KR102579131B1 - Carbon nanotube-MOF sheet, manufacturing method thereof, and lithium-sulfur secondary battery comprising the same - Google Patents

Abstract

The present invention relates to a carbon nanotube composite sheet grown on Co-MOF-74, a manufacturing method thereof, and a lithium sulfur secondary battery containing the same. According to an embodiment of the present invention, a carbon nanotube composite sheet grown on Co-MOF-74, which can suppress the elution of polysulfide during the reaction of a lithium sulfur secondary battery, and a lithium sulfur secondary battery utilizing the same can be provided. .

Description

Carbon nanotube-MOF sheet, manufacturing method thereof, and lithium-sulfur secondary battery comprising the same {Carbon nanotube-MOF sheet, manufacturing method thereof, and lithium-sulfur secondary battery comprising the same}

The present invention relates to a carbon nanotube sheet grown with MOF that can be used as an intermediate layer in the structure of a lithium sulfur battery, a manufacturing method thereof, and a lithium sulfur secondary battery using the same.

A lithium sulfur battery is a secondary battery that is charged and discharged through an electrochemical reaction between lithium ions and sulfur during battery operation. During discharge, lithium ions that move from the lithium metal anode to the sulfur anode meet with the sulfur of the anode to form lithium polysulfide (Li ₂ Sx, x=4-8), and ultimately Li ₂ S ₂ or Li ₂ S is formed. Through this reaction, the lithium-sulfur secondary battery produces a high capacity of 1675 mAh/g and has an energy density of 2600 Wh/kg, which has improved performance compared to other secondary batteries currently used. Lithium sulfur secondary batteries with such high capacity and high energy density characteristics can be applied to various fields such as electric vehicles and large-capacity energy storage systems.

A lithium sulfur battery generally consists of a cathode, an anode, a separator, and an electrolyte.

The cathode is a material that can emit lithium ions, and lithium metal is mainly used.

The positive electrode uses a mixture of sulfur as an active material, a conductive material to supplement the electrical conductivity of sulfur, which has low electrical conductivity, and a binder material to improve the contact between these materials.

The separator prevents physical electrical contact between the cathode and anode.

The electrolyte prevents electrons from moving between the cathode and anode, but only allows ions to move.

Meanwhile, in the case of lithium sulfur batteries, there are two major problems. First, lithium polysulfide (Li ₂ Sx, x=4-8) generated during discharge dissolves in the electrolyte. As a result, active material is lost from the positive electrode, and the eluted lithium polysulfides reach the negative electrode, damaging the surface of the lithium metal negative electrode and reducing battery performance. This phenomenon of polysulfide formation and movement is called the shuttle reaction, and is considered the biggest problem in lithium sulfur batteries.

Another problem is that in the case of lithium sulfur batteries, the reaction up to Li ₂ S does not occur completely, so the actual capacity is not developed as much as the theoretical capacity. The reason is that the reaction from Li ₂ S _{2 ,} which is an insulator, to Li ₂ S is difficult.

In an attempt to solve the shuttle phenomenon, research was conducted to physically block the movement of polysulfide by producing an interlayer. The middle layer is mainly made of carbon materials, such as carbon nanotubes or reduced graphene oxide. This intermediate layer is located between the anode, the sulfur electrode, and the separator, and improves the performance of the lithium sulfur secondary battery by physically preventing the movement of polysulfide.

However, this technology has limitations in that it mainly uses carbon materials to only physically block it. To overcome these limitations, research has been conducted on materials that can chemically adsorb to lithium polysulfide. However, these materials have the property of adsorbing lithium polysulfide only on the surface, and because their surface area is small, they have a limitation in that they are not utilized efficiently compared to the amount used.

Therefore, to prevent the shuttle phenomenon, both physical and chemical solutions must be provided at the same time. Additionally, in the case of materials that chemically bond, it is necessary to develop materials that can be efficiently utilized by having a large surface area.

To solve another problem, which is that the reaction from Li ₂ S ₂ to Li ₂ S does not occur easily, previous research has shown that cobalt sulfide materials were used to promote the reaction. However, these materials have the problem that the manufacturing process is difficult. Therefore, it is necessary to develop materials that can be utilized by easily synthesizing cobalt-based materials that can promote the reaction from Li ₂ S ₂ to Li ₂ S.

Korean Patent Publication No. 10-1684645, “Method for manufacturing vanadium oxide xerogel/carbon nanocomposite, lithium-sulfur secondary battery positive electrode comprising the same, and method for manufacturing the same”

One object of the present invention is to provide an intermediate layer for a lithium sulfur secondary battery that can suppress the migration of lithium polysulfide to the negative electrode that occurs during a lithium sulfur secondary battery reaction.

Another object of the present invention is to improve the capacity of lithium sulfur secondary batteries.

Another object of the present invention is to provide a method for manufacturing carbon nanotubes including MOF that can be used as an intermediate layer for lithium sulfur secondary batteries.

The carbon nanotube-MOF sheet for lithium sulfur secondary batteries of the present invention includes a sheet independently positioned between the positive electrode and the separator in a lithium sulfur secondary battery including a positive electrode, a separator, and a negative electrode, and the sheet is attached to the carbon nanotube. It is characterized by being formed by growing a metal-organic framework (MOF).

The carbon nanotube-MOF sheet for lithium sulfur secondary batteries of the present invention is characterized in that the metal-organic framework is Co-MOF-74.

Co-MOF-74 of the present invention is characterized by a surface area of 1300 m ² /g to 1356 m ² /g.

The carbon nanotube-MOF sheet for lithium-sulfur secondary batteries of the present invention prevents the MOF from chemically adsorbing lithium polysulfide, which is produced by the reaction of sulfur and lithium during discharging of the lithium-sulfur secondary battery, and moving it to the negative electrode. It is characterized by preventing.

Co-MOF-74 of the present invention is characterized by increasing the capacity of the lithium sulfur secondary battery by promoting the reaction from Li ₂ S ₂ to Li ₂ S.

The method for producing a carbon nanotube-MOF sheet of the present invention includes the steps of dispersing multi-layered carbon nanotubes, a cobalt compound, and an organic linker in a mixed solvent to prepare a mixed solution, reacting the mixed solution, and then centrifuging. Obtaining a synthetic material, washing the synthetic material, sonicating the washed synthetic material, and filtering and drying the sonicated synthetic material to produce a carbon nanotube-MOF sheet. Includes.

The organic linker in the carbon nanotube-MOF sheet manufacturing method of the present invention is characterized by being 2,5-dihydroxyterephthalic acid.

The mixed solvent in the carbon nanotube-MOF sheet manufacturing method of the present invention includes dimethylformamide, ethanol, and distilled water, and the dimethylformamide: ethanol: The volume ratio of distilled water (DI water) is 49.5:10:0.5.

In the step of reacting the mixed solution in the method for producing a carbon nanotube-MOF sheet of the present invention, the mixed solution is performed at 60°C to 150°C for 5 to 24 hours.

The lithium sulfur secondary battery of the present invention includes a positive electrode containing sulfur, a negative electrode containing lithium metal (Lithium), an electrolyte layer located between the positive electrode and the negative electrode, and a physical layer provided within the electrolyte layer between the positive electrode and the negative electrode. It includes a separator that prevents phosphorus contact and a carbon nanotube-MOF sheet that is independently positioned between the anode and the separator and is formed by growing Co-MOF-74.

According to an embodiment of the present invention, it is possible to provide a lithium sulfur secondary battery in which an intermediate layer made using carbon nanotubes grown with Co-MOF-74 is independently positioned between the anode and the separator.

According to the present invention, in order to effectively suppress the phenomenon of lithium polysulfide moving to the cathode, lithium containing an intermediate layer grown on carbon nanotubes with Co-MOF-74, a material with a large surface area and capable of adsorbing lithium polysulfide, is grown. A sulfur secondary battery can be provided.

According to the present invention, the middle layer made only of a conventional carbon material only physically adsorbs lithium polysulfide and suppresses the movement of the polysulfide, but the middle layer containing Co-MOF-74 of the present invention has a large surface area to absorb the polysulfide. Adsorption efficiency is excellent because it physically and chemically adsorbs to suppress the movement of the polysulfide.

According to the present invention, Co-MOF-74 containing cobalt can be synthesized in a relatively simple method, and Co-MOF-74 synthesized in this way promotes the reaction from Li ₂ S ₂ to Li ₂ S, thereby producing secondary lithium sulfur. The actual capacity of the battery can be improved.

Figure 1 is a scanning electron microscope (SEM) image of a carbon nanotube sheet on which Co-MOF-74 for a lithium sulfur secondary battery was grown according to Example 1 of the present invention.
Figure 2 is an X-ray diffraction (XRD) graph according to Example 1 of the present invention.
Figure 3 is a graph showing the results of testing the lithium sulfur secondary batteries of Example 2 and Comparative Example 1 of the present invention in a current density environment of 0.5C.
Figure 4 is a graph showing the voltage profile of the lithium sulfur secondary battery of Example 2 and Comparative Example 1 of the present invention.
Figure 5 is a flow chart showing a method of manufacturing a carbon nanotube sheet on which Co-MOF-74 for a lithium sulfur secondary battery was grown according to Example 1 of the present invention.
Figure 6 schematically illustrates the structure and effects of a lithium sulfur secondary battery designed according to Example 2 of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings and the contents described in the accompanying drawings, but the present invention is not limited or limited by the embodiments.

The terminology used herein is for describing embodiments and is not intended to limit the invention. As used herein, singular forms also include plural forms, unless specifically stated otherwise in the context. As used herein, “comprises” and/or “comprising” refers to the presence of one or more other components, steps, operations and/or elements. or does not rule out addition.

As used herein, “embodiment,” “example,” “aspect,” “example,” etc. should be construed to mean that any aspect or design described is better or advantageous than other aspects or designs. It's not like that.

Additionally, the term 'or' means an inclusive OR 'inclusive or' rather than an exclusive OR 'exclusive or'. That is, unless otherwise stated or clear from the context, the expression 'x uses a or b' means any of the natural inclusive permutations.

Additionally, a part of an act, layer, area, component request, etc., is said to be "on" or "on" another part, not only when it is directly on top of the other part, but also when there are other acts, layers, areas, or components in between. Also includes cases where etc. are included.

Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings.

The metal-organic framework (MOF) according to an embodiment of the present invention is formed by cross-connecting the coordination bonds of the inorganic nodes (metal ions or metal oxide clusters) and the multitopic organic linker. It refers to a porous material that forms a primary, secondary, or three-dimensional framework.

In addition, the carbon nanotube-MOF sheet for lithium sulfur secondary batteries has the characteristic of being able to exist independently within the secondary battery, and refers to a material having a composite structure in which MOF is dispersed on the surface of carbon nanotubes.

The carbon nanotube-MOF has a sheet form and simultaneously serves as an intermediate layer in a lithium sulfur secondary battery. That is, the sheet and the intermediate layer have the same meaning.

Figure 1 is a scanning electron microscope (SEM) image of a carbon nanotube-MOF sheet for a lithium sulfur secondary battery according to Example 1 of the present invention.

Referring to Figure 1, it can be seen that MOFs, expressed as bright irregular dots in a scanning electron microscope (SEM) image, are distributed in carbon nanotubes expressed as bright lines in the image.

The MOF represented by the bright irregular dot in Figure 1 is Co-MOF-74. Co-MOF-74 means that the metal portion of the MOF is cobalt.

Figure 1 (a) is an image of a carbon nanotube-MOF sheet for a lithium sulfur secondary battery according to an embodiment of the present invention observed through a scanning electron microscope at a magnification of 25,000 times.

Figure 1(b) is an image of the carbon nanotube-MOF sheet observed through the equipment at a magnification of 50,000 times. The scale at the bottom shows that the width of the minor axis of the MOF is approximately 800 nm.

The middle layer of existing carbon materials serves to inhibit the movement of lithium polysulfide generated during the lithium-sulfur battery reaction. MOF (Metal-Organic Framework) is a porous material with a skeletal structure formed by linking metal ions and organic molecules. It has a much larger surface area than other nanocompounds due to its numerous pores, and the presence of lithium polysulfide on the surface and inside of the MOF also increases. Adsorption occurs. For reference, the surface area of Co-MOF-74 has a value of 1300 m ² /g to 1356 m ² /g. Therefore, when carbon nanotubes grown with MOF are used in lithium sulfur secondary batteries, the adsorption efficiency can be considered to be superior to that of using only existing carbon materials.

In addition, the middle layer of existing carbon materials has the limitation of only physically blocking lithium polysulfide generated during the lithium-sulfur battery reaction. However, carbon nanotube-MOF sheets can adsorb lithium polysulfide physically and chemically.

First, MOF physically adsorbs polysulfide through its porous structure. And MOF chemically bonds with polysulfide through a Lewis acid-base reaction. This is due to the characteristics of the metal ion portion of the MOF, which is a Lewis acid, and polysulfide, which is a Lewis base.

The carbon nanotube-MOF sheet is in the form of an independent sheet and is located between the anode and the separator, serving as an intermediate layer and preventing lithium polysulfide from moving to the cathode. The sheet exists as a separate sheet without being contacted or coated with other materials, which has the advantage of making it easy to replace the middle layer in the secondary battery. In addition, by separating the sheet manufacturing process and the electrode manufacturing process, it has the advantage of making it easy to modify the process in sheet manufacturing or electrode manufacturing later.

Figure 5 is a flowchart showing a method of manufacturing a carbon nanotube sheet on which Co-MOF-74 for lithium sulfur secondary batteries was grown according to an embodiment of the present invention.

Referring to Figure 5, the method for producing a carbon nanotube-MOF sheet for a lithium sulfur secondary battery according to Example 1 of the present invention is to prepare a mixed solution by dispersing multilayer carbon nanotubes, a metal compound, and an organic connector in a mixed solvent. Step (S100), reacting the mixed solution and then centrifuging to obtain a synthetic material (S200), washing the synthetic material (S300), sonicating the synthetic material (S400), and filtering the synthetic material. and drying to form a carbon nanotube-MOF sheet (S500).

In the method for producing a carbon nanotube-MOF sheet for a lithium sulfur secondary battery according to Example 1 of the present invention, the step (S100) of preparing a mixed solution by dispersing the multilayer carbon nanotubes, a metal compound, and an organic linker in a mixed solvent is 2. , 5-Dihydroxyterephthalic acid (2,5-Dihydroxyterephthalic acid) is used as an organic linker to mix multi-layered carbon nanotubes and metal compounds and disperse them in a mixed solvent.

The multi-layered carbon nanotubes have activated carboxyl groups. Here, the negative (-) carboxyl group attracts positively ionized cobalt, creating a site where MOF will be formed.

In this case, the metal compound is a cobalt compound, and the cobalt compound may be cobalt(II) nitrate hexahydrate. The cobalt(II) nitric acid hexahydrate has the advantage that cobalt ions are easily separated when ionized in a solvent.

In addition, the Organic Linker includes 2,5-Dihydroxyterephthalic acid, 2-methylimidazole, 3Hydroxypicolinic acid, and polycarboxylic acid. It may be a rate-based material.

Additionally, the mixed solvent includes dimethylformamide, ethanol, and distilled water. At this time, each solvent can be mixed at a volume ratio of 49:10:1 to 49.5:10:0.5, preferably at a volume ratio of 49.5:10:0.5. The size of the MOF varies depending on the solvent ratio, and when mixed at the volume ratio of 49.5:10:0.5, the pore size of the MOF can be manufactured with the smallest size. This increases the specific surface area and improves the adsorption rate.

In the step (S200) of reacting the mixed solution and then centrifuging to obtain a synthetic material, the mixed solution is reacted at 60°C to 150°C for 5 to 24 hours using autoclave equipment. Preferably, the reaction proceeds at 120°C for 24 hours. Then, centrifugation is performed at 3000 rpm for 10 minutes using a centrifuge to obtain carbon nanotubes on which the synthetic material Co-MOF-74 has grown.

In the step of washing the synthetic material (S300), the synthetic material is washed with dimethylformamide using a centrifuge, and then washed with methanol using a centrifuge. The remaining cobalt ions and organic linkers that failed to react are removed.

In the method of manufacturing a carbon nanotube-MOF sheet for a lithium-sulfur secondary battery according to Example 2 of the present invention, the step (S400) of ultrasonically treating a synthetic material involves adding the synthetic material to methanol and using an ultrasonicator. The synthetic material is dispersed by ultrasonication for 20 minutes. At this time, due to the high reactivity and structural instability of MOF, there is a risk of deformation during general storage, so it is stored by dispersing it in methanol.

In the step of filtering and drying the synthetic material to form a carbon nanotube-MOF sheet (S500), the mixed solution prepared in the ultrasonic treatment step (S400) is vacuum filtered, washed, and dried at room temperature to form a carbon nanotube-MOF. This is the step of forming a sheet. Washing is performed to remove foreign substances that may occur during the filtration step. If dried at high temperature during the drying stage, the carbon nanotube-MOF sheet on the filter paper may curl, so dry slowly at room temperature. Drying is carried out for 12 to 16 hours.

At this time, the pore size of the MOF averages 1.6 nm, and in one example, the pore size of the MOF was made small and dense by adjusting the solvent ratio during the production of the MOF, which increases the surface area and improves the catalytic function.

The weight ratio of components in the carbon nanotube-MOF sheet is carbon nanotube:MOF = 35:65. Because the ratio of MOF is high compared to carbon nanotubes, the adsorption of polysulfide and reduction of Li ₂ S ₂ in the battery can be more effective.

A vacuum filter has a structure formed of an upper space and a lower space bordering the filter membrane. For vacuum filter treatment, the mixed solution is injected into the upper space of the vacuum filter device, and the vacuum pump connected to the lower space is operated to move the substances that pass through the filter membrane in the solution to the lower space, and the substances that do not pass are stored in the upper part of the filter membrane. This is the process of forming a sheet by leaving the remaining material.

Hereinafter, the present invention will be described in more detail through examples and comparative examples. These examples are for illustrating the present invention in more detail, and the scope of the present invention is not limited by these examples.

[Example 1] Production of carbon nanotube sheet grown on Co-MOF-74

0.05g of multiwalled carbon nanotube with activated carboxyl group, 0.49mmol of cobalt(II) nitrate hexahydrate, and 2,5-dihydroxyterephthalic acid (2, Prepare a mixed solution by dispersing 0.15 mmol of 5-Dihydroxyterephthalic acid) in a solvent containing dimethylformamide, ethanol, and distilled water in a volume ratio of 49.5:10:0.5, respectively.

The mixed solution is reacted at 120°C for 24 hours using autoclave equipment.

Thereafter, the mixed solution is centrifuged at 3000 rpm for 10 minutes to obtain a synthesized material.

The synthesized material is washed three times with dimethylformamide using a centrifuge. Afterwards, the product is washed five times with methanol using a centrifuge.

Disperse the carbon nanotubes on which Co-MOF-74 was grown in 10 mL of methanol to 3 mg. Afterwards, ultrasonication is performed for 20 minutes.

10 ml of the sonicated solution is introduced into a vacuum filter device and then subjected to a vacuum filter process to form a carbon nanotube sheet. Afterwards, 10 ml of methanol is injected at a time and the vacuum filter process is repeated to perform the cleaning process three times.

After the washing process was completed, the carbon nanotube composite sheet on which Co-MOF-74 was grown was dried at room temperature, and a carbon nanotube sheet on which Co-MOF-74 was grown was synthesized.

Figure 2 is an X-ray diffraction (XRD) graph according to Example 1 of the present invention.

The first figure is an X-ray diffraction graph of Example 1 (Co-MOF-74 @ MWCNT), the second figure is an X-ray diffraction graph of MWCNT (Multiwalled carbon nanotube), and the third figure is an am. Since the peak of the second graph and the peak of the third graph both appear on the first graph, it can be seen that the synthesis of the carbon nanotube sheet on which Co-MOF-74 was grown was carried out as designed.

In this way, when the carbon nanotube sheet on which Co-MOF-74 grown according to an embodiment of the present invention is used as an intermediate layer in a lithium sulfur secondary battery, the active material is lost from the positive electrode by chemically/physically adsorbing lithium polysulfide. It can prevent developed and eluted lithium polysulfide from reaching the negative electrode, damaging the surface of the lithium metal negative electrode and deteriorating battery performance.

[Example 2] Manufacturing a lithium sulfur secondary battery using a carbon nanotube sheet grown on Co-MOF-74 as an intermediate layer

In order to use the carbon nanotube composite sheet on which Co-MOF-74 synthesized in [Example 1] was grown as an intermediate layer, a lithium sulfur secondary battery having the structure of FIG. 6 was manufactured.

The sulfur anode is made of a slurry using sulfur, Super P conductive material, PVDF (Polyvinylidene Fluoride) binder, and NMP (N-methyl-2-pyrrolidone) solvent with a mass ratio of 6:3:1. , Tape casting is performed on aluminum foil with a thickness of 100 μm. Afterwards, it is dried in a vacuum oven environment at 50°C. [Example 1] was placed on the sulfur anode prepared in this way, a polyimide separator was placed on it, and a lithium metal anode was placed on it to manufacture a lithium sulfur secondary battery.

[Comparative Example 1] Manufacturing a lithium sulfur secondary battery without an intermediate layer

The lithium sulfur secondary battery of [Comparative Example 1] was manufactured in the same manner as [Example 2] except that the carbon nanotube composite sheet on which Co-MOF-74 was grown was not placed in the lithium sulfur secondary battery of [Example 2]. did.

Figure 3 is a graph showing the results of testing the lithium sulfur secondary batteries of Example 2 and Comparative Example 1 of the present invention in a current density environment of 0.5C.

Example 2 is a lithium sulfur secondary battery inserting a carbon nanotube sheet to which Co-MOF-74 is added, and Comparative Example 1 is a lithium sulfur secondary battery inserting a carbon nanotube sheet to which no intermediate layer is added. Additionally, in the graph of FIG. 3, an empty circle represents charging capacity, and a filled circle represents discharging capacity. The red circle represents Example 2, and the black circle represents Comparative Example 1.

Referring to FIG. 3, the capacity of the lithium sulfur secondary battery of Example 2 at 5 cycles (excluding the initial stabilization stage) is 1359 mAh/g, and the capacity at 50 cycles is 1195 mAh/g, an increase of about 12%. Shows capacity reduction rate. On the other hand, the lithium sulfur secondary battery of Comparative Example 1 had a capacity of 845 mAh/g at 5 cycles excluding the initial stabilization stage, and a capacity of 542 mAh/g at 50 cycles, showing a capacity reduction rate of about 36%.

In conclusion, when using the carbon nanotube sheet to which Co-MOF-74 was added according to Example 2 of the present invention, the capacity reduction rate at 50 cycles compared to the capacity at 5 cycles was smaller than that in Comparative Example 1. It can be seen that in Example 2 of the present invention, the effect of suppressing the movement of lithium polysulfide to the negative electrode is improved, which leads to improved battery performance.

Additionally, comparing the capacities developed in 10 cycles, Example 2 shows 1297 mAh/g and Comparative Example 1 shows 771 mAh/g. This is an increase of about 68% in the capacity of Example 2 compared to the capacity of Comparative Example 1. In other words, it can be seen that when a carbon nanotube sheet to which Co-MOF-74 is added is used, the capacity of the lithium sulfur secondary battery increases compared to the case where Co-MOF-74 is not added.

Figure 4 is a graph showing the voltage profile of the lithium sulfur secondary battery of Example 2 and Comparative Example 1 of the present invention.

In the graph of FIG. 4, the blue dotted box is the section where the reaction from Li ₂ S ₂ to Li ₂ S occurs. The red line means Example 2, and the black line means Comparative Example 1.

Referring to the graph in the blue dotted box of FIG. 4, the graph of Comparative Example 1 has a steep slope and the voltage decreases to 0. This means that the reaction from Li ₂ S ₂ to Li ₂ S does not occur easily and the reaction is terminated. In this case, the battery of Comparative Example 1 has a capacity of 800 mAh/g or less. On the other hand, the battery of Example 2 shows a relatively gentle slope in the graph and the voltage decreases. This means that the reaction from Li ₂ S ₂ to Li ₂ S was promoted and the reaction occurred relatively more, and the battery at this time had a capacity of about 1300 mAh/g.

In other words, when using a carbon nanotube sheet to which Co-MOF-74 is added, cobalt (Co) on the sheet promotes the reaction from Li ₂ S ₂ to Li ₂ S, thereby increasing the capacity of the lithium sulfur secondary battery compared to the carbon nanotube sheet. It can be seen that it increased compared to when only 1,000 mAh was used, and was closer to the theoretical capacity of 1675 mAh/g.

Figure 6 schematically illustrates the structure and effects of a lithium sulfur secondary battery designed according to Example 2 of the present invention.

Referring to FIG. 6, the lithium sulfur secondary battery using the carbon nanotube-MOF sheet according to Example 2 of the present invention includes a negative electrode containing lithium metal, a positive electrode containing sulfur, and the negative electrode and the positive electrode. It has a structure that includes a separator that is located between the anode and the cathode to prevent physical contact between the anode and the cathode, and a carbon nanotube sheet on which Co-MOF-74 is grown, which is located independently between the separator and the anode.

At this time, referring to the arrow in FIG. 6, when discharging a lithium-sulfur secondary battery, lithium polysulfide (Li ₂ Sx, It can be confirmed that it is suppressed from reaching.

When using only conventional carbon materials as an interlayer, there is a limitation in that it only physically blocks lithium polysulfide, and even when using a material that can chemically adsorb lithium polysulfide, adsorption occurs only on the surface of the material. There is a limit. However, when MOF, a porous material, is introduced into a carbon nanotube sheet as in the embodiment of the present invention, physical and chemical adsorption proceeds using a large surface area, making it possible to effectively hold lithium polysulfide.

As a result, by placing a carbon nanotube-MOF sheet between the anode and the separator in a lithium-sulfur secondary battery, the shuttle phenomenon can be further reduced compared to a simple carbon material intermediate layer. In addition, the performance of the battery can be improved by preventing the loss of the active material of the positive electrode and damage to the surface of the negative electrode.

Additionally, the arrow in FIG. 6 indicates that the reaction from Li ₂ S ₂ to Li ₂ S is promoted.

The reactions that occur in lithium sulfur secondary batteries are as follows. In the first step, S ₈ is reduced to Li ₂ S ₄ through a stepwise reaction of lithium polysulfide, which provides 25% of the theoretical capacity of the cell. This reaction involves a series of soluble polysulfides and is a solution-mediated reaction-dominant, fast-dynamic step.

The relatively low dynamic liquid-solid reaction in which Li ₂ S ₄ is further reduced to Li ₂ S ₂ in the second step reduces the theoretical capacity of the cell by 25% due to the high energy barrier for solid Li ₂ S ₂ nucleation. % is provided.

The final step, the solid-solid conversion reaction from Li ₂ S ₂ to Li ₂ S, is a rate-controlling step and provides 50% of the theoretical capacity of the battery. However, at this stage, interconversion is significantly reduced due to slowing of solid-state diffusion, increasing resistance, which causes a sharp decrease in discharge voltage, premature termination of discharge reaction, and lower sulfur utilization, resulting in a gap between the theoretical capacity of the battery and the actual developed capacity. A large deviation occurs.

At this time, when cobalt is used in a lithium-sulfur secondary battery, the reversible specific capacity increases and soluble polysulfide cannot be formed during the charging/discharging process, so excellent cycling performance can be achieved. In other words, cobalt not only acts as an effective polysulfide immobilizer through chemical adsorption to limit the diffusion of polysulfide, but also acts as a catalyst for the conversion from Li ₂ S ₂ to LiS ₂ . Therefore, the capacity development of the lithium sulfur secondary battery can be developed close to the theoretical capacity.

The carbon nanotube (Co-MOF-74 @ MWCNT) grown from the Co-MOF-74 is an independent sheet that does not contact the separator and serves as an intermediate layer in a lithium sulfur secondary battery. Having the form of an independent sheet has the effect of making it easy to replace the sheet and modifying the sheet manufacturing and electrode manufacturing processes.

According to the present invention, in order to physically and chemically prevent the movement of lithium polysulfide, a carbon nanotube sheet on which MOF is grown is independently placed between the anode and the separator.

In this way, a lithium sulfur secondary battery using a carbon nanotube-MOF sheet as an intermediate layer can effectively suppress the movement of lithium polysulfide compared to batteries using only existing carbon materials, and a MOF made of cobalt can inhibit the reduction reaction of Li ₂ S ₂ promotes As a result, the performance and capacity of lithium sulfur secondary batteries can be improved.

As described above, although the present invention has been described with reference to limited embodiments and drawings, the present invention is not limited to the above embodiments, and various modifications and variations can be made from these descriptions by those skilled in the art. This is possible. Therefore, the scope of the present invention should not be limited to the described embodiments, but should be determined by the claims and equivalents thereof as well as the claims described later.

Claims (10)

anode; separation membrane; And in a lithium sulfur secondary battery including a negative electrode,
Includes a sheet independently positioned between the anode and the separator,
The sheet is formed by growing a metal-organic framework (MOF) on carbon nanotubes,
A carbon nanotube-MOF sheet for a lithium sulfur secondary battery, wherein the metal-organic framework is Co-MOF-74.
delete According to paragraph 1,
A carbon nanotube-MOF sheet for a lithium sulfur secondary battery, characterized in that the surface area of the Co-MOF-74 is 1300 m ² /g to 1356 m ² /g.
According to paragraph 1,
The sheet is carbon for a lithium-sulfur secondary battery, wherein the MOF chemically adsorbs lithium polysulfide, which is generated by the reaction of sulfur and lithium during discharging of the lithium-sulfur secondary battery, and prevents it from moving to the negative electrode. Nanotube-MOF sheets.
According to paragraph 1,
The Co-MOF-74 is a carbon nanotube-MOF sheet for a lithium sulfur secondary battery, characterized in that it increases the capacity of the lithium sulfur secondary battery by promoting the reaction from Li ₂ S ₂ to Li ₂ S.
Preparing a mixed solution by dispersing multi-layered carbon nanotubes, a cobalt compound, and an organic linker in a mixed solvent;
reacting the mixed solution and centrifuging it to obtain a synthetic material;
washing the synthetic material;
Sonicating and dispersing the washed synthetic material in methanol; and
A method for producing a carbon nanotube-MOF sheet for a lithium sulfur secondary battery, comprising filtering and drying the ultrasonic treated synthetic material to produce a carbon nanotube-MOF sheet.
According to clause 6,
A method for producing a carbon nanotube-MOF sheet for a lithium sulfur secondary battery, wherein the organic linker is 2,5-dihydroxyterephthalic acid.
According to clause 6,
The mixed solvent includes dimethylformamide, ethanol, and distilled water, and the volume ratio of dimethylformamide:ethanol:distilled water is 49:10:1 to 49.5:10:0.5. Method for producing a carbon nanotube-MOF sheet for lithium sulfur secondary batteries.
According to clause 6,
In the step of reacting the mixed solution, a method of producing a carbon nanotube-MOF sheet for a lithium sulfur secondary battery, characterized in that the mixed solution is carried out at 60°C to 150°C for 5 to 24 hours.
Anode containing sulfur;
A negative electrode containing lithium metal (Lithium);
An electrolyte layer located between the anode and the cathode;
A separator provided in the electrolyte layer to prevent physical contact between the anode and the cathode; and
A lithium sulfur secondary battery that is independently positioned between the positive electrode and the separator and includes a carbon nanotube-MOF sheet formed by growing Co-MOF-74.