene, the formed elemental sulfur can be uniformly dispersed in the conductive network constructed by GA, the load is stable, the dissolution of elemental sulfur in the circulation process is reduced, and the swelling problem of the positive electrode of the lithium-sulfur battery is reduced. The Chinese patent CN 111029544A is prepared by preparing a modified three-dimensional graphene/polyaniline composite materialElemental sulfur is coated in the modified three-dimensional graphene/polyaniline composite material by a melt impregnation method, so that the dissolution and shuttling of polysulfide in an electrochemical reaction are inhibited, the mechanical strength of the anode material is improved, the conductivity of the anode material is improved, and the volume change of a sulfur anode in the charging and discharging processes is inhibited. The chinese patent CN 109037657 a utilizes graphene aerogel loaded titanium dioxide nanoparticle composite material as a material for containing active substance sulfur in the positive electrode of a lithium sulfur battery, and can solve the problems of inherent insulation of the sulfur positive electrode and a discharge product, large volume change during charge and discharge, and "shuttle effect" caused by polysulfide dissolving into electrolyte.
However, the graphene aerogel prepared by the method has a poor internal pore structure, the pore diameter and the distribution uniformity are not easy to control, and the performance consistency of the prepared cathode material is difficult to guarantee.
Disclosure of Invention
The invention mainly aims to provide a nitrogen-doped graphene aerogel and a preparation method thereof, a lithium-sulfur battery positive electrode material and a preparation method thereof, and a lithium-sulfur battery, so as to solve the problem that the pore size and distribution of the graphene aerogel in the prior art are not uniform enough, and the performance of the lithium-sulfur battery positive electrode material is influenced.
In order to achieve the above object, according to one aspect of the present invention, there is provided a method for preparing a nitrogen-doped graphene aerogel, comprising the steps of: step S1, mixing the soft template solvent, the graphene oxide suspension and the emulsifier to form a graphene oxide emulsion; wherein the soft template solvent is selected from one or more of cyclohexane, n-hexane, n-butanol, dichloromethane and tetrahydrofuran; step S2, adding an auxiliary reducing agent into the oxidized graphene emulsion, and then carrying out hydrothermal reduction reaction to obtain graphene hydrogel; wherein the auxiliary reducing agent comprises ethylenediamine and/or ammonia water; step S3, freezing and drying the graphene hydrogel to obtain graphene aerogel; step S4, calcining the graphene aerogel in an inert atmosphere to obtain the nitrogen-doped graphene aerogel.
Further, the soft template solvent comprises cyclohexane and n-butyl alcohol, and the volume ratio of the cyclohexane to the n-butyl alcohol is (5-8) to (4-6).
Further, in the step S1, the concentration of the graphene oxide suspension is 2-8 mg/mL, the volume content of the soft template solvent in the graphene oxide emulsion is 45-78%, and the weight content of the emulsifier in the graphene oxide emulsion is 0.5-1.5%.
Further, the auxiliary reducing agent is ethylenediamine and/or ammonia water, or further comprises one or more of the following substances besides ethylenediamine and/or ammonia water: sodium bisulfite, sodium ascorbate; the emulsifier is selected from sodium lauryl sulfate.
Further, in step S2, the amount of the auxiliary reducing agent is 0.5 to 3 μ L per mg of graphene oxide; in the hydrothermal reduction reaction process, the reaction temperature is 120-280 ℃, and the reaction time is 2-48 h.
Further, in the step S4, in the process of calcining the graphene aerogel, the calcining temperature is 500 to 700 ℃, and the calcining time is 2 to 6 hours.
According to another aspect of the present invention, there is also provided a nitrogen-doped graphene aerogel, which is prepared by the above preparation method.
According to another aspect of the present invention, there is also provided a lithium-sulfur battery cathode material, which includes a graphene aerogel and elemental sulfur loaded on the graphene aerogel, wherein the graphene aerogel is the nitrogen-doped graphene aerogel or is prepared by the preparation method.
According to another aspect of the invention, a preparation method of the lithium-sulfur battery cathode material is also provided, which comprises the steps of dipping the graphene aerogel into a solution containing elemental sulfur, and drying to obtain the lithium-sulfur battery cathode material.
According to another aspect of the invention, the lithium-sulfur battery comprises a positive pole piece, wherein the positive pole piece comprises a current collector and a positive active layer located on the surface of the current collector, the positive active layer comprises a binder and a positive material, and the positive material is the above-mentioned lithium-sulfur battery positive material or the lithium-sulfur battery positive material prepared by the above-mentioned preparation method.
The invention provides a preparation method of nitrogen-doped graphene aerogel, which comprises the steps of preparing a graphene oxide emulsion in advance by using a soft template solvent, a graphene suspension and an emulsifier, carrying out hydrothermal reduction reaction on the graphene oxide emulsion to obtain graphene hydrogel, and removing solvent components (including the soft template solvent and water entering the graphene oxide suspension) in the gel in a freeze drying mode to form the graphene aerogel. Because the auxiliary reducing agent adopted in the reduction process contains ethylenediamine and/or ammonia water, the auxiliary reducing agent can simultaneously assist in reducing graphene oxide and act as a nitrogen dopant, so that in the final calcination step, the nitrogen-doped graphene aerogel is formed.
According to the invention, one or more of cyclohexane, n-hexane, n-butanol, dichloromethane and tetrahydrofuran are used as a soft template solvent, and the addition of an emulsifier enables graphene oxide to exist in the form of emulsion, and the aperture and the shape of the graphene in the self-assembly process can be regulated, so that the aerogel with controllable aperture, higher porosity, lower density and more uniform pores is prepared. The added soft template solvent is a micromolecular template, is different from an organic polymer template and a metal template, does not need to be subjected to template etching after the subsequent hydrothermal reduction reaction, and can be removed by directly utilizing a freeze drying mode. The reasons of the above aspects promote that the nitrogen-doped graphene aerogel prepared by the invention has a better pore structure, uniform pore size and distribution, high specific surface area and high porosity. The nitrogen-doped graphene aerogel is applied to the positive electrode material of the lithium-sulfur battery, and the performance of the battery can be effectively improved.
Drawings
The accompanying drawings, which are incorporated in and constitute a part of this application, illustrate embodiments of the invention and, together with the description, serve to explain the invention and not to limit the invention. In the drawings:
fig. 1 shows appearance photographs of the graphene wet gels prepared in example 1 and comparative example 1, wherein (a) is a photograph corresponding to comparative example 1, and (b) is a photograph corresponding to example 1;
fig. 2 shows appearance photographs of nitrogen-doped graphene aerogels prepared in example 1 and comparative example 1, in which (a) is a photograph corresponding to comparative example 1 and (b) is a photograph corresponding to example 1;
fig. 3 shows SEM photographs of the nitrogen-doped graphene aerogels prepared in example 1 and comparative example 1, in which (a) and (b) correspond to photographs at different magnifications in comparative example 1, and (c) and (d) correspond to photographs at different magnifications in example 1.
Detailed Description
It should be noted that the embodiments and features of the embodiments in the present application may be combined with each other without conflict. The present invention will be described in detail below with reference to the embodiments with reference to the attached drawings.
As described in the background of the invention section, the graphene aerogel in the prior art has the problems of insufficient uniform pore size and distribution, poor pore structure, and influence on the performance of the lithium-sulfur battery cathode material.
In order to solve the above problems, the present invention provides a method for preparing a nitrogen-doped graphene aerogel, comprising the following steps: step S1, mixing and stirring the soft template solvent, the graphene oxide suspension and the emulsifier to form a graphene oxide emulsion; wherein the soft template solvent is selected from one or more of cyclohexane, n-hexane, n-butanol, dichloromethane and tetrahydrofuran; step S2, adding a reducing agent and a co-reducing agent into the oxidized graphene emulsion, and then carrying out hydrothermal reduction reaction to obtain graphene hydrogel; wherein the auxiliary reducing agent comprises ethylenediamine and/or ammonia water; step S3, freezing and drying the graphene hydrogel to obtain graphene aerogel; step S4, calcining the graphene aerogel in an inert atmosphere to obtain the nitrogen-doped graphene aerogel.
The preparation method comprises the steps of preparing a graphene oxide emulsion in advance by using a soft template solvent, a graphene suspension and an emulsifier, carrying out hydrothermal reduction reaction on the graphene oxide emulsion to obtain graphene hydrogel, and removing solvent components (including water entering the soft template solvent and the graphene oxide suspension) in the gel in a freeze drying mode to form the graphene aerogel. Because the auxiliary reducing agent adopted in the reduction process contains ethylenediamine and/or ammonia water, the auxiliary reducing agent can simultaneously assist in reducing graphene oxide and act as a nitrogen dopant, so that in the final calcination step, the nitrogen-doped graphene aerogel is formed.
According to the invention, one or more of cyclohexane, n-hexane, n-butanol, dichloromethane and tetrahydrofuran are used as a soft template solvent, and the addition of an emulsifier enables graphene oxide to exist in the form of emulsion, and the aperture and the shape of the graphene in the self-assembly process can be regulated, so that the aerogel with controllable aperture, higher porosity, lower density and more uniform pores is prepared. The added soft template solvent is a micromolecular template, is different from an organic polymer template and a metal template, does not need to be subjected to template etching after the subsequent hydrothermal reduction reaction, and can be removed by directly utilizing a freeze drying mode. The reasons of the above aspects promote that the nitrogen-doped graphene aerogel prepared by the invention has a better pore structure, uniform pore size and distribution, high specific surface area and high porosity. The nitrogen-doped graphene aerogel is applied to the positive electrode material of the lithium-sulfur battery, and the performance of the battery can be effectively improved. Specifically, by using the graphene aerogel prepared by the invention and a composite material formed by the graphene aerogel and sulfur as a positive electrode material of a lithium-sulfur battery, the graphene aerogel has good electrical conductivity, and meanwhile, the shuttle effect is effectively inhibited, and the electrochemical performance (such as cycle performance) of the battery is improved.
In addition to the above advantages, compared with other template methods such as metal template, organic polymer template, foam template, inorganic material template and other preparation processes, the method for preparing the graphene aerogel by using the soft template method has the advantages of simple process, no need of using concentrated sulfuric acid or hydrofluoric acid, environmental friendliness and the like.
Compared with a suspension form, the hydrothermal reduction reaction is carried out in the form of graphene oxide emulsion, so that the reaction can be carried out in more stable and more ordered micro units, and the uniformity of the pore shape, size and distribution of the final aerogel product can be better controlled. In order to further improve the pore structure of the nitrogen-doped graphene aerogel and further improve the performance of the lithium-sulfur battery after the nitrogen-doped graphene aerogel is applied to a positive electrode material, in a preferred embodiment, the soft template solvent comprises cyclohexane and n-butanol, and the weight ratio of the cyclohexane to the n-butanol is (5-8) to (4-6). And meanwhile, the cyclohexane and the n-butanol are used, and the ratio of the cyclohexane to the n-butanol is controlled in the range, so that the stability of the graphene oxide emulsion can be further improved, the subsequent hydrothermal reduction reaction can be carried out more stably, and the air structure of the final aerogel is further promoted.
In a preferred embodiment, in step S1, the concentration of the graphene oxide suspension is 2 to 8mg/mL, the volume content of the soft template solvent in the graphene oxide emulsion is 45 to 78%, and the weight content of the emulsifier in the graphene oxide emulsion is 0.5 to 1.5%. The dosage of each component is controlled within the range, the emulsion is more stable, the graphene oxide is more uniformly dispersed in different emulsion particles, the size of the emulsion particles is more appropriate, and the pore distribution and size of the aerogel can be better regulated and controlled.
The auxiliary reducing agent is added to assist the hydrothermal reduction of graphene oxide into graphene, and for the purpose of further improving the reduction effect, in a preferred embodiment, the auxiliary reducing agent is ethylenediamine and/or ammonia water, or includes one or more of the following substances in addition to ethylenediamine and/or ammonia water: sodium bisulfite and sodium ascorbate. For example, ethylenediamine and/or ammonia water may be used as the auxiliary reducing agent, and a mixture thereof with sodium hydrogen sulfite or sodium ascorbate may be used as the auxiliary reducing agent. More preferably, in step S2, the amount of the auxiliary reducing agent is 0.5 to 3 μ L per mg of graphene oxide; in the hydrothermal reduction reaction process, the reaction temperature is 120-280 ℃, and the reaction time is 2-48 h.
In the actual reaction process, the soft template solvent, the graphene oxide suspension and the emulsifier can be mixed in a beaker, and then stirred by a high-speed homogenizer to form a graphene oxide emulsion; then adding an auxiliary reducing agent into the graphene oxide emulsion, stirring again, placing the mixture into a polytetrafluoroethylene lining, and then placing the polytetrafluoroethylene lining into a reaction kettle to perform hydrothermal reduction reaction.
To further improve the stability of the graphene oxide emulsion, in a preferred embodiment the emulsifier is selected from sodium dodecyl sulfate.
Before freeze-drying, the graphene hydrogel is preferably soaked with an ethanol aqueous solution, for example, an ethanol aqueous solution with a volume concentration of 20 vol% of ethanol may be used for soaking. The purpose of soaking is to replace the template solvent in the hydrogel pores, so that the structure is more stable, and the structural collapse in the subsequent freeze drying process is further prevented. Secondly, the soaked graphene hydrogel can be freeze-dried to obtain graphene aerogel. In order to dope nitrogen, the graphene aerogel obtained in the freeze drying process is calcined in an inert atmosphere. In order to perform nitrogen doping more fully and remove residual small molecule impurities, in a preferred embodiment, in the step S4, during the process of calcining the graphene aerogel, the calcining temperature is 500 to 700 ℃, and the calcining time is 2 to 6 hours. The specific calcining process can be carried out in a high-temperature tube furnace, and the inert gas can be nitrogen, argon and the like.
According to another aspect of the present invention, there is also provided a nitrogen-doped graphene aerogel, which is prepared by the above preparation method. As described above, the graphene aerogel with controllable pore size, higher porosity, lower density and more uniform pores can be prepared by using the soft template method. The composite material formed by the composite material and sulfur is used as a lithium-sulfur battery positive electrode material, so that the lithium-sulfur battery positive electrode material has better conductivity, and meanwhile, the shuttle effect is inhibited, and the battery performance (such as cycle performance) is improved.
According to another aspect of the present invention, there is also provided a lithium-sulfur battery cathode material, which includes a graphene aerogel and elemental sulfur loaded on the graphene aerogel, wherein the graphene aerogel is the nitrogen-doped graphene aerogel or is prepared by the preparation method. The nitrogen-doped graphene aerogel is controllable in pore size, higher in porosity, lower in density and more uniform in pore, and is more suitable for being used as a positive electrode material of a lithium-sulfur battery after being compounded with sulfur.
According to another aspect of the present invention, there is also provided a method for preparing the above-mentioned positive electrode material for a lithium-sulfur battery, comprising: and (3) dipping the graphene aerogel into a solution containing elemental sulfur, and drying to obtain the lithium-sulfur battery positive electrode material. The dipping-drying process is preferably carried out for multiple times, and specifically, a solution containing elemental sulfur can be dropwise added onto the nitrogen-doped graphene aerogel, and the loading amount of sulfur can be controlled through multiple cycles of dropwise addition, adsorption saturation and drying. It should be noted that, as the nitrogen-doped graphene aerogel prepared by the method has a complete pore structure and more uniform size and distribution, the composite material formed after sulfur loading is more stable, and the sulfur content in the formed aerogel/sulfur composite material can reach 65-99 wt%.
According to another aspect of the present invention, there is also provided a lithium sulfur battery, including a positive electrode plate, which includes a current collector and a positive active layer located on a surface of the current collector, where the positive active layer includes a binder and a positive electrode material, and the positive electrode material is the above-mentioned positive electrode material of the lithium sulfur battery, or is the positive electrode material of the lithium sulfur battery prepared by the above-mentioned preparation method.
The specific positive electrode plate can be prepared by a method commonly used in the art, for example, the positive electrode material and the binder can be mixed and ground, then placed in a solvent to form positive electrode slurry, and then coated on a current collector and dried to form the positive electrode plate. The specific binder is preferably polyvinylidene fluoride (PVDF). The proportion of the positive electrode material of the lithium-sulfur battery is preferably 65-99% of the weight of the positive electrode active layer.
The present application is described in further detail below with reference to specific examples, which should not be construed as limiting the scope of the invention as claimed.
Example 1
1. Method for synthesizing nitrogen-doped graphene aerogel by template method
Adding cyclohexane, n-butanol and a graphene oxide suspension (the concentration is 6mg/ml) into a beaker according to the volume ratio of 20:20:15, then adding sodium dodecyl sulfate, and then stirring by a high-speed homogenizer at 12000rpm to obtain the graphene oxide emulsion. Wherein the weight content of the sodium dodecyl sulfate is 0.75 wt%, the volume content of the cyclohexane is about 36%, and the volume content of the n-butanol is about 36%.
And adding 50 mu L of auxiliary reducing agent ethylenediamine (equivalent to 1.5 mu L of added graphene oxide per milligram) into 20ml of graphene oxide emulsion, stirring again, adding into a polytetrafluoroethylene lining, and then placing into a reaction kettle for hydrothermal reduction for 6h at the temperature of 150 ℃ to obtain the graphene hydrogel.
And taking out the graphene hydrogel in the lining, soaking the graphene hydrogel in 20 vol% ethanol water solution, and freeze-drying the graphene hydrogel in a freeze dryer for 48 hours. And placing the freeze-dried graphene aerogel in a high-temperature tube furnace, and calcining for 6 hours at 650 ℃ under the protection of argon inert gas to obtain the nitrogen-doped graphene aerogel synthesized by the template method.
2. Preparation of positive pole piece
And (2) dropwise adding a sulfur solution which is saturated and dissolved in carbon disulfide onto the nitrogen-doped graphene aerogel prepared in the step (1), repeatedly and circularly dropwise adding, adsorbing, saturating and drying until the sulfur content is 80 +/-1 wt%, grinding the obtained composite material and a binding agent polyvinylidene fluoride (PVDF) according to a mass ratio of 0.95:5, placing the ground composite material and the binding agent PVDF into a solvent, and coating the mixture on a current collector to prepare the pole piece.
Example 2
1. Method for synthesizing nitrogen-doped graphene aerogel by template method
Adding cyclohexane, n-butanol and a graphene oxide suspension (the concentration is 6mg/ml) into a beaker according to the volume ratio of 20:15:10, then adding sodium dodecyl sulfate, and then stirring by a high-speed homogenizer at 12000rpm to obtain the graphene oxide emulsion. Wherein the amount of sodium dodecyl sulfate is 0.75 wt%, the volume content of cyclohexane is about 44%, and the volume content of n-butanol is about 33%.
And adding 42 mu L of auxiliary reducing agent ethylenediamine (equivalent to 1.6 mu L of added graphene oxide per milligram) into 20ml of graphene oxide emulsion, stirring again, adding into a polytetrafluoroethylene lining, and then placing into a reaction kettle for hydrothermal reduction for 6h at the temperature of 150 ℃ to obtain the graphene hydrogel.
And taking out the graphene hydrogel in the lining, soaking the graphene hydrogel in 20% water and ethanol solution, and freeze-drying the graphene hydrogel in a freeze dryer for 48 hours. And placing the freeze-dried graphene aerogel in a high-temperature tube furnace, and calcining for 6 hours at 650 ℃ under the protection of argon inert gas to obtain the nitrogen-doped graphene aerogel synthesized by the template method.
2. Preparation of positive pole piece
And (2) dropwise adding a sulfur solution which is saturated and dissolved in carbon disulfide onto the graphene aerogel prepared in the step (1), grinding the obtained composite material and a binding agent polyvinylidene fluoride (PVDF) according to a mass ratio of 0.95:5, placing the ground composite material and the binding agent polyvinylidene fluoride (PVDF) into a solvent, and coating the ground composite material and the binding agent polyvinylidene fluoride (PVDF) on a current collector to prepare the pole piece through a mode of repeated cyclic dropwise adding-adsorption saturation-drying until the sulfur content is 80 +/-1%.
Example 3
1. Method for synthesizing nitrogen-doped graphene aerogel by template method
Adding cyclohexane, n-butanol and graphene oxide suspension (6mg/ml) into a beaker according to the volume ratio of 20:20:15, then adding sodium dodecyl sulfate, and then stirring by a high-speed homogenizer at 12000rpm to obtain the graphene oxide emulsion. Wherein the content of sodium dodecyl sulfate is 0.75 wt%, the volume content of cyclohexane is about 36%, and the volume content of n-butanol is about 36%.
And adding 35 mu L of ethylenediamine (equivalent to 1.05 mu L of each mg of graphene oxide) and 25 mu L of sodium bisulfite (equivalent to 0.75 mu L of each mg of graphene oxide) into 20ml of graphene oxide emulsion respectively, stirring again, adding into a polytetrafluoroethylene lining, and then placing into a reaction kettle for hydrothermal reduction for 6 hours at the temperature of 150 ℃ to obtain the graphene hydrogel.
And taking out the graphene hydrogel in the lining, soaking the graphene hydrogel in 20% ethanol solution, and freeze-drying the graphene hydrogel in a freeze dryer for 48 hours. And placing the freeze-dried graphene aerogel in a high-temperature tube furnace, and calcining for 6 hours at 650 ℃ under the protection of inert gas to obtain the nitrogen-doped graphene aerogel synthesized by the template method.
2. Preparation of positive pole piece
And (2) dropwise adding a sulfur solution which is saturated and dissolved in carbon disulfide onto the graphene aerogel prepared in the step (1), grinding the obtained composite material and a binding agent polyvinylidene fluoride (PVDF) according to a mass ratio of 0.95:5, placing the ground composite material and the binding agent polyvinylidene fluoride (PVDF) into a solvent, and coating the ground composite material and the binding agent polyvinylidene fluoride (PVDF) on a current collector to prepare the pole piece through a mode of repeated cyclic dropwise adding-adsorption saturation-drying until the sulfur content is 80 +/-1%.
Example 4
1. Method for synthesizing nitrogen-doped graphene aerogel by template method
Adding n-hexane and graphene oxide suspension (with the concentration of 6mg/ml) into a beaker according to the proportion of 40:15, then adding sodium dodecyl sulfate, and then stirring by a high-speed homogenizer at 12000rpm to obtain the graphene oxide emulsion. Wherein the weight content of the sodium dodecyl sulfate is 0.75 wt%, and the volume content of the n-hexane is about 72%.
2. The subsequent synthesis and treatment of the graphene wet gel, and the preparation schemes of the composite material and the pole piece are the same as those in the example 1.
Example 5
1. Method for synthesizing nitrogen-doped graphene aerogel by template method
Adding cyclohexane and graphene oxide suspension (the concentration is 6mg/ml) into a beaker according to the proportion of 40:15, then adding sodium dodecyl sulfate, and then stirring by a high-speed homogenizer at 12000rpm to obtain the graphene oxide emulsion. Wherein the weight content of the sodium dodecyl sulfate is 0.75 wt%, and the volume content of the cyclohexane is about 72%.
2. The subsequent synthesis and treatment of the graphene wet gel, and the preparation schemes of the composite material and the pole piece are the same as those in the example 1.
Example 6
1. Method for synthesizing nitrogen-doped graphene aerogel by template method
Adding cyclohexane, n-butanol and a graphene oxide suspension (the concentration is 8mg/ml) into a beaker according to the volume ratio of 20:16:10, then adding sodium dodecyl sulfate, and then stirring by a high-speed homogenizer at 12000rpm to obtain the graphene oxide emulsion. Wherein the weight content of the sodium dodecyl sulfate is 0.5 wt%, the volume content of the cyclohexane is about 43%, and the volume content of the n-butanol is 35%.
Adding 55 mu L of auxiliary reducing agent ethylenediamine (equivalent to 0.6 mu L of added graphene oxide per milligram) into 20ml of graphene oxide emulsion, stirring again, adding into a polytetrafluoroethylene lining, and then placing into a reaction kettle for hydrothermal reduction for 6h at the temperature of 150 ℃ to obtain the graphene hydrogel.
And taking out the graphene hydrogel in the lining, soaking the graphene hydrogel in 20% ethanol solution, and freeze-drying the graphene hydrogel in a freeze dryer for 48 hours. And placing the freeze-dried graphene aerogel in a high-temperature tube furnace, and calcining for 6 hours at 650 ℃ under the protection of argon inert gas to obtain the nitrogen-doped graphene aerogel synthesized by the template method.
2. The preparation schemes of the subsequent composite material and the pole piece are the same as those of the example 1.
Example 7
1. Method for synthesizing nitrogen-doped graphene aerogel by template method
Adding cyclohexane, n-butanol and a graphene oxide suspension (the concentration is 2mg/ml) into a beaker according to the proportion of 12:9:25, then adding sodium dodecyl sulfate, and then stirring by a high-speed homogenizer at 12000rpm to obtain the graphene oxide emulsion. Wherein the weight content of the sodium dodecyl sulfate is 1.5 wt%, the volume content of the cyclohexane is 26%, and the volume content of the n-butanol is 19%.
And adding 30 mu L of auxiliary reducing agent ethylenediamine (equivalent to 1.4 mu L of added graphene oxide per milligram) into 20ml of graphene oxide emulsion, stirring again, adding into a polytetrafluoroethylene lining, and then placing into a reaction kettle for hydrothermal reduction for 3h at the temperature of 180 ℃ to obtain the graphene hydrogel.
2. The subsequent synthesis and treatment of the graphene wet gel, and the preparation schemes of the composite material and the pole piece are the same as those in the example 1.
Example 8
1. Method for synthesizing nitrogen-doped graphene aerogel by template method
Adding cyclohexane, n-butanol and a graphene oxide suspension (the concentration is 6mg/ml) into a beaker according to the volume ratio of 20:20:15, then adding sodium dodecyl sulfate, and then stirring by a high-speed homogenizer at 12000rpm to obtain the graphene oxide emulsion. Wherein the weight content of the sodium dodecyl sulfate is 0.75 wt%, the volume content of the cyclohexane is about 36%, and the volume content of the n-butanol is about 36%.
And adding 100 mu L of auxiliary reducing agent ammonia water (the concentration is 25%) into 20ml of graphene oxide emulsion (namely adding 3 mu L of auxiliary reducing agent ammonia water per milligram of graphene oxide), stirring again, adding the mixture into a polytetrafluoroethylene lining, and then placing the polytetrafluoroethylene lining into a reaction kettle for hydrothermal reduction for 6h at the temperature of 150 ℃ to obtain the graphene hydrogel.
2. The subsequent preparation schemes of wet gel, composite material and pole piece are the same as example 1.
Example 9
1. Method for synthesizing nitrogen-doped graphene aerogel by template method
Adding cyclohexane, n-butanol and a graphene oxide suspension (the concentration is 6mg/ml) into a beaker according to the volume ratio of 20:20:15, then adding sodium dodecyl sulfate, and then stirring by a high-speed homogenizer at 12000rpm to obtain the graphene oxide emulsion. Wherein the weight content of the sodium dodecyl sulfate is 0.75 wt%, the volume content of the cyclohexane is about 36%, and the volume content of the n-butanol is about 36%.
And adding 17 mu L of auxiliary reducing agent ammonia water (the concentration is 25%) into 20ml of graphene oxide emulsion (equivalent to adding 0.5 mu L of auxiliary reducing agent ammonia water into each milligram of graphene oxide), stirring again, adding into a polytetrafluoroethylene lining, and then placing into a reaction kettle for hydrothermal reduction for 12h at the temperature of 150 ℃ to obtain the graphene hydrogel.
2. The subsequent preparation schemes of the wet gel composite material and the pole piece are the same as the example 1.
Example 10
1. Method for synthesizing nitrogen-doped graphene aerogel by template method
Adding cyclohexane, n-butanol and a graphene oxide suspension (the concentration is 6mg/ml) into a beaker according to the proportion of 20:20:15, then adding sodium dodecyl sulfate, and then stirring by a high-speed homogenizer at 12000rpm to obtain the graphene oxide emulsion. Wherein, the weight content of the lauryl sodium sulfate is 0.75 wt%, the weight content of the cyclohexane is 36 wt%, and the weight content of the n-butanol is 36 wt%.
And adding 135 mu L of auxiliary reducing agent ammonia water (the concentration is 25%) into 20ml of graphene oxide emulsion (equivalent to adding 4 mu L of auxiliary reducing agent ammonia water per milligram of graphene oxide), stirring again, adding the mixture into a polytetrafluoroethylene lining, and then placing the polytetrafluoroethylene lining into a reaction kettle for hydrothermal reduction for 12h at the temperature of 150 ℃ to obtain the graphene hydrogel.
2. The subsequent preparation schemes of the wet gel composite material and the pole piece are the same as the example 1.
Comparative example 1
Taking 20ml of graphene oxide suspension (6mg/ml), and then placing the graphene oxide suspension in a reaction kettle for hydrothermal reduction for 6 hours at the temperature of 150 ℃. And taking out the graphene hydrogel in the lining, soaking the graphene hydrogel in 20% water and ethanol solution, and freeze-drying the graphene hydrogel in a freeze dryer for 48 hours. And placing the freeze-dried graphene aerogel in a high-temperature tube furnace, and calcining for 6 hours at 650 ℃ under the protection of inert gas to obtain the self-assembled graphene aerogel. The subsequent preparation schemes of the composite material and the pole piece are the same as those of the example 1.
Test method and related test data
1. Aerogel density test: the densities of the nitrogen-doped graphene aerogels prepared in the examples and comparative examples (comparative example corresponds to graphene aerogel) were tested by a drainage method.
2. Specific surface area test: the nitrogen-doped graphene aerogels prepared in the examples and comparative examples (the comparative example corresponds to graphene aerogel) were tested for specific surface area using a full-automatic specific surface area and porosity analyzer (mic 2020).
3. Conductivity of the pole piece: the volume resistivity of the positive electrode plates prepared in the examples and the comparative examples was measured by a four-probe method.
4. And (3) morphology testing: observing the pore structures of the graphene wet gel (wet gel before freeze drying) and the nitrogen-doped graphene aerogel (comparative example corresponds to graphene aerogel) prepared in example 1 and comparative example 1 by using an SEM electron microscope;
5. the positive electrode pieces prepared in the examples and the comparative examples were manufactured into CR2032 coin cells for testing. All cells were assembled in a glove box filled with argon. The button cell takes metal lithium as a negative electrode, and the electrolyte is 1.0M LiPF6In an amount of 40. mu.L, and a septum of Celgard 2400, andand (3) a positive pole piece. And (5) standing the assembled battery for 6 hours, and then performing cycle test on the performance. The test voltage range is 1.7V-2.8V, and the cycle rate is 0.5C.
The density and specific surface area results of the graphene aerogels prepared in the different examples and comparative examples are shown in table 1:
TABLE 1
The graphene aerogel prepared by the template method has lower density and higher specific surface area, and can improve more active sites for sulfur.
The results of the volume resistivity test of the positive electrode sheets prepared in the different examples and comparative examples are shown in table 2:
TABLE 2
| Example 1 | Example 2 | Example 3 | Example 4 | Example 5 | |
| Volume resistivity of pole piece (omega/□) | 38 | 32 | 25 | 23 | 19 |
| Example 6 | Example 7 | Example 8 | Example 9 | Example 10 | |
| Volume resistivity of pole piece (omega/□) | 24 | 29 | 19 | 13 | 24 |
| Comparative example 1 | |||||
| Volume resistivity of pole piece (omega/□) | 218 |
It should be noted that the volume resistivity of the pole piece depends on two factors of the integrity and compactness of the pore structure, and the case of lower resistivity is also brought about by too low density of the aerogel. The resistivity in comparative example 1 is much higher than that in the example, in fact, because the graphene is a compact structure and does not form a network pore structure. The graphene aerogel prepared in the embodiment of the invention has a network pore structure, so the resistivity is obviously lower than that of the comparative example. Especially, in examples 1, 2, 3, 6 and 7, a more preferable soft template solvent, an auxiliary reducing agent and a more preferable formula or condition are adopted, so that the prepared graphene aerogel has a more preferable network pore structure, can maintain relatively high resistivity, and can also improve the cycle performance of the battery (see below).
Fig. 1 shows appearance photographs of the graphene wet gels prepared in example 1 and comparative example 1, wherein (a) is a photograph corresponding to comparative example 1, and (b) is a photograph corresponding to example 1; fig. 2 shows photographs of the appearances of the nitrogen-doped graphene aerogels prepared in example 1 and comparative example 1 (comparative example corresponds to graphene aerogel), where (a) is the photograph corresponding to comparative example 1, and (b) is the photograph corresponding to example 1.
Fig. 3 shows SEM photographs of the nitrogen-doped graphene aerogels prepared in example 1 and comparative example 1 (comparative example corresponds to graphene aerogel), wherein (a) and (b) correspond to photographs at different magnifications in comparative example 1, and (c) and (d) correspond to photographs at different magnifications in example 1.
The cycle performance results of the positive electrode sheets prepared in the different examples and comparative examples after being used in the lithium sulfur battery are shown in table 3:
TABLE 3
| Example 1 | Example 2 | Example 3 | Example 4 | Example 5 | |
| Capacity retention of 100 circles (%) | 83.29％ | 86.17％ | 84.28％ | 82.66％ | 83.53％ |
| 200 circles Capacity Retention (%) | 80.08％ | 82.56％ | 79.46％ | 78.25％ | 75.56％ |
| 300 circles Capacity Retention (%) | 77.56％ | 79.87％ | 77.18％ | 73.24％ | 72.18％ |
| 400-cycle capacity retention (%) | 75.38％ | 76.58％ | 75.02％ | 70.42％ | 70.02％ |
| Example 6 | Example 7 | Example 8 | Example 9 | Example 10 | |
| Capacity retention of 100 circles (%) | 89.25％ | 88.28％ | 82.28％ | 82.98％ | 81.28％ |
| 200 circles Capacity Retention (%) | 85.66％ | 82.46％ | 78.94％ | 88.46％ | 80.46％ |
| 300 circles Capacity Retention (%) | 82.12％ | 76.18％ | 74.25％ | 75.48％ | 73.24％ |
| 400-cycle capacity retention (%) | 78.03％ | 73.02％ | 72.22％ | 72.05％ | 71.00％ |
| Comparative example 1 | |||||
| Capacity retention of 100 circles (%) | 79.31％ | ||||
| 200 circles Capacity Retention (%) | 75.54％ | ||||
| 300 circles Capacity Retention (%) | 71.25％ | ||||
| 400-cycle capacity retention (%) | 65.35％ |
Test results show that the conductivity of the cathode material of the lithium-sulfur battery prepared by the method is greatly improved, and the cycle performance of the lithium-sulfur battery can be effectively improved.
The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (10)
1. The preparation method of the nitrogen-doped graphene aerogel is characterized by comprising the following steps of:
step S1, mixing the soft template solvent, the graphene oxide suspension and the emulsifier to form a graphene oxide emulsion; wherein the soft template solvent is selected from one or more of cyclohexane, normal hexane, normal butanol, dichloromethane and tetrahydrofuran;
step S2, adding an auxiliary reducing agent into the graphene oxide emulsion, and then carrying out hydrothermal reduction reaction to obtain graphene hydrogel; wherein the auxiliary reducing agent comprises ethylenediamine and/or ammonia water;
step S3, freezing and drying the graphene hydrogel to obtain graphene aerogel;
step S4, calcining the graphene aerogel in an inert atmosphere to obtain the nitrogen-doped graphene aerogel.
2. The method according to claim 1, wherein the soft template solvent comprises cyclohexane and n-butanol in a volume ratio of (5-8) to (4-6).
3. The preparation method according to claim 1, wherein in the step S1, the concentration of the graphene oxide suspension is 2 to 8mg/mL, the volume content of the soft template solvent in the graphene oxide emulsion is 45 to 78%, and the weight content of the emulsifier in the graphene oxide emulsion is 0.5 to 1.5%.
4. The method according to any one of claims 1 to 3, wherein the auxiliary reducing agent is ethylenediamine and/or aqueous ammonia, or further comprises one or more of the following substances in addition to ethylenediamine and/or aqueous ammonia: sodium bisulfite, sodium ascorbate; the emulsifier is selected from sodium dodecyl sulfate.
5. The method according to any one of claims 1 to 3, wherein in the step S2, the auxiliary reducing agent is used in an amount of 0.5 to 3 μ L per mg of graphene oxide; in the hydrothermal reduction reaction process, the reaction temperature is 120-280 ℃, and the reaction time is 2-48 h.
6. The preparation method according to any one of claims 1 to 3, wherein in the step S4, in the process of calcining the graphene aerogel, the calcining temperature is 500-700 ℃, and the calcining time is 2-6 h.
7. A nitrogen-doped graphene aerogel, which is prepared by the preparation method of any one of claims 1 to 6.
8. A lithium-sulfur battery cathode material, comprising a graphene aerogel and elemental sulfur loaded on the graphene aerogel, wherein the graphene aerogel is the nitrogen-doped graphene aerogel in claim 7 or the nitrogen-doped graphene aerogel prepared by the preparation method in any one of claims 1 to 6.
9. The preparation method of the lithium-sulfur battery cathode material as claimed in claim 8, wherein the graphene aerogel is immersed in the solution containing the elemental sulfur and dried to obtain the lithium-sulfur battery cathode material.
10. A lithium-sulfur battery comprises a positive pole piece, and is characterized in that the positive pole piece comprises a current collector and a positive active layer positioned on the surface of the current collector, the positive active layer comprises a binder and a positive material, and the positive material is the lithium-sulfur battery positive material in claim 8 or the lithium-sulfur battery positive material prepared by the preparation method in claim 9.
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